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

Facile Fabrication of WO₃ Nanoplates Thin Films with Dominant Crystal Facet of (002) for Water Splitting

Single crystalline orthorhombic phase tungsten trioxide monohydrate (<i>O</i>-WO₃·H₂O, space group: <i>Pmnb</i>) nanoplates with a clear morphology and uniform size distribution have been synthesized by the hydrothermal method and fabricated on the surface of fluorine doped tin oxide (FTO) coated glass substrates with selective exposure of the crystal facet by the finger rubbing method. The rubbing method can easily arrange the <i>O</i>-WO₃·H₂O nanoplates along the (020) facet on the FTO substrate. The <i>O</i>-WO₃·H₂O nanoplate can be converted to monoclinic phase WO₃ (γ-WO₃, space group: <i>P</i>21/<i>n</i>) with dominant crystal facet of (002) without destroying the plate structure. Crystal morphologies, structures, and components of the powders and films have been determined by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman, X-ray photoelectron spectroscopy, etc. The band gap energies of the <i>O</i>-WO₃·H₂O and γ-WO₃ nanoplates were determined as ca. 2.26 and 2.49 eV, respectively. Photoelectrochemical properties of the films with (002) dominant crystal facet have also been checked for discussion of further application in water oxidation. The advantage of (002) facet dominant film was investigated by comparing to one spin-coated γ-WO₃ thin film with the same thickness via photoelectrochemical characterizations such as photocurrent, incident photon to current efficiency, and electrochemical impedance spectroscopy