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

Tuning the electrocatalytic properties of MoS₂ layers can 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 electrodes by 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 evidenced by atomic force microscopy and scanning tunneling microscopy. This morphology of the MoS₂ layers at the nanoscale induces some specific features in the Raman and photoluminescence spectra compared to exfoliated or microcrystalline MoS₂ layers. Moreover, the S-vacancy content in the layers can be tuned during CVD growth by using Ar/H₂ mixtures as a carrier gas. Detailed optical microtransmittance and microreflectance spectroscopies, micro-Raman, and X-ray photoelectron spectroscopy measurements with sub-millimeter spatial resolution show that the obtained samples present an excellent homogeneity over areas in the cm² range. The electrochemical and photoelectrochemical properties of these MoS₂ layers were investigated using electrodes with relatively large areas (0.8 cm²). The prepared MoS₂ cathodes show outstanding Faradaic efficiencies as well as long-term stability in acidic solutions. In addition, we demonstrate that there is an optimal number of S-vacancies to improve the electrochemical and photoelectrochemical performances of MoS₂.