Aligned Heterointerface-Induced 1T-MoS Monolayer with Near-Ideal Gibbs Free for Stable Hydrogen Evolution Reaction.

1T-phase molybdenum disulfide (1T-MoS2) exhibits superior hydrogen evolution reaction (HER) over 2H-phase MoS2 (2H-MoS2). However, its thermodynamic instability is the main drawback impeding its practical application. In this work, a stable 1T-MoS2 monolayer formed at edge-aligned 2H-MoS2 and a reduced graphene oxide heterointerface (EA-2H/1T/RGO) using a precursor-in-solvent synthesis strategy are reported. Theoretical prediction indicates that the edge-aligned layer stacking can induce heterointerfacial charge transfer, which results in a phase transition of the interfacial monolayer from 2H to 1T that realizes thermodynamic stability based on the adhesion energy between MoS2 and graphene. As an electrocatalyst for HER, EA-2H/1T/RGO displays an onset potential of -103 mV versus RHE, a Tafel slope of 46 mV dec(-1) and 10 h stability in acidic electrolyte. The unexpected activity of EA-2H/1T/RGO beyond 1T-MoS2 is due to an inherent defect caused by the gliding of S atoms during the phase transition from 2H to 1T, leading the Gibbs free energy of hydrogen adsorption (Delta G(H*)) to decrease from 0.13 to 0.07 eV, which is closest to the ideal value (0.06 eV) of 2H-MoS2. The presented work provides fundamental insights into the impressive electrochemical properties of HER and opens new avenues for phase transitions at 2D/2D hybrid interfaces.