(a)
(b)
(c)
(d)
Figure 4: SEM images of lyophilized GO solution and three typical GO hydrogels: (a) GO solution, (b) GO/PVP hydrogel with 1 mg mL−1 PVP, (c) GO/PDDA hydrogel with 0.1 mg mL−1 PDDA, and (d) GO/Ca2+ hydrogel with 9 mM Ca2+. , and scale [111].