Natural flake graphite powder

All the reagents were of analytical grade except EY (bioreagent) and were used without further purification. Graphite oxide was synthesized from natural flake graphite powder (Sinopharm Chemical Reagent Co. Ltd) by the modified Hummers method [53 (link)–54 (link)]. In a typical method, natural graphite (2 g), NaNO₃ (1 g), and H₂SO₄ (46 mL) were mixed in an ice bath, then KMnO₄ (6 g) was slowly added to this mixture, and was stirred continuously for 30 min. Then the resulting mixture was heated to a temperature of 35 °C and was stirred for 2 h. 90 mL of distilled water was added slowly to the mixture for 20 min. Then the mixture was rapidly heated to 98 °C and kept stirring for 15 min. 144 mL of distilled water and 20 mL of H₂O₂ were added to the mixture. After the reaction, the obtained yellow-brown dispersion of graphite oxide was washed with 5% HCl and water until pH 5 and dried in an oven at 60 °C. 0.5 g of graphite oxide powder was added into 1 L of distilled water, and the dispersion was treated with ultrasound (KQ-800KDB, KunShan Ultrasonic Instrument Co. Ltd) for 2 h until the solution became clear to obtain a graphene oxide (GO) solution.

Except EY (bioreagent), analytical grade reagents were used without further purification. GO was synthesized by the modified Hummers method14 (link) using natural flake graphite powder (Sinopharm Chemical Reagent Co. Ltd). A certain volume of NiCl₂ solution (10 mM) was dropwise added into 200 mL GO solution (1.0 mg mL⁻¹) prepared by 30 min ultrasonication, and then kept stirring for 2 h. The water of suspensions was removed in an oven at 80 °C. The obtained GO/NiCl₂ powders were put into a quartz tube, and treated in reduction ambient [N₂ (95%) + H₂ (5%)] at 500 °C for 2 h to obtain the GN composites. The mass ratios of Ni to GO were 2.0, 6.0 and 8.0 (wt%), and the resulting samples were labelled as GN2, GN6, and GN8, respectively. The metallic Ni, RGO and rGN6 composite (RGO/NiCl₂ as precursor) were also prepared for the purpose of comparison under the same experimental conditions.