Direct visualization of defect-mediated dissociation of water on TiO2(110)

The chemistry of metal oxide surfaces has long been thought to be dominated by reactions involving defects^1,2. These are minority sites such as oxygen vacancies. Thus far, it has proved difficult to obtain direct experimental evidence to support this idea, although some progress has been made^3,4,5. Here, we use the scanning tunnelling microscope (STM) to image the reaction of water molecules with bridging-oxygen vacancies on a model oxide surface, rutile TiO₂(110). In a form of single-molecule chemistry, individual oxygen vacancies are observed being transformed into OH species as a water molecule dissociates in the vacancy. We use the STM tip to selectively desorb individual H atoms, whilst leaving the vacancies intact. This allows us to distinguish between vacancies and OH, which have a similar appearance in STM. In a very clear way, these results validate the view that defects can play a key role in oxide surface reactions.