Band-gap shrinked NiO@Co₃O₄ nanotubes as high-performance supercapacitor electrodes

Novel band-gap shrinked NiO@Co₃O₄ nanotube arrays on Ni foam have been fabricated via a two-step synthesis route. Hexagonal NiO nanotube arrays are obtained by electrodeposition of Ni nanoparticles onto ZnO nanoprisms followed by selective dissolution of ZnO. The inside and outside surfaces of the NiO nanotubes are homogeneously covered with Co₃O₄ nanosheets via hydrothermal treatment, which enlarges the surface area and accelerates electron transferring. Benefiting from the hierarchically hollow structure and the synergistic effects, the obtained NiO@Co₃O₄ nanotube arrays deliver a superior specific capacitance of 1769.2 F/g at 1 A/g and outstanding cycling stability (87.5% capacitance retention after 10,000 cycles at 20 A/g). Density functional theory calculations reveal that the disappearance of the band-gap of the NiO@Co₃O₄ composite is one intrinsic reason that the conductivity and the capacitive charge storage performance are enhanced significantly. The as-prepared NiO@Co₃O₄ nanotube arrays are highly promising electrodes for high-performance supercapacitors.