## 3 Results and Discussion  
TiO\({}_{2}\) is a polymorphous material and has three crystalline phases, namely anatase, brookite, and rutile [29]. Rutile is the most stable polymorph, but anatase and brookite are also common [30]. Therefore, XRD was used to analyze the crystallinity of the synthesized TiO\({}_{2}\) NFs and to determine the formed phase. It should be noted that the choice of calcination temperature was based on our previous study [15], where using thermogravimetric analysis (TGA), differential thermal analysis (DTA) and XRD experiments we showed that calcinations at 500 and 600 \({}^{\circ}\)C resulted in the formation of different TiO\({}_{2}\) phases. Furthermore, other researchers have reported the calcination of TiO\({}_{2}\) NFs at 500 \({}^{\circ}\)C [28; 31], and 600 \({}^{\circ}\)C [32]. In the initial stages of ES, Ti(OiPr)\({}_{4}\) is hydrolyzed by reacting with moisture in the air to form ultrafine particles of amorphous TiO\({}_{2}\) in the PVP matrix. Accordingly, the as-spun NFs do not show any crystallinity [33]. However, amorphous TiO\({}_{2}\) can be crystallized by calcination at a high temperature. Figure 2a shows the XRD pattern of TiO\({}_{2}\) NFs calcined at 500 \({}^{\circ}\)C for 6 h. It shows peaks centered at 25.5, 37.9, 49.1, 55.2, 63.2, 68.8, and 76.2\({}^{\circ}\) corresponding to the (101), (002), (200), (211), (204), (116), and (101) planes of TiO\({}_{2}\) with tetragonal anatase phase (JCPDS No. 21-1272), respectively. The broadening of the diffraction peaks is due to the very small size of the TiO\({}_{2}\) nanocrystals [33]. In the literature, it is reported that anatase can be crystallized as the first crystalline phase from amorphous TiO\({}_{2}\). From a thermodynamic point of view, anatase is the phase with the lowest total energy, and a lower surface energy accelerates the nucleation kinetics and facilitates the formation of anatase [34]. Representative XRD patterns of TiO\({}_{2}\) calcined NFs at 600 \({}^{\circ}\)C and 500 \({}^{\circ}\)C and as-spun TiO\({}_{2}\) NFs are shown in Figure 2. It shows peaks located at 27.5, 36.2, 41.3, 54.4, 56.7, and 69.2\({}^{\circ}\), which can be attributed to the (110), (101), (111), (211), (220), and (301) planes of TiO\({}_{2}\) with rutile structure, respectively, which is consistent with the literature [35]. Thus, by controlling the calcination temperature from 500 to 600 \({}^{\circ}\)C, it is possible to obtain crystalline phases with anatase (at 500 \({}^{\circ}\)C) or rutile structures (at 600 \({}^{\circ}\)C). Figure 3a,b show the FE-SEM images of the as-spun NFs at different magnifications. It is evident that long and continuous NFs were formed. From the high-magnification FE-SEM image, the diameter of the as-spun NFs without calcination was estimated to be 720 nm. The large diameter of the as-spun NFs is mainly due to the existence of water and the PVP matrix. Figure 3c,d show the FE-SEM images of NFs crystallized at 500 \({}^{\circ}\)C for 6 h. It is evident that the NFs retained their morph  
Figure 1: Schematic illustration of electrospinning (ES) apparatus. and, from the high-magnification image, the diameter of NFs calcined at 500 \({}^{\circ}\)C was estimated to be 150 nm. Therefore, the diameter decreased significantly in comparison with the as-spun NFs. This is due to the evaporation of the solvent and PVP molecules at high temperatures. From a comparison between the high-magnification images of the as-spun NFs (Figure 3b) and NFs calcined at 500 \({}^{\circ}\)C, it can be observed that a change of surface morphology occurred from smooth to grainy, owing to the evaporation of the solvent and polymer molecules from the surfaces; consequently, a grainy structure remained on the surface of TiO\({}_{2}\) NFs calcined at 500 \({}^{\circ}\)C. Other studies have also reported the formation of a grainy structure of NFs after calcination [36; 37].