## 3 Results and Discussion
### Mechanism of the Formation of Flower-Like ZnO Superstructures  
The synthesis of ZnO flower-like structures was performed directly onto TOCgel surfaces through hydrothermal routes with some modification from the previous literature [34,41]. Indeed, the growth of ZnO superstructures onto the TOCgel surface can be related to the treatment of TOCgel with both Zinc nitrate and HMT solutions, which led to the nucleation and the growth of discrete seeds at the cellulosic fiber surface. Herein, HMT was used as a pH buffer by a slow release of OH- ions in the reaction medium to facilitate the controlled growth of ZnO particles though thermal decomposition as follows [58]:  
\[C_{6}H_{12}N_{4}+\ 6\ H_{2}O\ \leftrightarrow\ 6\ HCHO\ +\ 4\ NH_{3} \tag{1}\]  
\[NH_{3}+\ H_{2}O\ \rightarrow\ NH_{4}^{+}+\ HO^{-} \tag{2}\]  
Figure 5: FTIR spectra of TOCgel/ZnO composite, pure TOCgel, and pure ZnO. Figure 6: XPS survey spectra of the TOCgel/ZnO composite and the pure TOCgel (**A**); and high-resolution XPS scan of Zn 2p in the TOCgel/ZnO composite (**B**). \[Zn^{2+}+4\ HO^{-}\to Zn(OH)_{4}{}^{2-\frac{A}{2}}\!\!\to ZnO_{(solid)}+\text{water }+2 HO^{-} \tag{3}\]  
As mentioned in Equations (1)-(3), HMT releases slowly hydroxide ions in the medium, and then, zinc cations bound to OH\({}^{-}\) through coordination or electrostatic interaction, forming the complex Zn(OH)\({}^{2-}\) which contribute to the formation of ZnO nucleated via thermal treatment of the solution of Zn(OH)\({}^{4-}\) onto the TOCgel surface. In this study, the presence of polar moieties (COO\({}^{-}\) and OH) at the surface of TOCgel fibers should be responsible for the improvement of the heterogeneous nucleation of ZnO particles. The origin of this improvement lies especially in electrostatic forces that occur between COO\({}^{-}\) moieties of TOCgel and Zn\({}^{2+}\) ions introduced in the reaction medium. With the reaction proceeding, ZnO nanoparticles formed onto TOCgel surfaces would be aggregated into larger structures in order to minimize the total surface energy, which results in the formation of numerous spherical ZnO aggregations in the medium due to the Ostwald ripening [59]. Moreover, the very high concentration of carboxylate anions presented in the reaction medium could be responsible for the formation of such ZnO nanosheets. Indeed, with the reaction proceeding, the initial fast nucleation of ZnO leads to the decrease of the concentration of ZnO\({}^{2-}\) monomer. Thus, the adsorption of COO\({}^{-}\) ions on the positively charged Zn-(0001) plane would dominate in the competition with ZnO\({}^{2-}\) growth units. According to the literature [60, 61, 34, 62], the excess of COO\({}^{-}\) species stabilizes the surface charge and the structure of Zn-(0001) surfaces to some extent, retard growth along the _c_-axis and promotes the fast growth along the _a_-axis (0110) which results in the formation of ZnO nanosheets with a {2110} planar surface as illustrated in Figure 7.  
After that, these primary ZnO nanosheets (average thickness of \(\sim\)80 nm) might have many crystalline boundaries on their surfaces, which displayed more defects than other regions and were thermodynamically unstable. Consequently, they would tend to further diminish their energy through surface reconstruction, which would provide active sites for secondary heterogeneous nucleation and growth according to the literature [63, 34]. Figure 7: Schematic illustration of the proposed mechanism explaining the formation flower-like ZnO superstructures. In this context, the presence of these actives sites would promote the adsorption of carboxylate moieties of TOCgel (as a surfactant) especially onto the reactive planes of ZnO, usually the (001) plane [64], which would affect the secondary nucleation.