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

• HybridNiO@Co 3 O 4 NTAs were fabricated via two-stepdeposition and etching method. • NiO@Co 3 O 4 NTAselectrode has a specific capacitance of 1769.2 F/g and 87.5% capacitance retention. • Unique composites structure of hierarchically hollow NiO tubes and wrapped Co 3 O 4 nanosheets. • Synergistic effects lead to the improved capacitance behavior. • DFT calculations reveal band-gap disappearance for the mechanism of conductivity improvement. [Display omitted] Novel band-gap shrinked NiO@Co 3 O 4 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 Co 3 O 4 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@Co 3 O 4 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@Co 3 O 4 composite is one intrinsic reason that the conductivity and the capacitive charge storage performance are enhanced significantly. The as-prepared NiO@Co 3 O 4 nanotube arrays are highly promising electrodes for high-performance supercapacitors. [ABSTRACT FROM AUTHOR]