Water induced ultrathin Mo₂C nanosheets with high-density grain boundaries for enhanced hydrogen evolution

Grain boundary controlling is an effective approach for manipulating the electronic structure of electrocatalysts to improve their hydrogen evolution reaction performance. However, probing the direct effect of grain boundaries as highly active catalytic hot spots is very challenging. Herein, we demonstrate a general water-assisted carbothermal reaction strategy for the construction of ultrathin Mo₂C nanosheets with high-density grain boundaries supported on N-doped graphene. The polycrystalline Mo₂C nanosheets are connected with N-doped graphene through Mo–C bonds, which affords an ultra-high density of active sites, giving excellent hydrogen evolution activity and superior electrocatalytic stability. Theoretical calculations reveal that the d_z² orbital energy level of Mo atoms is controlled by the MoC₃ pyramid configuration, which plays a vital role in governing the hydrogen evolution activity. The d_z² orbital energy level of metal atoms exhibits an intrinsic relationship with the catalyst activity and is regarded as a descriptor for predicting the hydrogen evolution activity.