Correlation of TiO₂ photocatalytic activity and diffuse reflectance spectra

Photocatalytic activity of TiO₂ samples prepared by aqueous hydrolysis of TiCl₄ and deposition with NH₃ in gaseous acetone photocatalytic oxidation has been found dependent on calcination temperature. The activity reaches its maximum at calcination temperature 450°C and then decreases, all the samples being anatase. The photocatalytic activity of the samples in a series of different calcination temperatures correlates well with reflectance of the samples in the visible light region. The increase in calcination temperature from 320 to 500°C results in the growth of TiO₂ crystallites, decrease in BET surface area, slight changes in lattice parameters, and removal of weak surface Bronsted acid centers that are speculated to be OH groups. TiO₂ prepared by deposition with NaOH exhibits much higher photocatalytic activity. The only paramagnetic species detected in the TiO₂ samples was NO that resulted from oxidation of lattice-captured NH₃. No NO was detected in TiO₂ prepared by deposition with NaOH. The concentration of NO increases steadily with calcination temperature as indicated by ESR spectra. The observed correlation of diffuse reflectance and photocatalytic activity was attributed to opposite effects of the increase of crystallinity and removal of surface water and hydroxyl groups.