Charge carrier dynamics of standard TiO2 catalysts revealed by femtosecond diffuse reflectance spectroscopy

Dynamics of charge carriers generated by femtosecond UV (160 fs, 390 nm) excitation in five standard TiO2 photocatalytic powders (JRC-TIO-1, -2, -3, -4, and -5 supplied by the Catalysis Society of Japan) in a vacuum and air was investigated by means of time-resolved femtosecond diffuse reflectance spectroscopy, and was discussed from the viewpoints of crystal structure (anatase and rutile), particle size, and surrounding condition around the particles. Anatase TiO2 catalysts showed very rapid (less than 1 ps) and very slow electron-hole recombination processes, while rutile ones did not show any rapid decay. For catalysts composed of small particles; slower electron-hole recombination was observed in air compared with that in a vacuum, which was well explained in terms of upward band bending near the surface due to adsorption of oxygen. The relationship between the observed charge carrier dynamics and photocatalytic reactivity is also discussed.