A Unified Picture of Water Oxidation on Bare and Gallium Oxide- Covered Hematite from Density Functional Theory

Hematite is a promising catalyst for the photoelectrochemical water oxidation reaction, which however displays a low overall efficiency. To improve it, a systematic understanding of the underlying photocatalytic mechanisms is desirable but difficult to obtain by experimental techniques alone. Here, we have investigated the oxidation of water on the most stable terminations of the bare hematite, as well as in the presence of a monolayer of Ga2O3, by first-principles density functional theory-based methods. Although several surface terminations are very close in surface energy, they all yield a very similar overpotential of ∼0.8 V on the bare surface and ∼0.95 V on the Ga2O3-covered surface. Moreover, on all the relevant terminations, the overpotential-determining reaction step is the same, involving the dehydrogenation of a surface-adsorbed hydroxyl species. The reaction mechanism is largely independent of the atomistic details of the surface termination and crucially involves the formation of reaction intermediates involving lattice oxygen bound to adsorbed oxygen from water (O*−Os). It seems likely not only that different surface terminations coexist but also that they transform into one another during reaction conditions, even at the steady state. We also shed light on the important role of midgap states of hematite in the water oxidation cycle. In presence of the O*−Os species, midgap states that localize on this species can act as long-lived hole traps and fall prey to parasitic recombination processes. Without the O*−Os species, the surface states of hematite compete with the states on the active water oxidation species to attract the holes. The Ga2O3 layer passivates these competing surface states, making holes available at the active sites for water oxidation. Though Ga2O3-covered hematite has an overpotential higher than that of bare hematite, it nevertheless plays a very important role in making the holes available at the active site.