Nanosized TiO₂ has received increased attention as photocatalyst for environmental purification and for self-cleaning effects. TiO2 exists in three different crystalline habits: rutile, anatase and brookite. Thermohydrolysis of TiCl₄ in suitable aqueous solutions allows to obtain nanocrystalline pure anatase, brookite and rutile or various mixtures of the three phases.
In this study different routes were followed to obtain the various samples: TiCl₄ was hydrolyzed in pure water, in HCl solutions or in NaCl solutions. TiCl₄ was added at room temperature under stirring and the resulting solutions were heated at 100°C in an oven. The composition of the TiO₂ powders strongly depended on the acidity of the medium. Only rutile was formed at low pHs whereas binary or ternary mixtures of the three phases were obtained in H2O or in chloride solutions. Pure brookite nanoparticles were easily separated by peptization from the binary mixtures of brookite and rutile.
The various TiO₂ powders were characterized by X-ray diffraction, thermal analysis, diffuse reflectance spectroscopy and BET surface area determinations. The reactivity of all samples, both pure phases and mixtures, was tested for the photocatalytic degradation of 4-nitrophenol.
All the powders obtained by thermolysis of TiCl₄ were active under UV irradiation. Brookite revealed a good photocatalytic activity whereas that of rutile was scarce. The highest activities were generally obtained with powders consisting of more than one crystalline phase. Some samples exhibited similar or higher activities than commercial active Degussa P25.