Structure of a model TiO2 photocatalytic interface

TL;DR

This study reveals the atomic-level structure of liquid water interacting with TiO2(110), highlighting hydroxyl and water layers, and proposes a mechanism involving O2 and H2O adsorption for hydroxyl formation, advancing understanding of photocatalytic processes.

Contribution

It provides the first atomic-level structural characterization of liquid water on TiO2(110) and suggests a new mechanism for hydroxyl overlayer formation involving O2 and H2O adsorption.

Findings

Hydroxyl molecules form an ordered overlayer on TiO2(110).

Molecular water exists in the second layer above hydroxyls.

A proposed mechanism involves mixed O2 and H2O adsorption on defect sites.

Abstract

The interaction of water with TiO2 is crucial to many of its practical applications, including photocatalytic water splitting. Following the first demonstration of this phenomenon 40 years ago there have been numerous studies of the rutile single-crystal TiO2(110) interface with water. This has provided an atomic-level understanding of the water-TiO2 interaction. However, nearly all of the previous studies of water/TiO2 interfaces involve water in the vapour phase. Here, we explore the interfacial structure between liquid water and a rutile TiO2(110) surface pre-characterized at the atomic level. Scanning tunnelling microscopy and surface X-ray diffraction are used to determine the structure, which is comprised of an ordered array of hydroxyl molecules with molecular water in the second layer. Static and dynamic density functional theory calculations suggest that a possible mechanism for formation of the hydroxyl overlayer involves the mixed adsorption of O2 and H2O on a partially defected surface. The quantitative structural properties derived here provide a basis with which to explore the atomistic properties and hence mechanisms involved in TiO2 photocatalysis.