2.1. Chemicals
The GO gel (1 wt%) was purchased from Aladdin Co. Ltd . Nickel nitrate hexahydrate, 2-aminoethanol, hydrazine hydrate and hydrochloric acid were purchased from Sinopharm Chemical Reagent Co. Ltd . The other reagents were purchased from Aldrich Co. Ltd . All chemicals were not purified before usage. A Millipore direct-Q system was used to produce the ultrapure water of 18.2 MΩ, which was used throughout all experiments.