Abstract: Two-dimensional (2D) materials have come to light due to their unique thickness that owns abundant exposed edges with enhanced electrocatalytic properties. 2D molybdenum disulfide (MoS₂ ) nanosheet has aroused considerable attention due to its tunable surface chemistry and high electrochemical sur-face area. Nonetheless, several shortcomings associated with MoS₂, such as its naturally existing semi-conducting 2H phase, which has limited active sites due to the inert basal plane, restrict its application in water electrocatalysis. Taking into account the benefits of the 1T/2H phase of MoS₂, as well as the importance of engineering 2D/2D heterojunction interface for boosted electrocatalysis, metallic Ti₃ C₂ T_x was integrated with 1T/2H MoS₂ to develop 2D/2D 1T/2H MoS₂ /Ti₃ C₂ T_x heterostructured nanocompos-ites. Herein, with only 25 % of the intercalating agent, 1T/2H MoS₂ with the highest 1T phase content of ∼82 % was successfully synthesized. It was further incorporated with 1 wt% of Ti₃ C₂ T_x through a com-bination of ultrasonication and mechanical stirring process. The 1T/2H MoS₂ (25D)/ Ti₃ C₂ T_x-1 (MTC-1) manifested outstanding electrocatalytic performance with an overpotential and Tafel slope of 280 mV (83.80 mV dec^-1 ) and 300 mV (117.2 mV dec^-1 ), for catalyzing acidic and alkaline medium HER, respec-tively. Pivotally, the as-prepared catalysts also illustrated long-term stability for more than 40 h. The coupling method for the 2D nanosheets is crucial to suppress the oxidation of Ti₃ C₂ T_x and the restack-ing issue of 2D nanosheets. The superior HER activity is ascribed to the synergistic effect between the heterostructure, enhancing the electronic structure and charge separation capability. The intrinsic prop-erty of the catalyst further confirms by turnover frequency (TOF) calculation. As such, this research paves the way for designing high-efficiency 2D electrocatalysts and sheds light on the further advancement of tunable 2D electrocatalysts for robust water splitting and beyond.

Key words: 2D/2D electrocatalysts, Phase engineering, pH universal, Molybdenum disulfide Ti₃ C₂ T_x