SD (98%), SM2 (98%), SMM (98%), SMZ (98%), urea, n-butyl titanate, n-butanol, and ammonium dihydrogen phosphate were all purchased from Aladdin Reagent Co., Ltd (Shanghai, China). Potassium dichromate (K2Cr2O7, AR, 99.7%), sodium oxalate (Na2C2O4, AR, 99.7%), ethanol (AR, 99.7%) and isopropyl alcohol (IPA, AR, 99.7%) were obtained from Sinopharm Chemical Reagents Co., Ltd (Shanghai, China). All reagents were not purified further before use, and deionized water was used throughout the experiments.