Microscopic insights into the catalytic mechanisms of monolayer MoS₂ and its heterostructures in hydrogen evolution reaction

Exploring high-efficient catalysts for hydrogen evolution reaction (HER) has become very urgent for resolving the energy related issues. Recently, two-dimensional layered MoS₂ and its heterostructures with graphene or other traditional photocatalysts have presented great potentials for electrocatalytic and photocatalytic HER applications. On-site investigations of the atomic-scale structures and local electronic properties of the catalytically active sites are the key points for understanding the internal mechanisms, which however are hard to be achieved from the practical systems. Hereby, this review focuses on the recent progresses on the on-site scanning tunneling microscopy/spectroscopy investigations of the atomic structures and electronic properties of the ultrahigh-vacuum deposited and chemical vapor deposition (CVD) synthesized monolayer MoS₂ and MoS₂/graphene vertical stacks on the electrodes of Au(111) and Au foils. The correlations between the respective HER activities, edge types and edge electronic states are comparatively introduced. Secondly, this review also introduces the photocatalytic HER applications of CVD-grown MoS₂/WS₂ and WS₂/MoS₂ vertical stacks on Au foils, mainly considering of their type-II band alignments and the novel interlayer charge transfer behaviors. Finally, future research directions are also proposed for in-depth understanding of the catalytic mechanism, as well as for exploring more efficient HER catalysts.