Hydrothermal synthesis of nanosized anatase and rutile TiO2 using amorphous phase TiO2

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.