## 3 Results and Discussion
With the increase of calcination temperature to 600 \({}^{\circ}\)C, the sintering and coalescence of nanograins in individual NFs became more evident and, consequently, the diameter of NFs decreased to 130 nm as shown in the FE-SEM images of Figure 3e,f with different magnifications. As shown schematically in Figure 4, before calcination, TiO\({}_{2}\) NFs were amorphous with smooth surfaces without any grains. During the calcination, the diameter significantly decreased because of the evaporation of the polymer species and solvents as well as the decomposition of precursors. Simultaneously, nanosized grains started to grow. With the increase of calcination temperature, the nanograins coalesced with each other owing to the increase of diffusion rate, resulting in a significantly larger growth of nanograins and smaller fiber diameter [38; 39]. In the literature, many researchers have reported the effect of applied potential on the final morphology or diameter of synthesized NFs [40; 41; 42]. If the applied potential is lower than a threshold value, no stable liquid jets will be formed [43]. Once a threshold voltage is exceeded, a stable jet of liquid is directed toward the collector. We changed the applied potential by maintaining the positive voltage constant at +10 V and changing the negative voltage (\(-\)5, \(-\)10, and \(-\)20 V). As shown in Figure 5, with the increase of applied potential, the diameter first decreased and thereafter increased. As shown in Figure 5a,b, by applying the voltages of +10 and \(-\)5 V, the final diameter of TiO\({}_{2}\) NFs after calcination at 600 \({}^{\circ}\)C was 200 nm, and with the increase of negative voltage to \(-\)10 V, it decreased to 130 nm (Figure 5c,d); moreover, with the further increase of negative voltage to \(-\)20 V (Figure 5e,f), the final diameter increased to 195 nm. Figure 2: X-ray diffraction (XRD) patterns of TiO\({}_{2}\) nanofibers (NFs) calcined at 500 \({}^{\circ}\)C for 6 h and 600 \({}^{\circ}\)C for 6 h, and as-spun TiO\({}_{2}\) NFs. Figure 5g shows the relationship between the diameters of TiO\({}_{2}\) NFs and applied voltage. To explain the changes of diameter with the change of applied voltage, the following forces acting on  
Figure 4: Schematic illustration of the growth of nanograins in NFs with the increase of calcination temperature (modified from Reference [33]). Figure 5: Low- and high-magnification FE-SEM images of synthesized TiO\({}_{2}\) NFs prepared with different applied voltages: (**a,b**) 10 and \(-\)5 kV; (**c,d**) 10 and \(-\)10 kV; (**e,f**) 10 and \(-\)20 kV; (**g**) Diameter of obtained TiO\({}_{2}\) NFs as a function of applied voltage. Figure 3: Low- and high-magnification FE-SEM images of synthesized TiO\({}_{2}\) NFs. (**a,b**) As-spun NFs; (**c,d**) NFs calcined at 500 \({}^{\circ}\)C for 6 h; (**e,f**) NFs calcined at 600 \({}^{\circ}\)C for 6 h.  
a charged jet should be considered [44]: (i) gravitational force; (ii) electrostatic force responsible for the acceleration of the formed jet to the collector; (iii) Coulombic force stretching the jet; (iv) viscoelastic force; (v) surface tension (both viscoelastic force and surface tension prevent the stretching of the charged jet); and (vi) drag force owing to the friction between the charged jet and the surrounding atmosphere. The effect of applied potential on the diameters of NFs can be determined using the relationships among the Coulombic and viscoelastic forces and surface tension. At low applied potentials (\(-5\) kV), the Coulombic force is smaller than the viscoelastic and surface tension. Consequently, NFs with large diameters are formed. At moderate applied potentials (\(-10\) kV), all three forces are balanced, resulting in the decrease of diameters of NFs. With a further increase in the applied potentials (\(-20\) kV), the Coulombic force becomes much greater than the viscoelastic force and surface tension.