Origin of the Adsorption-State-Dependent Photoactivity of Methanol on TiO2(110)

Methanol has long been used as a hole scavenger in photocatalysis to improve charge separation. Although the prototypical methanol/TiO2(110) system shows salient adsorption-state-dependent photoactivity, the underlying reason still remains unclear. Through surface-sensitive ultraviolet photoelectron spectroscopy measurements, we found that methoxy anions and methanol strengthen and flatten the originally upward band bending of TiO2(110), respectively. Intensified upward band bending leads to charge separation in the depletion region and hole accumulation at the surface. Furthermore, density functional theory calculations show that the hole transfer at the CH3O–/TiO2 interface is thermodynamically allowed and that on CH3OH/TiO2 is unfavorable. The improved charge separation together with the allowed interfacial hole transfer is found to be responsible for the superior photoactivity of the methoxy anion over methanol on TiO2. These results reconcile the existing contradiction in the understanding of the charge transfer at the CH3O–/TiO2 interface based on the photoemission measured energy levels. Our work suggests that the redox potential level, rather than the vertical energy (for example, the HOMO of adsorbates) measured by spectroscopy, should be used to evaluate the possibility of the heterogeneous interfacial charge transfer.