Preparation of nanocrystalline TiO2 monoliths by using modified supercritical carbon dioxide

Abstract

A flow supercritical fluid crystallization with and without modifier/s was applied to prepare nanocrystalline TiO2 monoliths without subsequent thermal treatment. The effect of processing conditions (temperature, pressure, water concentration, type of modifier: water or ethanol and water) on the crystallization, texture and purity of TiO2 was investigated. Gel blocks for processing were prepared by nitric acid-catalysed sol-gel synthesis. It was revealed that one- or multi-step processing by scCO(2) with modifier leads to direct crystallization of TiO2 monoliths of anatase-brookite crystal structure even at mild processing temperature of 40 degrees C and 30 MPa. 15 wt. % of water was determined to be the optimal concentration of water in scCO(2) as a crystallization agent. Processing temperature affected the anatase and brookite crystallite-size. Processing pressure affected the crystallization and purity of monolithic TiO2. The pre-drying as well as the post-drying by scCO(2) at 40 degrees C visibly improved the solidity of TiO2 monolith and stabilized brookite crystallites.