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Template-free fabrication of SnO2 hollow spheres with photoluminescence from Sni
Materials Letters 64 (2010) 2033–2035
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
Template-free fabrication of SnO2 hollow spheres with photoluminescence from Sni Weiwei Yan a, Ming Fang a,⁎, Xiaoli Tan b, Mao Liu a, Peisheng Liu c, Xiaoye Hu a, Lide Zhang a,⁎ a b c
Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanostructure, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, PR China Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, 230031, Hefei, PR China Jiangsu Key Laboratory of ASCI Design, Nantong University, Nantong 226019, PR China
a r t i c l e
i n f o
Article history: Received 28 March 2010 Accepted 29 June 2010 Available online 6 July 2010 Keywords: SnO2 Hollow sphere Interstitial Sn2+ Luminescence Nanomaterials
a b s t r a c t SnO2 hollow spheres with interstitial Sn2+ defect were fabricated by the hydrothermal method without any surfactant or polymer, whose shell is constructed by two layers of tetragonal prism nanorod arrays. The growth mechanism of the hollow spheres was investigated and attributed to the nucleation and arrangement of SnO2 tetragonal prism nanorods on the surface of the hydrothermal reaction formed NO bubbles in the aqueous solution. After illumination by 275 nm wavelength light, narrow peak emissions centered at about 587–626 nm have been found in the photoluminescence spectrum, which have been ascribed to the interstitial Sn2+ defect in the SnO2 hollow spheres. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental details
Recently, noticeable efforts have been put into the fabrication of nanomaterials with controlled morphology and modulated properties [1]. Porous nanomaterials have been found having important applications in many areas, including ion exchange, molecular separation, catalysis, chromatography, microelectronics, and energy storage [2–4]. Those hollow spheres with novel properties may greatly enlarge the application field of the nanomaterials. SnO2 is one of the most intensively studied materials owing to its myriad of technologically important applications in gas sensors, transparent conducting electrodes, Li-ion rechargeable batteries, pollutant treatment and field effect (FE) sensors [5–8]. Different SnO2 nanostructures have been reported, such as nanoparticles, nanorods/belts/ arrays, nanotubes, nanodisks, nanoboxes, and hollow spheres [9–18]. Yet the methods utilized are usually sophisticated, and the nanostructures seldom exhibited any particular interesting optical, nonlinear optical or electro-optical functionality, and most of them were polycrystal [19]. So developing the simple template-free method to prepare semiconductor micro/nanostructures with novel properties was of importance. Herein, we report the fabrication of SnO2 hollow spheres with interstitial Sn2+ defect by utilizing a simple hydrothermal method. Unlike the conventional template-direction methods [20–22], no surfactants or polymer were used.
In a typical procedure, 1.5 g tin powder, 3 g sodium nitrate and 3 g sodium hydroxide were added into 50 ml water. After 5 min hand shaking, the mixture was transferred into a Teflon-lined autoclave, and held in an electric oven at 190 °C for 6 h. The autoclave was then cooled rapidly using tap water. The white powder was harvested by filtration and washed with de-ionized water before drying. After that, the product was annealed at 800 °C for 1 h in the atmosphere. All the characterization experiments were performed at room temperature. The phase was identified by X-ray diffraction (XRD) using a Philips X'Pert Pro MPD with Cu Ka (λ = 1.5406 Å) radiation. High Resolution Transmission Electron Microscopy (HRTEM; JEOL JEM-2010, 200 kV) and Field Emission Scanning Electron Microscopy (FESEM; FEI Sirion-200) were used for the morphological investigation. Photoluminescence (PL) spectrum was obtained using a Photoluminescence Spectrometer (Edinburgh FLS 920).
⁎ Corresponding authors. Tel.: +86 551 5591476. E-mail addresses: [email protected] (M. Fang), [email protected] (L. Zhang). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.06.070
3. Results and discussions Fig. 1a is the XRD pattern. All the diffraction peaks can be indexed to the tetragonal rutile SnO2 (JCPDS Card No. 41-1445). The final products are SnO2 hollow spheres from Fig. 1b–d, whose outsidediameter is about 1–3 μm and has an average of about 2.14 μm by measuring about 30 hollow spheres. A cavity can be clearly seen from a broken sphere shown in Fig. 1c. The shell of the hollow sphere is constructed by SnO2 nanorods in curved arrangements. The nanorods are 15–50 nm in diameter and show the square morphology from the top view (the inset in Fig. 1d). Another nanorod layer inside the shell can be clearly seen from Fig. 1d and the inset in Fig. 1c, which
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W. Yan et al. / Materials Letters 64 (2010) 2033–2035
the nanorod axis indicates the c-axis preferential growth direction. The result of energy-dispersive X-ray spectroscopy (EDX) declares the composition of Sn and O (the inset in Fig. 2d). The extra Cu and C signals originate from the TEM grid. Small quantity of urchin-like nanostructures is found together with the hollow spheres (Fig. 3a), which are also constructed by nanoprisms. And the content of the nanourchin has been estimated to be less than 30%. To ascertain the growth mechanism, experiments under different conditions were performed. If the reaction time was decreased to 2 h, a white product can be found on the bottom of the autoclave. However, it disappeared after shaking. This means the white product is some soluble salt. Moreover, all the tin powder dissolved in the first 1 h. If NaNO3 was absent, nothing changed with the tin powders. So, NaOH itself could hardly react with Sn under the hydrothermal conditions. But a film constructed by SnO2 nanosheets can be found on the surface of the melted tin particle without NaOH (Fig. 3b). These results indicated a slow reaction between NaNO3 and tin powders, and the SnO2 hollow spheres should grow in a different way from the ZnO hollow spheres [23]. From these observations, it is reasonable to propose the following equations, where Sn was oxidized to Na2[Sn(OH)6] (Eq. (1)), and then decomposed to SnO2, HNO3 and NO (Eq. (3)). It is obvious that the addition of NaOH could consume HNO3 and accelerate the reaction. Fig. 1. (a) XRD patterns of the product. The FESEM images of (b, c) the SnO2 hollow spheres, (d) the shell of the sphere containing the inset of the top view.
constructed the shell of the hollow spheres together with the outside layer. Fig. 2a shows the TEM image of the hollow sphere. The shell is about 220–400 nm in thickness, which agrees well with the results of FESEM. Fig. 2b is the TEM image of the nanorod with a sharp end, and the corresponding HRTEM image is shown in Fig. 2c. The clear lattice fringes and the spots in the SAED pattern confirm the high crystallinity. The horizontal and vertical interplanar spacings are 3.44 Å and 3.28 Å, which respectively correspond to the (001) and (110) planes of rutile SnO2. The perpendicularity of the (001) plane to
Fig. 2. TEM images of (a) the SnO2 hollow spheres, and (b) the nanorods. (c) HRTEM image and (d) SAED pattern containing the inset of the EDX spectroscopy of the spheres.
3Sn þ 6NaNO3 þ 10H2 O ¼ 3Na2 ½SnðOHÞ6 þ 2HNO3 þ 4NO↑
ð1Þ
3Sn þ 4NaNO3 þ 2NaOH þ 8H2 O ¼ 3Na2 ½SnðOHÞ6 þ 4NO↑
ð2Þ
Na2 ½SnðOHÞ6 ¼ SnO2 ↓ þ 2NaOH þ 2H2 O
ð3Þ
The NO bubbles act as the template in the formation of the hollow spheres. In the hydrothermal condition, SnO2 was firstly deposited on the surface of the NO bubbles. To achieve the lowest surface energy, SnO2 grew along the [001] direction and exposed the (110) surface. The NO bubbles in the center are not stable, which can easily escape during the fluctuation of the solution. Then solutions containing Na2[Sn(OH)6] might get into the cavity and start the growth of
Fig. 3. The FESEM images of (a) the urchin-like nanostructure, (b) the product without NaOH and (c) schematic representation of the formation mechanism of SnO2 hollow spheres: a NO bubbles; b SnO2 precipitated on the bubbles surface; c the double layered hollow SnO2 spheres.
W. Yan et al. / Materials Letters 64 (2010) 2033–2035
2035
tetragonal nanorod arrays in curved arrangements. The formation mechanism was investigated and discussed, and the cavity was attributed to the nucleation of SnO2 on the surface of the inborn NO bubbles in the solution. Narrow photoemission peaks centered at 587, 592, 598, 609, 617 and 626 nm were also found and ascribed to the Sni defect. Acknowledgments This work was financially supported by the Ministry of Science and Technology of China (Grant No. 2005CB623603) and the Director Fund of the Institute of Solid State Physics (084NY11311-6). References
Fig. 4. The emission spectrum of the SnO2 hollow nanostructures.
the tetragonal prism pointing to the center. As a result, the double layered hollow spheres were formed (Fig. 3c). Some SnO2 grew without the NO bubbles, then form the urchin-like nanostructures. The PL spectrum of the SnO2 hollow structures excited by 275 nm wavelength light at room temperature is shown in Fig. 4. The peak centered at 418 nm derives from the neutral electronic configurations, singly and doubly charged with the in-plane oxygen vacancies [24,25]. Another broad peak was centered at about 608 nm corresponding to the energy level of defect centers located in the band gap [24,26]. Some narrow peaks centered at 587, 592, 598, 609, 617 and 626 nm occurred, which had never been reported in the pure SnO2 by the other researchers as far as we knew. According to our previous study [27–29], these peaks prove the existence of the interstitial Sn2+ defect, and should been assigned as D3, D2, D1, C3, C2 and C1 band emissions from Sni, respectively. The Sni should originate from the incomplete oxidization of tin powders to Na2[Sn(OH)4], codeposit with SnO2 as SnO, and get into the interstitial site while annealing. 4. Conclusions A template-free hydrothermal method was used to fabricate SnO2 hollow spheres, whose shells are constructed by two layers of SnO2
[1] Aizawa M, Cooper AM, Malac M, Buriak JM. Nano Lett 2005;5:815. [2] Yin YD, Rioux RM, Erdonmez CK, Hughes S, Somorjai GA, Alivisatos AP. Science 2004;304:711. [3] Caruso F, Shi XY, Caruso RA, Susha A. Adv Mater 2001;13:740. [4] Wong MS, Cha JN, Choi KS, Deming TJ, Stucky GD. Nano Lett 2002;2:583. [5] Zhang LD, Fang M. Nano Today 2010;5:128. [6] Idota Y, Kubota T, Matsufuji A, Maekawa Y, Miyasaka T. Science 1997;276:1395. [7] Luo SH, Chu PK, Di ZF, Zhang M, Liu WL, Lin CL, et al. Appl Phys Lett 2006;88 013109-1. [8] Kolmakov A, Klenov DO, Lilach Y, Stemmer S, Moskovits M. Nano Lett 2005;5:667. [9] Duan JH, Yang SG, Liu HW, Gong JF, Huang HB, Zhao XN, et al. J Am Chem Soc 2005;127:6180. [10] Liu Y, Dong J, Liu ML. Adv Mater 2004;16:353. [11] Jiang LH, Sun GQ, Zhou ZH, Sun SG, Wang Q, Yan SY, et al. J Phys Chem B 2005;109: 8774. [12] Yu JG, Guo HG, Davis SA, Mann S. Adv Funct Mater 2006;16:2035. [13] Cheng B, Russell JM, Shi WS, Zhang L, Samulski ET. J Am Chem Soc 2004;126:5972. [14] Wang Y, Lee JY, Zeng HC. Chem Mater 2005;17:3899. [15] Pan ZW, Dai ZR, Wang ZL. Science 2001;291:1947. [16] Dai ZR, Pan ZW, Wang ZL. J Am Chem Soc 2002;124:8673. [17] Yang HG, Zeng HC. Angew Chem Int Ed 2004;43:5930. [18] Wang Y, Lee JY, Deivaraj TC. J Phys Chem B 2004;108:13589. [19] Peng Q, Dong YJ, Li YD. Angew Chem Int Ed 2003;42:3027. [20] Zhang YX, Li GH, Wu YC, Luo YY, Zhang LD. J Phys Chem B 2005;109:5478. [21] Fujikawa S, Kunitake T. Langmuir 2003;19:6545. [22] Wu DZ, Ge XW, Zhang ZC, Wang MZ, Zhang SL. Langmuir 2004;20:5192. [23] Liu B, Zeng HC. J Am Chem Soc 2004;126:16744. [24] Luo SH, Chu PK, Liu WL, Zhang M, Lin CL. Appl Phys Lett 2006;88 183112-1. [25] Cox DF, Fryberger TB, Semancik S. Phys Rev B 1988;38:2072. [26] Chang SS, Park DK. Mater Sci Eng B 2002;95:55. [27] Fang M, Zhang LD, Tan XL, Hu XY, Yan WW, Liu PS. J Phys Chem C 2009;113:9676. [28] Fang M, Tan XL, Cheng BC, Zhang LD. J Mater Chem 2009;19:1320. [29] Fang M, Tan XL, Cao XL, Zhang LD, Liu PS, Jiang Z. J Phys D 2007;40:7648.
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
Template-free fabrication of SnO2 hollow spheres with photoluminescence from Sni Weiwei Yan a, Ming Fang a,⁎, Xiaoli Tan b, Mao Liu a, Peisheng Liu c, Xiaoye Hu a, Lide Zhang a,⁎ a b c
Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanostructure, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, PR China Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, 230031, Hefei, PR China Jiangsu Key Laboratory of ASCI Design, Nantong University, Nantong 226019, PR China
a r t i c l e
i n f o
Article history: Received 28 March 2010 Accepted 29 June 2010 Available online 6 July 2010 Keywords: SnO2 Hollow sphere Interstitial Sn2+ Luminescence Nanomaterials
a b s t r a c t SnO2 hollow spheres with interstitial Sn2+ defect were fabricated by the hydrothermal method without any surfactant or polymer, whose shell is constructed by two layers of tetragonal prism nanorod arrays. The growth mechanism of the hollow spheres was investigated and attributed to the nucleation and arrangement of SnO2 tetragonal prism nanorods on the surface of the hydrothermal reaction formed NO bubbles in the aqueous solution. After illumination by 275 nm wavelength light, narrow peak emissions centered at about 587–626 nm have been found in the photoluminescence spectrum, which have been ascribed to the interstitial Sn2+ defect in the SnO2 hollow spheres. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental details
Recently, noticeable efforts have been put into the fabrication of nanomaterials with controlled morphology and modulated properties [1]. Porous nanomaterials have been found having important applications in many areas, including ion exchange, molecular separation, catalysis, chromatography, microelectronics, and energy storage [2–4]. Those hollow spheres with novel properties may greatly enlarge the application field of the nanomaterials. SnO2 is one of the most intensively studied materials owing to its myriad of technologically important applications in gas sensors, transparent conducting electrodes, Li-ion rechargeable batteries, pollutant treatment and field effect (FE) sensors [5–8]. Different SnO2 nanostructures have been reported, such as nanoparticles, nanorods/belts/ arrays, nanotubes, nanodisks, nanoboxes, and hollow spheres [9–18]. Yet the methods utilized are usually sophisticated, and the nanostructures seldom exhibited any particular interesting optical, nonlinear optical or electro-optical functionality, and most of them were polycrystal [19]. So developing the simple template-free method to prepare semiconductor micro/nanostructures with novel properties was of importance. Herein, we report the fabrication of SnO2 hollow spheres with interstitial Sn2+ defect by utilizing a simple hydrothermal method. Unlike the conventional template-direction methods [20–22], no surfactants or polymer were used.
In a typical procedure, 1.5 g tin powder, 3 g sodium nitrate and 3 g sodium hydroxide were added into 50 ml water. After 5 min hand shaking, the mixture was transferred into a Teflon-lined autoclave, and held in an electric oven at 190 °C for 6 h. The autoclave was then cooled rapidly using tap water. The white powder was harvested by filtration and washed with de-ionized water before drying. After that, the product was annealed at 800 °C for 1 h in the atmosphere. All the characterization experiments were performed at room temperature. The phase was identified by X-ray diffraction (XRD) using a Philips X'Pert Pro MPD with Cu Ka (λ = 1.5406 Å) radiation. High Resolution Transmission Electron Microscopy (HRTEM; JEOL JEM-2010, 200 kV) and Field Emission Scanning Electron Microscopy (FESEM; FEI Sirion-200) were used for the morphological investigation. Photoluminescence (PL) spectrum was obtained using a Photoluminescence Spectrometer (Edinburgh FLS 920).
⁎ Corresponding authors. Tel.: +86 551 5591476. E-mail addresses: [email protected] (M. Fang), [email protected] (L. Zhang). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.06.070
3. Results and discussions Fig. 1a is the XRD pattern. All the diffraction peaks can be indexed to the tetragonal rutile SnO2 (JCPDS Card No. 41-1445). The final products are SnO2 hollow spheres from Fig. 1b–d, whose outsidediameter is about 1–3 μm and has an average of about 2.14 μm by measuring about 30 hollow spheres. A cavity can be clearly seen from a broken sphere shown in Fig. 1c. The shell of the hollow sphere is constructed by SnO2 nanorods in curved arrangements. The nanorods are 15–50 nm in diameter and show the square morphology from the top view (the inset in Fig. 1d). Another nanorod layer inside the shell can be clearly seen from Fig. 1d and the inset in Fig. 1c, which
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W. Yan et al. / Materials Letters 64 (2010) 2033–2035
the nanorod axis indicates the c-axis preferential growth direction. The result of energy-dispersive X-ray spectroscopy (EDX) declares the composition of Sn and O (the inset in Fig. 2d). The extra Cu and C signals originate from the TEM grid. Small quantity of urchin-like nanostructures is found together with the hollow spheres (Fig. 3a), which are also constructed by nanoprisms. And the content of the nanourchin has been estimated to be less than 30%. To ascertain the growth mechanism, experiments under different conditions were performed. If the reaction time was decreased to 2 h, a white product can be found on the bottom of the autoclave. However, it disappeared after shaking. This means the white product is some soluble salt. Moreover, all the tin powder dissolved in the first 1 h. If NaNO3 was absent, nothing changed with the tin powders. So, NaOH itself could hardly react with Sn under the hydrothermal conditions. But a film constructed by SnO2 nanosheets can be found on the surface of the melted tin particle without NaOH (Fig. 3b). These results indicated a slow reaction between NaNO3 and tin powders, and the SnO2 hollow spheres should grow in a different way from the ZnO hollow spheres [23]. From these observations, it is reasonable to propose the following equations, where Sn was oxidized to Na2[Sn(OH)6] (Eq. (1)), and then decomposed to SnO2, HNO3 and NO (Eq. (3)). It is obvious that the addition of NaOH could consume HNO3 and accelerate the reaction. Fig. 1. (a) XRD patterns of the product. The FESEM images of (b, c) the SnO2 hollow spheres, (d) the shell of the sphere containing the inset of the top view.
constructed the shell of the hollow spheres together with the outside layer. Fig. 2a shows the TEM image of the hollow sphere. The shell is about 220–400 nm in thickness, which agrees well with the results of FESEM. Fig. 2b is the TEM image of the nanorod with a sharp end, and the corresponding HRTEM image is shown in Fig. 2c. The clear lattice fringes and the spots in the SAED pattern confirm the high crystallinity. The horizontal and vertical interplanar spacings are 3.44 Å and 3.28 Å, which respectively correspond to the (001) and (110) planes of rutile SnO2. The perpendicularity of the (001) plane to
Fig. 2. TEM images of (a) the SnO2 hollow spheres, and (b) the nanorods. (c) HRTEM image and (d) SAED pattern containing the inset of the EDX spectroscopy of the spheres.
3Sn þ 6NaNO3 þ 10H2 O ¼ 3Na2 ½SnðOHÞ6 þ 2HNO3 þ 4NO↑
ð1Þ
3Sn þ 4NaNO3 þ 2NaOH þ 8H2 O ¼ 3Na2 ½SnðOHÞ6 þ 4NO↑
ð2Þ
Na2 ½SnðOHÞ6 ¼ SnO2 ↓ þ 2NaOH þ 2H2 O
ð3Þ
The NO bubbles act as the template in the formation of the hollow spheres. In the hydrothermal condition, SnO2 was firstly deposited on the surface of the NO bubbles. To achieve the lowest surface energy, SnO2 grew along the [001] direction and exposed the (110) surface. The NO bubbles in the center are not stable, which can easily escape during the fluctuation of the solution. Then solutions containing Na2[Sn(OH)6] might get into the cavity and start the growth of
Fig. 3. The FESEM images of (a) the urchin-like nanostructure, (b) the product without NaOH and (c) schematic representation of the formation mechanism of SnO2 hollow spheres: a NO bubbles; b SnO2 precipitated on the bubbles surface; c the double layered hollow SnO2 spheres.
W. Yan et al. / Materials Letters 64 (2010) 2033–2035
2035
tetragonal nanorod arrays in curved arrangements. The formation mechanism was investigated and discussed, and the cavity was attributed to the nucleation of SnO2 on the surface of the inborn NO bubbles in the solution. Narrow photoemission peaks centered at 587, 592, 598, 609, 617 and 626 nm were also found and ascribed to the Sni defect. Acknowledgments This work was financially supported by the Ministry of Science and Technology of China (Grant No. 2005CB623603) and the Director Fund of the Institute of Solid State Physics (084NY11311-6). References
Fig. 4. The emission spectrum of the SnO2 hollow nanostructures.
the tetragonal prism pointing to the center. As a result, the double layered hollow spheres were formed (Fig. 3c). Some SnO2 grew without the NO bubbles, then form the urchin-like nanostructures. The PL spectrum of the SnO2 hollow structures excited by 275 nm wavelength light at room temperature is shown in Fig. 4. The peak centered at 418 nm derives from the neutral electronic configurations, singly and doubly charged with the in-plane oxygen vacancies [24,25]. Another broad peak was centered at about 608 nm corresponding to the energy level of defect centers located in the band gap [24,26]. Some narrow peaks centered at 587, 592, 598, 609, 617 and 626 nm occurred, which had never been reported in the pure SnO2 by the other researchers as far as we knew. According to our previous study [27–29], these peaks prove the existence of the interstitial Sn2+ defect, and should been assigned as D3, D2, D1, C3, C2 and C1 band emissions from Sni, respectively. The Sni should originate from the incomplete oxidization of tin powders to Na2[Sn(OH)4], codeposit with SnO2 as SnO, and get into the interstitial site while annealing. 4. Conclusions A template-free hydrothermal method was used to fabricate SnO2 hollow spheres, whose shells are constructed by two layers of SnO2
[1] Aizawa M, Cooper AM, Malac M, Buriak JM. Nano Lett 2005;5:815. [2] Yin YD, Rioux RM, Erdonmez CK, Hughes S, Somorjai GA, Alivisatos AP. Science 2004;304:711. [3] Caruso F, Shi XY, Caruso RA, Susha A. Adv Mater 2001;13:740. [4] Wong MS, Cha JN, Choi KS, Deming TJ, Stucky GD. Nano Lett 2002;2:583. [5] Zhang LD, Fang M. Nano Today 2010;5:128. [6] Idota Y, Kubota T, Matsufuji A, Maekawa Y, Miyasaka T. Science 1997;276:1395. [7] Luo SH, Chu PK, Di ZF, Zhang M, Liu WL, Lin CL, et al. Appl Phys Lett 2006;88 013109-1. [8] Kolmakov A, Klenov DO, Lilach Y, Stemmer S, Moskovits M. Nano Lett 2005;5:667. [9] Duan JH, Yang SG, Liu HW, Gong JF, Huang HB, Zhao XN, et al. J Am Chem Soc 2005;127:6180. [10] Liu Y, Dong J, Liu ML. Adv Mater 2004;16:353. [11] Jiang LH, Sun GQ, Zhou ZH, Sun SG, Wang Q, Yan SY, et al. J Phys Chem B 2005;109: 8774. [12] Yu JG, Guo HG, Davis SA, Mann S. Adv Funct Mater 2006;16:2035. [13] Cheng B, Russell JM, Shi WS, Zhang L, Samulski ET. J Am Chem Soc 2004;126:5972. [14] Wang Y, Lee JY, Zeng HC. Chem Mater 2005;17:3899. [15] Pan ZW, Dai ZR, Wang ZL. Science 2001;291:1947. [16] Dai ZR, Pan ZW, Wang ZL. J Am Chem Soc 2002;124:8673. [17] Yang HG, Zeng HC. Angew Chem Int Ed 2004;43:5930. [18] Wang Y, Lee JY, Deivaraj TC. J Phys Chem B 2004;108:13589. [19] Peng Q, Dong YJ, Li YD. Angew Chem Int Ed 2003;42:3027. [20] Zhang YX, Li GH, Wu YC, Luo YY, Zhang LD. J Phys Chem B 2005;109:5478. [21] Fujikawa S, Kunitake T. Langmuir 2003;19:6545. [22] Wu DZ, Ge XW, Zhang ZC, Wang MZ, Zhang SL. Langmuir 2004;20:5192. [23] Liu B, Zeng HC. J Am Chem Soc 2004;126:16744. [24] Luo SH, Chu PK, Liu WL, Zhang M, Lin CL. Appl Phys Lett 2006;88 183112-1. [25] Cox DF, Fryberger TB, Semancik S. Phys Rev B 1988;38:2072. [26] Chang SS, Park DK. Mater Sci Eng B 2002;95:55. [27] Fang M, Zhang LD, Tan XL, Hu XY, Yan WW, Liu PS. J Phys Chem C 2009;113:9676. [28] Fang M, Tan XL, Cheng BC, Zhang LD. J Mater Chem 2009;19:1320. [29] Fang M, Tan XL, Cao XL, Zhang LD, Liu PS, Jiang Z. J Phys D 2007;40:7648.