Effect of Surface Deposited Pt on the Photoactivity of TiO2

The roles of deposited Pt clusters and adsorbed O-2 in the photoactivity of anatase TiO2 (101) surfaces have been studied using density functional theory. O-2 only adsorbs to TiO2 surfaces when excess negative charge is available to form O-Ti bonds, which can be provided by a photoexcited electron or subsurface oxygen vacancy, in which cases the adsorption energies are -0.94 and -2.52 eV, respectively. When O-2 adsorbs near a subsurface defect, it scavenges extra electron density and creates a hole that can annihilate excited electrons. In aqueous solutions, O-2 interactions with the TiO2 surface are rare because water preferentially adsorbs at the surface. Pt clusters on TiO2 significantly enhance O-2 adsorption providing many adsorption sites with adsorption energies up to -1.69 eV, stronger than the -0.52 eV adsorption energy of H2O on the Pt cluster. Consequently, Pt increases the rate of electron scavenging by O-2 relative to that of undoped TiO2 leading to enhanced photocatalytic performance. Pt states completely bridge the band gap and act as electron-hole recombination centers, which are deleterious to the photoactivity of TiO2. The initial rise and subsequent fall in TiO2's photoactivity with Pt loading results from the competition between enhanced electron scavenging due to increased O-2 adsorption and increased electron-hole recombination.