Ferromagnetism of Single-Crystalline Cu₂O Induced through Poly(<i>N</i>‑vinyl-2-pyrrolidone) Interaction Triggering d‑Orbital Alteration

Ferromagnetic-like properties of cuprous oxide (Cu₂O) are induced through its interaction with chemisorbed surfactant poly­(<i>N</i>-vinyl-2-pyrrolidone) (PVP), which alters the intrinsic d¹⁰ configuration of Cu ions. Structural and magnetism-related properties of intact Cu₂O crystals (i-Cu₂O) and those capped with PVP (c-Cu₂O) were examined using various analytical instruments. SEM, TEM (corresponding selected area electron diffraction (SAED)), and XRD of i-Cu₂O and c-Cu₂O showed cubic and hexagonal shapes of single crystallinity with facets of {200} and {111}, respectively, resulting from the differential growth rates of the original identical crystals along the facets over time. Bulk magnetic susceptibility (χ) of i-Cu₂O and c-Cu₂O at room temperature in field-dependent magnetization and the difference in their magnetic moment in temperature-dependent magnetization showed diamagnetic and ferromagnetic properties, respectively. The difference in the fluorescence mode of X-ray absorption near edge structure (XANES) spectra between i-Cu₂O and c-Cu₂O, showing no quadruple pre-edge peak for the transition 1s → 3d in Cu­(II) ions with d⁹ electronic configuration, indicates an orbital alteration on the surface of c-Cu₂O caused by an interaction with PVP. Two peaks for c-Cu₂O at higher binding energies in O 1s X-ray photoelectron spectroscopy may be indicative of the ligand-to-metal charge transfer (LMCT) from O atoms of PVP to Cu ions of Cu₂O, generating a chemical interaction through coordination bonding. Large hyperfine splitting constants in electron paramagnetic resonance (EPR) spectra of c-Cu₂O support this interpretation, with septet hyperfine splitting suggestive of Cu–Cu interactions on the surface of c-Cu₂O via the interaction with O atoms of PVP. These results demonstrate that PVP capping of Cu₂O crystal (c-Cu₂O) induces ferromagnetism of Cu­(I) ions through coordination with O atoms of chemically adsorbed PVP. This may induce LMCT and Cu–Cu interactions that lead to changes in electronic configurations, deriving the ferromagnetic moments of c-Cu₂O