Highly active photocatalytic TiO2 powdwers obtained by thermohydrolysis of TiCl4 in water

Highly active photocatalytic TiO2 samples were synthesized by thermohydrolysis of TiCl4 in water at 100 °C. Rutile, binary mixtures of anatase and rutile or anatase and brookite, or ternary mixtures of anatase, brookite and rutile were obtained depending on the TiCl4/H2O ratio. Rietveld refinements were employed to evaluate the crystalline phases and composition of the mixtures. The effect of the aging time on the phase composition was also studied. The band gap values of the samples were obtained by the diffuse reflectance spectra. The position of the flat band potentials of anatase, brookite and rutile was determined measuring the photovoltage as a function of the suspension pH. From these data, the relative positions of the energy bands of the three semiconductors were estimated. 4-nitrophenol photodegradation was used to evaluate the photoactivity of the various samples. Some powders were more active than Degussa P25. The most efficient samples consisted of a ternary mixture of anatase, brookite and rutile. The high photocatalytic activity was explained by the presence of junctions among different polymorphic TiO2 phases that enhance the separation of the photogenerated electron-hole pairs, hindering their recombination.