Two-dimensional FeS2-encapsulated Au: a quasi-epitaxial heterojunction for synergistic catalytic activity under photoelectrocatalytic water reduction

We report the development of two-dimensional pyrite-type FeS2 nanosheets with tunable bulk size controlled by sulfide-mediated thermal diffusion under inert atmosphere. We also synthesized Au heterostructures that are encapsulated by FeS2, where the sulfide concentration and annealing time lead to eccentric aggregation of the FeS2. p-type Si coupled with the as-prepared catalysts exhibited enhanced photocathodic performance under an acidic medium. With FeS2, the optimized photocathodic performance was obtained by achieving a geometric photocurrent of 10 mA cm(-2) at a positive applied potential of 0.08 V (vs. reversible hydrogen electrode (RHE)). As the size of the FeS2 increased, the cathodic photocurrent also increased, which indicates a longer carrier lifetime and slow charge recombination on the catalyst surface. It was found that the direction of the FeS2 growth is aligned with the equivalent (111) plane of the Au, resulting in a unique Au-FeS2 heterostructure. The optimized heterostructure photocatalyst showed highly improved cathodic performance with an anodic shift of the cell voltage by 0.26 V (vs. RHE) compared with bare FeS2. This study demonstrates the synergistic effect of the generation of hot electrons on Au and fast charge flow across the FeS2 layer, which facilitates fast electron-hole separation and enhances the hydrogen evolution reaction.