Preparation of Metal Oxide Thin Films by the Surface Fixation Method

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Thin or coating films of metal oxides were prepared by the surface fixation (SF) method which utilizes the reaction of metal alkoxides with surface hydroxyls of the substrate.1. SiO_2-MgO system : The SiO_2-overlayer were deposited on MgO surface by the vapor-phase SF method using tetramethoxysilane. At coverages less than monolayer, Lewis acid sites, the amount and the strength of which were greater than that of silica gel, were generated possibly because of the electronic and structural interaction through the Mg-O-Si bond formed at the interface. The formation of oxide thin films at or less than monolayer was found to yield new characteristics or functions at the surface or interface.2. SiO_2-zeolite system : The SF methods using tetramethoxysilane in the gas-phase and cyclic dimethylsiloxane oligomer in the liquid-phase produced the SiO_2-overlayer selectively on the external surface of zeolites, which resulted in the control of pore-opening size and the gas adsorbability of zeolites. The control of pore-opening size could be achieved more easily by the liquid-phase method than the vapor-phase method.3. SnO_2-porous Vycor glass system : The formation of SnO_2 thin film on the porous glass by the liquid-phase SF method using tinpropoxide resulted in the appearance of electrical conductivity and the control of gas permeability. The electrical conductivity changed upon contact with in flammable gases, implying that the present SnO_2-porous glass system can be used as a gas sensor element.In conclusions, the surface fixation method makes it possible to deposit thin or coating films of metal oxides stepwise in a chemically controlled manner because the method is based on the chemical reaction at the surface of the substrates, and therefore the property of not only the deposited films but also the substrates can be well controlled.