Controllable synthesis of Co3O4 from nanosize to microsize with large-scale exposure of active crystal planes and their excellent rate capability in supercapacitors based on the crystal plane effect

Co3O4 nanorods, nanobelts, nanosheets and cubic/octahedral nanoparticles have been successfully synthesized with tunable size from the nanoscale to the microscale, accompanied by a variation in the nature of the exposed crystal planes. The products are formed by thermal treatment of Co(CO3)0.5(OH)·0.11H2O nanorod, nanobelt, nanosheet and nanocubic/nanooctahedral precursors at 250 °C. Detailed characterization, including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photo-electron spectroscopy (XPS), and nitrogen adsorption and desorption isotherms, revealed that the as-prepared nanorods, nanobelts, and nanosheet Co3O4 samples are single crystalline and mesoporous in nature with a predominance of exposed high-energy (110) crystal planes. They exhibited excellent electrochemical properties in supercapacitors, showing higher capacitance and better rate capability than conventional cubic/octahedral Co3O4 nanoparticles having exposed low-energy (100) and (111) planes. No decay in capacitance was observed when the scan rate was increased from 5 mV/s to 100 mV/s, or from 1 A/g to 10 A/g. The maximum value of the specific capacitance was calculated to be 162.8 F/g and the capacitance retention reached as high as 90%. Their excellent performance in supercapacitors is believed to result from the large-area exposure of active (110) crystal planes. The Co3O4 nanosheets showed the best performance due to their larger surface area and ability to provide a better pathway for charge transfer, and are promising electrode materials for application in practical supercapacitors.