Fig. 4: Spectroscopy characterization of Mo₂C hybrids.

a XRD patterns of H-Mo₂C/NG and Mo₂C/NG. The peaks near 2θ = 26° and 43° are assigned to the (002) and (100) reflections of graphene, respectively, and the weak peaks at 37° and 53° are considered to be the (\(\overline{2}\)11) and (\(\overline{2}\)22) planes of MoO₂ (PDF#32-0671), respectively. b The high-resolution Mo 3d XPS spectra of H-Mo₂C/NG and Mo₂C/NG. Mo⁴⁺ and Mo⁶⁺ can be attributed to MoO₂ and MoO₃ caused by the oxidation of molybdenum species in ambient atmosphere, respectively. The Mo²⁺ and Mo³⁺ are assigned to carbides and nitrides, respectively, which are known to serve as active sites for HER⁶. c C K-edge XANES spectra of H-Mo₂C/NG and Mo₂C/NG. The characteristic resonances of C = C π* and C–C σ* originating from NG support. d Mo K-edge XANES spectra of H-Mo₂C/NG, H-Mo₂C/NG-7.14, Mo₂C/NG, and the reference samples. H-Mo₂C/NG-7.14 and Mo₂C/NG have a higher half-edge energy than that of bulk Mo₂C reference, which arises from the charge-transfer from Mo to C atoms on the NG-supported Mo₂C⁸. e Mo K-edge Fourier transform EXAFS spectra of H-Mo₂C/NG, H-Mo₂C/NG-7.14, Mo₂C/NG, and the reference samples. f EPR spectra of H-Mo₂C/NG and Mo₂C/NG. H-Mo₂C/NG shows the EPR signals at g₂ = 1.930 (Mo–N species³⁶), g₁ = 1.870 and g₄ = 2 .056 (Mo⁵⁺ species³⁷), followed by g₅ = 2.111 and g₆ = 2.156 (the resonance on conduction electrons³⁸).