**Crystal Growth by Electrodeposition with Supercritical Carbon Dioxide Emulsion**

Masato Sone, Tso-Fu Mark Chang and Hiroki Uchiyama

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/54070

### **1. Introduction**

#### **1.1. Electroplating with supercritical carbon dioxide emulsion**

#### *1.1.1. Introduction*

### *1.1.1.1. Supercritical Carbon Dioxide*

A supercritical fluid (SCF) is any substance at a temperature and pressure above its critical point, as shown in Fig. 1, where distinct liquid and gas phases do not exist [1]. It can effuse through solids like a gas, and dissolve materials like a liquid. In addition, close to the critical point, small changes in pressure or temperature result in large changes in density, allowing many properties of a SCF to be fine-tuned between a gas and a liquid. SCFs are suitable as a substitute for organic solvents in a range of industrial and laboratory processes.

CO2 is non-polar, combining with the low surface tension property when it is in super‐ critical state; it is often used in extraction of organics in the food industries [2]. The ex‐ tremely low surface tension also makes supercritical CO2 (sc-CO2) an ideal medium in drying of nano-porous structures [3]. CO2 is non-toxic, and the critical point is relatively low when comparing with the other solvents, therefore, sc-CO2 is an important commer‐ cial and industrial solvent. Critical temperature pressure of sc-CO2 are 304.5K and 7.39 MPa, respectively. Comparison for the critical conditions of some commonly used sol‐ vents is shown in Table 1.

© 2013 Sone et al.; licensee InTech. This is an open access article 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. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

However, electrical conductivity and metal salts solubility are both very low in sc-CO2 [1], which are the basic requirements in electrochemistry. The limitations could be overcome by addition of a surfactant to form an emulsion composed of an aqueous electrolyte, sc-CO2, and the surfactant [9,10]. Two types of the emulsion could be formed depending on the type and concentration of surfactant used and concentration of CO2 in the system. One is H2O in CO2, where the continuous phase (CP) is CO2 and the dispersed phase (DP) is H2O. The oth‐ er one is CO2 in H2O, where the CP is H2O and the DP is CO2 [11,12]. Structure of the DP in CO2 in H2O emulsion is similar to micelles in mixture of oil and water. CO2 in H2O emulsion is usually used for application in electrochemical reaction because the higher solubility of metal salts and electrical conductivity CO2 in H2O emulsion when compared with H2O in

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Surface smoothening, grain refinement, and hardness enhancement are the effects of ap‐ plying sc-CO2 emulsion (SCE) in electrodeposition of Ni [13-15]. Surface of the Ni films fabricated by electrodeposition with SCE (EP-SCE) is defect-free and pinhole free as shown in Fig. 2(a), and defects are found on the Ni films when only the aqueous electro‐ lyte is used at ambient pressure as shown in Fig. 2(b) [13]. Ni films is electrodeposited with the aqueous electrolyte only at an elevated pressure, 42 MPa, to confirm the surface smoothening effect is not caused by the high pressure, and many defects are found on the Ni film as shown in Fig. 2(c). In order to further confirm the smooth surface is caused by SCE, emulsion made of n-hexane is studied [14]. Properties of n-hexane are considered to be close to sc-CO2, such as electrical conductivity. Surface conditions of the Ni films fabricated by EP-SCE are much better than the Ni films fabricated by electrode‐ position with n-Hexane. The results indicate that only emulsion made of sc-CO2 is effec‐

**Figure 2.** Ni films electrodeposited from (a)SCE, (b) the aqueous electrolyte only at atmospheric pressure and (c) the

For improvement in mechanical properties, grain refinement is believed to be the main cause as shown in Fig. 3 [15-17]. Ni films fabricated by EP-SCE are reported to have grain size in nano-scale. Because of the nano-grains, wear properties of the Ni films could be improved significantly [18]. Chung and Tsai proposed the grain refinement and

*1.1.1.3. Electrodeposition with Supercritical Carbon Dioxide Emulsion (EP-SCE)*

tive in increasing smoothness of the Ni films electrodeposited.

CO2 emulsion [11,12].

aqueous electrolyte only at 42 MPa.

**Figure 1.** Phase diagram of a single substance.



#### *1.1.1.2. Supercritical Carbon Dioxide Emulsion*

Electrodeposition is a key technology for fabricating micro components used in micro-elec‐ tro-mechanical systems (MEMS) [4,5]. Application of sc-CO2 in electrodeposition process is believed to solve the problems encounter in miniaturization of the devices [6], such as re‐ ducing usage of organic solvents in the cleaning process, drying of the nano-structures after the electrodeposition process, and minimize problems caused by evolution of H2. Evolution of H2 is an inevitable size reaction when performing electrodeposition reaction with an aqueous electrolyte. H2 gas bubbles adsorbed on the surface of cathode is one of the major causes for defects found in the electrodeposited materials [7]. CO2 is non-polar, solubility of H2 is high in CO2 [8]. Therefore, desorption of H2 gas bubbles from the surface of cathode could significantly enhanced in sc-CO2.

However, electrical conductivity and metal salts solubility are both very low in sc-CO2 [1], which are the basic requirements in electrochemistry. The limitations could be overcome by addition of a surfactant to form an emulsion composed of an aqueous electrolyte, sc-CO2, and the surfactant [9,10]. Two types of the emulsion could be formed depending on the type and concentration of surfactant used and concentration of CO2 in the system. One is H2O in CO2, where the continuous phase (CP) is CO2 and the dispersed phase (DP) is H2O. The oth‐ er one is CO2 in H2O, where the CP is H2O and the DP is CO2 [11,12]. Structure of the DP in CO2 in H2O emulsion is similar to micelles in mixture of oil and water. CO2 in H2O emulsion is usually used for application in electrochemical reaction because the higher solubility of metal salts and electrical conductivity CO2 in H2O emulsion when compared with H2O in CO2 emulsion [11,12].

#### *1.1.1.3. Electrodeposition with Supercritical Carbon Dioxide Emulsion (EP-SCE)*

**Figure 1.** Phase diagram of a single substance.

336 Advanced Topics on Crystal Growth

**Table 1.** Critical conditions of commonly used solvents

*1.1.1.2. Supercritical Carbon Dioxide Emulsion*

could significantly enhanced in sc-CO2.

**Fluid Critical Temperature (K) Critical Pressure (MPa)**

Electrodeposition is a key technology for fabricating micro components used in micro-elec‐ tro-mechanical systems (MEMS) [4,5]. Application of sc-CO2 in electrodeposition process is believed to solve the problems encounter in miniaturization of the devices [6], such as re‐ ducing usage of organic solvents in the cleaning process, drying of the nano-structures after the electrodeposition process, and minimize problems caused by evolution of H2. Evolution of H2 is an inevitable size reaction when performing electrodeposition reaction with an aqueous electrolyte. H2 gas bubbles adsorbed on the surface of cathode is one of the major causes for defects found in the electrodeposited materials [7]. CO2 is non-polar, solubility of H2 is high in CO2 [8]. Therefore, desorption of H2 gas bubbles from the surface of cathode

CO2 304.1 7.39 NH3 405.5 11.35 H2O 647.3 22.12 n-Pentane 469.7 3.37 Toluene 591.8 4.10

Surface smoothening, grain refinement, and hardness enhancement are the effects of ap‐ plying sc-CO2 emulsion (SCE) in electrodeposition of Ni [13-15]. Surface of the Ni films fabricated by electrodeposition with SCE (EP-SCE) is defect-free and pinhole free as shown in Fig. 2(a), and defects are found on the Ni films when only the aqueous electro‐ lyte is used at ambient pressure as shown in Fig. 2(b) [13]. Ni films is electrodeposited with the aqueous electrolyte only at an elevated pressure, 42 MPa, to confirm the surface smoothening effect is not caused by the high pressure, and many defects are found on the Ni film as shown in Fig. 2(c). In order to further confirm the smooth surface is caused by SCE, emulsion made of n-hexane is studied [14]. Properties of n-hexane are considered to be close to sc-CO2, such as electrical conductivity. Surface conditions of the Ni films fabricated by EP-SCE are much better than the Ni films fabricated by electrode‐ position with n-Hexane. The results indicate that only emulsion made of sc-CO2 is effec‐ tive in increasing smoothness of the Ni films electrodeposited.

**Figure 2.** Ni films electrodeposited from (a)SCE, (b) the aqueous electrolyte only at atmospheric pressure and (c) the aqueous electrolyte only at 42 MPa.

For improvement in mechanical properties, grain refinement is believed to be the main cause as shown in Fig. 3 [15-17]. Ni films fabricated by EP-SCE are reported to have grain size in nano-scale. Because of the nano-grains, wear properties of the Ni films could be improved significantly [18]. Chung and Tsai proposed the grain refinement and improvement in mechanical strength are also caused by C impurity in the Ni film from decomposition of CO2 in the electrolyte; evidence of the C impurity is detected from Xray photoelectron spectra [19].

respect to the volume of the aqueous electrolyte were varied from 10 to 50 vol% and 0 to 2.0 vol%, respectively. Cu plates with width and length at 1.0X2.0 cm2 were used as the working substrate, and Ni plates with width and length at 1.0X2.0 cm2 were used as

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The high-pressure experimental apparatus is shown in Fig. 4. Temperature variation of each run was confirmed to be less than 1.0 K. Maximum working temperature and maximum pressure were 424 K and 50 MPa, respectively. The reaction chamber was a stainless steel 316 vessel (PEEK coating on the inner wall) with a volume of 50 ml, kept in a temperature controlled air bath. There were holes at chamber cap for inflow and outflow of CO2 and wir‐ ing. Through the holes, platinum wires inserted in PEEK tube were used to position the sub‐ strates and connected to a programmable power supply. A magnetic agitator with a cross-

**Figure 4.** a) CO2 gas tank, (b) liquidization unit, (c) liquidization pump, (d) high-pressure pump, (e) thermal bath, (f) reaction cell, with PEEK coating on the inner wall, (g) substrates, (h) stirrer, (i) programmable power supply, (j) back

Cu plates were treated with 10 wt% degreasing solution and 10 wt% HCl solution for 1 min and 10 sec, respectively, prior to the reaction. Distance between the two substrates was 2.5 cm. Samples were electrodeposited at a constant temperature of 323 K with pressure vary‐

shaped magnetic-stirrer-bar was placed in the reaction chamber for mixing.

the counter electrode.

*1.1.2.2. Experimental apparatus*

pressure regulator, (k) trap, (l) thermometer.

*1.1.2.3. Electrodeposition*

Many studies have been reported on application of SCE in electrodeposition. However, ad‐ ditive such as brightener is often used in the electrolyte in these studies, and properties of the materials electrodeposited could be influenced by the additives [20,21]. Therefore, we studied and proposed a mechanism called periodic-plating-characteristic (PPC) to be the main cause for the effects observed in the metal films fabricated by EP-SCE. Physical proper‐ ties of SCE are expected to affect the PPC and properties of the metal films electrodeposited, and physical properties of SCE could be controlled by varying experimental pressure, vol‐ ume fraction of CO2 and surfactant in the system. In this study, physical properties of SCE are adjusted to study the influence on PPC and properties of the Ni films electrodeposited.

**Figure 3.** Effect of grain size on hardness.

#### *1.1.2. Experimental section*

#### *1.1.2.1. Materials*

CO2 with a minimum purity of 99.9 % was used. Composition of the additive-free Watts bath was NiSO4•6H2O (300 g/l), NiCl2•6H2O (50 g/l), and H3BO3 (50 g/l). pH of the addi‐ tive-free Watts bath was 3.31. A non-ionic surfactant, polyoxyethylene lauryl ether (C12H25(OCH2CH2)15OH) was used to form the emulsion. Volume fraction of CO2 with re‐ spect to the total volume of the reaction chamber and volume fraction of surfactant with respect to the volume of the aqueous electrolyte were varied from 10 to 50 vol% and 0 to 2.0 vol%, respectively. Cu plates with width and length at 1.0X2.0 cm2 were used as the working substrate, and Ni plates with width and length at 1.0X2.0 cm2 were used as the counter electrode.

#### *1.1.2.2. Experimental apparatus*

improvement in mechanical strength are also caused by C impurity in the Ni film from decomposition of CO2 in the electrolyte; evidence of the C impurity is detected from X-

Many studies have been reported on application of SCE in electrodeposition. However, ad‐ ditive such as brightener is often used in the electrolyte in these studies, and properties of the materials electrodeposited could be influenced by the additives [20,21]. Therefore, we studied and proposed a mechanism called periodic-plating-characteristic (PPC) to be the main cause for the effects observed in the metal films fabricated by EP-SCE. Physical proper‐ ties of SCE are expected to affect the PPC and properties of the metal films electrodeposited, and physical properties of SCE could be controlled by varying experimental pressure, vol‐ ume fraction of CO2 and surfactant in the system. In this study, physical properties of SCE are adjusted to study the influence on PPC and properties of the Ni films electrodeposited.

CO2 with a minimum purity of 99.9 % was used. Composition of the additive-free Watts bath was NiSO4•6H2O (300 g/l), NiCl2•6H2O (50 g/l), and H3BO3 (50 g/l). pH of the addi‐ tive-free Watts bath was 3.31. A non-ionic surfactant, polyoxyethylene lauryl ether (C12H25(OCH2CH2)15OH) was used to form the emulsion. Volume fraction of CO2 with re‐ spect to the total volume of the reaction chamber and volume fraction of surfactant with

ray photoelectron spectra [19].

338 Advanced Topics on Crystal Growth

**Figure 3.** Effect of grain size on hardness.

*1.1.2. Experimental section*

*1.1.2.1. Materials*

The high-pressure experimental apparatus is shown in Fig. 4. Temperature variation of each run was confirmed to be less than 1.0 K. Maximum working temperature and maximum pressure were 424 K and 50 MPa, respectively. The reaction chamber was a stainless steel 316 vessel (PEEK coating on the inner wall) with a volume of 50 ml, kept in a temperature controlled air bath. There were holes at chamber cap for inflow and outflow of CO2 and wir‐ ing. Through the holes, platinum wires inserted in PEEK tube were used to position the sub‐ strates and connected to a programmable power supply. A magnetic agitator with a crossshaped magnetic-stirrer-bar was placed in the reaction chamber for mixing.

**Figure 4.** a) CO2 gas tank, (b) liquidization unit, (c) liquidization pump, (d) high-pressure pump, (e) thermal bath, (f) reaction cell, with PEEK coating on the inner wall, (g) substrates, (h) stirrer, (i) programmable power supply, (j) back pressure regulator, (k) trap, (l) thermometer.

#### *1.1.2.3. Electrodeposition*

Cu plates were treated with 10 wt% degreasing solution and 10 wt% HCl solution for 1 min and 10 sec, respectively, prior to the reaction. Distance between the two substrates was 2.5 cm. Samples were electrodeposited at a constant temperature of 323 K with pressure vary‐ ing from 9 to 18 MPa and current density from 0.01 to 0.20 A/cm2 . The deposition time was 30 min for all the samples. According to Faraday's Law, ca. 6 μm of Ni film would be elec‐ trodeposited if 100 % efficiency is achieved.

#### *1.1.3. Effects of pressure*

Physical properties of sc-CO2, such as density, surface tension and viscosity, could be direct‐ ly adjusted by pressure, which is also expected to affect physical properties of SCE. Pressure could also influence homogeneity of the emulsion, size of the DP, and population or concen‐ tration of the DP in the emulsion.

Circular marks/defects with diameter ranged from several to several tens of micrometers were found on surface of the Ni films electrodeposited at 9.0 MPa as shown in Fig. 5 (a) [22]. Size in diameter and total number of the circular marks decreased with increase in pressure. Overall surface uniformity of the Ni films improved significantly when pressure was rose to 18.0 MPa as shown in Fig. 5(c). Surface of the Ni films fabricated by EP-SCE was composed of nano-scaled particles as shown in Fig. 5, which was very different from electric field ori‐ ented conical-shape morphology of the Ni films electrodeposited without SCE at atmospher‐ ic pressure [23]. Average roughness (Ra)of the Ni films decreased from 19.22 to 12.10 nm as pressure was increased from 9.0 to 18.0 MPa, shown in Fig. 6. The relatively large standard deviation of Ra for the Ni films electrodeposited at 9.0 MPa was caused by the circular marks found on the surface. Standard deviation of Ra decreased significantly as surface uniformity of the Ni films improved with increase in pressure.

Chemical reaction in a reaction medium containing SCE is highly dependent on homogenei‐ ty of SCE [1]. High homogeneity of SCE is a prerequisite for fabrication of smooth film when applying SCE in electrodeposition. Homogeneity of SCE can be referred as stability, average size, and size distribution of the DP in the emulsion. SCE is a dynamically emulsified sys‐ tem; size of the DP could be continuously fluctuating in the emulsion. Stability of the DP is considered to be high when size fluctuation of the DP and tendency for phase separation to occur are both low. Quantitatively, the stability is high when interfacial tension between sc-CO2 and the aqueous solution (*γ*) is low [24]. Growth in size of the DP is more likely to occur when *γ* is high, and continue growth in size of the DP would lead to phase separation. Creaming velocity (*us*) is another parameter that could be used to quantify stability of the DP [25]. Creaming is more likely to occur when *us* is high, and occurrence of creaming will lead to phase separation. *us* could be calculated using properties of the emulsion. Equation of *us* is shown in the following:

$$
\mu\_s = \frac{2r^2 \Delta \rho g}{9\,\mu\_\odot} \tag{1}
$$

**Figure 5.** Ni films electroplated with SCE and pressure at (a) 9.0 MPa, (b) 12.0 MPa, and (c) 18.0 MPa.

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where *r* is radius of the DP, *Δρ* is density difference between the DP and the CP, *g* is gravity, and *μc* is viscosity of the CP.

ing from 9 to 18 MPa and current density from 0.01 to 0.20 A/cm2

trodeposited if 100 % efficiency is achieved.

of the Ni films improved with increase in pressure.

*1.1.3. Effects of pressure*

340 Advanced Topics on Crystal Growth

tration of the DP in the emulsion.

of *us* is shown in the following:

and *μc* is viscosity of the CP.

30 min for all the samples. According to Faraday's Law, ca. 6 μm of Ni film would be elec‐

Physical properties of sc-CO2, such as density, surface tension and viscosity, could be direct‐ ly adjusted by pressure, which is also expected to affect physical properties of SCE. Pressure could also influence homogeneity of the emulsion, size of the DP, and population or concen‐

Circular marks/defects with diameter ranged from several to several tens of micrometers were found on surface of the Ni films electrodeposited at 9.0 MPa as shown in Fig. 5 (a) [22]. Size in diameter and total number of the circular marks decreased with increase in pressure. Overall surface uniformity of the Ni films improved significantly when pressure was rose to 18.0 MPa as shown in Fig. 5(c). Surface of the Ni films fabricated by EP-SCE was composed of nano-scaled particles as shown in Fig. 5, which was very different from electric field ori‐ ented conical-shape morphology of the Ni films electrodeposited without SCE at atmospher‐ ic pressure [23]. Average roughness (Ra)of the Ni films decreased from 19.22 to 12.10 nm as pressure was increased from 9.0 to 18.0 MPa, shown in Fig. 6. The relatively large standard deviation of Ra for the Ni films electrodeposited at 9.0 MPa was caused by the circular marks found on the surface. Standard deviation of Ra decreased significantly as surface uniformity

Chemical reaction in a reaction medium containing SCE is highly dependent on homogenei‐ ty of SCE [1]. High homogeneity of SCE is a prerequisite for fabrication of smooth film when applying SCE in electrodeposition. Homogeneity of SCE can be referred as stability, average size, and size distribution of the DP in the emulsion. SCE is a dynamically emulsified sys‐ tem; size of the DP could be continuously fluctuating in the emulsion. Stability of the DP is considered to be high when size fluctuation of the DP and tendency for phase separation to occur are both low. Quantitatively, the stability is high when interfacial tension between sc-CO2 and the aqueous solution (*γ*) is low [24]. Growth in size of the DP is more likely to occur when *γ* is high, and continue growth in size of the DP would lead to phase separation. Creaming velocity (*us*) is another parameter that could be used to quantify stability of the DP [25]. Creaming is more likely to occur when *us* is high, and occurrence of creaming will lead to phase separation. *us* could be calculated using properties of the emulsion. Equation

> <sup>2</sup> 2 9 *<sup>s</sup> C r g <sup>u</sup>* r

m

where *r* is radius of the DP, *Δρ* is density difference between the DP and the CP, *g* is gravity,

<sup>D</sup> <sup>=</sup> (1)

. The deposition time was

**Figure 5.** Ni films electroplated with SCE and pressure at (a) 9.0 MPa, (b) 12.0 MPa, and (c) 18.0 MPa.

**Figure 6.** Ra and grain size of Ni films fabricated by EP-SCE at different pressure.

Both *γ* and *us* are highly related to density of CO2 in SCE [24,25], and it has been reported that decrease in *γ* is observed when physical properties of the DP (mostly composed of CO2) is adjusted to close to physical properties of the CP. For SCE, density and viscosity of the DP are both much lower than those of the CP, and both density and viscosity of the DP can be increased by increasing pressure. In addition, increase in density of the DP can also reduce *Δρ* in equation (1), which leads to decrease in *us*. Therefore, both *γ* and *us* are lowered and the stability is improved with increase in pressure. At 320K, density of sc-CO2 increases from 320 to 770 kg/m3 and viscosity from 24 to 65 μPa·s when pressure is increased from 9.0 to 18.0 MPa [1,26].

**Figure 7.** XRD patterns of the Cu plate and Ni films fabricated by EP-SCE with different conditions.

the DP. Einstein-Stokes equation is shown in the following:

time in pulse plating is decreased [27-29].

*1.1.4. Effects of carbon dioxide volume fraction*

Decrease in size of the DP is expected to cause decrease in on-time of PPC. According to Ein‐ stein-Stokes equation, diffusion constant (*D*) of the DP is increased with a decrease in size of

where *kB* is Boltzmann's constant, *T* is the absolute temperature, *η* is viscosity of the reaction medium, and *r* is the radius of the DP. Movement of the DP in SCE is faster with increase in *D*, and frequency of a particular region to have contact with the DP would be increased by faster mobility of the DP. This would be like decreasing on-time in pulse plating. Reduction in surface roughness and grain size of Ni film electrodeposited has been reported when on-

Desorption of H2 gas bubbles from surface of cathode is also promoted with increase in pressure, because solubility of H2 in sc-CO2 is increased with increase in pressure. Combin‐ ing the improved homogeneity, the PPC, the promoted H2 gas bubbles desorption, *Ra* and grain size of the Ni film could be significantly reduced with increase in pressure for EP-SCE.

Increasing volume fraction of CO2 could increase total amount or concentration of the DP in SCE if enough surfactant is provided to maintain homogeneity of the DP. Desorption of H2

*<sup>r</sup>* <sup>=</sup> (2)

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 6 *Bk T <sup>D</sup>* ph

Major peak in XRD patterns of Ni films electrodeposited with SCE was (111) peak, which was caused by texture of the Cu plate, XRD patterns were shown in Fig. 7. Increase in pres‐ sure did not have significant influence on position of the (111) peak and relative intensity and position of both (200) and (220) peaks. Grain size calculated from Scherrer equation showed that grain refinement was observed with increase in pressure. Grain size decreased from 10.53 to 8.38 nm when pressure was increased from 9.0 to 18.0 MPa, shown in Fig. 6. Grain refinement is believed to be caused by the PPC when applying SCE in electrodeposi‐ tion reaction, where adsorption and desorption of the DP from surface of the working elec‐ trode would cause a reaction-on and –off phenomenon, respectively.

More uniform size distribution of the DP can lead to more uniform on-time and off-time of PPC. Lee *et al*. and Dhanuka *et al*. both reported that average size of the DP reduced and the size distribution became more uniform with increase in pressure through dynamic light scattering measurement and direct SEM observation [12,24]. In addition, high pressure also favors monomer salvation over aggregates, which prevents aggregation of the DP.

**Figure 7.** XRD patterns of the Cu plate and Ni films fabricated by EP-SCE with different conditions.

Decrease in size of the DP is expected to cause decrease in on-time of PPC. According to Ein‐ stein-Stokes equation, diffusion constant (*D*) of the DP is increased with a decrease in size of the DP. Einstein-Stokes equation is shown in the following:

$$D = \frac{k\_B T}{6\pi\eta r} \tag{2}$$

where *kB* is Boltzmann's constant, *T* is the absolute temperature, *η* is viscosity of the reaction medium, and *r* is the radius of the DP. Movement of the DP in SCE is faster with increase in *D*, and frequency of a particular region to have contact with the DP would be increased by faster mobility of the DP. This would be like decreasing on-time in pulse plating. Reduction in surface roughness and grain size of Ni film electrodeposited has been reported when ontime in pulse plating is decreased [27-29].

Desorption of H2 gas bubbles from surface of cathode is also promoted with increase in pressure, because solubility of H2 in sc-CO2 is increased with increase in pressure. Combin‐ ing the improved homogeneity, the PPC, the promoted H2 gas bubbles desorption, *Ra* and grain size of the Ni film could be significantly reduced with increase in pressure for EP-SCE.

#### *1.1.4. Effects of carbon dioxide volume fraction*

**Figure 6.** Ra and grain size of Ni films fabricated by EP-SCE at different pressure.

trode would cause a reaction-on and –off phenomenon, respectively.

to 18.0 MPa [1,26].

342 Advanced Topics on Crystal Growth

Both *γ* and *us* are highly related to density of CO2 in SCE [24,25], and it has been reported that decrease in *γ* is observed when physical properties of the DP (mostly composed of CO2) is adjusted to close to physical properties of the CP. For SCE, density and viscosity of the DP are both much lower than those of the CP, and both density and viscosity of the DP can be increased by increasing pressure. In addition, increase in density of the DP can also reduce *Δρ* in equation (1), which leads to decrease in *us*. Therefore, both *γ* and *us* are lowered and the stability is improved with increase in pressure. At 320K, density of sc-CO2 increases from 320 to 770 kg/m3 and viscosity from 24 to 65 μPa·s when pressure is increased from 9.0

Major peak in XRD patterns of Ni films electrodeposited with SCE was (111) peak, which was caused by texture of the Cu plate, XRD patterns were shown in Fig. 7. Increase in pres‐ sure did not have significant influence on position of the (111) peak and relative intensity and position of both (200) and (220) peaks. Grain size calculated from Scherrer equation showed that grain refinement was observed with increase in pressure. Grain size decreased from 10.53 to 8.38 nm when pressure was increased from 9.0 to 18.0 MPa, shown in Fig. 6. Grain refinement is believed to be caused by the PPC when applying SCE in electrodeposi‐ tion reaction, where adsorption and desorption of the DP from surface of the working elec‐

More uniform size distribution of the DP can lead to more uniform on-time and off-time of PPC. Lee *et al*. and Dhanuka *et al*. both reported that average size of the DP reduced and the size distribution became more uniform with increase in pressure through dynamic light scattering measurement and direct SEM observation [12,24]. In addition, high pressure also

favors monomer salvation over aggregates, which prevents aggregation of the DP.

Increasing volume fraction of CO2 could increase total amount or concentration of the DP in SCE if enough surfactant is provided to maintain homogeneity of the DP. Desorption of H2 gas bubbles from the surface of cathode is promoted when concentration of the DP is in‐ creased. Better desorption of H2 gas bubbles then leads to improvement in uniformity of the surface and decrease in Ra of the Ni films electrodeposited. Ra decreased from 30.18 to 16.79 nm with CO2 volume fraction increased from 10.0 to 50.0 vol%, shown in Fig. 8. Decrease in Ra is also contributed by decrease in on-time of the PPC, because frequency of adsorption and desorption between the DP and the working electrode is increased with increase in con‐ centration of the DP. This decrease in on-time also leads to decrease in grain size of the Ni films electrodeposited. Grain size was found to reduce from 9.41 to 8.02 nm when CO2 vol‐ ume fraction was increased from 10.0 to 50.0 vol%, shown in Fig. 8.

Ni film electrodeposited without addition of the surfactant. Ra of the Ni films decreased dra‐ matically from 89.67 to 18.93 nm as volume fraction of the surfactant increased from 0 to 0.1 vol% as shown in Fig. 9, and Ra was 16.90 and 17.04 nm when 1.0 and 2.0 vol% of the surfac‐ tant, respectively, was used. Increasing volume fraction of the surfactant is expected to cause reduction in size of the DP, because γ is reduced with increase in volume fraction of the surfactant [24], therefore, on-time of the PPC is decreased and causes grain size to de‐ crease. Grain size decreased from 15.23 to 8.81 nm for surfactant volume fraction from 0 to 2.0 vol%., shown in Fig. 9. Volume fraction of CO2 was fixed for the samples prepared with various volume fraction of the surfactant. Both Ra and grain size of the Ni films did not de‐ crease much from 1.0 to 2.0 vol% of the surfactant used. This indicates concentration of the

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**Figure 9.** Ra and grain size of Ni films fabricated by EP-SCE with different surfactant volume fraction.

Direct observation of PPC is extremely difficult because of the electrolyte is actually com‐ posed of heterogeneous emulsion and the high pressure. Indirect evidence of PPC was re‐ ported in Rahman et al.'s work [30] as shown in Fig. 10. The porous structures were expected to be caused by adsorption of the dispersed phase (DP), since electrodeposition re‐ action would be restrained in the region on the working electrode where it is covered by the DP. However, direct observation of PPC in EP-SCE is still required, but direct observation is extremely difficult because of the high pressure environment and heterogeneous emulsified electrolyte. In addition, size of the DP or the micelle is usually in the micro- or even nanoscale range [31,32]. This increases the difficulty for observation of the periodically reaction-

surfactant is close to saturation at ca. 1.0 vol%.

**1.2. Periodic-plating-characteristic**

on and –off phenomenon directly.

**Figure 8.** Ra and grain size of Ni films fabricated by EP-SCE with different CO2 volume fraction.

Defects were found on the surface for Ni films electrodeposited with CO2 volume fraction higher than 50.0 vol%. The defects were caused by adsorption of the DP on the surface of cathode when homogeneity of the emulsion is too low. Increase in size and size distribution of the DP occur when CO2 volume fraction is increased while temperature, pressure, and volume fraction of the surfactant remain fixed in the system [12]. Growth in size of the DP is an indication of poor homogeneity, and continues increasing volume fraction of CO2 in the system would eventually lead to phase separation. Some portion at the upper part of the Cu plate was found to be not electrodeposited with Ni when volume fraction of CO2 was in‐ creased to 60.0 vol%. When 60.0 vol% of CO2 was used, ca. 13% of the surface was found to be not covered by electrodeposited Ni, the value increased to 26% and 50% when CO2 vol‐ ume fraction was increased to 70.0 and 80.0 vol%, respectively.

#### *1.1.5. Effects of the surfactant volume fraction*

Usage of the surfactant in the system allows formation of the DP. Uniformity of the surface improved significantly when 0.1 vol% of the surfactant was used when comparing with the Ni film electrodeposited without addition of the surfactant. Ra of the Ni films decreased dra‐ matically from 89.67 to 18.93 nm as volume fraction of the surfactant increased from 0 to 0.1 vol% as shown in Fig. 9, and Ra was 16.90 and 17.04 nm when 1.0 and 2.0 vol% of the surfac‐ tant, respectively, was used. Increasing volume fraction of the surfactant is expected to cause reduction in size of the DP, because γ is reduced with increase in volume fraction of the surfactant [24], therefore, on-time of the PPC is decreased and causes grain size to de‐ crease. Grain size decreased from 15.23 to 8.81 nm for surfactant volume fraction from 0 to 2.0 vol%., shown in Fig. 9. Volume fraction of CO2 was fixed for the samples prepared with various volume fraction of the surfactant. Both Ra and grain size of the Ni films did not de‐ crease much from 1.0 to 2.0 vol% of the surfactant used. This indicates concentration of the surfactant is close to saturation at ca. 1.0 vol%.

**Figure 9.** Ra and grain size of Ni films fabricated by EP-SCE with different surfactant volume fraction.

#### **1.2. Periodic-plating-characteristic**

gas bubbles from the surface of cathode is promoted when concentration of the DP is in‐ creased. Better desorption of H2 gas bubbles then leads to improvement in uniformity of the surface and decrease in Ra of the Ni films electrodeposited. Ra decreased from 30.18 to 16.79 nm with CO2 volume fraction increased from 10.0 to 50.0 vol%, shown in Fig. 8. Decrease in Ra is also contributed by decrease in on-time of the PPC, because frequency of adsorption and desorption between the DP and the working electrode is increased with increase in con‐ centration of the DP. This decrease in on-time also leads to decrease in grain size of the Ni films electrodeposited. Grain size was found to reduce from 9.41 to 8.02 nm when CO2 vol‐

ume fraction was increased from 10.0 to 50.0 vol%, shown in Fig. 8.

344 Advanced Topics on Crystal Growth

**Figure 8.** Ra and grain size of Ni films fabricated by EP-SCE with different CO2 volume fraction.

ume fraction was increased to 70.0 and 80.0 vol%, respectively.

*1.1.5. Effects of the surfactant volume fraction*

Defects were found on the surface for Ni films electrodeposited with CO2 volume fraction higher than 50.0 vol%. The defects were caused by adsorption of the DP on the surface of cathode when homogeneity of the emulsion is too low. Increase in size and size distribution of the DP occur when CO2 volume fraction is increased while temperature, pressure, and volume fraction of the surfactant remain fixed in the system [12]. Growth in size of the DP is an indication of poor homogeneity, and continues increasing volume fraction of CO2 in the system would eventually lead to phase separation. Some portion at the upper part of the Cu plate was found to be not electrodeposited with Ni when volume fraction of CO2 was in‐ creased to 60.0 vol%. When 60.0 vol% of CO2 was used, ca. 13% of the surface was found to be not covered by electrodeposited Ni, the value increased to 26% and 50% when CO2 vol‐

Usage of the surfactant in the system allows formation of the DP. Uniformity of the surface improved significantly when 0.1 vol% of the surfactant was used when comparing with the Direct observation of PPC is extremely difficult because of the electrolyte is actually com‐ posed of heterogeneous emulsion and the high pressure. Indirect evidence of PPC was re‐ ported in Rahman et al.'s work [30] as shown in Fig. 10. The porous structures were expected to be caused by adsorption of the dispersed phase (DP), since electrodeposition re‐ action would be restrained in the region on the working electrode where it is covered by the DP. However, direct observation of PPC in EP-SCE is still required, but direct observation is extremely difficult because of the high pressure environment and heterogeneous emulsified electrolyte. In addition, size of the DP or the micelle is usually in the micro- or even nanoscale range [31,32]. This increases the difficulty for observation of the periodically reactionon and –off phenomenon directly.

The periodically adsorption and desorption of the DP is expected to cause a difference in the actual surface area available for the electrochemical reaction. Therefore, a fluctuation in the potential response would be expected if a constant current is applied to the system if the dif‐ ference in the actual surface area available is significant enough. Based on these assump‐ tions, an electroanalytical method, chronopotentiometry (CP), and a modified working electrode are used to directly observed the PPC.

Fluctuation in potential response was not observed for conventional electrodeposition as shown in Fig. 12(a). When EP-SCE was used, fluctuation in the potential response was ob‐ served as shown in Fig. 12(b). The fluctuation indicates the change on the surface condition of the working electrode, and this is most likely to be caused by adsorption and desorption of the DP. The surfactant used for form SCE was added to the Ni electrolyte without using the sc-CO2 to confirm the cause of the fluctuation, and the no fluctuation was observed as shown in Fig. 12(c). This result showed the fluctuation was not caused solely by the surfac‐ tant. Hence, we could confirm that the fluctuation observed in Fig. 12(b) is a direct observa‐

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**Figure 12.** Working electrode used for direct observation of PPC.(a) Conventional electrodeposition, (b) ESCE, and (c)

In fabrication of micro-structures used for Micro-Electro-Mechanical systems, electrodeposi‐ tion with a template made up of photoresist patterns on top of a conductive substrate is of‐ ten used as shown in Fig. 13. The photoresist patterns are used to confine dimensions of the structures electrodeposited on the conductive substrate. However, transport of H2 gas bub‐ bles away from the reaction site is less efficient at the bottom/holes of the photoresist pat‐ terns, and H2 gas bubbles remained at the bottom of the photoresist patterns would cause formation of defects in the micro-structures fabricated. There are several methods to elimi‐ nate the problems caused by evolution of H2. The most often applied method is reducing the evolution rate of H2 using a lower current density, but growth rate of the micro-structures is

conventional electrodeposition with addition of the surfactant. Current density applied was 0.020 A/cm2.

tion of PPC.

**1.3. Application of EP-SCE**

**Figure 10.** Porous Ni film fabricated by Ni ESCE.

In order to magnify the fluctuation in the potential response obtained from CP, small con‐ tact area between the working electrode and the electrolyte is used as shown in Fig. 11. Area of the working electrode having contact with the electrolyte would be only the tip of the Ø0.5 mm Cu wire, and the area is 0.196 mm2 .

**Figure 11.** Working electrode used for direct observation of PPC.

Fluctuation in potential response was not observed for conventional electrodeposition as shown in Fig. 12(a). When EP-SCE was used, fluctuation in the potential response was ob‐ served as shown in Fig. 12(b). The fluctuation indicates the change on the surface condition of the working electrode, and this is most likely to be caused by adsorption and desorption of the DP. The surfactant used for form SCE was added to the Ni electrolyte without using the sc-CO2 to confirm the cause of the fluctuation, and the no fluctuation was observed as shown in Fig. 12(c). This result showed the fluctuation was not caused solely by the surfac‐ tant. Hence, we could confirm that the fluctuation observed in Fig. 12(b) is a direct observa‐ tion of PPC.

**Figure 12.** Working electrode used for direct observation of PPC.(a) Conventional electrodeposition, (b) ESCE, and (c) conventional electrodeposition with addition of the surfactant. Current density applied was 0.020 A/cm2.

#### **1.3. Application of EP-SCE**

The periodically adsorption and desorption of the DP is expected to cause a difference in the actual surface area available for the electrochemical reaction. Therefore, a fluctuation in the potential response would be expected if a constant current is applied to the system if the dif‐ ference in the actual surface area available is significant enough. Based on these assump‐ tions, an electroanalytical method, chronopotentiometry (CP), and a modified working

In order to magnify the fluctuation in the potential response obtained from CP, small con‐ tact area between the working electrode and the electrolyte is used as shown in Fig. 11. Area of the working electrode having contact with the electrolyte would be only the tip of the

.

electrode are used to directly observed the PPC.

346 Advanced Topics on Crystal Growth

**Figure 10.** Porous Ni film fabricated by Ni ESCE.

Ø0.5 mm Cu wire, and the area is 0.196 mm2

**Figure 11.** Working electrode used for direct observation of PPC.

In fabrication of micro-structures used for Micro-Electro-Mechanical systems, electrodeposi‐ tion with a template made up of photoresist patterns on top of a conductive substrate is of‐ ten used as shown in Fig. 13. The photoresist patterns are used to confine dimensions of the structures electrodeposited on the conductive substrate. However, transport of H2 gas bub‐ bles away from the reaction site is less efficient at the bottom/holes of the photoresist pat‐ terns, and H2 gas bubbles remained at the bottom of the photoresist patterns would cause formation of defects in the micro-structures fabricated. There are several methods to elimi‐ nate the problems caused by evolution of H2. The most often applied method is reducing the evolution rate of H2 using a lower current density, but growth rate of the micro-structures is also decreased with low current density. Removal of the H2 gas bubbles from the reaction site is known to be improved by application of SCE. In this way, high current density and high growth rate of the micro-structures could be assured by application of EP-SCE.

**1.4. Conclusion**

**dioxide emulsion**

*2.1.1. Introduction*

Physical properties of SCE are found to be affected by pressure, volume fraction of CO2 and the surfactant. Improvement in homogeneity of SCE is achieved by increasing pres‐ sure. Therefore, size and size distribution of the DP are reduced as shown in Fig. 15(a) and (c). Since volume fraction of CO2 in the system is fixed, therefore, reduction in the size would lead to increase in concentration of the DP, and this is expected to cause de‐ crease in on-time of PPC and decrease in *Ra* and grain size. Increase in volume fraction of CO2 leads to increase in concentration of the DP, and the size and size distribution would remain small if enough surfactant is present in the system, shown in Fig. 15(b) to (c). Growth in size of the DP would occur if insufficient amount of the surfactant is present to stabilize CO2 introduced into the system, and continue growth in size of the DP would lead to phase separation as shown in Fig. 15(d). Phase separation could be prevented by increasing pressure or volume fraction of the surfactant, which lowers γ and *us*. Improvement in stability and reduction in size and size distribution of the DP are achieved with increase in volume fraction of the surfactant as shown in Fig. 15(e) and (c). This is why both Ra and grain size is lowered with increase in volume fraction of the surfactant used. We also studied and proposed a mechanism called periodic-plat‐ ing-characteristic (PPC) to be the main cause for the effects observed in the metal films fabricated by EP-SCE. Moreover, we directly observed PPC by an electroanalytical meth‐ od, chronopotentiometry (CP), and a modified working electrode. On these experimental

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results, we applied EP-SCE on fabrication of microstructure and succeeded.

**2.1. Electroless plating in a supercritical CO2 emulsion (ELP-SCE)**

**2. Crystal growth by electroless Ni-P plating using supercritical carbon**

Electroless plating (ELP) has a low processing temperature, a high metal-ion transportation density, the ability to deposit on electrically nonconductive materials, a more uniform thick‐ ness for products of any shape, and a simple deposition mechanism.[33,34] A wet process such as ELP can serve as a superior alternative for a three-dimensional (3D) integration tech‐ nology. The disadvantages of ELP are the high viscosity of the solution and anomalous growths in the plating film caused by the pretreatment condition. These disadvantages have

The key requirement for forming a uniform, conformal thin film over a complex 3D micro/ nanostructure will be to improve the transport properties. To this end, we propose a techni‐ que based on the criteria mentioned above. Specifically, we proposed an ELP method using dense carbon dioxide (CO2) beyond the critical point as a solvent. The changeable density of dense CO2 enables excellent control of the intermolecular interactions, and the high density

interfered with the formation of microstructures for electronic devices and MEMS.

**Figure 13.** Procedures for fabrication micro-structures.

Ni micro-structures fabrication by conventional electrodeposition with a current density are defective, shown in Fig. 14(a) and (b). These defects were caused by H2 gas bubbles remained inside the photoresist patterns. On the other hand, when EP-SCE was applied, defect-free Ni micro-structures could be obtained even when a high current density was used, shown in Fig. 14(c), and the average growth rate of the Ni micro-structures was about 5.1 μm/min.

**Figure 14.** Ni micro-structures fabricated by (a) conventional electrodepositon at 0.100 A/cm2 for 3 min, (b) conven‐ tional electrodepositon and (c) ESCE at 0.100 A/cm2 for 5 min.

### **1.4. Conclusion**

also decreased with low current density. Removal of the H2 gas bubbles from the reaction site is known to be improved by application of SCE. In this way, high current density and

Ni micro-structures fabrication by conventional electrodeposition with a current density are defective, shown in Fig. 14(a) and (b). These defects were caused by H2 gas bubbles remained inside the photoresist patterns. On the other hand, when EP-SCE was applied, defect-free Ni micro-structures could be obtained even when a high current density was used, shown in Fig. 14(c), and the average growth rate of the Ni micro-structures was

**Figure 14.** Ni micro-structures fabricated by (a) conventional electrodepositon at 0.100 A/cm2 for 3 min, (b) conven‐

high growth rate of the micro-structures could be assured by application of EP-SCE.

**Figure 13.** Procedures for fabrication micro-structures.

tional electrodepositon and (c) ESCE at 0.100 A/cm2 for 5 min.

about 5.1 μm/min.

348 Advanced Topics on Crystal Growth

Physical properties of SCE are found to be affected by pressure, volume fraction of CO2 and the surfactant. Improvement in homogeneity of SCE is achieved by increasing pres‐ sure. Therefore, size and size distribution of the DP are reduced as shown in Fig. 15(a) and (c). Since volume fraction of CO2 in the system is fixed, therefore, reduction in the size would lead to increase in concentration of the DP, and this is expected to cause de‐ crease in on-time of PPC and decrease in *Ra* and grain size. Increase in volume fraction of CO2 leads to increase in concentration of the DP, and the size and size distribution would remain small if enough surfactant is present in the system, shown in Fig. 15(b) to (c). Growth in size of the DP would occur if insufficient amount of the surfactant is present to stabilize CO2 introduced into the system, and continue growth in size of the DP would lead to phase separation as shown in Fig. 15(d). Phase separation could be prevented by increasing pressure or volume fraction of the surfactant, which lowers γ and *us*. Improvement in stability and reduction in size and size distribution of the DP are achieved with increase in volume fraction of the surfactant as shown in Fig. 15(e) and (c). This is why both Ra and grain size is lowered with increase in volume fraction of the surfactant used. We also studied and proposed a mechanism called periodic-plat‐ ing-characteristic (PPC) to be the main cause for the effects observed in the metal films fabricated by EP-SCE. Moreover, we directly observed PPC by an electroanalytical meth‐ od, chronopotentiometry (CP), and a modified working electrode. On these experimental results, we applied EP-SCE on fabrication of microstructure and succeeded.

### **2. Crystal growth by electroless Ni-P plating using supercritical carbon dioxide emulsion**

### **2.1. Electroless plating in a supercritical CO2 emulsion (ELP-SCE)**

#### *2.1.1. Introduction*

Electroless plating (ELP) has a low processing temperature, a high metal-ion transportation density, the ability to deposit on electrically nonconductive materials, a more uniform thick‐ ness for products of any shape, and a simple deposition mechanism.[33,34] A wet process such as ELP can serve as a superior alternative for a three-dimensional (3D) integration tech‐ nology. The disadvantages of ELP are the high viscosity of the solution and anomalous growths in the plating film caused by the pretreatment condition. These disadvantages have interfered with the formation of microstructures for electronic devices and MEMS.

The key requirement for forming a uniform, conformal thin film over a complex 3D micro/ nanostructure will be to improve the transport properties. To this end, we propose a techni‐ que based on the criteria mentioned above. Specifically, we proposed an ELP method using dense carbon dioxide (CO2) beyond the critical point as a solvent. The changeable density of dense CO2 enables excellent control of the intermolecular interactions, and the high density and diffusivity of the material assure that the plating films can form over nanoscale areas with outstanding reliability. Supercritical carbon dioxide (sc–CO2), however, has been found to be unsuitable as a medium for plating reactions. Metal salts are generally soluble in wa‐ ter, but water and CO2 tend to mix poorly. The problem can be solved by emulsifying sc– CO2 and a plating solution, then adding a nonionic surfactant.[13] The ELP technique we propose in this study uses a dense CO2 beyond the critical point. The ELP reaction in Elec‐ troless Plating in a Supercritical CO2 Emulsion (ELP-SCE) [35] takes place in an emulsion containing dense CO2. We discuss the surface morphology of the film plated by ELP-SCE, which turns out to have various advantages over the surface morphology of film plated by conventional ELP.

**Table 2.** Coating Obtained by Electroless Plating

*2.1.2.1. Materials*

*2.1.2. Electroless plating reaction and experimental method*

The ELP methods currently known can be used to deposit 12 different metals (Table 2). [34] Decomposition products (phosphorous and boron) in the reducing agent precipitate as the metal deposits, leaving films of the respective alloys. Two or more metals can be deposited at once without much difficulty. ELP methods are used for the deposition of more than 50 alloys of different qualitative compositions, mostly based on nickel, cobalt, and copper. For this study we selected an electroless nickel-phosphorus (Ni-P) plating process. The most widespread type of Ni-P plating uses hypophosphite as the reducing agent.[33,34]4

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major properties of electroless Ni-P, namely, its hardness, wear resistance, and corrosion re‐ sistance, have led to its technical application in many industries, from electronics, automak‐ ing, aerospace, and machinery to oil and gas production, power generation, printing, and textiles. The process is generally applied with a stable, acidic Ni-P plating solution, as con‐

CO2 with a minimum purity of 99.99% was purchased from Nippon Tansan Co., Ltd. The experiments were performed with a nonionic surfactant polyoxyethylene lauryl ether (C12H25(OCH2CH2)15OH) supplied by Toshin Yuka Kogyo. The electroless Ni–P plating solu‐ tion had a chemical composition of nickel chloride (9%), sodium hypophosphite (12%), com‐ plexing agent (12%), and ion-exchanged water (67%) (Okuno Chemical Industries Co., Ltd.). The substrate was a 99.99% pure film of copper measuring 10×20 mm (Mitsubishi Shindoh Co., Ltd). The substrate was washed with acetone and rinsed in deionized water before each reaction. The grease was removed from the sample by successive dipping in a 10 wt % solu‐ tion of NaOH and a 10 wt % solution HCl followed by rinsing in deionized water. The sam‐ ple was added to an activator solution consisting of hydrogen chloride (18%), palladium chloride (0.04%), and ion-exchanged water (81.96%) (Okuno Chemical Industries Co., Ltd.)

tact with CO2 acidifies water due to the formation and dissociation of carbonic acid.

Three

**Figure 15.** Illustration of conditions of the DP in SCE: (a) at relatively low pressure, (b) at relatively low CO2 volume fraction, (c) at relatively high pressure, medium CO2 volume fraction, and high surfactant volume fraction, (d) at rela‐ tively high CO2 volume fraction, and (e) at relatively low surfactant volume fraction. .


#### **Table 2.** Coating Obtained by Electroless Plating

#### *2.1.2. Electroless plating reaction and experimental method*

The ELP methods currently known can be used to deposit 12 different metals (Table 2). [34] Decomposition products (phosphorous and boron) in the reducing agent precipitate as the metal deposits, leaving films of the respective alloys. Two or more metals can be deposited at once without much difficulty. ELP methods are used for the deposition of more than 50 alloys of different qualitative compositions, mostly based on nickel, cobalt, and copper. For this study we selected an electroless nickel-phosphorus (Ni-P) plating process. The most widespread type of Ni-P plating uses hypophosphite as the reducing agent.[33,34]4 Three major properties of electroless Ni-P, namely, its hardness, wear resistance, and corrosion re‐ sistance, have led to its technical application in many industries, from electronics, automak‐ ing, aerospace, and machinery to oil and gas production, power generation, printing, and textiles. The process is generally applied with a stable, acidic Ni-P plating solution, as con‐ tact with CO2 acidifies water due to the formation and dissociation of carbonic acid.

#### *2.1.2.1. Materials*

and diffusivity of the material assure that the plating films can form over nanoscale areas with outstanding reliability. Supercritical carbon dioxide (sc–CO2), however, has been found to be unsuitable as a medium for plating reactions. Metal salts are generally soluble in wa‐ ter, but water and CO2 tend to mix poorly. The problem can be solved by emulsifying sc– CO2 and a plating solution, then adding a nonionic surfactant.[13] The ELP technique we propose in this study uses a dense CO2 beyond the critical point. The ELP reaction in Elec‐ troless Plating in a Supercritical CO2 Emulsion (ELP-SCE) [35] takes place in an emulsion containing dense CO2. We discuss the surface morphology of the film plated by ELP-SCE, which turns out to have various advantages over the surface morphology of film plated by

**Figure 15.** Illustration of conditions of the DP in SCE: (a) at relatively low pressure, (b) at relatively low CO2 volume fraction, (c) at relatively high pressure, medium CO2 volume fraction, and high surfactant volume fraction, (d) at rela‐

tively high CO2 volume fraction, and (e) at relatively low surfactant volume fraction. .

conventional ELP.

350 Advanced Topics on Crystal Growth

CO2 with a minimum purity of 99.99% was purchased from Nippon Tansan Co., Ltd. The experiments were performed with a nonionic surfactant polyoxyethylene lauryl ether (C12H25(OCH2CH2)15OH) supplied by Toshin Yuka Kogyo. The electroless Ni–P plating solu‐ tion had a chemical composition of nickel chloride (9%), sodium hypophosphite (12%), com‐ plexing agent (12%), and ion-exchanged water (67%) (Okuno Chemical Industries Co., Ltd.). The substrate was a 99.99% pure film of copper measuring 10×20 mm (Mitsubishi Shindoh Co., Ltd). The substrate was washed with acetone and rinsed in deionized water before each reaction. The grease was removed from the sample by successive dipping in a 10 wt % solu‐ tion of NaOH and a 10 wt % solution HCl followed by rinsing in deionized water. The sam‐ ple was added to an activator solution consisting of hydrogen chloride (18%), palladium chloride (0.04%), and ion-exchanged water (81.96%) (Okuno Chemical Industries Co., Ltd.) at 303 K, then rinsed in deionized water. This pretreatment was applied to all copper sub‐ strates regardless of the condition of the ELP.

agitating ternary system stirred at a speed of 500 rpm. The reaction commenced from the start of agitation. In the Results and Discussion of this chapter we describe the properties of the plated film fabricated by ELP-SCE in comparison with those of film fabricated by con‐ ventional ELP. The conventional method was performed under the following experimental conditions: 353 K, atmospheric pressure, a plating solution with a chemical composition

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An optical microscope (Digital Microscope VHX-500, Keyence. Co., Ltd.) and scanning elec‐ tron microscope (FE-SEM, S-4500, Hitachi High-Technologies Co., Ltd.) were used to study the surfaces of the plated Ni–P films. A surface texture measuring instrument (Surfcom 480A, Tokyo Seimitsu Co., Ltd.) with a diamond-tipped detector (2 *μ*m tip radius) was used to measure the average surface roughness (*Ra*) to a minimum height resolution of 1 nm (height measurement range: 80 *μ*m). The average *Ra* was calculated from measurements at five points or more. The film thickness was measured directly from a cross-sectional scan‐ ning electron microscopy (SEM) image of a plated Ni–P film fabricated by a focused ion beam system (FB-2100, Hitachi High-Technologies Co., Ltd.). The phosphorus composition of the fabricated film was measured by an FE-SEM (S-4300SE, Hitachi High-Technologies Co. Ltd.) equipped for energy-dispersive X-ray spectroscopy (EDX). An accelerating voltage of 20 kV with a collecting time of more than 300 s was applied. X-ray diffraction (XRD) anal‐ ysis (2θ-ω scans) was performed at room temperature (RT, 298 K) using a PANalytical X'pert Pro Galaxy system equipped with an X'celerator module. The X-ray source was

identical that used in ELP-SCE, constant stirring at a speed of 50 rpm.

CuKα, and the tube voltage and current were 45 kV and 40 mA, respectively.

Fig. 17 shows optical microscope images of a copper substrate and Ni-P films plated over the substrate by ELP-SCE at 15 MPa and 6 MPa. Polishing trace was observable on the cop‐ per substrate before pretreatment. The film plated at 15 MPa was uniformly bright and cov‐ ered both the front and back of the substrate (Fig. 17 (b)). The uniform brightness and coverage were attributable to the exact position of the substrate and the uniformity of the emulsion. The formation of emulsion was unstable in the film plated at 6 MPa, beyond the critical point, so the film was thin and even unformed in portions (Fig. 17 (c)). The surface

If CO2 exceeds a critical point, the density will rise rapidly. While the CO2 density at 323 K and 6 MPa is only 0.1 kg L-1, it reaches 0.7 kg L-1 at the higher pressure of 15 MPa.8 More‐ over, Sone et al. reported that in a ternary system of water, surfactant, and CO2, the CO2 formation in the water emulsion by the surfactant is affected not only by the temperature and pressure, but also agitation.4 They also found, in a similar experiment, that no emulsion was formed below the critical point. With ELP-SCE, a stable emulsion forms when the CO2

roughness (*Ra*) of the plating films was 0.030 μm at 15 MPa and 0.059 μm at 6 MPa.

density approaches the liquid phase, and a uniform plating film can be deposited.

*2.1.3. Electroless Ni-P plating in an emulsion of supercritical CO2*

*2.1.2.3. Analysis*

**Figure 16.** Experimental apparatus used for batch reaction in our electroless plating experiments beyond the critical point of CO2; (a) CO2 cylinder; (b) cooler and high pressure pump; (c) temperature controlled air bath; (d) reactor with magnetic stirrer; and (e) trap; BPR: back-pressure regulator; PI: pressure indicator; TI: temperature indicator; V: valves.

#### *2.1.2.2. Experimental apparatus and procedure*

Fig. 16 shows the high-pressure experimental apparatus (Japan Spectra Company) used for the ELP.[35] The temperature variation of each run was confirmed to be less than 1.0 K. The maximum working temperature and the maximum pressure were 424 K and 50 MPa, re‐ spectively. The reactor was a stainless steel 316 vessel with an internal volume of 50 mL, kept in a temperature-controlled air bath. A magnetic agitator with a cross-magnetic stirrer bar was placed within the reactor and the activated substrate was attached to the reactor with stainless wires. A plating reaction within a reactor starts only upon contact between the substrate and plating solution. As such, the substrate position in a reactor affects both the reproducibility and surface morphology of the ELP film. Yan et al. demonstrated the disper‐ sion behaviors of a ternary system composed of dense CO2, an electroplating (EP) solution, and a surfactant.[14] Their experiment was performed in a high-pressure view cell with an internal volume of 45 mL. In the absence of stirring, two separated phases, namely, a trans‐ parent upper CO2 phase and a clear green lower phase (the nickel EP solution) were ob‐ served at 323 K and 10 MPa. The ternary system with the CO2 volume fraction of 0.2 was stirred at 400 rpm. The CO2 dispersed into the plating solution with stirring, and the light scattering from the small CO2 drops in the solution increased the turbidity of the system. In our experiment, before the ELP reaction, the electroless Ni–P plating solution (30 mL) and surfactant (surfactant concentration: 1.0 wt % to the ELP solution) were placed in the reactor at atmospheric pressure. Next, liquid CO2 was pumped into the cell by a high performance liquid chromatography (HPLC) pump until a predetermined pressure was reached. The ELP reaction was performed at a temperature of 353 K and a pressure of 15 MPa in a constantly agitating ternary system stirred at a speed of 500 rpm. The reaction commenced from the start of agitation. In the Results and Discussion of this chapter we describe the properties of the plated film fabricated by ELP-SCE in comparison with those of film fabricated by con‐ ventional ELP. The conventional method was performed under the following experimental conditions: 353 K, atmospheric pressure, a plating solution with a chemical composition identical that used in ELP-SCE, constant stirring at a speed of 50 rpm.

### *2.1.2.3. Analysis*

at 303 K, then rinsed in deionized water. This pretreatment was applied to all copper sub‐

**BPR**

**PI**

**TI**

**Figure 16.** Experimental apparatus used for batch reaction in our electroless plating experiments beyond the critical point of CO2; (a) CO2 cylinder; (b) cooler and high pressure pump; (c) temperature controlled air bath; (d) reactor with magnetic stirrer; and (e) trap; BPR: back-pressure regulator; PI: pressure indicator; TI: temperature indicator; V: valves.

Fig. 16 shows the high-pressure experimental apparatus (Japan Spectra Company) used for the ELP.[35] The temperature variation of each run was confirmed to be less than 1.0 K. The maximum working temperature and the maximum pressure were 424 K and 50 MPa, re‐ spectively. The reactor was a stainless steel 316 vessel with an internal volume of 50 mL, kept in a temperature-controlled air bath. A magnetic agitator with a cross-magnetic stirrer bar was placed within the reactor and the activated substrate was attached to the reactor with stainless wires. A plating reaction within a reactor starts only upon contact between the substrate and plating solution. As such, the substrate position in a reactor affects both the reproducibility and surface morphology of the ELP film. Yan et al. demonstrated the disper‐ sion behaviors of a ternary system composed of dense CO2, an electroplating (EP) solution, and a surfactant.[14] Their experiment was performed in a high-pressure view cell with an internal volume of 45 mL. In the absence of stirring, two separated phases, namely, a trans‐ parent upper CO2 phase and a clear green lower phase (the nickel EP solution) were ob‐ served at 323 K and 10 MPa. The ternary system with the CO2 volume fraction of 0.2 was stirred at 400 rpm. The CO2 dispersed into the plating solution with stirring, and the light scattering from the small CO2 drops in the solution increased the turbidity of the system. In our experiment, before the ELP reaction, the electroless Ni–P plating solution (30 mL) and surfactant (surfactant concentration: 1.0 wt % to the ELP solution) were placed in the reactor at atmospheric pressure. Next, liquid CO2 was pumped into the cell by a high performance liquid chromatography (HPLC) pump until a predetermined pressure was reached. The ELP reaction was performed at a temperature of 353 K and a pressure of 15 MPa in a constantly

**d**

**c e**

strates regardless of the condition of the ELP.

352 Advanced Topics on Crystal Growth

**V V**

**b**

**a**

*2.1.2.2. Experimental apparatus and procedure*

An optical microscope (Digital Microscope VHX-500, Keyence. Co., Ltd.) and scanning elec‐ tron microscope (FE-SEM, S-4500, Hitachi High-Technologies Co., Ltd.) were used to study the surfaces of the plated Ni–P films. A surface texture measuring instrument (Surfcom 480A, Tokyo Seimitsu Co., Ltd.) with a diamond-tipped detector (2 *μ*m tip radius) was used to measure the average surface roughness (*Ra*) to a minimum height resolution of 1 nm (height measurement range: 80 *μ*m). The average *Ra* was calculated from measurements at five points or more. The film thickness was measured directly from a cross-sectional scan‐ ning electron microscopy (SEM) image of a plated Ni–P film fabricated by a focused ion beam system (FB-2100, Hitachi High-Technologies Co., Ltd.). The phosphorus composition of the fabricated film was measured by an FE-SEM (S-4300SE, Hitachi High-Technologies Co. Ltd.) equipped for energy-dispersive X-ray spectroscopy (EDX). An accelerating voltage of 20 kV with a collecting time of more than 300 s was applied. X-ray diffraction (XRD) anal‐ ysis (2θ-ω scans) was performed at room temperature (RT, 298 K) using a PANalytical X'pert Pro Galaxy system equipped with an X'celerator module. The X-ray source was CuKα, and the tube voltage and current were 45 kV and 40 mA, respectively.

### *2.1.3. Electroless Ni-P plating in an emulsion of supercritical CO2*

Fig. 17 shows optical microscope images of a copper substrate and Ni-P films plated over the substrate by ELP-SCE at 15 MPa and 6 MPa. Polishing trace was observable on the cop‐ per substrate before pretreatment. The film plated at 15 MPa was uniformly bright and cov‐ ered both the front and back of the substrate (Fig. 17 (b)). The uniform brightness and coverage were attributable to the exact position of the substrate and the uniformity of the emulsion. The formation of emulsion was unstable in the film plated at 6 MPa, beyond the critical point, so the film was thin and even unformed in portions (Fig. 17 (c)). The surface roughness (*Ra*) of the plating films was 0.030 μm at 15 MPa and 0.059 μm at 6 MPa.

If CO2 exceeds a critical point, the density will rise rapidly. While the CO2 density at 323 K and 6 MPa is only 0.1 kg L-1, it reaches 0.7 kg L-1 at the higher pressure of 15 MPa.8 More‐ over, Sone et al. reported that in a ternary system of water, surfactant, and CO2, the CO2 formation in the water emulsion by the surfactant is affected not only by the temperature and pressure, but also agitation.4 They also found, in a similar experiment, that no emulsion was formed below the critical point. With ELP-SCE, a stable emulsion forms when the CO2 density approaches the liquid phase, and a uniform plating film can be deposited.

According to an as-deposited coating surface composition analysis by EDX, the Ni-P film formed by ELP-SCE was composed of 20 wt % phosphorus. When the phosphorus content increases, the microstructure of an electroless Ni-P deposit changes from a mixture of amor‐ phous and nanocrystalline phases to a fully amorphous phase.[36] The structure of our Ni-P film was confirmed to be amorphous by XRD. Amorphous profiles with a wide angular range of 40-45° (2θ) appear nearby a 2θ position corresponding to the Ni {111} plane. The copper substrate underneath was responsible for the Cu diffraction peaks in the profiles of the Ni-P film formed by ELP-SCE. The peaks appear because the coated-deposit was too

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**30 35 40 45 50 55 60**

**Cu {200}**

**Cu {111}**

**Angle of 2θ (degree)**

We used an optical microscope to observe the surface features of a Ni–P film formed by con‐ ventional ELP, another Ni–P film formed by ELP-SCE, and a pure copper substrate (Fig. 19). The only clear difference in defects between the two Ni–P films was a polishing trace on the copper substrate. As is widely known, thin films fabricated by ELP have smaller pinholes

**Figure 18.** XRD spectrum of Ni-P film plated from ELP-SCE at 353K and 15 MPa for 180 min.

*2.1.4. Surface morphology of Ni-P film by conventional ELP and ELP-SCE*

thin (1.0 μm less) to totally absorb the penetration of the X-ray beam (Fig. 18).

**Intensity (a.u.)**

and cracks than those fabricated by EP.

**Figure 17.** Optical microscopy images of (a) pure Cu substrate and Ni–P films plated from ELP-SCE at 353 K for 60 min (b) at 15 MPa and (c) at 6 MPa.

According to an as-deposited coating surface composition analysis by EDX, the Ni-P film formed by ELP-SCE was composed of 20 wt % phosphorus. When the phosphorus content increases, the microstructure of an electroless Ni-P deposit changes from a mixture of amor‐ phous and nanocrystalline phases to a fully amorphous phase.[36] The structure of our Ni-P film was confirmed to be amorphous by XRD. Amorphous profiles with a wide angular range of 40-45° (2θ) appear nearby a 2θ position corresponding to the Ni {111} plane. The copper substrate underneath was responsible for the Cu diffraction peaks in the profiles of the Ni-P film formed by ELP-SCE. The peaks appear because the coated-deposit was too thin (1.0 μm less) to totally absorb the penetration of the X-ray beam (Fig. 18).

**Figure 18.** XRD spectrum of Ni-P film plated from ELP-SCE at 353K and 15 MPa for 180 min.

#### *2.1.4. Surface morphology of Ni-P film by conventional ELP and ELP-SCE*

**Figure 17.** Optical microscopy images of (a) pure Cu substrate and Ni–P films plated from ELP-SCE at 353 K for 60 min

(b) at 15 MPa and (c) at 6 MPa.

354 Advanced Topics on Crystal Growth

We used an optical microscope to observe the surface features of a Ni–P film formed by con‐ ventional ELP, another Ni–P film formed by ELP-SCE, and a pure copper substrate (Fig. 19). The only clear difference in defects between the two Ni–P films was a polishing trace on the copper substrate. As is widely known, thin films fabricated by ELP have smaller pinholes and cracks than those fabricated by EP.

tional ELP. The only particles observed were extremely fine, with diameters of several tens

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**(a)**

**(b)**

**(c)**

353 K and atmospheric pressure for 2 min, and (c) substrate activated by catalytic Pd.

× 30000 1. 00μm

**Figure 20.** SEM images of Ni–P films plated from (a) ELP-SCE at 353 K and 15 MPa for 180 min, (b) conventional ELP at

Fig. 21 shows how variations in the reaction time influence the surface features of the Ni–P films formed by conventional ELP and by ELP-SCE. The plating films formed by conven‐ tional plating for 5.0 and 30 min had thicknesses of 0.8 and 4.9 μm, respectively. The plating films formed by ELP-SCE for 360 and 540 min had thicknesses of 0.9 and 1.0 μm, respective‐ ly. The SEM images reveal nodules on all of the surfaces of the Ni–P films fabricated by con‐ ventional ELP (Fig. 20 (b), 21 (c), and 21 (d)). The images also show that the nodules

× 30000 1. 00μm

× 30000 1. 00μm

of nm or less (Fig. 20 (a)).

**Figure 19.** Optical microscopy images of Ni–P films plated from (a) ELP-SCE at 353 K and 15 MPa for 180 min, (b) con‐ ventional ELP at 353 K and atmospheric pressure for 2 min, and (c) pure copper substrate.

SEM observations reveal clear differences between the surface morphologies of the Ni–P films and the substrate activated by catalytic Pd, as shown in Fig. 20. The thickness of the Ni–P film fabricated by conventional ELP was 0.3 μm, while that fabricated by ELP-SCE was 0.8 μm. Nodules were observed on the Ni–P film fabricated by conventional ELP in the early stage of the reaction, as shown in Fig. 20 (b). This nodule formation was the result of concen‐ trated nickel reactions over a localized area on Pd nucleus on the surface of the substrate as shown in Fig. 20(c). Earlier reports also have confirmed that the nickel nucleus appears and grows on the Pd nucleus on the surface of the substrate.[37-39] These nodules become a seri‐ ous problem when they form in films fabricated by ELP on fine electronic devices and MEMS. Meanwhile, the thin film fabricated by ELP-SCE was free from nodules and pin‐ holes, but its thickness was still more than double that of the thin film fabricated by conven‐ tional ELP. The only particles observed were extremely fine, with diameters of several tens of nm or less (Fig. 20 (a)).

**Figure 20.** SEM images of Ni–P films plated from (a) ELP-SCE at 353 K and 15 MPa for 180 min, (b) conventional ELP at 353 K and atmospheric pressure for 2 min, and (c) substrate activated by catalytic Pd.

**Figure 19.** Optical microscopy images of Ni–P films plated from (a) ELP-SCE at 353 K and 15 MPa for 180 min, (b) con‐

SEM observations reveal clear differences between the surface morphologies of the Ni–P films and the substrate activated by catalytic Pd, as shown in Fig. 20. The thickness of the Ni–P film fabricated by conventional ELP was 0.3 μm, while that fabricated by ELP-SCE was 0.8 μm. Nodules were observed on the Ni–P film fabricated by conventional ELP in the early stage of the reaction, as shown in Fig. 20 (b). This nodule formation was the result of concen‐ trated nickel reactions over a localized area on Pd nucleus on the surface of the substrate as shown in Fig. 20(c). Earlier reports also have confirmed that the nickel nucleus appears and grows on the Pd nucleus on the surface of the substrate.[37-39] These nodules become a seri‐ ous problem when they form in films fabricated by ELP on fine electronic devices and MEMS. Meanwhile, the thin film fabricated by ELP-SCE was free from nodules and pin‐ holes, but its thickness was still more than double that of the thin film fabricated by conven‐

ventional ELP at 353 K and atmospheric pressure for 2 min, and (c) pure copper substrate.

356 Advanced Topics on Crystal Growth

Fig. 21 shows how variations in the reaction time influence the surface features of the Ni–P films formed by conventional ELP and by ELP-SCE. The plating films formed by conven‐ tional plating for 5.0 and 30 min had thicknesses of 0.8 and 4.9 μm, respectively. The plating films formed by ELP-SCE for 360 and 540 min had thicknesses of 0.9 and 1.0 μm, respective‐ ly. The SEM images reveal nodules on all of the surfaces of the Ni–P films fabricated by con‐ ventional ELP (Fig. 20 (b), 21 (c), and 21 (d)). The images also show that the nodules increased, both in size (from several hundred nm to over several μm) and in number, as the reaction time increased.

under the same activation processing conditions used for conventional ELP. We also found that the film thickness conferred a strong influence on the surface roughness. The thin film formed by ELP-SCE was very smooth, though it was still more than twice as thick as the film fabricated by conventional ELP. These results demonstrate that ELP-SCE suppressed the deposition reaction of the locally concentrated nickel. We can also

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**Figure 22.** Roughness curves of the surfaces of Ni–P films plated by (a) ELP-SCE at 353 K and 15 MPa for 180 min, (b)

The conventional ELP as performed at 353 K and atmospheric pressure. ELP-SCE was per‐ formed at 353 K and 15 MPa. Fig. 23 shows the relationship between the surface roughness and reaction time. The surface of the Ni–P film fabricated by the conventional ELP rough‐ ened as the reaction time increased. The roughness of the Ni–P film formed by ELP-SCE, meanwhile, showed no dependence on the reaction time. Nodules appeared on the surfaces of the Ni–P films fabricated by conventional ELP at all reaction times, and the nodules grew as the reaction times increased. No nodules were observed on the surfaces of any of the Ni– P films fabricated by ELP-SCE, even at the maximum reaction times. The aforementioned re‐ sults confirm that the excellent smoothness of ELP-SCE film mitigated the influence of the substrate pretreatment and was independent of the reaction time. Fig. 20, meanwhile, shows

ELP-SCE produced a thin film with high smoothness and outstanding uniformity. The Ni–P film fabricated by conventional ELP with a reaction time of 5 min had a thickness of 0.8 μm, or about the same thickness as ELP-SCe film fabricated with a reaction time of 180 min. The *Ra* of the ELP-SCE film was 0.03 μm, while that of the conventional ELP film was 0.06 μm.

ELP films are generally smoother than EP films and have fewer defects.[40] Even with ELP, however, defects such as microscopic nodules, pits, and pinholes are difficult to suppress. [41-43] Although suppression of a through-hole like a pinhole need a thick film, more nod‐ ules form in a thicker film. Nodules also easily form when the underlayer has projecting parts, foreign objects, and nuclear growth sites. Conventional suppression of nodule method

conventional ELP at 353 K and atmospheric pressure for 2 min, and (c) substrate activated by catalytic Pd.

a suppression of the growth of the nodules generated by conventional ELP.

see, in Fig. 20 and Fig. 21, that no nodules were formed.

**Figure 21.** SEM images of Ni–P films plated from ELP-SCE at 353 K and 15 MPa for reaction times of (a) 360 min and (b) 540 min, and conventional ELP at 353 K and atmospheric pressure for (c) 5 min and (d) 30 min.

Meanwhile, the Ni–P film fabricated by ELP-SCE was free of nodules (Fig. 20 (a)) and had an extremely uniform surface (Figs. 21 (a) and (b)). These results differed considerably from the changes in the surface features of conventional ELP films brought about by adjustments in the reaction time and the processing methods for the substrate activation by Pd. [37-39] We also found that our ELP technique could fabricate superb, highly uniform plated films even when the substrate pretreatment, reaction temperature, and chemical composition of the electroless Ni–P plating solution were all identical to those use d in conventional ELP. On this basis, we surmise that our ELP technique may be effective in suppressing the growth of nodules.

Fig. 22 shows the roughness curves on the surfaces of the Ni–P films formed by ELP-SCE, by conventional ELP, and by surface activation of the substrate. The evaluation length of the surface roughness measurement was 1.250 mm. The activated substrate had an *Ra* of 0.040 μm. The conventional ELP had an *Ra* of 0.048 μm and a rougher surface than the activated substrate. Previous reports have shown how activation processing changes the surface morphologies and deposition behaviors of electroless Ni–P films. [37-39] Meanwhile, ELP-SCE formed a film with improved smoothness (*Ra* of 0.030 μm) under the same activation processing conditions used for conventional ELP. We also found that the film thickness conferred a strong influence on the surface roughness. The thin film formed by ELP-SCE was very smooth, though it was still more than twice as thick as the film fabricated by conventional ELP. These results demonstrate that ELP-SCE suppressed the deposition reaction of the locally concentrated nickel. We can also see, in Fig. 20 and Fig. 21, that no nodules were formed.

increased, both in size (from several hundred nm to over several μm) and in number, as the

**Figure 21.** SEM images of Ni–P films plated from ELP-SCE at 353 K and 15 MPa for reaction times of (a) 360 min and

Meanwhile, the Ni–P film fabricated by ELP-SCE was free of nodules (Fig. 20 (a)) and had an extremely uniform surface (Figs. 21 (a) and (b)). These results differed considerably from the changes in the surface features of conventional ELP films brought about by adjustments in the reaction time and the processing methods for the substrate activation by Pd. [37-39] We also found that our ELP technique could fabricate superb, highly uniform plated films even when the substrate pretreatment, reaction temperature, and chemical composition of the electroless Ni–P plating solution were all identical to those use d in conventional ELP. On this basis, we surmise that our ELP technique may be effective in suppressing the

Fig. 22 shows the roughness curves on the surfaces of the Ni–P films formed by ELP-SCE, by conventional ELP, and by surface activation of the substrate. The evaluation length of the surface roughness measurement was 1.250 mm. The activated substrate had an *Ra* of 0.040 μm. The conventional ELP had an *Ra* of 0.048 μm and a rougher surface than the activated substrate. Previous reports have shown how activation processing changes the surface morphologies and deposition behaviors of electroless Ni–P films. [37-39] Meanwhile, ELP-SCE formed a film with improved smoothness (*Ra* of 0.030 μm)

(b) 540 min, and conventional ELP at 353 K and atmospheric pressure for (c) 5 min and (d) 30 min.

reaction time increased.

358 Advanced Topics on Crystal Growth

growth of nodules.

**Figure 22.** Roughness curves of the surfaces of Ni–P films plated by (a) ELP-SCE at 353 K and 15 MPa for 180 min, (b) conventional ELP at 353 K and atmospheric pressure for 2 min, and (c) substrate activated by catalytic Pd.

The conventional ELP as performed at 353 K and atmospheric pressure. ELP-SCE was per‐ formed at 353 K and 15 MPa. Fig. 23 shows the relationship between the surface roughness and reaction time. The surface of the Ni–P film fabricated by the conventional ELP rough‐ ened as the reaction time increased. The roughness of the Ni–P film formed by ELP-SCE, meanwhile, showed no dependence on the reaction time. Nodules appeared on the surfaces of the Ni–P films fabricated by conventional ELP at all reaction times, and the nodules grew as the reaction times increased. No nodules were observed on the surfaces of any of the Ni– P films fabricated by ELP-SCE, even at the maximum reaction times. The aforementioned re‐ sults confirm that the excellent smoothness of ELP-SCE film mitigated the influence of the substrate pretreatment and was independent of the reaction time. Fig. 20, meanwhile, shows a suppression of the growth of the nodules generated by conventional ELP.

ELP-SCE produced a thin film with high smoothness and outstanding uniformity. The Ni–P film fabricated by conventional ELP with a reaction time of 5 min had a thickness of 0.8 μm, or about the same thickness as ELP-SCe film fabricated with a reaction time of 180 min. The *Ra* of the ELP-SCE film was 0.03 μm, while that of the conventional ELP film was 0.06 μm.

ELP films are generally smoother than EP films and have fewer defects.[40] Even with ELP, however, defects such as microscopic nodules, pits, and pinholes are difficult to suppress. [41-43] Although suppression of a through-hole like a pinhole need a thick film, more nod‐ ules form in a thicker film. Nodules also easily form when the underlayer has projecting parts, foreign objects, and nuclear growth sites. Conventional suppression of nodule method prepares the smoothness and cleanness of an underlayer, while nonlinear diffusion adds re‐ active species that interfere with film growth over the projecting parts of a plating film (see Fig. 24).[44] Further, the pulse electroplating controls the thickness of a diffusion layer and is available to suppress nodule growth. [45]

ELP-SCE formed very smooth thin films whose thicknesses suppressed both nodules and pinholes without exceeding even 1 μm. The pH, reaction temperature, pretreatment, stirring speed, additive, and reactive species concentration in the plating solution all influence the deposition behavior of the ELP film. Yet in our current work we used the same plating solu‐ tion, reaction temperature, and pretreatment for both ELP-SCE and conventional ELP.

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The Ni–P film plated by ELP-SCE was free of pits and pinholes because the hydrogen bubbles produced by the electrolysis of the water were dissolved in the dispersed sc– CO2 phase of the emulsion.[13] Moreover, ELP-SCE plates the film under high pressure. High-pressure plating failed to deliver good results because hydrogen bubbles were less buoyant in the high-pressure system than at atmospheric pressure. Hence, the larger bubbles prevent the metal from covering the substrate. This, a characteristic effect of plating techniques that use sc-CO2 emulsion, suppresses the formation of pits or pinholes

**Figure 25.** a) Pinhole formation in conventional electroless plating. (b) Suppressed pinhole formation in ELP-SCE. M:

The best feature of ELP-SCE is its ability to suppress nodules and other abnormal growths formed by the plating reaction. The electroless metal deposition occurs by repeated 3D nu‐ cleation at catalytic sites on the substrate.[46] In 3D growth of the deposited Ni under a lownucleation-density condition, the deposited Ni grows and the surface roughness increases. Further, an activation processing technique with Pd catalyst can be used to influence the growth of the Ni–P film.[38] In the Ni–P films fabricated by conventional ELP in our current experiments, the activation processing roughened the surface and nodule growth was con‐ firmed. Nodule suppression is only attainable when the abovementioned factors exert their effects at the reaction site of the plating. That is, the growth suppression factor of the plating reaction and the state of the diffusion layer that conveys the film onto the substrate must both be influenced. ELP-SCE differs from the conventional method in three ways: the stir‐ ring speed, the addition of the surfactant for emulsion formation, and the decrease of the pH by the CO2 dissolution in the plating solution. Henceforth, we will also need to consider

via the mechanism shown in Fig. 25.

metal, Red: reducing agent, and sc-CO2: supercritical carbon dioxide.

**Figure 23.** Relationship between Ra and reaction time. Ra of the Ni–P film made by ELP-SCE and by conventional ELP are plotted as circles and triangles, respectively. The dotted line shows Ra of the activated substrate and the fine dot‐ ted line shows the surface roughness of the pure Cu substrate.

**Figure 24.** a) Linear O2 diffusion to a large activated area. (b) Linear and nonlinear O2 diffusion to a small pattern of nuclei.15

ELP-SCE formed very smooth thin films whose thicknesses suppressed both nodules and pinholes without exceeding even 1 μm. The pH, reaction temperature, pretreatment, stirring speed, additive, and reactive species concentration in the plating solution all influence the deposition behavior of the ELP film. Yet in our current work we used the same plating solu‐ tion, reaction temperature, and pretreatment for both ELP-SCE and conventional ELP.

prepares the smoothness and cleanness of an underlayer, while nonlinear diffusion adds re‐ active species that interfere with film growth over the projecting parts of a plating film (see Fig. 24).[44] Further, the pulse electroplating controls the thickness of a diffusion layer and

**Figure 23.** Relationship between Ra and reaction time. Ra of the Ni–P film made by ELP-SCE and by conventional ELP are plotted as circles and triangles, respectively. The dotted line shows Ra of the activated substrate and the fine dot‐

**Figure 24.** a) Linear O2 diffusion to a large activated area. (b) Linear and nonlinear O2 diffusion to a small pattern of

is available to suppress nodule growth. [45]

360 Advanced Topics on Crystal Growth

ted line shows the surface roughness of the pure Cu substrate.

nuclei.15

The Ni–P film plated by ELP-SCE was free of pits and pinholes because the hydrogen bubbles produced by the electrolysis of the water were dissolved in the dispersed sc– CO2 phase of the emulsion.[13] Moreover, ELP-SCE plates the film under high pressure. High-pressure plating failed to deliver good results because hydrogen bubbles were less buoyant in the high-pressure system than at atmospheric pressure. Hence, the larger bubbles prevent the metal from covering the substrate. This, a characteristic effect of plating techniques that use sc-CO2 emulsion, suppresses the formation of pits or pinholes via the mechanism shown in Fig. 25.

**Figure 25.** a) Pinhole formation in conventional electroless plating. (b) Suppressed pinhole formation in ELP-SCE. M: metal, Red: reducing agent, and sc-CO2: supercritical carbon dioxide.

The best feature of ELP-SCE is its ability to suppress nodules and other abnormal growths formed by the plating reaction. The electroless metal deposition occurs by repeated 3D nu‐ cleation at catalytic sites on the substrate.[46] In 3D growth of the deposited Ni under a lownucleation-density condition, the deposited Ni grows and the surface roughness increases. Further, an activation processing technique with Pd catalyst can be used to influence the growth of the Ni–P film.[38] In the Ni–P films fabricated by conventional ELP in our current experiments, the activation processing roughened the surface and nodule growth was con‐ firmed. Nodule suppression is only attainable when the abovementioned factors exert their effects at the reaction site of the plating. That is, the growth suppression factor of the plating reaction and the state of the diffusion layer that conveys the film onto the substrate must both be influenced. ELP-SCE differs from the conventional method in three ways: the stir‐ ring speed, the addition of the surfactant for emulsion formation, and the decrease of the pH by the CO2 dissolution in the plating solution. Henceforth, we will also need to consider how the collision phenomenon influences the plating film of the CO2 phase. We will need to collect more evidence to formulate a detailed mechanism for our process. Even so, our ex‐ periments have demonstrated that ELP-SCE produces more outstanding results than con‐ ventional ELP, forming thin films with high smoothness and superb uniformity.

thickness of diffusion layer close to surface of substrate and transport property of reactive

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In this chapter, we discuss what influences the characteristic reaction field of ELP-SCE to the plated film growth. In addition, a novel direct observation of nodule is proposed to clarify the nodule suppression mechanism of ELP-SCE. [48] At first, the Ni-P plated film is plated on Cu substrate by the conventional ELP method, and then re-plating by ELP-SCE or con‐ ventional ELP method is conducted on the Ni-P plated film in which nodules were formed by conventional ELP. Moreover, morphology of the nodules at a selected position in the Ni-

materials at surface of substrate for ELP-SCE is different from conventional method.

P plated film is compared before and after the re-plating.

**Table 3.** Bath composition and operating conditions of electroless Ni - P films

*2.2.2. Base Ni-P (BNP) film preparation for direct observation of nodule*

The substrate was a film of 99.99% pure copper measuring 10×20 mm (Mitsubishi Shindoh Co., Ltd.). The plated film was made by using the ELP solution shown in 2.1.2 at tempera‐ ture 353K and reaction time 5 minutes. Process procedures from step 1 to step 8 (Pretreat‐ ment-A) was shown in Table 3. The activation agent, the degreasing agent and the ELP solution were purchased from the Okuno Chemical Co., Ltd. The plating solution was kept in a glass beaker in a temperature-controlled water bath agitated with a magnetic agitator and a cross-magnetic stirrer bar. Agitation speed was 50 rpm. The substrate was inserted to the beaker by stainless steel wires. A lot of nodules were formed on the surface for the plat‐ ed film made with this condition, and the phosphorus content was 14wt% (Fig.26). The plat‐ ed film is amorphous in as deposited condition and strongly support the observations made from X-ray diffraction (XRD) measurements (Fig.27). When plating was performed again, influence from the substrates is a little because it is an amorphous plated film. Thereafter,

the Ni-P film made under this process condition is called base Ni-P film (BNP film).

### *2.1.5. Conclusion*

This chapter has proposed ELP-SCE, a hybrid technique combining ELP and supercritical fluid technology. The ELP reactions are carried out in an emulsion of sc-CO2 and an ELP sol‐ ution with surfactant. ELP-SCE formed a uniform Ni–P film free from the pinholes that typi‐ cally form from the hydrogen bubbles produced by the electrolysis of water, and free from the nodules that form from the nuclear growth in the ELP reaction.

### **2.2. Direct observation of nodule growth on electroless Ni-P deposition in supercritical CO2 emulsion**

### *2.2.1. Introduction*

The plated film obtained by ELP-SCE was extremely uniform, smooth and free from pin‐ holes and nodules. The film growth speed of ELP-SCE was slower than the conventional ELP. It is reported that an effect of pulse electroplating-like mechanism by adsorption and desorption of the supercritical CO2 (sc-CO2) phase from the plated film, called as "Periodic-Plating-Characteristic (PPC)" is a cause in the smoothing mechanism of the electroplating using a supercritical carbon dioxide emulsion (EP-SCE).[22] PPC might not be the only cause of film smoothing and nodule suppression in ELP-SCE since the film formation mech‐ anisms of electroplating and ELP are different, though the nodule formation of ELP-SCE can be affected by fast cycle of adsorption and desorption of dispersed CO2 phases to a minute convex part, and for the growth to be suppressed.

In addition, PPC effect itself cannot completely explain the effect of higher P content in Ni-P film by ELP-SCE, and the film growth speed is slower than conventional ELP. As one of the factors of the phenomenon of ELP-SCE, CO2 dissolves in the plating solution and causes de‐ crease of the pH. When the pH of the plating solution decreases, it is expected to cause P content to increase in the plated film and decrease in the film growth speed.[33] Thus, in previous study we discussed that the increase of the proton concentration in the plating sol‐ ution caused the effect of suppression of nodule growth.[47] However, it is necessary to clar‐ ify not only by an indirect method of observing the surface of the plated film via SEM or AFM, but also a direct method of observing growth of one nodule, in order to discuss the suppression mechanism of nodule growth of ELP-SCE clearly. Moreover, the slow film growth speed of ELP-SCE cannot be explained by only making a low pH plating solution. When a plated film fabricated by ELP in the pH=4.0 solution alone with agitation at the same speed used in ELP-SCE, the formation of the plated film was insufficient. For ELP-SCE, the sc-CO2 phase distributed in the plating solution causes viscosity of plating solution to be low, and, as a result, plating under a high-speed agitation was enabled. This means thickness of diffusion layer close to surface of substrate and transport property of reactive materials at surface of substrate for ELP-SCE is different from conventional method.

In this chapter, we discuss what influences the characteristic reaction field of ELP-SCE to the plated film growth. In addition, a novel direct observation of nodule is proposed to clarify the nodule suppression mechanism of ELP-SCE. [48] At first, the Ni-P plated film is plated on Cu substrate by the conventional ELP method, and then re-plating by ELP-SCE or con‐ ventional ELP method is conducted on the Ni-P plated film in which nodules were formed by conventional ELP. Moreover, morphology of the nodules at a selected position in the Ni-P plated film is compared before and after the re-plating.


**Table 3.** Bath composition and operating conditions of electroless Ni - P films

how the collision phenomenon influences the plating film of the CO2 phase. We will need to collect more evidence to formulate a detailed mechanism for our process. Even so, our ex‐ periments have demonstrated that ELP-SCE produces more outstanding results than con‐

This chapter has proposed ELP-SCE, a hybrid technique combining ELP and supercritical fluid technology. The ELP reactions are carried out in an emulsion of sc-CO2 and an ELP sol‐ ution with surfactant. ELP-SCE formed a uniform Ni–P film free from the pinholes that typi‐ cally form from the hydrogen bubbles produced by the electrolysis of water, and free from

**2.2. Direct observation of nodule growth on electroless Ni-P deposition in supercritical**

The plated film obtained by ELP-SCE was extremely uniform, smooth and free from pin‐ holes and nodules. The film growth speed of ELP-SCE was slower than the conventional ELP. It is reported that an effect of pulse electroplating-like mechanism by adsorption and desorption of the supercritical CO2 (sc-CO2) phase from the plated film, called as "Periodic-Plating-Characteristic (PPC)" is a cause in the smoothing mechanism of the electroplating using a supercritical carbon dioxide emulsion (EP-SCE).[22] PPC might not be the only cause of film smoothing and nodule suppression in ELP-SCE since the film formation mech‐ anisms of electroplating and ELP are different, though the nodule formation of ELP-SCE can be affected by fast cycle of adsorption and desorption of dispersed CO2 phases to a minute

In addition, PPC effect itself cannot completely explain the effect of higher P content in Ni-P film by ELP-SCE, and the film growth speed is slower than conventional ELP. As one of the factors of the phenomenon of ELP-SCE, CO2 dissolves in the plating solution and causes de‐ crease of the pH. When the pH of the plating solution decreases, it is expected to cause P content to increase in the plated film and decrease in the film growth speed.[33] Thus, in previous study we discussed that the increase of the proton concentration in the plating sol‐ ution caused the effect of suppression of nodule growth.[47] However, it is necessary to clar‐ ify not only by an indirect method of observing the surface of the plated film via SEM or AFM, but also a direct method of observing growth of one nodule, in order to discuss the suppression mechanism of nodule growth of ELP-SCE clearly. Moreover, the slow film growth speed of ELP-SCE cannot be explained by only making a low pH plating solution. When a plated film fabricated by ELP in the pH=4.0 solution alone with agitation at the same speed used in ELP-SCE, the formation of the plated film was insufficient. For ELP-SCE, the sc-CO2 phase distributed in the plating solution causes viscosity of plating solution to be low, and, as a result, plating under a high-speed agitation was enabled. This means

ventional ELP, forming thin films with high smoothness and superb uniformity.

the nodules that form from the nuclear growth in the ELP reaction.

convex part, and for the growth to be suppressed.

*2.1.5. Conclusion*

362 Advanced Topics on Crystal Growth

**CO2 emulsion**

*2.2.1. Introduction*

#### *2.2.2. Base Ni-P (BNP) film preparation for direct observation of nodule*

The substrate was a film of 99.99% pure copper measuring 10×20 mm (Mitsubishi Shindoh Co., Ltd.). The plated film was made by using the ELP solution shown in 2.1.2 at tempera‐ ture 353K and reaction time 5 minutes. Process procedures from step 1 to step 8 (Pretreat‐ ment-A) was shown in Table 3. The activation agent, the degreasing agent and the ELP solution were purchased from the Okuno Chemical Co., Ltd. The plating solution was kept in a glass beaker in a temperature-controlled water bath agitated with a magnetic agitator and a cross-magnetic stirrer bar. Agitation speed was 50 rpm. The substrate was inserted to the beaker by stainless steel wires. A lot of nodules were formed on the surface for the plat‐ ed film made with this condition, and the phosphorus content was 14wt% (Fig.26). The plat‐ ed film is amorphous in as deposited condition and strongly support the observations made from X-ray diffraction (XRD) measurements (Fig.27). When plating was performed again, influence from the substrates is a little because it is an amorphous plated film. Thereafter, the Ni-P film made under this process condition is called base Ni-P film (BNP film).

A rectangular shape shown in Fig.28. was fabricated by focused ion beam system (FIB) (ac‐ celerating voltage 40kV) on the surface of BNP film. The fabricating area was made by four rectangles of 50×20 μm to make a square observation area of 50×50 μm. The fabrication pro‐ gram was set so that the gallium (Ga) ion beam was not irradiated to the observation area to prevent influence from irradiated Ga ion of FIB to the plated Ni-P film growth at the obser‐ vation area. The observation area was measured by atomic force microscopy (AFM) before re-plating, and nodules that become observation candidates were decided on the observa‐

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**Figure 28.** a) SIM and (b) AFM image of rectangular shaped Ni-P film plated by conventional ELP at 353 K and atmos‐

tion area.

pheric pressure for 5min.

**Figure 26.** SIM image of Ni-P film by conventional ELP at 353 K and atmospheric pressure for 5min (with a film thick‐ ness of 0.8 μm).

**Figure 27.** XRD spectrum of Ni-P film plated from conventional ELP at 353K and atmospheric pressure for 5 min.

A rectangular shape shown in Fig.28. was fabricated by focused ion beam system (FIB) (ac‐ celerating voltage 40kV) on the surface of BNP film. The fabricating area was made by four rectangles of 50×20 μm to make a square observation area of 50×50 μm. The fabrication pro‐ gram was set so that the gallium (Ga) ion beam was not irradiated to the observation area to prevent influence from irradiated Ga ion of FIB to the plated Ni-P film growth at the obser‐ vation area. The observation area was measured by atomic force microscopy (AFM) before re-plating, and nodules that become observation candidates were decided on the observa‐ tion area.

**Figure 26.** SIM image of Ni-P film by conventional ELP at 353 K and atmospheric pressure for 5min (with a film thick‐

**Cu {111}**

**Cu {200}**

**30 35 40 45 50 55 60**

**Angle of 2θ (degree)**

**Figure 27.** XRD spectrum of Ni-P film plated from conventional ELP at 353K and atmospheric pressure for 5 min.

ness of 0.8 μm).

364 Advanced Topics on Crystal Growth

**Intensity (a.u.)**

The activation treatment using Pd has a big influence on the surface morphology of plated film. This is undesirable to observe the morphological change of the fine nodules[39]. In this study, a pretreatment for the re-plating on the rectangular shaped BNP film was processed in order from step 1 to 6 of Table 4 (Pretreatment-B). In the result of the surface texture measuring instrument, Ra of the plated film was 0.029 μm before and after the pretreatment-B, and the AFM measurement result did not have a substantial change in the surface mor‐ phology either (Fig. 29).

the base film was measured by an FESEM (S-4300SE, Hitachi High-technologies Co., Ltd.) equipped for energy-dispersive X-ray spectroscopy (EDX). An accelerating voltage of 20 kV with a collecting time of more than 300 s was applied. The surface morphology of plated film was examined using an atomic force microscopy (AFM, SPA-400, Seiko Instruments., Inc.) with a calibrated 20 μm xy-scan and 10 μm z-scan range PZT-scanner. A surface tex‐ ture measuring instrument (Surfcom 480A, Tokyo Seimitsu Co., Ltd.) with a diamond-tip‐ ped detector (2 μm tip radius) was used to measure the average surface roughness (Ra) to a minimum resolution of 1 nm for height (height measurement range: 80 μm). The average Ra was calculated from measurements at three points. 2θ-ω X-ray diffraction (XRD) analysis was performed at room temperature (RT, 298 K) using a PANalytical X'pert Pro Galaxy sys‐ tem equipped with an X'celerator module. The X-ray source was CuKα, and the tube volt‐

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**Figure 29.** AFM image of (a) Ni-P film by conventional ELP at 353 K and atmospheric pressure for 5min, and (b) After

age and the current are 45 kV and 40mA, respectively.

pretreatment-B.


**Table 4.** Composition of solutions and producers of catalyzing process.

### *2.2.3. ELP-SCE on pretreated BNP film and on pretreated copper substrate*

The rectangular shaped BNP film after pretreatment-B was plated by the conventional ELP and ELP-SCE again respectively. The conventional ELP was the same as the fabrication con‐ dition of BNP film. Details on the apparatus and plating method of ELP-SCE can be found in 2.1.2. The observation area after re-plating was measured by AFM, and morphology change of the nodule was analyzed. For re-plating, not only rectangular shaped BNP film after pre‐ treatment-B but also the copper substrate after pretreatment-A was performed regardless of each plating method.

### *2.2.4. Material characterization*

Focused ion beam system (FIB, FB-2100, Hitachi High-technologies Co., Ltd.) has scanning ion microscope (SIM). The liquid-metal ion sources of this instrument used Ga ion sources. SIM was used to observe the surfaces of the plated Ni-P films. A cross section of the plated Ni-P film was fabricated by FIB and the thickness of the plated film could be measured di‐ rectly from the SIM image on the screen. Moreover, FIB was used for fabricating of the area for carrying out direct observation of the nodule growth. The phosphorous composition of the base film was measured by an FESEM (S-4300SE, Hitachi High-technologies Co., Ltd.) equipped for energy-dispersive X-ray spectroscopy (EDX). An accelerating voltage of 20 kV with a collecting time of more than 300 s was applied. The surface morphology of plated film was examined using an atomic force microscopy (AFM, SPA-400, Seiko Instruments., Inc.) with a calibrated 20 μm xy-scan and 10 μm z-scan range PZT-scanner. A surface tex‐ ture measuring instrument (Surfcom 480A, Tokyo Seimitsu Co., Ltd.) with a diamond-tip‐ ped detector (2 μm tip radius) was used to measure the average surface roughness (Ra) to a minimum resolution of 1 nm for height (height measurement range: 80 μm). The average Ra was calculated from measurements at three points. 2θ-ω X-ray diffraction (XRD) analysis was performed at room temperature (RT, 298 K) using a PANalytical X'pert Pro Galaxy sys‐ tem equipped with an X'celerator module. The X-ray source was CuKα, and the tube volt‐ age and the current are 45 kV and 40mA, respectively.

The activation treatment using Pd has a big influence on the surface morphology of plated film. This is undesirable to observe the morphological change of the fine nodules[39]. In this study, a pretreatment for the re-plating on the rectangular shaped BNP film was processed in order from step 1 to 6 of Table 4 (Pretreatment-B). In the result of the surface texture measuring instrument, Ra of the plated film was 0.029 μm before and after the pretreatment-B, and the AFM measurement result did not have a substantial change in the surface mor‐

phology either (Fig. 29).

366 Advanced Topics on Crystal Growth

each plating method.

*2.2.4. Material characterization*

**Table 4.** Composition of solutions and producers of catalyzing process.

*2.2.3. ELP-SCE on pretreated BNP film and on pretreated copper substrate*

The rectangular shaped BNP film after pretreatment-B was plated by the conventional ELP and ELP-SCE again respectively. The conventional ELP was the same as the fabrication con‐ dition of BNP film. Details on the apparatus and plating method of ELP-SCE can be found in 2.1.2. The observation area after re-plating was measured by AFM, and morphology change of the nodule was analyzed. For re-plating, not only rectangular shaped BNP film after pre‐ treatment-B but also the copper substrate after pretreatment-A was performed regardless of

Focused ion beam system (FIB, FB-2100, Hitachi High-technologies Co., Ltd.) has scanning ion microscope (SIM). The liquid-metal ion sources of this instrument used Ga ion sources. SIM was used to observe the surfaces of the plated Ni-P films. A cross section of the plated Ni-P film was fabricated by FIB and the thickness of the plated film could be measured di‐ rectly from the SIM image on the screen. Moreover, FIB was used for fabricating of the area for carrying out direct observation of the nodule growth. The phosphorous composition of

**Figure 29.** AFM image of (a) Ni-P film by conventional ELP at 353 K and atmospheric pressure for 5min, and (b) After pretreatment-B.

#### *2.2.5. Direct observation of nodule growth in conventional method*

Copper substrate after pretreatment-A performed with the re-plating procedure had 0.8 μm of the plated film thickness, and it showed a lot of nodules as that of the BNP film as shown in Fig. 30 (1-a). This means that decomposition of a plating bath did not occur, when re-plat‐ ing is performed. Film thickness increased from 0.8 μm to 1.7 μm as a result of re-plating on BNP film after pretreatment-B by conventional ELP, and the number of nodules on the sur‐ face increased from the initial state as shown in Fig. 30 (1-b). It was possible to re-plating on BNP film with Pretreatment-B. Also, surface observation and film thickness of the plating films by the ELP-SCE and the conventional method using the plating solution with adjusted pH were shown in Figs. 30 (2-a), (2-b), (3-a), and (3-b). The stability of the plating solutions in each method were also confirmed by the observations of the plated surfaces. The plated film thickness is 0.6 μm, and the film has grown up on the copper substrate after Pretreat‐ ment-A set up simultaneously (Fig. 30 (2-a)). The film became a smooth film, although there were ditches resulted from the polishing ditches of the copper substrate. The change from initial film thickness of 0.8 μm could not be observed by SIM observation as a result of replating on BNP film after Pretreatment-B by ELP-SCE (Fig. 30 (2-b)). The phenomenon in Fig. 30 (2-b) is not peculiar and it also happens when the Ni-P film is obtained after Pretreat‐ ment-B without Pd activation and re-plated with low pH bath. A similar phenomenon was confirmed for the re-plating with conventional electroless plating adjusted to pH=4.0 by adding HCl (10wt%). The film was formed as for the copper substrate after Pretreatment-B (Fig. 30 (3-a)) but the film growth was difficult to be confirmed by the SIM observation from Fig. 30 (3-b).

**Figure 30.** SIM image of Re-Ni-P films plated on the Cu substrate by (1-a) conventional ELP at 353 K and atmospheric pressure for 5 min, (2-a) ELP-SCE at 353 K and 15MPa for 180 min and (3-a) conventional ELP at 353 K and atmospher‐ ic pressure for 20 min and pH of plating solution is 4.0. (b) means SIM image that plates the BNP film being processed

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The effect of PPC proposed with EP-SCE and the mechanical agitation are the causes for de‐ crease in the thickness of the diffusion layer of ELP-SCE. PPC for the sc-CO2 phase to repeat adsorption and desorption on the surface of the plated film could appear as similar as that of the pulse electro deposition (PED).[52] Actually, Rahman have succeeded by using the ef‐ fect of PPC in the sc-CO2 emulsion and the perfluorocarbon surfactant in forming the porous film.[30] Size of the pores are roughly several μm or less depends on the size of the sc-CO2 phase, which can be controlled by pressure, the amount of CO2, and the amount of the sur‐ factant. Moreover, the influence of the mechanical agitation is an important factor in the dif‐ fusion layer control. In the PED, the mechanical agitation and the duty cycle are optimized, and the thickness of the diffusion layer is controlled.[53] In ELP-SCE, the plating reaction field is made by making sc-CO2 and the plating solution in an emulsion to lowered the vis‐ cosity. This gives the possibility to exert a difference in influence from the mechanical agita‐

An extraordinary effect was confirmed for the morphological change of nodule (Fig. 32). This is the first report on of direct observation of the effect of nodule growth suppression by

for pretreatment-A put at the same time as each (a) samples.

tion like the conventional on the growth of the plated film.

*2.2.6. Suppression of nodule growth in ELP-SCE*

The AFM observation of the BNP film fabricated by FIB before and after re-plating with con‐ ventional ELP was shown in Fig. 31. Before re-plating, the film was measured with AFM while raising the magnification from Fig. 31 (1-a) to (1-c), and nodules at a specific position were decided by three places. After re-plating, the change in the morphology of the specific nodule was observed with AFM from Fig. 31 (2-a) to (2-c). The height of nodule before replating was 40 nm or less, and the width was about 500 nm. The measurement of the width of nodule was conducted along the direction of the dotted line arrow along the ditch in Fig. 31 (1-d) and (2-d). The shape of nodule was not changed as the film thickness increased to twice that of the initial thickness. The prior growth of nodules can be considered to come from spherical diffusion layer surrounding the neighboring nodule cores on the surface. The localized c oncentration by spherical diffusion occurs at the convex part when the thickness of the diffusion layer is the same or thicker than that of the convex part on the surface, and the thickness of the diffusion layer in neighborhood on the plated film greatly influences the surface-roughness of the plated film.[49,50] The thickness of typical Nernst diffusion layer was reported to be about 0.2 mm, and thickness could be about 0.02 mm when agitation is added.[49] In this experimental condition, it was considered that the Nernst diffusion layer was larger than enough to the size of these nodules in resulting the spherical diffusion[51], and led to a surface morphology like Fig. 31 (2-d). Moreover, this evaluating method is an effective direct observation method to study the growth mechanism and the surface mor‐ phology of the plated film including nodule at a specific position.

**Figure 30.** SIM image of Re-Ni-P films plated on the Cu substrate by (1-a) conventional ELP at 353 K and atmospheric pressure for 5 min, (2-a) ELP-SCE at 353 K and 15MPa for 180 min and (3-a) conventional ELP at 353 K and atmospher‐ ic pressure for 20 min and pH of plating solution is 4.0. (b) means SIM image that plates the BNP film being processed for pretreatment-A put at the same time as each (a) samples.

The effect of PPC proposed with EP-SCE and the mechanical agitation are the causes for de‐ crease in the thickness of the diffusion layer of ELP-SCE. PPC for the sc-CO2 phase to repeat adsorption and desorption on the surface of the plated film could appear as similar as that of the pulse electro deposition (PED).[52] Actually, Rahman have succeeded by using the ef‐ fect of PPC in the sc-CO2 emulsion and the perfluorocarbon surfactant in forming the porous film.[30] Size of the pores are roughly several μm or less depends on the size of the sc-CO2 phase, which can be controlled by pressure, the amount of CO2, and the amount of the sur‐ factant. Moreover, the influence of the mechanical agitation is an important factor in the dif‐ fusion layer control. In the PED, the mechanical agitation and the duty cycle are optimized, and the thickness of the diffusion layer is controlled.[53] In ELP-SCE, the plating reaction field is made by making sc-CO2 and the plating solution in an emulsion to lowered the vis‐ cosity. This gives the possibility to exert a difference in influence from the mechanical agita‐ tion like the conventional on the growth of the plated film.

#### *2.2.6. Suppression of nodule growth in ELP-SCE*

*2.2.5. Direct observation of nodule growth in conventional method*

Fig. 30 (3-b).

368 Advanced Topics on Crystal Growth

Copper substrate after pretreatment-A performed with the re-plating procedure had 0.8 μm of the plated film thickness, and it showed a lot of nodules as that of the BNP film as shown in Fig. 30 (1-a). This means that decomposition of a plating bath did not occur, when re-plat‐ ing is performed. Film thickness increased from 0.8 μm to 1.7 μm as a result of re-plating on BNP film after pretreatment-B by conventional ELP, and the number of nodules on the sur‐ face increased from the initial state as shown in Fig. 30 (1-b). It was possible to re-plating on BNP film with Pretreatment-B. Also, surface observation and film thickness of the plating films by the ELP-SCE and the conventional method using the plating solution with adjusted pH were shown in Figs. 30 (2-a), (2-b), (3-a), and (3-b). The stability of the plating solutions in each method were also confirmed by the observations of the plated surfaces. The plated film thickness is 0.6 μm, and the film has grown up on the copper substrate after Pretreat‐ ment-A set up simultaneously (Fig. 30 (2-a)). The film became a smooth film, although there were ditches resulted from the polishing ditches of the copper substrate. The change from initial film thickness of 0.8 μm could not be observed by SIM observation as a result of replating on BNP film after Pretreatment-B by ELP-SCE (Fig. 30 (2-b)). The phenomenon in Fig. 30 (2-b) is not peculiar and it also happens when the Ni-P film is obtained after Pretreat‐ ment-B without Pd activation and re-plated with low pH bath. A similar phenomenon was confirmed for the re-plating with conventional electroless plating adjusted to pH=4.0 by adding HCl (10wt%). The film was formed as for the copper substrate after Pretreatment-B (Fig. 30 (3-a)) but the film growth was difficult to be confirmed by the SIM observation from

The AFM observation of the BNP film fabricated by FIB before and after re-plating with con‐ ventional ELP was shown in Fig. 31. Before re-plating, the film was measured with AFM while raising the magnification from Fig. 31 (1-a) to (1-c), and nodules at a specific position were decided by three places. After re-plating, the change in the morphology of the specific nodule was observed with AFM from Fig. 31 (2-a) to (2-c). The height of nodule before replating was 40 nm or less, and the width was about 500 nm. The measurement of the width of nodule was conducted along the direction of the dotted line arrow along the ditch in Fig. 31 (1-d) and (2-d). The shape of nodule was not changed as the film thickness increased to twice that of the initial thickness. The prior growth of nodules can be considered to come from spherical diffusion layer surrounding the neighboring nodule cores on the surface. The localized c oncentration by spherical diffusion occurs at the convex part when the thickness of the diffusion layer is the same or thicker than that of the convex part on the surface, and the thickness of the diffusion layer in neighborhood on the plated film greatly influences the surface-roughness of the plated film.[49,50] The thickness of typical Nernst diffusion layer was reported to be about 0.2 mm, and thickness could be about 0.02 mm when agitation is added.[49] In this experimental condition, it was considered that the Nernst diffusion layer was larger than enough to the size of these nodules in resulting the spherical diffusion[51], and led to a surface morphology like Fig. 31 (2-d). Moreover, this evaluating method is an effective direct observation method to study the growth mechanism and the surface mor‐

phology of the plated film including nodule at a specific position.

An extraordinary effect was confirmed for the morphological change of nodule (Fig. 32). This is the first report on of direct observation of the effect of nodule growth suppression by ELP-SCE. Decrease in size of nodules and initiation of nucleation on the surface at random were observed though the plated film as the film grew up. The ditch parts were on the sub‐ strate before re-plating, which were filled after re-plating as shown in Fig.32 Random gener‐ ation of the refined nucleus and leveling effect are similar to the phenomenon that happens because of PPC.[22] The effect of the leveling of ELP-SCE could be caused by the fact that the thickness of the diffusion layer is as thin as the size of these nodules. On the other hand, we discussed the influence of the proton for the growth suppression of nodule in the previ‐ ous chapter. The re-plating experiment was done by conventional ELP that used the plating bath with pH adjusted to 4.0 by adding HCl (10 wt%) to confirm the influence of diffusion layer thickness of ELP-SCE and the proton. New nodules were observed to be hardly formed after re-plating as shown in Fig. 33. The plated film was observed to be from 183 nm to 254 nm as a result of measuring the width of ditch before and after re-plating, because of this re-plating. The growth of the convex part was suppressed under a plating condition even when it was thought that there was a thicker diffusion layer where spherical diffusion happened. However, all peculiar phenomena of ELP-SCE were not able to be shown. That is, the thickness change in the diffusion layer in addition to the influence of the proton greatly influences in ELP-SCE.

**Figure 32.** AFM images (1-a) of rectangular shaped Ni-P plating film plated by conventional ELP at 353 K and atmos‐ pheric pressure for 5min, and (2-a) of re-plated Ni-P film plated by ELP-SCE at 353 K and 15 MPa for 180 min on the (1 a) film. (number-b) is 2D image of (number-a). The dotted arrow indicates the direction where the width of nodules

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Two influences mentioned above on the plating reaction field might be the formation factors of the refined nuclei observed. In the PED, the nucleation is promoted because the plating reaction can be performed at high overpotential, and it becomes easy for a minute crystalli‐ zation to occur. On the other hand, the ELP that was not applied by the external power is the base reaction in ELP-SCE, and the electron necessary for the nucleation is only supplied by the reducing agent oxidized on the plated film. Also, crystallization overpotential that is higher than nuclear growth are necessary for the nucleation in the electrocrystallization.[54] Therefore, it will be necessary to supply a large amount of reducing agents to a reaction sur‐ face in ELP-SCE. The small film growth speed of ELP-SCE might have been caused by the reaction filed where the nucleation occur more frequently than nuclear growth. It causes high P content with plated film by ELP-SCE, moreover, because not only the reducing agent but also other reactive species such as the metal ions and protons are supplied easily volu‐

minously so far because of PPC and the mechanical agitation.

was measured.

**Figure 31.** AFM images (1-a, b, c, d) of rectangular shaped Ni-P film plated by conventional ELP at 353 K and atmos‐ pheric pressure for 5 min, and (2-a, b, c, d) of re-plated Ni-P film plated by conventional ELP at 353 K and atmospheric pressure for 5 min on the (1-a) film. The magnification rises from (a) to (c). (d) is 2D image of (c). The dotted arrow indicates the direction where the width of nodules was measured.

ELP-SCE. Decrease in size of nodules and initiation of nucleation on the surface at random were observed though the plated film as the film grew up. The ditch parts were on the sub‐ strate before re-plating, which were filled after re-plating as shown in Fig.32 Random gener‐ ation of the refined nucleus and leveling effect are similar to the phenomenon that happens because of PPC.[22] The effect of the leveling of ELP-SCE could be caused by the fact that the thickness of the diffusion layer is as thin as the size of these nodules. On the other hand, we discussed the influence of the proton for the growth suppression of nodule in the previ‐ ous chapter. The re-plating experiment was done by conventional ELP that used the plating bath with pH adjusted to 4.0 by adding HCl (10 wt%) to confirm the influence of diffusion layer thickness of ELP-SCE and the proton. New nodules were observed to be hardly formed after re-plating as shown in Fig. 33. The plated film was observed to be from 183 nm to 254 nm as a result of measuring the width of ditch before and after re-plating, because of this re-plating. The growth of the convex part was suppressed under a plating condition even when it was thought that there was a thicker diffusion layer where spherical diffusion happened. However, all peculiar phenomena of ELP-SCE were not able to be shown. That is, the thickness change in the diffusion layer in addition to the influence of the proton greatly

**Figure 31.** AFM images (1-a, b, c, d) of rectangular shaped Ni-P film plated by conventional ELP at 353 K and atmos‐ pheric pressure for 5 min, and (2-a, b, c, d) of re-plated Ni-P film plated by conventional ELP at 353 K and atmospheric pressure for 5 min on the (1-a) film. The magnification rises from (a) to (c). (d) is 2D image of (c). The dotted arrow

indicates the direction where the width of nodules was measured.

influences in ELP-SCE.

370 Advanced Topics on Crystal Growth

**Figure 32.** AFM images (1-a) of rectangular shaped Ni-P plating film plated by conventional ELP at 353 K and atmos‐ pheric pressure for 5min, and (2-a) of re-plated Ni-P film plated by ELP-SCE at 353 K and 15 MPa for 180 min on the (1 a) film. (number-b) is 2D image of (number-a). The dotted arrow indicates the direction where the width of nodules was measured.

Two influences mentioned above on the plating reaction field might be the formation factors of the refined nuclei observed. In the PED, the nucleation is promoted because the plating reaction can be performed at high overpotential, and it becomes easy for a minute crystalli‐ zation to occur. On the other hand, the ELP that was not applied by the external power is the base reaction in ELP-SCE, and the electron necessary for the nucleation is only supplied by the reducing agent oxidized on the plated film. Also, crystallization overpotential that is higher than nuclear growth are necessary for the nucleation in the electrocrystallization.[54] Therefore, it will be necessary to supply a large amount of reducing agents to a reaction sur‐ face in ELP-SCE. The small film growth speed of ELP-SCE might have been caused by the reaction filed where the nucleation occur more frequently than nuclear growth. It causes high P content with plated film by ELP-SCE, moreover, because not only the reducing agent but also other reactive species such as the metal ions and protons are supplied easily volu‐ minously so far because of PPC and the mechanical agitation.

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[13] Yoshida H, Sone M, Wakabayashi H, Yan H, Abe K, Tao XT, Mizushima A, Ichihara S, Miyata S. New electroplating method of nickel in emulsion of supercritical carbon dioxide and electrolyte solution to enhance uniformity and hardness of plated film.

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**Figure 33.** AFM images (1-a) of rectangular shaped Ni-P plating film plated by conventional ELP at 353 K and atmos‐ pheric pressure for 5min, and (2-a) of re-plated Ni-P film plated by conventional ELP at 353 K and atmospheric pres‐ sure for 20 min and pH 4.0 of plating solution on the (1-a) film. (number-b) is 2D image of (number-a). The dotted arrow indicates the direction where the width of nodules was measured

### *2.2.7. Conclusion*

We examined a direct observation of selected nodule growth in ELP reaction using AFM on a square sample substrate of 50×50 μm fabricated by FIB. The Ni-P plated film is plated by the conventional method and ELP-SCE again on the Ni-P plated film with nodule formed. Changes in fine nodules and other areas at a specific position in the surface morphology were compared before and after the re-plating. In ELP-SCE, the dominant growth of nodules was suppressed and the nucleation occurred on the other surface of the nodules, although the convex part of nodules grew dominantly in conventional ELP using the electrolyte only.

### **Author details**

Masato Sone\* , Tso-Fu Mark Chang and Hiroki Uchiyama

\*Address all correspondence to: msone@pi.titech.ac.jp

Precision & Intelligence Laboratory, Tokyo Institute of Technology, Yokohama, Japan

### **References**

**Figure 33.** AFM images (1-a) of rectangular shaped Ni-P plating film plated by conventional ELP at 353 K and atmos‐ pheric pressure for 5min, and (2-a) of re-plated Ni-P film plated by conventional ELP at 353 K and atmospheric pres‐ sure for 20 min and pH 4.0 of plating solution on the (1-a) film. (number-b) is 2D image of (number-a). The dotted

We examined a direct observation of selected nodule growth in ELP reaction using AFM on a square sample substrate of 50×50 μm fabricated by FIB. The Ni-P plated film is plated by the conventional method and ELP-SCE again on the Ni-P plated film with nodule formed. Changes in fine nodules and other areas at a specific position in the surface morphology were compared before and after the re-plating. In ELP-SCE, the dominant growth of nodules was suppressed and the nucleation occurred on the other surface of the nodules, although the convex part of nodules grew dominantly in conventional ELP using the electrolyte only.

arrow indicates the direction where the width of nodules was measured

, Tso-Fu Mark Chang and Hiroki Uchiyama

Precision & Intelligence Laboratory, Tokyo Institute of Technology, Yokohama, Japan

\*Address all correspondence to: msone@pi.titech.ac.jp

*2.2.7. Conclusion*

372 Advanced Topics on Crystal Growth

**Author details**

Masato Sone\*


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**Chapter 12**

**Inorganic Nanostructures Decorated Graphene**

Hong Ngee Lim, Nay Ming Huang, Chin Hua Chia and Ian Harrison

http://dx.doi.org/10.5772/54321

**1. Introduction**

Additional information is available at the end of the chapter

**1.1. Why use graphene for the assembly of nanostructures?**

make it an indispensable material in various kinds of synthesis processes.

Graphene, with zero energy gap between the highest occupied molecular orbit and the lowest unoccupied molecular orbit (HOMO-LUMO), offers a unique two-dimensional (2-D) envi‐ ronment for fast electron transport and has potential applications in electronic devices [1, 2]. Other consequences of the band structure is the opacity which is wavelength independent [3], and thermal conductivity [4]. The four edges of a graphene sheet provide significant number of centres for fast heterogeneous electron transfer [1], when compared to single-walled carbon nanotubes (SWCNTs) for which heterogeneous electron transfer occurs only at the two ends of the nanotube [5]. Consequently, graphene sheets may have wider applicability in electro‐ chemistry [6]. While graphite is brittle, graphene's flexibility is beneficial for use in electro‐ mechanical devices [7] and energy storage devices [8]. In the energy storage devices, its weight is of extremely important and the specific area per unit weight is an important figure of merit. Graphene exhibits a theoretical surface area of 2630 m2 g-1, which is ~ 260 times greater than graphite and twice that of CNTs [9]. Thus, graphene provides a way of enhancing the electro‐ chemical catalytic activity of materials by greatly increasing the high surface area [10]. The intriguing electronic, optical, electrochemical, mechanical and thermal properties of graphene

Various inorganic nanostructures have been prepared over the last two decades due to the special properties of nanostructured materials. Previously, 2-D ZnO nanoplates [11], 1-D PbS nanorods [12], 0-D semiconductor materials [13] and core-shell magnetic nanoparticles have been produced [14]. However, heavy aggregation of the nanostructures, resulted by van der Waals forces between the particles, may limit their special properties and cause structural instability, thus reduce their applicability. To prevent this clustering from occurring, nano‐ composites, consisting of the nanoparticles embedded within a matrix compound, can be used.

> © 2013 Lim et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

© 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

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

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


## **Inorganic Nanostructures Decorated Graphene**

Hong Ngee Lim, Nay Ming Huang, Chin Hua Chia and Ian Harrison

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/54321

**1. Introduction**

[42] Tashiro K, Yamamoto S, Hashimoto Y, Kawashima S, Honma H, Initial deposition morphologies of electroless nickel-phosphorus plating on a nonconductor and a con‐

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D118.

376 Advanced Topics on Crystal Growth

#### **1.1. Why use graphene for the assembly of nanostructures?**

Graphene, with zero energy gap between the highest occupied molecular orbit and the lowest unoccupied molecular orbit (HOMO-LUMO), offers a unique two-dimensional (2-D) envi‐ ronment for fast electron transport and has potential applications in electronic devices [1, 2]. Other consequences of the band structure is the opacity which is wavelength independent [3], and thermal conductivity [4]. The four edges of a graphene sheet provide significant number of centres for fast heterogeneous electron transfer [1], when compared to single-walled carbon nanotubes (SWCNTs) for which heterogeneous electron transfer occurs only at the two ends of the nanotube [5]. Consequently, graphene sheets may have wider applicability in electro‐ chemistry [6]. While graphite is brittle, graphene's flexibility is beneficial for use in electro‐ mechanical devices [7] and energy storage devices [8]. In the energy storage devices, its weight is of extremely important and the specific area per unit weight is an important figure of merit. Graphene exhibits a theoretical surface area of 2630 m2 g-1, which is ~ 260 times greater than graphite and twice that of CNTs [9]. Thus, graphene provides a way of enhancing the electro‐ chemical catalytic activity of materials by greatly increasing the high surface area [10]. The intriguing electronic, optical, electrochemical, mechanical and thermal properties of graphene make it an indispensable material in various kinds of synthesis processes.

Various inorganic nanostructures have been prepared over the last two decades due to the special properties of nanostructured materials. Previously, 2-D ZnO nanoplates [11], 1-D PbS nanorods [12], 0-D semiconductor materials [13] and core-shell magnetic nanoparticles have been produced [14]. However, heavy aggregation of the nanostructures, resulted by van der Waals forces between the particles, may limit their special properties and cause structural instability, thus reduce their applicability. To prevent this clustering from occurring, nano‐ composites, consisting of the nanoparticles embedded within a matrix compound, can be used.

© 2013 Lim et al.; licensee InTech. This is an open access article 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. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

These nanocomposites preserve the unique properties of the nanoparticles whilst often having the additional performance benefits of the matrix compound itself.

**Inorganic Nanostructure**

*Metal oxide*

ZnO

Pd 13±2 nm

**Morphology and**

Fe3O4 20, 30 and 40 nm In situ chemical synthesis

SnO2 4-5 nm One-step wet chemical

nanorods, an approximated diameter of ~90 nm and length of

~3 μm

method

**Dimension Synthesis Method Potential Application Reference**

storage, production of fuel cell [29]

Inorganic Nanostructures Decorated Graphene

http://dx.doi.org/10.5772/54321

379

oxidation [32]

Sensors, supercapacitors, drug delivery systems, waste water

Detection of cadmium(II), lead(II), copper(II), and mercury(II) [39]

conductors, catalysis [47]

[20]

treatment

Hydrothermal Solar cells, gas sensors, transparent

An anode catalyst for formic acid electrooxidation [30]

Au 30 nm Cyclic voltammetry scanning Detection of mercury [28]

Pd 5–7 nm Laser irradiation CO oxidation [31] Pt 5–7 nm Laser irradiation CO oxidation [31]

Ag2O 45 nm In situ oxidation route Supercapacitor [33] CoO 5–7 nm Laser irradiation CO oxidation [31] CuO < 20 nm In situ chemical synthesis Glucose biosensor [34] CuO 30 nm Hydrothermal Anode for lithium-ion batteries [21]

Fe3O4 12.5 nm Gas/liquid interface reactionAnode for lithium-ion batteries [35] Mn3O4 10 nm Ultrasonication Supercapacitors [36] NiO 32 nm Hydrothermal Anode for lithium-ion batteries [37] PbO image not shown Electrochemical route Detection of trace arsenic [38] SnO2 10 nm Microwave Detection of mercury(II) [19]

SnO2 2–3 nm Microwave Supercapacitor [32] SnO2 4–5 nm Microwave autoclave Anode for lithium-ion batteries [40] SnO2 2–6 nm Gas/liquid interface reactionAnode for lithium-ion batteries [41] SnO2 4–6 nm In situ chemical synthesis Anode for lithium-ion batteries [42] TiO2 ~20 nm Hydrothermal Detection of mercury [43] TiO2 4–5 nm Sonochemical Photocatalyst [44] TiO2 (P25) ~30 nm Hydrothermal Photocatalyst [45] TiO2 (P25) ~30 nm Hydrothermal Photoelectrocatalytic degradation [46]

Ni Single-layered Electroless Ni-plating Electrodes, sensors, hydrogen-

Electrochemical codeposition

PtRu 2 nm Microwave Electrocatalysts for methanol

With large surface area and the unique properties given above, graphene is an attractive choice as the matrix for inorganic nanostructures [15]. Functionalization of graphene sheets with various nanostructures can further enhance the properties of graphene. Heterostructures consisting of nanostructures distributed on the surface of graphene could potentially display not only the unique properties of nanostructures [16] and those of graphene [2, 17, 18], but also additional novel functionalities and properties due to the interaction between them. Moreover, the growth of the nanostructures can take place easily when graphene is used as a matrix due to the planar structure of graphene, in comparison to the hollow tubal-shaped CNTs. The particle size and size distribution of nanostructures are small and narrow when graphene is used as a support for the growth of nanostructures [19-21].

### **2. Inorganic nanostructures decorated graphene**

Graphene nanocomposites can be made from graphene oxide (GO) which is essentially a graphene sheet containing oxy-functional groups, such as epoxy, hydroxyl, carbonyl, and carboxylic, on the surface. The reactive functional groups provide a means of attaching the nanoparticles to the graphene sheet [22, 23]. The GO starting material can be simply and economically synthesized using chemical oxidation of graphite [24]. Graphene manufactured using this highly scalable synthesis route is also popularly known as reduced GO (rGO). Using GO as a support for metallic ions, many types of different nanocomposites have been made. The overview of the types of materials and synthesis methods pertaining to the graphenebased nanostructures are presented in Table 1 along with the potential application for the nanocomposites.



These nanocomposites preserve the unique properties of the nanoparticles whilst often having

With large surface area and the unique properties given above, graphene is an attractive choice as the matrix for inorganic nanostructures [15]. Functionalization of graphene sheets with various nanostructures can further enhance the properties of graphene. Heterostructures consisting of nanostructures distributed on the surface of graphene could potentially display not only the unique properties of nanostructures [16] and those of graphene [2, 17, 18], but also additional novel functionalities and properties due to the interaction between them. Moreover, the growth of the nanostructures can take place easily when graphene is used as a matrix due to the planar structure of graphene, in comparison to the hollow tubal-shaped CNTs. The particle size and size distribution of nanostructures are small and narrow when graphene is

Graphene nanocomposites can be made from graphene oxide (GO) which is essentially a graphene sheet containing oxy-functional groups, such as epoxy, hydroxyl, carbonyl, and carboxylic, on the surface. The reactive functional groups provide a means of attaching the nanoparticles to the graphene sheet [22, 23]. The GO starting material can be simply and economically synthesized using chemical oxidation of graphite [24]. Graphene manufactured using this highly scalable synthesis route is also popularly known as reduced GO (rGO). Using GO as a support for metallic ions, many types of different nanocomposites have been made. The overview of the types of materials and synthesis methods pertaining to the graphenebased nanostructures are presented in Table 1 along with the potential application for the

**Dimension Synthesis Method Potential Application Reference**

and catalysis

reduction Solar energy conversion [26]

reduction Solar energy conversion [26]

AgAu 50–200 nm In situ chemical synthesis Electrochemical immunosensing [27]

Antibacterial agent, nanofluids for cooling technology, water treatments, surface-enhanced Raman scattering (SERS), transparent and conductive film, electrochemical immunosensor,

[25]

the additional performance benefits of the matrix compound itself.

used as a support for the growth of nanostructures [19-21].

**2. Inorganic nanostructures decorated graphene**

**Morphology and**

Ag 16.9±3.5 nm Rapid thermal treatment

Ag ~420 nm Laser assisted photocatalytic

Au 30-70 nm Laser assisted photocatalytic

nanocomposites.

378 Advanced Topics on Crystal Growth

**Inorganic Nanostructure**

*Metal*


agent (Equation 1), effectively increasing the oxidation state of the Fe ions from Fe2+ to Fe3+. This is followed by the reaction of the Fe3+ ions, in an alkaline condition, into Fe3O4 nanopar‐ ticles (Equation 2) on the surface of the rGO. The complete stoichiometry is depicted by Equation 3. During the redox reaction, the polar oxygenated functional groups on the GO sheets serve as the anchoring sites for the Fe3O4 nanoparticles, consequently preventing serious

**Figure 1.** Schematic illustration of the formation of Fe3O4/rGO nanocomposite via a one-step in situ chemical deposi‐

tion method [20].

2+ 3+ 2Fe + GO 2Fe + rGO ® (1)

Inorganic Nanostructures Decorated Graphene

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381

34 2 Fe + 2Fe + 8OH Fe O + 4H O ® (2)

34 2 3Fe + GO 8OH Fe O + 4H O + rGO ® (3)

agglomeration of the magnetic nanoparticles (Figure 1b).

2+ 3+ -

( ) 2+ -

**Table 1.** A summary of inorganic nanostructures decorated graphene and their potential applications. (Note: If the inorganic nanostructures are nanoparticles, the shape is not mentioned under the column of Morphology and Dimension.)

### **3. Interaction of nanostructures with graphene**

The widely accepted mechanism for the synthesis of inorganic nanostructures decorated graphene is the attraction of the positively-charged metal ions by the polarised bonds of the functional groups on the GO. The attachment of the metal ions to the surface and edges of the GO results in a redox reaction and the formation of nucleation sites, which eventually leads to the growth of nanostructures on the 2-D graphene sheets. An example of this process is the redox hybridization process which occurs between GO and Fe2+ to form Fe3O4/rGO nanocom‐ posite and this is shown in Figure 1 [58]. The Fe2+ ions are first attached to the surface by the functional groups on the surface of the GO sheets (Figure 1a). The GO acts as an oxidizing agent (Equation 1), effectively increasing the oxidation state of the Fe ions from Fe2+ to Fe3+. This is followed by the reaction of the Fe3+ ions, in an alkaline condition, into Fe3O4 nanopar‐ ticles (Equation 2) on the surface of the rGO. The complete stoichiometry is depicted by Equation 3. During the redox reaction, the polar oxygenated functional groups on the GO sheets serve as the anchoring sites for the Fe3O4 nanoparticles, consequently preventing serious agglomeration of the magnetic nanoparticles (Figure 1b).

**Inorganic Nanostructure**

380 Advanced Topics on Crystal Growth

*Metal oxides*

Bi2WO6

La2Ti2O7

*Others*

Ag/TiO2

Core-shell Fe@Fe2O3@ Si-S-O

Fe2O3-ZnO

Dimension.)

**Morphology and**

nanoparticles containing square nanoplates, 100−300 nm

nanosheets, comprehensively integrated

TiO2 layer coexists

Fe2O3 ~50 nm,

30–40 nm

SnSb quasi-spherical,

mean of 22 nm One-pot

**3. Interaction of nanostructures with graphene**

with Ag

**Dimension Synthesis Method Potential Application Reference**

Reflux Photocatalyst [49]

irradiation Photocatalyst [50]

sensor [48]

Photoelectrochemical conversion [52]

Chromium removal [54]

ZrO2 ~42 nm Electrochemical route Enzymeless methyl parathion

Expansion and UV

NiFe2O4 6.5 nm Hydrothermal Anode for lithium-ion batteries [51] Pd-CoO 5–7 nm Laser irradiation CO oxidation [31]

> Dipping-lifting in sol-gel solution, reducing process and interface reaction

CdS 7.5–20 nm Solvothermal Photocatalyst [53]

FeS2 spherical, ~50 nm Hydrothermal Solar energy conversion [56]

**Table 1.** A summary of inorganic nanostructures decorated graphene and their potential applications. (Note: If the inorganic nanostructures are nanoparticles, the shape is not mentioned under the column of Morphology and

The widely accepted mechanism for the synthesis of inorganic nanostructures decorated graphene is the attraction of the positively-charged metal ions by the polarised bonds of the functional groups on the GO. The attachment of the metal ions to the surface and edges of the GO results in a redox reaction and the formation of nucleation sites, which eventually leads to the growth of nanostructures on the 2-D graphene sheets. An example of this process is the redox hybridization process which occurs between GO and Fe2+ to form Fe3O4/rGO nanocom‐ posite and this is shown in Figure 1 [58]. The Fe2+ ions are first attached to the surface by the functional groups on the surface of the GO sheets (Figure 1a). The GO acts as an oxidizing

ZnO <10 nm Hydrothermal Photocatalyst [55]

Solvothermal Anode for lithium-ion batteries [57]

thermodecomposition

$$\text{2Fe}^{2+} + \text{GO} \rightarrow \text{2Fe}^{3+} + \text{rGO} \tag{1}$$

$$\text{Fe}^{2+} + 2\text{Fe}^{3+} + 8\text{OH}^{\cdot} \rightarrow \text{Fe}\_3\text{O}\_4 + 4\text{H}\_2\text{O} \tag{2}$$

$$\text{\textbulletFe}^{2+} + \text{GO} \left( 8 \text{OH}^{\cdot} \right) \rightarrow \text{Fe}\_3\text{O}\_4 + 4 \text{H}\_2\text{O} + \text{rGO} \tag{3}$$

**Figure 1.** Schematic illustration of the formation of Fe3O4/rGO nanocomposite via a one-step in situ chemical deposi‐ tion method [20].

In the case of metal/graphene, the nanocomposites are produced through simultaneous reduction of GO and metal ions [29, 31, 32]. In the absence of a reducing environment as in pure water but under the illumination of a radiating energy such as laser, the reduction mechanism of the metal ions (M2+) involving the photogenerated electrons from GO are shown by Equations 4–7, where GO is partially reduced as portrayed by Equations 6 and 7 [26].

$$\text{GO} + hv \rightarrow \text{GO} \left( h^+ + e^- \right) \tag{4}$$

sheets, the ZnO nanoflowers appear on both sides of the graphene support [61]. Another method based around microwave heating has been used to make SnO2/graphene nanocom‐ posite [19] (Figure 2g). At a higher magnification (Figure 2h), the SnO2 nanoparticles were observed to uniformly adhere on the graphene sheets, with high density [19]. A simple, costeffective, efficient, and green in situ deposition to synthesize Fe3O4 nanoparticles on graphene [20] (Figure 2i) has also been explored. The magnetic property of the Fe3O4/graphene nano‐ composite allows the separation of composite from the solution by applying an external

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**Figure 2.** Electron micrographs of various inorganic nanostructures decorating graphene; (a) Ag, (b) FeS2, (c) CuO, (d) TiO2, (e) ZnO, (f) a higher magnification showing flower-like shape ZnO nanoarchitecture, (g) SnO2, (h) a higher magni‐ fication revealing SnO2 nanoparticles, and (i) Fe3O4. (j) Photo image of Fe3O4/graphene dispersed uniformly in water

magnetic field (Figure 2j).

and attracted under the external magnetic field.

$$\text{4h}^+ + \text{2H}\_2\text{O} \rightarrow \text{O}\_2 + \text{4H}^+\tag{5}$$

$$\text{M}^+ \text{+GO} \left(\text{e}^-\right) \rightarrow \text{GO} + \text{M} \tag{6}$$

$$\text{rGO} + 4\text{e}^- + 4\text{H}^+ \rightarrow \text{rGO} + 2\text{H}\_2\text{O} \tag{7}$$

In the presence of a reducing agent such as sodium borohydride, NaBH4 used for the synthesis of Ni/graphene, Ni ions and the COOH groups on the surface of the GO sheets were reduced to Ni metal and CH2OH, respectively. The corresponding equation may proceed as represented by Equation 8 [29].

$$4\text{Ni}\_2^{2+} + \text{BH}\_4^- + 8\text{OH}^- \rightarrow 4\text{Ni} + \text{BO}\_2^- + 6\text{H}\_2\text{O} \text{ } - \text{COOH} \xrightarrow{\{\text{BH}\_4^-\}} - \text{CH}\_2\text{OH} \tag{8}$$

The incorporation of nanoparticles could be through chemisorption, physisorption, electro‐ static interaction, van der Waals or covalent bonding with rGO [21, 22]. The attachment of nanostructures onto the surface of the graphene reduces the attractive interactions between the rGO sheets and, minimizes the aggregation and restacking of rGO during the reduction process [53]. Moreover, trace quantity of nanostructures on the basal planes of rGO allows uniform dispersion of the nanocomposite in polar solvents, which is otherwise impossible for rGO [20].

### **4. A new class of graphene-based inorganic nanostructures**

The morphological structure of graphene nanocomposites is varied and depends on the synthesis route. Ag/graphene was synthesized using a thermal expansion method and yielded uniformly distributed Ag nanoparticles on graphene sheets [25] (Figure 2a). Whereas the hydrothermal approach gave an assemble pyrite structured FeS2 nanospheres [56] (Figure 2b), CuO nanospheres [59] (Figure 2c), TiO2 nanoparticles [43] (Figure 2d) and ZnO nano‐ flowers [60] (Figures 2e and 2f). In the last example, some of the ZnO nanoflowers were brighter in the SEM image than others and seemed to be enveloped by a thin film of graphene. Since the functional groups, such as hydroxyl and epoxy groups, are attached to both sides of GO sheets, the ZnO nanoflowers appear on both sides of the graphene support [61]. Another method based around microwave heating has been used to make SnO2/graphene nanocom‐ posite [19] (Figure 2g). At a higher magnification (Figure 2h), the SnO2 nanoparticles were observed to uniformly adhere on the graphene sheets, with high density [19]. A simple, costeffective, efficient, and green in situ deposition to synthesize Fe3O4 nanoparticles on graphene [20] (Figure 2i) has also been explored. The magnetic property of the Fe3O4/graphene nano‐ composite allows the separation of composite from the solution by applying an external magnetic field (Figure 2j).

In the case of metal/graphene, the nanocomposites are produced through simultaneous reduction of GO and metal ions [29, 31, 32]. In the absence of a reducing environment as in pure water but under the illumination of a radiating energy such as laser, the reduction mechanism of the metal ions (M2+) involving the photogenerated electrons from GO are shown by Equations 4–7, where GO is partially reduced as portrayed by Equations 6 and 7 [26].

In the presence of a reducing agent such as sodium borohydride, NaBH4 used for the synthesis of Ni/graphene, Ni ions and the COOH groups on the surface of the GO sheets were reduced to Ni metal and CH2OH, respectively. The corresponding equation may proceed as represented

The incorporation of nanoparticles could be through chemisorption, physisorption, electro‐ static interaction, van der Waals or covalent bonding with rGO [21, 22]. The attachment of nanostructures onto the surface of the graphene reduces the attractive interactions between the rGO sheets and, minimizes the aggregation and restacking of rGO during the reduction process [53]. Moreover, trace quantity of nanostructures on the basal planes of rGO allows uniform dispersion of the nanocomposite in polar solvents, which is otherwise impossible for

The morphological structure of graphene nanocomposites is varied and depends on the synthesis route. Ag/graphene was synthesized using a thermal expansion method and yielded uniformly distributed Ag nanoparticles on graphene sheets [25] (Figure 2a). Whereas the hydrothermal approach gave an assemble pyrite structured FeS2 nanospheres [56] (Figure 2b), CuO nanospheres [59] (Figure 2c), TiO2 nanoparticles [43] (Figure 2d) and ZnO nano‐ flowers [60] (Figures 2e and 2f). In the last example, some of the ZnO nanoflowers were brighter in the SEM image than others and seemed to be enveloped by a thin film of graphene. Since the functional groups, such as hydroxyl and epoxy groups, are attached to both sides of GO

2 4 2 2 <sup>2</sup> 4 8 4 6, *BH Ni BH OH Ni BO H O COOH CH OH* - +- - - + + ® + + - ¾¾¾¾®- (8)

<sup>4</sup> 2 [ ]

**4. A new class of graphene-based inorganic nanostructures**

by Equation 8 [29].

382 Advanced Topics on Crystal Growth

rGO [20].

( ) – GO GO *hv h e* <sup>+</sup> +® + (4)

2 2 4 2H O O + 4H *<sup>h</sup>*<sup>+</sup> + ® (5)

( ) M + GO e GO + M + – ® (6)

– + GO + 4e + 4H rGO + 2H O ® <sup>2</sup> (7)

+

**Figure 2.** Electron micrographs of various inorganic nanostructures decorating graphene; (a) Ag, (b) FeS2, (c) CuO, (d) TiO2, (e) ZnO, (f) a higher magnification showing flower-like shape ZnO nanoarchitecture, (g) SnO2, (h) a higher magni‐ fication revealing SnO2 nanoparticles, and (i) Fe3O4. (j) Photo image of Fe3O4/graphene dispersed uniformly in water and attracted under the external magnetic field.

rGOhasproventobeaneffectivematrixfortheadhesionofnanostructuresduetotherichcontent of oxide functional groups on the basal planes and edges of the 2-D material. Good evidence of rGO being an accomplished support is depicted through an FESEM image of Fe3O4/graphene, in which the nanocomposite was prepared at low concentration of Fe2+ ions. The micrograph of the sample shows no evidence for the formation of Fe3O4 nanoparticles (Figure 3a). This is in contrast to the sample prepared using a higher concentration of Fe2+ ions where nanoparticles are apparent(Figure 2i).To investigate this observationfurther, elementalmappingofC,O, and Fe using energy dispersive x-ray (EDX) analysis (Figure 3b) was undertaken (Figure 3c). The areaofbright contrast correlateswiththeFe signalmap.This result, coupledwiththeXRDresult [20], provides evidence for the presence of Fe3O4 on the surface of the graphene. The exact form of the Fe3O4 cannot be determined. It is possible that a layer of Fe3O4 has formed on the surface of the rGO or more likely that very small nanoparticles have formed.

negatively charged electron cloud surrounding the oxygen atoms of the GO sheets is the initial step in the nucleation of the nanostructure on the graphene sheet. Since oxygen atoms are uniformly distributed in the starting graphene oxide sheets, a uniform decoration of nano‐ structures on the graphene sheets is produced after the reaction. Some metal oxides such as Fe3O4 and SnO2 can grow on the surface of rGO at room temperature [20, 39]. However, by increasing the reaction temperature, the high temperature calcination step may be able to be eliminated e.g. SnO2/graphene [42]. The reduction of GO may not occur during the deposition of the nanostructures on the surface of the GO, for example in the synthesis of MnO2/GO [61]. To ensure the GO is completely reduced, additional reducing agents can be used. For example, in the synthesis of nanocrystal Ag/graphene, hydrazine and ammonia solution in the presence

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In situ chemical synthesis has also been described as a one-pot thermodecomposition route. When graphene sheets are directly used, a stabilizing agent such as sodium dodecylbenzene‐ sulfonate (SDBS) is required along with a high reaction temperature to encourage the assembly of the nanostructure. The formation of Fe nanostructures on graphene is an example of this. [54]. Similarly, CuO/graphene was synthesized through the stabilization of graphene sheet with ethylene glycol at a high temperature [34]. For some materials, however, the high reaction temperature is not required to promote the assembly. Ag2O/graphene nanocomposites are an example and these were prepared in the presence of N-Methyl-2-pyrrolidone (NMP) under ambient conditions [33]. The stabilizing agents also function as a size- and shape-controlling agent and a reducing agent. In FeS2/graphene, the stabilization agent constraints the growth of FeS2 to a gelatin micelles geometry, resulting in well-formed nanoparticles [56]. Variations of the in situ chemical synthesis include the electrode-less Ni-plating on graphene sheets [29] and the attachment of Bi2WO6 nanoparticles and nanoplates onto GO by refluxing [49].

Hydrothermal synthesis is an efficient inorganic synthesis approach for the formation of a variety of nanomaterials, catalysts, ion-conductors, and zeolites [62] under controlled tem‐ perature and pressure. This synthesis route overcomes the drawbacks of high processing temperatures and long reaction times compared to conventional aqueous chemical processing conditions [63]. It has been recognized as an environmentally friendly process because the reaction uses aqueous solutions as a reaction medium and it is carried out in an autoclave, which is a closed system. This method can also be used for the preparation of high-purity, highly crystalline, ultrafine and homogeneous powders of various single and multi-compo‐ nent powders [64, 65]. The autoclave used in the hydrothermal synthesis, as mentioned previously, is a closed system and so raising its temperature increases the pressure inside the vessel above the critical pressure for water which enhances the dissolution of thermodynam‐ ically unstable compounds. The high heat energy and pressure in the autoclave facilitates fracture of the macronucleus to form nano-sized particles [66]. This method has been used for the synthesis of CuO/graphene [21], NiO/graphene [37], ZnO/graphene [47], and FeS2/

of polyvinyl alcohol were used [27].

**5.2. Hydrothermal**

graphene nano-composites [56].

**Figure 3.** Fe3O4/graphene prepared at a low concentration of Fe2+ ions: (a) FESEM image, (b) EDX spectrum, and (c) elemental mapping [20].

### **5. Growth processes of inorganic nanostructures on graphene sheets**

#### **5.1. In situ chemical synthesis**

In situ chemical synthesis is a robust route for the formation of graphene decorated with inorganic nanostructures. The attraction of the positively charged metal ions towards the negatively charged electron cloud surrounding the oxygen atoms of the GO sheets is the initial step in the nucleation of the nanostructure on the graphene sheet. Since oxygen atoms are uniformly distributed in the starting graphene oxide sheets, a uniform decoration of nano‐ structures on the graphene sheets is produced after the reaction. Some metal oxides such as Fe3O4 and SnO2 can grow on the surface of rGO at room temperature [20, 39]. However, by increasing the reaction temperature, the high temperature calcination step may be able to be eliminated e.g. SnO2/graphene [42]. The reduction of GO may not occur during the deposition of the nanostructures on the surface of the GO, for example in the synthesis of MnO2/GO [61]. To ensure the GO is completely reduced, additional reducing agents can be used. For example, in the synthesis of nanocrystal Ag/graphene, hydrazine and ammonia solution in the presence of polyvinyl alcohol were used [27].

In situ chemical synthesis has also been described as a one-pot thermodecomposition route. When graphene sheets are directly used, a stabilizing agent such as sodium dodecylbenzene‐ sulfonate (SDBS) is required along with a high reaction temperature to encourage the assembly of the nanostructure. The formation of Fe nanostructures on graphene is an example of this. [54]. Similarly, CuO/graphene was synthesized through the stabilization of graphene sheet with ethylene glycol at a high temperature [34]. For some materials, however, the high reaction temperature is not required to promote the assembly. Ag2O/graphene nanocomposites are an example and these were prepared in the presence of N-Methyl-2-pyrrolidone (NMP) under ambient conditions [33]. The stabilizing agents also function as a size- and shape-controlling agent and a reducing agent. In FeS2/graphene, the stabilization agent constraints the growth of FeS2 to a gelatin micelles geometry, resulting in well-formed nanoparticles [56]. Variations of the in situ chemical synthesis include the electrode-less Ni-plating on graphene sheets [29] and the attachment of Bi2WO6 nanoparticles and nanoplates onto GO by refluxing [49].

#### **5.2. Hydrothermal**

rGOhasproventobeaneffectivematrixfortheadhesionofnanostructuresduetotherichcontent of oxide functional groups on the basal planes and edges of the 2-D material. Good evidence of rGO being an accomplished support is depicted through an FESEM image of Fe3O4/graphene, in which the nanocomposite was prepared at low concentration of Fe2+ ions. The micrograph of the sample shows no evidence for the formation of Fe3O4 nanoparticles (Figure 3a). This is in contrast to the sample prepared using a higher concentration of Fe2+ ions where nanoparticles are apparent(Figure 2i).To investigate this observationfurther, elementalmappingofC,O, and Fe using energy dispersive x-ray (EDX) analysis (Figure 3b) was undertaken (Figure 3c). The areaofbright contrast correlateswiththeFe signalmap.This result, coupledwiththeXRDresult [20], provides evidence for the presence of Fe3O4 on the surface of the graphene. The exact form of the Fe3O4 cannot be determined. It is possible that a layer of Fe3O4 has formed on the surface

**Figure 3.** Fe3O4/graphene prepared at a low concentration of Fe2+ ions: (a) FESEM image, (b) EDX spectrum, and (c)

In situ chemical synthesis is a robust route for the formation of graphene decorated with inorganic nanostructures. The attraction of the positively charged metal ions towards the

**5. Growth processes of inorganic nanostructures on graphene sheets**

elemental mapping [20].

384 Advanced Topics on Crystal Growth

**5.1. In situ chemical synthesis**

of the rGO or more likely that very small nanoparticles have formed.

Hydrothermal synthesis is an efficient inorganic synthesis approach for the formation of a variety of nanomaterials, catalysts, ion-conductors, and zeolites [62] under controlled tem‐ perature and pressure. This synthesis route overcomes the drawbacks of high processing temperatures and long reaction times compared to conventional aqueous chemical processing conditions [63]. It has been recognized as an environmentally friendly process because the reaction uses aqueous solutions as a reaction medium and it is carried out in an autoclave, which is a closed system. This method can also be used for the preparation of high-purity, highly crystalline, ultrafine and homogeneous powders of various single and multi-compo‐ nent powders [64, 65]. The autoclave used in the hydrothermal synthesis, as mentioned previously, is a closed system and so raising its temperature increases the pressure inside the vessel above the critical pressure for water which enhances the dissolution of thermodynam‐ ically unstable compounds. The high heat energy and pressure in the autoclave facilitates fracture of the macronucleus to form nano-sized particles [66]. This method has been used for the synthesis of CuO/graphene [21], NiO/graphene [37], ZnO/graphene [47], and FeS2/ graphene nano-composites [56].

The hydrothermal systhesis methology is not restricted to pure aqueous solutions. The addition of other solvents like ethanol can be used to enhance the dispersion of gel-like GO [43]. For example, TiO2 nanoparticles could be chemically bonded to the surface of rGO [45, 46]. Likewise, a one pot synthesis of Fe2O3 nanoparticles, Zn salt and GO produced Fe2O3-ZnO/ graphene nanocomposites, in which the Fe2O3 nanoparticles were chemically bonded to the graphene sheets. The 50 nm sized Fe2O3 nanoparticles were covered with ZnO nanoparticles that are less than 10 nm in size [55].

the unoxidized portions of GO [38]. It does, however, swell in an aqueous solution as water

The formation of the actual graphene based GCE is problematic because it is difficult to obtain a uniform dispersion of graphene in a solvent. The graphene sheets when in solution tend to form irreversible agglomerates or even restack to graphite through strong *π*-*π* stacking and van der Waals interaction. If graphene is to be used for the modification of GCE, it must first be dispersed in a stabilizer to form a homogenous dispersion before being dropped cast on the GCE surface. The inorganic nanostructures on graphene are formed by cyclic voltammetry in the appropriate salt solution. An example of this method is the synthesis Ag/graphene [28]. Electrochemical co-deposition is another route to prepare inorganic nanostructure/graphene. Pd/graphene is an example, where a solution containing GO and a metal salt underwent

There have been several novel processing routes that have been reported. A Ag precursor was

Ag/graphene [25]. This method produced high purity nanocomposite because the process did not require any reducing or stabilizing agents. The extreme condition spontaneously reduced the Ag ion to Ag metal, and GO to rGO. Inorganic nanostructure/graphene has also been prepared using irradiation (lasers [31], [26], and exposure to UV [50]). Layer-by-layer inorganic structure/graphene can be prepared by dipping-lifting in sol-gel solution [52]. Finally, ultrasonication of the starting solution was able to generate of Mn3O4/graphene nanocompo‐

**6. Recent advances in the applications of inorganic nanostructure decorated**

Graphite-based lithium-ion batteries suffer from poor charge–discharge performance and consequently have poor power performance. In many applications like electric or hybrid cars, there will be periods of high power demand, for example accelerating to overtake a car, and this leads to increased heat dissipation in the cell and accelerated aging of the battery. The use of graphene electrodes significantly improves the power performance of the battery and has resulted in a significant number of papers. The versatility of graphene to accommodate lithium ions on both sides of its single atomic sheet provides high energy storage capacity above 600 mAhg–1, which is higher than the theoretical capacity of graphite (372 mAhg–1). Recently, the use of metal oxides and metal alloys graphene nanocomposite has improved the energy storage of graphene based electrodes even more (700–4000 mAhg–1) [37]. The extra capacity has been attributed to the synergistic effect between the nanoparticles and graphene sheets in the

C for 20s to produce

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physically grounded with dry GO before the product was heated at 1000o

nanocomposite, providing extra sites for the storage of lithium ions [35].

molecules bond to the GO.

voltammetry cycling simultaneously [30].

**5.5. Other unusual strategic routes**

**6.1. Anode for lithium-ion batteries**

sites [36].

**graphene**

If the solutions are not aqueous based, the method is known as the solvothermal process, for instance absolute ethanol was used as a medium for the synthesis of SnSb/graphene [57]. For the synthesis of CdS/graphene, dimethyl sulfoxide (DMSO) not only acted as a reaction medium but also the source of sulphide and a reducing agent [67]. SnO2/graphene was prepared via a gas–liquid interfacial reaction, in a process similar to hydrothermal method. During the reaction, a two-phase liquid was vaporized, which allowed the Sn4+ to react with the ammonia at gas/liquid interface to produce Sn(OH)4 along with in situ deposited onto the graphene sheets. The Sn(OH)4 subsequently decomposed to SnO2 on the graphene sheets [41]. Fe3O4/graphene has also been prepared using the same process [35].

### **5.3. Microwave heating**

Microwave heating is believed to be more depending on the molecular properties and the reaction conditions than conventional heating [68]. Microwave syntheses have been increas‐ ingly used in the preparation of high monodispersity nanoparticles of oxides such as SnO2, CeO2 and ZrO2 [69]. Utilizing microwave energy for the thermal treatment generally leads to very fine particles in the nanocrystalline regime mainly caused by the shorter synthesis time and a highly focused localised heating. The particle size often falls in the range of 15 nm – 35 nm [70]. Microwave heating also has been widely used for the fabrication of inorganic nanostructure/graphene where the particles were less than 10 nm with a narrow size distri‐ bution. In this synthesis, the graphene sheets obviously played an important role in constrain‐ ing the growth of nanostructures [32, 71]. Hydrothermal method has also been combined with microwave heating to ensure a complete reduction of GO [40].

### **5.4. Electrodeposition**

Electrodeposition provides a facile procedure and offers precise control of the thickness of the resulting film [72]. In addition to the speed of polymerization, which can be controlled by the current density [73], this method also enables mild processing conditions at room temperature [74], without toxic or excess chemicals [75]. GO is coated on the surface of a glassy carbon electrode (GCE) before being immersed into a salt solution. By cycling the potential, the metal salt is oxidized to metal oxide and the GO is reduced to graphene. This synthesis methodology has been used in the preparation of PbO/graphene [38] and ZrO2/graphene [48].

Although it is known to be hydrophilic, GO does not peel off from the GCE surface when it is placed in aqueous electrolytes because of the interaction of hydrophobic regions of GCE and the unoxidized portions of GO [38]. It does, however, swell in an aqueous solution as water molecules bond to the GO.

The formation of the actual graphene based GCE is problematic because it is difficult to obtain a uniform dispersion of graphene in a solvent. The graphene sheets when in solution tend to form irreversible agglomerates or even restack to graphite through strong *π*-*π* stacking and van der Waals interaction. If graphene is to be used for the modification of GCE, it must first be dispersed in a stabilizer to form a homogenous dispersion before being dropped cast on the GCE surface. The inorganic nanostructures on graphene are formed by cyclic voltammetry in the appropriate salt solution. An example of this method is the synthesis Ag/graphene [28].

Electrochemical co-deposition is another route to prepare inorganic nanostructure/graphene. Pd/graphene is an example, where a solution containing GO and a metal salt underwent voltammetry cycling simultaneously [30].

### **5.5. Other unusual strategic routes**

The hydrothermal systhesis methology is not restricted to pure aqueous solutions. The addition of other solvents like ethanol can be used to enhance the dispersion of gel-like GO [43]. For example, TiO2 nanoparticles could be chemically bonded to the surface of rGO [45, 46]. Likewise, a one pot synthesis of Fe2O3 nanoparticles, Zn salt and GO produced Fe2O3-ZnO/ graphene nanocomposites, in which the Fe2O3 nanoparticles were chemically bonded to the graphene sheets. The 50 nm sized Fe2O3 nanoparticles were covered with ZnO nanoparticles

If the solutions are not aqueous based, the method is known as the solvothermal process, for instance absolute ethanol was used as a medium for the synthesis of SnSb/graphene [57]. For the synthesis of CdS/graphene, dimethyl sulfoxide (DMSO) not only acted as a reaction medium but also the source of sulphide and a reducing agent [67]. SnO2/graphene was prepared via a gas–liquid interfacial reaction, in a process similar to hydrothermal method. During the reaction, a two-phase liquid was vaporized, which allowed the Sn4+ to react with the ammonia at gas/liquid interface to produce Sn(OH)4 along with in situ deposited onto the graphene sheets. The Sn(OH)4 subsequently decomposed to SnO2 on the graphene sheets [41].

Microwave heating is believed to be more depending on the molecular properties and the reaction conditions than conventional heating [68]. Microwave syntheses have been increas‐ ingly used in the preparation of high monodispersity nanoparticles of oxides such as SnO2, CeO2 and ZrO2 [69]. Utilizing microwave energy for the thermal treatment generally leads to very fine particles in the nanocrystalline regime mainly caused by the shorter synthesis time and a highly focused localised heating. The particle size often falls in the range of 15 nm – 35 nm [70]. Microwave heating also has been widely used for the fabrication of inorganic nanostructure/graphene where the particles were less than 10 nm with a narrow size distri‐ bution. In this synthesis, the graphene sheets obviously played an important role in constrain‐ ing the growth of nanostructures [32, 71]. Hydrothermal method has also been combined with

Electrodeposition provides a facile procedure and offers precise control of the thickness of the resulting film [72]. In addition to the speed of polymerization, which can be controlled by the current density [73], this method also enables mild processing conditions at room temperature [74], without toxic or excess chemicals [75]. GO is coated on the surface of a glassy carbon electrode (GCE) before being immersed into a salt solution. By cycling the potential, the metal salt is oxidized to metal oxide and the GO is reduced to graphene. This synthesis methodology

Although it is known to be hydrophilic, GO does not peel off from the GCE surface when it is placed in aqueous electrolytes because of the interaction of hydrophobic regions of GCE and

has been used in the preparation of PbO/graphene [38] and ZrO2/graphene [48].

Fe3O4/graphene has also been prepared using the same process [35].

microwave heating to ensure a complete reduction of GO [40].

that are less than 10 nm in size [55].

386 Advanced Topics on Crystal Growth

**5.3. Microwave heating**

**5.4. Electrodeposition**

There have been several novel processing routes that have been reported. A Ag precursor was physically grounded with dry GO before the product was heated at 1000o C for 20s to produce Ag/graphene [25]. This method produced high purity nanocomposite because the process did not require any reducing or stabilizing agents. The extreme condition spontaneously reduced the Ag ion to Ag metal, and GO to rGO. Inorganic nanostructure/graphene has also been prepared using irradiation (lasers [31], [26], and exposure to UV [50]). Layer-by-layer inorganic structure/graphene can be prepared by dipping-lifting in sol-gel solution [52]. Finally, ultrasonication of the starting solution was able to generate of Mn3O4/graphene nanocompo‐ sites [36].

### **6. Recent advances in the applications of inorganic nanostructure decorated graphene**

### **6.1. Anode for lithium-ion batteries**

Graphite-based lithium-ion batteries suffer from poor charge–discharge performance and consequently have poor power performance. In many applications like electric or hybrid cars, there will be periods of high power demand, for example accelerating to overtake a car, and this leads to increased heat dissipation in the cell and accelerated aging of the battery. The use of graphene electrodes significantly improves the power performance of the battery and has resulted in a significant number of papers. The versatility of graphene to accommodate lithium ions on both sides of its single atomic sheet provides high energy storage capacity above 600 mAhg–1, which is higher than the theoretical capacity of graphite (372 mAhg–1). Recently, the use of metal oxides and metal alloys graphene nanocomposite has improved the energy storage of graphene based electrodes even more (700–4000 mAhg–1) [37]. The extra capacity has been attributed to the synergistic effect between the nanoparticles and graphene sheets in the nanocomposite, providing extra sites for the storage of lithium ions [35].

In any battery application, the degradation of the storage capacity with repeated chargedischarge cycles is extremely important. The cycling performance of graphene inorganic nanostructures far exceeds that of their individual counterparts [21, 35]. Moreover, graphene inorganic nanostructures also displayed excellent reversible specific capacities at a broad range of current densities [40-42]. The good lithium cycling performance is ascribed to the structural integrity of the composite electrodes. The nanocystals located on the surface of graphene prevented the agglomeration of graphene sheets. Likewise, the graphene sheets hindered the direct contact among the adjacent nanocrystals. Minimizing the aggregation of nanocrystals and graphene sheets during discharge/charge cycling gives rise to high surface area, excellent electronic conductivity of the electrodes by forming an efficient electrically conductive network, and high carrier mobility. This heterogeneous construction also provides buffering spaces against the volume changes of nanocrystals during the lithium insertion/extraction processes.

polymers including polypyrole [100] and polyaniline [101, 102] usually show poor capacitance

Recently, graphene has been used as a supercapacitor electrode material due to its high surface area, excellent stability and good conductivity [103-106]. To effectively overcome the shortage of low specific capacitance, the 2-D structured graphene has been hybridized with pseudoca‐ pacitors for the preparation of supercapacitors [32, 107-109]. The specific capacitance and electrochemical stability of graphene inorganic structures is enhanced tremendously in comparison to their individual counterparts [33, 36]. The improvement of the supercapacitive behaviour is attributable to the different double-layer and pseudocapacitive contributions.

Organic effluents from industries, agricultural activities and the rapid increase in human waste as a result of the rapid population increase represents some of the most serious environmental pollutants. It is estimated that around four billion people worldwide have no or little access to clean and sanitized water supply, resulting in death, by severe waterborne diseases, of millions of people annually [110]. Photodegradation of organic pollutants has attracted increasing attention during the past decade as it appears to be a viable decontamination process with widespread application, regardless of the state (gas or liquid) or chemical nature of the process target [45, 111]. Photocatalytic oxidation is an economical process owing to the fact that it involves only a photocatalyst and light source [112]. This process does not yield toxic intermediate product, making it suitable for cleaning water environment that contains low to

When a photocatalyst is illuminated with the light of sufficient energy, electrons in the valence band of the photocatalyst are excited into the conduction band, therefore, creating the negative

superoxide radical anions, *O* •+, on the photocatalyst surface. Any organic contaminants at or near to the photocatalyst surface are oxidized by the generated radicals [114, 115]. The most commonly used photocatalysts are titanium dioxide and zinc oxide, which are semiconduc‐ tors. The drawback of these photocatalysts is the quick electron-hole recombination on the

To manipulate the rate of electron-hole recombination, the presence of non-metal species like carbon, nitrogen, boron and fluorine are targeted to minimize photogenerated electron-hole recombination rate, thus improving quantum efficiency and expanding their useful range of operation into visible light wavelengths [116, 117]. Graphene, a uniform and thin transparent

Nanocrystals are not stable and prone to aggregate, which results in a reduced surface area and so limits the likelihood of the photoinduced electron–hole pairs interacting with water molecules to produce the radicals thereby decreasing the application efficiency. This can be overcome by using graphene as a supporting matrix for the photocatalyst particles, as well

surface of semiconductors, which hampers the hydrogen evolution efficiency.

pairs will initiate a series of reactions and produce hydroxyl radicals, *H O* • and

). When the photocatalyst is in contact with water, the

Inorganic Nanostructures Decorated Graphene

http://dx.doi.org/10.5772/54321

389

behavior.

**6.3. Photocatalysts**

medium contaminants concentration [113].

conducting material, is a potential carbon source.

electron-positive hole pairs (*e* <sup>−</sup> −*h* <sup>+</sup>

*e* <sup>−</sup> −*h* <sup>+</sup>

Regardless of the type of inorganic nanocrystal, the graphene nanocomposites have generally displayed a large irreversible capacity in the first cycle [51, 57]. It is widely reported that this phenomenon may be attributed to electrolyte decomposition and formation of solid electrolyte interface (SEI) film [76-78]. Irreversible capacity is the occurrence of naturally non-recoverable charge capacity, in which the kinetics will vary depending on the battery chemistry, electrode composition and design, electrolyte formulation and impurities, and on the storage tempera‐ ture [79].

### **6.2. Supercapacitors**

With the increase in affluence in developing countries, the energy needs are increasing and energy sustainability is of significant concern especially when the depletion of fossil fuels is also factored in [61]. Supercapacitors, also known as ultracapacitors or electrochemical supercapacitors, have several important characteristics, including prolonged life cycle [80, 81], higher power density than batteries [82] and higher energy density [83] than conventional capacitors which have driven their use in pulse power and power backup applications [84]. According to the mechanism of charge storage, supercapacitors can be classified as i) electric double layer capacitors (EDLCs) where charge is stored at the electrode/electrolyte interface, and ii) pseudocapacitors where the charge is stored mainly by Faradaic reactions on the surface of the electrode materials [85, 86].

Carbon-based materials are commonly used in EDLCs as electrodes because of its high electrical conductivity and outstanding long-term electrochemical stability as a result of the extraordinary chemical stability of carbon [87]. Carbon-based materials such as activated carbon [88], xerogels [89], carbon nanotubes [90], mesoporous carbon [91] and carbide-derived carbons [92] have all been investigated for use as electrodes in EDLC. However, the limited charge accumulation in electrical double layer restricts the specific capacitance of EDLCs to a relatively small range of values between 90 and 250 F/g [93]. Meanwhile, pseudocapacitors like metal oxides such as RuO2 [94-96], NiO [96, 97], Co3O4 [98], MnO2 [99], or conducting polymers including polypyrole [100] and polyaniline [101, 102] usually show poor capacitance behavior.

Recently, graphene has been used as a supercapacitor electrode material due to its high surface area, excellent stability and good conductivity [103-106]. To effectively overcome the shortage of low specific capacitance, the 2-D structured graphene has been hybridized with pseudoca‐ pacitors for the preparation of supercapacitors [32, 107-109]. The specific capacitance and electrochemical stability of graphene inorganic structures is enhanced tremendously in comparison to their individual counterparts [33, 36]. The improvement of the supercapacitive behaviour is attributable to the different double-layer and pseudocapacitive contributions.

### **6.3. Photocatalysts**

In any battery application, the degradation of the storage capacity with repeated chargedischarge cycles is extremely important. The cycling performance of graphene inorganic nanostructures far exceeds that of their individual counterparts [21, 35]. Moreover, graphene inorganic nanostructures also displayed excellent reversible specific capacities at a broad range of current densities [40-42]. The good lithium cycling performance is ascribed to the structural integrity of the composite electrodes. The nanocystals located on the surface of graphene prevented the agglomeration of graphene sheets. Likewise, the graphene sheets hindered the direct contact among the adjacent nanocrystals. Minimizing the aggregation of nanocrystals and graphene sheets during discharge/charge cycling gives rise to high surface area, excellent electronic conductivity of the electrodes by forming an efficient electrically conductive network, and high carrier mobility. This heterogeneous construction also provides buffering spaces against the volume changes of nanocrystals during the lithium insertion/extraction

Regardless of the type of inorganic nanocrystal, the graphene nanocomposites have generally displayed a large irreversible capacity in the first cycle [51, 57]. It is widely reported that this phenomenon may be attributed to electrolyte decomposition and formation of solid electrolyte interface (SEI) film [76-78]. Irreversible capacity is the occurrence of naturally non-recoverable charge capacity, in which the kinetics will vary depending on the battery chemistry, electrode composition and design, electrolyte formulation and impurities, and on the storage tempera‐

With the increase in affluence in developing countries, the energy needs are increasing and energy sustainability is of significant concern especially when the depletion of fossil fuels is also factored in [61]. Supercapacitors, also known as ultracapacitors or electrochemical supercapacitors, have several important characteristics, including prolonged life cycle [80, 81], higher power density than batteries [82] and higher energy density [83] than conventional capacitors which have driven their use in pulse power and power backup applications [84]. According to the mechanism of charge storage, supercapacitors can be classified as i) electric double layer capacitors (EDLCs) where charge is stored at the electrode/electrolyte interface, and ii) pseudocapacitors where the charge is stored mainly by Faradaic reactions on the surface

Carbon-based materials are commonly used in EDLCs as electrodes because of its high electrical conductivity and outstanding long-term electrochemical stability as a result of the extraordinary chemical stability of carbon [87]. Carbon-based materials such as activated carbon [88], xerogels [89], carbon nanotubes [90], mesoporous carbon [91] and carbide-derived carbons [92] have all been investigated for use as electrodes in EDLC. However, the limited charge accumulation in electrical double layer restricts the specific capacitance of EDLCs to a relatively small range of values between 90 and 250 F/g [93]. Meanwhile, pseudocapacitors like metal oxides such as RuO2 [94-96], NiO [96, 97], Co3O4 [98], MnO2 [99], or conducting

processes.

388 Advanced Topics on Crystal Growth

ture [79].

**6.2. Supercapacitors**

of the electrode materials [85, 86].

Organic effluents from industries, agricultural activities and the rapid increase in human waste as a result of the rapid population increase represents some of the most serious environmental pollutants. It is estimated that around four billion people worldwide have no or little access to clean and sanitized water supply, resulting in death, by severe waterborne diseases, of millions of people annually [110]. Photodegradation of organic pollutants has attracted increasing attention during the past decade as it appears to be a viable decontamination process with widespread application, regardless of the state (gas or liquid) or chemical nature of the process target [45, 111]. Photocatalytic oxidation is an economical process owing to the fact that it involves only a photocatalyst and light source [112]. This process does not yield toxic intermediate product, making it suitable for cleaning water environment that contains low to medium contaminants concentration [113].

When a photocatalyst is illuminated with the light of sufficient energy, electrons in the valence band of the photocatalyst are excited into the conduction band, therefore, creating the negative electron-positive hole pairs (*e* <sup>−</sup> −*h* <sup>+</sup> ). When the photocatalyst is in contact with water, the *e* <sup>−</sup> −*h* <sup>+</sup> pairs will initiate a series of reactions and produce hydroxyl radicals, *H O* • and superoxide radical anions, *O* •+, on the photocatalyst surface. Any organic contaminants at or near to the photocatalyst surface are oxidized by the generated radicals [114, 115]. The most commonly used photocatalysts are titanium dioxide and zinc oxide, which are semiconduc‐ tors. The drawback of these photocatalysts is the quick electron-hole recombination on the surface of semiconductors, which hampers the hydrogen evolution efficiency.

To manipulate the rate of electron-hole recombination, the presence of non-metal species like carbon, nitrogen, boron and fluorine are targeted to minimize photogenerated electron-hole recombination rate, thus improving quantum efficiency and expanding their useful range of operation into visible light wavelengths [116, 117]. Graphene, a uniform and thin transparent conducting material, is a potential carbon source.

Nanocrystals are not stable and prone to aggregate, which results in a reduced surface area and so limits the likelihood of the photoinduced electron–hole pairs interacting with water molecules to produce the radicals thereby decreasing the application efficiency. This can be overcome by using graphene as a supporting matrix for the photocatalyst particles, as well as an electron acceptor, to improve the efficiency of the degradation of organic pollutants [67] and durability for consecutive photodegradation cycling [45, 49]. In addition, the giant π-conjugation of graphene and two-dimensional planar structure are the driving forces for the non-covalent adsorption between aromatic molecules and aromatic regions of the graphene [45].

high mechanical strength but ultra-light weight, rich electronic properties, and excellent chemical and thermal stability [130] and when coupled with graphene, they provided an attractive nanocomposite for the fabrication of electrochemical sensors [33, 131]. In this regard, inorganic nanostructure decorated graphene has been proven to be an effective tool to detect low amounts of biomarker [27], pesticide [48], heavy metals [28, 38, 39], and glucose [34], with

Inorganic Nanostructures Decorated Graphene

http://dx.doi.org/10.5772/54321

391

Besides the four major applications stated above, graphene inorganic nanostructures have also been investigated for other purposes. These include direct formic acid fuel cells, which are widely considered to be one of the most attractive power sources [30]; providing a favourable catalytic pathway for the formation of CO2 [31] to form CO [132]; and producing electrocata‐ lytic activity for methanol oxidation [32]. Other uses of the structures is to produce a photo‐ current under UV light or visible light illumination to meet the demand of renewable and clean energy source [52], assisting the conversion of solar energy into hydrogen via the water splitting process [133], and removing chromium(IV) in water through adsorption [54].

**7. What does the future hold for inorganic nanostructure decorated**

scientific community and industrialists who seek new partnerships and advances.

This work was supported by the Exploratory Research Grant Scheme (ER016-2011A), High Impact Research Grants from the University of Malaya (UM.C/625/1/HIR/030) and High Impact Research Grants from the Ministry of Higher Education of Malaysia (UM.C/625/1/HIR/

The significant potential of graphene-based inorganic nanostructure in solving many of today's problems is evident from the amount of effort that has been devoted to exploring the synthesis of the materials and the investigation of the materials in real-life applications. The cutting-edge research on graphene-based nanoinorganic materials has yet to mature. Drawing comparisons to silicone research, the interest will continue to grow until commercial products using graphene are realized. The simple and scalable production of GO, a derivative of graphene, that is rich in oxygenous functional groups, is encouragement for researchers to modify the surface of the one-atom thick carbon layers with a variety of inorganic nanostruc‐ tures to cater for the commercial demands and needs. The quality of inorganic nanostructure decorated graphene at atomic level is assured by systematic characterization using state-ofthe-art instruments. The graphene research offers novel and exciting opportunities for the

high sensitivity, selectivity, stability and reproducibility.

**6.5. Other interesting applications**

**graphene?**

**Acknowledgements**

MOHE/05).

Many photocatalysts have a wide band gap and so require UV light for operation. Since ultraviolet (UV) light accounts for only a small fraction (5%) of the Sun's energy as compared to visible light (45%); any shift in the optical response of a photocatalyst from the UV to the visible spectral range will have a profound positive effect on the photodegradation efficiency of a photocatalyst [118]. Incorporation of carbon is known to be able to reduce the band gap energy of a photocatalyst [116]. Photocatalysis using graphene inorganic structure could take place under the irradiation of visible light due to zero band semiconductor with symmetric *K* and *K'* [49].

On the other hand, the photoelectrocatalytic process takes advantage of the photocatalytic process by applying a biased voltage across a photo-electrode on which the photocata‐ lysts are supported. An enhancement of the photocurrents using graphene nanocomposite electrodes is attributed to the enhanced migration efficiency of the photo-induced elec‐ trons and enhanced adsorption activity of the aromatic molecules [46]. Photoelectrocatalyt‐ ic activity is dependent on the optimal value of graphene content, as superfluous graphene will reduce the absorption efficiency of light by a photocatalyst. Experiments have shown an increase in the degradation of the aromatic molecules by the graphene nanocomposite as the applied potential was increased relative to a reference electrode [119-124]. This was explained by the potential causing band bending close to the electrode surface reducing the potential barriers and improving the mobility of the carrier across the electrode, and hence minimizing the probability of recombination of electrons and holes and elevating the photoelectrocatalysis efficiency [124].

The overall excellent photocatalytic performance of graphene inorganic nanostructures is reportedly attributed to enhanced adsorptivity, extended light absorption range, efficient charge separation and transportation [45].

### **6.4. Sensing platform**

Many analytical methods with high sensitivity and low detection limit have been established for determination of organic and inorganic matters [125]. However, they are time consuming, expensive, require complicated instruments and a skilled operator, which are unsuitable for on-line or in-field monitoring [126]. In contrast, electrochemical analysis, which has the advantages of quick response, cheap instrumentation, low power consumption, simplified operation, time-saving, high sensitivity and selectivity, is widely applied in applications such as gas sensing, chemical sensing and biosensing [27, 127-129].

Direct detection using bare electrode yields poor sensitivity to the target material [128]. Inorganic nanoparticles are versatile and sensitive tracers because of their high surface area, high mechanical strength but ultra-light weight, rich electronic properties, and excellent chemical and thermal stability [130] and when coupled with graphene, they provided an attractive nanocomposite for the fabrication of electrochemical sensors [33, 131]. In this regard, inorganic nanostructure decorated graphene has been proven to be an effective tool to detect low amounts of biomarker [27], pesticide [48], heavy metals [28, 38, 39], and glucose [34], with high sensitivity, selectivity, stability and reproducibility.

### **6.5. Other interesting applications**

as an electron acceptor, to improve the efficiency of the degradation of organic pollutants [67] and durability for consecutive photodegradation cycling [45, 49]. In addition, the giant π-conjugation of graphene and two-dimensional planar structure are the driving forces for the non-covalent adsorption between aromatic molecules and aromatic regions of the

Many photocatalysts have a wide band gap and so require UV light for operation. Since ultraviolet (UV) light accounts for only a small fraction (5%) of the Sun's energy as compared to visible light (45%); any shift in the optical response of a photocatalyst from the UV to the visible spectral range will have a profound positive effect on the photodegradation efficiency of a photocatalyst [118]. Incorporation of carbon is known to be able to reduce the band gap energy of a photocatalyst [116]. Photocatalysis using graphene inorganic structure could take place under the irradiation of visible light due to zero band semiconductor with symmetric *K*

On the other hand, the photoelectrocatalytic process takes advantage of the photocatalytic process by applying a biased voltage across a photo-electrode on which the photocata‐ lysts are supported. An enhancement of the photocurrents using graphene nanocomposite electrodes is attributed to the enhanced migration efficiency of the photo-induced elec‐ trons and enhanced adsorption activity of the aromatic molecules [46]. Photoelectrocatalyt‐ ic activity is dependent on the optimal value of graphene content, as superfluous graphene will reduce the absorption efficiency of light by a photocatalyst. Experiments have shown an increase in the degradation of the aromatic molecules by the graphene nanocomposite as the applied potential was increased relative to a reference electrode [119-124]. This was explained by the potential causing band bending close to the electrode surface reducing the potential barriers and improving the mobility of the carrier across the electrode, and hence minimizing the probability of recombination of electrons and holes and elevating the

The overall excellent photocatalytic performance of graphene inorganic nanostructures is reportedly attributed to enhanced adsorptivity, extended light absorption range, efficient

Many analytical methods with high sensitivity and low detection limit have been established for determination of organic and inorganic matters [125]. However, they are time consuming, expensive, require complicated instruments and a skilled operator, which are unsuitable for on-line or in-field monitoring [126]. In contrast, electrochemical analysis, which has the advantages of quick response, cheap instrumentation, low power consumption, simplified operation, time-saving, high sensitivity and selectivity, is widely applied in applications such

Direct detection using bare electrode yields poor sensitivity to the target material [128]. Inorganic nanoparticles are versatile and sensitive tracers because of their high surface area,

graphene [45].

390 Advanced Topics on Crystal Growth

and *K'* [49].

photoelectrocatalysis efficiency [124].

charge separation and transportation [45].

as gas sensing, chemical sensing and biosensing [27, 127-129].

**6.4. Sensing platform**

Besides the four major applications stated above, graphene inorganic nanostructures have also been investigated for other purposes. These include direct formic acid fuel cells, which are widely considered to be one of the most attractive power sources [30]; providing a favourable catalytic pathway for the formation of CO2 [31] to form CO [132]; and producing electrocata‐ lytic activity for methanol oxidation [32]. Other uses of the structures is to produce a photo‐ current under UV light or visible light illumination to meet the demand of renewable and clean energy source [52], assisting the conversion of solar energy into hydrogen via the water splitting process [133], and removing chromium(IV) in water through adsorption [54].

### **7. What does the future hold for inorganic nanostructure decorated graphene?**

The significant potential of graphene-based inorganic nanostructure in solving many of today's problems is evident from the amount of effort that has been devoted to exploring the synthesis of the materials and the investigation of the materials in real-life applications. The cutting-edge research on graphene-based nanoinorganic materials has yet to mature. Drawing comparisons to silicone research, the interest will continue to grow until commercial products using graphene are realized. The simple and scalable production of GO, a derivative of graphene, that is rich in oxygenous functional groups, is encouragement for researchers to modify the surface of the one-atom thick carbon layers with a variety of inorganic nanostruc‐ tures to cater for the commercial demands and needs. The quality of inorganic nanostructure decorated graphene at atomic level is assured by systematic characterization using state-ofthe-art instruments. The graphene research offers novel and exciting opportunities for the scientific community and industrialists who seek new partnerships and advances.

### **Acknowledgements**

This work was supported by the Exploratory Research Grant Scheme (ER016-2011A), High Impact Research Grants from the University of Malaya (UM.C/625/1/HIR/030) and High Impact Research Grants from the Ministry of Higher Education of Malaysia (UM.C/625/1/HIR/ MOHE/05).

### **Author details**

Hong Ngee Lim1,2, Nay Ming Huang3 , Chin Hua Chia4 and Ian Harrison5

1 Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, UPM Serdang, Se‐ langor, Malaysia

[8] Pumera, M. Electrochemistry of graphene: New horizons for sensing and energy

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http://dx.doi.org/10.5772/54321

393

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2 Functional Device Laboratory, Institute of Advanced Technology, Universiti Putra Malay‐ sia, UPM Serdang, Selangor, Malaysia

3 Department of Physics, Faculty of Science, University of Malaya, Kuala Lumpur, Malaysia

4 School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Ma‐ laysia, Bangi, Selangor, Malaysia

5 Faculty of Engineering, The University of Nottingham Malaysia Campus, Jalan Broga, Se‐ menyih, Selangor, Malaysia

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**Chapter 13**

**Metal Chalcogenides Tetrahedral Molecular Clusters:**

In recent years, the development of crystalline porous materials based on metal chalcoge‐ nides attracts scientific attention for their adjustable porous structures and potential applica‐ tions in technology. In contrast to oxygen, for which only the di- and tri-nuclear

teristic strong tendency of sulfur and the other elements of Group 16 is reflected in the wide

ed as salts from polar solvents in the presence of suitable counter cations. [1]The choice of sulfides has many obvious advantages in the crystallization chemistry:[2] (a) In comparison with oxide and fluoride ions, the S2-ion has a much largerionic radius, which favors the tet‐ rahedral coordination withcationsand allows the discovery of sulfide homologues of zeo‐ lites. (b) The higher polar ability of the S2-ion shows more flexibility for the structure of tetrahedra angles. For example, the tetrahedraT-S-T angle ranges from109°–161°. But the range of angle for tetrahedra T-O-T is 140°–145°. (T = tetrahedra metal atom, such as In). Ob‐ viously, the frameworks with higher flexibility will have better ability to accommodate vari‐ ous shapes of the templates, and the arrangement of tetrahedralunits in the dense matter can

Nowadays, chemists use inorganic clusters as molecular building blocks to create open framework with cavities and channels, including porous semiconductor, fast ion exchanger, shape- and size-selective catalysis, and optoelectronic applications. Among these clusters, only the metal chalcogenides tetrahedral molecular clusters can serve as artificial tetrahedral atoms, and assemble the tetrahedral clusters into porous open-framework through inorganic

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

2- (X = S, Se, Te). The polychalcogenideXn

© 2013 Ou and Yang; licensee InTech. This is an open access article 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.

© 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

2-, are known in zeolite frameworks, the charac‐

2-are easily isolat‐

2-, O2

‧-and O3

**Crystal Engineering and Properties**

Chun-Chang Ou and Chung-Sung Yang

http://dx.doi.org/10. 5772/52660

homopolyatomic anions, i. e. O2

range of polychalcogenide ions Xn

remain their original architectures.

chalcogenides ligands.

**1. Introduction**

Additional information is available at the end of the chapter

