Oxidation of metallic two-dimensional transition metal dichalcogenides: 1T-MoS2 and 1T-TaS2

Molybdenum disulphide (MoS2) and tantalum disulphide (TaS2) are well-known members of the transition metal dichalcogenide (TMD) family. They occur naturally in hexagonal (2H) forms but can also be synthesized in metallic trigonal (1T) phases that are active hydrogen evolution catalysts and exhibit interesting phenomena such as superconductivity and charge density waves. Sparse experimental evidence indicates that 2D crystals of metallic TMDs degrade rapidly in air via an unknown oxidation mechanism. Here we present a computational study on oxidation at the edges and surfaces of 1T-MoS2 and 1T-TaS2 monolayers based on density functional theory calculations. Our results suggest that both 1T-MoS2 and 1T-TaS2 are very susceptible to oxidation because there are negligible energetic barriers to the dissociation of oxygen molecules at edge sites. However, further oxidation requires the substitution of sulphur atoms by oxygen, the rate of which is limited in the case of 1T-TaS2 because the detachment of SO2 groups is disfavored by 1.2 eV. Conversely, oxygen molecules adsorbed on 1T-MoS2 dissociate directly at molybdenum atoms at the edge, forming intermediate S-O-Mo structures that facilitate the subsequent formation and detachment of SO2. Oxidation is not harmful to the catalysis of the hydrogen evolution reaction on 1T-MoS2 but is detrimental to that on 1T-TaS2, which explains some apparently inconsistent experimental results. These results provide new insights into the mechanism of ambient oxidative degradation of metallic TMDs and the effect of oxidation on their catalytic properties.