Preparation of nanocrystalline titania thin films by using pure and water-modified supercritical carbon dioxide

Abstract

tProcessing by pure and water-modified (30 wt.%) supercritical carbon dioxide and by subcritical waterwere utilized for the direct preparation of highly pure TiO2anatase thin films without any subsequentthermal treatment. One step processing was compared with the two or three step processing combin-ing pure and modified CO2. The effect of temperature (40–150◦C) and the amount of CO2(100–200 g)passed through the high-pressure column on the (micro)structure and the purity of TiO2thin films wereexamined at pressure of 30 MPa. Prepared thin films were characterized with respect to the structuralproperties and purity by Raman spectroscopy. The most promising thin films were analysed with respectto microstructural properties by means of X-ray diffraction to determine the phase composition, thecrystallite-size and the crystallite-size distribution. High temperature had a positive effect on the crys-tallization as well as the purity of TiO2thin films during the one step and multi-step processing. When TiO2thin films were exposed to water-modified supercritical CO2and temperature of 150◦C under pressureof 30 MPa, the desired crystalline structure of anatase was obtained. The anatase crystallites growth wasmainly influenced by the presence of water. Anatase crystallites sizes of 2–12 nm were obtained depend-ing on the processing method on both investigated substrates (soda-lime glass and monocrystalline Si)on which the TiO2thin films were deposited. Using one step or multi-step processing by water-modifiedsupercritical CO2any undesirable effects such as Na+ions diffusion from the soda-lime glass substrateto the one-layer TiO2film, having negative effect on crystallization of anatase, did not take place. Theuniversality of developed processing by pure and water-modified supercritical CO2for preparation ofTiO2anatase thin films was successfully confirmed for two different substrates.