## 3 Results and Discussions
### Photocatalytic Activity  
Under visible light irradiation at ambient temperature, the photocatalytic activity of the as-synthesized Co\({}_{3}\)O\({}_{4}\) NPs has been tested for the oxidation of organic dyes such as methylene blue (M.B.). Figure 8 displays the UV-vis spectra of M.B. aqueous solution under visible light irradiation (>420 nm) in the presence of Co\({}_{3}\)O\({}_{4}\) NPs as a photocatalyst over various time intervals. The UV-visible spectrum of M.B. dye showed a maximum absorption peak at 662 nm. However, in the presence of Co\({}_{3}\)O\({}_{4}\) NPs, the absorption peak of the M.B. dye solution decreased with the increase in time due to the continuous photocatalytic degradation of M.B. molecules in the system. The dye color started to fade considerably after 25 min of visible light irradiation, and after 175 min of exposure, the color reduction reached 93.45%. Therefore, the photocatalytic experiments show that the as-prepared Co\({}_{3}\)O\({}_{4}\) NPs have high visible-light photocatalytic activity and can treat dyes in textile wastewater.  
The photocatalytic degradation process of Co\({}_{3}\)O\({}_{4}\) NPs involves the use of light energy to initiate a chemical reaction that breaks down organic compounds into smaller, harmless molecules. This is associated with the breakdown of the chromophoric group and the transformation of dye into low-molecular-weight by-products [73]. The dye degradation is mainly due to generating electrons and holes (e\({}^{-}\) & h\({}^{+}\)) on the catalyst surface under irradiation [74]. Under visible light, Co\({}_{3}\)O\({}_{4}\) NPs undergo charge separation, and electrons in the valence band of Co\({}_{3}\)O\({}_{4}\) can be excited to its conduction band (e\({}^{-}\) C.B.), causing the generation of holes in the valance band (h\({}^{+}\) V.B.) simultaneously (Equation (2)). Thus, the OH- generated from the reaction (Equation (3)) is the main factor for the photodegradation of the dye because it is a strong oxidizing species, known to be an oxidation process. These ROSs can then react with organic molecules, such as pollutants or contaminants, breaking them down into smaller, less harmful compounds such as carbon dioxide and water.  
\[\text{Co}_{3}\text{O}_{4}+\text{hv}\rightarrow\text{e}^{-}\text{CB}+\text{h} ^{+}\text{VB} \tag{2}\]  
\[\text{H}_{2}\text{O}+\text{h}^{+}\text{VB}\rightarrow\text{H}^{+}+\text{OH} \tag{3}\]  
\[\text{MB}+\text{OH}\cdot\rightarrow\text{degradation products (CO}_{2}\text{, H}_{2}\text{O, NO}_{3}\text{, etc.)} \tag{4}\]  
The average C.R. degradation percentage was calculated using the following equation.  
\[\text{Degradation efficiency (\%)}=\text{(Co}-\text{Ct)}/\text{Co}\times 100 \tag{5}\]  
Figure 8: Evolution of UV–vis absorption spectrum of methylene blue (M.B.) under visible light irradiation using Co\({}_{3}\)O\({}_{4}\) NPs.  
where Co was the initial concentration of the M.B. dye and Ct was the concentration of the dye after photocatalytic degradation. To optimize the photocatalytic degradation process of Co\({}_{3}\)O\({}_{4}\) NPs, several factors need to be considered, including the type of organic compound being degraded, the intensity and wavelength of the light source, the size and shape of the nanoparticles, and the presence of other compounds in the environment.