## 2 Materials and Methods
### Preparation of Nanocomite  
Fe\({}_{3}\)O\({}_{4}\) was prepared using the solvothermal method. Here, 2.7 g of FeCl\({}_{3}\)\(\cdot\)6H\({}_{2}\)O was dissolved in 80 mL of ethylene glycol and stirred magnetically until fully dissolved in a beaker. Then, we poured 2 g of polyethylene glycol-4000 into the above solution and stirred magnetically until it was completely dissolved. Finally, 7.2 g of sodium acetate was poured into a beaker and stirred magnetically until it was dissolved. After complete dissolution, the solution was poured into a reaction kettle and heated at 180 \({}^{\circ}\)C for 10 h, then washed 3 times with ethanol, placed in a vacuum drying oven, and heated at 40 \({}^{\circ}\)C until completely dried. Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\) was prepared using the sol-gel method. Here, 0.2 g of Fe\({}_{3}\)O\({}_{4}\) powder was added into 80 mL of absolute ethanol and ultrasonically dispersed for 30 min until the Fe\({}_{3}\)O\({}_{4}\) powder was fully mixed with the absolute ethanol. Next, 20 mL of ultrapure water was added into the above suspension, then 5 mL of ammonia was dropped slowly into the mixture, and the suspension was ultrasonically dispersed and mechanically stirred for 30 min and 30 min, respectively. After that, 4 mL of tetraethyl orthosilicate was dropped slowly into the above suspension, and the mixture was continually mechanically stirred for 6 h. Finally, the solution was magnetically separated by permanent magnets, the upper layer was poured out, and the black magnetic fluid was washed three times with absolute ethanol and placed into the vacuum drying box and dried at 40 \({}^{\circ}\)C.  
Preparation of Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\)@TiO\({}_{2}\) (FS-NT (0 mL), where '0 mL' represents the amount of N source in NH\({}_{4}\)OH, which was 0 mL) nanomaterials was carried out using the sol-gel method. Here, 7 mL of tetrabutyl titanate was added dropwise into 120 mL of absolute ethanol and ultrasonically dispersed for 10 min. Then, 0.2 g of Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\) was added into the above suspension and continued to be ultrasonicated for 20 min. The beaker containing the suspension was then placed in a water bath heated to a temperature of 40 \({}^{\circ}\)C, and the suspension was mechanically stirred for 8 h. After the reaction, the solid powder and solution in the beaker were magnetically separated with a magnet. The separated solid powder was washed three times with absolute ethanol, and then the washed powder was dried in a vacuum drying oven at 40 \({}^{\circ}\)C for 10 h. After drying, the FS-NT (0 mL) was placed in an atmosphere furnace for annealing. Argon was selected as the protective gas and the annealing temperature was set at 450 \({}^{\circ}\)C and the heating time was 3 h.  
A batch of Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\)@N-TiO\({}_{2}\) (FS-NT (2-5 mL), where '2-5 mL' represents the amount of N source in NH\({}_{4}\)OH, which was 2, 3, 4, and 5 mL, respectively) doped with different nitrogen contents was prepared using the sol-gel method. FS-NT (2-5 mL) with different nitrogen contents only required changing the amount of NH\({}_{4}\)OH added, and the sample preparation process was the same. The preparation of FS-NT (2-5 mL) used the process of preparing FS-NT (3 mL) as an example. Firstly, mixture A (7 mL of tetrabutyl titanate was added into 120 mL of absolute ethanol and dispersed by ultrasound for 10 min, then 0.27 g of Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\) was added into the above solution and dispersed by ultrasound for 15 min) and solution B (3 mL of NH\({}_{4}\)OH and 2 mL of deionized water were added into 20 mL of absolute ethanol) were prepared. Then, solution B was slowly added dropwise to mixture A. The mixture of A and B was stirred for 7 h. The magnetic separation was carried out with a magnet.