## 3 Results and Discussion
### FeOx/g-C\({}_{3}\)N\({}_{4}\) Composite Characterisation  
Figure 2 presents the FTIR spectra comparison between the FeOx iron vrchins film only, (FeOx/g-C\({}_{3}\)N\({}_{4}\))\(in\) and (FeOx/g-C\({}_{3}\)N\({}_{4}\))\(ex\) composites. Both the composite materials  
present the C-N and C=N vibration states in the wavenumbers range between 1200 and 1700 cm\({}^{-1}\). A triazine peak is also observed at 804 cm\({}^{-1}\). Sharper peak formation in the mentioned range is more evident in the (FeO\(\times\)/g-C\({}_{3}\)N\({}_{4}\))\(ex\) composite in comparison to (FeO\(\times\)/g-C\({}_{3}\)N\({}_{4}\))\(in\). This can possibly be attributed to the higher mass of g-C\({}_{3}\)N\({}_{4}\) on top of the FeOx supporting material. The corresponding FeOx spectrum involves the unresolved peaks below 1000 cm\({}^{-1}\), which are related to Fe-O stretching vibrations [54]. In Figure 2, a cluster of weak bands found at around 1500 cm\({}^{-1}\) correspond to the vibration modes of carbonates resulting from interaction of the surface with CO\({}_{2}\) from the air. This is commonly observed in iron oxide surfaces exposed to air humidity, which leads to the formation of hydroxides that are, in turn, susceptible to carbonate formation [55]. Micro-Raman analysis was also employed to investigate the properties of the (FeOx/g-C\({}_{3}\)N\({}_{4}\))\(in\) and (FeOx/g-C\({}_{3}\)N\({}_{4}\))\(ex\) as well as the reference FeOx support material. The Raman spectra (Figure 3) show the peaks of g-C\({}_{3}\)N\({}_{4}\) identified at 211, 359, 471, 588, 706, 768, 984, 1117, 1151, 1232, and 1312 cm\({}^{-1}\), respectively. For the (FeOx/g-C\({}_{3}\)N\({}_{4}\))\(ex\), spectrum shows essentially the g-C\({}_{3}\)N\({}_{4}\) bulk nature due to the complete coverage of the FeOx thin film. The (FeOx/g-C\({}_{3}\)N\({}_{4}\))\(in\) spectrum presents the g-C\({}_{3}\)N\({}_{4}\) with the characteristic peaks shifted with respect to the bulk powder g-C\({}_{3}\)N\({}_{4}\), such as the 521, 749, 810, 999, 1156, 1266, and 1346 cm\({}^{-1}\) and small traces of FeOx at 298 and 1386 cm\({}^{-1}\) being the other peaks in overlap with g-C\({}_{3}\)N\({}_{4}\). The shifting on the FeOx/g-C\({}_{3}\)N\({}_{4}\) (in situ) sample is attributed to the atomic lattice compressive stress because of synthesis procedure. In particular, in situ synthesis induces stress modifications on the FeOx because of the melamine thermal conversion to C3N4 on its surface that indeed favours strong adhesion with the substrate. On the other hand, in the ex situ preparation condition, the interaction between the already prepared g-C3N4 powder and the FeOx surface is milder, as proved by the poor adhesion [56]. Peaks assignments are presented in Supplementary Table S1. Figure 2: FTIR spectra comparisons between FeOx and the composites FeOx/g-C\({}_{3}\)N\({}_{4}\). Figure 4 presents the GIXRD patterns for both (FeO\(\times\)/g-C\({}_{3}\)N\({}_{4}\))\(in\) and (FeO\(\times\)/g-C\({}_{3}\)N\({}_{4}\))\(ex\). (FeO\(\times\)/g-C\({}_{3}\)N\({}_{4}\))\(ex\) clearly presents the peaks at 13.1\({}^{\circ}\) and 27.5\({}^{\circ}\) corresponding to the (100) and (002) crystal planes of the g-C\({}_{3}\)N\({}_{4}\) (JCPDS No. 87-1526). The diffraction peaks typical of hematite are labelled (012), (104), (113), (024), (116), and (122) (JCPDS No. 33-0664), and the observed traces of magnetite phase are labelled (111), (220), (311), (222), and (400) (JCPDS No. 19-0629). According to the growth mechanism of the urchins during the oxidizing annealing, the most oxidized iron phase (Fe\({}_{2}\)O\({}_{3}\)) is expected to most likely be present on the surface of the nanostructured film, in contact with the solution and with the g-C\({}_{3}\)N\({}_{4}\). On the other hand, in the (FeO\(\times\)/g-C\({}_{3}\)N\({}_{4}\))\(in\) there is clear evidence of hematite and traces of magnetite.