Low-temperature Synthesis of Heterostructures of Transition Metal Dichalcogenide Alloys (W_xMo_1-xS₂) and Graphene with Superior Catalytic Performance for Hydrogen Evolution

Large-area (∼cm²) films of vertical heterostructures formed by alternating graphene and transition-metal dichalcogenide (TMD) alloys are obtained by wet chemical routes followed by a thermal treatment at low temperature. In particular, we synthesized stacked graphene and W_xMo_1-xS₂ alloy phases that were used as hydrogen evolution catalysts. We observed a Tafel slope of 38.7 mV dec^-1 and 96 mV onset potential (at current density of 10 mA cm^-2) when the heterostructure alloy was annealed at 300 °C. These results indicate that heterostructures formed by graphene and W_0.4Mo_0.6S₂ alloys are far more efficient than WS₂ and MoS₂ by at least a factor of 2, and they are superior compared to other reported TMD systems. This strategy offers a cheap and low temperature synthesis alternative able to replace Pt in the hydrogen evolution reaction (HER). Furthermore, the catalytic activity of the alloy is stable over time, i.e., the catalytic activity does not experience a significant change even after 1000 cycles. Using density functional theory calculations, we found that this enhanced hydrogen evolution in the W_xMo_1-xS₂ alloys is mainly due to the lower energy barrier created by a favorable overlap of the d-orbitals from the transition metals and the s-orbitals of H₂; with the lowest energy barrier occurring for the W_0.4Mo_0.6S₂ alloy. Thus, it is now possible to further improve the performance of the "inert" TMD basal plane via metal alloying, in addition to the previously reported strategies such as creation of point defects, vacancies and edges. The synthesis of graphene/W_0.4Mo_0.6S₂ produced at relatively low temperatures is scalable and could be used as an effective low cost Pt-free catalyst.