Phase transformation and precipitation behavior of niobium component out of niobium-doped anatase-type TiO2 nanoparticles synthesized via hydrothermal crystallization

The precipitation behavior of niobium component out of niobium-doped anatase-type TiO2 and structural change in the course of heating were investigated. The samples were directly formed under hydrothermal conditions at 240 °C for 5 h in the presence of aqueous ammonia via crystallization from co-precipitates that were obtained from precursor solutions of TiOSO4 and NbCl5. The as-prepared niobium-doped anatase-type titania nanoparticles showed bluish color and absorption in the visible region, which was confirmed to be due to the presence of Ti(III) in the solid solutions using electron paramagnetic resonance measurement. The niobium-doped anatase-type titania existed stably without an appearance of any other phases after heating up to 500 °C for 1 h. In the course of heating at 500–800 °C, continual and clear decrease in the lattice parameters a0 and c0 of the anatase was observed, which was followed by the precipitation of Nb2O5 and TiNb2O7 out of the niobium-doped anatase, but the anatase phase was maintained without anatase-to-rutile phase transformation up to 850–1,000 °C. The anatase-to-rutile phase transformation was gradually retarded when the niobium content increased.