## 3 Results and Discussion  
### Characterization of Bi@Fe/CN  
SEM images were used to examine the surface morphology of the samples. The samples were irregular block granules with rough surfaces, as seen in Figure 1. It is obvious that there are defects on the surface of Bi@Fe/CN, which may be due to the bismuth iron oxide doped on the surface. The SSA of Bi@Fe/CN-1, Bi@Fe/CN-2, and Bi@Fe/CN-3 were 19.604, 37.073, and 43.964 m\({}^{2}\)/g, separately (Figure 2a). Additionally, the average pore diameters of Bi@Fe/CN-1, Bi@Fe/CN-2, and Bi@Fe/CN-3 were 3.72, 3.72, and 5.43 nm, respectively (Figure 2b). Obviously, Bi@Fe/CN-3 showed the largest pore size and SSA due to the higher hydrothermal temperature. This may be attributed to the decrease in the microcrystalline particle size of bismuth ferrite with the increase in hydrothermal temperature [23]. Generally, a larger SSA and favorable pore structure may provide more reaction sites for pollutants and oxidants in the reaction process, thereby increasing the removal efficiency of SMX [24]. The XRD patterns of the different samples are shown in Figure 3a. The presence of Bi\({}_{2}\)Fe\({}_{4}\)O\({}_{9}\) and g-C\({}_{3}\)N\({}_{4}\) was found in Bi@Fe/CN-3 (JCPDS #87-1522 and JCPDS #25-0090). Specifically, the peaks at 20 = 24.5\({}^{\circ}\), 27.5\({}^{\circ}\), and 47.7\({}^{\circ}\) were observed in Bi@Fe/CN-3, which correspond to the (101), (110), and (112) crystal planes of g-C\({}_{3}\)N\({}_{4}\), respectively. The result shows that the graphitic structure of g-C\({}_{3}\)N\({}_{4}\) remained even after the synthesis of the composite catalyst and subsequent pyrolysis [25]. Additionally, the diffraction peaks at 20 = 14.8\({}^{\circ}\), 28.2\({}^{\circ}\), 28.9\({}^{\circ}\), 33.7\({}^{\circ}\), and 47.0\({}^{\circ}\), which stood for the crystal faces of Bi\({}_{2}\)Fe\({}_{4}\)O\({}_{9}\) (001), (121), (211), (130), and (141), respectively, have been found in Bi@Fe/CN-3. Interestingly, the 14.8\({}^{\circ}\) peak was not present in Bi@Fe/CN-1 and Bi@Fe/CN-2; instead, the characteristic peaks at 27.7\({}^{\circ}\) (310), 30.4\({}^{\circ}\) (222), 32.9\({}^{\circ}\) (321), 52.4\({}^{\circ}\) (530), and 55.6\({}^{\circ}\) (611) corresponding to Bi\({}_{25}\)FeO\({}_{40}\) (JCPDS #46-0416) could be observed in Figure S1. This was attributed to the  
Furthermore, FTIR was used to study the surface functional groups of each sample. As depicted in Figure 3b, all samples exhibited stretching vibration peaks of the CN conjugated ring in the range of 1200-1700 cm\({}^{-1}\)[27]. The sharp peak at 808 cm\({}^{-1}\) represented the out-of-plane stretching vibration peak of the C\(-\)N heterocyclic ring [28]. Besides, the peak at 489 cm\({}^{-1}\) is attributed to the Bi\(-\)O stretching vibration in the BiO\({}_{6}\) octahedron, while the peak at 628 cm\({}^{-1}\) is related to the Fe\(-\)O in the FeO\({}_{6}\) octahedron in the system [29]. The presence of the mentioned characteristic peaks indicates that the g-C\({}_{3}\)N\({}_{4}\) functional group structure still remains in Bi@Fe/CN. XPS spectra were used to examine the elemental composition and valence on the surface of Bi@Fe/CN. As shown in XPS surveys of Bi@Fe/CN (Figure 4a), the characteristic peaks of the C, N, O, Fe, and Bi elements were observed. The peaks at the binding energies of 284.7, 285.9, 287.0, and 288.3 eV correspond to C\(-\)C\(-\)C\(-\)C, C\(-\)O/C\(-\)N, C\(-\)O, and N\(-\)C\(-\)O, respectively (Figure 4b) [30]. The N 1s XPS spectrum (Figure 4c) was deconvoluted into three peaks at the binding energies of 398.8, 399.4, and 401.4 eV, corresponding to pyridinic N, pyrrolic N, and graphitic N, respectively [31]. As shown in Figure 4d, the O 1s XPS spectrum can be fitted with three components: the lattice oxygen of the metal oxide at the binding energies of 529.7 eV [32], C\(-\)O (531.3 eV) [33], and C\(-\)O (531.9 eV) [34].