In order to observe the effect of heat treatment on the structure of ZnO nanofiber, the optimized composite fibers (see Fig. 3a4) were annealed at 500 °C with different heat flow rates during calcination period, namely, 5 °C, 25 °C, 50 °C, 75 °C per minute. A representative SEM image is presented in Fig. 3b. During calcination of composite nanofibers, two things happened simultaneously, namely, polymer molecules escaped after decomposition while ZnO nanoparticles come together and crystallized. The formation of crystalline ZnO nanofibers is confirmed through XRD.