Copper oxide nanoparticles were successfully synthesised via cost effective approach using aqueous extracts obtained from red flowers of Euphorbia pulcherrima. Green precipitates were obtained post-drying which were thermally oxidised to pure single phase monoclinic CuO at 500 °C as per the XRD analysis. Further characterization of un-annealed and annealed samples was carried out using Scanning Electron Microscope (SEM), Optical Microscope (OPM), High Resolution Transmission Electron Microscope (HRTEM), Energy-Dispersive X-ray spectroscopy (EDS), Fourier Transform Infrared spectroscopy (FTIR/ATR), Thermogravimetric analysis (TGA). Zeta potential, X-ray Photoelectron Spectroscopy (XPS), Diffuse Reflectance (DR), UV–Vis and Photoluminescence measurements (PL). Electrochemical properties were investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS).

SEM and OPM analysis on un-annealed and annealed samples reveal elongate and dense cubic agglomerate nanoparticles respectively. EDS spectra confirmed high purity phase composition of CuO. HRTEM analysis of annealed samples revealed a densed cubic nanoparticles, while selected electron area diffraction (SAED) and XRD patterns shows well-crystalline nanoparticles. FTIR/ATR spectra display peaks at 480 and 525 cm⁻¹, which are characteristic peaks of CuO. Zeta potential measurement of annealed samples recorded a value of −6.58 mV. TGA analysis indicated the thermal stability of CuO while XPS analysis confirmed synthesis of pure single phase CuO. Value of energy band gap obtained from diffuse reflectance spectrum was 1.3 eV. From electrochemical study, maximum areal capacitance achieved was 39.45 mF/cm² at a scan rate of 20 mV/s. This shows that Euphorbia pulcherrima mediated copper oxide nanoparticles is promising material for electrochemical energy storage application.