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Pd nanotube electrodes with improved electrocatalytic stability for formic acid electrooxidation
Materials Chemistry and Physics 134 (2012) 567e570
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Pd nanotube electrodes with improved electrocatalytic stability for formic acid electrooxidation You-Jung Song, Young-Woo Lee, Sang-Beom Han, Kyung-Won Park* Department of Chemical Engineering, Soongsil University, Seoul 156-743, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 December 2011 Received in revised form 25 February 2012 Accepted 8 March 2012
We report Pd nanotube electrodes fabricated by means of electrodeposition method as a function of applied current density. Transmission electron microscopy and scanning electron microscopy images show that the higher applied current density results in the rougher surface structure. From the X-ray diffraction patterns, it is observed that the smaller grain size of the Pd nanotube electrodes may be attributed to the higher applied current density. In the electrochemical data, it is found that the Pd nanotube electrode with the smaller grain size results in much higher electrocatalytic properties for formic acid electrooxidation. The Pd nanotube electrode electrodeposited under the highest applied current density exhibits the improved electrocatalytic stability in comparison with Pd catalyst supported by carbon black (Pd/C, E-TEK, Co.). Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Electrodeposition Pd nanotube array Formic acid electrooxidation Stability
1. Introduction Palladium has been of major interest as an anode catalyst for direct formic acid fuel cells (DFAFCs) as promising power sources [1e8]. It has been reported that electrochemical characteristics of anode materials are dramatically dependent on their particle size, crystallinity, surface area and morphology [9e12]. In particular, the ordered surface structure of one-dimensional nanostructured catalysts could affect electrochemical reactions and electrocatalytic activity [13,14]. Among a variety of synthetic approaches for one-dimensional nanostructure arrays, an anodic aluminum oxide (AAO) membrane-based method has received an attractive attention because its uniform and reproducible porous structure as an ideal template can provide highly ordered nanotube or nanowire-type arrays [15e17]. It has been well known that the variation of applied current density or voltage in the electrodeposition system could extremely affect the structural characteristics of the deposited electrodes [18]. Herein, Pd nanotube anode catalysts were obtained by means of electrodeposition method through porous AAO membrane. The nanotube structures were characterized by transmission electron microscopy, scanning electron microscopy and X-ray diffraction
* Corresponding author. Tel.: þ82 2 820 0613; fax: þ82 2 812 5378. E-mail address: [email protected] (K.-W. Park). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.03.048
method. To characterize and compare electrochemical properties and catalytic activity for formic acid electrooxidation, voltammograms of Pd nanotube anode catalysts were obtained using a potentiostat. 2. Experimental Electrodeposition was carried out as a function of applied current density at 25 C in an aqueous solution containing 0.2 M H3BO3 and 25 mM PdCl2 using Pt wire and Ag/AgCl used as a counter and a reference electrode, respectively. A thin Au layer was sputtered on one side of the porous alumina templates (Whatman, 200 nm pore size) for 2 min in Ar atmosphere to be served as a working electrode. After an Ag plate was polished and cleaned, a section of AAO film was immobilized on the surface of Ag plate which is served as a current collector electrode. The Pd nanotubes were electrodeposited for 12 h by using different current densities of 1, 3 and 5 mA cm2. The Pd nanotube arrays embedded in AAO were dipped into 3 M NaOH solution for l h to completely remove the AAO and washed in distilled water several times. The morphology and size of the Pd nanotube arrays were observed by means of scanning electron microscopy (SEM, JEOL JSM-6360A) and transmission electron microscopy (TEM, PhillipsF20). In the SEM analysis, the spot size was 40 nm and the free working distance (FWD) 11 13 mm. The TEM investigation was carried out at an accelerating voltage of 200 kV and Cu grids were
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Y.-J. Song et al. / Materials Chemistry and Physics 134 (2012) 567e570
used as substrates. The crystal structures of the Pd nanotubes were confirmed by X-ray diffraction (XRD, Rigaku X-ray diffractometer equipped with a Cu Ka source at 40 kV and 100 mA). To evaluate electrochemical properties and catalytic activity for formic acid electrooxidation of the Pd nanotube array electrodes, voltammograms were obtained in a solution of 0.1 M HClO4 with or without 0.5 M HCOOH at 25 C. A Pt wire and Ag/AgCl were used as a counter and a reference electrode, respectively. All potentials are reported with respect to Ag/AgCl. 3. Results and discussion Fig. 1 shows TEM and SEM images of Pd array electrodes electrodeposited using different current densities of 1, 3 and 5 mA cm2 for 12 h, denoted as Pd-NT-1, Pd-NT-3 and Pd-NT5, respectively. All the samples exhibit tubular arrays having an average diameter of w220 nm and different lengths with applied current densities. As described in the literature, the very thin metal layer as a working electrode on the AAO back side can produce nanotube arrays whereas nanowire arrays have been fabricated using the blocked metal layer [19e21]. The Pd-NT-1 (Fig. 1(a)e(c)) electrodeposited at relatively low applied current density of 1 mA cm2 consists of arrays with smooth and dense surface. However, as the applied current densities for tube array growth increase, the Pd-NT-3 (Fig. 1(d)e(f)) and Pd-NT-5 (Fig. 1(g)e(i)) consist of arrays with relatively rough surface. In particular, the tube arrays of the Pd-NT-5 electrodeposited under the highest
Fig. 2. XRD patterns of (a) Pd-NT-1, (b) Pd-NT-3 and (c) Pd-NT-5. The asterisk mark represents the XRD patterns of sputtered Au.
applied current density of 5 mA cm2 display the roughest surface. This suggests that the fast tube array growth by higher current density may result in rougher surface structure. To characterize crystal structures of Pd nanotube arrays, the XRD patterns of the Pd-NT electrodes are obtained as shown in Fig. 2. In the
Fig. 1. TEM and FE-SEM images of as-prepared Pd nanotube electrodes: Pd-NT-1 ((a),(b),(c)), Pd-NT-3 ((d),(e),(f)) and Pd-NT-5 ((g),(h),(i)).
Y.-J. Song et al. / Materials Chemistry and Physics 134 (2012) 567e570
Fig. 3. (a) CVs of Pd catalysts in Ar-saturated 0.1 M HClO4 with a scan rate of 50 mV s1 (b) CVs of Pd catalysts for formic acid electrooxidation at 25 C in 0.5 M HCOOH and 0.1 M HClO4 with a scan rate of 20 mV s1.
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XRD patterns, the reflections corresponding to the planes (111) (200) and (220), characteristic of the face-centered cubic structure of Pd are present with XRD peaks related to AAO templates. The diffraction peak width of Pd-NT-1 is less broad than those of Pd-NT-3 and Pd-NT-5. The Pd-NT-5 exhibits the broadest peaks implying that the grain size of PdNT-5 is smaller than those of Pd-NT-3 and Pd-NT-5. According to the calculation of particle sizes based on the full width half maximum of (220) peaks, the average grain sizes of Pd-NT-1, Pd-NT-3, and Pd-NT-5 are 12.6, 11.5, and 8.9 nm, respectively. This implies that smaller grain size of electrodes may be due to the increased applied current density for relatively fast tube array growth. To characterize catalytic properties of Pd nanotubes toward the formic acid electrooxidation, the nanotube electrodes were compared using Pd nanoparticles deposited on carbon black (Pd/C, E-TEK, Co.) as a typical catalyst. Fig. 3(a) shows cyclic voltammograms (CVs) of Pd-NT electrodes in Ar-saturated 0.1 M HClO4 with a scan rate of 50 mV s1 with palladium loadings of 28 mg cm2. From the hydrogen adsorption areas of Pd, the order of electrochemical active surface area (EASA) of Pd-NT electrodes is Pd-NT-1 < Pd-NT3 < Pd-NT-5. The high EASA of the Pd-NT-5 is attributed to small average grain sizes of Pd-NT-5 compared to Pd-NT-1 and Pd-NT-3. Thus, it is expected that the Pd-NT-5 may exhibit excellent electrocatalytic properties for formic acid electrooxidation. As indicated in Fig. 3(b), the electrocatalytic mass activity of the Pd-NT-5 in formic acid electrooxidation is much higher than those of Pd-NT-1 and PdNT-3. However, the Pd catalyst (20 wt%) supported by carbon black (Pd/C, E-TEK Co.) having the highest EASA exhibits the highest electrocatalytic activity in comparison with Pd-NT electrodes. To characterize the electrocatalytic stability for formic acid electrooxidation, the stability test was performed by applying oxidation potential of 0.2 V for 1 h in 0.5 M HCOOH and 0.1 M HClO4 at 25 C. The Pd-NT-5 exhibits nearly maintained electrocatalytic activity after the stability test representing such an excellent stability for formic acid electrooxidation (Fig. 4(a)). As indicated in Fig. 4(b) and (c), the size and morphology of the Pd-NT-5 still remain after the stability test suggesting much improved catalytic stability.
Fig. 4. (a) CVs of Pd-NT-5 before and after stability test in 0.5 M HCOOH and 0.1 M HClO4 at 25 C. TEM images and TED patterns of Pd-NT-5 (b) before and (c) After stability test. (d) CVs of Pd/C before and after stability test in 0.5 M HCOOH and 0.1 M HClO4 at 25 C. TEM images and TED patterns of Pd/C (e) before and (f) After stability test.
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In contrast, the electrocatalytic activity of the Pd/C after the stability test significantly deteriorates (Fig. 4(d)). Furthermore, the Pd/C shows increased particle size and non-uniform size distribution resulting in deteriorated catalytic activity (Fig. 4(e) and (f)). It is likely that the improved electrocatalytic stability of the Pd-NT-5 toward formic acid electrooxidation may result from the less aggregated Pd catalysts during the catalytic reaction in comparison with the Pd/C. 4. Conclusions Pd nanotube array electrodes have been fabricated by means of electrodeposition method as a function of applied current density. The smaller grain size of electrodes may be attributed to the increased applied current density for relatively fast tube array growth. The Pd nanotube electrode with the smaller grain size resulted in much higher EASA due to rougher surface structure and higher oxidation current for formic acid electrooxidation. In particular, the Pd nanotube electrode electrodeposited under the highest applied current density showed such an improved electrocatalytic stability maintaining the morphology of catalysts after the stability test in comparison with the Pd/C. Acknowledgments This work was supported by the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Knowledge
Economy, Republic of Korea (No. 20104010100610) and the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2010-0022892). References [1] S. Yang, X. Zhang, H. Mi, X. Ye, J. Power Sources 175 (2008) 26. [2] Y.-W. Lee, J.-K. Oh, H.-S. Kim, J.-K. Lee, S.-B. Han, W. Choi, K.-W. Park, J. Power Sources 195 (2010) 5896. [3] B. Liu, H.Y. Li, L. Die, X.H. Zhang, Z. Fan, J.H. Chen, J. Power Sources 186 (2009) 62. [4] D. Tu, B. Wu, B. Wang, C. Deng, Y. Gao, Appl. Catal. B-Environ. 103 (2011) 163. [5] V. Mazumder, S. Sun, J. Am. Chem. Soc. 131 (2009) 4588. [6] W. Zhou, J.Y. Lee, J. Phys. Chem. C 112 (2008) 3789. [7] Z. Liu, L. Hong, M.P. Tham, T.H. Lim, H. Jiang, J. Power Sources 161 (2006) 831. [8] H. Li, G. Sun, Q. Jiang, M. Zhu, S. Sun, Q. Xin, Electrochem. Commun. 9 (2007) 1410. [9] D.-J. Chen, Z.-Y. Zhou, Q. Wang, D.-M. Xiang, N. Tian, S.-G. Sun, Chem. Commun. 46 (2010) 4252. [10] S.S. Kim, C. Kim, H. Lee, Top. Catal. 53 (2010) 686. [11] L. Su, W. Jia, A. Schemphf, Y. Ding, Y. Lei, J. Phys. Chem. C 113 (2009) 16174. [12] H. Meng, S. Sun, J.-P. Masse, J.-P. Dodelet, Chem. Mater. 20 (2008) 6998. [13] Y. Lu, W. Chen, J. Phys, Chem. C 114 (2010) 21190. [14] D. Wen, S. Guo, S. Dong, E. Wang, Biosens. Bioelectron. 26 (2010) 1056. [15] X.-Z. Gong, J.-N. Tang, J.-Q. Li, Y.-K. Liang, Trans. Nonferrous Met. Soc. China 18 (2008) 642. [16] K. Kim, M. Kim, S.M. Cho, Mater. Chem. Phys. 96 (2006) 278. [17] A. Saedi, M. Ghorbani, Mater. Chem. Phys. 91 (2005) 417. [18] Y.-J. Song, J.-Y. Kim, K.-W. Park, Cryst. Growth Des. 9 (2009) 505. [19] J. Fu, S. Cherevko, C.-H. Chung, Electrochem. Commun. 10 (2008) 514. [20] L. Liu, E. Pippel, Angew. Chem. Int. Ed. 50 (2011) 2729. [21] A.D. Franklin, M.R. Maschmann, M. DaSilva, D.B. Janes, T.S. Fisher, T.D. Sands, J. Vac. Sci. Technol. B 25 (2007) 343.
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Pd nanotube electrodes with improved electrocatalytic stability for formic acid electrooxidation You-Jung Song, Young-Woo Lee, Sang-Beom Han, Kyung-Won Park* Department of Chemical Engineering, Soongsil University, Seoul 156-743, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 December 2011 Received in revised form 25 February 2012 Accepted 8 March 2012
We report Pd nanotube electrodes fabricated by means of electrodeposition method as a function of applied current density. Transmission electron microscopy and scanning electron microscopy images show that the higher applied current density results in the rougher surface structure. From the X-ray diffraction patterns, it is observed that the smaller grain size of the Pd nanotube electrodes may be attributed to the higher applied current density. In the electrochemical data, it is found that the Pd nanotube electrode with the smaller grain size results in much higher electrocatalytic properties for formic acid electrooxidation. The Pd nanotube electrode electrodeposited under the highest applied current density exhibits the improved electrocatalytic stability in comparison with Pd catalyst supported by carbon black (Pd/C, E-TEK, Co.). Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Electrodeposition Pd nanotube array Formic acid electrooxidation Stability
1. Introduction Palladium has been of major interest as an anode catalyst for direct formic acid fuel cells (DFAFCs) as promising power sources [1e8]. It has been reported that electrochemical characteristics of anode materials are dramatically dependent on their particle size, crystallinity, surface area and morphology [9e12]. In particular, the ordered surface structure of one-dimensional nanostructured catalysts could affect electrochemical reactions and electrocatalytic activity [13,14]. Among a variety of synthetic approaches for one-dimensional nanostructure arrays, an anodic aluminum oxide (AAO) membrane-based method has received an attractive attention because its uniform and reproducible porous structure as an ideal template can provide highly ordered nanotube or nanowire-type arrays [15e17]. It has been well known that the variation of applied current density or voltage in the electrodeposition system could extremely affect the structural characteristics of the deposited electrodes [18]. Herein, Pd nanotube anode catalysts were obtained by means of electrodeposition method through porous AAO membrane. The nanotube structures were characterized by transmission electron microscopy, scanning electron microscopy and X-ray diffraction
* Corresponding author. Tel.: þ82 2 820 0613; fax: þ82 2 812 5378. E-mail address: [email protected] (K.-W. Park). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.03.048
method. To characterize and compare electrochemical properties and catalytic activity for formic acid electrooxidation, voltammograms of Pd nanotube anode catalysts were obtained using a potentiostat. 2. Experimental Electrodeposition was carried out as a function of applied current density at 25 C in an aqueous solution containing 0.2 M H3BO3 and 25 mM PdCl2 using Pt wire and Ag/AgCl used as a counter and a reference electrode, respectively. A thin Au layer was sputtered on one side of the porous alumina templates (Whatman, 200 nm pore size) for 2 min in Ar atmosphere to be served as a working electrode. After an Ag plate was polished and cleaned, a section of AAO film was immobilized on the surface of Ag plate which is served as a current collector electrode. The Pd nanotubes were electrodeposited for 12 h by using different current densities of 1, 3 and 5 mA cm2. The Pd nanotube arrays embedded in AAO were dipped into 3 M NaOH solution for l h to completely remove the AAO and washed in distilled water several times. The morphology and size of the Pd nanotube arrays were observed by means of scanning electron microscopy (SEM, JEOL JSM-6360A) and transmission electron microscopy (TEM, PhillipsF20). In the SEM analysis, the spot size was 40 nm and the free working distance (FWD) 11 13 mm. The TEM investigation was carried out at an accelerating voltage of 200 kV and Cu grids were
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Y.-J. Song et al. / Materials Chemistry and Physics 134 (2012) 567e570
used as substrates. The crystal structures of the Pd nanotubes were confirmed by X-ray diffraction (XRD, Rigaku X-ray diffractometer equipped with a Cu Ka source at 40 kV and 100 mA). To evaluate electrochemical properties and catalytic activity for formic acid electrooxidation of the Pd nanotube array electrodes, voltammograms were obtained in a solution of 0.1 M HClO4 with or without 0.5 M HCOOH at 25 C. A Pt wire and Ag/AgCl were used as a counter and a reference electrode, respectively. All potentials are reported with respect to Ag/AgCl. 3. Results and discussion Fig. 1 shows TEM and SEM images of Pd array electrodes electrodeposited using different current densities of 1, 3 and 5 mA cm2 for 12 h, denoted as Pd-NT-1, Pd-NT-3 and Pd-NT5, respectively. All the samples exhibit tubular arrays having an average diameter of w220 nm and different lengths with applied current densities. As described in the literature, the very thin metal layer as a working electrode on the AAO back side can produce nanotube arrays whereas nanowire arrays have been fabricated using the blocked metal layer [19e21]. The Pd-NT-1 (Fig. 1(a)e(c)) electrodeposited at relatively low applied current density of 1 mA cm2 consists of arrays with smooth and dense surface. However, as the applied current densities for tube array growth increase, the Pd-NT-3 (Fig. 1(d)e(f)) and Pd-NT-5 (Fig. 1(g)e(i)) consist of arrays with relatively rough surface. In particular, the tube arrays of the Pd-NT-5 electrodeposited under the highest
Fig. 2. XRD patterns of (a) Pd-NT-1, (b) Pd-NT-3 and (c) Pd-NT-5. The asterisk mark represents the XRD patterns of sputtered Au.
applied current density of 5 mA cm2 display the roughest surface. This suggests that the fast tube array growth by higher current density may result in rougher surface structure. To characterize crystal structures of Pd nanotube arrays, the XRD patterns of the Pd-NT electrodes are obtained as shown in Fig. 2. In the
Fig. 1. TEM and FE-SEM images of as-prepared Pd nanotube electrodes: Pd-NT-1 ((a),(b),(c)), Pd-NT-3 ((d),(e),(f)) and Pd-NT-5 ((g),(h),(i)).
Y.-J. Song et al. / Materials Chemistry and Physics 134 (2012) 567e570
Fig. 3. (a) CVs of Pd catalysts in Ar-saturated 0.1 M HClO4 with a scan rate of 50 mV s1 (b) CVs of Pd catalysts for formic acid electrooxidation at 25 C in 0.5 M HCOOH and 0.1 M HClO4 with a scan rate of 20 mV s1.
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XRD patterns, the reflections corresponding to the planes (111) (200) and (220), characteristic of the face-centered cubic structure of Pd are present with XRD peaks related to AAO templates. The diffraction peak width of Pd-NT-1 is less broad than those of Pd-NT-3 and Pd-NT-5. The Pd-NT-5 exhibits the broadest peaks implying that the grain size of PdNT-5 is smaller than those of Pd-NT-3 and Pd-NT-5. According to the calculation of particle sizes based on the full width half maximum of (220) peaks, the average grain sizes of Pd-NT-1, Pd-NT-3, and Pd-NT-5 are 12.6, 11.5, and 8.9 nm, respectively. This implies that smaller grain size of electrodes may be due to the increased applied current density for relatively fast tube array growth. To characterize catalytic properties of Pd nanotubes toward the formic acid electrooxidation, the nanotube electrodes were compared using Pd nanoparticles deposited on carbon black (Pd/C, E-TEK, Co.) as a typical catalyst. Fig. 3(a) shows cyclic voltammograms (CVs) of Pd-NT electrodes in Ar-saturated 0.1 M HClO4 with a scan rate of 50 mV s1 with palladium loadings of 28 mg cm2. From the hydrogen adsorption areas of Pd, the order of electrochemical active surface area (EASA) of Pd-NT electrodes is Pd-NT-1 < Pd-NT3 < Pd-NT-5. The high EASA of the Pd-NT-5 is attributed to small average grain sizes of Pd-NT-5 compared to Pd-NT-1 and Pd-NT-3. Thus, it is expected that the Pd-NT-5 may exhibit excellent electrocatalytic properties for formic acid electrooxidation. As indicated in Fig. 3(b), the electrocatalytic mass activity of the Pd-NT-5 in formic acid electrooxidation is much higher than those of Pd-NT-1 and PdNT-3. However, the Pd catalyst (20 wt%) supported by carbon black (Pd/C, E-TEK Co.) having the highest EASA exhibits the highest electrocatalytic activity in comparison with Pd-NT electrodes. To characterize the electrocatalytic stability for formic acid electrooxidation, the stability test was performed by applying oxidation potential of 0.2 V for 1 h in 0.5 M HCOOH and 0.1 M HClO4 at 25 C. The Pd-NT-5 exhibits nearly maintained electrocatalytic activity after the stability test representing such an excellent stability for formic acid electrooxidation (Fig. 4(a)). As indicated in Fig. 4(b) and (c), the size and morphology of the Pd-NT-5 still remain after the stability test suggesting much improved catalytic stability.
Fig. 4. (a) CVs of Pd-NT-5 before and after stability test in 0.5 M HCOOH and 0.1 M HClO4 at 25 C. TEM images and TED patterns of Pd-NT-5 (b) before and (c) After stability test. (d) CVs of Pd/C before and after stability test in 0.5 M HCOOH and 0.1 M HClO4 at 25 C. TEM images and TED patterns of Pd/C (e) before and (f) After stability test.
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In contrast, the electrocatalytic activity of the Pd/C after the stability test significantly deteriorates (Fig. 4(d)). Furthermore, the Pd/C shows increased particle size and non-uniform size distribution resulting in deteriorated catalytic activity (Fig. 4(e) and (f)). It is likely that the improved electrocatalytic stability of the Pd-NT-5 toward formic acid electrooxidation may result from the less aggregated Pd catalysts during the catalytic reaction in comparison with the Pd/C. 4. Conclusions Pd nanotube array electrodes have been fabricated by means of electrodeposition method as a function of applied current density. The smaller grain size of electrodes may be attributed to the increased applied current density for relatively fast tube array growth. The Pd nanotube electrode with the smaller grain size resulted in much higher EASA due to rougher surface structure and higher oxidation current for formic acid electrooxidation. In particular, the Pd nanotube electrode electrodeposited under the highest applied current density showed such an improved electrocatalytic stability maintaining the morphology of catalysts after the stability test in comparison with the Pd/C. Acknowledgments This work was supported by the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Knowledge
Economy, Republic of Korea (No. 20104010100610) and the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2010-0022892). References [1] S. Yang, X. Zhang, H. Mi, X. Ye, J. Power Sources 175 (2008) 26. [2] Y.-W. Lee, J.-K. Oh, H.-S. Kim, J.-K. Lee, S.-B. Han, W. Choi, K.-W. Park, J. Power Sources 195 (2010) 5896. [3] B. Liu, H.Y. Li, L. Die, X.H. Zhang, Z. Fan, J.H. Chen, J. Power Sources 186 (2009) 62. [4] D. Tu, B. Wu, B. Wang, C. Deng, Y. Gao, Appl. Catal. B-Environ. 103 (2011) 163. [5] V. Mazumder, S. Sun, J. Am. Chem. Soc. 131 (2009) 4588. [6] W. Zhou, J.Y. Lee, J. Phys. Chem. C 112 (2008) 3789. [7] Z. Liu, L. Hong, M.P. Tham, T.H. Lim, H. Jiang, J. Power Sources 161 (2006) 831. [8] H. Li, G. Sun, Q. Jiang, M. Zhu, S. Sun, Q. Xin, Electrochem. Commun. 9 (2007) 1410. [9] D.-J. Chen, Z.-Y. Zhou, Q. Wang, D.-M. Xiang, N. Tian, S.-G. Sun, Chem. Commun. 46 (2010) 4252. [10] S.S. Kim, C. Kim, H. Lee, Top. Catal. 53 (2010) 686. [11] L. Su, W. Jia, A. Schemphf, Y. Ding, Y. Lei, J. Phys. Chem. C 113 (2009) 16174. [12] H. Meng, S. Sun, J.-P. Masse, J.-P. Dodelet, Chem. Mater. 20 (2008) 6998. [13] Y. Lu, W. Chen, J. Phys, Chem. C 114 (2010) 21190. [14] D. Wen, S. Guo, S. Dong, E. Wang, Biosens. Bioelectron. 26 (2010) 1056. [15] X.-Z. Gong, J.-N. Tang, J.-Q. Li, Y.-K. Liang, Trans. Nonferrous Met. Soc. China 18 (2008) 642. [16] K. Kim, M. Kim, S.M. Cho, Mater. Chem. Phys. 96 (2006) 278. [17] A. Saedi, M. Ghorbani, Mater. Chem. Phys. 91 (2005) 417. [18] Y.-J. Song, J.-Y. Kim, K.-W. Park, Cryst. Growth Des. 9 (2009) 505. [19] J. Fu, S. Cherevko, C.-H. Chung, Electrochem. Commun. 10 (2008) 514. [20] L. Liu, E. Pippel, Angew. Chem. Int. Ed. 50 (2011) 2729. [21] A.D. Franklin, M.R. Maschmann, M. DaSilva, D.B. Janes, T.S. Fisher, T.D. Sands, J. Vac. Sci. Technol. B 25 (2007) 343.