Nanostructuring Titania: Control over Nanocrystal Structure, Size, Shape, and Organization

Abstract

Control over crystal structure, size, shape, and organization of TiO₂ nanocrystals has been achieved by means of wet chemistry. Hydrolysis and polycondensation of titanium alkoxide [Ti(OR)₄] has been performed in the presence of tetramethylammonium hydroxide (Me₄NOH). This base both catalyzes the reaction and provides an organic cation that stabilizes the anatase polyanionic cores formed in this medium. These anatase clusters are organized so as to favour self-assembly into intermediate nanocrystals, which, in turn, self-assemble into superlattices. This self-assembling process has been exploited for the processing of highly structured titania films. Furthermore, larger anatase TiO₂ nanocrystals of different sizes and shapes have been obtained by adjusting the relative concentrations of titanium alkoxide and Me₄NOH, the reaction temperature, and the pressure. HRTEM, XRD, and EXAFS have been used to characterize the various samples and to elucidate the growth of titania anatase. Our observations are in accordance with theoretically predicted condensation and growth pathways. The formation of mesoscopic structures through a self-assembling process of the multiply charged polytitanate anions in the presence of Me₄N⁺ is also discussed.