Size effects and odd-even effects in MoS₂ nanosheets: first-principles studies

Molybdenum disulfide (MoS₂) nanostructures have been widely used as catalysts in the petroleum refinery industry for hydrodesulfurization process, in which the sulfur vacancies play a critical role in determining the catalytic activity. Here we report size effects and odd-even effects on the formation of sulfur vacancies in the triangular MoS₂ nanosheets using first-principles calculations. By modeling four types of edge structures of MoS₂ nanosheets, S-terminated edges are found to be energetically more favorable than Mo-terminated edges, and are then selected for studying energetics of sulfur vacancies. Two types of sulfur dimer vacancies at center (V_S@Cen) and at corner (V_S@Cnr) of the edges of S-terminated MoS₂ nanosheets are modeled, respectively. Our results reveal a strong odd-even effect in the formation of sulfur dimer vacancies, particularly for small MoS₂ nanosheets, in terms of the size of nanosheets that is defined by the number of Mo atoms on the edge. V_S@Cen dimer vacancy has a low formation energy at even-number but high formation energy at odd-number, while V_S@Cnr dimer vacancy exhibits a complete opposite trend. These results indicate that small MoS₂ nanosheets can exhibit unique materials properties for catalytic applications.