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Electrochemical rebuilding of pure gold surface into flower-like nanostructured gold films
Materials Letters 82 (2012) 202–204
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Electrochemical rebuilding of pure gold surface into flower-like nanostructured gold films Shili Xu, Yuan Yao, Zelin Li, Hefang Zhang, Fuli Huang, Wei Huang ⁎ Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China), College of Chemistry and Chemical Engineering, Hunan Normal University, Lushan Road, Changsha 410081 China
a r t i c l e
i n f o
Article history: Received 26 April 2012 Accepted 18 May 2012 Available online 26 May 2012 Keywords: Flower-like gold Anodization Rebuilding Electrocatalysis Nanoparticles Thin films
a b s t r a c t A facile, green method has been developed to fabricate a three-dimensional (3D) flower-like nanostructured gold film (FNGF) by anodization of pure gold substrate in a mixed solution containing potassium chloride (KCl) and ascorbic acid (AA). The formation of FNGFs might be related to three main interfacial processes: gold electrodissolution, chemical reduction of AuCl4−, and self-assembly of gold nanoparticles (Au NPs). The effects of step potentials and time on film roughness have also been investigated. The as-prepared 3D FNGF exhibits high electrocatalytic activities toward the glucose oxidation and hydrogen peroxide reduction in comparison with the Au NP modified glass carbon (GC) electrode. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Flower-like nanostructured noble metals have attracted intensive interests because of their wide applications in electrocatalysis [1], electroanalysis [2], surface-enhanced Raman spectroscopy [3,4], and superhydrophobicity [5]. These materials have usually been prepared by bottom-up methods such as chemical reduction [4,6], electroreduction [3], and photoreduction–centrifugation [7]. Generally, such methods require noble metal ions as precursors and many of them need surfactants. At present, some electrochemical approaches have been employed to fabricate porous or dendritic noble films from bulk noble metals or alloys [8–14]. However, research on preparing flower-like gold nanostructures directly with pure gold substrate can hardly be found in the literature. Herein, we report a facile and green approach to fabricate a 3D FNGF within several minutes at room temperature, simply by potentiostatically anodizing a pure gold substrate in a KCl/AA solution. The formation principle and electrocatalytic applications of the 3D FNGF are also presented.
mercurous sulfate electrode (SMSE) were employed as the working, counter and reference electrode, respectively. Prior to use the working electrode was polished with 1200# metallographic paper and then rinsed with ultrasonic waves. To fabricate FNGFs, the gold disk electrode was treated potentiostatically in 0.2 M (M= mol dm− 3) KCl + 1 M AA for a given period of time. The roughness factor (R) for the FNGFs was
2. Materials and methods Electrochemical experiments were carried out with a CHI 660C electrochemical station (Chenhua Instruments, Shanghai, China). A gold disk (d= 1 mm, purity≥ 99.99%), a platinum wire, and a saturated ⁎ Corresponding author. Tel./fax: + 86 731 88872531. E-mail address: [email protected] (W. Huang). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.05.068
Fig. 1. (a) linear scan voltammograms for the smooth Au electrode (geometric area: 0.00785 cm−2) in 0.2 M KCl, 1 M AA + 0.2 M K2SO4 and 0.2 M KCl + 1 M AA solutions; (b) current-time curve for the smooth Au electrode at 0.8 V in 0.2 M KCl + 1 M AA.
S. Xu et al. / Materials Letters 82 (2012) 202–204
characterized by cyclic voltammetry (CV) [15] in 0.5 M H2SO4. A 3D FNGF prepared at 0.8 V for 250 s was used to test the electrocatalytic activities toward the reduction of 0.1 M H2O2 and oxidation of 0.1 M glucose in 0.1 M phosphate buffer solution (PBS) + 0.1 M Na2SO4 (pH = 7.2). For comparison, 14-nm-diameter gold nanoparticles (Au NPs) were synthesized by a conventional citrate-reduction method [16] and then modified onto a 3-mm-diameter glass carbon (GC) electrode (denoted as Au NPs/GC). All solutions were freshly prepared using ultrapure water and analytical grade chemicals. All experiments were performed at room temperature (ca. 20 °C). The morphologies of the FNGFs were characterized by a field emission scanning electron microscope (FESEM, Nova NanoSEM 230). X-ray diffraction (XRD) patterns were obtained with a Dmax Rapid IIR diffractometer. 3. Results and discussion In 0.2 M KCl solution, gold anodic dissolution starts at 0.4 V and reaches a limiting current (~0.20 mA) plateau ranging from 0.65 to 0.78 V, followed by passivation at 0.8 V and above, which is similar to that in HCl solution [8,9] (Fig. 1a). AA electrooxidation in K2SO4 medium
203
commences at − 0.27 V, peaks at 0.15 and 0.4 V [17], and then maintains a near stable current (~0.53 mA) within a wide potential range from 0.4 V to 1 V (Fig. 1a). Interestingly, several distinct characteristics can be observed in a mixed solution of 0.2 M KCl+ 1 M AA (Fig. 1a). Firstly, the onset potential is shifted positively to −0.15 V and the first peak vanishes for AA oxidation due to the suppression effect of specific adsorption of Cl−. Secondly, the potential for gold passivation also rises to a higher value of 1 V. Both AA and in situ-released H+ during AA electrooxidation [17] could suppress the formation of gold oxide film and thus the gold surface is always depassivated. Thirdly, the oxidation current at potentials from 0.6 to 1 V is markedly enhanced, possibly resulting from two parallel electrode reactions, i.e., AA electrooxidation and gold electrodissolution. By further observation, the limiting current (~0.93 mA) is somewhat larger than the sum of two parallel parts (~(0.2 + 0.53) mA), and meanwhile the gold surface becomes darkened at potentials above 0.6 V, indicating that a nanostructured surface with high electrochemical activity is formed. Reducing AuCl4− ions to Au atoms by AA [18] and redeposition of Au NPs might be responsible for the formation of such a surface. Inspiredly, we have fabricated a uniform 3D FNGF (Fig. 2c) successfully within only 5 min by applying a dissolution potential on gold in 0.2 M KCl + 1 M AA solution. Current chaos
Fig. 2. SEM images of pure gold electrode treated in 1 M AA + 0.2 M KCl solution with a potential step of (a–d) 0.8 V or (e–f) 0.6 V for (a–b) 5 s and (c–f) 250 s, respectively; (g) XRD patterns of the 3D FNGF prepared at 0.8 V for 250 s (top curve); (h) The dependence of R on the step potentials (E) for 250 s or step time (t) at 0.8 V.
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S. Xu et al. / Materials Letters 82 (2012) 202–204
occurring in the I-t curves (Fig. 1b) during the fabrication of 3D FNGF might be related to the aperiodic fluctuations of active surface area due to the two main opposing processes of anodic dissolution and gold redeposition. Fig. 2a–d shows the surface morphology of FNGFs prepared at 0.8 V with different time. Many pits can be observed on the gold surface in 5 s due to its active electrodissolution (Fig. 2b). Meanwhile, some nonuniform particles scatter on the electrode surface (Fig. 2a), including NPs within 100 nm, primary nanoflowers (hundreds of nm), and larger nanostructured flowers (~1 μm): the latter two are comprised of many nanoflakes with the thickness within 10 nm (Fig. 2b). This morphology profile could show the different stages in forming flower-like gold nanostructures. A 3D microporous film is fully developed up to 250 s, with the surface being assembled densely with uniform nanostructured flowers (Fig. 2c). Such a flower-like 3D porous nanoarchitecture could be expected to fascinate fast mass/charge transfer in electrocatalysis. As the step potential is lowered to 0.6 V, a similar film can show up except that it gets thinner (Fig. 2e) and the particle size is a little smaller (Fig. 2f), which could be caused by a lower concentration of AuCl4− under a slower dissolution rate of gold. XRD patterns (Fig. 2g) show a (111) preferred orientation for the as-prepared 3D FNGF, noting that the intensity ratio (3.0) between the Au(111) and Au(200) diffraction peaks is higher than that (1.9) for the standard gold sample (JCPDS 89–3697). The Cl − adsorption on Au(111) facets during FNGFs growth should be responsible for this [3]. The R increases at first and reaches a maximum (~66) when the step time varies from 0 to 250 s or step potential from 0.6 to 0.8 V (Fig. 2h). Then it keeps nearly constant due to the limited mass transport of Cl − though the inner layer. So an optimized condition of 0.8 V and 250 s was chosen to fabricate the 3D FNGF electrode used for the electrocatalytic applications. Based on the above results, the formation principle of 3D NFGF is illustrated schematically in Fig. 3, involving an electrochemical rebuilding process. Once a step potential above 0.6 V is imposed, the pure gold surface is anodically dissolved into soluble AuCl4−. Immediately, the as-released AuCl4− is reduced to Au atoms by AA [18] and then many Au NPs form. The newly-produced Au NPs have high surface energy and can easily be assembled into flower-like particles. Strong electrolytes, especially the mass of H + released in situ during AA electro/chemical oxidation, can accelerate the assembly process [16]. Note that electrodissolution should happen on both the gold substrate and the newly-deposited particles in a short time. Then the dissolution of substrate gradually becomes more difficult with increasing time, as Cl − can hardly diffuse through the thick assembly layer. Thus, not much more AuCl4− released from the substrate can be used in the assembly process. Finally, dissolution of the upper layer and redeposition reach a dynamic equilibrium and a 3D FNGF with high roughness forms. As shown in the voltammograms in Fig. 4, notably enhanced catalytic current density as high as 2.8 times toward the glucose electrooxidation (Fig. 4A) and markedly positively-shifted potential (ΔE≈330 mV) toward the H2O2 electroreduction (Fig. 4B) have been observed respectively for the 3D FNGF electrode (Fig. 4b) compared with the Au NPs/GC electrode (Fig. 4a).
Fig. 4. The voltammograms of (a) Au NPs/GC and (b) 3D FNGF electrode in 0.1 M PBS + 0.1 M Na2SO4 + (A) 0.1 M glucose or (B) 0.1 M H2O2. The current densities were normalized to (A) the electrochemical active surface area (0.535 cm 2 for Au NPs/GC and 0.505 cm2 for 3D FNGF) and (B) the geometric area, respectively.
4. Conclusions We have developed a method for facile fabrication of the 3D FNGF by anodization of pure gold substrate in a KCl + AA solution via electrochemical rebuilding. The formation of the 3D FNGF mainly involves active dissolution of gold, chemical reduction of in-situ released AuCl4− and self-assembly of newly-produced Au NPs. The as-prepared 3D FNGF exhibits high electrocatalytic activities toward the reduction of hydrogen peroxide and the oxidation of glucose. We expected that this approach would be applicable to some other metals/alloys. Acknowledgments We appreciate financial supports from the National Natural Science Foundation of China (Grant nos. 21173075 and 21003045) and the Ph.D. Programs Foundation of the Education Ministry of China (Grant no. 20104306110003). We are grateful to Prof. S. Jenkins for improving our manuscript. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
Fig. 3. Schematic illustration of the fabrication principle of 3D FNGF.
Jena BK, Raj CR. Langmuir 2007;23:4064–70. Das AK, Raj CR. J Electroanal Chem 2010;638:189–94. Guo SJ, Wang L, Wang EK. Chem Commun 2007:3163–5. Liang HY, Li ZP, Wang WZ, Wu YS, Xu HX. Adv Mater 2009;21:4614–8. Cao ZW, Xiao DB, Kang LT, Wang ZL, Zhang SX, Ma Y, et al. Chem Commun 2008: 2692–4. Hong LJ, Li Q, Lin H, Li Y. Mater Res Bull 2009;44:1201–4. Wang L, Wei G, Guo CL, Sun LL, Sun YJ, Song YH, et al. Colloids Surf A Physicochem Eng Asp 2008;312:148–53. Deng YP, Huang W, Chen X, Li ZL. Electrochem Commun 2008;10:810–3. Xia Y, Huang W, Zheng JF, Niu ZJ, Li ZL. Biosens Bioelectron 2011;26:3555–61. Nishio K, Masuda H. Angew Chem Int Ed 2011;50:1–6. Jia FL, Yu CF, Deng KJ, Zhang LZ. J Phys Chem C 2007;111:8424–31. Zhao W, Xu JJ, Shi CG, Chen HY. Electrochem Commun 2006;8:773–8. Chen YJ, Sun SG, Chen SP, Li JT, Gong H. Langmuir 2004;20:9920–5. Huang W, Wang MH, Zheng JF, Li ZL. J Phys Chem C 2009;113:1800–5. Trasatti S, Petrii OA. Pure Appl Chem 1991;63:711–34. Zhang YR, Xu YZ, Xia Y, Huang W, Liu FA, Yang YC, et al. J Colloid Interface Sci 2011;359:536–41. Xing XK, Shao MJ, Hsiao MW, Adzic RR, Liu CC. J Electroanal Chem 1992;339: 211–25. Gou LF, Murphy CJ. Chem Mater 2005;17:3668–72.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Electrochemical rebuilding of pure gold surface into flower-like nanostructured gold films Shili Xu, Yuan Yao, Zelin Li, Hefang Zhang, Fuli Huang, Wei Huang ⁎ Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China), College of Chemistry and Chemical Engineering, Hunan Normal University, Lushan Road, Changsha 410081 China
a r t i c l e
i n f o
Article history: Received 26 April 2012 Accepted 18 May 2012 Available online 26 May 2012 Keywords: Flower-like gold Anodization Rebuilding Electrocatalysis Nanoparticles Thin films
a b s t r a c t A facile, green method has been developed to fabricate a three-dimensional (3D) flower-like nanostructured gold film (FNGF) by anodization of pure gold substrate in a mixed solution containing potassium chloride (KCl) and ascorbic acid (AA). The formation of FNGFs might be related to three main interfacial processes: gold electrodissolution, chemical reduction of AuCl4−, and self-assembly of gold nanoparticles (Au NPs). The effects of step potentials and time on film roughness have also been investigated. The as-prepared 3D FNGF exhibits high electrocatalytic activities toward the glucose oxidation and hydrogen peroxide reduction in comparison with the Au NP modified glass carbon (GC) electrode. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Flower-like nanostructured noble metals have attracted intensive interests because of their wide applications in electrocatalysis [1], electroanalysis [2], surface-enhanced Raman spectroscopy [3,4], and superhydrophobicity [5]. These materials have usually been prepared by bottom-up methods such as chemical reduction [4,6], electroreduction [3], and photoreduction–centrifugation [7]. Generally, such methods require noble metal ions as precursors and many of them need surfactants. At present, some electrochemical approaches have been employed to fabricate porous or dendritic noble films from bulk noble metals or alloys [8–14]. However, research on preparing flower-like gold nanostructures directly with pure gold substrate can hardly be found in the literature. Herein, we report a facile and green approach to fabricate a 3D FNGF within several minutes at room temperature, simply by potentiostatically anodizing a pure gold substrate in a KCl/AA solution. The formation principle and electrocatalytic applications of the 3D FNGF are also presented.
mercurous sulfate electrode (SMSE) were employed as the working, counter and reference electrode, respectively. Prior to use the working electrode was polished with 1200# metallographic paper and then rinsed with ultrasonic waves. To fabricate FNGFs, the gold disk electrode was treated potentiostatically in 0.2 M (M= mol dm− 3) KCl + 1 M AA for a given period of time. The roughness factor (R) for the FNGFs was
2. Materials and methods Electrochemical experiments were carried out with a CHI 660C electrochemical station (Chenhua Instruments, Shanghai, China). A gold disk (d= 1 mm, purity≥ 99.99%), a platinum wire, and a saturated ⁎ Corresponding author. Tel./fax: + 86 731 88872531. E-mail address: [email protected] (W. Huang). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.05.068
Fig. 1. (a) linear scan voltammograms for the smooth Au electrode (geometric area: 0.00785 cm−2) in 0.2 M KCl, 1 M AA + 0.2 M K2SO4 and 0.2 M KCl + 1 M AA solutions; (b) current-time curve for the smooth Au electrode at 0.8 V in 0.2 M KCl + 1 M AA.
S. Xu et al. / Materials Letters 82 (2012) 202–204
characterized by cyclic voltammetry (CV) [15] in 0.5 M H2SO4. A 3D FNGF prepared at 0.8 V for 250 s was used to test the electrocatalytic activities toward the reduction of 0.1 M H2O2 and oxidation of 0.1 M glucose in 0.1 M phosphate buffer solution (PBS) + 0.1 M Na2SO4 (pH = 7.2). For comparison, 14-nm-diameter gold nanoparticles (Au NPs) were synthesized by a conventional citrate-reduction method [16] and then modified onto a 3-mm-diameter glass carbon (GC) electrode (denoted as Au NPs/GC). All solutions were freshly prepared using ultrapure water and analytical grade chemicals. All experiments were performed at room temperature (ca. 20 °C). The morphologies of the FNGFs were characterized by a field emission scanning electron microscope (FESEM, Nova NanoSEM 230). X-ray diffraction (XRD) patterns were obtained with a Dmax Rapid IIR diffractometer. 3. Results and discussion In 0.2 M KCl solution, gold anodic dissolution starts at 0.4 V and reaches a limiting current (~0.20 mA) plateau ranging from 0.65 to 0.78 V, followed by passivation at 0.8 V and above, which is similar to that in HCl solution [8,9] (Fig. 1a). AA electrooxidation in K2SO4 medium
203
commences at − 0.27 V, peaks at 0.15 and 0.4 V [17], and then maintains a near stable current (~0.53 mA) within a wide potential range from 0.4 V to 1 V (Fig. 1a). Interestingly, several distinct characteristics can be observed in a mixed solution of 0.2 M KCl+ 1 M AA (Fig. 1a). Firstly, the onset potential is shifted positively to −0.15 V and the first peak vanishes for AA oxidation due to the suppression effect of specific adsorption of Cl−. Secondly, the potential for gold passivation also rises to a higher value of 1 V. Both AA and in situ-released H+ during AA electrooxidation [17] could suppress the formation of gold oxide film and thus the gold surface is always depassivated. Thirdly, the oxidation current at potentials from 0.6 to 1 V is markedly enhanced, possibly resulting from two parallel electrode reactions, i.e., AA electrooxidation and gold electrodissolution. By further observation, the limiting current (~0.93 mA) is somewhat larger than the sum of two parallel parts (~(0.2 + 0.53) mA), and meanwhile the gold surface becomes darkened at potentials above 0.6 V, indicating that a nanostructured surface with high electrochemical activity is formed. Reducing AuCl4− ions to Au atoms by AA [18] and redeposition of Au NPs might be responsible for the formation of such a surface. Inspiredly, we have fabricated a uniform 3D FNGF (Fig. 2c) successfully within only 5 min by applying a dissolution potential on gold in 0.2 M KCl + 1 M AA solution. Current chaos
Fig. 2. SEM images of pure gold electrode treated in 1 M AA + 0.2 M KCl solution with a potential step of (a–d) 0.8 V or (e–f) 0.6 V for (a–b) 5 s and (c–f) 250 s, respectively; (g) XRD patterns of the 3D FNGF prepared at 0.8 V for 250 s (top curve); (h) The dependence of R on the step potentials (E) for 250 s or step time (t) at 0.8 V.
204
S. Xu et al. / Materials Letters 82 (2012) 202–204
occurring in the I-t curves (Fig. 1b) during the fabrication of 3D FNGF might be related to the aperiodic fluctuations of active surface area due to the two main opposing processes of anodic dissolution and gold redeposition. Fig. 2a–d shows the surface morphology of FNGFs prepared at 0.8 V with different time. Many pits can be observed on the gold surface in 5 s due to its active electrodissolution (Fig. 2b). Meanwhile, some nonuniform particles scatter on the electrode surface (Fig. 2a), including NPs within 100 nm, primary nanoflowers (hundreds of nm), and larger nanostructured flowers (~1 μm): the latter two are comprised of many nanoflakes with the thickness within 10 nm (Fig. 2b). This morphology profile could show the different stages in forming flower-like gold nanostructures. A 3D microporous film is fully developed up to 250 s, with the surface being assembled densely with uniform nanostructured flowers (Fig. 2c). Such a flower-like 3D porous nanoarchitecture could be expected to fascinate fast mass/charge transfer in electrocatalysis. As the step potential is lowered to 0.6 V, a similar film can show up except that it gets thinner (Fig. 2e) and the particle size is a little smaller (Fig. 2f), which could be caused by a lower concentration of AuCl4− under a slower dissolution rate of gold. XRD patterns (Fig. 2g) show a (111) preferred orientation for the as-prepared 3D FNGF, noting that the intensity ratio (3.0) between the Au(111) and Au(200) diffraction peaks is higher than that (1.9) for the standard gold sample (JCPDS 89–3697). The Cl − adsorption on Au(111) facets during FNGFs growth should be responsible for this [3]. The R increases at first and reaches a maximum (~66) when the step time varies from 0 to 250 s or step potential from 0.6 to 0.8 V (Fig. 2h). Then it keeps nearly constant due to the limited mass transport of Cl − though the inner layer. So an optimized condition of 0.8 V and 250 s was chosen to fabricate the 3D FNGF electrode used for the electrocatalytic applications. Based on the above results, the formation principle of 3D NFGF is illustrated schematically in Fig. 3, involving an electrochemical rebuilding process. Once a step potential above 0.6 V is imposed, the pure gold surface is anodically dissolved into soluble AuCl4−. Immediately, the as-released AuCl4− is reduced to Au atoms by AA [18] and then many Au NPs form. The newly-produced Au NPs have high surface energy and can easily be assembled into flower-like particles. Strong electrolytes, especially the mass of H + released in situ during AA electro/chemical oxidation, can accelerate the assembly process [16]. Note that electrodissolution should happen on both the gold substrate and the newly-deposited particles in a short time. Then the dissolution of substrate gradually becomes more difficult with increasing time, as Cl − can hardly diffuse through the thick assembly layer. Thus, not much more AuCl4− released from the substrate can be used in the assembly process. Finally, dissolution of the upper layer and redeposition reach a dynamic equilibrium and a 3D FNGF with high roughness forms. As shown in the voltammograms in Fig. 4, notably enhanced catalytic current density as high as 2.8 times toward the glucose electrooxidation (Fig. 4A) and markedly positively-shifted potential (ΔE≈330 mV) toward the H2O2 electroreduction (Fig. 4B) have been observed respectively for the 3D FNGF electrode (Fig. 4b) compared with the Au NPs/GC electrode (Fig. 4a).
Fig. 4. The voltammograms of (a) Au NPs/GC and (b) 3D FNGF electrode in 0.1 M PBS + 0.1 M Na2SO4 + (A) 0.1 M glucose or (B) 0.1 M H2O2. The current densities were normalized to (A) the electrochemical active surface area (0.535 cm 2 for Au NPs/GC and 0.505 cm2 for 3D FNGF) and (B) the geometric area, respectively.
4. Conclusions We have developed a method for facile fabrication of the 3D FNGF by anodization of pure gold substrate in a KCl + AA solution via electrochemical rebuilding. The formation of the 3D FNGF mainly involves active dissolution of gold, chemical reduction of in-situ released AuCl4− and self-assembly of newly-produced Au NPs. The as-prepared 3D FNGF exhibits high electrocatalytic activities toward the reduction of hydrogen peroxide and the oxidation of glucose. We expected that this approach would be applicable to some other metals/alloys. Acknowledgments We appreciate financial supports from the National Natural Science Foundation of China (Grant nos. 21173075 and 21003045) and the Ph.D. Programs Foundation of the Education Ministry of China (Grant no. 20104306110003). We are grateful to Prof. S. Jenkins for improving our manuscript. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
Fig. 3. Schematic illustration of the fabrication principle of 3D FNGF.
Jena BK, Raj CR. Langmuir 2007;23:4064–70. Das AK, Raj CR. J Electroanal Chem 2010;638:189–94. Guo SJ, Wang L, Wang EK. Chem Commun 2007:3163–5. Liang HY, Li ZP, Wang WZ, Wu YS, Xu HX. Adv Mater 2009;21:4614–8. Cao ZW, Xiao DB, Kang LT, Wang ZL, Zhang SX, Ma Y, et al. Chem Commun 2008: 2692–4. Hong LJ, Li Q, Lin H, Li Y. Mater Res Bull 2009;44:1201–4. Wang L, Wei G, Guo CL, Sun LL, Sun YJ, Song YH, et al. Colloids Surf A Physicochem Eng Asp 2008;312:148–53. Deng YP, Huang W, Chen X, Li ZL. Electrochem Commun 2008;10:810–3. Xia Y, Huang W, Zheng JF, Niu ZJ, Li ZL. Biosens Bioelectron 2011;26:3555–61. Nishio K, Masuda H. Angew Chem Int Ed 2011;50:1–6. Jia FL, Yu CF, Deng KJ, Zhang LZ. J Phys Chem C 2007;111:8424–31. Zhao W, Xu JJ, Shi CG, Chen HY. Electrochem Commun 2006;8:773–8. Chen YJ, Sun SG, Chen SP, Li JT, Gong H. Langmuir 2004;20:9920–5. Huang W, Wang MH, Zheng JF, Li ZL. J Phys Chem C 2009;113:1800–5. Trasatti S, Petrii OA. Pure Appl Chem 1991;63:711–34. Zhang YR, Xu YZ, Xia Y, Huang W, Liu FA, Yang YC, et al. J Colloid Interface Sci 2011;359:536–41. Xing XK, Shao MJ, Hsiao MW, Adzic RR, Liu CC. J Electroanal Chem 1992;339: 211–25. Gou LF, Murphy CJ. Chem Mater 2005;17:3668–72.