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.
Lithium intercalation in MoS 2 drives the transformation from its semiconducting 2H to metallic 1T/1T’ phase, allowing the fabrication of low-resistance edge contacts in transistors and diodes. However, due to the presence of chemically reactive lithium, the properties of the resulting Li x MoS 2 are sensitive to processing methods, raising additional concerns about its stability and performance when applied in electronic devices, which have rarely been discussed. Here, the processing method of lithium-intercalated MoS 2 in n-butyllithium solution is investigated for optimal edge contact performance in transistors. The stability of Li x MoS 2 when exposed to various solvents, ambient environment (air and vacuum) and thermal stress (from room temperature to 300 °C) is studied in terms of its electrical conductivity and phase ratio. It is found that exposure to water causes immediate degradation of the electrical conductivity of Li x MoS 2 , while under thermal stress, irreversible degradation occurs when the temperature exceeds 150 °C. Therefore, avoiding thermal stress during device fabrication is essential. Following this, an optimal procedure is proposed to fabricate low-resistance 1T-Li x MoS 2 edge contacts, achieving a contact resistance of ≈420 Ω µm, which also displays suppressed defect states in edge-contacted transistors.
危险品化学品经营许可证（带存储）