- Email: [email protected]
Preparation of one-dimensional nanostructured ZnO
Particuology 8 (2010) 383–385
Contents lists available at ScienceDirect
Particuology journal homepage: www.elsevier.com/locate/partic
Short communication
Preparation of one-dimensional nanostructured ZnO Xiuping Jiang a,∗ , Youzhi Liu a , Yanyang Gao b , Xuejun Zhang b , Lihong Shi c a
Research Center of Shanxi Province for High Gravity Chemical Engineering and Technology, North University of China, College Road 3, Taiyuan 030051, China School of Science, North University of China, Taiyuan 030051, China c Shanxi Meteorological Science Institute, Taiyuan 030002, China b
a r t i c l e
i n f o
Article history: Received 23 July 2009 Received in revised form 18 December 2009 Accepted 12 May 2010 Keywords: Nano-ZnO Sol–gel method Ethylenediamine
a b s t r a c t Rod-like ZnO particles were synthesized via a sol–gel method by adding ethylene diamine (EDA) to the reaction system of Zn(Ac)2 ·2H2 O and H2 C2 O4 ·2H2 O. The crystal phase and morphology of the products were characterized by XRD (X-ray powder diffractometer) and TEM (transmission electron microscope). Rod-like ZnO belongs to the hexagonal Wurtzite system, with diameters and lengths of about 20–200 nm and 0.2–1.5 m, respectively. Experimental results showed that the morphology of nano-ZnO can be controlled by modulating the quantities of EDA added into the reaction system and that EDA plays an important role in the formation of rod-like ZnO particles. The growth mechanism of the rod-like nanoZnO was briefly discussed. The proposed facile, reproducible, effective and low-cost synthesis promises future large-scale preparation of nanostructured ZnO for application in nanotechnology. © 2010 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction As a wide direct band gap (ca. 3.37 eV) semiconductor oxide with large exciton binding energy (60 meV), zinc oxide (ZnO) has become an important functional material with unique properties of near-UV emission, optical transparency, and electric conductivity (Choi et al., 2005; Wang, 2005), possessing potential applications such as laser diodes (Aoki, Hatanaka, & Look, 2000), solar cells (Rensmo et al., 1997) and sensors (Rodriguez, Jirsak, Dvorak, Sambasivan, & Fischer, 2000). Regardless of application, the morphology and particle size of ZnO play important roles, thus calling for novel routes for fabricating ultrafine ZnO particles with special morphologies. One-dimensional ZnO nanostructures such as nanowire, nanotube and nanorod have elicited considerable attention because of their versatile applications in electronic and optoelectronic devices, resulting in numerous methods and techniques such as hydrothermal method (Chu, Zeng, & Jiang, 2006; Liu & Zeng, 2003; Song et al., 2004; Zhang, Tian, Wang, & Zhao, 2008), catalytic growth (Lyu et al., 2002), evaporation at high temperature (Wang et al., 2002), pyrolysis (Xu, Xu, Liu, & Wang, 2002), template method (Li, Meng, Zhang & Phillipp, 2000; Park, Kim, Jung, & Yi, 2002; Vayssieres, Keis, Lindquist, & Hagfeldt, 2001), etc. However, most of the methods create ZnO nanostructures by using a catalyst, e.g., gold, which is not desirable because of its high cost.
∗ Corresponding author. Tel.: +86 351 3921986; fax: +86 351 3921497. E-mail address: [email protected] (X. Jiang).
In this work, one-dimensional nanostructured ZnO with rodlike morphology was successfully synthesized from Zn(Ac)2 ·2H2 O and H2 C2 O4 ·2H2 O by a sol–gel method in the presence of EDA, which not only provides mild reaction conditions, but is also easy to scale up for production. 2. Experimental All chemicals were analytical grade reagents and used as received without further purification. The water used in this work was distilled and deionized. A typical procedure is as follows. 0.025 mol Zn(Ac)2 ·2H2 O and EDA (at a given molar ratio to the Zn(Ac)2 ·2H2 O) were dissolved into 100 mL absolute ethanol under refluxing with agitation for 2 h at 80 ◦ C in a four-neck flask (250 mL). Subsequently, 0.025 mol H2 C2 O4 ·2H2 O dissolved in 50 mL absolute ethanol were added dropwise into the mixed solution with vigorous agitation, to result in a white solution, which was maintained at the same temperature with vigorous stirring for 0.5 h, and then filtered to give a white gelatin which was collected and rinsed twice with distilled water and absolute ethanol and dried in vacuum at 80 ◦ C for 1 h, and then, the resulting products were calcined in a muffle furnace at 500 ◦ C for 2 h. The products were collected and rinsed twice again with absolute ethanol and distilled water and then dried in vacuum at 80 ◦ C for 1 h. Finally, a loose white powder was obtained. The phase structure of the obtained samples was characterized by XRD (Japan Rigaku D/max-␥B, 35 kV × 30 mA, Cu-K␣, = 0.154178 nm). The morphology of the obtained samples was
1674-2001/$ – see front matter © 2010 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.partic.2010.05.005
384
X. Jiang et al. / Particuology 8 (2010) 383–385
Fig. 1. XRD pattern of nano-ZnO prepared with EDA.
characterized by TEM (HITACHI H–600–2, 2000 kV of accelerating voltage, 100,000 times). 3. Results and discussion Fig. 1 shows typical XRD spectra of nano-ZnO prepared with EDA in the experiments. All the diffraction peaks are well assigned to those of standard hexagonal phase ZnO (JCPDS card No. 36-1451). No characteristic peaks of impurities were observed. The results showed that the as-prepared product is single-phase hexagonal ZnO. The morphology and microstructure of the products are further analyzed by TEM in Fig. 2, showing the rod-like nano-ZnO structure grown with directional alignment and enclosing numerous small spherical particles. Wu, Zheng and Ding (2001) found such similar structure in the preparation of rod-like nano-PbCl2 , that is, nanoparticles were all linearly arranged under the guidance of additives through the combined actions of macroscopic agitation and microscopic molecular thermodynamic movement to gradually form the rod-like nanostructure. Fig. 2(a) shows a typical TEM image of a ZnO rod obtained at R = 0.1 (R being the molar ratio of EDA to Zn(Ac)2 ·2H2 O), 280–320 nm in diameter and 2.5–7 m in length, consisting of 40 nm nanoparticles. When R was raised to 0.2, mainly rod-like
nano-ZnO, 20–200 nm in diameter and 0.2–1.5 m in length, was obtained, both smaller than those for the case of R = 0.1, and mostly needle-like in shape, while the nanoparticles are about 16–40 nm in size, as shown in Fig. 2(b). When R was further increased to 0.5, the as-obtained products shown in Fig. 2(c) are rod-like nanoZnO or groups of clusters similar to rod-like nano-ZnO, and the ends of the nanorods are brachycephaly or doliform, the diameters of the nanoparticles are about 30 nm and the diameters of the nanorods are more uniform, about 480 nm in diameter and 4.2 m in length. When R was adjusted to 1, no rod-like nanoparticles were obtained. The morphology of nano-ZnO thus depends on the amount of EDA added, and varies with the amount of the additive. In order to confirm the crucial role of EDA in the reaction process, other organic compounds such as citric acid and sodium tartrate were added, resulting no longer, however, in rod-like nano-ZnO formation. Our previous work showed (Jiang, Gao, & Jia, 2008), too, that without any additive spherical ZnO nanoparticles were obtained. Obviously, EDA plays an important role in changing spherical nanoparticles into nanorods. The present experiments led to the following possible growth mechanism of ZnO nanorods in the light of the undeniable role of additives used. Similar to the synthesis of CdS with EDA (Zhao, Hou, Huang, & Li, 2003), a template mechanism is proposed to explain the nucleation and growth of rod-like nano-ZnO in the experiments. Chelation between EDA and Zn2+ cations appears to be the key factor influencing the morphology of ZnO crystals. The EDA molecule containing two nitrogen atoms is a good chelating ligand which has been used extensively as a structuredirecting reagent in synthesizing certain semiconductors. Likewise, the EDA groups may serve as chelating ligands to the Zn2+ cations as follows. nZn2+ + 2n(en) n[Zn(en)2 ]2+
(en = ethylenediamine)
The Zn2+ cations and the EDA groups could be linked through chelation, thus self-assembling to a chain structure. Moreover, the nucleation and growth of nano-ZnO depend on the decomposition of n[Zn(en)2 ]2+ . Subsequently, as calcination goes on, n[Zn(en)2 ]2+ decomposes while ZnO nanopartiles assemble along the chain structure at the same time, thus, forming the rod-like nano-ZnO. It can thus be inferred that EDA acts as a ligand to form relatively stable Zn2+ complexes, leading to the chain structure which influences the preferential growth towards rod-like structures.
Fig. 2. TEM images of rod-like nano-ZnO prepared with different EDAs to Zn(Ac)2 ·2H2 O ratios: (a) R = 0.1, (b) R = 0.2 and (c) R = 0.5.
X. Jiang et al. / Particuology 8 (2010) 383–385
4. Conclusions ZnO nanorods were successfully synthesized via a sol–gel method by using EDA as an additive. Through changing the molar ratio of EDA to Zn(Ac)2 ·2H2 O in absolute ethanol solution, ZnO nanorods with different diameters and lengths were formed. EDA plays a significant role in controlling the shape and size of the ZnO crystals during the crystal growth. The proposed method is simple and easy to control, thus highly promising for industrial application as well as extendible to the synthesis of other similar metal oxides nanorods. References Aoki, T., Hatanaka, Y., & Look, D. C. (2000). ZnO diode fabricated by excimer-laser doping. Applied Physics Letters, 76, 3257–3258. Choi, S. H., Kim, E. G., Park, J. N., An, K. J., Lee, N. Y., Kim, S. C., et al. (2005). Large-scale synthesis of hexagonal pyramid-shaped ZnO nanocrystals from thermolysis of Zn–oleate complex. The Journal of Physical Chemistry B, 109, 14,792–14,794. Chu, D. W., Zeng, Y. P., & Jiang, D. L. (2006). Preparation of ZnO nanorods via surfactant assisted hydrothermal synthesis. Journal of Inorganic Materials, 21(3), 571–575 (in Chinese). Jiang, X. P., Gao, Y. Y., & Jia, S. Y. (2008). Preparation of one-dimensional nanostructure ZnO. Journal of North University of China (Nature Science Edition), 29(2), 147–150 (in Chinese). Li, Y., Meng, G. W., Zhang, L. D., & Phillipp, F. (2000). Ordered semiconductor ZnO nanowire arrays and their photoluminescence properties. Applied Physics Letters, 76, 2011–2013. Liu, B., & Zeng, H. C. (2003). Hydrothermal synthesis of ZnO nanorods in the diameter regime of 50 nm. Journal of the American Chemical Society, 125(15), 4430–4431.
385
Lyu, S. C., Zhang, Y., Ruh, H., Lee, H. J., Shim, H. W., Suh, E. K., et al. (2002). Low temperature growth and photoluminescence of well-aligned zinc oxide nanowires. Chemical Physics Letters, 363, 134–138. Park, W. I., Kim, D. H., Jung, S. W., & Yi, G. C. (2002). Metalorganic vapor-phase epitaxial growth of vertically well-aligned ZnO nanorods. Applied Physics Letters, 80, 4232–4234. Rensmo, H., Keis, K., Lindstrom, H., Sodergren, S., Solbrand, A., Hagfeldt, A., et al. (1997). High light-to-energy conversion efficiencies for solar cells based on nanostructured ZnO electrodes. The Journal of Physical Chemistry B, 101(14), 2598–2601. Rodriguez, J. A., Jirsak, T., Dvorak, J., Sambasivan, S., & Fischer, D. (2000). Reaction of NO2 with Zn and ZnO: Photoemission, XANES, and density functional studies on the formation of NO3 . The Journal of Physical Chemistry B, 104(2), 319–328. Song, X. C., Xu, Z. D., Chen, W. X., Han, G., Liu, B., & Zheng, Y. F. (2004). Synthesis and characterization of ZnO nanorods. Chinese Journal of Inorganic Chemistry, 20(2), 186–190 (in Chinese). Vayssieres, L., Keis, K., Lindquist, S. E., & Hagfeldt, A. (2001). Purpose-built anisotropic metal oxide material: 3D highly oriented microrod array of ZnO. The Journal of Physical Chemistry B, 105(17), 3350–3352. Wang, Z. L. (2005). Self-assembled nanoarchitectures of polar nanobelts/nanowires. Journal of Materials Chemistry, 15, 1021–1024. Wang, Y. W., Zhang, L. D., Wang, G. Z., Peng, X. S., Chu, Z. Q., & Liang, C. H. (2002). Catalytic growth of semiconducting zinc oxide nanowires and their photoluminescence properties. Journal of Crystal Growth, 234, 171–175. Wu, Q. S., Zheng, N. W., & Ding, Y. P. (2001). Synthesis and formation mechanism of PbCl2 nanowires by micelle-template method. Chemical Journal of Chinese Universities, 22(6), 898–900 (in Chinese). Xu, C. K., Xu, G. D., Liu, Y. K., & Wang, G. H. (2002). A simple and novel route for the preparation of ZnO nanorods. Solid State Communications, 122, 175–179. Zhang, Y. H., Tian, Y. W., Wang, Z. F., & Zhao, Y. X. (2008). Synthesis and morphology control of ZnO crystallite in special system. Journal of Bohai University (Natural Science Edition), 29(3), 208–213 (in Chinese). Zhao, Q. T., Hou, L. S., Huang, R. A., & Li, S. Z. (2003). Surfactant-assisted growth and characterization of CdS nanorods. Inorganic Chemistry Communications, 6, 1459–1462.
Contents lists available at ScienceDirect
Particuology journal homepage: www.elsevier.com/locate/partic
Short communication
Preparation of one-dimensional nanostructured ZnO Xiuping Jiang a,∗ , Youzhi Liu a , Yanyang Gao b , Xuejun Zhang b , Lihong Shi c a
Research Center of Shanxi Province for High Gravity Chemical Engineering and Technology, North University of China, College Road 3, Taiyuan 030051, China School of Science, North University of China, Taiyuan 030051, China c Shanxi Meteorological Science Institute, Taiyuan 030002, China b
a r t i c l e
i n f o
Article history: Received 23 July 2009 Received in revised form 18 December 2009 Accepted 12 May 2010 Keywords: Nano-ZnO Sol–gel method Ethylenediamine
a b s t r a c t Rod-like ZnO particles were synthesized via a sol–gel method by adding ethylene diamine (EDA) to the reaction system of Zn(Ac)2 ·2H2 O and H2 C2 O4 ·2H2 O. The crystal phase and morphology of the products were characterized by XRD (X-ray powder diffractometer) and TEM (transmission electron microscope). Rod-like ZnO belongs to the hexagonal Wurtzite system, with diameters and lengths of about 20–200 nm and 0.2–1.5 m, respectively. Experimental results showed that the morphology of nano-ZnO can be controlled by modulating the quantities of EDA added into the reaction system and that EDA plays an important role in the formation of rod-like ZnO particles. The growth mechanism of the rod-like nanoZnO was briefly discussed. The proposed facile, reproducible, effective and low-cost synthesis promises future large-scale preparation of nanostructured ZnO for application in nanotechnology. © 2010 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction As a wide direct band gap (ca. 3.37 eV) semiconductor oxide with large exciton binding energy (60 meV), zinc oxide (ZnO) has become an important functional material with unique properties of near-UV emission, optical transparency, and electric conductivity (Choi et al., 2005; Wang, 2005), possessing potential applications such as laser diodes (Aoki, Hatanaka, & Look, 2000), solar cells (Rensmo et al., 1997) and sensors (Rodriguez, Jirsak, Dvorak, Sambasivan, & Fischer, 2000). Regardless of application, the morphology and particle size of ZnO play important roles, thus calling for novel routes for fabricating ultrafine ZnO particles with special morphologies. One-dimensional ZnO nanostructures such as nanowire, nanotube and nanorod have elicited considerable attention because of their versatile applications in electronic and optoelectronic devices, resulting in numerous methods and techniques such as hydrothermal method (Chu, Zeng, & Jiang, 2006; Liu & Zeng, 2003; Song et al., 2004; Zhang, Tian, Wang, & Zhao, 2008), catalytic growth (Lyu et al., 2002), evaporation at high temperature (Wang et al., 2002), pyrolysis (Xu, Xu, Liu, & Wang, 2002), template method (Li, Meng, Zhang & Phillipp, 2000; Park, Kim, Jung, & Yi, 2002; Vayssieres, Keis, Lindquist, & Hagfeldt, 2001), etc. However, most of the methods create ZnO nanostructures by using a catalyst, e.g., gold, which is not desirable because of its high cost.
∗ Corresponding author. Tel.: +86 351 3921986; fax: +86 351 3921497. E-mail address: [email protected] (X. Jiang).
In this work, one-dimensional nanostructured ZnO with rodlike morphology was successfully synthesized from Zn(Ac)2 ·2H2 O and H2 C2 O4 ·2H2 O by a sol–gel method in the presence of EDA, which not only provides mild reaction conditions, but is also easy to scale up for production. 2. Experimental All chemicals were analytical grade reagents and used as received without further purification. The water used in this work was distilled and deionized. A typical procedure is as follows. 0.025 mol Zn(Ac)2 ·2H2 O and EDA (at a given molar ratio to the Zn(Ac)2 ·2H2 O) were dissolved into 100 mL absolute ethanol under refluxing with agitation for 2 h at 80 ◦ C in a four-neck flask (250 mL). Subsequently, 0.025 mol H2 C2 O4 ·2H2 O dissolved in 50 mL absolute ethanol were added dropwise into the mixed solution with vigorous agitation, to result in a white solution, which was maintained at the same temperature with vigorous stirring for 0.5 h, and then filtered to give a white gelatin which was collected and rinsed twice with distilled water and absolute ethanol and dried in vacuum at 80 ◦ C for 1 h, and then, the resulting products were calcined in a muffle furnace at 500 ◦ C for 2 h. The products were collected and rinsed twice again with absolute ethanol and distilled water and then dried in vacuum at 80 ◦ C for 1 h. Finally, a loose white powder was obtained. The phase structure of the obtained samples was characterized by XRD (Japan Rigaku D/max-␥B, 35 kV × 30 mA, Cu-K␣, = 0.154178 nm). The morphology of the obtained samples was
1674-2001/$ – see front matter © 2010 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.partic.2010.05.005
384
X. Jiang et al. / Particuology 8 (2010) 383–385
Fig. 1. XRD pattern of nano-ZnO prepared with EDA.
characterized by TEM (HITACHI H–600–2, 2000 kV of accelerating voltage, 100,000 times). 3. Results and discussion Fig. 1 shows typical XRD spectra of nano-ZnO prepared with EDA in the experiments. All the diffraction peaks are well assigned to those of standard hexagonal phase ZnO (JCPDS card No. 36-1451). No characteristic peaks of impurities were observed. The results showed that the as-prepared product is single-phase hexagonal ZnO. The morphology and microstructure of the products are further analyzed by TEM in Fig. 2, showing the rod-like nano-ZnO structure grown with directional alignment and enclosing numerous small spherical particles. Wu, Zheng and Ding (2001) found such similar structure in the preparation of rod-like nano-PbCl2 , that is, nanoparticles were all linearly arranged under the guidance of additives through the combined actions of macroscopic agitation and microscopic molecular thermodynamic movement to gradually form the rod-like nanostructure. Fig. 2(a) shows a typical TEM image of a ZnO rod obtained at R = 0.1 (R being the molar ratio of EDA to Zn(Ac)2 ·2H2 O), 280–320 nm in diameter and 2.5–7 m in length, consisting of 40 nm nanoparticles. When R was raised to 0.2, mainly rod-like
nano-ZnO, 20–200 nm in diameter and 0.2–1.5 m in length, was obtained, both smaller than those for the case of R = 0.1, and mostly needle-like in shape, while the nanoparticles are about 16–40 nm in size, as shown in Fig. 2(b). When R was further increased to 0.5, the as-obtained products shown in Fig. 2(c) are rod-like nanoZnO or groups of clusters similar to rod-like nano-ZnO, and the ends of the nanorods are brachycephaly or doliform, the diameters of the nanoparticles are about 30 nm and the diameters of the nanorods are more uniform, about 480 nm in diameter and 4.2 m in length. When R was adjusted to 1, no rod-like nanoparticles were obtained. The morphology of nano-ZnO thus depends on the amount of EDA added, and varies with the amount of the additive. In order to confirm the crucial role of EDA in the reaction process, other organic compounds such as citric acid and sodium tartrate were added, resulting no longer, however, in rod-like nano-ZnO formation. Our previous work showed (Jiang, Gao, & Jia, 2008), too, that without any additive spherical ZnO nanoparticles were obtained. Obviously, EDA plays an important role in changing spherical nanoparticles into nanorods. The present experiments led to the following possible growth mechanism of ZnO nanorods in the light of the undeniable role of additives used. Similar to the synthesis of CdS with EDA (Zhao, Hou, Huang, & Li, 2003), a template mechanism is proposed to explain the nucleation and growth of rod-like nano-ZnO in the experiments. Chelation between EDA and Zn2+ cations appears to be the key factor influencing the morphology of ZnO crystals. The EDA molecule containing two nitrogen atoms is a good chelating ligand which has been used extensively as a structuredirecting reagent in synthesizing certain semiconductors. Likewise, the EDA groups may serve as chelating ligands to the Zn2+ cations as follows. nZn2+ + 2n(en) n[Zn(en)2 ]2+
(en = ethylenediamine)
The Zn2+ cations and the EDA groups could be linked through chelation, thus self-assembling to a chain structure. Moreover, the nucleation and growth of nano-ZnO depend on the decomposition of n[Zn(en)2 ]2+ . Subsequently, as calcination goes on, n[Zn(en)2 ]2+ decomposes while ZnO nanopartiles assemble along the chain structure at the same time, thus, forming the rod-like nano-ZnO. It can thus be inferred that EDA acts as a ligand to form relatively stable Zn2+ complexes, leading to the chain structure which influences the preferential growth towards rod-like structures.
Fig. 2. TEM images of rod-like nano-ZnO prepared with different EDAs to Zn(Ac)2 ·2H2 O ratios: (a) R = 0.1, (b) R = 0.2 and (c) R = 0.5.
X. Jiang et al. / Particuology 8 (2010) 383–385
4. Conclusions ZnO nanorods were successfully synthesized via a sol–gel method by using EDA as an additive. Through changing the molar ratio of EDA to Zn(Ac)2 ·2H2 O in absolute ethanol solution, ZnO nanorods with different diameters and lengths were formed. EDA plays a significant role in controlling the shape and size of the ZnO crystals during the crystal growth. The proposed method is simple and easy to control, thus highly promising for industrial application as well as extendible to the synthesis of other similar metal oxides nanorods. References Aoki, T., Hatanaka, Y., & Look, D. C. (2000). ZnO diode fabricated by excimer-laser doping. Applied Physics Letters, 76, 3257–3258. Choi, S. H., Kim, E. G., Park, J. N., An, K. J., Lee, N. Y., Kim, S. C., et al. (2005). Large-scale synthesis of hexagonal pyramid-shaped ZnO nanocrystals from thermolysis of Zn–oleate complex. The Journal of Physical Chemistry B, 109, 14,792–14,794. Chu, D. W., Zeng, Y. P., & Jiang, D. L. (2006). Preparation of ZnO nanorods via surfactant assisted hydrothermal synthesis. Journal of Inorganic Materials, 21(3), 571–575 (in Chinese). Jiang, X. P., Gao, Y. Y., & Jia, S. Y. (2008). Preparation of one-dimensional nanostructure ZnO. Journal of North University of China (Nature Science Edition), 29(2), 147–150 (in Chinese). Li, Y., Meng, G. W., Zhang, L. D., & Phillipp, F. (2000). Ordered semiconductor ZnO nanowire arrays and their photoluminescence properties. Applied Physics Letters, 76, 2011–2013. Liu, B., & Zeng, H. C. (2003). Hydrothermal synthesis of ZnO nanorods in the diameter regime of 50 nm. Journal of the American Chemical Society, 125(15), 4430–4431.
385
Lyu, S. C., Zhang, Y., Ruh, H., Lee, H. J., Shim, H. W., Suh, E. K., et al. (2002). Low temperature growth and photoluminescence of well-aligned zinc oxide nanowires. Chemical Physics Letters, 363, 134–138. Park, W. I., Kim, D. H., Jung, S. W., & Yi, G. C. (2002). Metalorganic vapor-phase epitaxial growth of vertically well-aligned ZnO nanorods. Applied Physics Letters, 80, 4232–4234. Rensmo, H., Keis, K., Lindstrom, H., Sodergren, S., Solbrand, A., Hagfeldt, A., et al. (1997). High light-to-energy conversion efficiencies for solar cells based on nanostructured ZnO electrodes. The Journal of Physical Chemistry B, 101(14), 2598–2601. Rodriguez, J. A., Jirsak, T., Dvorak, J., Sambasivan, S., & Fischer, D. (2000). Reaction of NO2 with Zn and ZnO: Photoemission, XANES, and density functional studies on the formation of NO3 . The Journal of Physical Chemistry B, 104(2), 319–328. Song, X. C., Xu, Z. D., Chen, W. X., Han, G., Liu, B., & Zheng, Y. F. (2004). Synthesis and characterization of ZnO nanorods. Chinese Journal of Inorganic Chemistry, 20(2), 186–190 (in Chinese). Vayssieres, L., Keis, K., Lindquist, S. E., & Hagfeldt, A. (2001). Purpose-built anisotropic metal oxide material: 3D highly oriented microrod array of ZnO. The Journal of Physical Chemistry B, 105(17), 3350–3352. Wang, Z. L. (2005). Self-assembled nanoarchitectures of polar nanobelts/nanowires. Journal of Materials Chemistry, 15, 1021–1024. Wang, Y. W., Zhang, L. D., Wang, G. Z., Peng, X. S., Chu, Z. Q., & Liang, C. H. (2002). Catalytic growth of semiconducting zinc oxide nanowires and their photoluminescence properties. Journal of Crystal Growth, 234, 171–175. Wu, Q. S., Zheng, N. W., & Ding, Y. P. (2001). Synthesis and formation mechanism of PbCl2 nanowires by micelle-template method. Chemical Journal of Chinese Universities, 22(6), 898–900 (in Chinese). Xu, C. K., Xu, G. D., Liu, Y. K., & Wang, G. H. (2002). A simple and novel route for the preparation of ZnO nanorods. Solid State Communications, 122, 175–179. Zhang, Y. H., Tian, Y. W., Wang, Z. F., & Zhao, Y. X. (2008). Synthesis and morphology control of ZnO crystallite in special system. Journal of Bohai University (Natural Science Edition), 29(3), 208–213 (in Chinese). Zhao, Q. T., Hou, L. S., Huang, R. A., & Li, S. Z. (2003). Surfactant-assisted growth and characterization of CdS nanorods. Inorganic Chemistry Communications, 6, 1459–1462.