Cobalt nitrate hexahydrate

Nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O, 99.0%), cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O), ferric nitrate nonahydrate (Fe(NO₃)₃·9H₂O) and ammonium molybdate ((NH₄)₆Mo₇O₂₄·4H₂O) were ordered from Sinopharm Chemical Reagent Co., Ltd. The other nitroarenes were obtained from Aladdin Reagent Co.

100 mg of GO was dispersed in 40 mL deionized water. Then 1.454 g of cobalt nitrate hexahydrate (Co(NO₃)·6H₂O, 98.5%, Sinopharm Chemical Reagent Co., Ltd) was added into the GO aqueous solution and stirred for 20 min. 2-Methylimidazole (2-mlm, 99%, Sinopharm Chemical Reagent Co., Ltd) was dispersed in another 60 mL deionized water and, after it completely dissolved, it was quickly added to the above mentioned mixture, and stirred for 30 min. The composite of Co-MOF and GO (denoted as Co-MOF–GO) was obtained after centrifugation and dried overnight at 60 °C. Then, Co-MOF–GO was mixed with sublimed sulfur (S, 99.5%, Sinopharm Chemical Reagent Co., Ltd) at a mass ratio of 1 : 3, then heated in a furnace filled with argon at 800 °C for 2 h (heating rate 5 °C min⁻¹) to obtain the Co_1−xS/Co₉S₈ and graphene hybrid (denoted as Co_1−xS/Co₉S₈–G). Finally, the Co_1−xS/Co₉S₈–G powder was annealed in a muffle furnace at 300 °C for 3 h (heating rate 2 °C min⁻¹) to obtain Co₃S₄–G.