Determining the binding strength of phenolic anchoring groups on hydrated WO₃ surfaces

WO₃ has many properties that make it a promising anode material for photoelectrochemical water splitting. However, its practical use is limited by its moderate efficiency. Anchoring a co-catalyst to the WO₃ surface can be a way of substantially improving in particular the kinetics of the oxygen evolution reaction, and thus the overall efficiency. This requires identifying anchoring groups that reliably bind to WO₃ in the presence of water. Based on first-principles calculations we here determine whether catechol, resorcinol or hydroquinone bind to a hydrated R45° (001) WO₃ surface. Furthermore, we calculate the binding to clusters of WO₃ to study the effects of non-ideal surface structures, and we gain insight into the electronic interaction between a ligand and the cluster. We discuss different levels of approximation to calculate the anchoring of a molecule to a surface. For all adsorbates studied here, we find that dispersive and especially covalent binding to a hydrated surface is in energetic competition with surrounding water molecules. However, structural defects in the WO₃ surface have the potential to enable covalent binding, for which strong electronic interaction is to be expected. The theoretical results for catechol are in agreement with an experimental Raman and contact angle analysis with a porous WO₃ photoanode.