Self-ordering water molecules at TiO₂ interfaces: Advances in structural classification

In the five-decade search for efficient photocatalysts to convert natural sunlight into hydrogen via photoelectrochemical (PEC) dissociation of water, the underlying physics and chemistry of PEC processes taking place at metal-oxide photocatalysts remains relatively poorly understood and is an active area of research by both theorists and experimentalists. This is surely the case for water structuring at metal-oxide surfaces, including their "self-ordering." In this work, we apply classical molecular-dynamics techniques to investigate and classify the structure and ordering of water layers at two TiO₂ surfaces-anatase ⟨101⟩ and rutile ⟨110⟩. We are interested in identifying and classifying layers using local order parameters to distinguish the layered-water superstructure from bulk-like water configurations as observed in liquid water and common ice polymorphs. In particular, we look for the formation of regions with reduced molecular mobility and assess whether they are "ice-like," as has been proposed in recent interpretations in the literature, or, instead, how these interfacial-water structures might be otherwise described. We leverage quantitative and order-parameter analysis techniques to categorize the structural properties of layers of water molecules formed and compare them to both cubic and hexagonal polytypes of bulk ice I, as well as bulk liquid water. In doing so, we propose a general structural recognition/classification framework suitable for identifying and describing molecules at any condensed-state-water interface.