## 3 Results and Discussion
### Photocatalytic Activity  
The photocatalytic activity of the prepared materials was evaluated at the proof-of-concept level in CIP degradation. First the g-C\({}_{3}\)N\({}_{4}\) dispersed powder and coating samples were tested. The adsorption of powders and the poor photocatalytic degradation of CIP in films and powders was observed (Figure S13). Also, the (FeO\(\times\)/g-C\({}_{3}\)N\({}_{4}\))\(ex\) thin films show only 5% CIP disappearance after 180 min irradiation (Figure S14). The observed inferior CIP removal of (FeO\(\times\)/g-C\({}_{3}\)N\({}_{4}\))\(ex\) was due to the weak g-C\({}_{3}\)N\({}_{4}\) adhesion with consequent partial detachment during the experiment, as pointed out by SEM. Therefore, we have not performed further in-depth analysis of these samples. The photocatalytic activity of the (FeO\(\times\)/g-C\({}_{3}\)N\({}_{4}\))\(in\) thin films were compared in the photocatalytic removal of CIP through visible irradiation. Results are reported in Figure 13, and the corresponding UV-Vis spectra obtained are reported in Supplementary Figure S16. Low adsorption (less than 5%) is observed in the dark during the equilibrium period. Upon visible illumination, the system (FeO\(\times\)/g-C\({}_{3}\)N\({}_{4}\))\(in\) shows a better performance and leads to a 14% of CIP disappearance after 180 min of irradiation with a kinetic constant of 8.28 \(\times\) 10\({}^{-4}\) min\({}^{-1}\). The results obtained via (FeO\(\times\)/g-C\({}_{3}\)N\({}_{4}\))\(in\) are of relevance if one considers the control experiments reported in Figure S15. It is observed that both thermolysis at 50 \({}^{\circ}\)C and visible irradiation in the absence of a photocatalyst (photolysis) do not cause any decrease in CIP concentration. This demonstrates that the presence of the composite (FeO\(\times\)/g-C\({}_{3}\)N\({}_{4}\))\(in\) is mandatory for achieving CIP degradation and that degradation is not a consequence of the temperature increase from the heat developed by the radiation system or photolysis. Figure 13: (**a**) removal yields and (**b**) corresponding reaction kinetics of the CIP upon irradiation of (FeO\(\times\)/g-C\({}_{3}\)N\({}_{4}\))\(in\) immersed in aqueous solutions containing CIP (C\({}_{0}\) = 10 ppm). Given the results obtained from (FeOx/g-C\({}_{3}\)N\({}_{4}\))\(in\), the photo-Fenton reaction was tested with the addition of H\({}_{2}\)O\({}_{2}\). Results are reported in Figure 14, where experiments performed with FeOx alone and in the presence of H\({}_{2}\)O\({}_{2}\) 5 \(\times\) 10\({}^{-4}\) M are reported for comparison. The irradiation of FeOx alone (black) caused a disappearance of CIP of about 15%. When H\({}_{2}\)O\({}_{2}\) (5 \(\times\) 10\({}^{-4}\) M) is present, the material showed a higher CIP disappearance in the dark (17%), possibly due to an initial Fenton reaction, and upon irradiation a disappearance of 35% of CIP is observed with moderate improvements of the degradation after the initial 60 min of illumination on both photo-Fenton cases (FeOx + H\({}_{2}\)O\({}_{2}\) and (FeOx/g-C\({}_{3}\)N\({}_{4}\))\(in\) + H\({}_{2}\)O\({}_{2}\)). This moderate enhancement of degradation is attributed to slight temperature changes and is consistent with the literature [74]. The calculated kinetic constant is k = 1.6 \(\times\) 10\({}^{-3}\) min\({}^{-1}\). A comparison of the results of FeOx (Figure 14) with (FeOx/g-C\({}_{3}\)N\({}_{4}\))\(in\) (Figure 13) shows that their photocatalytic activities are very similar. However, the addition of H\({}_{2}\)O\({}_{2}\) (5 \(\times\) 10\({}^{-4}\) M) to the aqueous solution of CIP in which (FeOx/g-C\({}_{3}\)N\({}_{4}\))\(in\) is present had a booster effect: in fact, 60% of CIP disappearance is achieved after irradiation, a value significantly higher than that observed with FeOx and (5 \(\times\) 10\({}^{-4}\) M) H\({}_{2}\)O\({}_{2}\) alone (6%).