An official website of the United States government
Abstract X-ray photoelectron spectroscopy (XPS) shows that dramatic changes in the core level binding energies can provide strong indications of transitions between more dielectric and more metallic CoFe2O4and NiCo2O4thin films. These significant variations in the XPS core level binding energies are possible with a combination of annealing and oxygen exposure; however, the behaviors of the CoFe2O4and NiCo2O4thin films are very different. The XPS Co and Fe 2p3/2core levels for the CoFe2O4thin film at room temperature show large photovoltaic surface charging, leading to binding energy shifts, characteristic of a highly dielectric (or insulating) surface at room temperature. The photovoltaic charging, observed in the XPS binding energies of the Co and Fe 2p3/2core levels, decreases with increasing temperature. The XPS core level binding energies of CoFe2O4thin film saturated at lower apparent binding energies above 455 K. This result shows that the prepared CoFe2O4thin film can be dielectric at room temperature but become more metallic at elevated temperatures. The dielectric nature of the CoFe2O4thin film was restored only when the film was annealed in sufficient oxygen, indicating that oxygen vacancies play an important role in the transition of the film from dielectric (or insulating) to metallic. In contrast, the XPS studies of initially metallic NiCo2O4thin film demonstrated that annealing NiCo2O4thin film led to a more dielectric or insulating film. The original more metallic character of the NiCo2O4film was restored when the NiCo2O4was annealed in sufficient oxygen. Effective activation energies are estimated for the carriers from a modified Arrhenius-type model applied to the core level binding energy changes of the CoFe2O4and NiCo2O4thin films, as a function of temperature. The origin of the carriers, however, is not uniquely identified. This work illustrates routes to regulate the surface metal-to-insulator transition of dielectric oxides, especially in the case of insulating NiCo2O4thin film that can undergo reversible metal-to-insulator transition with temperature.
more »
« less
Local coordination geometry within cobalt spinel oxides mediates photoinduced polaron formation
Comparison of the optical properties of Co3O4and ZnCo2O4elucidates fundamental differences in mechanisms of photoinduced polaron formation based on the presence or absence of substitutional lattice defects arising from cation inversion.
more »
« less
- PAR ID:
- 10592353
- Publisher / Repository:
- Royal Society of Chemistry
- Date Published:
- Journal Name:
- Chemical Science
- ISSN:
- 2041-6520
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
The 1:1 cocrystal of 5-fluorocytosine (5FC) and 4-hydroxybenzaldehyde (4HB), C4H4FN3O·C7H6O2has been synthesized and its structure characterized by single-crystal X-ray diffraction and Hirshfeld surface analysis. The compound crystallizes in the monoclinicP21/cspace group. A robust supramolecular architecture is stabilized by N—H...O, N—H...N, C—H...O and C—H...F hydrogen bonds, formingR22(8),R44(22),R66(32), andR88(34) ring motifs. The N—H...O and N—H...N hydrogen bonds form strong directional interactions, contributing to theR22(8) andR88(34) motifs through dimeric and extended ring structures. O—H...O interactions link 5FC and 4HB molecules, generating anR66(32) ring that enhances the packing. Weaker C—H...F bonds help form theR44(22) tetrameric motif, supporting the overall three-dimensional supramolecular framework. Additionally, C—F...π interactions between the fluorine atom and the aromatic ring add further to the crystal cohesion. Hirshfeld surface analysis and two-dimensional fingerprint plots confirm that O...H/H...O contacts are the most significant, highlighting the central role of hydrogen bonding in the stability and organization of the crystal structure.more » « less
-
Abstract Spinel compounds are of great interest in both fundamental and application-oriented perspectives due to the geometric magnetic frustration inherent to their lattice and the resulting complex magnetic states. Here, we applied x-ray diffraction, magnetization, heat capacity and powder inelastic neutron scattering measurements, along with theoretical calculations, to study the exotic properties of chromite-spinel oxides CoCr2O4and MnCr2O4. The temperature dependence of the phonon spectra provides an insight into the correlation between oxygen motion and the magnetic order, as well as the magnetoelectric effect in the ground state of MnCr2O4. Moreover, spin-wave excitations in CoCr2O4and MnCr2O4are compared with Heisenberg model calculations. This approach enables the precise determination of exchange energies and offers a comprehensive understanding of the spin dynamics and relevant exchange interactions in complicated spiral spin ordering.more » « less
-
Abstract In an effort to reconcile the various interpretations for the cation components of the 2p3/2observed in x-ray photoelectron spectroscopy (XPS) of several spinel oxide materials, the XPS spectra of both spinel alloy nanoparticles and crystalline thin films are compared. We observed that different components of the 2p3/2core level XPS spectra, of these inverse spinel thin films, are distinctly surface and bulk weighted, indicating surface-to-bulk core level shifts in the binding energies. Surface-to-bulk core level shifts in binding energies of Ni and Fe 2p3/2core levels of NiFe2O4thin film are observed in angle-resolved XPS. The ratio between surface-weighted components and bulk-weighted components of the Ni and Fe core levels shows appreciable dependency on photoemission angle, with respect to surface normal. XPS showed that the ferrite nanoparticles NixCo1−xFe2O4(x= 0.2, 0.5, 0.8, 1) resemble the surface of the NiFe2O4thin film. Surface-to-bulk core level shifts are also observed in CoFe2O4and NiCo2O4thin films but not as significantly as in NiFe2O4thin film. Estimates of surface stoichiometry of some spinel oxide nanoparticles and thin films suggested that the apportionment between cationic species present could be farther from expectations for thin films as compared to what is seen with nanoparticles.more » « less
-
Abstract Emissions of methane (CH4) and nitrous oxide (N2O) from soils to the atmosphere can offset the benefits of carbon sequestration for climate change mitigation. While past study has suggested that both CH4and N2O emissions from tidal freshwater forested wetlands (TFFW) are generally low, the impacts of coastal droughts and drought‐induced saltwater intrusion on CH4and N2O emissions remain unclear. In this study, a process‐driven biogeochemistry model, Tidal Freshwater Wetland DeNitrification‐DeComposition (TFW‐DNDC), was applied to examine the responses of CH4and N2O emissions to episodic drought‐induced saltwater intrusion in TFFW along the Waccamaw River and Savannah River, USA. These sites encompass landscape gradients of both surface and porewater salinity as influenced by Atlantic Ocean tides superimposed on periodic droughts. Surprisingly, CH4and N2O emission responsiveness to coastal droughts and drought‐induced saltwater intrusion varied greatly between river systems and among local geomorphologic settings. This reflected the complexity of wetland CH4and N2O emissions and suggests that simple linkages to salinity may not always be relevant, as non‐linear relationships dominated our simulations. Along the Savannah River, N2O emissions in the moderate‐oligohaline tidal forest site tended to increase dramatically under the drought condition, while CH4emission decreased. For the Waccamaw River, emissions of both CH4and N2O in the moderate‐oligohaline tidal forest site tended to decrease under the drought condition, but the capacity of the moderate‐oligohaline tidal forest to serve as a carbon sink was substantially reduced due to significant declines in net primary productivity and soil organic carbon sequestration rates as salinity killed the dominant freshwater vegetation. These changes in fluxes of CH4and N2O reflect crucial synergistic effects of soil salinity and water level on C and N dynamics in TFFW due to drought‐induced seawater intrusion.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1039/d5sc01909e
