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

Molybdenum disulfide (MoS) nanostructures have been widely used as catalysts in the petroleum refinery industry for the hydrodesulfurization process, in which 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 MoS nanosheets using first-principles calculations. By modeling four types of edge structures of MoS 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 the center (V@Cen) and at the corner (V@Cnr) of the edges of S-terminated MoS nanosheets are modeled, respectively. Our results reveal a strong odd-even effect on the formation of sulfur dimer vacancies, particularly for small MoS nanosheets, in terms of the size of nanosheets that is defined by the number of Mo atoms on the edge. The V@Cen dimer vacancy has a low formation energy at an even-number but a high formation energy at an odd-number, while the V@Cnr dimer vacancy exhibits a complete opposite trend. These results indicate that small MoS nanosheets can exhibit unique material properties for catalytic applications.