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.