## 3 Results and Discussion
However, as there is both longitudinal and transverse stretching and the latter can lead to the increase of diameter of NFs, the increased diameter of NFs obtained in this study can be attributed to the dominance of transverse stretching relative to longitudinal stretching. Furthermore, with the increase of applied potential, the formed jet can accelerate faster toward the collector; thus, the solvent would not have sufficient time to evaporate, eventually leading to the formation of NFs with a larger diameter [44]. It should be noted that by comparing between Figure 5a,c,e, it can be seen that under the lowest applied voltage, some beads appear. This may be due to the fact that the low applied voltage leads to the viscoelastic force which can lead to the formation of some beads. It can be observed that the TiO\({}_{2}\) NFs produced at the lowest applied voltage (\(-5\) kV) were aligned, whereas with the increase of voltage, disordered TiO\({}_{2}\) NFs were obtained. As schematically shown in Figure 6, the as-spun charged NFs can be aligned if they are affected by pulling forces. Electrostatic forces pull the two ends of the NFs toward the two collectors and consequently align the NFs. Under the effect of pulling forces, the NFs turn perpendicularly toward the edges of the collectors. Subsequently, the two ends of the NFs are deposited on the collectors. If these steps are repeated, arrays of uniaxially aligned NFs can be obtained across the gap [18]. The effects of voltage can be ascribed to the \"whipping\" instability. During ES, the rapid growth of a nonaxisymmetric or whipping instability resulted in the bending of NFs and permitted the electrical forces to elongate the jet. With the increase of the applied voltage, whipping instability increased accordingly, leading to the entanglement and knotting of the as-spun NFs and rendered the alignment of NFs more difficult. Hence, the alignment of NFs is not possible at high applied voltages [18]. In the subsequent step, while the positive and negative voltages were fixed at +15 and \(-10\) kV, respectively, the distance between the needle and Al collector was changed (20, 30, and 40 cm). Figure 7a,b show the low- and high-magnification FE-SEM micrographs of TiO\({}_{2}\) NFs with the  
Figure 6: Schematic illustration of the process of alignment of NFs (modified from Reference [25]). needle-to-Al-collector distance of 20 cm after calcination at 600 \({}^{\circ}\)C, respectively. The synthesized NFs were long and continuous with an approximate diameter of 150 nm. With the increase of needle-to-Al-collector distance to 30 cm, as shown in Figure 7c,d, the diameter increased to 185 nm, and with the further increase of needle-to-Al-collector distance to 40 cm, the diameter increased to 245 nm (Figure 7e,f). It is expected that, with the increase of needle-to-collector distance, the solvent has more time to evaporate; therefore, a smaller diameter should be obtained [45]. However, the experimental results exhibit a contrasting trend, as shown in Figure 7g. With the increase of needle-to-collector distance, the strength of the electric field, defined as the applied voltage divided by the distance between the needle and collector, decreased. Thus, it can be concluded that, with the decrease of electric field, the viscoelastic force increased, which prevented the stretching of NFs, resulting in the increase of the diameter of NFs. From Figure 7a, c, and e, it can be seen that at a short distance, some beads appear. This is likely due to the fact that the electrospinning solution reaches the Al collector before the full evaporation of the solvent [45]. From Figures 5 and 7, it may be noticed that with the increase of either applied voltage or needle-to-Al-collector distance, the number of the produced NFs was decreased. This can be explained partly the fact that if the distance is long, some formed NFs may be unable to reach to the Al collector. However, more works should be carried out to clarify the reason behind the phenomena that occur with the increase of applied voltages.