## 3 Results and Discussion
### Photocatalytic Activity
(**a**) HfO\({}_{2}\)-graphene (50 wt%) nanocomposites, (**b**) different dosages of catalysts, (**c**) c/c\({}_{0}\) vs. time, (**d**) number of recycling tests, and (**e**) scavenger test photodegradation of MB solution by all the catalysts. photocatalytic performance does not decrease significantly after five runs. The prepared catalyst was stable and reusable UV light illumination. The important reaction involved in MB dye decomposition, undergoing UV light illumination, was detected by scavengers' impact on the degradation of oxidizing species. The results are shown in Figure 7e. Different scavengers, such as benzoquinone for O2*\({}^{-}\), triethanolamine for h+, isopropyl alcohol for OH, and ethanol for e', have been utilized to trap the active species for decomposition. The decomposition performance was low to a maximum extent for benzoquinone. This result depicted the significant key role of O2*\({}^{-}\) in the photodecomposition of MB dye. However, the decomposition is affected by triethanolamine, depicting the role of h+ in the decomposition. The photocatalytic degradation is mainly based on O2*\({}^{-}\), with HfO2 under UV light illumination. The following equations show attainable reaction steps of the photocatalytic degradation mechanism under UV light irradiation. \[\text{HfO}_{2}+\text{hv}\rightarrow\text{HfO}_{2}\left(\text{h}^{+}+\text{e}^ {-}\right) \tag{5}\]  
\[\text{HfO}_{2}\left(\text{e}^{-}\right)+\text{Graphene}\rightarrow\text{HfO}_ {2}+\text{Graphene}\left(\text{e}^{-}\right) \tag{6}\]  
\[\text{Graphene}\left(\text{e}^{-}\right)+\text{O}_{2}\rightarrow\text{O}^{2 }+\text{Graphene} \tag{7}\]  
\[\text{HfO}_{2}\left(\text{h}^{+}\right)+\text{OH}^{-}\rightarrow\text{HfO}_ {2}+\bullet\text{OH} \tag{8}\]  
\[\text{HfO}_{2}\left(\text{h}^{+}\right)+\bullet\text{OH}+\text{O}_{2}+\text{ dyes}\rightarrow\text{CO}_{2}+\text{H}_{2}\text{O}+\text{ mineralization product} \tag{9}\]  
The enhanced photocatalytic activity under UV light illumination can be attributed to the effective separation of photogenerated electrons inhibiting recombination in the HfO2-graphene composite. Under UV illumination, e\({}^{-}\) is excited from the VB to the CB of HfO2. As a result, the electron-hole pairs are generated on the HfO2 surface (Equation (5)). These photogenerated electrons and holes play a crucial role in pollutant degradation. The previous report proposed graphene as an electron acceptor in the composite [59; 60]. As a result, the negatively charged electron is transferred into the graphene through the percolation mechanism (Equation (6)). Then, these negatively charged graphene sheets can activate the dissolved oxygen to produce O2*\({}^{-}\) (Equation (7)). In addition, the positively charged holes present in the HfO2 can react with the adsorbed water to form OH (Equation (8)). Finally, the reactive active species, such as holes, OH*, and O2\({}^{-}\), were directly involved in the oxidation processes, leading to the degradation of methylene blue (Equation (9)) [61]. Figure 8 shows the degradation of the photographic image of the MB dye solution under the UV-light irradiation. From Figure 8, it was found that the gradual degradation of MB dye takes place. At 180 min of irradiation, MB in the presence of HfO2-graphene nanocomposite was completely degraded. Figure 8: Photographic image of MB dye solution in the presence of HfO2-graphene nanocomposite after 180 min of UV-light irradiation. [PERSON] et al. demonstrated that graphene-SnO\({}_{2}\) composites spent nearly 360 min, even for the partial photodegradation of MB dye under UV light illumination [37]. [PERSON] et al. synthesized the P25-graphene composite that achieved 85% of the degradation of the initial MB dye under UV light irradiation, even for less than 60 min [36]. The graphene-TiO\({}_{2}\) composite displayed the degradation of methylene blue by 75% when illuminated with sunlight for 180 min. BiVO4-graphene catalyst degraded the methylene blue by 89% after illumination with visible light for 180 min [62]. [PERSON] et al.