Fig. 1

A TEM of ZnO nanostructures obtained by mechanochemical process [21], B TEM of ZnO using solvothermal method [22], C, D, E Formation mechanism of ZnO nanoflowers and its SEM micrograph synthesized by precipitation process [23], F, G SEM images of the ultrathin anodic aluminum oxide membrane after sol filling and annealing treatment in which ZnO nanotubes are injected by sol–gel into the pores of membrane [24]