The Role of Surface Deposited Pt on the Photoactivity of TiO2

The role of Pt clusters on defect free anatase TiO₂ (101) surfaces has been studied using ab initio pseudopotential calculations based on density functional theory. The calculated electronic structure of Pt clusters supported on (101) anatase shows that excited electrons and holes are trapped by Pt nearly equally (upon addition of an extra electron 0.57 e- localizes on the Pt cluster, while the addition of a hole causes the loss of 0.59 e- from the Pt cluster). Furthermore, Pt states completely bridge the band gap and act as  electron-hole recombination centers, which is deleterious to the photoactivity of TiO₂. In addition, calculations show that O₂ will not adsorb on the TiO₂ surface unless an extra electron or subsurface defect exists. On the other hand, the Pt cluster was found to significantly enhance the adsorption of O₂, stemming from the high O₂ adsorption energy of -1.69 eV and the many adsorption sites a Pt cluster presents. This results in enhanced photocatalytic performance because O₂ can scavenge electrons more efficiently from Pt than from TiO₂, which increases the interfacial electron transfer rate. The initial increase and subsequent decrease in TiO₂’s photoactivity with increasing Pt loading can be explained by the competition between increased O₂ adsorption and electron-hole recombination. At Pt loadings above the optimum level the increased electron-hole recombination outweighs the increased O₂ adsorption, which is limited by its concentration and diffusion in solution.