Surface Modification of TiO2 with Metal Oxides for Water Splitting

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 studies predict that alkaline earth metal oxide modified TiO2 can enhance water activation and the free energies of oxygen evolution reaction (OER).DFT simulations were performed on extended rutile (110) and anatase (101) surfaces modified with alkaline earth oxide (AEO) nanoclusters of composition (MO)4 and (MO)12 to explore coverage and dispersion effects in the nanocluster modification of TiO2 1. We use the VASP5.4 code with a cut off energy of 400 eV, the PW91 approximation to the exchange-correlation functional and the DFT+U approach to describe the partially occupied 3d states in reduced TiO2, with U = 4.5 eV.We use models of rutile (110) modified with MgO nanoclusters of composition (MgO)4 and (MgO)12 to model dispersion/coverage effects. The nanoclusters bind strongly the rutile surface, forming new interfacial bonds. Computed formation energies of oxygen vacancies show that these heterostructures are easily reducible, with reductions of ca. 2 eV in the cost of forming oxygen vacancies, with electron localization on the TiO2 substrate. The computed free energy for the OER steps , using the standard computational model for structures without and with surface hydroxyls shows that moderating stability of the *OH intermediate in the water oxidation steps, reduces the overpotential compared to pure rutile (110) and competitive with hematite. The most favourable water adsorption mode for OER is at the MgO-rutile interface.Work on other surface modified TiO2 materials includes CeO2 and MnOx, where these are also reducible and activate water to produce hydroxyls. Comparison is made with experimental results from collaborators.