Trace amounts of water-induced distinct growth behaviors of NiO nanostructures on graphene in CO2-expanded ethanol and their applications in lithium-ion batteries.

In this work, we have developed a new method to grow NiO nanomaterials on the surface of graphene nanosheets (GNSs). The morphologies of NiO nanomaterials grown on GNSs could be tailored by trace amounts of water introduced into the mixed solvents of CO2-expanded ethanol (CE). Small and uniform Ni-salt nanoparticles (Ni-salt-NPs) were grown on the surface of graphene oxide (GO) through the decomposition of nickel nitrate directly in CE. However, when trace amounts of water were introduced into the mixed solvents, Ni-salt nanoflakes arrays (Ni-salt-NFAs) were grown on the surface of GO with almost perpendicular direction. After thermal treatment in N2 atmosphere, these Ni-salt @GO composites were converted to NiO@GNSs composites. The forming mechanisms of the NiO-NPs@GNSs and NiO-NFAs@GNSs were discussed by series comparative experiments. The presence of the trace amounts of water affected the chemical composition and structure of the precursors formed in CE and the growth behaviors on the surface of GNSs. When used as anode materials for lithium-ion batteries, the NiO-NPs@GNSs composite exhibited better cycle and rate performance compared with the NiO-NFAs@GNSs.