Hybrid Nanomaterials

**3**

**Chapter 1**

Nanomaterials

*Rafael Vargas-Bernal*

recent advances of this decade.

**2. Basic concepts**

**1. Introduction**

Introductory Chapter: Hybrid

One of the most exciting research fields in recent decades in the area of materials engineering is that of hybrid nanomaterials. These materials possess extraordinary physical and chemical properties derived from their size in the nanoscale. Among the reasons for this technological and scientific trend are its multidisciplinarity and the combination of the best attributes of both inorganic and organic chemistry, which give rise to multifunctional materials based on an approximation of building blocks. In addition, there is the possibility of incorporating the physical and biological sciences to produce biomimetic approaches to create unique materials derived from the requirements of emerging technologies that lead to the development of a current driving force to perform unprecedented research of materials, devices, and applications. So far, although many reviews, articles, and books are being continuously published, the scientific literature continues to surprise with new contributions and different views of researchers around the world. The purpose of this chapter is to present an updated introduction of hybrid nanomaterials and their

Hybrid nanomaterials are defined as unique chemical conjugates of organic and/or inorganic materials [1]. That is, these are mixtures of two or more inorganic components, two or more organic components, or at least one of both types of components. The resulting material is not a simple mixture of its components but a synergistic material with properties and performance to develop applications with unique properties, which are determined by the interface of the components at the molecular or supramolecular level. Its functionality is associated with the improvement of physicochemical properties. For the electrochemical or biochemical properties through the optimization mainly of magnetic, electronic, optical, and thermal properties or a combination of them, since the mechanical properties are rather directing towards flexibility, which is not considered as a demand [1], see **Figure 1**. The inclusion of nano-sized materials has further expanded the extraordinary properties of hybrid materials thanks to the challenge of having greater options for multifunctional materials. In the last two decades, the development of multifunctional applications is receiving a lot of attention thanks to the chemical and physical properties of the materials, which are giving rise to developments of high adding value. These materials are classified as innovative advanced materials applicable to a huge diversity of applications including optics, electronics, sensors, ionics, energy conversion and storage, mechanics, membranes, protective coatings, catalysis, etc. [1].

#### **Chapter 1**

## Introductory Chapter: Hybrid Nanomaterials

*Rafael Vargas-Bernal*

#### **1. Introduction**

One of the most exciting research fields in recent decades in the area of materials engineering is that of hybrid nanomaterials. These materials possess extraordinary physical and chemical properties derived from their size in the nanoscale. Among the reasons for this technological and scientific trend are its multidisciplinarity and the combination of the best attributes of both inorganic and organic chemistry, which give rise to multifunctional materials based on an approximation of building blocks. In addition, there is the possibility of incorporating the physical and biological sciences to produce biomimetic approaches to create unique materials derived from the requirements of emerging technologies that lead to the development of a current driving force to perform unprecedented research of materials, devices, and applications. So far, although many reviews, articles, and books are being continuously published, the scientific literature continues to surprise with new contributions and different views of researchers around the world. The purpose of this chapter is to present an updated introduction of hybrid nanomaterials and their recent advances of this decade.

#### **2. Basic concepts**

Hybrid nanomaterials are defined as unique chemical conjugates of organic and/or inorganic materials [1]. That is, these are mixtures of two or more inorganic components, two or more organic components, or at least one of both types of components. The resulting material is not a simple mixture of its components but a synergistic material with properties and performance to develop applications with unique properties, which are determined by the interface of the components at the molecular or supramolecular level. Its functionality is associated with the improvement of physicochemical properties. For the electrochemical or biochemical properties through the optimization mainly of magnetic, electronic, optical, and thermal properties or a combination of them, since the mechanical properties are rather directing towards flexibility, which is not considered as a demand [1], see **Figure 1**. The inclusion of nano-sized materials has further expanded the extraordinary properties of hybrid materials thanks to the challenge of having greater options for multifunctional materials. In the last two decades, the development of multifunctional applications is receiving a lot of attention thanks to the chemical and physical properties of the materials, which are giving rise to developments of high adding value. These materials are classified as innovative advanced materials applicable to a huge diversity of applications including optics, electronics, sensors, ionics, energy conversion and storage, mechanics, membranes, protective coatings, catalysis, etc. [1].

#### *Hybrid Nanomaterials - Flexible Electronics Materials*

**Figure 1.**

*Physical properties of the components of a hybrid material.*

The unique versatility of these materials allows designing materials with tunable properties, with improved performance and properties to their long-established counterparts in the market. The diversity of organic and inorganic components that can be incorporated into these materials is from sizes of a few angstroms to thousands of angstroms, so these can be categorized among molecular species, in nano- and/or supramolecular sizes, or with extended structure. Some of the illustrative examples of hybrid materials are [1]:


**5**

**Figure 2.**

*components.*

*Introductory Chapter: Hybrid Nanomaterials DOI: http://dx.doi.org/10.5772/intechopen.92012*

• Polymer-coated inorganic nanoparticles

• Fiber-reinforced nanocomposites, etc.

• Silica-embedded bioactive species

chemistry and molecular engineering.

**3. Classification**

• Organically substituted polysiloxanes and polysilsesquioxanes

In the range of nanomaterials and supramolecular materials, there is a greater variety of possible cases, which leads to a wide continuous set, whose size ranges from molecules to solid-state materials. Moreover, the chemical nature of the components as well as the interaction between them leads to different possibilities of structure, degree of organization, and properties. In the design of this type of materials, it is transcendent to tune the nature, extent, and accessibility of internal surfaces [2–4]. Globally, the trend most used for the application of new materials is to predict and control their chemical and physical properties. In particular, the manipulation of atoms and molecules in nano-sized materials is related to nano-

In accordance with the chemical origin of the interface or links established between the components in a hybrid material, these materials can be categorized as is shown in **Figure 2** [1, 2]. A first class of hybrid materials are those based on the synergy of the phases through weak chemical interactions based on Coulomb forces, London dispersion forces, hydrogen bonds, and dipole–dipole forces. A second class of hybrid materials are those based on the synergy of the phases through strong chemical bonds such as Lewis acid–base, covalent, or ionic-covalent bonds. The latter class of materials depends on the relative stability of the synergy between its components, since it determines the types of organic functionalization or the type of complex organic ligatures based on transition metal cations required

In addition, hybrid materials can be classified as organic-in-inorganic (organic moieties used to modify inorganic materials) or inorganic-in-organic (inorganic constituents used to modify organic materials or matrices), as illustrated in **Figure 3** [3]. Hybrid materials based on the first approach can be subdivided into two types, namely, (1) inorganic materials modified by organic moieties and (2) colloidal polymers stabilized by organic moieties. On the other hand, inorganic materials are

*Basic classification of hybrid materials according to the types of chemical interaction between their* 

to anchor the organic components to inorganic components.


*Hybrid Nanomaterials - Flexible Electronics Materials*

illustrative examples of hybrid materials are [1]:

*Physical properties of the components of a hybrid material.*

• Intergrowth organic–inorganic perovskites

• Organically grafted inorganic phases

chalcogenides, and 2D materials

• Mixed organic–inorganic polymers

• Silica plus organic polymer

• Polymer-clay composites

• Biomineral-type composites

materials

• Sol–gel silica modified with organic molecules

• Organically modified mesoporous materials

• Poly-oligo-silsesquixane-loaded polymers

• Active organic molecules doped into conductive polymers

• Polymer-supported inorganic clusters or nanoparticles

• Polymer-magnetic nanoparticle composites

• Active organic molecules intercalated into layered silicates, oxides,

• Polymers intercalated in layered silicates, oxides, chalcogenides, and 2D

• Donor-acceptor perovskites

**Figure 1.**

The unique versatility of these materials allows designing materials with tunable properties, with improved performance and properties to their long-established counterparts in the market. The diversity of organic and inorganic components that can be incorporated into these materials is from sizes of a few angstroms to thousands of angstroms, so these can be categorized among molecular species, in nano- and/or supramolecular sizes, or with extended structure. Some of the

**4**


In the range of nanomaterials and supramolecular materials, there is a greater variety of possible cases, which leads to a wide continuous set, whose size ranges from molecules to solid-state materials. Moreover, the chemical nature of the components as well as the interaction between them leads to different possibilities of structure, degree of organization, and properties. In the design of this type of materials, it is transcendent to tune the nature, extent, and accessibility of internal surfaces [2–4]. Globally, the trend most used for the application of new materials is to predict and control their chemical and physical properties. In particular, the manipulation of atoms and molecules in nano-sized materials is related to nanochemistry and molecular engineering.

### **3. Classification**

In accordance with the chemical origin of the interface or links established between the components in a hybrid material, these materials can be categorized as is shown in **Figure 2** [1, 2]. A first class of hybrid materials are those based on the synergy of the phases through weak chemical interactions based on Coulomb forces, London dispersion forces, hydrogen bonds, and dipole–dipole forces. A second class of hybrid materials are those based on the synergy of the phases through strong chemical bonds such as Lewis acid–base, covalent, or ionic-covalent bonds. The latter class of materials depends on the relative stability of the synergy between its components, since it determines the types of organic functionalization or the type of complex organic ligatures based on transition metal cations required to anchor the organic components to inorganic components.

In addition, hybrid materials can be classified as organic-in-inorganic (organic moieties used to modify inorganic materials) or inorganic-in-organic (inorganic constituents used to modify organic materials or matrices), as illustrated in **Figure 3** [3]. Hybrid materials based on the first approach can be subdivided into two types, namely, (1) inorganic materials modified by organic moieties and (2) colloidal polymers stabilized by organic moieties. On the other hand, inorganic materials are

#### **Figure 2.**

*Basic classification of hybrid materials according to the types of chemical interaction between their components.*

**Figure 3.**

*Types of hybrid materials based on the adding of inorganic and organic components and vice versa.*

modified via surface charge or functionalization with ligatures. Among the colloidal particles that can be used are nanoparticles, nanobars, nanotubes, nanostars, nanoflowers, etc., which must be electrostatically stabilized to be uniformly distributed throughout the material, preventing the formation of clusters. The incorporated inorganic constituents have small particle or structure morphologies, and these are made of clays, ceramics, minerals, metals, semiconductors, carbon-based nanomaterials, and two-dimensional materials. These materials are integrated into organic materials or matrices of either chemical or biological type. Chemical matrices can be polymers, monomers, synthetic molecules, etc., and the chemical materials derived from them are layers by layers (LbL), hydrogels, brushes, and copolymer blocks, both in the form of vehicles or coatings. The biological matrices used belong to one of the following groups: bacteria, microorganisms, molecules, polysaccharides, proteins, nucleic acids, carbohydrates, and lipids.

A more recent classification of hybrid materials is based on their functionality [1]. Three different types of materials can be identified: (1) structurally hybridized materials, (2) functionally hybridized materials, and (3) hybridized materials in their chemical bond.

#### **4. Applications**

Many applications have emerged by taking hybrid materials to the commercialization stage, and another huge amount is in its research and prototype phase to become emerging applications, as shown in **Figure 4** [4]. Among the numerous applications of organic–inorganic hybrid materials are:

**7**

*Introductory Chapter: Hybrid Nanomaterials DOI: http://dx.doi.org/10.5772/intechopen.92012*

• Spin-on-glass materials

• Electrical insulators

• Smart textiles

**Figure 4.**

• Green tires

• Biosensors

• Fuel cells

• Solar cells

• Actuators

• Optical chemical sensors

• Supercapacitors

• Flame retardants

• Automotive parts

• Dental products

• Acoustic and thermal insulators

*Applications of hybrid nanomaterials to flexible electronics (adapted from [4]).*

• Controlled-release biocapsules

• Biocatalysts and/or photocatalysts

• Hybrid anti-cancer nanoparticles

• Contrast agents for magnetic resonance imaging (MRI)


*Introductory Chapter: Hybrid Nanomaterials DOI: http://dx.doi.org/10.5772/intechopen.92012*


#### **Figure 4.**

*Hybrid Nanomaterials - Flexible Electronics Materials*

rides, proteins, nucleic acids, carbohydrates, and lipids.

applications of organic–inorganic hybrid materials are:

their chemical bond.

**4. Applications**

**Figure 3.**

• Gas barriers

• Packaging

• Sealants

• Hybrid pigments

• Decorative coatings

• Scratch-resistant coatings

• Anti-corrosion coatings

• Hair care products

modified via surface charge or functionalization with ligatures. Among the colloidal particles that can be used are nanoparticles, nanobars, nanotubes, nanostars, nanoflowers, etc., which must be electrostatically stabilized to be uniformly distributed throughout the material, preventing the formation of clusters. The incorporated inorganic constituents have small particle or structure morphologies, and these are made of clays, ceramics, minerals, metals, semiconductors, carbon-based nanomaterials, and two-dimensional materials. These materials are integrated into organic materials or matrices of either chemical or biological type. Chemical matrices can be polymers, monomers, synthetic molecules, etc., and the chemical materials derived from them are layers by layers (LbL), hydrogels, brushes, and copolymer blocks, both in the form of vehicles or coatings. The biological matrices used belong to one of the following groups: bacteria, microorganisms, molecules, polysaccha-

*Types of hybrid materials based on the adding of inorganic and organic components and vice versa.*

A more recent classification of hybrid materials is based on their functionality [1]. Three different types of materials can be identified: (1) structurally hybridized materials, (2) functionally hybridized materials, and (3) hybridized materials in

Many applications have emerged by taking hybrid materials to the commercialization stage, and another huge amount is in its research and prototype phase to become emerging applications, as shown in **Figure 4** [4]. Among the numerous

**6**

*Applications of hybrid nanomaterials to flexible electronics (adapted from [4]).*


Tag sensors for realizing radiofrequency identification (RFID) based on inkjet printing nanomaterials are easily stamped on textile, plastic, paper, glass, and metallic surfaces [6]. For example, by means of hybrid materials, using titania and silica, it is possible to develop templates onto polymer and/or glass substrates.

Electrochemical energy storage using supercapacitors is an option to power portable and/or wearable electronic devices. For this application, nanomaterials such as metal–organic frameworks (MOFs), metal nitrides, MXenes, and phosphorene are mixed with organic materials to improve electrode performance [7].

A huge variety of hybrid materials has been proposed for implementing electrodes for rechargeable batteries by means of inorganic polymers and materials such as graphene, carbon nanotubes, or their combination [8]. These storage devices are used to power portable applications.

Hybrid materials such as conductive polymers combined with nanostructured transition metal oxides, graphene, and/or carbon nanotubes are being used for the design of electrodes for solar cells [9].

#### **5. Conclusions**

Around the world, different research groups are continuously presenting new strategies, studies, and applications related to the technological development of novel hybrid materials. These materials promote continuous innovation in various technological sectors as presented in this chapter. Although there are numerous advances so far, the real possibilities can only be limited by the imagination of engineers and scientists around the world. The contributions of biologists, chemists, physicists, and materials scientists take advantage of both the integration and the miniaturization of electronic devices to develop emerging technologies in various areas of electronics as presented in the chapter. In this way, it can be affirmed that the area of hybrid nanomaterials is experiencing continuous growth as a topic of scientific research, incorporating more and more research groups worldwide thanks to the diversity of approaches that drive technological innovation.

#### **Acknowledgements**

The author appreciates the support of CONRICYT for access to the database to cite technical documents included in this chapter.

#### **Thanks**

The author wants to thank his wife and son for their support and time to edit this book. The author appreciates the support of Rebekah Pribetic working for Intech as Author Service Manager.

**9**

**Author details**

Rafael Vargas-Bernal

Guanajuato, México

Higher Technological Institute of Irapuato, Carretera Irapuato-Silao Km,

© 2020 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,

\*Address all correspondence to: ravargas@itesi.edu.mx

provided the original work is properly cited.

*Introductory Chapter: Hybrid Nanomaterials DOI: http://dx.doi.org/10.5772/intechopen.92012* *Introductory Chapter: Hybrid Nanomaterials DOI: http://dx.doi.org/10.5772/intechopen.92012*

*Hybrid Nanomaterials - Flexible Electronics Materials*

• Organic light-emitting diodes (OLEDs)

• Photochromic and/or electrochromic coatings, etc. [5]

mixed with organic materials to improve electrode performance [7].

Tag sensors for realizing radiofrequency identification (RFID) based on inkjet printing nanomaterials are easily stamped on textile, plastic, paper, glass, and metallic surfaces [6]. For example, by means of hybrid materials, using titania and silica, it is possible to develop templates onto polymer and/or glass substrates.

Electrochemical energy storage using supercapacitors is an option to power portable and/or wearable electronic devices. For this application, nanomaterials such as metal–organic frameworks (MOFs), metal nitrides, MXenes, and phosphorene are

A huge variety of hybrid materials has been proposed for implementing electrodes for rechargeable batteries by means of inorganic polymers and materials such as graphene, carbon nanotubes, or their combination [8]. These storage devices are

Hybrid materials such as conductive polymers combined with nanostructured transition metal oxides, graphene, and/or carbon nanotubes are being used for the

Around the world, different research groups are continuously presenting new strategies, studies, and applications related to the technological development of novel hybrid materials. These materials promote continuous innovation in various technological sectors as presented in this chapter. Although there are numerous advances so far, the real possibilities can only be limited by the imagination of engineers and scientists around the world. The contributions of biologists, chemists, physicists, and materials scientists take advantage of both the integration and the miniaturization of electronic devices to develop emerging technologies in various areas of electronics as presented in the chapter. In this way, it can be affirmed that the area of hybrid nanomaterials is experiencing continuous growth as a topic of scientific research, incorporating more and more research groups worldwide thanks to the diversity of approaches that drive technological

The author appreciates the support of CONRICYT for access to the database to

The author wants to thank his wife and son for their support and time to edit this book. The author appreciates the support of Rebekah Pribetic working for Intech as

• Flexible hybrid batteries

• Microlenses and waveguides

used to power portable applications.

design of electrodes for solar cells [9].

**5. Conclusions**

innovation.

**Thanks**

**Acknowledgements**

Author Service Manager.

cite technical documents included in this chapter.

**8**

#### **Author details**

Rafael Vargas-Bernal Higher Technological Institute of Irapuato, Carretera Irapuato-Silao Km, Guanajuato, México

\*Address all correspondence to: ravargas@itesi.edu.mx

© 2020 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.

### **References**

[1] Gómez-Romero P, Sanchez C. Functional Hybrid Materials. Weinheim, Germany: Wiley-VCH; 2004. p. 434

[2] Kickelbick G. Hybrid Materials: Synthesis, Characterization and Applications. Germany Wiley-VCH: Weinheim; 2007. p. 517

[3] Saveleva MS, Eftekhari K, Abalymov A, Douglas TEL, Volodkin D, Parakhonskyiy V, et al. Hierarchy of hybrid materials – The place of inorganics-in-organics in it, their composition and applications. Frontiers in Chemistry. 2019;**7**:179. DOI: 10.3389/fchem.2019.00179

[4] Wu W. Inorganic nanomaterials for printed electronics: A review. Nanoscale. 2017;**9**(22):7342-7372. DOI: 10.1039/c7nr01604b

[5] Sanchez C, Bellevile P, Popall M, Nicole L. Applications of advanced hybrid organic-inorganic nanomaterials: From laboratory to market. Chemical Society Reviews. 2011;**40**(2):696-753. DOI: 10.1039/c0cs00136h

[6] Singh R, Singh E, Nalwa HS. Inkjet printed nanomaterials based flexible radio frequency identification (RFID) tag sensors for the Internet of nano things. RSC Advances. 2017;**7**(77):48597-48630. DOI: 10.1039/ c7ra07191d

[7] Dubal DP, Chodankar NR, Km DH, Gomez-Romero P. Towards flexible solid-state supercapacitors for smart and wearable electronics. Chemical Society Reviews. 2018;**47**(6):2065-2129. DOI: 10.1039/c7cs00505a

[8] Peng HJ, Huang JQ, Zhang Q. A review of flexible lithium-sulfur and analogous alkali metal-chalcogen rechargeable batteries. Chemical Society Reviews. 2017;**46**(17):5237-5288. DOI: 10.1039/c7cs00139h

[9] Li L, Wu Z, Yuan S, Zhang XB. Advances and challenges for flexible energy storage and conversion devices and systems. Energy & Environmental Science. 2014;**7**(7):2101-2122. DOI: 10.1039/c4ee00318g

**Chapter 2**

**Abstract**

Black Phosphorus

discussed based on quantum transport.

out due to a large charge carrier scattering rate.

quantum hall effect

**1. Introduction**

**11**

Electronic Transport in Few-Layer

Subjected to an adequately high magnetic field, Landau levels (LLs) form to alter the electronic transport behavior of a semiconductor. Especially in twodimensional (2D) limit, quantum Hall effect sheds light on a variety of intrinsic properties of 2D electronic systems. With the raising quality of field effect transistors (FET) based on few-layer black phosphorus (BP), electronic transport in quantum limit (quantum transport) has been extensively studied in literatures. This chapter investigates the electronic transport in few-layer BP, especially in quantum limit. At the beginning of this chapter, a brief introduction to the background of LL, edge state, and quantum Hall effect will be delivered. We then examine the fabrication of high-quality FET based on BP and their electronic performances followed by exploring the magnetoresistances of these high-quality devices which reveal Shubnikov-de Haas (SdH) oscillations and quantum Hall effect in BP. Intrinsic parameters like effective mass, Landé g-factor, and so on are

**Keywords:** black phosphorus, field effect transistor, mobility, Landau level,

Developing high-quality functional materials such as high-mobility semiconductors is an essential process to explore fundamental condensed matter research. The recent rediscovery of BP as a new member of 2D materials with theoretically predicted high mobility and widely tunable electronic bandgap makes it promising for various electronic devices and for probing interesting physical phenomena [1–4]. However, there are various obstacles to overcome in order to achieve the theoretically high carrier mobility in few-layer BP. Those obstacles include degradation of flake quality in atmosphere environment, electronic scatterings (against charge impurities, crystal defeats, and so on), high contact resistance, and so on. In devices without high enough mobility, new physical phenomena can be smeared

Since Landau level (LL) is crucial to understanding the physical phenomena in quantum transport process, we would like to deliver a brief idea about it here. For clarity, we will neglect the spin degeneracy at first and will come to it when necessary. Consider an electronic system without electronic interactions in an applied magnetic field along *z* direction and the magnetic field strength is *B*:

*Gen Long, Xiaolong Chen, Shuigang Xu and Ning Wang*

#### **Chapter 2**

## Electronic Transport in Few-Layer Black Phosphorus

*Gen Long, Xiaolong Chen, Shuigang Xu and Ning Wang*

#### **Abstract**

Subjected to an adequately high magnetic field, Landau levels (LLs) form to alter the electronic transport behavior of a semiconductor. Especially in twodimensional (2D) limit, quantum Hall effect sheds light on a variety of intrinsic properties of 2D electronic systems. With the raising quality of field effect transistors (FET) based on few-layer black phosphorus (BP), electronic transport in quantum limit (quantum transport) has been extensively studied in literatures. This chapter investigates the electronic transport in few-layer BP, especially in quantum limit. At the beginning of this chapter, a brief introduction to the background of LL, edge state, and quantum Hall effect will be delivered. We then examine the fabrication of high-quality FET based on BP and their electronic performances followed by exploring the magnetoresistances of these high-quality devices which reveal Shubnikov-de Haas (SdH) oscillations and quantum Hall effect in BP. Intrinsic parameters like effective mass, Landé g-factor, and so on are discussed based on quantum transport.

**Keywords:** black phosphorus, field effect transistor, mobility, Landau level, quantum hall effect

#### **1. Introduction**

Developing high-quality functional materials such as high-mobility semiconductors is an essential process to explore fundamental condensed matter research. The recent rediscovery of BP as a new member of 2D materials with theoretically predicted high mobility and widely tunable electronic bandgap makes it promising for various electronic devices and for probing interesting physical phenomena [1–4]. However, there are various obstacles to overcome in order to achieve the theoretically high carrier mobility in few-layer BP. Those obstacles include degradation of flake quality in atmosphere environment, electronic scatterings (against charge impurities, crystal defeats, and so on), high contact resistance, and so on. In devices without high enough mobility, new physical phenomena can be smeared out due to a large charge carrier scattering rate.

Since Landau level (LL) is crucial to understanding the physical phenomena in quantum transport process, we would like to deliver a brief idea about it here. For clarity, we will neglect the spin degeneracy at first and will come to it when necessary. Consider an electronic system without electronic interactions in an applied magnetic field along *z* direction and the magnetic field strength is *B*:

**10**

*Hybrid Nanomaterials - Flexible Electronics Materials*

[9] Li L, Wu Z, Yuan S, Zhang XB. Advances and challenges for flexible energy storage and conversion devices and systems. Energy & Environmental Science. 2014;**7**(7):2101-2122. DOI:

10.1039/c4ee00318g

[1] Gómez-Romero P, Sanchez C.

[2] Kickelbick G. Hybrid Materials: Synthesis, Characterization and Applications. Germany Wiley-VCH:

Volodkin D, Parakhonskyiy V, et al. Hierarchy of hybrid materials – The place of inorganics-in-organics in it, their composition and applications. Frontiers in Chemistry. 2019;**7**:179. DOI: 10.3389/fchem.2019.00179

[4] Wu W. Inorganic nanomaterials for printed electronics: A review. Nanoscale. 2017;**9**(22):7342-7372.

[5] Sanchez C, Bellevile P, Popall M, Nicole L. Applications of advanced hybrid organic-inorganic nanomaterials: From laboratory to market. Chemical Society Reviews. 2011;**40**(2):696-753.

[6] Singh R, Singh E, Nalwa HS. Inkjet printed nanomaterials based flexible radio frequency identification (RFID) tag sensors for the Internet of nano things. RSC Advances.

2017;**7**(77):48597-48630. DOI: 10.1039/

[7] Dubal DP, Chodankar NR, Km DH, Gomez-Romero P. Towards flexible solid-state supercapacitors for smart and wearable electronics. Chemical Society Reviews. 2018;**47**(6):2065-2129.

[8] Peng HJ, Huang JQ, Zhang Q. A review of flexible lithium-sulfur and analogous alkali metal-chalcogen rechargeable batteries. Chemical Society Reviews. 2017;**46**(17):5237-5288. DOI:

DOI: 10.1039/c7nr01604b

DOI: 10.1039/c0cs00136h

DOI: 10.1039/c7cs00505a

10.1039/c7cs00139h

c7ra07191d

Weinheim; 2007. p. 517

[3] Saveleva MS, Eftekhari K, Abalymov A, Douglas TEL,

Functional Hybrid Materials. Weinheim, Germany: Wiley-VCH; 2004. p. 434

**References**

*Hybrid Nanomaterials - Flexible Electronics Materials*

$$\mathcal{B} = \begin{pmatrix} 0 \\ 0 \\ B \end{pmatrix} \tag{1}$$

opposite sides is broken, i.e., more electrons are filled in the edge states in one boundary. As we have discussed before, the energy of electrons is quantized. Con-

Alternatively, quantum Hall effect can be understood based on Berry phase [6]. Consider an adiabatic system in which the eigenstate evolves with external parameters slowly. When the external parameters make up a loop and come back to itself, the eigenstate should also come back to itself (let us rule out the possibility of degeneracy here) but with a different phase, because the eigen energy of eigenstates with different phases is invariant. This phase is the so-called Berry phase. This Berry phase manifests itself in an electronic system through weak localization effect, A-B effect, and so on. All these effects come from the dynamics of electrons. For Bloch electron under the perturbation of a weak electric field *E*, its velocity *v*

*<sup>h</sup>* , where *ν* ¼ 1*,* 2*,* 3… is integer.

*v k*ð Þ¼ *<sup>∂</sup>ε*ð Þ *<sup>k</sup>*

transverse contribution to velocity, the Hall conductivity is given by

*<sup>σ</sup>xy* <sup>¼</sup> *<sup>e</sup>*<sup>2</sup> ℏ ð

magnetic field is zero). Therefore, the Hall conductivity is

<sup>ℏ</sup>∂*<sup>k</sup>* � *<sup>e</sup>*

*BZ*

where the integration is over the entire Brillouin zone and the result of this integration is the Chern number *ν* of the Brillouin zone (ν = 0 when external

*σxy* ¼ *ν*

This reaches the same conclusion with previous analysis, i.e., the Hall conduc-

In this chapter, we would introduce the detailed work on BP from the characterization of the electronic transport process and figure out the main limitation to high-quality devices, followed by efforts to conquer the impediment. Once devices with high enough quality can be fabricated, we will go to the electronic transport in quantum limit (cryogenic temperature and high magnetic field). Realization of quantum transport process in BP allows people to determine some critical parameters (effective mass, Landé g-factor, and so on) which are crucial for seeking

In 2014, several groups first demonstrated the FET based on few-layer BP (**Figure 1a**) [1, 3, 7, 8], giving rise to the renaissance of BP. As a semiconducting layered material, BP has a high carrier mobility at room temperature, with the value up to 1000 cm2V�1s�<sup>1</sup> at the first demonstration of its FET, owing to its relatively

*e*2

where *ε* is the energy of electron and **Ω** is the Berry curvature. The first term is the result of band dispersion. The second term which is absent in classic Bloch theorem is more interesting. It describes a velocity perpendicular to the applied electric field which will give rise to the Hall effect without magnetic field. With this

> *d*2 *k*

*VH* is quantized. Analysis into more details

<sup>ℏ</sup> *<sup>E</sup>* � **<sup>Ω</sup>**ð Þ *<sup>k</sup>* (6)

ð Þ <sup>2</sup>*<sup>π</sup>* <sup>2</sup> <sup>Ω</sup>*kxky* (7)

*<sup>h</sup>* (8)

sequently, the Hall conductance *<sup>σ</sup><sup>H</sup>* <sup>¼</sup> *Ichannel*

*Electronic Transport in Few-Layer Black Phosphorus DOI: http://dx.doi.org/10.5772/intechopen.89149*

with a given wave vector *k* becomes

leads to *<sup>σ</sup><sup>H</sup>* <sup>¼</sup> *<sup>ν</sup> <sup>e</sup>*<sup>2</sup>

tivity is quantized.

**13**

engineering applications of this material.

**2. High-quality FET based on BP**

In Landau gauge (the wave function is gauge invariant),

$$\mathbf{B} = \nabla \times \mathbf{A} = \nabla \times \begin{pmatrix} \mathbf{0} \\ \mathbf{B} \mathbf{x} \\ \mathbf{0} \end{pmatrix} \tag{2}$$

where *A* is the electromagnetic vector potential. The Hamiltonian of this system is then

$$H = \frac{1}{2m}(\mathbf{P} - e\mathbf{A})^2 = \frac{p\_x^2}{2m} + \frac{1}{2m}\left(p\_y - e\mathbf{B}\mathbf{x}\right) \tag{3}$$

where *e* stands for electron charge, *P* is the canonical momentum, and *m* is the mass of charge carriers. Solving this Hamiltonian we get,

$$\begin{aligned} E\_n &= \hbar o(n + 1/2) \\ o &= \frac{e\mathcal{B}}{m} \end{aligned} \tag{4}$$

where *ω* is the cyclotron frequency and ℏ is the reduced Planck constant. Clearly, the eigen energy of electrons are separated and equally spaced. The energy gap between two adjacent LLs is ℏ*ω*. Furthermore, by studying the electronic systems in finite regime, the degeneracy (*gL*) of LL per unit area is derived to be,

$$\mathbf{g}\_L = \frac{eB}{2\pi\hbar} \tag{5}$$

Now we take the spin of electron into consideration. Besides the formation of LLs, the energy of electron is also split by Zeeman effect *EZ* ¼ *gμBB*, where *μ<sup>B</sup>* ¼ *e*ℏ*=*2*m*<sup>0</sup> (*m*<sup>0</sup> is free electron mass) is the Bohr magneton and *g* is the Landé g-factor. Interestingly, the Zeeman splitting coincides precisely with the energy gap between LLs *EC* ¼ ℏ*ω* ¼ *e*ℏ*B=m* [5], for free electrons (*g* ¼ 2, *m* ¼ *m*0). It means that spin-up electrons in *nth* LL exhibit the same energy with spin down ones in ð Þ *n* þ 1 *th* LLs. However, this coincidence usually does not happen in real material systems because of the deviations of effective mass from *m*<sup>0</sup> and g-factor from 2. Nevertheless, this coincidence can be realized by tilting the magnetic field to adjust the ratio between *EC* and *EZ* which would be discussed hereafter.

We conclude now that under applied magnetic field, electrons cycle in quantized traces with quantized energy in one direction. However, when these traces intersect the edge of the electron system, these electrons cannot complete the full cyclotron. Instead, they bounce back and enter another adjacent traces because the cycling direction is fixed. As a result, electrons near the edge are in a skipping motion along one-dimensional boundary moving in a single direction, which is referred to as chiral edge states. Due to the absence of scattering, the edge states carry electronic current with no voltage drop, i.e., *Rxx* ¼ *Vxx=Ichannel* ¼ 0. Electrons near different edges move in different directions to ensure that the net current, in the absence of electric field, is zero.

When an in-plane electric field is applied, electrons are accelerated along one direction. As a result, the balance between the occupation of edge states in two

*Electronic Transport in Few-Layer Black Phosphorus DOI: http://dx.doi.org/10.5772/intechopen.89149*

*B* ¼

*B* ¼ ∇ � *A* ¼ ∇ �

<sup>2</sup>*<sup>m</sup>* ð Þ *<sup>P</sup>* � *<sup>e</sup><sup>A</sup>* <sup>2</sup> <sup>¼</sup> *<sup>p</sup>*<sup>2</sup>

where *A* is the electromagnetic vector potential. The Hamiltonian of this

*x* 2*m* þ 1

where *e* stands for electron charge, *P* is the canonical momentum, and *m* is the

*En* ¼ ℏ*ω*ð Þ *n* þ 1*=*2 *<sup>ω</sup>* <sup>¼</sup> *<sup>e</sup><sup>B</sup> m*

where *ω* is the cyclotron frequency and ℏ is the reduced Planck constant. Clearly, the eigen energy of electrons are separated and equally spaced. The energy gap between two adjacent LLs is ℏ*ω*. Furthermore, by studying the electronic systems in finite regime, the degeneracy (*gL*) of LL per unit area is derived to be,

*gL* <sup>¼</sup> *eB*

Now we take the spin of electron into consideration. Besides the formation of LLs, the energy of electron is also split by Zeeman effect *EZ* ¼ *gμBB*, where *μ<sup>B</sup>* ¼ *e*ℏ*=*2*m*<sup>0</sup> (*m*<sup>0</sup> is free electron mass) is the Bohr magneton and *g* is the Landé g-factor. Interestingly, the Zeeman splitting coincides precisely with the energy gap between LLs *EC* ¼ ℏ*ω* ¼ *e*ℏ*B=m* [5], for free electrons (*g* ¼ 2, *m* ¼ *m*0). It means that spin-up electrons in *nth* LL exhibit the same energy with spin down ones in ð Þ *n* þ 1 *th* LLs. However, this coincidence usually does not happen in real material systems because of the deviations of effective mass from *m*<sup>0</sup> and g-factor from 2. Nevertheless, this coincidence can be realized by tilting the magnetic field to adjust the ratio between

We conclude now that under applied magnetic field, electrons cycle in quantized traces with quantized energy in one direction. However, when these traces intersect the edge of the electron system, these electrons cannot complete the full cyclotron. Instead, they bounce back and enter another adjacent traces because the cycling direction is fixed. As a result, electrons near the edge are in a skipping motion along one-dimensional boundary moving in a single direction, which is referred to as chiral edge states. Due to the absence of scattering, the edge states carry electronic current with no voltage drop, i.e., *Rxx* ¼ *Vxx=Ichannel* ¼ 0. Electrons near different edges move in different directions to ensure that the net current, in

When an in-plane electric field is applied, electrons are accelerated along one direction. As a result, the balance between the occupation of edge states in two

In Landau gauge (the wave function is gauge invariant),

*<sup>H</sup>* <sup>¼</sup> <sup>1</sup>

*Hybrid Nanomaterials - Flexible Electronics Materials*

*EC* and *EZ* which would be discussed hereafter.

the absence of electric field, is zero.

**12**

mass of charge carriers. Solving this Hamiltonian we get,

system is then

0 0 *B* 1

0 *Bx* 0

1

<sup>2</sup>*<sup>m</sup> py* � *eBx* � �

<sup>2</sup>*π*<sup>ℏ</sup> (5)

0

B@

CA (1)

CA (2)

(3)

(4)

0

B@

opposite sides is broken, i.e., more electrons are filled in the edge states in one boundary. As we have discussed before, the energy of electrons is quantized. Consequently, the Hall conductance *<sup>σ</sup><sup>H</sup>* <sup>¼</sup> *Ichannel VH* is quantized. Analysis into more details leads to *<sup>σ</sup><sup>H</sup>* <sup>¼</sup> *<sup>ν</sup> <sup>e</sup>*<sup>2</sup> *<sup>h</sup>* , where *ν* ¼ 1*,* 2*,* 3… is integer.

Alternatively, quantum Hall effect can be understood based on Berry phase [6]. Consider an adiabatic system in which the eigenstate evolves with external parameters slowly. When the external parameters make up a loop and come back to itself, the eigenstate should also come back to itself (let us rule out the possibility of degeneracy here) but with a different phase, because the eigen energy of eigenstates with different phases is invariant. This phase is the so-called Berry phase. This Berry phase manifests itself in an electronic system through weak localization effect, A-B effect, and so on. All these effects come from the dynamics of electrons. For Bloch electron under the perturbation of a weak electric field *E*, its velocity *v* with a given wave vector *k* becomes

$$\boldsymbol{\nu}(\boldsymbol{k}) = \frac{\partial \boldsymbol{\varepsilon}(\boldsymbol{k})}{\hbar \partial \boldsymbol{k}} - \frac{\boldsymbol{e}}{\hbar} \boldsymbol{E} \times \boldsymbol{\Omega}(\boldsymbol{k}) \tag{6}$$

where *ε* is the energy of electron and **Ω** is the Berry curvature. The first term is the result of band dispersion. The second term which is absent in classic Bloch theorem is more interesting. It describes a velocity perpendicular to the applied electric field which will give rise to the Hall effect without magnetic field. With this transverse contribution to velocity, the Hall conductivity is given by

$$
\sigma\_{\text{xy}} = \frac{e^2}{\hbar} \int\_{BZ} \frac{d^2k}{(2\pi)^2} \Omega\_{k\_x k\_y} \tag{7}
$$

where the integration is over the entire Brillouin zone and the result of this integration is the Chern number *ν* of the Brillouin zone (ν = 0 when external magnetic field is zero). Therefore, the Hall conductivity is

$$
\sigma\_{\text{xy}} = \nu \frac{e^2}{h} \tag{8}
$$

This reaches the same conclusion with previous analysis, i.e., the Hall conductivity is quantized.

In this chapter, we would introduce the detailed work on BP from the characterization of the electronic transport process and figure out the main limitation to high-quality devices, followed by efforts to conquer the impediment. Once devices with high enough quality can be fabricated, we will go to the electronic transport in quantum limit (cryogenic temperature and high magnetic field). Realization of quantum transport process in BP allows people to determine some critical parameters (effective mass, Landé g-factor, and so on) which are crucial for seeking engineering applications of this material.

#### **2. High-quality FET based on BP**

In 2014, several groups first demonstrated the FET based on few-layer BP (**Figure 1a**) [1, 3, 7, 8], giving rise to the renaissance of BP. As a semiconducting layered material, BP has a high carrier mobility at room temperature, with the value up to 1000 cm2V�1s�<sup>1</sup> at the first demonstration of its FET, owing to its relatively

The environmental instability of BP flakes makes it a challenge to fabricate high performance BP-based devices, since most experiments reported on BP relied on mechanically exfoliated flakes. To prevent the degradation, several kinds of passivation were developed. Atomic layer-deposited *AlOx* overlayers effectively suppress ambient degradation, preserving the intrinsic high carrier mobility and on-off ratios in BP FETs [16]. Alternatively, encapsulating BP by a polymer superstrate, such as PMMA, can also suppress the oxidation [8, 17]. Moreover, it was demonstrated that hexagonal boron nitride (*h*BN) can be effectively used for passivation of BP [9, 18–21]. The devices fabricated by *h*BN-encapsulated BP showed air-stable performance and hysteresis-free transport characteristics in ambient conditions, without observable decrease in carrier mobility or on-off ratio after few weeks' exposure in air [9, 18]. Furthermore, *h*BN is an insulating layered material, providing atomically thin clean surface and dielectric environment. Before the rediscovery of BP, it was already reported that *h*BN can significantly improve the mobility of graphene-based

devices [22, 23]. Researchers found that by applying the *h*BN encapsulation

**2.2 Ohmic contact and ambipolar transport in BP**

*Electronic Transport in Few-Layer Black Phosphorus DOI: http://dx.doi.org/10.5772/intechopen.89149*

cryogenic temperatures.

**Figure 2.**

**15**

*reproduced with permission from NPG [25].*

technique in graphene to BP (**Figure 2**), the mobility of BP-based FETs can increase to several 1000 cm2V1s<sup>1</sup> [9, 24], later even up to 45,000 cm2V1s<sup>1</sup> [10] at

As that in other semiconducting devices, the electrical performance of devices based on BP is significantly affected by the electrical contacts. Low contact resistance is critical to achieving high mobility, high on-off ratio, and large photo response in BP. The main issue in BP-based FET is the existence of large Schottky barrier in the contacts, which limits the current injection and its potential for applications. Several kinds of contact engineering have been identified toward highquality electrical contacts. By choosing various metals with different work functions to match BP, different performance of BP transistors can be achieved [26, 27]. The aluminum-contacted BP displays ambipolar characteristic with nearly symmetric electron and hole mobility of 950 cm2V1s<sup>1</sup> in 13 nm flake and unipolar *n*-type

*The cross-section HR-TEM image of the* h*BN/BP/*h*BN structure. The right panel shows the element analysis for nitrogen (blue), phosphorus (green), and oxygen (red). Scale bars: 4/10 nm (left/right). This figure is*

#### **Figure 1.**

*High-mobility BP FET. (a) Schematic of FET device based on encapsulated BP. The right panel shows the crystal structure of BP. (b) Transfer curves of high-mobility BP devices at cryogenic temperature accompanied with the corresponding FET mobility. (c) Temperature dependence of FET mobility and hall mobility. Panel (a) is reproduced with permission from NPG [12]. Panels (b) and (c) are reproduced with permission from ACS [10].*

weak electron-phonon interaction [3]. Meanwhile, the FET's current on-off ratio exceeds 105, which indicates the promising electronic application of BP-based devices [2, 3, 9]. Up to now, the record room temperature hole mobility achieved in experiments arrives 5200 cm2V1s1, almost approaching the theoretical phonon scattering limitation ranging between 4800 and 6400 cm2V1s<sup>1</sup> for few-layer BP [10]. The high mobility and desirable current saturation of BP at room temperature offer advantages for high-frequency electronic and optoelectronic applications [11]. At cryogenic temperatures, the FET hole mobility can be even up to 45,000 cm2V1s<sup>1</sup> as shown in **Figure 1b** and **c** [10], the highest value among the presently reported 2D semiconductors. The ultrahigh mobility in BP at low temperature makes it as a unique platform for studying the quantum transport in 2D semiconductors.

As the most famous elemental layered material beyond graphene, BP provides the needed bandgap for FET applications, especially in infrared region. Unlike semiconducting transition metal dichalcogenides (TMDCs), the bandgap in BP is direct for all number of layers, which makes this material particularly promising for optoelectronic applications. Moreover, the bandgap of BP is widely tunable achieved by layer number, electric field, strain, and alloying. Combined with its strong anisotropic, BP may allow for the exploration of new exotic phenomena and multifunction devices.

#### **2.1 Realization of air-stable devices based on BP**

Although bulk BP is considered as the most stable allotrope of phosphorus, it is reported that few-layer BP is unstable in ambient conditions. Significant surface roughening over the time after exfoliation can be observed by AFM [8]. The investigation of water condensation on BP flakes reveals that BP is very hygroscopic and tends to uptake moisture from air [13]. Long-time exposure to air can even etch BP away. The in situ Raman and transmission electron spectroscopy studies of the degradation of BP show that the degradation in air mainly arises from the photoassisted oxidation reaction with oxygen dissolved in the adsorbed water on its surface [14]. Theoretical work revealed that BP presents a strong dipolar moment out of plane which makes it very hydrophilic [15].

*Electronic Transport in Few-Layer Black Phosphorus DOI: http://dx.doi.org/10.5772/intechopen.89149*

The environmental instability of BP flakes makes it a challenge to fabricate high performance BP-based devices, since most experiments reported on BP relied on mechanically exfoliated flakes. To prevent the degradation, several kinds of passivation were developed. Atomic layer-deposited *AlOx* overlayers effectively suppress ambient degradation, preserving the intrinsic high carrier mobility and on-off ratios in BP FETs [16]. Alternatively, encapsulating BP by a polymer superstrate, such as PMMA, can also suppress the oxidation [8, 17]. Moreover, it was demonstrated that hexagonal boron nitride (*h*BN) can be effectively used for passivation of BP [9, 18–21]. The devices fabricated by *h*BN-encapsulated BP showed air-stable performance and hysteresis-free transport characteristics in ambient conditions, without observable decrease in carrier mobility or on-off ratio after few weeks' exposure in air [9, 18]. Furthermore, *h*BN is an insulating layered material, providing atomically thin clean surface and dielectric environment. Before the rediscovery of BP, it was already reported that *h*BN can significantly improve the mobility of graphene-based devices [22, 23]. Researchers found that by applying the *h*BN encapsulation technique in graphene to BP (**Figure 2**), the mobility of BP-based FETs can increase to several 1000 cm2V1s<sup>1</sup> [9, 24], later even up to 45,000 cm2V1s<sup>1</sup> [10] at cryogenic temperatures.

#### **2.2 Ohmic contact and ambipolar transport in BP**

As that in other semiconducting devices, the electrical performance of devices based on BP is significantly affected by the electrical contacts. Low contact resistance is critical to achieving high mobility, high on-off ratio, and large photo response in BP. The main issue in BP-based FET is the existence of large Schottky barrier in the contacts, which limits the current injection and its potential for applications. Several kinds of contact engineering have been identified toward highquality electrical contacts. By choosing various metals with different work functions to match BP, different performance of BP transistors can be achieved [26, 27]. The aluminum-contacted BP displays ambipolar characteristic with nearly symmetric electron and hole mobility of 950 cm2V1s<sup>1</sup> in 13 nm flake and unipolar *n*-type

#### **Figure 2.**

*The cross-section HR-TEM image of the* h*BN/BP/*h*BN structure. The right panel shows the element analysis for nitrogen (blue), phosphorus (green), and oxygen (red). Scale bars: 4/10 nm (left/right). This figure is reproduced with permission from NPG [25].*

weak electron-phonon interaction [3]. Meanwhile, the FET's current on-off ratio exceeds 105, which indicates the promising electronic application of BP-based devices [2, 3, 9]. Up to now, the record room temperature hole mobility achieved in experiments arrives 5200 cm2V1s1, almost approaching the theoretical phonon scattering limitation ranging between 4800 and 6400 cm2V1s<sup>1</sup> for few-layer BP [10]. The high mobility and desirable current saturation of BP at room temperature offer advantages for high-frequency electronic and optoelectronic applications [11].

*High-mobility BP FET. (a) Schematic of FET device based on encapsulated BP. The right panel shows the crystal structure of BP. (b) Transfer curves of high-mobility BP devices at cryogenic temperature accompanied with the corresponding FET mobility. (c) Temperature dependence of FET mobility and hall mobility. Panel (a) is reproduced with permission from NPG [12]. Panels (b) and (c) are reproduced with permission from*

At cryogenic temperatures, the FET hole mobility can be even up to 45,000

optoelectronic applications. Moreover, the bandgap of BP is widely tunable achieved by layer number, electric field, strain, and alloying. Combined with its strong anisotropic, BP may allow for the exploration of new exotic phenomena and

**2.1 Realization of air-stable devices based on BP**

*Hybrid Nanomaterials - Flexible Electronics Materials*

out of plane which makes it very hydrophilic [15].

semiconductors.

**Figure 1.**

*ACS [10].*

multifunction devices.

**14**

cm2V1s<sup>1</sup> as shown in **Figure 1b** and **c** [10], the highest value among the presently reported 2D semiconductors. The ultrahigh mobility in BP at low temperature makes it as a unique platform for studying the quantum transport in 2D

As the most famous elemental layered material beyond graphene, BP provides the needed bandgap for FET applications, especially in infrared region. Unlike semiconducting transition metal dichalcogenides (TMDCs), the bandgap in BP is direct for all number of layers, which makes this material particularly promising for

Although bulk BP is considered as the most stable allotrope of phosphorus, it is reported that few-layer BP is unstable in ambient conditions. Significant surface roughening over the time after exfoliation can be observed by AFM [8]. The investigation of water condensation on BP flakes reveals that BP is very hygroscopic and tends to uptake moisture from air [13]. Long-time exposure to air can even etch BP away. The in situ Raman and transmission electron spectroscopy studies of the degradation of BP show that the degradation in air mainly arises from the photoassisted oxidation reaction with oxygen dissolved in the adsorbed water on its surface [14]. Theoretical work revealed that BP presents a strong dipolar moment

behavior in 3 nm flake, while the palladium-contacted BP shows *p*-type dominated transport behaviour in thick flakes. Besides traditional metallic contacts, graphene can also be used as a contact medium to BP. Graphene has an atomically flat surface, and its work function can be tuned by gate voltages, which leads to the elimination of Schottky barrier height using graphene electrodes [18]. In addition, to reduce the contact resistance, monolayer *h*BN can be inserted between metal films and BP by van der Waals transfer technique [20]. The monolayer *h*BN acts as a tunnel barrier with relatively high tunnel conductivity. If bilayer *h*BN is used as the tunnel barrier, it results in a higher contact resistance with a factor of 10 than that in monolayer *h*BN. In *h*BN-encapsulated BP devices, selective area etching technique was used to open the window to make the contact [9]. By using appropriate etching recipe, the top *h*BN can be quickly etched, while BP layer still survives, ensuring the metals directly contact with the plasma-treated BP. Low contact resistance down to several kΩ *μ*m can be achieved even at low temperature. Surprisingly, this selective area etching technique can be effectively applied to other 2D semiconductor such as TMDCs with high performance [28].

Few-layer BP has a direct bandgap with the size around 0.3 eV. The small bandgap of few-layer BP facilitates the injections of both electron and hole, resulting in ambipolar characteristic being easily observed in BP-based transistors. The ambipolar transport in BP can exploit both *n*-type and *p*-type in a single transistor, with the mobility of both electron and hole at a high value of several thousand cm2V1s<sup>1</sup> [29], which provides the promising applications in complementary metal-oxide semiconductors and 2D material-based memory devices [30]. The possibility of manipulation in a single ambipolar transistor without external doping or ion implantation process simplifies the BP-based nanotechnology in feature's industrial application. The ambipolar functionality and high mobility of BP help the realization of flexible ambipolar inverter, frequency doubler, inverting and noninverting analogy amplifiers, and amplitude-modulated demodulator [31]. Furthermore, the ambipolar operation of BP can be locally controlled by electrostatic gating to form gate-defined PN junction [32]. Under illumination, these PN junctions show strong photocurrent due to photovoltaic effect, attractive for energy harvesting in the near-infrared.

thickness of BP decreases from bulk to monolayer, its direct bandgap was predicted to increase roughly from 0.3 to 2.0 eV [34]. This prediction has been experimentally confirmed by the measurements of optical absorption [35]. The optical bandgaps of monolayer, bilayer, trilayer, and bulk BPs were determined to be 1.73, 1.15, 0.83, and 0.35 eV, respectively. The bandgap of few-layer BP is also widely tunable by electric fields. By using an in situ potassium doping technique, a vertical electric field from dopants modulates the bandgap of few-layer BP and even tunes the system to an anisotropic Dirac semimetal state, owing to the giant Stark effect [36]. For practical applications, the dynamic tuning of bandgap can be realized by the dual-gate FET configuration in BP-based devices [37]. For example, the transport measurement in a 10-nm-thick BP flake shows that its bandgap can be continuously tuned from 0.3 eV to below 0.05 eV, using a moderate displacement field of 1.1 Vnm1. Local-strain engineering was demonstrated to be another way to control the bandgap in BP. When BP is subjected to a periodic strain, remarkable shift of optical absorption edge up to 0.7 eV was detected [38]. When BP-based FETs fabricated flexible substrates, the applied mechanical strain continuously modulates its bandgap, significantly altering the density of thermally activated carriers. As a result, large piezo-resistive effects were observed at room temperature [39]. Another way to tune the bandgap of BP includes alloying. Successful synthesis of layered black arsenic-phosphorus with tunable compositions allows the bandgap of

*Anisotropic transport of BP. This figure is reproduced with permission from NPG [1].*

*Electronic Transport in Few-Layer Black Phosphorus DOI: http://dx.doi.org/10.5772/intechopen.89149*

alloyed BP to be changed from 0.3 to 0.15 eV [40], making black arsenic-

**3. SdH oscillation and quantum hall effect in BP**

[41, 42].

**17**

**Figure 3.**

phosphorus as a promising candidate for long-wavelength infrared photodetectors

Upon subjection to a magnetic field, LLs form in an electronic system. However, high scattering rate of charge carriers leads to a broadening of LLs. The LLs manifest itself only when this broadening is lower than the energy gap between two adjacent LLs. The achievement of high-quality devices (high carrier mobility and low scattering rate) suppresses this broadening and finally allows the observation of LL with laboratory reachable magnetic fields. By the end of 2014, Li et al. and Nathaniel et al*.* reported the first observation of Shubnikov-de Haas (SdH) oscillations in BP hole system [3, 24] (**Figure 4a**). Later on Li et al. observed the quantum Hall effect in the same system (**Figure 4b**) [43]. **Figure 4a** shows Δ*Rxx* (a smooth background subtracted from *Rxx*) and striking patterns of SdH oscillations,

#### **2.3 Strong transport anisotropy in BP**

BP has a puckered honeycomb lattice, yielding strong in-plane anisotropy. The unusual anisotropic structure of BP results in its strong in-plane anisotropic electrical and optical properties. BP-based devices with electrode probes fabricated at various angles have been used to probe the electrical conductivity and carrier mobility along different direction (**Figure 3**) [1, 7]. Higher hole mobility and conductivity were found along light effective mass directions (*x*-direction) [1, 9]. The experimental results verified the theoretical calculations, which predicted that the effective mass in few-layer BP along *x* and *y* directions are *mx* 0*:*14 *m*<sup>0</sup> and *my* 0*:*89 *m*0, respectively [2]. Not only electrical properties but also optical properties of BP show strong in-plane anisotropy, such as optical absorption and polarized Raman spectrum in few-layer BP [1] and photoluminescence in monolayer BP [33].

#### **2.4 Widely tunable electronic bandgap**

Due to the strong layer interaction and quantum confinement of the charge carriers in the out-of-plane direction, BP shows stronger thickness-dependent bandgap compared with other 2D semiconductors, such as TMDCs. When the

*Electronic Transport in Few-Layer Black Phosphorus DOI: http://dx.doi.org/10.5772/intechopen.89149*

behavior in 3 nm flake, while the palladium-contacted BP shows *p*-type dominated transport behaviour in thick flakes. Besides traditional metallic contacts, graphene can also be used as a contact medium to BP. Graphene has an atomically flat surface, and its work function can be tuned by gate voltages, which leads to the elimination of Schottky barrier height using graphene electrodes [18]. In addition, to reduce the contact resistance, monolayer *h*BN can be inserted between metal films and BP by van der Waals transfer technique [20]. The monolayer *h*BN acts as a tunnel barrier with relatively high tunnel conductivity. If bilayer *h*BN is used as the tunnel barrier, it results in a higher contact resistance with a factor of 10 than that in monolayer *h*BN. In *h*BN-encapsulated BP devices, selective area etching technique was used to open the window to make the contact [9]. By using appropriate etching recipe, the top *h*BN can be quickly etched, while BP layer still survives, ensuring the metals directly contact with the plasma-treated BP. Low contact resistance down to several kΩ *μ*m can be achieved even at low temperature. Surprisingly, this selective area etching technique can be effectively applied to other 2D semiconductor such as

Few-layer BP has a direct bandgap with the size around 0.3 eV. The small bandgap of few-layer BP facilitates the injections of both electron and hole, resulting in ambipolar characteristic being easily observed in BP-based transistors. The ambipolar transport in BP can exploit both *n*-type and *p*-type in a single transistor, with the mobility of both electron and hole at a high value of several thousand cm2V1s<sup>1</sup> [29], which provides the promising applications in complementary metal-oxide semiconductors and 2D material-based memory devices [30]. The possibility of manipulation in a single ambipolar transistor without external doping or ion implantation process simplifies the BP-based nanotechnology in feature's industrial application. The ambipolar functionality and high mobility of BP help the realization of flexible ambipolar inverter, frequency doubler, inverting and noninverting analogy amplifiers, and amplitude-modulated demodulator [31]. Furthermore, the ambipolar operation of BP can be locally controlled by electrostatic gating to form gate-defined PN junction [32]. Under illumination, these PN junctions show strong photocurrent due to photovoltaic effect, attractive for energy

BP has a puckered honeycomb lattice, yielding strong in-plane anisotropy. The unusual anisotropic structure of BP results in its strong in-plane anisotropic electrical and optical properties. BP-based devices with electrode probes fabricated at various angles have been used to probe the electrical conductivity and carrier mobility along different direction (**Figure 3**) [1, 7]. Higher hole mobility and conductivity were found along light effective mass directions (*x*-direction) [1, 9]. The experimental results verified the theoretical calculations, which predicted that the effective mass in few-layer BP along *x* and *y* directions are *mx* 0*:*14 *m*<sup>0</sup> and *my* 0*:*89 *m*0, respectively [2]. Not only electrical properties but also optical properties of BP show strong in-plane anisotropy, such as optical absorption and polarized Raman spec-

trum in few-layer BP [1] and photoluminescence in monolayer BP [33].

Due to the strong layer interaction and quantum confinement of the charge carriers in the out-of-plane direction, BP shows stronger thickness-dependent bandgap compared with other 2D semiconductors, such as TMDCs. When the

TMDCs with high performance [28].

*Hybrid Nanomaterials - Flexible Electronics Materials*

harvesting in the near-infrared.

**2.3 Strong transport anisotropy in BP**

**2.4 Widely tunable electronic bandgap**

**16**

**Figure 3.** *Anisotropic transport of BP. This figure is reproduced with permission from NPG [1].*

thickness of BP decreases from bulk to monolayer, its direct bandgap was predicted to increase roughly from 0.3 to 2.0 eV [34]. This prediction has been experimentally confirmed by the measurements of optical absorption [35]. The optical bandgaps of monolayer, bilayer, trilayer, and bulk BPs were determined to be 1.73, 1.15, 0.83, and 0.35 eV, respectively. The bandgap of few-layer BP is also widely tunable by electric fields. By using an in situ potassium doping technique, a vertical electric field from dopants modulates the bandgap of few-layer BP and even tunes the system to an anisotropic Dirac semimetal state, owing to the giant Stark effect [36]. For practical applications, the dynamic tuning of bandgap can be realized by the dual-gate FET configuration in BP-based devices [37]. For example, the transport measurement in a 10-nm-thick BP flake shows that its bandgap can be continuously tuned from 0.3 eV to below 0.05 eV, using a moderate displacement field of 1.1 Vnm1. Local-strain engineering was demonstrated to be another way to control the bandgap in BP. When BP is subjected to a periodic strain, remarkable shift of optical absorption edge up to 0.7 eV was detected [38]. When BP-based FETs fabricated flexible substrates, the applied mechanical strain continuously modulates its bandgap, significantly altering the density of thermally activated carriers. As a result, large piezo-resistive effects were observed at room temperature [39]. Another way to tune the bandgap of BP includes alloying. Successful synthesis of layered black arsenic-phosphorus with tunable compositions allows the bandgap of alloyed BP to be changed from 0.3 to 0.15 eV [40], making black arsenicphosphorus as a promising candidate for long-wavelength infrared photodetectors [41, 42].

#### **3. SdH oscillation and quantum hall effect in BP**

Upon subjection to a magnetic field, LLs form in an electronic system. However, high scattering rate of charge carriers leads to a broadening of LLs. The LLs manifest itself only when this broadening is lower than the energy gap between two adjacent LLs. The achievement of high-quality devices (high carrier mobility and low scattering rate) suppresses this broadening and finally allows the observation of LL with laboratory reachable magnetic fields. By the end of 2014, Li et al. and Nathaniel et al*.* reported the first observation of Shubnikov-de Haas (SdH) oscillations in BP hole system [3, 24] (**Figure 4a**). Later on Li et al. observed the quantum Hall effect in the same system (**Figure 4b**) [43]. **Figure 4a** shows Δ*Rxx* (a smooth background subtracted from *Rxx*) and striking patterns of SdH oscillations,

#### **Figure 4.**

*Quantum transport in BP. (a) Rxx versus Vg and magnetic field B. (b) Hall resistance Rxy* ¼ *Vxy=I variation with magnetic field at cryogenic temperature. The quantized plateaus demonstrate the quantum Hall effect in BP two-dimensional hole gas. This figure is reproduced with permission from NPG [43, 44].*

appearing along straight lines, are clearly resolved. When the Fermi level of the 2D hole system is located at the center of certain LLs (at the middle of two adjacent LLs), the system exhibits high resistance (low resistance). Hence the oscillations are employed as a powerful tool to monitor the evolution of Fermi energy with gate voltage considering that the energy gap between two adjacent LLs is ℏ*ω*.

More quantitatively, according to the Lifshitz-Kosevich model, the temperature dependence of oscillation amplitude follows [45].

$$
\Delta R \propto \frac{\lambda(T)}{\sinh \left( \lambda(T) \right)} \tag{9}
$$

**Figure 5.**

*Electronic Transport in Few-Layer Black Phosphorus DOI: http://dx.doi.org/10.5772/intechopen.89149*

**Figure 6.**

**19**

*with permission from ACS [10].*

*Effective mass of holes in BP. (a) ΔRxx as a function of 1/B at different temperatures. The inset shows the FFT result at selected temperatures. (b) Evolution of oscillation amplitudes with temperatures at different magnetic fields. The solid lines represent the fitting result of temperature reduction factor. (c) Effective mass of holes obtained from the fitting results shown in panel b. This figure is reproduced with permission from NPG [44].*

*Spin-selective quantum scattering process (a, b) Oscillation component ΔRxx at selected gate voltages (panel a) and temperatures (panel b). The blue dashed lines present the fitting results of Eq. (10). (c, d) Quantum scattering times for up- and down-spin orientations obtained from the fitting results. This figure is reproduced*

where *<sup>λ</sup>*ð Þ¼ *<sup>T</sup>* <sup>2</sup>*π*<sup>2</sup>*kBTm*<sup>∗</sup> *<sup>=</sup>*ℏ*eB* is the thermal damping factor, *kB* stands for Boltzmann constant, ℏ denotes the reduced Plank constant, and *m*<sup>∗</sup> is the effective cyclotron mass. **Figure 5a** shows the oscillation component Δ*Rxx* evolves with magnetic field at few different temperatures. The obtained temperature dependence of the oscillation amplitudes accompanied with the fitting result of thermal reduction factor is shown in **Figure 5b** [44]. **Figure 5c** displays the extracted effective cyclotron mass from the fitting result [44]. Similar values of effective cyclotron mass have been reported by different groups [9, 10, 24, 27].

As device quality further improved, Zeeman splitting can be resolved in laboratory-accessible magnetic field [10], and alternative SdH oscillation amplitudes were observed. By reproducing the oscillation component with high-order spin-resolved LK formula

$$\Delta R\_{\text{xx}} = 2R\_0 \sum\_{r\_\uparrow, \uparrow, \downarrow} \frac{r\lambda(T)}{\sinh\left(\lambda(T)\right)} \exp\left(-r \frac{\pi}{ar\tau\_\uparrow, \downarrow}\right) \cos\left(r\phi\_{\uparrow\uparrow, \downarrow}\right) \tag{10}$$

where *ω* is the cyclotron frequency, *λ*ð Þ *T* stands for the thermal factor, *ϕ*↑*,* <sup>↓</sup> is the corrected Berry phase taking Zeeman splitting into consideration, and *τ*↑*,* <sup>↓</sup> is the spin-resolved quantum scattering times. The extracted spin-dependent quantum scattering time at different temperatures and gate voltages are presented in

*Electronic Transport in Few-Layer Black Phosphorus DOI: http://dx.doi.org/10.5772/intechopen.89149*

#### **Figure 5.**

appearing along straight lines, are clearly resolved. When the Fermi level of the 2D hole system is located at the center of certain LLs (at the middle of two adjacent LLs), the system exhibits high resistance (low resistance). Hence the oscillations are employed as a powerful tool to monitor the evolution of Fermi energy with gate

*Quantum transport in BP. (a) Rxx versus Vg and magnetic field B. (b) Hall resistance Rxy* ¼ *Vxy=I variation with magnetic field at cryogenic temperature. The quantized plateaus demonstrate the quantum Hall effect in*

More quantitatively, according to the Lifshitz-Kosevich model, the temperature

*sinh* ð Þ *<sup>λ</sup>*ð Þ *<sup>T</sup>* (9)

cos *rϕ*↑*,* <sup>↓</sup>

� � (10)

<sup>Δ</sup>*R*<sup>∝</sup> *<sup>λ</sup>*ð Þ *<sup>T</sup>*

where *<sup>λ</sup>*ð Þ¼ *<sup>T</sup>* <sup>2</sup>*π*<sup>2</sup>*kBTm*<sup>∗</sup> *<sup>=</sup>*ℏ*eB* is the thermal damping factor, *kB* stands for Boltzmann constant, ℏ denotes the reduced Plank constant, and *m*<sup>∗</sup> is the effective cyclotron mass. **Figure 5a** shows the oscillation component Δ*Rxx* evolves with magnetic field at few different temperatures. The obtained temperature dependence of the oscillation amplitudes accompanied with the fitting result of thermal reduction factor is shown in **Figure 5b** [44]. **Figure 5c** displays the extracted effective cyclotron mass from the fitting result [44]. Similar values of effective

voltage considering that the energy gap between two adjacent LLs is ℏ*ω*.

*BP two-dimensional hole gas. This figure is reproduced with permission from NPG [43, 44].*

cyclotron mass have been reported by different groups [9, 10, 24, 27].

*rλ*ð Þ *T*

As device quality further improved, Zeeman splitting can be resolved in laboratory-accessible magnetic field [10], and alternative SdH oscillation amplitudes were observed. By reproducing the oscillation component with high-order

sinh ð Þ *<sup>λ</sup>*ð Þ *<sup>T</sup>* exp �*<sup>r</sup> <sup>π</sup>*

where *ω* is the cyclotron frequency, *λ*ð Þ *T* stands for the thermal factor, *ϕ*↑*,* <sup>↓</sup> is the corrected Berry phase taking Zeeman splitting into consideration, and *τ*↑*,* <sup>↓</sup> is the spin-resolved quantum scattering times. The extracted spin-dependent quantum scattering time at different temperatures and gate voltages are presented in

*ωτ*↑*,* <sup>↓</sup> � �

dependence of oscillation amplitude follows [45].

*Hybrid Nanomaterials - Flexible Electronics Materials*

spin-resolved LK formula

**18**

**Figure 4.**

Δ*Rxx* ¼ 2*R*<sup>0</sup>

X *r,* <sup>↑</sup>*,* <sup>↓</sup>

*Effective mass of holes in BP. (a) ΔRxx as a function of 1/B at different temperatures. The inset shows the FFT result at selected temperatures. (b) Evolution of oscillation amplitudes with temperatures at different magnetic fields. The solid lines represent the fitting result of temperature reduction factor. (c) Effective mass of holes obtained from the fitting results shown in panel b. This figure is reproduced with permission from NPG [44].*

#### **Figure 6.**

*Spin-selective quantum scattering process (a, b) Oscillation component ΔRxx at selected gate voltages (panel a) and temperatures (panel b). The blue dashed lines present the fitting results of Eq. (10). (c, d) Quantum scattering times for up- and down-spin orientations obtained from the fitting results. This figure is reproduced with permission from ACS [10].*

While when the band is empty, it will not screen the electronic field, and the wave function of electrons penetrates all the way to the opposite surface. Based on this screening effect, wide quantum wells have been formed in few-layer BP which can be switched between double layers of charge carriers and single layer of charge carriers [55]. The switch in one single device from double quantum wells and single quantum well has never been realized in other systems. Besides, one can tune the distance between the double layers of charge carriers by selecting *h*BN flakes with different thicknesses. The realization of tunable quantum wells paves the way for using 2D materials as wide quantum wells for investigating phenomena, such as LL hybridization, inter-well Coulomb interactions, or multicomponent quantum Hall

In this chapter, we mainly focus on the development of high-quality FET and the electronic transport in quantum limitations based on BP. Although few-layer BP degrades in the atmosphere, encapsulation with BN flakes was proven to be a reliable strategy to realize air-stable devices based on BP. The realization of Ohmic contact through selective etching technique lays the foundation for high-quality devices. Thanks to the rather small electronic bandgaps for few-layer BP flakes, by aligning the work function of contact metals with the band edge of BP, ambipolarconducting channels were realized in single field effect devices. In devices with high carrier mobility, the scattering rate of charge carriers is low leading to limited broadening of LLs. As a consequence, the LLs become resolvable in laboratory reachable magnetic field at cryogenic temperature. Based on the SdH oscillations and quantum Hall effect, a spin-selective quantum scattering process was

established in BP 2DHG. Intrinsic parameters such as effective mass and Landé gfactor were measured. The realization of high-quality air-stable BP devices promises

The authors acknowledge the financial support from the Research Grants Council of Hong Kong (Project No. FB417- UoM-HKUST and C7036-17 W).

ferromagnetism in this highly anisotropic system.

*Electronic Transport in Few-Layer Black Phosphorus DOI: http://dx.doi.org/10.5772/intechopen.89149*

its potential in next-generation electronic applications.

The authors declare no conflict of interest.

**4. Conclusions**

**Acknowledgements**

**Conflict of interest**

**21**

**Figure 7.**

*Coincidence of BP LLs (a) Schematic of spin-resolved LLs evolve with increasing tilting angle at fixed perpendicular magnetic field. The vertical dashed line indicates the coincidence angle θc. (b) The evolution of SdH oscillation component with increasing tilting angles. This figure is reproduced with permission from ACS [10].*

**Figure 6c** and **d**, respectively. The charge carriers in BP two-dimensional hole gas (2DHG) show the spin-selective scattering behavior on a phenomenological level. Similar scattering process was also reported in GaAs [46] and ZnO [47] systems. Recently, an analogy called valley-selective scattering process was observed in MoS2 [48]. The mechanism behind this spin-selective scattering process remains to be further addressed.

The resolvability of Zeeman splitting makes it possible to determine the Landé g-factor of BP electronic systems via the coincidence method taking advantage that the Zeeman energy *EZ* ¼ *gμBBtotal* depends on the total magnetic field, while the cyclotron energy *EC* <sup>¼</sup> <sup>ℏ</sup>*eB*⊥*=m*<sup>∗</sup> is determined by the field component *<sup>B</sup>*<sup>⊥</sup> perpendicular to the 2DHG plane. When the *EZ* is a multiple integer of *EC*, i.e., *EZ* ¼ *iEC* (*<sup>i</sup>* is an integer number), the spin-resolved LLs overlap. Hence *<sup>χ</sup><sup>s</sup>* <sup>¼</sup> *gm*<sup>∗</sup> <sup>¼</sup> <sup>2</sup>*<sup>i</sup>* cos *<sup>θ</sup><sup>c</sup>* is valid at coincidence angle *θ<sup>c</sup>* as indicated in **Figure 7a** [10, 49]. **Figure 7b** presents the measurement of coincidence angle, and the extracted spin susceptibility *χ<sup>s</sup>* is � 0.64 which leads to a Landé g-factor of *g* ¼ 2*:*47 [10]. Later on, the same method was applied to electron-doped BP to extract a spin susceptibility of � 1.1 and Landé g-factor of � 2.8 [29, 50].

Making use of a magnetic field up to 45 T, fractional quantum state at filling factor *<sup>ν</sup>* ¼ � <sup>4</sup> <sup>3</sup> accompanied with quantum feature at �0.56�0.1 was observed in 2018 [51] making BP the second exfoliated van der Waals crystals to exhibit fractional quantum Hall effect after graphene.

When the electronic band of a semiconductor is occupied by induced electrons (e.g., electrons induced by gate voltage), it will screen the electronic field hence prohibiting the wave function of electrons from penetrating the topmost few layers [52–54].

*Electronic Transport in Few-Layer Black Phosphorus DOI: http://dx.doi.org/10.5772/intechopen.89149*

While when the band is empty, it will not screen the electronic field, and the wave function of electrons penetrates all the way to the opposite surface. Based on this screening effect, wide quantum wells have been formed in few-layer BP which can be switched between double layers of charge carriers and single layer of charge carriers [55]. The switch in one single device from double quantum wells and single quantum well has never been realized in other systems. Besides, one can tune the distance between the double layers of charge carriers by selecting *h*BN flakes with different thicknesses. The realization of tunable quantum wells paves the way for using 2D materials as wide quantum wells for investigating phenomena, such as LL hybridization, inter-well Coulomb interactions, or multicomponent quantum Hall ferromagnetism in this highly anisotropic system.

#### **4. Conclusions**

In this chapter, we mainly focus on the development of high-quality FET and the electronic transport in quantum limitations based on BP. Although few-layer BP degrades in the atmosphere, encapsulation with BN flakes was proven to be a reliable strategy to realize air-stable devices based on BP. The realization of Ohmic contact through selective etching technique lays the foundation for high-quality devices. Thanks to the rather small electronic bandgaps for few-layer BP flakes, by aligning the work function of contact metals with the band edge of BP, ambipolarconducting channels were realized in single field effect devices. In devices with high carrier mobility, the scattering rate of charge carriers is low leading to limited broadening of LLs. As a consequence, the LLs become resolvable in laboratory reachable magnetic field at cryogenic temperature. Based on the SdH oscillations and quantum Hall effect, a spin-selective quantum scattering process was established in BP 2DHG. Intrinsic parameters such as effective mass and Landé gfactor were measured. The realization of high-quality air-stable BP devices promises its potential in next-generation electronic applications.

#### **Acknowledgements**

**Figure 6c** and **d**, respectively. The charge carriers in BP two-dimensional hole gas (2DHG) show the spin-selective scattering behavior on a phenomenological level. Similar scattering process was also reported in GaAs [46] and ZnO [47] systems. Recently, an analogy called valley-selective scattering process was observed in MoS2 [48]. The mechanism behind this spin-selective scattering process remains to be

*Coincidence of BP LLs (a) Schematic of spin-resolved LLs evolve with increasing tilting angle at fixed perpendicular magnetic field. The vertical dashed line indicates the coincidence angle θc. (b) The evolution of SdH oscillation component with increasing tilting angles. This figure is reproduced with permission from*

*Hybrid Nanomaterials - Flexible Electronics Materials*

The resolvability of Zeeman splitting makes it possible to determine the Landé g-factor of BP electronic systems via the coincidence method taking advantage that the Zeeman energy *EZ* ¼ *gμBBtotal* depends on the total magnetic field, while the cyclotron energy *EC* <sup>¼</sup> <sup>ℏ</sup>*eB*⊥*=m*<sup>∗</sup> is determined by the field component *<sup>B</sup>*<sup>⊥</sup> perpendicular to the 2DHG plane. When the *EZ* is a multiple integer of *EC*, i.e., *EZ* ¼ *iEC* (*<sup>i</sup>* is an integer number), the spin-resolved LLs overlap. Hence *<sup>χ</sup><sup>s</sup>* <sup>¼</sup> *gm*<sup>∗</sup> <sup>¼</sup> <sup>2</sup>*<sup>i</sup>* cos *<sup>θ</sup><sup>c</sup>* is valid at coincidence angle *θ<sup>c</sup>* as indicated in **Figure 7a** [10, 49]. **Figure 7b** presents the measurement of coincidence angle, and the extracted spin susceptibility *χ<sup>s</sup>* is � 0.64 which leads to a Landé g-factor of *g* ¼ 2*:*47 [10]. Later on, the same method was applied to electron-doped BP to extract a spin susceptibility of � 1.1 and Landé

Making use of a magnetic field up to 45 T, fractional quantum state at filling

2018 [51] making BP the second exfoliated van der Waals crystals to exhibit frac-

When the electronic band of a semiconductor is occupied by induced electrons (e.g., electrons induced by gate voltage), it will screen the electronic field hence prohibiting the wave function of electrons from penetrating the topmost few layers [52–54].

<sup>3</sup> accompanied with quantum feature at �0.56�0.1 was observed in

further addressed.

**Figure 7.**

*ACS [10].*

g-factor of � 2.8 [29, 50].

tional quantum Hall effect after graphene.

factor *<sup>ν</sup>* ¼ � <sup>4</sup>

**20**

The authors acknowledge the financial support from the Research Grants Council of Hong Kong (Project No. FB417- UoM-HKUST and C7036-17 W).

#### **Conflict of interest**

The authors declare no conflict of interest.

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*Electronic Transport in Few-Layer Black Phosphorus DOI: http://dx.doi.org/10.5772/intechopen.89149*

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#### **Author details**

Gen Long1,2†, Xiaolong Chen1,3†, Shuigang Xu4† and Ning Wang<sup>1</sup> \*

1 The Hong Kong University of Science and Technology, Hong Kong, China

2 University of Geneva, Geneva, Switzerland

3 Southern University of Science and Technology, Shenzhen, China

4 University of Manchester, Manchester, UK

\*Address all correspondence to: phwang@ust.hk

† These authors contributed equally.

© 2020 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.

*Electronic Transport in Few-Layer Black Phosphorus DOI: http://dx.doi.org/10.5772/intechopen.89149*

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**Author details**

**22**

Gen Long1,2†, Xiaolong Chen1,3†, Shuigang Xu4† and Ning Wang<sup>1</sup>

3 Southern University of Science and Technology, Shenzhen, China

2 University of Geneva, Geneva, Switzerland

*Hybrid Nanomaterials - Flexible Electronics Materials*

4 University of Manchester, Manchester, UK

† These authors contributed equally.

provided the original work is properly cited.

\*Address all correspondence to: phwang@ust.hk

1 The Hong Kong University of Science and Technology, Hong Kong, China

© 2020 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,

\*

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[37] Deng B, Vy T, Xie Y, Jiang H, Li C, Guo Q, et al. Efficient electrical control of thin-film black phosphorus bandgap. Nature Communications. 2017;**8**:14474

[38] Quereda J, San-Jose P, Parente V, Vaquero-Garzon L, Molina-Mendoza AJ, Agraït N, et al. Strong modulation of optical properties in black phosphorus through strain-engineered rippling. Nano Letters. 2016;**16**(5):2931-2937

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photodetectors based on black arsenic phosphorus. Science Advances. 2017;

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phosphorus transistors with type control

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[31] Zhu W, Yogeesh MN, Yang S, Aldave SH, Kim J-S, Sonde S, et al. Flexible black phosphorus ambipolar

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Communications. 2017;**8**(1):1672

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Lee YH. High-performance n-type black

phosphorus mid-infrared photodetector. Nature

[20] Cao Y, Mishchenko A, Yu GL, Khestanova E, Rooney AP, Prestat E, et al. Quality heterostructures from twodimensional crystals unstable in air by their assembly in inert atmosphere. Nano Letters. 2015;**15**(8):4914-4921

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[36] Kim J, Su Baik S, Ryu SH, Sohn Y, Park S, Park B-G, et al. Observation of tunable band gap and anisotropic Dirac semimetal state in black phosphorus. Science. 2015;**349**(6249):723-726

[37] Deng B, Vy T, Xie Y, Jiang H, Li C, Guo Q, et al. Efficient electrical control of thin-film black phosphorus bandgap. Nature Communications. 2017;**8**:14474

[38] Quereda J, San-Jose P, Parente V, Vaquero-Garzon L, Molina-Mendoza AJ, Agraït N, et al. Strong modulation of optical properties in black phosphorus through strain-engineered rippling. Nano Letters. 2016;**16**(5):2931-2937

[39] Zhang Z, Li L, Horng J, Wang NZ, Yang F, Yijun Y, et al. Strain-modulated bandgap and piezo-resistive effect in black phosphorus field-effect transistors. Nano Letters. 2017;**17**(10): 6097-6103

[40] Liu B, Köpf M, Abbas AN, Wang X, Guo Q, Jia Y, et al. Black arsenic– phosphorus: Layered anisotropic infrared semiconductors with highly tunable compositions and properties. Advanced Materials. 2015;**27**(30): 4423-4429

[41] Long M, Gao A, Wang P, Xia H, Ott C, Pan C, et al. Room temperature high-detectivity mid-infrared photodetectors based on black arsenic phosphorus. Science Advances. 2017; **3**(6):e1700589

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transistors. Nano Letters. 2018;**18**(10): 6611-6616

[51] Yang J, Tran S, Wu J, Che S, Stepanov P, Taniguchi T, et al. Integer and fractional quantum hall effect in ultrahigh quality few-layer black phosphorus transistors. Nano Letters. 2018;**18**(1):229-234

[52] Long G, Xu S, Zhang T, Wu Z, Wong WK, Han T, et al. Charge density wave phase transition on the surface of electrostatically doped multilayer graphene. Applied Physics Letters. 2016; **109**(18):183107

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**27**

**Chapter 3**

**Abstract**

tion areas in the future.

**1. Introduction**

(about 2620 m2

g<sup>−</sup><sup>1</sup>

graphene, carbon nanotube, carbon nanofiber

Vapor Deposition

*Hua-Fei Li, Shuguang Deng and Gui-Ping Dai*

Synthesis of Three-Dimensional

Nanocarbon Hybrids by Chemical

Carbon nanomaterials such as graphene, carbon nanotube (CNT), and carbon nanofiber (CNF) have received tremendous attentions in the past two decades due to their extraordinary mechanical strength and thermal and electrical properties. Recently, it indicates that three-dimensional (3D) nanocarbon hybrids overcome the weakness of individual low-dimensional nanocarbon materials and exhibit unique properties among carbon nanomaterials. Efforts have thus been made to acquire synergistic integration of one-dimensional (1D) and two-dimensional (2D) carbon nanomaterials. Meanwhile, chemical vapor deposition (CVD) is a widespread and effective method of fabricating three-dimensional nanocarbon hybrids compared with other synthetic methods. In this case, a number of 3D nanocarbon hybrids are synthesized by using different precursors at diverse temperature, and the nanocarbon hybrids are expected to be a promising choice for various applica-

**Keywords:** chemical vapor deposition, three-dimensional nanocarbon hybrids,

As the typical 1D carbon materials, carbon nanotubes (CNTs) (**Figure 1a**) and carbon nanofibers (CNFs) have been widely investigated in the past two decades because of their merits, such as outstanding mechanical strength, large surface-to-volume ratio, and extraordinary electrical conductivity [1–4]. At the

outstanding physical and chemical properties such as high specific surface areas

Nevertheless, their physical and chemical performances inevitably decrease compared to the theoretical prediction result from the existence of the van der Waals interaction, generating easy self-aggregation and stacking during the synthesis process [8]. Therefore, the 3D nanocarbon hybrids (such as CNT/graphene, CNF/ graphene, CNT/CNF hybrids) are studied by a large number of the research groups, aiming at overcoming these shortcomings and a synergistic integration of their inherent properties in the new hybrid materials [9–11]. These nanocarbon hybrids have an interconnected network of carbon structure, resulting in a synergistic effect in enhanced conductivity in comparison with the individual components,

), great lightweight, and fast electron transport kinetics [5–7].

carbon, displays

same time, graphene (**Figure 1b**), a recently discovered 2D sp2

#### **Chapter 3**

transistors. Nano Letters. 2018;**18**(10):

*Hybrid Nanomaterials - Flexible Electronics Materials*

[51] Yang J, Tran S, Wu J, Che S, Stepanov P, Taniguchi T, et al. Integer and fractional quantum hall effect in ultrahigh quality few-layer black phosphorus transistors. Nano Letters.

[52] Long G, Xu S, Zhang T, Wu Z, Wong WK, Han T, et al. Charge density wave phase transition on the surface of electrostatically doped multilayer graphene. Applied Physics Letters. 2016;

[53] Ye JT, Zhang YJ, Akashi R, Bahramy MS, Arita R, Iwasa Y.

Superconducting dome in a gate-tuned band insulator. Science. 2012;**338**(6111):

[54] Lu JM, Zheliuk O, Leermakers I, Yuan NF, Zeitler U, Law KT, et al. Evidence for two-dimensional Ising superconductivity in gated MoS2. Science. 2015;**350**(6266):1353-1357

[55] Tran S, Yang J, Gillgren N, Espiritu T, Shi Y, Watanabe K, et al. Surface transport and quantum hall effect in ambipolar black phosphorus double quantum wells. Science Advances. 2017;**3**(6):e1603179

6611-6616

2018;**18**(1):229-234

**109**(18):183107

1193-1196

**26**

## Synthesis of Three-Dimensional Nanocarbon Hybrids by Chemical Vapor Deposition

*Hua-Fei Li, Shuguang Deng and Gui-Ping Dai*

#### **Abstract**

Carbon nanomaterials such as graphene, carbon nanotube (CNT), and carbon nanofiber (CNF) have received tremendous attentions in the past two decades due to their extraordinary mechanical strength and thermal and electrical properties. Recently, it indicates that three-dimensional (3D) nanocarbon hybrids overcome the weakness of individual low-dimensional nanocarbon materials and exhibit unique properties among carbon nanomaterials. Efforts have thus been made to acquire synergistic integration of one-dimensional (1D) and two-dimensional (2D) carbon nanomaterials. Meanwhile, chemical vapor deposition (CVD) is a widespread and effective method of fabricating three-dimensional nanocarbon hybrids compared with other synthetic methods. In this case, a number of 3D nanocarbon hybrids are synthesized by using different precursors at diverse temperature, and the nanocarbon hybrids are expected to be a promising choice for various application areas in the future.

**Keywords:** chemical vapor deposition, three-dimensional nanocarbon hybrids, graphene, carbon nanotube, carbon nanofiber

#### **1. Introduction**

As the typical 1D carbon materials, carbon nanotubes (CNTs) (**Figure 1a**) and carbon nanofibers (CNFs) have been widely investigated in the past two decades because of their merits, such as outstanding mechanical strength, large surface-to-volume ratio, and extraordinary electrical conductivity [1–4]. At the same time, graphene (**Figure 1b**), a recently discovered 2D sp2 carbon, displays outstanding physical and chemical properties such as high specific surface areas (about 2620 m2 g<sup>−</sup><sup>1</sup> ), great lightweight, and fast electron transport kinetics [5–7]. Nevertheless, their physical and chemical performances inevitably decrease compared to the theoretical prediction result from the existence of the van der Waals interaction, generating easy self-aggregation and stacking during the synthesis process [8]. Therefore, the 3D nanocarbon hybrids (such as CNT/graphene, CNF/ graphene, CNT/CNF hybrids) are studied by a large number of the research groups, aiming at overcoming these shortcomings and a synergistic integration of their inherent properties in the new hybrid materials [9–11]. These nanocarbon hybrids have an interconnected network of carbon structure, resulting in a synergistic effect in enhanced conductivity in comparison with the individual components,

and the special 3D structure significantly provides a variety of applications, such as field-effect transistors [12, 13], electron field emitters [14–16], sensors [17–20], fuel cell [21–23], batteries [24, 25], and supercapacitors [26–30].

To date, a number of techniques and methods have been utilized for the fabrication of nanocarbon hybrid, such as mixing process of surface-treated carbon materials (including solution processing [31, 32], vacuum filtration [33, 34], layerby-layer self-assembly method [35, 36]), hydrothermal method [37], multi-step approaches using combinations of decorated carbon materials and CVD [10], and multi-step and one-step chemical vapor deposition [38–48]. Among all the nanocarbon hybrid fabrication approaches reported, CVD techniques are considered as the most versatile and promising way for nanocarbon composite production with reasonable structure and mechanical strength, which has attracted tremendous research attention during the recent decades. As a sophisticated synthesis method for both laboratory research and industry production, conventional CVD (shown in **Figure 2**) is applied in many areas, such as thin-film coating, crystal growth, and

**Figure 1.** *Schematic diagrams of graphene and carbon nanotube.*

**29**

**Figure 3.**

*Classification of 3D nanocarbon hybrids synthesis techniques.*

*Synthesis of Three-Dimensional Nanocarbon Hybrids by Chemical Vapor Deposition*

[9, 28] and thereafter reassembly of carbon atoms into sp2

dimensional carbonaceous nanomaterials is still a big challenge.

powder production and also suitable for the synthesis of nanocarbon materials. The mechanism of conventional CVD generally includes two steps, initially thermal decomposition of gaseous precursor [10], organic solvents [47], or solid feedstock

under the effective catalysis such as Cu [8], Fe [12], Co [28, 44], Ni [24], or their mixture [11, 49] at high temperature. Compared with other approaches, CVD technique significantly fabricated well-interconnected three-dimensional nanocarbon materials without needing sophisticated chemical routes with solvents and highly toxic agents during synthesis process. Zhu et al. [50] reported that the seamless, covalently bonded three-dimensional nanocarbon architecture was fabricated on the surface of Cu foil via simple two-step CVD methods. It is worth noting that although different aforementioned methods are employed for the production of nanocarbon hybrids, a facile and simple approach for controllable growth of three-

In this chapter, we present a summary of the researches about nanocarbon hybrid in recent years, with a focus on the popular fabrication techniques.

Moreover, the merits and demerits and effect of experimental parameters of these CVD methods are presented in detail. Finally, we discuss the development trend, challenges, and performance applications of nanocarbon hybrids in the further.

**2. Preparation techniques of three-dimensional nanocarbon hybrids**

Up to now, varied approaches have been used for the fabrication of 3D nanocarbon hybrids, and the preparation technology generally could be categorized into four different approaches (shown in **Figure 3**): mixing process of surface-treated carbon materials, hydrothermal method, multi-step approaches using combinations of decorated carbon materials and CVD, and multi-step and one-step chemical vapor deposition. In addition, early researches on the construction of 3D hybrids focus on mixing process, which includes solution processing, vacuum filtration, and layer-by-layer self-assembly methods. Compared to other methods, hydrothermal route is an appropriate way to the mass preparation of graphene-carbon nanotube hybrids because of the easy operation and mild experimental environment. Moreover, the composites consisted of carbon nanotube and carbon nanofiber mainly produced by utilizing the multi-step approaches using combinations of decorated carbon materials and CVD method. Especially, multi-step and one-step

carbon nanostructures

*DOI: http://dx.doi.org/10.5772/intechopen.89671*

**Figure 2.** *Schematic diagrams of CVD technique.*

*Synthesis of Three-Dimensional Nanocarbon Hybrids by Chemical Vapor Deposition DOI: http://dx.doi.org/10.5772/intechopen.89671*

powder production and also suitable for the synthesis of nanocarbon materials. The mechanism of conventional CVD generally includes two steps, initially thermal decomposition of gaseous precursor [10], organic solvents [47], or solid feedstock [9, 28] and thereafter reassembly of carbon atoms into sp2 carbon nanostructures under the effective catalysis such as Cu [8], Fe [12], Co [28, 44], Ni [24], or their mixture [11, 49] at high temperature. Compared with other approaches, CVD technique significantly fabricated well-interconnected three-dimensional nanocarbon materials without needing sophisticated chemical routes with solvents and highly toxic agents during synthesis process. Zhu et al. [50] reported that the seamless, covalently bonded three-dimensional nanocarbon architecture was fabricated on the surface of Cu foil via simple two-step CVD methods. It is worth noting that although different aforementioned methods are employed for the production of nanocarbon hybrids, a facile and simple approach for controllable growth of threedimensional carbonaceous nanomaterials is still a big challenge.

In this chapter, we present a summary of the researches about nanocarbon hybrid in recent years, with a focus on the popular fabrication techniques. Moreover, the merits and demerits and effect of experimental parameters of these CVD methods are presented in detail. Finally, we discuss the development trend, challenges, and performance applications of nanocarbon hybrids in the further.

#### **2. Preparation techniques of three-dimensional nanocarbon hybrids**

Up to now, varied approaches have been used for the fabrication of 3D nanocarbon hybrids, and the preparation technology generally could be categorized into four different approaches (shown in **Figure 3**): mixing process of surface-treated carbon materials, hydrothermal method, multi-step approaches using combinations of decorated carbon materials and CVD, and multi-step and one-step chemical vapor deposition. In addition, early researches on the construction of 3D hybrids focus on mixing process, which includes solution processing, vacuum filtration, and layer-by-layer self-assembly methods. Compared to other methods, hydrothermal route is an appropriate way to the mass preparation of graphene-carbon nanotube hybrids because of the easy operation and mild experimental environment. Moreover, the composites consisted of carbon nanotube and carbon nanofiber mainly produced by utilizing the multi-step approaches using combinations of decorated carbon materials and CVD method. Especially, multi-step and one-step

**Figure 3.** *Classification of 3D nanocarbon hybrids synthesis techniques.*

*Hybrid Nanomaterials - Flexible Electronics Materials*

cell [21–23], batteries [24, 25], and supercapacitors [26–30].

and the special 3D structure significantly provides a variety of applications, such as field-effect transistors [12, 13], electron field emitters [14–16], sensors [17–20], fuel

To date, a number of techniques and methods have been utilized for the fabrication of nanocarbon hybrid, such as mixing process of surface-treated carbon materials (including solution processing [31, 32], vacuum filtration [33, 34], layerby-layer self-assembly method [35, 36]), hydrothermal method [37], multi-step approaches using combinations of decorated carbon materials and CVD [10], and multi-step and one-step chemical vapor deposition [38–48]. Among all the nanocarbon hybrid fabrication approaches reported, CVD techniques are considered as the most versatile and promising way for nanocarbon composite production with reasonable structure and mechanical strength, which has attracted tremendous research attention during the recent decades. As a sophisticated synthesis method for both laboratory research and industry production, conventional CVD (shown in **Figure 2**) is applied in many areas, such as thin-film coating, crystal growth, and

**28**

**Figure 2.**

**Figure 1.**

*Schematic diagrams of graphene and carbon nanotube.*

*Schematic diagrams of CVD technique.*

chemical vapor deposition is considered as a simple and promising way to build 3D hybrids with hierarchical structure and stability.

#### **2.1 Mixing process of surface-treated carbon materials**

As the early hybridization approaches, solution processing, vacuum filtration, layer-by-layer self-assembly methods, and so on could be classified into the facile mixing process. Altogether 1D carbon nanomaterial incorporation of 2D nanomaterial with a facile mixing process exhibits a synergistic effect in enhanced properties. However, the nanocarbon hybrids are synthesized by utilizing various methods to mix modified carbon-based feedstocks, which generally need sophisticated chemical routes with solvents and highly toxic agents [31, 33–36]. Furthermore, this kind of techniques suffers from poor controllability, leading to the restriction of practical application due to the aggregation and stacking of carbon-based materials [51].

#### **2.2 Hydrothermal method**

With regard to hydrothermal method, firstly, the carbon feedstocks are dissolved and the mixed solution is transferred into a heating instrument. Secondly, the hydrothermal treatment is performed at low temperature, and the final product is obtained after centrifugation, washing, and freeze-drying process. Although this method has merits of mild conditions and scale-up synthesis, it is not suitable for the industrial production due to the time-consuming fault and defective products. Besides, the obtained 3D nanostructures are chiefly based on weak interconnection between individual nanocarbon components instead of owning powerful bonding, leading to robust 3D architecture [52].

#### **2.3 Multi-step approaches using combinations of decorated carbon materials and CVD**

Chemical vapor deposition is considered as the most promising approach of the preparation of graphene, CNTs, and CNFs on the substrate surface. Thus, it is always employed to facilitate the growth of CNTs on the decorated carbon materials, leading to the 3D hierarchical composite. For example, most of the reported 3D carbon nanotube/carbon nanofiber hierarchical composites are typically prepared by a multi-step route, which first needs electrospinning technique and post-carbonization for the preparation of CNFs, followed by decorating the CNFs with metal catalyst nanoparticles, and eventually the CNT growth is promoted by using toxic organic gases or solvent as carbon source during the CVD process [10, 22]. This kind of CVD-based methods has distinctive advantages: efficiency, convenience, and high yield. However, the stable and suitable decorated carbon materials that always need sophisticated pretreatment are vital to the construction of 3D carbon hybrids.

#### **2.4 Multi-step synthesis by chemical vapor deposition**

Multi-step chemical vapor depositions have been utilized in recent years to integrate individual 1D with 2D carbon nanomaterials to achieve controllable configurations of 3D nanostructures. Recently, Tang et al. successfully fabricated graphene-carbon nanotube composite on exfoliated vermiculite (EV) substrate by the multi-step CVD method (as shown in **Figure 4**). The whole CVD process could be divided into two steps: firstly, the aligned CNTs are synthesized at 650°C by using C2H4 as carbon source, and, secondly, the uniform graphene sheet directly

**31**

**Figure 5.**

**Figure 4.**

*Synthesis of Three-Dimensional Nanocarbon Hybrids by Chemical Vapor Deposition*

grows on the surface of substrate at a higher temperature of 950°C by utilizing the hydrocarbon—CH4, resulting in the in situ synthesis of graphene-carbon nanotubegraphene sandwiches [53]. In other methods of the successful fabrication of 3D hybrids, the obtained component materials are always entangled with each other, and the ordered 3D packing architecture is hardly available. Nevertheless, this multi-step way successfully integrates low-dimensional materials into 3D ordered, controllable, and well-connected structures [50]. Additionally, the morphology and nanostructure could be well controlled by adjusting the experimental parameters due to the separated CVD processes. It is a pity that the multi-step process always requires strict growth conditions and large consumption of power (high tempera-

Recently, tremendous efforts have been made to produce 3D nanocarbon hybrid

via simultaneously in situ growing of 1D and 2D carbon nanomaterials on the surface of substrate during the CVD method. For example, Dong et al. (illustrated in **Figure 5**) reported that graphene/carbon nanotube hybrids were synthesized by a facile single-step CVD route employing ethanol (C2H5OH) as feedstock on the surface of Cu substrate decorated with Si nanoparticles, and the property and shape of hybrid could be varied by adjusting the fabrication environment (e.g., Si nanoparticles, temperature, and annealing time). The single-step route has the merits of better electrical conductivity and lesser defect density than the multi-step methods [43]. Additionally, although this one-step process effectively decreases the

*DOI: http://dx.doi.org/10.5772/intechopen.89671*

ture) for the growth of nanocarbon materials.

**2.5 One-step synthesis by chemical vapor deposition**

*Scheme for the two-step CVD synthesis of graphene/carbon nanotubes hybrids.*

*Scheme for the one-step CVD synthesis of graphene/carbon nanotubes hybrids.*

*Synthesis of Three-Dimensional Nanocarbon Hybrids by Chemical Vapor Deposition DOI: http://dx.doi.org/10.5772/intechopen.89671*

grows on the surface of substrate at a higher temperature of 950°C by utilizing the hydrocarbon—CH4, resulting in the in situ synthesis of graphene-carbon nanotubegraphene sandwiches [53]. In other methods of the successful fabrication of 3D hybrids, the obtained component materials are always entangled with each other, and the ordered 3D packing architecture is hardly available. Nevertheless, this multi-step way successfully integrates low-dimensional materials into 3D ordered, controllable, and well-connected structures [50]. Additionally, the morphology and nanostructure could be well controlled by adjusting the experimental parameters due to the separated CVD processes. It is a pity that the multi-step process always requires strict growth conditions and large consumption of power (high temperature) for the growth of nanocarbon materials.

#### **2.5 One-step synthesis by chemical vapor deposition**

Recently, tremendous efforts have been made to produce 3D nanocarbon hybrid via simultaneously in situ growing of 1D and 2D carbon nanomaterials on the surface of substrate during the CVD method. For example, Dong et al. (illustrated in **Figure 5**) reported that graphene/carbon nanotube hybrids were synthesized by a facile single-step CVD route employing ethanol (C2H5OH) as feedstock on the surface of Cu substrate decorated with Si nanoparticles, and the property and shape of hybrid could be varied by adjusting the fabrication environment (e.g., Si nanoparticles, temperature, and annealing time). The single-step route has the merits of better electrical conductivity and lesser defect density than the multi-step methods [43]. Additionally, although this one-step process effectively decreases the

**Figure 4.** *Scheme for the two-step CVD synthesis of graphene/carbon nanotubes hybrids.*

**Figure 5.** *Scheme for the one-step CVD synthesis of graphene/carbon nanotubes hybrids.*

*Hybrid Nanomaterials - Flexible Electronics Materials*

hybrids with hierarchical structure and stability.

**2.2 Hydrothermal method**

leading to robust 3D architecture [52].

**2.4 Multi-step synthesis by chemical vapor deposition**

**and CVD**

**2.1 Mixing process of surface-treated carbon materials**

chemical vapor deposition is considered as a simple and promising way to build 3D

As the early hybridization approaches, solution processing, vacuum filtration, layer-by-layer self-assembly methods, and so on could be classified into the facile mixing process. Altogether 1D carbon nanomaterial incorporation of 2D nanomaterial with a facile mixing process exhibits a synergistic effect in enhanced properties. However, the nanocarbon hybrids are synthesized by utilizing various methods to mix modified carbon-based feedstocks, which generally need sophisticated chemical routes with solvents and highly toxic agents [31, 33–36]. Furthermore, this kind of techniques suffers from poor controllability, leading to the restriction of practical application due to the aggregation and stacking of carbon-based materials [51].

With regard to hydrothermal method, firstly, the carbon feedstocks are dissolved and the mixed solution is transferred into a heating instrument. Secondly, the hydrothermal treatment is performed at low temperature, and the final product is obtained after centrifugation, washing, and freeze-drying process. Although this method has merits of mild conditions and scale-up synthesis, it is not suitable for the industrial production due to the time-consuming fault and defective products. Besides, the obtained 3D nanostructures are chiefly based on weak interconnection between individual nanocarbon components instead of owning powerful bonding,

**2.3 Multi-step approaches using combinations of decorated carbon materials** 

Chemical vapor deposition is considered as the most promising approach of the preparation of graphene, CNTs, and CNFs on the substrate surface. Thus, it is always employed to facilitate the growth of CNTs on the decorated carbon materials, leading to the 3D hierarchical composite. For example, most of the reported 3D carbon nanotube/carbon nanofiber hierarchical composites are typically prepared by a multi-step route, which first needs electrospinning technique and post-carbonization for the preparation of CNFs, followed by decorating the CNFs with metal catalyst nanoparticles, and eventually the CNT growth is promoted by using toxic organic gases or solvent as carbon source during the CVD process [10, 22]. This kind of CVD-based methods has distinctive advantages: efficiency, convenience, and high yield. However, the stable and suitable decorated carbon materials that always need sophisticated pretreatment are vital to the construction of 3D carbon

Multi-step chemical vapor depositions have been utilized in recent years to integrate individual 1D with 2D carbon nanomaterials to achieve controllable configurations of 3D nanostructures. Recently, Tang et al. successfully fabricated graphene-carbon nanotube composite on exfoliated vermiculite (EV) substrate by the multi-step CVD method (as shown in **Figure 4**). The whole CVD process could be divided into two steps: firstly, the aligned CNTs are synthesized at 650°C by using C2H4 as carbon source, and, secondly, the uniform graphene sheet directly

**30**

hybrids.

consumption of power, they still need high temperature, flammable gases, or toxic chemicals for the in-situ growth of 3D architecture.

#### **3. Effect of experimental parameters of CVD methods**

#### **3.1 Effect of catalyst nanoparticles**

It is known that substrate is the important part in the conventional CVD method, and the choice of substrate is essential to the morphology, nanostructure, and applications for carbonaceous nanomaterials. We generally use single transition metal substrate (Fe, Co, Ni, Cu, palladium (Pd) [41], ruthenium (Ru) [54]) as the catalyst for the preparation of graphene, and Fe, Co, Ni, and Cu are of great interest, because of the low cost and availability.

Remarkably, to build the uniform 3D architecture, single metal substrate is not enough for the CVD growth process. Hence, substrate embedded with metal nanoparticles serves as the bifunctional catalyst to facilitate the synthesis of different dimensional carbon materials, and the crucial issue for the in situ growth of 3D hybrids depends on the stability of catalyst nanoparticles during the deposition process. In CVD methods, the metal nanoparticles for the growth of hybrids could be obtained by a variety of ways, such as spin coating [47], electron evaporation [50], template etching [55, 56], and so on. Moreover, the covalent C–C bonding between different dimensional carbon materials, which is of paramount importance for 3D nanostructure, is probable to be achieved by such methods [57–59]. Nguyen et al. fabricated graphene/carbon nanotube composite by employing the Cu substrate-embedded Fe nanoparticles as the catalyst in the simple CVD approach [15]. In which Cu foil served as the template for the graphene sheet preparation. Additionally iron nanoparticles served as the catalyst for the CNT preparation. Besides, similar report indicated that the diameter, density, and quality of CNTs of composite could be defined by the size of the catalyst nanoparticles [45]. And various densities of catalyst nanoparticles had a different effect on the purity, thermal stability, and defects of 3D carbon hybrids [59].

#### **3.2 Effect of growth temperature**

Although low-dimensional carbon nanomaterials' nanostructure and diameter in 3D architecture is directly related to the size and nature of catalyst nanoparticles, it also could be indirectly determined by adjusting growth temperature in CVD technique. The different growth of CNF/CNT hybrid was fabricated due to the different carbon source decomposition and diffusion rate at various growth temperatures in the study of Park et al. [24]. Furthermore, the growth temperature is also crucial for the defects and properties of 3D carbon nanomaterials. Lin et al. [57] found that at different growing temperatures, the various architecture of sample could be produced by indirectly changing the number of layers of graphene and packing density of CNTs. And the ratio of the ID/IG (Raman spectroscopy analysis), defects, and surface area increased with the decrease of growth temperature, leading to the increased specific capacitance. As a result, it is crucial to seek the appropriate growth temperature for the growth of well-developed 3D composite.

#### **3.3 Effect of carrier gas**

In the CVD approach, hydrogen (H2), argon (Ar), and nitrogen (N2) are utilized for the growth of carbon materials in the high-temperature annealing process, and

**33**

**Figure 6.**

*Synthesis of Three-Dimensional Nanocarbon Hybrids by Chemical Vapor Deposition*

rate of H2 etching optimized the 3D nanocarbon formation.

*Schematics illustrating direct CNT growth on planar graphene under H2 etching.*

So far, quite a few investigations have been dedicated to the fabrication of 3D carbonaceous hybrids by using various carbon sources, and studies have illustrated that the carbon sources can also be basically classified into the three categories: hydrocarbon compounds (CH4 [45, 58], C2H2 [11, 50], C2H4 [10], C3H8 [43]), liquid

**3.4 Effect of carbon source**

the influence of variety of gases in the conventional CVD process is different. As for Ar and N2, they serve as the carrier gas to introduce the vapor into the CVD furnace under a suitable flow rate. As for H2, it has multifunctional effects in the practical CVD environment. First, it is believed that H2 removes surface impurities (such as S and P) and defects which can cause local variations of carbon solubility in the metal substrate in the high-temperature process [13, 60]. And it also enables the reduction process of metal oxides for producing enough catalyst nanoparticles at the high temperature [10, 22, 24]. Yan et al. [49] fabricated mesoscopic 3D composite comprised of graphene and CNTs under the effect of Ni-Co catalysts which was produced at 800°C in H2 atmosphere. Unlike the conventional CVD synthesis of individual 1D or 2D carbon nanostructure, H2 also plays an important role in building nanostructure of 3D hierarchical hybrids, especially for graphene/CNT composite. For example, there were two simultaneous reactions appearing during the construction of 3D graphene/CNT hybrids in the previous report [8]. On one time, the methane decomposed with the increasing temperature and thereafter facilitates the CNT growth out of islands of metal catalyst. Simultaneously, hydrogenation process appeared on the surface of graphene sheet (shown in **Figure 6**). In this process, graphene sheet was effectively etched under the atmosphere of H2 and transformed into CH4 at the point of connection with the catalyst nanoparticles (Ni nanoparticle + C graphene +2H2 → Ni + CH4) [61]. Furthermore, the morphology of the hybrids was adjusted via varying the H2 flow rate to change the two contrary reactions in the CVD method. Consequently, the high density of CNTs grown on the surface of graphene sheet under the suitable flow rate of H2, implying that the

*DOI: http://dx.doi.org/10.5772/intechopen.89671*

*Synthesis of Three-Dimensional Nanocarbon Hybrids by Chemical Vapor Deposition DOI: http://dx.doi.org/10.5772/intechopen.89671*

the influence of variety of gases in the conventional CVD process is different. As for Ar and N2, they serve as the carrier gas to introduce the vapor into the CVD furnace under a suitable flow rate. As for H2, it has multifunctional effects in the practical CVD environment. First, it is believed that H2 removes surface impurities (such as S and P) and defects which can cause local variations of carbon solubility in the metal substrate in the high-temperature process [13, 60]. And it also enables the reduction process of metal oxides for producing enough catalyst nanoparticles at the high temperature [10, 22, 24]. Yan et al. [49] fabricated mesoscopic 3D composite comprised of graphene and CNTs under the effect of Ni-Co catalysts which was produced at 800°C in H2 atmosphere. Unlike the conventional CVD synthesis of individual 1D or 2D carbon nanostructure, H2 also plays an important role in building nanostructure of 3D hierarchical hybrids, especially for graphene/CNT composite. For example, there were two simultaneous reactions appearing during the construction of 3D graphene/CNT hybrids in the previous report [8]. On one time, the methane decomposed with the increasing temperature and thereafter facilitates the CNT growth out of islands of metal catalyst. Simultaneously, hydrogenation process appeared on the surface of graphene sheet (shown in **Figure 6**). In this process, graphene sheet was effectively etched under the atmosphere of H2 and transformed into CH4 at the point of connection with the catalyst nanoparticles (Ni nanoparticle + C graphene +2H2 → Ni + CH4) [61]. Furthermore, the morphology of the hybrids was adjusted via varying the H2 flow rate to change the two contrary reactions in the CVD method. Consequently, the high density of CNTs grown on the surface of graphene sheet under the suitable flow rate of H2, implying that the rate of H2 etching optimized the 3D nanocarbon formation.

#### **3.4 Effect of carbon source**

*Hybrid Nanomaterials - Flexible Electronics Materials*

**3.1 Effect of catalyst nanoparticles**

est, because of the low cost and availability.

stability, and defects of 3D carbon hybrids [59].

**3.2 Effect of growth temperature**

chemicals for the in-situ growth of 3D architecture.

**3. Effect of experimental parameters of CVD methods**

consumption of power, they still need high temperature, flammable gases, or toxic

It is known that substrate is the important part in the conventional CVD method, and the choice of substrate is essential to the morphology, nanostructure, and applications for carbonaceous nanomaterials. We generally use single transition metal substrate (Fe, Co, Ni, Cu, palladium (Pd) [41], ruthenium (Ru) [54]) as the catalyst for the preparation of graphene, and Fe, Co, Ni, and Cu are of great inter-

Remarkably, to build the uniform 3D architecture, single metal substrate is not enough for the CVD growth process. Hence, substrate embedded with metal nanoparticles serves as the bifunctional catalyst to facilitate the synthesis of different dimensional carbon materials, and the crucial issue for the in situ growth of 3D hybrids depends on the stability of catalyst nanoparticles during the deposition process. In CVD methods, the metal nanoparticles for the growth of hybrids could be obtained by a variety of ways, such as spin coating [47], electron evaporation [50], template etching [55, 56], and so on. Moreover, the covalent C–C bonding between different dimensional carbon materials, which is of paramount importance for 3D nanostructure, is probable to be achieved by such methods [57–59]. Nguyen et al. fabricated graphene/carbon nanotube composite by employing the Cu substrate-embedded Fe nanoparticles as the catalyst in the simple CVD approach [15]. In which Cu foil served as the template for the graphene sheet preparation. Additionally iron nanoparticles served as the catalyst for the CNT preparation. Besides, similar report indicated that the diameter, density, and quality of CNTs of composite could be defined by the size of the catalyst nanoparticles [45]. And various densities of catalyst nanoparticles had a different effect on the purity, thermal

Although low-dimensional carbon nanomaterials' nanostructure and diameter in 3D architecture is directly related to the size and nature of catalyst nanoparticles, it also could be indirectly determined by adjusting growth temperature in CVD technique. The different growth of CNF/CNT hybrid was fabricated due to the different carbon source decomposition and diffusion rate at various growth temperatures in the study of Park et al. [24]. Furthermore, the growth temperature is also crucial for the defects and properties of 3D carbon nanomaterials. Lin et al. [57] found that at different growing temperatures, the various architecture of sample could be produced by indirectly changing the number of layers of graphene and packing density of CNTs. And the ratio of the ID/IG (Raman spectroscopy analysis), defects, and surface area increased with the decrease of growth temperature, leading to the increased specific capacitance. As a result, it is crucial to seek the appropriate growth temperature for the growth of well-developed 3D composite.

In the CVD approach, hydrogen (H2), argon (Ar), and nitrogen (N2) are utilized for the growth of carbon materials in the high-temperature annealing process, and

**32**

**3.3 Effect of carrier gas**

So far, quite a few investigations have been dedicated to the fabrication of 3D carbonaceous hybrids by using various carbon sources, and studies have illustrated that the carbon sources can also be basically classified into the three categories: hydrocarbon compounds (CH4 [45, 58], C2H2 [11, 50], C2H4 [10], C3H8 [43]), liquid

**Figure 6.** *Schematics illustrating direct CNT growth on planar graphene under H2 etching.*

carbon sources (ethanol [47], pyridine [22], toluene [41]), and solid feedstock (melamine [49], Prussian blue [9], camphor [62]) and so on. According to the relevant reports, diverse carbonaceous hybrids choose various carbon sources as feedstock for the basic supply of 3D architecture. With respect to CNT/CNF hybrids, hydrocarbon compounds are always considered as feedstocks of CNTs on the surface of obtained CNFs. With respect to CNT/graphene or CNF/graphene composites, hydrocarbon compounds, liquid carbon sources, and solid feedstock are all used as precursors for the growth of hierarchical architecture. Notably, for the synthesis of graphene, the present CVD technique requires high growth temperature, typically 1000°C [63–65]. Since it is more environment-friendly, convenient, and economical for industrial fabrication, a low-temperature route is greatly desirable. Liquid and solid carbon sources decompose at a lower temperature relative to major gaseous carbon sources. Therefore, liquid and solid feedstock could be a better choice for the growth of 3D CNT/graphene or CNF/graphene hybrids because of the quick carbon diffusivity through metal catalysts and covering on the surface at lower temperature. Moreover, during the dehydrogenation process of liquid or solid carbon sources, the overall dehydrogenation barrier and nucleation barrier are much lower than that of gaseous carbon source from the relevant report [66]. Recently, low temperature (800°C) one-step CVD synthesis of 3D hybrids composed by CNTs and graphene sheet are demonstrated by using melamine as the single solid carbon source [56]. Nevertheless, 3D hybrid growth at lower temperature still remains a challenge.

#### **4. Development trend and application prospect of three-dimensional nanocarbon hybrids**

Three-dimensional nanocarbon hybrids have been used for a variety of applications, for example, transparent and flexible electrodes and field-effect transistors [12–16, 47], sensors [17–20], fuel cell [67], batteries [9, 11, 44, 55], supercapacitors [10, 50, 51], and so on.

#### **4.1 Three-dimensional nanocarbon hybrids in transparent and flexible electrodes and field-effect transistors**

Because of the outstanding mechanical, electrical, and thermal properties, low dimensional nanocarbon materials have recently attracted enormous interest for potential application in transparent and flexible nanoelectronics [68–70]. Furthermore, 3D graphene-based hybrids which offset shortcomings of pure graphene received a large number of attentions in particular for two applications: transparent and flexible electrodes and field-effect transistors. Kim et al. [13] successful synthesized single-walled carbon nanotubes (SWCNT)/graphene hybrids on the Cu foil coated with CNTs. Notably, compared to pure CNT (58.78 ± 36.17 cm2 /V s) and graphene (341.7 ± 259.4 cm2 /V s), SWCNT-graphene hybrids possessed higher field-effect mobilities (μ) (394.46 ± 176.27 cm<sup>2</sup> /V s) and better output characteristics (**Figure 7**), suggesting that the electrical conductivity of this hybrids dramatically increased compared to individual carbon material. As for transparent and flexible device applications, the hybrids showed the low sheet resistance (300 Ω/sq) with 96.4% optical transparency which is largely lower than the monolayer graphene (∼1 kΩ/sq) grown by CVD method, indicating that composite is a promising material in developing high-performance transparent and flexible devices. Additionally, the hybrids possessed improved mechano-electrical property result from the CNT growth and obtained hybrid demonstrated that at an

**35**

ible capacity (300 mA h g<sup>−</sup><sup>1</sup>

*Synthesis of Three-Dimensional Nanocarbon Hybrids by Chemical Vapor Deposition*

applied field of 4.0 V/μm, the hybrid exhibited a current density of 1.33 mA cm<sup>−</sup><sup>2</sup> [15], implying superiority than that of pure CNT materials on indium tin oxide films (ITO) glass [71]. And it probably replaces the ITO films, the most common transparent and flexible electrodes, as an alternative material with properties including high on/off ratios and outstanding electrical conductivity for high-

*Output characteristics (IDS − VDS) of graphene, SWCNT-graphene hybrid film, and SWCNT. Reproduced* 

In the practical application, the higher active and stable catalysts are crucial to the high electrochemical performance of fuel cell. Compared to the pure Pt-graphene cathode material, the Pt-3D nanocarbon composite cathode exhibits much smaller oxygen reduction reaction (ORR) charge transfer resistance and higher maximum power density in the direct methanol fuel cell [23] and proton exchange membrane fuel cells [21]. Moreover, due to the expensive cost and poor durability, as the spread anode and cathode electro-catalysts for ORR, Pt-based materials are hampered in the commercialization. Significantly, the CNT/CNF composite acts as the effective Pt-free ORR catalyst with a comparable activity, cheap price, and better thermal stability and durability, and the unique 3D network results in the enhanced electrochemical performance [22], implying 3D hybrid materials are becoming increasing competitive in the fuel cell applications.

Carbon-based materials (such as CNTs, CNFs, graphene), with their merits

resistance and contact resistances in contrast with the individual CNF material [11].

), outstanding cycling stability, and lower electrolyte

coulumbic efficiencies, and low-capacity fade, are excellent choices as electrode materials of lithium-ion batteries [72, 73]. Nevertheless, the cycling performance and high-rate capability of individual material are not as satisfactory as expected, possibly owing to the large contact resistance of easy self-aggregation and stacking. Moreover, hybrids consisting of various low dimensional carbon materials, which favor different oriented diffusion of the lithium ion and the 3D nanocarbon architecture, are beneficial to the electrons' collection and transport around the cycling process, leading to high electrical conductivity and chemical stability. For example, the 3D nanocarbon hybrid anode exhibited significantly enhanced revers-

, high-

of reversible lithium-carbon reaction, low-intercalation potential with Li+

**4.2 Three-dimensional nanocarbon hybrids in fuel cell and batteries**

*DOI: http://dx.doi.org/10.5772/intechopen.89671*

performance flexible device in the future.

**Figure 7.**

*with permission from ref. [13].*

*Synthesis of Three-Dimensional Nanocarbon Hybrids by Chemical Vapor Deposition DOI: http://dx.doi.org/10.5772/intechopen.89671*

**Figure 7.** *Output characteristics (IDS − VDS) of graphene, SWCNT-graphene hybrid film, and SWCNT. Reproduced with permission from ref. [13].*

applied field of 4.0 V/μm, the hybrid exhibited a current density of 1.33 mA cm<sup>−</sup><sup>2</sup> [15], implying superiority than that of pure CNT materials on indium tin oxide films (ITO) glass [71]. And it probably replaces the ITO films, the most common transparent and flexible electrodes, as an alternative material with properties including high on/off ratios and outstanding electrical conductivity for highperformance flexible device in the future.

#### **4.2 Three-dimensional nanocarbon hybrids in fuel cell and batteries**

In the practical application, the higher active and stable catalysts are crucial to the high electrochemical performance of fuel cell. Compared to the pure Pt-graphene cathode material, the Pt-3D nanocarbon composite cathode exhibits much smaller oxygen reduction reaction (ORR) charge transfer resistance and higher maximum power density in the direct methanol fuel cell [23] and proton exchange membrane fuel cells [21]. Moreover, due to the expensive cost and poor durability, as the spread anode and cathode electro-catalysts for ORR, Pt-based materials are hampered in the commercialization. Significantly, the CNT/CNF composite acts as the effective Pt-free ORR catalyst with a comparable activity, cheap price, and better thermal stability and durability, and the unique 3D network results in the enhanced electrochemical performance [22], implying 3D hybrid materials are becoming increasing competitive in the fuel cell applications.

Carbon-based materials (such as CNTs, CNFs, graphene), with their merits of reversible lithium-carbon reaction, low-intercalation potential with Li+ , highcoulumbic efficiencies, and low-capacity fade, are excellent choices as electrode materials of lithium-ion batteries [72, 73]. Nevertheless, the cycling performance and high-rate capability of individual material are not as satisfactory as expected, possibly owing to the large contact resistance of easy self-aggregation and stacking. Moreover, hybrids consisting of various low dimensional carbon materials, which favor different oriented diffusion of the lithium ion and the 3D nanocarbon architecture, are beneficial to the electrons' collection and transport around the cycling process, leading to high electrical conductivity and chemical stability. For example, the 3D nanocarbon hybrid anode exhibited significantly enhanced reversible capacity (300 mA h g<sup>−</sup><sup>1</sup> ), outstanding cycling stability, and lower electrolyte resistance and contact resistances in contrast with the individual CNF material [11].

*Hybrid Nanomaterials - Flexible Electronics Materials*

ture still remains a challenge.

**nanocarbon hybrids**

**electrodes and field-effect transistors**

[10, 50, 51], and so on.

(58.78 ± 36.17 cm2

carbon sources (ethanol [47], pyridine [22], toluene [41]), and solid feedstock (melamine [49], Prussian blue [9], camphor [62]) and so on. According to the relevant reports, diverse carbonaceous hybrids choose various carbon sources as feedstock for the basic supply of 3D architecture. With respect to CNT/CNF hybrids, hydrocarbon compounds are always considered as feedstocks of CNTs on the surface of obtained CNFs. With respect to CNT/graphene or CNF/graphene composites, hydrocarbon compounds, liquid carbon sources, and solid feedstock are all used as precursors for the growth of hierarchical architecture. Notably, for the synthesis of graphene, the present CVD technique requires high growth temperature, typically 1000°C [63–65]. Since it is more environment-friendly, convenient, and economical for industrial fabrication, a low-temperature route is greatly desirable. Liquid and solid carbon sources decompose at a lower temperature relative to major gaseous carbon sources. Therefore, liquid and solid feedstock could be a better choice for the growth of 3D CNT/graphene or CNF/graphene hybrids because of the quick carbon diffusivity through metal catalysts and covering on the surface at lower temperature. Moreover, during the dehydrogenation process of liquid or solid carbon sources, the overall dehydrogenation barrier and nucleation barrier are much lower than that of gaseous carbon source from the relevant report [66]. Recently, low temperature (800°C) one-step CVD synthesis of 3D hybrids composed by CNTs and graphene sheet are demonstrated by using melamine as the single solid carbon source [56]. Nevertheless, 3D hybrid growth at lower tempera-

**4. Development trend and application prospect of three-dimensional** 

**4.1 Three-dimensional nanocarbon hybrids in transparent and flexible** 

/V s) and graphene (341.7 ± 259.4 cm2

better output characteristics (**Figure 7**), suggesting that the electrical conductivity of this hybrids dramatically increased compared to individual carbon material. As for transparent and flexible device applications, the hybrids showed the low sheet resistance (300 Ω/sq) with 96.4% optical transparency which is largely lower than the monolayer graphene (∼1 kΩ/sq) grown by CVD method, indicating that composite is a promising material in developing high-performance transparent and flexible devices. Additionally, the hybrids possessed improved mechano-electrical property result from the CNT growth and obtained hybrid demonstrated that at an

hybrids possessed higher field-effect mobilities (μ) (394.46 ± 176.27 cm<sup>2</sup>

/V s), SWCNT-graphene

/V s) and

Because of the outstanding mechanical, electrical, and thermal properties, low dimensional nanocarbon materials have recently attracted enormous interest for potential application in transparent and flexible nanoelectronics [68–70]. Furthermore, 3D graphene-based hybrids which offset shortcomings of pure graphene received a large number of attentions in particular for two applications: transparent and flexible electrodes and field-effect transistors. Kim et al. [13] successful synthesized single-walled carbon nanotubes (SWCNT)/graphene hybrids on the Cu foil coated with CNTs. Notably, compared to pure CNT

Three-dimensional nanocarbon hybrids have been used for a variety of applications, for example, transparent and flexible electrodes and field-effect transistors [12–16, 47], sensors [17–20], fuel cell [67], batteries [9, 11, 44, 55], supercapacitors

**34**

Additionally, by building 3D carbon network, at current densities of 0.36, 0.6, 1.2, 2.4, and 6 mA/cm<sup>2</sup> , the rate performance of graphene/CNF hybrids reached about 420, 385, 329, 229, and 189 mA h g<sup>−</sup><sup>1</sup> , (as shown in **Figure 8**), which were superior to those of other pure nanocarbon performances [44]. Therefore, hybridization of the different low-dimensional carbon nanomaterials is an effective route to provide fast ion/electron transfer and higher Li storage capability, and the hierarchical 3D carbonaceous architecture is also promising for Li-ion battery applications in the future.

Because of the large energy density, capacity (1673 mA h g<sup>−</sup><sup>1</sup> ), low cost, and environmental benignity of sulfur, lithium-sulfur (Li-S) batteries are investigated by a large number of research groups. However, the "shuttling effect" which always triggers an inevitable sulfur loss in practical Li-S battery applications, leading to an increase in internal resistance, low cycling capacity, and poor coulombic efficiency. To solve this problem, porous carbon materials, e.g., CNTs and graphene, also have been utilized to capture and encapsulate sulfur, blocking the high solubility of polysulfides during the Li-S battery applications [74, 75]. And compared to pure CNT which is always hindered by problems of easy self-aggregation, enormous interface resistance, and poor S-storage ability, the 3D hybrids composed of CNTs and graphene are more suitable for the cathode of high-rate performance for Li-S batteries. The hybrid structure exhibits unique advantages: (i) the well-connected junction between the CNTs and graphene sheets enable rapid electron transfer; (ii) robust nanostructure provides flexibility and mechanical robustness, which effectively buffers volume changes during the cycling process [9]. Zhao et al. reported that graphene/CNT composite cathode possessed remarkable performance: a reversible capacity (928 mA h g<sup>−</sup><sup>1</sup> ) at 1 C capacity and at a high current rate of 5 C, the capacity as high as about 650 mA h g<sup>−</sup><sup>1</sup> could be obtained even after 100 cycles with a coulombic efficiency of about 92% in Li-S battery applications [76]. Furthermore, it is worth noting that electrochemical performance and catalytic activity have significantly improved nitrogen doping according to a relevant report, thus nitrogen-doped 3D hybrids also applied in the Li-S batteries. Tang et al. employed glucose and dicyandiamide as the carbon and nitrogen feedstocks to prepare the nitrogen-doped nanocarbon hybrid by a one-step chemical vapor deposition process technique, and the result (1314 mA h g<sup>−</sup><sup>1</sup> at 0.2 C, a capacity retention

#### **Figure 8.**

*Comparison of the rate capabilities of CNF/GNS, GNS, CNT, commercial natural graphite discharged at C/5, CNF (30 nm in diameter), CNF/natural graphite, and natural graphite spheres. Reproduced with permission from ref. [44].*

**37**

**Figure 9.**

*of 0.3 a g<sup>−</sup><sup>1</sup>*

*with permission from ref. [30].*

*Synthesis of Three-Dimensional Nanocarbon Hybrids by Chemical Vapor Deposition*

3D nanostructure has the potential toward promising Li-S batteries.

**4.3 Three-dimensional nanocarbon hybrids in supercapacitors**

) at 10 mV s<sup>−</sup><sup>1</sup>

tor was superior to the graphene electrode (99.6 μF cm<sup>−</sup><sup>2</sup>

ion diffusion/transport resistance (shown in **Figure 9**).

of 97% after 200 cycles at a high rate of 2 C) exhibited the improved cyclic and rate performances [9]. These experimental results also indicate that the nitrogen-doped

A variety of nanocarbon materials, e.g., CNTs, graphene, or mesoporous and activated carbon possess enormous specific surface areas yet are limited by low performance owing to aggregation and internal resistance, leading to decreased capacitance than theoretical prediction [77]. To overcome the aforementioned disadvantages, 3D composites are considered as attractive materials for supercapacitor application by inhibiting the agglomeration and improving the electrolyte electrode accessibility and the electrode conductivity. Relevant report demonstrated that the

supercapacitor based on polyaniline/carbon nanotube/carbon nanofiber (PANI/ CNT/CNF) electrode [30]. Compared with pure PANI/CNF, the hybrids showed higher specific capacitance and energy density, superior rate capability, and lower

Meanwhile, due to the merits—high theoretical capacity, low cost, and natural abundance—diverse potential metal oxides, e.g., RuO2 [78], MnO2 [79], NiO [80],

*(A) GCD curves of CNF, CNT/CNF, PANI/CNF, and PANI/CNT/CNF film electrodes at a current density* 

*Specific capacitance vs. current density for PANI/CNF and PANI/CNT/CNF film electrodes. (D) Charging/*

*discharging cycling stability of PANI/CNT/CNF film electrodes at a current density of 15 a g<sup>−</sup><sup>1</sup>*

*, respectively. (B) GCD curves of PANI/CNT/CNF film electrodes at different current densities. (C)* 

of 3D CNT/graphene-based supercapaci-

) [8]. Zhou et al. fabricated

*. Reproduced* 

*DOI: http://dx.doi.org/10.5772/intechopen.89671*

capacitance (653.7 μF cm<sup>−</sup><sup>2</sup>

*Synthesis of Three-Dimensional Nanocarbon Hybrids by Chemical Vapor Deposition DOI: http://dx.doi.org/10.5772/intechopen.89671*

of 97% after 200 cycles at a high rate of 2 C) exhibited the improved cyclic and rate performances [9]. These experimental results also indicate that the nitrogen-doped 3D nanostructure has the potential toward promising Li-S batteries.

#### **4.3 Three-dimensional nanocarbon hybrids in supercapacitors**

A variety of nanocarbon materials, e.g., CNTs, graphene, or mesoporous and activated carbon possess enormous specific surface areas yet are limited by low performance owing to aggregation and internal resistance, leading to decreased capacitance than theoretical prediction [77]. To overcome the aforementioned disadvantages, 3D composites are considered as attractive materials for supercapacitor application by inhibiting the agglomeration and improving the electrolyte electrode accessibility and the electrode conductivity. Relevant report demonstrated that the capacitance (653.7 μF cm<sup>−</sup><sup>2</sup> ) at 10 mV s<sup>−</sup><sup>1</sup> of 3D CNT/graphene-based supercapacitor was superior to the graphene electrode (99.6 μF cm<sup>−</sup><sup>2</sup> ) [8]. Zhou et al. fabricated supercapacitor based on polyaniline/carbon nanotube/carbon nanofiber (PANI/ CNT/CNF) electrode [30]. Compared with pure PANI/CNF, the hybrids showed higher specific capacitance and energy density, superior rate capability, and lower ion diffusion/transport resistance (shown in **Figure 9**).

Meanwhile, due to the merits—high theoretical capacity, low cost, and natural abundance—diverse potential metal oxides, e.g., RuO2 [78], MnO2 [79], NiO [80],

#### **Figure 9.**

*Hybrid Nanomaterials - Flexible Electronics Materials*

420, 385, 329, 229, and 189 mA h g<sup>−</sup><sup>1</sup>

mance: a reversible capacity (928 mA h g<sup>−</sup><sup>1</sup>

rate of 5 C, the capacity as high as about 650 mA h g<sup>−</sup><sup>1</sup>

sition process technique, and the result (1314 mA h g<sup>−</sup><sup>1</sup>

2.4, and 6 mA/cm2

future.

Additionally, by building 3D carbon network, at current densities of 0.36, 0.6, 1.2,

to those of other pure nanocarbon performances [44]. Therefore, hybridization of the different low-dimensional carbon nanomaterials is an effective route to provide fast ion/electron transfer and higher Li storage capability, and the hierarchical 3D carbonaceous architecture is also promising for Li-ion battery applications in the

environmental benignity of sulfur, lithium-sulfur (Li-S) batteries are investigated by a large number of research groups. However, the "shuttling effect" which always triggers an inevitable sulfur loss in practical Li-S battery applications, leading to an increase in internal resistance, low cycling capacity, and poor coulombic efficiency. To solve this problem, porous carbon materials, e.g., CNTs and graphene, also have been utilized to capture and encapsulate sulfur, blocking the high solubility of polysulfides during the Li-S battery applications [74, 75]. And compared to pure CNT which is always hindered by problems of easy self-aggregation, enormous interface resistance, and poor S-storage ability, the 3D hybrids composed of CNTs and graphene are more suitable for the cathode of high-rate performance for Li-S batteries. The hybrid structure exhibits unique advantages: (i) the well-connected junction between the CNTs and graphene sheets enable rapid electron transfer; (ii) robust nanostructure provides flexibility and mechanical robustness, which effectively buffers volume changes during the cycling process [9]. Zhao et al. reported that graphene/CNT composite cathode possessed remarkable perfor-

100 cycles with a coulombic efficiency of about 92% in Li-S battery applications [76]. Furthermore, it is worth noting that electrochemical performance and catalytic activity have significantly improved nitrogen doping according to a relevant report, thus nitrogen-doped 3D hybrids also applied in the Li-S batteries. Tang et al. employed glucose and dicyandiamide as the carbon and nitrogen feedstocks to prepare the nitrogen-doped nanocarbon hybrid by a one-step chemical vapor depo-

*Comparison of the rate capabilities of CNF/GNS, GNS, CNT, commercial natural graphite discharged at C/5, CNF (30 nm in diameter), CNF/natural graphite, and natural graphite spheres. Reproduced with permission* 

Because of the large energy density, capacity (1673 mA h g<sup>−</sup><sup>1</sup>

, the rate performance of graphene/CNF hybrids reached about

, (as shown in **Figure 8**), which were superior

) at 1 C capacity and at a high current

could be obtained even after

at 0.2 C, a capacity retention

), low cost, and

**36**

**Figure 8.**

*from ref. [44].*

*(A) GCD curves of CNF, CNT/CNF, PANI/CNF, and PANI/CNT/CNF film electrodes at a current density of 0.3 a g<sup>−</sup><sup>1</sup> , respectively. (B) GCD curves of PANI/CNT/CNF film electrodes at different current densities. (C) Specific capacitance vs. current density for PANI/CNF and PANI/CNT/CNF film electrodes. (D) Charging/ discharging cycling stability of PANI/CNT/CNF film electrodes at a current density of 15 a g<sup>−</sup><sup>1</sup> . Reproduced with permission from ref. [30].*

and Co3O4 [81] are regarded as the potential materials for pseudocapacitors. Particularly, as one of the most promising pseudocapacitor materials, when the MnO2 combined with 3D carbon hybrid, the drawbacks such as weak conductivity, low specific surface area, and brittleness of metal oxide electrodes are effectively alleviated, resulting in higher electrochemical performances [26, 27]. Wang et al. synthesized nanocarbon hierarchical composites (CNTs/CNFs) decorated with MnO2 for flexible supercapacitors [10]. And the 3D nanocarbon hybrid/MnO2 electrodes showed large better specific capacitance, cycling stability, maximum energy density, and rate capability than the CNF/MnO2 electrodes. These enhanced electrochemical performances of hybridized-based electrodes indicate that the designed hierarchical structures of composites support a large special surface area for the reaction between electrolyte ions and metal oxides. Simultaneously, the special 3D nanostructures improve the electrode nanomaterials' electronic conductivity and facilitate transport channels for electrolyte ions. It is no doubt that 3D nanocarbon hybrids will have a crucial impact on the emerging materials of high-performance supercapacitor applications.

#### **5. Conclusion and further prospects**

To combine the merits of each building block, 3D nanocarbon structures (CNT/ graphene, CNF/graphene, CNT/CNF hybrids) have been prepared by a variety of methods. The synthesis procedure, merits, and demerits of different approaches reported in the literatures are discussed in this chapter. Among them, chemical vapor deposition is regarded as the most promising fabrication method, and nicely hybrid architectures are achievable by this way. Nevertheless, there are various drawbacks and challenges in the practice synthesis. One of the great challenges in the CVD synthesis of the three-dimensional nanocarbon hybrids is convenience or simpleness when compared to preparation methods of individual nanocarbon materials. A simple scalable CVD method to fabricate controllable architecture of 3D nanocarbon hybrid is still crucial to industrial production. Furthermore, a variety of applications have been presented in this chapter. Compared to individual nanocarbon components, the superior performances of 3D nanocarbon hybrids signify their promising and wide application in the future, and 3D hybrid electrode materials are becoming more competitive in energy storage applications. It is worth mentioning that studies on the growth mechanism of 3D nanostructure which is necessary for the full understanding of CVD growth process is seldom reported from the relevant literatures. And some crucial problem still remained to be solved, particularly the interactions between various individual components and structure control in the future.

**39**

**Author details**

, Shuguang Deng2

provided the original work is properly cited.

\* and Gui-Ping Dai3

1 Institute for Advanced Study, Nanchang University, Nanchang, Jiangxi, China

3 Department of Chemical Engineering, School of Environmental and Chemical

\*Address all correspondence to: shuguang.deng@asu.edu and nanodai@gmail.com

© 2019 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,

2 Department of Chemical Engineering, School for Engineering of Matter,

Transport and Energy, Arizona State University, Tempe, USA

Engineering, Nanchang University, Nanchang, Jiangxi, China

\*

Hua-Fei Li1

*Synthesis of Three-Dimensional Nanocarbon Hybrids by Chemical Vapor Deposition*

*DOI: http://dx.doi.org/10.5772/intechopen.89671*

*Synthesis of Three-Dimensional Nanocarbon Hybrids by Chemical Vapor Deposition DOI: http://dx.doi.org/10.5772/intechopen.89671*

#### **Author details**

*Hybrid Nanomaterials - Flexible Electronics Materials*

supercapacitor applications.

control in the future.

**5. Conclusion and further prospects**

and Co3O4 [81] are regarded as the potential materials for pseudocapacitors. Particularly, as one of the most promising pseudocapacitor materials, when the MnO2 combined with 3D carbon hybrid, the drawbacks such as weak conductivity, low specific surface area, and brittleness of metal oxide electrodes are effectively alleviated, resulting in higher electrochemical performances [26, 27]. Wang et al. synthesized nanocarbon hierarchical composites (CNTs/CNFs) decorated with MnO2 for flexible supercapacitors [10]. And the 3D nanocarbon hybrid/MnO2 electrodes showed large better specific capacitance, cycling stability, maximum energy density, and rate capability than the CNF/MnO2 electrodes. These enhanced electrochemical performances of hybridized-based electrodes indicate that the designed hierarchical structures of composites support a large special surface area for the reaction between electrolyte ions and metal oxides. Simultaneously, the special 3D nanostructures improve the electrode nanomaterials' electronic conductivity and facilitate transport channels for electrolyte ions. It is no doubt that 3D nanocarbon hybrids will have a crucial impact on the emerging materials of high-performance

To combine the merits of each building block, 3D nanocarbon structures (CNT/ graphene, CNF/graphene, CNT/CNF hybrids) have been prepared by a variety of methods. The synthesis procedure, merits, and demerits of different approaches reported in the literatures are discussed in this chapter. Among them, chemical vapor deposition is regarded as the most promising fabrication method, and nicely hybrid architectures are achievable by this way. Nevertheless, there are various drawbacks and challenges in the practice synthesis. One of the great challenges in the CVD synthesis of the three-dimensional nanocarbon hybrids is convenience or simpleness when compared to preparation methods of individual nanocarbon materials. A simple scalable CVD method to fabricate controllable architecture of 3D nanocarbon hybrid is still crucial to industrial production. Furthermore, a variety of applications have been presented in this chapter. Compared to individual nanocarbon components, the superior performances of 3D nanocarbon hybrids signify their promising and wide application in the future, and 3D hybrid electrode materials are becoming more competitive in energy storage applications. It is worth mentioning that studies on the growth mechanism of 3D nanostructure which is necessary for the full understanding of CVD growth process is seldom reported from the relevant literatures. And some crucial problem still remained to be solved, particularly the interactions between various individual components and structure

**38**

Hua-Fei Li1 , Shuguang Deng2 \* and Gui-Ping Dai3 \*

1 Institute for Advanced Study, Nanchang University, Nanchang, Jiangxi, China

2 Department of Chemical Engineering, School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, USA

3 Department of Chemical Engineering, School of Environmental and Chemical Engineering, Nanchang University, Nanchang, Jiangxi, China

\*Address all correspondence to: shuguang.deng@asu.edu and nanodai@gmail.com

© 2019 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.

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[61] Ci LJ, Song L, Jariwala D, Elías AL, Gao W, Terrones M, et al. Graphene shape control by multistage cutting and transfer. Advanced Materials. 2009;**21**(44):4487-4491

[62] Shinde SM, Kalita G, Sharma S, Papon R, Yusop MZ, Tanemura M. Synthesis of a three dimensional structure of vertically aligned carbon nanotubes and graphene from a single solid carbon source. RSC Advances. **2014**;**4**(26):13355-13360

[63] Li X et al. Large-area synthesis of high-quality and uniform graphene films on copper foils. Science. 2009;**324**(5932):1312-1314

[64] Lee S, Lee K, Zhong ZH. Wafer scale homogeneous bilayer graphene films by chemical vapor deposition. Nano Letters. 2010;**10**(11):4702-4707

[65] Li XS, Cai WW, Colombo L, Ruoff RS. Evolution of graphene growth on Ni and Cu by carbon isotope labeling. Nano Letters. 2009;**9**(12):4268-4272

[66] Li ZC, Wu P, Wang CX, Fan XD, Zhang WH, Zhai XF, et al. Lowtemperature growth of graphene by chemical vapor deposition using solid and liquid carbon sources. ACS Nano. 2011;**5**:3385-3390

[67] Li SS, Luo YH, Lv W, Yu WJ, Wu S, Hou PX, et al. Vertically aligned carbon nanotubes grown on Graphene paper as electrodes in lithium-ion batteries and dye-sensitized solar cells. Advanced Energy Materials. 2011;**1**(4):486-490

[68] Javey A, Guo J, Wang Q, Lundstrom M, Dai H. Ballistic carbon nanotube field-effect transistors. Nature. 2003;**424**(6949):654-657

[69] Kakade BA, Pillai VK, Late DJ, Chavan PG, Sheini FJ, More MA, et al. High current density, low threshold field emission from functionalized carbon

nanotube bucky paper. Applied Physics Letters. 2010;**97**(7):073102

[70] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al. Electric field effect in atomically thin carbon films. Science. 2004;**306**(5696):666-669

[71] Yang H, Shang X, Li Z, Qu S, Gu Z, Xu Y, et al. Synthesis of largearea single-walled carbon nanotube films on glass substrate and their field electron emission properties. Materials Chemistry and Physics. 2010;**124**(1):78-82

[72] Obrovac MN, Christensen L. Structural changes in silicon anodes during lithium insertion/extraction. Electrochemical and Solid-State Letters. 2004;**7**(5):A93-A96

[73] Winter M, Besenhard JO, Spahr ME, Novák P. Insertion electrode materials for rechargeable lithium batteries. Advanced Materials. 1998;**10**(10):725-763

[74] Zhou G, Wang DW, Li F, Hou PX, Yin LC, Liu C, et al. A flexible nanostructured Sulphur–carbon nanotube cathode with high rate performance for Li-S batteries. Energy & Environmental Science. 2012;**5**(10):8901-8906

[75] Wang HL, Yang Y, Liang YY, Robinson JT, Li YG, Jackson A, et al. Graphene-wrapped sulfur particles as a rechargeable lithium–sulfur battery cathode material with high capacity and cycling stability. Nano Letters. 2011;**11**(7):2644-2647

[76] Zhao MQ, Liu XF, Zhang Q, Tian GL, Huang JQ, Zhu WC, et al. Graphene/single-walled carbon nanotube hybrids: One-step catalytic growth and applications for highrate Li–S batteries. ACS Nano. 2012;**6**(12):10759-10769

**45**

*Synthesis of Three-Dimensional Nanocarbon Hybrids by Chemical Vapor Deposition*

*DOI: http://dx.doi.org/10.5772/intechopen.89671*

[78] Wu ZS, Wang DW, Ren W, Zhao J, Zhou G, Li F, et al. Anchoring hydrous RuO2 on graphene sheets for highperformance electrochemical capacitors.

[79] Lv P, Feng YY, Li Y, Feng W. Carbon fabric-aligned carbon nanotube/MnO2/ conducting polymers ternary composite electrodes with high utilization and mass loading of MnO2 for supercapacitors. Journal of Power Sources.

[80] Liu J, Jiang J, Bosman M, Fan HJ. Three-dimensional tubular arrays of MnO2–NiO nanoflakes with high areal pseudocapacitance. Journal of Materials Chemistry. 2012;**22**(6):2419-2426

[81] Liu JP, Jiang J, Cheng CW,

2011;**23**(18):2076-2081

Li HX, Zhang JX, Gong H, et al. Co3O4 nanowire@MnO2 ultrathin nanosheet core/shell arrays: A new class of high-performance Pseudocapacitive materials. Advanced Materials.

Advanced Functional Materials.

[77] Zhang LL, Zhao XS. Carbonbased materials as supercapacitor electrodes. Chemical Society Reviews.

2009;**38**(9):2520-2531

2010;**20**(20):3595-3602

2012;**220**:160-168

*Synthesis of Three-Dimensional Nanocarbon Hybrids by Chemical Vapor Deposition DOI: http://dx.doi.org/10.5772/intechopen.89671*

[77] Zhang LL, Zhao XS. Carbonbased materials as supercapacitor electrodes. Chemical Society Reviews. 2009;**38**(9):2520-2531

*Hybrid Nanomaterials - Flexible Electronics Materials*

nanotube bucky paper. Applied Physics

Letters. 2010;**97**(7):073102

2004;**306**(5696):666-669

2010;**124**(1):78-82

2004;**7**(5):A93-A96

1998;**10**(10):725-763

2012;**5**(10):8901-8906

2011;**11**(7):2644-2647

[71] Yang H, Shang X, Li Z, Qu S, Gu Z, Xu Y, et al. Synthesis of largearea single-walled carbon nanotube films on glass substrate and their field electron emission properties. Materials Chemistry and Physics.

[72] Obrovac MN, Christensen L. Structural changes in silicon anodes during lithium insertion/extraction. Electrochemical and Solid-State Letters.

[73] Winter M, Besenhard JO,

[74] Zhou G, Wang DW, Li F,

[75] Wang HL, Yang Y, Liang YY, Robinson JT, Li YG, Jackson A, et al. Graphene-wrapped sulfur particles as a rechargeable lithium–sulfur battery cathode material with high capacity and cycling stability. Nano Letters.

[76] Zhao MQ, Liu XF, Zhang Q, Tian GL, Huang JQ, Zhu WC, et al. Graphene/single-walled carbon nanotube hybrids: One-step catalytic growth and applications for highrate Li–S batteries. ACS Nano. 2012;**6**(12):10759-10769

Spahr ME, Novák P. Insertion electrode materials for rechargeable lithium batteries. Advanced Materials.

Hou PX, Yin LC, Liu C, et al. A flexible nanostructured Sulphur–carbon nanotube cathode with high rate performance for Li-S batteries. Energy & Environmental Science.

[70] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al. Electric field effect in atomically thin carbon films. Science.

[61] Ci LJ, Song L, Jariwala D, Elías AL, Gao W, Terrones M, et al. Graphene shape control by multistage cutting and transfer. Advanced Materials.

[62] Shinde SM, Kalita G, Sharma S, Papon R, Yusop MZ, Tanemura M. Synthesis of a three dimensional structure of vertically aligned carbon nanotubes and graphene from a single solid carbon source. RSC Advances.

[63] Li X et al. Large-area synthesis of high-quality and uniform graphene films on copper foils. Science. 2009;**324**(5932):1312-1314

[64] Lee S, Lee K, Zhong ZH. Wafer scale homogeneous bilayer graphene films by chemical vapor deposition. Nano Letters. 2010;**10**(11):4702-4707

Ruoff RS. Evolution of graphene growth on Ni and Cu by carbon isotope labeling. Nano Letters. 2009;**9**(12):4268-4272

[66] Li ZC, Wu P, Wang CX, Fan XD, Zhang WH, Zhai XF, et al. Lowtemperature growth of graphene by chemical vapor deposition using solid and liquid carbon sources. ACS Nano.

[67] Li SS, Luo YH, Lv W, Yu WJ, Wu S, Hou PX, et al. Vertically aligned carbon nanotubes grown on Graphene paper as electrodes in lithium-ion batteries and dye-sensitized solar cells. Advanced Energy Materials. 2011;**1**(4):486-490

Lundstrom M, Dai H. Ballistic carbon nanotube field-effect transistors. Nature. 2003;**424**(6949):654-657

[69] Kakade BA, Pillai VK, Late DJ, Chavan PG, Sheini FJ, More MA, et al. High current density, low threshold field emission from functionalized carbon

[68] Javey A, Guo J, Wang Q,

2011;**5**:3385-3390

[65] Li XS, Cai WW, Colombo L,

2009;**21**(44):4487-4491

**2014**;**4**(26):13355-13360

**44**

[78] Wu ZS, Wang DW, Ren W, Zhao J, Zhou G, Li F, et al. Anchoring hydrous RuO2 on graphene sheets for highperformance electrochemical capacitors. Advanced Functional Materials. 2010;**20**(20):3595-3602

[79] Lv P, Feng YY, Li Y, Feng W. Carbon fabric-aligned carbon nanotube/MnO2/ conducting polymers ternary composite electrodes with high utilization and mass loading of MnO2 for supercapacitors. Journal of Power Sources. 2012;**220**:160-168

[80] Liu J, Jiang J, Bosman M, Fan HJ. Three-dimensional tubular arrays of MnO2–NiO nanoflakes with high areal pseudocapacitance. Journal of Materials Chemistry. 2012;**22**(6):2419-2426

[81] Liu JP, Jiang J, Cheng CW, Li HX, Zhang JX, Gong H, et al. Co3O4 nanowire@MnO2 ultrathin nanosheet core/shell arrays: A new class of high-performance Pseudocapacitive materials. Advanced Materials. 2011;**23**(18):2076-2081

**47**

**Chapter 4**

**Abstract**

*Amita Somya*

metal phosphates

**1. Introduction**

Hybrid Ion Exchangers

Hybrid ion exchangers are of recent origin in the field of ion exchange chemistry. They have shown excellent chemical, mechanical and thermal stability conversant to both organic and inorganic counterparts. Very recently, new classes of ion exchangers have been studied by combining surfactants and inorganic metal phosphates. This article highlights the salient features of metal phosphates as ion exchangers, various development stages with the modifications, with an emphasis on the recent developments in the field of analytical chemistry, particularly surfactant-based hybrid fibrous and non-fibrous metal phosphates as ion exchangers. Surfactants or surface-active agents when present in the matrix of inorganic metal phosphates not only enhance their ion-exchange capacity but, also the selective adsorption of metal ions. Therefore, these materials are of great importance in industrial and environmental applications.

**Keywords:** hybrid ion exchangers, stability, surfactants, metal phosphates, inorganic

Analytical chemistry, broadly conceived, underlines and contributes to almost all branches of chemistry as an experimental science. It plays an important role in nearly all aspects of chemistry, such as, agricultural, clinical, environmental, forensic, manufacturing, metallurgical and pharmaceutical chemistry. The goal of a chemical analysis is to provide information about the composition of a sample of matter. The discipline of analytical chemistry consists of qualitative and quantitative analyses. The former deals with the identification of elements, ions or compounds present in a sample, while the latter deals with the determination of how much of one or more constituents is present; whether the sample is solid, liquid, gas or a mixture. Analytical methods are ordinarily classified according to the property that is observed in the final measurement process. Some more important of these properties as well as the names of the methods based upon these properties are given in **Table 1**. Prior to chemical analysis, separations are extremely important in analytical chemistry. The aim of an analytical separation is, usually, to eliminate or reduce interferences so that quantitative analytical information can be obtained about complex mixtures. There is a variety of separation methods that are in common use, including precipitation, distillation, solvent extraction, crystallization, dialysis, ion-exchange, chromatography, electrophoresis, field flow fractionation etc.

Of all the different types of separation methods, chromatography has the unique position of being applicable to all types of problems in all branches of science. This technique provides a very efficient method for the identification, separation, determination and purification of chemical compounds. It has undergone explosive growth in the last 30–40 years. The chromatographic technique was first invented by a Russian

## **Chapter 4** Hybrid Ion Exchangers

*Amita Somya*

### **Abstract**

Hybrid ion exchangers are of recent origin in the field of ion exchange chemistry. They have shown excellent chemical, mechanical and thermal stability conversant to both organic and inorganic counterparts. Very recently, new classes of ion exchangers have been studied by combining surfactants and inorganic metal phosphates. This article highlights the salient features of metal phosphates as ion exchangers, various development stages with the modifications, with an emphasis on the recent developments in the field of analytical chemistry, particularly surfactant-based hybrid fibrous and non-fibrous metal phosphates as ion exchangers. Surfactants or surface-active agents when present in the matrix of inorganic metal phosphates not only enhance their ion-exchange capacity but, also the selective adsorption of metal ions. Therefore, these materials are of great importance in industrial and environmental applications.

**Keywords:** hybrid ion exchangers, stability, surfactants, metal phosphates, inorganic metal phosphates

#### **1. Introduction**

Analytical chemistry, broadly conceived, underlines and contributes to almost all branches of chemistry as an experimental science. It plays an important role in nearly all aspects of chemistry, such as, agricultural, clinical, environmental, forensic, manufacturing, metallurgical and pharmaceutical chemistry. The goal of a chemical analysis is to provide information about the composition of a sample of matter.

The discipline of analytical chemistry consists of qualitative and quantitative analyses. The former deals with the identification of elements, ions or compounds present in a sample, while the latter deals with the determination of how much of one or more constituents is present; whether the sample is solid, liquid, gas or a mixture. Analytical methods are ordinarily classified according to the property that is observed in the final measurement process. Some more important of these properties as well as the names of the methods based upon these properties are given in **Table 1**.

Prior to chemical analysis, separations are extremely important in analytical chemistry. The aim of an analytical separation is, usually, to eliminate or reduce interferences so that quantitative analytical information can be obtained about complex mixtures. There is a variety of separation methods that are in common use, including precipitation, distillation, solvent extraction, crystallization, dialysis, ion-exchange, chromatography, electrophoresis, field flow fractionation etc.

Of all the different types of separation methods, chromatography has the unique position of being applicable to all types of problems in all branches of science. This technique provides a very efficient method for the identification, separation, determination and purification of chemical compounds. It has undergone explosive growth in the last 30–40 years. The chromatographic technique was first invented by a Russian


#### **Table 1.**

*Analytical techniques and principal applications.*


**49**

**Figure 1.**

*Stages of development of ion exchangers and sorbents.*

*Hybrid Ion Exchangers*

*DOI: http://dx.doi.org/10.5772/intechopen.92116*

botanist Mikhil Tswett in 1906, at the University of Warsaw. He coined the term 'Chromatography' from the Greek words 'Chromatos' and 'graphy' which mean 'color' and 'to write' respectively. The International Union of Pure and Applied Chemists (IUPAC) has drafted a recommended definition of chromatography: 'Chromatography is a physical method of separation in which the components to be separated are distributed between two phases, one of which is a stationary phase, while the other is a mobile phase' [1]. Since its discovery, this technique has undergone tremendous modifications and nowadays various types of chromatographic techniques have been developed for separating almost any kind of given mixture, whether coloured or colourless into its constituents and to test the purity of these constituents. The applications of chromatography have extensively been used in the last 50 years, owing not only to the development of several new types of chromatographic techniques, but also due to the growing needs of the scientists for better methods of separating the complex mixtures or metal

ions [2]. The different chromatographic methods are summarized in **Table 2**.

#### **Table 2.**

*A classification of the principal chromatographic techniques.*

#### *Hybrid Ion Exchangers DOI: http://dx.doi.org/10.5772/intechopen.92116*

*Hybrid Nanomaterials - Flexible Electronics Materials*

1. Gravimetry Weight of pure analyte

2. Titrimetry Volume of standard reagent

**S.N. Technique Stationary phase Mobile** 

6. Thermal analysis Chemical/physical changes in the

8. Radiochemical analysis Characteristic ionizing nuclear

Cellulose-water complex

**S.N. Technique Property measured Principal areas of application**

solution reacting with the analyte

Wavelength and intensity of electromagnetic radiation emitted or absorbed by the analyte

4. Mass spectrometry Mass of analyte or fragments of it Qualitative or structural for

Various physico-chemical properties of separated analytes

analyte when heated or cooled

Electrical properties of the analyte in solution

radiation emitted by the analyte

or compound of known stoichiometry

Silica, cellulose, ion-exchange resin, controlled porosity

Solid or bonded phase

Controlled porosity

Ion-exchange resin or bonded-phase

or bonded-phase

solid

solid

1. Paper chromatography (PC)

3. Atomic and molecular spectrometry

5. Chromatography and electrophoresis

7. Electro-chemical analysis

chromatography (TLC)

*Analytical techniques and principal applications.*

chromatography (GLC)

chromatography (GSC)

chromatography (SEC)

chromatography (IEC)

9. Chiral chromatography (CC)

8. Ion chromatography (IC) Ion-exchange resin

*A classification of the principal chromatographic techniques.*

5. High-performance liquid chromatography (HPLC)

2. Thin layer

**Table 1.**

3. Gas-liquid

4. Gas-solid

6. Size-exclusion

7. Ion-exchange

**phase**

Liquid Gas Column Partition

Solid Gas Column Adsorption

**Format Principal sorption mechanism**

Quantitative for major or minor

Quantitative for major or minor

Qualitative, quantitative or structural for major down to trace level components

major down to trace level components isotope ratio

Qualitative and quantitative separations of mixtures at major to trace levels

Characterization of single or mixed major/minor components

Qualitative and quantitative for major to trace level components

Qualitative and quantitative at major to trace levels

components

components

(adsorption, ion-exchange, exclusion)

(partition, ion-exchange, exclusion)

(adsorption)

Liquid Planar Partition

Liquid Planar Adsorption

Liquid Column Modified partition

Liquid Column Exclusion

Liquid Column Ion-exchange

Liquid Column Ion-exchange

Solid chiral selector Liquid Column Selective adsorption

**48**

**Table 2.**

botanist Mikhil Tswett in 1906, at the University of Warsaw. He coined the term 'Chromatography' from the Greek words 'Chromatos' and 'graphy' which mean 'color' and 'to write' respectively. The International Union of Pure and Applied Chemists (IUPAC) has drafted a recommended definition of chromatography: 'Chromatography is a physical method of separation in which the components to be separated are distributed between two phases, one of which is a stationary phase, while the other is a mobile phase' [1]. Since its discovery, this technique has undergone tremendous modifications and nowadays various types of chromatographic techniques have been developed for separating almost any kind of given mixture, whether coloured or colourless into its constituents and to test the purity of these constituents. The applications of chromatography have extensively been used in the last 50 years, owing not only to the development of several new types of chromatographic techniques, but also due to the growing needs of the scientists for better methods of separating the complex mixtures or metal ions [2]. The different chromatographic methods are summarized in **Table 2**.

**Figure 1.**

*Stages of development of ion exchangers and sorbents.*

Out of these several chromatographic methods, ion-exchange has gained great attention by analysts in practice. The phenomenon of ion-exchange is not of recent origin. It has an interesting historical background. Various time spans may be identified for the development of ion-exchange technique. **Figure 1** summarizes the various stages of the development of ion exchangers and sorbents.

Initially, the ion exchangers were mostly used for water softening, but later on they were widely employed in various fields such as syntheses and some preparative works. The use of ion exchangers provided the new methods for analysts, which not only met the requirements of modern laboratories but also led to the solution of previously insolvable problems. Thus, the ion-exchange process has been established as an analytical tool in laboratories and industries. An interest in ion-exchange operations in industries is increasing day by day as their field of applications is expanding and today, it is an extremely valuable supplement to other procedures such as filtration, distillation and adsorption. All over the world, various plants are in operation, accomplishing tasks that range from the recovery of metals from industrial wastes to the separation of rare earths, and from catalysis of organic reactions to decontamination of water in cooling systems of nuclear reactors.

#### **2. Ion exchangers**

Ion exchangers are insoluble solid materials or immiscible liquids (in case of liquid ion exchangers) containing exchangeable ions. These ions can be exchanged for a stoichiometrically equivalent amount of other ions of the same sign on contacting with an electrolyte solution. Depending upon their ability of exchanging cations, anions or both, the ion exchangers may be categorized as 'cation', 'anion' or 'amphoteric' ion exchangers, respectively. A cation exchanger comprises a matrix with negative charge while an anion exchanger comprises a matrix with positive charge. The negative or positive charge on the matrix is compensated by the oppositely charged counter ions, which are mobile in nature. A typical ion-exchange reaction may be represented as follows:

$$\mathsf{A} \mathsf{ -- A} \star \mathsf{B} \text{ (a.e.}\\\text{) } \xleftarrow{\qquad} \mathsf{R} \mathsf{ -- B} \star \mathsf{R} \quad \overleftarrow{\qquad} \quad \mathsf{ (b.e.)} \mathsf{ -- A} \mathsf{ -- B}$$

**51**

*Hybrid Ion Exchangers*

ing agents.

properties.

'inorganic' in nature.

the matrix.

**2.1 Organic ion exchangers**

**2.2 Inorganic ion exchangers**

ated substances.

an appreciable extent.

marily a reflection of differences in affinity.

*DOI: http://dx.doi.org/10.5772/intechopen.92116*

• Selectivity for one ion relative to another, including the cases in which the varying affinities of the ions are modified by the use of complexing and chelat-

• Donnan exclusion—the ability to exclude ions but not, in general, undissoci-

• Screening effect—the inability of very large ions or polymers to be adsorbed to

• Differences in migration rates of adsorbed substances down a column—pri-

• Ionic mobility restricted to the exchangeable ions and counter ions only.

• Miscellaneous properties—swelling, surface area and other mechanical

On the basis of the nature of matrix, an ion exchanger may be 'organic' or

Organic ion exchangers, commonly known as 'ion-exchange resins', are well known for their uniformity, chemical and mechanical stability and for the easy control over their ion-exchange property through synthetic methods. Organic resins have wide applications in analytical chemistry because of their high stability in the wide range of pH and reproducibility in the results, but their instability under the conditions of high temperature and strong radiation led to a major switch for the development of inorganic ion-exchange materials. The matrix of inorganic ion exchanger is more reactive than that of organic resins and hence, the selectivity for the metal ion depends both on adsorption characteristics of the matrix and the nature of the ionogenic groups attached to

Inorganic ion exchangers are capable of being stable at elevated temperatures

and in the presence of strong radiations and, hence, they have wide-ranging applications in nuclear researches such as radioisotope separations, nuclear waste treatments etc. They are used in the determination and detection of metals in pharmaceutical and biological products, analysis of alloys and rocks, as ion selective electrodes, as packing materials in ion-exchange chromatography and as catalysts. They also find applications in environmental analysis [3]. The widespread importance of inorganic ion exchangers in practical applications, and scientific interest in their nature and properties, has precipitated a wealth of published literature on the subject. Good starting points for further basic information are classic books like those of Clearfield [4], Amphlett [5] and Qureshi and Varshney [6]. These books have provided a complete picture and thorough insight into this field and

• Equivalence of exchange.

where 'A' and 'B' are the replaceable counter ions, 'R' is the structural unit (matrix) of the ion exchanger and 'aq' stands for the aqueous phase. This process is reversible, that is, it can be reversed by suitably changing the concentration of the ions in solution.

The actual utility of an ion exchanger depends chiefly on its ion-exchange characteristics such as ion-exchange capacity, pH-titration, concentration, elution and distribution behaviour. The ion-exchange capacity depends on hydrated ionic radii and selectivity. The selectivity of any ion exchanger, in turn, is influenced by the nature of its functional group and degree of its cross linking. Ion exchangers, having groups that are capable of complex formation with some particular ions, will adsorb these ions more strongly. As the degree of cross linking increases, the exchanger becomes more selective towards ions of different sizes. The elution of H+ ions from a column of ion exchanger depends on the concentration of the eluant while an optimum concentration of the eluant, necessary for maximum elution of H+ ions, depends on the nature of ionogenic groups present in the exchanger, which depends upon the pKa values of the acids used in preparation. The efficiency of an ion exchanger depends on the following fundamental exchange reactions:

• Equivalence of exchange.

*Hybrid Nanomaterials - Flexible Electronics Materials*

Out of these several chromatographic methods, ion-exchange has gained great attention by analysts in practice. The phenomenon of ion-exchange is not of recent origin. It has an interesting historical background. Various time spans may be identified for the development of ion-exchange technique. **Figure 1** summarizes the

Initially, the ion exchangers were mostly used for water softening, but later on they were widely employed in various fields such as syntheses and some preparative works. The use of ion exchangers provided the new methods for analysts, which not only met the requirements of modern laboratories but also led to the solution of previously insolvable problems. Thus, the ion-exchange process has been established as an analytical tool in laboratories and industries. An interest in ion-exchange operations in industries is increasing day by day as their field of applications is expanding and today, it is an extremely valuable supplement to other procedures such as filtration, distillation and adsorption. All over the world, various plants are in operation, accomplishing tasks that range from the recovery of metals from industrial wastes to the separation of rare earths, and from catalysis of organic reactions to decontamination of water in cooling systems of nuclear

Ion exchangers are insoluble solid materials or immiscible liquids (in case of liquid ion exchangers) containing exchangeable ions. These ions can be exchanged for a stoichiometrically equivalent amount of other ions of the same sign on contacting with an electrolyte solution. Depending upon their ability of exchanging cations, anions or both, the ion exchangers may be categorized as 'cation', 'anion' or 'amphoteric' ion exchangers, respectively. A cation exchanger comprises a matrix with negative charge while an anion exchanger comprises a matrix with positive charge. The negative or positive charge on the matrix is compensated by the oppositely charged counter ions, which are mobile in nature. A typical ion-exchange reaction

where 'A' and 'B' are the replaceable counter ions, 'R' is the structural unit (matrix) of the ion exchanger and 'aq' stands for the aqueous phase. This process is reversible, that is, it can be reversed by suitably changing the concentration of the

The actual utility of an ion exchanger depends chiefly on its ion-exchange characteristics such as ion-exchange capacity, pH-titration, concentration, elution and distribution behaviour. The ion-exchange capacity depends on hydrated ionic radii and selectivity. The selectivity of any ion exchanger, in turn, is influenced by the nature of its functional group and degree of its cross linking. Ion exchangers, having groups that are capable of complex formation with some particular ions, will adsorb these ions more strongly. As the degree of cross linking increases, the exchanger becomes more selective towards ions of different sizes. The elution of

 ions from a column of ion exchanger depends on the concentration of the eluant while an optimum concentration of the eluant, necessary for maximum elution of

 ions, depends on the nature of ionogenic groups present in the exchanger, which depends upon the pKa values of the acids used in preparation. The efficiency of an

ion exchanger depends on the following fundamental exchange reactions:

(1)

various stages of the development of ion exchangers and sorbents.

**50**

H+

H+

reactors.

**2. Ion exchangers**

may be represented as follows:

ions in solution.


On the basis of the nature of matrix, an ion exchanger may be 'organic' or 'inorganic' in nature.

#### **2.1 Organic ion exchangers**

Organic ion exchangers, commonly known as 'ion-exchange resins', are well known for their uniformity, chemical and mechanical stability and for the easy control over their ion-exchange property through synthetic methods. Organic resins have wide applications in analytical chemistry because of their high stability in the wide range of pH and reproducibility in the results, but their instability under the conditions of high temperature and strong radiation led to a major switch for the development of inorganic ion-exchange materials. The matrix of inorganic ion exchanger is more reactive than that of organic resins and hence, the selectivity for the metal ion depends both on adsorption characteristics of the matrix and the nature of the ionogenic groups attached to the matrix.

#### **2.2 Inorganic ion exchangers**

Inorganic ion exchangers are capable of being stable at elevated temperatures and in the presence of strong radiations and, hence, they have wide-ranging applications in nuclear researches such as radioisotope separations, nuclear waste treatments etc. They are used in the determination and detection of metals in pharmaceutical and biological products, analysis of alloys and rocks, as ion selective electrodes, as packing materials in ion-exchange chromatography and as catalysts. They also find applications in environmental analysis [3]. The widespread importance of inorganic ion exchangers in practical applications, and scientific interest in their nature and properties, has precipitated a wealth of published literature on the subject. Good starting points for further basic information are classic books like those of Clearfield [4], Amphlett [5] and Qureshi and Varshney [6]. These books have provided a complete picture and thorough insight into this field and

widespread importance of inorganic ion exchangers. Important advances in this field have been reviewed by a number of workers/researchers at various stages of its development, such as Fuller [7], Qureshi et al. [8], Vesely and Pekarek [9], Clearfield [10, 11], Alberti et al. [12, 13], Alberti and Costantino [13], Marinsky [14], Varshney [15–20], Ivanov [21] and Terres-Rojas [22]. Dyer [23–25] has dealt with the theories involved zeolite molecular sieves, which have principles underlying the inorganic ion exchangers. Alberti (Itly) and Clearfield (USA) devoted most of their studies on the crystalline inorganic ion exchangers.

Inorganic ion exchangers are generally the oxides, hydroxides and insoluble acid salts of polyvalent metals, heteropolyacid salts and insoluble metal ferrocyanides. These materials are generally produced by combining the oxides of elements of III, IV, V and VI groups of the periodic table. A large number of such materials have been synthesized by mixing phosphoric, arsenic, molybdic, antimonic and vanadic acids with titanium, zirconium, tin, thorium, cerium, iron, antimony, chromium, niobium, tantalum, bismuth, nickel, cobalt, etc. However, the majority of work has been carried out on zirconium, titanium, tin, niobium and tantalum. Out of the above, metal phosphates have been found to have good chemical stability, reproducibility in ion-exchange behaviour and selectivity for certain metal ions.

#### **2.3 Hybrid ion exchangers**

Since organic ion exchangers were found to be unstable at elevated temperatures and under strong radiations, inorganic ion exchangers were taken as alternatives for such cases. However, the main drawback of inorganic ion-exchange materials has been that they are not very much reproducible in ion-exchange behaviour. Further, they are found not to be chemically and mechanically very stable perhaps due to their inorganic nature. Thus, to overcome these shortcomings, an interest was developed to obtain some organic-based inorganic ion exchangers. These exchangers were termed as 'hybrid ion exchangers' as they consist of both the organic and inorganic counterparts and have the properties not seen in purely organic or purely inorganic materials. This new class of ion exchangers has been prepared in these laboratories by incorporating a polymeric or monomeric organic species into the inorganic ion-exchange matrix [26–30]. The hybrid ion exchangers have shown an improvement in a number of ways. One of them is its granulometric properties that make it more suitable for the application in column operations. The binding with an organic species also introduces better mechanical properties in the end product, that is, hybrid ion-exchange materials. Hybrid ion exchangers can be prepared as three-dimensional porous materials in which layers are cross linked or as layered compounds containing sulphonic acid, carboxylic acid or amino groups.

The reactivity of both organic and inorganic precursors is usually quite different and phase separation tends to occur. The properties of hybrid materials do not depend only on organic and inorganic components but also on the interface between both phases. The general tendency is therefore, to increase interfacial interactions by creating an intimate mixing, or interpenetration between organic and inorganic networks. Moreover, the formation of chemical bonds between organic and inorganic species would prevent phase separation, allowing the synthesis of molecular composites or organic-inorganic copolymers. Hybrid materials can, thus, be divided into two classes [31].

• Class I corresponds to hybrid systems in which weak interactions such as van der Waals forces or hydrogen bonds or electrostatic interactions are created

**53**

**Table 3.**

*Hybrid Ion Exchangers*

*DOI: http://dx.doi.org/10.5772/intechopen.92116*

**2.4 Hybrid metal phosphates**

**S.N***.* **Name of the materials**

1. Polyacrylonitrile thorium(IV) phosphate

> cerium(IV) phosphate

thorium(IV) phosphate

4. Acrylonitrile cerium (IV) phosphate

> cerium(IV) phosphate

thorium(IV) phosphate

7. Pectin cerium(IV) phosphate

8. Pectin thorium(IV) phosphate

10. Pyridine cerium(IV) phosphate

11. Pyridine thorium(IV) phosphate

*Different types of hybrid metal phosphates and their important properties.*

12. n-Butyl acetate cerium(IV) phosphate

9. Cellulose acetate thorium(IV) phosphate

2. Polystyrene

3. Polystyrene

5. Acrylamide

6. Acrylamide

between organic and inorganic phases. This class involves mainly small

components are bonded through strong covalent chemical bonds.

Hybrid organo-inorganic phosphates open up a land of opportunities in materials science and ion-exchange chemistry. These nanocomposites bridge high-temperature materials such as glasses and ceramics with very fragile species such as organic compounds or biomolecules. In last 10–15 years, some hybrid ionexchange materials have been synthesized in the laboratories, such as acrylamide

> **Ion-exchange capacity for Na+ (meq/ dry g)**

**Selectivity X-ray nature References**

crystalline

crystalline

[39]

[41]

3.90 Pb(II) Microcrystalline [36]

2.95 Hg(II) Microcrystalline [37]

4.52 Cd(II) Crystalline [38]

2.60 Hg(II) Crystalline [40]

1.78 Hg(II) Amorphous [42]

2.15 Pb(II) Amorphous [42]

1.70 Pb(II) Amorphous [43]

2.00 Hg(II) Amorphous [44]

2.10 Pb(II) Amorphous [45]

2.25 Hg(II) Amorphous [46]

2.86 Hg(II) Poorly

2.00 Pb(II) Poorly

• Class II corresponds to hybrid compounds where both organic and inorganic

organic species embedded within an oxide matrix.

between organic and inorganic phases. This class involves mainly small organic species embedded within an oxide matrix.

• Class II corresponds to hybrid compounds where both organic and inorganic components are bonded through strong covalent chemical bonds.

#### **2.4 Hybrid metal phosphates**

*Hybrid Nanomaterials - Flexible Electronics Materials*

**2.3 Hybrid ion exchangers**

acid or amino groups.

thus, be divided into two classes [31].

of their studies on the crystalline inorganic ion exchangers.

widespread importance of inorganic ion exchangers. Important advances in this field have been reviewed by a number of workers/researchers at various stages of its development, such as Fuller [7], Qureshi et al. [8], Vesely and Pekarek [9], Clearfield [10, 11], Alberti et al. [12, 13], Alberti and Costantino [13], Marinsky [14], Varshney [15–20], Ivanov [21] and Terres-Rojas [22]. Dyer [23–25] has dealt with the theories involved zeolite molecular sieves, which have principles underlying the inorganic ion exchangers. Alberti (Itly) and Clearfield (USA) devoted most

Inorganic ion exchangers are generally the oxides, hydroxides and insoluble acid salts of polyvalent metals, heteropolyacid salts and insoluble metal ferrocyanides. These materials are generally produced by combining the oxides of elements of III, IV, V and VI groups of the periodic table. A large number of such materials have been synthesized by mixing phosphoric, arsenic, molybdic, antimonic and vanadic acids with titanium, zirconium, tin, thorium, cerium, iron, antimony, chromium, niobium, tantalum, bismuth, nickel, cobalt, etc. However, the majority of work has been carried out on zirconium, titanium, tin, niobium and tantalum. Out of the above, metal phosphates have been found to have good chemical stability, reproducibility in ion-exchange behaviour and selectivity for certain metal ions.

Since organic ion exchangers were found to be unstable at elevated temperatures and under strong radiations, inorganic ion exchangers were taken as alternatives for such cases. However, the main drawback of inorganic ion-exchange materials has been that they are not very much reproducible in ion-exchange behaviour. Further, they are found not to be chemically and mechanically very stable perhaps due to their inorganic nature. Thus, to overcome these shortcomings, an interest was developed to obtain some organic-based inorganic ion exchangers. These exchangers were termed as 'hybrid ion exchangers' as they consist of both the organic and inorganic counterparts and have the properties not seen in purely organic or purely inorganic materials. This new class of ion exchangers has been prepared in these laboratories by incorporating a polymeric or monomeric organic species into the inorganic ion-exchange matrix [26–30]. The hybrid ion exchangers have shown an improvement in a number of ways. One of them is its granulometric properties that make it more suitable for the application in column operations. The binding with an organic species also introduces better mechanical properties in the end product, that is, hybrid ion-exchange materials. Hybrid ion exchangers can be prepared as three-dimensional porous materials in which layers are cross linked or as layered compounds containing sulphonic acid, carboxylic

The reactivity of both organic and inorganic precursors is usually quite different and phase separation tends to occur. The properties of hybrid materials do not depend only on organic and inorganic components but also on the interface between both phases. The general tendency is therefore, to increase interfacial interactions by creating an intimate mixing, or interpenetration between organic and inorganic networks. Moreover, the formation of chemical bonds between organic and inorganic species would prevent phase separation, allowing the synthesis of molecular composites or organic-inorganic copolymers. Hybrid materials can,

• Class I corresponds to hybrid systems in which weak interactions such as van der Waals forces or hydrogen bonds or electrostatic interactions are created

**52**

Hybrid organo-inorganic phosphates open up a land of opportunities in materials science and ion-exchange chemistry. These nanocomposites bridge high-temperature materials such as glasses and ceramics with very fragile species such as organic compounds or biomolecules. In last 10–15 years, some hybrid ionexchange materials have been synthesized in the laboratories, such as acrylamide


#### **Table 3.**

*Different types of hybrid metal phosphates and their important properties.*

and pyridine-based zirconium and tin phosphates [32–34], acrylonitrile-based zirconium phosphate [35]. These materials have shown promising ion-exchange characteristics and have been utilized in the separation of metal ions due to their selectivity towards different metals ions. Metal phosphates, such as tin(IV) phosphate, cerium(IV) phosphate, zirconium(IV) phosphate etc. were found very good ion exchangers and intercalating agents too. The whole idea to convert them to hybrid ion exchangers has been to enhance the interlayer distances by introducing organic species in the metrics of these metal phosphates, resulting in improved ionexchange properties. These metal-organic phosphates, or hybrid ion exchangers, correspond to metal-organic frameworks where features of organic and inorganic counterparts are revealed in terms of ion-exchange capacity, thermal, chemical and mechanical stability in addition to metal ion selectivity. However, structures of these metal-organic phosphates could not be described as these materials have been found amorphous or poorly crystalline. **Table 3** summarizes their ion-exchange capacities and selectivity towards metal ions.

#### **3. Surfactants**

Surfactants constitute the most important group of detergent components. 'Surfactant' is an abbreviation for surface-active agents (shown in **Figure 2**), which literally means 'active at the surface'. The surface can be between solid and liquid, between air and liquid and between two different immiscible liquids. The unique property of surfactants is 'adsorption', which occurs at liquid/solid, liquid/liquid and at air/liquid interfaces. At air-water interfaces and in water, or similarly strongly hydrogen-bonded solvents, they self-associate at concentrations above the critical micelle concentration (CMC) to form association colloids, known as 'micelles' [47].

Thus, 'surfactants, surface-active agents [48] or, detergents are amphiphilic, organic or organo-metallic compounds which form association colloids or micelles in solution'. Amphiphilic substances or amphiphiles are comprised of a hydrophobic portion, usually a long alkyl chain, attached to hydrophilic or water solubilityenhancing functional groups. Actually, surfactant molecule consists of two parts: a water-hating (hydrophobic) part and a water-loving (hydrophilic) part. The following figure shows the basic structure of a surfactant molecule.

There are three basic concepts that need to be well understood in order to explain the majority of observed phenomena; these are solubility, adsorption of a surfactant at a surface and the formation of micelles in solution. These three phenomena differentiate a surfactant from other chemical entities. It is the abnormal solubility characteristics of surfactants which offers the adsorption on surfaces/interfaces and formation of micelles. Surfactants reduce surface tensions when dissolved in water or water solutions, reduce interfacial tensions between two liquids, or between a liquid and a solid [49]. When a surfactant molecule is introduced into water, the water-hating

**55**

*Hybrid Ion Exchangers*

*DOI: http://dx.doi.org/10.5772/intechopen.92116*

• In the removal of the soils from surfaces.

positing onto the surface.

• Anionic surfactants [50]

• Cationic surfactants [51]

• Nonionic surfactants [52]

• Gemini surfactants [54]

• Amphoteric surfactants [53]

and observe the break in the plot [56–58].

part tends to escape by attaching itself to any available surface other than water. At the same time, the water-loving part tries to remain in water. As a result, surfactants get strongly 'adsorbed' to many surfaces, such as fabric, soil, glass and where the water and air meet (i.e., water/air interface). This tendency of surfactants is useful:

• In holding soil particles in suspension form and preventing them from rede-

• In reducing surface tension of water and allowing the water to spread out.

The unusual properties of aqueous surfactant solutions can be ascribed to the presence of a hydrophilic head group and a hydrophobic chain (or tail) in the molecule. The polar or ionic head group usually interacts strongly with an aqueous environment, in which case it is solvated via dipole-dipole or ion-dipole interactions. Depending on the chemical structure of the hydrophilic moiety bound to the hydrophobic portion, the surfactants may be categorized into following types:

**3.1 Micelle formation and critical micelle concentration (CMC)**

The formation of micelles in aqueous solution is generally viewed as a compromise between the tendency for alkyl chains to avoid energetically unfavourable contacts with water, and the desire for the polar part to maintain contact with aqueous environment. In dilute aqueous solution, at concentration generally less than 10<sup>−</sup><sup>4</sup>

the behaviour of ionic surfactants parallels that of strong electrolytes while the behaviour of nonionic surfactants often resembles that of the simple organic molecules. At higher surfactant concentrations, however, a pronounced deviation from 'ideal' behaviour in dilute solution occurs—this deviation generally being considerably larger than that exhibited by simple strong electrolytes. Thus, the adsorption of a surfactant from solution onto a surface depends upon the concentration [53]. Each surfactant has a characteristic CMC value. The most obvious evidence of micellar growth is probably the dramatic increase in viscosity with increasing concentration, which is observed in several surfactant solutions. Micellar growth is favoured by decreasing the temperature, adding electrolyte and lengthening the surfactant chain

length and is, furthermore, very sensitive to the nature of the counter ion.

The physico-chemical properties of surfactants vary significantly below and above the CMC value [55]. Below the CMC value, the physico-chemical properties of ionic surfactants resemble those of a strong electrolyte. Above the CMC value, these properties change dramatically, indicating a highly co-operative association process. The general way of obtaining the CMC value of a surfactant micelle is to plot an appropriate physico-chemical property versus the surfactant concentration

M,

**Figure 2.** *Structure of Surfactant monomer.*

#### *Hybrid Ion Exchangers DOI: http://dx.doi.org/10.5772/intechopen.92116*

*Hybrid Nanomaterials - Flexible Electronics Materials*

capacities and selectivity towards metal ions.

**3. Surfactants**

and pyridine-based zirconium and tin phosphates [32–34], acrylonitrile-based zirconium phosphate [35]. These materials have shown promising ion-exchange characteristics and have been utilized in the separation of metal ions due to their selectivity towards different metals ions. Metal phosphates, such as tin(IV) phosphate, cerium(IV) phosphate, zirconium(IV) phosphate etc. were found very good ion exchangers and intercalating agents too. The whole idea to convert them to hybrid ion exchangers has been to enhance the interlayer distances by introducing organic species in the metrics of these metal phosphates, resulting in improved ionexchange properties. These metal-organic phosphates, or hybrid ion exchangers, correspond to metal-organic frameworks where features of organic and inorganic counterparts are revealed in terms of ion-exchange capacity, thermal, chemical and mechanical stability in addition to metal ion selectivity. However, structures of these metal-organic phosphates could not be described as these materials have been found amorphous or poorly crystalline. **Table 3** summarizes their ion-exchange

Surfactants constitute the most important group of detergent components. 'Surfactant' is an abbreviation for surface-active agents (shown in **Figure 2**), which literally means 'active at the surface'. The surface can be between solid and liquid, between air and liquid and between two different immiscible liquids. The unique property of surfactants is 'adsorption', which occurs at liquid/solid, liquid/liquid and at air/liquid interfaces. At air-water interfaces and in water, or similarly strongly hydrogen-bonded solvents, they self-associate at concentrations above the critical micelle concentration (CMC) to form association colloids, known as 'micelles' [47]. Thus, 'surfactants, surface-active agents [48] or, detergents are amphiphilic, organic or organo-metallic compounds which form association colloids or micelles in solution'. Amphiphilic substances or amphiphiles are comprised of a hydrophobic portion, usually a long alkyl chain, attached to hydrophilic or water solubilityenhancing functional groups. Actually, surfactant molecule consists of two parts: a water-hating (hydrophobic) part and a water-loving (hydrophilic) part. The

following figure shows the basic structure of a surfactant molecule.

There are three basic concepts that need to be well understood in order to explain the majority of observed phenomena; these are solubility, adsorption of a surfactant at a surface and the formation of micelles in solution. These three phenomena differentiate a surfactant from other chemical entities. It is the abnormal solubility characteristics of surfactants which offers the adsorption on surfaces/interfaces and formation of micelles. Surfactants reduce surface tensions when dissolved in water or water solutions, reduce interfacial tensions between two liquids, or between a liquid and a solid [49]. When a surfactant molecule is introduced into water, the water-hating

**54**

**Figure 2.**

*Structure of Surfactant monomer.*

part tends to escape by attaching itself to any available surface other than water. At the same time, the water-loving part tries to remain in water. As a result, surfactants get strongly 'adsorbed' to many surfaces, such as fabric, soil, glass and where the water and air meet (i.e., water/air interface). This tendency of surfactants is useful:


The unusual properties of aqueous surfactant solutions can be ascribed to the presence of a hydrophilic head group and a hydrophobic chain (or tail) in the molecule. The polar or ionic head group usually interacts strongly with an aqueous environment, in which case it is solvated via dipole-dipole or ion-dipole interactions. Depending on the chemical structure of the hydrophilic moiety bound to the hydrophobic portion, the surfactants may be categorized into following types:


#### **3.1 Micelle formation and critical micelle concentration (CMC)**

The formation of micelles in aqueous solution is generally viewed as a compromise between the tendency for alkyl chains to avoid energetically unfavourable contacts with water, and the desire for the polar part to maintain contact with aqueous environment. In dilute aqueous solution, at concentration generally less than 10<sup>−</sup><sup>4</sup> M, the behaviour of ionic surfactants parallels that of strong electrolytes while the behaviour of nonionic surfactants often resembles that of the simple organic molecules. At higher surfactant concentrations, however, a pronounced deviation from 'ideal' behaviour in dilute solution occurs—this deviation generally being considerably larger than that exhibited by simple strong electrolytes. Thus, the adsorption of a surfactant from solution onto a surface depends upon the concentration [53]. Each surfactant has a characteristic CMC value. The most obvious evidence of micellar growth is probably the dramatic increase in viscosity with increasing concentration, which is observed in several surfactant solutions. Micellar growth is favoured by decreasing the temperature, adding electrolyte and lengthening the surfactant chain length and is, furthermore, very sensitive to the nature of the counter ion.

The physico-chemical properties of surfactants vary significantly below and above the CMC value [55]. Below the CMC value, the physico-chemical properties of ionic surfactants resemble those of a strong electrolyte. Above the CMC value, these properties change dramatically, indicating a highly co-operative association process. The general way of obtaining the CMC value of a surfactant micelle is to plot an appropriate physico-chemical property versus the surfactant concentration and observe the break in the plot [56–58].

Depending upon the chemical structure of the surfactant, its micelle can be cationic, anionic, zwitterionic or nonionic. The electrostatic character of the micelles depends in some cases on the pH of the aqueous solution due to protonation equilibria. Zwitterionic surfactants, of course, also can become either cationic or anionic, and several types of nonionic surfactants can also form anionic or cationic micelles in the appropriate pH range. Micelles are not static species but rather exist in a dynamic equilibrium. The micelle may be represented as a globular, cylindrical or ellipsoidal cluster [59] of individual surfactant molecules in equilibrium with its monomer. The reverse orientation of the hydrophilic and hydrophobic part of the surfactant in a hydrocarbon medium leads to the formation of reversed micelles [60].

#### **3.2 Surfactant adsorption and surface properties**

The effectiveness of surfactant adsorption is mainly determined by surfactant concentration, surfactant functional group, alkyl hydrocarbon chain length, environment etc. When the surfactant concentration is well below the CMC value, individual surfactant molecules tend to adsorb on exposed interfaces to reduce surface tension. As the concentration of surfactant approaches the CMC value, surfactant molecules form dimers, and multiple molecules aggregate, micelles. Once the CMC is reached, any additional surfactant molecules added to the system will be incorporated into new or existing aggregates. Thus, further increase of surfactant concentration above the CMC value results in bilayer or multilayer formation at interface. The adsorption tendency of the surfactants at the surfaces imparts the properties of foaming, wetting, emulsification, dispersing of solids and detergency. The adsorption increases the concentration of surfactant at the surfaces. Surfactant adsorption is a consideration in any application where surfactants come in contact with a surface or interface. It is from solutions that surfactants then preferentially adsorb to interfaces and, because of their amphiphilic nature, preferentially segregate at interfaces. There are a number of areas of applications where surfactant adsorption is important including ore floatation, improved oil recovery, soil remediation, detergency, surfactant-based separation processes and wetting. Surfactant adsorption may occur due to electrostatic interactions, van der Waals interaction, hydrogen-bonding and/ or solvation and desolvation of adsorbate and adsorbent species [61].

The hydrophobic forces that drive surfactants to segregate at air-water interfaces are essentially the same that drive surfactant adsorption onto solid surfaces. However, they can be differing in the chemical forces associated with the solid surfaces. Ionic surfactants tend to adsorb onto oppositely charged solid surfaces due to electrostatic forces while adsorption of ionic surfactants on a like charged substrate, being less understood, can occur via hydrogen-bonding or attractive dispersion forces [62], as in the case for nonionic surfactants.

The number of industrial applications of surfactants is huge, and represents the subject of several book series. However, in analytical chemistry, surfactants have been recognized as being very useful in improving analytical technology, for example, in chromatography [63] and luminescence spectroscopy [64]. The use of surfactants in chromatography, particularly in ion-exchange, is of our interest. It is well known that surfactants are composed of two parts—hydrophobic and hydrophilic, which are oppositely charged, and the surfactants also act as ion exchangers. When their solutions are kept in contact with solid material, they are adsorbed on the solid surface with their hydrophilic part remaining in the solutions due to their surface-active property and they make that surface 'active'. It is clear from the above discussion that they play an important role in the adsorption behaviour of the molecules.

**57**

*Hybrid Ion Exchangers*

care and laundry products.

*DOI: http://dx.doi.org/10.5772/intechopen.92116*

**3.3 Surfactant-based hybrid ion exchangers**

The widespread utility of surfactants in practical applications and scientific interest regarding their nature and properties have precipitated a wealthy literature [65, 66] on the subject. One of the predominant reasons for the ubiquitous applications of surfactants has been their remarkable ability to influence the properties of the surfaces and interfaces. Surfactants are widely used in various industrial applications [67–70] such as petroleum, pharmaceuticals, agro-chemicals, processing of foods, paints, coatings, adhesives, lubricants, in photographic films, personal

Varshney et al. [71, 72] used surfactants as media in the adsorption studies of some alkaline earths and heavy metal ions on inorganic and hybrid ion exchangers and observed that the presence of surfactants in aqueous media increases the adsorption of metal ions on the surface on ion-exchange materials. Hence, exceptional high adsorption of the said metal ions has opened the doors in the field of material science. It was thought worthwhile to incorporate the surfactants in the matrix of inorganic ion exchangers to see how they could change the characteristics of the ion exchangers. Very recently, some hybrid fibrous and non-fibrous metal phosphates have been synthesized by combining surfactants and inorganic ion exchangers (metal phosphates) [73–81] by the researchers. Surfactants based ion exchangers also correspond to metal organic frameworks as surfactants being an organic counterpart introduced in the inorganic metal phosphates by the bonding in between the layers of metal phosphates. At this level too, structure could not been explained, reason being

Somya et al. [73–75] have probably first used surfactants in the synthesis some novel hybrid fibrous and non-fibrous metal phosphates by introducing surfactants (anionic, cationic and nonionic) in the matrix of inorganic metal phosphates. They have explored the ion-exchange studies such as ion-exchange, pH-titration, concentration, elution and thermal behaviour in addition to adsorption studies for some alkaline earths and heavy metal ions. Those materials were found to be selective for certain metal ions and, on that basis, some binary separations have been performed in the laboratory providing their potential role in environmental and analytical chemistry. The introduction of surfactants in the matrix of inorganic ion exchangers has been characterized by some physico-chemical studies like, IR, X-ray diffrac-

Later on, Iqbal [78–81] have synthesized the same class of hybrid metal phosphates by combining sodium dodecyl benzene sulphonate and sodium bis (2-ethylhexyl) sulphosuccinate in the matrix of cerium (IV) and tin (IV) phosphates. They have explored some ion-exchange studies in addition to physico-chemical characterization like IR, XRD, SEM, TGA/DTA/DTG, elemental studies and differential pulse polarography. These materials have shown selective adsorption for certain metal ions. Hence, binary separations have been done by using columns of the synthesized materials. Most of the surfactant based metal phosphates were found

As per the studies done, so far, it is clearly indicated that surfactants have played

a key role in synthesis of new class of hybrid metal phosphates as ion-exchange materials. They have enhanced not only the ion-exchange capacities of the inorganic metal phosphates when present in their matrix but also the adsorption of metal ions. These metal-organic hybrid materials have shown selectivity towards

the amorphous and poorly crystalline nature of these materials.

tion, elemental, SEM and TGA/DTA/DTG studies.

amorphous or poorly crystalline.

**4. Conclusion**

*Hybrid Nanomaterials - Flexible Electronics Materials*

**3.2 Surfactant adsorption and surface properties**

micelles [60].

Depending upon the chemical structure of the surfactant, its micelle can be cationic, anionic, zwitterionic or nonionic. The electrostatic character of the micelles depends in some cases on the pH of the aqueous solution due to protonation equilibria. Zwitterionic surfactants, of course, also can become either cationic or anionic, and several types of nonionic surfactants can also form anionic or cationic micelles in the appropriate pH range. Micelles are not static species but rather exist in a dynamic equilibrium. The micelle may be represented as a globular, cylindrical or ellipsoidal cluster [59] of individual surfactant molecules in equilibrium with its monomer. The reverse orientation of the hydrophilic and hydrophobic part of the surfactant in a hydrocarbon medium leads to the formation of reversed

The effectiveness of surfactant adsorption is mainly determined by surfactant concentration, surfactant functional group, alkyl hydrocarbon chain length, environment etc. When the surfactant concentration is well below the CMC value, individual surfactant molecules tend to adsorb on exposed interfaces to reduce surface tension. As the concentration of surfactant approaches the CMC value, surfactant molecules form dimers, and multiple molecules aggregate, micelles. Once the CMC is reached, any additional surfactant molecules added to the system will be incorporated into new or existing aggregates. Thus, further increase of surfactant concentration above the CMC value results in bilayer or multilayer formation at interface. The adsorption tendency of the surfactants at the surfaces imparts the properties of foaming, wetting, emulsification, dispersing of solids and detergency. The adsorption increases the concentration of surfactant at the surfaces. Surfactant adsorption is a consideration in any application where surfactants come in contact with a surface or interface. It is from solutions that surfactants then preferentially adsorb to interfaces and, because of their amphiphilic nature, preferentially segregate at interfaces. There are a number of areas of applications where surfactant adsorption is important including ore floatation, improved oil recovery, soil remediation, detergency, surfactant-based separation processes and wetting. Surfactant adsorption may occur due to electrostatic interactions, van der Waals interaction, hydrogen-bonding and/

or solvation and desolvation of adsorbate and adsorbent species [61].

dispersion forces [62], as in the case for nonionic surfactants.

The hydrophobic forces that drive surfactants to segregate at air-water interfaces are essentially the same that drive surfactant adsorption onto solid surfaces. However, they can be differing in the chemical forces associated with the solid surfaces. Ionic surfactants tend to adsorb onto oppositely charged solid surfaces due to electrostatic forces while adsorption of ionic surfactants on a like charged substrate, being less understood, can occur via hydrogen-bonding or attractive

The number of industrial applications of surfactants is huge, and represents the subject of several book series. However, in analytical chemistry, surfactants have been recognized as being very useful in improving analytical technology, for example, in chromatography [63] and luminescence spectroscopy [64]. The use of surfactants in chromatography, particularly in ion-exchange, is of our interest. It is well known that surfactants are composed of two parts—hydrophobic and hydrophilic, which are oppositely charged, and the surfactants also act as ion exchangers. When their solutions are kept in contact with solid material, they are adsorbed on the solid surface with their hydrophilic part remaining in the solutions due to their surface-active property and they make that surface 'active'. It is clear from the above discussion that they play an important role in the adsorption behaviour of the

**56**

molecules.

#### **3.3 Surfactant-based hybrid ion exchangers**

The widespread utility of surfactants in practical applications and scientific interest regarding their nature and properties have precipitated a wealthy literature [65, 66] on the subject. One of the predominant reasons for the ubiquitous applications of surfactants has been their remarkable ability to influence the properties of the surfaces and interfaces. Surfactants are widely used in various industrial applications [67–70] such as petroleum, pharmaceuticals, agro-chemicals, processing of foods, paints, coatings, adhesives, lubricants, in photographic films, personal care and laundry products.

Varshney et al. [71, 72] used surfactants as media in the adsorption studies of some alkaline earths and heavy metal ions on inorganic and hybrid ion exchangers and observed that the presence of surfactants in aqueous media increases the adsorption of metal ions on the surface on ion-exchange materials. Hence, exceptional high adsorption of the said metal ions has opened the doors in the field of material science. It was thought worthwhile to incorporate the surfactants in the matrix of inorganic ion exchangers to see how they could change the characteristics of the ion exchangers. Very recently, some hybrid fibrous and non-fibrous metal phosphates have been synthesized by combining surfactants and inorganic ion exchangers (metal phosphates) [73–81] by the researchers. Surfactants based ion exchangers also correspond to metal organic frameworks as surfactants being an organic counterpart introduced in the inorganic metal phosphates by the bonding in between the layers of metal phosphates. At this level too, structure could not been explained, reason being the amorphous and poorly crystalline nature of these materials.

Somya et al. [73–75] have probably first used surfactants in the synthesis some novel hybrid fibrous and non-fibrous metal phosphates by introducing surfactants (anionic, cationic and nonionic) in the matrix of inorganic metal phosphates. They have explored the ion-exchange studies such as ion-exchange, pH-titration, concentration, elution and thermal behaviour in addition to adsorption studies for some alkaline earths and heavy metal ions. Those materials were found to be selective for certain metal ions and, on that basis, some binary separations have been performed in the laboratory providing their potential role in environmental and analytical chemistry. The introduction of surfactants in the matrix of inorganic ion exchangers has been characterized by some physico-chemical studies like, IR, X-ray diffraction, elemental, SEM and TGA/DTA/DTG studies.

Later on, Iqbal [78–81] have synthesized the same class of hybrid metal phosphates by combining sodium dodecyl benzene sulphonate and sodium bis (2-ethylhexyl) sulphosuccinate in the matrix of cerium (IV) and tin (IV) phosphates. They have explored some ion-exchange studies in addition to physico-chemical characterization like IR, XRD, SEM, TGA/DTA/DTG, elemental studies and differential pulse polarography. These materials have shown selective adsorption for certain metal ions. Hence, binary separations have been done by using columns of the synthesized materials. Most of the surfactant based metal phosphates were found amorphous or poorly crystalline.

#### **4. Conclusion**

As per the studies done, so far, it is clearly indicated that surfactants have played a key role in synthesis of new class of hybrid metal phosphates as ion-exchange materials. They have enhanced not only the ion-exchange capacities of the inorganic metal phosphates when present in their matrix but also the adsorption of metal ions. These metal-organic hybrid materials have shown selectivity towards

certain metals. Hence, these materials open a door with ample opportunities for the researchers in the field of analytical and environmental science where they can be used in water pollution control.

#### **Acknowledgements**

The author (A. Somya) is highly thankful to the Department of Applied Chemistry, Aligarh Muslim University, Aligarh for providing the research facilities. The author is also thankful to the Council of Scientific and Industrial Research, New Delhi for providing funds under Senior Research Fellowship scheme [09/112(0408)2K8-EMR-I]. The author wants to express special thanks to the Chancellor, Pro-Chancellor, Vice-Chancellor and Dean, School of Engineering, Presidency University, Bengaluru for giving enthusiastic and constant support.

#### **Author details**

Amita Somya Department of Chemistry, School of Engineering, Presidency University, Bengaluru, Karnataka, India

\*Address all correspondence to: somya.amita@gmail.com

© 2020 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.

**59**

1995;**34**:2865

*Hybrid Ion Exchangers*

**References**

1992;**37**(1):21

Elsevier; 1991. p. 413

Press, Inc.; 1982

1991

1971;**14**:45

1972;**7**:615

*DOI: http://dx.doi.org/10.5772/intechopen.92116*

[12] Alberti G, Casciola M, Costantino U. Inorganic ion-exchange membranes made of acid salts of tetravalent metals. A short review. Journal of Membrane

[13] Alberti G, Costantino U. Recent progress in the field of synthetic inorganic exchangers having a layered or fibrous structure. Journal of Chromatography. 1974;**102**:5

[14] Marinsky JA. Reactive and Functional Polymers. 1995;**27**(2):107

[15] Varshney KG, Gupta U. Tin(IV) antimonate as a lead-selective cation exchanger: Synthesis, characterization, and analytical applications. Bulletin of the Chemical Society of Japan.

[16] Varshney KG, Mohammad A. Chemical

and Environmental Research.

[17] Varshney KG. In: Sinha AK, Srivastava KN, Sinha BK, Pandey SK, Agarwal NK, editors. Environment Management in Developing Countries, Water and its Managements. Vol II;

[18] Varshney KG. Chemical and

[20] Varshney KG. Solid State Phenomena. 2003;**90-91**:445

1996;**208**(1):23

1997;**69**:567

[21] Ivanov VA, Timofeevskaya VD, Gorshkov VI, Drozdeva NV. Journal of Radioanalytical and Nuclear Chemistry.

[22] Terres-Rojas E, Dominguez JM, Sales MA, Lopez E. Chemical Industries.

Environmental Research. 1994;**3&4**:301

[19] Varshney KG. In: Srivastava MM, Srivastava S, editors. Recent Trends in Chemistry. New Delhi: Discovery Publishing House; 2003. p. 467

Science. 1983;**16**:137

1990;**63**:1515

1992;**1**(4):353

1993. p. 64

[1] Ettre LS. Nomenclature for Chromatography. Pure and Applied

[2] Qureshi M, Zafar S, Ahmad TS, Rahman N. Chemia Analityczna.

[3] Varshney KG. New development in ion-exchange. In: Abe M, Kataoha T, Kodansha ST, editors. Proceeding Int. Conf. Ion-exchange (ICEE-91). Tokyo:

[4] Clearfield A. Inorganic Ion-exchange Materials. Boca Raton, Florida: CRC

Exchangers. Amsterdam: Elsevier; 1964

[6] Qureshi M, Varshney KG. Inorganic Ion-exchangers in Chemical Analysis. Boca Raton, Florida: CRC Press, Inc.;

[7] Fuller MJ. Inorganic ion-exchange chromatography on oxides and hydrous oxides. Chromatographic Reviews.

[8] Qureshi M, Qureshi SZ, Gupta JP, Rathore HS. Recent progress in ionexchange studies on insoluble salts of polybasic metals. Separation Science.

[9] Vesely V, Pekarek V. Synthetic inorganic ion exchangers-1: Hydrous oxides and acidic salts of multivalent

[10] Clearfield A. Role of ion exchange in solid-state chemistry. Chemical

metals. Talanta. 1972;**19**:1245

[11] Clearfield A. Inorganic ion exchangers: A technology ripe for development. I and EC Research.

Reviews. 1988;**88**:125

[5] Amphlett CB. Inorganic Ion-

Chemistry. 1993;**65**(4):819

### **References**

*Hybrid Nanomaterials - Flexible Electronics Materials*

used in water pollution control.

**Acknowledgements**

certain metals. Hence, these materials open a door with ample opportunities for the researchers in the field of analytical and environmental science where they can be

The author (A. Somya) is highly thankful to the Department of Applied Chemistry, Aligarh Muslim University, Aligarh for providing the research facilities. The author is also thankful to the Council of Scientific and Industrial Research, New Delhi for providing funds under Senior Research Fellowship scheme [09/112(0408)2K8-EMR-I]. The author wants to express special thanks to the Chancellor, Pro-Chancellor, Vice-Chancellor and Dean, School of Engineering, Presidency University, Bengaluru for giving enthusiastic and constant support.

**58**

**Author details**

Bengaluru, Karnataka, India

provided the original work is properly cited.

Department of Chemistry, School of Engineering, Presidency University,

© 2020 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,

\*Address all correspondence to: somya.amita@gmail.com

Amita Somya

[1] Ettre LS. Nomenclature for Chromatography. Pure and Applied Chemistry. 1993;**65**(4):819

[2] Qureshi M, Zafar S, Ahmad TS, Rahman N. Chemia Analityczna. 1992;**37**(1):21

[3] Varshney KG. New development in ion-exchange. In: Abe M, Kataoha T, Kodansha ST, editors. Proceeding Int. Conf. Ion-exchange (ICEE-91). Tokyo: Elsevier; 1991. p. 413

[4] Clearfield A. Inorganic Ion-exchange Materials. Boca Raton, Florida: CRC Press, Inc.; 1982

[5] Amphlett CB. Inorganic Ion-Exchangers. Amsterdam: Elsevier; 1964

[6] Qureshi M, Varshney KG. Inorganic Ion-exchangers in Chemical Analysis. Boca Raton, Florida: CRC Press, Inc.; 1991

[7] Fuller MJ. Inorganic ion-exchange chromatography on oxides and hydrous oxides. Chromatographic Reviews. 1971;**14**:45

[8] Qureshi M, Qureshi SZ, Gupta JP, Rathore HS. Recent progress in ionexchange studies on insoluble salts of polybasic metals. Separation Science. 1972;**7**:615

[9] Vesely V, Pekarek V. Synthetic inorganic ion exchangers-1: Hydrous oxides and acidic salts of multivalent metals. Talanta. 1972;**19**:1245

[10] Clearfield A. Role of ion exchange in solid-state chemistry. Chemical Reviews. 1988;**88**:125

[11] Clearfield A. Inorganic ion exchangers: A technology ripe for development. I and EC Research. 1995;**34**:2865

[12] Alberti G, Casciola M, Costantino U. Inorganic ion-exchange membranes made of acid salts of tetravalent metals. A short review. Journal of Membrane Science. 1983;**16**:137

[13] Alberti G, Costantino U. Recent progress in the field of synthetic inorganic exchangers having a layered or fibrous structure. Journal of Chromatography. 1974;**102**:5

[14] Marinsky JA. Reactive and Functional Polymers. 1995;**27**(2):107

[15] Varshney KG, Gupta U. Tin(IV) antimonate as a lead-selective cation exchanger: Synthesis, characterization, and analytical applications. Bulletin of the Chemical Society of Japan. 1990;**63**:1515

[16] Varshney KG, Mohammad A. Chemical and Environmental Research. 1992;**1**(4):353

[17] Varshney KG. In: Sinha AK, Srivastava KN, Sinha BK, Pandey SK, Agarwal NK, editors. Environment Management in Developing Countries, Water and its Managements. Vol II; 1993. p. 64

[18] Varshney KG. Chemical and Environmental Research. 1994;**3&4**:301

[19] Varshney KG. In: Srivastava MM, Srivastava S, editors. Recent Trends in Chemistry. New Delhi: Discovery Publishing House; 2003. p. 467

[20] Varshney KG. Solid State Phenomena. 2003;**90-91**:445

[21] Ivanov VA, Timofeevskaya VD, Gorshkov VI, Drozdeva NV. Journal of Radioanalytical and Nuclear Chemistry. 1996;**208**(1):23

[22] Terres-Rojas E, Dominguez JM, Sales MA, Lopez E. Chemical Industries. 1997;**69**:567

[23] Dyer A. An Introduction to Zeolite Molecular Sieves. Avon: Bath Press Ltd.; 1988

[24] Dyer A, Townsend RP. The mobility of cations in synthetic zeolites with the faujasite framework — V: The self-diffusion of zinc into X and Y zeolites. Journal of Inorganic and Nuclear Chemistry. 1973;**35**:3001

[25] Dyer A, Hudson MJ, Williams PA. Progress in ion-exchange: Advances and applications. In: Proceedings of the Ion-Ex '95 Conference. Vol. 196. Special Publication by Royal Society of Chemistry; 1997. p. 498

[26] Chetverina RB, Boichinova ES. Zhurnal Pikladnoi Khimii. 1977;**50**:1181

[27] Malik WU, Srivastava SK, Kumar S. Ion-exchange behaviour of pyridinium tungstoarsenate. Talanta. 1976;**23**:323

[28] Chetverina RB, Baichinova ES. Ibid. 1977;**50**:1183

[29] Niwas R, Khan AA, Varshney KG. Synthesis and ion exchange behaviour of polyaniline Sn(IV) arsenophosphate: A polymeric inorganic ion exchanger. Colloids and Surfaces, A: Physiochemical and Engineering Aspects. 1999;**150**:7

[30] Varshney KG, Pandith AH. Chemical and Environmental Research. 1996;**5**(1-4):141

[31] Sanchez C, Ribot F. Design of hybrid organic-inorganic materials synthesized via sol-gel chemistry. New Journal of Chemistry. 1994;**18**:1007

[32] Varshney KG, Jain V, Tayal N. Synthesis and ion-exchange behaviour of acrylamide zirconium(IV) phosphate: A novel crystalline and Hg(II) selective hybrid inorganic ion exchanger. Indian Journal of Chemical Technology. 2003;**10**:186

[33] Varshney KG, Jain V, Agrawal A, Mojumdar SC. Pyridine based zirconium(IV) and tin(IV) phosphates as new and novel intercalated ion exchangers. Journal of Thermal Analysis and Calorimetry. 2006;**86**:3,609

[34] Varshney KG, Gupta P. Synthesis and characterization of a mercury selective phase of acrylamide tin(IV) phosphate hybrid ion exchanger: Separation of Hg(II) from Cd(II), Pb(II) and Sr(II). Indian Journal of Chemistry. 2003;**42A**:2974

[35] Varshney KG, Pandith AH. Synthesis and ion exchange behavior of acrylonitrile-based zirconium phosphate - A new hybrid cation exchanger. Journal of the Indian Chemical Society. 2001;**78**:250

[36] Varshney KG, Tayal N, Khan AA, Niwas R. Synthesis, characterization and analytical applications of lead (II) selective polyacrylonitrile thorium (IV) phosphate: A novel fibrous ion exchanger. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 2001;**181**:123

[37] Varshney KG, Tayal N, Gupta P, Agrawal A, Drabik M. Synthesis, ionexchange and physico-chemical studies on a polystyrene cerium(IV) phosphate hybrid fibrous ion exchanger. Indian Journal of Chemistry. 2004;**43A**:2586

[38] Varshney KG, Tayal N. Polystyrene thorium(IV) phosphate as a new crystalline and cadmium selective fibrous ion exchanger. Synthesis characterization and analytical applications. Langmuir. 2001;**17**:2589

[39] Varshney KG, Tayal N, Gupta U. Acrylonitrile based cerium (IV) phosphate as a new mercury selective fibrous ion-exchanger. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 1998;**145**:71

**61**

*Hybrid Ion Exchangers*

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*DOI: http://dx.doi.org/10.5772/intechopen.92116*

[47] Tanford C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes. 2nd ed. New York: Wiley;

[48] Rosen MJ. Surfactants and Interfacial Phenomena. New York:

[49] Cullum DC. Introduction of Surfactant Analysis. 1st ed. UK: Chapman and Hall; 1994

[50] Linfield WM, editor. Anionic Surfactants. New York: Dekker M; 1973

Surfactants. New York; 1970

[52] Schick MJ, editor. Nonionic

[53] Porter MR. Handbook of

[51] Jungermann E. Dekker M. Cationic

Surfactants. New York: Dekker M; 1967

Surfactants. 2nd ed. UK: Chapman and

[54] Menger FM, Littau CA. Gemini surfactants: A new class of selfassembling molecules. Journal of the American Chemical Society.

[55] Mukerjee P, Banerjee K. A study of the surface pH of micelles using solubilized indicator dyes. The Journal of Physical Chemistry. 1964;**68**:3567

[56] Preston WC. Some correlating principles of detergent action. The Journal of Physical and Colloid

Tamamushi BI, Isemura T. Colloidal Surfactants, Some Physico-Chemical Properties. New York: Academia Press;

[58] Mukerjee P, Mysels KJ. Critical Micelle Concentrations of Aqueous Surfactant Systems. Washington, DC: National Bureau of Standards, NSRDS-

Chemistry. 1948;**52**:84

1963

NBS 36; 1971

[57] Shinoda K, Nakagawa T,

1980

Wiley; 1978

Hall; 1994

1993;**115**:10083

Tayal N. Synthesis, characterization and applications of a new phase of crystalline and mercury selective acrylamide cerium(IV) phosphate: A novel fibrous ion exchanger. Indian Journal of Chemistry. 2003;**42A**:89

[41] Mojumdar SC, Varshney KG, Agrawal A. Hybrid fibrous ion exchange materials: Past, present and future. Research Journal of Chemistry and Environment. 2006;**10**(2):85

[42] Varshney KG, Agrawal A, Mojumdar SC. Pectin based cerium (IV) and thorium (IV) phosphates as novel hybrid fibrous ion exchangers synthesis, characterization and thermal behaviour. Journal of Thermal Analysis and Calorimetry.

[43] Varshney KG, Drabik M, Agrawal A. Cellulose acetate based thorium(IV) phosphate as a new and novel hybrid fibrous cation exchanger: Synthesis, characterization and thermal analysis. Indian Journal of Chemistry.

[44] Varshney KG, Agrawal A, Mojumdar SC. Pyridine based cerium(IV) phosphate hybrid fibrous ion exchanger. Journal of Thermal Analysis and Calorimetry.

[45] Varshney KG, Agrawal A, Mojumdar SC. Pyridine based thorium(IV) phosphate hybrid fibrous ion exchanger. Journal of Thermal Analysis and Calorimetry.

[46] Varshney KG, Rafiquee MZA, Somya A, Drabik M. Synthesis and characterization of a Hg(II) selective n-butyl acetate cerium(IV) phosphate as a new intercalated fibrous ion exchanger: Effect of surfactants on the adsorption behaviour. Indian Journal of

Chemistry. 2006;**45A**:1856

2005;**81**:183

2006;**45A**:2045

2007;**90**(3):731

2007;**90**(3):721

*Hybrid Ion Exchangers DOI: http://dx.doi.org/10.5772/intechopen.92116*

*Hybrid Nanomaterials - Flexible Electronics Materials*

[33] Varshney KG, Jain V,

2006;**86**:3,609

Agrawal A, Mojumdar SC. Pyridine based zirconium(IV) and tin(IV) phosphates as new and novel intercalated ion exchangers. Journal of Thermal Analysis and Calorimetry.

[34] Varshney KG, Gupta P. Synthesis and characterization of a mercury selective phase of acrylamide tin(IV) phosphate hybrid ion exchanger: Separation of Hg(II) from Cd(II), Pb(II) and Sr(II). Indian Journal of

Chemistry. 2003;**42A**:2974

Aspects. 2001;**181**:123

[35] Varshney KG, Pandith AH. Synthesis and ion exchange behavior of acrylonitrile-based zirconium phosphate - A new hybrid cation exchanger. Journal of the Indian Chemical Society. 2001;**78**:250

[36] Varshney KG, Tayal N, Khan AA, Niwas R. Synthesis, characterization and analytical applications of lead (II) selective polyacrylonitrile thorium (IV) phosphate: A novel fibrous ion exchanger. Colloids and Surfaces A: Physicochemical and Engineering

[37] Varshney KG, Tayal N, Gupta P, Agrawal A, Drabik M. Synthesis, ionexchange and physico-chemical studies on a polystyrene cerium(IV) phosphate hybrid fibrous ion exchanger. Indian Journal of Chemistry. 2004;**43A**:2586

[38] Varshney KG, Tayal N. Polystyrene thorium(IV) phosphate as a new crystalline and cadmium selective fibrous ion exchanger. Synthesis characterization and analytical applications. Langmuir. 2001;**17**:2589

[39] Varshney KG, Tayal N,

1998;**145**:71

Gupta U. Acrylonitrile based cerium (IV) phosphate as a new mercury selective fibrous ion-exchanger. Colloids and Surfaces A: Physicochemical and Engineering Aspects.

[23] Dyer A. An Introduction to Zeolite Molecular Sieves. Avon: Bath Press Ltd.;

[24] Dyer A, Townsend RP. The mobility of cations in synthetic zeolites with the faujasite

framework — V: The self-diffusion of zinc into X and Y zeolites. Journal of Inorganic and Nuclear Chemistry.

[25] Dyer A, Hudson MJ, Williams PA. Progress in ion-exchange: Advances and applications. In: Proceedings of the Ion-Ex '95 Conference. Vol. 196. Special Publication by Royal Society of

[26] Chetverina RB, Boichinova ES. Zhurnal Pikladnoi Khimii. 1977;**50**:1181

[28] Chetverina RB, Baichinova ES. Ibid.

[29] Niwas R, Khan AA, Varshney KG. Synthesis and ion exchange behaviour of polyaniline Sn(IV) arsenophosphate:

A polymeric inorganic ion exchanger. Colloids and Surfaces, A: Physiochemical and Engineering

[30] Varshney KG, Pandith AH.

Chemical and Environmental Research.

[31] Sanchez C, Ribot F. Design of hybrid organic-inorganic materials synthesized via sol-gel chemistry. New Journal of

Tayal N. Synthesis and ion-exchange behaviour of acrylamide zirconium(IV) phosphate: A novel crystalline and Hg(II) selective hybrid inorganic ion exchanger. Indian Journal of Chemical

Aspects. 1999;**150**:7

1996;**5**(1-4):141

Chemistry. 1994;**18**:1007

[32] Varshney KG, Jain V,

Technology. 2003;**10**:186

[27] Malik WU, Srivastava SK, Kumar S. Ion-exchange behaviour of pyridinium tungstoarsenate. Talanta.

1988

1973;**35**:3001

1976;**23**:323

1977;**50**:1183

Chemistry; 1997. p. 498

**60**

[40] Varshney KG, Gupta P, Tayal N. Synthesis, characterization and applications of a new phase of crystalline and mercury selective acrylamide cerium(IV) phosphate: A novel fibrous ion exchanger. Indian Journal of Chemistry. 2003;**42A**:89

[41] Mojumdar SC, Varshney KG, Agrawal A. Hybrid fibrous ion exchange materials: Past, present and future. Research Journal of Chemistry and Environment. 2006;**10**(2):85

[42] Varshney KG, Agrawal A, Mojumdar SC. Pectin based cerium (IV) and thorium (IV) phosphates as novel hybrid fibrous ion exchangers synthesis, characterization and thermal behaviour. Journal of Thermal Analysis and Calorimetry. 2005;**81**:183

[43] Varshney KG, Drabik M, Agrawal A. Cellulose acetate based thorium(IV) phosphate as a new and novel hybrid fibrous cation exchanger: Synthesis, characterization and thermal analysis. Indian Journal of Chemistry. 2006;**45A**:2045

[44] Varshney KG, Agrawal A, Mojumdar SC. Pyridine based cerium(IV) phosphate hybrid fibrous ion exchanger. Journal of Thermal Analysis and Calorimetry. 2007;**90**(3):731

[45] Varshney KG, Agrawal A, Mojumdar SC. Pyridine based thorium(IV) phosphate hybrid fibrous ion exchanger. Journal of Thermal Analysis and Calorimetry. 2007;**90**(3):721

[46] Varshney KG, Rafiquee MZA, Somya A, Drabik M. Synthesis and characterization of a Hg(II) selective n-butyl acetate cerium(IV) phosphate as a new intercalated fibrous ion exchanger: Effect of surfactants on the adsorption behaviour. Indian Journal of Chemistry. 2006;**45A**:1856

[47] Tanford C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes. 2nd ed. New York: Wiley; 1980

[48] Rosen MJ. Surfactants and Interfacial Phenomena. New York: Wiley; 1978

[49] Cullum DC. Introduction of Surfactant Analysis. 1st ed. UK: Chapman and Hall; 1994

[50] Linfield WM, editor. Anionic Surfactants. New York: Dekker M; 1973

[51] Jungermann E. Dekker M. Cationic Surfactants. New York; 1970

[52] Schick MJ, editor. Nonionic Surfactants. New York: Dekker M; 1967

[53] Porter MR. Handbook of Surfactants. 2nd ed. UK: Chapman and Hall; 1994

[54] Menger FM, Littau CA. Gemini surfactants: A new class of selfassembling molecules. Journal of the American Chemical Society. 1993;**115**:10083

[55] Mukerjee P, Banerjee K. A study of the surface pH of micelles using solubilized indicator dyes. The Journal of Physical Chemistry. 1964;**68**:3567

[56] Preston WC. Some correlating principles of detergent action. The Journal of Physical and Colloid Chemistry. 1948;**52**:84

[57] Shinoda K, Nakagawa T, Tamamushi BI, Isemura T. Colloidal Surfactants, Some Physico-Chemical Properties. New York: Academia Press; 1963

[58] Mukerjee P, Mysels KJ. Critical Micelle Concentrations of Aqueous Surfactant Systems. Washington, DC: National Bureau of Standards, NSRDS-NBS 36; 1971

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[70] Karsa DR. Industrial Applications of Surfactants. 4th ed. Cambridge, UK: Royal Society of Chemistry; 1999

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**65**

Section 2

Flexible Electronics

## Section 2
