*2.2. Preparation of ZnO Particles*

For preparation of ZnO particles [31], 878 mg (0.004 mol) Zn(CH3COO)2·2H2O was added into 40 mL diethylene glycol with continued ultrasonic until dissolved completely, followed by transferring into a teflon-lined stainless steel autoclave (45 mL). Then, the autoclave was heated to 160 ◦C from room temperature in an oven and maintained for 5 h. After the reaction, the white power was obtained by centrifugation. Then, the power was washed with ethanol and water in turn, and finally dried in the oven at 100 ◦C for 5 h.
