Surface Modification of Rutile TiO2 with Metal Chalcogenides for Hydrogen Evolution

Abstract

The hydrogen evolution reaction (HER) is a key reaction in water splitting and there is an intensive search for suitable HER catalysts that will promote this reaction and also show good stability, while not using precious metals. We have studied the surface modification of rutile TiO2 with metal chalcogenide nanoclusters for the promotion of the hydrogen evolution reaction (HER) using density functional theory corrected for on-site Coulomb interactions (DFT+U). Recently, metal chalcogenides have emerged as potential catalysts for the HER due to the favourable interaction of H at chalcogen sites, particularly those that are undercoordinated. In developing materials for HER the key descriptor has been the adsorption energy of hydrogen at the catalysts, with the target free energy being close to 0 eV. Analogous to noble metal loading, surface modification of TiO2 with nanoclusters of potential HER co-catalysts, based on earth-abundant materials, is an emerging strategy. This approach combines the desirable properties of a stable titania photocatalyst with active sites provided by the low-coordinated chalcogen ions of the supported clusters. Our models consist of M4X4 (M = Sn, Zn; X = S, Se) nanoclusters at the rutile (110) surface and we examine the Gibb’s free energy of H adsorption at the modified surface which is a useful material descriptor in assessing the performance of a HER catalyst. These calculations take into account changes in the entropy and the zero-point energies (ZPEs) of the reactants and products, which our results show are material specific. As expected, the interaction of H is more favourable at the titania surface and forms stable hydroxyls. However, after subsequent adsorption events the surface sites become saturated and H adsorption at chalcogen sites exhibit Gibb’s free energies in the active range, particularly for MS modifiers relative to MSe.