Preparation of nanocrystalline titania thin films by using pure and water-modified supercritical carbon dioxide
Abstract
Processing by pure and water-modified (30 wt.%) supercritical carbon dioxide and by subcritical water were utilized for the direct preparation of highly pure TiO2 anatase thin films without any subsequent thermal treatment. One step processing was compared with the two or three step processing combining pure and modified CO2.
The effect of temperature (40-150 degrees C) and the amount of CO2 (100-200g) passed through the high-pressure column on the (micro)structure and the purity of TiO2 thin films were examined at pressure of 30 MPa. Prepared thin films were characterized with respect to the structural properties and purity by Raman spectroscopy.
The most promising thin films were analysed with respect to microstructural properties by means of X-ray diffraction to determine the phase composition, the crystallite-size and the crystallite-size distribution. High temperature had a positive effect on the crystallization as well as the purity of TiO2 thin films during the one step and multi-step processing.
When TiO2 thin films were exposed to water-modified supercritical CO2 and temperature of 150 degrees C under pressure of 30 MPa, the desired crystalline structure of anatase was obtained. The anatase crystallites growth was mainly influenced by the presence of water.
Anatase crystallites sizes of 2-12 nm were obtained depending on the processing method on both investigated substrates (soda-lime glass and monocrystalline Si) on which the TiO2 thin films were deposited. Using one step or multi-step processing by water-modified supercritical CO2 any undesirable effects such as Na* ions diffusion from the soda-lime glass substrate to the one-layer TiO2 film, having negative effect on crystallization of anatase, did not take place.
The universality of developed processing by pure and water-modified supercritical CO2 for preparation of TiO2 anatase thin films was successfully confirmed for two different substrates.