Low temperature synthesis of heterostructures of transition metal dichalcogenide alloys (WxMo1-xS2) and graphene with superior catalytic performance for hydrogen evolution

TL;DR

This study presents a low-temperature, scalable method to synthesize graphene and W$_x$Mo$_{1-x}$S$_2$ alloy heterostructures that exhibit superior catalytic performance for hydrogen evolution, outperforming traditional TMD catalysts and replacing platinum.

Contribution

The paper introduces a novel low-temperature synthesis approach for graphene/TMD alloy heterostructures with enhanced catalytic activity for HER, demonstrating improved efficiency and stability over existing catalysts.

Findings

Heterostructures show a Tafel slope of 38.7 mV/dec and onset potential of 96 mV at 10 mA/cm$^2$

W$_{0.4}$Mo$_{0.6}$S$_2$ alloy exhibits at least twice the efficiency of WS$_2$ and MoS$_2$

Catalytic activity remains stable after 1000 cycles.

Abstract

Large-area ($\sim$cm$^2$) 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 (300 $^\circ$C). In particular, we synthesized stacked graphene and W$_x$Mo$_{1-x}$S$_2$ 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 is annealed at 300 $^o$C. These results indicate that heterostructure formed by graphene and W$_{0.4}$Mo$_{0.6}$S$_2$ alloys are far more efficient than WS$_2$ and MoS$_2$ by at least a factor of two, and it is superior than any other reported TMD system. 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$_x$Mo$_{1-x}$S$_2$ 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 H$_2$, with the lowest energy barrier occurring for W$_{0.4}$Mo$_{0.6}$S$_2$ 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 of creation of point defects, vacancies and edges. The synthesis of graphene/W$_{0.4}$Mo$_{0.6}$S$_2$ produced at relatively low temperatures is scalable and could be used as an effective low cost Pt-free catalyst.