2H/1T Phase Transition of Multilayer MoS₂ by Electrochemical Incorporation of S Vacancies

The phase transition of multilayer MoS₂ nanosheets from semiconducting 2H to metallic 1T (2H/1T) has been realized mainly by chemical methods (e.g., Li intercalation). Here, we develop a simple yet effective method, cyclic voltammetry, to successfully tune the 2H/1T phase transition of multilayer MoS₂ nanosheets without using intercalation species. The phase transition is triggered by the electrochemical incorporation of S vacancies (obtained by electrochemical etching), which on the one hand injects electrons into the framework of S-Mo-S and on the other hand facilitates the sliding of S planes. Density functional theory calculations show that O doping in the framework of S-Mo-S decreases the energy barrier for forming S vacancies and stabilizes the 1T-phase by occupying the 4d orbital of Mo. Our calculations further show that the presence of S vacancies and O incorporation not only reduces the bandgap of MoS₂ , indicating an increased conductivity, but also decreases the hydrogen adsorption free energy, implying significant improvement of hydrogen evolution reaction (HER) activity. Indeed, the overpotential and Tafel plot of the electrochemically treated MoS₂ nanosheets are decreased respectively by 174 mV and 25 mV/dec at a cathodic current density of 10 mA cm^-2 compared with pristine 2H-MoS₂ nanosheets. The HER experiment also reveals the order of catalytical activity for the studied phases and structural defects: 1T-MoS₂ > S vacancies > O doping >2H-MoS₂ . Our study has provided a new route to control the phase transition of multilayer MoS₂ nanosheets with promising applications potentially in catalysis and optoelectronics.