Single-layer MoS formation by sulfidation of molybdenum oxides in different oxidation states on Au(111).

The sulfidation of a MoO precursor into MoS is an important step in the preparation of catalysts for the hydrodesulfurization process that is widely utilized in oil refineries. Molybdenum oxides are also the most commonly used precursors for MoS growth in, e.g., the synthesis of novel two-dimensional materials. In the present study, we investigate the transformation of MoO into MoS on a model Au(111) surface through sulfidation in HS gas atmosphere using in situ scanning tunneling microscopy and X-ray photoemission spectroscopy. We find that progressive annealing steps of physical vapor deposited MoO powder allow us to control the stoichiometry and oxidation state of the precursor oxide. Subsequently, we investigate the sulfidation of the compounds ranging from pure low-oxygen Mo to fully oxidized MoO oxide sulfidation using two different methods. We find that the prerequisite for the efficient formation of MoS is that Mo stays in the highest Mo state before sulfidation, whereas the presence of the reduced MoO phase impedes the MoS growth. We also find that it is more efficient to form MoS by post-sulfidation of MoO rather than its reactive deposition in HS gas, which leads to rather stable amorphous oxysulfide phases.