Photocatalysis: Importance of the Aqueous Interface

The interaction of water on TiO₂ is crucial to elucidate photocatalytic water splitting for hydrogen fuel generation. Anatase and rutile are the most commonly occurring polymorphs of TiO₂. Although the interaction of water on rutile surfaces has been extensively probed, similar work on anatase is lacking.

Led by Prof. Geoff Thornton of the London Centre for Nanotechnology, a multi-institutional team of experimentalists and theoreticians from the UK, Spain, France and USA present a quantitative structural analysis of the interface between natural (reduced) anatase TiO₂ (101) (A₁₀₁) and water. Surface X-ray diffraction results for the room temperature interface with an ultra-thin water film (10 ±2 monolayers) and bulk water are discussed by reference to density functional theory calculations.

This study shows that room temperature aqueous interfaces with reduced A₁₀₁ have a mixture of molecular H₂O (25%) and OH_t (75%) bound to Ti_5c in the contact layer. OH_t formation from water dissociation is accompanied by the formation of OH_br. On the basis of previous calculations, the reduced state of the anatase will play a crucial role in the formation of this contact layer since it provides the excess electrons needed for dissociation. Upon water exposure to the as-prepared surface, the surface atoms contract towards the bulk and adopt a relatively more bulk-like appearance when compared to the as-prepared surface.

This study highlights the importance of the substrate environment in determining its reactivity. For A₁₀₁, the aqueous interface is reactive, whereas the UHV substrate is inert at room temperature. Since the aqueous interface is relevant in photocatalysis, it also highlights the importance of studies in realistic environments. The authors present an atomistic experimental and theoretical perspective of the A₁₀₁ / H₂O interface which will allow for future investigations pertaining to understanding the precise mechanism of photocatalysis on TiO₂. Ultimately this will drive future materials design and engineering to optimise the photocatalytic process on TiO₂.

The full article in Journal of Physical Chemistry Letters: Water Dissociates at the Aqueous Interface with Reduced Anatase TiO₂ (101)