Improving electrocatalytic activity of 2H-MoS₂ nanosheets obtained by liquid phase exfoliation: Covalent surface modification versus interlayer interaction

Liquid phase exfoliation (LPE) is a promising method for synthesizing two-dimensional (2D) 2H-MoS₂ due to the balance obtained in the resulting yield and quality. Improvements in the electrocatalytic activity of 2D 2H-MoS₂ are typically attributed to an increased number of active sites at the plane edges. However, the conclusion need be carefully redefined in the case of LPE, because the covalent surface modification and solvent intercalation between layers can affect the electrocatalytic activity as well. In this study, bulk 2H-MoS₂ was exfoliated in three typical solvents, including N, N-dimethyl formamide (DMF), formamide, and 1-methyl-2-pyrrolidinone (NMP), to synthesize 2H-MoS₂ nanosheets. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectrum were used to confirm the possibility of surface and interface functionalization of the 2H-MoS₂ nanosheets. Further, the effects on the electrocatalytic activity of 2H-MoS₂ nanosheets were investigated mainly by density functional theory periodic calculations based on the analysis of electronic energy band structures. The results have demonstrated that 2H-MoS₂ nanosheets prepared in formamide exhibit smaller average size and thickness, and higher electrocatalytic activity. The surface functionalization of the 2H-MoS₂ nanosheets is indeed generated by Mo-N covalent bonds due to the presence of S vacancies. Such covalent functionalization leads to a slight decrease of energy band gap with 0.03 eV in a (3 × 3 × 1) 2H-MoS₂ supercell. In contrast, the interface functionalization formed by the intercalation of solvent molecules (e.g., formamide) between layers can cause a remarkable decrease in the band gap of (1 × 1 × 2) 2H-MoS₂, the phase transition from semiconductor to semimetal, higher carrier mobility, and smaller effective mass (m_e = 0.052 m₀).