Fig. 1 Different-dimensional sp2-hybrid carbon nanomaterials, graphene forms (powder, film, and monolith), and concept of 3D graphene

Fig. 2 Scheme of blowing process

Fig. 3 (a) Kelvin structure and (b) Weaire-Phelan structure for honeycomb, (c) scheme of blowing kinetics

Fig. 4 B-C-N-O light-element system suitable for blowing

Fig. 5 (a) Scheme of chemical blowing process; (b, c) Photo and optical image of SG; (d) Optical image of a graphene membrane and graphitic struts of SG; (e, f) Scanning electron microscope (SEM) images of intermediate polymeric bubbles and their thin walls; (g) Optical image of a large few-layered graphene membrane taken from SG; (h) Atomic force microscope (AFM) image of an individual graphene membrane; (i) High-resolution transmission electron microscope (HRTEM) images of SG; (j-m) SEM images of strutted graphene grown at heating rates of 1, 4, 20 and 100 ℃/min[24,55]

Fig. 6 (a) Synthesis scheme of foaming based on PMP precursor; (b) SEM images of N-P-O-co-doped graphene foam[67]

Fig. 7 Synthesis schemes and SEM/HRTEM images of loaded graphene foam (a, b) Fe2O3[35]; (c, d) Co2P[36]; (e, g) NiFeP[73]

Fig. 8 (a) Synthesis scheme and (b-d) TEM images of 3D strutted g-C3N4 foam heated at different rates[87]

Fig. 9 (a) Synthesis scheme, (b) SEM, (c) optical, (d) AFM and (e) HRTEM images of BN foam by foaming of AB[29]; (f) Synthesis scheme and (g, h) SEM images of BN foam using boric acid and PEO[97]

Fig. 10 Abundant applications based on 2D-material foams