Hydrogen evolution on different facets of δ1- MoN and δ3-MoN: Considering the adsorbed oxygen and hydroxyl by Surface Pourbaix diagrams

δ₁-and δ₃-MoN, the two most stable phases of molybdenum nitride, show a potential application in hydrogen evolution as their excellent corrosion resistance and high conductivity. However, we still lack the theoretical study about HER on their different surfaces. In order to simulate the realistic condition during heterogeneous catalysis, detailed atomic structure of each MoN surface (the 1O and 1OH) is determined by surface Pourbaix diagram. Further exploration of hydrogen evolution shows that the 1O and 1OH could change the catalytic site of one MoN surface, and weak the hydrogen adsorption ability. This is attributed to the downshift of Mo d-band center in top layer caused by the 1O and 1OH. And the hydrogen adsorption ability on catalytic sites of N and O atoms also follow the p-band center theory, respectively. It is interesting that several δ₁-and δ₃-MoN surfaces demonstrate comparable exchange current density, e.g., (101) and (001)N of δ₁-MoN, and (110) of δ₃-MoN with 1.76, 1.97 and 0.17 mA cm^?2, respectively. This work is expected to contribute to the theoretical understanding of HER on different δ₁-and δ₃-MoN surfaces, and provide guidance for corresponding experiments on them.