Au-MoS₂ Hybrids as Hydrogen Evolution Electrocatalysts

Core-shell nanoparticles provide a unique morphology to exploit electronic interactions between dissimilar materials, conferring upon them new or improved functionalities. MoS₂ is a layered transition-metal disulfide that has been studied extensively for the hydrogen evolution reaction (HER) but still suffers from low electrocatalytic activity due to its poor electronic conductivity. To understand the fundamental aspects of the MoS₂-Au hybrids with regard to their electrocatalytic activity, a single to a few layers of MoS₂ were deposited over Au nanoparticles via a versatile procedure that allows for complete encapsulation of Au nanoparticles of arbitrary geometries. High-resolution transmission electron microscopy of the Au@MoS₂ nanoparticles provides direct evidence for the core-shell morphology and also reveals the presence of morphological defects and irregularities in the MoS₂ shell that are known to be more active for HER than the pristine MoS₂ basal plane. Electrochemical measurements show a significant improvement in the HER activity of Au@MoS₂ nanoparticles relative to freestanding MoS₂ or Au-decorated MoS₂. The best electrochemical performance was demonstrated by the Au nanostars - the largest Au core employed here - encapsulated in a MoS₂ shell. Density-functional theory calculations show that charge transfer occurs from the Au to the MoS₂ layers, producing a more conductive catalyst layer and a better electrode for electrochemical HER. The strategies to further improve the catalytic properties of such hybrid nanoparticles are discussed.