Preparation of Metal Oxide Thin Films by the Surface Fixation Method

通过表面固定（SF）方法制备金属氧化物薄膜或涂层，该方法利用金属醇盐与基底表面羟基的反应. SiO_2-MgO系统：采用四甲氧基硅烷气相SF法在MgO表面沉积SiO_2覆盖层。当覆盖率小于单层时，由于界面上形成的Mg-O-Si键的电子和结构相互作用，产生了数量和强度均大于硅胶的刘易斯酸中心。氧化物薄膜的单层或更少的形成被发现在表面或界面产生新的特性或功能. SiO_2-沸石体系：采用气相四甲氧基硅烷和液相环状二甲基硅氧烷低聚物的SF法，在沸石外表面选择性地形成SiO_2覆盖层，从而控制沸石的孔径大小和气体吸附性能。液相法比气相法更容易实现孔径的控制. SnO_2-多孔Vycor玻璃体系：采用异丙醇锡液相SF法在多孔玻璃上形成SnO_2薄膜，使其具有导电性，并可控制其透气性。SnO_2-多孔玻璃体系在可燃气体中的电导率发生了变化，表明该体系可以用作气敏元件.总之，表面固定法是基于在基底表面的化学反应，使化学控制的方式逐步存款金属氧化物薄膜或涂层成为可能，从而不仅可以很好地控制沉积薄膜的性能，而且可以很好地控制衬底的性能。

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.