**Applications**

**Chapter 9**

Provisional chapter

**Convective Transport Characteristics of Nanofluids in**

DOI: 10.5772/intechopen.72291

Metal foams can be well used as ideal materials for various efficient heat transfer devices due to light weight, high specific, and high thermal conductivity. Nanofluids have higher thermal conductivities than traditional fluid, so it can be used as an efficient heat transfer characteristics medium. This paper focuses on heat transfer of nanofluid, metal foam and the combination of the two. The physical properties of nanofluid and metal foam are summarized. The characteristics of flow and heat transfer are introduced. This work creates a close connection between scientific research and practical applications of

Keywords: metal foam, nanofluid, heat transfer, forced convection, natural convection,

Metal foam owns the advantages of light weight, high specific surface area, high thermal conductivity and relatively high permeability. Owing to recent advances in manufacturing technologies, metal foam becomes commercially available. The metal foam can be well used as an ideal material for manufacturing efficient heat transfer devices: heat exchangers, heat sinks, solar collectors and catalyst reformers. The practical structure of metal foam (copper) is shown in Figure 1 and the schematic diagram of convective heat transfer through metal foam is shown in Figure 2. From Figures 1 and 2, the high specific surface area of metal foam, and lots of pores can be found in metal foams. When fluid flowing through metal foam, the high specific surface area can provide a large surface area to the heat transfer. So the metal foam is a very good extended surface of heat transfer. Nanofluid is a special kind of engineered colloids made of a base fluid and nanoparticles (1–100 nm). Nanofluid owns the higher thermal conductivity and single-phase heat transfer coefficient than the base fluid does, so it can be

> © The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

Convective Transport Characteristics of Nanofluids

**Light-Weight Metal Foams with High Porosity**

in Light-Weight Metal Foams with High Porosity

Huijin Xu, Zhanbin Xing, Fuqiang Wang and

Huijin Xu, Zhanbin Xing, Fuqiang Wang and

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

this dual heat transfer enhancement method.

http://dx.doi.org/10.5772/intechopen.72291

Changying Zhao

Changying Zhao

Abstract

phase change

1. Introduction

#### **Convective Transport Characteristics of Nanofluids in Light-Weight Metal Foams with High Porosity** Convective Transport Characteristics of Nanofluids in Light-Weight Metal Foams with High Porosity

DOI: 10.5772/intechopen.72291

Huijin Xu, Zhanbin Xing, Fuqiang Wang and Changying Zhao Huijin Xu, Zhanbin Xing, Fuqiang Wang and Changying Zhao

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.72291

#### Abstract

Metal foams can be well used as ideal materials for various efficient heat transfer devices due to light weight, high specific, and high thermal conductivity. Nanofluids have higher thermal conductivities than traditional fluid, so it can be used as an efficient heat transfer characteristics medium. This paper focuses on heat transfer of nanofluid, metal foam and the combination of the two. The physical properties of nanofluid and metal foam are summarized. The characteristics of flow and heat transfer are introduced. This work creates a close connection between scientific research and practical applications of this dual heat transfer enhancement method.

Keywords: metal foam, nanofluid, heat transfer, forced convection, natural convection, phase change

#### 1. Introduction

Metal foam owns the advantages of light weight, high specific surface area, high thermal conductivity and relatively high permeability. Owing to recent advances in manufacturing technologies, metal foam becomes commercially available. The metal foam can be well used as an ideal material for manufacturing efficient heat transfer devices: heat exchangers, heat sinks, solar collectors and catalyst reformers. The practical structure of metal foam (copper) is shown in Figure 1 and the schematic diagram of convective heat transfer through metal foam is shown in Figure 2. From Figures 1 and 2, the high specific surface area of metal foam, and lots of pores can be found in metal foams. When fluid flowing through metal foam, the high specific surface area can provide a large surface area to the heat transfer. So the metal foam is a very good extended surface of heat transfer. Nanofluid is a special kind of engineered colloids made of a base fluid and nanoparticles (1–100 nm). Nanofluid owns the higher thermal conductivity and single-phase heat transfer coefficient than the base fluid does, so it can be

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

distribution, and eproduction in any medium, provided the original work is properly cited.

Figure 1. The structure of convective heat transfer through metal foams.

In this chapter, flow and thermal transport of nanofluids in metal foams are presented. The recent advances for metal foams, nanofluids, and the combination of nanofluids and metal foams are reviewed. The performance of forced convection, natural convection, and phase change heat transfer of nanofluids in metal foams are analyzed. The contents and the brief introduction for this chapter are shown in the following. Although there is great application potential of nanofluids in thermal science, little attention has been paid to the effect of nonuniform nanoparticle concentration on convective heat transfer of nanofluid in metal foam based on local thermal non-equilibrium (LTNE) model. Hence, laminar convective heat transfer of nanofluid in metal foam with fully developed hydraulic and thermal fields is discussed. The

Convective Transport Characteristics of Nanofluids in Light-Weight Metal Foams with High Porosity

http://dx.doi.org/10.5772/intechopen.72291

149

flow and heat transfer characteristics of nanofluids to this case are discussed as well.

Over the last several decades, flow and heat transfer in metal foam have been studied experimentally, theoretically or numerically by many scholars. In this section, the properties and characteristics of metallic porous media are firstly presented and the recent research progress

2. Thermal transport in metal foams

Figure 3. Nanofluid.

Figure 4. Nanoparticles.

on thermal transport in porous media is reviewed.

Figure 2. The schematic of convective heat transfer through metal foams.

used as an efficient heat transfer medium. The micrograph of the nanofluid is shown in Figures 3 and 4 shows the schematic diagram of nanoparticles. Nanoparticle is very small and microscopic effects are very obvious. With the addition of nanoparticles, the physical properties of the nanofluid are changed, which made the nanofluid beneficial to heat transfer and attracts widespread attention of scholars. A tremendous number of investigations on the nanofluid can be found in literatures. Furthermore, a lot of experimental researches were conducted on the convective heat transfer of nanofluids, most of which showed that the nanofluid is able to enhance the convective heat transfer. The advantage of the nanofluid and that of the porous foam can be combined as one to further enhance the heat transfer of thermal equipment. For nanofluids flowing through metal foams, some studies have been reported.

Convective Transport Characteristics of Nanofluids in Light-Weight Metal Foams with High Porosity http://dx.doi.org/10.5772/intechopen.72291 149

Figure 3. Nanofluid.

Figure 4. Nanoparticles.

used as an efficient heat transfer medium. The micrograph of the nanofluid is shown in Figures 3 and 4 shows the schematic diagram of nanoparticles. Nanoparticle is very small and microscopic effects are very obvious. With the addition of nanoparticles, the physical properties of the nanofluid are changed, which made the nanofluid beneficial to heat transfer and attracts widespread attention of scholars. A tremendous number of investigations on the nanofluid can be found in literatures. Furthermore, a lot of experimental researches were conducted on the convective heat transfer of nanofluids, most of which showed that the nanofluid is able to enhance the convective heat transfer. The advantage of the nanofluid and that of the porous foam can be combined as one to further enhance the heat transfer of thermal equipment. For nanofluids flowing through metal foams, some studies have been reported.

Figure 1. The structure of convective heat transfer through metal foams.

148 Novel Nanomaterials - Synthesis and Applications

Figure 2. The schematic of convective heat transfer through metal foams.

In this chapter, flow and thermal transport of nanofluids in metal foams are presented. The recent advances for metal foams, nanofluids, and the combination of nanofluids and metal foams are reviewed. The performance of forced convection, natural convection, and phase change heat transfer of nanofluids in metal foams are analyzed. The contents and the brief introduction for this chapter are shown in the following. Although there is great application potential of nanofluids in thermal science, little attention has been paid to the effect of nonuniform nanoparticle concentration on convective heat transfer of nanofluid in metal foam based on local thermal non-equilibrium (LTNE) model. Hence, laminar convective heat transfer of nanofluid in metal foam with fully developed hydraulic and thermal fields is discussed. The flow and heat transfer characteristics of nanofluids to this case are discussed as well.

#### 2. Thermal transport in metal foams

Over the last several decades, flow and heat transfer in metal foam have been studied experimentally, theoretically or numerically by many scholars. In this section, the properties and characteristics of metallic porous media are firstly presented and the recent research progress on thermal transport in porous media is reviewed.

#### 2.1. Pressure drop and permeability

The Darcy model is the first model to describe the percolation theory of porous media. In 1856, Darcy proposed a linear relationship between seepage velocity and pressure drop. Although the theory is simple and easy to understand, but it is very limited. Forchheimer modified the Darcy model by adding the inertia terms associated with the velocity square the equation, but still cannot be applied to the turbulent region. Brinkman considered the effective viscous dissipation term and modified the Darcy model, and found that the results are closer to the molecular diffusion at large porosity [1, 2]. In 1952, Ergun [3] proposed the empirical formula for permeability of porous medium:

$$K = \frac{d\_t^2 \varepsilon^3}{150(1 - \varepsilon)^2} \tag{1}$$

porous media immersion heat transfer. After that, many scholars studied the effective thermal conductivity of porous media. Most studies of effective thermal conductivity are mainly focu-

Convective Transport Characteristics of Nanofluids in Light-Weight Metal Foams with High Porosity

The above model is mainly concentrated on sand, cylindrical, spherical packing bed and fiber insulation blanket, but the estimation of the effective thermal conductivity of metal foam has a large deviation from the experimental result [11]. Calmidi and Mahajan [4] respectively measured the effective thermal conductivity of ERG aluminum foam at the low temperature (ignoring radiation heat transfer) by using air and water as the flow phase. Boomsma and Poulikakos [12] proposed an efficient thermal conductivity model for predicting the three-dimensional ideal cellular structure of metal foam, which is in good agreement with the experimental data of Calmidi and Mahajan [4]. Bhattacharya and Mahajan [5] used the circular cylinder model to modify the Calmidi's correlation. Hadim and North [13] generalized the correlation coefficient of the thermal conductivity model proposed by Wakao et al. [14], and make it applicable to calculate the effective thermal diffusivity and the stagnation thermal conductivity of sintered porous media. The formulas

Time Researcher Empirical formula Equation numbers

<sup>þ</sup> ð Þ <sup>1</sup>�<sup>r</sup> <sup>b</sup> L kf <sup>þ</sup> <sup>b</sup> ð Þ<sup>L</sup> <sup>2</sup> <sup>3</sup>ð Þ �kf þ

> 3 p

kf ¼ αlkl þ αvkv (11)

ffiffi 2 p ð Þ �2<sup>e</sup> <sup>2</sup> <sup>2</sup>πλ<sup>2</sup> <sup>1</sup>�2<sup>e</sup> ffiffi 2

um

um

� �kf (15)

� �kf (16)

h i � � , r ¼ 0:09

<sup>e</sup><sup>2</sup> þ

ffi 3 p <sup>2</sup> �t=L kf � � � � �<sup>1</sup> (13)

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffi 2 <sup>p</sup> <sup>2</sup>�<sup>5</sup> ffi 2 p <sup>8</sup> e3�2ε � � <sup>π</sup> <sup>3</sup>�<sup>4</sup> ffiffi 2 <sup>p</sup> ð Þ <sup>e</sup>�<sup>e</sup>

<sup>p</sup> ð Þ � ��<sup>1</sup>

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

r h i � �

<sup>2</sup>e2þλπð Þ <sup>1</sup>�<sup>e</sup> <sup>þ</sup> <sup>3</sup>e�2<sup>λ</sup>

s

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi <sup>3</sup>þð Þ <sup>1</sup>�<sup>ε</sup> ffiffi 3 p ð Þ �<sup>5</sup>

ð Þ ke <sup>y</sup> <sup>¼</sup> <sup>ε</sup>kf <sup>þ</sup> ð Þ <sup>1</sup> � <sup>ε</sup> ks <sup>þ</sup> <sup>0</sup>:1PrRed <sup>u</sup>

ffi 3 p <sup>2</sup> �<sup>b</sup> L kf <sup>þ</sup> <sup>b</sup> ð Þ<sup>L</sup> <sup>4</sup><sup>r</sup> 3 ffi 3 <sup>p</sup> ð Þ �kf

(10)

(12)

(14)

� � � � �<sup>1</sup> (8)

3 <sup>p</sup> <sup>r</sup> <sup>b</sup> ð Þ<sup>L</sup> kf <sup>þ</sup> <sup>1</sup>þ<sup>b</sup> ð Þ<sup>L</sup> kf 3

> <sup>r</sup>2<sup>þ</sup> <sup>2</sup>ffi 3 <sup>p</sup> ð Þ <sup>1</sup>�<sup>ε</sup> <sup>2</sup>�<sup>r</sup> <sup>1</sup><sup>þ</sup> <sup>4</sup>ffi

2 <sup>3</sup> <sup>2</sup>�<sup>r</sup> <sup>1</sup><sup>þ</sup> <sup>4</sup>ffi 3 p

kse <sup>¼</sup> <sup>2</sup>ffiffi 3 <sup>p</sup> <sup>r</sup> <sup>b</sup> ð Þ<sup>L</sup> <sup>1</sup>þ<sup>b</sup> ð Þ<sup>L</sup> ks 3 <sup>þ</sup> ð Þ <sup>1</sup>�<sup>r</sup> <sup>b</sup> L 2 <sup>3</sup> <sup>b</sup> ð Þ<sup>L</sup> ks þ ffi 3 p <sup>2</sup> �<sup>b</sup> L 4r 3 ffi 3 <sup>p</sup> <sup>b</sup> ð Þ<sup>L</sup> ks � � � � �<sup>1</sup> (9)

ks <sup>¼</sup> <sup>1</sup>ffiffi 2 <sup>p</sup> <sup>4</sup><sup>λ</sup>

t <sup>L</sup> <sup>¼</sup> � ffiffi 3 <sup>p</sup> �

1982 Wakao [14] ð Þ ke <sup>x</sup> <sup>¼</sup> <sup>ε</sup>kf <sup>þ</sup> ð Þ <sup>1</sup> � <sup>ε</sup> ks <sup>þ</sup> <sup>0</sup>:5PrRed <sup>u</sup>

Table 2. Models for predicting effective thermal conductivity of porous media.

e ¼ 0:339, λ ¼

3 <sup>p</sup> <sup>t</sup>=<sup>L</sup> kf þð Þ ð Þ ks�kf <sup>=</sup><sup>3</sup> <sup>þ</sup>

> p <sup>1</sup><sup>þ</sup> <sup>1</sup>ffi 3 <sup>p</sup> �8=3

b <sup>L</sup> ¼ �rþ

ke ¼ εkf þ ð Þ 1 � ε ks (7)

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151

sed on the volume fraction of each component:

were given in Table 2.

1992 Calmidi and Mahajan [4] kfe <sup>¼</sup> <sup>2</sup>ffiffi

2001 Boomsma and Poulikakos [12] kec

2002 Bhattacharya and Mahajan [5] kfe <sup>¼</sup> <sup>2</sup>ffiffi

In 1998, Calmidi and Mahajan [4] proposed the empirical formula of the metal foam permeability based on the experiment:

$$\frac{K}{d\_p^2} = 0.00073(1 - \varepsilon)^{-0.224} \left( \frac{1.18}{1 - e^{-(1 - \varepsilon)/0.04}} \sqrt{\frac{1 - \varepsilon}{3\pi}} \right)^{-1.11}.\tag{2}$$

Bhattacharya and Mahajan [5] and Plessis et al. [6] proposed an empirical formula of the permeability coefficient and the inertia coefficient by using a foam sample with a pore size of 45–100 PPI and a porosity of 0.973–0.978, and employed water and glycerol as the liquid phase. Many scholars studied the pressure drop and the permeability of the flow in porous media, and the formulas were given in Table 1.

#### 2.2. Effective thermal conductivity

The effective thermal conductivity is of great significance for the study of heat transfer mechanism in porous media. Maxwell [10] firstly studied the effective thermal conductivity of


Table 1. Models for predicting pressure drop and permeability of flow in porous media.

porous media immersion heat transfer. After that, many scholars studied the effective thermal conductivity of porous media. Most studies of effective thermal conductivity are mainly focused on the volume fraction of each component:

2.1. Pressure drop and permeability

150 Novel Nanomaterials - Synthesis and Applications

for permeability of porous medium:

ability based on the experiment:

K d2 p

media, and the formulas were given in Table 1.

dl <sup>2</sup> <sup>¼</sup> <sup>ε</sup><sup>2</sup>

2002 Fourie and Plessis [9] <sup>f</sup> Fourie <sup>¼</sup> ð Þ <sup>3</sup> � <sup>χ</sup> ð Þ <sup>χ</sup> � <sup>1</sup> CDχ<sup>1</sup>:<sup>5</sup>

K p <sup>L</sup>ru<sup>2</sup> <sup>¼</sup> <sup>f</sup> <sup>¼</sup> <sup>1</sup>

Redp

f ¼ 0:22, Redp

Table 1. Models for predicting pressure drop and permeability of flow in porous media.

2.2. Effective thermal conductivity

1994 Plessis et al. [6] <sup>K</sup>

2000 Paek et al. [7] <sup>Δ</sup><sup>p</sup> ffiffi

2006 Liu et al. [8] <sup>f</sup> <sup>¼</sup> <sup>22</sup> <sup>1</sup> � <sup>ε</sup>

The Darcy model is the first model to describe the percolation theory of porous media. In 1856, Darcy proposed a linear relationship between seepage velocity and pressure drop. Although the theory is simple and easy to understand, but it is very limited. Forchheimer modified the Darcy model by adding the inertia terms associated with the velocity square the equation, but still cannot be applied to the turbulent region. Brinkman considered the effective viscous dissipation term and modified the Darcy model, and found that the results are closer to the molecular diffusion at large porosity [1, 2]. In 1952, Ergun [3] proposed the empirical formula

<sup>K</sup> <sup>¼</sup> <sup>d</sup><sup>2</sup>

<sup>¼</sup> <sup>0</sup>:00073 1ð Þ � <sup>ε</sup> �0:<sup>224</sup> <sup>1</sup>:<sup>18</sup>

In 1998, Calmidi and Mahajan [4] proposed the empirical formula of the metal foam perme-

Bhattacharya and Mahajan [5] and Plessis et al. [6] proposed an empirical formula of the permeability coefficient and the inertia coefficient by using a foam sample with a pore size of 45–100 PPI and a porosity of 0.973–0.978, and employed water and glycerol as the liquid phase. Many scholars studied the pressure drop and the permeability of the flow in porous

The effective thermal conductivity is of great significance for the study of heat transfer mechanism in porous media. Maxwell [10] firstly studied the effective thermal conductivity of

Time Researcher Empirical formula Equation numbers

ReK <sup>þ</sup> <sup>0</sup>:<sup>105</sup> <sup>¼</sup> <sup>μ</sup>

<sup>ε</sup><sup>2</sup> ð Þ <sup>3</sup>�<sup>χ</sup> � ffiffi K p dl

þ 0:22, 30 < Redp < 300

<sup>r</sup><sup>u</sup> ffiffi K

24ε<sup>3</sup> CD <sup>¼</sup> <sup>1</sup> <sup>þ</sup> <sup>10</sup>=Re<sup>0</sup>:<sup>667</sup> � � <sup>¼</sup> <sup>1</sup> <sup>þ</sup> <sup>10</sup> <sup>r</sup>udð Þ <sup>χ</sup> � <sup>1</sup> <sup>=</sup>2με � ��0:<sup>667</sup>

<sup>36</sup>χ χð Þ �<sup>1</sup> , F <sup>¼</sup> <sup>2</sup>:05χ χð Þ �<sup>1</sup>

f ε3 150 1ð Þ � ε

1 � e�ð Þ <sup>1</sup>�<sup>ε</sup> <sup>=</sup>0:<sup>04</sup>

<sup>2</sup> (1)

: (2)

(3)

(5)

(6)

ffiffiffiffiffiffiffiffiffiffiffi 1 � ε 3π

<sup>p</sup> <sup>þ</sup> <sup>0</sup>:<sup>105</sup> (4)

! r �1:<sup>11</sup>

$$k\_{\varepsilon} = \varepsilon k\_{\hat{f}} + (1 - \varepsilon)k\_{\text{s}} \tag{7}$$

The above model is mainly concentrated on sand, cylindrical, spherical packing bed and fiber insulation blanket, but the estimation of the effective thermal conductivity of metal foam has a large deviation from the experimental result [11]. Calmidi and Mahajan [4] respectively measured the effective thermal conductivity of ERG aluminum foam at the low temperature (ignoring radiation heat transfer) by using air and water as the flow phase. Boomsma and Poulikakos [12] proposed an efficient thermal conductivity model for predicting the three-dimensional ideal cellular structure of metal foam, which is in good agreement with the experimental data of Calmidi and Mahajan [4]. Bhattacharya and Mahajan [5] used the circular cylinder model to modify the Calmidi's correlation. Hadim and North [13] generalized the correlation coefficient of the thermal conductivity model proposed by Wakao et al. [14], and make it applicable to calculate the effective thermal diffusivity and the stagnation thermal conductivity of sintered porous media. The formulas were given in Table 2.


Table 2. Models for predicting effective thermal conductivity of porous media.

#### 2.3. Convective heat transfer coefficient

Lu et al. [15, 16] studied the forced convection characteristics of shell-and-tube heat exchangers filled with high porosity metal foams. Qu et al. [17] experimentally studied the natural convection of air in a open-call copper foam, and found that there is a turning point in the Grashof number for small porosity (ε = 0.9). Guo [18] numerically simulated the laminar forcedconvection heat transfer in a porous medium flat plate channel with constant heat flux and analyzed the flow and heat transfer performance. Fand et al. [19] immersed the porous medium in water or silicone oil with the porous medium randomly stacked by glass spheres. Many researchers studied convective heat transfer of flow in porous media, the formulas for predicting Nusselt number were given at Table 3.

Because of the difference between the thermal conductivity of the fluid and that of the metal foam, the heat is diffused at a different rate between the two phases. So some researchers hold that the solid and fluid phases have different temperatures, namely LTNE model. Convective heat transfer performance in metal foams was numerically investigated based on the local thermal equilibrium (LTE) model and the LTNE model and the velocity and temperature fields was obtained.

The steady forced convective heat transfer in a tube fully filled with metal foam is numerically considered under the boundary condition of a uniform temperature. Effects of porosity on mean Nusselt number with LTE/LTNE models are shown in Figure 5. The LTE and LTNE Nusselt numbers are both decreased with an increase in foam porosity. The relative deviation is reduced by increasing porosity, due to the greatly decreased solid effective thermal resistance. When porosity is greater than 95%, the relative deviation between LTE/LTNE Nusselt numbers is lower than 20%. For ε >95%, the LTE model can be treated as a rapid estimation tool for thermal performance of metal foams.

Difference between solid and fluid thermal conductivities is the most significant quantity for metal foam LTNE effect. Figure 6 presents the effects of thermal conductivity ratio on mean Nusselt numbers with LTE/LTNE models. The Nu difference for LTE/LTNE models is reduced when thermal conductivity ratio is increased, which is attributed to that kfe/kse is increased

> with an increase in thermal conductivity ratio. When fluid thermal conductivity equals to solid thermal conductivity (k<sup>r</sup> = 1), LTE Nusselt number coincides with LTNE Nusselt number, in which condition with the LTNE effect is negligible and the LTE assumption holds. In addition, the relative deviation gradually decreases to zero with an increase in thermal conductivity ratio. The relative deviation is reduced to 20% when thermal conductivity ratio is increased to 0.148. The LTE model can be roughly used for prediction of heat transfer in porous foams

Convective Transport Characteristics of Nanofluids in Light-Weight Metal Foams with High Porosity

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153

Figure 6. Effect of thermal conductivity ratio on mean Nusselt number with LTE/LTNE models.

The concept of nanofluid, by adding nanoparticles into a base fluid, is firstly proposed in 1995 [24]. Since then, lots of work has been done on the transport phenomena of nanofluid. In this

instead of LTNE model in the range kf/k<sup>s</sup> > 0.148.

Figure 5. Effect of porosity on mean Nusselt number.

3. Transport phenomena in nanofluids


Table 3. Models for predicting Nusselt number of heat transfer in porous media.

Convective Transport Characteristics of Nanofluids in Light-Weight Metal Foams with High Porosity http://dx.doi.org/10.5772/intechopen.72291 153

Figure 5. Effect of porosity on mean Nusselt number.

2.3. Convective heat transfer coefficient

152 Novel Nanomaterials - Synthesis and Applications

predicting Nusselt number were given at Table 3.

tool for thermal performance of metal foams.

2003 Boomsma et al. [20] Nu <sup>¼</sup> <sup>q</sup>

2007 Arisetty et al. [21] Nu <sup>¼</sup> <sup>2</sup>Rh

2000 Calmidi and Mahajan [22] Nusf <sup>¼</sup> <sup>h</sup>sfd<sup>f</sup>

1982 Fand et al. [19] NuPr<sup>0</sup>:<sup>0877</sup> <sup>¼</sup> <sup>0</sup>:618Ra<sup>0</sup>:<sup>698</sup> <sup>þ</sup> <sup>8</sup>:<sup>54</sup> � <sup>10</sup><sup>6</sup>

Table 3. Models for predicting Nusselt number of heat transfer in porous media.

was obtained.

Lu et al. [15, 16] studied the forced convection characteristics of shell-and-tube heat exchangers filled with high porosity metal foams. Qu et al. [17] experimentally studied the natural convection of air in a open-call copper foam, and found that there is a turning point in the Grashof number for small porosity (ε = 0.9). Guo [18] numerically simulated the laminar forcedconvection heat transfer in a porous medium flat plate channel with constant heat flux and analyzed the flow and heat transfer performance. Fand et al. [19] immersed the porous medium in water or silicone oil with the porous medium randomly stacked by glass spheres. Many researchers studied convective heat transfer of flow in porous media, the formulas for

Because of the difference between the thermal conductivity of the fluid and that of the metal foam, the heat is diffused at a different rate between the two phases. So some researchers hold that the solid and fluid phases have different temperatures, namely LTNE model. Convective heat transfer performance in metal foams was numerically investigated based on the local thermal equilibrium (LTE) model and the LTNE model and the velocity and temperature fields

The steady forced convective heat transfer in a tube fully filled with metal foam is numerically considered under the boundary condition of a uniform temperature. Effects of porosity on mean Nusselt number with LTE/LTNE models are shown in Figure 5. The LTE and LTNE Nusselt numbers are both decreased with an increase in foam porosity. The relative deviation is reduced by increasing porosity, due to the greatly decreased solid effective thermal resistance. When porosity is greater than 95%, the relative deviation between LTE/LTNE Nusselt numbers is lower than 20%. For ε >95%, the LTE model can be treated as a rapid estimation

Difference between solid and fluid thermal conductivities is the most significant quantity for metal foam LTNE effect. Figure 6 presents the effects of thermal conductivity ratio on mean Nusselt numbers with LTE/LTNE models. The Nu difference for LTE/LTNE models is reduced when thermal conductivity ratio is increased, which is attributed to that kfe/kse is increased

Time Researcher Empirical formula Equation

Dhyd

C2 <sup>0</sup>:<sup>173</sup>

df Pr<sup>0</sup>:<sup>37</sup> <sup>¼</sup> CT <sup>V</sup>df

<sup>k</sup><sup>c</sup> <sup>¼</sup> mc Tð Þ <sup>c</sup>,outlet�Tc,inlet <sup>A</sup>conð Þ <sup>T</sup>pl�Tc,inlet

> ευ <sup>0</sup>:<sup>5</sup>

Nu <sup>¼</sup> <sup>1</sup>:2Re<sup>0</sup>:<sup>43</sup>Pr<sup>1</sup>=<sup>3</sup>, <sup>20</sup> <sup>&</sup>lt; Re <sup>&</sup>lt; <sup>240</sup> (23)

Dhyd kc

NuPr<sup>0</sup>:<sup>0877</sup> <sup>¼</sup> <sup>0</sup>:766Ra<sup>0</sup>:<sup>374</sup> <sup>C</sup><sup>1</sup> <sup>d</sup>

<sup>A</sup>conð Þ <sup>T</sup>pl�Tc,inlet

<sup>k</sup><sup>f</sup> <sup>¼</sup> <sup>2</sup>Rq<sup>w</sup> <sup>k</sup>fð Þ <sup>T</sup><sup>w</sup> �Tf,<sup>b</sup>

<sup>k</sup><sup>f</sup> <sup>¼</sup> CTRe<sup>0</sup>:<sup>5</sup>

2005 Brito and Rodríguez [23] Nu <sup>¼</sup> <sup>1</sup>:1Re<sup>0</sup>:<sup>43</sup>Pr<sup>1</sup>=<sup>3</sup>, <sup>10</sup> <sup>&</sup>lt; Re <sup>&</sup>lt; <sup>100</sup> (22)

numbers

(19)

(20)

Ge � sechRa,ð Þ <sup>0</sup>:<sup>001</sup> <sup>&</sup>lt; Remax <sup>¼</sup> <sup>3</sup> (17)

Pr<sup>0</sup>:<sup>37</sup> (21)

ð Þ <sup>3</sup> <sup>&</sup>lt; Remax <sup>¼</sup> <sup>100</sup> (18)

Figure 6. Effect of thermal conductivity ratio on mean Nusselt number with LTE/LTNE models.

with an increase in thermal conductivity ratio. When fluid thermal conductivity equals to solid thermal conductivity (k<sup>r</sup> = 1), LTE Nusselt number coincides with LTNE Nusselt number, in which condition with the LTNE effect is negligible and the LTE assumption holds. In addition, the relative deviation gradually decreases to zero with an increase in thermal conductivity ratio. The relative deviation is reduced to 20% when thermal conductivity ratio is increased to 0.148. The LTE model can be roughly used for prediction of heat transfer in porous foams instead of LTNE model in the range kf/k<sup>s</sup> > 0.148.

#### 3. Transport phenomena in nanofluids

The concept of nanofluid, by adding nanoparticles into a base fluid, is firstly proposed in 1995 [24]. Since then, lots of work has been done on the transport phenomena of nanofluid. In this work, the basic features of nanofluid are comprehensively presented. Nanofluid is a new type heat exchange medium which is made by mixing highly conductive nanoparticles and the traditional heat transfer fluid. Due to the addition of nanoparticles, the density, the thermal conductivity and the viscosity of nanofluid are obviously different from those of traditional media, and can be used as a more efficient heat exchange medium.

#### 3.1. Thermal conductivity

Lee et al. [25] have measured the thermal conductivity of four nanofluids: copper oxide and water, copper oxide and ethylene glycol, alumina and water, alumina and ethylene glycol. Li and Xuan [26] analyzed the mechanisms of nanofluids to improve the thermal conductivity. Xie et al. [27] measured the thermal conductivity of the alumina nanoparticle suspension. The influence of pH value of suspension, specific surface area of the dispersed system, crystallization of solid phase and thermal conductivity of the base fluid on the nanofluid thermal conductivity was studied. Eastman et al. [28] measured the thermal conductivity of the copper nanofluid and found that the thermal conductivity of nanoparticles is increased obviously. Guo [29] used KD-2 thermal analyzer to measure the thermal conductivity of the nanofluid. Using the temperature oscillation technique, Das et al. [30] prove that the thermal conductivity of copper oxide/water and alumina/water increases with an increase in the temperature and a decrease in the particle size. Patel et al. [31] have also obtained similar conclusions through experiments. Ebrahimnia-Bajestan et al. [32] applied nanofluids to the solar system, and studied the laminar convection heat transfer of the TiO2/water nanofluid in a tube by experimental and numerical methods. Many researchers proposed models of nanofluids thermal conductivity base on experimental study [33–47], and the formulas were given in Table 4.

#### 3.2. Viscosity and friction factor

At present, there is no suitable theory to predict the viscosity of nanofluids accurately. Einstein [48] proved that the relative viscosity of the suspension is a simple function of the volume fraction of suspended particles. Scholars revised the formula in different aspects, and put forward their correction models respectively [39, 49]. With a least-square curve fitting, Maïga et al. [50] proposed a correlation based on some experimental data available in the open literature. Shafahi et al. [51] indicated that the nanofluid viscosity is a function of the temperature and proposed the correlations. Scholars proposed correlations of different nanoparticle types. The formulas of nanofluids viscosity were given in Table 5.

Xuan et al. [34, 53] found that the friction factor of nanofluids is almost the same as that of water at the same velocity, and is independent of the volume fraction of nanoparticles. Therefore, the friction factor of nanofluids is calculated with a single-phase model:

$$
\lambda\_{\rm nf} = \frac{\Delta P\_{\rm nf} d}{L} \frac{2g}{\nu\_{\rm m}^2} \tag{24}
$$

convective heat transfer of nanofluids. Xuan et al. [34, 53] established an experimental system to measure the convective heat transfer coefficient of nanofluid and the laminar flow and turbulent flow friction factors in the channel. In nanofluid, the nanoparticles undergo thermophoretic motion with in the temperature gradient field. Researchers have taken more and more attention to the thermophoretic motion of nanoparticles [54, 55]. For the heating of the side wall in a rectangular channel, Berkovski-Polevikov's coefficients have good agreement with the experimental data with length-width ratio between 1 and 10, and MacGregor-Emery's

<sup>μ</sup>nf <sup>¼</sup> <sup>μ</sup>bf <sup>1</sup> <sup>þ</sup> <sup>5</sup>:45<sup>f</sup> <sup>þ</sup> <sup>108</sup>:2f<sup>2</sup> � � for titania

Time Researcher Empirical formula Equation numbers

f

f kbf

<sup>þ</sup> <sup>r</sup>npfcp,np 2kbf

<sup>2</sup>knfþkbf <sup>þ</sup> <sup>f</sup>ð Þ <sup>k</sup>nf�kIr ð Þ <sup>2</sup>kIrþknp �<sup>α</sup>ð Þ <sup>k</sup>np�kIr ð Þ <sup>2</sup>kIrþknf <sup>α</sup>ð Þ <sup>2</sup>knfþkbf ð Þ <sup>2</sup>kIrþknp �2<sup>α</sup>ð Þ <sup>k</sup>np�kIr ð Þ <sup>k</sup>Irþknf

<sup>¼</sup> <sup>0</sup>:<sup>991</sup> <sup>þ</sup> <sup>0</sup>:276T<sup>f</sup> <sup>þ</sup> <sup>77</sup>:6f<sup>2</sup> <sup>þ</sup> <sup>3641</sup>:2Tf<sup>2</sup> <sup>þ</sup>

<sup>d</sup>np � �<sup>0</sup>:<sup>369</sup> <sup>k</sup>np

Time Researcher Empirical formula Equation numbers

<sup>k</sup>bf � �<sup>0</sup>:<sup>7476</sup>

<sup>d</sup>np � � � <sup>0</sup>:025<sup>f</sup> <sup>k</sup>np

<sup>T</sup><sup>2</sup> � <sup>2</sup>:<sup>3</sup> � <sup>10</sup>�<sup>4</sup><sup>T</sup> <sup>þ</sup> <sup>3</sup>:<sup>9</sup> � <sup>10</sup>�<sup>2</sup> for <sup>f</sup> <sup>¼</sup> <sup>4</sup>%

<sup>k</sup>npþð Þ <sup>n</sup>�<sup>1</sup> <sup>k</sup>bfþð Þ <sup>k</sup>bf�knp <sup>f</sup> <sup>k</sup>bf, n <sup>¼</sup> <sup>3</sup>=<sup>ψ</sup> (26)

Convective Transport Characteristics of Nanofluids in Light-Weight Metal Foams with High Porosity

ffiffiffiffiffiffiffiffiffiffiffiffi kBT

<sup>υ</sup> (32)

Pr<sup>0</sup>:<sup>66</sup>Re<sup>0</sup>:<sup>4</sup> (27)

<sup>3</sup>πrclμbf <sup>q</sup> (30)

<sup>¼</sup> <sup>0</sup>, <sup>α</sup> <sup>¼</sup> <sup>d</sup>np dnpþt � �<sup>3</sup> (31)

0:00217 sin ð Þ T � f

Pr<sup>0</sup>:<sup>9955</sup>Re<sup>1</sup>:<sup>2321</sup> (35)

<sup>0</sup>:<sup>613</sup> � � (36)

(25)

155

http://dx.doi.org/10.5772/intechopen.72291

(28)

(29)

(33)

(34)

(39)

(40)

<sup>k</sup>npþ2kbf�ð Þ <sup>k</sup>np�kbf <sup>f</sup> <sup>k</sup>bf

Tfr � �<sup>10</sup> <sup>k</sup>np <sup>k</sup>bf � �<sup>0</sup>:<sup>03</sup>

> Ð ∞ 0 kclð Þr n rð Þ <sup>k</sup>clð Þþ<sup>r</sup> <sup>2</sup>kbfdr

<sup>k</sup>npþ2kbf�ð Þ <sup>k</sup>np�kbf ð Þ <sup>1</sup>þ<sup>β</sup> <sup>3</sup>

<sup>k</sup>bf <sup>¼</sup> <sup>1</sup> <sup>þ</sup> <sup>4</sup>:<sup>4</sup> <sup>T</sup>

ð Þþ 1�f 3f Ð ∞ 0 kcl n rð Þ <sup>k</sup>clð Þþ<sup>r</sup> <sup>2</sup>kbfdr <sup>k</sup>bf

<sup>k</sup>bf <sup>¼</sup> <sup>k</sup>npþ2kbf�2<sup>f</sup>ð Þ <sup>k</sup>bf�knp <sup>k</sup>npþ2kbfþ<sup>f</sup>ð Þ <sup>k</sup>bf�knp

<sup>1</sup>�fþ2<sup>f</sup> <sup>k</sup>np

<sup>k</sup>bf <sup>¼</sup> <sup>1</sup> <sup>þ</sup> <sup>64</sup>:7f<sup>0</sup>:<sup>746</sup> <sup>d</sup>bf

<sup>k</sup>bf <sup>¼</sup> <sup>1</sup> <sup>þ</sup> <sup>1</sup>:011<sup>f</sup> <sup>þ</sup> <sup>2</sup>:438<sup>f</sup> <sup>47</sup>

<sup>k</sup>np�kbf lnkbf <sup>þ</sup>knp 2kbf

<sup>1</sup>�fþ2<sup>f</sup> <sup>k</sup>bf <sup>k</sup>np�kbf lnkbf <sup>þ</sup>knp 2kbf

� <sup>6</sup>:<sup>01</sup> � <sup>10</sup>�<sup>6</sup>T<sup>2</sup> � <sup>3647</sup>:1T<sup>f</sup> sin <sup>f</sup>

1906 Einstein [48] μnf ¼ ð Þ 1 þ 2:5f μbf (37) 2005 Maïga et al. [50] <sup>μ</sup>nf <sup>¼</sup> <sup>1</sup> <sup>þ</sup> <sup>1</sup>:73<sup>f</sup> <sup>þ</sup> <sup>123</sup>f<sup>2</sup> � �μbf (38)

2010 Shafahi et al. [51] <sup>μ</sup>nf <sup>¼</sup> <sup>2</sup>:<sup>9</sup> � <sup>10</sup>�<sup>7</sup>T<sup>2</sup> � <sup>2</sup>:<sup>0</sup> � <sup>10</sup>�<sup>4</sup><sup>T</sup> <sup>þ</sup> <sup>3</sup>:<sup>4</sup> � <sup>10</sup>�<sup>2</sup> for <sup>f</sup> <sup>¼</sup> <sup>1</sup>% <sup>μ</sup>nf <sup>¼</sup> <sup>3</sup>:<sup>4</sup> � <sup>10</sup>�<sup>7</sup>

2013 Yang et al. [52] <sup>μ</sup>nf <sup>¼</sup> <sup>μ</sup>bf <sup>1</sup> <sup>þ</sup> <sup>39</sup>:11<sup>f</sup> <sup>þ</sup> <sup>533</sup>:9f<sup>2</sup> � � for alumina

α � � <sup>k</sup>nf�kbf

kbf

Table 4. Models for predicting nanofluid thermal conductivity.

Table 5. Models for predicting nanofluid viscocity.

1873 Maxwell [33] <sup>k</sup>nf <sup>¼</sup> <sup>k</sup>npþ2kbfþ<sup>2</sup>ð Þ <sup>k</sup>np�kbf <sup>f</sup>

2011 Lee et al. [35] <sup>k</sup>nf

2013 Hadadian [39] <sup>k</sup>nf

2005 Xue and Xu [40] <sup>1</sup> � <sup>f</sup>

2005 Xue [44] <sup>k</sup>nf <sup>¼</sup> <sup>k</sup>bf

2016 Esfe et al. [45] knf

2005 Chon et al. [46] <sup>k</sup>nf

2011 Khanafer and Vafai [47] <sup>k</sup>nf

2002 Keblinski et al. [42] Rednp <sup>¼</sup> CRMdnp

2003 Wang et al. [37] <sup>k</sup>nf <sup>¼</sup> ð Þþ <sup>1</sup>�<sup>f</sup> <sup>3</sup><sup>f</sup>

1999 Lee et al. [25] <sup>k</sup>nf <sup>¼</sup> <sup>k</sup>npþð Þ <sup>n</sup>�<sup>1</sup> <sup>k</sup>bf�ð Þ <sup>n</sup>�<sup>1</sup> ð Þ <sup>k</sup>bf�knp <sup>f</sup>

2004 Yu et al. [38] <sup>k</sup>nf <sup>¼</sup> <sup>k</sup>npþ2kbfþ<sup>2</sup>ð Þ <sup>k</sup>np�kbf ð Þ <sup>1</sup>þ<sup>β</sup> <sup>3</sup>

#### 3.3. Convective heat transfer

As a new type heat exchanging medium, the nanofluid has a very pronounced enhancement effect on the convective heat transfer. Scholars have carried out a series of studies on the


Table 4. Models for predicting nanofluid thermal conductivity.

work, the basic features of nanofluid are comprehensively presented. Nanofluid is a new type heat exchange medium which is made by mixing highly conductive nanoparticles and the traditional heat transfer fluid. Due to the addition of nanoparticles, the density, the thermal conductivity and the viscosity of nanofluid are obviously different from those of traditional

Lee et al. [25] have measured the thermal conductivity of four nanofluids: copper oxide and water, copper oxide and ethylene glycol, alumina and water, alumina and ethylene glycol. Li and Xuan [26] analyzed the mechanisms of nanofluids to improve the thermal conductivity. Xie et al. [27] measured the thermal conductivity of the alumina nanoparticle suspension. The influence of pH value of suspension, specific surface area of the dispersed system, crystallization of solid phase and thermal conductivity of the base fluid on the nanofluid thermal conductivity was studied. Eastman et al. [28] measured the thermal conductivity of the copper nanofluid and found that the thermal conductivity of nanoparticles is increased obviously. Guo [29] used KD-2 thermal analyzer to measure the thermal conductivity of the nanofluid. Using the temperature oscillation technique, Das et al. [30] prove that the thermal conductivity of copper oxide/water and alumina/water increases with an increase in the temperature and a decrease in the particle size. Patel et al. [31] have also obtained similar conclusions through experiments. Ebrahimnia-Bajestan et al. [32] applied nanofluids to the solar system, and studied the laminar convection heat transfer of the TiO2/water nanofluid in a tube by experimental and numerical methods. Many researchers proposed models of nanofluids thermal conductiv-

ity base on experimental study [33–47], and the formulas were given in Table 4.

types. The formulas of nanofluids viscosity were given in Table 5.

fore, the friction factor of nanofluids is calculated with a single-phase model:

At present, there is no suitable theory to predict the viscosity of nanofluids accurately. Einstein [48] proved that the relative viscosity of the suspension is a simple function of the volume fraction of suspended particles. Scholars revised the formula in different aspects, and put forward their correction models respectively [39, 49]. With a least-square curve fitting, Maïga et al. [50] proposed a correlation based on some experimental data available in the open literature. Shafahi et al. [51] indicated that the nanofluid viscosity is a function of the temperature and proposed the correlations. Scholars proposed correlations of different nanoparticle

Xuan et al. [34, 53] found that the friction factor of nanofluids is almost the same as that of water at the same velocity, and is independent of the volume fraction of nanoparticles. There-

> <sup>λ</sup>nf <sup>¼</sup> <sup>Δ</sup>Pnf<sup>d</sup> L

As a new type heat exchanging medium, the nanofluid has a very pronounced enhancement effect on the convective heat transfer. Scholars have carried out a series of studies on the

2g u2 m

(24)

media, and can be used as a more efficient heat exchange medium.

3.1. Thermal conductivity

154 Novel Nanomaterials - Synthesis and Applications

3.2. Viscosity and friction factor

3.3. Convective heat transfer


Table 5. Models for predicting nanofluid viscocity.

convective heat transfer of nanofluids. Xuan et al. [34, 53] established an experimental system to measure the convective heat transfer coefficient of nanofluid and the laminar flow and turbulent flow friction factors in the channel. In nanofluid, the nanoparticles undergo thermophoretic motion with in the temperature gradient field. Researchers have taken more and more attention to the thermophoretic motion of nanoparticles [54, 55]. For the heating of the side wall in a rectangular channel, Berkovski-Polevikov's coefficients have good agreement with the experimental data with length-width ratio between 1 and 10, and MacGregor-Emery's coefficient has good agreement with the experimental data with length-width ratio greater than 10 [56]. Maïga et al. [50] considered the influence of the nanoparticle volume fraction and the Reynolds number on the average convective heat transfer coefficient of water-based nanofluid. Sakai et al. [57] improved the Buongiorno model for the convective heat transfer of nanofluids, so that it can be applied to continuity equations, momentum equations and energy equations without the effect of nanoparticle volume fraction distribution. Jia and Wang [58] improved the Eubank-Proctor model and fitted out a coefficient of mixture flow considering natural convection. Yang et al. [59] made two kinds nanofluids using graphite nanoparticles, and measured the laminar-flow convective heat transfer coefficient in a horizontal tube heat exchanger. Formulas for Nusselt number of nanofluids convection were given in Table 6.

Buongiorno [60] proposed a mathematical model on the non-uniform volume traction of nanoparticles. He assumed incompressible flow, no chemical reactions, negligible external forces, dilute mixture, negligible viscous dissipation, negligible radiative heat transfer, and LTE between nanoparticles and the base fluid [52, 60].

The forced convective heat transfer of the nanofluid in a plain tube at the full development section was studied by the numerical method. Figure 7 is the effect of nanoparticle volume fraction on the Nusselt number and kw/k<sup>B</sup> (k<sup>w</sup> is the thermal conductivity of the wall and k<sup>B</sup> is the average thermal conductivity). It can be seen that the Nusselt number increases at first and then decreases with an increase in the nanoparticle volume fraction, and there is a maximum value of the Nusselt number with a suitable nanoparticle volume fraction. The increase in the nanoparticle volume traction leads to a more uniform velocity, which is beneficial to the convective heat transfer. In addition, the high nanoparticle volume fraction can increases the thermal conductivity, so an increase in the nanoparticle volume traction is beneficial to the convective heat transfer. The ratio of the wall thermal conductivity to the average thermal conductivity decreases with an increase in the nanoparticle volume fraction. When the ration of kw/k<sup>B</sup> decreases, the Nusselt number is reduced. The effect of nanoparticle volume traction is

more obvious with a larger kw/kB, and the effect of kw/k<sup>B</sup> is more obvious with larger nanopar-

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157

Figure 7. Effect of nanoparticle volume fraction on mean Nusselt number.

Figure 8 is the relationship between the Nusselt number and NBT. NBT is a dimensionless parameter related to the Brown motion and the thermophoretic motion. It can be found that the nanofluid is unbenefited for heat transfer with low NBT (<0.2). There is a maximum Nusselt number when NBT is from about 0.4 to 0.5. Then the Nusselt number decreases with an increase in NBT. When the NBT is close to 10 or greater, the Nusselt number tends to be constant. The enhancement via the Brown diffusion motion causes nanoparticle to disturb the

ticle volume traction.

Figure 8. Effect of NBT on mean Nusselt number.


Table 6. Models for predicting Nusselt number of nanofluid convective heat transfer.

Convective Transport Characteristics of Nanofluids in Light-Weight Metal Foams with High Porosity http://dx.doi.org/10.5772/intechopen.72291 157

Figure 7. Effect of nanoparticle volume fraction on mean Nusselt number.

coefficient has good agreement with the experimental data with length-width ratio greater than 10 [56]. Maïga et al. [50] considered the influence of the nanoparticle volume fraction and the Reynolds number on the average convective heat transfer coefficient of water-based nanofluid. Sakai et al. [57] improved the Buongiorno model for the convective heat transfer of nanofluids, so that it can be applied to continuity equations, momentum equations and energy equations without the effect of nanoparticle volume fraction distribution. Jia and Wang [58] improved the Eubank-Proctor model and fitted out a coefficient of mixture flow considering natural convection. Yang et al. [59] made two kinds nanofluids using graphite nanoparticles, and measured the laminar-flow convective heat transfer coefficient in a horizontal tube heat exchanger. Formulas for Nusselt number of nanofluids convection were given in Table 6.

Buongiorno [60] proposed a mathematical model on the non-uniform volume traction of nanoparticles. He assumed incompressible flow, no chemical reactions, negligible external forces, dilute mixture, negligible viscous dissipation, negligible radiative heat transfer, and

The forced convective heat transfer of the nanofluid in a plain tube at the full development section was studied by the numerical method. Figure 7 is the effect of nanoparticle volume fraction on the Nusselt number and kw/k<sup>B</sup> (k<sup>w</sup> is the thermal conductivity of the wall and k<sup>B</sup> is the average thermal conductivity). It can be seen that the Nusselt number increases at first and then decreases with an increase in the nanoparticle volume fraction, and there is a maximum value of the Nusselt number with a suitable nanoparticle volume fraction. The increase in the nanoparticle volume traction leads to a more uniform velocity, which is beneficial to the convective heat transfer. In addition, the high nanoparticle volume fraction can increases the thermal conductivity, so an increase in the nanoparticle volume traction is beneficial to the convective heat transfer. The ratio of the wall thermal conductivity to the average thermal conductivity decreases with an increase in the nanoparticle volume fraction. When the ration of kw/k<sup>B</sup> decreases, the Nusselt number is reduced. The effect of nanoparticle volume traction is

Time Researcher Empirical formula Equation

Nunf <sup>¼</sup> <sup>0</sup>:0059 1:<sup>0</sup> <sup>þ</sup> <sup>7</sup>:6286f<sup>0</sup>:<sup>6886</sup>Pe<sup>0</sup>:<sup>001</sup>

W

W � ��0:<sup>09</sup>, 2 ≤ <sup>H</sup>

Nu <sup>¼</sup> <sup>0</sup>:28Re<sup>0</sup>:<sup>35</sup>Pr<sup>0</sup>:<sup>36</sup> for constant wall temperature

ho <sup>¼</sup> AoΔTlm Q

<sup>¼</sup> <sup>2</sup>:<sup>11</sup> Gznf <sup>þ</sup> <sup>0</sup>:<sup>574</sup> GrnfPrnf <sup>d</sup>

0:2þPr � �<sup>0</sup>:<sup>29</sup> <sup>H</sup>

0:2þPr � �<sup>0</sup>:<sup>28</sup> <sup>H</sup>

> nf <sup>L</sup> D � �<sup>1</sup>=<sup>3</sup> <sup>μ</sup>nf μw � ��0:<sup>14</sup>

Nu <sup>¼</sup> <sup>0</sup>:<sup>22</sup> PrRa

μnf � �<sup>0</sup>:<sup>14</sup>

<sup>Ω</sup> <sup>¼</sup> NunfPr�1=<sup>3</sup>

Table 6. Models for predicting Nusselt number of nanofluid convective heat transfer.

2005 Maïga et al. [50] Nu <sup>¼</sup> <sup>0</sup>:086Re<sup>0</sup>:<sup>55</sup>Pr<sup>0</sup>:<sup>5</sup> for constant heat flux

<sup>U</sup> <sup>¼</sup> <sup>1</sup> hnf Ai Ao � � <sup>þ</sup> Do <sup>2</sup><sup>k</sup> ln Do Di <sup>þ</sup> <sup>1</sup>

� �

� �

� �<sup>0</sup>:<sup>13</sup>, 1 ≤ <sup>H</sup>

np

np

� �<sup>0</sup>:<sup>75</sup> h i<sup>1</sup>=<sup>3</sup>

Re<sup>0</sup>:<sup>333</sup> nf Pr<sup>0</sup>:<sup>4</sup>

Re<sup>0</sup>:<sup>9238</sup> nf Pr<sup>0</sup>:<sup>4</sup>

<sup>W</sup> <sup>≤</sup> <sup>10</sup>, Pr <sup>≤</sup> 105, Ra <sup>≤</sup> 1013

L

<sup>W</sup> <sup>≤</sup> <sup>2</sup>, <sup>10</sup>�<sup>3</sup> <sup>≤</sup> Pr <sup>≤</sup> 105, 103 <sup>≤</sup> PrRa

nf for laminar flow

, Gznf <sup>¼</sup> <sup>d</sup>

nf for turbulent flow

0:2þPr H W � ��<sup>3</sup>

<sup>L</sup>RenfPrnf

numbers

(41)

(42)

(43)

(44)

(45)

LTE between nanoparticles and the base fluid [52, 60].

156 Novel Nanomaterials - Synthesis and Applications

2000 Xuan and Roetzel [53] Nunf <sup>¼</sup> <sup>0</sup>:4328 1:<sup>0</sup> <sup>þ</sup> <sup>11</sup>:285f<sup>0</sup>:<sup>754</sup>Pe<sup>0</sup>:<sup>218</sup>

2011 Corcione [56] Nu <sup>¼</sup> <sup>0</sup>:<sup>18</sup> PrRa

2015 Jia and Wang [58] Nunf <sup>μ</sup>wnf

2005 Yang et al. [59] <sup>1</sup>

more obvious with a larger kw/kB, and the effect of kw/k<sup>B</sup> is more obvious with larger nanoparticle volume traction.

Figure 8 is the relationship between the Nusselt number and NBT. NBT is a dimensionless parameter related to the Brown motion and the thermophoretic motion. It can be found that the nanofluid is unbenefited for heat transfer with low NBT (<0.2). There is a maximum Nusselt number when NBT is from about 0.4 to 0.5. Then the Nusselt number decreases with an increase in NBT. When the NBT is close to 10 or greater, the Nusselt number tends to be constant. The enhancement via the Brown diffusion motion causes nanoparticle to disturb the

Figure 8. Effect of NBT on mean Nusselt number.

flow more effectively, causing local turbulence to enhance the heat transfer between nanoparticles and the base liquid. Nanoparticles will move to the cold region (wall) by thermophoresis diffusion. For large nanoparticle aggregating, nanoparticle of other areas is too small, so it has little heat transfer enhancement with too large NBT.

In Xu et al. [70], velocity and temperature fields are numerically obtained. The effects of some key parameters on flow and heat transfer of nanofluid in porous media are analyzed. For the nanofluid flowing through metal foams, the nanoparticle volume fraction is a most important parameter, the effect of which on pressure drop is shown in Figure 9. As can be seen, with the increase in volume fraction, the pressure drop per unit length gradually increases and the increasing amplitude for pressure drop also increases. This is attributed to that the dynamic viscosity and the density of nanofluid are increased sharply with the increase in volume fraction. Figure 10 shows the effect of nanoparticle volume fraction on heat transfer for two different nanoparticles (Al2O3 and TiO3). As the nanoparticle volume fraction increases, Nusselt number gradually increases but the increasing amplitude is reduced. This is attributed to that the thermal conductivity increasing amplitude is decreased with an increase in

Convective Transport Characteristics of Nanofluids in Light-Weight Metal Foams with High Porosity

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159

Figure 9. Effects of nanoparticle volume fraction on the overall pressure drop.

Figure 10. Effects of nanoparticle volume fraction on heat transfer.

#### 4. Convection of nanofluids in metal foams

Even though metal foams own excellent thermal performance, poor heat conduction ability of most heat transfer fluids restricts further heat transfer improvement in metal foams, for which the combination of the metal foam and the nanofluid with highly conductive nanoparticles is a promising solution. In this chapter, the transport characteristics of nanofluids flowing through metal foams. In this chapter, the recent advances on the forced convection and natural convection of nanofluids in porous foams will be firstly reviewed and the latest research concerns from the perspective of fundamental research will be put forward.

#### 4.1. Experimental data

Cheng [61] tested the heat transfer performance of the heat pipe with different nanofluid volume fractions and liquid filling rates, and also tested the heat transfer performance of the screen suction core heat pipe. Hajipour et al. [62] studied the mixed convection of alumina/ water nanofluid in a vertical square channel partially filled with open metal foams under the constant wall heat flux using the experimental and numerical method. Goodarzi et al. [63] studied the laminar and turbulent mixing flow and heat transfer of Cu/water nanofluids in a shallow rectangular cavity using a two-phase mixture model. Mao [64] studied the generation, fusion and detachment of boiling bubbles on the smooth plate and foam metal surface. Nazari et al. [65] studied the influence of the interaction between nanofluid and porous medium of extended surface on the heat exchanger thermal performance, and the forced convection of alumina/water nanofluids in a circular tube filled with metal foams was studied experimentally with isothermal boundary conditions.

#### 4.2. Modeling the forced convective heat transfer

Matin and Pop [66] studied the force convection heat transfer of nanofluids in a horizontal porous medium channel at fully developed section with constant heat flux. Xu et al. [67] investigated the dual heat transfer enhancement of nanofluids flowing in a metal foam channel by numerical method based on the local non thermal equilibrium model. Mahdi et al. [68] summarized the influence of the porosity, permeability, inertial coefficient and effective heat exchange coefficient of porous media, and also studied the effect of thermodynamic parameters of nanofluids. Sivasankaran and Narrein [69] proposed a numerical simulation of laminar pulsating heat transfer and hydraulic characteristics of alumina/water nanofluid in a threedimensional spiral microchannel radiator, using the modified viscosity equation and the twophase mixing model.

In Xu et al. [70], velocity and temperature fields are numerically obtained. The effects of some key parameters on flow and heat transfer of nanofluid in porous media are analyzed. For the nanofluid flowing through metal foams, the nanoparticle volume fraction is a most important parameter, the effect of which on pressure drop is shown in Figure 9. As can be seen, with the increase in volume fraction, the pressure drop per unit length gradually increases and the increasing amplitude for pressure drop also increases. This is attributed to that the dynamic viscosity and the density of nanofluid are increased sharply with the increase in volume fraction. Figure 10 shows the effect of nanoparticle volume fraction on heat transfer for two different nanoparticles (Al2O3 and TiO3). As the nanoparticle volume fraction increases, Nusselt number gradually increases but the increasing amplitude is reduced. This is attributed to that the thermal conductivity increasing amplitude is decreased with an increase in

Figure 9. Effects of nanoparticle volume fraction on the overall pressure drop.

flow more effectively, causing local turbulence to enhance the heat transfer between nanoparticles and the base liquid. Nanoparticles will move to the cold region (wall) by thermophoresis diffusion. For large nanoparticle aggregating, nanoparticle of other areas is

Even though metal foams own excellent thermal performance, poor heat conduction ability of most heat transfer fluids restricts further heat transfer improvement in metal foams, for which the combination of the metal foam and the nanofluid with highly conductive nanoparticles is a promising solution. In this chapter, the transport characteristics of nanofluids flowing through metal foams. In this chapter, the recent advances on the forced convection and natural convection of nanofluids in porous foams will be firstly reviewed and the latest research concerns

Cheng [61] tested the heat transfer performance of the heat pipe with different nanofluid volume fractions and liquid filling rates, and also tested the heat transfer performance of the screen suction core heat pipe. Hajipour et al. [62] studied the mixed convection of alumina/ water nanofluid in a vertical square channel partially filled with open metal foams under the constant wall heat flux using the experimental and numerical method. Goodarzi et al. [63] studied the laminar and turbulent mixing flow and heat transfer of Cu/water nanofluids in a shallow rectangular cavity using a two-phase mixture model. Mao [64] studied the generation, fusion and detachment of boiling bubbles on the smooth plate and foam metal surface. Nazari et al. [65] studied the influence of the interaction between nanofluid and porous medium of extended surface on the heat exchanger thermal performance, and the forced convection of alumina/water nanofluids in a circular tube filled with metal foams was studied experimen-

Matin and Pop [66] studied the force convection heat transfer of nanofluids in a horizontal porous medium channel at fully developed section with constant heat flux. Xu et al. [67] investigated the dual heat transfer enhancement of nanofluids flowing in a metal foam channel by numerical method based on the local non thermal equilibrium model. Mahdi et al. [68] summarized the influence of the porosity, permeability, inertial coefficient and effective heat exchange coefficient of porous media, and also studied the effect of thermodynamic parameters of nanofluids. Sivasankaran and Narrein [69] proposed a numerical simulation of laminar pulsating heat transfer and hydraulic characteristics of alumina/water nanofluid in a threedimensional spiral microchannel radiator, using the modified viscosity equation and the two-

too small, so it has little heat transfer enhancement with too large NBT.

from the perspective of fundamental research will be put forward.

4. Convection of nanofluids in metal foams

158 Novel Nanomaterials - Synthesis and Applications

4.1. Experimental data

phase mixing model.

tally with isothermal boundary conditions.

4.2. Modeling the forced convective heat transfer

Figure 10. Effects of nanoparticle volume fraction on heat transfer.

nanoparticle volume fraction. Due to thermal conductivity of Al2O3 is higher than that of TiO3, Nusselt number of Al2O3 is higher than that of TiO3 as shown in Figure 10. From Figure 10, the maximum heat transfer augmentation of nanofluid is about 3.8% for Al2O3 and 3.0% for TiO3, which is very useful for further improving thermal performance of metal foam heat exchangers and heat sinks, especially for high heat-flux applications.

#### 4.3. Modeling the natural convective heat transfer

Sun and Pop [71] studied the steady natural convection of the water-based nanofluid in a right triangle shell filled a porous medium using the numerical method. It is found that the average Nusselt number can be increased by increasing the nanoparticle volume fraction under a low Rayleigh number, but the average Nusselt number decreases with an increase in the nanoparticle volume fraction under a high Rayleigh number. Sherement [72] established a Buongiorno mathematical model for the three-dimensional natural convection of nanofluids in porous media, and considered that the heterogeneous models of nanoparticles are more suitable. Bhadauria and Agarwal [73] proposed a detailed model of the nanofluid saturated porous layer.

A lattice Boltzmann (LB) model for the nanofluid natural convection in a porous medium was established by using the volume-averaging method. Figures 11 and 12 show the velocity and temperature distributions for Ra = 104 and 106 , respectively. The other parameters are set to Da = 0.0001, ε = 0.6, and f = 0.5%. For Ra = 104 , the temperature is very uniform in the y direction, and the heat is transferred mainly by heat conduction. Streamlines are nearly in parallel with the gravitational direction. In Figure 12, the high temperature region at the upper left side and the low temperature region the bottom right side respectively diffuse heat with a

quicker speed than other region. The change of the heat transfer regime from the heat conduc-

Figure 13 shows the effect of the Rayleigh number on the average Nusselt number with

of Nuave is mild for the small Rayleigh number, and Nuave increases sharply for large Rayleigh numbers. The effect of Ra on the flow and Nuave is more obvious for the high Darcy number.

. With an increase in the Rayleigh number, the increasing amplitude

.

Convective Transport Characteristics of Nanofluids in Light-Weight Metal Foams with High Porosity

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161

tion to the natural convection is clearly shown in Figures 11 and 12.

Figure 12. The distributions of the flow and the temperature for Ra = 106

Figure 13. Effect of the Rayleigh number on the heat transfer.

Da = 10<sup>3</sup> and Da = 10<sup>4</sup>

Figure 11. The distributions of the flow and the temperature for Ra = 104 .

Convective Transport Characteristics of Nanofluids in Light-Weight Metal Foams with High Porosity http://dx.doi.org/10.5772/intechopen.72291 161

Figure 12. The distributions of the flow and the temperature for Ra = 106 .

nanoparticle volume fraction. Due to thermal conductivity of Al2O3 is higher than that of TiO3, Nusselt number of Al2O3 is higher than that of TiO3 as shown in Figure 10. From Figure 10, the maximum heat transfer augmentation of nanofluid is about 3.8% for Al2O3 and 3.0% for TiO3, which is very useful for further improving thermal performance of metal foam heat exchan-

Sun and Pop [71] studied the steady natural convection of the water-based nanofluid in a right triangle shell filled a porous medium using the numerical method. It is found that the average Nusselt number can be increased by increasing the nanoparticle volume fraction under a low Rayleigh number, but the average Nusselt number decreases with an increase in the nanoparticle volume fraction under a high Rayleigh number. Sherement [72] established a Buongiorno mathematical model for the three-dimensional natural convection of nanofluids in porous media, and considered that the heterogeneous models of nanoparticles are more suitable. Bhadauria and Agarwal [73] proposed a detailed model of the nanofluid saturated porous

A lattice Boltzmann (LB) model for the nanofluid natural convection in a porous medium was established by using the volume-averaging method. Figures 11 and 12 show the velocity and

direction, and the heat is transferred mainly by heat conduction. Streamlines are nearly in parallel with the gravitational direction. In Figure 12, the high temperature region at the upper left side and the low temperature region the bottom right side respectively diffuse heat with a

.

, respectively. The other parameters are set to

, the temperature is very uniform in the y

gers and heat sinks, especially for high heat-flux applications.

4.3. Modeling the natural convective heat transfer

160 Novel Nanomaterials - Synthesis and Applications

temperature distributions for Ra = 104 and 106

Da = 0.0001, ε = 0.6, and f = 0.5%. For Ra = 104

Figure 11. The distributions of the flow and the temperature for Ra = 104

layer.

quicker speed than other region. The change of the heat transfer regime from the heat conduction to the natural convection is clearly shown in Figures 11 and 12.

Figure 13 shows the effect of the Rayleigh number on the average Nusselt number with Da = 10<sup>3</sup> and Da = 10<sup>4</sup> . With an increase in the Rayleigh number, the increasing amplitude of Nuave is mild for the small Rayleigh number, and Nuave increases sharply for large Rayleigh numbers. The effect of Ra on the flow and Nuave is more obvious for the high Darcy number.

Figure 13. Effect of the Rayleigh number on the heat transfer.

increase in the nanoparticle volume traction, and it is beneficial for promoting the heat trans-

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163

Phase change heat transfer of nanofluid in porous foams is a relatively new theme. In this chapter, the basic scientific problems for this topic will be firstly presented and then the recent research advances will be reviewed. The future research points will also be

Boiling heat transfer is used in a variety of industrial processes and applications, such as refrigeration, power generation, heat exchangers, cooling of high-power electronics components and cooling of nuclear reactors [74]. The use of nanofluids for boiling heat transfer enhancement is a promising solution that is currently being explored by many researchers for

Lee and Mudawar [75] have undertaken an experimental study to explore the benefits of using alumina/water nanofluid for microchannel cooling applications. They revealed the enhancement of the heat transfer coefficient for single-phase laminar flow. However, in the two-phase regime, the nanofluids caused the surface deposition in micro-channels, and large agglomerates of nanoparticles were formed. Kim et al. [76] investigated the subcooled flow boiling using dilute alumina, zinc oxide and diamond water-based nanofluids. Kim et al. [77] studied the pool boiling by experiment with water-based nanofluids containing Al2O3, ZrO2 and SiO2 nanoparticles. An irregular porous structure was formed at the surface. You et al. [78] measured the CHF in pool boiling using a flat, square copper heater submerged into nanofluids at a sub-atmospheric pressure of 2.89 psia. Nanoparticle deposition was observed by Bang and Chang [79], who also measured a CHF enhancement of 50% with aluminawater nanofluids on a stainless steel plate. Zhu et al. [80] developed a boiling heat transfer coefficient correlation of the refrigerant/lubricating oil mixture on the surface of the metal

Several researchers have noticed the nano-deposition at the heater surface, which can alter the surface area, the surface wettability and the bubble nucleation. The nucleation site density, the bubble departure diameter and the bubble frequency are all affected by the nanofluid boiling. It was found by several researchers [77, 78] that bubble diameters increase during boiling with nanofluids, but the nucleation site density decreases with the addition of nanoparticles into the

Liquid-solid phase change in porous media is frequently encountered in lots of natural and engineering systems. Over the past several decades, this problem has been extensively investigated analytically, experimentally and numerically [81]. Thermal management systems based

fer, which leads to the increased average Nusselt number.

5. Phase change heat transfer

5.1. Liquid-gas phase change heat transfer

discussed.

pool boiling applications.

foam surface.

base fluid.

5.2. Liquid-solid phase change heat transfer

Figure 14. Effect of the Darcy number on the heat transfer.

Figure 14 shows the detailed relationship between the Darcy number and the average Nusselt number. The average Nusselt number almost does not change when Da is less than 10<sup>5</sup> . As Da increases from 10<sup>7</sup> to 10<sup>6</sup> , Nuave only increases by 0.03% for Ra = 10<sup>6</sup> and by 0.14% for Ra = 107 . Natural convection almost can be ignored and there is only heat conduction when the Darcy number is less than 10<sup>5</sup> .

Figure 15 shows the effect of the nanoparticle volume traction on Nuave with different thermal conductivities of nanoparticles (kp). Physical characteristics of nanofluids differ for different nanoparticle volume tractions. The thermal conductivity of the nanofluid increases with an

Figure 15. Effect of the nanoparticle concentration on the heat transfer.

increase in the nanoparticle volume traction, and it is beneficial for promoting the heat transfer, which leads to the increased average Nusselt number.

## 5. Phase change heat transfer

Phase change heat transfer of nanofluid in porous foams is a relatively new theme. In this chapter, the basic scientific problems for this topic will be firstly presented and then the recent research advances will be reviewed. The future research points will also be discussed.

#### 5.1. Liquid-gas phase change heat transfer

Figure 14 shows the detailed relationship between the Darcy number and the average Nusselt number. The average Nusselt number almost does not change when Da is less than 10<sup>5</sup>

Figure 15 shows the effect of the nanoparticle volume traction on Nuave with different thermal conductivities of nanoparticles (kp). Physical characteristics of nanofluids differ for different nanoparticle volume tractions. The thermal conductivity of the nanofluid increases with an

.

Figure 15. Effect of the nanoparticle concentration on the heat transfer.

. Natural convection almost can be ignored and there is only heat conduction when

, Nuave only increases by 0.03% for Ra = 10<sup>6</sup> and by 0.14% for

increases from 10<sup>7</sup> to 10<sup>6</sup>

the Darcy number is less than 10<sup>5</sup>

162 Novel Nanomaterials - Synthesis and Applications

Figure 14. Effect of the Darcy number on the heat transfer.

Ra = 107

. As Da

Boiling heat transfer is used in a variety of industrial processes and applications, such as refrigeration, power generation, heat exchangers, cooling of high-power electronics components and cooling of nuclear reactors [74]. The use of nanofluids for boiling heat transfer enhancement is a promising solution that is currently being explored by many researchers for pool boiling applications.

Lee and Mudawar [75] have undertaken an experimental study to explore the benefits of using alumina/water nanofluid for microchannel cooling applications. They revealed the enhancement of the heat transfer coefficient for single-phase laminar flow. However, in the two-phase regime, the nanofluids caused the surface deposition in micro-channels, and large agglomerates of nanoparticles were formed. Kim et al. [76] investigated the subcooled flow boiling using dilute alumina, zinc oxide and diamond water-based nanofluids. Kim et al. [77] studied the pool boiling by experiment with water-based nanofluids containing Al2O3, ZrO2 and SiO2 nanoparticles. An irregular porous structure was formed at the surface. You et al. [78] measured the CHF in pool boiling using a flat, square copper heater submerged into nanofluids at a sub-atmospheric pressure of 2.89 psia. Nanoparticle deposition was observed by Bang and Chang [79], who also measured a CHF enhancement of 50% with aluminawater nanofluids on a stainless steel plate. Zhu et al. [80] developed a boiling heat transfer coefficient correlation of the refrigerant/lubricating oil mixture on the surface of the metal foam surface.

Several researchers have noticed the nano-deposition at the heater surface, which can alter the surface area, the surface wettability and the bubble nucleation. The nucleation site density, the bubble departure diameter and the bubble frequency are all affected by the nanofluid boiling. It was found by several researchers [77, 78] that bubble diameters increase during boiling with nanofluids, but the nucleation site density decreases with the addition of nanoparticles into the base fluid.

#### 5.2. Liquid-solid phase change heat transfer

Liquid-solid phase change in porous media is frequently encountered in lots of natural and engineering systems. Over the past several decades, this problem has been extensively investigated analytically, experimentally and numerically [81]. Thermal management systems based on latent heat storage of phase change materials (PCMs) can be widely used. Many researches are focused on demonstrating the performance improvement over pure PCM-based thermal management systems and the free and forced-convection heat transfer phenomena inside the porous media [82, 83].

Acknowledgements

Author details

References

89-94

329-336

Huijin Xu1,2\*, Zhanbin Xing2

nologies. 1997;85:55

toral Science Foundation of China (No. 2015 M570363).

\*Address all correspondence to: hjxu1015@gmail.com

China University of Petroleum (Huadong), Qingdao, China

This work is supported by the National Natural Science Foundation of China (No. 51406238), the Fundamental Research Funds for the Central Universities (No. 17CX02047), the Foundation for Outstanding Young Scientist in Shandong Province (No. BS2014NJ009), and the Postdoc-

Convective Transport Characteristics of Nanofluids in Light-Weight Metal Foams with High Porosity

http://dx.doi.org/10.5772/intechopen.72291

165

, Fuqiang Wang<sup>3</sup> and Changying Zhao<sup>1</sup>

2 Department of Energy and Power Engineering, College of Pipeline and Civil Engineering,

3 School of Automobile Engineering, Harbin Institute of Technology at Weihai, Weihai, China

[1] Zheng KC, Wen Z, Wang ZS, Guo-Feng L, Liu XL, Wu WF. Review on forced convection heat transfer in porous media (in Chinese). Acta Physica Sinica. 2012;61(1):1-11

[2] Amhalhel G, Furmański P. Problems of modeling flow and heat transfer in porous media. Biuletyn Instytutu Techniki Cieplnej Politechniki Warsza Wskiej. Journal of Power Tech-

[3] Ergun S. Fluid flow through packed columns. Chemical Engineering Progress. 1952;48(2):

[4] Calmidi VV, Mahajan RL. The effective thermal conductivity of high porosity fibrous

[5] Bhattacharya AC, Mahajan R. Thermophysical properties of high porosity metal foams.

[6] Plessis PD, Montillet A, Comiti J, Legrand J. Pressure drop prediction for flow through high porosity metallic foams. Chemical Engineering Science. 1994;49:3545-3553

[7] Paek JW, Kang BH, Kim SY, Hyun JM. Effective thermal conductivity and permeability of aluminum foam materials. International Journal of Thermophysics. 2000;21(2):453-464 [8] Liu JF, WT W, Chiu WC, Hsieh WH. Measurement and correlation of friction characteristic of flow through foam matrixes. Experimental Thermal and Fluid Science. 2006;30(4):

International Communications in Heat and Mass Transfer. 2002;45:1017-1031

metal foams. Journal of Heat Transfer. 1992;121(2):466-471

1 China-UK Low Carbon College, Shanghai Jiao Tong University, Shanghai, China

Hong and Herling [84] experimentally studied the effect of surface area density on the performance of paraffin-infiltrated aluminum foams with pore sizes from 500 to 2 mm. Lafdi et al. [85] also conducted an experimental study with paraffin-infiltrated aluminum foams and found that both pore size and porosity affected the performance of the system. Tian and Zhao [86] performed similar experiments with copper foams. With the advancement of fabrication techniques for microcellular metal foams [87], the effect of pore size and porosity becomes more interesting due to the extremely large surface area enabled by metal foams with small pore size. Numerical models were also developed to predict the temperature profile of PCM metal foam systems. These models had an origin in Boomsma and Poulikakos [12], where the effective thermal conductivity (ke) of an infiltrated porous metal foam was estimated based on a tetrakaidecahedron pore model. Tao et al. [88] investigated the latent heat storage (LHS) performance of metal foams/paraffin composite phase change material (CPCM) using lattice Boltzmann method. Gao et al. [89] also used the lattice Boltzmann method to simulate solid-liquid phase change with natural convection in porous media under LTNE conditions.

A new sort of nanofluid phase change material (PCM) is developed by suspending a small amount of nanoparticles in melting paraffin by Wu et al. [90]. Zheng et al. [91] found that Ag/1-Tetradecanol showed remarkably high thermal conductivity and reasonably high phase change enthalpy. Khodadadi et al. [92] numerically simulated the solidification of Cu/H2O nanofluids in a vertical square enclosure. Guo [93] numerically obtained that room with alumina/paraffin as PCM ceiling is a good way of saving the required cool energy in summer. Wu et al. [94] investigated the effects of Cu nanoparticles on the thermal conductivity and the phase change heat transfer of Cu/paraffin PCM by the Hot Disk thermal constants analyzer and infrared monitoring methods respectively. The results show that adding nanoparticles is an efficient way to enhance the phase change heat transfer of PCM.

#### 6. Summary

Metal foams and nanofluids are greatly potential for the application of practical thermal applications since they are beneficial for heat transfer enhancement. A review of previous study for different convective flow and heat transfer regimes about the metal foam and the nanofluid is presented in this article. The effects of several parameters in metal foam and nanofluid properties, thermal boundary conditions, and flow and heat transfer characteristics were analyzed. Previous studies have shown that nanofluid and metal foam can enhance heat transfer. Some suggestions for future works should be paid attention to, as turbulent flow of nanofluids flow in metal foams, new models for the heat transfer of nanofluids in metal foams, the micro effect of nanofluid, the non-Newtonian effect of nanofluids, and the slip effect of nanofluid in metal foams.

## Acknowledgements

on latent heat storage of phase change materials (PCMs) can be widely used. Many researches are focused on demonstrating the performance improvement over pure PCM-based thermal management systems and the free and forced-convection heat transfer phenomena inside the

Hong and Herling [84] experimentally studied the effect of surface area density on the performance of paraffin-infiltrated aluminum foams with pore sizes from 500 to 2 mm. Lafdi et al. [85] also conducted an experimental study with paraffin-infiltrated aluminum foams and found that both pore size and porosity affected the performance of the system. Tian and Zhao [86] performed similar experiments with copper foams. With the advancement of fabrication techniques for microcellular metal foams [87], the effect of pore size and porosity becomes more interesting due to the extremely large surface area enabled by metal foams with small pore size. Numerical models were also developed to predict the temperature profile of PCM metal foam systems. These models had an origin in Boomsma and Poulikakos [12], where the effective thermal conductivity (ke) of an infiltrated porous metal foam was estimated based on a tetrakaidecahedron pore model. Tao et al. [88] investigated the latent heat storage (LHS) performance of metal foams/paraffin composite phase change material (CPCM) using lattice Boltzmann method. Gao et al. [89] also used the lattice Boltzmann method to simulate solid-liquid phase change with natural convection in porous media under

A new sort of nanofluid phase change material (PCM) is developed by suspending a small amount of nanoparticles in melting paraffin by Wu et al. [90]. Zheng et al. [91] found that Ag/1-Tetradecanol showed remarkably high thermal conductivity and reasonably high phase change enthalpy. Khodadadi et al. [92] numerically simulated the solidification of Cu/H2O nanofluids in a vertical square enclosure. Guo [93] numerically obtained that room with alumina/paraffin as PCM ceiling is a good way of saving the required cool energy in summer. Wu et al. [94] investigated the effects of Cu nanoparticles on the thermal conductivity and the phase change heat transfer of Cu/paraffin PCM by the Hot Disk thermal constants analyzer and infrared monitoring methods respectively. The results show that adding nanoparticles is an efficient way to enhance the phase change heat

Metal foams and nanofluids are greatly potential for the application of practical thermal applications since they are beneficial for heat transfer enhancement. A review of previous study for different convective flow and heat transfer regimes about the metal foam and the nanofluid is presented in this article. The effects of several parameters in metal foam and nanofluid properties, thermal boundary conditions, and flow and heat transfer characteristics were analyzed. Previous studies have shown that nanofluid and metal foam can enhance heat transfer. Some suggestions for future works should be paid attention to, as turbulent flow of nanofluids flow in metal foams, new models for the heat transfer of nanofluids in metal foams, the micro effect of nanofluid, the non-Newtonian effect of nanofluids, and the slip effect of

porous media [82, 83].

164 Novel Nanomaterials - Synthesis and Applications

LTNE conditions.

transfer of PCM.

6. Summary

nanofluid in metal foams.

This work is supported by the National Natural Science Foundation of China (No. 51406238), the Fundamental Research Funds for the Central Universities (No. 17CX02047), the Foundation for Outstanding Young Scientist in Shandong Province (No. BS2014NJ009), and the Postdoctoral Science Foundation of China (No. 2015 M570363).

## Author details

Huijin Xu1,2\*, Zhanbin Xing2 , Fuqiang Wang<sup>3</sup> and Changying Zhao<sup>1</sup>

\*Address all correspondence to: hjxu1015@gmail.com

1 China-UK Low Carbon College, Shanghai Jiao Tong University, Shanghai, China

2 Department of Energy and Power Engineering, College of Pipeline and Civil Engineering, China University of Petroleum (Huadong), Qingdao, China

3 School of Automobile Engineering, Harbin Institute of Technology at Weihai, Weihai, China

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**Chapter 10**

**Provisional chapter**

**Nanotechnologies in Cultural Heritage - Materials and**

This chapter aims to evaluate the nanomaterials that can be used to diagnostic, conservation and restoration of different artifacts and monuments and that can contribute to solving the problems which could appear during weathering processes of them. The nanotechnology, as a new and revolutionary area in science, can improve the traditional methods currently used for restoration and preservation in cultural heritage and can contribute to the creation of new highly specialized methods for diagnostic and treatment of different artifacts or even monuments. With a smaller size, a higher penetrability, viscosity, thermal and magnetic properties, in comparison with the traditional materials, the nanomaterials can contribute to solve the problems deriving from specific phenomena that could appear during the intervention and to identify the potential newly formed products in the treated materials. In this chapter, some aspects about the nanomaterials used for conservation and restoration of stone and paper artifacts are evidenced and

**Keywords:** nanomaterial, nanotechnology, cultural heritage, hydroxyapatite

**Nanotechnologies in Cultural Heritage - Materials and** 

DOI: 10.5772/intechopen.71950

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

The term "preservation" may have different meanings depending on the field in which it is applied [1]. The knowledge of the preservation of artworks on different artifacts is not limited to historical and semiotic analyses. Preservation nowadays requires an interdisciplinary team with a solid knowledge of materials science, chemistry, physics, microbiology, art history and nanotechnology in order to contribute and offer solutions to prevent the natural aging of some artifacts (paper documents, stone, paints, etc.) and to provide useful and basic

**Instruments for Diagnosis and Treatment**

**Instruments for Diagnosis and Treatment**

Rodica-Mariana Ion, Sanda-Maria Doncea and

Rodica-Mariana Ion, Sanda-Maria Doncea and

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.71950

Daniela Ţurcanu-Caruțiu

**Abstract**

discussed.

**1. Introduction**

Daniela Ţurcanu-Caruțiu

**Provisional chapter**

#### **Nanotechnologies in Cultural Heritage - Materials and Instruments for Diagnosis and Treatment Instruments for Diagnosis and Treatment**

**Nanotechnologies in Cultural Heritage - Materials and** 

DOI: 10.5772/intechopen.71950

Rodica-Mariana Ion, Sanda-Maria Doncea and Daniela Ţurcanu-Caruțiu Daniela Ţurcanu-Caruțiu Additional information is available at the end of the chapter

Rodica-Mariana Ion, Sanda-Maria Doncea and

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.71950

#### **Abstract**

This chapter aims to evaluate the nanomaterials that can be used to diagnostic, conservation and restoration of different artifacts and monuments and that can contribute to solving the problems which could appear during weathering processes of them. The nanotechnology, as a new and revolutionary area in science, can improve the traditional methods currently used for restoration and preservation in cultural heritage and can contribute to the creation of new highly specialized methods for diagnostic and treatment of different artifacts or even monuments. With a smaller size, a higher penetrability, viscosity, thermal and magnetic properties, in comparison with the traditional materials, the nanomaterials can contribute to solve the problems deriving from specific phenomena that could appear during the intervention and to identify the potential newly formed products in the treated materials. In this chapter, some aspects about the nanomaterials used for conservation and restoration of stone and paper artifacts are evidenced and discussed.

**Keywords:** nanomaterial, nanotechnology, cultural heritage, hydroxyapatite

#### **1. Introduction**

The term "preservation" may have different meanings depending on the field in which it is applied [1]. The knowledge of the preservation of artworks on different artifacts is not limited to historical and semiotic analyses. Preservation nowadays requires an interdisciplinary team with a solid knowledge of materials science, chemistry, physics, microbiology, art history and nanotechnology in order to contribute and offer solutions to prevent the natural aging of some artifacts (paper documents, stone, paints, etc.) and to provide useful and basic

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

predictions about the degradation of the cultural patrimony and can come up with viable solutions [2, 3].

One of the directions for the use of nanoparticles, nowadays, is the preservation of cultural heritage. The application of nanotechnologies to different artifacts has recently proved the huge potential of this science to apply in the field of preservation of universal cultural heritage [12, 13]. Solid nanodispersions, micelles, gels or microemulsions offer new viable solutions for the restoration and preservation of works of art. Some recently concerns are related to the synthesis and the application of nanoparticles of Ca and Mg or hydroxyapatite to paper and stone preservation [14]. In this chapter, the most used methods of nanoparticle synthesis and some of their recent applications for the preservation of artifacts are presented. The novelty of this area resides in a type of cultural heritage material (stone, paper) and starts with the main degradation paths and discussing protocols for the application of innovative nanomaterial-based tools for cleaning, consolidation or deacidification, which represent the majority of the case studies encountered in restoration and conserva-

Nanotechnologies in Cultural Heritage - Materials and Instruments for Diagnosis and Treatment

http://dx.doi.org/10.5772/intechopen.71950

175

The main types of atmospheric pollutants which could affect the stony monuments are shown

Usually, calcium carbonate can take three forms of polymorphs: calcite, aragonite and vaterite [15]. It is known that in the calcite variety (calcium carbonate variety detected by X-ray diffraction (XRD)) dissolution is affected by the presence of foreign substances Mg2+ (from

**Nr.crt. Pollutant Source Effect on stone** 1 Carbon dioxide and carbon monoxide Internal combustion engines Acid rain, soil

2 Nitrogen and sulfur oxides Combustion of fossil fuels by motor

O, called epsomite), which is considered one of the major cations in seawater and

plants

activity

vehicles or thermoelectric power

Combustion of fossil fuels, the evaporation of fuels, solvents used in various industrial processes

Combustion of fossil fuels, human

pollution

Acid rain, soil pollution

Acid rain, soil pollution

Acid rain, soil pollution

tion procedures.

*2.1.1. Influence of pollutants*

**2.1. Stone**

in **Table 1**.

MgSO4

·7H2

**2. Artifacts conservation and restoration**

3 Volatile organic compounds (hydrocarbons and

4 Suspended particulates—very small particles similar with gas may contain iron oxides, heavy metals (lead, cadmium, manganese, chromium), asbestos fibers or other pollutants

hydrocarbon derivatives)

**Table 1.** Pollutant source and effects on stone.

Generally speaking, the term "nano" defines an extremely small entity in the order of 10−9 m [4].

The applications of nanotechnologies in the field of preservation of cultural heritage could include:


The present chapter means to complete a survey on the use of nanotechnology for the preservation and restoration of the stone monuments and different damaged paper and stone artifacts. The scientific principles behind numerous nanomaterials on different types of common movable and fixed artistic substrates are discussed. Compared to traditional methods, these new tools have the benefit of considerably less impact on both the operators and the environment. Different types of nanoparticles currently used to produce conservation treatments with enhanced material properties are discussed.

#### **1.1. Nanomaterials for restoration and conservation of cultural heritage**

Nanostructures represent a stage of matter between agglomerated molecules and structures and are typically characterized by a large surface area that affects their physicochemical properties. Innovative applications are of nanostructures and are based on two types of unique properties associated with nanostructures [11]:


One of the directions for the use of nanoparticles, nowadays, is the preservation of cultural heritage. The application of nanotechnologies to different artifacts has recently proved the huge potential of this science to apply in the field of preservation of universal cultural heritage [12, 13]. Solid nanodispersions, micelles, gels or microemulsions offer new viable solutions for the restoration and preservation of works of art. Some recently concerns are related to the synthesis and the application of nanoparticles of Ca and Mg or hydroxyapatite to paper and stone preservation [14]. In this chapter, the most used methods of nanoparticle synthesis and some of their recent applications for the preservation of artifacts are presented. The novelty of this area resides in a type of cultural heritage material (stone, paper) and starts with the main degradation paths and discussing protocols for the application of innovative nanomaterial-based tools for cleaning, consolidation or deacidification, which represent the majority of the case studies encountered in restoration and conservation procedures.

## **2. Artifacts conservation and restoration**

#### **2.1. Stone**

predictions about the degradation of the cultural patrimony and can come up with viable

Generally speaking, the term "nano" defines an extremely small entity in the order of 10−9 m [4]. The applications of nanotechnologies in the field of preservation of cultural heritage could

• Diagnosis of the damaged surfaces to obtain physicochemical and structural information on the materials that form the historic heritage and for identifying the surface damaging

• New instruments and diagnostic methods, in order to make an informed decision on the

• Innovative methods for cleaning surfaces affected by polluting substances and black crusts [7].

• New products for consolidation and protection of natural and artificial stones (compatible

• Materials and innovative processes against raising of humidity and against sulfation for the conservation and/or restoration interventions on art manufactures of different

The present chapter means to complete a survey on the use of nanotechnology for the preservation and restoration of the stone monuments and different damaged paper and stone artifacts. The scientific principles behind numerous nanomaterials on different types of common movable and fixed artistic substrates are discussed. Compared to traditional methods, these new tools have the benefit of considerably less impact on both the operators and the environment. Different types of nanoparticles currently used to produce conservation treatments

Nanostructures represent a stage of matter between agglomerated molecules and structures and are typically characterized by a large surface area that affects their physicochemical properties. Innovative applications are of nanostructures and are based on two types of unique

**2.** Changes in reactivity and mechanical properties due to small physical dimensions and a large area of the specific surface. The advantages of small granule sizes in comparison with the agglomerated materials include: a low sintering temperature, super-elasticity,

solutions [2, 3].

174 Novel Nanomaterials - Synthesis and Applications

products) [9].

nature [10].

type: alveolarization, fractures and so on [5].

with enhanced material properties are discussed.

properties associated with nanostructures [11]:

materials to use during the further phases of restoration [6].

• Treatment of surfaces (protection, waterproofing, self-cleaning) [8].

**1.1. Nanomaterials for restoration and conservation of cultural heritage**

**1.** New optical properties due to the generation of quantum effects.

improved diffusion, improved dielectric and tribological properties.

include:

#### *2.1.1. Influence of pollutants*

The main types of atmospheric pollutants which could affect the stony monuments are shown in **Table 1**.

Usually, calcium carbonate can take three forms of polymorphs: calcite, aragonite and vaterite [15]. It is known that in the calcite variety (calcium carbonate variety detected by X-ray diffraction (XRD)) dissolution is affected by the presence of foreign substances Mg2+ (from MgSO4 ·7H2 O, called epsomite), which is considered one of the major cations in seawater and


**Table 1.** Pollutant source and effects on stone.

groundwater. Also, SO4 2− (from atmospheric pollution responsible for calcite conversion in gypsum) and NO3 (responsible for solubilizing the stone in the wall) are the most damaging pollutants or different artifacts [16]. Processes such as crystallization and salt dissolution contribute to new pores, which are responsible for an accelerated damaging process through the microcracks generated in the stone. The dissolution rate significantly increases in the presence of NaCl solutions, due to electrostatic reasons. Calcium carbonate (CaCO<sup>3</sup> ) can be found in soils, rocks and sediments. Among the minerals, calcium carbonate is one of the most sensitive ones to weathering [17]. Due to rapid weather destruction, small amounts of CaCO<sup>3</sup> can dominate the geochemical behavior of aquatic systems. Given the sensitivity of this rock, it is necessary to study the influence of climatic and environmental factors on this rock [18].

surface size. Daniele and Taglieri [13] studied the morphology of quartz aggregates in granite and showed that the fragmented dimensions of the quartz aggregates are different depending

Nanotechnologies in Cultural Heritage - Materials and Instruments for Diagnosis and Treatment

**a.** Sedimentation and cementation under natural pressure of particles of natural rocks erod-

**b.** Precipitation from natural solutions—gypsum, limestone, dolomite and travertine; sedimentation and consolidation of residues of dead organisms (shells, shells)—diatomite, limestone and chalk. The limestone is derived from marine sediments and fossilized organic lakes (such as crinoids or brachiopods), consisting of calcium carbonate or calcite. Sandstones are the result of sedimentation of sand together with silica or calcium carbonate; the silica in the tiles may have the same adverse effect encountered at the granite. Both sedimentary rocks—limestone and sandstone—are easier to cut than granite, which has made them quite often used in the masonry of historical buildings [23, 24]. On the Romanian territory, there are many masonry made of siliceous sandstone or calcareous sandstone, the first being those with higher compressive strength. The mineralogical composition and crystal structure of the stone monuments before and after the conservation treatment can be

diffraction peaks at 2ϑ: 29.5, 39.5, 47.6, 48.58, etc., corresponding to the calcite phase, which shows that in the water the precipitated calcium carbonate crystals were formed mainly of

In **Figure 2**, there are Fourier transform infrared (FTIR) peaks corresponding to the vaterite

Very clear bands could be observed in FTIR spectra, which show significant differences between not-treated and treated limestones, especially at the bands assigned to carbonate

(**Figure 1**) has a series of

http://dx.doi.org/10.5772/intechopen.71950

177

on the type of granite. The sedimentary rocks are those originating:

ed by wind, rain, sunbathing, conglomerate and sandstone.

determined by X-ray diffraction (XRD) [25, 26]. A sample of CaCO<sup>3</sup>

phase generated inside and outside of the damaged limestone surfaces.

calcite.

groups and to sulfate bands (**Figure 3**).

**Figure 1.** X-ray diffraction of calcite.

The pollutant effects on buildings and monuments degradation require a multidisciplinary approach using conventional and unconventional methods to achieve a good understanding of the mechanisms and consequences of such pollution [6]. It has been established that the hydraulic properties of stones and their traction resistance are the most important parameters that control the stone resistance to decomposition, and these parameters are widely used to estimate their durability.

The effects of the environment on some monuments (Basarabi-Murfatlar Churches) have been assessed through various analytical investigations: thermal analysis, XRD, EDXRF and ion chromatography. When the temperature is less than 120°C, a weight loss due to the absorbed water is recorded, especially in the absence of hydrated salts. In the temperature range 200– 600°C, a loss of chemically bonded water is registered, but no other compounds subjected to weight loss. After 600°C, CO2 loss is observed, due to the decomposition of carbonates, higher for the most degraded stones [19].

Stone surface alteration (determined by scanning electron microscopy (SEM) analysis coupled with diffused X-ray spectroscopy (SEM-EDX)) could be classified in several types:


Kruhl and Nega [21] investigated the fragmented form of quartz particles and found that the size of the fragments decreases as temperature increases during suture formation, while Bernal et al. [22] concluded that the processes of degradation cause the fragmentation of the surface size. Daniele and Taglieri [13] studied the morphology of quartz aggregates in granite and showed that the fragmented dimensions of the quartz aggregates are different depending on the type of granite. The sedimentary rocks are those originating:


In **Figure 2**, there are Fourier transform infrared (FTIR) peaks corresponding to the vaterite phase generated inside and outside of the damaged limestone surfaces.

Very clear bands could be observed in FTIR spectra, which show significant differences between not-treated and treated limestones, especially at the bands assigned to carbonate groups and to sulfate bands (**Figure 3**).

**Figure 1.** X-ray diffraction of calcite.

groundwater. Also, SO4

176 Novel Nanomaterials - Synthesis and Applications

estimate their durability.

for the most degraded stones [19].

oxides present in the medium.

conditions:

ticles on the surface.

gypsum) and NO3

2− (from atmospheric pollution responsible for calcite conversion in

) can be found in

can

(responsible for solubilizing the stone in the wall) are the most damaging

pollutants or different artifacts [16]. Processes such as crystallization and salt dissolution contribute to new pores, which are responsible for an accelerated damaging process through the microcracks generated in the stone. The dissolution rate significantly increases in the presence

soils, rocks and sediments. Among the minerals, calcium carbonate is one of the most sensitive ones to weathering [17]. Due to rapid weather destruction, small amounts of CaCO<sup>3</sup>

dominate the geochemical behavior of aquatic systems. Given the sensitivity of this rock, it is necessary to study the influence of climatic and environmental factors on this rock [18].

The pollutant effects on buildings and monuments degradation require a multidisciplinary approach using conventional and unconventional methods to achieve a good understanding of the mechanisms and consequences of such pollution [6]. It has been established that the hydraulic properties of stones and their traction resistance are the most important parameters that control the stone resistance to decomposition, and these parameters are widely used to

The effects of the environment on some monuments (Basarabi-Murfatlar Churches) have been assessed through various analytical investigations: thermal analysis, XRD, EDXRF and ion chromatography. When the temperature is less than 120°C, a weight loss due to the absorbed water is recorded, especially in the absence of hydrated salts. In the temperature range 200– 600°C, a loss of chemically bonded water is registered, but no other compounds subjected to weight loss. After 600°C, CO2 loss is observed, due to the decomposition of carbonates, higher

Stone surface alteration (determined by scanning electron microscopy (SEM) analysis coupled with diffused X-ray spectroscopy (SEM-EDX)) could be classified in several types:

**a.** Surface damage caused by sulfur and calcium and by the calcite reaction with the sulfur

**b.** Alteration of the surface caused by deposits, when no chemical reactions occurred. These deposits are composed of dust from the anthropogenic particles. These degraded layers are mainly caused by the epigenetic formation of gypsum. In areas with high traffic, the sulfur oxide content is considerably higher [20]. There are two different mechanisms of destruction in the process of damaging the stones under the action of environmental

• Chemical alteration of calcite and precipitation on gypsum that catches atmospheric par-

Kruhl and Nega [21] investigated the fragmented form of quartz particles and found that the size of the fragments decreases as temperature increases during suture formation, while Bernal et al. [22] concluded that the processes of degradation cause the fragmentation of the

• Physical phenomenon of deposition of atmospheric particles on the surface.

of NaCl solutions, due to electrostatic reasons. Calcium carbonate (CaCO<sup>3</sup>

**Figure 2.** FTIR spectra of inside and outside treated limestone surface.

#### **2.2. Causes of natural stone degradation**

**The natural stone** is subjected to a slow and continuous process of deterioration (known as alteration— or degradation, or even "decay"), which is a phenomenon caused by physicochemical causes other than mechanical actions: moisture; crystallization of salts in the mass of the material; deposition of pollutants on the surface of the rock, acting through chemical and/or biological processes; high temperature variations during day and night (strong heat cycles - temperature drop) or accidental fire action; and erosion due to strong winds (process without significant mechanical stress) [25–27].

pollutants, such as mineral salts, combustion gases, powdered plant residues and microorganisms, **Table 2**. The tensions in the surfaces that delineate the pores due to the salts crystallized here, known as "stone efflorescence," represents a great danger for the durability of a historical structure. The most common salts in the efflorescence phenomenon of masonry are sulfates, carbonates and nitrates (sodium, magnesium, calcium, potassium), generated from the atmo-

Exposition to moisture in the form of fog, mist or dew. Diesel engines are one of the most damaging particle sources

**Freeze-thaw cycles**. If the mortar pores are filled with water, a pressure will be exerted on the pore walls of the mortar due to the increase in the volume (by 9%) of the frozen water. Thus, prolonged freeze-thaw cycles will progressively degrade mortars with moisture. The most

The nanomaterials are adequate materials for the architectural heritage, due to their consolidation and protection capacity of damaged building materials. The nanoparticles are able of self-cleaning coatings for a preventive protection system for historical surfaces, preserving the original aspect of treated elements, decreasing the deposition of pollutants and soiling and reducing the onset of external degradation processes due to soiling phenomena. The nanoparticles must have the following attributes: termal stability, biologically and chemically inertness, nontoxic, low cost, stable toward visible or near UV light, good adaptability to various environment and good adsorption in solar spectrum. In addition, these treatments can also have water repellent properties, which favor this self-cleaning action. On the other hand, the presence of soluble salts is recognized as an important decay agent of stone heritage.

), which are transformed in (HNO3

, H2 SO4

Construction, surface water mitigating through the shoulder, ditches or imbibing through in the paved surface of the pavement

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179

Severe alveolar weathering, granular disintegration,

granular disintegration,

salt weathering

efflorescences and subflorescences in buildings, monuments and quarry

efflorescences and subflorescences in buildings, monuments and quarry

Ongoing decay mechanisms, such as

and H2

CO<sup>3</sup> ) in

spheric pollutants (SO2

4 Black crusts - soiling on buildings. This aids the production of gypsum, which crystallizes on the surface

contact with the mortars [24, 29].

**Table 2.** Causes of natural stone degradation.

, NO2

sensitive to this phenomenon is lime mortars [30].

**2.3. Type of inorganic nano-consolidants**

and CO<sup>2</sup>

**Nr.crt. Factor Source Effect on stone**

groundwater,

and thenardite (Na2SO4)

Nanotechnologies in Cultural Heritage - Materials and Instruments for Diagnosis and Treatment

3 Freeze-thaw cycles Weather seasons Severe alveolar weathering,

1 Moisture Rain, freeze-thaw cycles,

2 Salt crystallization Mirabilite (Na2SO4• 10H2O)

**Moisture** is one of the main factors that attack stones through the capillary phenomenon, and limestone, dolomite and marble are most affected by this external factor [28]. During 10 years of exposure to a 100 cm column of atmospheric precipitation, calcareous rocks can reduce their thickness by 0.2 mm. Moisture can also affect the rocks that contain the tiny black, feldspar and tremolite. In the case of granite, the moisture from the pores can lead to the splitting of the stone into layers of 1–3 mm thick, through the expansion-contraction cycles (important to the reversible frost-thaw phenomena, because the freezing volume increases with 9%).

**Salt crystallization** is one of the main phenomena of the destruction of porous materials, which occurs by penetrating the material pores by the aqueous solution containing dissolved

**Figure 3.** FTIR spectra of not-treated and treated limestones.


**Table 2.** Causes of natural stone degradation.

**2.2. Causes of natural stone degradation**

178 Novel Nanomaterials - Synthesis and Applications

**Figure 2.** FTIR spectra of inside and outside treated limestone surface.

without significant mechanical stress) [25–27].

**Figure 3.** FTIR spectra of not-treated and treated limestones.

**The natural stone** is subjected to a slow and continuous process of deterioration (known as alteration— or degradation, or even "decay"), which is a phenomenon caused by physicochemical causes other than mechanical actions: moisture; crystallization of salts in the mass of the material; deposition of pollutants on the surface of the rock, acting through chemical and/or biological processes; high temperature variations during day and night (strong heat cycles - temperature drop) or accidental fire action; and erosion due to strong winds (process

**Moisture** is one of the main factors that attack stones through the capillary phenomenon, and limestone, dolomite and marble are most affected by this external factor [28]. During 10 years of exposure to a 100 cm column of atmospheric precipitation, calcareous rocks can reduce their thickness by 0.2 mm. Moisture can also affect the rocks that contain the tiny black, feldspar and tremolite. In the case of granite, the moisture from the pores can lead to the splitting of the stone into layers of 1–3 mm thick, through the expansion-contraction cycles (important to the reversible frost-thaw phenomena, because the freezing volume increases with 9%).

**Salt crystallization** is one of the main phenomena of the destruction of porous materials, which occurs by penetrating the material pores by the aqueous solution containing dissolved pollutants, such as mineral salts, combustion gases, powdered plant residues and microorganisms, **Table 2**. The tensions in the surfaces that delineate the pores due to the salts crystallized here, known as "stone efflorescence," represents a great danger for the durability of a historical structure. The most common salts in the efflorescence phenomenon of masonry are sulfates, carbonates and nitrates (sodium, magnesium, calcium, potassium), generated from the atmospheric pollutants (SO2 , NO2 and CO<sup>2</sup> ), which are transformed in (HNO3 , H2 SO4 and H2 CO<sup>3</sup> ) in contact with the mortars [24, 29].

**Freeze-thaw cycles**. If the mortar pores are filled with water, a pressure will be exerted on the pore walls of the mortar due to the increase in the volume (by 9%) of the frozen water. Thus, prolonged freeze-thaw cycles will progressively degrade mortars with moisture. The most sensitive to this phenomenon is lime mortars [30].

#### **2.3. Type of inorganic nano-consolidants**

The nanomaterials are adequate materials for the architectural heritage, due to their consolidation and protection capacity of damaged building materials. The nanoparticles are able of self-cleaning coatings for a preventive protection system for historical surfaces, preserving the original aspect of treated elements, decreasing the deposition of pollutants and soiling and reducing the onset of external degradation processes due to soiling phenomena. The nanoparticles must have the following attributes: termal stability, biologically and chemically inertness, nontoxic, low cost, stable toward visible or near UV light, good adaptability to various environment and good adsorption in solar spectrum. In addition, these treatments can also have water repellent properties, which favor this self-cleaning action. On the other hand, the presence of soluble salts is recognized as an important decay agent of stone heritage. Thus, in the last few years, the study of the application of nanoparticles as a desulfating agent for stone, mortars and wall paintings is being carried out [31].

option for this monument [34]. This is the reason for finding other optimal materials. Also, a reduced penetration depth and a limited solubility of lime in water are causing chromatic alteration of stone surface [35, 36]. Except the metallic hydroxides above mentioned, for stone consolidation could be used hydroxyapatite (HAp) [37, 38]. It is a natural mineral form of

> )6 (OH)2

Hydrolysis of acid-catalyzed cellulose is the main source of paper degradation. It is well known that the degradation process resides in the manufacture of low quality paper. At the beginning of the eighteenth century, papermaking technology changed and the paper began to be made of increasingly reactive materials (wood pulp) and acidic substances (rosin, used for sizing, chlorine for bleaching and so on). After a long period of exposure to environmental conditions (e.g., temperature, humidity, light), these substances accelerate chemical degradation of the paper. The global effect is the rapid decrease in the resistance to degradation of paper documents, especially on paper made since the eighteenth century [41–43]. There is a general consensus on the inevitable treatment of deacidification in paper preservation. Deacidification involves a complete neutralization of the paper and, in most cases, the introduction of an alkaline reservoir that opposes the acidity assault in the environment (e.g., pollution) [44]. In this context, many studies have been developed addressing acidity elimination processes [45]. The best methods of deacidification are based on nonaqueous solvents. Less polar fluids minimize the risk of ink solubility. Among these most important deacidification methods are the Wei T'o method and the Bookkeeper

Recently, a new method has been proposed based on alcohols dispersed in calcium hydroxide nanoparticles that give good results in deacidification of the paper. The above-mentioned studies have also been extended to the treatment of paper with magnesium hydroxide nanoparticles (brucellosis) because it has been shown that magnesium reduces the rate of

(MgO), precipitating magnesium salts with an alkaline solution and electrolyzing an aqueous

powder is used as a precursor for the synthesis of magnesium oxide, the size of the hydroxide particles, the shape and degree of agglomeration of these are key parameters for many applications. An example is the sintering stage in the ceramic manufacturing process. Magnesium-based

is usually accomplished by the first two methods [47]. When magnesium hydroxide

oxidative degradation of the cellulose substrate due to exposure to light.

Magnesium hydroxide nanoparticles, Mg(OH)2

solution of a bounce magnesium.

and has the ability to readily accept in its

(brucite), can be obtained by hydrating Mg

ions, with the chemical composi-

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181

ion may be replaced by fluoride,

3

Nanotechnologies in Cultural Heritage - Materials and Instruments for Diagnosis and Treatment

calcium apatite having the formula Ca10(PO4

**3. Paper**

method [46].

Mg(OH)2

structure numerous substitution ions for both Ca2+ and PO4

tion altering and the morphological structure [39]. The OH<sup>−</sup>

chloride or carbonate ions, producing fluorouracil or chlorapatite [40].

**3.1. Considerations on the methods of conserving the historical paper**

The principle of the inorganic materials is to create an insoluble "paste" that fills the pores of the stone. There is a large area of such consolidants, as it is shown in **Table 3**.

Some inorganic materials, such as calcium hydroxide Ca(OH)<sup>2</sup> , magnesium hydroxide Mg(OH)2 , barium hydroxide Ba(OH)<sup>2</sup> , strontium hydroxide Sr(OH)2 and hydroxyapatite (HAp), have already been used as consolidants for different damaged carbonate stones [32]. Calcium hydroxide has been used as nanoparticles (130–300 nm), dispersed in alcohols, as nanosols (50–250 nm), as "pastelike" in ethanol and as calcium hydroxide microparticles (1–3 mm) [33]. But, due to their low porosity, high moisture content of the substrate, oversaturation of the material and quick evaporation of the solvent, calcium hydroxide is not an optimal


**Table 3.** Consolidants used for stone and their effects.

option for this monument [34]. This is the reason for finding other optimal materials. Also, a reduced penetration depth and a limited solubility of lime in water are causing chromatic alteration of stone surface [35, 36]. Except the metallic hydroxides above mentioned, for stone consolidation could be used hydroxyapatite (HAp) [37, 38]. It is a natural mineral form of calcium apatite having the formula Ca10(PO4 )6 (OH)2 and has the ability to readily accept in its structure numerous substitution ions for both Ca2+ and PO4 3 ions, with the chemical composition altering and the morphological structure [39]. The OH<sup>−</sup> ion may be replaced by fluoride, chloride or carbonate ions, producing fluorouracil or chlorapatite [40].

## **3. Paper**

Thus, in the last few years, the study of the application of nanoparticles as a desulfating agent

The principle of the inorganic materials is to create an insoluble "paste" that fills the pores of

(HAp), have already been used as consolidants for different damaged carbonate stones [32]. Calcium hydroxide has been used as nanoparticles (130–300 nm), dispersed in alcohols, as nanosols (50–250 nm), as "pastelike" in ethanol and as calcium hydroxide microparticles (1–3 mm) [33]. But, due to their low porosity, high moisture content of the substrate, oversaturation of the material and quick evaporation of the solvent, calcium hydroxide is not an optimal

, strontium hydroxide Sr(OH)2

layer of higher hardness. In general, these solutions are no

treatment to make it more effective. The process remains

based, but experts believe the problem of the durability of the treated stone, the application to historical structures

Superficial penetration into the pores of the stone. Danger of

All of these builders have increased the resistance of the treated stone layer, but unfortunately they are unstable in color under the action of the already mentioned agents

Polyvinyl acetate may cause a glossy-glassy appearance on the surface of the stone. On the other hand, if the polymers were insufficiently diluted in the solvent, layers were formed which could represent a screen for retaining the moisture and salts in the stone, that is, exactly the opposite of what was intended by applying the waterproofing treatment

The best materials with a reinforcing function A deep penetration into the pores of silky tiles. The penetration of the alkoxylans in the stone occurs at a depth of 20–25 mm, which means much more than the inorganic

, magnesium hydroxide

and hydroxyapatite

the stone. There is a large area of such consolidants, as it is shown in **Table 3**.

1 Alcalo-silicates Deposition of silica in the limestone mass

2 Silico-fluoride Silicone tiles can form a cemented crust on the outside, a

3 Alkaline hydroxides The consolidation effect is still low, requiring repeat

4 Strontium and barium hydroxides These solutions seem to be more effective than calcium-

builders

9 The polyurethanes Treatment is very effective, but warmth and light produce

opposite effects

longer recommended

relatively uneconomical

microorganism population

for stone, mortars and wall paintings is being carried out [31].

, barium hydroxide Ba(OH)<sup>2</sup>

180 Novel Nanomaterials - Synthesis and Applications

**Nr.crt. Consolidant Effect**

5 Inorganic builders (zinc stearate and

6 Alcosilanes (or alkoxylans). Increased

7 Acrylic polymers (methyl-methacrylate, methyl-acrylate, ethyl-methacrylate and

hydrofluoric acid )

butyl-methacrylate).

acetate)

8 Vinyl polymers (polyvinyl chloride,

polyvinylchloride-chlorinate, polyvinyl

**Table 3.** Consolidants used for stone and their effects.

aluminum stearate, aluminum sulfate, phosphoric acid, phosphate and

mechanical strength has also been reported with approx. 20% of the silicon tiles treated, which is already performing

Mg(OH)2

Some inorganic materials, such as calcium hydroxide Ca(OH)<sup>2</sup>

#### **3.1. Considerations on the methods of conserving the historical paper**

Hydrolysis of acid-catalyzed cellulose is the main source of paper degradation. It is well known that the degradation process resides in the manufacture of low quality paper. At the beginning of the eighteenth century, papermaking technology changed and the paper began to be made of increasingly reactive materials (wood pulp) and acidic substances (rosin, used for sizing, chlorine for bleaching and so on). After a long period of exposure to environmental conditions (e.g., temperature, humidity, light), these substances accelerate chemical degradation of the paper. The global effect is the rapid decrease in the resistance to degradation of paper documents, especially on paper made since the eighteenth century [41–43]. There is a general consensus on the inevitable treatment of deacidification in paper preservation. Deacidification involves a complete neutralization of the paper and, in most cases, the introduction of an alkaline reservoir that opposes the acidity assault in the environment (e.g., pollution) [44]. In this context, many studies have been developed addressing acidity elimination processes [45]. The best methods of deacidification are based on nonaqueous solvents. Less polar fluids minimize the risk of ink solubility. Among these most important deacidification methods are the Wei T'o method and the Bookkeeper method [46].

Recently, a new method has been proposed based on alcohols dispersed in calcium hydroxide nanoparticles that give good results in deacidification of the paper. The above-mentioned studies have also been extended to the treatment of paper with magnesium hydroxide nanoparticles (brucellosis) because it has been shown that magnesium reduces the rate of oxidative degradation of the cellulose substrate due to exposure to light.

Magnesium hydroxide nanoparticles, Mg(OH)2 (brucite), can be obtained by hydrating Mg (MgO), precipitating magnesium salts with an alkaline solution and electrolyzing an aqueous solution of a bounce magnesium.

Mg(OH)2 is usually accomplished by the first two methods [47]. When magnesium hydroxide powder is used as a precursor for the synthesis of magnesium oxide, the size of the hydroxide particles, the shape and degree of agglomeration of these are key parameters for many applications. An example is the sintering stage in the ceramic manufacturing process. Magnesium-based alkaline compounds are of great importance in the preservation of cultural heritage. Such an area of interest is also the treatments for deacidification of acidic paper in order to preserve it. Both the Wei T'o and Bookkeeper methods are based on the use of magnesium compounds that generate Mg(OH)2 "in situ" after hydrolysis. Unfortunately, some studies have demonstrated a limited homogeneous distribution of alkaline reservoirs [48]. The purpose of the new studies was to synthesize magnesium nanoparticles following the procedure similar to that of calcium hydroxide, which yielded very good results. Unlike the Wei T'o and Bookkeeper methods, these nanoparticles once deposited on the paper immediately acquire the role of deacidifier or buffer. Synthetic pathways for Mg(OH)2 nanoparticles have an important role both theoretically and practically. Several papers have shown that the precipitation of metallic hydroxides in their salts is strongly affected by variation in synthesis parameters. These include high-temperature reactions, reagent concentration and aging time. In particular, it was demonstrated that temperatures above 100°C promote the formation of nanoparticles in nonaqueous media. Some studies also reported significant effects of organic solvents related to the shape and size of precipitated particles. The obtained particles were characterized for determination of their chemical nature by: FT-TR spectroscopy, specific surface area (SA) measurement of dry powder, X-ray diffraction (XRD) characterization and shape and size characterization by SEM and TEM electronic microscopy with scanning or electron transmission. Applying deacidification of nanoparticle paper is compared to Wei T'o [49].

physicochemical compatibility with the support, and after conversion into carbonates, they

Nanotechnologies in Cultural Heritage - Materials and Instruments for Diagnosis and Treatment

Nanoparticles dispersed in alcohols can be applied to the paper by spraying, impregnating or dipping [51]. This method produces in situ hydroxides and requires dispersants to stabilize the system. The solvents used are volatile, environmentally friendly and with low surface tension so that they act as carrier solids for solid particles and ensure the homogeneity and

and CaO and MgO reagents were used. The oxides in turn were obtained from the corresponding carbonates, which were initially milled and then milled to a size of 100 μm. Then,

The process consists of suspending in a mixture of isopropyl alcohol and water in each of the two stoichiometric proportions of the respective oxides according to the equations given above. The hydrolysis reaction was carried out at 80°C for about 30 min (using deionized

The resulting particles were used to treat paper with HAp in isopropanol, and the historical paper was manually sprayed. The pulverized sheets were from a book printed in Romania, in Bucharest, from a private collection from the end of the nineteenth century. Untreated paper was

and examined by scanning electron microscopy (SEM) [37, 38]. Initially untreated, unwritten, unprinted and uncolored paper was investigated by SEM and AFM, too, as a reference (**Figure 5**). The cellular microarchitecture of cellulose was investigated by SEM. The micrographs obtained for this sample revealed a densely packed cellulose fiber network which, on a microscopic scale, inside the sheet of paper appears randomly oriented without having a majority

the carbonates were calcined at 1000°C. Both carbonates were of analytical purity.

*3.1.2. Treatment of paper document with suspension of alkaline hydroxide nanoparticles*

taken as a reference. The paper was then treated with the nanoparticles of Ca(OH)<sup>2</sup>

**Figure 5.** SEM images for not-damaged and damaged cellulose substrates.

nanoparticles was made in heterogeneous medium,

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183

or Mg(OH)2

work as alkaline reservoirs without producing any undesirable side effect [50].

penetration of nanoparticles into the depth of cellulose fibers [52].

and Mg(OH)2

The synthesis of Ca(OH)<sup>2</sup>

water).

In some cases, during the degradation process, it is possible to identify some odorous compounds such as vanillin or vanilic acid, well identified by gas chromatography, **Figure 4**.

#### *3.1.1. Synthesis of nanoparticles of alkaline hydroxides*

This process can be considerably stopped or slowed down by deacidification treatment. The excellent deacidification agents are Mg(OH)<sup>2</sup> and Ca(OH)<sup>2</sup> because they provide very good

**Figure 4.** GC-MS chromatogram of the damaged paper.

physicochemical compatibility with the support, and after conversion into carbonates, they work as alkaline reservoirs without producing any undesirable side effect [50].

alkaline compounds are of great importance in the preservation of cultural heritage. Such an area of interest is also the treatments for deacidification of acidic paper in order to preserve it. Both the Wei T'o and Bookkeeper methods are based on the use of magnesium compounds that

a limited homogeneous distribution of alkaline reservoirs [48]. The purpose of the new studies was to synthesize magnesium nanoparticles following the procedure similar to that of calcium hydroxide, which yielded very good results. Unlike the Wei T'o and Bookkeeper methods, these nanoparticles once deposited on the paper immediately acquire the role of deacidifier or buffer.

practically. Several papers have shown that the precipitation of metallic hydroxides in their salts is strongly affected by variation in synthesis parameters. These include high-temperature reactions, reagent concentration and aging time. In particular, it was demonstrated that temperatures above 100°C promote the formation of nanoparticles in nonaqueous media. Some studies also reported significant effects of organic solvents related to the shape and size of precipitated particles. The obtained particles were characterized for determination of their chemical nature by: FT-TR spectroscopy, specific surface area (SA) measurement of dry powder, X-ray diffraction (XRD) characterization and shape and size characterization by SEM and TEM electronic microscopy with scanning or electron transmission. Applying deacidification of nanoparticle paper is

In some cases, during the degradation process, it is possible to identify some odorous compounds such as vanillin or vanilic acid, well identified by gas chromatography, **Figure 4**.

This process can be considerably stopped or slowed down by deacidification treatment. The

and Ca(OH)<sup>2</sup>

because they provide very good

"in situ" after hydrolysis. Unfortunately, some studies have demonstrated

nanoparticles have an important role both theoretically and

generate Mg(OH)2

Synthetic pathways for Mg(OH)2

182 Novel Nanomaterials - Synthesis and Applications

compared to Wei T'o [49].

*3.1.1. Synthesis of nanoparticles of alkaline hydroxides*

excellent deacidification agents are Mg(OH)<sup>2</sup>

**Figure 4.** GC-MS chromatogram of the damaged paper.

Nanoparticles dispersed in alcohols can be applied to the paper by spraying, impregnating or dipping [51]. This method produces in situ hydroxides and requires dispersants to stabilize the system. The solvents used are volatile, environmentally friendly and with low surface tension so that they act as carrier solids for solid particles and ensure the homogeneity and penetration of nanoparticles into the depth of cellulose fibers [52].

The synthesis of Ca(OH)<sup>2</sup> and Mg(OH)2 nanoparticles was made in heterogeneous medium, and CaO and MgO reagents were used. The oxides in turn were obtained from the corresponding carbonates, which were initially milled and then milled to a size of 100 μm. Then, the carbonates were calcined at 1000°C. Both carbonates were of analytical purity.

The process consists of suspending in a mixture of isopropyl alcohol and water in each of the two stoichiometric proportions of the respective oxides according to the equations given above. The hydrolysis reaction was carried out at 80°C for about 30 min (using deionized water).

#### *3.1.2. Treatment of paper document with suspension of alkaline hydroxide nanoparticles*

The resulting particles were used to treat paper with HAp in isopropanol, and the historical paper was manually sprayed. The pulverized sheets were from a book printed in Romania, in Bucharest, from a private collection from the end of the nineteenth century. Untreated paper was taken as a reference. The paper was then treated with the nanoparticles of Ca(OH)<sup>2</sup> or Mg(OH)2 and examined by scanning electron microscopy (SEM) [37, 38]. Initially untreated, unwritten, unprinted and uncolored paper was investigated by SEM and AFM, too, as a reference (**Figure 5**).

The cellular microarchitecture of cellulose was investigated by SEM. The micrographs obtained for this sample revealed a densely packed cellulose fiber network which, on a microscopic scale, inside the sheet of paper appears randomly oriented without having a majority

**Figure 5.** SEM images for not-damaged and damaged cellulose substrates.

direction of microfiber orientation. The fibers are homogeneous and seem to come from plant fibers, perhaps cotton or linen. The size of the fibers is different, some of them are whole, others are broken. Some fibers have inlay that might be salt crystals. The presence of mineral crystals in paper can be considered as a consequence of how it is made (**Figure 6**). The presence of luminous areas on the image is a consequence of either the presence of a thicker part of the glue material or a rupture of the paper [53].

On a macroscopic examination of the sprayed sample of nanoparticles of Mg(OH)2 , no negative influence on the optical parameters of the paper is observed. Some white deposits of Mg (OH)2 were formed on the paper surface more pronounced in this case than in the case of Ca (OH)2 , although both consolidants have the same concentration and volume applied to the same type of paper. Not all Mg (OH)2 nanoparticles neutralize the acidity of the paper and the unreacted quantity will be carbonated over time in the presence of atmospheric CO<sup>2</sup> .

#### *3.1.3. Hydroxyapatite nanoparticles: synthesis and characterization*

Hydroxyapatite (HAp) was obtained by the modified precipitation chemical method, and the synthesized substance was analyzed by spectral techniques: atomic force microscopy (AFM), scanning electron microscopy (SEM) (**Figure 7**) X-ray diffraction (XRD) and spectroscopy in Fourier transform infrared (FTIR).

As a method of synthesis, a modified wet precipitation method has been chosen because this is more advantageous due to the ease with which it is achieved, the low working temperature, the relatively important percentage of the pure product and the synthesis equipment that is not expensive. It has been found that well-crystallized products with a low degree of sintering have been obtained, but relatively high calcination temperatures were required and the application of this long-term treatment, 4 h, was to obtain a finished product with the desired parameters [3]. Both X-ray diffraction and infrared spectroscopy showed the high degree of purity of the reaction products [54]. The study of SEM and AFM images was very consistent with the results obtained by other analysis techniques: thermal analysis [46]. It has been determined that the crystal size for HAp synthesized is 70–nm. It has also been concluded that a sintering temperature of the synthesis product above 850°C leads to the occurrence of a by-product reaction, namely tricalcium phosphate, and for its conversion into Hap, it is necessary to calcinate at 1200°C for 4 h.

Nanotechnologies in Cultural Heritage - Materials and Instruments for Diagnosis and Treatment

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185

Nanomaterials bring huge enhancements of improvement endeavors for various applications, due to the extensive scale nanomaterials for auxiliary applications. The structure-handling property acquires an imperative segment in the cultural heritage. The present paper plans to do a review of the condition of workmanship on the use of some nanomaterials to the preservation and rebuilding of the stony and paper cultural artifacts. With a smaller size, a higher penetrability, viscosity, thermal and magnetic properties, in comparison with the traditional materials, the nanomaterials can contribute to solve the problems deriving from specific phenomena that could appear during the intervention and to identify the potential newly formed products in the treated materials. In this chapter, some aspects about the nanomaterials used for conservation and restoration of stone and paper artifacts are evidenced and discussed. Distinctive sorts of nanoparticles right now used to create preservation with upgraded material properties and novel functionalities have been discussed and exemplified in this chapter

, hydroxyapatite), both for their synthesis, characteriza-

**4. Conclusions**

(Ca(OH)<sup>2</sup>

, Mg(OH)2

, Ba(OH)<sup>2</sup>

**Figure 7.** SEM images of not-treated and treated papers with Hap.

, Sr(OH)2

tion and specific applications for paper and stone surfaces.

As synthesis reagents were used, calcium nitrate tetrahydrate, Ca(NO<sup>3</sup> ) 2 ·4H2 O, and dibasic ammonium phosphate, (NH4 ) 2 HPO4 , were previously dissolved each in deionized water. Then, the solution of Ca(NO<sup>3</sup> ) 2 ·4H2 O was added dropwise over the (NH4 ) 2 HPO4 solution, which was stirred vigorously at room temperature for about 1 h until a milky and somewhat gelatinous precipitate was obtained and further stirred for further 1 h to increase the reaction rate and homogenize the mixture [20]. The mixture was synthesized at 100°C for 24 h. Then, the precipitate was washed and filtered on a glass filter. After filtration, the compact and sticky cake was dried at 80°C in a furnace. The dried powder was then ground into a mortar and then calcined in an alumina crucible for 4 h [27].

**Figure 6.** AFM images of not-damaged and damaged cellulose substrates.

Nanotechnologies in Cultural Heritage - Materials and Instruments for Diagnosis and Treatment http://dx.doi.org/10.5772/intechopen.71950 185

**Figure 7.** SEM images of not-treated and treated papers with Hap.

As a method of synthesis, a modified wet precipitation method has been chosen because this is more advantageous due to the ease with which it is achieved, the low working temperature, the relatively important percentage of the pure product and the synthesis equipment that is not expensive. It has been found that well-crystallized products with a low degree of sintering have been obtained, but relatively high calcination temperatures were required and the application of this long-term treatment, 4 h, was to obtain a finished product with the desired parameters [3]. Both X-ray diffraction and infrared spectroscopy showed the high degree of purity of the reaction products [54]. The study of SEM and AFM images was very consistent with the results obtained by other analysis techniques: thermal analysis [46]. It has been determined that the crystal size for HAp synthesized is 70–nm. It has also been concluded that a sintering temperature of the synthesis product above 850°C leads to the occurrence of a by-product reaction, namely tricalcium phosphate, and for its conversion into Hap, it is necessary to calcinate at 1200°C for 4 h.

#### **4. Conclusions**

direction of microfiber orientation. The fibers are homogeneous and seem to come from plant fibers, perhaps cotton or linen. The size of the fibers is different, some of them are whole, others are broken. Some fibers have inlay that might be salt crystals. The presence of mineral crystals in paper can be considered as a consequence of how it is made (**Figure 6**). The presence of luminous areas on the image is a consequence of either the presence of a thicker part

tive influence on the optical parameters of the paper is observed. Some white deposits of Mg

Hydroxyapatite (HAp) was obtained by the modified precipitation chemical method, and the synthesized substance was analyzed by spectral techniques: atomic force microscopy (AFM), scanning electron microscopy (SEM) (**Figure 7**) X-ray diffraction (XRD) and spectroscopy in

which was stirred vigorously at room temperature for about 1 h until a milky and somewhat gelatinous precipitate was obtained and further stirred for further 1 h to increase the reaction rate and homogenize the mixture [20]. The mixture was synthesized at 100°C for 24 h. Then, the precipitate was washed and filtered on a glass filter. After filtration, the compact and sticky cake was dried at 80°C in a furnace. The dried powder was then ground into a mortar

were formed on the paper surface more pronounced in this case than in the case of Ca

, although both consolidants have the same concentration and volume applied to the

nanoparticles neutralize the acidity of the paper and the

, were previously dissolved each in deionized water.

O was added dropwise over the (NH4

) 2 ·4H2

> ) 2 HPO4

, no nega-

.

O, and dibasic

solution,

On a macroscopic examination of the sprayed sample of nanoparticles of Mg(OH)2

unreacted quantity will be carbonated over time in the presence of atmospheric CO<sup>2</sup>

of the glue material or a rupture of the paper [53].

*3.1.3. Hydroxyapatite nanoparticles: synthesis and characterization*

) 2 HPO4

and then calcined in an alumina crucible for 4 h [27].

**Figure 6.** AFM images of not-damaged and damaged cellulose substrates.

)2 ·4H2

As synthesis reagents were used, calcium nitrate tetrahydrate, Ca(NO<sup>3</sup>

same type of paper. Not all Mg (OH)2

184 Novel Nanomaterials - Synthesis and Applications

Fourier transform infrared (FTIR).

ammonium phosphate, (NH4

Then, the solution of Ca(NO<sup>3</sup>

(OH)2

(OH)2

Nanomaterials bring huge enhancements of improvement endeavors for various applications, due to the extensive scale nanomaterials for auxiliary applications. The structure-handling property acquires an imperative segment in the cultural heritage. The present paper plans to do a review of the condition of workmanship on the use of some nanomaterials to the preservation and rebuilding of the stony and paper cultural artifacts. With a smaller size, a higher penetrability, viscosity, thermal and magnetic properties, in comparison with the traditional materials, the nanomaterials can contribute to solve the problems deriving from specific phenomena that could appear during the intervention and to identify the potential newly formed products in the treated materials. In this chapter, some aspects about the nanomaterials used for conservation and restoration of stone and paper artifacts are evidenced and discussed. Distinctive sorts of nanoparticles right now used to create preservation with upgraded material properties and novel functionalities have been discussed and exemplified in this chapter (Ca(OH)<sup>2</sup> , Mg(OH)2 , Ba(OH)<sup>2</sup> , Sr(OH)2 , hydroxyapatite), both for their synthesis, characterization and specific applications for paper and stone surfaces.

## **Acknowledgements**

This chapter received financial support from MCI-UEFISCDI by the projects: PNII 261/2014, PN 16.31.02.04.04, 11 BM/2016 and 31CI/2017.

[6] López-Arce P, Gomez-Villalba LS, Fernández-Valle ME, Álvarez de Buergo M, Fort R. Influence of porosity and relative humidity on consolidation of dolostone with calcium hydroxide nanoparticles: Effectiveness assessment with non-destructive techniques.

Nanotechnologies in Cultural Heritage - Materials and Instruments for Diagnosis and Treatment

http://dx.doi.org/10.5772/intechopen.71950

187

[7] Vázquez-Calvo C, Ávarez de Buergo M, Fort R, Varas-Muriel MJ. Characterization of patinas by means of microscopic techniques. Materials Characterization. 2007;**58**(11-12):

[8] Daniele V, Taglieri G, Quaresima R. The nanolimes in cultural heritage conservation: Characterisation and analysis of the carbonatation process. Journal of Cultural Heritage.

[9] Ferreira Pinto AP, Delgado-Rodrigues J. Stone consolidation: The role of treatment procedures. Journal of Cultural Heritage. 2008;**9**(1):38-53. http://dx.doi.org/10.1016/j.culher.

[10] Naidu S, Sassoni E, Scherer GW. New treatment for corrosion-resistant coatings for marble and consolidation of limestone. In: Stefanaggi M, Vergès-Belmin V, editors. Jardins de Pierres – Conservation of Stone in Parks, Gardens and Cemeteries, Paris, 22-24 June

[12] Baglioni P, Chelazzi D, Giorgi R. Nanotechnologies in the conservation of cultural heritage. A compendium of materials and techniques. Edit. Springer; 2015. ISBN: 978-94-017-

[13] Daniele V, Taglieri G. Nanolime suspensions applied on natural lithotypes: The influence of concentration and residual water content on carbonatation process and on treat-

[14] Ion RM, Teodorescu S, Ştirbescu RM, Dulamă ID, Şuică-Bunghez IR, Bucurică IA, Fierăscu RC, Fierascu I, Ion ML. Effects of the restoration mortar on chalk stone buildings. IOP Conference Series: Materials Science and Engineering; 2016; Jessy. 2016:012038

[15] Lopez-Arce P, Zornoza-Indart A. Carbonation acceleration of calcium hydroxide nanoparticles: induced by yeast fermentation. Applied Physics A. 2015;**120**(4):1475-1495

[16] Ion RM, Fierascu RC, Leahu M, Ion ML, Turcanu D. Nanomaterials for conservation and preservation of historical monuments. In: Proc EWCHP; Bolzano. 2013. pp. 97-104 [17] Ion RM, Bunghez RI, Pop SF, Fierascu RC, Ion ML, Leahu M. Chemical weathering of chalk stone materials from Basarabi churches. Metalurgia International. 2013;**18**(1):89-93

[18] López-Arce P, Gomez-Villalba LS, Martínez-Ramírez S, Álvarez de Buergo M, Fort R. Influence of relative humidity on the carbonation of calcium hydroxide nanoparticles and the formation of calcium carbonate polymorphs. Powder Technology. 2011;**205**(1):263-269.

DOI: http://dx.doi.org/10.1016/j.powtec.2010.09.026

Materials Characterization. 2010;**61**(2):168-184

2011. pp. 289-294; 2011. ISBN: 2-905430-17-6

2007.06.004

9303-2

1119-1132. DOI: http://dx.doi.org/10.1016/j.matchar.2007.04.024

[11] Ion RM. Nano Crystalline Materials. Bucharest: FMR Ed; 2003. 189 p

ment effectiveness. Journal of Cultural Heritage. 2010;**11**(1):102-106

2008;**9**(3):294-301. DOI: http://dx.doi.org/10.1016/j.culher.2007.10.007

### **Author details**

Rodica-Mariana Ion1,2\*, Sanda-Maria Doncea1 and Daniela Ţurcanu-Caruțiu<sup>3</sup>

\*Address all correspondence to: rodica\_ion2000@yahoo.co.uk

1 ICECHIM, Research Center for Scientific Investigations and Conservation/Preservation of Industrial, Cultural and Medical Heritage (SCI-HERITAG), Bucharest, Romania

2 Materials Engineering Department, Research Center "Nanomaterials for Mechanical Microsystems", Valahia University, Targoviste, Romania

3 Center of Expertise of Artworks by Advanced Instrumental Methods (CEOAMIA), Ovidius University, Constanța, Romania

#### **References**


[6] López-Arce P, Gomez-Villalba LS, Fernández-Valle ME, Álvarez de Buergo M, Fort R. Influence of porosity and relative humidity on consolidation of dolostone with calcium hydroxide nanoparticles: Effectiveness assessment with non-destructive techniques. Materials Characterization. 2010;**61**(2):168-184

**Acknowledgements**

186 Novel Nanomaterials - Synthesis and Applications

**Author details**

PN 16.31.02.04.04, 11 BM/2016 and 31CI/2017.

Rodica-Mariana Ion1,2\*, Sanda-Maria Doncea1

University, Constanța, Romania

07 October 2017]

2001;**43**:258-266

conbuildmat.2015.05.100

**References**

\*Address all correspondence to: rodica\_ion2000@yahoo.co.uk

Microsystems", Valahia University, Targoviste, Romania

This chapter received financial support from MCI-UEFISCDI by the projects: PNII 261/2014,

1 ICECHIM, Research Center for Scientific Investigations and Conservation/Preservation of

3 Center of Expertise of Artworks by Advanced Instrumental Methods (CEOAMIA), Ovidius

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**Chapter 11**

**Provisional chapter**

**Nanofibers and Electrospinning Method**

**Nanofibers and Electrospinning Method**

DOI: 10.5772/intechopen.72060

As a result of their peculiar features like extremely high surface area to weight ratio, low density, as well as high pore volume and controllable pore size, which may not be present in other structures, nanofibers have taken centre stage in nanotechnology. The indicated properties, consequently, make non-woven nanofibers as appropriate materials for wide-spread applications. Electrospinning, because of its high productivity, simplicity, low cost, reproducibility and its potentialities of being utilised at the industrial level is regarded as one of the most potential processes in nanotechnology. This method implies the application of high voltage electric field aiming to extract very thin fibres from a polymeric fluid stream (solution or melt) potentially deliverable through a millimetre-scale needle. Electrospinning, as a technique, is reliant on various processing standards like solution properties and processing parameters. Consequently, altering these parameters could exert a considerable degree of influence on the nanofiber size, shape and morphology. Thus, by controlling those parameters well, specific fibres can be produced to benefit

Electrospinning is a simple and comprehensive process for generating an ultrafine fibre from varieties of materials which include polymer, composite and ceramic. The electrospinning setup consists of three major components namely, high voltage power supply, syringe with metal needle and a conductive collector. It is, in fact, very sophisticated, but a simple, processing mechanism of producing nanofiber. The electrospinning process can be classified into several techniques like vibration electrospinning, magneto-electrospinning, siro-electrospinning and bubble electrospinning, according to Liu et al. [1]. As the charge liquid jet moves from the

**Keywords:** nanofibers, application, electrospinning, nanostructure

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Nabeel Zabar Abed Al-Hazeem

Nabeel Zabar Abed Al-Hazeem

http://dx.doi.org/10.5772/intechopen.72060

**Abstract**

various applications.

**1. Introduction**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter


#### **Nanofibers and Electrospinning Method Nanofibers and Electrospinning Method**

#### Nabeel Zabar Abed Al-Hazeem Nabeel Zabar Abed Al-Hazeem

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.72060

**Abstract**

[45] Samanta A. Synthesis of nano calcium hydroxide in aqueous medium. Journal American

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[50] Mosini V, Calvini P, Mattogno G, Righini G. Derivative infrared spectroscopy and electron spectroscopy for chemical analysis of ancient paper documents. Cellulose

[51] Doncea SM, Ion RM, Nuta A, Somoghi R, Ghiurea M. Optical methods of investigation for book papers conservation with nanoparticles. In: SPIE US, editor. Proceeding of

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[54] Taglieri G, Mondelli C, Daniele V, Pusceddu E, Trapananti A. Synthesis and X-ray diffraction analyses of calcium hydroxide nanoparticles in aqueous suspension. Advances in Materials Physics and Chemistry. 2013;**3**:108-112. http://dx.doi.org/10.4236/ampc.2013.

Ceramic Society. 2016;**795**:787-795

190 Novel Nanomaterials - Synthesis and Applications

of Cultural Heritage. 2013;235-329

Journal of Physics. 2008;**53**(5-6):781-791

Chemistry and Technology. 1990;**24**:263-272

dx.doi.org/10.1016/j.ceramint.2014.04.073

Special. 2009;83-86

SPIE; 2010. p. 78211 F

s00339-011-6457-2

31A013

mentation Science and Technology. 2010;**38**(1):96-106

As a result of their peculiar features like extremely high surface area to weight ratio, low density, as well as high pore volume and controllable pore size, which may not be present in other structures, nanofibers have taken centre stage in nanotechnology. The indicated properties, consequently, make non-woven nanofibers as appropriate materials for wide-spread applications. Electrospinning, because of its high productivity, simplicity, low cost, reproducibility and its potentialities of being utilised at the industrial level is regarded as one of the most potential processes in nanotechnology. This method implies the application of high voltage electric field aiming to extract very thin fibres from a polymeric fluid stream (solution or melt) potentially deliverable through a millimetre-scale needle. Electrospinning, as a technique, is reliant on various processing standards like solution properties and processing parameters. Consequently, altering these parameters could exert a considerable degree of influence on the nanofiber size, shape and morphology. Thus, by controlling those parameters well, specific fibres can be produced to benefit various applications.

DOI: 10.5772/intechopen.72060

**Keywords:** nanofibers, application, electrospinning, nanostructure

#### **1. Introduction**

Electrospinning is a simple and comprehensive process for generating an ultrafine fibre from varieties of materials which include polymer, composite and ceramic. The electrospinning setup consists of three major components namely, high voltage power supply, syringe with metal needle and a conductive collector. It is, in fact, very sophisticated, but a simple, processing mechanism of producing nanofiber. The electrospinning process can be classified into several techniques like vibration electrospinning, magneto-electrospinning, siro-electrospinning and bubble electrospinning, according to Liu et al. [1]. As the charge liquid jet moves from the

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

syringe tip to the collector, the mode of current flow changes from ohmic to convective flow as the charge moves instead to the fibre surface.

A slow acceleration is a characteristic of the ohmic flow, since the geometry of the Taylor cone is controlled by the ratio of the surface tension to electrostatic repulsion [2]. After successfully addressing the ohmic flow, the jet travels at a rapid acceleration, which includes the transition zone from liquid to dry solid. In the end, the jet penetrates the collector [3–5]. The name 'Taylor Cone' simply represents the conical shape formed at the needle tip see (**Figure 1**).

In 1964, Sir Geoffrey Ingram Taylor described this cone [7] as a continuation of the study by Zeleny [7] on the formation of a cone-jet of glycerine exposed to high electric fields. Several others including Wilson and Taylor, Nolan and Macky [7] continued research in this area. However, it was Taylor who investigated further into the reactions between droplets and electric fields. Taylor's result is based on two assumptions: (1) that the surface of the cone is an equipotential surface and (2) that the cone exists in steady state equilibrium. Immediately after being discharged and the Taylor cone activated, the polymer jet goes through a whipping process [8] in which the solvent evaporates, precipitating a charged polymer fibre, which lays itself at random on a grounded collecting metal screen. As far as the melt is concerned, the discharged jet solidifies when it travels in the air and is collected on the grounded metal screen [7]. Fridrikh et al. [9] theorised that the terminal diameter of the 'whipping' jet (ht ) is controlled by flow rate (Q), electric current (I) and fluid surface tension (γ) as given by the equation

$$\mathbf{h}\_{\mathbf{i}} = \left(\gamma \overline{\varepsilon} \frac{\mathbf{Q}^2}{\mathbf{I}^2} \frac{2}{\pi (2 \ln \mathbf{x} - 3)}\right)^{\frac{1}{3}} \tag{1}$$

**2. Electrospinning setups**

The electrospinning apparatus is really a simple idea, carrying only three main components: a high voltage power supply, a polymer solution reservoir (e.g., a syringe, with a small diameter needle) with or without a flow control pump, and a metal collecting screen. A high voltage power supply with adjustable control can well provide up to 50-kV DC output and, depending on the number of electrospinning jets, the multiple outputs that function independently, are necessitated. The polymeric solution is kept in a reservoir and connected to a power supply to establish a charged polymer jet. Charging the polymer solution could be done either with a syringe with a metal needle or a capillary with a metal tip in the polymer solution. If the syringe is not placed horizontally, polymer flow can be driven by gravity. However, to remove the experimental variables, a syringe pump is engaged to control the precise flow rate. The fibre collecting screen is expected to be conductive and it can either be a stationary plate or a rotating platform or substrate. The plate can produce non-woven fibres, whereas a

Nanofibers and Electrospinning Method http://dx.doi.org/10.5772/intechopen.72060 193

Presently, two standard electrospinning setups are available namely the vertical and horizontal, with three new electrospinning setups with different angles for the study of the effect of the gravity. As a result of the increasing interest in this technology, many research groups have developed sophisticated mechanisms by which more complex nanofibers structures, can be fabricated in a more controlled and efficient manner [10, 11], see (**Figure 2**). For instance, motor-controlled multiple jets and fibre-collecting targets provide avenue for producing a single nanofibrous scaffold consisting of multiple layers, with each layer obtained from a different polymer type. Furthermore, this technology can be used to manufacture polymer composite scaffolds where the fibres of each layer represent a combination of various polymer types.

rotating platform can produce both nonwoven and aligned fibres.

**Figure 2.** Electrospinning setup and controllable electrospinning process parameters [12].

**2.1. Electrospinning process**

where ε ¯ is the dielectric constant, (x) is the displacement, Eq. (1) offers a prediction that the terminal diameter of the whipping jet is controlled by the flow rate, electric current and the surface tension of the fluid, disregards the elastic effects and fluid evaporation, and also makes an assumption about the minimal jet thinning after the saturation of the whipping instability takes place.

**Figure 1.** Illustration of Taylor Cone formations from the Syringe Needle Tip [6].

## **2. Electrospinning setups**

#### **2.1. Electrospinning process**

syringe tip to the collector, the mode of current flow changes from ohmic to convective flow

A slow acceleration is a characteristic of the ohmic flow, since the geometry of the Taylor cone is controlled by the ratio of the surface tension to electrostatic repulsion [2]. After successfully addressing the ohmic flow, the jet travels at a rapid acceleration, which includes the transition zone from liquid to dry solid. In the end, the jet penetrates the collector [3–5]. The name 'Taylor Cone' simply represents the conical shape formed at the needle tip see

In 1964, Sir Geoffrey Ingram Taylor described this cone [7] as a continuation of the study by Zeleny [7] on the formation of a cone-jet of glycerine exposed to high electric fields. Several others including Wilson and Taylor, Nolan and Macky [7] continued research in this area. However, it was Taylor who investigated further into the reactions between droplets and electric fields. Taylor's result is based on two assumptions: (1) that the surface of the cone is an equipotential surface and (2) that the cone exists in steady state equilibrium. Immediately after being discharged and the Taylor cone activated, the polymer jet goes through a whipping process [8] in which the solvent evaporates, precipitating a charged polymer fibre, which lays itself at random on a grounded collecting metal screen. As far as the melt is concerned, the discharged jet solidifies when it travels in the air and is collected on the grounded metal screen [7]. Fridrikh et al. [9] theorised that the terminal diameter of the 'whipping' jet (ht

controlled by flow rate (Q), electric current (I) and fluid surface tension (γ) as given by the

\_\_\_\_\_\_\_\_ 2 π(2 lnx − 3))

¯ is the dielectric constant, (x) is the displacement, Eq. (1) offers a prediction that the terminal diameter of the whipping jet is controlled by the flow rate, electric current and the surface tension of the fluid, disregards the elastic effects and fluid evaporation, and also makes an assumption about the minimal jet thinning after the saturation of the whipping

\_\_1 3

¯ Q2 \_\_\_ I2

) is

(1)

as the charge moves instead to the fibre surface.

192 Novel Nanomaterials - Synthesis and Applications

ht = (γε

**Figure 1.** Illustration of Taylor Cone formations from the Syringe Needle Tip [6].

(**Figure 1**).

equation

where ε

instability takes place.

The electrospinning apparatus is really a simple idea, carrying only three main components: a high voltage power supply, a polymer solution reservoir (e.g., a syringe, with a small diameter needle) with or without a flow control pump, and a metal collecting screen. A high voltage power supply with adjustable control can well provide up to 50-kV DC output and, depending on the number of electrospinning jets, the multiple outputs that function independently, are necessitated. The polymeric solution is kept in a reservoir and connected to a power supply to establish a charged polymer jet. Charging the polymer solution could be done either with a syringe with a metal needle or a capillary with a metal tip in the polymer solution. If the syringe is not placed horizontally, polymer flow can be driven by gravity. However, to remove the experimental variables, a syringe pump is engaged to control the precise flow rate. The fibre collecting screen is expected to be conductive and it can either be a stationary plate or a rotating platform or substrate. The plate can produce non-woven fibres, whereas a rotating platform can produce both nonwoven and aligned fibres.

Presently, two standard electrospinning setups are available namely the vertical and horizontal, with three new electrospinning setups with different angles for the study of the effect of the gravity. As a result of the increasing interest in this technology, many research groups have developed sophisticated mechanisms by which more complex nanofibers structures, can be fabricated in a more controlled and efficient manner [10, 11], see (**Figure 2**). For instance, motor-controlled multiple jets and fibre-collecting targets provide avenue for producing a single nanofibrous scaffold consisting of multiple layers, with each layer obtained from a different polymer type. Furthermore, this technology can be used to manufacture polymer composite scaffolds where the fibres of each layer represent a combination of various polymer types.

**Figure 2.** Electrospinning setup and controllable electrospinning process parameters [12].

#### **3. Mechanism and technique for nanofiber formation**

Electrospinning process has been studied extensively [13]. The mechanism takes place when the surface tension of the solution is overcome by an applied electric field, thereby ejecting tiny jets from the surface. Taylor [14] identified a critical voltage at which this breakdown would occur:

$$\mathbf{V}\_c^2 = 4 \frac{\mathbf{H}^2}{\mathbf{L}^2} \left( \ln \frac{2\mathbf{L}}{\mathbf{R}} - \frac{3}{2} \right) (0.117 \,\pi \eta \mathbf{R}) \tag{2}$$

where Vc is the critical voltage, H is the separation between the capillary and the ground, L is the length of the capillary, R is the radius of the capillary and γ is the surface tension of the liquid. A similar relationship by Carson et al. [15] had progressed for the potential electrostatic spraying from a hemispherical drop pendant from a capillary tube:

$$\mathbf{V} = \Im 00 \sqrt{20 \eta \mathbf{r}} \tag{3}$$

where [η] is the intrinsic viscosity of the polymer which is the ratio of the specific viscosity to the concentration at an infinite dilution and C is the concentration of the polymer solu-

of the entanglement of polymer chains in a solution. As far as highly diluted solutions are

solution. There is low probability for individual molecules to be entangled with each other.

When Be is greater than one, the polymer concentration as well as the level of molecular entanglement are enhanced, leading to more favourable conditions for the fibre formation [20]. The experiment implied that as the solution viscosity increased the fibre diameter also increased (approximately proportionally) to the jet length. Baumgarten detailed the relationship between fibre diameter and solution viscosity expressed by the following equation:

> √ \_\_

where D is the fibre diameter and η is the solution viscosity in poise. It is also proves that fibre diameter is highly dependent on the applied electric field. An increase in the applied voltage increases the electrostatic stress, which, in turn, produces smaller diameter fibres [7, 21]. According to Huang et al. [22] further increase in the electric field, is a critical value achieved when the electrostatic repulsive force deals with the surface tension and the charged jet of the fluid is ejected from the end of the Taylor cone. In the process, the solvent evaporates, contributing to the formation of charged polymer fibre. As for the melt, the discharged jet solidifies when it moves in the air. Based on findings of the study conducted by Pham et al. [23], the shape of the base is dependent on the surface tension of the liquid and the force of the electric field; jets can be ejected from surfaces that are mostly flat if the electric field is high enough. The solvent in the polymer jet evaporates in the movement to the collecting screen, thereby increasing the surface charge on the jet. As it passes via the electric field, this increase in the surface charge induces instability in the polymer jet [24]. The polymer jet divides geometrically, first into two jets, and then into many more as the process repeats itself in order to compensate for this instability. Nanofibers are formed from the spinning force action, given

is less than unity, the polymer molecules are sparsely distributed in the

also describes the level

Nanofibers and Electrospinning Method http://dx.doi.org/10.5772/intechopen.72060 195

η (5)

tion. Intrinsic viscosity is dependent on polymer molecular weight. B*<sup>e</sup>*

**Figure 3.** Schematic illustration of the effects by electric field applied to a solution in a capillary.

D = <sup>2</sup>

concerned, when B*<sup>e</sup>*

where v is electric field, γ is the surface tension of the liquid and r is the radius of the pendant drop [15]. As he investigated a small range of fluids, Taylor determined a 49.3° equilibrium angle-balanced surface tension with electrostatic forces, and he had manipulated this value in his derivation. Taylor cones are required for electrospinning because they define the onset of subtle velocity gradients in the fibre forming process. When V > Vc, a thin jet of solution will explode from the cone surface and travel towards the nearest electrode of opposite polarity, or electrical ground. As a way to describe this, electrospinning jet is a string of charged elements connected by a viscoelastic medium, with one end attached to the point of origin and the other end is let free. When a polymer solution, held in a capillary by its surface tension, is subjected to an electrical field, a charge is induced on the liquid surface [16]. Mutual charge repulsion occurs in a force opposing the surface tension forces and shear stresses are set up in the fluid. With increased intensity in the electrical field, ions in the like-polarity solution will be forced to aggregate at the surface of the drop. The length of the stable jet increases with increasing voltage. After the viscoelastic jet starts flowing away from the Taylor's cone, initially it traverses a linear path. The jet gradually starts to bend away from this linear path and complex shape changes may occur from repulsive forces generated in the charged elements inside the electrospinning jet [17]. The jet may undergo substantial reductions in cross sectional area and spiralling loops may grow from it. This phenomenon is often referred to as whipping instability. Consequently, the hemispherical surface of the solution at the tip of the capillary elongates to form a cone, called the Taylor cone (**Figure 3**). At remarkably high electrical fields (V > Vc), a charged jet of solution is ejected from the tip of the Taylor cone and will travel to an electrode of opposite polarity (or electrical ground).

A dimensionless parameter called the Berry number (B*<sup>e</sup>* ) [18, 19] was used by various research groups as a processing index for controlling the diameter of fibres. B*<sup>e</sup>* number is defined as:

$$\mathbf{B}\_{\epsilon} = \eta \mathbf{C} \tag{4}$$

**Figure 3.** Schematic illustration of the effects by electric field applied to a solution in a capillary.

**3. Mechanism and technique for nanofiber formation**

<sup>2</sup> = 4 H2 \_\_\_

spraying from a hemispherical drop pendant from a capillary tube:

V = 300 √

L2 (ln\_\_\_ 2L

would occur:

Vc

194 Novel Nanomaterials - Synthesis and Applications

(or electrical ground).

A dimensionless parameter called the Berry number (B*<sup>e</sup>*

groups as a processing index for controlling the diameter of fibres. B*<sup>e</sup>*

B*<sup>e</sup>* = C (4)

Electrospinning process has been studied extensively [13]. The mechanism takes place when the surface tension of the solution is overcome by an applied electric field, thereby ejecting tiny jets from the surface. Taylor [14] identified a critical voltage at which this breakdown

<sup>R</sup> <sup>−</sup> \_\_3

where Vc is the critical voltage, H is the separation between the capillary and the ground, L is the length of the capillary, R is the radius of the capillary and γ is the surface tension of the liquid. A similar relationship by Carson et al. [15] had progressed for the potential electrostatic

where v is electric field, γ is the surface tension of the liquid and r is the radius of the pendant drop [15]. As he investigated a small range of fluids, Taylor determined a 49.3° equilibrium angle-balanced surface tension with electrostatic forces, and he had manipulated this value in his derivation. Taylor cones are required for electrospinning because they define the onset of subtle velocity gradients in the fibre forming process. When V > Vc, a thin jet of solution will explode from the cone surface and travel towards the nearest electrode of opposite polarity, or electrical ground. As a way to describe this, electrospinning jet is a string of charged elements connected by a viscoelastic medium, with one end attached to the point of origin and the other end is let free. When a polymer solution, held in a capillary by its surface tension, is subjected to an electrical field, a charge is induced on the liquid surface [16]. Mutual charge repulsion occurs in a force opposing the surface tension forces and shear stresses are set up in the fluid. With increased intensity in the electrical field, ions in the like-polarity solution will be forced to aggregate at the surface of the drop. The length of the stable jet increases with increasing voltage. After the viscoelastic jet starts flowing away from the Taylor's cone, initially it traverses a linear path. The jet gradually starts to bend away from this linear path and complex shape changes may occur from repulsive forces generated in the charged elements inside the electrospinning jet [17]. The jet may undergo substantial reductions in cross sectional area and spiralling loops may grow from it. This phenomenon is often referred to as whipping instability. Consequently, the hemispherical surface of the solution at the tip of the capillary elongates to form a cone, called the Taylor cone (**Figure 3**). At remarkably high electrical fields (V > Vc), a charged jet of solution is ejected from the tip of the Taylor cone and will travel to an electrode of opposite polarity

\_\_\_\_\_

<sup>2</sup>)(0.117R) (2)

20 r (3)

) [18, 19] was used by various research

number is defined as:

where [η] is the intrinsic viscosity of the polymer which is the ratio of the specific viscosity to the concentration at an infinite dilution and C is the concentration of the polymer solution. Intrinsic viscosity is dependent on polymer molecular weight. B*<sup>e</sup>* also describes the level of the entanglement of polymer chains in a solution. As far as highly diluted solutions are concerned, when B*<sup>e</sup>* is less than unity, the polymer molecules are sparsely distributed in the solution. There is low probability for individual molecules to be entangled with each other.

When Be is greater than one, the polymer concentration as well as the level of molecular entanglement are enhanced, leading to more favourable conditions for the fibre formation [20]. The experiment implied that as the solution viscosity increased the fibre diameter also increased (approximately proportionally) to the jet length. Baumgarten detailed the relationship between fibre diameter and solution viscosity expressed by the following equation:

$$\mathbf{D} = \sqrt[2]{\eta} \tag{5}$$

where D is the fibre diameter and η is the solution viscosity in poise. It is also proves that fibre diameter is highly dependent on the applied electric field. An increase in the applied voltage increases the electrostatic stress, which, in turn, produces smaller diameter fibres [7, 21]. According to Huang et al. [22] further increase in the electric field, is a critical value achieved when the electrostatic repulsive force deals with the surface tension and the charged jet of the fluid is ejected from the end of the Taylor cone. In the process, the solvent evaporates, contributing to the formation of charged polymer fibre. As for the melt, the discharged jet solidifies when it moves in the air. Based on findings of the study conducted by Pham et al. [23], the shape of the base is dependent on the surface tension of the liquid and the force of the electric field; jets can be ejected from surfaces that are mostly flat if the electric field is high enough. The solvent in the polymer jet evaporates in the movement to the collecting screen, thereby increasing the surface charge on the jet. As it passes via the electric field, this increase in the surface charge induces instability in the polymer jet [24]. The polymer jet divides geometrically, first into two jets, and then into many more as the process repeats itself in order to compensate for this instability. Nanofibers are formed from the spinning force action, given by the electrostatic force on the continuous splitting of the polymer droplets. They are deposited one layer after another on the metal target plate, forming a non-woven nanofibrous mat. The mechanisms, by which nanofibers are formed, much less controlled, have not been fully elucidated yet, although the electrospraying/electrospinning technology has been used for such a long time. Not much of theoretical clarity has been achieved, although, several studies have been carried out to investigate the mechanism of fibre formation to reproducibly control scaffold design. A uniform fibrous structure is created only under optimised operating during the process electrospinning. The structural morphology of the nanofibers is affected by both extrinsic and intrinsic parameters [16]. In order to produce inform nanofibers, external parameters, like environmental humidity and temperature, in addition to intrinsic parameters, including applied voltage, working distance and conductivity and viscosity of the polymer solution need to be optimised. The intrinsic parameters are more critical in determining the nanofiber structure in general sense.

1% to 5% by weight as shown in **Figure 4**, and below 3% for beads or beaded fibres and 3% and above for continuous nanofibers [32]. A bead that contains nanofibers structure was created below this range. Spherical beads had become longer and turned into spindle-shaped ones, with increased viscosity, and the number of beads in the structure was decreased. In the same manner, Liu et al. also talked about that a different particular range of viscosity rendered appropriate for the formation of uniform nanofibers composed of cellulose [33]. Recent studies done by Deitzel et al. [34] and Demir et al. [35], in addition, have illustrated that a more viscous polymer solution can well create larger fibres. To sum up, these studies have been able

Nanofibers and Electrospinning Method http://dx.doi.org/10.5772/intechopen.72060 197

to prove that there is a polymer-specific, optimal viscosity value for electrospinning.

*4.1.2. Conductivity*

(e) 1% [32].

According to Sill and von Recum [36], polymer concentration determines the spinnability of a solution. For chain entanglements to occur, the solution must have high enough polymer concentration. However, the solution should not be either too diluted or too concentrated. Both, the viscosity and surface tension of the solution are affected by the polymer concentration.

The charge carrying capacity of polymer solutions with high conductivity is greater than solutions with low conductivity. Therefore, the fibre jet produced from a solution of high conductivity will tend to have higher tensile force when exposed to an applied voltage. Through

**Figure 4.** FESEM of electrospinning of nanofibers with different concentrations PLA. (a) 5%, (b) 4%, (c) 3%, (d) 2%, and

### **4. Operating parameters for electrospinning**

Three main parameters, which are solution parameters, process parameters and ambient parameters, tend to affect the electrospinning process. These operating parameters play a big role in determining the desired quality of the electrospun fibre produced [25, 26]. The most preferred in many applications is a fibre with diameter within 10–1000 nm in scale and a smooth surface morphology. The solution properties are difficult to alter, according to Lu and Ding [25], since the relationship between one parameter will drag the other parameters; and in addition, they are very difficult to isolate as one controllable parameter. Li and Wang [27] discussed the effects of working parameters that govern the electrospinning process and the process, and discovered that these parameters could affect the fibre morphologies and diameters. In their study on the effects of the parameters on nanofiber diameter, Thompson et al. [28] found out that the jet radius could leave an impact to the production of the electrospun fibre. In their study, a number of parameters significantly affected the fibre formation compared to other parameters. For instance, the first electrospinning method by Formhals [29] had had some technical disadvantages since it was not easy to dry the fibres entirely after electrospinning as the spinning and collection zones have a very short distance; this resulted in a less aggregated web structure [29]. After half a decade, however Formhals [30], in his pioneering work, changed the distance between the nozzle and the collecting device in order to give more drying time for the electrospun fibres at a longer distance [31].

#### **4.1. Polymer/solution parameters**

#### *4.1.1. Viscosity/concentration*

The most critical factor in controlling the structural morphology of the nanofibrous structure is polymer solution viscosity, a parameter that is directly proportional to the concentration of the polymer solution. For fibre formation, polymer viscosity should be in a particular range, depending on the type of polymer and solvent used. An electrospinning method by Zeng J et al. was used to fabricate PLA nanofiber, with different concentrations or viscosities ranging from 1% to 5% by weight as shown in **Figure 4**, and below 3% for beads or beaded fibres and 3% and above for continuous nanofibers [32]. A bead that contains nanofibers structure was created below this range. Spherical beads had become longer and turned into spindle-shaped ones, with increased viscosity, and the number of beads in the structure was decreased. In the same manner, Liu et al. also talked about that a different particular range of viscosity rendered appropriate for the formation of uniform nanofibers composed of cellulose [33]. Recent studies done by Deitzel et al. [34] and Demir et al. [35], in addition, have illustrated that a more viscous polymer solution can well create larger fibres. To sum up, these studies have been able to prove that there is a polymer-specific, optimal viscosity value for electrospinning.

According to Sill and von Recum [36], polymer concentration determines the spinnability of a solution. For chain entanglements to occur, the solution must have high enough polymer concentration. However, the solution should not be either too diluted or too concentrated. Both, the viscosity and surface tension of the solution are affected by the polymer concentration.

#### *4.1.2. Conductivity*

by the electrostatic force on the continuous splitting of the polymer droplets. They are deposited one layer after another on the metal target plate, forming a non-woven nanofibrous mat. The mechanisms, by which nanofibers are formed, much less controlled, have not been fully elucidated yet, although the electrospraying/electrospinning technology has been used for such a long time. Not much of theoretical clarity has been achieved, although, several studies have been carried out to investigate the mechanism of fibre formation to reproducibly control scaffold design. A uniform fibrous structure is created only under optimised operating during the process electrospinning. The structural morphology of the nanofibers is affected by both extrinsic and intrinsic parameters [16]. In order to produce inform nanofibers, external parameters, like environmental humidity and temperature, in addition to intrinsic parameters, including applied voltage, working distance and conductivity and viscosity of the polymer solution need to be optimised. The intrinsic parameters are more critical in determining

Three main parameters, which are solution parameters, process parameters and ambient parameters, tend to affect the electrospinning process. These operating parameters play a big role in determining the desired quality of the electrospun fibre produced [25, 26]. The most preferred in many applications is a fibre with diameter within 10–1000 nm in scale and a smooth surface morphology. The solution properties are difficult to alter, according to Lu and Ding [25], since the relationship between one parameter will drag the other parameters; and in addition, they are very difficult to isolate as one controllable parameter. Li and Wang [27] discussed the effects of working parameters that govern the electrospinning process and the process, and discovered that these parameters could affect the fibre morphologies and diameters. In their study on the effects of the parameters on nanofiber diameter, Thompson et al. [28] found out that the jet radius could leave an impact to the production of the electrospun fibre. In their study, a number of parameters significantly affected the fibre formation compared to other parameters. For instance, the first electrospinning method by Formhals [29] had had some technical disadvantages since it was not easy to dry the fibres entirely after electrospinning as the spinning and collection zones have a very short distance; this resulted in a less aggregated web structure [29]. After half a decade, however Formhals [30], in his pioneering work, changed the distance between the nozzle and the collecting device in order

to give more drying time for the electrospun fibres at a longer distance [31].

The most critical factor in controlling the structural morphology of the nanofibrous structure is polymer solution viscosity, a parameter that is directly proportional to the concentration of the polymer solution. For fibre formation, polymer viscosity should be in a particular range, depending on the type of polymer and solvent used. An electrospinning method by Zeng J et al. was used to fabricate PLA nanofiber, with different concentrations or viscosities ranging from

the nanofiber structure in general sense.

196 Novel Nanomaterials - Synthesis and Applications

**4.1. Polymer/solution parameters**

*4.1.1. Viscosity/concentration*

**4. Operating parameters for electrospinning**

The charge carrying capacity of polymer solutions with high conductivity is greater than solutions with low conductivity. Therefore, the fibre jet produced from a solution of high conductivity will tend to have higher tensile force when exposed to an applied voltage. Through

**Figure 4.** FESEM of electrospinning of nanofibers with different concentrations PLA. (a) 5%, (b) 4%, (c) 3%, (d) 2%, and (e) 1% [32].

observation, an increase in the solution conductivity brings about a substantial decrease in the nanofiber diameter; and also, evidently the radius of the nanofiber jet is inversely related to the cube root of the electrical conductivity of the solution [7, 37, 38].

The conductivity of a given cell has a connection with the molar conductivity following Eq. (6) [39], where k is the conductivity with units of mS/cm, c is the ion concentration with units of mol/L and therefore, the molar conductivity (Λ) has units of S cm<sup>2</sup> /mol.

$$
\Lambda = \frac{\mathbf{k}}{\mathbf{c}} \tag{6}
$$

produced nanofibers with the smallest diameter. The ion size was also found to ascertain the nanofiber diameter. Ions with smaller radii had higher charge density, and thus they gave

Nanofibers and Electrospinning Method http://dx.doi.org/10.5772/intechopen.72060 199

Molecular weight of the polymer also leaves a great effect on the morphologies of the electrospinning fibre. The entanglement of polymer chains in solutions, namely the solution viscosity, is principally a reflection of the molecular weight. Keeping the concentration fixed, and lowering the molecular weight of the polymer have the ability to form beads instead of the smooth fibre. Smooth fibre will be obtained by increasing the molecular weight. What is also worth noting is that too high molecular weight favours micro-ribbon formation even with low concentration [42, 43]. Çiğdem A et al. [44] studied the impact of the molecular weight (MW) on the fibre structure of electrospun poly(vinylalcohol) (PVA) which has molecular weights that range from 89000 to 186,000 g/mol when dissolved in water, as can be seen in **Figure 6**.

As the function of solvent compositions of the solution, surface tension is an important factor in electrospinning. Yang etal. [45] conducted a study on the influence of surface tensions on the morphologies of electrospun products with PVP as model with ethanol, N,N-dimethylformamide (DMF) and dichloromethane (MC) as solvents. In the process, it was discovered that different solutions may contribute different surface tensions. With the concentration fixed, and the surface tension of the solution, reduced beaded fibres can be converted into smooth fibres.

For a particular polymer to solubilise and be transformed into nanofibers via the process of electrospinning, the choice of solvent is very important. The solubility of the polymer in the solvent and the boiling point of the solvent, which altogether indicate its volatility, are two major aspects worth considering when it comes to choosing a solvent. Volatile solvents are

**Figure 6.** FESEM showing the typical structure in the electrospun PVA polymer for various molecular weights. (a) 89000–98,000 g/mol; (b) ~ 125,000 g/mol; and (c) 146,000–186,000 g/mol (solution concentration: 25 wt.%) [44].

greater forces of elongation on the electrospun nanofibers [41].

*4.1.3. Molecular weight*

*4.1.4. Surface tension*

*4.1.5. Solvent selection*

Chitral and Shesha [40] had published the results of a comprehensive investigation of the effects of change in the conductivity of polyethylene oxide (PEO)/water solution on the electrospinning process and fibre morphology. The effects of the conductivity of PEO solution on the jet current and jet path were elaborated further, with the addition of NaCl to the solution results in the formation of protrusion on the fibre surface, as shown in **Figure 5**. The effects of the conductivity of polyethylene oxide (PEO) solution on the jet current and jet path were also considered.

Zong and colleagues [37] had done a study to see the effect of adding varying kinds of salts to poly(L-lactic acid) (PLLA) solutions in electrospinning. KH<sup>2</sup> PO4 , NaH<sup>2</sup> PO4 and NaCl were studied, and each was added in separation at 1% W/V to PLLA solutions. The resulting electrospun nanofibers were smooth, bead-free and they also had smaller diameters than those of the nanofibers electrospun from solutions that did not have a salt. While KH<sup>2</sup> PO4 − which contains solutions produced nanofibers with the largest diameter, those containing NaCl

**Figure 5.** FESEM images of samples of electrospinning PEO/NaCl fibres for a range of conductivities. (a) 5 g/0 g, (b) 5 g/0.1 g, (c) 5 g/0.2 g, (d) 5 g/0.5 g, (e) 5 g/1.25 g, and (f) 5 g/2 g [40].

produced nanofibers with the smallest diameter. The ion size was also found to ascertain the nanofiber diameter. Ions with smaller radii had higher charge density, and thus they gave greater forces of elongation on the electrospun nanofibers [41].

#### *4.1.3. Molecular weight*

observation, an increase in the solution conductivity brings about a substantial decrease in the nanofiber diameter; and also, evidently the radius of the nanofiber jet is inversely related to

The conductivity of a given cell has a connection with the molar conductivity following Eq. (6) [39], where k is the conductivity with units of mS/cm, c is the ion concentration with units

Chitral and Shesha [40] had published the results of a comprehensive investigation of the effects of change in the conductivity of polyethylene oxide (PEO)/water solution on the electrospinning process and fibre morphology. The effects of the conductivity of PEO solution on the jet current and jet path were elaborated further, with the addition of NaCl to the solution results in the formation of protrusion on the fibre surface, as shown in **Figure 5**. The effects of the conductivity of polyethylene oxide (PEO) solution on the jet current and jet path were also considered. Zong and colleagues [37] had done a study to see the effect of adding varying kinds of salts

studied, and each was added in separation at 1% W/V to PLLA solutions. The resulting electrospun nanofibers were smooth, bead-free and they also had smaller diameters than those

contains solutions produced nanofibers with the largest diameter, those containing NaCl

**Figure 5.** FESEM images of samples of electrospinning PEO/NaCl fibres for a range of conductivities. (a) 5 g/0 g, (b)

5 g/0.1 g, (c) 5 g/0.2 g, (d) 5 g/0.5 g, (e) 5 g/1.25 g, and (f) 5 g/2 g [40].

of the nanofibers electrospun from solutions that did not have a salt. While KH<sup>2</sup>

/mol.

<sup>c</sup> (6)

PO4

, NaH<sup>2</sup>

PO4

and NaCl were

PO4 − which

the cube root of the electrical conductivity of the solution [7, 37, 38].

of mol/L and therefore, the molar conductivity (Λ) has units of S cm<sup>2</sup>

to poly(L-lactic acid) (PLLA) solutions in electrospinning. KH<sup>2</sup>

Λ = k\_\_

198 Novel Nanomaterials - Synthesis and Applications

Molecular weight of the polymer also leaves a great effect on the morphologies of the electrospinning fibre. The entanglement of polymer chains in solutions, namely the solution viscosity, is principally a reflection of the molecular weight. Keeping the concentration fixed, and lowering the molecular weight of the polymer have the ability to form beads instead of the smooth fibre. Smooth fibre will be obtained by increasing the molecular weight. What is also worth noting is that too high molecular weight favours micro-ribbon formation even with low concentration [42, 43]. Çiğdem A et al. [44] studied the impact of the molecular weight (MW) on the fibre structure of electrospun poly(vinylalcohol) (PVA) which has molecular weights that range from 89000 to 186,000 g/mol when dissolved in water, as can be seen in **Figure 6**.

#### *4.1.4. Surface tension*

As the function of solvent compositions of the solution, surface tension is an important factor in electrospinning. Yang etal. [45] conducted a study on the influence of surface tensions on the morphologies of electrospun products with PVP as model with ethanol, N,N-dimethylformamide (DMF) and dichloromethane (MC) as solvents. In the process, it was discovered that different solutions may contribute different surface tensions. With the concentration fixed, and the surface tension of the solution, reduced beaded fibres can be converted into smooth fibres.

#### *4.1.5. Solvent selection*

For a particular polymer to solubilise and be transformed into nanofibers via the process of electrospinning, the choice of solvent is very important. The solubility of the polymer in the solvent and the boiling point of the solvent, which altogether indicate its volatility, are two major aspects worth considering when it comes to choosing a solvent. Volatile solvents are

**Figure 6.** FESEM showing the typical structure in the electrospun PVA polymer for various molecular weights. (a) 89000–98,000 g/mol; (b) ~ 125,000 g/mol; and (c) 146,000–186,000 g/mol (solution concentration: 25 wt.%) [44].

the more favourable choice as they assist the dehydration of the nanofibers during trajectory from the capillary tip to the collector surface, because of their lower boiling point, and thus causing a rapid evaporation rate. Nonetheless, highly volatile solvents that have very low boiling points should be prevented as they may evaporate at the capillary tip and further leading to the clogging and the obstruction of the flow-rate of the polymer solution. Solvents that have high boiling points may not dehydrate entirely before reaching the collector, thus resulting in ribbon-like, flat, nanofiber morphologies or conglutination of nanofibers at the boundaries [36, 46]. The ability of the electrospinning polyvinylpyrrolidone (PVP) by Yang and Coworkers [45], was investigated with different solvents. The solvents examined were MC, ethanol and DMF, while the beaded nanofibers were formed from DCM and DMF solutions of PVP, the use of ethanol produced PVP nanofibers. Nanofibers electrospun from an integration of the ethanol and DMF had small diameters of 20 nm, while a combination of ethanol and DCM resulted in the formation of nanofibers with diameters as large as 300 nm (see **Figure 7**). It is therefore conclusive that nanofiber morphology and porosity may be regulated by the defensible use of solvents or a combination of solvents.

**4.2. Electrospinning parameters**

change in high voltage as shown in **Figure 8**.

**Figure 8.** SEM images of the PVDF nanofibers prepared at different applied voltages [48].

The amount of charge per unit surface area of the polymer droplet which constitutes charge density is determined by the applied voltage, working distance and the conductivity of the polymer solution. Applied voltage is used to provide the driving force to spin fibres by imparting charge to the polymer droplet. The working distance which is the distance between the tip of syringe and the collecting plate, in addition to the applied voltage, can influence the structural morphology of nanofibers. Demir et al. [35] suggested that when higher voltages are applied, more polymer is ejected to form a larger diameter fibre. Similarly, high voltage conditions also created a rougher fibre structure. To reduce bead formation, Zong et al. [37] proposed an approach to increase charge density on the surface of the droplet by adding salt particles. However, they concluded that high-charge density produced thinner fibres, a finding not corroborated by Demir et al. [35]. The study by Pham et al. [23] shows that in the state of low voltages or field strengths, typically, a drop is suspended at the needle tip, and a jet will originate from the Taylor cone producing bead-free spinning (under the assumption that the force of the electric field is sufficient to address the surface tension). Hao Shao et al. [48] studied the effect of high voltage, and observed change in the morphology as a result of

Nanofibers and Electrospinning Method http://dx.doi.org/10.5772/intechopen.72060 201

*4.2.1. Voltage supply*

**Figure 7.** TEM images of PEO nanofibers electrospun different solvent. (a) chloroform (3%), (b) Ethanol (4%), (c) DMC (5%), (d) Water (7%) [47].

#### **4.2. Electrospinning parameters**

#### *4.2.1. Voltage supply*

the more favourable choice as they assist the dehydration of the nanofibers during trajectory from the capillary tip to the collector surface, because of their lower boiling point, and thus causing a rapid evaporation rate. Nonetheless, highly volatile solvents that have very low boiling points should be prevented as they may evaporate at the capillary tip and further leading to the clogging and the obstruction of the flow-rate of the polymer solution. Solvents that have high boiling points may not dehydrate entirely before reaching the collector, thus resulting in ribbon-like, flat, nanofiber morphologies or conglutination of nanofibers at the boundaries [36, 46]. The ability of the electrospinning polyvinylpyrrolidone (PVP) by Yang and Coworkers [45], was investigated with different solvents. The solvents examined were MC, ethanol and DMF, while the beaded nanofibers were formed from DCM and DMF solutions of PVP, the use of ethanol produced PVP nanofibers. Nanofibers electrospun from an integration of the ethanol and DMF had small diameters of 20 nm, while a combination of ethanol and DCM resulted in the formation of nanofibers with diameters as large as 300 nm (see **Figure 7**). It is therefore conclusive that nanofiber morphology and porosity may be regu-

**Figure 7.** TEM images of PEO nanofibers electrospun different solvent. (a) chloroform (3%), (b) Ethanol (4%), (c) DMC (5%),

(d) Water (7%) [47].

lated by the defensible use of solvents or a combination of solvents.

200 Novel Nanomaterials - Synthesis and Applications

The amount of charge per unit surface area of the polymer droplet which constitutes charge density is determined by the applied voltage, working distance and the conductivity of the polymer solution. Applied voltage is used to provide the driving force to spin fibres by imparting charge to the polymer droplet. The working distance which is the distance between the tip of syringe and the collecting plate, in addition to the applied voltage, can influence the structural morphology of nanofibers. Demir et al. [35] suggested that when higher voltages are applied, more polymer is ejected to form a larger diameter fibre. Similarly, high voltage conditions also created a rougher fibre structure. To reduce bead formation, Zong et al. [37] proposed an approach to increase charge density on the surface of the droplet by adding salt particles. However, they concluded that high-charge density produced thinner fibres, a finding not corroborated by Demir et al. [35]. The study by Pham et al. [23] shows that in the state of low voltages or field strengths, typically, a drop is suspended at the needle tip, and a jet will originate from the Taylor cone producing bead-free spinning (under the assumption that the force of the electric field is sufficient to address the surface tension). Hao Shao et al. [48] studied the effect of high voltage, and observed change in the morphology as a result of change in high voltage as shown in **Figure 8**.

**Figure 8.** SEM images of the PVDF nanofibers prepared at different applied voltages [48].

#### *4.2.2. Needle diameter (nozzle)*

The size of needle has a certain effect on the nanofibers diameters. It was discovered that a reduction in the diameters of the electrospun nanofibers was caused by a decrease in the internal diameter of the needle. The nozzle (usually the syringe needle set up) determines the amount of polymer melt that comes out, which, in turn, affects the size of the drop being formed and also the pressure or the amount of force required by the pump to push the melt out. If the polymer melt is less viscous, it can easily flow out of the nozzle. The polymer melt is usually a thick highly viscous fluid. So, if the nozzle is too small, and the melt is too viscous, the melt cannot be forced out. Therefore, an appropriate nozzle should be used. Different types of nozzles or spinnerets have been used over the years [49]. The effect of needle diameters on the resulting electrospun poly(methyl methacrylate) (PMMA) average nanofiber diameters was evaluated for three different needle gauges by Javier Macossay et al. [49]. These fibres presented regular surface morphologies, with a few nanofiber bundles being evident in **Figure 9**.

#### *4.2.3. Distance between tip and collector*

The distance from the needle to the collector is very important, because by decreasing it, the electrical field increases instead; and also the stretching force does and the time at which the fibre undergoes the field is lower, causing sufficient evaporation of the solvent of the fibres. The result is that when there is reduction in the distance between the needle and the collector the fibres grow and may be subject to structural deformities like blobs. The high voltage was determined at 15 kV and the distance from the tip of the needle to the collector is in the range from 9 cm to 21 cm. The fibres' morphology was assessed from the SEM images of **Figure 10**; for the shortest distance (9 cm), the fibres came together at their intersections following the incomplete evaporation of the solvent before the jet arrived at the collector. For the other four distances used, the fibres appeared similar and the mean fibre diameter increased a little with the distance to the collector. The distance established between the tip and the collector exerted a direct influence in flight time and electric field strength. For the fibres to form, the electrospinning jet must be given ample time for most of the solvents to be evaporated. The electric field strength will increase at the same time and this will increase the acceleration of

the jet to the collector. As a result, there may not be enough time for solvents to evaporate

Nanofibers and Electrospinning Method http://dx.doi.org/10.5772/intechopen.72060 203

**Figure 10.** SEM images of the PVDF nanofibers prepared at different spinning distances [48].

Another important parameter process is the flow rate of the polymer solution within the syringe. For the polymer solution to have enough time for polarization, lower flow rate is more preferred. If the flow rate is very high, bead fibres with thick diameter will form instead of smooth fibres with thin diameters owing to the short drying time before reaching the collector, and also due to low stretching forces. There is a corresponding rise in the fiber diameter or blobs size, as a result of greater volume of solution ejected from the needle tip, when there is increase in the feed rate. As shown in **Figure 11**, Shamim Z et al. [50] indicated that when the flow rate is decreased with other parameters kept constant; there is a decrease in the blobs size and an increase in nanofiber diameter. The inference here is that, with the decrease

in flow rate, blobs size could get smaller until the non-beaded structure is obtained.

The formation of nanofibers can be classified into woven and non-woven nanofibers. The type of collector used plays a big role in differentiating the type or nanofiber alignments. The use

when they reach the collector.

*4.2.4. Flow rate*

*4.2.5. Collector*

**Figure 9.** SEM of PMMA nanofibers, utilizing internal diameter needle. (a) 0.83 mm, (b) a 0.4 mm, (c) a 0.1 mm [49].

**Figure 10.** SEM images of the PVDF nanofibers prepared at different spinning distances [48].

the jet to the collector. As a result, there may not be enough time for solvents to evaporate when they reach the collector.

#### *4.2.4. Flow rate*

*4.2.2. Needle diameter (nozzle)*

202 Novel Nanomaterials - Synthesis and Applications

*4.2.3. Distance between tip and collector*

The size of needle has a certain effect on the nanofibers diameters. It was discovered that a reduction in the diameters of the electrospun nanofibers was caused by a decrease in the internal diameter of the needle. The nozzle (usually the syringe needle set up) determines the amount of polymer melt that comes out, which, in turn, affects the size of the drop being formed and also the pressure or the amount of force required by the pump to push the melt out. If the polymer melt is less viscous, it can easily flow out of the nozzle. The polymer melt is usually a thick highly viscous fluid. So, if the nozzle is too small, and the melt is too viscous, the melt cannot be forced out. Therefore, an appropriate nozzle should be used. Different types of nozzles or spinnerets have been used over the years [49]. The effect of needle diameters on the resulting electrospun poly(methyl methacrylate) (PMMA) average nanofiber diameters was evaluated for three different needle gauges by Javier Macossay et al. [49]. These fibres presented regular surface morphologies, with a few nanofiber bundles being evident in **Figure 9**.

The distance from the needle to the collector is very important, because by decreasing it, the electrical field increases instead; and also the stretching force does and the time at which the fibre undergoes the field is lower, causing sufficient evaporation of the solvent of the fibres. The result is that when there is reduction in the distance between the needle and the collector the fibres grow and may be subject to structural deformities like blobs. The high voltage was determined at 15 kV and the distance from the tip of the needle to the collector is in the range from 9 cm to 21 cm. The fibres' morphology was assessed from the SEM images of **Figure 10**; for the shortest distance (9 cm), the fibres came together at their intersections following the incomplete evaporation of the solvent before the jet arrived at the collector. For the other four distances used, the fibres appeared similar and the mean fibre diameter increased a little with the distance to the collector. The distance established between the tip and the collector exerted a direct influence in flight time and electric field strength. For the fibres to form, the electrospinning jet must be given ample time for most of the solvents to be evaporated. The electric field strength will increase at the same time and this will increase the acceleration of

**Figure 9.** SEM of PMMA nanofibers, utilizing internal diameter needle. (a) 0.83 mm, (b) a 0.4 mm, (c) a 0.1 mm [49].

Another important parameter process is the flow rate of the polymer solution within the syringe. For the polymer solution to have enough time for polarization, lower flow rate is more preferred. If the flow rate is very high, bead fibres with thick diameter will form instead of smooth fibres with thin diameters owing to the short drying time before reaching the collector, and also due to low stretching forces. There is a corresponding rise in the fiber diameter or blobs size, as a result of greater volume of solution ejected from the needle tip, when there is increase in the feed rate. As shown in **Figure 11**, Shamim Z et al. [50] indicated that when the flow rate is decreased with other parameters kept constant; there is a decrease in the blobs size and an increase in nanofiber diameter. The inference here is that, with the decrease in flow rate, blobs size could get smaller until the non-beaded structure is obtained.

#### *4.2.5. Collector*

The formation of nanofibers can be classified into woven and non-woven nanofibers. The type of collector used plays a big role in differentiating the type or nanofiber alignments. The use

**Figure 11.** SEM images of PVA different flow rate. (a) 0.1 ml/h, (b) 0.5 ml/h, (c) 1 ml/h, (d) 1.5 ml/h [51].

of oriented collector, as well as static double grounded collector [2], constitutes the methods adopted in developing aligned woven nanofibers. The rotating drum collector is used for collecting the aligned arrays nanofibers, while the rotating disk is used for collecting uniaxially aligned nanofibers. The alignment fibres obtained from the rotating drum correspond to the rotational speed applied on the drum [2]. This type of electrospinning method is more complex because the speed of the rotation needs to be very properly controlled to produce nanofibers with such a good alignment. The rotating disk collector can also serve to collect continuous nanofibers, since they can very much attract the large electrical field applied at the edge of the disk [2].

As seen in **Figure 12**, SEM images of diverse collectors for many reports, have been developed including the wire mesh studied by Wang X et al. [52], pin studied by Sundaray B et al. [53], grids studied by Li D et al. [54], parallel or gridded bar and rotating rods or wheel studied by Xu CY et al. [55], and liquid bath studied by Ki CS et al. [56]. Kim et al. [57] proved that the different types of composition used in the collector affected the structure of the poly (L-lactide) (PLLA) and poly (lactide-co-glycolide) (PLA50GA50) fibres.

#### **4.3. Ambient parameters**

Fibre diameters and morphologies such as humidity and temperature could also be affected by ambient parameters. Increasing temperature, as noted by Mituppatham et al. [58] for instance, favours the thinner fibre diameter with polyamide-6 fibres for the inverse relationship between the solution viscosity and the temperature, as shown in **Figure 13**. With regards to humidity, low humidity could dry the solvent totally and increase the velocity of the solvent evaporation. On the contrary, high humidity will lead to thick fibre diameter because the charges on the jet can be neutralised and the stretching forces

**Figure 13.** SEM images of the electrospun PA-6-32 fibers under different temperatures. (a) 30ᴼ and (b) 60ᴼ [58].

**Figure 12.** SEM images of the different electrospun products with various types of collectors. (a) Wire Screen, (b) Pin,

Nanofibers and Electrospinning Method http://dx.doi.org/10.5772/intechopen.72060 205

The variety of humidity can also affect the surface morphologies of electrospun PS fibres, as recently show by Casper et al. [59]. Nezarati et al. [60] observed that low humidity (5% RH) resulted in beads connected by thin fibres, but increasing the RH (20–75% RH) resulted in smooth, uniform fibres for poly(ethylene glycol) (PEG). In addition as relative humidity was

increased from 50–75%, fibre density decreased, see (**Figure 14**).

(c) Gridded Bar, (d) Parallel Bar, (e) Rotating Wheel, and (f) Liquid Bath [52–56].

become small.

**Figure 12.** SEM images of the different electrospun products with various types of collectors. (a) Wire Screen, (b) Pin, (c) Gridded Bar, (d) Parallel Bar, (e) Rotating Wheel, and (f) Liquid Bath [52–56].

of oriented collector, as well as static double grounded collector [2], constitutes the methods adopted in developing aligned woven nanofibers. The rotating drum collector is used for collecting the aligned arrays nanofibers, while the rotating disk is used for collecting uniaxially aligned nanofibers. The alignment fibres obtained from the rotating drum correspond to the rotational speed applied on the drum [2]. This type of electrospinning method is more complex because the speed of the rotation needs to be very properly controlled to produce nanofibers with such a good alignment. The rotating disk collector can also serve to collect continuous nanofibers, since they can very much attract the large electrical field applied at the

**Figure 11.** SEM images of PVA different flow rate. (a) 0.1 ml/h, (b) 0.5 ml/h, (c) 1 ml/h, (d) 1.5 ml/h [51].

As seen in **Figure 12**, SEM images of diverse collectors for many reports, have been developed including the wire mesh studied by Wang X et al. [52], pin studied by Sundaray B et al. [53], grids studied by Li D et al. [54], parallel or gridded bar and rotating rods or wheel studied by Xu CY et al. [55], and liquid bath studied by Ki CS et al. [56]. Kim et al. [57] proved that the different types of composition used in the collector affected the structure of the poly (L-lactide)

Fibre diameters and morphologies such as humidity and temperature could also be affected by ambient parameters. Increasing temperature, as noted by Mituppatham et al. [58] for instance, favours the thinner fibre diameter with polyamide-6 fibres for the inverse relationship between the solution viscosity and the temperature, as shown in **Figure 13**. With regards to humidity, low humidity could dry the solvent totally and increase the velocity of the solvent evaporation. On the contrary, high humidity will lead to thick

(PLLA) and poly (lactide-co-glycolide) (PLA50GA50) fibres.

edge of the disk [2].

204 Novel Nanomaterials - Synthesis and Applications

**4.3. Ambient parameters**

**Figure 13.** SEM images of the electrospun PA-6-32 fibers under different temperatures. (a) 30ᴼ and (b) 60ᴼ [58].

fibre diameter because the charges on the jet can be neutralised and the stretching forces become small.

The variety of humidity can also affect the surface morphologies of electrospun PS fibres, as recently show by Casper et al. [59]. Nezarati et al. [60] observed that low humidity (5% RH) resulted in beads connected by thin fibres, but increasing the RH (20–75% RH) resulted in smooth, uniform fibres for poly(ethylene glycol) (PEG). In addition as relative humidity was increased from 50–75%, fibre density decreased, see (**Figure 14**).

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### **5. Conclusions**

In this chapter, nanofibers are successfully grown using the electrospinning method at different parameters. These nanofibers, fabricated on several substrates, were investigated using FESEM, observations revealed the formation of nanofibers. Effect of the parameters on the morphology and the diameter was shown. The optimal condition was selected to study the effects of parameters on surface morphology and diameter through the study of characterisation of semiconductor/polymer NFs through the above observations and structural. The semiconductor/polymer nanofibers thin films have offered interesting due to having multi-applications. The sensor, solar cell, supercapacitors, drug delivered, filtering, and more of that its success through been fabricated nanofibers using electrospinning. The electrospinning will not use for fabricating nanofibers only in future work, however, will expand to produce other nanostructures such as nanorods, due to the electronic process has the ability to control the parameters.

## **Author details**

Nabeel Zabar Abed Al-Hazeem1,2\*

\*Address all correspondence to: nabeelnano333@gmail.com

1 Gifted School in Anbar, Gifted Guardianship Committee, Ministry of Education, Iraq

2 Institute of Nano-Optoelectronics Research and Technology Laboratory (iNOR), USM – School of Physics, Penang, Malaysia

#### **References**

**5. Conclusions**

206 Novel Nanomaterials - Synthesis and Applications

**Author details**

Nabeel Zabar Abed Al-Hazeem1,2\*

USM – School of Physics, Penang, Malaysia

\*Address all correspondence to: nabeelnano333@gmail.com

In this chapter, nanofibers are successfully grown using the electrospinning method at different parameters. These nanofibers, fabricated on several substrates, were investigated using FESEM, observations revealed the formation of nanofibers. Effect of the parameters on the morphology and the diameter was shown. The optimal condition was selected to study the effects of parameters on surface morphology and diameter through the study of characterisation of semiconductor/polymer NFs through the above observations and structural. The semiconductor/polymer nanofibers thin films have offered interesting due to having multi-applications. The sensor, solar cell, supercapacitors, drug delivered, filtering, and more of that its success through been fabricated nanofibers using electrospinning. The electrospinning will not use for fabricating nanofibers only in future work, however, will expand to produce other nanostructures such as nanorods, due to the electronic process has the ability to control the parameters.

**Figure 14.** SEM images of poly(ethylene glycol) (PEG) electrospun at relative humidity (RH) ranging from 5 to 75% [60].

1 Gifted School in Anbar, Gifted Guardianship Committee, Ministry of Education, Iraq

2 Institute of Nano-Optoelectronics Research and Technology Laboratory (iNOR),


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**Chapter 12**

**Provisional chapter**

**Palladium (II) Oxide Nanostructures as Promising**

**Palladium (II) Oxide Nanostructures as Promising** 

DOI: 10.5772/intechopen.72323

One of the most important environment monitoring problems is the detection of oxidizing gases in the ambient air. Negative impact of noxious oxidizing gases (ozone and nitrogen oxides) on human health, sensitive vegetation, and ecosystems is very serious. For this reason, palladium (II) oxide nanostructures have been employed for oxidizing gas detection. Thin and ultrathin films of palladium (II) oxide were prepared by thermal oxidation at dry oxygen of previously formed pure palladium layers on polished

At ozone and nitrogen dioxide detection, PdO films prepared by oxidation at *T* = 870 K have demonstrated good values of sensitivity, signal stability, operation speed, and reproducibility of sensor response. In comparison with other materials, palladium (II) oxide thin and ultrathin films have some advantages at gas sensor fabrication. Firstly, for oxidizing gas detection, PdO films with *p*-type conductivity are more perspective than the material with *n*-type conductivity. Secondly, at ambient conditions, palladium (II) oxide is insoluble in water and does not react with it. These facts are favorable for the fabrication of gas detectors because they make possible to minimize the air humidity influence on PdO sensor response values. Thirdly, the synthesis procedure of PdO films is rather simple and is compatible with planar processes of microelectronic

**Keywords:** palladium (II) oxide, nanostructure, gas sensor, ozone, nitrogen dioxide

/Si (100), optical quality quartz, and amorphous carbon/KCl substrates.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

Nowadays, the detection of oxidizing gases in the ambient air is one of the most important environment monitoring problems for industrialized countries. During the last 25 years, the

**Materials for Gas Sensors**

**Materials for Gas Sensors**

Vasily N. Popov and Petre Badica

Vasily N. Popov and Petre Badica

http://dx.doi.org/10.5772/intechopen.72323

**Abstract**

poly-Al2

industry.

**1. Introduction**

O3 , SiO2

Alexander M. Samoylov, Stanislav V. Ryabtsev,

Alexander M. Samoylov, Stanislav V. Ryabtsev,

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter


**Provisional chapter**

#### **Palladium (II) Oxide Nanostructures as Promising Materials for Gas Sensors Materials for Gas Sensors**

**Palladium (II) Oxide Nanostructures as Promising** 

DOI: 10.5772/intechopen.72323

Alexander M. Samoylov, Stanislav V. Ryabtsev, Vasily N. Popov and Petre Badica Vasily N. Popov and Petre Badica Additional information is available at the end of the chapter

Alexander M. Samoylov, Stanislav V. Ryabtsev,

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.72323

#### **Abstract**

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210 Novel Nanomaterials - Synthesis and Applications

One of the most important environment monitoring problems is the detection of oxidizing gases in the ambient air. Negative impact of noxious oxidizing gases (ozone and nitrogen oxides) on human health, sensitive vegetation, and ecosystems is very serious. For this reason, palladium (II) oxide nanostructures have been employed for oxidizing gas detection. Thin and ultrathin films of palladium (II) oxide were prepared by thermal oxidation at dry oxygen of previously formed pure palladium layers on polished poly-Al2 O3 , SiO2 /Si (100), optical quality quartz, and amorphous carbon/KCl substrates. At ozone and nitrogen dioxide detection, PdO films prepared by oxidation at *T* = 870 K have demonstrated good values of sensitivity, signal stability, operation speed, and reproducibility of sensor response. In comparison with other materials, palladium (II) oxide thin and ultrathin films have some advantages at gas sensor fabrication. Firstly, for oxidizing gas detection, PdO films with *p*-type conductivity are more perspective than the material with *n*-type conductivity. Secondly, at ambient conditions, palladium (II) oxide is insoluble in water and does not react with it. These facts are favorable for the fabrication of gas detectors because they make possible to minimize the air humidity influence on PdO sensor response values. Thirdly, the synthesis procedure of PdO films is rather simple and is compatible with planar processes of microelectronic industry.

**Keywords:** palladium (II) oxide, nanostructure, gas sensor, ozone, nitrogen dioxide

#### **1. Introduction**

Nowadays, the detection of oxidizing gases in the ambient air is one of the most important environment monitoring problems for industrialized countries. During the last 25 years, the

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

steady increase in concentration of nitrogen dioxide and tropospheric (low level) ozone is observed. As it is known, three out of six common air pollutants (also called "criteria pollutants") are oxidizing gases: sulfur dioxide, nitrogen oxides, and tropospheric ozone [1, 2]. One part of ecologists is sure that increase in the content of low-level ozone in atmospheric air is caused mainly by an intensification of industrial production, motor and air transport. Undoubtedly, ozone gas is applied in many fields such as food, pharmaceutical, textile, and chemical industries, water treatment, and purification of gases. However, there is an opinion that emergence of tropospheric ozone in ambient air is a consequence of the "greenhouse" effect [3].

For these reasons, various types of the binary, ternary and quaternary metal-oxide semiconductors have been widely applied for oxidizing gas detection. In most cases, for this purpose,

The study of palladium (II) oxide nanostructures as materials for gas sensor fabrication was started only since 2014. The assumption to use palladium (II) oxide, which is a *p*-type semiconductor with the energy band gap Δ*E*<sup>g</sup> = 2.2–2.7 eV [36–38], as the material for the detection of toxic and highly inflammable gas in ambient air has not been accidental for some reasons [39, 40]. Firstly, for a long time, palladium and its compounds in (+2) oxidation state were exploited as very effective catalysts for oxidation reactions of hydrocarbons, including automobile catalytic converters and the catalytic combustion of methane in advanced gas turbines. In catalytic converters, the key processes are the complete oxidation of any hydrocarbon in the exhaust gas stream, the simultaneous oxidation of carbon monoxide, and reduction of nitrogen oxides. Secondly, due to the extremely high catalytic activity, palladium and palladium (II) oxide were applied as additives to improve gas-sensing performance of tin dioxide

capable to lower the detection limit of oxidizing gases, became more active.

traditional *n*-type conductivity materials using for the same purpose.

**2. Fabrication of palladium (II) oxide nanostructures**

[35] are used traditionally. In recent years, the search of the materials, which would be

 to a wide range of gases [41–43]. Thirdly, the opinion that long recovery process and high stability could be referred to the main disadvantages of the oxidizing gas sensors based on tin dioxide has been expressed earlier [44, 45]. Fourthly, the metal oxide semiconductors with *p*-type conductivity are more perspective for oxidizing gas detection than the materials with *n*-type conductivity. In this case, the chemical adsorption of oxidizing gas molecule on *p*-type semiconductors surface leads to decrease in the sensor resistance that has simplified the detection process [35, 36]. Thus, at oxidizing gas detection, palladium (II) oxide nanostructures should demonstrate the increase in sensor response value in comparison with the

Initially, the sensing properties to oxidizing gases of palladium (II) oxide nanostructures were tested on ultrathin and thin films at detection of ozone and nitrogen dioxide [46, 47]. The procedure of PdO thin and ultrathin films synthesis was realized by two stages. First, the initial palladium films (thickness 5–30 nm) were formed by thermal sublimation of palladium foil (purity is 99.99%) in high vacuum chamber evacuated to 5 × 10−7 Torr using a turbo molecular pump. In vacuum chamber, the condensation of Pd metal vapors was performed on different

amorphous carbon (**Figure 2**). The values of tungsten heater temperature in order to fabricate initial palladium films with average rate within interval 0.01–0.016 nm per second were determined as a result of Pd films cross-sections by high-resolution scanning and transmission

The substructure of initial palladium layers was studied by an X-ray analysis and the HEED method. As it is shown in **Figure 3a** and **3b**, the initial Pd films were polycrystalline and

/Si (100), Si (100), optical quality quartz, and KCl (100) with buffer layer of

[14–20], ZnO [21–26], WO3

[27–32], In2

http://dx.doi.org/10.5772/intechopen.72323

Palladium (II) Oxide Nanostructures as Promising Materials for Gas Sensors

O3

[33, 34], and

213

the *n*-type semiconductors such as SnO2

TiO2

SnO2

substrates: SiO2

electron microscopy (HR STEM) study.

Under sunlight, the interaction of ozone, nitrogen oxides, and volatile hydrocarbons can produce many toxic organic compounds (**Figure 1**). By the action of sunlight, oxygen atoms freed from nitrogen dioxide attack oxygen molecules to make ozone. Nitrogen oxide can combine with ozone to reform nitrogen dioxide, and the cycle repeats.

Moreover, at interaction with ozone, the ultraviolet component of sunlight leads to the formation of excess quantity of the reactive oxygen species (ROS): oxygen ions, free radicals, and peroxides. In living bodies, even the trace amounts of ROC can provoke an oxidative stress. For human, the oxidative stress is a reason for atherosclerosis, hypertension, Alzheimer's disease, diabetes, and geromorphism [4–9]. The negative impact on human health of an aspiration of noxious oxidizing gases (ozone and nitrogen oxides) is more serious, particularly for children, the elderly, and people who suffer from lung diseases [1, 2]. Nitrogen oxides and tropospheric ozone can also have harmful effects on sensitive vegetation and ecosystems [10–13].

**Figure 1.** The chemical reactions of tropospheric ozone under sunlight.

For these reasons, various types of the binary, ternary and quaternary metal-oxide semiconductors have been widely applied for oxidizing gas detection. In most cases, for this purpose, the *n*-type semiconductors such as SnO2 [14–20], ZnO [21–26], WO3 [27–32], In2 O3 [33, 34], and TiO2 [35] are used traditionally. In recent years, the search of the materials, which would be capable to lower the detection limit of oxidizing gases, became more active.

steady increase in concentration of nitrogen dioxide and tropospheric (low level) ozone is observed. As it is known, three out of six common air pollutants (also called "criteria pollutants") are oxidizing gases: sulfur dioxide, nitrogen oxides, and tropospheric ozone [1, 2]. One part of ecologists is sure that increase in the content of low-level ozone in atmospheric air is caused mainly by an intensification of industrial production, motor and air transport. Undoubtedly, ozone gas is applied in many fields such as food, pharmaceutical, textile, and chemical industries, water treatment, and purification of gases. However, there is an opinion that emergence of tropospheric ozone in ambient air is a consequence of the "greenhouse"

Under sunlight, the interaction of ozone, nitrogen oxides, and volatile hydrocarbons can produce many toxic organic compounds (**Figure 1**). By the action of sunlight, oxygen atoms freed from nitrogen dioxide attack oxygen molecules to make ozone. Nitrogen oxide can combine

Moreover, at interaction with ozone, the ultraviolet component of sunlight leads to the formation of excess quantity of the reactive oxygen species (ROS): oxygen ions, free radicals, and peroxides. In living bodies, even the trace amounts of ROC can provoke an oxidative stress. For human, the oxidative stress is a reason for atherosclerosis, hypertension, Alzheimer's disease, diabetes, and geromorphism [4–9]. The negative impact on human health of an aspiration of noxious oxidizing gases (ozone and nitrogen oxides) is more serious, particularly for children, the elderly, and people who suffer from lung diseases [1, 2]. Nitrogen oxides and tropospheric ozone can also have harmful effects on sensitive

with ozone to reform nitrogen dioxide, and the cycle repeats.

**Figure 1.** The chemical reactions of tropospheric ozone under sunlight.

vegetation and ecosystems [10–13].

212 Novel Nanomaterials - Synthesis and Applications

effect [3].

The study of palladium (II) oxide nanostructures as materials for gas sensor fabrication was started only since 2014. The assumption to use palladium (II) oxide, which is a *p*-type semiconductor with the energy band gap Δ*E*<sup>g</sup> = 2.2–2.7 eV [36–38], as the material for the detection of toxic and highly inflammable gas in ambient air has not been accidental for some reasons [39, 40]. Firstly, for a long time, palladium and its compounds in (+2) oxidation state were exploited as very effective catalysts for oxidation reactions of hydrocarbons, including automobile catalytic converters and the catalytic combustion of methane in advanced gas turbines. In catalytic converters, the key processes are the complete oxidation of any hydrocarbon in the exhaust gas stream, the simultaneous oxidation of carbon monoxide, and reduction of nitrogen oxides. Secondly, due to the extremely high catalytic activity, palladium and palladium (II) oxide were applied as additives to improve gas-sensing performance of tin dioxide SnO2 to a wide range of gases [41–43]. Thirdly, the opinion that long recovery process and high stability could be referred to the main disadvantages of the oxidizing gas sensors based on tin dioxide has been expressed earlier [44, 45]. Fourthly, the metal oxide semiconductors with *p*-type conductivity are more perspective for oxidizing gas detection than the materials with *n*-type conductivity. In this case, the chemical adsorption of oxidizing gas molecule on *p*-type semiconductors surface leads to decrease in the sensor resistance that has simplified the detection process [35, 36]. Thus, at oxidizing gas detection, palladium (II) oxide nanostructures should demonstrate the increase in sensor response value in comparison with the traditional *n*-type conductivity materials using for the same purpose.

#### **2. Fabrication of palladium (II) oxide nanostructures**

Initially, the sensing properties to oxidizing gases of palladium (II) oxide nanostructures were tested on ultrathin and thin films at detection of ozone and nitrogen dioxide [46, 47]. The procedure of PdO thin and ultrathin films synthesis was realized by two stages. First, the initial palladium films (thickness 5–30 nm) were formed by thermal sublimation of palladium foil (purity is 99.99%) in high vacuum chamber evacuated to 5 × 10−7 Torr using a turbo molecular pump. In vacuum chamber, the condensation of Pd metal vapors was performed on different substrates: SiO2 /Si (100), Si (100), optical quality quartz, and KCl (100) with buffer layer of amorphous carbon (**Figure 2**). The values of tungsten heater temperature in order to fabricate initial palladium films with average rate within interval 0.01–0.016 nm per second were determined as a result of Pd films cross-sections by high-resolution scanning and transmission electron microscopy (HR STEM) study.

The substructure of initial palladium layers was studied by an X-ray analysis and the HEED method. As it is shown in **Figure 3a** and **3b**, the initial Pd films were polycrystalline and

highly dispersive with random orientation of grains irrespective of the substrate nature

Prepared Pd nanostructures on different substrates were annealed at dry oxygen atmosphere for 1 h for layers with thickness 5–15 nm and for 2 h for layers with thickness 30 ± 5 nm at temperatures *T*ox = 510, 570, 670, 770, 870, and 1070 K. The dehumidification of oxygen at pressure 120–130 kPa (1.2–1.3 Bar) was carried out by gas flow passage through gas bubbler with

concentrated sulfuric acid and further through silica tube full of ground zeolite [48].

**3. Phase composition and crystal structure of palladium (II) oxide** 

changes. The increase in intensities of palladium reflexes was found only.

X-ray diffraction (XRD) patterns of samples prepared by oxidation of Pd films on SiO<sup>2</sup>

(100) wafers at dry oxygen atmosphere at *T*ox = 510, 570, 770, 870, and 970 K are shown in

It is necessary to note that in **Figure 4**, the values of XRD reflex intensities are presented in a logarithmic scale because the intensity of Si (400) peak practically exceeds the intensity of palladium and palladium (II) oxide peaks by two orders of magnitude owing to a small thickness of the prepared films. The comparison of the as-grown Pd films XRD patterns with XRD patterns of Pd film after the annealing at *T*ox = 510 K (**Figure 4a**) did not reveal any quality

Thus, it has been established that the annealing of Pd layers at *T*ox < 570 K (**Figure 4a**) did not result in the change of their phase constitution. The annealing at *T*ox = 570 K resulted in the formation of two phase films (**Figure 4b**). XRD patterns have shown the presence of Pd with

According to XRD results, the rise of the oxidation temperature up to *T*ox = 770 K and *T*ox = 870 K led to the formation of the homogenous polycrystalline PdO films. It has been determined (**Figure 4c** and **4d**) that palladium (II) oxide films were characterized by tetragonal crystal lat-

peaks became sharper and higher with the oxidizing temperature increasing from *T*ox = 770 K up to *T*ox = 970 K. Moreover, the peaks of palladium (II) oxide prepared by oxidation at *T*ox = 970 K are much sharper and higher than those for films oxidized at *T*ox = 870 K. This fact can be interpreted as one of the evidences of the crystalline perfection enhancement of palladium (II) oxide films and the grain size enlargement with the increase in the oxidation

/*mmc* and PtS structure type). XRD patterns show (**Figure 4c**–**4e**) that the

/Si

215

/Si (100), optical quality quartz, and amorphous carbon/KCl). The analysis of *bright-field (***Figure 3c***) and dark-field (***Figure 3d**) TEM images proves that palladium crystalline grains form a continuous coating without an axial texture with very low density of micropores. On bright-field image, the light contrast (**Figure 3c**) testifies to the decrease of film thickness at

Palladium (II) Oxide Nanostructures as Promising Materials for Gas Sensors

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(SiO2

grain borders [48].

**nanostructures**

PdO simultaneously.

tice (space group *P*42

temperature.

**Figure 4**.

**Figure 2.** High-resolution TEM image of Pd/SiO<sup>2</sup> /Si (100) heterostructure cross-section prepared by focused ion beam (FIB) technique.

**Figure 3.** Experimental results of initial Pd films (thickness ~10 nm) crystal structure study: (a) XRD patterns of Pd film deposited on Si (100) substrate; (b) HEED patterns of Pd film deposited on amorphous carbon/KCl substrate; (c) bright-field TEM image of Pd film deposited on amorphous carbon/KCl substrate; (d) dark-field TEM image of Pd film deposited on amorphous carbon/KCl substrate.

highly dispersive with random orientation of grains irrespective of the substrate nature (SiO2 /Si (100), optical quality quartz, and amorphous carbon/KCl). The analysis of *bright-field (***Figure 3c***) and dark-field (***Figure 3d**) TEM images proves that palladium crystalline grains form a continuous coating without an axial texture with very low density of micropores. On bright-field image, the light contrast (**Figure 3c**) testifies to the decrease of film thickness at grain borders [48].

Prepared Pd nanostructures on different substrates were annealed at dry oxygen atmosphere for 1 h for layers with thickness 5–15 nm and for 2 h for layers with thickness 30 ± 5 nm at temperatures *T*ox = 510, 570, 670, 770, 870, and 1070 K. The dehumidification of oxygen at pressure 120–130 kPa (1.2–1.3 Bar) was carried out by gas flow passage through gas bubbler with concentrated sulfuric acid and further through silica tube full of ground zeolite [48].

## **3. Phase composition and crystal structure of palladium (II) oxide nanostructures**

**Figure 2.** High-resolution TEM image of Pd/SiO<sup>2</sup>

214 Novel Nanomaterials - Synthesis and Applications

Pd film deposited on amorphous carbon/KCl substrate.

(FIB) technique.

/Si (100) heterostructure cross-section prepared by focused ion beam

**Figure 3.** Experimental results of initial Pd films (thickness ~10 nm) crystal structure study: (a) XRD patterns of Pd film deposited on Si (100) substrate; (b) HEED patterns of Pd film deposited on amorphous carbon/KCl substrate; (c) bright-field TEM image of Pd film deposited on amorphous carbon/KCl substrate; (d) dark-field TEM image of X-ray diffraction (XRD) patterns of samples prepared by oxidation of Pd films on SiO<sup>2</sup> /Si (100) wafers at dry oxygen atmosphere at *T*ox = 510, 570, 770, 870, and 970 K are shown in **Figure 4**.

It is necessary to note that in **Figure 4**, the values of XRD reflex intensities are presented in a logarithmic scale because the intensity of Si (400) peak practically exceeds the intensity of palladium and palladium (II) oxide peaks by two orders of magnitude owing to a small thickness of the prepared films. The comparison of the as-grown Pd films XRD patterns with XRD patterns of Pd film after the annealing at *T*ox = 510 K (**Figure 4a**) did not reveal any quality changes. The increase in intensities of palladium reflexes was found only.

Thus, it has been established that the annealing of Pd layers at *T*ox < 570 K (**Figure 4a**) did not result in the change of their phase constitution. The annealing at *T*ox = 570 K resulted in the formation of two phase films (**Figure 4b**). XRD patterns have shown the presence of Pd with PdO simultaneously.

According to XRD results, the rise of the oxidation temperature up to *T*ox = 770 K and *T*ox = 870 K led to the formation of the homogenous polycrystalline PdO films. It has been determined (**Figure 4c** and **4d**) that palladium (II) oxide films were characterized by tetragonal crystal lattice (space group *P*42 /*mmc* and PtS structure type). XRD patterns show (**Figure 4c**–**4e**) that the peaks became sharper and higher with the oxidizing temperature increasing from *T*ox = 770 K up to *T*ox = 970 K. Moreover, the peaks of palladium (II) oxide prepared by oxidation at *T*ox = 970 K are much sharper and higher than those for films oxidized at *T*ox = 870 K. This fact can be interpreted as one of the evidences of the crystalline perfection enhancement of palladium (II) oxide films and the grain size enlargement with the increase in the oxidation temperature.

**Figure 4.** X-ray diffraction patterns of palladium film deposited on SiO<sup>2</sup> /Si (100) substrate after oxidation in dry oxygen at different temperatures: *a*—*T*ox = 510 K; *b*—*T*ox = 570 K; *c*—*T*ox = 770 K; *d*—*T*ox = 870 K; and *e*—*T*ox = 970 K.

**4. Electrical properties of palladium (II) oxide nanostructures**

**Figure 5.** HEED patterns (*a*) and bright-field TEM image (*b*) of PdO film after oxidation at *T*ox = 870 K.

510 Pd Pd Pd 570 Pd + PdO Pd + PdO Pd + PdO

670 — PdO — PdO PdO PdO PdO PdO PdO PdO PdO PdO

**Table 1.** Results of phase composition study of Pd films after oxidation at *T* = 500–970 K.

**Oxidation temperature** *T***ox, K Phase composition**

Seebeck effect study and by calculation of the electromotive force *E*emf values:

constantan thermocouples have been used to measure a temperature difference.

exceed 7%.

The type of conductivity of PdO films synthesized at *T*ox = 770–970 K was determined by the

**X-ray analysis HEED TEM microdiffraction**

Palladium (II) Oxide Nanostructures as Promising Materials for Gas Sensors

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217

*E*emf = −*S* ∇ *T*, (1)

where *S* is thermo-power (Seebeck coefficient), and ∇*T* is the temperature gradient. Copper-

Experimental values of *E*emf have proved the *p*-type conductivity for all homogeneous palladium (II) oxide films (**Figure 6**). The fact of *p*-type conductivity of PdO bulk samples was reported in the previous publications [36, 37]. The values of thermo power (Seebeck coefficient) *S* have been calculated using the Eq. (1). Depending on thickness of palladium oxide films and oxidation temperature, the Seebeck coefficient values changed within the limits from +120 to +220 μV/K. The relative error at thermo-power measurement did not

High energy electron diffraction (HEED) technique was used as an alternative method to study PdO film phase composition (**Figure 5**).

**Table 1** compares the results of X-ray analysis (layers on SiO<sup>2</sup> /Si substrates), the HEED method (layers on optical quartz and Al<sup>2</sup> O3 substrates), and TEM micro diffraction (layers on amorphous carbon/KCl). An examination of the data presented in **Table 1** shows that the X-ray analysis, HEED method, and TEM micro diffraction gave the identical results for the films oxidized at temperatures *T*ox = 510, 570, 770, 870, and 970 K. The results of these two methods confirm that: (1) the annealing of Pd films at *T*ox = 510 K does not induce the changes in their phase composition; (2) after annealing of Pd films at the *T*ox = 570 K, the partial oxidation takes place and gives two phase samples – a mixture of Pd and PdO; and (3) after the annealing of palladium layers at *T*ox = 770–970 K, the total oxidation gives homogeneous PdO films [48].

Palladium (II) Oxide Nanostructures as Promising Materials for Gas Sensors http://dx.doi.org/10.5772/intechopen.72323 217

**Figure 5.** HEED patterns (*a*) and bright-field TEM image (*b*) of PdO film after oxidation at *T*ox = 870 K.


**Table 1.** Results of phase composition study of Pd films after oxidation at *T* = 500–970 K.

High energy electron diffraction (HEED) technique was used as an alternative method to

on amorphous carbon/KCl). An examination of the data presented in **Table 1** shows that the X-ray analysis, HEED method, and TEM micro diffraction gave the identical results for the films oxidized at temperatures *T*ox = 510, 570, 770, 870, and 970 K. The results of these two methods confirm that: (1) the annealing of Pd films at *T*ox = 510 K does not induce the changes in their phase composition; (2) after annealing of Pd films at the *T*ox = 570 K, the partial oxidation takes place and gives two phase samples – a mixture of Pd and PdO; and (3) after the annealing of palladium layers at *T*ox = 770–970 K, the total oxidation gives homo-

O3

at different temperatures: *a*—*T*ox = 510 K; *b*—*T*ox = 570 K; *c*—*T*ox = 770 K; *d*—*T*ox = 870 K; and *e*—*T*ox = 970 K.

/Si substrates), the HEED

/Si (100) substrate after oxidation in dry oxygen

substrates), and TEM micro diffraction (layers

study PdO film phase composition (**Figure 5**).

**Figure 4.** X-ray diffraction patterns of palladium film deposited on SiO<sup>2</sup>

method (layers on optical quartz and Al<sup>2</sup>

216 Novel Nanomaterials - Synthesis and Applications

geneous PdO films [48].

**Table 1** compares the results of X-ray analysis (layers on SiO<sup>2</sup>

#### **4. Electrical properties of palladium (II) oxide nanostructures**

The type of conductivity of PdO films synthesized at *T*ox = 770–970 K was determined by the Seebeck effect study and by calculation of the electromotive force *E*emf values:

$$E\_{\rm omf} = -\mathbf{S} \cdot \nabla \cdot T,\tag{1}$$

where *S* is thermo-power (Seebeck coefficient), and ∇*T* is the temperature gradient. Copperconstantan thermocouples have been used to measure a temperature difference.

Experimental values of *E*emf have proved the *p*-type conductivity for all homogeneous palladium (II) oxide films (**Figure 6**). The fact of *p*-type conductivity of PdO bulk samples was reported in the previous publications [36, 37]. The values of thermo power (Seebeck coefficient) *S* have been calculated using the Eq. (1). Depending on thickness of palladium oxide films and oxidation temperature, the Seebeck coefficient values changed within the limits from +120 to +220 μV/K. The relative error at thermo-power measurement did not exceed 7%.

**Figure 6.** Electromotive force *E*emf dependence upon the temperature gradient for PdO film prepared by oxidation at *T*ox = 870 K (thickness ~ 30 nm).

In view of *p*-type conductivity, palladium (II) oxide films are characterized with the cation deficiency regarding the stoichiometric 1:1 ratio. Thus, for PdO, the Kröger-Vink defect reactions can be written as follows:

**a.** with the cations in deficiency on the lattice sites:

$$\rm{Pd\_{pd}^\*} + \rm{O\_{O}^\*} + \frac{1}{2}\rm{O\_2}^{\
eg\circ} \rightleftharpoons \rm{Pd\_{pd}^\*} + \rm{V\_{Pd}^\*} + 2\rm{O\_{O}^\*} + 2\,\rm{h}^- \tag{2}$$

oxidizing oxygen molecules of synthetic air (SA) by a pen-ray ultraviolet (UV) lamp calibrated

ozone was blown directly on the sensor placed on the top of holder within the test chamber. The operating temperature *T*d of the sensor ranging from room temperature to 670 K was controlled by chromel-alumel thermocouple. The measurement started after the sample resistance

Sensor response *S* was determined as the ratio of the sensor resistance in synthetic air *R*<sup>0</sup>

As it possible to see in **Figures 7** and **8**, at rather low operation temperature *T*d, the sensors based on thin (*T*<sup>d</sup> = 490 K) and ultrathin (*T*<sup>d</sup> = 448 K) PdO films show good sensitivity to rather low concentrations of ozone. **Figures 7** and **8** show that at process of ozone quantitative detection at SA atmosphere within concentration interval 100–250 ppb, palladium (II) oxide films have demonstrated high values of sensor response, signal stability, and reproducibility of sensor response also. This fact was proved by the results of multiple measure-

characterized by higher values of sensor response (on the average in 7–8 times) in comparison with ultrathin films at the same ozone concentrations (**Figure 9**). It is possible to explain

**Figure 7.** Time dependence of PdO ultrathin film (thickness ~ 10 nm) sensor resistance *R* at ozone different concentrations

concentration range between 0.03 ppb and 800 ppb. The synthetic air containing

*<sup>R</sup>* (4)

per minute, respectively. The gas flow rate was

concentration were performed in flow path conditions with

Palladium (II) Oxide Nanostructures as Promising Materials for Gas Sensors

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concentrations. PdO films with thickness of about 35 nm are

to

219

to give the O3

achieved a steady value [50, 51].

the sensor resistance in gas *R*:

The measurements of NO2

ment cycles with the same O<sup>3</sup>

(operation temperature *T*<sup>d</sup> = 448 K).

the rates of 300 cm3

*<sup>S</sup>* <sup>=</sup> *<sup>R</sup>*\_\_0

measured by controllers produced by Bronkhorst.

and O3

per minute and 2.4 dm3

**b.** with the anions in excess on the interstitial sites:

$$\rm{Pd\_{pd}^\* + O\_O^\* + \frac{1}{2}O\_2^{\prime \text{(gs)}} \rightleftharpoons \rm{Pd\_{pd}^\* + O\_O^\* + O\_l^{\prime} + 2} h^{\prime} \tag{3}$$

The results obtained in the present work correlate with the capacitance voltage characteristics of PdO films on silicon [49]. Previously it was found that within the band gap of PdO films, one single energy state is realized only [49]. Therefore, only one type of point defects, which have generated holes, dominates in palladium (II) oxide films. The experimental study of the point defects nature will be the subject of further investigations.

#### **5. Gas sensor properties of palladium (II) oxide nanostructures**

Ozone and nitrogen dioxide sensitivity has been measured using the specially fabricated test samples of gas sensors based on thin and ultrathin PdO films oxidized at *T*ox = 870 K. During sensor response measurements of PdO, ultrathin and thin films prepared by oxidation at *T*ox = 873 K, synthetic air, calibrated gas mixtures with fixed nitrogen dioxide concentration, and standardized ozone generator produced by Optec were used. The ozone gas was generated by oxidizing oxygen molecules of synthetic air (SA) by a pen-ray ultraviolet (UV) lamp calibrated to give the O3 concentration range between 0.03 ppb and 800 ppb. The synthetic air containing ozone was blown directly on the sensor placed on the top of holder within the test chamber. The operating temperature *T*d of the sensor ranging from room temperature to 670 K was controlled by chromel-alumel thermocouple. The measurement started after the sample resistance achieved a steady value [50, 51].

Sensor response *S* was determined as the ratio of the sensor resistance in synthetic air *R*<sup>0</sup> to the sensor resistance in gas *R*:

$$S = \frac{R\_0}{R} \tag{4}$$

The measurements of NO2 and O3 concentration were performed in flow path conditions with the rates of 300 cm3 per minute and 2.4 dm3 per minute, respectively. The gas flow rate was measured by controllers produced by Bronkhorst.

As it possible to see in **Figures 7** and **8**, at rather low operation temperature *T*d, the sensors based on thin (*T*<sup>d</sup> = 490 K) and ultrathin (*T*<sup>d</sup> = 448 K) PdO films show good sensitivity to rather low concentrations of ozone. **Figures 7** and **8** show that at process of ozone quantitative detection at SA atmosphere within concentration interval 100–250 ppb, palladium (II) oxide films have demonstrated high values of sensor response, signal stability, and reproducibility of sensor response also. This fact was proved by the results of multiple measurement cycles with the same O<sup>3</sup> concentrations. PdO films with thickness of about 35 nm are characterized by higher values of sensor response (on the average in 7–8 times) in comparison with ultrathin films at the same ozone concentrations (**Figure 9**). It is possible to explain

In view of *p*-type conductivity, palladium (II) oxide films are characterized with the cation deficiency regarding the stoichiometric 1:1 ratio. Thus, for PdO, the Kröger-Vink defect reac-

**Figure 6.** Electromotive force *E*emf dependence upon the temperature gradient for PdO film prepared by oxidation at

(*gas*) ⇄ PdPd

(*gas*) ⇄ PdPd

The results obtained in the present work correlate with the capacitance voltage characteristics of PdO films on silicon [49]. Previously it was found that within the band gap of PdO films, one single energy state is realized only [49]. Therefore, only one type of point defects, which have generated holes, dominates in palladium (II) oxide films. The experimental study of the

Ozone and nitrogen dioxide sensitivity has been measured using the specially fabricated test samples of gas sensors based on thin and ultrathin PdO films oxidized at *T*ox = 870 K. During sensor response measurements of PdO, ultrathin and thin films prepared by oxidation at *T*ox = 873 K, synthetic air, calibrated gas mixtures with fixed nitrogen dioxide concentration, and standardized ozone generator produced by Optec were used. The ozone gas was generated by

<sup>×</sup> + VPd

<sup>×</sup> + OO

″ + 2 OO

<sup>×</sup> + O*<sup>i</sup>*

<sup>×</sup> + 2 *h*·· (2)

″ + 2 *h*·· (3)

tions can be written as follows:

218 Novel Nanomaterials - Synthesis and Applications

*T*ox = 870 K (thickness ~ 30 nm).

PdPd

PdPd

**a.** with the cations in deficiency on the lattice sites:

**b.** with the anions in excess on the interstitial sites:

<sup>×</sup> + OO <sup>×</sup> + \_\_1 <sup>2</sup> O2

<sup>×</sup> + OO <sup>×</sup> + \_\_1 <sup>2</sup> O2

point defects nature will be the subject of further investigations.

**5. Gas sensor properties of palladium (II) oxide nanostructures**

**Figure 7.** Time dependence of PdO ultrathin film (thickness ~ 10 nm) sensor resistance *R* at ozone different concentrations (operation temperature *T*<sup>d</sup> = 448 K).

**Figure 8.** Time dependence of PdO thin film (thickness ~ 35 nm) sensor resistance *R* at ozone different concentrations (operation temperature *T*<sup>d</sup> = 490 K).

this fact that the contribution in integrated conductivity of near-surface layers with high defects density is essentially higher for ultrathin PdO films than for films with thickness of about 35 nm (**Figure 9**). It is necessary to emphasis that the established feature demands a detailed study.

It has been established that PdO thin and ultrathin film sensors gave the stable signal, and the resistance values reliably returned to the baseline at SA atmosphere [50, 51]. It is necessary to note that the recovery period is quite long (600–700 s). It is necessary to note that the similar sensor behavior is typical for other materials used oxidizing gas detection. Usually in this case, the long recovery period is explained by the absence of oxidizing gas immediate interaction with oxygen molecules adsorbed on sensor material surface. At reducing gas detection, the direct interaction with oxygen molecules takes place; therefore, the recovery time is quite short. Moreover, the recovery time depends significantly on the operating temperature.

The sensitivity of palladium (II) oxide ultrathin films to nitrogen dioxide (another toxic oxidizing gas) has also been tested (**Figure 10**). As it can be seen in **Figure 10**, at the process of NO2 quantitative detection within concentration interval 500 ppb–200 ppm, PdO ultrathin films have demonstrated good values of sensor response, signal stability, and reproducibility of sensor response [51]. It is necessary to note that the recovery period at NO2 detection is longer than that at O3 detection (**Figures 7**, **8**, and **10**).

During the determination of ozone (concentration ϕ = 100 ppb) and nitrogen dioxide (concentration ϕ = 10 ppm), the temperature dependences of PdO ultrathin film sensor response *S* are presented in **Figure 11**.

It is found that within interval of operation temperature 323 < *T*<sup>d</sup> < 623 K, the maximum values of response *S* have been observed at *T*<sup>d</sup> = 448 K (NO<sup>2</sup> detection) and at *T*<sup>d</sup> = 490 K (O<sup>3</sup> detection).

As it can be seen in **Figure 11**, approximately equal values of sensor response *S* of palladium

**Figure 10.** Time dependence of PdO ultrathin film (thickness ~ 10 nm) sensor resistance *R* at nitrogen dioxide different

**Figure 9.** Dependence of PdO ultrathin and thin film sensor response *S* at ozone different concentrations: 1—Ultrathin film (thickness ~ 10 nm, operation temperature *T*<sup>d</sup> = 448 K) and 2—Thin film (thickness ~ 35 nm, operation temperature

Palladium (II) Oxide Nanostructures as Promising Materials for Gas Sensors

http://dx.doi.org/10.5772/intechopen.72323

221

) = 0.1 ppm and

(II) oxide films are realized at different concentration of oxidizing gases: φ(O3

φ(NO2

*T*<sup>d</sup> = 490 K).

) = 10 ppm.

concentrations (operation temperature *T*<sup>d</sup> = 448 K).

**Figure 9.** Dependence of PdO ultrathin and thin film sensor response *S* at ozone different concentrations: 1—Ultrathin film (thickness ~ 10 nm, operation temperature *T*<sup>d</sup> = 448 K) and 2—Thin film (thickness ~ 35 nm, operation temperature *T*<sup>d</sup> = 490 K).

this fact that the contribution in integrated conductivity of near-surface layers with high defects density is essentially higher for ultrathin PdO films than for films with thickness of about 35 nm (**Figure 9**). It is necessary to emphasis that the established feature demands a

**Figure 8.** Time dependence of PdO thin film (thickness ~ 35 nm) sensor resistance *R* at ozone different concentrations

It has been established that PdO thin and ultrathin film sensors gave the stable signal, and the resistance values reliably returned to the baseline at SA atmosphere [50, 51]. It is necessary to note that the recovery period is quite long (600–700 s). It is necessary to note that the similar sensor behavior is typical for other materials used oxidizing gas detection. Usually in this case, the long recovery period is explained by the absence of oxidizing gas immediate interaction with oxygen molecules adsorbed on sensor material surface. At reducing gas detection, the direct interaction with oxygen molecules takes place; therefore, the recovery time is quite short. Moreover, the recovery time depends significantly on the operating temperature.

The sensitivity of palladium (II) oxide ultrathin films to nitrogen dioxide (another toxic oxidizing gas) has also been tested (**Figure 10**). As it can be seen in **Figure 10**, at the process of

During the determination of ozone (concentration ϕ = 100 ppb) and nitrogen dioxide (concentration ϕ = 10 ppm), the temperature dependences of PdO ultrathin film sensor response *S* are

It is found that within interval of operation temperature 323 < *T*<sup>d</sup> < 623 K, the maximum values

detection) and at *T*<sup>d</sup> = 490 K (O<sup>3</sup>

of sensor response [51]. It is necessary to note that the recovery period at NO2

detection (**Figures 7**, **8**, and **10**).

 quantitative detection within concentration interval 500 ppb–200 ppm, PdO ultrathin films have demonstrated good values of sensor response, signal stability, and reproducibility

detection is

detection).

detailed study.

(operation temperature *T*<sup>d</sup> = 490 K).

220 Novel Nanomaterials - Synthesis and Applications

NO2

longer than that at O3

presented in **Figure 11**.

of response *S* have been observed at *T*<sup>d</sup> = 448 K (NO<sup>2</sup>

**Figure 10.** Time dependence of PdO ultrathin film (thickness ~ 10 nm) sensor resistance *R* at nitrogen dioxide different concentrations (operation temperature *T*<sup>d</sup> = 448 K).

As it can be seen in **Figure 11**, approximately equal values of sensor response *S* of palladium (II) oxide films are realized at different concentration of oxidizing gases: φ(O3 ) = 0.1 ppm and φ(NO2 ) = 10 ppm.

At interaction with PdO surface ozone molecules are more active than nitrogen dioxide ones. This interaction is accompanied by more essential increase in the hole density of palladium (II) oxide ultrathin films. In general case, the surface interaction of PdO nanostructures can be

<sup>×</sup> + OO

According to Eq. (5), oxygen atom is integrated with palladium (II) oxide structure and O2 molecule is desorbed from the surface. As result of this reaction (5), two holes are formed.

From this point of view, it is possible to explain high efficiency of palladium (II) oxide films at ozone detection. The attempt to distinguish the real reason of PdO nanostructures' higher

> **Magnetic susceptibility χ × 106 , cm3 /mol**

**Figure 12.** Dependence of PdO sensor response *S* upon the ozone concentration in synthetic air (operating temperature

<sup>×</sup> + O″*i* + 2 *h*·· + O2

Palladium (II) Oxide Nanostructures as Promising Materials for Gas Sensors

http://dx.doi.org/10.5772/intechopen.72323

**Dipole moment**  μ **× 1030**

0.66D

0.39D

**, C·m**

(*gas*) (5)

223

**PEL, ppm**

(0.2 mg/m3)

(9 mg/m3)

0.1

5

(*gas*) ⇄ PdPd

**Magnetic Properties**

**Table 2.** Physicochemical properties and permissible exposure limit (PEL) of ozone and nitrogen dioxide [52–56].

written within the framework of Kröger-Vink notation:

<sup>×</sup> + O3

**Space group symmetry**

O3 48.00 C2v Diamagnetic +6.7 2.2

NO2 46.0055 C2v Paramagnetic +150.0 1.3

<sup>×</sup> + OO

PdPd

**Molecule Molar mass** 

**M, g × mol−1**

*T*<sup>d</sup> = 490 K (220°C).

**Figure 11.** Dependence of PdO ultrathin film (thickness ~ 10 nm) sensor response *S* upon the operation temperature *T*d at detection of ozone (O3 concentration 0.1 ppm) and nitrogen dioxide (NO2 concentration 10 ppm).

#### **6. Discussion**

Data presented in **Table 2** show that physical properties (molecular mass, electric dipole moment) and chemical properties (structure of molecule, high oxidative activity) of detected gases are very similar. According to experimental evidence from microwave spectroscopy, ozone and nitrogen dioxide are bent molecules with *C*2v symmetry (**Table 2**). O3 and NO2 are the polar molecules with a dipole moment of 0.66 and 0.39 D, respectively.

Data in **Table 2** show that ozone and nitrogen dioxide molecules essentially differ with magnetic properties only. Ozone is diamagnetic, which means that its electrons are all paired. Unlike ozone, the ground electronic state of nitrogen dioxide is a doublet state. Owing to since nitrogen atom has one unpaired electron NO2 molecule is paramagnetic.

Nevertheless, at detection of ozone and nitrogen dioxide, the temperature that has matched the maximum values of sensor response differs only 25°. Thus, there is prerequisite for the increase in selectivity of palladium (II) oxide sensors at O3 and NO2 detection after studying in detail that oxidation procedure conditions influence on microstructure and stoichiometry deviation.

Moreover, at ambient conditions, palladium (II) oxide is insoluble in water and does not react with it. As the bottom sediment, the palladium (II) hydroxide is formed only at interaction of soluble palladium (II) salt and alkali [52]. These facts are favorable for fabrication of gas detectors because they make possible to minimize the air humidity influence on PdO sensor response values.

To estimate such perspective for palladium (II) oxide nanostructures, we have allowed the speculative extrapolation of experimental data to the point that corresponds to zero ozone concentration (**Figure 12**). At ozone concentration *φ* = 10 ppb, which corresponds to 0.1 × PEL (permissible exposure limit), sensor response *S* would be about 2 (*S* ~ 2 is an open circle in **Figure 12**). The extrapolated sensitivity value at concentration *φ*(O3 ) = 0.1 × PEL arouses hope that palladium (II) oxide films will be used in fabrication of ozone sensors.

At interaction with PdO surface ozone molecules are more active than nitrogen dioxide ones. This interaction is accompanied by more essential increase in the hole density of palladium (II) oxide ultrathin films. In general case, the surface interaction of PdO nanostructures can be written within the framework of Kröger-Vink notation:

$$\mathrm{Pd}^{\ast}\_{\mathrm{Pd}} + \mathrm{O}^{\ast}\_{\mathrm{O}} + \mathrm{O}\_{\mathrm{g}}^{\mathrm{(gas)}} \rightleftharpoons \mathrm{Pd}^{\ast}\_{\mathrm{Pd}} + \mathrm{O}^{\ast}\_{\mathrm{O}} + \mathrm{O}^{\ast}\_{\mathrm{i}} + 2\,\mathrm{h}^{\cdot} + \mathrm{O}\_{\mathrm{2}}^{\cdot \text{(gas)}}\tag{5}$$

According to Eq. (5), oxygen atom is integrated with palladium (II) oxide structure and O2 molecule is desorbed from the surface. As result of this reaction (5), two holes are formed.

From this point of view, it is possible to explain high efficiency of palladium (II) oxide films at ozone detection. The attempt to distinguish the real reason of PdO nanostructures' higher


**6. Discussion**

detection of ozone (O3

222 Novel Nanomaterials - Synthesis and Applications

response values.

Data presented in **Table 2** show that physical properties (molecular mass, electric dipole moment) and chemical properties (structure of molecule, high oxidative activity) of detected gases are very similar. According to experimental evidence from microwave spectroscopy,

**Figure 11.** Dependence of PdO ultrathin film (thickness ~ 10 nm) sensor response *S* upon the operation temperature *T*d at

Data in **Table 2** show that ozone and nitrogen dioxide molecules essentially differ with magnetic properties only. Ozone is diamagnetic, which means that its electrons are all paired. Unlike ozone, the ground electronic state of nitrogen dioxide is a doublet state. Owing to since

Nevertheless, at detection of ozone and nitrogen dioxide, the temperature that has matched the maximum values of sensor response differs only 25°. Thus, there is prerequisite for the increase

that oxidation procedure conditions influence on microstructure and stoichiometry deviation. Moreover, at ambient conditions, palladium (II) oxide is insoluble in water and does not react with it. As the bottom sediment, the palladium (II) hydroxide is formed only at interaction of soluble palladium (II) salt and alkali [52]. These facts are favorable for fabrication of gas detectors because they make possible to minimize the air humidity influence on PdO sensor

To estimate such perspective for palladium (II) oxide nanostructures, we have allowed the speculative extrapolation of experimental data to the point that corresponds to zero ozone concentration (**Figure 12**). At ozone concentration *φ* = 10 ppb, which corresponds to 0.1 × PEL (permissible exposure limit), sensor response *S* would be about 2 (*S* ~ 2 is an open circle in

**Figure 12**). The extrapolated sensitivity value at concentration *φ*(O3

that palladium (II) oxide films will be used in fabrication of ozone sensors.

molecule is paramagnetic.

concentration 10 ppm).

and NO2

and NO2

detection after studying in detail

) = 0.1 × PEL arouses hope

are

ozone and nitrogen dioxide are bent molecules with *C*2v symmetry (**Table 2**). O3

the polar molecules with a dipole moment of 0.66 and 0.39 D, respectively.

concentration 0.1 ppm) and nitrogen dioxide (NO2

nitrogen atom has one unpaired electron NO2

in selectivity of palladium (II) oxide sensors at O3

**Table 2.** Physicochemical properties and permissible exposure limit (PEL) of ozone and nitrogen dioxide [52–56].

**Figure 12.** Dependence of PdO sensor response *S* upon the ozone concentration in synthetic air (operating temperature *T*<sup>d</sup> = 490 K (220°C).

sensitivity to ozone, it should be looked for ozone's extremely high oxidizing ability. As it can be seen in **Table 2**, the PEL value of ozone is smaller than the similar characteristic of nitrogen dioxide by 50 times practically. This fact is the indirect evidence of ozone-exclusive oxidizing activity. The difference in sensitivity of palladium (II) oxide nanostructures at ozone and nitrogen dioxide detection will be a subject of the subsequent experiments and discussions. Now, it is possible to designate the direction of these future researches only. It is reasonable to assume that under ozone molecules impact, the metastable nanoclusters are formed on the surface of PdO, in which the oxidation states of palladium are higher than (II), for example, (III) or (IV).

[3] Amos P. K. Tai, Maria Val Martin, Colette L. Heald. Threat to future global food security from climate change and ozone air pollution. Nature Climate Change. Letters. Published

Palladium (II) Oxide Nanostructures as Promising Materials for Gas Sensors

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#### **7. Conclusion**

The results of X-ray analysis, HEED, and HR TEM have demonstrated the possibility of the synthesis of homogeneous nanocrystalline thin and ultrathin films of palladium (II) oxide on different substrates. The very first examinations of sensitivity to different nitrogen dioxide and ozone concentration at rather low operating temperature have shown the high values of sensor response, signal stability, operation speed, and reproducibility of PdO films sensor response. The possibility of work at quite low temperatures will allow decreasing in the energy consumption of the analytical instruments. The detection of O3 and NO2 by palladium (II) oxide sensors can be applied in the fields of the human health and environment protection. Because the synthesis procedure is rather simple and compatible with planar processes of the microelectronic industry PdO nanostructures have a good perspective to be one of the main materials for commercial fabrication of oxidizing gases (ozone, nitrogen dioxide, chlorine etc.) sensors.

#### **Author details**

Alexander M. Samoylov1 \*, Stanislav V. Ryabtsev1 , Vasily N. Popov1 and Petre Badica2

\*Address all correspondence to: samoylov@chem.vsu.ru


#### **References**


[3] Amos P. K. Tai, Maria Val Martin, Colette L. Heald. Threat to future global food security from climate change and ozone air pollution. Nature Climate Change. Letters. Published online: 27-07-2014. DOI:10.1038/nclimate2317

sensitivity to ozone, it should be looked for ozone's extremely high oxidizing ability. As it can be seen in **Table 2**, the PEL value of ozone is smaller than the similar characteristic of nitrogen dioxide by 50 times practically. This fact is the indirect evidence of ozone-exclusive oxidizing activity. The difference in sensitivity of palladium (II) oxide nanostructures at ozone and nitrogen dioxide detection will be a subject of the subsequent experiments and discussions. Now, it is possible to designate the direction of these future researches only. It is reasonable to assume that under ozone molecules impact, the metastable nanoclusters are formed on the surface of PdO, in which the oxidation states of palladium are higher than (II), for example, (III) or (IV).

The results of X-ray analysis, HEED, and HR TEM have demonstrated the possibility of the synthesis of homogeneous nanocrystalline thin and ultrathin films of palladium (II) oxide on different substrates. The very first examinations of sensitivity to different nitrogen dioxide and ozone concentration at rather low operating temperature have shown the high values of sensor response, signal stability, operation speed, and reproducibility of PdO films sensor response. The possibility of work at quite low temperatures will allow decreasing in the energy con-

sensors can be applied in the fields of the human health and environment protection. Because the synthesis procedure is rather simple and compatible with planar processes of the microelectronic industry PdO nanostructures have a good perspective to be one of the main materials for commercial fabrication of oxidizing gases (ozone, nitrogen dioxide, chlorine etc.) sensors.

[1] Estimating Mortality Risk Reduction and Economic Benefits from Controlling Ozone Air Pollution. Report of the US National Academies of Sciences. Washington: The National

[2] Health Aspects of Air Pollution with Particulate Matter, Ozone and Nitrogen Dioxide. Report on a WHO Working Group, 13-15-01-2003. Bonn, Germany: ©World Health

and NO2

, Vasily N. Popov1

by palladium (II) oxide

and Petre Badica2

sumption of the analytical instruments. The detection of O3

\*Address all correspondence to: samoylov@chem.vsu.ru

\*, Stanislav V. Ryabtsev1

1 Voronezh State University, Universitetskaya, Voronezh, Russian Federation 2 National Institute of Materials Physics, Atomistilor, Magurele, Ilfov, Romania

**7. Conclusion**

224 Novel Nanomaterials - Synthesis and Applications

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[41] Yamazoe N, Kurokawa Y, Seiyama T. Effects of additives on semiconductor gas sensors. Sensors and Actuators B. 1983;**4**:283-289

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**Chapter 13**

**Provisional chapter**

**Functionalized Carbon Nanomaterials in Drug Delivery:**

Carbon nanotubes (CNTs) have attracted substantial research interest in biomedical sciences and bionanotechnology, rendered from its unique structure, electronic, mechanical, and optical properties. Despite the diverse potential applications, the integration of CNTs in biomedical research is one of the most challenging areas where nanotubes fall under much scrutiny. Pristine nanotubes are highly hydrophobic, and non-dispersible in most of the common aqueous and organic solvents and to render nanotubes biocompatible, functionalization is one of the key prerequisites. In this regard, covalent and noncovalent functionalization are the two widely adopted approaches for co-tethering biologically active molecules on the CNTs. Likewise, the hollow cavity of the nanotube facilitates in the endohedral encapsulation of biomolecules, peptides, DNA oligonucleotides, and proteins, thereby retaining the physiological attributes of the biological molecules. The chapter focuses on the emerging approaches to the functionalization of single-wall CNTs (SWCNTs) and the potential application of functionalized SWCNTs in tuberculosis and cancer chemotherapy using state-of-the-art density func-

tional theory, molecular docking and molecular dynamics simulation methods.

**Keywords:** carbon nanotubes, drug delivery, molecular dynamics, density functional

**Functionalized Carbon Nanomaterials in Drug Delivery:** 

DOI: 10.5772/intechopen.71889

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

hybridized), graphite, graphene, fullerenes, and carbon nanotubes (*sp2*

and reproduction in any medium, provided the original work is properly cited.

Carbon is the most versatile element in the periodic table that forms the basis of all kinds of life on earth. Elemental carbon displays a complex allotropy depending on the nature of hybrid-

hybridized). Graphite is the most common allotrope of carbon and the word graphite in Greek

**Emergent Perspectives from Application**

**Emergent Perspectives from Application**

Additional information is available at the end of the chapter

**1.1. Carbon: The fundamental building block of life**

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.71889

Nabanita Saikia

**Abstract**

theory

**1. Introduction**

ization; diamond (*sp3*

Nabanita Saikia

**Provisional chapter**

### **Functionalized Carbon Nanomaterials in Drug Delivery: Emergent Perspectives from Application Emergent Perspectives from Application**

**Functionalized Carbon Nanomaterials in Drug Delivery:** 

DOI: 10.5772/intechopen.71889

#### Nabanita Saikia Additional information is available at the end of the chapter

Nabanita Saikia

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.71889

#### **Abstract**

Carbon nanotubes (CNTs) have attracted substantial research interest in biomedical sciences and bionanotechnology, rendered from its unique structure, electronic, mechanical, and optical properties. Despite the diverse potential applications, the integration of CNTs in biomedical research is one of the most challenging areas where nanotubes fall under much scrutiny. Pristine nanotubes are highly hydrophobic, and non-dispersible in most of the common aqueous and organic solvents and to render nanotubes biocompatible, functionalization is one of the key prerequisites. In this regard, covalent and noncovalent functionalization are the two widely adopted approaches for co-tethering biologically active molecules on the CNTs. Likewise, the hollow cavity of the nanotube facilitates in the endohedral encapsulation of biomolecules, peptides, DNA oligonucleotides, and proteins, thereby retaining the physiological attributes of the biological molecules. The chapter focuses on the emerging approaches to the functionalization of single-wall CNTs (SWCNTs) and the potential application of functionalized SWCNTs in tuberculosis and cancer chemotherapy using state-of-the-art density functional theory, molecular docking and molecular dynamics simulation methods.

**Keywords:** carbon nanotubes, drug delivery, molecular dynamics, density functional theory

#### **1. Introduction**

#### **1.1. Carbon: The fundamental building block of life**

Carbon is the most versatile element in the periodic table that forms the basis of all kinds of life on earth. Elemental carbon displays a complex allotropy depending on the nature of hybridization; diamond (*sp3* hybridized), graphite, graphene, fullerenes, and carbon nanotubes (*sp2* hybridized). Graphite is the most common allotrope of carbon and the word graphite in Greek

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

means '*to write*'. *Graphene* an acronym for the 2D layered graphite, is the mother of all carbon materials [1], as a graphene sheet can be wrapped to form 0D fullerenes, rolled to form 1D nanotubes, or stacked to form 3D graphite as depicted in **Figure 1**. The unearthing of "*groundbreaking experiments regarding the two-dimensional material graphene*" by Geim and Novoselov in 2010, heralded graphene as the next generation carbon material [2].

CNTs are hexagonally arranged, honey-combed lattice of carbon atoms formed by the rolling of graphene into seamless cylindrical structures (see **Figure 2a**). Nanotubes like graphene have a high diameter to length ratio (aspect ratio) [3] and demonstrate high electrical, mechanical, and thermal conductivity along with structural stability [4–7]. CNTs are broadly classified as single-wall CNTs (SWCNTs) and multi-wall CNTs (MWCNTs). The SWCNT comprise of a single graphene sheet, with diameter ~ 0.5–1.5 nm and length of ~100 μm [8], while MWCNT is formed from the co-axial stacking of SWCNTs, with diameter ~1.4–100 nm, length between 1 nm-μm, and internuclear distance of 0.3–0.4 nm between the co-axial tubes. The representation of a zigzag (*m* = 0), armchair (*n* = *m*), and chiral (n ≠ *m*) nanotube is depicted in **Figure 2b**-**d**. The (*n*, *m*) indices render remarkable electronic properties to the CNTs [9] and the *sp*<sup>2</sup> hybridization along the tubular axis makes it chemically inert by nature.

> The unique electronic properties exhibited by CNT are governed by the quantum confinement of electrons where the periodic boundary conditions come into interplay. Because of the quantum confinement, electrons can propagate along the tube axis: forward and backward, along with the conservation of energy and momentum. Unlike metals which have a smooth density of states (DOS), CNTs are characterized by many van Hove singularities [10], and the DOS depends on diameter and chirality of the nanotube [11]. The conducting properties of CNT is an inverse function of its diameter, that is, with increase in diameter, band gap between the valence and conduction bands decreases and at a certain point both the bands overlap to give rise to metallic nanotubes. Semiconducting nanotubes on the other hand (with similar diameter as metallic nanotubes) possess similar van Hove singularities near the Fermi level [12].

> **Figure 2.** (a) A graphene sheet depicting the (b) zigzag and (c) armchair and (d) chiral CNT based on rolling of carbon

Functionalized Carbon Nanomaterials in Drug Delivery: Emergent Perspectives from Application

http://dx.doi.org/10.5772/intechopen.71889

233

atoms along chiral vectors through the circumference (OA) of nanotube.

These unprecedented properties have largely contributed to the extensive biomedical research, especially as nanocapsules for therapeutic drugs, proteins, and gene delivery [13]. CNTs find application in bionanotechnology and pharmaceutical sciences, and current drug delivery modules have been incorporating CNTs for improved target specific detection and treatment of diseases. Although the potential application of carbon nanomaterials, particularly CNTs are farfetched, the concerns raised over the effect of large-scale synthesis of CNTs to the environment, biocompatibility, toxicity, biodegradation and remediation cannot be undermined and needs to be addressed thoroughly. Hence, a detailed *in vivo* and *in vitro* toxicity analyses is mandatory in understanding CNT-based therapeutic regimes for sustained drug delivery

Some of the questions that underlie the importance of the study are (i) understanding the mechanism of CNT uptake followed by the subsequent release of therapeutic molecules, (ii) *in vivo* biocompatibility, and (iii) long-term practical implication to direct exposure to the

applications.

**Figure 1.** Formation of SWCNT, fullerene and graphite from a single graphene monolayer.

means '*to write*'. *Graphene* an acronym for the 2D layered graphite, is the mother of all carbon materials [1], as a graphene sheet can be wrapped to form 0D fullerenes, rolled to form 1D nanotubes, or stacked to form 3D graphite as depicted in **Figure 1**. The unearthing of "*groundbreaking experiments regarding the two-dimensional material graphene*" by Geim and Novoselov in

CNTs are hexagonally arranged, honey-combed lattice of carbon atoms formed by the rolling of graphene into seamless cylindrical structures (see **Figure 2a**). Nanotubes like graphene have a high diameter to length ratio (aspect ratio) [3] and demonstrate high electrical, mechanical, and thermal conductivity along with structural stability [4–7]. CNTs are broadly classified as single-wall CNTs (SWCNTs) and multi-wall CNTs (MWCNTs). The SWCNT comprise of a single graphene sheet, with diameter ~ 0.5–1.5 nm and length of ~100 μm [8], while MWCNT is formed from the co-axial stacking of SWCNTs, with diameter ~1.4–100 nm, length between 1 nm-μm, and internuclear distance of 0.3–0.4 nm between the co-axial tubes. The representation of a zigzag (*m* = 0), armchair (*n* = *m*), and chiral (n ≠ *m*) nanotube is depicted in **Figure 2b**-**d**.

The (*n*, *m*) indices render remarkable electronic properties to the CNTs [9] and the *sp*<sup>2</sup>

hybrid-

2010, heralded graphene as the next generation carbon material [2].

232 Novel Nanomaterials - Synthesis and Applications

ization along the tubular axis makes it chemically inert by nature.

**Figure 1.** Formation of SWCNT, fullerene and graphite from a single graphene monolayer.

**Figure 2.** (a) A graphene sheet depicting the (b) zigzag and (c) armchair and (d) chiral CNT based on rolling of carbon atoms along chiral vectors through the circumference (OA) of nanotube.

The unique electronic properties exhibited by CNT are governed by the quantum confinement of electrons where the periodic boundary conditions come into interplay. Because of the quantum confinement, electrons can propagate along the tube axis: forward and backward, along with the conservation of energy and momentum. Unlike metals which have a smooth density of states (DOS), CNTs are characterized by many van Hove singularities [10], and the DOS depends on diameter and chirality of the nanotube [11]. The conducting properties of CNT is an inverse function of its diameter, that is, with increase in diameter, band gap between the valence and conduction bands decreases and at a certain point both the bands overlap to give rise to metallic nanotubes. Semiconducting nanotubes on the other hand (with similar diameter as metallic nanotubes) possess similar van Hove singularities near the Fermi level [12].

These unprecedented properties have largely contributed to the extensive biomedical research, especially as nanocapsules for therapeutic drugs, proteins, and gene delivery [13]. CNTs find application in bionanotechnology and pharmaceutical sciences, and current drug delivery modules have been incorporating CNTs for improved target specific detection and treatment of diseases. Although the potential application of carbon nanomaterials, particularly CNTs are farfetched, the concerns raised over the effect of large-scale synthesis of CNTs to the environment, biocompatibility, toxicity, biodegradation and remediation cannot be undermined and needs to be addressed thoroughly. Hence, a detailed *in vivo* and *in vitro* toxicity analyses is mandatory in understanding CNT-based therapeutic regimes for sustained drug delivery applications.

Some of the questions that underlie the importance of the study are (i) understanding the mechanism of CNT uptake followed by the subsequent release of therapeutic molecules, (ii) *in vivo* biocompatibility, and (iii) long-term practical implication to direct exposure to the physiological environment. Although theoretical and/or experimental studies have attempted to address the main questions like mechanism of drug-nanotube interaction, the preferable binding sites of drugs onto nanotubes, drug activity under confinement, and change in redox properties of drugs under the physiological conditions, these studies are rather limited in predicting the likelihood of using CNT as carrier vehicles for the long-term storage and release of therapeutic and biologically active molecules *in vivo*.

CNTs are generally hydrophobic with low solubility in most of the common aqueous and organic solvents and the hydrophobicity is accounted to the size, structure, and bundling effect which restricts the uptake and assimilation within the biological environment [25]. Functionalization assists in reducing the bundling effect which arises due to the van der Waals (vdW) attractive forces between adjacent nanotube surfaces and is efficient in increasing the biocompatibility thereby facilitating cellular internalization and trafficking. It has been reported that the functionalized nanotubes (*f*CNT) exhibit better biocompatibility with reduced *in vivo* and *in vitro* toxicity [26–28]. The extent of functionalization depends on the nature and reactivity of sidewall (curvature), number of functional groups that can be cotethered along the sidewall, and steric hindrance between functional groups and nanotube sidewall. The subsequent sections discuss some of the adopted approaches in the functional-

Functionalized Carbon Nanomaterials in Drug Delivery: Emergent Perspectives from Application

http://dx.doi.org/10.5772/intechopen.71889

235

Some of the alternative schemes to functionalization of CNT is through covalent method using 1,3-dipolar cycloaddition [29], [2 + 1] cycloaddition of dichlorocarbene, silylene, germylene [30], hydroboration [31], arylation, hydrogenation by Birch reduction [32], carboxylic acid groups [33], Diels-Alder reaction, esterification of carboxylated nanotubes [34] and fluorination reactions [35]. Experimental and theoretical studies have shown that the extent of covalent functionalization depends on the curvature of nanotube as an increase in curvature

The solubility of CNT can be enhanced by the covalent functionalization using 1,3-DC reactions.

imine (CHNNH) are the commonly used functional groups for 1,3-DC reaction. The intrinsic physical properties of CNTs such as photoluminescence and Raman scattering decreases upon covalent functionalization, due to chemical bond formation between the functional group and

using two-layered ONIOM (B3LYP/6-31G\*:AM1) approach. Likewise, experimental studies by Prato and co-workers [37] substantiated the theoretical findings on the 1,3-DC functionalization of CNTs. 1,3-DC functionalization was also achieved through the addition of ozone wherein ozone adds onto the end caps and kink regions rather than nanotube sidewall due to increased strain and loss in conjugation. Lu et al. [38] reported the 1,3-DC reaction of ozone onto nano-

Prato and co-workers [39] investigated the 1,3-DC reaction of azomethine ylide on both SWCNT and MWCNT. The water-soluble amine functionalized CNT was highly suitable for immobilization of biomolecules, and purification of pristine nanotubes during syntheses. Attachment of peptide molecules onto covalently functionalized SWCNT was reported by Prato and co-workers [40]. The C-terminal group of peptide chain was attached to N-terminal

carbon atoms. Theoretical studies by Lu et al. [30] reported the reaction energies (*E*<sup>r</sup>

), nitrone (CH2

) and retro barrier height values of a series of 1,3-dipolar molecules on (5, 5) SWCNT

N(H)O), nitrile ylide (CHNCH2

), nitrile

), barrier

ization of SWCNTs at the level of experiment and theory.

**2.1. Solubilization of CNT through covalent functionalization**

decreases the reactivity toward sidewall functionalization [36].

), ozone (O3

tube sidewalls using two-layered ONIOM (B3LYP/6-31G\*:AM1) method.

NHCH2

*2.1.1. 1,3-dipolar Cycloaddition (DC)*

Azomethine ylide (CH2

heights (*E*<sup>a</sup>

The chapter in a very comprehensive yet succinct way addresses the potential applications of SWCNTs in drug delivery, managing to draw a fine line between the scopes of application and practical viability of integrating carbon nanomaterials in biomedical research. Herein, we report the theoretical aspects of modeling novel SWCNT-based drug delivery systems using the covalent and noncovalent functionalization schemes. Nanotubes of varying chirality and length are considered for functionalization, drug loading, and targeting onto the active binding sites of receptor proteins. With the successful incorporation of CNTs in cancer therapy, we propose a novel approach toward integrating CNTs in Tuberculosis (TB) therapy. To the best of our knowledge, theoretical studies on the potential application of CNTs in pharmaceutical sciences pertaining to TB and other bacterial diseases have not been discussed extensively. We address the recent theoretical advancements using the state-of-the-art density functional theory (DFT), molecular docking, and molecular dynamics (MD) simulation methods. Molecular docking serves as an instrumental tool in computer-aided drug design for predicting the preferred binding mode of a ligand to a receptor (protein). Docking studies help characterize the protein binding cavity, understand the orientation of ligand with respect to the receptor protein, and the nature of interaction between the protein with functionalized nanomaterial, which can aid in the structure-based design of novel drug delivery systems for future experimental studies.

## **2. Functionalization of CNTs**

Traditional approaches to drug delivery function over a broad spectrum, resulting which, specificity toward drug administration and delivery are rarely accomplished. Development of polymer-based nanocomposite materials has enabled the successful engineering of drug delivery modules via the incorporation of nanomaterials and nanoparticles as nanocapsules for sustained release of therapeutics in a dosage-dependent manner. Nanotechnology, on the other hand, has revolutionized the pharmaceutical sector with the assimilation of functionalized nanomaterials like CNTs in drug, gene delivery, and tissue engineering [14, 15]. CNTs play dual role by rendering directionality in targeting the tumor (malignant) cells and facilitating the controlled mediated release of therapeutic molecules. The application of CNTs as carrier payloads for anticancer drugs cisplatin [16], carboplatin [17], doxorubicin (DOX) [18–20], mechlorethamine [21], paclitaxel [22] and antitubercular drugs like isoniazid (INH) [23], rifampicin, pyrazinamide (PZA) [24] have been reported.

With the inherent limitations in application of pristine, unmodified nanotubes, functionalization is the collective approach toward tailoring nanotubes electronic properties. Pristine CNTs are generally hydrophobic with low solubility in most of the common aqueous and organic solvents and the hydrophobicity is accounted to the size, structure, and bundling effect which restricts the uptake and assimilation within the biological environment [25]. Functionalization assists in reducing the bundling effect which arises due to the van der Waals (vdW) attractive forces between adjacent nanotube surfaces and is efficient in increasing the biocompatibility thereby facilitating cellular internalization and trafficking. It has been reported that the functionalized nanotubes (*f*CNT) exhibit better biocompatibility with reduced *in vivo* and *in vitro* toxicity [26–28]. The extent of functionalization depends on the nature and reactivity of sidewall (curvature), number of functional groups that can be cotethered along the sidewall, and steric hindrance between functional groups and nanotube sidewall. The subsequent sections discuss some of the adopted approaches in the functionalization of SWCNTs at the level of experiment and theory.

#### **2.1. Solubilization of CNT through covalent functionalization**

Some of the alternative schemes to functionalization of CNT is through covalent method using 1,3-dipolar cycloaddition [29], [2 + 1] cycloaddition of dichlorocarbene, silylene, germylene [30], hydroboration [31], arylation, hydrogenation by Birch reduction [32], carboxylic acid groups [33], Diels-Alder reaction, esterification of carboxylated nanotubes [34] and fluorination reactions [35]. Experimental and theoretical studies have shown that the extent of covalent functionalization depends on the curvature of nanotube as an increase in curvature decreases the reactivity toward sidewall functionalization [36].

#### *2.1.1. 1,3-dipolar Cycloaddition (DC)*

physiological environment. Although theoretical and/or experimental studies have attempted to address the main questions like mechanism of drug-nanotube interaction, the preferable binding sites of drugs onto nanotubes, drug activity under confinement, and change in redox properties of drugs under the physiological conditions, these studies are rather limited in predicting the likelihood of using CNT as carrier vehicles for the long-term storage and release of

The chapter in a very comprehensive yet succinct way addresses the potential applications of SWCNTs in drug delivery, managing to draw a fine line between the scopes of application and practical viability of integrating carbon nanomaterials in biomedical research. Herein, we report the theoretical aspects of modeling novel SWCNT-based drug delivery systems using the covalent and noncovalent functionalization schemes. Nanotubes of varying chirality and length are considered for functionalization, drug loading, and targeting onto the active binding sites of receptor proteins. With the successful incorporation of CNTs in cancer therapy, we propose a novel approach toward integrating CNTs in Tuberculosis (TB) therapy. To the best of our knowledge, theoretical studies on the potential application of CNTs in pharmaceutical sciences pertaining to TB and other bacterial diseases have not been discussed extensively. We address the recent theoretical advancements using the state-of-the-art density functional theory (DFT), molecular docking, and molecular dynamics (MD) simulation methods. Molecular docking serves as an instrumental tool in computer-aided drug design for predicting the preferred binding mode of a ligand to a receptor (protein). Docking studies help characterize the protein binding cavity, understand the orientation of ligand with respect to the receptor protein, and the nature of interaction between the protein with functionalized nanomaterial, which can aid in the structure-based design of novel drug delivery systems for future experi-

Traditional approaches to drug delivery function over a broad spectrum, resulting which, specificity toward drug administration and delivery are rarely accomplished. Development of polymer-based nanocomposite materials has enabled the successful engineering of drug delivery modules via the incorporation of nanomaterials and nanoparticles as nanocapsules for sustained release of therapeutics in a dosage-dependent manner. Nanotechnology, on the other hand, has revolutionized the pharmaceutical sector with the assimilation of functionalized nanomaterials like CNTs in drug, gene delivery, and tissue engineering [14, 15]. CNTs play dual role by rendering directionality in targeting the tumor (malignant) cells and facilitating the controlled mediated release of therapeutic molecules. The application of CNTs as carrier payloads for anticancer drugs cisplatin [16], carboplatin [17], doxorubicin (DOX) [18–20], mechlorethamine [21], paclitaxel [22] and antitubercular drugs like isoniazid (INH)

With the inherent limitations in application of pristine, unmodified nanotubes, functionalization is the collective approach toward tailoring nanotubes electronic properties. Pristine

therapeutic and biologically active molecules *in vivo*.

234 Novel Nanomaterials - Synthesis and Applications

mental studies.

**2. Functionalization of CNTs**

[23], rifampicin, pyrazinamide (PZA) [24] have been reported.

The solubility of CNT can be enhanced by the covalent functionalization using 1,3-DC reactions. Azomethine ylide (CH2 NHCH2 ), ozone (O3 ), nitrone (CH2 N(H)O), nitrile ylide (CHNCH2 ), nitrile imine (CHNNH) are the commonly used functional groups for 1,3-DC reaction. The intrinsic physical properties of CNTs such as photoluminescence and Raman scattering decreases upon covalent functionalization, due to chemical bond formation between the functional group and carbon atoms. Theoretical studies by Lu et al. [30] reported the reaction energies (*E*<sup>r</sup> ), barrier heights (*E*<sup>a</sup> ) and retro barrier height values of a series of 1,3-dipolar molecules on (5, 5) SWCNT using two-layered ONIOM (B3LYP/6-31G\*:AM1) approach. Likewise, experimental studies by Prato and co-workers [37] substantiated the theoretical findings on the 1,3-DC functionalization of CNTs. 1,3-DC functionalization was also achieved through the addition of ozone wherein ozone adds onto the end caps and kink regions rather than nanotube sidewall due to increased strain and loss in conjugation. Lu et al. [38] reported the 1,3-DC reaction of ozone onto nanotube sidewalls using two-layered ONIOM (B3LYP/6-31G\*:AM1) method.

Prato and co-workers [39] investigated the 1,3-DC reaction of azomethine ylide on both SWCNT and MWCNT. The water-soluble amine functionalized CNT was highly suitable for immobilization of biomolecules, and purification of pristine nanotubes during syntheses. Attachment of peptide molecules onto covalently functionalized SWCNT was reported by Prato and co-workers [40]. The C-terminal group of peptide chain was attached to N-terminal side group to form a supramolecular complex of peptide wrapped nanotubes. Gallo et al. [23], incorporated *f*SWCNT and fullerenes as nanovectors for the functionalization of INH drug. Armchair (5, 5) SWCNT was functionalized via 1,3-DC reaction of azomethine ylide with the polyethylene glycol (PEG) oligomer tailored to the INH drug. Increasing the number of functionalized units leads to an increase in HOMO-LUMO energy gap and global hardness, and decrease in binding (−3.52 to −6.65 eV) and solvation energy (−31.60 to −49.99 eV) values. An increase in global hardness with increase in functionalization suggests a net stabilization of the complex. It is noteworthy to mention that the optimum length of PEG oligomer used as a linker for the 1,3-DC functionalization is essential as longer PEG chains can interfere with drug administration, block the interaction between the nanotube and cell lines of the body, cellular uptake of drug, and degrade the therapeutic activity of drug molecules [41]. The PEG units with superior hydrophilicity, biocompatibility, and low immunogenicity can resist the opsonisation and increase the retention time of the nanotube-drug conjugate system *in vivo* [42, 43].

The structure, electronic properties, and reactivity of a series of 1,3-DC functionalized armchair (*n, m*) and zigzag (*n, 0*) SWCNTs with antitubercular drugs 2-methyl heptyl isonicotinate (MHI) and PZA via. PEG linker was investigated using first-principles DFT calculations [44–46]. With increase in sidewall functionalization, the global hardness and HOMO-LUMO energy gap decreases suggesting an overall decrease in stability of the complex, which is indicative of the localized induced deformation in the nanotube at the site of covalent attachment. On the other hand, the solubility of bare INH and PZA drugs was enhanced in presence of nanotube support. We showed that the optimum length and chirality of the nanotube is central to understand the electronic properties of functionalized nanotubes, particularly from a drug delivery perspective.

#### *2.1.2. Functionalization using organic acids*

Covalent functionalization of CNTs using carboxylic (−COOH) group was realized through oxidation with strong organic acids like H2 SO<sup>4</sup> /HNO3 [47], phosphates, and sulfur-containing units. Acid functionalized CNTs are highly soluble in water under a wide range of pH and exhibit a significant reduction in the aggregation of nanotube bundles. The dispersibility facilitates in the sidewall, endohedral and end tip functionalization of CNT with organic acids and different functional groups. The sidewall functionalization of CNTs via cycloaddition reaction with azide, ozone, transition metal oxides, and carbenes [48] is illustrated in **Figure 3**.

Lu et al. [31] using two-layered ONIOM (B3LYP/6-31G\*:AM1) approach reported the reaction pathway and site selectivity for [2 + 1] cycloaddition of dichlorocarbene, silylene, germylene, and oxycarbonylnitrene onto (5, 5) SWCNT. Dichlorocarbene addition occurs preferentially at the 1,2-pair site. The silylene addition at 1,2-pair site was predicted to be exothermic (−20.7 kcal mol−1) and follows a barrier less reaction pathway. Germylene addition was exothermic by 8.5 kcal mol−1, lower than dichlorocarbene and silylene and proceeds in absence of a transition state pathway. Oxycarbonylnitrene addition onto 1,2-pair site of SWCNT was exothermic by 66.2 kcal mol−1, higher than the other three cycloaddition groups. The transition state had an activation barrier of 7.2 kcal mol−1, which suggested that the cycloaddition reaction was facile in nature.

**2.2. Noncovalent functionalization of CNTs**

Although covalent functionalization improves the solubility of CNT, it modifies the intrinsic electronic properties by deforming the C-C bond length, perturbing the *π*-delocalization, and shortening the length of the nanotube. Noncovalent functionalization provides the alternative

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**Figure 3.** Schemes for sidewall functionalization of SWCNT using covalent bonds. Adopted from Ref. [48].

Functionalized Carbon Nanomaterials in Drug Delivery: Emergent Perspectives from Application http://dx.doi.org/10.5772/intechopen.71889 237

**Figure 3.** Schemes for sidewall functionalization of SWCNT using covalent bonds. Adopted from Ref. [48].

#### **2.2. Noncovalent functionalization of CNTs**

side group to form a supramolecular complex of peptide wrapped nanotubes. Gallo et al. [23], incorporated *f*SWCNT and fullerenes as nanovectors for the functionalization of INH drug. Armchair (5, 5) SWCNT was functionalized via 1,3-DC reaction of azomethine ylide with the polyethylene glycol (PEG) oligomer tailored to the INH drug. Increasing the number of functionalized units leads to an increase in HOMO-LUMO energy gap and global hardness, and decrease in binding (−3.52 to −6.65 eV) and solvation energy (−31.60 to −49.99 eV) values. An increase in global hardness with increase in functionalization suggests a net stabilization of the complex. It is noteworthy to mention that the optimum length of PEG oligomer used as a linker for the 1,3-DC functionalization is essential as longer PEG chains can interfere with drug administration, block the interaction between the nanotube and cell lines of the body, cellular uptake of drug, and degrade the therapeutic activity of drug molecules [41]. The PEG units with superior hydrophilicity, biocompatibility, and low immunogenicity can resist the opsonisation and increase the retention time of the nanotube-drug conjugate system *in vivo* [42, 43].

The structure, electronic properties, and reactivity of a series of 1,3-DC functionalized armchair (*n, m*) and zigzag (*n, 0*) SWCNTs with antitubercular drugs 2-methyl heptyl isonicotinate (MHI) and PZA via. PEG linker was investigated using first-principles DFT calculations [44–46]. With increase in sidewall functionalization, the global hardness and HOMO-LUMO energy gap decreases suggesting an overall decrease in stability of the complex, which is indicative of the localized induced deformation in the nanotube at the site of covalent attachment. On the other hand, the solubility of bare INH and PZA drugs was enhanced in presence of nanotube support. We showed that the optimum length and chirality of the nanotube is central to understand the electronic properties of functionalized nanotubes, particularly from

Covalent functionalization of CNTs using carboxylic (−COOH) group was realized through

units. Acid functionalized CNTs are highly soluble in water under a wide range of pH and exhibit a significant reduction in the aggregation of nanotube bundles. The dispersibility facilitates in the sidewall, endohedral and end tip functionalization of CNT with organic acids and different functional groups. The sidewall functionalization of CNTs via cycloaddition reaction with azide, ozone, transition metal oxides, and carbenes [48] is illustrated in **Figure 3**.

Lu et al. [31] using two-layered ONIOM (B3LYP/6-31G\*:AM1) approach reported the reaction pathway and site selectivity for [2 + 1] cycloaddition of dichlorocarbene, silylene, germylene, and oxycarbonylnitrene onto (5, 5) SWCNT. Dichlorocarbene addition occurs preferentially at the 1,2-pair site. The silylene addition at 1,2-pair site was predicted to be exothermic (−20.7 kcal mol−1) and follows a barrier less reaction pathway. Germylene addition was exothermic by 8.5 kcal mol−1, lower than dichlorocarbene and silylene and proceeds in absence of a transition state pathway. Oxycarbonylnitrene addition onto 1,2-pair site of SWCNT was exothermic by 66.2 kcal mol−1, higher than the other three cycloaddition groups. The transition state had an activation barrier of 7.2 kcal mol−1, which suggested that the cycloaddition

/HNO3

[47], phosphates, and sulfur-containing

SO<sup>4</sup>

a drug delivery perspective.

236 Novel Nanomaterials - Synthesis and Applications

reaction was facile in nature.

*2.1.2. Functionalization using organic acids*

oxidation with strong organic acids like H2

Although covalent functionalization improves the solubility of CNT, it modifies the intrinsic electronic properties by deforming the C-C bond length, perturbing the *π*-delocalization, and shortening the length of the nanotube. Noncovalent functionalization provides the alternative approach to improving the solubility of nanotubes without deforming the *π*-delocalization. For example, exohedral wrapping with polymeric molecules like PEG [49], polymers [50], ss-DNA, and endohedral filling can help in increasing the solubility. Likewise, the polymer molecules can form a surface coating via *π*-stacking interactions, mediated by weak vdW forces, and hydrophobic interactions. The following subsection discusses some of the widely adopted approaches to the noncovalent functionalization of CNTs.

*2.2.2. Functionalization using biomolecules and nucleobases*

interaction energy tend to be on the higher side [67].

*2.2.3. Noncovalent functionalization using polymers*

cally perturbing the overall electronic properties.

strong organic acids, leading to the formation of carboxylated SWCNT.

Functionalization of CNTs with biomolecules is useful in the development of nanobio composite devices. Immobilization of DNA in DNA-based biosensors is possible with the incorporation of CNTs in nucleic acid sensing, gene therapy, and biosensor fabrication [59–61]. DNA because of the base pairing sequence facilitates in the alignment of nanotube assembly [62]. Rodger and co-workers [63] investigated the interaction of CNT with DNA using linear dichroism (LD) method. DNA/CNT hybrid exhibited higher LD signals, higher than the sum of the LD spectrum of individual DNA and SWCNT. Jung et al. [64] developed methods for covalent linking of DNA oligonucleotides onto SWCNT films which were later immobilized onto solid surfaces. The carboxylated/aminated DNA oligonucleotides were covalently attached to functionalized SWCNT, the length of which was controlled via oxidation with

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Li et al. [65] investigated the self-assembly of CNT and gold nanoparticles into multicomponent structures using DNA oligonucleotides. The CNT pre-functionalized with -COOH facilitated in the grafting of ssDNA strands and multiple assemblies of nanotubes were thus possible using this technique. In another combined theoretical and experimental study by Sood and co-workers [66], interaction of DNA nucleobases namely adenine (A), guanine (G), cytosine (C), thymine (T) with (5, 5) SWCNT was reported. The *ab initio* studies showed that binding energies of nucleobases onto SWCNT was governed mainly by vdW forces and follow the order: C > G > A > T, respectively. Likewise, the binding energies of A, G, C, T, U nucleobases on (7, 0) SWCNT was predicted to follow the order: G > C > A > T > U, and bears a monotonic dependence on nanotube diameter; that is, nanotubes with small diameter due to low curvature exhibits low interaction energy whereas for nanotubes with high diameter the

Polymer wrapping of CNTs mediated via noncovalent functionalization toward the synthesis of highly dispersed, stable and reinforced functional dispersants in aqueous and organic solvents was reviewed by Fujigaya and Nakashima [68]. The polymer wrapping forms a thermodynamically stable coating and any unbound polymer could be removed via dialysis, ultra-centrifugation or chromatographic separation techniques. Similarly, noncovalent functionalization of CNTs using polyethylene glycol (PEG) PEGylated-phospholipid chains forms an effective means of high loading of the drug and biomolecules at the free end of PEG chain and onto nanotube sidewall. PEGylated-phospholipid chain facilitates the high loading (about ~400%) of drug molecules onto CNT and characterizes as a potent carrier vehicle in drug delivery applications. PEG tailored SWCNT exhibit no toxicity for over several months, which was further substantiated from time-dependent assays performed onto mice. Drugs which normally remained insoluble within the biological systems, upon conjugation with PEG modified CNT revealed high solubility as well as retention time within the body. The noncovalent functionalization retained the physical properties of nanotubes without drasti-

First-principles studies on the interaction of conjugated polymers with (8, 0) SWCNT and (10 × 10) graphene sheet was investigated by Jilili et al. [69], to confirm the experimental

#### *2.2.1. Functionalization via π-π stacking*

The *π-π* stacking of organic molecules namely pyrene, anthracene, and porphyrin increases the solubility of pristine nanotubes and facilitates in the binding of proteins, polysaccharides, and peptides. Dai and co-workers [51] investigated the noncovalent functionalization of CNT, wherein succinimidyl ester group was co-tethered onto pyrene rings via butanoic acid side chains, facilitating the immobilization of proteins. The amide group of the protein replaces the N-hydroxysuccinimide group that propagates the transportation of biomolecules. Falvo and co-workers [52] reported the functionalization of MWCNT with streptavidin protein, wherein the MWCNT pre-functionalized with 1-pyrene butanoic acid succidymidyl ester (1-pbase) was co-tethered on the nanotube sidewall. The pyrenes formed *π-π* bonds with the MWCNT sidewall under the influence of which MWCNT undergoes a phase transfer with 1-base acting as a phase transfer catalyst.

The noncovalent functionalization of pyrene on SWCNT was investigated by Cosnier and co-workers [53] for application as modified electrodes in biosensing devices. Calomel electrode was taken as the reference and Pt electrode (5 mm diameter) modified by casting 20 *μl* THF solution of pristine SWCNT and B-doped SWCNT polished with 20 *μm* diamond paste as the counter electrode. The SWCNT/pyrene-biotin and B-SWCNT/pyrene-biotin was incubated in 20 *μl* avidin solutions for 20 min, and the response time for glucose sensing was measured using amperometric response technique. The enzyme-modified SWCNT electrodes were incorporated as electrodes for glucose sensing. In-situ polymerization of MWCNT with polyimide (PI) results in the π-stacking interactions between the imide and aromatic benzene rings of CNT with subsequent wrapping of PI along the nanotube circumferential axis [54]. Polymer wrapping improves the thermal stability and renders the conjugated complex suitable for nanoelectronics devices with improved electronic, thermal and optical properties.

Noncovalent functionalization of porphyrin molecules with SWCNTs have been extensively studied as high yield light-harvesting systems with tunable electronic properties for biological and optoelectronic applications [55, 56]. Roquelet et al. [57], reported an efficient method for the synthesis of porphyrin/SWCNT complex utilizing a micelle-swelling technique in presence of organic solvent. The organic solvent leads to swelling of the micelle facilitating the interaction of porphyrin molecules to the micelle core and SWCNT. Dispersion corrected DFT calculations on the structure, electronic and optical properties of SWCNT functionalized tetraphenylporphyrin (TPP) molecule showed that diameter rather than chirality of the nanotube stabilizes the π-π stacking of TPP molecule [58]. The optical absorption of TPP was not affected by the diameter or chirality of CNT and the optical spectra showed the absorption of π-stacked TPP at almost the same position as the isolated TPP, indicating that the TPP absorption properties were preserved in the complex.

#### *2.2.2. Functionalization using biomolecules and nucleobases*

approach to improving the solubility of nanotubes without deforming the *π*-delocalization. For example, exohedral wrapping with polymeric molecules like PEG [49], polymers [50], ss-DNA, and endohedral filling can help in increasing the solubility. Likewise, the polymer molecules can form a surface coating via *π*-stacking interactions, mediated by weak vdW forces, and hydrophobic interactions. The following subsection discusses some of the widely adopted

The *π-π* stacking of organic molecules namely pyrene, anthracene, and porphyrin increases the solubility of pristine nanotubes and facilitates in the binding of proteins, polysaccharides, and peptides. Dai and co-workers [51] investigated the noncovalent functionalization of CNT, wherein succinimidyl ester group was co-tethered onto pyrene rings via butanoic acid side chains, facilitating the immobilization of proteins. The amide group of the protein replaces the N-hydroxysuccinimide group that propagates the transportation of biomolecules. Falvo and co-workers [52] reported the functionalization of MWCNT with streptavidin protein, wherein the MWCNT pre-functionalized with 1-pyrene butanoic acid succidymidyl ester (1-pbase) was co-tethered on the nanotube sidewall. The pyrenes formed *π-π* bonds with the MWCNT sidewall under the influence of which MWCNT undergoes a phase transfer with 1-base acting

The noncovalent functionalization of pyrene on SWCNT was investigated by Cosnier and co-workers [53] for application as modified electrodes in biosensing devices. Calomel electrode was taken as the reference and Pt electrode (5 mm diameter) modified by casting 20 *μl* THF solution of pristine SWCNT and B-doped SWCNT polished with 20 *μm* diamond paste as the counter electrode. The SWCNT/pyrene-biotin and B-SWCNT/pyrene-biotin was incubated in 20 *μl* avidin solutions for 20 min, and the response time for glucose sensing was measured using amperometric response technique. The enzyme-modified SWCNT electrodes were incorporated as electrodes for glucose sensing. In-situ polymerization of MWCNT with polyimide (PI) results in the π-stacking interactions between the imide and aromatic benzene rings of CNT with subsequent wrapping of PI along the nanotube circumferential axis [54]. Polymer wrapping improves the thermal stability and renders the conjugated complex suitable for nanoelectronics devices with improved electronic, thermal

Noncovalent functionalization of porphyrin molecules with SWCNTs have been extensively studied as high yield light-harvesting systems with tunable electronic properties for biological and optoelectronic applications [55, 56]. Roquelet et al. [57], reported an efficient method for the synthesis of porphyrin/SWCNT complex utilizing a micelle-swelling technique in presence of organic solvent. The organic solvent leads to swelling of the micelle facilitating the interaction of porphyrin molecules to the micelle core and SWCNT. Dispersion corrected DFT calculations on the structure, electronic and optical properties of SWCNT functionalized tetraphenylporphyrin (TPP) molecule showed that diameter rather than chirality of the nanotube stabilizes the π-π stacking of TPP molecule [58]. The optical absorption of TPP was not affected by the diameter or chirality of CNT and the optical spectra showed the absorption of π-stacked TPP at almost the same position as the isolated TPP, indicating that the TPP absorption properties were preserved

approaches to the noncovalent functionalization of CNTs.

*2.2.1. Functionalization via π-π stacking*

238 Novel Nanomaterials - Synthesis and Applications

as a phase transfer catalyst.

and optical properties.

in the complex.

Functionalization of CNTs with biomolecules is useful in the development of nanobio composite devices. Immobilization of DNA in DNA-based biosensors is possible with the incorporation of CNTs in nucleic acid sensing, gene therapy, and biosensor fabrication [59–61]. DNA because of the base pairing sequence facilitates in the alignment of nanotube assembly [62]. Rodger and co-workers [63] investigated the interaction of CNT with DNA using linear dichroism (LD) method. DNA/CNT hybrid exhibited higher LD signals, higher than the sum of the LD spectrum of individual DNA and SWCNT. Jung et al. [64] developed methods for covalent linking of DNA oligonucleotides onto SWCNT films which were later immobilized onto solid surfaces. The carboxylated/aminated DNA oligonucleotides were covalently attached to functionalized SWCNT, the length of which was controlled via oxidation with strong organic acids, leading to the formation of carboxylated SWCNT.

Li et al. [65] investigated the self-assembly of CNT and gold nanoparticles into multicomponent structures using DNA oligonucleotides. The CNT pre-functionalized with -COOH facilitated in the grafting of ssDNA strands and multiple assemblies of nanotubes were thus possible using this technique. In another combined theoretical and experimental study by Sood and co-workers [66], interaction of DNA nucleobases namely adenine (A), guanine (G), cytosine (C), thymine (T) with (5, 5) SWCNT was reported. The *ab initio* studies showed that binding energies of nucleobases onto SWCNT was governed mainly by vdW forces and follow the order: C > G > A > T, respectively. Likewise, the binding energies of A, G, C, T, U nucleobases on (7, 0) SWCNT was predicted to follow the order: G > C > A > T > U, and bears a monotonic dependence on nanotube diameter; that is, nanotubes with small diameter due to low curvature exhibits low interaction energy whereas for nanotubes with high diameter the interaction energy tend to be on the higher side [67].

#### *2.2.3. Noncovalent functionalization using polymers*

Polymer wrapping of CNTs mediated via noncovalent functionalization toward the synthesis of highly dispersed, stable and reinforced functional dispersants in aqueous and organic solvents was reviewed by Fujigaya and Nakashima [68]. The polymer wrapping forms a thermodynamically stable coating and any unbound polymer could be removed via dialysis, ultra-centrifugation or chromatographic separation techniques. Similarly, noncovalent functionalization of CNTs using polyethylene glycol (PEG) PEGylated-phospholipid chains forms an effective means of high loading of the drug and biomolecules at the free end of PEG chain and onto nanotube sidewall. PEGylated-phospholipid chain facilitates the high loading (about ~400%) of drug molecules onto CNT and characterizes as a potent carrier vehicle in drug delivery applications. PEG tailored SWCNT exhibit no toxicity for over several months, which was further substantiated from time-dependent assays performed onto mice. Drugs which normally remained insoluble within the biological systems, upon conjugation with PEG modified CNT revealed high solubility as well as retention time within the body. The noncovalent functionalization retained the physical properties of nanotubes without drastically perturbing the overall electronic properties.

First-principles studies on the interaction of conjugated polymers with (8, 0) SWCNT and (10 × 10) graphene sheet was investigated by Jilili et al. [69], to confirm the experimental observation that polymers are suitable for noncovalent functionalization. The GGA approximation was predicted to be inadequate in describing the physisorbed systems, whereas LDA and vdW corrected GGA yield conclusive results. The electronic structure of SWCNT/graphene was maintained around the Fermi energy with negligible charge transfer between the conjugates. The interaction of polymer-SWCNT/graphene was of weak vdW type with minimal effects on the physical and electronic properties of SWCNT/graphene, important for an effective noncovalent functionalization.

poverty, homelessness, synergy with HIV/AIDS pandemic, multi-drug resistant (MDR), and

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Streptomycin was the first antitubercular drug discovered in 1943 [90] and since then several therapeutic drugs like para-amino salicylic acid (1946), isoniazid (INH) (1952), pyrazinamide (PZA) (1952), cycloserine (1952), ethionamide (1956), rifampicin (1957) and ethambutol (1962) have been discovered. The TB therapy involves a combination of four first-line drugs namely; INH, PZA, rifampicin, and ethambutol administered for a period of 2 months followed by minimum 4 months' treatment regimen of INH and rifampicin [91]. PZA (pyrazine-2-carboxamide) is one of the first-line drugs used in TB treatment recommended by the WHO. PZA is metabolized into its active form (pyrazoic acid) by the amidase activity of *M. tuberculosis* nicotinamidase/pyrazinamidase (MtPncA) encoded by the pncA gene [92]. The administration of PZA in high dosage can cause minor to detrimental health problems and the antibiotic resistance of bacteria under prolonged exposure triggers the need for better drug delivery

We performed DFT, molecular docking and MD studies on the SWCNT-mediated PZA delivery onto the active site of *M. Tuberculosis* pncA enzyme [93]. The DFT calculations predict that the covalent functionalization was thermodynamically favored with negative binding energy values. The decrease in binding energy of PZA/SWCNT with increase in nanotube diameter illustrates that the curvature of nanotube plays an important role in determining the reactivity, and nanotubes with narrow diameter are thermodynamically favorable compared to tubes with larger diameter. The molecular docking studies supported the DFT results thereby establishing that, incorporation of SWCNT facilitates in target specific delivery of PZA within the binding site of pncA as shown in **Figure 4**. The narrow diameter nanotubes were better docked compared to the larger diameter nanotubes and length of PEG chain was predicted to be reasonably adequate for the delivery of PZA within the binding site of pncA. The presence of nanotube did not result in any structural deformation in pncA, rather the incorporation of SWCNT facilitated in the stabilization of

Noncovalent functionalization of SWCNT and boron nitride nanotubes (BNNTs) with PZA was investigated using DFT and MD simulation (see **Figure 5**) to comprehend the role of nanotube chirality on the electronic properties of the complexes [94]. BNNTs are structural analogs of CNTs with a wide band gap of ~5.5 eV, high chemical, and thermal stability. The potential application of BNNTs is rather limited in terms of its high chemical stability and poor dispersibility. The theoretical results predict the modification in electronic structure of both SWCNTs and BNNTs with the enhancement of electronic states, significant lowering in HOMO-LUMO energy gap and the presence of new dispersionless states within the band gap. Depending on the nanotube chirality, PZA exhibits a preferential selectivity for adsorption, which is further confirmed from the band structure, DOS, total projected DOS,

The functionalized nanotube facilitates in the loading and delivery of a PZA onto the active entering pathway of pncA without the nanotube affecting the structural conformation of

extensively drug resistant (XDR) stains of *M. tuberculosis* [89].

methods to directly bind with the TB bacteria.

PZA conjugated complex.

and frontier orbital analysis.

## **3. Functionalized carbon Nanomaterials in drug delivery**

Drug delivery is a process of administering drugs in a controlled, sustained manner to achieve maximal therapeutic efficacy upon transdermal administration. The foremost objectives in developing novel drug delivery systems are to improve the therapeutic competence by [1] increasing bioavailability, [2] preventing toxicity, harmful side-effects by increasing the persistence of a drug, [3] reducing drug exposure toward non-target cells, and [4] minimizing drug degradation and loss [70–73]. Drug delivery systems are designed to improve the pharmacological and therapeutic profile of drug molecules with an ability to cross the cell membrane upon administration [74, 75]. The most important characteristic of SWCNT as a drug delivery system is its ability to penetrate the cell membrane [76], and facilitate the intracellular internalization and trafficking within the cell cytoplasm [77]. A major breakthrough in nanoscience was the advent of CNTs as one of the most sought-after materials for designing novel drug delivery carrier modules to comply with the biotechnological and pharmaceutical objectives. CNT due to its needle-like cylindrical structure can easily penetrate the cell membrane and enter the cell nuclei, while the cell does not recognize it as an intruder.

Functionalized CNTs can act as carriers for antimicrobial agents like amphotericin B [78, 79] and transport it within the mammalian cells. This reduces the antifungal toxicity as compared to the toxicity of free drug (40% of the cells being killed by CNTs-free formulation compared to no cell death by CNTs formulation). The surface-engineered CNTs can capture the pathogenic bacteria in liquid media [80, 81]. In addition, SWCNTs exhibit unique optical properties such as near-infrared region (NIR) fluorescence, and Raman scattering. The fluorescence range spans the entire biological tissue transparent window and is, therefore, promising for drug detection and biological imaging [82–84].

#### **3.1. SWCNTs in tuberculosis therapy**

The science of bacteriology is credited to the contributions of Louis Pasteur and Robert Koch. It was the discovery of *Mycobacterium tuberculosis* by Koch that revolutionized medical history [85]. TB is a chronic disease caused by the infection of *Mycobacterium tuberculosis* [86] and is a leading cause of mortality worldwide. The World Health Organization (WHO) 2017 annual report prompted to 10.4 million new TB cases, of which, India and Indonesia alone accounted for a third of the world's TB-burden [87]. In 2016, a total of 9287 new TB cases were reported in the United States [88]. The drastic widespread of TB is mainly accounted to poverty, homelessness, synergy with HIV/AIDS pandemic, multi-drug resistant (MDR), and extensively drug resistant (XDR) stains of *M. tuberculosis* [89].

observation that polymers are suitable for noncovalent functionalization. The GGA approximation was predicted to be inadequate in describing the physisorbed systems, whereas LDA and vdW corrected GGA yield conclusive results. The electronic structure of SWCNT/graphene was maintained around the Fermi energy with negligible charge transfer between the conjugates. The interaction of polymer-SWCNT/graphene was of weak vdW type with minimal effects on the physical and electronic properties of SWCNT/graphene, important for an

Drug delivery is a process of administering drugs in a controlled, sustained manner to achieve maximal therapeutic efficacy upon transdermal administration. The foremost objectives in developing novel drug delivery systems are to improve the therapeutic competence by [1] increasing bioavailability, [2] preventing toxicity, harmful side-effects by increasing the persistence of a drug, [3] reducing drug exposure toward non-target cells, and [4] minimizing drug degradation and loss [70–73]. Drug delivery systems are designed to improve the pharmacological and therapeutic profile of drug molecules with an ability to cross the cell membrane upon administration [74, 75]. The most important characteristic of SWCNT as a drug delivery system is its ability to penetrate the cell membrane [76], and facilitate the intracellular internalization and trafficking within the cell cytoplasm [77]. A major breakthrough in nanoscience was the advent of CNTs as one of the most sought-after materials for designing novel drug delivery carrier modules to comply with the biotechnological and pharmaceutical objectives. CNT due to its needle-like cylindrical structure can easily penetrate the cell mem-

brane and enter the cell nuclei, while the cell does not recognize it as an intruder.

Functionalized CNTs can act as carriers for antimicrobial agents like amphotericin B [78, 79] and transport it within the mammalian cells. This reduces the antifungal toxicity as compared to the toxicity of free drug (40% of the cells being killed by CNTs-free formulation compared to no cell death by CNTs formulation). The surface-engineered CNTs can capture the pathogenic bacteria in liquid media [80, 81]. In addition, SWCNTs exhibit unique optical properties such as near-infrared region (NIR) fluorescence, and Raman scattering. The fluorescence range spans the entire biological tissue transparent window and is, therefore, promising for

The science of bacteriology is credited to the contributions of Louis Pasteur and Robert Koch. It was the discovery of *Mycobacterium tuberculosis* by Koch that revolutionized medical history [85]. TB is a chronic disease caused by the infection of *Mycobacterium tuberculosis* [86] and is a leading cause of mortality worldwide. The World Health Organization (WHO) 2017 annual report prompted to 10.4 million new TB cases, of which, India and Indonesia alone accounted for a third of the world's TB-burden [87]. In 2016, a total of 9287 new TB cases were reported in the United States [88]. The drastic widespread of TB is mainly accounted to

**3. Functionalized carbon Nanomaterials in drug delivery**

effective noncovalent functionalization.

240 Novel Nanomaterials - Synthesis and Applications

drug detection and biological imaging [82–84].

**3.1. SWCNTs in tuberculosis therapy**

Streptomycin was the first antitubercular drug discovered in 1943 [90] and since then several therapeutic drugs like para-amino salicylic acid (1946), isoniazid (INH) (1952), pyrazinamide (PZA) (1952), cycloserine (1952), ethionamide (1956), rifampicin (1957) and ethambutol (1962) have been discovered. The TB therapy involves a combination of four first-line drugs namely; INH, PZA, rifampicin, and ethambutol administered for a period of 2 months followed by minimum 4 months' treatment regimen of INH and rifampicin [91]. PZA (pyrazine-2-carboxamide) is one of the first-line drugs used in TB treatment recommended by the WHO. PZA is metabolized into its active form (pyrazoic acid) by the amidase activity of *M. tuberculosis* nicotinamidase/pyrazinamidase (MtPncA) encoded by the pncA gene [92]. The administration of PZA in high dosage can cause minor to detrimental health problems and the antibiotic resistance of bacteria under prolonged exposure triggers the need for better drug delivery methods to directly bind with the TB bacteria.

We performed DFT, molecular docking and MD studies on the SWCNT-mediated PZA delivery onto the active site of *M. Tuberculosis* pncA enzyme [93]. The DFT calculations predict that the covalent functionalization was thermodynamically favored with negative binding energy values. The decrease in binding energy of PZA/SWCNT with increase in nanotube diameter illustrates that the curvature of nanotube plays an important role in determining the reactivity, and nanotubes with narrow diameter are thermodynamically favorable compared to tubes with larger diameter. The molecular docking studies supported the DFT results thereby establishing that, incorporation of SWCNT facilitates in target specific delivery of PZA within the binding site of pncA as shown in **Figure 4**. The narrow diameter nanotubes were better docked compared to the larger diameter nanotubes and length of PEG chain was predicted to be reasonably adequate for the delivery of PZA within the binding site of pncA. The presence of nanotube did not result in any structural deformation in pncA, rather the incorporation of SWCNT facilitated in the stabilization of PZA conjugated complex.

Noncovalent functionalization of SWCNT and boron nitride nanotubes (BNNTs) with PZA was investigated using DFT and MD simulation (see **Figure 5**) to comprehend the role of nanotube chirality on the electronic properties of the complexes [94]. BNNTs are structural analogs of CNTs with a wide band gap of ~5.5 eV, high chemical, and thermal stability. The potential application of BNNTs is rather limited in terms of its high chemical stability and poor dispersibility. The theoretical results predict the modification in electronic structure of both SWCNTs and BNNTs with the enhancement of electronic states, significant lowering in HOMO-LUMO energy gap and the presence of new dispersionless states within the band gap. Depending on the nanotube chirality, PZA exhibits a preferential selectivity for adsorption, which is further confirmed from the band structure, DOS, total projected DOS, and frontier orbital analysis.

The functionalized nanotube facilitates in the loading and delivery of a PZA onto the active entering pathway of pncA without the nanotube affecting the structural conformation of

pncA as shown in **Figure 6**. The incorporation of nanotube yields better docking scores for PZA then the drug being administered in bare form. Although covalent functionalization aids in achieving target specific delivery of PZA within the active site of pncA, noncovalent functionalization was predicted to be effective for engineering nanotube structure and electronic

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Zanella et al. [95] performed theoretical studies on the interaction of non-steroid anti-inflammatory drug nimesulide with pristine and Si-doped capped SWCNT. The adsorption of nimesulide on Si-doped capped SWCNT exhibit a higher binding energy of 1.8 eV compared to pristine capped SWCNT (0.32 eV) which was due to the high reactive bonding sites on Si atom. The strong interaction of nimesulide with Si-doped SWCNT served as better drug

Wang and co-workers [96] performed MD studies to investigate the mechanism of encapsulation of nifedipine drug within (10, 10) SWCNT. Their studies showed that the internal

**Figure 6.** (a) Docked PZA/pncA, (b) electrostatic surface (c) PZA/(5, 5) SWCNT docked onto pncA, (d) electrostatic surface, (e) PZA/(8, 0) SWCNT docked onto pncA, (f) electrostatic surface, (g) docked PZA/(5, 5) BNNT with pncA,

(h) electrostatic surface. Adopted from Ref. [94].

properties for successful drug delivery applications.

delivery carriers in comparison to pristine capped SWCNT.

**Figure 4.** The docked conformation and hydrophobic surface of sidewall functionalized PZA/SWCNT within the active binding site of pncA protein, (a–b) PZA/(9,0) SWCNT (3 unit cells), (c–d) PZA/(9,0) SWCNT (5 unit cells), (e–f) edge functionalized PZA/(9,0) SWCNT (5 unit cells). Reprinted with permission from Ref. [93]. Copyright 2017, Elsevier.

**Figure 5.** (a) Adsorption sites in the model SWCNT, (b) Adsorption sites in a model (5, 5) BNNT; optimized geometries of (c) PZA, (d) (5, 5) CNT, (e) (8, 0) CNT and (f) (5, 5) BNNT. Adopted from Ref. [94].

pncA as shown in **Figure 6**. The incorporation of nanotube yields better docking scores for PZA then the drug being administered in bare form. Although covalent functionalization aids in achieving target specific delivery of PZA within the active site of pncA, noncovalent functionalization was predicted to be effective for engineering nanotube structure and electronic properties for successful drug delivery applications.

Zanella et al. [95] performed theoretical studies on the interaction of non-steroid anti-inflammatory drug nimesulide with pristine and Si-doped capped SWCNT. The adsorption of nimesulide on Si-doped capped SWCNT exhibit a higher binding energy of 1.8 eV compared to pristine capped SWCNT (0.32 eV) which was due to the high reactive bonding sites on Si atom. The strong interaction of nimesulide with Si-doped SWCNT served as better drug delivery carriers in comparison to pristine capped SWCNT.

Wang and co-workers [96] performed MD studies to investigate the mechanism of encapsulation of nifedipine drug within (10, 10) SWCNT. Their studies showed that the internal

**Figure 4.** The docked conformation and hydrophobic surface of sidewall functionalized PZA/SWCNT within the active binding site of pncA protein, (a–b) PZA/(9,0) SWCNT (3 unit cells), (c–d) PZA/(9,0) SWCNT (5 unit cells), (e–f) edge functionalized PZA/(9,0) SWCNT (5 unit cells). Reprinted with permission from Ref. [93]. Copyright 2017, Elsevier.

**Figure 5.** (a) Adsorption sites in the model SWCNT, (b) Adsorption sites in a model (5, 5) BNNT; optimized geometries

of (c) PZA, (d) (5, 5) CNT, (e) (8, 0) CNT and (f) (5, 5) BNNT. Adopted from Ref. [94].

242 Novel Nanomaterials - Synthesis and Applications

**Figure 6.** (a) Docked PZA/pncA, (b) electrostatic surface (c) PZA/(5, 5) SWCNT docked onto pncA, (d) electrostatic surface, (e) PZA/(8, 0) SWCNT docked onto pncA, (f) electrostatic surface, (g) docked PZA/(5, 5) BNNT with pncA, (h) electrostatic surface. Adopted from Ref. [94].

adsorption of nifedipine was more stable than external adsorption by 5.3 to 7.8 kcal/mol. In solvent phase, the encapsulation of nifedipine was impeded due to competitive vdW and hydrophobic interactions in SWCNT-nifedipine-water complex. Encapsulation of nifedipine orients the distribution of water molecules inside SWCNT accompanied by the H-bond formation between water molecules and oxygen atom of nifedipine. During the encapsulation process, SWNT undergoes weak fluctuations due to the oscillatory behavior of nifedipine encapsulated within the CNT.

with topoisomerase I (top 1) CPT interacts through π stacking with AT and GC base pairs of DNA. The optimum interacting distance of CPT from AT and GC bases was calculated at 3.87 and 3.38 Å, from the central aromatic rings (**Figure 7b**). The re-rank score of bare CPT drug

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Likewise, for the docking of CPT/8 × 8 graphene with Top1 (**Figure 8a**) CPT gets docked between the AT and GC base pairs. However, graphene gets docked along the phosphate backbone of the ds-DNA helix as shown from **Figure 8b** indicating a strong interaction between the polar phosphate groups of the DNA helix. Compared to the docking of bare CPT drug, presence of graphene stabilizes the intercalation of CPT between the AT and GC base

The docking of CPT/GO with Top1 as illustrated in **Figure 9a**, depicts CPT to get docked between AT and GC base pairs of DNA, mediated by π-π stacking interaction similar to that observed for bare CPT and CPT/8 × 8 graphene. However, in the presence of GO, GO undergoes strong interactions with DNA bases and gets docked between the DNA helix and the interaction is stabilized by intermolecular H-bond between polar functional units on the basal plane of GO and DNA nucleobases (**Figure 9b**). The molecular docking studies on bare CPT and CPT functionalized graphene and GO systems showed that the interaction of CPT with Top1 is mediated by π-stacking interaction between the aromatic rings of CPT and the A and C bases of DNA. In presence of graphene and GO, CPT undergoes a similar trend in adsorption while the graphene and GO nanomaterial gets docked along the phosphate backbone

**Figure 7.** (a) Secondary structure of Top1 protein with the CPT drug docked within the DNA, (b) interacting distance between CPT and the DNA base pairs of top 1. Reprinted with permission from Ref. [99]. Copyright 2017, Elsevier.

was calculated as −89.01 a.u. with an H-bond score of −2.53 a.u. as shown in **Table 1**.

pairs, as observed from the increase in re-rank score values.

indicating a strong preferential interaction with DNA.

#### **4. SWCNTs in cancer therapy**

Platinum-based Phase II and Phase III anticancer drugs hold promise in the treatment of cancer with new drugs being discovered, some of which are still under clinical trials. The two main limitations in use of Pt-based anticancer drugs are [1] the anticancer drugs undergo poor circulation in tissue cells and its activity is reduced with time due to the complex formation with plasma and tissue cells, and [2] tumor cells demonstrate resistance toward Pt-based drugs under prolonged exposure, rendering them ineffective as potent anti-tumor agents. Lippard and co-workers [16, 17] incorporated capped *f*CNT as longboat delivery vehicles for cisplatin anticancer drug through clathrin-dependent endocytosis and measured the changes in redox potential before and after release of the drug. The substituted c,c,t [Pt(NH3 ) 2 Cl2 (OEt) (O2 CCH2 CH2 COOH)] pro-drug was attached to SWCNT functionalized with phospholipid tethered amine with PEG to solubilize the nanotube. Burger et al. [97] investigated the encapsulation of cisplatin in a phospholipid formulation. The lipid-coated cisplatin nanocapsules exhibit drug-lipid ratio and *in vitro* cytotoxicity 1000 times higher than free cisplatin. This method thus formed an effective approach in drug delivery and the means of producing lipidbased nanocapsules for encapsulating different bio- and therapeutic molecules. Hilter and Hill [98] suggested three preferred orientations of cisplatin toward the entry into CNT and probable interactions using mechanical principles and mathematical modeling. The atomic interaction between nanotubes and cisplatin was calculated using hybrid-discrete-continuum approximation. In this approximation, cisplatin was taken as a collection of discrete atoms and the CNT was treated as a continuum body of repeating carbon atoms. Non-bonded interaction, suction, and acceptance energies were calculated using the Lennard-Jones (LJ) potential. For nanotube radius of 5.3 Å, cisplatin exhibited maximum suction energy, depending on the orientation of nanotubes as a function of radii.

We performed density functional studies on the noncovalent functionalization of non-Pt-based anticancer drug camptothecin (CPT) on graphene-based nanomaterials and its prototypes, including graphene oxide (GO) [99]. The noncovalent adsorption of CPT induces a significant strain within the nanosheets and the interaction was thermodynamically favored from energetics perspective. In case of GO, surface incorporation of functional groups resulted in significant crumpling along the basal plane and the interaction was mediated by H-bonding rather than π-π stacking. The molecular docking studies of CPT onto Top1 (**Figure 7a**) showed CPT to be stacked between the Watson Crick AT and GC base pairs and the interaction was mediated via π-π stacking (**Figure 7b**). For the binding of CPT functionalized graphene and GO with topoisomerase I (top 1) CPT interacts through π stacking with AT and GC base pairs of DNA. The optimum interacting distance of CPT from AT and GC bases was calculated at 3.87 and 3.38 Å, from the central aromatic rings (**Figure 7b**). The re-rank score of bare CPT drug was calculated as −89.01 a.u. with an H-bond score of −2.53 a.u. as shown in **Table 1**.

adsorption of nifedipine was more stable than external adsorption by 5.3 to 7.8 kcal/mol. In solvent phase, the encapsulation of nifedipine was impeded due to competitive vdW and hydrophobic interactions in SWCNT-nifedipine-water complex. Encapsulation of nifedipine orients the distribution of water molecules inside SWCNT accompanied by the H-bond formation between water molecules and oxygen atom of nifedipine. During the encapsulation process, SWNT undergoes weak fluctuations due to the oscillatory behavior of nifedipine

Platinum-based Phase II and Phase III anticancer drugs hold promise in the treatment of cancer with new drugs being discovered, some of which are still under clinical trials. The two main limitations in use of Pt-based anticancer drugs are [1] the anticancer drugs undergo poor circulation in tissue cells and its activity is reduced with time due to the complex formation with plasma and tissue cells, and [2] tumor cells demonstrate resistance toward Pt-based drugs under prolonged exposure, rendering them ineffective as potent anti-tumor agents. Lippard and co-workers [16, 17] incorporated capped *f*CNT as longboat delivery vehicles for cisplatin anticancer drug through clathrin-dependent endocytosis and measured the changes

in redox potential before and after release of the drug. The substituted c,c,t [Pt(NH3

tethered amine with PEG to solubilize the nanotube. Burger et al. [97] investigated the encapsulation of cisplatin in a phospholipid formulation. The lipid-coated cisplatin nanocapsules exhibit drug-lipid ratio and *in vitro* cytotoxicity 1000 times higher than free cisplatin. This method thus formed an effective approach in drug delivery and the means of producing lipidbased nanocapsules for encapsulating different bio- and therapeutic molecules. Hilter and Hill [98] suggested three preferred orientations of cisplatin toward the entry into CNT and probable interactions using mechanical principles and mathematical modeling. The atomic interaction between nanotubes and cisplatin was calculated using hybrid-discrete-continuum approximation. In this approximation, cisplatin was taken as a collection of discrete atoms and the CNT was treated as a continuum body of repeating carbon atoms. Non-bonded interaction, suction, and acceptance energies were calculated using the Lennard-Jones (LJ) potential. For nanotube radius of 5.3 Å, cisplatin exhibited maximum suction energy, depending on

We performed density functional studies on the noncovalent functionalization of non-Pt-based anticancer drug camptothecin (CPT) on graphene-based nanomaterials and its prototypes, including graphene oxide (GO) [99]. The noncovalent adsorption of CPT induces a significant strain within the nanosheets and the interaction was thermodynamically favored from energetics perspective. In case of GO, surface incorporation of functional groups resulted in significant crumpling along the basal plane and the interaction was mediated by H-bonding rather than π-π stacking. The molecular docking studies of CPT onto Top1 (**Figure 7a**) showed CPT to be stacked between the Watson Crick AT and GC base pairs and the interaction was mediated via π-π stacking (**Figure 7b**). For the binding of CPT functionalized graphene and GO

COOH)] pro-drug was attached to SWCNT functionalized with phospholipid

) 2 Cl2 (OEt)

encapsulated within the CNT.

244 Novel Nanomaterials - Synthesis and Applications

(O2 CCH2

CH2

**4. SWCNTs in cancer therapy**

the orientation of nanotubes as a function of radii.

Likewise, for the docking of CPT/8 × 8 graphene with Top1 (**Figure 8a**) CPT gets docked between the AT and GC base pairs. However, graphene gets docked along the phosphate backbone of the ds-DNA helix as shown from **Figure 8b** indicating a strong interaction between the polar phosphate groups of the DNA helix. Compared to the docking of bare CPT drug, presence of graphene stabilizes the intercalation of CPT between the AT and GC base pairs, as observed from the increase in re-rank score values.

The docking of CPT/GO with Top1 as illustrated in **Figure 9a**, depicts CPT to get docked between AT and GC base pairs of DNA, mediated by π-π stacking interaction similar to that observed for bare CPT and CPT/8 × 8 graphene. However, in the presence of GO, GO undergoes strong interactions with DNA bases and gets docked between the DNA helix and the interaction is stabilized by intermolecular H-bond between polar functional units on the basal plane of GO and DNA nucleobases (**Figure 9b**). The molecular docking studies on bare CPT and CPT functionalized graphene and GO systems showed that the interaction of CPT with Top1 is mediated by π-stacking interaction between the aromatic rings of CPT and the A and C bases of DNA. In presence of graphene and GO, CPT undergoes a similar trend in adsorption while the graphene and GO nanomaterial gets docked along the phosphate backbone indicating a strong preferential interaction with DNA.

**Figure 7.** (a) Secondary structure of Top1 protein with the CPT drug docked within the DNA, (b) interacting distance between CPT and the DNA base pairs of top 1. Reprinted with permission from Ref. [99]. Copyright 2017, Elsevier.


**Table 1.** The re-rank scores and H-bond scores for the best docked conformations of CPT and CPT/8 × 8 graphene, and CPT/8 × 8 GO sheets, respectively.

Boucetta et al. [20] investigated the supramolecular MWCNT-DOX-copolymer complex for anticancer activities. Since DOX, a clinically acclaimed anticancer drug belonging to the family of anthracyclines exhibit fluorescence properties, its uptake and interaction with nanotubes upon administration can be monitored using fluorescent spectrophotometry. The copolymer coated MWCNT formed supramolecular complexes with DOX via *π-*stacking and revealed enhanced cytotoxicity leading to highly efficient cell killing efficiency. Likewise, Liu et al. reported the use of DOX loaded PEG functionalized CNT for targeted delivery of anticancer drugs in tumor

cells [22]. The SWCNT was pre-functionalized with PEG and DOX and a fluorescence probe (fluorescein) was loaded onto the nanotube via *π* stacking. The loading and subsequent release of DOX were found to be pH dependent; decrease in pH from 9 to 5 showed a decrease in DOX loading. Under acidic conditions (pH 5.5), DOX exhibited increased hydrophilicity and solubility with lysosomes and endosomes, facilitating the release of drug molecules from the nanotube. On decreasing the pH, surface loading of DOX onto nanotube surface lowered and at lower acidic pH, amine group of DOX underwent protonation resulting in increased solubility

**Figure 9.** (a) Secondary structure of Top1 protein with the CPT/8 × 8 GO sheet docked within the DNA, (b) the binding of CPT/8 × 8 GO sheet with DNA base pairs of top 1. Reprinted with permission from Ref. [99]. Copyright 2017, Elsevier.

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Carbon nanotubes have proffered as one of the novel functional materials of the 21st century, broadening the theoretical and experimental perspectives in research to explore its novel and intriguing properties. Due to the conjugated *π*-electron backbone and curvature (properties very similar to fullerene and graphene) they are highly reactive and depending on the size, length and (*n*, *m*) indices, the electronic properties can be tuned to fit the desired functionality. Since the synthesis of CNT yields a mixture of both metallic and semiconducting tubes of varying diameter and chirality, separation and purification of nanotubes pose a major problem which restricts the applicability. Secondly, nanotubes are highly

of DOX molecules.

**5. Summary**

**Figure 8.** (a) Secondary structure of Top1 with the CPT/8 × 8 graphene sheet docked within the DNA, (b) binding of CPT/8 × 8 graphene sheet with DNA base pairs of top 1. Reprinted with permission from Ref. [99]. Copyright 2017, Elsevier.

Functionalized Carbon Nanomaterials in Drug Delivery: Emergent Perspectives from Application http://dx.doi.org/10.5772/intechopen.71889 247

**Figure 9.** (a) Secondary structure of Top1 protein with the CPT/8 × 8 GO sheet docked within the DNA, (b) the binding of CPT/8 × 8 GO sheet with DNA base pairs of top 1. Reprinted with permission from Ref. [99]. Copyright 2017, Elsevier.

cells [22]. The SWCNT was pre-functionalized with PEG and DOX and a fluorescence probe (fluorescein) was loaded onto the nanotube via *π* stacking. The loading and subsequent release of DOX were found to be pH dependent; decrease in pH from 9 to 5 showed a decrease in DOX loading. Under acidic conditions (pH 5.5), DOX exhibited increased hydrophilicity and solubility with lysosomes and endosomes, facilitating the release of drug molecules from the nanotube. On decreasing the pH, surface loading of DOX onto nanotube surface lowered and at lower acidic pH, amine group of DOX underwent protonation resulting in increased solubility of DOX molecules.

#### **5. Summary**

Boucetta et al. [20] investigated the supramolecular MWCNT-DOX-copolymer complex for anticancer activities. Since DOX, a clinically acclaimed anticancer drug belonging to the family of anthracyclines exhibit fluorescence properties, its uptake and interaction with nanotubes upon administration can be monitored using fluorescent spectrophotometry. The copolymer coated MWCNT formed supramolecular complexes with DOX via *π-*stacking and revealed enhanced cytotoxicity leading to highly efficient cell killing efficiency. Likewise, Liu et al. reported the use of DOX loaded PEG functionalized CNT for targeted delivery of anticancer drugs in tumor

**Figure 8.** (a) Secondary structure of Top1 with the CPT/8 × 8 graphene sheet docked within the DNA, (b) binding of CPT/8 × 8 graphene sheet with DNA base pairs of top 1. Reprinted with permission from Ref. [99]. Copyright 2017,

**Table 1.** The re-rank scores and H-bond scores for the best docked conformations of CPT and CPT/8 × 8 graphene, and

**Re-rank score nanosheet**

−89.10 95.87 −2.57 0.00

**H-bond score CPT**

**H-bond score nanosheet**

**System Re-rank score** 

246 Novel Nanomaterials - Synthesis and Applications

8 × 8 graphene/CPT docked onto

CPT/8 × 8 GO sheets, respectively.

Top1

Elsevier.

**CPT**

CPT\_Top1 −89.01 — −2.53 —

8 × 8 GO/CPT docked onto Top1 −90.21 126.21 −2.29 −4.44

Carbon nanotubes have proffered as one of the novel functional materials of the 21st century, broadening the theoretical and experimental perspectives in research to explore its novel and intriguing properties. Due to the conjugated *π*-electron backbone and curvature (properties very similar to fullerene and graphene) they are highly reactive and depending on the size, length and (*n*, *m*) indices, the electronic properties can be tuned to fit the desired functionality. Since the synthesis of CNT yields a mixture of both metallic and semiconducting tubes of varying diameter and chirality, separation and purification of nanotubes pose a major problem which restricts the applicability. Secondly, nanotubes are highly hydrophobic and non-dispersible in most of the common aqueous and organic solvents and tend to aggregate in bundles. To improve nanotube dispersibility, surface modification via functionalization is thus a sought-after approach and covalent and noncovalent functionalization methods can reduce the bundling effect and hydrophobicity. Covalent functionalization although renders high stability to the nanotubes, it tends to distort the structural and inherent electronic properties. Noncovalent functionalization, on the other hand, retains the intrinsic properties of the nanotube, as it forms a surface coating on the nanotube sidewall, and facilitates the uptake of drugs, biomolecules, peptides, proteins, DNA, RNA, and genes within the biological systems.

[4] Dresselhaus MS, Dresselhaus G, Jorio A. Unusual properties and structures of carbon

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Although CNTs demonstrate practical applicability in all facets of science, be it biology, physics, medicine, nanotechnology, catalysis, or materials science, its long-term implications need to be assessed from the perspective of human health to environmental risks. The long-term fate of CNTs released into the environment depends on the structural, morphological and synthetic treatments [100]. Methods of reducing toxicity *in vivo* and *in vitro* can be envisaged via functionalization of CNT. Proper assessment and in-depth study are essential to render nanotubes useful for diverse and environmentally benign applications.

We investigated the potential application of SWCNTs, graphene-based nanomaterials and its prototypes in TB and cancer chemotherapy using conventional DFT methods supported by molecular docking and MD simulation on the nature of interaction of therapeutic drug functionalized SWCNTs/graphene with the binding site of the protein. The functionalization of SWCNTs with therapeutic drugs using covalent and noncovalent schemes were adopted to investigate the drug binding with the nanotube and the stability of the conjugated complexes. DFT results supported by molecular docking and MD simulation helps in contemplating the feasibility of SWCNT-based novel drug delivery in cancer and TB therapy.

#### **Author details**

Nabanita Saikia

Address all correspondence to: nabanita16@gmail.com

Department of Physics, Michigan Technological University, Houghton, Michigan, United States

#### **References**


[4] Dresselhaus MS, Dresselhaus G, Jorio A. Unusual properties and structures of carbon nanotubes. Annual Review of Materials Research. 2004;**34**:247

hydrophobic and non-dispersible in most of the common aqueous and organic solvents and tend to aggregate in bundles. To improve nanotube dispersibility, surface modification via functionalization is thus a sought-after approach and covalent and noncovalent functionalization methods can reduce the bundling effect and hydrophobicity. Covalent functionalization although renders high stability to the nanotubes, it tends to distort the structural and inherent electronic properties. Noncovalent functionalization, on the other hand, retains the intrinsic properties of the nanotube, as it forms a surface coating on the nanotube sidewall, and facilitates the uptake of drugs, biomolecules, peptides, proteins, DNA, RNA, and genes

Although CNTs demonstrate practical applicability in all facets of science, be it biology, physics, medicine, nanotechnology, catalysis, or materials science, its long-term implications need to be assessed from the perspective of human health to environmental risks. The long-term fate of CNTs released into the environment depends on the structural, morphological and synthetic treatments [100]. Methods of reducing toxicity *in vivo* and *in vitro* can be envisaged via functionalization of CNT. Proper assessment and in-depth study are essential to render

We investigated the potential application of SWCNTs, graphene-based nanomaterials and its prototypes in TB and cancer chemotherapy using conventional DFT methods supported by molecular docking and MD simulation on the nature of interaction of therapeutic drug functionalized SWCNTs/graphene with the binding site of the protein. The functionalization of SWCNTs with therapeutic drugs using covalent and noncovalent schemes were adopted to investigate the drug binding with the nanotube and the stability of the conjugated complexes. DFT results supported by molecular docking and MD simulation helps in contemplating the feasibility of SWCNT-based novel drug delivery in cancer and TB

nanotubes useful for diverse and environmentally benign applications.

Address all correspondence to: nabanita16@gmail.com

Department of Physics, Michigan Technological University, Houghton, Michigan,

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**Author details**

Nabanita Saikia

United States

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**Chapter 14**

**Provisional chapter**

**Ultralight Paper-Based Electrodes for Energy**

**Ultralight Paper-Based Electrodes for Energy** 

DOI: 10.5772/intechopen.74098

In this chapter, we briefly introduce our recent work related to the topic of ultralight paper-based electrodes for energy applications. Herein, the ultralight paper-based counter electrodes containing commercial poly(3,4-ethylenedioxythiophene):polystyrenesul fonate (PEDOT:PSS) and homemade graphene dots (GDs) are synthesized for preparing flexible dye-sensitized solar cells (DSSCs). Because the GDs/PEDOT:PSS composite can well fill the porosity of paper substrate, the flexible DSSC with GDs/PEDOT:PSScoated paper electrode exhibits much higher cell efficiency than that of DSSC using paper electrode with Pt. The features of lightweight, low-cost, space-saving (high flexibility), high machinability (easy-cutting) and environmental friendly would make the GDs/ PEDOT:PSS-coated paper electrodes highly potential in portable/wearable electronic

**Keywords:** conducting polymer, flexible electronic, graphene dot, paper electrode

Flexible and lightweight electronics have attracted much attention because of their high potential to be integrated into wearable commercial products [1]. Among all the flexible substrates, the printed paper substrate, composed of cellulose fibers, has been considered as the most promising one due to the following features: environmental friendly, low-cost, lightweight and easy for roll-to-roll processing [2]. Currently, the printed paper has been widely used as a substrate for transistors [3], displays [4], sensors [5], memories [6], batteries [7] and supercapacitors [8], etc. Furthermore, Professor Wang's group reported a paper-based triboelectric nanogenerator having origami configurations for harvesting mechanical energy [9].

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.74098

**Applications**

**Applications**

Chuan-Pei Lee

Chuan-Pei Lee

**Abstract**

applications.

**1. Introduction**

#### **Ultralight Paper-Based Electrodes for Energy Applications Ultralight Paper-Based Electrodes for Energy Applications**

DOI: 10.5772/intechopen.74098

#### Chuan-Pei Lee Chuan-Pei Lee

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.74098

#### **Abstract**

In this chapter, we briefly introduce our recent work related to the topic of ultralight paper-based electrodes for energy applications. Herein, the ultralight paper-based counter electrodes containing commercial poly(3,4-ethylenedioxythiophene):polystyrenesul fonate (PEDOT:PSS) and homemade graphene dots (GDs) are synthesized for preparing flexible dye-sensitized solar cells (DSSCs). Because the GDs/PEDOT:PSS composite can well fill the porosity of paper substrate, the flexible DSSC with GDs/PEDOT:PSScoated paper electrode exhibits much higher cell efficiency than that of DSSC using paper electrode with Pt. The features of lightweight, low-cost, space-saving (high flexibility), high machinability (easy-cutting) and environmental friendly would make the GDs/ PEDOT:PSS-coated paper electrodes highly potential in portable/wearable electronic applications.

**Keywords:** conducting polymer, flexible electronic, graphene dot, paper electrode

#### **1. Introduction**

Flexible and lightweight electronics have attracted much attention because of their high potential to be integrated into wearable commercial products [1]. Among all the flexible substrates, the printed paper substrate, composed of cellulose fibers, has been considered as the most promising one due to the following features: environmental friendly, low-cost, lightweight and easy for roll-to-roll processing [2]. Currently, the printed paper has been widely used as a substrate for transistors [3], displays [4], sensors [5], memories [6], batteries [7] and supercapacitors [8], etc. Furthermore, Professor Wang's group reported a paper-based triboelectric nanogenerator having origami configurations for harvesting mechanical energy [9].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*e<sup>−</sup>*

*I3*

*1.5 I<sup>−</sup> + TiO2 − dye+* → *0.5 I3*

(dye\*) into the conduction band of the TiO<sup>2</sup>

is reduced to I−

the nonconductive counter anion, PSS−

Under light illumination, a photo-excited electron is injected from the excited state of the dye

film by a driving chemical diffusion gradient, and is collected at a conductive glass substrate. After passing through an external circuit, the electron is reintroduced into the DSSC at the Pt

However, the most used electrocatalyst on the CE of DSSCs is Pt, which is an expensive noble metal and is rare on earth. To further reduce the cost of fabrication of DSSCs on industrial scale, it is better to develop metal-free electrocatalytic materials for the CEs of DSSCs. Accordingly, conducting polymers (e.g., poly(3,4-ethylene dioxythiophene) (PEDOT) [17],

role (PPy) [19], and polyaniline (PANI) [20]) and carbonaceous materials (such as carbon black (CB) [21], graphite [22], carbon nanotube (CNT) [23] and graphene [24]) have become the most promising electrocatalysts for the CE of DSSCs since they are metal element-free, have low material cost and possess good electrocatalytic activity. A water-dispersible conducting polymer, poly(3,4-ethylene dioxythiophene):poly(4-styrene sulfonate) (PEDOT:PSS), has attracted much attention as the catalytic CEs of DSSCs mainly due to its exceptional advantage of aqueous solution processibility [25]. Nevertheless, pristine PEDOT:PSS films are plagued by low

conductivity (i.e., <1 S cm−1) and poor electrocatalytic activity for the reduction of I

film as well as the poor catalytic surface area of its flat film [26]. Inert solvents [27] or carbon materials [28] have been employed to improve conductivity and catalytic surface areas of PEDOT:PSS films. For example, by introducing CB into the PEDOT:PSS-based CEs for their DSSCs, *η* can achieve to 7.01% [28]. Multiwall CNT-PEDOT:PSS composite CE for DSSCs exhibits *η* of 6.50% [29]. A catalytic film composited of graphene and PEDOT:PSS for the use of CE in a DSSC had reached 4.50% efficiency [30]; however, a perfect graphene sheet usually possesses limited active sites for electrocatalytic reaction in spite of its extraordinarily high electrical conductivity [31]. Several strategies are employed to increase the electrocatalytic active sites on graphene sheets, such as chemical functionalization [32], heteroatom doping (e.g., nitrogen-doped graphene) [33] and nanosized graphene pieces (e.g., graphene dots (GDs) [34]). Among these graphene nanostructures, GDs have attracted great attention and been widely applied in bioimaging [35], LEDs [36] and photovoltaics [37] due to their unique properties of quantum confinement and edge effects [38, 39]. Moreover, nanometer size and rich oxygen-containing group of GDs facilitate them to be well dispersed in most solvents [40],

. Immediately, I−

poly(3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4]-dioxepine) (PProDOT-Et<sup>2</sup>

the circle of a DSSC and to refresh the DSSC without other chemical side reactions.

*Electron reception*

*Interception reaction*

CE, where I3

−

*(TiO2 film)* → *e<sup>−</sup>(TCO)* (3)

Ultralight Paper-Based Electrodes for Energy Applications

http://dx.doi.org/10.5772/intechopen.74098

*<sup>−</sup> + 2 e<sup>−</sup> (Pt CE)* → *3I<sup>−</sup>* (4)

*<sup>−</sup> + TiO2 − dye* (5)

) to complete

259

) [18], polypyr-

3 − due to

. The injected electron percolates through the TiO<sup>2</sup>

, disturbing the conduction path of PEDOT inside the

regenerates the oxidized dye (dye<sup>+</sup>

**Figure 1.** Schematic sketch of a DSSC.

Professor Hu's group developed highly flexible organic light-emitting diodes (OLEDs) on biodegradable and transparent paper substrates [10]. They also demonstrated a range of electronic devices on transparent paper substrate, including organic solar cell [11], touch screen [12] and thin film transistor [13].

For the applications in electrochemical devices, the main challenges of using printed paper as electrode substrates are how to make it to become conductive and electrochemical active via nonsintering processes, as well as to make a conductive layer on it uniformly and continuously because paper-based substrates are nonconductive, porous and rough due to its fibrous nature [14]. Up to now, the printed paper has not been used as the electrode substrates for the third-generation solar cell, namely dye-sensitized solar cell (DSSC). DSSCs have been extensively studied and widely developed in the last two decade since they do not rely on expensive fabrication processes and can be possibly prepared by using roll-to-roll methods [15]. Whereas, flexible DSSCs are handy, convenient for transportation and suitable for setting up at complex environments, which endow them high competitiveness in solar cell markets [16].

As shown in **Figure 1**, a traditional DSSC is consisted of a counter electrode (CE) with Pt catalyst, an I− /I3 − -based electrolyte and a dye-sensitized TiO<sup>2</sup> photoanode. Briefly, the basic sequence of the working principle of a DSSC is shown below:

*Activation*

$$\text{TiO}\_2-\text{dye} \rightarrow \text{TiO}\_2-\text{dye}^\* \tag{1}$$

*Electron injection*

$$\text{TiO}\_2-\text{dye}^\* \rightarrow \text{TiO}\_2-\text{dye}^\* + e^\cdot (\text{TiO}\_2\text{film}) \tag{2}$$

$$e^{\cdot} \text{(TiO}\_{2}\text{film)} \rightarrow e^{\cdot} \text{(TCO)}\tag{3}$$

*Electron reception*

$$\rm I\_3^- + 2\ e^- (Pt\ CE^\circ \rightarrow \ 3I^- \tag{4}$$

*Interception reaction*

Professor Hu's group developed highly flexible organic light-emitting diodes (OLEDs) on biodegradable and transparent paper substrates [10]. They also demonstrated a range of electronic devices on transparent paper substrate, including organic solar cell [11], touch screen

For the applications in electrochemical devices, the main challenges of using printed paper as electrode substrates are how to make it to become conductive and electrochemical active via nonsintering processes, as well as to make a conductive layer on it uniformly and continuously because paper-based substrates are nonconductive, porous and rough due to its fibrous nature [14]. Up to now, the printed paper has not been used as the electrode substrates for the third-generation solar cell, namely dye-sensitized solar cell (DSSC). DSSCs have been extensively studied and widely developed in the last two decade since they do not rely on expensive fabrication processes and can be possibly prepared by using roll-to-roll methods [15]. Whereas, flexible DSSCs are handy, convenient for transportation and suitable for setting up at complex environments, which endow them high competitiveness in solar cell markets [16]. As shown in **Figure 1**, a traditional DSSC is consisted of a counter electrode (CE) with Pt

photoanode. Briefly, the basic

*(TiO2 film)* (2)


*TiO2 − dye* → *TiO2 − dye*<sup>∗</sup> (1)

sequence of the working principle of a DSSC is shown below:

*TiO2 − dye*<sup>∗</sup> → *TiO2 − dye+ + e<sup>−</sup>*

[12] and thin film transistor [13].

**Figure 1.** Schematic sketch of a DSSC.

258 Novel Nanomaterials - Synthesis and Applications

catalyst, an I−

*Electron injection*

*Activation*

/I3 −

$$1.5\ I^{-} + TiO\_{z} - dye^{\*} \rightarrow 0.5\ I\_{3}^{-} + TiO\_{z} - dye \tag{5}$$

Under light illumination, a photo-excited electron is injected from the excited state of the dye (dye\*) into the conduction band of the TiO<sup>2</sup> . The injected electron percolates through the TiO<sup>2</sup> film by a driving chemical diffusion gradient, and is collected at a conductive glass substrate. After passing through an external circuit, the electron is reintroduced into the DSSC at the Pt CE, where I3 − is reduced to I− . Immediately, I− regenerates the oxidized dye (dye<sup>+</sup> ) to complete the circle of a DSSC and to refresh the DSSC without other chemical side reactions.

However, the most used electrocatalyst on the CE of DSSCs is Pt, which is an expensive noble metal and is rare on earth. To further reduce the cost of fabrication of DSSCs on industrial scale, it is better to develop metal-free electrocatalytic materials for the CEs of DSSCs. Accordingly, conducting polymers (e.g., poly(3,4-ethylene dioxythiophene) (PEDOT) [17], poly(3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4]-dioxepine) (PProDOT-Et<sup>2</sup> ) [18], polypyrrole (PPy) [19], and polyaniline (PANI) [20]) and carbonaceous materials (such as carbon black (CB) [21], graphite [22], carbon nanotube (CNT) [23] and graphene [24]) have become the most promising electrocatalysts for the CE of DSSCs since they are metal element-free, have low material cost and possess good electrocatalytic activity. A water-dispersible conducting polymer, poly(3,4-ethylene dioxythiophene):poly(4-styrene sulfonate) (PEDOT:PSS), has attracted much attention as the catalytic CEs of DSSCs mainly due to its exceptional advantage of aqueous solution processibility [25]. Nevertheless, pristine PEDOT:PSS films are plagued by low conductivity (i.e., <1 S cm−1) and poor electrocatalytic activity for the reduction of I 3 − due to the nonconductive counter anion, PSS− , disturbing the conduction path of PEDOT inside the film as well as the poor catalytic surface area of its flat film [26]. Inert solvents [27] or carbon materials [28] have been employed to improve conductivity and catalytic surface areas of PEDOT:PSS films. For example, by introducing CB into the PEDOT:PSS-based CEs for their DSSCs, *η* can achieve to 7.01% [28]. Multiwall CNT-PEDOT:PSS composite CE for DSSCs exhibits *η* of 6.50% [29]. A catalytic film composited of graphene and PEDOT:PSS for the use of CE in a DSSC had reached 4.50% efficiency [30]; however, a perfect graphene sheet usually possesses limited active sites for electrocatalytic reaction in spite of its extraordinarily high electrical conductivity [31]. Several strategies are employed to increase the electrocatalytic active sites on graphene sheets, such as chemical functionalization [32], heteroatom doping (e.g., nitrogen-doped graphene) [33] and nanosized graphene pieces (e.g., graphene dots (GDs) [34]). Among these graphene nanostructures, GDs have attracted great attention and been widely applied in bioimaging [35], LEDs [36] and photovoltaics [37] due to their unique properties of quantum confinement and edge effects [38, 39]. Moreover, nanometer size and rich oxygen-containing group of GDs facilitate them to be well dispersed in most solvents [40], which benefits for various solution-processable applications. Therefore, the incorporation of GDs in PEDOT:PSS enable us to fabricate an efficient electrocatalytic film by using a simple solution-coating method under low temperature (<100°C).

In this chapter, an all-metal-free CE-containing GDs-PEDOT:PSS and printed paper is developed for flexible DSSCs, and it exhibits higher performance and bending stability than those of a paper electrode with sputtered Pt. The concurrent advantage in low material cost, simple fabrication processes, highly bending durability, lightweight, space-saving, high machinability and environmental friendly makes the GDs-PEDOT:PSS-coated paper electrode playing a crucial role in lightweight electronic devices. Most importantly, this GDs-PEDOT:PSS composite ink can be used in the printable processes for mass production of flexible electrodes.

> For the preparation of paper-based CEs, the commercial printing papers were used as the substrate of counter electrodes. The paper with a fixed coating area was immersed in the GDs-PEDOT:PSS solution (i.e., 30 V% GDs solution content) for 10 min, and it was took out and then dried under 60°C. The thus prepared GDs-PEDOT:PSS/paper electrode (left-hand side of **Figure 4**) was used as the CE for the studies on flexible DSSCs. For comparison purposes, the paper-based CE with sputtered Pt (right-hand side of **Figure 4**) was also prepared as the

For the preparation of the flexible photoanode of DSSCs, the dye-sensitized TiO<sup>2</sup>

of 2:1) uniformly. Then, the surface of the ITO-PEN substrate was coated with a TiOx

prepared on the conducting plastic substrate (13 Ω sq.−1, ITO-PEN) according to the previous

(P25) with 6 mL binary solution consisting of tert-butanol and deionized water (volume ratio

layer by spraying an ethanol solution (10 mL) containing titanium tetraisopropoxide (0.028 g)

PEN substrate through doctor blade technique [44]. Thereafter, an active area of 0.4 × 0.4 cm<sup>2</sup>

**Figure 4.** Pictures of CEs with 30 V% GDs-PEDOT:PSS composite (left) and with sputtered Pt (right) on paper substrates.

paste was synthesized by mixing 1 g TiO<sup>2</sup>

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261

paste, a 10-μm-thick film was coated on the treated ITO-

film was

powder

compact

standard CE.

**2.3. Fabrication of flexible dye-sensitized solar cells**

**Figure 3.** The GDs solutions under ambient light (left) and UV light (right) [41].

reports [42, 43]. First, the binder-free TiO<sup>2</sup>

on it. Using the binder-free TiO<sup>2</sup>

### **2. Experimental**

#### **2.1. Synthesis of graphene dots**

The GDs solution was prepared by using deionized water and glucose as the solvent and source, respectively. First, the as-prepared glucose solution (2.5 mL) was transferred to a glass bottle with 4 mL volume and a tightened cover. The synthesis reaction was carried out in a microwave oven (595 W) for 9 min; the glucose molecules are pyrolyzed and then converted to GDs as shown in **Figure 2**. Subsequently, the reaction bottle was cooled to ambient temperature, and the water-soluble GDs solution was thus prepared. **Figure 3** is the photographs of GDs solution taken under the illumination of visible light and UV light, showing the excitation wavelength-dependent fluorescence property of GDs.

#### **2.2. Preparation of paper-based counter electrodes**

The mixing solutions composed of 50 V% PEDOT:PSS aqueous solution and 50 V% binary solution consisting of GDs (X) and ethanol (Y) (X/Y = 3/2) were used for preparing the GDs-PEDOT:PSS composite inks.

**Figure 2.** Preparation of graphene dots via the microwave-assisted hydrothermal technique.

**Figure 3.** The GDs solutions under ambient light (left) and UV light (right) [41].

which benefits for various solution-processable applications. Therefore, the incorporation of GDs in PEDOT:PSS enable us to fabricate an efficient electrocatalytic film by using a simple

In this chapter, an all-metal-free CE-containing GDs-PEDOT:PSS and printed paper is developed for flexible DSSCs, and it exhibits higher performance and bending stability than those of a paper electrode with sputtered Pt. The concurrent advantage in low material cost, simple fabrication processes, highly bending durability, lightweight, space-saving, high machinability and environmental friendly makes the GDs-PEDOT:PSS-coated paper electrode playing a crucial role in lightweight electronic devices. Most importantly, this GDs-PEDOT:PSS composite ink can be used in the printable processes for mass production of flexible electrodes.

The GDs solution was prepared by using deionized water and glucose as the solvent and source, respectively. First, the as-prepared glucose solution (2.5 mL) was transferred to a glass bottle with 4 mL volume and a tightened cover. The synthesis reaction was carried out in a microwave oven (595 W) for 9 min; the glucose molecules are pyrolyzed and then converted to GDs as shown in **Figure 2**. Subsequently, the reaction bottle was cooled to ambient temperature, and the water-soluble GDs solution was thus prepared. **Figure 3** is the photographs of GDs solution taken under the illumination of visible light and UV light, showing the excita-

The mixing solutions composed of 50 V% PEDOT:PSS aqueous solution and 50 V% binary solution consisting of GDs (X) and ethanol (Y) (X/Y = 3/2) were used for preparing the GDs-

solution-coating method under low temperature (<100°C).

tion wavelength-dependent fluorescence property of GDs.

**Figure 2.** Preparation of graphene dots via the microwave-assisted hydrothermal technique.

**2.2. Preparation of paper-based counter electrodes**

**2. Experimental**

**2.1. Synthesis of graphene dots**

260 Novel Nanomaterials - Synthesis and Applications

PEDOT:PSS composite inks.

For the preparation of paper-based CEs, the commercial printing papers were used as the substrate of counter electrodes. The paper with a fixed coating area was immersed in the GDs-PEDOT:PSS solution (i.e., 30 V% GDs solution content) for 10 min, and it was took out and then dried under 60°C. The thus prepared GDs-PEDOT:PSS/paper electrode (left-hand side of **Figure 4**) was used as the CE for the studies on flexible DSSCs. For comparison purposes, the paper-based CE with sputtered Pt (right-hand side of **Figure 4**) was also prepared as the standard CE.

#### **2.3. Fabrication of flexible dye-sensitized solar cells**

For the preparation of the flexible photoanode of DSSCs, the dye-sensitized TiO<sup>2</sup> film was prepared on the conducting plastic substrate (13 Ω sq.−1, ITO-PEN) according to the previous reports [42, 43]. First, the binder-free TiO<sup>2</sup> paste was synthesized by mixing 1 g TiO<sup>2</sup> powder (P25) with 6 mL binary solution consisting of tert-butanol and deionized water (volume ratio of 2:1) uniformly. Then, the surface of the ITO-PEN substrate was coated with a TiOx compact layer by spraying an ethanol solution (10 mL) containing titanium tetraisopropoxide (0.028 g) on it. Using the binder-free TiO<sup>2</sup> paste, a 10-μm-thick film was coated on the treated ITO-PEN substrate through doctor blade technique [44]. Thereafter, an active area of 0.4 × 0.4 cm<sup>2</sup>

**Figure 4.** Pictures of CEs with 30 V% GDs-PEDOT:PSS composite (left) and with sputtered Pt (right) on paper substrates.

**Figure 5.** A flexible photoanode with dye-sensitized TiO<sup>2</sup> film on ITO-PEN substrate.

was selected from the TiO<sup>2</sup> films by scrapping. The as-prepared TiO<sup>2</sup> /ITO-PEN electrodes were gradually heated to 120°C under ambient conditions, and subsequently annealed at the respective temperatures for 60 min. After annealing and cooling to 80°C, the TiO<sup>2</sup> /ITO-PEN electrode was immediately immersed in a 5 × 10−4 M N719 dye solution for 60 min under 55°C. The thus prepared dye-sensitized TiO<sup>2</sup> /ITO-PEN photoanode (**Figure 5**) was coupled with paper-based CE (i.e., GDs-PEDOT:PSS or sputtered Pt) as the flexible DSSC.

#### **3. Results and discussion**

GDs are edge-bound nanosized graphene pieces and exhibit unique electronic and optical properties due to the quantum confinement and edge effects [38, 39]. **Figure 6(a)** shows the transmission electron microscopy (TEM) image of the monodispersed GDs, which exhibit uniform diameters of ~3.50 nm. As shown in the inset of **Figure 6(a)**, the high-resolution transmission electron microscopy (HRTEM) image indicates high crystallinity of GDs with a lattice spacing of 0.246 nm corresponding to the interplanar separation of graphene (1120). **Figure 6(b)** presents the atomic force microscope (AFM) image of the monodispersed GDs; the inset of **Figure 6(b)** reveals that the average height of GDs is around 2.90 nm. As shown in **Figure 6(c)**, two absorption peaks centered at 228 and 282 nm are observed in the ultraviolet-visible (UV-visible) spectrum of the diluted GD solution, which consists with the result in the previous literature [36]. **Figure 6(d)** shows the photoluminescence (PL) spectrum of the GD solution. The broad emission peaks centered at around 450, 460 and 537 nm are observed when the sample is excited by 300, 400 and 500 nm, respectively, and they show the decrease of PL intensity. The excitation wavelength-dependent intensity and emission wavelength observed here is a common phenomenon for carbonaceous quantum dots [36, 45]. The elemental distribution of the GDs is analyzed by energy-dispersive X-ray spectroscopy (EDS), and the elemental mappings of C and O are shown in **Figure 7**. Obviously, the C content is much higher than O content; the atomic ratio of C/O is 95.32/4.68, which demonstrated C is the dominant element in the GDs.

A mixing solution composed of 50 V% PEDOT:PSS solution, 30 V% GDs solution and 20 V% ethanol were used as the GDs-PEDOT:PSS composite ink. To explore the advantage of the GDs-PEDOT:PSS composite ink for the application in DSSCs, 30 V% GDs-PEDOT:PSS and Pt were separately coated onto the printed papers as the CEs of flexible DSSCs. **Figure 8(a)** and **(b)** shows the top-view SEM images of paper electrodes with sputtered Pt and with 30 V% GDs-PEDOT:PSS, respectively. The SEM images obviously show that the porosity of paper substrate

**Figure 7.** SEM image (a) and EDS spectrum (b) of a GD film. (c) Elemental C mapping of the image shown in (a). The

elemental mappings of C (c) and O (d) of the GD film [46].

**Figure 6.** (a) TEM image with the corresponding HRTEM image (inset) of GDs; (b) AFM image of the GDs and its

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263

corresponding height profile; (c) absorbance spectrum of the GDs; (d) PL spectra of GDs [41].

**Figure 6.** (a) TEM image with the corresponding HRTEM image (inset) of GDs; (b) AFM image of the GDs and its corresponding height profile; (c) absorbance spectrum of the GDs; (d) PL spectra of GDs [41].

was selected from the TiO<sup>2</sup>

262 Novel Nanomaterials - Synthesis and Applications

**3. Results and discussion**

55°C. The thus prepared dye-sensitized TiO<sup>2</sup>

**Figure 5.** A flexible photoanode with dye-sensitized TiO<sup>2</sup>

films by scrapping. The as-prepared TiO<sup>2</sup>

film on ITO-PEN substrate.

were gradually heated to 120°C under ambient conditions, and subsequently annealed at the

electrode was immediately immersed in a 5 × 10−4 M N719 dye solution for 60 min under

GDs are edge-bound nanosized graphene pieces and exhibit unique electronic and optical properties due to the quantum confinement and edge effects [38, 39]. **Figure 6(a)** shows the transmission electron microscopy (TEM) image of the monodispersed GDs, which exhibit uniform diameters of ~3.50 nm. As shown in the inset of **Figure 6(a)**, the high-resolution transmission electron microscopy (HRTEM) image indicates high crystallinity of GDs with a lattice spacing of 0.246 nm corresponding to the interplanar separation of graphene (1120). **Figure 6(b)** presents the atomic force microscope (AFM) image of the monodispersed GDs; the inset of **Figure 6(b)** reveals that the average height of GDs is around 2.90 nm. As shown in **Figure 6(c)**, two absorption peaks centered at 228 and 282 nm are observed in the ultraviolet-visible (UV-visible) spectrum of the diluted GD solution, which consists with the result in the previous literature [36]. **Figure 6(d)** shows the photoluminescence (PL) spectrum of the GD solution. The broad emission peaks centered at around 450, 460 and 537 nm are observed when the sample is excited by 300, 400 and 500 nm, respectively, and they show the decrease of PL intensity. The excitation wavelength-dependent intensity and emission wavelength observed here is a common phenomenon for carbonaceous quantum dots [36, 45]. The elemental distribution of the GDs is analyzed by energy-dispersive X-ray spectroscopy (EDS), and the elemental mappings of C and O are shown in **Figure 7**. Obviously, the C content is much higher than O content; the atomic ratio of C/O is

respective temperatures for 60 min. After annealing and cooling to 80°C, the TiO<sup>2</sup>

with paper-based CE (i.e., GDs-PEDOT:PSS or sputtered Pt) as the flexible DSSC.

95.32/4.68, which demonstrated C is the dominant element in the GDs.

/ITO-PEN electrodes

/ITO-PEN photoanode (**Figure 5**) was coupled

/ITO-PEN

**Figure 7.** SEM image (a) and EDS spectrum (b) of a GD film. (c) Elemental C mapping of the image shown in (a). The elemental mappings of C (c) and O (d) of the GD film [46].

A mixing solution composed of 50 V% PEDOT:PSS solution, 30 V% GDs solution and 20 V% ethanol were used as the GDs-PEDOT:PSS composite ink. To explore the advantage of the GDs-PEDOT:PSS composite ink for the application in DSSCs, 30 V% GDs-PEDOT:PSS and Pt were separately coated onto the printed papers as the CEs of flexible DSSCs. **Figure 8(a)** and **(b)** shows the top-view SEM images of paper electrodes with sputtered Pt and with 30 V% GDs-PEDOT:PSS, respectively. The SEM images obviously show that the porosity of paper substrate

approach for reducing the cost of the DSSCs. Moreover, the bending test is carried out to study the durability of the paper CEs. Both GDs/PEDOT:PSS-coated paper electrode and Pt-coated paper electrode are bended for several times (0, 50, 100 and 150 times) and then assembled with the flexible photoanodes to measure and record their corresponding photovoltaic performances. **Figure 9(a)** shows the bending time dependence of cell efficiency for the flexible DSSCs with various paper CEs; their corresponding photovoltaic parameters are shown in **Figure 9(b)**–**(d)**. It obviously shows that the GDs/PEDOT:PSS-coated paper CE shows unfailing performance even though it was bended for 150 times; on the contrary, the Pt-coated paper CE lost its original per-

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265

In summary, a GDs-PEDOT:PSS composite ink was synthesized for preparing the all-metalfree paper-based CEs for flexible DSSCs. The GDs-PEDOT:PSS/paper CE was fabricated through a low-cost and simple coating method (i.e., soak and dry), which could be easily scaled up to mass production. An all-flexible DSSC with GDs/PEDOT:PSS-coated paper CE exhibits much higher cell efficiency (4.91%) than that of cell using paper CE with sputtered Pt (1.70%), since the porosity of paper substrate can be well filled by GDs-PEDOT:PSS, which cannot be achieved by sputtered Pt. After bending for 150 cycles, the performance of GDs/ PEDOT:PSS-coated paper CE is still perfectly preserved; on the contrary, the paper CE with sputtered Pt lost its initial performance drastically. In conclusion, this GDs/PEDOT:PSScoated paper CE is lightweight, low-cost, space-saving (high flexibility), high machinability (easy-cutting) and environmental friendly, which shows the potential for the future applica-

This work was supported by the Ministry of Science and Technology (MOST) of Taiwan.

formance drastically.

**4. Conclusion**

tions on portable/wearable electronics.

The authors declare no competing financial interests.

Address all correspondence to: d96524014@ntu.edu.tw

Department of Applied Physics and Chemistry, University of Taipei, Taiwan

**Acknowledgements**

**Conflict of interest**

**Author details**

Chuan-Pei Lee

**Figure 8.** The top-view SEM images of paper substrates with (a) sputtered Pt and with (b) 30 V% GDs-PEDOT:PSS composite [46].

can be perfectly filled by GDs/PEDOT:PSS; however, that cannot be achieved by sputtering Pt, which means that the sputtered Pt layer could not provide a continuous electron transport route in a porous paper substrate. Above two paper, CEs (i.e., 30 V% GDs-PEDOT:PSS and sputtered Pt) are assembled with flexible dye/TiO<sup>2</sup> /ITO-PEN photoanodes for studying the pertinent photovoltaic performance.

**Figure 9** shows the photovoltaic parameters of the flexible DSSCs using various paper-based CEs with different bending times. As shown in **Figure 9(a)**, the flexible DSSC with GDs/PEDOT:PSScoated paper CE (*η* = 4.91%) exhibits three times higher cell efficiency than that of cell using Pt-coated paper CE (*η* = 1.70%), since the GDs/PEDOT:PSS composite can well fill the porosity of paper substrate. It is worth to mention that the development of an all-metal-free CE is an effective

**Figure 9.** The photovoltaic parameters of the flexible DSSCs using various paper-based CEs with different bending times. (a) Cell efficiency (*η*); (b) fill factor (*FF*); (c) open-circuit voltage (*V*OC); (d) short-circuit current density (*J* SC) [46].

approach for reducing the cost of the DSSCs. Moreover, the bending test is carried out to study the durability of the paper CEs. Both GDs/PEDOT:PSS-coated paper electrode and Pt-coated paper electrode are bended for several times (0, 50, 100 and 150 times) and then assembled with the flexible photoanodes to measure and record their corresponding photovoltaic performances. **Figure 9(a)** shows the bending time dependence of cell efficiency for the flexible DSSCs with various paper CEs; their corresponding photovoltaic parameters are shown in **Figure 9(b)**–**(d)**. It obviously shows that the GDs/PEDOT:PSS-coated paper CE shows unfailing performance even though it was bended for 150 times; on the contrary, the Pt-coated paper CE lost its original performance drastically.

#### **4. Conclusion**

can be perfectly filled by GDs/PEDOT:PSS; however, that cannot be achieved by sputtering Pt, which means that the sputtered Pt layer could not provide a continuous electron transport route in a porous paper substrate. Above two paper, CEs (i.e., 30 V% GDs-PEDOT:PSS and

**Figure 8.** The top-view SEM images of paper substrates with (a) sputtered Pt and with (b) 30 V% GDs-PEDOT:PSS

**Figure 9** shows the photovoltaic parameters of the flexible DSSCs using various paper-based CEs with different bending times. As shown in **Figure 9(a)**, the flexible DSSC with GDs/PEDOT:PSScoated paper CE (*η* = 4.91%) exhibits three times higher cell efficiency than that of cell using Pt-coated paper CE (*η* = 1.70%), since the GDs/PEDOT:PSS composite can well fill the porosity of paper substrate. It is worth to mention that the development of an all-metal-free CE is an effective

**Figure 9.** The photovoltaic parameters of the flexible DSSCs using various paper-based CEs with different bending times. (a) Cell efficiency (*η*); (b) fill factor (*FF*); (c) open-circuit voltage (*V*OC); (d) short-circuit current density (*J*

/ITO-PEN photoanodes for studying the

SC) [46].

sputtered Pt) are assembled with flexible dye/TiO<sup>2</sup>

pertinent photovoltaic performance.

264 Novel Nanomaterials - Synthesis and Applications

composite [46].

In summary, a GDs-PEDOT:PSS composite ink was synthesized for preparing the all-metalfree paper-based CEs for flexible DSSCs. The GDs-PEDOT:PSS/paper CE was fabricated through a low-cost and simple coating method (i.e., soak and dry), which could be easily scaled up to mass production. An all-flexible DSSC with GDs/PEDOT:PSS-coated paper CE exhibits much higher cell efficiency (4.91%) than that of cell using paper CE with sputtered Pt (1.70%), since the porosity of paper substrate can be well filled by GDs-PEDOT:PSS, which cannot be achieved by sputtered Pt. After bending for 150 cycles, the performance of GDs/ PEDOT:PSS-coated paper CE is still perfectly preserved; on the contrary, the paper CE with sputtered Pt lost its initial performance drastically. In conclusion, this GDs/PEDOT:PSScoated paper CE is lightweight, low-cost, space-saving (high flexibility), high machinability (easy-cutting) and environmental friendly, which shows the potential for the future applications on portable/wearable electronics.

#### **Acknowledgements**

This work was supported by the Ministry of Science and Technology (MOST) of Taiwan.

### **Conflict of interest**

The authors declare no competing financial interests.

#### **Author details**

Chuan-Pei Lee

Address all correspondence to: d96524014@ntu.edu.tw

Department of Applied Physics and Chemistry, University of Taipei, Taiwan

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**Chapter 15**

**Provisional chapter**

**Rationally Fabricated Nanomaterials for Desalination**

**Rationally Fabricated Nanomaterials for Desalination** 

Rationally designed nanomaterials from synthetic/biopolymers like chitosan, zeolites, graphene, nanometal/oxides, zerovalent metal/magnetic iron, OMS and nanocarbon/ carbon nanotube (CNT) utilized in desalination/purification are thoroughly discussed. Conventional desalination membrane/materials own inherent limitations; nevertheless, designed nanocomposite/hybrid/films address the new challenges/constraints and consequently aid the remediation of environmental/water pollution, thus denoting prospective nanotechnology/science. The morphology and chemical functionality of certain natural/synthetic polymers are altered/controlled rationally yielding advanced membranes/materials, for example, aquaporin, nanochannels, graphene and smart selfassemble block copolymer blends to cater futuristic desalination needs besides superseded conventional membrane limitations too. In a nut shell, advance nanotechnology via electrospinning, track-etching, phase inversion and interfacial polymerization yields structured composites/matrixes that conquer traditional barriers of conventional desalination and supplies treated/purified water. This review confers synthetic strategy and utility of nanomaterials that are procured via ordered/rational designing/self-assembly to be used in separation techniques including RO/FO antifouling membrane, superwet surface, oil-water/emulsion separation and multifunctional desalination nanodevices. **Keywords:** nanomaterial, designing, chitosan, OMS, desalination, water purification,

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

The ever growing population and economic expansion put potential crisis in supply, and the availability of fresh water as UN for the decade 2030 forecasts 40% high global water

DOI: 10.5772/intechopen.74738

**and Water Purification**

**and Water Purification**

http://dx.doi.org/10.5772/intechopen.74738

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Rajendra S. Dongre

Rajendra S. Dongre

**Abstract**

electrospun

**1. Introduction**

#### **Rationally Fabricated Nanomaterials for Desalination and Water Purification Rationally Fabricated Nanomaterials for Desalination and Water Purification**

DOI: 10.5772/intechopen.74738

Rajendra S. Dongre Rajendra S. Dongre

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.74738

#### **Abstract**

Rationally designed nanomaterials from synthetic/biopolymers like chitosan, zeolites, graphene, nanometal/oxides, zerovalent metal/magnetic iron, OMS and nanocarbon/ carbon nanotube (CNT) utilized in desalination/purification are thoroughly discussed. Conventional desalination membrane/materials own inherent limitations; nevertheless, designed nanocomposite/hybrid/films address the new challenges/constraints and consequently aid the remediation of environmental/water pollution, thus denoting prospective nanotechnology/science. The morphology and chemical functionality of certain natural/synthetic polymers are altered/controlled rationally yielding advanced membranes/materials, for example, aquaporin, nanochannels, graphene and smart selfassemble block copolymer blends to cater futuristic desalination needs besides superseded conventional membrane limitations too. In a nut shell, advance nanotechnology via electrospinning, track-etching, phase inversion and interfacial polymerization yields structured composites/matrixes that conquer traditional barriers of conventional desalination and supplies treated/purified water. This review confers synthetic strategy and utility of nanomaterials that are procured via ordered/rational designing/self-assembly to be used in separation techniques including RO/FO antifouling membrane, superwet surface, oil-water/emulsion separation and multifunctional desalination nanodevices.

**Keywords:** nanomaterial, designing, chitosan, OMS, desalination, water purification, electrospun

#### **1. Introduction**

The ever growing population and economic expansion put potential crisis in supply, and the availability of fresh water as UN for the decade 2030 forecasts 40% high global water

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

uptake hints intensifies water consumption [1]. Amid estimated 780 million that do not have access to safe water, UN reported that global populations will face water scarcity by 2050. Thus, obligatory purifications of existing polluted/unsafe water were achieved via desalination or any other techniques. Desalination via membrane technique was developed since 1960 which removes soluble salts and micro-pollutants that are not of concern in conventional treatments, nevertheless being expensive over conventional water purifications. It also has drawbacks like high costs/energy inputs, and greenhouse gas pollution puts constraints on the environment to discover rationally developed desalination materials. Conventional polyamide-based membranes own inherent drawbacks viz. limited permeability, less selectivity and low chemical stability, and affecting separation performance [2]. Nanotechnology aids in the design of smart/advanced materials/membranes via fabrication of one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) nanomaterials owing to improved efficiency for revolutionary desalination/water purification. Rationally designed nanomembranes trounce such demerits, for example, graphene membranes owing to exclusive features like tunable properties, extraordinary robustness, and advantageous leverage with a superior separation efficiency on par with CNT and biomimetic aquaporin membranes [3–5]. Fabricated 2-D membranes own fundamentally different separation capability due to peculiar features like punching nanometer/ultrathin pores, precisely controlled/manipulated shape/size super-strong impermeable monolayers, and facile industrial scale-up since they use cheap/biopolymeric feedstock/raw materials [6–9]. Certain nanomaterials such as 2-D graphenes, zeolites and molybdenum disulfides assembled via layer-stacking and own unique configurations with controlled interlayer spacing have fascinated research in making high-performance desalination membranes with advantages peculiarity like tangible manipulation for permeability and contaminant's selectivity. Further, 2-D materials innovatively converted into 3D nanomembrane to impart enhanced selectivity and minimized fouling, which find potential applicability in solar desalination devices [10]. Versatile pertinence of nanotechnology is exploited in fabrication of many superior organic and/or inorganic based nano-materials owing myriad functionality by virtue of inherent firm controlled shape and pore distribution with facile alterations in porosity, surface area: volume ratio, and optimized dimensions/shapes as designed prior to their synthesis. Desalination materials/membranes have been designed and developed via assorted methodologies reported in the study [6–12]. On the basis of structure, properties, and performance relationships, these smart nanomaterials are fabricated for water treatments [1–13], and so this review signifies/focuses on recent advances in synthetic strategy in its designing/fabrication besides purification applications.

resulting in high permeability as demonstrated for seawater at a substantially lower cost than conventional membrane technologies [10–14]. Pervaporation is performed via a designed membrane owing to preferential higher affinity with one component with a faster diffusion rate responsible for the separation of oil-water/mixtures. Pervaporation technique is advantageous in desalination due to 100% salt rejections and low-energy consumption attenuated by combined membrane permeation and evaporation. Several sophisticated advanced membranes were synthesized, for example, polyvinyl alcohol-based membrane owing to peculiar features viz. excellent film formation, hydrophilicity, and hydroxyl-induced swelling accountable for the effective desalination/separation of pseudo-liquid blend/mixtures [13, 14]. Nanosilica in maleic acid-polyvinyl alcohol yielding composite membranes shows enhanced water flux/diffusion coefficient and 99.9% salt rejections as desired for an efficient desalination [10]. Polyethylene terephthalate grafted in styrene obtains a membrane for pervaporation with an improved toluene selectivity [15]. PEBAX matrix of 100-μm dimension obtained for working at 50°C feed water temperature showed enhanced diffusion and viscosity reduction ruled by vacuum and thickness, which vitally determines pervaporation performance [10]. Commercial coalescing water filtration and adsorptive difficult emulsion separations for oily contaminants were achieved via fabricated nanofibers found to get partially deactivated besides membrane fouling, thus increasing their treatment cost [1–15]. Coalescing filtrations break stable/difficult oilwater and surfactant emulsions [16–18] that are viable on a coalescing medium/material used, as large droplets downstream settlement require less residence time. In this context, electrospun nanofibrous-based membranes own a larger surface area, vitally enhancing its coalescing filtra-

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273

Assorted nanoporous membranes/materials yield via diversified synthetic techniques [19] including phase inversion, interfacial polymerization, track-etching, and electrospinning as

Chemical stratification is performed to remove the solvent from liquid-polymer solution and converts homogeneous solution into porous solid membrane/films in a controlled fashion [1, 19]. This phase inversion is vastly reliant on both the types of solvent and polymers, besides accomplished via captivate precipitation and/or heat/vapor/evaporation-induced phase separations. Immersion precipitation and thermo-induced phase separation are applied for nanofiltration (NF), ultra-filtration (UF), and reverse osmosis (RO) membrane fabrications [10]. Membrane morphology and porosity are found to be controlled by the nature of solvent/oil-water mixtures. Phase inversion technique is used to yield superoleophobic poly(acrylic acid)-graft PVDF membrane that induces efficient desalination and major emulsion/oil-water separations.

A step-growth polycondensation occurs in immiscible solvents (aqueous solution, impregnates one monomer and organic solution containing a second monomer), for example, diamine and di-acid chloride solution yield polyamide membrane [1, 10]. This technique is used to fabricate

tion performance [1].

*1.2.1. Phase inversion*

described in the below section:

*1.2.2. Interfacial polymerization*

**1.2. Fabrication methods of nanomaterials/membranes**

#### **1.1. Assorted techniques for desalination**

#### *1.1.1. Adsorptive separation and pervaporation*

Desalination extracts/removes salts and mineral components from saline water [1–14]. It is an earliest form of treatment still popular throughout for the conversion of seawater (salinity due to dissolved salts at 10,000 ppm) to drinking water for addressing water scarcity. Desalinated water is better than river/groundwater, as it has less salt and lime-scale contents. This desalination technology has demanded the usage of assorted smart/fabricated materials involving through advancement in nanotechnology. Thermal-based desalination is achieved via adsorption onto fabricated porous silica surfaces owing to double-bond surface force affinity for pollutants, resulting in high permeability as demonstrated for seawater at a substantially lower cost than conventional membrane technologies [10–14]. Pervaporation is performed via a designed membrane owing to preferential higher affinity with one component with a faster diffusion rate responsible for the separation of oil-water/mixtures. Pervaporation technique is advantageous in desalination due to 100% salt rejections and low-energy consumption attenuated by combined membrane permeation and evaporation. Several sophisticated advanced membranes were synthesized, for example, polyvinyl alcohol-based membrane owing to peculiar features viz. excellent film formation, hydrophilicity, and hydroxyl-induced swelling accountable for the effective desalination/separation of pseudo-liquid blend/mixtures [13, 14]. Nanosilica in maleic acid-polyvinyl alcohol yielding composite membranes shows enhanced water flux/diffusion coefficient and 99.9% salt rejections as desired for an efficient desalination [10]. Polyethylene terephthalate grafted in styrene obtains a membrane for pervaporation with an improved toluene selectivity [15]. PEBAX matrix of 100-μm dimension obtained for working at 50°C feed water temperature showed enhanced diffusion and viscosity reduction ruled by vacuum and thickness, which vitally determines pervaporation performance [10]. Commercial coalescing water filtration and adsorptive difficult emulsion separations for oily contaminants were achieved via fabricated nanofibers found to get partially deactivated besides membrane fouling, thus increasing their treatment cost [1–15]. Coalescing filtrations break stable/difficult oilwater and surfactant emulsions [16–18] that are viable on a coalescing medium/material used, as large droplets downstream settlement require less residence time. In this context, electrospun nanofibrous-based membranes own a larger surface area, vitally enhancing its coalescing filtration performance [1].

#### **1.2. Fabrication methods of nanomaterials/membranes**

Assorted nanoporous membranes/materials yield via diversified synthetic techniques [19] including phase inversion, interfacial polymerization, track-etching, and electrospinning as described in the below section:

#### *1.2.1. Phase inversion*

uptake hints intensifies water consumption [1]. Amid estimated 780 million that do not have access to safe water, UN reported that global populations will face water scarcity by 2050. Thus, obligatory purifications of existing polluted/unsafe water were achieved via desalination or any other techniques. Desalination via membrane technique was developed since 1960 which removes soluble salts and micro-pollutants that are not of concern in conventional treatments, nevertheless being expensive over conventional water purifications. It also has drawbacks like high costs/energy inputs, and greenhouse gas pollution puts constraints on the environment to discover rationally developed desalination materials. Conventional polyamide-based membranes own inherent drawbacks viz. limited permeability, less selectivity and low chemical stability, and affecting separation performance [2]. Nanotechnology aids in the design of smart/advanced materials/membranes via fabrication of one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) nanomaterials owing to improved efficiency for revolutionary desalination/water purification. Rationally designed nanomembranes trounce such demerits, for example, graphene membranes owing to exclusive features like tunable properties, extraordinary robustness, and advantageous leverage with a superior separation efficiency on par with CNT and biomimetic aquaporin membranes [3–5]. Fabricated 2-D membranes own fundamentally different separation capability due to peculiar features like punching nanometer/ultrathin pores, precisely controlled/manipulated shape/size super-strong impermeable monolayers, and facile industrial scale-up since they use cheap/biopolymeric feedstock/raw materials [6–9]. Certain nanomaterials such as 2-D graphenes, zeolites and molybdenum disulfides assembled via layer-stacking and own unique configurations with controlled interlayer spacing have fascinated research in making high-performance desalination membranes with advantages peculiarity like tangible manipulation for permeability and contaminant's selectivity. Further, 2-D materials innovatively converted into 3D nanomembrane to impart enhanced selectivity and minimized fouling, which find potential applicability in solar desalination devices [10]. Versatile pertinence of nanotechnology is exploited in fabrication of many superior organic and/or inorganic based nano-materials owing myriad functionality by virtue of inherent firm controlled shape and pore distribution with facile alterations in porosity, surface area: volume ratio, and optimized dimensions/shapes as designed prior to their synthesis. Desalination materials/membranes have been designed and developed via assorted methodologies reported in the study [6–12]. On the basis of structure, properties, and performance relationships, these smart nanomaterials are fabricated for water treatments [1–13], and so this review signifies/focuses on recent advances in synthetic strategy in its designing/fabrication besides purification applications.

Desalination extracts/removes salts and mineral components from saline water [1–14]. It is an earliest form of treatment still popular throughout for the conversion of seawater (salinity due to dissolved salts at 10,000 ppm) to drinking water for addressing water scarcity. Desalinated water is better than river/groundwater, as it has less salt and lime-scale contents. This desalination technology has demanded the usage of assorted smart/fabricated materials involving through advancement in nanotechnology. Thermal-based desalination is achieved via adsorption onto fabricated porous silica surfaces owing to double-bond surface force affinity for pollutants,

**1.1. Assorted techniques for desalination**

272 Novel Nanomaterials - Synthesis and Applications

*1.1.1. Adsorptive separation and pervaporation*

Chemical stratification is performed to remove the solvent from liquid-polymer solution and converts homogeneous solution into porous solid membrane/films in a controlled fashion [1, 19]. This phase inversion is vastly reliant on both the types of solvent and polymers, besides accomplished via captivate precipitation and/or heat/vapor/evaporation-induced phase separations. Immersion precipitation and thermo-induced phase separation are applied for nanofiltration (NF), ultra-filtration (UF), and reverse osmosis (RO) membrane fabrications [10]. Membrane morphology and porosity are found to be controlled by the nature of solvent/oil-water mixtures. Phase inversion technique is used to yield superoleophobic poly(acrylic acid)-graft PVDF membrane that induces efficient desalination and major emulsion/oil-water separations.

#### *1.2.2. Interfacial polymerization*

A step-growth polycondensation occurs in immiscible solvents (aqueous solution, impregnates one monomer and organic solution containing a second monomer), for example, diamine and di-acid chloride solution yield polyamide membrane [1, 10]. This technique is used to fabricate ultra-thin films (10 nm to μm) to be used for RO and NF membranes [1–10]. A membrane skeletal morphology or a layer-by-layer build-up of a composite can be controlled by many factors like the type/concentration of monomer and solvents, and reaction time, besides posttreatment conditions [10, 19]. The study reported interfacial polymerization of diamine and acyl chloride onto cellulose nanocrystal layer yielding a triple-layered composite/membrane for nanofiltration/desalination [1]. Interfacial polymerized MCM-41-silica and graphene oxide onto polyamide surface yield ultra-films [10] that showed huge water flux and salt rejections in desalination.

Morphological/topographical features of such electrospun membranes can be altered by numerous methodologies namely molecular bonding, in situ polymerization, and dopants encroachment technology. Akin to surface modifications achieved via nanoparticle coating, chemical/ heat treatments, grafting, and interfacial polymerization were found to be efficient in coalescing filtration across commercial treatments. During the last decade, electrospun technique utilizes myriad polymeric feedstocks for devising nanofibrous membranes for pressure/thermal-driven microfiltration (MF), UF, NF, forward osmosis (FO) and coalescing filtrations besides adsorp-

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275

Electrospun membranes own 3D interconnected skeletons as critical for improved desalination media which is advantageous over traditional membranes in regard to the efficiency, price, and energy. Microporous polymeric progressive desalination membranes are fabricated via assorted techniques like film lithography, stretching, phase inversion, electrospinning owing to peculiarity viz. huge interconnective 3D porosity, adjustable pore-size distribution, high water flux, high surface area with molecular orientations and facile fiber-axis directional macroscales. These films/composites fascinated prominent field like water treatment/purification/separation, pressure-driven distillation, oil-water/marine oil spill cleanups, and RO/NF pretreatment feeds. Functionalized polymeric membranes can act as methanol fuel cell, separators for rechargeable lithium-ion batteries, pressureretarded/driven/osmosis used for bacteria/fungus culture media/suspended particles micro-filtration, dye solutions, and ultra-filtration [1–19]. Tailored polyvinylidenefluoridepolyacrylonitrile nanofiber membranes are characterized for chemical adsorption, liquid filtration, and extraction of harmful chemicals from contaminated water. Nanochitosan owns fast adsorption kinetics, high arsenic sorption capacity, and facile arsenic and other such anionic removal [20–23]. Electrospun polystyrene-polydopamine/PDA fibrous cross-linked with β-cyclodextrin coating was found to overcome existing limitations for the removal of anionic pollutants from water compared to non β-cyclodextrin/mere PDA fiber [21, 22]. Biohybrid membranes from polyvinyl alcohol and hydrocolloidal natural

gum yield via electrospinning remove assorted nanometal like Ag, Au, Pt, Fe<sup>3</sup>

from water [1, 21]. PVA/GK, amidoxime webs yield via methane plasma treatments and two-nozzle electrospinning, respectively, exhibited altered porosity and hydrophobicity besides elevated sorption capacity for the selective adsorption of uranium and vanadium, besides offering huge utility in tissue engineering and drug-delivery systems [1, 10, 19]. Self-assembly/phase separation techniques for nanomaterial synthesis control 3-D porosity without fiber orientation, while templating controls for fiber orientation over dimension arrangement using sacrificing agents [20]. Electrospinning fabricates tunable morphology and diameter in 1D to 3D nanofiber pores with a facile modification achieved via chemical grafting of rough surface [1, 19]. Chemical compositions, mechanical features, patterns, and membrane pore areas are dominant factors in desalinations that are attenuated by electrospinning, for example, polymethyl methacrylate fiber axis with 0.97-μm diameter found to affect cross-link adherence or extended orientation that ultimately regulates water flux. Thus, nanofiber augmented/controlled via electrospinning to manage fiber alignment yields aligned membranes for desalination utilities as shown in **Figure 1**. Still, many challenges require balancing the degree of orientation, nanofiber thickness, and productivity

O4

, and CuO

tive desalination [19].

#### *1.2.3. Track-etching*

Track-etching involves energetic heavy ion irradiation onto a substrate resulting in a lineardamaged track across irradiated polymer surfaces that yield nanoporous membrane [19]. This technique precisely augmented a pore-size distribution of nm to μm dimension and a pore density of 1–1010 cm−2 [10], for example, nanoporous silicon nitride membranes using porous nanosilicon templates [1, 10].

#### *1.2.4. Electrospinning*

Electrospinning technique uses high-voltage treatment to polymers that is outsized to overcome the surface tension of solution droplets, and the resulted charged liquid jet is then converted into ultrafine/nanofibrous at collection drums. The morphology and skeletal parameters like porosity, shape/size distribution, and ratios of the corresponding nanomaterials/membranes are controlled by adjusting the electric voltage and treatment conditions viz. polymeric solution's viscosity and flow of solution [19]. Illustrious nanofibrous PVDF and 2-D nanosheets (Ti3C2Tx MXene) are prepared for sieving cations and dyes from contaminated water [1, 10].

#### **1.3. Myriad nanomaterials/membranes for desalination**

Assorted nanoporous material-based membranes are categorized as inorganic, organic, and inorganic-organic hybrid as per the material compositions. Various inorganic membranes include Al<sup>2</sup> O3 -, TiO2 -, ZrO2 -, SiO2 -, TiO2 -SiO2 -, TiO2 -ZrO2 -, and Al<sup>2</sup> O3 -SiC-based ceramics, and 2-D matter like graphenes and carbon nanotubes. Organic membranes are obtained from polymers like polyvinyl alcohol, polyimide, polypropylene, polyethersulfone, cellulose acetate, cellulose nitrates, polysulfone, polyvinylidene fluoride, polyacrylonitrile, polytetrafluoroethylene, and biopolymers like chitin, chitosan, and so on. Chitosan-blended dendrimers showed highly efficient anionic dyes, heavy metals, and organic contaminants removal from water [1]. Hybrid membranes yield, using inorganic metal/metal oxide, carbon materials into a polymeric matrix [1–19].

#### *1.3.1. Electrospun membranes*

Electrospinning technique is found to control many parameters like porous tortuosity, pores size and/or shape deviations (straight or cylindrical) which are crucially reduced in nano-fibers synergistic amalgamation. Polystyrene-based nanofibrous smooth surface of 452 nm owns a peculiarity viz. uniform porosity with low tortuosity, no bead formation, and least surface roughness, establishing futuristic coalescing filtration media for oily emulsion separations. Morphological/topographical features of such electrospun membranes can be altered by numerous methodologies namely molecular bonding, in situ polymerization, and dopants encroachment technology. Akin to surface modifications achieved via nanoparticle coating, chemical/ heat treatments, grafting, and interfacial polymerization were found to be efficient in coalescing filtration across commercial treatments. During the last decade, electrospun technique utilizes myriad polymeric feedstocks for devising nanofibrous membranes for pressure/thermal-driven microfiltration (MF), UF, NF, forward osmosis (FO) and coalescing filtrations besides adsorptive desalination [19].

ultra-thin films (10 nm to μm) to be used for RO and NF membranes [1–10]. A membrane skeletal morphology or a layer-by-layer build-up of a composite can be controlled by many factors like the type/concentration of monomer and solvents, and reaction time, besides posttreatment conditions [10, 19]. The study reported interfacial polymerization of diamine and acyl chloride onto cellulose nanocrystal layer yielding a triple-layered composite/membrane for nanofiltration/desalination [1]. Interfacial polymerized MCM-41-silica and graphene oxide onto polyamide surface yield ultra-films [10] that showed huge water flux and salt rejections in desalination.

Track-etching involves energetic heavy ion irradiation onto a substrate resulting in a lineardamaged track across irradiated polymer surfaces that yield nanoporous membrane [19]. This technique precisely augmented a pore-size distribution of nm to μm dimension and a pore density of 1–1010 cm−2 [10], for example, nanoporous silicon nitride membranes using porous

Electrospinning technique uses high-voltage treatment to polymers that is outsized to overcome the surface tension of solution droplets, and the resulted charged liquid jet is then converted into ultrafine/nanofibrous at collection drums. The morphology and skeletal parameters like porosity, shape/size distribution, and ratios of the corresponding nanomaterials/membranes are controlled by adjusting the electric voltage and treatment conditions viz. polymeric solution's viscosity and flow of solution [19]. Illustrious nanofibrous PVDF and 2-D nanosheets (Ti3C2Tx MXene) are prepared for sieving cations and dyes from contaminated water [1, 10].

Assorted nanoporous material-based membranes are categorized as inorganic, organic, and inorganic-organic hybrid as per the material compositions. Various inorganic membranes include

like graphenes and carbon nanotubes. Organic membranes are obtained from polymers like polyvinyl alcohol, polyimide, polypropylene, polyethersulfone, cellulose acetate, cellulose nitrates, polysulfone, polyvinylidene fluoride, polyacrylonitrile, polytetrafluoroethylene, and biopolymers like chitin, chitosan, and so on. Chitosan-blended dendrimers showed highly efficient anionic dyes, heavy metals, and organic contaminants removal from water [1]. Hybrid membranes yield,

Electrospinning technique is found to control many parameters like porous tortuosity, pores size and/or shape deviations (straight or cylindrical) which are crucially reduced in nano-fibers synergistic amalgamation. Polystyrene-based nanofibrous smooth surface of 452 nm owns a peculiarity viz. uniform porosity with low tortuosity, no bead formation, and least surface roughness, establishing futuristic coalescing filtration media for oily emulsion separations.


O3



*1.2.3. Track-etching*

nanosilicon templates [1, 10].

274 Novel Nanomaterials - Synthesis and Applications


*1.3.1. Electrospun membranes*


**1.3. Myriad nanomaterials/membranes for desalination**




using inorganic metal/metal oxide, carbon materials into a polymeric matrix [1–19].

*1.2.4. Electrospinning*

Al<sup>2</sup> O3 -, TiO2 Electrospun membranes own 3D interconnected skeletons as critical for improved desalination media which is advantageous over traditional membranes in regard to the efficiency, price, and energy. Microporous polymeric progressive desalination membranes are fabricated via assorted techniques like film lithography, stretching, phase inversion, electrospinning owing to peculiarity viz. huge interconnective 3D porosity, adjustable pore-size distribution, high water flux, high surface area with molecular orientations and facile fiber-axis directional macroscales. These films/composites fascinated prominent field like water treatment/purification/separation, pressure-driven distillation, oil-water/marine oil spill cleanups, and RO/NF pretreatment feeds. Functionalized polymeric membranes can act as methanol fuel cell, separators for rechargeable lithium-ion batteries, pressureretarded/driven/osmosis used for bacteria/fungus culture media/suspended particles micro-filtration, dye solutions, and ultra-filtration [1–19]. Tailored polyvinylidenefluoridepolyacrylonitrile nanofiber membranes are characterized for chemical adsorption, liquid filtration, and extraction of harmful chemicals from contaminated water. Nanochitosan owns fast adsorption kinetics, high arsenic sorption capacity, and facile arsenic and other such anionic removal [20–23]. Electrospun polystyrene-polydopamine/PDA fibrous cross-linked with β-cyclodextrin coating was found to overcome existing limitations for the removal of anionic pollutants from water compared to non β-cyclodextrin/mere PDA fiber [21, 22]. Biohybrid membranes from polyvinyl alcohol and hydrocolloidal natural gum yield via electrospinning remove assorted nanometal like Ag, Au, Pt, Fe<sup>3</sup> O4 , and CuO from water [1, 21]. PVA/GK, amidoxime webs yield via methane plasma treatments and two-nozzle electrospinning, respectively, exhibited altered porosity and hydrophobicity besides elevated sorption capacity for the selective adsorption of uranium and vanadium, besides offering huge utility in tissue engineering and drug-delivery systems [1, 10, 19]. Self-assembly/phase separation techniques for nanomaterial synthesis control 3-D porosity without fiber orientation, while templating controls for fiber orientation over dimension arrangement using sacrificing agents [20]. Electrospinning fabricates tunable morphology and diameter in 1D to 3D nanofiber pores with a facile modification achieved via chemical grafting of rough surface [1, 19]. Chemical compositions, mechanical features, patterns, and membrane pore areas are dominant factors in desalinations that are attenuated by electrospinning, for example, polymethyl methacrylate fiber axis with 0.97-μm diameter found to affect cross-link adherence or extended orientation that ultimately regulates water flux. Thus, nanofiber augmented/controlled via electrospinning to manage fiber alignment yields aligned membranes for desalination utilities as shown in **Figure 1**. Still, many challenges require balancing the degree of orientation, nanofiber thickness, and productivity to be achieved by effective strategic controls like pore area, gap width, and orientation. Electrospinning coats nanofiber onto polymers/ceramic, imparting a high-specific surface and tunable porosity needs in separation [1, 19].

chemical treatment, for example, graphene nanopore provides hydrophilic sites with enhanced water flux achieved via precise nanopore edges with tunable properties and hydrophilicity in certain advanced desalination [19, 24]. Chemical vapor deposition is combined with focused

Rationally Fabricated Nanomaterials for Desalination and Water Purification

)-based membranes that carry effective ion separations as a function of pore size, chemistry, geometry, and hydrostatic pressure [1]. Mechanistic separations via reverse osmosis and capacitive deionization utilized many nanomaterials including zeolites, carbon nanotubes, and graphene to develop highly efficient and capable futuristic desalinations [1, 19, 24]. Advanced designing of nanomaterials paved new avenues and ability to manipulate nanomaterials like carbon nanotubes, nanowires, graphene, quantum dots, super-lattices, and nanoshells [1, 24]. Nanotechnology enables unique nanofabrication that controls macroscopic nontortuous ~1-nm pores specifically designed in MCM, carbon nanotubes, and graphene to offer well-designed size-selective, filtration membranes superior to conventional polymeric membranes (rigidity

Aluminosilicates are commonly termed as zeolites that possess 3–8-nm pore dimensions found to control morphological features that are well exploited for adsorption/ion exchangers in water treatments. Molecular dynamics stimulates a tight pore distribution as vital for absolute salt rejections and high water permeability, for example, ZK-4 molecular sieve-based RO membrane (4.4-nm) solvates salt and allows the passage of water to flow [1, 25, 26]. Mordenite-coated α-alumina zeolite (MFI, 5.6-nm) RO membranes exhibited hydrophobicity and lowers salt/ ionic transport due to fabricated interstitial-defective surfaces so as to control major ion transportations. Zeolites/molecular sieves can be directly coated onto ceramics and incorporated via laser-induced fragmentation to yield RO/FO membranes, for example, Linde-zeolite-A (4.4 nm) interfacial-embedded composites [25, 26] as shown in **Figure 2**. Further rising of zeolite weight % was found to enhance water permeability and salt rejections in resultant membranes compared to experimental thin-film membranes (without zeolite). Zeolite-coated membrane permeates high salt/water throughout since specific transport limits intrinsic pores, and it is

difficult to establish its exact role in water transport and salt rejection [10, 25, 26].

Assorted nanocarbon materials get vast popularity due to their specific morphology, physicochemical properties, and varied significant utilities. Thus, carbon-based materials are developed as nanoparticles, nanoonions, peapods, nanofibers, nanorings, nanowires, nanotubes, and fullerenes, owing to extensive analytical explicabilities. Intrinsic surface defects of nanostructured carbons were found to affect their stability, and mechanical and physicochemical properties, which further aids in rationally designed requisite materials like zero-dimensional fullerenes and diamond clusters, 1-D nanotubes, 2-D graphenes, 3D nanodiamond (<1 μm), and ultra-hard fullerite [1, 19, 24]. Nanocarbon materials have special advantages viz. facile functionality alterations, high carrier capacity, hydrophilic/hydrophobic incorporations besides

, MoTe2

http://dx.doi.org/10.5772/intechopen.74738

, WS2 , 277

electron beam to get sculptured single-layer assorted metal dichalcogenide (MoSe2

owing to size-selective nondesign porosity) for desalination [1, 19, 24].

WSe2

*1.3.3. Zeolites*

*1.3.4. Nanocarbons*

high chemical stability [1, 15, 24].

#### *1.3.2. Advanced nanomaterials*

Novel nanocomposites, films, hybrids, matrixes, and membranes were fabricated via constitutional morphological alterations to owe substantial water flux/permeability and salt rejection sought in desalination [1, 19]. Lucratively, R & D has designed nanoporous materials that accede to high water flux to keep salt/other contaminates away, thus paving the path for futuristic overwhelmed desalination. Single-layer nanoporous sulfur-coated molybdenum (MoS<sup>2</sup> ) sheets exhibited 70% higher water flux than graphene RO membranes due to unusual features like a fish-bone/hourglass architecture owing to a nozzle subnanoporosity withstanding necessary water pressure/volumes and robustness as energetic and economic than other counterparts [24]. Certain single-layer nanosheet (pore area of 20–60 Å<sup>2</sup> ), MFI-zeolite, polymeric high-flux RO, and graphene membranes owing to nanofiltration are significant due to vital modulated parameters like porosity, velocity distributions, permeation, and water density imparting 90% ion rejection with huge water transportation than conventional membranes [24]. Modern nanotechnology aids in designing opportunistic energy-efficient membranes/sheets/mats for efficient desalinations, for example, few A<sup>0</sup> to several nanoporous dimensionally drilled resultant molecular sieves/membranes that *control mass transportations* [1, 19]. B*oron nitride-doped carbon nanotubes and* single-atom-thick *graphene* are augmented for assorted hydrated ions/salt rejections performance than conventional zeolitic membranes in desalination [1]. Hydroxyl functionality gets altered via

**Figure 1.** Schematic application of electrospun nanofibrous membranes in various fields.

chemical treatment, for example, graphene nanopore provides hydrophilic sites with enhanced water flux achieved via precise nanopore edges with tunable properties and hydrophilicity in certain advanced desalination [19, 24]. Chemical vapor deposition is combined with focused electron beam to get sculptured single-layer assorted metal dichalcogenide (MoSe2 , MoTe2 , WS2 , WSe2 )-based membranes that carry effective ion separations as a function of pore size, chemistry, geometry, and hydrostatic pressure [1]. Mechanistic separations via reverse osmosis and capacitive deionization utilized many nanomaterials including zeolites, carbon nanotubes, and graphene to develop highly efficient and capable futuristic desalinations [1, 19, 24]. Advanced designing of nanomaterials paved new avenues and ability to manipulate nanomaterials like carbon nanotubes, nanowires, graphene, quantum dots, super-lattices, and nanoshells [1, 24]. Nanotechnology enables unique nanofabrication that controls macroscopic nontortuous ~1-nm pores specifically designed in MCM, carbon nanotubes, and graphene to offer well-designed size-selective, filtration membranes superior to conventional polymeric membranes (rigidity owing to size-selective nondesign porosity) for desalination [1, 19, 24].

#### *1.3.3. Zeolites*

) sheets

), MFI-zeolite, polymeric high-flux RO, and

to several nanoporous dimensionally drilled resultant molecular

to be achieved by effective strategic controls like pore area, gap width, and orientation. Electrospinning coats nanofiber onto polymers/ceramic, imparting a high-specific surface

Novel nanocomposites, films, hybrids, matrixes, and membranes were fabricated via constitutional morphological alterations to owe substantial water flux/permeability and salt rejection sought in desalination [1, 19]. Lucratively, R & D has designed nanoporous materials that accede to high water flux to keep salt/other contaminates away, thus paving the path for futuristic

exhibited 70% higher water flux than graphene RO membranes due to unusual features like a fish-bone/hourglass architecture owing to a nozzle subnanoporosity withstanding necessary water pressure/volumes and robustness as energetic and economic than other counterparts [24].

graphene membranes owing to nanofiltration are significant due to vital modulated parameters like porosity, velocity distributions, permeation, and water density imparting 90% ion rejection with huge water transportation than conventional membranes [24]. Modern nanotechnology aids in designing opportunistic energy-efficient membranes/sheets/mats for efficient desali-

sieves/membranes that *control mass transportations* [1, 19]. B*oron nitride-doped carbon nanotubes and* single-atom-thick *graphene* are augmented for assorted hydrated ions/salt rejections performance than conventional zeolitic membranes in desalination [1]. Hydroxyl functionality gets altered via

**Figure 1.** Schematic application of electrospun nanofibrous membranes in various fields.

overwhelmed desalination. Single-layer nanoporous sulfur-coated molybdenum (MoS<sup>2</sup>

and tunable porosity needs in separation [1, 19].

Certain single-layer nanosheet (pore area of 20–60 Å<sup>2</sup>

*1.3.2. Advanced nanomaterials*

276 Novel Nanomaterials - Synthesis and Applications

nations, for example, few A<sup>0</sup>

Aluminosilicates are commonly termed as zeolites that possess 3–8-nm pore dimensions found to control morphological features that are well exploited for adsorption/ion exchangers in water treatments. Molecular dynamics stimulates a tight pore distribution as vital for absolute salt rejections and high water permeability, for example, ZK-4 molecular sieve-based RO membrane (4.4-nm) solvates salt and allows the passage of water to flow [1, 25, 26]. Mordenite-coated α-alumina zeolite (MFI, 5.6-nm) RO membranes exhibited hydrophobicity and lowers salt/ ionic transport due to fabricated interstitial-defective surfaces so as to control major ion transportations. Zeolites/molecular sieves can be directly coated onto ceramics and incorporated via laser-induced fragmentation to yield RO/FO membranes, for example, Linde-zeolite-A (4.4 nm) interfacial-embedded composites [25, 26] as shown in **Figure 2**. Further rising of zeolite weight % was found to enhance water permeability and salt rejections in resultant membranes compared to experimental thin-film membranes (without zeolite). Zeolite-coated membrane permeates high salt/water throughout since specific transport limits intrinsic pores, and it is difficult to establish its exact role in water transport and salt rejection [10, 25, 26].

#### *1.3.4. Nanocarbons*

Assorted nanocarbon materials get vast popularity due to their specific morphology, physicochemical properties, and varied significant utilities. Thus, carbon-based materials are developed as nanoparticles, nanoonions, peapods, nanofibers, nanorings, nanowires, nanotubes, and fullerenes, owing to extensive analytical explicabilities. Intrinsic surface defects of nanostructured carbons were found to affect their stability, and mechanical and physicochemical properties, which further aids in rationally designed requisite materials like zero-dimensional fullerenes and diamond clusters, 1-D nanotubes, 2-D graphenes, 3D nanodiamond (<1 μm), and ultra-hard fullerite [1, 19, 24]. Nanocarbon materials have special advantages viz. facile functionality alterations, high carrier capacity, hydrophilic/hydrophobic incorporations besides high chemical stability [1, 15, 24].

[1]. Graphene is utilized potentially in the fabrication of RO/FO membrane fabrications [1] due to high breaking strength and impermeability aiding to create tunable pore size and ultrathin high flux membranes akin to molecular sieves. Molecular dynamic and impregnation techniques simulate the designing of pores (3–25 nm) in ordered carbon, subterminal pores in CNT that allows facile ionic transportation within electrolyte interfaces [20, 21]. The porosity of nanocarbon, CNT, and graphene can be tailored to direct specific adsorption/ultrafast ion

Rationally Fabricated Nanomaterials for Desalination and Water Purification

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279

Nanometals/metal oxides owe peculiar features like high specific surface area, short intraparticle diffusions, and facile compressibility, which are devoid of activated carbon; hence they are preferred for use in the remediation of metals, arsenic, radionuclides, and organics [1, 19]. Nanometal/oxide gets easily compressed into porous pellets, bead, and powder, for example, Solmete-X, Inc., USA, commercialized nanoselective resin *Arsen-Xnp* from nanoiron oxide-coated organic polymer for the removal of arsenate from water. Nanozerovalent iron adsorbs chlorinated polycyclic aromatic hydrocarbons and perchlorate from water due to its

contaminated water [1] besides being injected directly into systems as it gets easily removed

ity and low toxicity availability in disinfectants in water treatments [19]. Assorted properties, applications, and approach of nanomaterials in treatments [19] are summarized in **Table 1**.

Rational designing is done to get ultra-thin, dense active membranes/composites which own less salt rejections and ambient water flux utilized in much FO/RO desalination [1]. Nanomaterial's intrusion in contemporary polymeric skeleton offers extra physical/permeable barriers so as to attain two bulk phases as utilized in promising oil-water separations

rin-rapped polyamide, and polysulfone-zeolite's superior sieving fabricated myriad smart membranes [25] (**Figure 1**). Functionalized multi-wall carbon nanotube (MWCNT) polyamide-based RO membrane reported 200% water flux with at par salt rejection, amplified water flux [1–17], high hydrophilicity, and thermal stability [27]. Mesosilica MCM-41 incorporated polyamide-based RO membranes augmented water permeability/flux [16]. Biomimic amphiphilic tri-block vesicles enclosed aquaporin-Z and interfacial polymerize RO/FO mem brane impart complete salt rejection and 800 times water flux for seawater desalinations [15, 17]. Conventionally intense polarized membranes own depleted water flux and operation efficiency as serious issues in RO/FO separations [1]. Smart nanosupported bottleneck RO/FO membranes are proficient due to nanoporosity, low tortuosity, high mechanical strength [17], and huge water flux than conventional under similar conditions. Zeolite/nanosilica-polysulfone hydrogel (10 nm) was found to alleviate conventional membrane clogged owing [1, 15] to water flux enhancement and anti-fouling mitigation as achieved due to anti-adhesive surface hydrophilicity [15]. Grafted polyethylene glycol-based RO/FO membrane has great

O4

separates arsenic from

has high stabil-

, CNT, aquapo-

transports vital in separation/desalination [1, 19].

high specific surface area than granular iron [30]. Magnetic nano-Fe3

*1.3.6. Rationally designed composites/hybrid/biomimetics*

by increased osmotic pressure in FO osmosis [1, 19]. Nanosilver-coated TiO<sup>2</sup>

and desalination [1, 19]. Nanomaterial matrix/blend, for example, nano-SiO<sup>2</sup>

*1.3.5. Nanometal/metal oxides*

**Figure 2.** Some nanocarbon materials used in desalination/water purifications.

Allotropic carbon nanotube (CNT) contains rolled-up/cylindrical graphite layers arising as fascinated structural materials due to excellent thermal-electrical conductance, strength, and adsorption properties [1, 12]. CNTs are characterized as single-walled and multi-walled based on the built-up route. A high specific area and elevated available adsorption sites of CNT aid its adaptable surface chemistry. Hydrophobic CNT surfaces are benignly stabilized to avoid persistent contaminant adsorption/aggregation and for preconcentration/detections [15, 19] besides metals/ions too adsorbed onto CNTs through electrostatic attraction and chemical bonding. CNT-based desalinations are facile for point-of-use water purification, for example, plasma-modified ultra-long CNT-based membranes own specific enhanced salt, organic, and metal adsorptions compared to conventional carbon [19]. Futuristic techniques can equip with CNT so as to achieve superior desalination, disinfection, and filtration. USA developed CNTboron sponges to separate oil-water/emulsion besides oil spills removal aiding oil remediation [1]. Less CNT amount is available in mitigations of degradable antibiotics/pharmaceutical contaminants in point-of-use purification systems [19]. Facile, cheap, and compatibility features of CNT, nanometal, and zeolite nanomaterial-based pellet/bead are utilized for arsenic removal from water [1, 22]. Electrochemically active CNT-based membrane can remove salt, proteins, virus, dyes, and phenol from water [1]. Advanced nanotechnology controls carbon nanotube diameter [19] that aids fabrication of CNT-coated high flux RO membrane owing to labile hydrophobicity and surface roughness [13–17]. Rapid mass transport occurs in CNTcoated polystyrene/silicon nitride membranes that are designed via carboxylation offering high salt ions rejection due to salt anionic and carboxyl repulsions. Some CNT composites owe a low surface area of 210 m2 /g than the activated carbon of 1500 m2 /g; instead, the adsorptive capacity is more due to ordered nanoporosity, which enhances the adsorption of pollutants [1]. Graphene is utilized potentially in the fabrication of RO/FO membrane fabrications [1] due to high breaking strength and impermeability aiding to create tunable pore size and ultrathin high flux membranes akin to molecular sieves. Molecular dynamic and impregnation techniques simulate the designing of pores (3–25 nm) in ordered carbon, subterminal pores in CNT that allows facile ionic transportation within electrolyte interfaces [20, 21]. The porosity of nanocarbon, CNT, and graphene can be tailored to direct specific adsorption/ultrafast ion transports vital in separation/desalination [1, 19].

#### *1.3.5. Nanometal/metal oxides*

Allotropic carbon nanotube (CNT) contains rolled-up/cylindrical graphite layers arising as fascinated structural materials due to excellent thermal-electrical conductance, strength, and adsorption properties [1, 12]. CNTs are characterized as single-walled and multi-walled based on the built-up route. A high specific area and elevated available adsorption sites of CNT aid its adaptable surface chemistry. Hydrophobic CNT surfaces are benignly stabilized to avoid persistent contaminant adsorption/aggregation and for preconcentration/detections [15, 19] besides metals/ions too adsorbed onto CNTs through electrostatic attraction and chemical bonding. CNT-based desalinations are facile for point-of-use water purification, for example, plasma-modified ultra-long CNT-based membranes own specific enhanced salt, organic, and metal adsorptions compared to conventional carbon [19]. Futuristic techniques can equip with CNT so as to achieve superior desalination, disinfection, and filtration. USA developed CNTboron sponges to separate oil-water/emulsion besides oil spills removal aiding oil remediation [1]. Less CNT amount is available in mitigations of degradable antibiotics/pharmaceutical contaminants in point-of-use purification systems [19]. Facile, cheap, and compatibility features of CNT, nanometal, and zeolite nanomaterial-based pellet/bead are utilized for arsenic removal from water [1, 22]. Electrochemically active CNT-based membrane can remove salt, proteins, virus, dyes, and phenol from water [1]. Advanced nanotechnology controls carbon nanotube diameter [19] that aids fabrication of CNT-coated high flux RO membrane owing to labile hydrophobicity and surface roughness [13–17]. Rapid mass transport occurs in CNTcoated polystyrene/silicon nitride membranes that are designed via carboxylation offering high salt ions rejection due to salt anionic and carboxyl repulsions. Some CNT composites owe

**Figure 2.** Some nanocarbon materials used in desalination/water purifications.

278 Novel Nanomaterials - Synthesis and Applications

/g than the activated carbon of 1500 m2

capacity is more due to ordered nanoporosity, which enhances the adsorption of pollutants

/g; instead, the adsorptive

a low surface area of 210 m2

Nanometals/metal oxides owe peculiar features like high specific surface area, short intraparticle diffusions, and facile compressibility, which are devoid of activated carbon; hence they are preferred for use in the remediation of metals, arsenic, radionuclides, and organics [1, 19]. Nanometal/oxide gets easily compressed into porous pellets, bead, and powder, for example, Solmete-X, Inc., USA, commercialized nanoselective resin *Arsen-Xnp* from nanoiron oxide-coated organic polymer for the removal of arsenate from water. Nanozerovalent iron adsorbs chlorinated polycyclic aromatic hydrocarbons and perchlorate from water due to its high specific surface area than granular iron [30]. Magnetic nano-Fe3 O4 separates arsenic from contaminated water [1] besides being injected directly into systems as it gets easily removed by increased osmotic pressure in FO osmosis [1, 19]. Nanosilver-coated TiO<sup>2</sup> has high stability and low toxicity availability in disinfectants in water treatments [19]. Assorted properties, applications, and approach of nanomaterials in treatments [19] are summarized in **Table 1**.

#### *1.3.6. Rationally designed composites/hybrid/biomimetics*

Rational designing is done to get ultra-thin, dense active membranes/composites which own less salt rejections and ambient water flux utilized in much FO/RO desalination [1]. Nanomaterial's intrusion in contemporary polymeric skeleton offers extra physical/permeable barriers so as to attain two bulk phases as utilized in promising oil-water separations and desalination [1, 19]. Nanomaterial matrix/blend, for example, nano-SiO<sup>2</sup> , CNT, aquaporin-rapped polyamide, and polysulfone-zeolite's superior sieving fabricated myriad smart membranes [25] (**Figure 1**). Functionalized multi-wall carbon nanotube (MWCNT) polyamide-based RO membrane reported 200% water flux with at par salt rejection, amplified water flux [1–17], high hydrophilicity, and thermal stability [27]. Mesosilica MCM-41 incorporated polyamide-based RO membranes augmented water permeability/flux [16]. Biomimic amphiphilic tri-block vesicles enclosed aquaporin-Z and interfacial polymerize RO/FO mem brane impart complete salt rejection and 800 times water flux for seawater desalinations [15, 17]. Conventionally intense polarized membranes own depleted water flux and operation efficiency as serious issues in RO/FO separations [1]. Smart nanosupported bottleneck RO/FO membranes are proficient due to nanoporosity, low tortuosity, high mechanical strength [17], and huge water flux than conventional under similar conditions. Zeolite/nanosilica-polysulfone hydrogel (10 nm) was found to alleviate conventional membrane clogged owing [1, 15] to water flux enhancement and anti-fouling mitigation as achieved due to anti-adhesive surface hydrophilicity [15]. Grafted polyethylene glycol-based RO/FO membrane has great


**SN**

**Nanoporous** 

**Characteristics**

**Useful properties**

High reactivity, stable, durable, toxic, needs UV activation,

controllable nanosizes controlling by process conditions

Nanoparticle manipulations via magnetism/magnetically

tunable colloidality and shows superparamagnetism,

highly recyclable, easy magnetic separation, own very

large surface to volume ratio and biocompatible

Short intraparticle diffusivity, tunable pore size/surface

Needs variant nanodopant to

Heavy metallic anion removal, filters, slurry

reactors, palates, powders

Rationally Fabricated Nanomaterials for Desalination and Water Purification

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281

enhance capturing potential by

interconnections

chemistry, compressible, abrasion resistive, magnetic,

like calcinations

**Applications**

**Adverse properties**

Removes suspended fine

Water disinfections, antifouling processes

particles, needs ultraviolet

activation

Stabilization is needed to

Environmental remediation, cation sensors,

nanobeads adsorbents in water treatments,

groundwater remediations

enhance its potential

**materials**

10 11

Metals magnetic

nanoparticles/alloys

12

Nanometals/

nanometal oxides

**Table 1.**

Characteristics of the utility of nanomembrane/materials in water treatment processes.

reusable

Nano-TiO2


**SN**

**Nanoporous** 

**Characteristics**

**Useful properties**

Reliable, automated, charge-based repulsion, high

selectivity, low pressure, costly

**Applications**

**Adverse properties**

High energy, costly, membrane

Water hardness, color, odor reduction and

heavy metal removal, sea water desalination,

jam, concentrated polarization,

few nanoscale pore dimensions

Leakage of nanoparticle,

bulk nanomaterials needed

for oxidation, and composite

dependency

wastewater

Reverse osmosis and removal of

280 Novel Nanomaterials - Synthesis and Applications

micropollutants, bionanocomposite

membrane utility

**materials**

1 FO)

2

Nanocomposite

Large hydrophilicity, water permeability/flux, thermal/

mechanical robust, fouling resistant

membranes

3

Self-assembled

Homogeneous nanoporosity, tunable designed/tailoring

scales

Pore blockages/chocking,

Ultra-filtrations, filters, cartridges, nanofiber

composite membranes in water treatments

and high-performing direct contact

membrane distillations

Low pressure desalination, biomimetic

membrane for RO and FO filtrations, and

surface imprint and embed membrane

filtrations

leakages of nanofibers

Applied on laboratory/small

Ultra-filtrations, process scale-up.

membranes

4

Nanocarbon

High porous, high permeable, bacteriosidal,

superhydrophobic, tailored electrospun and sustain

high salinity, hydrophobic. Survive filtration under high

Assort molecular transportation, highly selective, tailored

Low pressure desalination,

mechanical weakness

to dense polyfilm, mechanically stable, regenerative/

self-healed

pressure/vacuum

5

Aquaporin

membranes

6

CNT: carbon

Assessable adsorption sites, vast reusability, high cost,

Production cost is very high,

Degradation of antibiotics, organic,

pharmaceutics and own high specific salt

adsorption

Organic and heavy metal removals,

biodegradable, biocompatible, for example,

chitosan/dendrite-based

Water disinfections, ion-exchange beds in

water purification/softening, solar thermal

collector, adsorptive refrigeration

As permeable reactive barriers, sediment

cap, groundwater remediation like PCB and

PAH degradations

own health risks

Dendrimer production steps

are complex and multistage

processes

Reduction in active surface

through immobilization

Stabilization is needed, that is,

surface modifications

nanotubes

7

Dendrimers/

Water-soluble bifunctional (inner hydrophobics and outer

hydrophilic absorptions), encapsulate molecules, reusable,

bioactive mimics, handy toxicity, nontoxic

Highly microporous molecular sieves, lodge variety of

cations, selective molecule sorts at size exclusions, control

ion release, regular molecular porosity

Permeable reactive barrier to filter out contaminants, high

specific surface area, and nZVI high mobility/reactivity

dendrons (arborol

cascade species)

8

Zeolites/

aluminosilicate

9

Nanozerovalent iron

(1–100 nm)

health risk

membranes

Nanofiltration (RO/

**Table 1.** Characteristics of the utility of nanomembrane/materials in water treatment processes. surface tensions, which are utilized for more wetability for hydrophobic foulants [1]. Certain smart nanomembranes, for example, graphene oxide-coupled polyamide, nanosilver-coated polysulfobetaine and polypeptide-grafted single-walled carbon nanotube owing peculiar features viz. bendable size/porosity, high defect density, irreversible bacterial cell adhesion, and antibacterial activity exploited in antimicrobial RO/FO and desalination [1–20].

Thus, nanotechnology improved flux efficiency via ordered pore/size/shape variations and physical barrier/selective charge-base repulsion as requisite for specific/establish emulsion separation/desalination [1–20]. Argonide corporation-USA developed nanoceram interlinked nanofibrous of 2–100 nm diameter and surface area of 300–600 m<sup>2</sup> /g to work as an electropositive filter cartridge [27]. Cellulose polymers tailored on nonwoven glass sheets are utilized for ultra-filtration of dirt, bacteria, viruses, and proteins [18]. Certain nanomaterials like fluorocarbon-coated tetramethyl orthosilicate and polyurethane/polylactic acid/polyethylene oxide-coated bio-films' superior features have exploited in desalination due to its mechanical strength, hydrophobicity, biocompatibility, and nontoxicity [1–19]. Biomimetic membranes akin to a*quaporin* are embedded in polymeric matrix/nanofilters [20] to withstand 10 bar high pressures and huge water flux of >100 L/(hm<sup>2</sup> ) involved in brackish water desalination. Nanotechnology aids in the fabrication of specific aquaporin-based membranes competitive with conventional membranes withstanding under all critical conditions like operating pressures of reverse osmosis, high temperature, acidic/alkaline range, and fouling-based corrosion [1]. Nano-Al<sup>2</sup> O3 /TiO2 zeolite, CNT, photo-catalytic, and nanobimetallic incorporation improve hydrophilicity, raise water permeability, enhance foul resistance, and elevate mechanical and thermal stability which impart high water permeation in RO/FO membrane desalination [1, 20]. Carbon intrude nanopolymeric matrices-based semi-permeable membranes raised the hydrophilicity and increased water permeability/salt permeation as utilized for reverse osmosis [1]. Trimesoyl chloride-metaphenylenediamine interfacial-coated polyethersulfone yields nano-NaX zeolite with 40–150 nm dimensions catering to effective RO/FO membrane [25, 26]. Nano-H<sup>2</sup> O Company-USA has commercialized *Quantum-Flux/WO-2006/098872-A3* matrix owing to more permeable efficiency with low fouling and no clogging for membrane-based reverse osmosis [27]. Such coated matrix membranes maintained a surface profile that carries immobilization of potential harmful nanoparticles, for example, P25-Evonik robust membrane [1, 27]. Smart material-based membranes overcome inherent conventional material limitations and address global challenges of water scarcity besides combating environmental pollution [1–20]. Molecular designing of CNT and aquaporin membranes highlighted surface modification and interfacial interactions with enhanced fouling resistivity as shown in **Figure 3**.

*1.3.8. Ordered mesosilica*

given in **Figure 4**.

Ordered mesosilica (OMS) are designed via eco-friendly synthetic paths to offer controlled pore-size network as explored in industrial applications [28]. Kuroda-Mobil Oil Company in

Rationally Fabricated Nanomaterials for Desalination and Water Purification

http://dx.doi.org/10.5772/intechopen.74738

283

porosity of 2–20 nm via amphiphilic co-polymeric pore/structure directing agent/precursor [1, 28]. Eco-sustainable mesosilica has achieved via greener syntheses from raw feedstock, and thus sol-gel is preferred [28]. Research on various applications of OMS (last decade) is

OMS has an ordered chemical/structural/textural distinctiveness which permits to exhibit pollutant adsorption selectivity in water purifications/desalinations [29]. Conventional petroleum-derived surfactants are replaced by renewable amphiphilic polysaccharides that impart an efficient porosity and a high pore volume in resultant OMS [28, 29]. Hydro-soluble polyionic micelles afford wide-ordered porosity needed for oil-water/emulsion separations as targeted OMS applicability differs by means of porosity and template removal/thermal calcinations/chemical extractions [1, 29]. Calcination temperature vitally shrinks shape/size, porosity, and morphology and affects micro-skeleton of OMS as mentioned in **Table 2**.

OMS manufacturing is expensive due to costly precursor usage like silicon-alkoxide/TEOS, so, it is developed in a more eco-synthetic way for large scale [28, 29]. Environmental impacts are lessened if biomass/recycled waste silica sources are used viz. fly/rice husk's ashes extracted silica. Several templating surfactants are recognized for lyotropic liquid crystalline phase formations. However, classical surfactants are substituted by new hydro-soluble/dissociable and recoverable porogens. The ecodesigned industrial ordered meso-silica synthesis

/g and a controlled

1990 had prepared ordered mesosilica with huge surface areas of >1000 m<sup>2</sup>

**Figure 3.** Molecular-level designed CNT and aquaporin-based membranes.

#### *1.3.7. Multifunctional nanodevices*

Advancement in nanoscience aids in the design of proactive/flexible synchronous and synergistic functionality in desired water treatments as achieved via nanodevice concepts like Fenton nanofiltration and self-floated solar device using well-ordered CNT, nanogold-derived hydrophobic membranes [1, 17]. Polypyrrole-coated stainless steel-based hydrophobic membranes enhance water evaporation than natural solar heating/point-of-use seawater desalination [44]. Molecular simulation theoretically designed smart nanomaterials/devices/systems for best ground-breaking solutions to the existing desalination/purification problems [1, 19].

**Figure 3.** Molecular-level designed CNT and aquaporin-based membranes.

#### *1.3.8. Ordered mesosilica*

surface tensions, which are utilized for more wetability for hydrophobic foulants [1]. Certain smart nanomembranes, for example, graphene oxide-coupled polyamide, nanosilver-coated polysulfobetaine and polypeptide-grafted single-walled carbon nanotube owing peculiar features viz. bendable size/porosity, high defect density, irreversible bacterial cell adhesion, and

Thus, nanotechnology improved flux efficiency via ordered pore/size/shape variations and physical barrier/selective charge-base repulsion as requisite for specific/establish emulsion separation/desalination [1–20]. Argonide corporation-USA developed nanoceram interlinked

positive filter cartridge [27]. Cellulose polymers tailored on nonwoven glass sheets are utilized for ultra-filtration of dirt, bacteria, viruses, and proteins [18]. Certain nanomaterials like fluorocarbon-coated tetramethyl orthosilicate and polyurethane/polylactic acid/polyethylene oxide-coated bio-films' superior features have exploited in desalination due to its mechanical strength, hydrophobicity, biocompatibility, and nontoxicity [1–19]. Biomimetic membranes akin to a*quaporin* are embedded in polymeric matrix/nanofilters [20] to withstand 10 bar

Nanotechnology aids in the fabrication of specific aquaporin-based membranes competitive with conventional membranes withstanding under all critical conditions like operating pressures of reverse osmosis, high temperature, acidic/alkaline range, and fouling-based corrosion

hydrophilicity, raise water permeability, enhance foul resistance, and elevate mechanical and thermal stability which impart high water permeation in RO/FO membrane desalination [1, 20]. Carbon intrude nanopolymeric matrices-based semi-permeable membranes raised the hydrophilicity and increased water permeability/salt permeation as utilized for reverse osmosis [1]. Trimesoyl chloride-metaphenylenediamine interfacial-coated polyethersulfone yields nano-NaX zeolite with 40–150 nm dimensions catering to effective RO/FO membrane [25, 26].

owing to more permeable efficiency with low fouling and no clogging for membrane-based reverse osmosis [27]. Such coated matrix membranes maintained a surface profile that carries immobilization of potential harmful nanoparticles, for example, P25-Evonik robust membrane [1, 27]. Smart material-based membranes overcome inherent conventional material limitations and address global challenges of water scarcity besides combating environmental pollution [1–20]. Molecular designing of CNT and aquaporin membranes highlighted surface modification and interfacial interactions with enhanced fouling resistivity as shown in **Figure 3**.

Advancement in nanoscience aids in the design of proactive/flexible synchronous and synergistic functionality in desired water treatments as achieved via nanodevice concepts like Fenton nanofiltration and self-floated solar device using well-ordered CNT, nanogold-derived hydrophobic membranes [1, 17]. Polypyrrole-coated stainless steel-based hydrophobic membranes enhance water evaporation than natural solar heating/point-of-use seawater desalination [44]. Molecular simulation theoretically designed smart nanomaterials/devices/systems for best

ground-breaking solutions to the existing desalination/purification problems [1, 19].

O Company-USA has commercialized *Quantum-Flux/WO-2006/098872-A3* matrix

zeolite, CNT, photo-catalytic, and nanobimetallic incorporation improve

/g to work as an electro-

) involved in brackish water desalination.

antibacterial activity exploited in antimicrobial RO/FO and desalination [1–20].

nanofibrous of 2–100 nm diameter and surface area of 300–600 m<sup>2</sup>

high pressures and huge water flux of >100 L/(hm<sup>2</sup>

[1]. Nano-Al<sup>2</sup>

Nano-H<sup>2</sup>

O3 /TiO2

282 Novel Nanomaterials - Synthesis and Applications

*1.3.7. Multifunctional nanodevices*

Ordered mesosilica (OMS) are designed via eco-friendly synthetic paths to offer controlled pore-size network as explored in industrial applications [28]. Kuroda-Mobil Oil Company in 1990 had prepared ordered mesosilica with huge surface areas of >1000 m<sup>2</sup> /g and a controlled porosity of 2–20 nm via amphiphilic co-polymeric pore/structure directing agent/precursor [1, 28]. Eco-sustainable mesosilica has achieved via greener syntheses from raw feedstock, and thus sol-gel is preferred [28]. Research on various applications of OMS (last decade) is given in **Figure 4**.

OMS has an ordered chemical/structural/textural distinctiveness which permits to exhibit pollutant adsorption selectivity in water purifications/desalinations [29]. Conventional petroleum-derived surfactants are replaced by renewable amphiphilic polysaccharides that impart an efficient porosity and a high pore volume in resultant OMS [28, 29]. Hydro-soluble polyionic micelles afford wide-ordered porosity needed for oil-water/emulsion separations as targeted OMS applicability differs by means of porosity and template removal/thermal calcinations/chemical extractions [1, 29]. Calcination temperature vitally shrinks shape/size, porosity, and morphology and affects micro-skeleton of OMS as mentioned in **Table 2**.

OMS manufacturing is expensive due to costly precursor usage like silicon-alkoxide/TEOS, so, it is developed in a more eco-synthetic way for large scale [28, 29]. Environmental impacts are lessened if biomass/recycled waste silica sources are used viz. fly/rice husk's ashes extracted silica. Several templating surfactants are recognized for lyotropic liquid crystalline phase formations. However, classical surfactants are substituted by new hydro-soluble/dissociable and recoverable porogens. The ecodesigned industrial ordered meso-silica synthesis

**Nomenclature Surfactant used Structure E-factor [30]**

(with *n* = 12, 14, 16 or 18

(with *n* = 12, 14, 16 or 18 and

H2*<sup>n</sup>*

H2*n*+1N+

P123 and F127, respectively *P*6*mm*, hexagonal and *Im*3*m*,

P123 *Ia*3*d*, cubic 25/4–6.2

P123 *P*6*m*, hexagonal —

<sup>d</sup>COK-12 in citrate/citric acid surfactant, E-factors are found to be lower than nanomaterials and bulk chemicals.

Copious economical amicable, mesoporous materials without the need of organic surfactants rather than metal cation impurities, residual natural lignins

Silicon alkoxides, uniform oligomers impart highly organized mesostructure at any pH, facile and cheap protocol, water as

are helpful

solvent

*P*6*mm*, hexagonal for

Rationally Fabricated Nanomaterials for Desalination and Water Purification

41.9/21–2.0 and 45.3/1 = 45.3

285

29.9/12

16/2.3–7.0

and *Ia*3*d*, cubic MCM-48

Wormhole framework

cubic, respectively,

*Fm*3*m,* cubic —

Polycondensation is uncontrolled under neutral/acidic conditions, strong acids and high temperatures

High energy input and expensive procedures, toxic precursors, needs catalysts and organic solvents only, byproduct alcohol during hydrolysis. Silicate oligomers with varied degrees of polymerization, polycondensation is uncontrolled under neutral/acidic

needed purification

conditions

*P*6*mm*, hexagonal 31.9/10

http://dx.doi.org/10.5772/intechopen.74738

MCM-41

structure

Alkyltrimethyl ammonium salt C*<sup>n</sup>*

Alkyltrimethyl ammonium C*<sup>n</sup>*

Uncharged amine surfactant

Polyethylene oxide-polybutylene oxide-polyethylene oxide tri-block copolymer B50-6600 (EO39BO47EO39,

**Table 3.** Ordered mesoporous materials, used surfactant, and their crystallographic structure.

**Origin Type Advantages Drawbacks**

+1N+ (CH<sup>3</sup> )3 X−

(CH<sup>3</sup> )3 X−

C*n* H2*n*+1NH<sup>2</sup>

Dow)

FSM-16 yield from layered silicate kanemite

Natural Natural clays, diatomite/

zeolite

Synthetic Silicon alkoxides Si(OR)4, for

fumed silica

KIT-6 by tri-block copolymer (EO20PO70EO20)-butanol.

kieselgur, siliceous sedimentary rock and minerals, besides

example, TEOS, TMOS, besides soluble silicates, for example, sodium silicate, colloidal and

X = X or Br)

and X = Cl or Br)

MCM-41/48: mobile composition of matter

Folder sheet mesoporous

Hexagonal mesoporous

Santa Barbara amorphous

Korea Advanced Institute of Science and Technology

FSM-16<sup>a</sup>

HMS

silica

KIT-6<sup>b</sup>

FDU-1<sup>c</sup>

COK-12<sup>d</sup> Centrum voor Oppervlaktechemie &

Katalyse

a

b

c

FuDan University

FDU-1 in NaCl salt.

SBA-15 and 16

**Figure 4.** Research publications on various field applications of mesoporous silica (last decade).


**Table 2.** Shape, size, pore-order morphology, and microskeletons in mesosilica.

which uses assisted self-assembly path for removal of template own profound impacts on surface, chemical and textural properties of OMS and thus affects their synthetic applications [29]. Calcination and solvent extraction eliminate organic templates during synthesis, and numerous template removals are studied to lessen process time and solvent usage. Assorted varied surfactants vitally fix sustainable synthetic paths as shown with waste generation (E-factor = kg waste/kg product) in **Table 3**.

These developed sustainable OMS syntheses rely on the choice of silica precursor, environmental impact, byproduct, and cost [29]. Green chemistry synthesis aspects escort silica from natural mineral deposits, biomass, or industrial wastes, and constitute huge resources that are commercially compared in **Table 4**.


a FSM-16 yield from layered silicate kanemite

b KIT-6 by tri-block copolymer (EO20PO70EO20)-butanol.

c FDU-1 in NaCl salt.

which uses assisted self-assembly path for removal of template own profound impacts on surface, chemical and textural properties of OMS and thus affects their synthetic applications [29]. Calcination and solvent extraction eliminate organic templates during synthesis, and numerous template removals are studied to lessen process time and solvent usage. Assorted varied surfactants vitally fix sustainable synthetic paths as shown with waste generation

**Figure 4.** Research publications on various field applications of mesoporous silica (last decade).

**Silica solids Particle shape Size dimensions Pore-size ordering**

DDAB-MSN Irregular 80–100 nm Ink-bottle porosity TDTHP-NS Spherical 100–500 nm Microporosity

MTAB-MSN Spherical 50–100 nm Low-stretched mesoporous channels CTAB-MSN Spherical 70–100 nm Well-ordered mesoporous channels

50–75 nm width

Well-ordered mesoporous channels

These developed sustainable OMS syntheses rely on the choice of silica precursor, environmental impact, byproduct, and cost [29]. Green chemistry synthesis aspects escort silica from natural mineral deposits, biomass, or industrial wastes, and constitute huge resources that are

(E-factor = kg waste/kg product) in **Table 3**.

STAB-MSN Rod-like 100–500 nm length

284 Novel Nanomaterials - Synthesis and Applications

**Table 2.** Shape, size, pore-order morphology, and microskeletons in mesosilica.

commercially compared in **Table 4**.

<sup>d</sup>COK-12 in citrate/citric acid surfactant, E-factors are found to be lower than nanomaterials and bulk chemicals.

**Table 3.** Ordered mesoporous materials, used surfactant, and their crystallographic structure.



anomalous liquid fast permeation into capillary high pressure created inside inter-layers

Oil-water separations are required if fuel spillage especially gasoline/diesel/petrol during transportation failures releases oil in aqueous systems to pose environmental hazardous, and thus emulsion separations are highly sought [1, 19]. Interface science and bionic knowledge have developed some 2-D membranes via surface micronanohierarchical structure grafting so as to impart unique/super-wetting characters viz. superhydrophobicity, superoleophobicity, and superamphiphilicity to be utilized for oil-water separation [1, 15]. Bio-inspired spatial hierarchical polytetrafluoroethylene-coated stainless steel mesh has self-cleaned superhydrophobic surfaces that impart a water contact angle of >150° and a diesel contact angle of ∼0° and responsibly perform excellent oil-water separations [15]. Such hydrogel-coated mesh is superior due to extraordinary water passage selectivity over oil, thus preventing oil-material contacts and avoiding membrane clogging caused by viscous oils besides allowing gravity-driven separation. Surface chemistry designed membranes like polyvinylpyridine-polydimethylsi-

loxane-polyurethane sponges, grafted polyacrylic acid, and sodium silicate-TiO2

and toggled superoleophilicity as benefited for emulsion/oil-water separation [15].

**1.5. Futuristic nanomaterials for desalination**

by future scientists.

steel mesh have irreversible encapsulation of low-surface-energy species, flexible wet-ability,

Advanced nanotechnologically designed/engineered nanoadsorbents, nanometals, nanomembranes, and photo-catalysts have vulnerable flexibility and adjustability with water treatment systems encompassing assorted micro-pollutants [1]. The compatible existing water treatment processes can be integrated simply in conventional modules. Nanomaterials are advantageous due to their ability to be integrated to various multifunctional membranes that enable both particle retention and contaminant mitigations compared to conventional materials used in water technologies [20]. Auxiliary usages of nanomaterials impart higher process efficiency and higher sorption rates. However, in order to minimize the health risk of nanomaterial's usage in water treatments, several regulatory norms need to be prepared for being adaptable to mass/large-scale utility. Still, nanostructured materials have offered potential innovations in serious contaminants degradation, decentralized water treatments, and point-of-use devices [1–20]. Nanotechnology assists in wastewater treatments for the mitigation of pathogens, organic and inorganic, heavy metals, and other toxic contaminants using nano-ZnO RO/FO nanofilms and polyrhodanine-encapsulated magnetic nanoparticle that are removed by contaminants up to ppb level [19]. There emerge innovative technologies in nanoscience; yet, many challenges posed by water purification need to be resolved

. Such ultrafast transportation via precise casing imparts single/double-layered graphene as enthusiastic representative in desalination [18] which has 100% LiCl, NaCl, and KCl salt rejections than traditional membranes [17–19]. Akin, two-dimensional inorganic matrixes like MXenes possess atomic-layered skeletons with controllable compositions as potential nanofiltration membranes. Nanofiltration assembly/devices were developed with specified and

, CNT, and noble-metals and Fenton agents [1, 19].

Rationally Fabricated Nanomaterials for Desalination and Water Purification

, K<sup>+</sup>

http://dx.doi.org/10.5772/intechopen.74738

, Mg2+, and

287

stainless

and used for seawater desalinations as it contains varied ions including Na+

Cl<sup>−</sup>

smartly designed nano-TiO2

**Table 4.** Comparison of different silica sources for the synthesis of silica-based mesoporous material.

Eco-designed OMS synthesis focused on assisted self-assembly involving assorted [28, 29] treatments via precipitation, liquid phase reactions, solid recovery, washing, and drying as shown in **Figure 5**.

#### **1.4. Major smart materials/membranes for desalination and oil-water/emulsion separations**

Nanomaterial-aided modus operandi have simulated membranes especially designed onto advanced nanostructures like CNT, graphene, graphene oxide and reduced graphene oxide for desalination [1, 19]. Graphene/graphene oxide/rGO showed unique performance and promising direction in futuristic water treatment/desalination [1–20]. Such stimulated membrane penetrates water via nanometer pores in single-layer graphene and offers salt nanofiltration with enhanced permeability than conventional RO [17, 19]. Graphene oxide membrane exhibits

**Figure 5.** Steps involved in precipitation and assisted self-assembly routes for OMS synthesis.

anomalous liquid fast permeation into capillary high pressure created inside inter-layers and used for seawater desalinations as it contains varied ions including Na+ , K<sup>+</sup> , Mg2+, and Cl<sup>−</sup> . Such ultrafast transportation via precise casing imparts single/double-layered graphene as enthusiastic representative in desalination [18] which has 100% LiCl, NaCl, and KCl salt rejections than traditional membranes [17–19]. Akin, two-dimensional inorganic matrixes like MXenes possess atomic-layered skeletons with controllable compositions as potential nanofiltration membranes. Nanofiltration assembly/devices were developed with specified and smartly designed nano-TiO2 , CNT, and noble-metals and Fenton agents [1, 19].

Oil-water separations are required if fuel spillage especially gasoline/diesel/petrol during transportation failures releases oil in aqueous systems to pose environmental hazardous, and thus emulsion separations are highly sought [1, 19]. Interface science and bionic knowledge have developed some 2-D membranes via surface micronanohierarchical structure grafting so as to impart unique/super-wetting characters viz. superhydrophobicity, superoleophobicity, and superamphiphilicity to be utilized for oil-water separation [1, 15]. Bio-inspired spatial hierarchical polytetrafluoroethylene-coated stainless steel mesh has self-cleaned superhydrophobic surfaces that impart a water contact angle of >150° and a diesel contact angle of ∼0° and responsibly perform excellent oil-water separations [15]. Such hydrogel-coated mesh is superior due to extraordinary water passage selectivity over oil, thus preventing oil-material contacts and avoiding membrane clogging caused by viscous oils besides allowing gravity-driven separation. Surface chemistry designed membranes like polyvinylpyridine-polydimethylsiloxane-polyurethane sponges, grafted polyacrylic acid, and sodium silicate-TiO2 stainless steel mesh have irreversible encapsulation of low-surface-energy species, flexible wet-ability, and toggled superoleophilicity as benefited for emulsion/oil-water separation [15].

#### **1.5. Futuristic nanomaterials for desalination**

Eco-designed OMS synthesis focused on assisted self-assembly involving assorted [28, 29] treatments via precipitation, liquid phase reactions, solid recovery, washing, and drying as

solutions

Plentiful cheap and nontoxic, acidity conferred by residual metal ions within matrix besides waste disposal

Has lower surface area and pore volume than other precursors, porous silica material regenerations as hard templates used for nanocasting

Nanomaterial-aided modus operandi have simulated membranes especially designed onto advanced nanostructures like CNT, graphene, graphene oxide and reduced graphene oxide for desalination [1, 19]. Graphene/graphene oxide/rGO showed unique performance and promising direction in futuristic water treatment/desalination [1–20]. Such stimulated membrane penetrates water via nanometer pores in single-layer graphene and offers salt nanofiltration with enhanced permeability than conventional RO [17, 19]. Graphene oxide membrane exhibits

**1.4. Major smart materials/membranes for desalination and oil-water/emulsion** 

**Figure 5.** Steps involved in precipitation and assisted self-assembly routes for OMS synthesis.

**Table 4.** Comparison of different silica sources for the synthesis of silica-based mesoporous material.

**Origin Type Advantages Drawbacks**

Industrial ashes, for example, coal, rice husk, and packaging resin waste, glasswares

286 Novel Nanomaterials - Synthesis and Applications

shown in **Figure 5**.

**separations**

Recycling wastes

> Advanced nanotechnologically designed/engineered nanoadsorbents, nanometals, nanomembranes, and photo-catalysts have vulnerable flexibility and adjustability with water treatment systems encompassing assorted micro-pollutants [1]. The compatible existing water treatment processes can be integrated simply in conventional modules. Nanomaterials are advantageous due to their ability to be integrated to various multifunctional membranes that enable both particle retention and contaminant mitigations compared to conventional materials used in water technologies [20]. Auxiliary usages of nanomaterials impart higher process efficiency and higher sorption rates. However, in order to minimize the health risk of nanomaterial's usage in water treatments, several regulatory norms need to be prepared for being adaptable to mass/large-scale utility. Still, nanostructured materials have offered potential innovations in serious contaminants degradation, decentralized water treatments, and point-of-use devices [1–20]. Nanotechnology assists in wastewater treatments for the mitigation of pathogens, organic and inorganic, heavy metals, and other toxic contaminants using nano-ZnO RO/FO nanofilms and polyrhodanine-encapsulated magnetic nanoparticle that are removed by contaminants up to ppb level [19]. There emerge innovative technologies in nanoscience; yet, many challenges posed by water purification need to be resolved by future scientists.

#### **1.6. Conclusions**

Rationally designed smart nanomaterials provide myriad scientific and technology growth to desalination/water purification. Still, biomaterials must be explored in this perspective that owes high permeation and low salt rejections in futuristic desalination. Thus, advanced nanotechnology aided commercially viable products/solutions that enhance/replace existing desalination/purification. Certain functionalized nanoporous biopolymeric membranes were found to cater to inherent challenges. Bio-polymer cross-linking fixes usual instability and imposes functionally cost-effective nanoporous biomaterials to be used in desalinations. Overall, rational fabrication highlighted "*design-for-purpose*" unlike *trial-and-error approach* starts with scientific perceptions by knowing inherent barriers, and thus the conceptual design of nanomaterial is proposed, as fed back to desalination problems for thorough usage in water treatments. Nanomaterials' performance must unambiguously be defined in water purifications and need redesigning under failure conditions with "thinking-outside-the-box" prospective as confronted by desalination. Till now, nanomaterials' rational designing offered more unprecedented opportunities for solving desalination challenges in a sustainable manner. A prospective-ordered designing should owe few aspects namely molecular dynamics/ simulation tools to extend problem definition and theoretically needs more multi-functional/ all-in-one nanomaterial for effective desalination/water purifications. Therefore, progressive and versatile nanomaterials/devices, which can work under ambient conditions with paramount desalinations/water purification performance, are expected in the near future. Such design-sophisticated material surfaces reversibly counter stimuli via innovative and promising exterior/interior changes and eminent environmental adaptability which displayed myriad functions viz. interfacial pollutant adsorption, omniphobic slippery coatings, responsive particle-stabilized emulsions, and self-healed surface membranes. This chapter described the strategic and valuable rational-designing idea for assorted biomimetic membranes/materials owing to environmental utilities.

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#### **Acknowledgements**

The author is thankful to the Head, Department of Chemistry, R.T.M. Nagpur University, Nagpur, for laboratory facilities and to the Vice Chancellor, Nagpur University, Nagpur, for the sanction of a research project under University Research Project Scheme, No. Dev./ RTMNURP/AH/1672 (9), dated 24 September 2016.

#### **Author details**

Rajendra S. Dongre

Address all correspondence to: rsdongre@hotmail.com

Department of Chemistry, R.T.M. Nagpur University, Nagpur, Maharashtra, India

### **References**

**1.6. Conclusions**

288 Novel Nanomaterials - Synthesis and Applications

owing to environmental utilities.

RTMNURP/AH/1672 (9), dated 24 September 2016.

Address all correspondence to: rsdongre@hotmail.com

**Acknowledgements**

**Author details**

Rajendra S. Dongre

Rationally designed smart nanomaterials provide myriad scientific and technology growth to desalination/water purification. Still, biomaterials must be explored in this perspective that owes high permeation and low salt rejections in futuristic desalination. Thus, advanced nanotechnology aided commercially viable products/solutions that enhance/replace existing desalination/purification. Certain functionalized nanoporous biopolymeric membranes were found to cater to inherent challenges. Bio-polymer cross-linking fixes usual instability and imposes functionally cost-effective nanoporous biomaterials to be used in desalinations. Overall, rational fabrication highlighted "*design-for-purpose*" unlike *trial-and-error approach* starts with scientific perceptions by knowing inherent barriers, and thus the conceptual design of nanomaterial is proposed, as fed back to desalination problems for thorough usage in water treatments. Nanomaterials' performance must unambiguously be defined in water purifications and need redesigning under failure conditions with "thinking-outside-the-box" prospective as confronted by desalination. Till now, nanomaterials' rational designing offered more unprecedented opportunities for solving desalination challenges in a sustainable manner. A prospective-ordered designing should owe few aspects namely molecular dynamics/ simulation tools to extend problem definition and theoretically needs more multi-functional/ all-in-one nanomaterial for effective desalination/water purifications. Therefore, progressive and versatile nanomaterials/devices, which can work under ambient conditions with paramount desalinations/water purification performance, are expected in the near future. Such design-sophisticated material surfaces reversibly counter stimuli via innovative and promising exterior/interior changes and eminent environmental adaptability which displayed myriad functions viz. interfacial pollutant adsorption, omniphobic slippery coatings, responsive particle-stabilized emulsions, and self-healed surface membranes. This chapter described the strategic and valuable rational-designing idea for assorted biomimetic membranes/materials

The author is thankful to the Head, Department of Chemistry, R.T.M. Nagpur University, Nagpur, for laboratory facilities and to the Vice Chancellor, Nagpur University, Nagpur, for the sanction of a research project under University Research Project Scheme, No. Dev./

Department of Chemistry, R.T.M. Nagpur University, Nagpur, Maharashtra, India


[16] Lee A, Elam JW, Darling SB. Membrane materials for water purification: Design, development, and application. Environmental Science: Water Research & Technology. 2016; *2*:17-42

**Chapter 16**

Provisional chapter

**Noble Metal-Based Nanocomposites for Fuel Cells**

DOI: 10.5772/intechopen.71949

Noble metal-based nanocomposites are attractive for a rich variety of electrocatalytic applications as they can exhibit not only a combination of the properties associated with each component but also synergy due to a strong coupling between different constituents. Using noble metal as the base component, a plenty of methods have been recently demonstrated for the synthesis of noble metal-based nanocomposites with novel structures (e.g., alloys, core-shell, skin and 1D/2D structures). In this chapter, an account of recent advances of synthetic approaches to noble metal-based nanocomposites with controlled structures, compositions and sizes are reviewed. The relationship between structures and electrochemical properties of these nanocomposites in fuel cell field is discussed. The potential future directions of research in the field are also addressed.

With the global rapid increase of energy demand and the depletion of fossil fuels, research on environment-friendly energy sources has attracted considerable attention in recent years. The real commercialization of fuel cells is a promising solution for the global problems of energy supply and clean environment. A fuel cell can convert chemical energy into electric energy by an electrochemical reaction of hydrogen-containing fuel with oxidant. Based on the electrolyte type, fuel cells are classified to be several kinds: proton exchange membrane fuel cells (PEMFCs), phosphoric acid fuel cells (PAFCs), solid acid fuel cells (SAFCs), alkaline fuel cells (AFCs) and high-temperature fuel cells. Among all kinds of fuel cells, only hydrogen PEMFC has been used in commercial vehicles (the Toyota Mirai) from 2014, due to its short start-up time, high-energy density and low working temperature. Thus, we will focus on PEMFCs in this chapter. Besides hydrogen, other fuels which are suitable for PEMFCs include alcohols (methanol, ethanol, glycol etc.) and formic acid. Compared with hydrogen, they have lower

> © The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

Keywords: noble metal, nanocomposites, electrochemical, fuel cell

Noble Metal-Based Nanocomposites for Fuel Cells

Hongpan Rong, Shuping Zhang, Sajid Muhammad and Jiatao Zhang

Hongpan Rong, Shuping Zhang, Sajid Muhammad and Jiatao Zhang

http://dx.doi.org/10.5772/intechopen.71949

Abstract

1. Introduction

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter


#### **Noble Metal-Based Nanocomposites for Fuel Cells** Noble Metal-Based Nanocomposites for Fuel Cells

DOI: 10.5772/intechopen.71949

Hongpan Rong, Shuping Zhang, Sajid Muhammad and Jiatao Zhang Hongpan Rong, Shuping Zhang, Sajid Muhammad and Jiatao Zhang

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.71949

#### Abstract

[16] Lee A, Elam JW, Darling SB. Membrane materials for water purification: Design, development, and application. Environmental Science: Water Research & Technology. 2016;

[17] Lalia BS, Kochkodan V, Hashaikeh R, Hilal N. A review on membrane fabrication: Structure, properties and performance relationship. Desalination. 2013;**326**:77-95

[18] Dervin S, Dionysiou DD, Pillai SC. 2D nanostructures for water purification: Graphene

[19] Wang Z, Ciacchi LC, Wei G. Recent advances in nanoporous membranes for water puri-

[20] Davis ME. Ordered porous materials emerging application. Nature. 2002;**417**:813-821

solutions PAMAM dendrimer with ethylenediamine core-terminal NH<sup>2</sup>

minerals. Journal of Cleaner Production. 2012;**29-30**:208-213

[21] Diallo MS, Goddard WA. Dendrimer enhanced ultrafiltration. Recovery of Cu(II) from

[22] Aredes S, Klein B, Pawlik M. The removal of arsenic from water using natural iron oxide

[23] Torrey JD et al. Oxidation behavior of zero-valent iron nanoparticles in mixed matrix water purification membranes. Environmental Science: Water Research & Technology.

[24] Lin SC, Buehler MJ. Mechanics and molecular filtration performance of graphyne nanoweb membranes for selective water purification. Nanoscale. 2013;**5**:11801

[25] Humplik T, Wang EN. Framework water capacity & infiltration pressure of MFI Zeolites.

[26] Gehrke I, Geiser A, Somborn-Schulz A. Nano-techn innovations for water treatment. Nanotechnology, Science Applications. 2015;**8**:1-17. DOI: 10.2147/NSA.S43773

[27] Porada S et al. Direct prediction of the desalination performance of porous carbon electrodes for capacitive deionization. Energy & Environmental Science. 2013;**6**:3700

[28] Corine G'r, Reboul J, Bonneb M, Lebeau B'n'd. Ecodesign of ordered mesoporous silica

[29] Fernandes FM, Coradin T, Aimé C. Self-assembly in biosilicification and biotemplated

[30] Sheldon RA. E factors, green chemistry & catalysis: Odyssey. Chemical Communications.

. Environmental

and beyond. Nanoscale. 2016;**8**:15115-15131

fication: Review. Nanomaterials. 2018;**8**(2):65

Science & Technology. 2005;**39**:S1366-S1377

2015;**1**(2):146-152. DOI: 10.1039/c4ew00068d

Microporous & Mesoporous Materials. 2014;**190**:84-91

materials. Chemical Society Reviews. 2013;**42**:4217

2008;(9):3352-3365. DOI: 10.1039/B803584A

silica materials: Review. Nanomaterials. 2014;**4**:792-812

*2*:17-42

290 Novel Nanomaterials - Synthesis and Applications

Noble metal-based nanocomposites are attractive for a rich variety of electrocatalytic applications as they can exhibit not only a combination of the properties associated with each component but also synergy due to a strong coupling between different constituents. Using noble metal as the base component, a plenty of methods have been recently demonstrated for the synthesis of noble metal-based nanocomposites with novel structures (e.g., alloys, core-shell, skin and 1D/2D structures). In this chapter, an account of recent advances of synthetic approaches to noble metal-based nanocomposites with controlled structures, compositions and sizes are reviewed. The relationship between structures and electrochemical properties of these nanocomposites in fuel cell field is discussed. The potential future directions of research in the field are also addressed.

Keywords: noble metal, nanocomposites, electrochemical, fuel cell

#### 1. Introduction

With the global rapid increase of energy demand and the depletion of fossil fuels, research on environment-friendly energy sources has attracted considerable attention in recent years. The real commercialization of fuel cells is a promising solution for the global problems of energy supply and clean environment. A fuel cell can convert chemical energy into electric energy by an electrochemical reaction of hydrogen-containing fuel with oxidant. Based on the electrolyte type, fuel cells are classified to be several kinds: proton exchange membrane fuel cells (PEMFCs), phosphoric acid fuel cells (PAFCs), solid acid fuel cells (SAFCs), alkaline fuel cells (AFCs) and high-temperature fuel cells. Among all kinds of fuel cells, only hydrogen PEMFC has been used in commercial vehicles (the Toyota Mirai) from 2014, due to its short start-up time, high-energy density and low working temperature. Thus, we will focus on PEMFCs in this chapter. Besides hydrogen, other fuels which are suitable for PEMFCs include alcohols (methanol, ethanol, glycol etc.) and formic acid. Compared with hydrogen, they have lower

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

energy density [1]. But they are cheap, plentiful, easily stored and transported. Therefore, direct methanol fuel cell (DMFC), direct ethanol fuel cell (DEFC), and direct formic acid fuel cell (DFAFC) are also promising in commercial applications.

strategies are established in colloidal reaction system, such as facet-selective capping (CO [38, 39], halide anions [40], amines [9], formaldehyde [30, 41], etc.), seed growth [42, 43] and

Noble Metal-Based Nanocomposites for Fuel Cells http://dx.doi.org/10.5772/intechopen.71949 293

In this chapter, we focus on the key factors behind the novel synthesis strategies and provide the discussions along the topic of composition. The very recent achievements of the controllable synthesis of noble metal-based nanocomposites are given and discussed on the structure-

At present, owing to the outstanding catalytic activity and superior resistant characteristics to corrosion, Pt-based nanoparticles (NPs) supported on porous carbon are still the most efficient catalysts in PEMFCs [15, 16, 18, 27]. In the past several years, to develop advanced Pt-based nanocatalysts for electrochemical reaction, researchers have proposed several synthesis strategies, which mainly include (1) alloying, (2) core-shell, (3) Pt-skin, (4) 1D or 2D structures and (5) porous, caged, hollow and frame structures. The integration of two or more strategies into one kind of Pt-based nanocomposites is beneficial for the construction of super-electrocatalysts

In the past decade, Pt-based nanostructures have been widely used to prepare excellent catalysts for diverse chemical reactions [45–47]. Compared with pure Pt nanocatalysts, Ptbased alloy and/or intermetallics exhibit higher activity and stability [48]. As since the surface electronic structure and d-band center position of Pt were changed by incorporating a second and/or third metal into Pt lattice. Owing to higher oxidation potentials of transition metals than Pt, they are always removed from the surface of Pt-based alloy by electrochemical leaching. A pure Pt shell with a thickness of more than several atomic layers is produced after the dealloying process [49]. Although the dealloyed Pt-based nanocatalysts show a higher electrochemical activity compared with Pt/C [50], they perform a poor long-term durability

For bimetallic and multi-metallic Pt-based nanocomposites, the exposed crystal facet has a close relationship with their electrocatalytic performance. Tian et al. and Stamenkovic et al. have demonstrated that the ORR activities on Pt decrease in the order of high-index facet (hkl) > (111) ≫ (100) in HClO4 solution [52]. Well-defined shapes of Pt-based nanocrystals (NCs) are, thus, of great interest for boosting electrochemical performance. It is believed that the surface energy of a fcc Pt NCs exposed with different facets increases in an order of (111) < (100) < (110) < (hkl) [53]. Accordingly, high-energy crystal facets and surface defects vanish quickly during the NC growth process due to the adatom incorporation on them. Therefore, it is a challenge to prepare NCs with high-index facets and high density of surface defects. Compared with alloy, intermetallic compounds show outstanding structure stability owing to the ordered atom arrangements. Rong et al. developed a kinetically controlled

since the re-deposition of leached transition metal onto the anode [51].

oxidative etching [44].

property relations in electrochemical reactions.

with excellent catalytic activity and stability.

2.1. Pt-based alloys and intermetallics

2. Pt-based nanocomposites

At the anode of a fuel cell, hydrogen-containing fuel is oxidized to produce electrons that are transferred to the cathode through an external circuit. At the cathode, oxygen is reduced to be water. Thus, catalysts are essential at both electrodes to promote the fuel oxidation reaction and oxygen reduction reaction (ORR). Most fuel oxidation and oxygen reduction reactions catalyzed by noble metal-based nanocomposites are performed in acidic solutions. The reason contains two aspects: (1) for DMFCs, DEFCs and DFAFCs, carbonates can form in alkaline electrolyte [1]; (2) hydrogen oxidation reaction on platinum (Pt) in acid is 2 orders of magnitude quicker than in alkaline electrolytes [2]. When the fuel is hydrogen, the hydrogen oxidation reaction (HOR) rate on Pt is extremely fast, and 0.05 mg cm�<sup>2</sup> Pt loading at the anode is enough. However, to achieve a desirable catalytic performance, the slow reaction rate of ORR at the cathode even on the best Pt-based catalyst needs much more Pt loading amount (~0.4 mg cm�<sup>2</sup> ) [3]. According to the data reported by Vesborg in 2012, Pt is one of the most expensive metals [4]. The large-scale application of fuel cell technology in transportation field or portable power generation is limited by the high cost of electrode catalysts. Numerous researches have been undertaken to improve the intrinsic activity of electrocatalysts to reduce the noble metal loading of the electrodes without compromising fuel cell performance.

The catalytic activity, selectivity and stability of noble metal-based nanocomposites are closely related to the factors including size, shape/morphology and composition [5–8]. In the past decade, various experimental methods have been developed to synthesize sizedependent, high catalytic performance noble metal-based nanocomposites with diverse morphologies, such as polyhedron, concave, wire, plate, belt/ribbon, dendrite/branch and cage/frame structures [9–21]. Smaller size means higher surface-area-to-volume ratio, higher atomic utilization efficiency and more catalytic active sites. During electrochemical process, small-size noble metal-based nanocomposites provide high electrocatalytic activity, but the challenge is the aggregation that could happen under electrocatalytic conditions, resulting in the poor performance of long-term stability test (e.g., commercial Pt/C). Alloying one or two kinds of transition metals with noble metal has become a valid strategy to develop excellent electrocatalysts [22–25]. The addition of the second/third metal not only changes the surface active sites by ensemble effect, but also alters the binding strength of reactants, intermediates, and products by electronic/strain effect [26–28]. Different noble metals polyhedrons/ concave expose atoms with different coordination numbers, thus have different surface energy and exhibit different catalytic performances [29–31]. 1D (wire) and 2D (plate, belt/ ribbon) noble metal-based nanostructures have high surface area (high atom utilization efficiency), high conductivity and large interfacial area contacting with the support in electrochemical reactions [12, 15, 32]. Dendrite and branch structures not only exhibit large surface area and high active sites, but also significantly relieve the aggregation happened in electrocatalytic stability test [9, 33, 34]. Highly open noble metal nanocages and nanoframes exhibit enhanced electrocatalytic properties due to their three-dimensional accessible surface atoms [20, 35, 36]. A nanosegregated noble metal skin on these open structures can further improve the catalytic performance [18, 37]. Thus, numerous shape-controlled synthetic

strategies are established in colloidal reaction system, such as facet-selective capping (CO [38, 39], halide anions [40], amines [9], formaldehyde [30, 41], etc.), seed growth [42, 43] and oxidative etching [44].

In this chapter, we focus on the key factors behind the novel synthesis strategies and provide the discussions along the topic of composition. The very recent achievements of the controllable synthesis of noble metal-based nanocomposites are given and discussed on the structureproperty relations in electrochemical reactions.

### 2. Pt-based nanocomposites

energy density [1]. But they are cheap, plentiful, easily stored and transported. Therefore, direct methanol fuel cell (DMFC), direct ethanol fuel cell (DEFC), and direct formic acid fuel

At the anode of a fuel cell, hydrogen-containing fuel is oxidized to produce electrons that are transferred to the cathode through an external circuit. At the cathode, oxygen is reduced to be water. Thus, catalysts are essential at both electrodes to promote the fuel oxidation reaction and oxygen reduction reaction (ORR). Most fuel oxidation and oxygen reduction reactions catalyzed by noble metal-based nanocomposites are performed in acidic solutions. The reason contains two aspects: (1) for DMFCs, DEFCs and DFAFCs, carbonates can form in alkaline electrolyte [1]; (2) hydrogen oxidation reaction on platinum (Pt) in acid is 2 orders of magnitude quicker than in alkaline electrolytes [2]. When the fuel is hydrogen, the hydrogen oxidation reaction (HOR) rate on Pt is extremely fast, and 0.05 mg cm�<sup>2</sup> Pt loading at the anode is enough. However, to achieve a desirable catalytic performance, the slow reaction rate of ORR at the cathode even on the best Pt-based catalyst needs much more Pt loading amount (~0.4 mg

) [3]. According to the data reported by Vesborg in 2012, Pt is one of the most expensive metals [4]. The large-scale application of fuel cell technology in transportation field or portable power generation is limited by the high cost of electrode catalysts. Numerous researches have been undertaken to improve the intrinsic activity of electrocatalysts to reduce the noble metal

The catalytic activity, selectivity and stability of noble metal-based nanocomposites are closely related to the factors including size, shape/morphology and composition [5–8]. In the past decade, various experimental methods have been developed to synthesize sizedependent, high catalytic performance noble metal-based nanocomposites with diverse morphologies, such as polyhedron, concave, wire, plate, belt/ribbon, dendrite/branch and cage/frame structures [9–21]. Smaller size means higher surface-area-to-volume ratio, higher atomic utilization efficiency and more catalytic active sites. During electrochemical process, small-size noble metal-based nanocomposites provide high electrocatalytic activity, but the challenge is the aggregation that could happen under electrocatalytic conditions, resulting in the poor performance of long-term stability test (e.g., commercial Pt/C). Alloying one or two kinds of transition metals with noble metal has become a valid strategy to develop excellent electrocatalysts [22–25]. The addition of the second/third metal not only changes the surface active sites by ensemble effect, but also alters the binding strength of reactants, intermediates, and products by electronic/strain effect [26–28]. Different noble metals polyhedrons/ concave expose atoms with different coordination numbers, thus have different surface energy and exhibit different catalytic performances [29–31]. 1D (wire) and 2D (plate, belt/ ribbon) noble metal-based nanostructures have high surface area (high atom utilization efficiency), high conductivity and large interfacial area contacting with the support in electrochemical reactions [12, 15, 32]. Dendrite and branch structures not only exhibit large surface area and high active sites, but also significantly relieve the aggregation happened in electrocatalytic stability test [9, 33, 34]. Highly open noble metal nanocages and nanoframes exhibit enhanced electrocatalytic properties due to their three-dimensional accessible surface atoms [20, 35, 36]. A nanosegregated noble metal skin on these open structures can further improve the catalytic performance [18, 37]. Thus, numerous shape-controlled synthetic

cell (DFAFC) are also promising in commercial applications.

292 Novel Nanomaterials - Synthesis and Applications

loading of the electrodes without compromising fuel cell performance.

cm�<sup>2</sup>

At present, owing to the outstanding catalytic activity and superior resistant characteristics to corrosion, Pt-based nanoparticles (NPs) supported on porous carbon are still the most efficient catalysts in PEMFCs [15, 16, 18, 27]. In the past several years, to develop advanced Pt-based nanocatalysts for electrochemical reaction, researchers have proposed several synthesis strategies, which mainly include (1) alloying, (2) core-shell, (3) Pt-skin, (4) 1D or 2D structures and (5) porous, caged, hollow and frame structures. The integration of two or more strategies into one kind of Pt-based nanocomposites is beneficial for the construction of super-electrocatalysts with excellent catalytic activity and stability.

#### 2.1. Pt-based alloys and intermetallics

In the past decade, Pt-based nanostructures have been widely used to prepare excellent catalysts for diverse chemical reactions [45–47]. Compared with pure Pt nanocatalysts, Ptbased alloy and/or intermetallics exhibit higher activity and stability [48]. As since the surface electronic structure and d-band center position of Pt were changed by incorporating a second and/or third metal into Pt lattice. Owing to higher oxidation potentials of transition metals than Pt, they are always removed from the surface of Pt-based alloy by electrochemical leaching. A pure Pt shell with a thickness of more than several atomic layers is produced after the dealloying process [49]. Although the dealloyed Pt-based nanocatalysts show a higher electrochemical activity compared with Pt/C [50], they perform a poor long-term durability since the re-deposition of leached transition metal onto the anode [51].

For bimetallic and multi-metallic Pt-based nanocomposites, the exposed crystal facet has a close relationship with their electrocatalytic performance. Tian et al. and Stamenkovic et al. have demonstrated that the ORR activities on Pt decrease in the order of high-index facet (hkl) > (111) ≫ (100) in HClO4 solution [52]. Well-defined shapes of Pt-based nanocrystals (NCs) are, thus, of great interest for boosting electrochemical performance. It is believed that the surface energy of a fcc Pt NCs exposed with different facets increases in an order of (111) < (100) < (110) < (hkl) [53]. Accordingly, high-energy crystal facets and surface defects vanish quickly during the NC growth process due to the adatom incorporation on them. Therefore, it is a challenge to prepare NCs with high-index facets and high density of surface defects. Compared with alloy, intermetallic compounds show outstanding structure stability owing to the ordered atom arrangements. Rong et al. developed a kinetically controlled method to tune the surface defect of Pt-based intermetallics [24]. Based on their proposed growth mechanism, large electronegativity difference, etching and diffusing processes are necessary for the synthesis of defect-rich cubic intermetallic Pt3Sn NCs. In this protocol, N,Ndimethylformamide (DMF) is solvent and poly(vinylpyrrolidone) (PVP) is surfactant. As shown in Figure 1, rich surface defects endow the defect-rich cubic Pt3Sn NCs with excellent catalytic activity for formic acid oxidation reaction, excellent stability caused by the structure stability of intermetallic compounds.

#### 2.2. Ultrathin Pt-based nanocomposites

#### 2.2.1. Ultrathin Pt-based nanowires (NWs)

Ultrathin Pt-based NWs with a diameter of few atomic layers present high ratio of surface atoms to bulk atoms, which could increase Pt utilization efficiency greatly. During the synthesis process of bimetallic and multi-metallic Pt-based NWs, there are two obstacles to overcome: (1) controlling the reduction and nucleation process of different kinds of metal precursors with different reduction potentials; (2) confining the growth along a certain direction and inhibiting the other two directions. While diverse efforts to develop such materials have been made, some achievements of the fabrication of ultrathin Pt-based NWs with few nanometers have been reported. Oleylamine (OAm) was widely used as soft template in the preparation of ultrathin Pt-based NWs. In 2007, Sun's group synthesized ultrathin Pt-Fe NWs with a diameter of 2–3 nm in a mixed solution of 1-octadecene (ODE) and OAm [54]. The length of Pt-Fe NWs ranged from 20 to 200 nm with the tuning volume ratio of OAm/ODE. They claimed that OAm self-organizes into reverse-micelle-like structure in ODE solution and higher ratio of OAm/ ODE resulted in the formation of longer NWs.

In the same OAm/ODE system, Li et al. prepared Pt/NiO core-shell NW first, subsequently reduced it into PtNi alloy NWs by thermal annealing and finally converted it into jagged Pt NWs through electrochemical dealloying (Figure 2). The jagged Pt NWs had shorter PtdPt bond length than regular Pt NWs and/or bulk Pt. Short PtdPt bond length resulted in a compressive strain on the surface, which produced the world record ORR properties. The specific and mass activity for ORR at 0.9 V vs. reversible hydrogen electrode (RHE) are 11.5 mA cm�<sup>2</sup> and 13.9 A/mgPt, respectively [16].

As shown in Figure 2, Huang's group used OAm as the solvent, reductant and surfactant, cetyltrimethylammonium chloride (CTAC) as structure-directing and glucose as reducing reagent to synthesize hierarchical platinum-cobalt (Co) NWs [14]. These PtCo NWs had highindex, Pt-rich facets and ordered intermetallic structure. The novel structure enabled the PtCo NWs excellent performance toward alcohol oxidation reactions and ORR (39.6/33.7 times of the specific/mass activities than commercial Pt/C for ORR). They used CTAC, Mo(CO)6 and Ni (acac)2 (acac = acetylacetonate) as structure-directing reagent to prepare subnanometer Pt NWs with a diameter of 0.8 nm [17]. They claimed that the carbonyl decomposed from Mo(CO)6, Ni2+ and proper amount of CTAC were important for the formation of 1D structure. With addition of a stronger reductant (glucose), they fabricated PtNi, PtCo and PtNiCo NWs with similar diameter. These subnanometer Pt alloy NWs showed enhanced ORR activity and stability (mass and specific activities of 4.20 A mgPt and 5.11 mA cm�<sup>2</sup> at 0.9 V vs. RHE). They

used this effective reaction system to synthesize a series of Pt-based NWs for electrochemical reactions: (1) PtNiPd core-shell NWs showed superior glycoloxidation reaction (EGOR), glycerol oxidation reaction (GOR) and ORR performances [55], (2) porous Pt3Ni NWs with extraordinary catalytic performance toward methanol oxidation reaction (MOR) and ORR [56], (3) hierarchical PtPb NWs exhibited higher activity and stability for MOR and ethanol oxidation reaction (EOR) than commercial Pt/C [57], (4) screw thread-like PtCu NWs showed excellent

Figure 1. (A–C) HAADF-STEM images and (D–F) corresponding structure model of (A and D) cubic, (B and E) concave cubic, and (C and F) defect-rich cubic intermetallic Pt3Sn NCs. (G) Cyclic voltammograms of formic acid oxidation in 0.1 M HClO4 + 1 M HCOOH (scan rate: 50 mV/s). (H) Loss of peak current density in forward scans as a function of cycling numbers (0.65–1.23 V vs. RHE, scan rate: 50 mV/s). Reprinted with permission from Ref. [24]. Copyright 2016

Noble Metal-Based Nanocomposites for Fuel Cells http://dx.doi.org/10.5772/intechopen.71949 295

Recently, Li's group realized the reduction of Mo in this reaction system by a hydrogen assisted solution route (HASR) [32]. Ultrathin Pt-Mo-Ni NWs with a diameter of ~2.5 nm were

properties for MOR and EOR [58].

Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

method to tune the surface defect of Pt-based intermetallics [24]. Based on their proposed growth mechanism, large electronegativity difference, etching and diffusing processes are necessary for the synthesis of defect-rich cubic intermetallic Pt3Sn NCs. In this protocol, N,Ndimethylformamide (DMF) is solvent and poly(vinylpyrrolidone) (PVP) is surfactant. As shown in Figure 1, rich surface defects endow the defect-rich cubic Pt3Sn NCs with excellent catalytic activity for formic acid oxidation reaction, excellent stability caused by the structure

Ultrathin Pt-based NWs with a diameter of few atomic layers present high ratio of surface atoms to bulk atoms, which could increase Pt utilization efficiency greatly. During the synthesis process of bimetallic and multi-metallic Pt-based NWs, there are two obstacles to overcome: (1) controlling the reduction and nucleation process of different kinds of metal precursors with different reduction potentials; (2) confining the growth along a certain direction and inhibiting the other two directions. While diverse efforts to develop such materials have been made, some achievements of the fabrication of ultrathin Pt-based NWs with few nanometers have been reported. Oleylamine (OAm) was widely used as soft template in the preparation of ultrathin Pt-based NWs. In 2007, Sun's group synthesized ultrathin Pt-Fe NWs with a diameter of 2–3 nm in a mixed solution of 1-octadecene (ODE) and OAm [54]. The length of Pt-Fe NWs ranged from 20 to 200 nm with the tuning volume ratio of OAm/ODE. They claimed that OAm self-organizes into reverse-micelle-like structure in ODE solution and higher ratio of OAm/

In the same OAm/ODE system, Li et al. prepared Pt/NiO core-shell NW first, subsequently reduced it into PtNi alloy NWs by thermal annealing and finally converted it into jagged Pt NWs through electrochemical dealloying (Figure 2). The jagged Pt NWs had shorter PtdPt bond length than regular Pt NWs and/or bulk Pt. Short PtdPt bond length resulted in a compressive strain on the surface, which produced the world record ORR properties. The specific and mass activity for ORR at 0.9 V vs. reversible hydrogen electrode (RHE) are

As shown in Figure 2, Huang's group used OAm as the solvent, reductant and surfactant, cetyltrimethylammonium chloride (CTAC) as structure-directing and glucose as reducing reagent to synthesize hierarchical platinum-cobalt (Co) NWs [14]. These PtCo NWs had highindex, Pt-rich facets and ordered intermetallic structure. The novel structure enabled the PtCo NWs excellent performance toward alcohol oxidation reactions and ORR (39.6/33.7 times of the specific/mass activities than commercial Pt/C for ORR). They used CTAC, Mo(CO)6 and Ni (acac)2 (acac = acetylacetonate) as structure-directing reagent to prepare subnanometer Pt NWs with a diameter of 0.8 nm [17]. They claimed that the carbonyl decomposed from Mo(CO)6, Ni2+ and proper amount of CTAC were important for the formation of 1D structure. With addition of a stronger reductant (glucose), they fabricated PtNi, PtCo and PtNiCo NWs with similar diameter. These subnanometer Pt alloy NWs showed enhanced ORR activity and stability (mass and specific activities of 4.20 A mgPt and 5.11 mA cm�<sup>2</sup> at 0.9 V vs. RHE). They

stability of intermetallic compounds.

294 Novel Nanomaterials - Synthesis and Applications

2.2. Ultrathin Pt-based nanocomposites 2.2.1. Ultrathin Pt-based nanowires (NWs)

ODE resulted in the formation of longer NWs.

11.5 mA cm�<sup>2</sup> and 13.9 A/mgPt, respectively [16].

Figure 1. (A–C) HAADF-STEM images and (D–F) corresponding structure model of (A and D) cubic, (B and E) concave cubic, and (C and F) defect-rich cubic intermetallic Pt3Sn NCs. (G) Cyclic voltammograms of formic acid oxidation in 0.1 M HClO4 + 1 M HCOOH (scan rate: 50 mV/s). (H) Loss of peak current density in forward scans as a function of cycling numbers (0.65–1.23 V vs. RHE, scan rate: 50 mV/s). Reprinted with permission from Ref. [24]. Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

used this effective reaction system to synthesize a series of Pt-based NWs for electrochemical reactions: (1) PtNiPd core-shell NWs showed superior glycoloxidation reaction (EGOR), glycerol oxidation reaction (GOR) and ORR performances [55], (2) porous Pt3Ni NWs with extraordinary catalytic performance toward methanol oxidation reaction (MOR) and ORR [56], (3) hierarchical PtPb NWs exhibited higher activity and stability for MOR and ethanol oxidation reaction (EOR) than commercial Pt/C [57], (4) screw thread-like PtCu NWs showed excellent properties for MOR and EOR [58].

Recently, Li's group realized the reduction of Mo in this reaction system by a hydrogen assisted solution route (HASR) [32]. Ultrathin Pt-Mo-Ni NWs with a diameter of ~2.5 nm were

can not only relieve the undesirable overbinding of oxygen atoms on catalyst, but also stabilize the undercoordinated active sites, further improving the electrochemical activity and stability. While the synthesis of ultrathin Pt-based NWs has many achievements, more innovative

Noble Metal-Based Nanocomposites for Fuel Cells http://dx.doi.org/10.5772/intechopen.71949 297

Since catalytic reactions take place on the surface of nanocatalysts, the alteration of surface composition and electronic structure directly causes different catalytic properties [59]. The physical and chemical properties of core-shell nanostructures can be altered by the design of core, shell and interface. The construction of Pt shell on the surface of nanocatalysts is another possible solution to prepare enhanced Pt-based catalysts with minimum usage of Pt. The catalytic properties of Pt based core-shell NCs have close relationship with the Pt shell thickness. The charge transfer between core and shell components changes the band structure of shell, thus influencing the catalytic performance [60]. The electronic and geometric properties of the top surface layer are affected by the core, and the influence decreases with the increased thickness of shell. For core-shell structures with thin shell, the different lattice constant between core and shell components results in the lattice strain and brings geometric effect [61]. Strong electrochemical activity enhancements have been observed for monolayer or a few atomic layer Pt shells. Thanks to the development of instrumental techniques, researchers can

So far, diverse synthetic strategies have been developed to prepare diverse inorganic core-shell structures: semiconductor@semiconductor [62], metal@metal [63, 64], semiconductor@metal [65],

Several synthetic strategies have been developed to fabricate core-shell structures, such as seedmediated growth, dealloying, galvanic replacement and one-pot synthesis. The most effective and widely used synthetic methodology is seed-mediated growth. Adzic and co-workers have fabricated various core-shell electrocatalysts with monolayer and ultrathin Pt shell via Cu under potential deposition (UPD) and pulse electrodeposition (PED). The core materials contain intermetallics (PtPb, PdPb, PdFe) [70], core-shell (Pd@PdAu) [71], alloy (AuNi) [72] and transition metal nitride (TiCuN) [73]. As shown in Figure 3, in an UPD process, they prepared core first, then dropped onto a flat glassy electrode to form a thin film. The electrodes were deposited Cu monolayer in a H2SO4 + CuSO4 solution, then rinsed in a K2PtCl4 + H2SO4 solution to replace Cu with Pt. The Cu deposition and replacement operations were conducted under an Ar atmosphere [70]. Due to the limited electrode area and critical synthesis conditions, UPD and PED are not suitable for the large-scale preparation of core-shell nanocatalysts with ultrathin Pt shell. Zhang's group reported the synthesis of Ag@Pt and Au@Ag@Pt particles with a controllable thin Pt shell by interface-mediated galvanic replacement. These sub-ten-nanometer core-shell NCs

Utilizing the different reduction potentials of different metal precursors, bimetallic core-shell structure could be prepared by one-pot synthetic process [75, 76]. However, it is still a challenge to control this process since alloys or heterostructures with uncontrolled morphologies are usually produced. Bu et al. synthesized PtPb@Pt core-shell nanoplate in OAm/ODE mixture and this

works are still desired to enrich this field.

2.2.2. Core-shell structures with ultrathin Pt shell

study ultrathin Pt shells closely and clearly.

showed better ORR performance than Ag [74].

metal@semiconductor [66–68], and even multishell structures [69].

Figure 2. Representative (a) STEM image, (b) TEM image, (c) TEM-EDS, (d) PXRD pattern and (e) STEM-ADF image and EDS elemental mappings of the hierarchical Pt3Co NWs. Inset in (a) is an enlarged STEM image. The composition is Pt/Co = 74.8/25.2, as revealed by ICP-AES. The scale bars in (a), inset of (a), (b) and (e) are 200, 20, 10 and 10 nm, respectively. Reprinted with permission from Ref. [14]. Copyright 2016 Nature Publishing Group.

successful synthesized in OAm. Hydrogen served not only as reductant, but also as structuredirecting species. A series of Pt-Mo-M (M = Fe, Co, Mn, Ru) NWs could be synthesized by this HASR method. The Pt-Mo-Ni NWs exhibited higher maximum power density than Pt/C in DEFC experiments.

In the above mentioned cases, soft template method is very effective in OAm system to construct ultrathin Pt-based NWs. The largest extent exposure of Pt atoms results in the extraordinary electrochemical activity. The presence of the second and third transition metal can not only relieve the undesirable overbinding of oxygen atoms on catalyst, but also stabilize the undercoordinated active sites, further improving the electrochemical activity and stability. While the synthesis of ultrathin Pt-based NWs has many achievements, more innovative works are still desired to enrich this field.

#### 2.2.2. Core-shell structures with ultrathin Pt shell

successful synthesized in OAm. Hydrogen served not only as reductant, but also as structuredirecting species. A series of Pt-Mo-M (M = Fe, Co, Mn, Ru) NWs could be synthesized by this HASR method. The Pt-Mo-Ni NWs exhibited higher maximum power density than Pt/C in

Figure 2. Representative (a) STEM image, (b) TEM image, (c) TEM-EDS, (d) PXRD pattern and (e) STEM-ADF image and EDS elemental mappings of the hierarchical Pt3Co NWs. Inset in (a) is an enlarged STEM image. The composition is Pt/Co = 74.8/25.2, as revealed by ICP-AES. The scale bars in (a), inset of (a), (b) and (e) are 200, 20, 10 and 10 nm,

respectively. Reprinted with permission from Ref. [14]. Copyright 2016 Nature Publishing Group.

In the above mentioned cases, soft template method is very effective in OAm system to construct ultrathin Pt-based NWs. The largest extent exposure of Pt atoms results in the extraordinary electrochemical activity. The presence of the second and third transition metal

DEFC experiments.

296 Novel Nanomaterials - Synthesis and Applications

Since catalytic reactions take place on the surface of nanocatalysts, the alteration of surface composition and electronic structure directly causes different catalytic properties [59]. The physical and chemical properties of core-shell nanostructures can be altered by the design of core, shell and interface. The construction of Pt shell on the surface of nanocatalysts is another possible solution to prepare enhanced Pt-based catalysts with minimum usage of Pt. The catalytic properties of Pt based core-shell NCs have close relationship with the Pt shell thickness. The charge transfer between core and shell components changes the band structure of shell, thus influencing the catalytic performance [60]. The electronic and geometric properties of the top surface layer are affected by the core, and the influence decreases with the increased thickness of shell. For core-shell structures with thin shell, the different lattice constant between core and shell components results in the lattice strain and brings geometric effect [61]. Strong electrochemical activity enhancements have been observed for monolayer or a few atomic layer Pt shells. Thanks to the development of instrumental techniques, researchers can study ultrathin Pt shells closely and clearly.

So far, diverse synthetic strategies have been developed to prepare diverse inorganic core-shell structures: semiconductor@semiconductor [62], metal@metal [63, 64], semiconductor@metal [65], metal@semiconductor [66–68], and even multishell structures [69].

Several synthetic strategies have been developed to fabricate core-shell structures, such as seedmediated growth, dealloying, galvanic replacement and one-pot synthesis. The most effective and widely used synthetic methodology is seed-mediated growth. Adzic and co-workers have fabricated various core-shell electrocatalysts with monolayer and ultrathin Pt shell via Cu under potential deposition (UPD) and pulse electrodeposition (PED). The core materials contain intermetallics (PtPb, PdPb, PdFe) [70], core-shell (Pd@PdAu) [71], alloy (AuNi) [72] and transition metal nitride (TiCuN) [73]. As shown in Figure 3, in an UPD process, they prepared core first, then dropped onto a flat glassy electrode to form a thin film. The electrodes were deposited Cu monolayer in a H2SO4 + CuSO4 solution, then rinsed in a K2PtCl4 + H2SO4 solution to replace Cu with Pt. The Cu deposition and replacement operations were conducted under an Ar atmosphere [70]. Due to the limited electrode area and critical synthesis conditions, UPD and PED are not suitable for the large-scale preparation of core-shell nanocatalysts with ultrathin Pt shell. Zhang's group reported the synthesis of Ag@Pt and Au@Ag@Pt particles with a controllable thin Pt shell by interface-mediated galvanic replacement. These sub-ten-nanometer core-shell NCs showed better ORR performance than Ag [74].

Utilizing the different reduction potentials of different metal precursors, bimetallic core-shell structure could be prepared by one-pot synthetic process [75, 76]. However, it is still a challenge to control this process since alloys or heterostructures with uncontrolled morphologies are usually produced. Bu et al. synthesized PtPb@Pt core-shell nanoplate in OAm/ODE mixture and this

enclosed with {111} facets toward ORR [85]. Compared with hydrophilic solvents such as water, ethylene glycol and DMF, systematic shape control of Pd NCs in hydrophobic solvent remains a challenging task. As shown in Figure 4, Niu et al. synthesized icosahedral, decahedral, octahedral, tetrahedral and triangular plates Pd NCs in the mixture of OAm, formaldehyde and toluene [30]. In this protocol, Pd(acac)2 was used as the metal precursor and the morphology of Pd was determined by the quantity of OAm. Different intermediates were formed with different amount of OAm under room temperature stirring and reduction rate decreased with the increase amount of OAm. Seeds were formed with the addition of formaldehyde at room temperature, which also acted as selective surfactant in this reaction

Noble Metal-Based Nanocomposites for Fuel Cells http://dx.doi.org/10.5772/intechopen.71949 299

Figure 4. (A) Proposed mechanism for formation of Pd NCs. (B–F) TEM images and (B–G) cyclic voltammograms of Pd

(C) decahedral Pd, (D) octahedral Pd, (E) tetrahedral Pd, (F) triangular platelike Pd, and (G) commercial Pd. Adapted with

. (B) Icosahedral Pd,

catalysts with different shapes in 0.1 M HClO4 and 2 M HCOOH solution at a scan rate of 50 mV s�<sup>1</sup>

permission from Ref. [30]. Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 3. Schematic illustration of the stepwise synthesis of monolayer Pt core-shell electrocatalysts. Reprinted with permission from Ref. [72]. Copyright 2016 American Chemistry Society.

nanoplate exhibited about eight-fold increase in mass activity for MOR and EOR and enhanced ORR performance compared with Pt catalyst. In their synthesis process, L-ascorbic acid was used as the reducing reagent. The Pt shell thickness was 0.8–1.2 nm (equal to four to six atomic layers). According to the well-established d-band model, tensile lattice strain upshifts the d-band center and thus results in a stronger bonding with adsorbates and a lower reaction rate. In the case of PtPb@Pt core-shell nanoplate, although Pb atom in the core is larger than Pt, the tensile strain did not decrease the electrochemical properties. They claimed that the surface strain effect on binding behavior was facet dependent, and the tensile strain on the exposed Pt(110) helped to decrease the PtdO bond strength.

Compared to monolayer Pt shell, a few atomic layer Pt shell can protect the transition metal atoms from leaching under electrochemical operation and have better stability. However, the stability of core-shell structured nanocatalysts with ultrathin Pt shell is still far away from satisfactory.

### 3. Other noble metal-based nanocomposites

#### 3.1. Pd-based nanocomposites

Palladium (Pd)-based nanocomposites have exhibited extraordinary performance in diverse applications such as hydrogenation reactions [5, 77], photothermal therapy [78], cross-coupling reactions [79] and electrochemical reactions [80–82]. Pd and Pt are in the same group and Pd-Pt nanostructures have showed excellent electrocatalytic properties [37, 83]. Considering Pt-based nanocomposites that have been discussed above, Pd-Pt nanostructures are not reviewed in this subsection.

The electrochemical reactions on Pd NCs are structure sensitive. For the low index planes of Pd, Kondo et al. reported that reduction current density of ORR at 0.9 V (RHE) increased in the order of (110) < (111) < (100) [84]. Shao et al. also demonstrated that the specific activity of Pd nanocubes exposed with {100} facets was about 10 times higher than that of Pd octahedral

enclosed with {111} facets toward ORR [85]. Compared with hydrophilic solvents such as water, ethylene glycol and DMF, systematic shape control of Pd NCs in hydrophobic solvent remains a challenging task. As shown in Figure 4, Niu et al. synthesized icosahedral, decahedral, octahedral, tetrahedral and triangular plates Pd NCs in the mixture of OAm, formaldehyde and toluene [30]. In this protocol, Pd(acac)2 was used as the metal precursor and the morphology of Pd was determined by the quantity of OAm. Different intermediates were formed with different amount of OAm under room temperature stirring and reduction rate decreased with the increase amount of OAm. Seeds were formed with the addition of formaldehyde at room temperature, which also acted as selective surfactant in this reaction

nanoplate exhibited about eight-fold increase in mass activity for MOR and EOR and enhanced ORR performance compared with Pt catalyst. In their synthesis process, L-ascorbic acid was used as the reducing reagent. The Pt shell thickness was 0.8–1.2 nm (equal to four to six atomic layers). According to the well-established d-band model, tensile lattice strain upshifts the d-band center and thus results in a stronger bonding with adsorbates and a lower reaction rate. In the case of PtPb@Pt core-shell nanoplate, although Pb atom in the core is larger than Pt, the tensile strain did not decrease the electrochemical properties. They claimed that the surface strain effect on binding behavior was facet dependent, and the tensile strain on the exposed

Figure 3. Schematic illustration of the stepwise synthesis of monolayer Pt core-shell electrocatalysts. Reprinted with

Compared to monolayer Pt shell, a few atomic layer Pt shell can protect the transition metal atoms from leaching under electrochemical operation and have better stability. However, the stability of core-shell structured nanocatalysts with ultrathin Pt shell is still far away from

Palladium (Pd)-based nanocomposites have exhibited extraordinary performance in diverse applications such as hydrogenation reactions [5, 77], photothermal therapy [78], cross-coupling reactions [79] and electrochemical reactions [80–82]. Pd and Pt are in the same group and Pd-Pt nanostructures have showed excellent electrocatalytic properties [37, 83]. Considering Pt-based nanocomposites that have been discussed above, Pd-Pt nanostructures are not reviewed in this

The electrochemical reactions on Pd NCs are structure sensitive. For the low index planes of Pd, Kondo et al. reported that reduction current density of ORR at 0.9 V (RHE) increased in the order of (110) < (111) < (100) [84]. Shao et al. also demonstrated that the specific activity of Pd nanocubes exposed with {100} facets was about 10 times higher than that of Pd octahedral

Pt(110) helped to decrease the PtdO bond strength.

permission from Ref. [72]. Copyright 2016 American Chemistry Society.

298 Novel Nanomaterials - Synthesis and Applications

3. Other noble metal-based nanocomposites

satisfactory.

subsection.

3.1. Pd-based nanocomposites

Figure 4. (A) Proposed mechanism for formation of Pd NCs. (B–F) TEM images and (B–G) cyclic voltammograms of Pd catalysts with different shapes in 0.1 M HClO4 and 2 M HCOOH solution at a scan rate of 50 mV s�<sup>1</sup> . (B) Icosahedral Pd, (C) decahedral Pd, (D) octahedral Pd, (E) tetrahedral Pd, (F) triangular platelike Pd, and (G) commercial Pd. Adapted with permission from Ref. [30]. Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

system. Growth process was observed at elevated temperature and the final morphology was decided by the co-action of surface energy, elastic strain energy and twin boundary energy. The maximum current densities of the prepared Pd NCs were in the order of icosahedral ≈octahedral > tetrahedral > decahedral > triangular plates toward formic acid oxidation reaction.

Among bimetallic Pd-base nanocatalysts for electrochemical reactions, PdCu NCs have received a great deal of attention due to the low cost and good performance. Yin et al. prepared PdCu NPs by emulsion-assisted ternary ethylene glycol system, which showed superior methanol oxidation activity than Pd in alkaline solution [86]. Mao et al. also synthesized PdCu NCs with tunable compositions in octadecylamine. Pd0.5Cu0.5 exhibited the highest mass activity for ethanol electro-oxidation in alkaline solution [8]. Gao et al. prepared PdCu nanocubes in OAm and the PdCu nanocubes showed 2 times mass activity than commercial Pt/C catalysts [87].

Although numerous Pd-based nanocomposites have been proven to show excellent electrocatalytic activities, but the stability is still a major issue for Pd-based electrocatalysts even in alkaline solution. Better performance of Pd-based nanocomposites is essential for the substitute of Pt in the application of fuel cell.

#### 3.2. Other noble metal-based nanocomposites

Generally speaking, besides Pt and Pd, noble metals also contain gold (Au), silver (Ag), rhodium (Rh), ruthenium (Ru), osmium (Os) and iridium (Ir). They are always used to modify the electronic structure of Pd and Pt in the fuel cell application and these nanocomposites will not be discussed below [88–92].

Since Haruta et al. reported the oxidation of CO by supported Au at low temperature, Aubased nanocomposites have received a great deal of attention in catalytic field [93]. Surfactants being on the surface may decrease the electrochemical activities. Yin et al. used reduced grapheme oxide (rGO) as reductant and support to confine the growth of Au so they prepared surfactant-free Au/rGO hybrids for ORR in alkaline solution [94]. In this method, utilizing the electrostatic interaction between negative-charged rGO and positive Au(III) ions also the coordination effects between heteroatoms at the defects of rGO and Au(III) ions, Au clusters with a diameter of 1.8 nm were reduced and anchored by rGO. Although the catalytic activity was not as good as Pt/C, Au/rGO exhibited relatively high onset potential, methanol tolerance, and extraordinary stability (Figure 5).

rotating disk electrode (RDE) data by Koutecky-Levich equation and demonstrated that O2 was reduced by a four-electron pathway. Gupta et al. studied the influence of anisotropic Ag on ORR activity in alkaline media [97]. They prepared Ag NPs in ethylene glycol with different ratio of anisotropic NPs by adding different amount of Cu(CH3COO)2H2O, which acted as etchant. They reported that sample containing higher ratio of anisotropic Ag NPs showed

Figure 5. (A) SEM, (B) TEM images of as-prepared Au/rGO hybrids. (C) Scheme of the ORR happened on Au/rGO hybrids. (D) RDE curves of commercial Pt/C, Au/rGO hybrids, Au NP/rGO hybrids, rGO sheets, Au clusters in O2 saturated 0.1 M KOH at a scanning rate of 50 mV/s at 1600 rpm. Reprinted with permission from Ref. [94] Copyright

Noble Metal-Based Nanocomposites for Fuel Cells http://dx.doi.org/10.5772/intechopen.71949 301

Zheng et al. studied the size effect of Ir/C toward hydrogen oxidation/evolution reaction (HOR/HER) in alkaline solution [98]. They prepared Ir/C with different sizes by heat treating commercial Ir/C at different temperature. Higher temperature resulted in larger size. The particle sizes of Ir/C changed from 3 to 12 nm. Different from the law of Pt NCs with the same size range, larger Ir/C had higher HOR/HER activities. Qin et al. utilized CeO2 and C to disperse Ir NPs; the mass and specific activities of Ir/CeO2-C toward HOR in alkaline conditions are 2.8- and 1.8-fold higher than Ir/C. They claimed that the addition of CeO2 decreased the agglomeration extent of Ir NPs, and the particle size changed from 1.1 to 3.4 nm after 2000

better ORR performance.

2012 American Chemistry Society.

potential cycles [99].

By introducing other metal components, the cost of Au can be reduced, the modification of crystallography and electronic structure may increase the catalytic performance. With abundant tip, edge and surface atoms, 2D Au-based nanocomposites have attracted increasing attention. Xu et al. prepared AuCu bimetallic NCs with tunable composition via a simple wet-chemical method [95]. The 2D AuCu triangular porous nanoprisms exhibited high electrocatalytic activity. The mass activities of Au1Cu1 for ethylene glycol and glycerol electrooxidation reactions were 3.0- and 3.9-fold improvements over those of pure Au, respectively.

Owing to the poor stability in acidic solution, ORR catalyzed by Ag was always demonstrated in alkaline solution [96, 97]. Tammeveski et al. prepared the Ag-based nanocomposites by sputter deposition of Ag on the surface of multi-walled carbon nanotube [96]. They analyzed

system. Growth process was observed at elevated temperature and the final morphology was decided by the co-action of surface energy, elastic strain energy and twin boundary energy. The maximum current densities of the prepared Pd NCs were in the order of icosahedral ≈octahedral > tetrahedral > decahedral > triangular plates toward formic acid oxidation reaction.

Among bimetallic Pd-base nanocatalysts for electrochemical reactions, PdCu NCs have received a great deal of attention due to the low cost and good performance. Yin et al. prepared PdCu NPs by emulsion-assisted ternary ethylene glycol system, which showed superior methanol oxidation activity than Pd in alkaline solution [86]. Mao et al. also synthesized PdCu NCs with tunable compositions in octadecylamine. Pd0.5Cu0.5 exhibited the highest mass activity for ethanol electro-oxidation in alkaline solution [8]. Gao et al. prepared PdCu nanocubes in OAm and the PdCu nanocubes showed 2 times mass activity than commercial Pt/C catalysts [87].

Although numerous Pd-based nanocomposites have been proven to show excellent electrocatalytic activities, but the stability is still a major issue for Pd-based electrocatalysts even in alkaline solution. Better performance of Pd-based nanocomposites is essential for the substi-

Generally speaking, besides Pt and Pd, noble metals also contain gold (Au), silver (Ag), rhodium (Rh), ruthenium (Ru), osmium (Os) and iridium (Ir). They are always used to modify the electronic structure of Pd and Pt in the fuel cell application and these nanocomposites will

Since Haruta et al. reported the oxidation of CO by supported Au at low temperature, Aubased nanocomposites have received a great deal of attention in catalytic field [93]. Surfactants being on the surface may decrease the electrochemical activities. Yin et al. used reduced grapheme oxide (rGO) as reductant and support to confine the growth of Au so they prepared surfactant-free Au/rGO hybrids for ORR in alkaline solution [94]. In this method, utilizing the electrostatic interaction between negative-charged rGO and positive Au(III) ions also the coordination effects between heteroatoms at the defects of rGO and Au(III) ions, Au clusters with a diameter of 1.8 nm were reduced and anchored by rGO. Although the catalytic activity was not as good as Pt/C, Au/rGO exhibited relatively high onset potential, methanol tolerance,

By introducing other metal components, the cost of Au can be reduced, the modification of crystallography and electronic structure may increase the catalytic performance. With abundant tip, edge and surface atoms, 2D Au-based nanocomposites have attracted increasing attention. Xu et al. prepared AuCu bimetallic NCs with tunable composition via a simple wet-chemical method [95]. The 2D AuCu triangular porous nanoprisms exhibited high electrocatalytic activity. The mass activities of Au1Cu1 for ethylene glycol and glycerol electrooxidation reactions were

Owing to the poor stability in acidic solution, ORR catalyzed by Ag was always demonstrated in alkaline solution [96, 97]. Tammeveski et al. prepared the Ag-based nanocomposites by sputter deposition of Ag on the surface of multi-walled carbon nanotube [96]. They analyzed

3.0- and 3.9-fold improvements over those of pure Au, respectively.

tute of Pt in the application of fuel cell.

300 Novel Nanomaterials - Synthesis and Applications

not be discussed below [88–92].

and extraordinary stability (Figure 5).

3.2. Other noble metal-based nanocomposites

Figure 5. (A) SEM, (B) TEM images of as-prepared Au/rGO hybrids. (C) Scheme of the ORR happened on Au/rGO hybrids. (D) RDE curves of commercial Pt/C, Au/rGO hybrids, Au NP/rGO hybrids, rGO sheets, Au clusters in O2 saturated 0.1 M KOH at a scanning rate of 50 mV/s at 1600 rpm. Reprinted with permission from Ref. [94] Copyright 2012 American Chemistry Society.

rotating disk electrode (RDE) data by Koutecky-Levich equation and demonstrated that O2 was reduced by a four-electron pathway. Gupta et al. studied the influence of anisotropic Ag on ORR activity in alkaline media [97]. They prepared Ag NPs in ethylene glycol with different ratio of anisotropic NPs by adding different amount of Cu(CH3COO)2H2O, which acted as etchant. They reported that sample containing higher ratio of anisotropic Ag NPs showed better ORR performance.

Zheng et al. studied the size effect of Ir/C toward hydrogen oxidation/evolution reaction (HOR/HER) in alkaline solution [98]. They prepared Ir/C with different sizes by heat treating commercial Ir/C at different temperature. Higher temperature resulted in larger size. The particle sizes of Ir/C changed from 3 to 12 nm. Different from the law of Pt NCs with the same size range, larger Ir/C had higher HOR/HER activities. Qin et al. utilized CeO2 and C to disperse Ir NPs; the mass and specific activities of Ir/CeO2-C toward HOR in alkaline conditions are 2.8- and 1.8-fold higher than Ir/C. They claimed that the addition of CeO2 decreased the agglomeration extent of Ir NPs, and the particle size changed from 1.1 to 3.4 nm after 2000 potential cycles [99].

Lin et al. reported a salt-templated method to prepare Rh (1.6 nm)/ultrathin carbon nanosheets, which displayed a comparable ORR activity and better durability to commercial Pt/C under basic conditions [100]. The salt template was green and easy to remove. In this approach, Na2SO4 particles acted as template and poured into prefabricated rhodium oleate. During the subsequent heat-treatment under N2, Rh NPs were reduced and anchored on the ultrathin carbon nanosheets transformed from oleate. After water rinsing, Na2SO4 particles were removed and Rh/ultrathin carbon nanosheets were obtained.

Author details

References

DOI: 10.1021/cr400389f

DOI: 10.1021/cs400282a

10.1002/chem.201501442

c2ra20839c

c2sc00004k

Hongpan Rong, Shuping Zhang, Sajid Muhammad and Jiatao Zhang\*

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Applications, School of Materials, Beijing Institute of Technology, Beijing, PR China

\*Address all correspondence to: zhangjt@bit.edu.cn

Owing to the strong RudO bond strength, metallic Ru show poor ORR activity. The addition of Se can transfer charge from Ru to Se, which reduces the oxygen binding energy on Ru and increases the ORR activity [3]. Cao et al. prepared Se-modified Ru NPs by heating the Ru black and Se in xylene under refluxing condition with Ar bubbling [101]. The ORR activity of Se-modified Ru NPs in H2SO4 was higher than that of clean and Se-modified Ru disk. Zaikovskii et al. fabricated Se-modified Ru/C by reacting Ru/C NPs with SeO2 followed by annealing [102]. They prepared RuSe/C with different Se coverage by tuning the ratio of Se/ Ru. Higher amount of Se caused lower extent of Ru oxidation, thus resulting in higher ORR activity. However, Ru is the active center for ORR and blocked shell of RuSe decreased ORR activity. Neergat et al. synthesized RuSe NPs by reducing RuCl3xH2O and SeO2 by NaBH4 in water at 80C [103]. They investigated the ORR performance of several nanocatalysts in the presence of methanol, and the ORR activities were in the order of Ir < Pt < Rh < IrSe < RhSe < RuSe.

### 4. Conclusions and outlooks

This chapter has demonstrated the significant developments that have been accomplished in the noble metal-based nanocomposites for fuel cell. For the past decade, great progress has been made in developing Pt- and Pd-based nanocomposites for electrochemical reactions in fuel cell. Surface electronic structure of Pt and Pd can be modified by incorporating a second and/or third metal, which significantly influences the electrochemical performance. Constructing Pt ultrathin structure is an effective strategy to improve the utilization of Pt and the specific activity of Ptbased nanocatalysts. Significant progresses have been made on Ir-, Rh-, and Ru-based nanocatalysts. However, their activities are still not comparable to those of Pt- and Pd-based catalysts, especially those of Pt-based nanocomposites. For the practical application, fuel cell is still a long way to go.

At present, most Pt-based catalysts are characterized by RDE technique, which is a fast screening technique. They need to be evaluated in a fuel cell environment before large-scale application. As a result, simple preparation method is very important for scale production of catalysts. Wet-chemical method is suitable for scale preparation. Researchers need to optimize the fabrication technology of the state-of-the-art noble metal-based electrocatalysts, and find a simple, green and low-cost preparation method. Developing in situ characterization technique for the electrochemical process will provide a basis for mechanism exploration. According to the excellent catalytic performance of binary noble metal structures, general preparation strategies and catalytic property studies for tri- and multi-metallic nanocomposites are urgent.

### Author details

Lin et al. reported a salt-templated method to prepare Rh (1.6 nm)/ultrathin carbon nanosheets, which displayed a comparable ORR activity and better durability to commercial Pt/C under basic conditions [100]. The salt template was green and easy to remove. In this approach, Na2SO4 particles acted as template and poured into prefabricated rhodium oleate. During the subsequent heat-treatment under N2, Rh NPs were reduced and anchored on the ultrathin carbon nanosheets transformed from oleate. After water rinsing, Na2SO4 particles

Owing to the strong RudO bond strength, metallic Ru show poor ORR activity. The addition of Se can transfer charge from Ru to Se, which reduces the oxygen binding energy on Ru and increases the ORR activity [3]. Cao et al. prepared Se-modified Ru NPs by heating the Ru black and Se in xylene under refluxing condition with Ar bubbling [101]. The ORR activity of Se-modified Ru NPs in H2SO4 was higher than that of clean and Se-modified Ru disk. Zaikovskii et al. fabricated Se-modified Ru/C by reacting Ru/C NPs with SeO2 followed by annealing [102]. They prepared RuSe/C with different Se coverage by tuning the ratio of Se/ Ru. Higher amount of Se caused lower extent of Ru oxidation, thus resulting in higher ORR activity. However, Ru is the active center for ORR and blocked shell of RuSe decreased ORR activity. Neergat et al. synthesized RuSe NPs by reducing RuCl3xH2O and SeO2 by NaBH4 in water at 80C [103]. They investigated the ORR performance of several nanocatalysts in the presence of methanol, and the ORR activities were in the order of Ir < Pt < Rh < IrSe <

This chapter has demonstrated the significant developments that have been accomplished in the noble metal-based nanocomposites for fuel cell. For the past decade, great progress has been made in developing Pt- and Pd-based nanocomposites for electrochemical reactions in fuel cell. Surface electronic structure of Pt and Pd can be modified by incorporating a second and/or third metal, which significantly influences the electrochemical performance. Constructing Pt ultrathin structure is an effective strategy to improve the utilization of Pt and the specific activity of Ptbased nanocatalysts. Significant progresses have been made on Ir-, Rh-, and Ru-based nanocatalysts. However, their activities are still not comparable to those of Pt- and Pd-based catalysts, especially those of Pt-based nanocomposites. For the practical application, fuel cell is still a long

At present, most Pt-based catalysts are characterized by RDE technique, which is a fast screening technique. They need to be evaluated in a fuel cell environment before large-scale application. As a result, simple preparation method is very important for scale production of catalysts. Wet-chemical method is suitable for scale preparation. Researchers need to optimize the fabrication technology of the state-of-the-art noble metal-based electrocatalysts, and find a simple, green and low-cost preparation method. Developing in situ characterization technique for the electrochemical process will provide a basis for mechanism exploration. According to the excellent catalytic performance of binary noble metal structures, general preparation strategies and catalytic property studies for tri- and multi-metallic nanocomposites are urgent.

were removed and Rh/ultrathin carbon nanosheets were obtained.

RhSe < RuSe.

way to go.

4. Conclusions and outlooks

302 Novel Nanomaterials - Synthesis and Applications

Hongpan Rong, Shuping Zhang, Sajid Muhammad and Jiatao Zhang\*

\*Address all correspondence to: zhangjt@bit.edu.cn

Beijing Key Laboratory of Construction-Tailorable Advanced Functional Materials and Green Applications, School of Materials, Beijing Institute of Technology, Beijing, PR China

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**Chapter 17**

**Provisional chapter**

**Capillary Electrophoresis in Nanotechnologies versus**

Nanomaterials are attracting an interest of many researches. All this attention is due the unique physical and chemical properties of nanomaterials differing significantly from the bulk materials mainly due to their size in range of nanometers. Capillary electrophoresis (CE) is a powerful, well-established analytical technique that provides numerous valuable benefits over other separation methods including high-performance liquid chromatography. The connection between CE and nanotechnology can be approached by two strategies: (i) CE analysis of nanomaterials and (ii) nanomaterials for CE improvement. The first perspective focuses on uses of CE as a method for characterization employed during nanomaterial production and modification as well as for monitoring their properties and interactions with other molecules. The second viewpoint deals with applications of nanomaterials for improving CE performance, mainly by enhancing efficiency of separation using nanomaterials as a stationary or pseudo-stationary phase and by enhancing detection sensitivity and/or selectivity in both optical and electrochemical detection. Moreover, applications of nanomaterials for sample preparation before CE analysis will be mentioned. This chapter aims at highlighting the symbiosis of CE and nanotechnology as a combination of modern,

**Capillary Electrophoresis in Nanotechnologies versus** 

DOI: 10.5772/intechopen.72015

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

**Nanotechnologies in Capillary Electrophoresis**

progressive field with well-known and reliable analytical method.

**Keywords:** capillary electrophoresis, nanomaterials, separation efficiency, sensitivity

The very first nano-scientist was a Roman potter, who made the Lycurgus Cup as the oldest known application of nanomaterials (fifth-fourth century B.C.). This cup was made from so-called "gold-ruby glass" containing tiny gold droplets (5–60 nm in size). Therefore, the glass appeared green in daylight (reflected light), but red when light was transmitted from the inside of the vessel [1]. In spite of the fact that we do not know the name of the potter, which is based on nano-research

**Nanotechnologies in Capillary Electrophoresis**

Vojtech Adam and Marketa Vaculovicova

Vojtech Adam and Marketa Vaculovicova

http://dx.doi.org/10.5772/intechopen.72015

**Abstract**

**1. Introduction**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter


**Provisional chapter**

### **Capillary Electrophoresis in Nanotechnologies versus Nanotechnologies in Capillary Electrophoresis Nanotechnologies in Capillary Electrophoresis**

**Capillary Electrophoresis in Nanotechnologies versus** 

DOI: 10.5772/intechopen.72015

Vojtech Adam and Marketa Vaculovicova Additional information is available at the end of the chapter

Vojtech Adam and Marketa Vaculovicova

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.72015

#### **Abstract**

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Nanomaterials are attracting an interest of many researches. All this attention is due the unique physical and chemical properties of nanomaterials differing significantly from the bulk materials mainly due to their size in range of nanometers. Capillary electrophoresis (CE) is a powerful, well-established analytical technique that provides numerous valuable benefits over other separation methods including high-performance liquid chromatography. The connection between CE and nanotechnology can be approached by two strategies: (i) CE analysis of nanomaterials and (ii) nanomaterials for CE improvement. The first perspective focuses on uses of CE as a method for characterization employed during nanomaterial production and modification as well as for monitoring their properties and interactions with other molecules. The second viewpoint deals with applications of nanomaterials for improving CE performance, mainly by enhancing efficiency of separation using nanomaterials as a stationary or pseudo-stationary phase and by enhancing detection sensitivity and/or selectivity in both optical and electrochemical detection. Moreover, applications of nanomaterials for sample preparation before CE analysis will be mentioned. This chapter aims at highlighting the symbiosis of CE and nanotechnology as a combination of modern, progressive field with well-known and reliable analytical method.

**Keywords:** capillary electrophoresis, nanomaterials, separation efficiency, sensitivity

#### **1. Introduction**

The very first nano-scientist was a Roman potter, who made the Lycurgus Cup as the oldest known application of nanomaterials (fifth-fourth century B.C.). This cup was made from so-called "gold-ruby glass" containing tiny gold droplets (5–60 nm in size). Therefore, the glass appeared green in daylight (reflected light), but red when light was transmitted from the inside of the vessel [1]. In spite of the fact that we do not know the name of the potter, which is based on nano-research

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© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

without the knowledge of nanoparticles, Richard Feynman opened "nano-window of twentyfirst century" with his lecture "There's plenty of room at the bottom". This lecture came to be looked upon as the starting point of nanoscience as we are already living in [2]. In 1974, Taniguchi used the term "Nanotechnology" for the first time. This term was defined as the technology, where dimensions, within the range of 0.1–100 nm, play a key role. At the nanolevel, gravity is less an issue while the strength of materials is a bigger one and also quantum size effect is a key aspect. Due to the unique size-dependent spectroscopic, electronic, and thermal features and also chemical properties, and ability to be functionalized, arising from the small sizes and large surface-to-volume ratio, nanomaterials found their applications not only in electronics, physics, and engineering but also in natural sciences. Although nanomaterials are greatly affecting numerous scientific fields, it can be perceived differently. In chemistry, the range of sizes has been associated with colloid solutions, micelles, polymeric molecules, and also large molecules, or aggregates of number of molecules. Recently, structures such as carbon nanotubes, silicon nanorods, and semiconductor quantum dots have been emerged as particularly interesting classes of nanomaterials. In physics and electrical engineering, nanoscience is most often associated with quantum behavior, and the behavior of electrons and photons in nanoscale structures. Biochemistry and biology is interested in nanostructures such as cells components. The most widely investigated biological structures including DNA, viruses, and subcellular organelles can be considered as nanostructures [3].

**2. Capillary electrophoresis**

CE increased rapidly in past few decades.

trokinetic chromatography.

**Figure 1.** Scheme of CE setup.

Capillary electrophoresis is an extremely powerful microcolumn separation technique, separating molecules based on their mobilities in the electric field. Its main advantages include

Capillary Electrophoresis in Nanotechnologies versus Nanotechnologies in Capillary…

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Classical CE separation takes place in a fused silica capillary with internal diameter of 20–100μm, where the voltage of up to ±30 kV is applied. The scheme of the setup is shown in **Figure 1**.

Diversity of detection modalities applicable in CE is very wide. Optical detection methods including photometric and fluorimetric detection are probably the most common ones currently used in CE; however, electrochemical detection including amperometric and contactless conductometric detection have unparalleled advantages. Especially for analysis of small inorganic ions, which do not absorb light, electrochemical detection is an alternative to the indirect optical approach. Also, popularity of mass spectrometric (MS) detection coupled to

From the very beginning, electromigration methods benefited from the use of certain sieving media, such as paper or gel. Moreover, since the separations have been transferred to capillary and/or chip, the addition of some kind of "stationary phase", sieving media or pseudo-stationary phase increased the number of applications. This step led to rise of several electrophoretic methods such as capillary electrochromatography or capillary micellar elec-

Low sensitivity, which is probably the main weakness of CE in connection with the most wide spread detection method—spectrophotometric detection—is caused by the short optical path-length (given by the capillary diameter) and the low sample volume that is injected.

high separation efficiency, short time of analysis, and low consumption of chemicals.

#### **1.1. From nano-pottery to modern analytical tools**

It is obvious that "nano" influences the whole scientific world including instrumental analytical chemistry. Due to above-mentioned unique properties, not only new approaches and assays are being developed, but also standard techniques have been upgraded and capillary electrophoresis (CE) belongs to the group of these highly affected methods. In 1981, and since then, this powerful analytical technique progressed significantly not only in instrumentation, but also in method development, data acquisition, and processing. The group of applications has also widened markedly. The applications of CE are covering huge number of analytes from inorganic ions [4–8] and organic molecules [9–11] to biomolecules such as proteins [12–14] and DNA [15–17]. The golden era of CE was in 1990s, during the Human Genome Project [18]. The sequencing of the whole human genome was successfully finished in 2006 identifying all 20,000–25,000 genes (approximately) in human DNA and determining the sequences of 3 billion base pairs that make up human DNA.

Next great boom of CE begun due to the micro-total analysis system (μTAS) concept [19]. Due to the relatively simple instrumentation and ease of miniaturization of CE, the fast growth of attention in microfluidics and particularly in chip-based CE [20–23] was observed. Even though CE provides rapid results with high efficiency and resolution, and sample consumption is low; advantages of high number of theoretical plates can be diminished by relatively low sensitivity of commonly used photometric detection systems [24]. Therefore, new approaches improving these weak sites are investigated and the use of nanomaterials is widely tested.
