Effect of Distance between Needle Tip and Collector on Nanofibers Morphology

Preparation of TiO2/PVP Nanocomposites

To study the effect of distance on the fiber morphology been chosen distances of 6 cm to 18 cm with the stability of all other parameters of the process of spinning, is investigated. Field emission scanning electron microscopy (FESEM) observations reveal the effect change the distance on fiber morphology. In this experiment 6 wt.% solution of Polyvinylpyrrolidone (PVP) in Ethanol and 0.4M titanium dioxide (TiO2) in Ethanol too , were used mats of fibres were electrospun at spinning distance of 6, 9, 12, 15 and 18 cm, high voltage 14 KV, needle (20 g), flow rate 1ml/h throughout the experiment.

Characterizations Techniques

Morphological surface observations and structural investigations were performed by field emission scanning electron microscopy (FESEM) model FEI Nova NanoSEM 450 and high resolution x-ray diffractometer (HR-XRD) using Philips X’Pert equipped with Cu-K radiation = 0.15419 nm), respectively. Raman spectra were measured at RT using JobinYvon HR 800 UV spectrometer system with 514.5 nm Ar+ laser as excitation source. Micro-Photoluminescence (m- PL) measurements were carried out at room temperature using HeCd laser with an excitation source of 325 nm. Atomic force microscopy (AFM), veeco NanoScope Analysis version 1.2

Structural and Morphological Characterization

Figure 1 shows the evolution of XRD patterns of TiO2/ PVP thin films recorded for different distances. It can be noticed that the crystal structure and orientation of the nanocrystalline TiO2/PVP films deposited on the glass substrate vary with the variation of solution flow rate. The position of diffractions peaks is not affected while the relative intensity varies considerably. The peaks were indexed within a tetragonal crystal structure of TiO2 anatase phase, in conformity with some previously reported results [11, 12, 13, 14, 15]. It is also important to note that the peaks are sharp and well defined. Independently of the distance, six diffraction peaks can be observed at 2θ of 25.31º, 37.78º, 48.10º, 53.96º, 55.20º and 62.79° which correspond to (101), (004), (200), (105), (211), (204) reflections of TiO2 phases [6]; it has a tetragonal structure with lattice parameters a= 3.7850 Å, c= 9.5140 Å, and β=90°), respectively. At distance of 6 cm (Figure. 1a), the emergence of (101) and (200) peaks were observed, while at distance of 9 cm (Figure. 1b), the emergence of (101), (200), (105) and (211) peaks could be seen. Finally, at 12, 15 and 18 cm (Figure. 1c, d and e), seven peaks appeared confirming the formation of TiO2 anatase phase (101), (004), (200), (105), (211) and (204). At 15cm. (Figure. 1d) the peaks appeared confirming the formation of TiO2 anatase phase with higher intensity than was previously the case. It can be concluded that with change in the distance, alteration in the structure (phase stability) contributed to the emergence/ disappearance of new peaks, which is an indication also of the degree of crystallinity (change of peaks’ intensity) [16].

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FESEM images as shown in Figure 5.6 showed that at the distance of 6 cm (Figure 2a) fiber structure was regular with a few fiber deposit and large diameter average. When the distance was increased, the structure is improved with decreasing in the diameter; this was due to the fact that distance was short and if there was not enough time for evaporating the solvent before deposition of the fibers on the collector, the fibers may merge. At distances of 9 and 12 cm Figure 2b and c, showed nanofibers more than in previous cases, but not many fibers because the distance was not enough; they have average diameter of 157 and 132 nm, respectively. The distance of 15 cm (Figure 2d) obtained the best results among the selected distances, where the fiber was formed on a regular basis with an average diameter of 69 nm less than what he got in the other distances. The stretching force was increased with decrease of the diameter and vice versa. These findings are consistent with R. Jalili et al and J. Ayutsede, et al. [17, 18]. At the distance of 18 cm (Figure 2e), there was a slight change in morphology and an increase in average fiber diameter of 84 nm. This was due to the decrease in the high voltage resulting in less stretching of the fibers before deposition on the collector [1, 19]. This change proves that the distance associated with the amount of high voltage, was not sufficient in this distance, which resulted in a decline in results from what it was when the distance was 15 cm.

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The distance and electric field strength has influence on the evaporation rate. The higher value of distance and electric field strength increase the evaporation rate. The distance should be appropriate to give enough time for the evaporation of the solvent, as well as to stretch before depositing on the collector [20, 21]. The distance had impact on the morphology and fibers diameter; an increase of distance had better results on morphology with lower average diameter.

AFM images of a 5 × 5 μm 3D area of each nanostructured TiO2/PVP composites are shown in Figure. 5.7 The roughness was found to be very sensitive to the variation of distance during electrospinning process; the roughness increases with longest distance 9, 10, 13, 19 and 14 nm for 6, 9, 12, 15 and 18 cm, respectively, see Figure 3a, b, c, d and e. This indicates that the quality of the TiO2/PVP surface can be easily improved by using an optimum distance; resulting in good increase of grain size and hence improved (higher) roughness of nanostructured TiO2/PVP thin films. The sample grown at 15 cm exhibited a higher surface roughness (19 nm) compared with the others because of the reason is that the distance 15 cm was the optimum with selected parameters during electrospinning. Therefore, the parameters of growth are suitable for controlling the surface morphology as well as for increasing the surface roughness and density of the material distribution on the surface.

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