Phase-segregated NiPx@FePyOz core@shell nanoparticles: ready-to-use nanocatalysts for electro- and photo-catalytic water oxidation through in situ activation by structural transformation and spontaneous ligand removal

Abstract

The high overpotential of the oxygen evolution reaction is a critical issue to be overcome to realize efficient overall water splitting and enable hydrogen generation powered by sunlight. Homogeneous and stable nanoparticles (NPs) dispersed in solvents are useful as both electrocatalysts and cocatalysts of photocatalysts for the electro- and photo-catalytic oxygen evolution reaction, respectively, through their adsorption on various electrode substrates. Here, phase-segregated NiP_x@FeP_yO_z core@shell NPs are selectively synthesized by the reaction of Fe(CO)₅ with amorphous NiP_x seed-NPs. The NiP_x@FeP_yO_z NPs on conductive substrates exhibit higher electrocatalytic activity in the oxygen evolution reaction than those of other metal phosphide-based catalysts. The NiP_x@FeP_yO_z NPs can also be used as a cocatalyst of an anodic BiVO₄ photocatalyst to boost the photocatalytic water oxidation reaction. The excellent catalytic activity and high stability of the NiP_x@FeP_yO_z NPs without any post-treatments are derived from in situ activation through both the structural transformation of NiP_x@FeP_yO_z into mixed hydroxide species, (Ni, Fe)O_xH_y, and the spontaneous removal of the insulating organic ligands from NPs to form a smooth and robust (Ni, Fe)O_xH_y/substrate heterointerface during the oxygen evolution reaction.