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

TL;DR

This study uses advanced quasiparticle techniques to accurately assign electronic levels in methanol on TiO2(110), clarifying surface chemistry and explaining photocatalytic activity differences.

Contribution

It provides a novel assignment of electronic levels to specific surface species and introduces a semi-quantitative model for predicting quasiparticle energies at interfaces.

Findings

Frontier levels correspond to chemisorbed methanol, not decomposition products.

The HOMO of methoxy is closer to the valence band, explaining higher photocatalytic activity.

Developed a semi-quantitative model for quasiparticle energy prediction.

Abstract

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