ESR Investigation into the effects of heat treatment and crystal structure on radicals produced over irradiated TiO2 powder

Electron spin resonance measurements were carried out at 77 K under irradiation for anatase TiO2 powders treated by heating at various temperatures in the air. For the untreated powder photoproduced holes were trapped at the surface forming Ti4+O-.Ti4+OH- radicals, while, for the heated powder, they were trapped as Ti4+O2-Ti4+O-. radicals at the surface. Photoproduced electrons were trapped as Ti3+ at the surface of the unheated powder, while they trapped at the inner part of the heat-treated powder. These differences could be explained by the desorption of surface hydroxyl groups and the change in the surface structure accompanied by the crystalline growth. No photoproduced radicals were detected for rutile TiO2 powder, which may explain the low photocatalytic activity of this crystalline structure. (C) 1997 Elsevier Science S.A.