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Cu2S nanostructures prepared by Cu-cysteine precursor templated route
Materials Letters 63 (2009) 1935–1938
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Cu2S nanostructures prepared by Cu-cysteine precursor templated route Ling Jiang, Ying-Jie Zhu ⁎ State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China; Graduate School of Chinese Academy of Sciences, PR China
a r t i c l e
i n f o
Article history: Received 9 March 2009 Accepted 4 June 2009 Available online 10 June 2009 Keywords: Nanomaterials Crystal growth Cu2S L-cysteine Precursor Template
a b s t r a c t A facile Cu-cysteine precursor templated route for the synthesis of Cu2S nanowires, dendritic-like and flowerlike nanostructures is reported. The Cu-cysteine precursors are prepared through the reaction between Cu2+, L-cysteine and ethanolamine at room temperature, and the morphologies of Cu-cysteine precursors can be controlled by adjusting the molar ratio of L-cysteine to Cu2+. The Cu-cysteine precursors are used as both templates and source materials for the subsequent preparation of polycrystalline Cu2S nanostructures by thermal treatment, and the morphologies of the precursors can be well preserved after the thermal transformation to Cu2S nanostructures. The samples are characterized using X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, energy dispersive spectroscopy and Fourier transform infrared spectroscopy. © 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Biomolecules, as life's basic building blocks, have unique structures and functions which make them excellent for the design and synthesis of structures in the nanometer or submicrometer regime [1–6]. L-cysteine (HSCH2CH(NH2)COOH) is cheap and unique in structure with sulfhydryl, amino, and carboxyl functional groups, which have a strong tendency to coordinate with metallic cations. L-cysteine has been exploited for the preparation of quantum dots and porous networks [7,8], nanowires and microrods [9–11], spherical nanostructures [12], flowerlike patterns [13], porous flowerlike nanospheres [14] and pagoda-like hierarchical architectures [15]. Recently, we reported a novel and facile L-cysteine-assisted preparation of lead chalcogenide polycrystalline nanotubes self-assembled from nanocrystals at room temperature [16], and this work inspired us to explore a simple room-temperature approach for the low-cost preparation of nanostructrured Cu-cysteine precursors. Herein, we report a facile room-temperature approach for the preparation of Cu-cysteine precursor nanostructures with nanowire, dendritic-like and flowerlike morphologies through the reaction between Cu2+, L-cysteine and ethanolamine in aqueous solution, and the morphologies of Cucysteine precursors can be controlled by adjusting the molar ratio of 2+ L-cysteine to Cu . The Cu-cysteine precursors are used as both templates and source materials for the preparation of Cu2S nanostructures with well preserved morphologies by thermal treatment.
All chemicals used in the experiments were analytical grade reagents and used as received without further purification. The preparation of Cu-cysteine precursors was carried out at room temperature. A parameter R is defined as the molar ratio of cysteine to CuCl2·2H2O. L-cysteine (0.06 g, R = 1; 0.12 g, R = 2; 0.24 g, R = 4) and 0.085 g CuCl2·2H2O were dissolved in 50 mL of deionized water, then 0.09 mL ethanolamine was added. The three typical precursors prepared at different molar ratios (R = 1, 2, and 4) are named as precursor A, precursor B, and precursor C, respectively. The mixture was stirred at room temperature for 1.5 h (precursors A and B) and 5 h (precursor C), the sky-blue floccules formed and were separated by centrifugation, washed with deionized water and absolute alcohol for several times and dried under vacuum. For the preparation of Cu2S nanostructures, the dried powders of Cu-cysteine precursors were heated in nitrogen with a heating rate of 10 °C min− 1 from room temperature to 300 °C for precursors A and B, and 350 °C for precursor C, then heating was terminated immediately and cooled to room temperature.
⁎ Corresponding author. Tel.: +86 21 52412616; fax: +86 21 52413122. E-mail address: [email protected] (Y.-J. Zhu). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.06.005
3. Results and discussion Fig. 1 shows X-ray powder diffraction (XRD, Rigaku D/MAX 2550V, CuKα) patterns and scanning electron microscopy (SEM, JEOL JSM6700F) micrographs of as-prepared Cu-cysteine precursors prepared using different molar ratios of L-cysteine to Cu2+ (defined as R) for different reaction time. The XRD patterns show that the as-prepared precursors are well crystallized and three kinds of Cu-cysteine precursors can be obtained under different experimental conditions.
1936
L. Jiang, Y.-J. Zhu / Materials Letters 63 (2009) 1935–1938
Fig. 1. XRD patterns and SEM micrographs of Cu-cysteine precursors obtained using different molar ratios of L-cysteine to Cu2+ (defined as R): (a) and (d) precursor A; (b) and (e) precursor B; (c) and (f) precursor C.
The morphologies of Cu-cysteine precursors (Fig. 1d,e,f) show the formation of nanowires (precursor A), dendritic nanostructures (precursor B) and flower-like nanostructures (precursor C). The morphology of the precursor varies greatly depending on the molar ratio of cysteine to Cu2+. The precursor A (Fig. 1d) consists of well dispersed nanowires with an average diameter of about 200 nm and lengths of up to 50 µm. The precursor B (Fig. 1e) is composed of dendritic-like nanostructures constructed by nanosheets with acerate ends. The precursor C (Fig. 1f) consists of flower-like nanostructures which are also constructed by nanosheets with acerate ends. The assembly degree of nanosheets in precursor C is higher than that in precursor B. Shindo and Brown [17] proposed the possible mechanism of the interaction between L-cysteine and bivalent metal cations based on infrared spectra. In the cysteine molecule, there are three functional groups, −NH2, −COOH, and −SH, which have a strong tendency to coordinate with metallic cations. Cysteine was used in the preparation of modified electrodes and biosensors [18]. Recently, Xie et al. [19] reported that Cu2+ ions could react with cysteine to form complexes at room temperature in non-alkaline aqueous solution. In the present work, Cu-cysteine precursors are obtained in an alkaline solution at room temperature, and ethanolamine plays an important role in the formation of the precursors. Fourier transform infrared (FTIR) spectroscopic study was carried out to provide preliminary proof for the functional groups and chemical bonding. Fig. 2 shows FTIR spectra of the Cu-cysteine precursors and pure cysteine. In the FTIR spectrum of pure cysteine, the peaks located at 1585 and 1544 cm− 1 are attributed to the diagnostic vibrations of acylamino (−CO−NH2−) group, indicating the presence of the amino acid. These two peaks shift to 1652 and 1618 cm− 1 in precursor A, 1627 cm− 1 in precursor B, 1625 and 1588 cm− 1 in precursor C. The peaks at 3304 and 3238 cm− 1 for precursors B and C, 3209 and
3140 cm− 1 for precursor A are the characteristic peaks of amido (−NH2) group. The characteristic signal of −SH appears at ca. 2553 cm− 1 in pure L-cysteine, but disappears in all the precursors, suggesting that the sulfur component of the precursors originates from the −SH group of the cysteine molecule [13]. A precursor thermal transformation route to the preparation of Cu2S nanostructures has been designed. The Cu-cysteine precursors with different morphologies are used as both templates and source materials for the subsequent preparation of Cu2S nanostructures with well preserved morphologies by thermal treatment. The thermal
Fig. 2. FTIR spectra of pure L-cysteine and precursors A, B and C.
L. Jiang, Y.-J. Zhu / Materials Letters 63 (2009) 1935–1938
1937
Fig. 3. DSC and TG curves for the as-prepared (a) precursor A; (b) precursor B and (c) precursor C.
behavior of the Cu-cysteine precursors in nitrogen was investigated by thermogravimetric (TG) and differential scanning calorimetric (DSC) analysis. Fig. 3 shows DSC-TG curves of precursors A (a), B (b) and C (c). For precursors A and B, the endothermal process starts at ~220 °C and ~200 °C, and both ends at around 250 °C, and the weight loss is
about 52% and 55%, respectively. We propose that two chemical reactions may take place during this process, including precursor decomposition and phase transformation from CuS to Cu2S. The Cucysteine precursors start to decompose at the beginning of the endothermal process, leading to the formation of intermediate CuS. At
Fig. 4. XRD patterns and TEM micrographs of Cu2S nanostructures obtained by thermal treatment in nitrogen of (a) and (d) precursor A; (b) and (e) precursor B; and (c) and (f) precursor C; (g) and (h) are high-resolution TEM (HRTEM) images of precursors B and C, respectively.
1938
L. Jiang, Y.-J. Zhu / Materials Letters 63 (2009) 1935–1938
higher temperatures above 220 °C, CuS undergoes a desulfurization to form cubic Cu2S. For precursor C, there are two endothermal peaks in the DSC curve, and the mass loss is around 55%. The XRD patterns of the samples prepared by the thermal decomposition of Cu-cysteine precursors are shown in Fig. 4a–c. The diffraction peaks of three samples can be indexed to a single phase of cubic Cu2S (JCPDS No. 53-0522), indicating that cubic Cu2S can be obtained by the thermal decomposition of any of three precursors. The morphologies of Cu2S nanostructures obtained by thermal treatment of Cu-cysteine precursors in nitrogen are shown in Fig. 4d–h. Fig. 4d shows that Cu2S obtained from precursor A consists of wire-like morphology, and each wire is composed of nanoparticles. The morphologies of Cu2S samples obtained from precursors B and C are dendritic-like and flowerlike, respectively, and these nanostructures are constructed by nanocrystals (Fig. 4e–h). The morphologies of the as-prepared Cu-cysteine precursors can be well preserved during thermal transformation to Cu2S nanostructures.
4. Conclusions A simple L-cysteine-assisted route has been developed to prepare three kinds of Cu-cysteine precursor nanostructures with wire, dendritic and flowerlike morphologies through the reaction between Cu2+ ions and L-cysteine in an alkaline solution at room temperature. The molar ratio of L-cysteine to Cu2+ plays an important role in controlling the morphologies of the precursors. The Cu-cysteine precursors are used as both templates and source materials for the preparation of Cu2S nanostructures with a variety of morphologies by thermal treatment,
and the morphologies of the precursors can be well preserved during the thermal transformation to Cu2S nanostructures. Acknowledgements Financial support from the National Natural Science Foundation of China (50772124, 50821004), the Program of Shanghai Subject Chief Scientist (07XD14031) and the Fund for Nano-Science and Technology from Science and Technology Commission of Shanghai (0852 nm05800) is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
Knez M, Bittner AM, Boes F, Wege C, Jeske H, Maiâ E, et al. Nano Lett 2003;3:1079–82. Shankar SS, Rai A, Ankamwar B, Singh A, Ahmad A, Sastry M. Nat Mater 2004;3:482–8. Lu QY, Gao F, Komarneni S. Adv Mater 2004;16:1629–32. Mao CB, Solis DJ, Reiss BD, Kottmann ST, Sweeney RY, Hayhurst A, et al. Science 2004;303:213–7. Nam KT, Kim DW, Yoo PJ, Chiang CY, Meethong N, Hammond PT, et al. Science 2006;312:885–8. Zhang B, Ye XC, Dai W, Hou WY, Zuo F, Xie Y. Nanotechnology 2006;17:385–90. Gao JH, Liang GL, Zhang B, Kuang Y, Zhang XX, Xu B. J Am Chem Soc 2007;129:1428–33. Zuo F, Zhang B, Tang XZ, Xie Y. Nanotechnology 2007;18:215608. Shen XF, Yan XP. Angew Chem Int Ed 2007;46:7659–63. Mondal K, Roy P, Srivastava SK. Cryst Growth Des 2008;8:1580–4. Liu XL, Zhu YJ. Mater Lett 2009;63:1085–8. Xiong SL, Xi BJ, Wang CM, Zou GF, Fei LF, Wang WZ, et al. Chem Eur J 2007;13:3076–81. Zhang B, Ye XC, Hou WY, Zhao Y, Xie Y. J Phys Chem B 2006;110:8978–85. Chen LY, Zhang ZD, Wang WZ. J Phys Chem C 2008;112:4117–23. Zuo F, Yan S, Zhang B, Zhao Y, Xie Y. J Phys Chem C 2008;112:2831–5. Tong H, Zhu YJ, Yang LX, Li L, Zhang L. Angew Chem Int Ed 2006;45:7739–42. Shindo H, Brown TL. J Am Chem Soc 1965;87:1904–9. Hernández P, Vicente J, Hernández L. Electroanalysis 2003;15:1625–31. Li BX, Xie Y, Xue Y. J Phys Chem C 2007;111:12181–7.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Cu2S nanostructures prepared by Cu-cysteine precursor templated route Ling Jiang, Ying-Jie Zhu ⁎ State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China; Graduate School of Chinese Academy of Sciences, PR China
a r t i c l e
i n f o
Article history: Received 9 March 2009 Accepted 4 June 2009 Available online 10 June 2009 Keywords: Nanomaterials Crystal growth Cu2S L-cysteine Precursor Template
a b s t r a c t A facile Cu-cysteine precursor templated route for the synthesis of Cu2S nanowires, dendritic-like and flowerlike nanostructures is reported. The Cu-cysteine precursors are prepared through the reaction between Cu2+, L-cysteine and ethanolamine at room temperature, and the morphologies of Cu-cysteine precursors can be controlled by adjusting the molar ratio of L-cysteine to Cu2+. The Cu-cysteine precursors are used as both templates and source materials for the subsequent preparation of polycrystalline Cu2S nanostructures by thermal treatment, and the morphologies of the precursors can be well preserved after the thermal transformation to Cu2S nanostructures. The samples are characterized using X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, energy dispersive spectroscopy and Fourier transform infrared spectroscopy. © 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Biomolecules, as life's basic building blocks, have unique structures and functions which make them excellent for the design and synthesis of structures in the nanometer or submicrometer regime [1–6]. L-cysteine (HSCH2CH(NH2)COOH) is cheap and unique in structure with sulfhydryl, amino, and carboxyl functional groups, which have a strong tendency to coordinate with metallic cations. L-cysteine has been exploited for the preparation of quantum dots and porous networks [7,8], nanowires and microrods [9–11], spherical nanostructures [12], flowerlike patterns [13], porous flowerlike nanospheres [14] and pagoda-like hierarchical architectures [15]. Recently, we reported a novel and facile L-cysteine-assisted preparation of lead chalcogenide polycrystalline nanotubes self-assembled from nanocrystals at room temperature [16], and this work inspired us to explore a simple room-temperature approach for the low-cost preparation of nanostructrured Cu-cysteine precursors. Herein, we report a facile room-temperature approach for the preparation of Cu-cysteine precursor nanostructures with nanowire, dendritic-like and flowerlike morphologies through the reaction between Cu2+, L-cysteine and ethanolamine in aqueous solution, and the morphologies of Cucysteine precursors can be controlled by adjusting the molar ratio of 2+ L-cysteine to Cu . The Cu-cysteine precursors are used as both templates and source materials for the preparation of Cu2S nanostructures with well preserved morphologies by thermal treatment.
All chemicals used in the experiments were analytical grade reagents and used as received without further purification. The preparation of Cu-cysteine precursors was carried out at room temperature. A parameter R is defined as the molar ratio of cysteine to CuCl2·2H2O. L-cysteine (0.06 g, R = 1; 0.12 g, R = 2; 0.24 g, R = 4) and 0.085 g CuCl2·2H2O were dissolved in 50 mL of deionized water, then 0.09 mL ethanolamine was added. The three typical precursors prepared at different molar ratios (R = 1, 2, and 4) are named as precursor A, precursor B, and precursor C, respectively. The mixture was stirred at room temperature for 1.5 h (precursors A and B) and 5 h (precursor C), the sky-blue floccules formed and were separated by centrifugation, washed with deionized water and absolute alcohol for several times and dried under vacuum. For the preparation of Cu2S nanostructures, the dried powders of Cu-cysteine precursors were heated in nitrogen with a heating rate of 10 °C min− 1 from room temperature to 300 °C for precursors A and B, and 350 °C for precursor C, then heating was terminated immediately and cooled to room temperature.
⁎ Corresponding author. Tel.: +86 21 52412616; fax: +86 21 52413122. E-mail address: [email protected] (Y.-J. Zhu). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.06.005
3. Results and discussion Fig. 1 shows X-ray powder diffraction (XRD, Rigaku D/MAX 2550V, CuKα) patterns and scanning electron microscopy (SEM, JEOL JSM6700F) micrographs of as-prepared Cu-cysteine precursors prepared using different molar ratios of L-cysteine to Cu2+ (defined as R) for different reaction time. The XRD patterns show that the as-prepared precursors are well crystallized and three kinds of Cu-cysteine precursors can be obtained under different experimental conditions.
1936
L. Jiang, Y.-J. Zhu / Materials Letters 63 (2009) 1935–1938
Fig. 1. XRD patterns and SEM micrographs of Cu-cysteine precursors obtained using different molar ratios of L-cysteine to Cu2+ (defined as R): (a) and (d) precursor A; (b) and (e) precursor B; (c) and (f) precursor C.
The morphologies of Cu-cysteine precursors (Fig. 1d,e,f) show the formation of nanowires (precursor A), dendritic nanostructures (precursor B) and flower-like nanostructures (precursor C). The morphology of the precursor varies greatly depending on the molar ratio of cysteine to Cu2+. The precursor A (Fig. 1d) consists of well dispersed nanowires with an average diameter of about 200 nm and lengths of up to 50 µm. The precursor B (Fig. 1e) is composed of dendritic-like nanostructures constructed by nanosheets with acerate ends. The precursor C (Fig. 1f) consists of flower-like nanostructures which are also constructed by nanosheets with acerate ends. The assembly degree of nanosheets in precursor C is higher than that in precursor B. Shindo and Brown [17] proposed the possible mechanism of the interaction between L-cysteine and bivalent metal cations based on infrared spectra. In the cysteine molecule, there are three functional groups, −NH2, −COOH, and −SH, which have a strong tendency to coordinate with metallic cations. Cysteine was used in the preparation of modified electrodes and biosensors [18]. Recently, Xie et al. [19] reported that Cu2+ ions could react with cysteine to form complexes at room temperature in non-alkaline aqueous solution. In the present work, Cu-cysteine precursors are obtained in an alkaline solution at room temperature, and ethanolamine plays an important role in the formation of the precursors. Fourier transform infrared (FTIR) spectroscopic study was carried out to provide preliminary proof for the functional groups and chemical bonding. Fig. 2 shows FTIR spectra of the Cu-cysteine precursors and pure cysteine. In the FTIR spectrum of pure cysteine, the peaks located at 1585 and 1544 cm− 1 are attributed to the diagnostic vibrations of acylamino (−CO−NH2−) group, indicating the presence of the amino acid. These two peaks shift to 1652 and 1618 cm− 1 in precursor A, 1627 cm− 1 in precursor B, 1625 and 1588 cm− 1 in precursor C. The peaks at 3304 and 3238 cm− 1 for precursors B and C, 3209 and
3140 cm− 1 for precursor A are the characteristic peaks of amido (−NH2) group. The characteristic signal of −SH appears at ca. 2553 cm− 1 in pure L-cysteine, but disappears in all the precursors, suggesting that the sulfur component of the precursors originates from the −SH group of the cysteine molecule [13]. A precursor thermal transformation route to the preparation of Cu2S nanostructures has been designed. The Cu-cysteine precursors with different morphologies are used as both templates and source materials for the subsequent preparation of Cu2S nanostructures with well preserved morphologies by thermal treatment. The thermal
Fig. 2. FTIR spectra of pure L-cysteine and precursors A, B and C.
L. Jiang, Y.-J. Zhu / Materials Letters 63 (2009) 1935–1938
1937
Fig. 3. DSC and TG curves for the as-prepared (a) precursor A; (b) precursor B and (c) precursor C.
behavior of the Cu-cysteine precursors in nitrogen was investigated by thermogravimetric (TG) and differential scanning calorimetric (DSC) analysis. Fig. 3 shows DSC-TG curves of precursors A (a), B (b) and C (c). For precursors A and B, the endothermal process starts at ~220 °C and ~200 °C, and both ends at around 250 °C, and the weight loss is
about 52% and 55%, respectively. We propose that two chemical reactions may take place during this process, including precursor decomposition and phase transformation from CuS to Cu2S. The Cucysteine precursors start to decompose at the beginning of the endothermal process, leading to the formation of intermediate CuS. At
Fig. 4. XRD patterns and TEM micrographs of Cu2S nanostructures obtained by thermal treatment in nitrogen of (a) and (d) precursor A; (b) and (e) precursor B; and (c) and (f) precursor C; (g) and (h) are high-resolution TEM (HRTEM) images of precursors B and C, respectively.
1938
L. Jiang, Y.-J. Zhu / Materials Letters 63 (2009) 1935–1938
higher temperatures above 220 °C, CuS undergoes a desulfurization to form cubic Cu2S. For precursor C, there are two endothermal peaks in the DSC curve, and the mass loss is around 55%. The XRD patterns of the samples prepared by the thermal decomposition of Cu-cysteine precursors are shown in Fig. 4a–c. The diffraction peaks of three samples can be indexed to a single phase of cubic Cu2S (JCPDS No. 53-0522), indicating that cubic Cu2S can be obtained by the thermal decomposition of any of three precursors. The morphologies of Cu2S nanostructures obtained by thermal treatment of Cu-cysteine precursors in nitrogen are shown in Fig. 4d–h. Fig. 4d shows that Cu2S obtained from precursor A consists of wire-like morphology, and each wire is composed of nanoparticles. The morphologies of Cu2S samples obtained from precursors B and C are dendritic-like and flowerlike, respectively, and these nanostructures are constructed by nanocrystals (Fig. 4e–h). The morphologies of the as-prepared Cu-cysteine precursors can be well preserved during thermal transformation to Cu2S nanostructures.
4. Conclusions A simple L-cysteine-assisted route has been developed to prepare three kinds of Cu-cysteine precursor nanostructures with wire, dendritic and flowerlike morphologies through the reaction between Cu2+ ions and L-cysteine in an alkaline solution at room temperature. The molar ratio of L-cysteine to Cu2+ plays an important role in controlling the morphologies of the precursors. The Cu-cysteine precursors are used as both templates and source materials for the preparation of Cu2S nanostructures with a variety of morphologies by thermal treatment,
and the morphologies of the precursors can be well preserved during the thermal transformation to Cu2S nanostructures. Acknowledgements Financial support from the National Natural Science Foundation of China (50772124, 50821004), the Program of Shanghai Subject Chief Scientist (07XD14031) and the Fund for Nano-Science and Technology from Science and Technology Commission of Shanghai (0852 nm05800) is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
Knez M, Bittner AM, Boes F, Wege C, Jeske H, Maiâ E, et al. Nano Lett 2003;3:1079–82. Shankar SS, Rai A, Ankamwar B, Singh A, Ahmad A, Sastry M. Nat Mater 2004;3:482–8. Lu QY, Gao F, Komarneni S. Adv Mater 2004;16:1629–32. Mao CB, Solis DJ, Reiss BD, Kottmann ST, Sweeney RY, Hayhurst A, et al. Science 2004;303:213–7. Nam KT, Kim DW, Yoo PJ, Chiang CY, Meethong N, Hammond PT, et al. Science 2006;312:885–8. Zhang B, Ye XC, Dai W, Hou WY, Zuo F, Xie Y. Nanotechnology 2006;17:385–90. Gao JH, Liang GL, Zhang B, Kuang Y, Zhang XX, Xu B. J Am Chem Soc 2007;129:1428–33. Zuo F, Zhang B, Tang XZ, Xie Y. Nanotechnology 2007;18:215608. Shen XF, Yan XP. Angew Chem Int Ed 2007;46:7659–63. Mondal K, Roy P, Srivastava SK. Cryst Growth Des 2008;8:1580–4. Liu XL, Zhu YJ. Mater Lett 2009;63:1085–8. Xiong SL, Xi BJ, Wang CM, Zou GF, Fei LF, Wang WZ, et al. Chem Eur J 2007;13:3076–81. Zhang B, Ye XC, Hou WY, Zhao Y, Xie Y. J Phys Chem B 2006;110:8978–85. Chen LY, Zhang ZD, Wang WZ. J Phys Chem C 2008;112:4117–23. Zuo F, Yan S, Zhang B, Zhao Y, Xie Y. J Phys Chem C 2008;112:2831–5. Tong H, Zhu YJ, Yang LX, Li L, Zhang L. Angew Chem Int Ed 2006;45:7739–42. Shindo H, Brown TL. J Am Chem Soc 1965;87:1904–9. Hernández P, Vicente J, Hernández L. Electroanalysis 2003;15:1625–31. Li BX, Xie Y, Xue Y. J Phys Chem C 2007;111:12181–7.