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

초록

Ferromagnetic-like properties of cuprous oxide (Cu2O) are induced through its interaction with chemisorbed surfactant poly(N-vinyl-2-pyrrolidone) (PVP), which alters the intrinsic d(10) 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 (chi) 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 d(9) 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 CuCu 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 CuCu interactions that lead to changes in electronic configurations, deriving the ferromagnetic moments of c-Cu2O.

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