Ultrathin nickel hydroxide and oxide nanosheets: Synthesis, characterizations and excellent supercapacitor performances

High-quality ultrathin two-dimensional nanosheets of α-Ni(OH) ₂ are synthesized at large scale via microwave-assisted liquid-phase growth under low-temperature atmospheric conditions. After heat treatment, non-layered NiO nanosheets are obtained while maintaining their original frame structure. The well-defined and freestanding nanosheets exhibit a micron-sized planar area and ultrathin thickness (<2 nm), suggesting an ultrahigh surface atom ratio with unique surface and electronic structure. The ultrathin 2D nanostructure can make most atoms exposed outside with high activity thus facilitate the surface-dependent electrochemical reaction processes. The ultrathin a-Ni(OH)₂ and NiO nanosheets exhibit enhanced supercapacitor performances. Particularly, the a-Ni(OH)₂ nanosheets exhibit a maximum specific capacitance of 4172.5 F g^-1 at a current density of 1 A g^-1. Even at higher rate of 16 A g^-1, the specific capacitance is still maintained at 2680 F g^-1 with 98.5% retention after 2000 cycles. Even more important, we develop a facile and scalable method to produce high-quality ultrathin transition metal hydroxide and oxide nanosheets and make a possibility in commercial applications.