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

摘要

Large-area ($\sim$cm$^2$) films of vertical heterostructures formed byalternating graphene and transition-metal dichalcogenide(TMD) alloys areobtained by wet chemical routes followed by a thermal treatment at lowtemperature (300 $^\circ$C). In particular, we synthesized stacked graphene andW$_x$Mo$_{1-x}$S$_2$ alloy phases that were used as hydrogen evolutioncatalysts. We observed a Tafel slope of 38.7 mV dec$^{-1}$ and 96 mV onsetpotential (at current density of 10 mA cm$^{-2}$) when the heterostructurealloy is annealed at 300 $^o$C. These results indicate that heterostructureformed by graphene and W$_{0.4}$Mo$_{0.6}$S$_2$ alloys are far more efficientthan WS$_2$ and MoS$_2$ by at least a factor of two, and it is superior thanany other reported TMD system. This strategy offers a cheap and low temperaturesynthesis 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 after1000 cycles. Using density functional theory calculations, we found that thisenhanced hydrogen evolution in the W$_x$Mo$_{1-x}$S$_2$ alloys is mainly due tothe lower energy barrier created by a favorable overlap of the d-orbitals fromthe transition metals and the s-orbitals of H$_2$, with the lowest energybarrier occurring for W$_{0.4}$Mo$_{0.6}$S$_2$ alloy. Thus, it is now possibleto further improve the performance of the "inert" TMD basal plane via metalalloying, in addition to the previously reported strategies of creation ofpoint defects, vacancies and edges. The synthesis ofgraphene/W$_{0.4}$Mo$_{0.6}$S$_2$ produced at relatively low temperatures isscalable and could be used as an effective low cost Pt-free catalyst.