MoS₂ Photoelectrodes for Hydrogen Production: Tuning the S-Vacancy Content in Highly Homogeneous Ultrathin Nanocrystals

Tuning the electrocatalytic properties of MoS2 layerscan be achieved through different paths, such as reducing their thickness,creating edges in the MoS2 flakes, and introducing S-vacancies.We combine these three approaches by growing MoS2 electrodesby using a special salt-assisted chemical vapor deposition (CVD) method.This procedure allows the growth of ultrathin MoS2 nanocrystals(1-3 layers thick and a few nanometers wide), as evidencedby atomic force microscopy and scanning tunneling microscopy. Thismorphology of the MoS2 layers at the nanoscale inducessome specific features in the Raman and photoluminescence spectracompared to exfoliated or microcrystalline MoS2 layers.Moreover, the S-vacancy content in the layers can be tuned duringCVD growth by using Ar/H-2 mixtures as a carrier gas. Detailedoptical microtransmittance and microreflectance spectroscopies, micro-Raman,and X-ray photoelectron spectroscopy measurements with sub-millimeterspatial resolution show that the obtained samples present an excellenthomogeneity over areas in the cm(2) range. The electrochemicaland photoelectrochemical properties of these MoS2 layerswere investigated using electrodes with relatively large areas (0.8cm(2)). The prepared MoS2 cathodes show outstandingFaradaic efficiencies as well as long-term stability in acidic solutions.In addition, we demonstrate that there is an optimal number of S-vacanciesto improve the electrochemical and photoelectrochemical performancesof MoS2.