## 2 Results and Discussion
### Photocatalytic Activity  
As mentioned in the literature, ZnO, in combination with other metal particles, can degrade various organic dyes in wastewater, including methylene blue (MB) [38] or rhodamine B [39], among others [64]. Consequently, we considered challenging to investigate the photocatalytic degradation of malachite green (MG) dye under irradiation with simulated solar light, by using L2 and L3 composite films as photocatalysts. To highlight the efficiency of photocatalysts L2 and L3, we firstly studied the photo-optical behavior of MG dye during the irradiation in the absence of any photocatalyst and when L1 film was used (Figure S3 SI). Although malachite green (MG) has a high photostability, it was observed that the absorption band at 616 nm was reduced with only 9% after 180 min of simulated solar light irradiation in the absence of any photocatalyst (Figure S3a). In the presence of L1 film (Figure S3b), there was a slight influence of about 3% (12% degradation after 60 min of irradiation) due to an absorption process, which produced a color change in this film from translucent white to a bluish color. This was more likely due to the absorption of the dye on the surface of the film, rather than a degradation process. Figure 9 illustrates the evolution of the UV-Vis absorption spectra of MG dye in aqueous solutions at a concentration of 10\({}^{-5}\) M, as was induced by composite films L2 and L3 under simulated solar light exposure, recorded after different irradiation times. The characteristic absorption band of the MG dye at 616 nm registered an absorbance decrease of 43% after 60 min of irradiation when assisted by L2 film (Figure 9a), while it totally disappeared when L3 hybrid film was used as photocatalyst after only 45 min exposure to simulated solar light (Figure 9b), proving 100% photodegradation activity of L3 film with no signature for intermediary products (Figure S4, SI). This change could be viewed with the naked eye (Figure 9b, inset). These experimental features highlighted that the L3 film, containing doped ZnO-SnO\({}_{2}\) particles, exhibited higher photocatalytic performance than the L2 film, which contained only ZnO particles. The superior photocatalytic response of ZnO-SnO\({}_{2}\) could be associated with the cubic shape of the particles incorporated in the polymer matrix, increased light absorption, and reduced electron-hole pair recombination. Figure 8: EDX analysis results for L1 (**a**) and the mapping images registered for L2 (**b**) and L3 (**c**) formulations. To further asses the performance of L2 and L3 photopolymerized hybrid films as photocatalysts, the kinetic rate constant K was determined by using the formula \(\rm{Kt}=\ln C_{0}/C_{t}\), where t is the irradiation time (min), \(\rm{C_{0}}\) is the initial concentration of MG dye, and \(\rm{C_{t}}\) represents the concentration after t min of irradiation, by assuming that the process follows pseudo-first-order kinetics [30]. Figure 9c,d show the \(\ln C_{0}/C_{t}\) plots as a function of the irradiation time. The kinetic rate constant for MG alone was \(\rm{K_{1}}=0.0005\) min\({}^{-1}\), and it increased to \(\rm{K_{2}}=0.002\) min\({}^{-1}\) for L1 film, \(\rm{K_{3}}=0.01\) min\({}^{-1}\) for L2 film, and \(\rm{K_{4}}=0.050\) min\({}^{-1}\) for L3 film. These results brought additional proof that the ZnO-SnO\({}_{2}\) cubes embedded in the polymer matrix exhibited a superior photocatalytic response compared to the equivalent samples containing ZnO star shapes. The mechanism followed by different dyes during the photocatalytic decomposition is described in detail in the literature [65; 66], as well as the reactions that occurred on the surface of the film [67]. To evaluate the reusability of the L3 composite film, repeated photocatalytic tests were performed using this sample. As illustrated in Figure 10, the photodegradation activity of the ZnO-SnO\({}_{2}\) composite was retained even after five consecutive cycles of reuse with an insignificant reduction of the photocatalytic performance.