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Formation of Amorphous Zinc Citrate Spheres and Their Conversion to Crystalline ZnO Nanostructures Seungho Cho, Ji-Wook Jang, Alum Jung, Sung-Hyun Lee, Jaeguen Lee, Jae Sung Lee, and Kun-Hong Lee* Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-Dong, Nam-Gu, Pohang, Gyungbuk, Korea 790-784 Received September 8, 2010. Revised Manuscript Received November 15, 2010 We report a method for synthesizing zinc citrate spheres at a low temperature (90 °C) under normal atmospheric pressure. The spherical structures were amorphous and had an average diameter of ∼1.7 μm. The amorphous zinc citrate spheres could be converted into crystalline ZnO nanostructures in aqueous solutions by heating at 90 °C for 1 h. By local dissolution of the zinc citrate spheres, nucleation and growth of ZnO occurred on the surfaces of the amorphous zinc citrate spheres. The morphologies and exposed crystal faces of the crystalline ZnO nanostructures (structure I: oblate spheroid; structure II: prolate spheroid; structure III: hexagonal disk; structure IV: sphere) could be controlled simply by varying the solution composition (solutions I, II, III, or IV) in which the as-prepared amorphous zinc citrate spheres were converted. The concentration of citrate anions and solution pH played a decisive role in determining the morphologies and exposed crystal faces of the crystalline ZnO nanostructures. On the basis of experimental results, we propose a plausible mechanism for the conversion of amorphous zinc citrate spheres into the variety of observed ZnO structures.
1. Introduction Over the past few decades, one of the important goals of materials science has been the development of methods for tuning the structures of materials to obtain specific morphologies and crystalline natures.1-3 The design of nanostructured materials with a variety of shapes has driven nanoscience and nanotechnology to develop techniques for tailoring the physical and chemical properties of materials.4-6 The properties are also closely related to the crystalline nature of the material, such as the crystallinity, crystallite orientations, defects, and exposed crystal faces. Therefore, an understanding of the growth mechanisms is indispensable for controlling the morphologies and the crystalline nature of nano- and microstructures and for producing delicate and welltuned materials that may be employed in cutting-edge technologies. From a practical standpoint, it is also highly desirable to develop total syntheses that require normal atmospheric pressures, thereby reducing the energy requirements and facilitating the process of scaling up for mass production. The citrate ligand is widespread in nature. With three carboxylate groups and one hydroxyl group, citric acid and citrates can provide anions ranging from the monovalent H3cit- to the tetradentate cit4-, depending on the pH of the solution.7,8 Citric *Corresponding author: E-mail:
[email protected].
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acid and citrates are important biological ligands for metal ions.9 Few studies have focused on the structure of crystalline zinc citrate complexes.8,10 To the best of our knowledge, formation of amorphous zinc citrate structures has not been reported. Zinc oxide (ZnO, space group = P63mc; a = 0.324 95 nm, c = 0.520 69 nm), which constitutes a new generation semiconductor material, is a II-VI semiconductor that has a wide direct band gap of 3.37 eV at room temperature and a large exciton binding energy of ∼60 meV. ZnO has useful characteristics, such as a large piezoelectric constant, and its electrical conductivity can be easily modified. ZnO has received considerable attention over the past few years because of these unique properties and has been used in a variety of applications.11-19 The properties of ZnO depend strongly on its structure, including morphology and aspect ratio, as well as on the size, orientation, density, and exposed faces of the crystal.20-23 Therefore, researchers are engaged in ongoing efforts to develop ZnO synthesis methods that offer uniformity and fine (11) Tominaga, K.; Umezu, N.; Mori, I.; Ushiro, T.; Moriga, T.; Nakabayashi, I. Thin Solid Films 1998, 334, 35. (12) Liu, C.; Zapien, J. A.; Yao, Y.; Meng, X.; Lee, C.-S.; Fan, S.; Lifshitz, Y.; Lee, S.-T. Adv. Mater. 2003, 15, 838. (13) Yang, K.; She, G.-W.; Wang, H.; Ou, X.-M.; Zhang, X.-H.; Lee, C.-S.; Lee, S.-T. J. Phys. Chem. C 2009, 113, 20169. (14) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (15) Arnold, M. S.; Avouris, P.; Pan, Z. W.; Wang, Z. L. J. Phys. Chem. B 2003, 107, 659. (16) Lee, C. J.; Lee, T. J.; Lyu, S. C.; Zhang, Y.; Ruh, H.; Lee, H. J. Appl. Phys. Lett. 2003, 81, 3648. (17) Wang, Z. L.; Song, J. H. Science 2006, 14, 242. (18) Zeng, H.; Cai, W.; Liu, P.; Xu, X.; Zhou, H.; Klingshirn, C.; Kalt, H. ACS Nano 2008, 2, 1661. (19) Zeng, H.; Xu, X.; Bando, Y.; Gautam, U. K.; Zhai, T.; Fang, X.; Liu, B.; Golberg, D. Adv. Funct. Mater. 2009, 19, 3165. (20) Wang, Z. L. Condens. Matter 2004, 16, R829. (21) Zhang, J.; Sun, L.; Yin, J.; Su, H.; Liao, C.; Yan, C. Chem. Mater. 2002, 14, 4172. (22) Zhao, Q.; Zhang, H. Z.; Zhu, Y. W.; Feng, S. Q.; Sun, X. C.; Xu, J.; Yu, D. P. Appl. Phys. Lett. 2005, 86, 203115. (23) Jang, E. S.; Won, J.-H.; Hwang, S.-J.; Choy, J.-H. Adv. Mater. 2006, 18, 3309.
Published on Web 12/09/2010
DOI: 10.1021/la103600c
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control over the morphologies and exposed crystal faces of the structures to enable exploration of potential uses of ZnO as a source of smart and functional materials.21,24 ZnO structures have been synthesized by a number of different methods, including thermal evaporation,15,25-27 molecular beam epitaxy,28 chemical vapor deposition (CVD),14,16,29 metal-organic chemical vapor deposition (MOCVD),30-32 hydrothermal synthesis,33-35 and sonochemical method.36 The conventional vapor-phase synthesis methods require complex procedures and rigid environmental conditions, such as high temperatures (500-1400 °C) and low or high pressures. Thus, the required heating and vacuum systems are sophisticated and expensive. In contrast, solution chemical approaches enable the growth of ZnO crystals at much lower temperatures (
