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Facile solvothermal synthesis of Cu2SnS3 architectures and their visible-light-driven photocatalytic properties
Materials Letters 89 (2012) 240–242
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Facile solvothermal synthesis of Cu2SnS3 architectures and their visible-light-driven photocatalytic properties Yu Tan n, Zhiqun Lin, Wenhui Ren, Woyun Long, Yong Wang, Xicheng Ouyang Science College of Hunan Agricultural University, Changsha 410128, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 July 2012 Accepted 24 August 2012 Available online 3 September 2012
In this paper, three dimensional (3D) Cu2SnS3 architectures were successfully synthesized via a simple solvothermal route. The structure, morphology and optical properties of the as-prepared samples were characterized by using an X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV–vis diffuse reflectance spectra (UV–vis DRS). The SEM and TEM image indicated that the flower Cu2SnS3 architectures consist of large number of uniform nanosheets. The band gap is about 2.42 eV, which have been confirmed by UV–vis DRS. Photocatalytic activities by photodegradation reaction of methylene blue (MB) were investigated under visible-light irradiation. The results indicate the potential applications of the Cu2SnS3 architectures in visible-light-driven photocatalysts. Published by Elsevier B.V.
Keywords: Semiconductors Cu2SnS3 Microstructure Photocatalytic activity
1. Introduction Photocatalysts are expected to play an important role in helping to solve many serious environmental and pollution challenges. Because of their electronic structure, semiconductor photocatalysts such as titanium oxide (TiO2) and zinc oxide (ZnO) have been applied to a variety of environmental processes such as remediation of organic contaminants and destruction of microorganisms [1–4]. However, with a wide band gap of 3.2 eV (TiO2) and 3.37 eV (ZnO), which are only active in the ultraviolet light region. While, it is not responsive to visible light (l 4400 nm), which accounts for around 46% of the total solar energy [5]. To make full use of solar energy, a decreased band gap would allow extending the photocatalytic application from the current UV light into the dominant visible light range. Thus narrow band gap semiconductor, such as CdS [6], In2S3 [7] metal sulfides have been proven to be a group of highly efficient catalysts for photochemical reactions, since photogenerated charge carriers can rapidly move to the surface of the catalysts, reducing or oxidizing organic molecules. Unfortunately, the production and use of Cd or In related compounds are harmful to human health, damaging the environment, hindered by the very cost and rareness of the Cd or In element. Thus, it is desirable to exploit low cost, good stability, nontoxicity, and novel visible lightsensitive semiconductor photocatalysts. The ternary Cu–Sn–S system, as an important ternary I–IV–VI group semiconductor with small or mid band-gap, have attracted great attention owing to their broad applications in photovoltaic devices, electrochemical properties and photocatalytic activities for H2 evolution under visible light irradiation [8–18]. In this letter, 3D
n
Corresponding author. Tel./fax: þ 86 731 84618071. E-mail address: [email protected] (Y. Tan).
0167-577X/$ - see front matter Published by Elsevier B.V. http://dx.doi.org/10.1016/j.matlet.2012.08.117
Cu2SnS3 architectures were successfully synthesized via a simple solvothermal route. The unique properties inherent to well-defined 3D hierarchical structures may offer extraordinary potential applications in photocatalysis. Herein, photocatalytic properties of the 3D Cu2SnS3 architectures were investigated by photodegradation of MB under visible light irradiation. To the best of our knowledge, this is the first report on the fabrication of Cu2SnS3 architectures and applications in visible-light-driven photocatalysts.
2. Experimental section All the reagents were of analytical grade and used without further purification. Typically, 0.5 mmol tin (IV) chloride pentahydrate (SnCl4 5H2O), 1 mmol copper chloride dehydrate (CuCl2 2H2O) and 1.5 mmol L-cysteine were dissolved in 30 mL of deionized water, and then the mixture underwent ultrasonic treatment for a few minutes to become a transparent solution. Later, the mixed solution was transferred into a 50 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 180 1C for 12 h, the vessel was then cooled to room temperature. The product was centrifuged and washed several times with deionized water and absolute ethanol, and then dried in a vacuum at 60 1C for 4 h. XRD measurements were carried out using a D-2500 XRD spectrometer (Rigaku) with a Cu Ka line of 0.1541 nm to characterize the crystal structure of the products. SEM micrographs were taken using a Hitachi S-4800 scanning electron microscope. TEM micrographs and high resolution transmission electron microscopy (HRTEM) was performed using JEOL 2010. UV–visible DRS were recorded on a UV–visible spectrophotometer (TU-1901 UV–vis spectrophotometer) equipped with a Labsphere diffuse reflectance accessory. The photocatalytic degradation of MB was carried out in an aqueous
Y. Tan et al. / Materials Letters 89 (2012) 240–242
solution at ambient temperature under visible light irradiation of a 100 W Xe lamp with a 420 nm cutoff filter. Briefly, a mixture of 20 mg Cu2SnS3 products and 50 mL 1 10 5 M MB solution (C16H18ClN3S 3H2O) was put in a Pyrex beaker (100 mL). Prior to orradiation, the solution was magnetically stirred in the dark for 1 h to ensure the adsorption equilibrium. Photocatalytic degradation was monitored by measuring the absorbance of solution using a TU-1901 UV–vis spectrophotometer. At given intervals of illumination, the samples of the reaction solution were taken out and analyzed.
3. Results and discussion The XRD pattern of the as-synthesized product is shown in Fig. 1. All the diffraction peaks can be indexed to the standard diffraction data of the corresponding Mohite Cu2SnS3 (space ˚ b¼11.51 A˚ and group: P1) with lattice parameters of a¼ 6.64 A, c¼ 19.93 A˚ (JCPDS card file no. 35-684). No other impurities, such as binary sulfides (SnS2 or CuS), oxides, or organic compounds related to reactants, were detected by XRD analysis. Interestingly, the (1 3 1) diffraction peak showed the strongest intensity in the pattern. These observations may indicate that their (1 3 1) planes tend to be preferentially oriented parallel to the surface of the supporting substrate. Fig. 2(a) shows a typical low-magnification SEM image of the as-grown Cu2SnS3 samples, which illustrates the uniformity flowerlike morphology with an average diameter of 1 mm. A magnified
241
SEM view (Fig. 2b) shows that these individual flower-shaped consist of large number of uniform nanosheets architecture as building blocks. These quantities of nanosheets with interpenetrating growth order are connected to each other to build flowerlike architectures. The TEM image (Fig. 2c) further confirms the sheetlike structure with a clear contrast difference in each individual Cu2SnS3 flowerlike nanostructures. The aggregation and/or assembly of the nanosheets give rise to numerous hierarchical pores on the nanoscale. The edge portion of the flowerlike nanostructures is lighter than that of the center and is comprised of 2D nanosheets. Fig. 2(d) shows a HRTEM image at the edge of individual flowerlike Cu2SnS3 architecture. This HRTEM image further supports the claim of crystallinity for Cu2SnS3. The periodic fringe spacing of 0.313 nm corresponds to the interplanar spacing between the (1 3 1) planes of Cu2SnS3, which is in well agreement with the XRD data. UV–vis diffuse reflectance measurement was used to reveal the energy structure and optical properties of the as-prepared Cu2SnS3 architecture. UV–vis DRS of as-prepared Cu2SnS3 powders is presented in Fig. 3. It could be seen that the Cu2SnS3 product has a steep absorption edge in the visible range, which indicated that the absorption relevant to the band gap was due to the intrinsic transition of the materials rather than the transition from impurity levels. The band gap, calculated from the onset of the absorption edge (inset of Fig. 3), is about 2.42 eV. Combined with a high surfaceto–volume ratio, the ability to absorb visible light makes these Cu2SnS3 architectures an effective photocatalyst for solar-driven applications. The photocatalytic activities of the Cu2SnS3 architectures were evaluated by the degradation of MB dye, a typical pollutant in the textile industry under visible light irradiation. The absorption spectrum (temporal evolution) of an aqueous solution of MB (initial concentration: 1 10 5 M, 50 mL) in the presence of 20 mg Cu2SnS3
Fig. 1. XRD patterns of Cu2SnS3 flowerlike architectures.
Fig. 3. UV–vis diffuse reflectance spectrum of Cu2SnS3 product. The inset showed the plots of (ahn)2 vs hn.
Fig. 2. (a) Low magnification SEM images of the Cu2SnS3 flowerlike architectures; (b) a magnified SEM images of the individual Cu2SnS3 flowerlike architectures; (c) TEM images Cu2SnS3 flowerlike architectures and (d) HRTEM image of the edge of individual flowerlike Cu2SnS3 architectures.
Fig. 4. (a) Temporal evolution of the absorption spectrum of MB in the presence of Cu2SnS3 flowerlike architectures under visible light irradiation.
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Y. Tan et al. / Materials Letters 89 (2012) 240–242
under under visible light irradiation of a 100 W Xe lamp is displayed in Fig. 4. When the light was turned on, the main peaks (663 nm) decreased continuously with increased irradiation time, indicating that the MB solution was decomposed completely in the present system (after 4 h). In our experiments, we found that trace amounts (20 mg) of the samples can accomplish good adsorption and degradation to the MB when irradiated by visible light, which is better or comparable to previous reports [19,20]. Due to the good adsorption and degradation effect of the Cu2SnS3 architectures exhibited in the organic dye solution under visible light irradiation, we can conclude that the obtained sample might be used in the wastewater treatment in industry.
4. Conclusions In summary, a simple solvothermal route can be successfully used for the preparation of Cu2SnS3 samples with the morphology of flowerlike architectures. The DRS spectra showed that the Cu2SnS3 architectures have broad visible light absorption spectrum. The as-obtained Cu2SnS3 architectures exhibited effective catalysis for oxidative decomposition of MB under visible light irradiation. The photocatalytic activities of the Cu2SnS3 architectures are better or comparable to previous reports and our results indicate the potential applications of the Cu2SnS3 architectures in visible-light-driven photocatalysts. These results provide new insights that might lead to the development of further highperformance photocatalysts and nanomaterials for energy and environmental applications that utilize solar energy.
Acknowledgments We thank the financial support of the National Natural Science Foundation of China (Grant no. 90606009).
References [1] Fujishima A, Honda K. Nature 1972;238:37–8. [2] Jang ES, Won JH, Hwang SJ, Choy JH. Adv Mater 2006;18:3309–12. [3] Sandro U, Alessandro D, Domenica S, Silvia B, Adriano Z, Carlo LJ. Am Chem Soc 2007;129:2822–8. [4] Lei AH, Qu BH, Zhou WC, Wang YG, Zhang QL, Zou BS. Mater Lett 2012;66:72–5. [5] Rehman S, Ullah R, Butt AM, Gohar NDJ. Hazard Mater 2009;170:560–9. [6] Wang C, Ao YH, Wang PF, Hou J, Qian J, Zhang SH. Mater Lett 2010;64:439–41. [7] Du WM, Zhu J, Li SX, Qian XF. Cryst Growth Des 2008;8:2130–6. [8] Xiong YJ, Xie Y, Du GA, Su HL. Inorg Chem 2002;41:2953–9. [9] Chen XY, Wang X, An CH, Liu JW, Qian YT. J. Crys. Growth 2003;256:368–76. [10] Wu CZ, Hu ZP, Wang CL, Sheng H, Yang JL, Xie Y. Appl Phys Lett 2007;91: 143104–6. [11] Pienack N, Nather C, Bensch W. Solid State Sci 2007;9:100–7. [12] Zheng L, Xu Y, Song Y, Wu CZ, Zhang M, Xie Y. Inorg Chem 2009;48:4003–9. [13] Fernandes PA, Salome PMP, Cunha AFJ. Phys D: Appl Phys 2010;43:215403–11. [14] Qu BH, Zhang M, Lei DN, Zeng YP, Chen YJ, Chen LB, et al. Nanoscale 2011;3:3646 10. [15] Qu BH, Li HX, Zhang M, Mei L, Chen LB, Wang YG, et al. Nanoscale 2011;3: 4389–93. [16] Berg DM, Djemour R, Gutay L, Siebentritt S, Dale PJ, Fontane X, et al. Appl Phys Lett 2012;100:192013–6. [17] Yi LX, Wang D, Gao MY. Cryst Eng Comm 2012;14:401–4. [18] Norako ME, Greaney MJ, Brutchey RLJ. Am Chem Soc 2012;134:23–6. [19] Cao TP, Li YJ, Wang CH, Zhang ZY, Zhang MY, Shao CL, et al. J Mater Chem 2011;21:6922–7. [20] Cao HQ, Xiao YJ, Zhang SC. Nanotechnology 2011;22:015604–11.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Facile solvothermal synthesis of Cu2SnS3 architectures and their visible-light-driven photocatalytic properties Yu Tan n, Zhiqun Lin, Wenhui Ren, Woyun Long, Yong Wang, Xicheng Ouyang Science College of Hunan Agricultural University, Changsha 410128, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 July 2012 Accepted 24 August 2012 Available online 3 September 2012
In this paper, three dimensional (3D) Cu2SnS3 architectures were successfully synthesized via a simple solvothermal route. The structure, morphology and optical properties of the as-prepared samples were characterized by using an X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV–vis diffuse reflectance spectra (UV–vis DRS). The SEM and TEM image indicated that the flower Cu2SnS3 architectures consist of large number of uniform nanosheets. The band gap is about 2.42 eV, which have been confirmed by UV–vis DRS. Photocatalytic activities by photodegradation reaction of methylene blue (MB) were investigated under visible-light irradiation. The results indicate the potential applications of the Cu2SnS3 architectures in visible-light-driven photocatalysts. Published by Elsevier B.V.
Keywords: Semiconductors Cu2SnS3 Microstructure Photocatalytic activity
1. Introduction Photocatalysts are expected to play an important role in helping to solve many serious environmental and pollution challenges. Because of their electronic structure, semiconductor photocatalysts such as titanium oxide (TiO2) and zinc oxide (ZnO) have been applied to a variety of environmental processes such as remediation of organic contaminants and destruction of microorganisms [1–4]. However, with a wide band gap of 3.2 eV (TiO2) and 3.37 eV (ZnO), which are only active in the ultraviolet light region. While, it is not responsive to visible light (l 4400 nm), which accounts for around 46% of the total solar energy [5]. To make full use of solar energy, a decreased band gap would allow extending the photocatalytic application from the current UV light into the dominant visible light range. Thus narrow band gap semiconductor, such as CdS [6], In2S3 [7] metal sulfides have been proven to be a group of highly efficient catalysts for photochemical reactions, since photogenerated charge carriers can rapidly move to the surface of the catalysts, reducing or oxidizing organic molecules. Unfortunately, the production and use of Cd or In related compounds are harmful to human health, damaging the environment, hindered by the very cost and rareness of the Cd or In element. Thus, it is desirable to exploit low cost, good stability, nontoxicity, and novel visible lightsensitive semiconductor photocatalysts. The ternary Cu–Sn–S system, as an important ternary I–IV–VI group semiconductor with small or mid band-gap, have attracted great attention owing to their broad applications in photovoltaic devices, electrochemical properties and photocatalytic activities for H2 evolution under visible light irradiation [8–18]. In this letter, 3D
n
Corresponding author. Tel./fax: þ 86 731 84618071. E-mail address: [email protected] (Y. Tan).
0167-577X/$ - see front matter Published by Elsevier B.V. http://dx.doi.org/10.1016/j.matlet.2012.08.117
Cu2SnS3 architectures were successfully synthesized via a simple solvothermal route. The unique properties inherent to well-defined 3D hierarchical structures may offer extraordinary potential applications in photocatalysis. Herein, photocatalytic properties of the 3D Cu2SnS3 architectures were investigated by photodegradation of MB under visible light irradiation. To the best of our knowledge, this is the first report on the fabrication of Cu2SnS3 architectures and applications in visible-light-driven photocatalysts.
2. Experimental section All the reagents were of analytical grade and used without further purification. Typically, 0.5 mmol tin (IV) chloride pentahydrate (SnCl4 5H2O), 1 mmol copper chloride dehydrate (CuCl2 2H2O) and 1.5 mmol L-cysteine were dissolved in 30 mL of deionized water, and then the mixture underwent ultrasonic treatment for a few minutes to become a transparent solution. Later, the mixed solution was transferred into a 50 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 180 1C for 12 h, the vessel was then cooled to room temperature. The product was centrifuged and washed several times with deionized water and absolute ethanol, and then dried in a vacuum at 60 1C for 4 h. XRD measurements were carried out using a D-2500 XRD spectrometer (Rigaku) with a Cu Ka line of 0.1541 nm to characterize the crystal structure of the products. SEM micrographs were taken using a Hitachi S-4800 scanning electron microscope. TEM micrographs and high resolution transmission electron microscopy (HRTEM) was performed using JEOL 2010. UV–visible DRS were recorded on a UV–visible spectrophotometer (TU-1901 UV–vis spectrophotometer) equipped with a Labsphere diffuse reflectance accessory. The photocatalytic degradation of MB was carried out in an aqueous
Y. Tan et al. / Materials Letters 89 (2012) 240–242
solution at ambient temperature under visible light irradiation of a 100 W Xe lamp with a 420 nm cutoff filter. Briefly, a mixture of 20 mg Cu2SnS3 products and 50 mL 1 10 5 M MB solution (C16H18ClN3S 3H2O) was put in a Pyrex beaker (100 mL). Prior to orradiation, the solution was magnetically stirred in the dark for 1 h to ensure the adsorption equilibrium. Photocatalytic degradation was monitored by measuring the absorbance of solution using a TU-1901 UV–vis spectrophotometer. At given intervals of illumination, the samples of the reaction solution were taken out and analyzed.
3. Results and discussion The XRD pattern of the as-synthesized product is shown in Fig. 1. All the diffraction peaks can be indexed to the standard diffraction data of the corresponding Mohite Cu2SnS3 (space ˚ b¼11.51 A˚ and group: P1) with lattice parameters of a¼ 6.64 A, c¼ 19.93 A˚ (JCPDS card file no. 35-684). No other impurities, such as binary sulfides (SnS2 or CuS), oxides, or organic compounds related to reactants, were detected by XRD analysis. Interestingly, the (1 3 1) diffraction peak showed the strongest intensity in the pattern. These observations may indicate that their (1 3 1) planes tend to be preferentially oriented parallel to the surface of the supporting substrate. Fig. 2(a) shows a typical low-magnification SEM image of the as-grown Cu2SnS3 samples, which illustrates the uniformity flowerlike morphology with an average diameter of 1 mm. A magnified
241
SEM view (Fig. 2b) shows that these individual flower-shaped consist of large number of uniform nanosheets architecture as building blocks. These quantities of nanosheets with interpenetrating growth order are connected to each other to build flowerlike architectures. The TEM image (Fig. 2c) further confirms the sheetlike structure with a clear contrast difference in each individual Cu2SnS3 flowerlike nanostructures. The aggregation and/or assembly of the nanosheets give rise to numerous hierarchical pores on the nanoscale. The edge portion of the flowerlike nanostructures is lighter than that of the center and is comprised of 2D nanosheets. Fig. 2(d) shows a HRTEM image at the edge of individual flowerlike Cu2SnS3 architecture. This HRTEM image further supports the claim of crystallinity for Cu2SnS3. The periodic fringe spacing of 0.313 nm corresponds to the interplanar spacing between the (1 3 1) planes of Cu2SnS3, which is in well agreement with the XRD data. UV–vis diffuse reflectance measurement was used to reveal the energy structure and optical properties of the as-prepared Cu2SnS3 architecture. UV–vis DRS of as-prepared Cu2SnS3 powders is presented in Fig. 3. It could be seen that the Cu2SnS3 product has a steep absorption edge in the visible range, which indicated that the absorption relevant to the band gap was due to the intrinsic transition of the materials rather than the transition from impurity levels. The band gap, calculated from the onset of the absorption edge (inset of Fig. 3), is about 2.42 eV. Combined with a high surfaceto–volume ratio, the ability to absorb visible light makes these Cu2SnS3 architectures an effective photocatalyst for solar-driven applications. The photocatalytic activities of the Cu2SnS3 architectures were evaluated by the degradation of MB dye, a typical pollutant in the textile industry under visible light irradiation. The absorption spectrum (temporal evolution) of an aqueous solution of MB (initial concentration: 1 10 5 M, 50 mL) in the presence of 20 mg Cu2SnS3
Fig. 1. XRD patterns of Cu2SnS3 flowerlike architectures.
Fig. 3. UV–vis diffuse reflectance spectrum of Cu2SnS3 product. The inset showed the plots of (ahn)2 vs hn.
Fig. 2. (a) Low magnification SEM images of the Cu2SnS3 flowerlike architectures; (b) a magnified SEM images of the individual Cu2SnS3 flowerlike architectures; (c) TEM images Cu2SnS3 flowerlike architectures and (d) HRTEM image of the edge of individual flowerlike Cu2SnS3 architectures.
Fig. 4. (a) Temporal evolution of the absorption spectrum of MB in the presence of Cu2SnS3 flowerlike architectures under visible light irradiation.
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Y. Tan et al. / Materials Letters 89 (2012) 240–242
under under visible light irradiation of a 100 W Xe lamp is displayed in Fig. 4. When the light was turned on, the main peaks (663 nm) decreased continuously with increased irradiation time, indicating that the MB solution was decomposed completely in the present system (after 4 h). In our experiments, we found that trace amounts (20 mg) of the samples can accomplish good adsorption and degradation to the MB when irradiated by visible light, which is better or comparable to previous reports [19,20]. Due to the good adsorption and degradation effect of the Cu2SnS3 architectures exhibited in the organic dye solution under visible light irradiation, we can conclude that the obtained sample might be used in the wastewater treatment in industry.
4. Conclusions In summary, a simple solvothermal route can be successfully used for the preparation of Cu2SnS3 samples with the morphology of flowerlike architectures. The DRS spectra showed that the Cu2SnS3 architectures have broad visible light absorption spectrum. The as-obtained Cu2SnS3 architectures exhibited effective catalysis for oxidative decomposition of MB under visible light irradiation. The photocatalytic activities of the Cu2SnS3 architectures are better or comparable to previous reports and our results indicate the potential applications of the Cu2SnS3 architectures in visible-light-driven photocatalysts. These results provide new insights that might lead to the development of further highperformance photocatalysts and nanomaterials for energy and environmental applications that utilize solar energy.
Acknowledgments We thank the financial support of the National Natural Science Foundation of China (Grant no. 90606009).
References [1] Fujishima A, Honda K. Nature 1972;238:37–8. [2] Jang ES, Won JH, Hwang SJ, Choy JH. Adv Mater 2006;18:3309–12. [3] Sandro U, Alessandro D, Domenica S, Silvia B, Adriano Z, Carlo LJ. Am Chem Soc 2007;129:2822–8. [4] Lei AH, Qu BH, Zhou WC, Wang YG, Zhang QL, Zou BS. Mater Lett 2012;66:72–5. [5] Rehman S, Ullah R, Butt AM, Gohar NDJ. Hazard Mater 2009;170:560–9. [6] Wang C, Ao YH, Wang PF, Hou J, Qian J, Zhang SH. Mater Lett 2010;64:439–41. [7] Du WM, Zhu J, Li SX, Qian XF. Cryst Growth Des 2008;8:2130–6. [8] Xiong YJ, Xie Y, Du GA, Su HL. Inorg Chem 2002;41:2953–9. [9] Chen XY, Wang X, An CH, Liu JW, Qian YT. J. Crys. Growth 2003;256:368–76. [10] Wu CZ, Hu ZP, Wang CL, Sheng H, Yang JL, Xie Y. Appl Phys Lett 2007;91: 143104–6. [11] Pienack N, Nather C, Bensch W. Solid State Sci 2007;9:100–7. [12] Zheng L, Xu Y, Song Y, Wu CZ, Zhang M, Xie Y. Inorg Chem 2009;48:4003–9. [13] Fernandes PA, Salome PMP, Cunha AFJ. Phys D: Appl Phys 2010;43:215403–11. [14] Qu BH, Zhang M, Lei DN, Zeng YP, Chen YJ, Chen LB, et al. Nanoscale 2011;3:3646 10. [15] Qu BH, Li HX, Zhang M, Mei L, Chen LB, Wang YG, et al. Nanoscale 2011;3: 4389–93. [16] Berg DM, Djemour R, Gutay L, Siebentritt S, Dale PJ, Fontane X, et al. Appl Phys Lett 2012;100:192013–6. [17] Yi LX, Wang D, Gao MY. Cryst Eng Comm 2012;14:401–4. [18] Norako ME, Greaney MJ, Brutchey RLJ. Am Chem Soc 2012;134:23–6. [19] Cao TP, Li YJ, Wang CH, Zhang ZY, Zhang MY, Shao CL, et al. J Mater Chem 2011;21:6922–7. [20] Cao HQ, Xiao YJ, Zhang SC. Nanotechnology 2011;22:015604–11.