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