We performed ab initio calculations on the basic set of MoS₂ single-layer materials, namely the 1H, 1T and 1T′ polytypes, to lay a theoretical framework on the emerging breakthrough-discoveries of high activity towards the hydrogen evolution reaction (HER) in exfoliated MoS₂ and related materials. Our calculations show that for exfoliated MoS₂, 1T′ is the most HER active polytype, with active sites both on the basal plane and at the edges of the layered grains. In comparison, the basal planes of the 1H and 1T polytypes are HER inactive and their edge-sites are not as active as those of the 1T′ polytype. We also found that 1T-MoS₂ is unstable and easily transforms into 1T′-MoS₂, and the 1T′ phase is metastable with a considerable barrier >0.7 eV to bar its transformation into the most stable 1H phase. Further, unlike the case of exfoliated WS₂, the HER activity of exfoliated MoS₂ is not so critically affected by the lattice strain. In addition, in contrast to the 1T′-WS₂, the 1T′-MoS₂ is not metallic but has a very small bandgap of 0.1–0.2 eV. Hence, 1T′-MoS₂ should have a high enough conductivity and other suitable properties for it to function as an electrochemical HER catalyst.

Manipulating electronic and magnetic properties of two-dimensional (2D) transitional-metal dichalcogenides (TMDs) MX2 by doping has raised a lot of attention recently. By performing the first-principles calculations, we have investigated the structural, electronic, and magnetic properties of transitional metal (TM)-doped MoS2 at low and high impurity concentrations. Our calculation result indicates that the five elements of V-, Mn-, Fe-, Co-, and Cu-doped monolayer MoS2 at low impurity concentration all give rise to the good diluted magnetic semiconductors. By studying various configurations with different TM-TM separations, we found that the impurity atoms prefer to stay together in the nearest neighboring (NN) configuration, in which the doped TM atoms are FM coupling except for Fe doping at 12 % concentration. For V, Mn, and Fe doping, the total magnetic moment is smaller than the local magnetic moment of the dopants because the induced spins on the nearby host atoms are antiparallel to that of the doped atoms. In contrast, Co and Cu doping both give the higher total magnetic moment. Especially, Cu doping induces strong ferromagnetism relative to the local spins. However, the atomic structures of Co- and Cu-doped MoS2 deviate from the original prismatic configuration, and the magnetic moments of the doped systems decrease at 12 % impurity concentration although both elements give higher magnetic moments at 8 % impurity concentration. Our calculations indicate that V and Mn are promising candidates for engineering and manipulating the magnetism of the 2D TMDs.