Abstract

Phase-pure TiO₂ nanocrystallites with narrow particle-size distributions were selectively prepared by hydrothermal processes starting from amorphous TiO₂. Autoclaving amorphous TiO₂ in the presence of HF and HCl as cooperative catalysts led to the formation of narrow-sized anatase TiO₂ with a regular crystalline surface. Use of nitric acid as a cooperative catalyst with HF also gave the anatase TiO₂ with a narrow size distribution but with a rather irregular crystalline surface. On the other hand, amorphous TiO₂ was converted to phase-pure rutile TiO₂ nanocrystallites by autoclaving in the presence of citric and nitric acids. Chelation of TiO₆ octahedra with citric acid and acidification with nitric acid are critical for the phase transition from amorphous to rutile. The shape of all rutile nanocrystallites was rod-like. The crystal growth of TiO₂ to phase-pure anatase or rutile was proposed to proceed via respective face-sharing and edge-sharing processes through dissolution–precipitation of the dissolved TiO₆ octahedra from the amorphous phase. The photocatalytic activity in the redox reaction of 2-propanol with oxygen was quite comparable among the synthesized anatase and rutile nanocrystallites.