Imaging Water Dissociation on TiO2(110): Evidence for Inequivalent Geminate OH Groups

Identical regions of partially reduced TiO2(110) surfaces with bridge-bonded oxygen vacancy (BBOV) concentrations of ∼ 10 % ML (1 ML = 5.2?1014 cm-2) were imaged using scanning tunneling microscopy (STM) before and after dosing H2O at ambient temperature (∼ 300 K). Provided the H2O coverage is less than the BBOV concentration, atomically resolved images confirm that the titration of individual BBOV by dissociative adsorption of H2O is strongly favored. The products are two hydroxyl species, one positioned at BBOV and denoted OHV and the other, denoted OHB, formed by protonation at either of the two nearest-neighbor bridge-bonded oxygen atoms. Proton hopping along the [001] direction is observed at ambient temperature, with a strong preference for OHB (∼ 10x) proton motion. This powerful imbalance demonstrates the inequality of OHV and OHB and suggests differences in their charge and/or binding configuration