Operando characterization of an amorphous molybdenum sulfide nanoparticle catalyst during the hydrogen evolution reaction

Molybdenum sulfide structures, particularly amorphous MoS₃ nanoparticles, are promising materials in the search for cost-effective and scalable water-splitting catalysts. Ex situ observations show that the nanoparticles exhibit a composition change from MoS₃ to defective MoS₂ when subjected to hydrogen evolution reaction (HER) conditions, raising questions regarding the active surface sites taking part in the reaction. We tracked the in situ transformation of amorphous MoS₃ nanoparticles under HER conditions through ambient pressure X-ray photoelectron spectroscopy and performed density functional theory studies of model MoS_x systems. We demonstrate that, under operating conditions, surface sites are converted from MoS₃ to MoS₂ in a gradual manner and that the electrolytic current densities are proportional to the extent of the transformation. We also posit that it is the MoS₂ edge-like sites that are active during HER, with the high activity of the catalyst being attributed to the increase in surface MoS₂ edge-like sites after the reduction of MoS₃ sites.