## 3 Results and Discussion
### Photocatalytic Activity
prepared TiO\({}_{2}-\)Graphene nanocomposites that degraded the MB dye under sunlight irradiation for 180 min; the degradation achieved was 75%. [PERSON] et al. prepared G- \(\alpha\)MoO\({}_{3}\) nanocomposite, and it degraded the MB dye under UV illumination, with a degradation percentage of 97% [63]. [PERSON] et.al. prepared GO-TiO\({}_{2}\) under UV-light irradiation, and its degradation achieved was 75% [64]. The present results were compared with the existing results in Table 1.  
To determine photocatalytic tetracycline degradation of HfO\({}_{2}\)-graphene nanocomposites (50 wt%), 15 mg of HfO\({}_{2}\)-graphene nanocomposites mixed with the 20 ppm tetracycline solution (100 mL, pH = 5.19) and treated under UV light irradiation. To prevent the physisorption of tetracycline on the photocatalysts during degradation, tetracycline solution and HfO\({}_{2}\)-graphene nanocomposites were stirred in the dark for 2 h with a magnetic stirrer. Following that, the reaction mixture was then exposed to UV light irradiation with continuous stirring for 180 min to examine degradation. The characteristic absorption peak at 357 nm is used to measure tetracycline degradation concentrations [65]. As a result, HfO\({}_{2}\)-graphene degrades tetracycline with high efficiency, as shown in Figure 9A. In the blank test, without the photocatalyst, tetracycline degradation efficiency was measured under UV light. The HfO\({}_{2}\)-graphene (50 wt%) photocatalyst degrades 81 \(\pm\) 2% of tetracycline after 180 min, suggesting that tetracycline can be degraded more efficiently under UV light (Figure 9A). The degradation kinetics of HfO\({}_{2}\)-graphene under UV light were further investigated based on the pseudo-first-order kinetic model:  
Figure 6:0  
where C\({}_{0}\) indicates initial tetracycline concentration, C\({}_{t}\) represents its concentration as a function of degradation time \(t\), and k represents the reaction rate constant. Based on the results, we have identified a pseudo-first-order kinetic model for tetracycline degradation in the presence of UV light (Figure 9B). To validate the HfO\({}_{2}\)-graphene nanocomposite impact on the degradation of tetracycline, we conducted a photodegradation experiment without HfO\({}_{2}\)-graphene. Nevertheless, it rarely shows any photodegradation of tetracycline without HfO\({}_{2}\)-graphene (Figure 9A). According to these results, the synthesized HfO\({}_{2}\)-graphene nanocomposite is an extremely photoactive material, and the degradation process takes place via photocatalysis. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Nanocomposites** & **Irradiation Source** & **Dye** & **Irradiation Time (min)** & **Degradation (\%)** & **References** \\ \hline P25-graphene & UV & MB dye & 60 & 85 & [36] \\ \hline Graphene-SnO\({}_{2}\) & UV & MB dye & 360 & 100 & [37] \\ \hline TiO\({}_{2}\)-graphene & sunlight & MB dye & 180 & 75 & [50] \\ \hline graphene-BiVO\({}_{4}\) & Visible light & MB dye & 180 & 89 & [62] \\ \hline G- \(\alpha\)MoO\({}_{3}\) & UV & MB dye & 180 & 97 & [63] \\ \hline GO-TiO\({}_{2}\) & UV & MO dye & 240 & 75 & [64] \\ \hline HfO\({}_{2}\)-graphene & UV & MB dye & 180 & 98.5 & This work \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of photocatalytic MB dye degradation efficiency of HfO\({}_{2}\)-graphene.