Core/Shell-Like Magnetic Structure and Optical Properties in CuO Nanoparticles Synthesized by Green Route

We report a core/shell-like magnetic structure at room temperature in nonstoichiometric CuO nanoparticles (NPs) synthesized by a green route using Phyllanthus niruri whole plant extract over leaf extract. The XRD pattern analyzed by Rietveld refinement confirms single phase formation of CuO NPs crystals in a monoclinic structure with space group C2/C. SEM and TEM microstructures reveal agglomeration of NPs, spherical in shape with an average particle size of 20 nm. The FTIR spectrum confirms the plant derivatives and its biomolecules and functional group associated with the CuO NPs. The core level XPS of Cu-2p and O-1s suggests the presence of oxygen vacancies and coexistence of mixed oxidation states of Cu, i.e., Cu2+ and Cu3+. UV–visible absorption spectra show enhancement in the optical band gap energy (Eg ∼ 2.9 eV) along with a strong absorption peak at wavelength ∼244 nm, which is not observed in previously reported literature [Kalarani, G.; Lekha, N. C.; Arunkumar, G. Ann. Romanian Soc. Cell Biol. 2021, 25, 12900; Hemashree, S.; Pruthvi, M. L.; Mahesh, M. K. Int. J. Res. Appl. Sci. Eng. Technol. 2023, 11, 459]. It is likely due to the surface plasmon resonance. The investigation of magnetization of CuO NPs synthesized by PN extract is entirely missing [Kalarani, G.; Lekha, N. C.; Arunkumar, G. Ann. Romanian Soc. Cell Biol. 2021, 25, 12900; Hemashree, S.; Pruthvi, M. L.; Mahesh, M. K. Int. J. Res. Appl. Sci. Eng. Technol. 2023, 11, 459]. M-H results reveal room temperature ferromagnetism at a low field (0.3 T) with a coercive field of 0.02 T along with the absence of saturation magnetization. The M-T result indicates a magnetic transition temperature beyond 350 K with a significant bifurcation between both branches of magnetic susceptibilities χFC and χZFC. Overall results suggest a core/shell-like magnetic structure, where the inner part exhibits a strong antiferromagnetic order and the outer surface behaves like a weak disordered ferromagnet due to uncompensated surface spins. The room temperature ferromagnetic ordering is likely due to the virtual hopping of electrons from the Cu2+(3d9) site to the Cu3+(3d8) site.