The limits of ground-state water splitting on ZnO surfaces: A density functional theory study

Abstract

The water (H2O) splitting reactions towards hydrogen (H2) and oxygen (O2) production in the ground state are here investigated on ZnO catalyst surfaces by means of density functional theory simulations. To this end, the Zn-terminated and O-terminated (0001) and (0001̅) surfaces, respectively, and the non-polar (101̅0) and (112̅0) ones, have been investigated. The reaction thermodynamics has been analysed, being endergonic at normal conditions, and only exergonic at high temperatures and with high partial pressures of reactants. The adsorption of H2O, H2, O2, and reaction intermediates OH, O, and H, underlines the oxophylic character of Zn-terminated (0001) surface, as well as the H-phylic character of O-terminated (0001̅) surface, while non-polar surfaces display both O- and H-phylic centers. The adsorption and co-adsorption strengths and elementary steps energy barriers along the reaction path pinpoint the key reaction limiting steps. The H2 formation step has a prohibitive barrier of 4.91 eV on the (0001̅) surface, and a more moderate barrier of 2.33 and 1.83 eV for the non-polar (101̅0) and (112̅0) surfaces respectively. On the (0001) surface, the rate limiting step is O2 formation, with an energy barrier of 4.94 eV. Regardless of the surface, a higher affinity towards water would be a way to improve the reaction catalysis. The possible modification of surfaces to reduce the energy costs of the limiting steps are discussed, including the use of light-triggered ZnO catalyst, as well as the simultaneous presence of different surfaces to better split the different reaction steps in an energetically more efficient fashion.