*2.5. Photocatalytic Degradation of Rose Bengal Dye*

The photocatalytic degradation of RB dye at different time intervals for 60 min is measured in the presence of Fe2O3 incorporated TiO2 photocatalysts nanocomposites, (TiO2)1−x(Fe2O3)x, where x = 0, 0.1, 0.5, and 1.0) as shown in Figure 9. In this work, 100 ppm aqueous solution of RB dye is taken without pH adjustment, and 150 mg of catalyst was added in 150 mL of dye solution to test photocatalytic activity. The as-prepared nanocomposites were suspended in dye solution (this refers to all the photocatalysts set that were prepared) via ultrasonication for 5 min, then was stirred in the dark for 1 h to reach adsorption–desorption equilibrium. Then the solution was irradiated with UV light (25 W, UV-LED, 365 nm) to perform the experiments. The concentration of RB dye solution was continuously monitored via UV-Vis spectrophotometer at stipulated time intervals. The absorbance of RB dye decreased with time during the photocatalytic degradation, clearly observable at 546 nm.

**Figure 9.** UV-Vis absorbance of the photocatalytic degradation of 100 ppm of Rose Bengal dye (60 min) over1gL−<sup>1</sup> of (**a**) PT, (**b**) 0.1F, (**c**) 0.5F and (**d**) PF samples.

Based on previous reports, the conduction band position of TiO2 and Fe2O3 was estimated to be −0.22 and +0.30 V (versus NHE at pH = 0) [25]. While the valence band position could be calculated based on the bandgap values estimated from the DRS measurements (3.15 eV for TiO2 and 1.91 eV for Fe2O3). Formation of the internal electric field upon light irradiation and Fermi energy level alignment between TiO2 and Fe2O3 has promoted the separation of photogenerated electron-hole pairs. Therefore, as the UV light falls on

the suspended photocatalyst on the dye solution, photogenerated electron/hole pairs are formed on the conduction and valence bands. The photogenerated electrons are known for their ability to react with the dissolved oxygen forming O2 •− radicals while the holes react with OH− groups forming OH• radicals. Those oxidation species react efficiently with dye resulting in the degradation of the dye molecules. The following equations could explain the possible degradation mechanism using our photocatalyst composite:

((TiO2)-(Fe2O3) photocatalyst + UV irritation <sup>→</sup> (TiO2)-(Fe2O3) <sup>×</sup> (hVB++ eCB<sup>−</sup>)

$$\text{OH}^- + \text{h}\_{\text{VB}}^+ \rightarrow \text{OH}^\bullet$$

$$\text{O}\_2 + \text{e}\_{\text{CB}}^- \rightarrow \text{O}\_2^{\bullet-}$$

$$\text{O}\_2\text{\bullet}^{\bullet-} + \text{H}^+ \rightarrow \text{HO}\_2\text{\bullet}^\bullet$$

$$\text{HO}\_2\text{\bullet}^\bullet + \text{H}^+ + \text{e}\_{\text{CB}}^- \rightarrow \text{H}\_2\text{O}\_2$$

$$\text{H}\_2\text{O}\_2 + \text{e}\_{\text{CB}}^- \rightarrow \text{OH}^- + \text{OH}^\bullet$$

$$\text{H}\_2\text{O}\_2 \rightarrow \text{H}^+ + \text{HO}\_2^\bullet$$

$$\text{H}\_2\text{O}\_2 + \text{UV} \text{ irration} \rightarrow 2[\text{OH}^\bullet]$$

$$\text{OH}^\bullet \text{ and } \text{h}\_{\text{VB}}^+ + \text{RB dye} \rightarrow \text{CO}\_2 + \text{H}\_2\text{O}$$

The maximum photocatalytic degradation of the RB dye reaches 97% in the PT sample, which is pure TiO2 NPs. Pure TiO2 is widely known for its high oxidation potential compared to other photocatalysts under UV irradiation. In contrast, the photocatalytic activity of the other samples decreased with increasing the iron oxide content. Interestingly, the RB dye removal via adsorption increased remarkably with increasing the iron oxide content to almost 80% with the 0.5F catalyst. However, both adsorption and photocatalytic removal activity decreased to the minimum in the case of the pure iron oxide sample PF as shown in Table 4 and Figure 9. The enhanced adsorption capacity of the samples can be explained based on the surface texture analysis (surface area and pore size distribution). The terms related to degradation were calculated using the following formulae and briefed in Table 4:

$$\% \text{ Removal efficiency} = [\text{C}\_{\text{o}} - \text{C}] \times 100 / \text{C} \tag{7}$$

$$-\ln\left(\mathbb{C}/\mathbb{C}\_{0}\right) = \text{kt} \tag{8}$$

where Co and C are the initial and final concentration of RB dye, respectively, k is the 1st order rate constant, and t is the irradiation time. All the RB dye degradation parameters are briefed in Table 4.


**Table 4.** Photocatalytic degradation parameters.

\* Both dark adsorption and photocatalytic reaction were conducted for 60 min where the Photocatalytic degradation (removal efficiency) was calculated based on the initial concentration of the Rose Bengal dye after the adsorption step (Initial II). \*\* R<sup>2</sup> is a correlation coefficient.
