(a)

(b)

(c)

(d)

Figure 1: (a) SEM images of the undoped NiO/PVP nanofibers before calcination. (b) SEM images of the undoped NiO/PVP nanofibers after calcination. (c) SEM images of the Fe-doped NiO/PVP composite nanofibers before calcination. (d) SEM images of the Fe-doped NiO/PVP composite nanofibers after calcination.