The structure of water on rutile TiO 2 (110) for applications in solar hydrogen production: towards a predictive model u
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The structure of water on rutile TiO2 (110) for applications in solar hydrogen production: towards a predictive model using hybrid-exchange density functional theory M. Patel 1 , G. Mallia 1 , and N. M. Harrison 1,2 1
Thomas Young Centre, Department of Chemistry, Imperial College London, UK 2 STFC Daresbury Laboratory, Daresbury, Warrington, UK
ABSTRACT Periodic hybrid-exchange density functional theory (DFT) simulations are used to develop a predictive model of the structure of water on the rutile TiO2 (110) surface (Θ ≤ 1 ML). A description of the adsorbed species is given: dissociated water molecules and either mixed or dissociative dimers. The behaviour of the adsorbates is rationalised by considering both direct intermolecular and surface-mediated interactions. Some of these results are then compared with those from water adsorption on the rutile SnO2 (110) surface, isostructural to TiO2 (110). Lastly, the electronic structure of the surface in contact with monolayer water (Θ = 1 ML) reveals the contributions of adsorbate states involved in the photocatalytic reaction that controls the water oxidation process. INTRODUCTION Current challenges in the hydrogen economy are the storage and transport of hydrogen, the efficient conversion of hydrogen into electrical energy in fuel cells, and the carbonfree and economically-viable generation of hydrogen. Photoelectrochemical water splitting over semiconductor materials such as titanium dioxide (TiO2 ) provides a carbon-free renewable route to hydrogen production [1]. The reaction mechanisms that occur at the crystal surfaces are debated [2, 3]: identifying and understanding them in detail could potentially lead to the suggestion of engineering and design rules for more efficient systems. TiO2 is a transition metal oxide that adopts a variety of crystal structures, three of which are rutile, anatase and brookite. The (110) surface of rutile TiO2 is the most stable [4], and is considered to be a quintessential model metal oxide system for the study of water chemistry. The question of whether water molecules are adsorbed molecularly or dissociatively on the rutile TiO2 (110) surface is a continuing issue, as well as the conditions under which each mode of adsorption takes place. The majority of experimental measurements support the concept that molecular adsorption dominates in the first layer of water (Θ ≤ 1 ML) on nearly-perfect surfaces at low temperatures (< 350 K) [5]. Various levels of theory have been applied for periodic and cluster simulations of water adsorption on this surface. Theoretical studies mostly predict that the dissociation of water is energetically favoured on the defect-free TiO2 (110) surface at all coverages up to one monolayer (1 ML) [6]. Many of these simulations are based on models characterised by constraints (either point group or translational symmetry): neighbouring adsorbates are restricted to be at fixed orientation and separation. The important role of intermolecular interactions has been emphasised, suggesting a more complex pic-
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