Fig. 1: Synthesis and characterization of the 3D porous VN.

a Schematics illustrating the synthesis of highly crystalline 3D porous TMNs. b The obtained VN sample. c, d SEM images of VN. e, f TEM images of VN. g HAADF image of VN. h HRTEM image of VN. i–l EDS mapping and spectrum of VN.