Synthesis and in situ sulfidation of molybdenum carbide MXene using fluorine-free etchant for electrocatalytic hydrogen evolution reactions

Synthesizing MXenes from M_n+1AX_n (MAX) phases using hazardous hydrogen fluoride is a common and effective method. However, fluorine termination on the basal planes and edges of the resulting MXenes is undesirable for the electrocatalytic hydrogen evolution reaction (HER), while oxygen (O), hydroxyl (OH), and sulfur (S) termination favors this reaction. Herein, we unveil a simple fluorine-free exfoliation and two-step vulcanization method for synthesizing molybdenum sulfide-modified molybdenum carbide (MoS₂/Mo₂CT_x MXene, T = OH, O, S) for the HER in alkaline medium. Microwave-assisted hydrothermal treatment of the MAX phase (Mo₃AlC₂) with sodium hydroxide-sodium sulfide as an etching solution and thioacetamide as a source of sulfide ions enabled the selective dissolution of the aluminum (Al) layer and sulfidation of the surface Mo atoms to form amorphous MoS₂. Thus, the vulcanization of Mo₂CT_x MXene resulted in the formation of MoS₂/Mo₂CT_x MXene. The MoS₂ formed on the surface of Mo₂CT_x provided enhanced stability by preventing oxidation. MoS₂/Mo₂CT_x exhibited enhanced electrocatalytic activity toward the HER, mainly due to the O, OH, and amorphous MoS₂ functionalities. The MoS₂ sites on the surface exhibited an overpotential of 110 ± 7 mV at a current density of 10 mA cm^-2 as a result of enhanced charge transfer and mass transfer. Thus, the sulfidation method demonstrated herein is capable of producing amorphous MoS₂ structures on Mo₂CT_x MXene, which could be applied for the surface modification of other molybdenum-based nanomaterials or electrocatalysts to improve stability and enhance electrocatalytic activity.