## 3 Results and Discussion
### SEM Analysis of TOCgel/ZnO Composites' Characterization  
Figure 2 exhibits typical scanning electron microscopy (SEM) images of TOCgel/ZnO composite films prepared with different concentrations of zinc nitrate hexahydrate in the presence of HMT through the hydrothermal route at room temperature. In the present work, the contribution of carboxylate moieties of TOCgel in the tuning of the shape of ZnO structures directly grown within the cellulosic fibrous structure was studied. In comparison with the pure TOCgel (Figure 2A), our results indicate that the amount and the morphology of well-dispersed ZnO clearly depend on the concentration of Zn\({}^{2+}\) introduced in the reaction. A low-magnification view of composite films shown in Figure 2B-E demonstrates the formation of uniform flower-like ZnO superstructures well-dispersed onto TOCgel surfaces and having an average diameter of about 5 um. However, when the concentration of Zn\({}^{2+}\) is too low (50 mM), the flower-like ZnO structure appears to be not completely achieved (Figure 2B). The increase of the Zn\({}^{2+}\) concentration (\(\geq\)10 mM) strongly enhances the formation of a large quantity of well-shaped flower-like ZnO superstructures within the fibrous 3D network of TOCgel (Figure 2C,D). In addition, the size of these ZnO superstructures remains unchanged and is equal to 5 um approximately. The dimension of these superstructures was in agreement with the work of [PERSON] _et al._, which synthetize flower-like ZnO superstructures with approximately 5 \(\upmu\)m diameter [52]. Moreover, higher concentration of Zn\({}^{2+}\) (\(\geq\) 40 mM) leads to higher amount of ZnO superstructures which appear more collapsed and denser as shown in Figure 2E. In this case, the surface of cellulose fibers is covered by a continuous and relatively homogeneous-deposited ZnO coating. However, the resulting thick coating ZnO render the final composite film brittle and could not be suitable due to its poor mechanical behavior (very brittle). Furthermore, to study the effect of carboxylate groups (COO-) of TOCgel (\(\sim\)1.2 mmol/g) on the morphology of ZnO architectures, we prepared a composite film based on native cellulose fibers (\(\sim\)0.05 mmol/g) and ZnO under reaction conditions similar to that used for the TOCgel/ZnO composite film. Interestingly, we did not observe any regular flowerlike morphology via SEM (Figure 3A) of this sample (lower carboxylate amount); rather, some aggregated structures with undefined morphologies were observed. In comparison with SEM images provided in Figures 2 and 3, it is evident that the flower-like ZnO superstructures are formed only in the presence of carboxylates, which indicate the crucial role of carboxylate moieties of TOCgel in nucleation and growth process of ZnO in a particular direction that ultimately results in the formation of such anisotropic flower-like morphology, which is in accordance with the results reported elsewhere by other researchers [41]. These results indicate that the flower-like ZnO architectures can be formed in the presence of ascorbate ions which are similar to carboxylate moieties of oxidized cellulose material (TOCgel). In addition, the presence of carboxylate functions onto  
Figure 2: SEM images of TOCgel/ZnO composite films prepared with different concentrations of Zn\({}^{2+}\): 0 mM (**A**); 5 mM (**B**); 10 mM (**C**); 20 mM (**D**) and 40 mM (**E**). cellulose fibers surfaces enhances the individualization state of cellulose nanofibers and consequently, facilitates the diffusion phenomena of chemical reactants (_i.e._, Zn\({}^{2+}\); HMT, OH\({}^{-}\)) in the cellulosic material. Consequently, this phenomenon leads to the formation of well-dispersed ZnO superstructures onto the TOCgel network. Moreover, we prepared pure ZnO using the same hydrothermal conditions in the presence of HMT and without the addition of cellulose fibers in the reaction medium.