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