Ultraviolet photoeffects on oxygen-hydrogen interstitial clusters in rutile TiO2

초록

Super-bandgap illumination of semiconductors affects the diffusion and reaction rates of interstitial atoms, but photoeffects on clusters of interstitials remain virtually unexplored. In prototypical metal oxides such as TiO2 and ZnO, oxygen interstitials (Oi) appear to form stable clusters below about 300 degrees C. Such cluster formation becomes very important when chemically prepared surfaces of binary oxides submerged in liquid water inject Oi into the solid. New kinetically dominated phenomena occur, such as strong isotopic fractionation, that are influenced by interstitial trapping in clusters. Judicious coordination of temperature and illumination allows optimization of competing kinetic effects, but little is known about the composition of Oi-containing clusters or their photoresponse. This study begins to fill that gap by combining simulations of Oi-Hi clusters by density functional theory (DFT) with self-diffusion measurements of 18O in submerged single-crystal rutile TiO2(110) under ultraviolet (UV) illumination. The simulations show that Oi-(Hi)x exists in several isomers, each with multiple charge states that can change upon illumination. The diffusion measurements suggest that UV changes the populations of Oi-containing cluster isomers deep in the solid, and that the details of these changes depend upon the application of potential bias. The results indicate that illumination alters the rate constants for formation or dissociation of (Oi)y-(Hi)x or alters the concentration of the reactant Hi.

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