*3.3. Synthesis of UCNPs@R-TiO2 Nanocomposite*

UCNPs@R-TiO2 were prepared as follows [11,20,40]. Firstly, the R-TiO2 nanoparticles (TiO2−x) were prepared as follows: 4 μL of titanium isopropoxide was dropped into a solution containing 1.2 mL of HF and 20 mL of isopropanol. After the obtained mixture was stirred for 10 min under an argon atmosphere, 5 μL of TiCl3 solution was added to it [11]. Then, 20 mg of UCNPs were also added to the obtained mixture under an ultrasonic bath and the mixture solution continued to sonicate for 30 min under an argon atmosphere. The obtained mixture was then encapsulated into a 50 mL autoclave flask, heated to 180 ◦C at a heating rate of 3 ◦C/min, and kept at 180 ◦C for 20 h under an argon atmosphere. The produced UCNPs@R-TiO2 (40%) nanocomposite was collected and washed, first with ethanol and then with ultrapure water. It was then dried at 60 ◦C for 10 h in a vacuum environment. The R-TiO2 nanoparticles and UCNPs@R-TiO2 nanocomposites with different mass ratios were prepared according to the above method by changing the amount of UCNPs.
