Crystal-Plane Dependence of Critical Concentration for Nucleation on Hydrothermal ZnO Nanowires Yong He, † Takeshi Yanagida,* ,† Kazuki Nagashima, † Fuwei Zhuge, † Gang Meng, † Bo Xu, † Annop Klamchuen, † Sakon Rahong, † Masaki Kanai, † Xiaomin Li, ‡ Masaru Suzuki, § Shoichi Kai, § and Tomoji Kawai* ,† † The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka Ibaraki, Osaka 567-0047, Japan ‡ State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China § Department of Applied Quantum Physics and Nuclear Engineering, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan * S Supporting Information ABSTRACT: Hydrothermal ZnO nanowires have shown great potential for various nanoscale device applications due to their fascinating properties and low- temperature processing. A preferential crystal growth of ZnO (0001) polar plane is essential and fundamental to realize the anisotropic nanowire growth. Here we demonstrate that a critical concentration for a nucleation strongly depends on a crystal plane, which plays an important role on an anisotropic growth of hydrothermal ZnO nanowires. We measure a growth rate of each crystal plane when varying a concentration of Zn ionic species by using a regular array structure. Selective anisotropic growth on (0001) plane emerges within a certain concentration range. Above the concentration range, a crystal growth on (101̅0) plane tends to simultaneously occur. This strong concentration dependence on the crystal plane is understood in terms of a critical concentration difference between (0001) plane and (101̅0) plane, which is related to the surface energy difference between the crystal planes. ■ INTRODUCTION ZnO nanowires formed via a hydrothermal method have attracted much attention due to their fascinating physical properties and the potentials for nanodevice applications, including light-emitting devices, solar cells, power generators, and others. 1-7 One of the reasons why the hydrothermal method has been so attractive even compared with other methods is that whole hydrothermal processes can be performed under a relatively low-temperature range less than 100 °C, 8,9 which is hardly attainable to other conventional vapor-phase methods including VLS mechanism. 10-27 This low-temperature synthesis is favorable, especially when integrating nanowires with other components (e.g., polymers) within the devices. 9,28 In the hydrothermal growth of ZnO nanowires, a preferential nucleation at ZnO (0001) polar plane is essential and fundamental to realize the anisotropic nanowire growth. 4,29,30 The effects of various experimental parameters, including pH, temperature, and ionic species, have been examined to understand their roles on the reaction scheme of hydrothermal growth. 30-45 For example, Yamabi and Imai reported the role of pH on the variation of ionic species during hydrothermal ZnO nanowire growth. 36 Xu et al. demonstrated the importance of temperature on the morphology of hydrothermal ZnO nanowires. 37 Joo et al. recently reported a face-selective electrostatic control of hydrothermal ZnO nanowires by intentionally adding positively charged ions during growth. 32 In most previous studies, the origin of anisotropic crystal growth in hydrothermal ZnO nanowires has been interpreted in terms of the variations of ionic species in aqueous solutions and their electrostatic interactions with ZnO crystal planes. 31,32,38,39 Here we demonstrate that a crystal-plane dependence on critical concentration for a nucleation plays an important role on an anisotropic growth of hydrothermal ZnO nanowires. We found the strong concentration dependence on the crystal growth of each ZnO crystal plane when varying only a concentration of Zn species under the same temperature and pH value. This strong concentration dependence on the crystal plane can be understood in terms of a critical concentration difference between (0001) plane and (101̅0) plane for a nucleation. Our findings highlight that controlling precisely a concentration during growth is essential to tailor the morphology of hydrothermal ZnO nanowires. Received: November 16, 2012 Revised: December 17, 2012 Published: December 21, 2012 Article pubs.acs.org/JPCC © 2012 American Chemical Society 1197 dx.doi.org/10.1021/jp3113232 | J. Phys. Chem. C 2013, 117, 1197-1203