A comprehensive understanding of water photooxidation on Ag3PO4 surfaces

Abstract

The oxygen evolution reaction (OER) is known to be the bottleneck of water-splitting. Ag₃PO₄ is a highly efficient visible light photocatalyst for dye degradation and water oxidation to O₂, with a higher OER rate than BiVO₄ and WO₃. Despite extensive studies on Ag₃PO₄, the surface properties including surface electronic states, reaction sites and mechanisms of OER on Ag₃PO₄ surfaces are not clear at present. Herein, we reported a comparative first-principles density functional theory study of the bulk, surface properties and the mechanism of OER on the three primary low index facets of Ag₃PO₄: (100), (110) and (111). We revealed for the first time that the rate-limiting step of the OER on Ag₃PO₄ (100), (110) and (111) surfaces is the dehydrogenation of HO* (HO* → O* + H⁺ + e⁻), which is different from most reported metal oxides and nitrides like TiO₂ and g-C₃N₄. The OER process on the (100) surface tends to proceed by following a different mechanism as that on the (110) and (111) surfaces. The illumination of the Ag₃PO₄ (100), (110), and (111) surfaces with solar light provides enough overpotential for the OER to proceed spontaneously. In particular, the free energy change of removal of the first proton from water on the Ag₃PO₄ (111) surface is much lower than that on (100) and (110) surfaces, giving an explanation for the experimentally observed higher catalytic activity of the (111) surface. The exposed phosphorus atoms on the Ag₃PO₄ (111) surface promote the dehydrogenation of H₂O and suppress the formation of mid-gap states. Our results are profound for understanding the underlying mechanism of the photocatalytic water oxidation process occurring on Ag₃PO₄ surfaces, and serve as a foundation for developing new high-performance Ag₃PO₄ based photocatalysts for water splitting and organic contaminant decomposition.