Direct Imaging of Site-Specific Photocatalytical Reactions of O2 on TiO2(110)

Photostimulated reactions of single O-2 molecules on reduced TiO2(110) surfaces were directly observed at an atomic level with high-resolution scanning tunneling microscopy at 50 K. Two distinct reactions of O-2 desorption and dissociation occur at different active sites of terminal Ti atoms and bridging O vacancies, respectively. Two reaction channels follow very different kinetics. While hole-mediated O-2 desorption is promptly and fully completed, electron-mediated O-2 dissociation is much slower and is quenched above some critical O-2 coverage. Evidently, the O-2 photochemistry on TiO2(110) is quite more complex than thought previously. Density functional theory calculations indicate that both coordination and charge state of an O-2 molecule chemisorbed at the specific site largely determine a particular reaction pathway.