EtOH was prepared by vigorously stirring at 65 C under nitrogen atmosphere. After 5 min, NH4OH and H2O were added.
The final solution was centrifuged at 12,000 rpm for 10 min and then rinsed by DI water and ethanol for several times to
ensure the removal of unreacted materials. The particles were dried at 75 C for 24 h to obtain the final NPs.