## 2 Results and Discussion
### UV-Vis DRS and Visible Light Photodegradation Studies  
UV-Vis diffuse reflectance spectra (UV-Vis DRS) of pristine TiO2, unwashed and washed TP2 are shown in Figure 7a. All samples have a bit of difference in the pattern in UV-Vis diffuse reflectance spectra. It is obvious that pristine TiO2 has the highest absorbance in UV-Vis spectra followed by washed TP2 and unwashed TP2 samples. Even though the modified TiO2 or TP2 has lower absorbance as compared to unmodified pristine TiO2, this result is expected because there are no bulk property modifications other than on the TP2 surface. Therefore, the chemical properties for TP2 and its band gap energy do not notably change even after surface alteration. According to [PERSON] et al. [49], from the solid state band theory, the absorption coefficient can be described as a function of incident photon energy; \((\alpha\hbar\
u)^{2}=\mathrm{A(h\
u}-E_{\mathrm{g}})\), where \(\alpha\) is the absorption coefficient (cm\({}^{-1}\)), A is a constant, h\(\
u\) (eV) is the energy of excitation and \(E_{\mathrm{g}}\) is the band gap energy. The band gap energy can be performed by plotting \((\alpha\hbar\
u)^{2}\) vs. h\(\
u\). The Tauc plot on the photon energy-axis gives the value of the direct band gap energy of semiconductors [50]. Figure 7b shows a graph of \((\alpha\hbar\
u)^{2}\) vs. h\(\
u\), named as [PERSON]'s graph plot. Theoretically, it seems that the band gap energy for all samples did not change and stayed approximat  
Figure 7: Graph plots for: (**a**) UV–Vis diffuse reflectance spectra (DRS); (**b**) [PERSON]’s equation; (**c**) percentage remaining of MB dye under visible light irradiation; and (**d**) adsorption study. Abs, absorption. the band gap energy of TiO\({}_{2}\) photocatalystyst in TP0 did not change to a visible light active response, and this was similarly observed in Figure 7c where no photodegradation of 12 mg\(\cdot\)L\({}^{-1}\) MB was observed for TP0 under visible light irradiation. Only decolorization of MB was observed as reported on the adsorption site of the porous surface of TP0 as proven by the adsorption graph in Figure 7d and discussed in our previous study [51]. Unwashed TP2 showed a low photocatalytic degradation of MB dye due to the C-C bond in PEG that makes TP2 become slightly active under the visible light condition. Surprisingly, TP2 shows an active photodegradation under visible light where a complete decolorization of 12 mg\(\cdot\)L\({}^{-1}\) MB was achieved at 75 min. Although there is no shifting occur in band gap energy of TP2 due to its sole-surface modifications, it is believed that the visible light active ability in TP2 is due to the formation of C=O resulting from the oxidation of C-O bond in PEG detected by XPS and FTIR spectrum, which have been discussed earlier in Figures 5 and 6. According to [PERSON] et al. [37], C=O bond can act as an electron injector that initiated the formation of hydroxyl radical, thus eventually degraded the MB dye pollutant. TP2 had undergone the photoluminescence (PL) analysis at low activation energy irradiation to observe the effect of C=O activation under visible light irradiation. The PL emission spectra can be used to reveal the efficiency of charge carrier trapping, immigration and transfer and to understand the fate of photo-induced electrons and holes in a semiconductor [52; 53]. It is known that the PL spectrum is the result of the recombination of excited electrons and holes where the lower PL intensity means a lower recombination rate of electron-hole pairs under light irradiation [54]. Figure 8 shows the PL spectra of pristine TiO\({}_{2}\), TP2 and unwashed samples of TP2 using the excitation wavelength of 500 nm. The photoluminescence spectrum in this study that was conducted under low excitation energy (500 nm) was meant to confirm that the excitation of electrons happened under a low energy level (visible light spectrum). It is shown that increased PL intensity leads to increased absorption of low excitation energy, resulting in higher photocatalytic activity [55].