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

Abstract

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.