TiO2 coating photocatalysts with nanostructure and preferred orientation showing excellent activity for decomposition of aqueous acetic acid

Abstract

TiO2 coatings are attractive materials for use as photocatalysts, photoelectrodes and solar cells. Structural modification, quantum size effects, and large surface-to-volume make the coatings interesting. We have described the effects of crystal strucmre [1], crystallinity [2] and morphology [3] of TiO2 coatings on photocatalytic properties, and shown that a TiO2 coating prepared from a chemically-modified alkoxide solution had characteristic structur.es [4]. Here we report the photocatalytic properties of TiO2 coatings prepared by the chemically modified alkoxide method, which have nanostructure and show preferred orientation along the c axis. We also report the effect of oxygen vacancies in the TiO2 coatings on the photocatalytic properties. Decomposition of acetic acid is a very important reaction because during decomposition acetic acid usually generates organic compounds containing more than two carbons. Decomposition of aqueous acetic acid by use of oxygen and photocatalysts is lcnown to proceed differently from the photo-Kolbe reaction [5] with a product of ethane. During the photocatalytic decomposition of aqueous acetic acid with oxygen, formaldehyde is generated as an intermediate compound and this oxidizes readily to carbon dioxide. Chemical oxygen demand (COD) decreases to zero characteristically in the process. Therefore, the photocatalytic decomposition of aqueous acetic acid with oxygen is a key reaction for purification and treatment of water. T iQ coatings were prepared by the chemically modified alkoxide method. The method has been described in detail elsewhere [4]. The precursor solution for the coatings was prepared from titanium tetraisopropoxide, anhydrous el:hanol, diethanol amine, water and polyethylene glycol (molecular weight; Mw = 2000). The concentration of titanium tetraisopropoxide in ethanol was 0.5 mol dm -3. The molar ratios of water and diethanolamine to titanium tetraisopropoxide were 1 and 2, respectively. The concentration of polyethylene glycol to titanium tetraisopropoxide was 20 wt%. Gel coating, which was prepared on a quartz glass plate by a dip-coating method, crystallized to anatase during elevation on