Ferromagnetism of Single-Crystalline Cu<sub>2</sub>O Induced through
Poly(<i>N</i>‑vinyl-2-pyrrolidone)
Interaction Triggering d‑Orbital Alteration
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Abstract
Ferromagnetic-like
properties of cuprous oxide (Cu<sub>2</sub>O)
are induced through its interaction with chemisorbed surfactant poly(<i>N</i>-vinyl-2-pyrrolidone) (PVP), which alters the intrinsic
d<sup>10</sup> configuration of Cu ions. Structural and magnetism-related
properties of intact Cu<sub>2</sub>O crystals (i-Cu<sub>2</sub>O)
and those capped with PVP (c-Cu<sub>2</sub>O) were examined using
various analytical instruments. SEM, TEM (corresponding selected area
electron diffraction (SAED)), and XRD of i-Cu<sub>2</sub>O and c-Cu<sub>2</sub>O 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-Cu<sub>2</sub>O
and c-Cu<sub>2</sub>O 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-Cu<sub>2</sub>O and c-Cu<sub>2</sub>O, showing no quadruple pre-edge peak for the transition 1s
→ 3d in Cu(II) ions with d<sup>9</sup> electronic configuration,
indicates an orbital alteration on the surface of c-Cu<sub>2</sub>O caused by an interaction with PVP. Two peaks for c-Cu<sub>2</sub>O 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 Cu<sub>2</sub>O, generating a chemical
interaction through coordination bonding. Large hyperfine splitting
constants in electron paramagnetic resonance (EPR) spectra of c-Cu<sub>2</sub>O support this interpretation, with septet hyperfine splitting
suggestive of Cu–Cu interactions on the surface of c-Cu<sub>2</sub>O via the interaction with O atoms of PVP. These results demonstrate
that PVP capping of Cu<sub>2</sub>O crystal (c-Cu<sub>2</sub>O) 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-Cu<sub>2</sub>O
