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 (BBO(V)) concentrations of approximately 10% ML (1 ML = 5.2 x 10(14) cm(-2)) were imaged using scanning tunneling microscopy (STM) before and after dosing H2O at ambient temperature (approximately 300 K). Atomically resolved images confirm that H2O adsorbs dissociatively on the BBO(V) sites, producing two hydroxyl species, one positioned at BBO(V) and denoted OH(V) and the other, denoted OH(B), formed by protonation at either of the two nearest-neighbor bridge-bonded oxygen atoms. Hydrogen hopping along the [001] direction is observed at ambient temperature, with a strong preference for OH(B) (approximately 10x) hydrogen motion. This powerful imbalance demonstrates the inequality of OH(V) and OH(B) and suggests differences in their charge and/or binding configuration.