## 3 Results and Discussion
### Photocatalytic Activity
Figure 14: Removal yields (**a**) and reaction kinetics (**b**) of the CIP disappearance upon the irradiation of FeOx (black), FeOx + H\({}_{2}\)O\({}_{2}\) (5 \(\times\) 10\({}^{-4}\) M) (red), and FeOx/g-C\({}_{3}\)N\({}_{4}\) composites + H\({}_{2}\)O\({}_{2}\) (5 \(\times\) 10\({}^{-4}\) M) (blue). The starting concentration of CIP is 10 ppm. Therefore, the photo-Fenton process further enhances the degradation efficiency of the (FeOx/g-C\({}_{3}\)N\({}_{4}\))_in_ composite material compared with the two parts alone. In addition, good adhesion over the glass slides was observed both during photocatalysis and the photo-Fenton reaction. An overview of the results obtained during the photocatalytic and photo-Fenton experiments are provided in Table 1.  
It has been observed that the (FeOx/g-C\({}_{3}\)N\({}_{4}\))_in_ composite material exhibits very important photoactivity during CIP removal under photo-Fenton conditions, showing an increase of one order of magnitude of the relative kinetic constant. This activity is higher than that of each counterpart alone. This picture is compatible with the arising of a heterojunction effect, such as a complete (where the solid-liquid interfaces of both materials are redox active) or partial (where only one, i.e., the FeOx conduction band is redox active) Z-scheme in the composite. Another possible explanation for the enhanced performance in the composite is that g-C\({}_{3}\)N\({}_{4}\) is simply acting as an electron acceptor, thus temporarily trapping electrons from the FeOx conduction band and inhibiting charge recombination [75]. We consider this last scenario unlikely, as the bandgap of g-C\({}_{3}\)N\({}_{4}\) is also compatible with visible absorption. A likely photocatalysis mechanism for the (FeOx/g-C\({}_{3}\)N\({}_{4}\))_in_ composite, presented in Figure 15, can be hypothesized based on our data and the current literature. It is known that the degradation of CIP in the presence of iron oxides and g-C\({}_{3}\)N\({}_{4}\) occurs mainly through the formation of active species, i.e., superoxide radicals (O\({}_{2}\)\({}^{-}\)), hydroxyl radical (OH), and holes (h+) [30; 48; 76; 77; 78; 79; 80]. In the case of (FeOx/g-C\({}_{3}\)N\({}_{4}\))_in_, the degradation of CIP is then expected to proceed via two different pathways depending on whether H\({}_{2}\)O\({}_{2}\) is present or not. Without H\({}_{2}\)O\({}_{2}\) (Figure 15a), the CIP degradation probably proceeds through the formation of a Z-scheme between FeOx and g-C\({}_{3}\)N\({}_{4}\) where primarily the (O\({}_{2}\)\({}^{-}\)) and (OH) active species are produced. Note that in this case, OH can only be produced through the reaction between h+ and water [78; 79; 80; 81; 82] and that the superoxide radicals are not very efficient in reacting with CIP [76; 83]. When H\({}_{2}\)O\({}_{2}\) is added, OH can also be produced through the reactions between CB electrons and H\({}_{2}\)O\({}_{2}\) and between superoxide and H\({}_{2}\)O\({}_{2}\) (Figure 15b) [76; 83; 76; 84; 85; 86; 87], resulting in a greater availability of highly active OH, which would explain the higher activity in the photo-Fenton mode. A detailed investigation of the composite materials working mechanism via transient UV-visible spectroscopy and using selective radical inhibitors will be undertaken in future investigations to confirm this mechanism hypothesis. Nevertheless, we consider the results reported here as a proof-of-concept that the in situ approach is a viable route for achieving heterojunction effects on thin-film photocatalysts for application in pollutant degradation. This is proven by data in Table 2 where a comparison is reported between the best results we obtained with the (FeOx/g-C\({}_{3}\)N\({}_{4}\))_in_ +H\({}_{2}\)O\({}_{2}\) (5 \(\times\) 10\({}^{-4}\) M) samples and literature data (90\(-\)97) for the removal of ciprofloxacin in water and additional information can also be found from literature [88; 89; 90; 91].