## 4 Conclusions  
TiO\({}_{2}\) and Fe\({}_{3}\)O\({}_{4}\)@TiO\({}_{2}\) were prepared by the solvothermal method. The structures and photocatalytic properties of TiO\({}_{2}\) and Fe\({}_{3}\)O\({}_{4}\)@TiO\({}_{2}\) were characterized. The photocatalytic rates of AR73 degradation by TiO\({}_{2}\) and Fe\({}_{3}\)O\({}_{4}\)@TiO\({}_{2}\) were compared under high-pressure mercury lamp irradiation. The complex of Fe\({}_{3}\)O\({}_{4}\)@TiO\({}_{2}\) can reduce the band gap to extend the light absorption range, maintain a certain proportion of (001) high-energy facet exposure, and make the absorption band of the photocatalyst move to the visible band. The band gaps of Fe\({}_{3}\)O\({}_{4}\)@TiO\({}_{2}\) and TiO\({}_{2}\) are 2.5 eV and 2.9 eV, respectively. As revealed by XRD analysis of TiO\({}_{2}\) and Fe\({}_{3}\)O\({}_{4}\)@TiO\({}_{2}\), Fe\({}_{3}\)O\({}_{4}\)@TiO\({}_{2}\) still has a certain proportion of (001) high-energy surface exposure. The percentages of (001) exposed surface of TiO\({}_{2}\) and Fe\({}_{3}\)O\({}_{4}\)@TiO\({}_{2}\) were estimated to be 12.1% and 25.2% by Raman spectroscopy, respectively. The photocatalytic degradation rates of Fe\({}_{3}\)O\({}_{4}\)@TiO\({}_{2}\) and TiO\({}_{2}\) at 24 min were 93.56% and 74.47%, respectively. The cycle experiment of photocatalytic degradation of Acid Red 73 by Fe\({}_{3}\)O\({}_{4}\)@TiO\({}_{2}\) showed that at the fifth cycle, the rate of dye degradation can reach 77.05%, but the rate of dye degradation can reach more than 90% after self-cleaning treatment. In the photocatalytic cycle experiment, the reason for the decrease in degradation rate is that some intermediates insoluble in water were adsorbed on the surface of the photocatalyst. However, the photocatalyst can realize self-cleaning under long-time irradiation from a high-pressure mercury lamp, and the intermediate products on its surface are degraded into small molecules and dissolved in water, improving the photocatalytic activity. According to the analysis of the results of the first-order kinetic equation model and photocatalytic degradation of AR73 by Fe\({}_{3}\)O\({}_{4}\)@TiO\({}_{2}\) and TiO\({}_{2}\), the photocatalytic effect of Fe\({}_{3}\)O\({}_{4}\)@TiO\({}_{2}\) is obviously better than that of TiO\({}_{2}\). The Fe\({}_{3}\)O\({}_{4}\)@TiO\({}_{2}\) photocatalyst can be recycled by magnetism and will not pollute the environment, so it has a certain development prospect in wastewater treatment.  
Conceptualization, L.S. and Z.Y.; methodology, L.S. and Q.Z.; software, X.O. and Q.Z.; validation, L.S., Z.Y. and Q.Z.; formal analysis, L.S., Z.Y. and Q.Z.; investigation, Z.Y. and J.M.; resources, L.S. and Z.Y.; data curation, L.S. and Q.Z.; writing--original draft preparation, L.S. and Z.Y.; writing--review and editing, L.S. and Z.Y.; visualization, X.S.; supervision, L.S. and W.G.; project administration, L.S., Z.Y. and W.X.; funding acquisition, L.S. and S.M. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China, grant number 11904268, and the Hubei Province Education Department of China, grant number Q20181702.  
Not applicable.  
The authors declare no conflict of interest.