Role of introduced Se element and induced anion vacancies in Mo(SSe)/G van der Waals heterostructure for enhanced hydrogen evolution reaction.

The Gibbs free energy of hydrogen adsorption at the edge of molybdenum disulfide (MoS) is close to that of Pt, meaning that MoS is the best candidate to replace Pt-based materials. However, easy agglomeration between layers to mask active sites, lack of catalytic activity in the basal planes, and poor electronic conductivity make MoS exhibit dissatisfactory hydrogen evolution reaction (HER) catalytic performance. Here, we successfully construct a van der Waals heterostructure stacked alternately with Mo(SSe) and graphene (Mo(SSe)/G) to enhance its catalytic ability. The introduction of Se into MoS and the thermal treatment induce the sample to generate more anion vacancies. Theoretical and experimental results demonstrate the constructed van der Waals heterostructure, the introduced Se element, and the increased anion vacancies are in favor of promoting the number of active sites and improving the electronic conductivity of the catalyst. Therefore, Mo(SSe)/G exhibits superior HER catalytic performance (the overpotentials of 137 mV and 136 mV at a current of 10 mA cm) and long-term stabilities (>90 h and 140 h at a current density of 20 mA cm) in both acidic and alkaline media.