All chemicals including cobalt nitrate hexahydrate (Co(NO3)2·6H2O, AR), copper nitrate trihydrate (Cu(NO3)2·3H2O, AR), sodium hypophosphite monohydrate (NaH2PO2·H2O, AR), potassium hydroxide (KOH, AR), hydrochloric acid (HCl, 37%), and ammonia borane (NH3BH3, 97%) were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd, China. Deionized water was used throughout this experiment.