Coverage Dependence of Methanol Dissociation on TiO<sub>2</sub>(110)

Although the photochemistry of methanol on TiO<sub>2</sub>(110) has been widely investigated as a prototypical model of the photocatalytic reaction of organic molecules, the most fundamental question of the adsorption state of methanol on TiO<sub>2</sub>(110) is still unclear. We have investigated the adsorption of methanol on TiO<sub>2</sub>(110) using sum frequency generation vibrational spectroscopy (SFG-VS) and density functional theory (DFT) calculations. The SFG results indicate the dissociation of methanol is highly dependent on the coverage. The DFT calculations suggest that the methanol prefers the partially dissociated structure at low coverage, whereas the second layer methanol, which is hydrogen-bonded to the bridge-bonded oxygen site, largely blocks the dissociation of the first layer methanol. Our results not only resolves a long-standing debate regarding the adsorption state of methanol on TiO<sub>2</sub>(110) but also provides a detailed insight into the adsorption structure and sheds light on the photochemistry on this surface at the molecular level.