In-Plane Heterojunctions Enable Multiphasic Two-Dimensional (2D) MoS₂ Nanosheets As Efficient Photocatalysts for Hydrogen Evolution from Water Reduction

Two-dimensional (2D) single-layer MoS₂ nanosheets are demonstrated as efficient photocatalysts for hydrogen evolution reaction (HER) from water reduction, thanks to specific in-plane heterojunctions constructed in the MoS₂ monolayer. These functional heterojunctions are formed among the different phases of chemically exfoliated MoS₂ monolayers: semiconducting 2H, metallic 1T, and quasi-metallic 1T′ phases. The proportion of the three MoS₂ phases can be systematically controlled via thermal annealing of the nanosheets. Interestingly, a volcano relationship is observed between the photocatalytic HER activity and the annealing temperature with an optimum activity obtained after annealing at 60 °C. First-principles calculations were integrated with experimental studies to shed light on the role of the multiphases of MoS₂ and reveal that optimum photocatalytic HER activity results from the formation of the in-plane heterojunctions between 1T′ MoS₂ and 2H MoS₂. Importantly, this facilitates not only balanced light absorption and charge generation by the 2H phase, efficient charge separation at the 1T′/2H interface, but also favorable HER over the basal sites of 1T′ MoS₂. Our work manifests how the confluence of the optical, electronic and chemical properties of 2D MoS₂ monolayers can be fully captured for efficient photocatalytic water reduction.