Single-layer MoS₂ formation by sulfidation of molybdenum oxides in different oxidation states on Au(111)

The sulfidation of a MoO₃ precursor into MoS₂ 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 MoS₂ growth in, e.g., the synthesis of novel two-dimensional materials. In the present study, we investigate the transformation of MoO_x into MoS₂ on a model Au(111) surface through sulfidation in H₂S gas atmosphere using in situ scanning tunneling microscopy and X-ray photoemission spectroscopy. We find that progressive annealing steps of physical vapor deposited MoO₃ 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 MoO₃ oxide sulfidation using two different methods. We find that the prerequisite for the efficient formation of MoS₂ is that Mo stays in the highest Mo⁶⁺ state before sulfidation, whereas the presence of the reduced MoO_x phase impedes the MoS₂ growth. We also find that it is more efficient to form MoS₂ by post-sulfidation of MoO_x rather than its reactive deposition in H₂S gas, which leads to rather stable amorphous oxysulfide phases.