First Principles Modelling of Surface Modified TiO2 for Water Activation and Oxygen Evolution

Abstract

The photocatalytic oxidation of water to molecular oxygen is a key step in the water splitting reaction. The development of robust catalysts with high efficiency in this half-reaction is crucial for achieving technological application of water splitting. Density Functional Theory simulations can provide key insights into potential catalysts for this reaction. In this contribution we describe our work on modelling of surface modified TiO2 focusing on water activation and the oxygen evolution mechanism on these heterostructures. Rutile and anatase TiO2 are modified with nanostructures of metal oxides, namely alkaline earth oxides, cerium oxide and manganese oxides; these are also being prepared and characterised at collaborators laboratories in the framework of an EU H2020 M-ERA.net 2 project RATOCAT.We find that nanoclusters of these oxides with different compositions and adsorption modes are strongly adsorbed and can induce a red shift in light absorption towards the visible region of the electromagnetic spectrum. A simple model of the photoexcited state allows the localisation of electrons and holes produce by excitation to be determined and understood. Typically, DFT studies neglect key issues issues such as reducibility of the oxide or the role of adsorbed hydroxyls on the properties and the water activation processes in OER. We include a detailed analysis of reduction of the heterostructures, in which we find that loss of oxygen is generally favourable. In addition, the surface hydroxylation (through adsorption and dissociation of water) and the cluster hydroxylation are examined. We find that a single water molecule will dissociate at the supported oxide nanocluster, with large energy gain that indicates hydroxyl groups will be present and must be accounted for.With a suitable model of a hydroxylated oxide heterostructure, including oxygen vacancies, we explore water activation and find for those structures with hydroxyls and single oxygen vacancies, the water adsorption is dissociative but with moderate energy gains. Too high an energy gain and the subsequent OER steps are not favourable. We determine the step with the highest free energy cost and use this to estimate an overpotential for these heterostructures as well as determining key aspects of these structures that drive favourable OER.Support for this work from Science Foundation Ireland through the H2020 M-ERA.net 2 co-fund program, Grant Number 16/M-ERA/3418 is acknowledged. We also acknowledge access to computing resources through the SFI funded Irish Centre for High End Computing, ICHEC.