Room-temperature growth (“farming”) and high-performance supercapacitor applications of highly crystalline CuO nanowires/graphene nanoplatelet nanopowders

We report a first-of-its-kind farming-like growth of nanopowders of CuO nanowires (NWs) via room-temperature thermal oxidation. Compared to conventional thermal annealing methods for producing copper oxide nanostructures, which require elevated temperatures (300–600 °C), the present method yields a large amount of highly crystalline CuO NW nanopowders at a much lower temperature (i.e., room temperature). Two-dimensional carbon nanostructures such as graphene nanoplatelets (GNPs) were used as supports for the growth of the CuO NWs. The GNPs were coated with Cu seed layers by the electroless plating method, which is suitable for mass production. After electroplating of Cu layers, the GNP supports were kept at room temperature and under constant humidity (50 or 60% relative humidity) for over 24 h, resulting in the dense wire-like morphology of copper oxide. Scanning electron microscopy, energy dispersive X-ray diffraction, X-ray diffraction, and Raman spectroscopy measurements revealed that the NWs consisted of highly crystalline monoclinic CuO. Once the NWs were formed, their morphology was stable for up to 168 h at room temperature. The as-prepared CuO nanopowders were tested as electrodes of electrochemical capacitors (or supercapacitors). In a three-electrode configuration, a working electrode made of CuO NWs exhibited an excellent mass-specific capacitance of 145 F g⁻¹ at 5 mV s⁻¹ in a 3 M KOH aqueous electrolyte. The growth of CuO nanopowders on GNPs illustrated in this study demonstrates a novel approach for the room-temperature synthesis of nanopowders, with promising applications in next-generation energy devices.