Atomistic understanding of extended defects in reduced tungsten oxide to guide the optimisation of properties for energy applications

Tungsten oxides are promising materials for a range of energy applications including water splitting for hydrogen production, rechargeable batteries for energy storage and thermoelectrics for energy generation (all technologies critical to addressing UN Sustainable Development Goal #7: Affordable and clean energy). Part of the interest in tungsten oxides is their rich and complex defect chemistry which provides considerable scope for tuning properties for such applications. In particular, reduction of tungsten oxide films has been observed to induce formation of unique extended defect structures which are correlated with significant changes in optical properties as well as electrical and thermal conductivity. However, a detailed atomistic understanding of the structure of these defects, their formation, and impact on electronic and thermal properties which could be used to guide materials optimisation is currently missing. The aim of this project is to deliver an understanding of the structure, electronic and thermal (phononic) properties of tungsten oxides containing a range of complex extended defects (line defects and grain boundary defects) controlled by processing (e.g., level of reduction) and choice of substrate. To achieve this, we will combine scanning transmission electron microscopy (which can provide atomically resolved images of defect structures) with complementary first principles materials modelling methods (density functional theory) which can provide predictive models of structure and associated electronic and thermal properties. By applying these techniques in a synergistic and tightly integrated manner we will uncover structure-property relationships at the atomic scale and identify design rules for optimising materials properties by defect and microstructure engineering. By addressing fundamental knowledge gaps concerning the properties of defects in reduced tungsten oxides this research will provide a solid foundation for future work aimed at materials optimisation for applications. In particular the prospect to engineer the electrical, optical and thermal properties by controlling these complex defect structures (e.g., via appropriate substrate selection or annealing procedure) holds significant promise. More fundamentally understanding these general issues is important for a far wider range of reducible oxides considered for energy applications. This collaborative project brings together strengths in first principles materials and defect modelling in the UK and advanced electron microscopy of defects in crystals in Japan. The research will benefit, as well as benefit from, the world-leading materials research communities in both countries as well as deliver impact internationally.