Effect of surface deposited Pt on the photoactivity of TiO ₂

The roles of deposited Pt clusters and adsorbed O ₂ in the photoactivity of anatase TiO ₂ (101) surfaces have been studied using density functional theory. O ₂ only adsorbs to TiO ₂ 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 TiO ₂ surface are rare because water preferentially adsorbs at the surface. Pt clusters on TiO ₂ 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 H ₂O on the Pt cluster. Consequently, Pt increases the rate of electron scavenging by O ₂ relative to that of undoped TiO ₂ 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 TiO ₂. The initial rise and subsequent fall in TiO ₂'s photoactivity with Pt loading results from the competition between enhanced electron scavenging due to increased O ₂ adsorption and increased electron-hole recombination.