Interplay between hydrogen bonding and electron solvation on hydrated TiO2 surface

At metal-oxide/protic-solvent interfaces, partially hydrated or ``wet electron'' states represent the lowest energy pathway for electron transfer.$^{1}$ Through a joint two-photo photoemission (2PP) experiment and density function theory (DFT) study, we identified the electronic states corresponding to the partially solvated or wet electron state in H$_{2}$O overlayers on rutile TiO$_{2}$(110) surface. We find that a network of dangling H atoms can stabilize photoexcited electrons, in so-called wet-electron states. The energies of the ``wet electron'' states correlate closely with the number and configuration of the dangling H atoms involved in stabilizing them. We also performed DFT calculations of H$_{2}$O and H covered anatase (101) surface. Comparing with rutile (110), anatase (101) surface accommodates weaker H$_{2}$O molecule-surface hydrogen bonding. Our calculated wet electron state on anatase (101) surface has a lower energy than on rutile. Moreover, the longer distance between the adjacent adsorbate sites and the lower binding energy of H$_{2}$O allow for greater freedom for the adsorbed molecules to undergo structural relaxation in solvation of injected electrons. These differences might contribute to the higher photocatalytic activity of anatase compared with rutile. [1] K. Onda B. Li, J. Zhao, K.D. Jordan, J. Yang and H. Petek, \textit{Science} \textbf{308}, 1154 (2005).