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Synthesis of flower-like CdS nanostructures by organic-free hydrothermal process and their optical properties
Materials Letters 61 (2007) 3507 – 3510 www.elsevier.com/locate/matlet
Synthesis of flower-like CdS nanostructures by organic-free hydrothermal process and their optical properties Hui Zhang, Deren Yang ⁎, Xiangyang Ma State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, People's Republic of China Received 13 June 2006; accepted 22 November 2006 Available online 8 December 2006
Abstract Flower-like CdS nanostructures consisting of sword-like nanorods have been prepared via organic-free hydrothermal method using thioacetamide (TAA) as sulfur source. X-ray diffraction pattern (XRD) shows that the obtained flower-like CdS nanostructures are of hexagonal phase. The selected area electron diffraction (SAED) pattern identifies that the flower-like CdS nanostructures are single crystalline in nature. Furthermore, the optical properties of flower-like CdS nanostructures have been characterized by ultraviolet–vis (UV–vis) and photoluminescence (PL) spectra. Finally, the investigation on the mechanism indicates that the internal structure and sulfur mass transport controlled by TAA play the critical role in the formation of flower-like CdS nanostructures. © 2006 Published by Elsevier B.V. Keywords: CdS; Flower-like; Hydrothermal process; TAA
1. Introduction In recent years, there has been considerable interest in the synthesis and functionalization of nanostructural materials due to their significant potential application and novel property [1–4]. There are usually two categories of methods including vapor-phase process and solution-phase route to prepare nanomaterials [5–11]. Among them, chemical solution route provides a more promising option due to its simpleness, practicality, large scale, controllability and low cost. It is well known that control of the size, morphology and structure represents some of key issues in nanotechnology fabrication because these parameters are some of the key elements that determine their properties. Lots of nanomaterials with various morphologies, which include not only nanotubes, nanowires, and nanobelts, but also some special nanostructures such as nanobridges, nanonails, nanodiskettes, star-shaped, and flower-like have been fabricated [12–18]. The effect of morphology of nanomaterials on their properties has also been observed [19]. ⁎ Corresponding author. E-mail address: [email protected] (D. Yang). 0167-577X/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.matlet.2006.11.105
Cadmium sulfide (CdS), a direct band gap material with Eg of 2.42 eV at room temperature can be widely used for photoelectronic devices. Much effect has been employed to synthesize various morphologies of CdS nanomaterials including nanowires, nanorods, nanotubes, hollow sphere, peanut and nanocable by various physical and chemical solutions [20–24]. However, it is inevitable to use template, high temperature or catalyst in the synthesis process. Previously, our group reported a thioglycolic acid (TGA) assisted hydrothermal process synthesis of CdS nanorods without any template or catalyst [12]. And it is revealed that TGA acts as the oriented growth reactant during above process. Furthermore, our previous results indicate that the crystal growth of CdS is also determined by mass transport process via control of decomposition rate of free S2− from the sulfur source in the solution [25]. Herein, we report the controllable growth of flower-like CdS nanostructure by organic-free hydrothermal process using thioacetamide (TAA) as sulfur source. Comparing to other sulfur source, the action of TAA on the formation of flower-like CdS nanostructures is revealed. The mechanism for the formation of flower-like CdS nanostructures has been preliminary presented.
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2. Experimental All chemicals were of analytical grade. Cadmium sulfate, TAA and sodium sulfide were purchased from the Shanghai Chemical Reagent Company and used as received without further purification. The flower-like CdS nanostructures were fabricated by organic-free hydrothermal process. In brief, 60 ml 0.01 M TAA was added into the aqueous solution of 60 ml 0.04 M CdSO4 under stirring. After 10 min stirring, the aforementioned solution was then transferred into a Teflon lined stainless steel autoclave, sealed and maintained at
Fig. 2. UV–vis absorption (a) and PL (b) spectra of flower-like CdS nanostructures.
200 °C for 20 h. Subsequently, the resulting yellow solid product was centrifuged, washed with distilled water and ethanol to remove the ions possibly remaining in the final product and finally dried at 60 °C in air. Moreover, sodium sulfide instead of TAA was used as sulfur source, while kept other conditions unchanged. The obtained samples were characterized by X-ray powder diffraction (XRD) using a Japan Rigaku D/max-ga X-ray diffractometer with graphite monochromatosed CuKa radiation (λ = 1.54178 Å). Transmission electron microscopy (TEM) observations were performed with a JEM 200 CX microscope operated at 160 kV. The field emission scan electron microscope (FESEM) images were obtained on Hitachi S4700. The optical absorption of above sample was examined by a PERKIN ELMER Lambda20 UV/VIS Spectrometer. The photoluminescence spectrum (PL) was achieved on a Hitachi 850 fluorescence spectrophotometer using a 350 nm excitation line. 3. Results and discussion Fig. 1. Morphological and structure characterization of flower-like CdS nanostructures: (a) XRD pattern; (b) FESEM image; (c) TEM image. The upper left inset corresponds to the SAED pattern of a rod-like CdS main core.
Fig. 1 shows the morphological and structural characterization of CdS nanostructures prepared by organic-free hydrothermal method. As indicated by the XRD pattern of CdS nanostructures shown in Fig. 1a,
H. Zhang et al. / Materials Letters 61 (2007) 3507–3510
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cadmium source is added into above solution. However, during the hydrothermal process, it is decomposed slowly to release free S2− and CdS nuclei are formed in suitable supersaturation and began to grow into CdS sheet according to the following reaction: CH3 CSNH2
hydrothermal
Y
CH3 CN þ H2 S
Cd2þ H2 SYCdS þ 2Hþ
Fig. 3. TEM image of CdS particles prepared by hydrothermal process using sodium sulfide as sulfur source.
all diffraction peaks can be indexed as the hexagonal CdS with lattice constants a = 4.14, c = 6.72 Å, which is very consistent with the values in the standard card (JCPDS 41-4019). No peaks related to Cd, CdO and other cadmium compound are observed. Fig. 1b shows the FESEM image of the CdS nanostructures by organic-free hydrothermal method using TAA as sulfur source. The morphology of flower-like can be observed from the FESEM image. The typical flower-like CdS nanostructures consisted of CdS nanorods with about several hundred nanometers in width and several micrometers in length, which protrude from the root of the flower-like CdS nanostructures. Individual CdS nanorod has a cusp end similar to a sword with the rough surface, which consists of CdS nanocrystals. The further characterization of abovementioned sample has been performed by TEM as shown in Fig. 1c. The morphology of flower-like CdS nanostructures has also been observed. However, due to the action of sonication during the preparation of TEM observation, the total morphology of flower-like is destroyed. The short CdS nanorods with several hundred nanometers in length and several ten nanometers in width grow in the two sides along the sword-like CdS main core just like fish-bone. The selected area electron diffraction (SAED) pattern performed on the sword-like CdS main core and inserted in Fig. 1c (left) shows high light diffraction spots. The above observation indicates that the flower-like CdS nanostructures are of high crystallinity and can be indexed as the hexagonal CdS phase, which is in accordance with the XRD results in Fig. 1a. The room temperature UV–vis absorption and PL spectra of flowerlike CdS nanostructures dispersed in ethanol are recorded and shown in Fig. 2. The blue shift to 494 nm can be found compared with the band gap of the characteristic absorption of bulk CdS probably due to the size effect as shown in Fig. 2a. The PL spectrum for the sample using a 350 nm excitation line is shown in Fig. 2b. Two emissions, i.e. one at 504 nm belongs to the band to band emission, the other at 720 nm induced by self-activated emission of CdS, have been observed from the flower-like CdS nanostructures, in agreement with the previously report [26]. As we know, the growth habit of crystals is mainly determined by the internal structure of a given crystal, and also affected by external conditions such as temperature, mass transport, capping molecule, and time. In this case, the internal structure and mass transport controlled by sulfur source are the key parameters. We employ TAA as sulfur source to control the nucleation and growth process, which was previously used to direct the shape of other sulfide [27]. Before the hydrothermal process, due to the very low decomposition of TAA resulting in low free S2+ concentration, almost no CdS nuclei are formed when
ð1Þ ð2Þ
Due to hexagonal structure of CdS, the growth rate along c axis is usually the fastest and the rod-like morphology is frequently obtained. In addition, for the slowly decomposition rate of TAA, enough sulfur source can be subsequently provided to proceed secondary growth, which is also reported previously [28]. Therefore, the rod-like subbranch of CdS is formed on the rod-like main core. The formation of flower-like of CdS nanostructures indicates that the nucleation and growth of CdS nanostructures are well controlled by using CdSO4 and TAA as starting agents. To make this point clear, Na2S was also used as sulfur sources keeping other conditions constant. It was observed that only small CdS particles are obtained shown in Fig. 3. As a result, the flower-like CdS nanostructures consisting of sword-like CdS nanorod are formed due to the internal structure and secondary growth controlled by sulfur mass transport. In summary, the flower-like CdS nanostructures have been prepared by organic-free hydrothermal process. The flower-like CdS nanostructures are single crystalline and hexagonal phase, which consist of sword-like CdS nanorods. The internal structure and secondary growth controlled by sulfur mass transport are the critical roles to synthesize the flow-like CdS nanostructures. It is reasonable to believe that the hydrothermal method presented here is desirable for fabrication of other sulfide nanostructures.
Acknowledgment The authors would like to appreciate the financial supports of the Natural Science Foundation of China (No. 60225010). Thanks Prof. Youwen Wang for the TEM measurements and Prof. Yifan Zheng for the FESEM measurements. We also thank Prof. Xianping Fan for UV–vis and PL measurement. References [1] X. Duan, Y. Huang, R. Agarwal, C.M. Lieber, Nature 421 (2003) 241. [2] M.S Fuhrer, J. Nygard, L. Shih, M. Forero, Young-Gui Yoon, M.S.C. Mazzoni, Hyoung Joon Choi, Science 288 (2000) 494. [3] Z.F. Ren, Z.P. Huang, J.W. Xu, J.H. Wang, P. Bush, M.P. Siegal, P.N. Provencio, Science 282 (1998) 1105. [4] W.Z. Pan, R.Z. Dai, Z.L. Wang, Science 291 (2001) 1947. [5] J.Y. Lao, J.Y. Huang, D.Z. Wang, Z.F. Ren, Nano Letters 2 (2002) 1287. [6] Zhengrong R. Tian, James A. Voigt, Jun Liu, Bonnie Mckenzie, Matthew J. Mcdermott, Journal of the American Chemical Society 124 (2002) 12954. [7] Changhui Ye, Guowen Meng, Yinhai Wang, Zhi Jiang, Lide Zhang, Journal of Physical Chemistry. B 106 (2002) 10338. [8] Hui Zhang, Yujie Ji, Xiangyang Ma, Jin Xu, Deren Yang, Nanotechnology 14 (2003) 974. [9] Hui Zhang, Xiangyang Ma, Jin Xu, Junjie Niu, Jian Sha, Deren Yang, Journal of Crystal Growth 246 (2002) 108. [10] H. Zhang, X.Y. Ma, J. Xu, J.J. Niu, D.R. Yang, Nanotechnology 14 (2003) 423. [11] Hui Zhang, Deren Yang, Yujie Ji, Xiangyang Ma, Jin Xu, Duanling Que, Journal of Physical Chemistry. B 108 (2004) 1179.
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[12] Hui Zhang, Xiangyang Ma, Yujie Ji, Jin Xu, Deren Yang, Chemical Physics Letters 377 (2003) 654. [13] Hui Zhang, Xiangyang Ma, Jin Xu, Deren Yang, Journal of Crystal Growth 263 (2004) 372. [14] Yujie Ji, Hui Zhang, Xiangyang Ma, Jin Xu, Deren Yang, J. Phys.: Condens. Matter 15 (2003) 661. [15] J.Y. Lao, J.Y. Huang, D.Z. Wang, Z.F. Ren, Nano Letters 3 (2003) 235. [16] Z.R. Dai, Z.W. Pan, Z.L. Wang, Journal of the American Chemical Society 124 (2002) 8673. [17] Yujie Ji, Xiangyang Ma, Hui Zhang, Jin Xu, Deren Yang, Journal of Physics. Condensed Matter 15 (2003) 7611. [18] Hui Zhang, Deren Yang, Xiangyang Ma, Yujie Ji, Jin Xu, Duanlin Que, Nanotechnology (in press). [19] J. Hulliger, Angewandte Chemie. International Edition in English 33 (1994) 143. [20] J. Zhan, X. Yang, D. Wang, S. Li, Y. Xie, Y. Xia, Y. Qian, Advanced Materials 12 (2000) 1348.
[21] Y. Li, H. Li, Y. Ding, Y. Qian, L. Yang, G. Zhou, Chemistry of Materials 10 (1998) 2301. [22] S. Yu, Y. Wu, J. Yang, Z. Han, Y. Xie, Y. Qian, X. Liu, Chemistry of Materials 10 (1998) 2309. [23] J. Huang, Y. Xie, B. Li, Y. Liu, Y. Qian, S. Zhang, Advanced Materials 12 (2000) 808. [24] Y. Xie, J. Huang, B. Li, Y. Liu, Y. Qian, Advanced Materials 12 (2000) 1523. [25] Hui Zhang, Deren Yang, Xiangyang Ma, Yujie Ji, ShenZhong Li, Duanlin Que, Materials Chemistry and Physics 93 (2005) 65. [26] Feng Gao, Qingyi Lu, Songhai Xie, Dongyuan Zhao, Advanced Materials 14 (2002) 1537. [27] Yurong Ma, Limin Qi, Jiming Ma, Humin Cheng, Crystal Growth and Design 4 (2004) 351. [28] Debao Wang, Dabin Yu, Maosong Mo, Xiangyang Liu, Yitai Qian, Solid State Communications 125 (2003) 475.
Synthesis of flower-like CdS nanostructures by organic-free hydrothermal process and their optical properties Hui Zhang, Deren Yang ⁎, Xiangyang Ma State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, People's Republic of China Received 13 June 2006; accepted 22 November 2006 Available online 8 December 2006
Abstract Flower-like CdS nanostructures consisting of sword-like nanorods have been prepared via organic-free hydrothermal method using thioacetamide (TAA) as sulfur source. X-ray diffraction pattern (XRD) shows that the obtained flower-like CdS nanostructures are of hexagonal phase. The selected area electron diffraction (SAED) pattern identifies that the flower-like CdS nanostructures are single crystalline in nature. Furthermore, the optical properties of flower-like CdS nanostructures have been characterized by ultraviolet–vis (UV–vis) and photoluminescence (PL) spectra. Finally, the investigation on the mechanism indicates that the internal structure and sulfur mass transport controlled by TAA play the critical role in the formation of flower-like CdS nanostructures. © 2006 Published by Elsevier B.V. Keywords: CdS; Flower-like; Hydrothermal process; TAA
1. Introduction In recent years, there has been considerable interest in the synthesis and functionalization of nanostructural materials due to their significant potential application and novel property [1–4]. There are usually two categories of methods including vapor-phase process and solution-phase route to prepare nanomaterials [5–11]. Among them, chemical solution route provides a more promising option due to its simpleness, practicality, large scale, controllability and low cost. It is well known that control of the size, morphology and structure represents some of key issues in nanotechnology fabrication because these parameters are some of the key elements that determine their properties. Lots of nanomaterials with various morphologies, which include not only nanotubes, nanowires, and nanobelts, but also some special nanostructures such as nanobridges, nanonails, nanodiskettes, star-shaped, and flower-like have been fabricated [12–18]. The effect of morphology of nanomaterials on their properties has also been observed [19]. ⁎ Corresponding author. E-mail address: [email protected] (D. Yang). 0167-577X/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.matlet.2006.11.105
Cadmium sulfide (CdS), a direct band gap material with Eg of 2.42 eV at room temperature can be widely used for photoelectronic devices. Much effect has been employed to synthesize various morphologies of CdS nanomaterials including nanowires, nanorods, nanotubes, hollow sphere, peanut and nanocable by various physical and chemical solutions [20–24]. However, it is inevitable to use template, high temperature or catalyst in the synthesis process. Previously, our group reported a thioglycolic acid (TGA) assisted hydrothermal process synthesis of CdS nanorods without any template or catalyst [12]. And it is revealed that TGA acts as the oriented growth reactant during above process. Furthermore, our previous results indicate that the crystal growth of CdS is also determined by mass transport process via control of decomposition rate of free S2− from the sulfur source in the solution [25]. Herein, we report the controllable growth of flower-like CdS nanostructure by organic-free hydrothermal process using thioacetamide (TAA) as sulfur source. Comparing to other sulfur source, the action of TAA on the formation of flower-like CdS nanostructures is revealed. The mechanism for the formation of flower-like CdS nanostructures has been preliminary presented.
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H. Zhang et al. / Materials Letters 61 (2007) 3507–3510
2. Experimental All chemicals were of analytical grade. Cadmium sulfate, TAA and sodium sulfide were purchased from the Shanghai Chemical Reagent Company and used as received without further purification. The flower-like CdS nanostructures were fabricated by organic-free hydrothermal process. In brief, 60 ml 0.01 M TAA was added into the aqueous solution of 60 ml 0.04 M CdSO4 under stirring. After 10 min stirring, the aforementioned solution was then transferred into a Teflon lined stainless steel autoclave, sealed and maintained at
Fig. 2. UV–vis absorption (a) and PL (b) spectra of flower-like CdS nanostructures.
200 °C for 20 h. Subsequently, the resulting yellow solid product was centrifuged, washed with distilled water and ethanol to remove the ions possibly remaining in the final product and finally dried at 60 °C in air. Moreover, sodium sulfide instead of TAA was used as sulfur source, while kept other conditions unchanged. The obtained samples were characterized by X-ray powder diffraction (XRD) using a Japan Rigaku D/max-ga X-ray diffractometer with graphite monochromatosed CuKa radiation (λ = 1.54178 Å). Transmission electron microscopy (TEM) observations were performed with a JEM 200 CX microscope operated at 160 kV. The field emission scan electron microscope (FESEM) images were obtained on Hitachi S4700. The optical absorption of above sample was examined by a PERKIN ELMER Lambda20 UV/VIS Spectrometer. The photoluminescence spectrum (PL) was achieved on a Hitachi 850 fluorescence spectrophotometer using a 350 nm excitation line. 3. Results and discussion Fig. 1. Morphological and structure characterization of flower-like CdS nanostructures: (a) XRD pattern; (b) FESEM image; (c) TEM image. The upper left inset corresponds to the SAED pattern of a rod-like CdS main core.
Fig. 1 shows the morphological and structural characterization of CdS nanostructures prepared by organic-free hydrothermal method. As indicated by the XRD pattern of CdS nanostructures shown in Fig. 1a,
H. Zhang et al. / Materials Letters 61 (2007) 3507–3510
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cadmium source is added into above solution. However, during the hydrothermal process, it is decomposed slowly to release free S2− and CdS nuclei are formed in suitable supersaturation and began to grow into CdS sheet according to the following reaction: CH3 CSNH2
hydrothermal
Y
CH3 CN þ H2 S
Cd2þ H2 SYCdS þ 2Hþ
Fig. 3. TEM image of CdS particles prepared by hydrothermal process using sodium sulfide as sulfur source.
all diffraction peaks can be indexed as the hexagonal CdS with lattice constants a = 4.14, c = 6.72 Å, which is very consistent with the values in the standard card (JCPDS 41-4019). No peaks related to Cd, CdO and other cadmium compound are observed. Fig. 1b shows the FESEM image of the CdS nanostructures by organic-free hydrothermal method using TAA as sulfur source. The morphology of flower-like can be observed from the FESEM image. The typical flower-like CdS nanostructures consisted of CdS nanorods with about several hundred nanometers in width and several micrometers in length, which protrude from the root of the flower-like CdS nanostructures. Individual CdS nanorod has a cusp end similar to a sword with the rough surface, which consists of CdS nanocrystals. The further characterization of abovementioned sample has been performed by TEM as shown in Fig. 1c. The morphology of flower-like CdS nanostructures has also been observed. However, due to the action of sonication during the preparation of TEM observation, the total morphology of flower-like is destroyed. The short CdS nanorods with several hundred nanometers in length and several ten nanometers in width grow in the two sides along the sword-like CdS main core just like fish-bone. The selected area electron diffraction (SAED) pattern performed on the sword-like CdS main core and inserted in Fig. 1c (left) shows high light diffraction spots. The above observation indicates that the flower-like CdS nanostructures are of high crystallinity and can be indexed as the hexagonal CdS phase, which is in accordance with the XRD results in Fig. 1a. The room temperature UV–vis absorption and PL spectra of flowerlike CdS nanostructures dispersed in ethanol are recorded and shown in Fig. 2. The blue shift to 494 nm can be found compared with the band gap of the characteristic absorption of bulk CdS probably due to the size effect as shown in Fig. 2a. The PL spectrum for the sample using a 350 nm excitation line is shown in Fig. 2b. Two emissions, i.e. one at 504 nm belongs to the band to band emission, the other at 720 nm induced by self-activated emission of CdS, have been observed from the flower-like CdS nanostructures, in agreement with the previously report [26]. As we know, the growth habit of crystals is mainly determined by the internal structure of a given crystal, and also affected by external conditions such as temperature, mass transport, capping molecule, and time. In this case, the internal structure and mass transport controlled by sulfur source are the key parameters. We employ TAA as sulfur source to control the nucleation and growth process, which was previously used to direct the shape of other sulfide [27]. Before the hydrothermal process, due to the very low decomposition of TAA resulting in low free S2+ concentration, almost no CdS nuclei are formed when
ð1Þ ð2Þ
Due to hexagonal structure of CdS, the growth rate along c axis is usually the fastest and the rod-like morphology is frequently obtained. In addition, for the slowly decomposition rate of TAA, enough sulfur source can be subsequently provided to proceed secondary growth, which is also reported previously [28]. Therefore, the rod-like subbranch of CdS is formed on the rod-like main core. The formation of flower-like of CdS nanostructures indicates that the nucleation and growth of CdS nanostructures are well controlled by using CdSO4 and TAA as starting agents. To make this point clear, Na2S was also used as sulfur sources keeping other conditions constant. It was observed that only small CdS particles are obtained shown in Fig. 3. As a result, the flower-like CdS nanostructures consisting of sword-like CdS nanorod are formed due to the internal structure and secondary growth controlled by sulfur mass transport. In summary, the flower-like CdS nanostructures have been prepared by organic-free hydrothermal process. The flower-like CdS nanostructures are single crystalline and hexagonal phase, which consist of sword-like CdS nanorods. The internal structure and secondary growth controlled by sulfur mass transport are the critical roles to synthesize the flow-like CdS nanostructures. It is reasonable to believe that the hydrothermal method presented here is desirable for fabrication of other sulfide nanostructures.
Acknowledgment The authors would like to appreciate the financial supports of the Natural Science Foundation of China (No. 60225010). Thanks Prof. Youwen Wang for the TEM measurements and Prof. Yifan Zheng for the FESEM measurements. We also thank Prof. Xianping Fan for UV–vis and PL measurement. References [1] X. Duan, Y. Huang, R. Agarwal, C.M. Lieber, Nature 421 (2003) 241. [2] M.S Fuhrer, J. Nygard, L. Shih, M. Forero, Young-Gui Yoon, M.S.C. Mazzoni, Hyoung Joon Choi, Science 288 (2000) 494. [3] Z.F. Ren, Z.P. Huang, J.W. Xu, J.H. Wang, P. Bush, M.P. Siegal, P.N. Provencio, Science 282 (1998) 1105. [4] W.Z. Pan, R.Z. Dai, Z.L. Wang, Science 291 (2001) 1947. [5] J.Y. Lao, J.Y. Huang, D.Z. Wang, Z.F. Ren, Nano Letters 2 (2002) 1287. [6] Zhengrong R. Tian, James A. Voigt, Jun Liu, Bonnie Mckenzie, Matthew J. Mcdermott, Journal of the American Chemical Society 124 (2002) 12954. [7] Changhui Ye, Guowen Meng, Yinhai Wang, Zhi Jiang, Lide Zhang, Journal of Physical Chemistry. B 106 (2002) 10338. [8] Hui Zhang, Yujie Ji, Xiangyang Ma, Jin Xu, Deren Yang, Nanotechnology 14 (2003) 974. [9] Hui Zhang, Xiangyang Ma, Jin Xu, Junjie Niu, Jian Sha, Deren Yang, Journal of Crystal Growth 246 (2002) 108. [10] H. Zhang, X.Y. Ma, J. Xu, J.J. Niu, D.R. Yang, Nanotechnology 14 (2003) 423. [11] Hui Zhang, Deren Yang, Yujie Ji, Xiangyang Ma, Jin Xu, Duanling Que, Journal of Physical Chemistry. B 108 (2004) 1179.
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[12] Hui Zhang, Xiangyang Ma, Yujie Ji, Jin Xu, Deren Yang, Chemical Physics Letters 377 (2003) 654. [13] Hui Zhang, Xiangyang Ma, Jin Xu, Deren Yang, Journal of Crystal Growth 263 (2004) 372. [14] Yujie Ji, Hui Zhang, Xiangyang Ma, Jin Xu, Deren Yang, J. Phys.: Condens. Matter 15 (2003) 661. [15] J.Y. Lao, J.Y. Huang, D.Z. Wang, Z.F. Ren, Nano Letters 3 (2003) 235. [16] Z.R. Dai, Z.W. Pan, Z.L. Wang, Journal of the American Chemical Society 124 (2002) 8673. [17] Yujie Ji, Xiangyang Ma, Hui Zhang, Jin Xu, Deren Yang, Journal of Physics. Condensed Matter 15 (2003) 7611. [18] Hui Zhang, Deren Yang, Xiangyang Ma, Yujie Ji, Jin Xu, Duanlin Que, Nanotechnology (in press). [19] J. Hulliger, Angewandte Chemie. International Edition in English 33 (1994) 143. [20] J. Zhan, X. Yang, D. Wang, S. Li, Y. Xie, Y. Xia, Y. Qian, Advanced Materials 12 (2000) 1348.
[21] Y. Li, H. Li, Y. Ding, Y. Qian, L. Yang, G. Zhou, Chemistry of Materials 10 (1998) 2301. [22] S. Yu, Y. Wu, J. Yang, Z. Han, Y. Xie, Y. Qian, X. Liu, Chemistry of Materials 10 (1998) 2309. [23] J. Huang, Y. Xie, B. Li, Y. Liu, Y. Qian, S. Zhang, Advanced Materials 12 (2000) 808. [24] Y. Xie, J. Huang, B. Li, Y. Liu, Y. Qian, Advanced Materials 12 (2000) 1523. [25] Hui Zhang, Deren Yang, Xiangyang Ma, Yujie Ji, ShenZhong Li, Duanlin Que, Materials Chemistry and Physics 93 (2005) 65. [26] Feng Gao, Qingyi Lu, Songhai Xie, Dongyuan Zhao, Advanced Materials 14 (2002) 1537. [27] Yurong Ma, Limin Qi, Jiming Ma, Humin Cheng, Crystal Growth and Design 4 (2004) 351. [28] Debao Wang, Dabin Yu, Maosong Mo, Xiangyang Liu, Yitai Qian, Solid State Communications 125 (2003) 475.