Ultraviolet photoeffects on oxygen-hydrogen interstitial clusters in rutile TiO₂

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 (O_i) appear to form stable clusters below about 300 °C. Such cluster formation becomes very important when chemically prepared surfaces of binary oxides submerged in liquid water inject O_i 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 O_i-containing clusters or their photoresponse. This study begins to fill that gap by combining simulations of O_i-H_i 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 O_i-(H_i)_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 O_i-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 (O_i)_y-(H_i)_x or alters the concentration of the reactant H_i.