## 3 Results and Discussion
### Photocatalytic Degradation  
The photocatalytic activity of pure ZnO (rod-like), TOCgel/ZnO (FL) and native cellulose/ZnO (aggregated nanoparticles) in the degradation of MB was performed in air at room temperature and under the exposure of the ultraviolet lamp for 2 h. The MB was chosen in this work due to their strong adsorption characteristics on many surfaces, good resistance to light degradation and a well, defined optical absorption band in the visible region [67].  
From Figure 10A, it can be observed that the absorption peak intensity of MB in the 500-700 nm regions (especially at 665 nm) decreases rapidly, compared to the intensity of the initial MB solution, because supported ZnO structures promotes the catalytic photodegradation [68]. This result was also confirmed by the significant change in the coloration of dried samples (from blue to violet) under the exposure to the UV light lamp for about two hours as represented in the optical images (Figure 10B). The violet color of irradiated sample could be related to the degradation state of MB molecules through the photocatalysis effect of flower-like ZnO superstructures embedded in the TOCgel/ZnO composite material. Of course, the coloration change and the decrease of the absorption peak of MB were more pronounced in the case of the pure ZnO, because this sample consist in a 100% ZnO particles. However, in the case of composites, the amount of ZnO was about 15%-20% (_w/w_) as determined by the thermal analysis (Figure 9).  
Figure 10: UV-Vis spectra of different supernatants of MB obtained after UV-irradiation for 2 h; (**A**) optical images of corresponding dried samples after photocatalytic process for pure ZnO and TOCgel materials (**B**).  
According to the literature [69], the photocatalytic process of ZnO can be elucidated by the photogeneration process as illustrated in the Figure 11.  
In fact, the UV radiation absorbed by ZnO induces the formation of electron-hole pairs through the photogeneration process. Then, the interaction between holes and hydroxyl groups of ZnO contributes to the formation of the strong oxidative hydroxyl radicals (\({}^{\circ}\)OH). Furthermore, H\({}_{2}\)O could also react with holes to form hydroxyl radicals. Thus, the strong oxidant property of the hydroxyl radical lead to the oxidation of organic materials, and of course to the degradation of MB as shown in Figure 10.  
In the present study, it appears interesting to appoint the effect of the morphology of ZnO structures onto their photocatalytic degradation efficiency in the degradation of MB solution.  
To further explain this, a further experiment was performed using other morphologies of ZnO under the same conditions to demonstrate the effect of ZnO morphologies on the photocatalytic efficiency of ZnO structures. In this context, we investigated the photocatalytic performance of native cellulose/ZnO composite in the degradation of MB solution under the UV irradiation for 2 h. As shown in Figure 10A, the decrease of the absorption peak (at 665 nm) appears to be more pronounced in the case of TOCgel/ZnO (FL) than that of native cellulose/ZnO (aggregated nanoparticles), which clearly indicates the influence of ZnO morphologies on the photocatalytic degradation process. Our results are in clear accordance with the literature in which flower-like ZnO structures exhibit the highest catalytic activity efficiency more than other structures such as spindelike, swordlike, commercial ZnO materials [13,52]. As shown in the SEM images (Figure 3); we demonstrated that the oxidized cellulose or TOCgel (rich in carboxylate groups) should be responsible for the formation of flower-like ZnO superstructures in the former composite, which was not the case of the oxidized cellulose.