Paper SS+EN-ThM1
Hindering Influence of Oxygen Vacancies for Photoactivity on TiO₂(110)

Thursday, October 31, 2013, 8:00 am, Room 201 A

In photoinduced processes, surface point-defects are expected to act as charge trapping and/or recombination centers. However, the direct impact of surface defects on photoreactivity is not well explored. We present the first observation of a suppressing effect of oxygen vacancy (V_O) defects on photoreactivity of TiO₂(110). Direct scanning tunneling microscopy imaging reveal a pronounced site-selectivity in the hole-mediated photooxidation of trimethyl acetate (TMA) on TiO₂(110) upon ultra-violet light irradiation, wherein the reaction readily occurs at regular Ti sites but is completely inhibited at V_O defects. Utilizing electron energy loss spectroscopy and density functional theory, we show that the lack of reactivity of TMA groups adsorbed at V_O’s cannot be attributed to either a less active adsorption conformation or electron transfer from the V_O defect. Instead, we propose that the excess unpaired electrons associated with the V_O promptly recombine with photoexcited holes approaching the surface, effectively ‘screening’ TMA species at V_O site. We also show that this screening effect is spatially short-ranged, being predominately localized at the V_O, and only mildly affecting TMA’s at adjacent Ti sites. The direct impact of O vacancies on TMA photoreactivity over TiO₂(110) is expected to have similar implications for other hole-mediated (e.g., photooxidation) reactions in which adsorption at or near electronic point-defects is possible. Furthermore, the localized influence of these defects on hole-mediated chemistry offers opportunities for additional study of site-selective photocatalysis on TiO₂. The presented results also demonstrate that structure–reactivity relationships, a customary subject in heterogeneous catalysis, are clearly relevant to photocatalysis.