Controllable hydrothermal-assisted synthesis of mesoporous Co₃O₄ nanosheets

Mesoporous cobalt oxide (Co₃O₄) nanosheets (about 15 nm in thickness and about 1 μm in width) with mostly pore sizes of 10-15 nm were obtained by heat treatment of hydrothermally-synthesized hydrothermal products at 300 °C. The compositions and morphologies of the hydrothermal products could be tailored by controlling the hydrothermal reaction temperature at a range of 120-200 °C and time. Quasi-sheet-like Co(OH)₂ is favored to form at 120 °C for 6 h through oriented attachment, and then slice sheets Co(NH₃)₅(ONO)(NO₃)₂ and Co(CN)₂·H₂O were formed at over 150 °C for 6 h in water due to thermodynamic instability. Their sheets become thinner and more decentralized with the extension of the hydrothermal reaction time. The (CH₃COO)₂Co precursor was hydrothermally synthesized in ethanol. All of hydrothermal products were transformed into porous Co₃O₄ with similar morphologies after heat treatment. Mesoporous Co₃O₄ nanosheets displayed a Brunauer-Emmett-Teller (BET) surface area of about 65 m² g^-1. Cyclic voltammetry (CV) results indicated mesoporous Co₃O₄ nanosheets have good electrochemical properties and show a specific capacitance (C_S) of 880 F g^-1 in 1 M KOH at a current density of 1 A g^-1.