MoS 2 Photoelectrodes for Hydrogen Production: Tuning the S-Vacancy Content in Highly Homogeneous Ultrathin Nanocrystals.

Tuning the electrocatalytic properties of MoS 2 layers can be achieved through different paths, such as reducing their thickness, creating edges in the MoS 2 flakes, and introducing S-vacancies. We combine these three approaches by growing MoS 2 electrodes by using a special salt-assisted chemical vapor deposition (CVD) method. This procedure allows the growth of ultrathin MoS 2 nanocrystals (1-3 layers thick and a few nanometers wide), as evidenced by atomic force microscopy and scanning tunneling microscopy. This morphology of the MoS 2 layers at the nanoscale induces some specific features in the Raman and photoluminescence spectra compared to exfoliated or microcrystalline MoS 2 layers. Moreover, the S-vacancy content in the layers can be tuned during CVD growth by using Ar/H 2 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 cm 2 range. The electrochemical and photoelectrochemical properties of these MoS 2 layers were investigated using electrodes with relatively large areas (0.8 cm 2 ). The prepared MoS 2 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 MoS 2 .