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Synthesis of SnO2 nano-sheets by a template-free hydrothermal method
Materials Letters 63 (2009) 2085–2088
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
Synthesis of SnO2 nano-sheets by a template-free hydrothermal method Yue Li a, Yanqun Guo a, Ruiqin Tan b,⁎, Ping Cui a, Yong Li a, Weijie Song a a b
Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, No. 519 Zhuangshi Road, Zhenhai District, Ningbo, 315201, PR China Faculty of Information Science and Engineering, Ningbo University, No. 818 Fenghua Road, Jiangbei District, Ningbo, 315211, PR China
a r t i c l e
i n f o
Article history: Received 10 April 2009 Accepted 25 June 2009 Available online 4 July 2009 Keywords: Nanomaterials Hydrothermal SnO2 Microstructure Nano-sheets
a b s t r a c t Square SnO2 nano-sheets have been successfully synthesized by a template-free hydrothermal method basing on the reaction between SnCl2 and NaOH in ethanol/water solution. The products have been characterized by X-ray diffraction, scanning electron microscope, and transmission electron microscope. The size and morphology of nano-sheets could be controlled by changing the ratio of ethanol to water. Compared experiments showed that the addition of NaOH was crucial to the formation of SnO2 nano-sheets, and the addition of glycol could induce the formation of cabbage-like structure. The nano-sheets show a strong absorption peak around 330 nm, corresponding to a band gap value of 3.75 eV. This blue shift of the band gap energy was possibly induced by the spatial confinement of excitons in the nano-sheet structures. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Synthesis of nano-materials with controlled morphology, size, and crystal structure is a key step towards nano-technological applications. As an n-type wide-band gap semiconductor, SnO2 is an important functional material which has been extensively used in gas sensors and optoelectronic devices [1,2]. Over the past few years, SnO2 with various geometrical morphologies have been reported, including hollow structures, nano-belt/wire/tubes/rods, nano-diskettes, nano-sheets, and nanoparticles [3–8]. Recently, SnO2 with two-dimensional structure has attracted much attention owing to their potential applications as gas sensors [9] and photocatalyst [10]. Many research groups have been making great effects to synthesize a novel SnO2 nano-structure. Dai and Pan reported that SnO2 diskettes were synthesized by evaporating SnO2 powders at elevated temperature [11]. Two-dimensional (2D) hierarchical SnO2 flower-like architectures were synthesized by a simply mild hydrothermal method based on the reaction between tin foil, NaOH and KBrO3 in Xie's group [12]. Hexagonal SnO2 nano-sheets were synthesized in ethanol/water solution by hydrothermal process [13]. Flower-like zincdoped SnO2 nano-crystals have been prepared by a simple hydrothermal process [14]. It is clear that the hydrothermal processes, especially starting from aqueous solutions are most widely investigated [15–18]. However, experimental trials for the SnO2 preparation by template-free routes are inadequate. Morphology control by using template-free hydrothermal routes for nano SnO2 synthesis is still challenging.
⁎ Corresponding author. Tel.: +86 574 87913375; fax: +86 574 86685163. E-mail address: [email protected] (R. Tan). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.06.060
In this report, we demonstrate a template-free hydrothermal process for the synthesis of SnO2 nano-sheets basing on the reaction between SnCl2 and NaOH in ethanol/water solution. Effects of the experimental condition on the microstructure of the SnO2 nano-sheets were investigated using XRD, SEM, and TEM. Optical properties of the SnO2 nano-sheets were studied using UV-vis spectroscopy. 2. Experimental SnCl2·2H2O and NaOH were selected as starting materials. All the reagents were purchased from Sinopharm chemical Reagent Co., were of analytical grade, and needed no further purification. Firstly 6 mmol SnCl2·2H2O was dissolved into 20 mL water, 5 mL ethanol and 15 mL water,10 mL ethanol and 10 mL water, respectively. Then 0.4 mol/L NaOH solution was dropped into each SnCl2 solution until pH= 13 under continuous magnetic stirring. The obtained mixture were transferred into a 100 mL Teflon-lined stainless autoclave, sealed and maintained at 180 °C for 12 h, and then cooled down to room temperature. The obtained precipitates were centrifuged and washed several times with water and ethanol respectively until Cl-ions could not be detected. The products were finally dried in the vacuum at 80 °C for 1 h. XRD analysis for phase identification and line broadening was performed using a Bruker AXS D8 advance diffractometer with Cu Kα radiation at a power of 1.6 kW. The diffraction pattern was calibrated using the standard spectra of corundum. The morphology and microstructure of the SnO2 nanoparticles were characterized by a Hitachi S4800 field emission scanning electron microscope and a FEI Tecnai G2 F20 field emission transmission electron microscope, respectively. UV-vis spectroscopy was recorded using a Perkin Elmer Lambda 950 UV-vis-NIR spectrometer using reflection mode.
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Y. Li et al. / Materials Letters 63 (2009) 2085–2088
3. Results and discussion
Fig. 1. A typical XRD pattern of the SnO2 nano-sheets.
Fig. 1 shows a typical XRD pattern of the as-synthesized product when the ratio of ethanol to water is 1:3. All the diffraction peaks can be perfectly indexed to a rutile SnO2 structure (JCPDS card, No. 411445, space group: P42/mnm, ao = 4.738 Å, co = 3.187 Å) and no impurity phase was observed. This indicated that the product was pure rutile SnO2. Other products synthesized under different water/ ethanol ratios revealed similar results. SEM images for this product are shown in Fig. 2(a) and (b). It could be observed that the product was composed of nano-sheets with 500– 600 nm in edge length, 100–200 nm in width and 2–3 nm in thickness. The TEM image, with SAED pattern and the high-resolution TEM image for a piece of nano-sheet are shown in Fig. 2 (c), respectively. The nano-sheets exhibited a single-crystalline structure with its (110) face dominated. Beltran et al. reported that the surface energy for rutile SnO2 followed the sequence (110) b (010) b (101) b (001) [19]. The nano-sheets obtained in this work revealed the lowest surface energy. Surprisingly, by changing the ethanol/water ratio, the size and
Fig. 2. SEM and TEM images of the obtained SnO2 nano-structures at ethanol/water ratio of: (a–c) 1:3, (d) 0:1, and (e) 1:1; and (f) the SEM image for the SnO2 obtained without NaOH.
Y. Li et al. / Materials Letters 63 (2009) 2085–2088
morphology of SnO2 nano-sheets could be controlled. Fig. 2 (d) and 2(e) shows the SEM images of the products obtained when the ethanol/ water ratio was 0:1 and 1:1. The final products were flower-like nanoarchitectures composed of irregular SnO2 nano-sheets. The polarity of solvents and the addition of NaOH were crucial for the formation of SnO2 nano-sheets. Water is a dipolar, amphiprotic solvent with a high dielectric constant while ethanol has a low dielectric constant, weak polarity, and low surface tension. The polarity of the water/ethanol solution would change with different volume ratio of ethanol to water. The degree of SnCl2 hydrolysis would be different and the observed morphology would change with variation of solvent. Different precipitates including basic chloride and hydrous oxide will be formed after the addition of NaOH, and SnO2 nano-sheets would be formed after the hydrothermal process. Fig. 2(f) shows the SEM image of the product obtained without using NaOH. It was observed that only nanoparticles with a diameter of 5 nm were formed. Donaldson et al. investigated the precipitate of the SnCl2–H2O system under different PHs in detail [20]. Imai et al. proposed that the intermediate precipitate can influence the morphology of the final product [21]. Our results indicated that NaOH played a key role for the SnO2 nano-sheet formation. When using glycol instead of ethanol in the experiments, a novel cabbage-like SnO2 product composed of nano-sheets was formed when the glycol/water ratio was 1:2. The corresponding XRD pattern and SEM images are shown in Fig. 3. The obtained product contains lots of two-dimensional nano-sheets, which are about several micrometres in length and 300–600 nm in width. The cabbage-like nano-sheets revealed the same rutile SnO2 structure. The formation of this novel morphology was resulted from the addition of glycol. To the
2087
Fig. 4. UV-vis spectra of the SnO2 nano-sheets at different ethanol/water ratio: (a) 0:1; (b) 1:1; (c) 1:3.
best of our knowledge, such cabbage-like morphology has not been reported. Fig. 4 shows the UV-vis spectra of the SnO2 nano-sheets. It was observed that there existed a strong absorption peak around 330 nm for all the SnO2 nano-sheets. The corresponding band gap value was
Fig. 3. The (a) XRD pattern and (b–e) SEM images of the cabbage-like SnO2 nano-sheets prepared with the addition of glycol by hydrothermal treatment at 180 °C for 24 h.
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Y. Li et al. / Materials Letters 63 (2009) 2085–2088
3.75 eV. The band gap values were calculated using the relation of Eg = 1240/λ, where λ was the wavelength of the absorption peak in nanometers. Lee et al. studied the size dependence of the band gap energies of the quantum-confined SnO2 particles. The band gap energy decreased from 4.01 eV to 3.68 eV when the mean particle radius increased from 1.75 nm to 4.2 nm [22]. Chen and Gao reported that the absorption bands of the nanorods with diameters N4.5 nm are very close to that of the bulk SnO2 (345 nm, BG energy 3.60 eV), which can be attributed to the fact that the diameters are much larger than the exciton Bohr radius (∼ 2.7 nm) of SnO2 [17]. An indirect band gap of 2.68 eV for the bare SnO2 (110) surface was obtained by theoretical calculation [19]. Compared with these results, the band gap energy of 3.75 eV measured in this work revealed a blue shift. Although the edge length and the width of the as-synthesized products are several hundred nanometers, the SnO2 nano-sheets almost fit the critical point of the weak and strong confinement regime curve. Here the special nano-sheets morphology might induce the relative blue shift of the band gap energy due to a spatial confinement of an exciton. 4. Conclusion In summary, a template-free hydrothermal method was developed for the synthesis of the SnO2 nano-sheets staring from SnCl2 and NaOH in ethanol/water solution. By changing the ethanol/water ratio, the size of nano-sheets and morphology could be controlled. NaOH played a crucial role during the formation of the SnO2 nano-sheets. The addition of glycol could induce the formation of cabbage-like SnO2. The nano-sheets showed a strong absorption peak around 330 nm, corresponding to a band gap value of 3.75 eV, which were different from bulk SnO2.
Acknowledgements The authors appreciate the financial supports from the “Hundred Talents Program” of the Chinese Academy of Sciences and the Ningbo Natural Science Foundation (No.2007A610027, No.2008A610047). This work is also sponsored by K.C. Wang Magna Found in Ningbo University. References [1] Sahm T, Rong W, Barsan N, Madler L, Weimar U. Sens Actuators, B 2007;127:63–8. [2] Samotaev NN, Vasiliev AA, Podlepetsky BI, Sokolov AV, Pisliakov AV. Sens Actuators, B 2007;127:242–7. [3] Wan HZ, Liang JB, Fan H, Xi BJ, Zhang MF, Xiong SL, et al. J Solid State Chem 2008;181:122–9. [4] Dai ZR, Pan ZW, Wang ZL. Solid State Commun 2001;118:351–4. [5] Wang WZ, Niu JZ, Ling A. J Cryst Growth 2008;310:351–5. [6] Krishnakumar T, Pinna NK, Kumari P, Perumal K, Jayaprakash R. Mater Lett 2008;62:3437–40. [7] Leite ER, Weber IT, Longo E, Varela JA. Adv Mater 2000;12:965–8. [8] Nayral C, Ould-Ely T, Maisonnat A, Chaudret B, Fau P, Lescouzères L, et al. Adv Mater 1999;11:61–3. [9] Ge JP, Wang J, Zhang HX, Wang X, Peng Q, Li YD. Sens Actuators, B 2006;113: 937–43. [10] Wang WW, Zhu YJ, Yang LX. Adv Funct Mater 2007;17:59–64. [11] Dai ZR, Pan ZW, Wang ZL. J Am Chem Soc 2002;124:8673–80. [12] Zhao QR, Li ZQ, Wu CZ, Xue B, Xie YJ. Nanopart Res 2006;8:1065–9. [13] Yang XH, Wang LL. Mater Lett 2007;61:3705–7. [14] Li ZR, Li XL, Zhang XX, Qian YT. J Cryst Growth 2006;291:258–61. [15] Lou XW, Wang Y, Yuan CL, Lee JY, Archer LA. Adv Mater 2006;18:2325–9. [16] Yang HG, Zeng HC. Angew Chem, Int Ed 2004;43:5930–3. [17] Chen DL, Gao L. Chem Phys Lett 2004;398:201–6. [18] Uchiyama H, Ohgi H, Imai H. Cryst Growth Des 2006;6:2186–90. [19] Beltran A, Andres J, Sambrano JR, Longo E. J Phys Chem, A 2008;112:8943–52. [20] Li LL, Chu Y, Liu Y, Dong LH, Huo L, Yang FY. Mater Lett 2006;60:2138–41. [21] Ohgi H, Maeda T, Hosono E, Fujihara S, Imai H. Cryst Growth Des 2005;5:1079–83. [22] Lee JH, Ribeiro C, Giraldi TR, Longo E, Leite ER, Varela JA. Appl Phys Lett 2004;84: 1745–8.
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
Synthesis of SnO2 nano-sheets by a template-free hydrothermal method Yue Li a, Yanqun Guo a, Ruiqin Tan b,⁎, Ping Cui a, Yong Li a, Weijie Song a a b
Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, No. 519 Zhuangshi Road, Zhenhai District, Ningbo, 315201, PR China Faculty of Information Science and Engineering, Ningbo University, No. 818 Fenghua Road, Jiangbei District, Ningbo, 315211, PR China
a r t i c l e
i n f o
Article history: Received 10 April 2009 Accepted 25 June 2009 Available online 4 July 2009 Keywords: Nanomaterials Hydrothermal SnO2 Microstructure Nano-sheets
a b s t r a c t Square SnO2 nano-sheets have been successfully synthesized by a template-free hydrothermal method basing on the reaction between SnCl2 and NaOH in ethanol/water solution. The products have been characterized by X-ray diffraction, scanning electron microscope, and transmission electron microscope. The size and morphology of nano-sheets could be controlled by changing the ratio of ethanol to water. Compared experiments showed that the addition of NaOH was crucial to the formation of SnO2 nano-sheets, and the addition of glycol could induce the formation of cabbage-like structure. The nano-sheets show a strong absorption peak around 330 nm, corresponding to a band gap value of 3.75 eV. This blue shift of the band gap energy was possibly induced by the spatial confinement of excitons in the nano-sheet structures. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Synthesis of nano-materials with controlled morphology, size, and crystal structure is a key step towards nano-technological applications. As an n-type wide-band gap semiconductor, SnO2 is an important functional material which has been extensively used in gas sensors and optoelectronic devices [1,2]. Over the past few years, SnO2 with various geometrical morphologies have been reported, including hollow structures, nano-belt/wire/tubes/rods, nano-diskettes, nano-sheets, and nanoparticles [3–8]. Recently, SnO2 with two-dimensional structure has attracted much attention owing to their potential applications as gas sensors [9] and photocatalyst [10]. Many research groups have been making great effects to synthesize a novel SnO2 nano-structure. Dai and Pan reported that SnO2 diskettes were synthesized by evaporating SnO2 powders at elevated temperature [11]. Two-dimensional (2D) hierarchical SnO2 flower-like architectures were synthesized by a simply mild hydrothermal method based on the reaction between tin foil, NaOH and KBrO3 in Xie's group [12]. Hexagonal SnO2 nano-sheets were synthesized in ethanol/water solution by hydrothermal process [13]. Flower-like zincdoped SnO2 nano-crystals have been prepared by a simple hydrothermal process [14]. It is clear that the hydrothermal processes, especially starting from aqueous solutions are most widely investigated [15–18]. However, experimental trials for the SnO2 preparation by template-free routes are inadequate. Morphology control by using template-free hydrothermal routes for nano SnO2 synthesis is still challenging.
⁎ Corresponding author. Tel.: +86 574 87913375; fax: +86 574 86685163. E-mail address: [email protected] (R. Tan). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.06.060
In this report, we demonstrate a template-free hydrothermal process for the synthesis of SnO2 nano-sheets basing on the reaction between SnCl2 and NaOH in ethanol/water solution. Effects of the experimental condition on the microstructure of the SnO2 nano-sheets were investigated using XRD, SEM, and TEM. Optical properties of the SnO2 nano-sheets were studied using UV-vis spectroscopy. 2. Experimental SnCl2·2H2O and NaOH were selected as starting materials. All the reagents were purchased from Sinopharm chemical Reagent Co., were of analytical grade, and needed no further purification. Firstly 6 mmol SnCl2·2H2O was dissolved into 20 mL water, 5 mL ethanol and 15 mL water,10 mL ethanol and 10 mL water, respectively. Then 0.4 mol/L NaOH solution was dropped into each SnCl2 solution until pH= 13 under continuous magnetic stirring. The obtained mixture were transferred into a 100 mL Teflon-lined stainless autoclave, sealed and maintained at 180 °C for 12 h, and then cooled down to room temperature. The obtained precipitates were centrifuged and washed several times with water and ethanol respectively until Cl-ions could not be detected. The products were finally dried in the vacuum at 80 °C for 1 h. XRD analysis for phase identification and line broadening was performed using a Bruker AXS D8 advance diffractometer with Cu Kα radiation at a power of 1.6 kW. The diffraction pattern was calibrated using the standard spectra of corundum. The morphology and microstructure of the SnO2 nanoparticles were characterized by a Hitachi S4800 field emission scanning electron microscope and a FEI Tecnai G2 F20 field emission transmission electron microscope, respectively. UV-vis spectroscopy was recorded using a Perkin Elmer Lambda 950 UV-vis-NIR spectrometer using reflection mode.
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3. Results and discussion
Fig. 1. A typical XRD pattern of the SnO2 nano-sheets.
Fig. 1 shows a typical XRD pattern of the as-synthesized product when the ratio of ethanol to water is 1:3. All the diffraction peaks can be perfectly indexed to a rutile SnO2 structure (JCPDS card, No. 411445, space group: P42/mnm, ao = 4.738 Å, co = 3.187 Å) and no impurity phase was observed. This indicated that the product was pure rutile SnO2. Other products synthesized under different water/ ethanol ratios revealed similar results. SEM images for this product are shown in Fig. 2(a) and (b). It could be observed that the product was composed of nano-sheets with 500– 600 nm in edge length, 100–200 nm in width and 2–3 nm in thickness. The TEM image, with SAED pattern and the high-resolution TEM image for a piece of nano-sheet are shown in Fig. 2 (c), respectively. The nano-sheets exhibited a single-crystalline structure with its (110) face dominated. Beltran et al. reported that the surface energy for rutile SnO2 followed the sequence (110) b (010) b (101) b (001) [19]. The nano-sheets obtained in this work revealed the lowest surface energy. Surprisingly, by changing the ethanol/water ratio, the size and
Fig. 2. SEM and TEM images of the obtained SnO2 nano-structures at ethanol/water ratio of: (a–c) 1:3, (d) 0:1, and (e) 1:1; and (f) the SEM image for the SnO2 obtained without NaOH.
Y. Li et al. / Materials Letters 63 (2009) 2085–2088
morphology of SnO2 nano-sheets could be controlled. Fig. 2 (d) and 2(e) shows the SEM images of the products obtained when the ethanol/ water ratio was 0:1 and 1:1. The final products were flower-like nanoarchitectures composed of irregular SnO2 nano-sheets. The polarity of solvents and the addition of NaOH were crucial for the formation of SnO2 nano-sheets. Water is a dipolar, amphiprotic solvent with a high dielectric constant while ethanol has a low dielectric constant, weak polarity, and low surface tension. The polarity of the water/ethanol solution would change with different volume ratio of ethanol to water. The degree of SnCl2 hydrolysis would be different and the observed morphology would change with variation of solvent. Different precipitates including basic chloride and hydrous oxide will be formed after the addition of NaOH, and SnO2 nano-sheets would be formed after the hydrothermal process. Fig. 2(f) shows the SEM image of the product obtained without using NaOH. It was observed that only nanoparticles with a diameter of 5 nm were formed. Donaldson et al. investigated the precipitate of the SnCl2–H2O system under different PHs in detail [20]. Imai et al. proposed that the intermediate precipitate can influence the morphology of the final product [21]. Our results indicated that NaOH played a key role for the SnO2 nano-sheet formation. When using glycol instead of ethanol in the experiments, a novel cabbage-like SnO2 product composed of nano-sheets was formed when the glycol/water ratio was 1:2. The corresponding XRD pattern and SEM images are shown in Fig. 3. The obtained product contains lots of two-dimensional nano-sheets, which are about several micrometres in length and 300–600 nm in width. The cabbage-like nano-sheets revealed the same rutile SnO2 structure. The formation of this novel morphology was resulted from the addition of glycol. To the
2087
Fig. 4. UV-vis spectra of the SnO2 nano-sheets at different ethanol/water ratio: (a) 0:1; (b) 1:1; (c) 1:3.
best of our knowledge, such cabbage-like morphology has not been reported. Fig. 4 shows the UV-vis spectra of the SnO2 nano-sheets. It was observed that there existed a strong absorption peak around 330 nm for all the SnO2 nano-sheets. The corresponding band gap value was
Fig. 3. The (a) XRD pattern and (b–e) SEM images of the cabbage-like SnO2 nano-sheets prepared with the addition of glycol by hydrothermal treatment at 180 °C for 24 h.
2088
Y. Li et al. / Materials Letters 63 (2009) 2085–2088
3.75 eV. The band gap values were calculated using the relation of Eg = 1240/λ, where λ was the wavelength of the absorption peak in nanometers. Lee et al. studied the size dependence of the band gap energies of the quantum-confined SnO2 particles. The band gap energy decreased from 4.01 eV to 3.68 eV when the mean particle radius increased from 1.75 nm to 4.2 nm [22]. Chen and Gao reported that the absorption bands of the nanorods with diameters N4.5 nm are very close to that of the bulk SnO2 (345 nm, BG energy 3.60 eV), which can be attributed to the fact that the diameters are much larger than the exciton Bohr radius (∼ 2.7 nm) of SnO2 [17]. An indirect band gap of 2.68 eV for the bare SnO2 (110) surface was obtained by theoretical calculation [19]. Compared with these results, the band gap energy of 3.75 eV measured in this work revealed a blue shift. Although the edge length and the width of the as-synthesized products are several hundred nanometers, the SnO2 nano-sheets almost fit the critical point of the weak and strong confinement regime curve. Here the special nano-sheets morphology might induce the relative blue shift of the band gap energy due to a spatial confinement of an exciton. 4. Conclusion In summary, a template-free hydrothermal method was developed for the synthesis of the SnO2 nano-sheets staring from SnCl2 and NaOH in ethanol/water solution. By changing the ethanol/water ratio, the size of nano-sheets and morphology could be controlled. NaOH played a crucial role during the formation of the SnO2 nano-sheets. The addition of glycol could induce the formation of cabbage-like SnO2. The nano-sheets showed a strong absorption peak around 330 nm, corresponding to a band gap value of 3.75 eV, which were different from bulk SnO2.
Acknowledgements The authors appreciate the financial supports from the “Hundred Talents Program” of the Chinese Academy of Sciences and the Ningbo Natural Science Foundation (No.2007A610027, No.2008A610047). This work is also sponsored by K.C. Wang Magna Found in Ningbo University. References [1] Sahm T, Rong W, Barsan N, Madler L, Weimar U. Sens Actuators, B 2007;127:63–8. [2] Samotaev NN, Vasiliev AA, Podlepetsky BI, Sokolov AV, Pisliakov AV. Sens Actuators, B 2007;127:242–7. [3] Wan HZ, Liang JB, Fan H, Xi BJ, Zhang MF, Xiong SL, et al. J Solid State Chem 2008;181:122–9. [4] Dai ZR, Pan ZW, Wang ZL. Solid State Commun 2001;118:351–4. [5] Wang WZ, Niu JZ, Ling A. J Cryst Growth 2008;310:351–5. [6] Krishnakumar T, Pinna NK, Kumari P, Perumal K, Jayaprakash R. Mater Lett 2008;62:3437–40. [7] Leite ER, Weber IT, Longo E, Varela JA. Adv Mater 2000;12:965–8. [8] Nayral C, Ould-Ely T, Maisonnat A, Chaudret B, Fau P, Lescouzères L, et al. Adv Mater 1999;11:61–3. [9] Ge JP, Wang J, Zhang HX, Wang X, Peng Q, Li YD. Sens Actuators, B 2006;113: 937–43. [10] Wang WW, Zhu YJ, Yang LX. Adv Funct Mater 2007;17:59–64. [11] Dai ZR, Pan ZW, Wang ZL. J Am Chem Soc 2002;124:8673–80. [12] Zhao QR, Li ZQ, Wu CZ, Xue B, Xie YJ. Nanopart Res 2006;8:1065–9. [13] Yang XH, Wang LL. Mater Lett 2007;61:3705–7. [14] Li ZR, Li XL, Zhang XX, Qian YT. J Cryst Growth 2006;291:258–61. [15] Lou XW, Wang Y, Yuan CL, Lee JY, Archer LA. Adv Mater 2006;18:2325–9. [16] Yang HG, Zeng HC. Angew Chem, Int Ed 2004;43:5930–3. [17] Chen DL, Gao L. Chem Phys Lett 2004;398:201–6. [18] Uchiyama H, Ohgi H, Imai H. Cryst Growth Des 2006;6:2186–90. [19] Beltran A, Andres J, Sambrano JR, Longo E. J Phys Chem, A 2008;112:8943–52. [20] Li LL, Chu Y, Liu Y, Dong LH, Huo L, Yang FY. Mater Lett 2006;60:2138–41. [21] Ohgi H, Maeda T, Hosono E, Fujihara S, Imai H. Cryst Growth Des 2005;5:1079–83. [22] Lee JH, Ribeiro C, Giraldi TR, Longo E, Leite ER, Varela JA. Appl Phys Lett 2004;84: 1745–8.