Insights into Photoexcited Electron Scavenging Processes on TiO₂ Obtained from Studies of the Reaction of O₂ with OH Groups Adsorbed at Electronic Defects on TiO₂(110)

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Abstract

In this study we show that molecular oxygen reacts with bridging OH (OHbr) groups formed as a result of water dissociation at oxygen vacancy defects on the surface of rutile TiO2(110). The electronic structure of an oxygen vacancy defect on TiO2(110) is essentially the same as that of electron trap states detected on photoexcited or sensitized TiO2 photocatalysts, being Ti3+ in nature. Electron energy loss spectroscopy (EELS) measurements, in agreement with valence band photoemission results in the literature, indicate that water dissociation at oxygen vacancy sites has little or no impact on the electronic structure of these sites. Temperature programmed desorption (TPD) measurements show that O2 adsorbed at 120 K reacts with near unity reaction probability with OHbr groups on TiO2(110) to form an unidentified intermediate that decomposes to generate terminal OH groups at nondefect sites. Commensurate with this process, the electronic defect associated with the original oxygen vacancy defect (Ti3+) is oxidized. Vibrational EELS results indicate that the reaction between O2 and OHbr occurs at about 230 K, whereas electronic EELS results suggest that charge is transferred away from the vacancies at 90 K. Detailed TPD experiments in which the precoverage of water was varied indicate that chemisorption of O2 at cation sites on the TiO2(110) surface is not required in order for the reaction between O2 and OHbr to occur, which implies a direct interaction between weakly bound (physisorbed) O2 and the OHbr groups. In agreement with this conclusion, we find that second-layer water, which selectively hydrogen-bonds to bridging O2- sites and bridging OH groups, blocks the reaction of O2 with OHbr groups and prevents oxidation of the vacancy-related Ti3+ electronic state. These results suggest that the electron scavenging role of O2 in photocatalysis may involve a direct reaction between O2 and trapped electrons located at bridging OH groups. Our studies suggest that the negative influence of high water concentrations in gas-phase heterogeneous photocatalysis studies results from hydrogen-bonded water blocking access of O2 to trapped electrons located at surface OH groups.

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