Low-temperature Synthesis of Heterostructures of Transition Metal Dichalcogenide Alloys (W_<i>x</i>Mo_1–<i>x</i>S₂) 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_<i>x</i>Mo_1–<i>x</i>S₂ 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>i.e.</i>, 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_<i>x</i>Mo_1–<i>x</i>S₂ 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 <i>via</i> 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