Richard Robinson

(Metastable Nanoparticle Surfaces for Tunable Catalysis Systems) The need for inexpensive high-activity materials in electrocatalytic applications such as water splitting has driven substantial research into the surface chemistry of non-noble metal catalysts, especially metal oxides. Great progress has been achieved in optimization of oxide electroactivity by the use of mixed metal oxides, which achieve the necessary physicochemical properties via controlled substitution of cation sites. Despite the obvious importance of the anionic partner in determining the catalyst electronic and surface structure, knowledge of the effects of anionic substitution in electrocatalysis is far less certain. In this talk I overview the creation of metastable phases in nanoparticles through anion exchange and show that, by controlling the relative proportions (e.g., oxide and sulfide), the activity of nanoparticle electrocatalysts can be substantially increased. For instance, in the cobalt oxide system we find that lightly doped CoOxS0.18 exhibits a metastable, S-substituted CoO structure and is 2-3 times more active in the hydrogen evolution reaction than either end-member of the oxide-sulfide series. We explain this result using density functional theory calculations, which reveal that S-substitution of the CoO surface increases the bonding strength of H to the surface, and reaches an optimum value at a low doping level, consistent with our observations. These results show that metastable nanoparticle catalysts are accessible and provide an important strategy for controlling the physicochemical properties of electrocatalyst surfaces for energy harvesting.