Level alignment of a prototypical photocatalytic system: Methanol on TiO2(110)

摘要

Photocatalytic and photovoltaic activity depends on the optimal alignment ofelectronic levels at the molecule/semiconductor interface. Establishing levelalignment experimentally is complicated by the uncertain chemical identity ofthe surface species. We address the assignment of the occupied and emptyelectronic levels for the prototypical photocatalytic system of methanol on arutile TiO2 (110) surface. Using many-body quasiparticle (QP) techniques weshow that the frontier levels measured in ultraviolet photoelectron and twophoton photoemission spectroscopy experiments can be assigned with confidenceto the molecularly chemisorbed methanol, rather than its decomposition product,the methoxy species. We find the highest occupied molecular orbital (HOMO) ofthe methoxy species is much closer to the valence band maximum, suggesting whyit is more photocatalytically active than the methanol molecule. We develop ageneral semi-quantitative model for predicting many-body QP energies based onthe appropriate description of electronic screening within the bulk, molecularor vacuum regions of the wavefunctions at molecule/semiconductor interfaces.