Small polarons and the Janus nature of $\text{TiO}_\text{2}(110)$

TL;DR

This study uses density functional theory to explore how small polarons on rutile TiO2(110) surfaces influence water adsorption, dissociation, and hydrogen bonding, revealing complex effects on surface chemistry relevant to catalysis and photovoltaics.

Contribution

It provides a detailed theoretical analysis of the multifaceted role of surface polarons on water interactions with rutile TiO2(110), a topic not previously well understood.

Findings

Polarons suppress water dissociation at polaronic sites.

Polarons facilitate water dissociation at non-polaronic sites.

Polarons strengthen hydrogen bonds, affecting water structure.

Abstract

Polarons are ubiquitous in many semiconductors and have been linked with conductivity and optical response of materials for photovoltaics and heterogeneous catalysis, yet how surface polarons influence adsorption remains unclear. Here, by modelling the surface of rutile titania using density functional theory, we reveal the effect of small surface polarons on water adsorption, dissociation, and hydrogen bonding. On the one hand the presence of such polarons significantly suppresses dissociation of water molecules that are bonded directly to polaronic sites. On the other hand, polarons facilitate water dissociation at certain non-polaronic sites. Furthermore, polarons strengthen hydrogen bonds, which in turn affects water dissociation in hydrogen bonded overlayer structures. This study reveals that polarons at the rutile surface have complex, multi-faceted, effects on water adsorption, dissociation and hydrogen bonding, highlighting the importance of polarons on water structure and dynamics on such surfaces. We expect that many of the physical properties of surface polarons identified here will apply more generally to surfaces and interfaces that can host small polarons, beyond titania.