## 3 Results and Discussion  
Physical and Optical Properties and Surface Chemistry of Fe\({}_{3}\)O\({}_{4}\), BiOI, and BF-5  
XRD analyzed the structures and phases of the obtained materials; the diffractograms are presented in Figure 3. The diffraction peaks of Fe\({}_{3}\)O\({}_{4}\) 20 = 30.1\({}^{\circ}\), 35.4\({}^{\circ}\), 43.0\({}^{\circ}\), 53.4\({}^{\circ}\), 56.9\({}^{\circ}\), and 62.5\({}^{\circ}\), attributable to the crystalline planes (JCPDS. 88-0315) (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0), showed a high purity of the magnetite obtained. The diffraction peaks of the individual BiOI were identified (Figure 3b), which appear at 20 = 9.6\({}^{\circ}\), 19.4\({}^{\circ}\), 29.6\({}^{\circ}\), 31.6\({}^{\circ}\), 45.4\({}^{\circ}\), 55.2\({}^{\circ}\), and 66.2\({}^{\circ}\), corresponding to the tetragonal phase of the semiconductor. According to the standard cards, (JCPDS.73-2062) and (JCPDS 88-0315), all diffraction peaks can be indexed to BiOI and Fe\({}_{3}\)O\({}_{4}\) by their crystalline planes (0 0 1), (0 1 2), (1 1 0), (0 2 0), (1 1 2), (2 2 0) and (3 1 1), (4 4 0), respectively. The results indicate a coexistence of BiOI and magnetite phases in the BF-5 material, observing an abundant BiOI phase with a high facet exposure (1 1 0) and a minority phase of Fe\({}_{3}\)O\({}_{4}\), as reported by several authors, even with stoichiometric ratios higher than those presented in this study [18; 22; 41; 42]. This can be seen in the planes (3 1 1) (4 4 0), visible in the magnetic material. In addition, there is no presence of pure bismuth (JCPDS 85-1329), evidencing that the materials were obtained without changing the original crystalline structure of BiOI. Figure 3: X-ray diffraction patterns of the synthesized materials: (**a**) Magnetic–Fe\({}_{3}\)O\({}_{4}\), (**b**) individual BiOI, (**c**) BiOI/Fe\({}_{3}\)O\({}_{4}\)–BF-5. To have greater clarity regarding the intensity and definition of the BF-5 signals, the average crystal sizes were estimated using XRD data from the samples synthesized by the Scherrer Equation (2), using the most intense peak widenings of the spectrum [43; 44]. \[L=\frac{K\lambda}{\beta\text{cos}\,\theta} \tag{2}\]  
where \(L\) represents the sizes of the nanoparticles, \(K\) represents the Scherrer constant equal to 0.9494; \(\lambda\) is the wavelength of the X-rays; \(\beta\) is the half-height width of the peak; and \(\theta\) is half of a diffraction peak [45; 46]. An average BF-5 crystal size of 15.9 nm was obtained, and the estimate of the pure BiOI crystallite was 17.9 nm. These findings conclude that reasonable morphology control was achieved in the solvothermal synthesis and that there is no higher crystallinity between the sample obtained and the pure materials, considering BF-5 as a promising nanomaterial for photocatalysis. The higher peak intensity observed in Figure 3c could only be attributed to a greater presence of the (1 1 0) BiOI face on the surface of BF-5. [PERSON] et al. [47] identified that the BiOI catalyst with the predominant facet (1 1 0) shows superior photocatalytic activity on the degradation of phenolic compounds compared to those with the predominant facet (0 0 1). Figure 4c,d show the morphology of the BiOI semiconductor. This material has microspheres in the form of rosettes with an average diameter of 2.8 \(\upmu\)m. Figure 4e,f show the BF-5 material synthesized in this study. The BiOI nanosheets coat the magnetite to form a three-dimensional structure with an approximate diameter between 200-450 nm, attached in the form of a cluster, as reported in the study by [PERSON] et al. [20]. It is observed that the Fe\({}_{3}\)O\({}_{4}\) nanoparticles are embedded and coated by the BiOI nanosheets, forming a heterostructure that benefits the photocatalytic process by keeping the semiconductor surface exposed to pollutants in water [48].