Surface Structure of TiO2 P25 Nanoparticles: Infrared Study of Hydroxy Groups on Coordinative Defect Sites

Surface hydroxy groups are known to play a significant role in the photocatalytic behavior of TiO2. The molecular or dissociative character of H2O adsorption on titania is strongly affected by the local surface structure, but dissociation, producing -OH, should prevail on surface sites with a high degree of coordinative unsaturation. Here we report an IR investigation of hydroxy groups on low coordination sites of TiO2 P25 (Degussa), a commercial titania powder considered a staple in the field of heterogeneous photocatalysis. The presence of coordinatively defective surface terminations well agrees with the roughness of the border of TiO2 particles observed by high resolution transmission electron microscopy. Furthermore, IR spectra of adsorbed CO were used to probe the Lewis acidity of surface Ti4+ ions exposed after the removal of water molecules and hydroxy groups. Evidence for two additional types of surface cationic sites with respect those reported in the literature was found.