Controlling anatase and rutile polymorph selectivity during low-temperature synthesis of mesoporous TiO2 films

Abstract

The polymorph selectivity of a microemulsion-based synthesis solution used for evaporation-induced self-assembly preparation of ordered mesoporous titania films was studied and unexpectedly found to differ from the polymorph selectivity obtained in the films prepared from the reaction solution. While titania nanocrystallites of the rutile phase form in the reaction solution prior to film formation, crystal growth was found to continue after evaporation-induced self-assembly of titania films, resulting in a shift in polymorph selectivity to the anatase phase. Evaporation of solvent and hydrochloric acid during film formation and subsequent aging increases the concentration of the titania precursor and possibly decreases the acidity of the prepared films, both effects that increase the rate of titania condensation, thus promoting the kinetically favoured formation of anatase. The rutile content of the films can be increased by employing an extended reaction time of the synthesis solution prior to the film formation showing that preformed rutile nanocrystallites are retained and incorporated into the titania film during its formation. The crystal growth in the films could be influenced by introducing hydrochloric acid vapour during aging of the formed films, resulting in a decreased formation of anatase nanocrystallites. Such control of the polymorph selectivity in mesoporous titania films is a major advancement towards the synthesis of ordered mesoporous titania films with photocatalytically optimal anatase/rutile ratios and towards the synthesis of pure rutile mesoporous titania.