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Synthesis and characterization of tin disulfide hexagonal nanoflakes via solvothermal decomposition
Materials Letters 67 (2012) 32–34
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis and characterization of tin disulfide hexagonal nanoflakes via solvothermal decomposition Jianmao Yang a, b,⁎, Qiwei Tian b, c, Zhigang Chen b, c, Xiaofeng Xu d, Liusheng Zha a, b a
Research Center for Analysis and Measurement, Donghua University, Shanghai 201620, China State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai, 201620, China College of Materials Science and Engineering, Donghua University, Shanghai 201620, China d Department of Applied Physics, Donghua University, Shanghai 201620, China b c
a r t i c l e
i n f o
Article history: Received 3 August 2011 Accepted 8 September 2011 Available online 16 September 2011 Keywords: Tin disulfide Nanoflakes Solvothermal decomposition Crystal growth Semiconductor
a b s t r a c t Tin disulfide (SnS2) hexagonal flakes with diameters in the range of 50−150 nm are synthesized by using SnCl2.2H2O and sodium diethyldithiocabamate as source materials via a solvothermal decomposition route. Asprepared SnS2 hexagonal nanoflakes are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS) and ultraviolet–visible (UV–vis) spectroscopy. The band gap energy of the SnS2 nanoflakes is measured to be 2.17 eV, and the conduction band (CB) and valence band (VB) levels of the SnS2 nanoflakes are calculated to be −4.34 eV and −6.55 eV respectively, showing them to be suitable for optional and electronic applications. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Metal chalcogenide semiconductor nanomaterials have attracted widespread scientific and technological interest due to their unique size-tunable optical and electronic properties [1–3] as well as their potential applications in solar cells [4,5], light-emitting diodes [6,7] and bio-labels [8], etc. Tin disulfide (SnS2) is one of n-type metal chalcogenide semiconductors with layered hexagonal cadmium iodide (CdI2) structure. It is composed of sheets of tin atoms sandwiched between two close-packed sheets of sulfur atoms [9]. In particular, SnS2 is low-cost and nontoxic, and its component elements are readily available. Currently, SnS2 nanomaterials have been widely used for a variety of industrial and technical applications, for example, semiconductor [10], Li-ion battery anode material [11], solar cells [1], etc. The current focus on their applications has increased an incentive for synthetic approaches that can offer a fastidious control over SnS2 nanomaterials. Therefore, it remains a primary objective to develop efficient routes for synthesizing high-quality SnS2 nanomaterials with superior properties. A variety of methods have been developed for the syntheses of SnS2, such as direct combination of the constituent elements [12], solid-state metathesis [10], vapor transport [13], these synthetic processes require high temperature or expensive instruments. Mild ⁎ Corresponding author at: Research Center for Analysis and Measurement, Donghua University, No. 2999 North Renmin Road, Songjiang District, Shanghai 201620, China. Tel.: + 86 21 67792624; fax: + 86 21 67792818. E-mail address: [email protected] (J. Yang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.09.044
syntheses approaches, hydrothermal and solvothermal methods [14,15], successive ionic layer adsorption and reaction (SILAR) [16], can produce relative big SnS2 crystals with several hundred nanometers long and wide, even to micrometers, so it is difficult to obtain pure nanocrystals (NCs) of this material. In the present work, we report a one-pot synthesis of hexagonal phase SnS2 nanoflakes using a molecular precursor approach. This approach is easily scaled up and has great synthetic reproducibility, by heating a molecular precursor (tin diethyldithiocabamate) in an organic solvent. The shape and structure of SnS2 nanoflakes have been investigated. In addition, their band-gap and band edge are determined by UV–vis absorption spectrum and cyclic voltammogram (CV). 2. Experimental section 2.1. Synthesis of tin disulfide Tin diethyldithiocabamate was synthesized as follows. Firstly, an ethanol solution (5 mL) containing SnCl2.2H2O (2.2565 g, 10 mmol) was added into another ethanol solution (5 mL) containing sodium diethyldithiocabamate (Na-DEDTC) (4.54 g, 20 mmol) under magnetic stirring for 15 min. Subsequently, the product of Sn-DEDTC2 was filtered, washed with ethanol, and dried at room temperature under vacuum. Then SnS2 nanoflakes were prepared by thermal decompositions of the molecular precursor (Sn-DEDTC2). At first, 20 mL of oleylamine was slowly heated to 120 °C under vacuum with magnetic stirring for 30 min to remove residual water and oxygen, during which the flask was purged periodically with dry nitrogen gas. Then another 5 mL of
J. Yang et al. / Materials Letters 67 (2012) 32–34
33
oleylamine containing 1 mmol of Sn-DEDTC2 was injected into the above solution, and heated to 320 °C for 30 min and then cooled to 60 °C, naturally, forming a black colored solution. By an addition of 30 mL ethanol, the black product was centrifuged, and further purified by being dispersed in 1 mL of chloroform and was precipitated again with excess ethanol. After vacuum desiccation, the final SnS2 products can be easily dispersed in several organic solvents such as hexane, toluene, and chloroform for characterizations. 2.2. Characterization The crystal structure of SnS2 sample was investigated by X-ray diffraction (XRD) analysis, using a Bruker D4 X-ray diffractometer with Cu Kα radiation (λ = 0.15418 nm). The size and morphology of SnS2 sample were determined using a Hitachi S-4800 field emission scanning electron microscope (FESEM) and JEOL JEM-2010F transmission electron microscope (TEM) at 200 kV. UV–visible absorption spectrum was measured by a Shimadzu UV-2550 ultraviolet–visible-near-infrared spectrophotometer. In the cyclic voltammogram measurement, acetonitrile solution containing tetrabutylammonium perchlorate (0.1 M) was used as a supporting electrolyte, and a conventional three electrode cell was used with platinum as the counter electrode and an Ag/AgCl reference electrode. The working electrode was prepared by drying 20 mL of the dispersion (10 mg mL− 1) of SnS2 nanoflakes in chloroform on a platinum electrode.
Fig. 2. SEM image of SnS2 hexagonal nanoflakes.
3. Results and discussion From the XRD pattern (Fig. 1) of the as-prepared SnS2 sample, it can be seen that all the peaks could be indexed as the pure hexagonal phase of SnS2 with cell constants a = 3.639 Å, c = 5.878 Å, which are in good agreement with the reported values (a= 3.649 Å, c = 5.899 Å, JCPDS Card No. 23-677). No impurity is detected in the XRD analysis. The strong and sharp diffraction peaks suggest that the products are well crystallized. The high magnification image clearly shows that SnS2 flakes have hexagonal shapes, and the mean diameter is in the range of 50– 150 nm (Fig. 2). X-ray energy dispersive spectrum (see supplementary material) of the as-synthesized SnS2 nanoflakes shows that the asprepared sample is composed of tin and sulfide with a stoichiometric proportion of ~1:2, similar to the result of the SnS2 nanobelts [17]. TEM image, Fig. 3a, shows that the as-prepared SnS2 material displays hexagonal shapes, consistent with the SEM image. A highresolution TEM image, Fig. 3b, shows that the fringes with a lattice spacing of ~0.1966 nm can be resolved, which corresponds to {003} planes of SnS2 crystal. The flake-like morphology is mainly caused by the intrinsic anisotropic growth of SnS2 crystals, and the growth mechanism is similar to that one given in the literature [18]. The selected area electron diffraction (SAED) pattern, Fig. 3c, confirms that the assynthesized SnS2 nanoflakes are structurally single-crystals. The optical properties of the SnS2 nanoflakes are of great importance for their optoelectronic applications. The optical absorption of the SnS2
Fig. 3. (a) TEM, (b) HRTEM images and (c) SAED of as-synthesized SnS2 nanoflakes.
nanoflakes was studied in the wavelength range of 300–800 nm. Fig. 4 shows the absorption spectrum of the as-synthesized SnS2 nanoflakes in ethanol, typically exhibiting a broad shoulder with a trail in the long-wavelength range. The broad spectrum absorption distinctly suggests that the as-synthesized SnS2 nanoflakes should be visible light responsive. To determine the band gap energy (Eg) of the SnS2 material, the dependence of absorption coefficient (α) on the photon energy equation is given as follows [19]: 1 = 2 αhν ¼ B hν−Eg where α is the optical absorption coefficient, hv is the photon energy, Eg is the band gap energy, and B is the constant having separated values for different transitions. The optical absorption coefficient (α) was deduced from the following formula [20]: α¼
Fig. 1. XRD pattern of SnS2 hexagonal nanoflakes.
4π k: λ
The plot of (αhv) 2 vs. hv for these SnS2 nanoflakes is shown in the inset in Fig. 4. The linear nature of the curve indicates the existence of direct transition. The band gap energy Eg is determined by extrapolating the straight curve portion of the plot to the energy axis. The band gap energy is estimated to be 2.17 eV, which is quite near to the reported value [16,21].
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J. Yang et al. / Materials Letters 67 (2012) 32–34
4. Conclusions In this paper, the simple synthetic method has been developed to synthesize SnS2 hexagonal nanoflakes with high crystal quality. The bandgap energy of the SnS2 nanoflakes is measured to be 2.17 eV, and the CB and VB levels of the SnS2 nanoplates are calculated to be −4.34 eV and −6.55 eV, respectively. This preparation approach can be carried out for mass production of multifunctional tin disulfide, especially for optic and electronic application. Acknowledgments
Fig. 4. UV–vis absorption spectrum and inset plot of (αhv)2 against (hv) of the as-synthesized SnS2 nanoflakes.
This work was supported by the National Natural Science Foundation of China (Grant Nos. 50902021, 51073033), the Fundamental Research Funds for the Central Universities in China (No. 2010 C03-2-1), and Scientific Starting Funds for Young Teachers of Donghua University (No. 228-10-0044019). Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10. 1016/j.matlet.2011.09.044.
References
Fig. 5. Cyclic voltammogram curves for the SnS2 nanoflakes and corresponding CB/VB energy levels (inset).
Previous results have shown that the CV measurements are helpful for determination of the conduction band (CB) and valence band (VB) of nanomaterials [22]. With Ag/AgCl reference electrode, it is expected that the SnS2 nanoplates have CB= −(Ered + 4.72) eV, where Ered is the onset potentials of reduction peaks. Fig. 5 shows the cyclic voltammogram of the as-synthesized SnS2 nanoplates. The voltammogram exhibits a reduction onset at ca. −0.38 eV versus Ag/AgCl reference electrode, and then CB can be determined to be −4.34 eV. Therefore the VB can be easily determined to be −6.55 eV according to the equation (Eg = ECB − EVB), as demonstrated in the inset in Fig. 5.
[1] Tan FR, Qu SC, Zeng XB, Zhang CS, Shi MJ, Wang ZJ, et al. Solid State Commun 2010;150:58–61. [2] Zhang YC, Du ZN, Li SY, Zhang M. Appl Catal B Environ 2010;95:153–9. [3] Gao S, Jia X, Yang S, Li Z, Jiang K. J Solid State Chem 2011;184:764–9. [4] Li HX, Zhang QL, Pan AL, Wang YG, Zou BS, Fan HJ. Chem Mater 2011;23: 1299–305. [5] Siddiki MK, Li J, Galipeau D, Qiao Q. Energy Environ Sci 2010;3:867–83. [6] Wang X, Li W, Sun K. J Mater Chem 2011;21:8558–65. [7] Wang Z, Liu J, Dai Y, Dong W, Zhang S, Chen J. Ind Eng Chem Res 2011;50: 7977–84. [8] Jalilian AR, Panahifar A, Mahmoudi M, Akhlaghi M, Simchi A. Radiochim Acta 2009;97:51–6. [9] Greenaway DL, Nitsche R. J Phys Chem Solids 1965;26:1445–58. [10] Sharp L, Soltz D, Parkinson BA. Cryst Growth Des 2006;6:1523–7. [11] Liu S, Yin X, Hao Q, Zhang M, Li L, Chen L, et al. Mater Lett 2010;64:2350–3. [12] Xiao H, Zhang YC. Mater Chem Phys 2008;112:742–4. [13] Yella A, Mugnaioli E, Panthofer M, Therese HA, Kolb U, Tremel W. Angew Chem Int Ed 2009;48:6426–30. [14] Kim TJ, Kim C, Son D, Choi M, Park B. J Power Sources 2007;167:529–35. [15] Lei Y, Song S, Fan W, Xing Y, Zhang H. J Phys Chem C 2009;113:1280–5. [16] Deshpande NG, Sagade AA, Gudage YG, Lokhande CD, Sharma R. J Alloys Compd 2007;436:421–6. [17] Ji Y, Zhang H, Ma X, Xu J, Yang D. J Phys Condens Matter 2003;15:L661–5. [18] Yang Q, Tang K, Wang C, Zhang D, Qian Y. J Solid State Chem 2002;164:106–9. [19] Lin YT, Shi JB, Chen YC, Chen CJ, Wu PF. Nanoscale Res Lett 2009;4:694–8. [20] Khelia C, Boubaker K, Amlouk M. J Alloys Compd 2011;509:929–35. [21] Sanchez-Juarez A, Tiburcio-Silver A, Ortiz A. Thin Solid Films 2005;480:452–6. [22] Yue W, Han S, Peng R, Shen W, Geng H, Wu F, et al. J Mater Chem 2010;20: 7570–8.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis and characterization of tin disulfide hexagonal nanoflakes via solvothermal decomposition Jianmao Yang a, b,⁎, Qiwei Tian b, c, Zhigang Chen b, c, Xiaofeng Xu d, Liusheng Zha a, b a
Research Center for Analysis and Measurement, Donghua University, Shanghai 201620, China State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai, 201620, China College of Materials Science and Engineering, Donghua University, Shanghai 201620, China d Department of Applied Physics, Donghua University, Shanghai 201620, China b c
a r t i c l e
i n f o
Article history: Received 3 August 2011 Accepted 8 September 2011 Available online 16 September 2011 Keywords: Tin disulfide Nanoflakes Solvothermal decomposition Crystal growth Semiconductor
a b s t r a c t Tin disulfide (SnS2) hexagonal flakes with diameters in the range of 50−150 nm are synthesized by using SnCl2.2H2O and sodium diethyldithiocabamate as source materials via a solvothermal decomposition route. Asprepared SnS2 hexagonal nanoflakes are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS) and ultraviolet–visible (UV–vis) spectroscopy. The band gap energy of the SnS2 nanoflakes is measured to be 2.17 eV, and the conduction band (CB) and valence band (VB) levels of the SnS2 nanoflakes are calculated to be −4.34 eV and −6.55 eV respectively, showing them to be suitable for optional and electronic applications. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Metal chalcogenide semiconductor nanomaterials have attracted widespread scientific and technological interest due to their unique size-tunable optical and electronic properties [1–3] as well as their potential applications in solar cells [4,5], light-emitting diodes [6,7] and bio-labels [8], etc. Tin disulfide (SnS2) is one of n-type metal chalcogenide semiconductors with layered hexagonal cadmium iodide (CdI2) structure. It is composed of sheets of tin atoms sandwiched between two close-packed sheets of sulfur atoms [9]. In particular, SnS2 is low-cost and nontoxic, and its component elements are readily available. Currently, SnS2 nanomaterials have been widely used for a variety of industrial and technical applications, for example, semiconductor [10], Li-ion battery anode material [11], solar cells [1], etc. The current focus on their applications has increased an incentive for synthetic approaches that can offer a fastidious control over SnS2 nanomaterials. Therefore, it remains a primary objective to develop efficient routes for synthesizing high-quality SnS2 nanomaterials with superior properties. A variety of methods have been developed for the syntheses of SnS2, such as direct combination of the constituent elements [12], solid-state metathesis [10], vapor transport [13], these synthetic processes require high temperature or expensive instruments. Mild ⁎ Corresponding author at: Research Center for Analysis and Measurement, Donghua University, No. 2999 North Renmin Road, Songjiang District, Shanghai 201620, China. Tel.: + 86 21 67792624; fax: + 86 21 67792818. E-mail address: [email protected] (J. Yang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.09.044
syntheses approaches, hydrothermal and solvothermal methods [14,15], successive ionic layer adsorption and reaction (SILAR) [16], can produce relative big SnS2 crystals with several hundred nanometers long and wide, even to micrometers, so it is difficult to obtain pure nanocrystals (NCs) of this material. In the present work, we report a one-pot synthesis of hexagonal phase SnS2 nanoflakes using a molecular precursor approach. This approach is easily scaled up and has great synthetic reproducibility, by heating a molecular precursor (tin diethyldithiocabamate) in an organic solvent. The shape and structure of SnS2 nanoflakes have been investigated. In addition, their band-gap and band edge are determined by UV–vis absorption spectrum and cyclic voltammogram (CV). 2. Experimental section 2.1. Synthesis of tin disulfide Tin diethyldithiocabamate was synthesized as follows. Firstly, an ethanol solution (5 mL) containing SnCl2.2H2O (2.2565 g, 10 mmol) was added into another ethanol solution (5 mL) containing sodium diethyldithiocabamate (Na-DEDTC) (4.54 g, 20 mmol) under magnetic stirring for 15 min. Subsequently, the product of Sn-DEDTC2 was filtered, washed with ethanol, and dried at room temperature under vacuum. Then SnS2 nanoflakes were prepared by thermal decompositions of the molecular precursor (Sn-DEDTC2). At first, 20 mL of oleylamine was slowly heated to 120 °C under vacuum with magnetic stirring for 30 min to remove residual water and oxygen, during which the flask was purged periodically with dry nitrogen gas. Then another 5 mL of
J. Yang et al. / Materials Letters 67 (2012) 32–34
33
oleylamine containing 1 mmol of Sn-DEDTC2 was injected into the above solution, and heated to 320 °C for 30 min and then cooled to 60 °C, naturally, forming a black colored solution. By an addition of 30 mL ethanol, the black product was centrifuged, and further purified by being dispersed in 1 mL of chloroform and was precipitated again with excess ethanol. After vacuum desiccation, the final SnS2 products can be easily dispersed in several organic solvents such as hexane, toluene, and chloroform for characterizations. 2.2. Characterization The crystal structure of SnS2 sample was investigated by X-ray diffraction (XRD) analysis, using a Bruker D4 X-ray diffractometer with Cu Kα radiation (λ = 0.15418 nm). The size and morphology of SnS2 sample were determined using a Hitachi S-4800 field emission scanning electron microscope (FESEM) and JEOL JEM-2010F transmission electron microscope (TEM) at 200 kV. UV–visible absorption spectrum was measured by a Shimadzu UV-2550 ultraviolet–visible-near-infrared spectrophotometer. In the cyclic voltammogram measurement, acetonitrile solution containing tetrabutylammonium perchlorate (0.1 M) was used as a supporting electrolyte, and a conventional three electrode cell was used with platinum as the counter electrode and an Ag/AgCl reference electrode. The working electrode was prepared by drying 20 mL of the dispersion (10 mg mL− 1) of SnS2 nanoflakes in chloroform on a platinum electrode.
Fig. 2. SEM image of SnS2 hexagonal nanoflakes.
3. Results and discussion From the XRD pattern (Fig. 1) of the as-prepared SnS2 sample, it can be seen that all the peaks could be indexed as the pure hexagonal phase of SnS2 with cell constants a = 3.639 Å, c = 5.878 Å, which are in good agreement with the reported values (a= 3.649 Å, c = 5.899 Å, JCPDS Card No. 23-677). No impurity is detected in the XRD analysis. The strong and sharp diffraction peaks suggest that the products are well crystallized. The high magnification image clearly shows that SnS2 flakes have hexagonal shapes, and the mean diameter is in the range of 50– 150 nm (Fig. 2). X-ray energy dispersive spectrum (see supplementary material) of the as-synthesized SnS2 nanoflakes shows that the asprepared sample is composed of tin and sulfide with a stoichiometric proportion of ~1:2, similar to the result of the SnS2 nanobelts [17]. TEM image, Fig. 3a, shows that the as-prepared SnS2 material displays hexagonal shapes, consistent with the SEM image. A highresolution TEM image, Fig. 3b, shows that the fringes with a lattice spacing of ~0.1966 nm can be resolved, which corresponds to {003} planes of SnS2 crystal. The flake-like morphology is mainly caused by the intrinsic anisotropic growth of SnS2 crystals, and the growth mechanism is similar to that one given in the literature [18]. The selected area electron diffraction (SAED) pattern, Fig. 3c, confirms that the assynthesized SnS2 nanoflakes are structurally single-crystals. The optical properties of the SnS2 nanoflakes are of great importance for their optoelectronic applications. The optical absorption of the SnS2
Fig. 3. (a) TEM, (b) HRTEM images and (c) SAED of as-synthesized SnS2 nanoflakes.
nanoflakes was studied in the wavelength range of 300–800 nm. Fig. 4 shows the absorption spectrum of the as-synthesized SnS2 nanoflakes in ethanol, typically exhibiting a broad shoulder with a trail in the long-wavelength range. The broad spectrum absorption distinctly suggests that the as-synthesized SnS2 nanoflakes should be visible light responsive. To determine the band gap energy (Eg) of the SnS2 material, the dependence of absorption coefficient (α) on the photon energy equation is given as follows [19]: 1 = 2 αhν ¼ B hν−Eg where α is the optical absorption coefficient, hv is the photon energy, Eg is the band gap energy, and B is the constant having separated values for different transitions. The optical absorption coefficient (α) was deduced from the following formula [20]: α¼
Fig. 1. XRD pattern of SnS2 hexagonal nanoflakes.
4π k: λ
The plot of (αhv) 2 vs. hv for these SnS2 nanoflakes is shown in the inset in Fig. 4. The linear nature of the curve indicates the existence of direct transition. The band gap energy Eg is determined by extrapolating the straight curve portion of the plot to the energy axis. The band gap energy is estimated to be 2.17 eV, which is quite near to the reported value [16,21].
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J. Yang et al. / Materials Letters 67 (2012) 32–34
4. Conclusions In this paper, the simple synthetic method has been developed to synthesize SnS2 hexagonal nanoflakes with high crystal quality. The bandgap energy of the SnS2 nanoflakes is measured to be 2.17 eV, and the CB and VB levels of the SnS2 nanoplates are calculated to be −4.34 eV and −6.55 eV, respectively. This preparation approach can be carried out for mass production of multifunctional tin disulfide, especially for optic and electronic application. Acknowledgments
Fig. 4. UV–vis absorption spectrum and inset plot of (αhv)2 against (hv) of the as-synthesized SnS2 nanoflakes.
This work was supported by the National Natural Science Foundation of China (Grant Nos. 50902021, 51073033), the Fundamental Research Funds for the Central Universities in China (No. 2010 C03-2-1), and Scientific Starting Funds for Young Teachers of Donghua University (No. 228-10-0044019). Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10. 1016/j.matlet.2011.09.044.
References
Fig. 5. Cyclic voltammogram curves for the SnS2 nanoflakes and corresponding CB/VB energy levels (inset).
Previous results have shown that the CV measurements are helpful for determination of the conduction band (CB) and valence band (VB) of nanomaterials [22]. With Ag/AgCl reference electrode, it is expected that the SnS2 nanoplates have CB= −(Ered + 4.72) eV, where Ered is the onset potentials of reduction peaks. Fig. 5 shows the cyclic voltammogram of the as-synthesized SnS2 nanoplates. The voltammogram exhibits a reduction onset at ca. −0.38 eV versus Ag/AgCl reference electrode, and then CB can be determined to be −4.34 eV. Therefore the VB can be easily determined to be −6.55 eV according to the equation (Eg = ECB − EVB), as demonstrated in the inset in Fig. 5.
[1] Tan FR, Qu SC, Zeng XB, Zhang CS, Shi MJ, Wang ZJ, et al. Solid State Commun 2010;150:58–61. [2] Zhang YC, Du ZN, Li SY, Zhang M. Appl Catal B Environ 2010;95:153–9. [3] Gao S, Jia X, Yang S, Li Z, Jiang K. J Solid State Chem 2011;184:764–9. [4] Li HX, Zhang QL, Pan AL, Wang YG, Zou BS, Fan HJ. Chem Mater 2011;23: 1299–305. [5] Siddiki MK, Li J, Galipeau D, Qiao Q. Energy Environ Sci 2010;3:867–83. [6] Wang X, Li W, Sun K. J Mater Chem 2011;21:8558–65. [7] Wang Z, Liu J, Dai Y, Dong W, Zhang S, Chen J. Ind Eng Chem Res 2011;50: 7977–84. [8] Jalilian AR, Panahifar A, Mahmoudi M, Akhlaghi M, Simchi A. Radiochim Acta 2009;97:51–6. [9] Greenaway DL, Nitsche R. J Phys Chem Solids 1965;26:1445–58. [10] Sharp L, Soltz D, Parkinson BA. Cryst Growth Des 2006;6:1523–7. [11] Liu S, Yin X, Hao Q, Zhang M, Li L, Chen L, et al. Mater Lett 2010;64:2350–3. [12] Xiao H, Zhang YC. Mater Chem Phys 2008;112:742–4. [13] Yella A, Mugnaioli E, Panthofer M, Therese HA, Kolb U, Tremel W. Angew Chem Int Ed 2009;48:6426–30. [14] Kim TJ, Kim C, Son D, Choi M, Park B. J Power Sources 2007;167:529–35. [15] Lei Y, Song S, Fan W, Xing Y, Zhang H. J Phys Chem C 2009;113:1280–5. [16] Deshpande NG, Sagade AA, Gudage YG, Lokhande CD, Sharma R. J Alloys Compd 2007;436:421–6. [17] Ji Y, Zhang H, Ma X, Xu J, Yang D. J Phys Condens Matter 2003;15:L661–5. [18] Yang Q, Tang K, Wang C, Zhang D, Qian Y. J Solid State Chem 2002;164:106–9. [19] Lin YT, Shi JB, Chen YC, Chen CJ, Wu PF. Nanoscale Res Lett 2009;4:694–8. [20] Khelia C, Boubaker K, Amlouk M. J Alloys Compd 2011;509:929–35. [21] Sanchez-Juarez A, Tiburcio-Silver A, Ortiz A. Thin Solid Films 2005;480:452–6. [22] Yue W, Han S, Peng R, Shen W, Geng H, Wu F, et al. J Mater Chem 2010;20: 7570–8.