Selective Preparation of SnO2 and SnO Crystals with Controlled Morphologies in an Aqueous Solution System Hiroaki Uchiyama, Hirotoshi Ohgi, and Hiroaki Imai* Department of Applied Chemistry, Faculty of Science and Technology, Keio UniVersity, 3-14-1 Hiyoshi, Kohoku, Yokohama 223-8522, Japan
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 9 2186-2190
ReceiVed June 3, 2006
ABSTRACT: The oxidation state of tin oxide crystals grown in an aqueous solution of Sn(II) was successfully controlled. Rutiletype SnO2 and tetragonal SnO were selectively produced under acidic conditions below pH 3.3 and alkaline conditions above pH 13.1, respectively. The nanostructures, including the grain size and shape, and the mesoscopic assembled states of the crystals were changed with the concentration of the Sn(II) species and a subtle variation of the pH. This versatile fabrication process for functional oxides is based on the difference in the stability of Sn(II) and Sn(IV) depending on the pH in an aqueous solution system. Introduction Metal oxides have been used in various devices for electric, optoelectronic, magnetic, and sensing applications. The performance of these devices is influenced by the nanostructure and crystallinity. Recently, the crystal growth of oxide materials from an aqueous solution has been focused on techniques for the controlled fabrication of various nanostructures at near room temperatures.1-7 Through the aqueous routes, functional oxide crystals and a metastable phase were successfully prepared without additional heat treatments. A wide variety of nanostructures have been reported using the solution approach. Although many morphological studies have been conducted, the control of the oxidation state of metal ions has hardly been investigated. The oxidation state is essential for the crystal structure and the functionality of the oxide materials. Magnetic bacteria produce alignments of magnetite (Fe3O4) nanoparticles through the oxidation of Fe(II) in magnetosome.4,5 A fabrication technique of MxFe3-xO4 (M ) Fe, Co, Ni, etc.) films in an aqueous solution also was achieved through the partial oxidation of Fe(II) on a substrate by an oxidizing agent.6,7 However, the strict control of the oxidation state is generally difficult in an aqueous solution. Therefore, it is challenging to fabricate metal oxide materials by controlling the oxidation state of multivalent metal ions in a solution. Tin oxides, such as rutile-type SnO2 and tetragonal SnO, have received great attention as functional materials. Rutile-type SnO2 is widely used for various devices, such as transparent electrodes8,9 and gas sensors,10,11 because of its excellent optical and electrical properties. Tetragonal SnO is expected to be used as an anode material for lithium ion rechargeable batteries.12 The theoretical capacity of SnO (875 mA‚h/g) and SnO2 (783 mA‚h/g) is superior to that of graphite (372 mA‚h/g). Therefore, the preparation of morphologically controlled SnO2 and SnO crystals would be important for such applications. The morphological control of thermodynamically stable SnO2 has been widely studied, and many types of SnO2, such as tubes,13 rods,14 spheres,15 and plates,2 were prepared by chemical vapor deposition,13,16 sol-gel,17 hydrothermal,14,15 and nonaqueous methods.11,18 On the other hand, the preparation of SnO is relatively difficult because Sn(II) is easily oxidized to Sn(IV) in the atmosphere.19,20 Various morphologies of crystalline SnO, such as diskettes,21 plates,22 and whiskers,23 were obtained by * To whom correspondence should be addressed. Phone: +81-45-5661556. Fax: +81-45-566-1551. E-mail:
[email protected].
sonication10,24 and hydrothermal methods.23,25 The preparation of SnO was performed at low temperatures (
