## 3 Results and Discussion
### XPS Analysis  
XPS was used to analyse the chemical bonding and composition of all the prepared materials. Figure 10 presents the full survey scan of the FeOx iron urchins, (FeOx/g-C\({}_{3}\)N\({}_{4}\))\(in\), and (FeOx/g-C\({}_{3}\)N\({}_{4}\))\(ex\) composites that are compared to the FeOx as a support material only. The XPS survey spectra reveals presence of Fe, O, N, and C.  
Figure 9: (FeOx/g-C\({}_{3}\)N\({}_{4}\))\(in\) TEM and HRTEM analysis (**a**). Bright field TEM image of a crystalline petal whose SAED diffraction pattern is given in (**c**). The intensity profile obtained from (**c**) is shown in (**b**): gray lines indicate gross intensity (solid line) and the subtracted background (dashed line), and the red line is the resulting net intensity. Figure 11 shows Fe 2p\({}_{3/2}\) core level spectra of FeOx and its composites, the Fe 2p survey can be found in the Supplementary Materials (Figure S12a). The Fe 2p\({}_{3/2}\) core level main peak position (without curve fitting) is located at around 711 eV binding energy (BE) with spin-orbit splitting and a satellite at 719 eV. The peak at 711 eV was further deconvoluted into three peaks, namely 709.8 eV, 710.8 eV, and 712 eV, which correspond to +3 oxidation state of alpha hematite [62; 63; 64]. This clearly indicates the presence of dominating alpha Fe2O3 phase (hematite) with the +3 oxidation state on the surface of FeOx and (FeOx/g-C3N4)\(ex\) samples. For the XPS spectra of (FeOx/g-C3N4)\(in\), the main BE positions at 711.5 eV with spin-orbit splitting indicate the presence of a Fe3O4 phase. Therefore, the presence of both hematite and Fe3O4 cannot be excluded in the samples, thus further supporting our previously reported (Figure 4) XRD results. In Figure 12a, the C1s spectrum of the composite materials contains two peaks at 285 eV and 288.9 eV for (FeOx/g-C3N4)\(in\), which arise from C-N and sp\({}^{2}\)-hybridized C in N=C-N. However, for (FeOx/g-C3N4)\(ex\), the peaks originate at 285.1 eV and 289.3 eV [65; 66]. Thus, the C 1s core level spectra of both composite films, i.e., (FeOx/g-C3N4)\(ex\) and (FeOx/g-C3N4)\(in\), clearly show the presence of the C-N bonding structure coming from g-C3N4 with specific contributions as shown in adjacent peaks resulting from the fitted components. A slight modification of peak positions related to g-C3N4 was observed in case of the (FeOx/g-C3N4)\(in\) composite film, which could be attributed to interface effects with FeOx [65; 66; 67; 68; 69; 70]. The N 1s spectrum (Figure 12b) related to (FeOx/g-C3N4)\(in\) is deconvoluted into three different peaks at 399.8, 401.5, and 402.1 eV, which are attributed, respectively, to sp\({}^{2}\)-hybridized N in C=N-C, sp\({}^{3}\)-hybridized N in N-C3, and C-N-H from amino groups and \(\pi\) excitations in heterocycles. The N 1s core level spectra also clearly indicate the respective bonding contributions (reported with fitted components) originating from the g-C3N4 chemical structure. It is also interesting to see that the fitted components with respective chemical structures show modifications in the intensity as well. This is not fully understood yet but probably results from minor changes in the atomic lattice arrangement. Also, the O 1s spectra reported in the Figure S12b, identifies the presence of Fe-O chemical bonding as in Fe oxides with two peaks at 530 eV and 532.6 eV arising from the lattice oxygen in FeOx and the related hydroxide-oxyhydroxide system [71; 72]. The small peak  
Figure 10: XPS survey scan for all elements in the FeOx urchin and its composites with g-C3N4. around 532 eV is associated with the presence of adsorbed water on the surface of the samples [73]. Figure 11: XPS spectra of the Fe 2p\({}_{3/2}\) core level and deconvolution peaks of the iron urchin and its composites with g-C\({}_{3}\)N\({}_{4}\). All the spectra were normalized to the maximum of the peak and curve fitting was achieved using [PERSON] background and Voigt profiles.