Temperature-controlled propolis-mediated synthesis of CuO nanoparticles: structure–optical–photocatalytic relationships for sunlight-driven methylene blue degradation

Green, propolis-mediated synthesis offers a low-impact route to metal-oxide photocatalysts, yet the role of synthesis temperature on intrinsic structure–property–performance relationships is not well defined. Here, CuO nanoparticles were biosynthesized using propolis extract at four controlled reaction temperatures (60, 70, 80, 90 °C) while keeping precursor chemistry and post-treatment constant. Samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV–visible spectroscopy to probe crystal structure, morphology, and electronic/optical disorder. XRD confirmed that the monoclinic CuO phase altered with the synthesis temperature, including crystallite size, lattice strain, defect density, and specific surface area. TEM showed quasi-spherical, polycrystalline particles with an average particle size of ~ 26.5 nm. Based on optical analysis, increasing the synthesis temperature of CuO nanoparticles from 60 to 90 °C resulted in a nonlinear change in the direct/indirect band gaps (1.40–1.51 eV) and affected Urbach energy, refractive index, optical conductivity, and dielectric parameters, highlighting the impact of temperature on electronic-structure disorder. Photocatalytic performance was evaluated by sunlight-assisted degradation of methylene blue; all samples followed pseudo-first-order kinetics. The material synthesized at 70 °C delivered the highest degradation (63.92%) and rate constant (k = 0.01464 min⁻ 1 ). These results demonstrate that synthesis temperature alone can be used to tune structural defects and optical properties of propolis-derived CuO nanoparticles, enabling optimization of eco-friendly photocatalysts for sunlight-driven dye remediation without additional dopants or co-catalysts.