Preparation of Metal Oxide Nanowires by Hydrothermal Synthesis in Supercritical Water
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N11.19.1
Preparation of Metal Oxide Nanowires by Hydrothermal Synthesis in Supercritical Water Yukiya Hakuta, Hiromichi Hayashi and Kunio Arai Supercritical Fluid Research Center National Institute of Advanced Industrial Science and Technology (AIST) Nigatake 4-2-1, Miyagino-ku, Sendai 983-8551, JAPAN ABSTRACT Three kinds of single metal oxide (MnO2, ZnO, and AlO(OH)) and one complex metal oxide (K2O⋅6TiO2) having nano structure of wire, rod and ribbon were rapidly synthesized by hydrothermal synthesis in supercritical water. Aqueous Mn(NO3)2, Zn(NO)2, Al(NO3)3 solutions and mixtures of TiO2 sols and KOH solutions were used as starting materials, respectively. Syntheses of these nanostructured materials were performed by a flow type apparatus. We investigated the relationship between reaction parameters (temperature, pH and reaction time) and morphologies of the products. Reaction temperatures were 350, 400, and 420 °C. Reaction time is in the range of 1.8 - 116 s. The product was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that particle morphologies strongly depend on pH for MnO2 and ZnO. MnO2 nanowires with 10 nm in diameter and ZnO nanorods with 50 nm in diameter were obtained from acidic metal salt solutions. In the case of AlO(OH), temperature and time were key parameters for crystal growth. In the case of K2O6TiO2, larger fibrous particles with 50 nm in diameter were obtained at higher reaction temperature.
INTRODUCTION One-dimension materials such as rod, wire and ribbon are of interest in wide fields of semiconductor, electric, optical devices due to these specific physical and chemical properties that are different form bulk materials. Formation of carbon nanotube or some metal nanowires (e.g. Ag, Si) has shown tremendous success [1]. Recently production of nanorod or nanowire of metal oxides has been reported by many researchers. Two methods are establishing to produce nanostructured metal oxide. One is anodic alumina template (AAT) techniques. Production of nanorods or nanowires of NiO[2], SiO2[3], TiO2[4], ZrO2[5], In2O3[6-8], LiCoO2[9], LiNiO2[10] was reported by AAT methods. The other is epitaxial growth method by which several metal oxide nanowires such as TiO2[11] and ZnO[12-14] were successfully synthesized. Besides these methods, hydrothermal synthesis is one of the promising methods for nano structure materials, too and ZnO [15] and ZnWO4 [16] nanorods were synthesized by hydrothermal routes. Considering from the practical mass production, these methods have some problems such as requirement of complex multi-step treatments and use of expensive raw materials. It is desirable to develop simple and practical method for the production of nanostructure materials. One of methods for producing metal oxide nanoparticles is hydrothermal synthesis in supercritical water using a flow type apparatus [17]. Immediate formation of metal oxide nano particles was achieved by feeding aqueous metal salt solution into supercritical water in a flow reactor.
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