## 3 Results
### Photocatalytic Performance Evaluation  
The photocatalytic reaction of TiO\({}_{2}\)/ZnO is schematically shown in Figure 9. The electrons in the valence band can move to the conduction band and generate holes in the valence band by ultraviolet (UV) rays in sunlight. In heterojunction of ZnO with an energy band gap of 3.2 eV and TiO\({}_{2}\) with an energy band gap of 3.4 eV, the electrons in the conduction band of ZnO move to the conduction band of TiO\({}_{2}\); then the holes move from the valence band of TiO\({}_{2}\) to the valence band of ZnO. Electrons transferred to the TiO\({}_{2}\) conduction band react with O\({}_{2}\) to form a superoxide anion (O\({}_{2}\)^). The holes transferred to the ZnO valence band react to form a strong hydroxyl radical (OH\({}^{\bullet}\)). Such reactions continuously occur under UV irradiation, and high active radicals such as O\({}_{2}\)^ and OH\({}^{\bullet}\) can promote the photodegradation of organic pollutants [46; 47; 48; 49]. Photodegradation characteristics using methylene blue were investigated for photocatalytic performance evaluation of TiO\({}_{2}\)/ZnO nanofibers. TiO\({}_{2}\) nanofibers were used as a reference material to compare with TiO\({}_{2}\)/ZnO nanofibers. All nanofibers were fabricated by the electrospinningning process. Figure 8: X-ray diffraction (XRD) pattern of TiO\({}_{2}\)/ZnO nanofibers. Figure 7: Average diameters of TiO\({}_{2}\)/ZnO nanofibers according to the amount of acetic acid. Figure 10 shows the FE-SEM images of TiO\({}_{2}\) nanofibers and TiO\({}_{2}\)/ZnO-1 and TiO\({}_{2}\)/ZnO-2 nanofibers used in methylene blue photolysis experiments for photocatalytic performance evaluation. The microstructure in all nanofibers shows a typical one-dimensional nanofiber structure and their shapes are similar to each other. The diameters of TiO\({}_{2}\), TiO\({}_{2}\)/ZnO-1, and TiO\({}_{2}\)/ZnO-2 nanofibers are 400 nm, 600 nm, and 400 nm, respectively. The diameters of TiO\({}_{2}\) nanofibers and TiO\({}_{2}\)/ZnO-1 and TiO\({}_{2}\)/ZnO-2 nanofibers are summarized in Table 3.  
Figure 11 shows the absorption spectra to evaluate the photocatalytic performance of TiO\({}_{2}\)/ZnO nanofibers observed by UV-Vis analysis. Figure 11a-d are the spectra of methylene blue without nanofibers, with TiO\({}_{2}\) nanofibers, with TiO\({}_{2}\)/ZnO-1 nanofibers, and with TiO\({}_{2}\)/ZnO-2 nanofibers, \begin{table}
\begin{tabular}{c c} \hline \hline
**Nanofibers** & **Diameter (nm)** \\ \hline TiO\({}_{2}\) & 400 \(\pm\) 32 \\ TiO\({}_{2}\)/ZnO-1 & 600 \(\pm\) 27 \\ TiO\({}_{2}\)/ZnO-2 & 400 \(\pm\) 13 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The nanofibers used in the methylene blue decomposition experiments for photocatalytic performance evaluation. Figure 10: FE-SEM images of electrospinning process conditions of nanofibers used in the decomposition of methylene blue for photocatalytic performance evaluation: (**A**) TiO\({}_{2}\) nanofibers, (**B**) TiO\({}_{2}\)/ZnO-1 nanofibers, (**C**) TiO\({}_{2}\)/ZnO-2 nanofibers. Figure 9: Illustration of photocatalytic mechanism in TiO\({}_{2}\)/ZnO heterojunction [46]. respectively. A total of 0.5 g of nanofiber was added in a 200 mL methylene blue solution (Figure 11b-d). All spectra have peaks at wavelength of 291 nm, 611 nm, and 656 nm. The intensity of the peaks gradually decreased with time because some factors such as UV, TiO\({}_{2}\) nanofibers, and TiO\({}_{2}\)/ZnO nanofibers, cause methylene blue to decompose. Since methylene blue shows a low absorbance when photolyzed by photocatalytic reaction, it is possible to know whether the photocatalytic reaction has occurred or not [50; 51]. To make sure methylene blue decomposed by photocatalytic reaction, the concentration of methylene blue on time was measured and normalized. The curves of normalized value (C/Co) were plotted as shown in Figure 12. In methylene blue solution without TiO\({}_{2}\) nanofibers, the concentration very slowly decreased with time and only UV irradiation decomposed the methylene blue.