Size Dependence of Lattice Parameter and Electronic Structure in CeO2 Nanoparticles

  • Damien Prieur*
    Damien Prieur
    Helmholtz Zentrum Dresden-Rossendorf (HZDR), Institute of Resource Ecology, PO Box 510119, 01314 Dresden, Germany
    The Rossendorf Beamline at ESRF−The European Synchrotron, CS40220, 38043 Grenoble Cedex 9 France
    *Email: [email protected]
  • Walter Bonani*
    Walter Bonani
    European Commission, Joint Research Centre, P.O. Box 2340, D-76125 Karlsruhe, Germany
    *E-mail: [email protected]
  • Karin Popa
    Karin Popa
    European Commission, Joint Research Centre, P.O. Box 2340, D-76125 Karlsruhe, Germany
    More by Karin Popa
  • Olaf Walter
    Olaf Walter
    European Commission, Joint Research Centre, P.O. Box 2340, D-76125 Karlsruhe, Germany
    More by Olaf Walter
  • Kyle W. Kriegsman
    Kyle W. Kriegsman
    Department of Chemistry and Alexandra Navrotsky Institute for Experimental Thermodynamics, Washington State University, Pullman, Washington 99164, United States
  • Mark H. Engelhard
    Mark H. Engelhard
    Environmental Molecular Sciences Laboratory, Pacific Northwest National Lab, Richland, Washington 99352, United States
  • Xiaofeng Guo
    Xiaofeng Guo
    Department of Chemistry and Alexandra Navrotsky Institute for Experimental Thermodynamics, Washington State University, Pullman, Washington 99164, United States
    More by Xiaofeng Guo
  • Rachel Eloirdi
    Rachel Eloirdi
    European Commission, Joint Research Centre, P.O. Box 2340, D-76125 Karlsruhe, Germany
  • Thomas Gouder
    Thomas Gouder
    European Commission, Joint Research Centre, P.O. Box 2340, D-76125 Karlsruhe, Germany
  • Aaron Beck
    Aaron Beck
    Institute for Nuclear Waste Disposal, Karlsruhe Institute of Technology, P.O. Box 3640, 76021 Karlsruhe, Germany
    More by Aaron Beck
  • Tonya Vitova
    Tonya Vitova
    Institute for Nuclear Waste Disposal, Karlsruhe Institute of Technology, P.O. Box 3640, 76021 Karlsruhe, Germany
    More by Tonya Vitova
  • Andreas C. Scheinost
    Andreas C. Scheinost
    Helmholtz Zentrum Dresden-Rossendorf (HZDR), Institute of Resource Ecology, PO Box 510119, 01314 Dresden, Germany
    The Rossendorf Beamline at ESRF−The European Synchrotron, CS40220, 38043 Grenoble Cedex 9 France
  • Kristina Kvashnina
    Kristina Kvashnina
    Helmholtz Zentrum Dresden-Rossendorf (HZDR), Institute of Resource Ecology, PO Box 510119, 01314 Dresden, Germany
    The Rossendorf Beamline at ESRF−The European Synchrotron, CS40220, 38043 Grenoble Cedex 9 France
  • , and 
  • Philippe Martin
    Philippe Martin
    CEA, DEN, DMRC, SFMA, LCC, F30207 Bagnols sur Cèze cedex, France
Cite this: Inorg. Chem. 2020, 59, 8, 5760–5767
Publication Date (Web):April 1, 2020
https://doi.org/10.1021/acs.inorgchem.0c00506
Copyright © 2020 American Chemical Society
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Supporting Info (1)»

Abstract

Intrinsic properties of a compound (e.g., electronic structure, crystallographic structure, optical and magnetic properties) define notably its chemical and physical behavior. In the case of nanomaterials, these fundamental properties depend on the occurrence of quantum mechanical size effects and on the considerable increase of the surface to bulk ratio. Here, we explore the size dependence of both crystal and electronic properties of CeO2 nanoparticles (NPs) with different sizes by state-of-the art spectroscopic techniques. X-ray diffraction, X-ray photoelectron spectroscopy, and high-energy resolution fluorescence-detection hard X-ray absorption near-edge structure (HERFD-XANES) spectroscopy demonstrate that the as-synthesized NPs crystallize in the fluorite structure and they are predominantly composed of CeIV ions. The strong dependence of the lattice parameter with the NPs size was attributed to the presence of adsorbed species at the NPs surface thanks to Fourier transform infrared spectroscopy and thermogravimetric analysis measurements. In addition, the size dependence of the t2g states in the Ce LIII XANES spectra was experimentally observed by HERFD-XANES and confirmed by theoretical calculations.

Synopsis

In CeO2 NPs, the lattice parameter and the electronic structure are size-dependent.

Introduction

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CeO2-based nanoparticles (NPs) offer unique redox properties that open promising possibilities for applications in catalysis, (1,2) energy storage, (3,4) biomedicine, (5) and nuclear activities. (6) Quantum mechanical size effects, combined with a considerable increase of the surface to bulk ratio, are responsible for the unique properties of nanometer-sized particles, including electronic and geometric structure, and optical and magnetic properties. (7,8) A thorough understanding of the dependence of these properties on particle size is of great importance notably for the design of next generation materials.
In this context, our work focuses on studying the size dependence of both crystallographic and electronic structures of CeO2 NPs, as these two fundamental properties are of technological importance and theoretical interest and broad prospects. (2) The change of unit cell dimensions with decreasing particle size has been previously reported, but remains a subject of discussion. Different hypotheses have been put forward to explain this phenomenon: surface stress induced by the presence of sorbed species and partial reduction of CeIV to CeIII. (9−11) Furthermore, the electronic structure and its size dependence is of fundamental interest as energetic and catalytic properties notably lie on it. (2)
In the present work, the crystal structure of as-synthesized CeO2 NPs was characterized by using X-ray diffraction (XRD) and transmission electron microscopy (TEM) giving access to their size and lattice parameter. Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) was performed to determine species potentially absorbed at the surface. Furthermore, we probed the electronic structure of CeO2 NPs using both high-resolution X-ray photoelectron spectroscopy (XPS) and high-energy resolution fluorescence-detection hard X-ray absorption near-edge structure (HERFD-XANES) spectroscopy at the Ce LIII edge. Thanks to the use of an X-ray emission spectrometer, such inner-shell spectroscopy provides an element-selective probe of the electronic state and allows observing spectral features with significantly enhanced energy resolution compared to usual data limited by Ce LIII edge core hole lifetime broadening (Supporting Information Figure S1). (12,13)

Experimental Methods

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Please note that a careful attention should be made for the characterization of these NPs material to avoid any energy-induced transformations.

Synthesis Methods

Nanoparticles Obtained by Hydrothermal Treatment

Ceria NPs samples were synthesized by alkaline precipitation of cerium ammonium sulfate precursor followed by hydrothermal condensation in a pressurized autoclave at different temperatures. (14,15) In detail, a 1 M Ce(IV) solution was prepared by dissolving cerium(IV) ammonium sulfate dehydrate (Alfa Aesar) in deionized water; cerium(IV) hydroxide was directly precipitated by adding an excess of ammonium hydroxide (Sigma-Aldrich, 25% in water) under constant stirring for 3 h. A yellow cerium(IV) hydroxide precipitate was recovered by centrifugation, repeatedly washed with deionized water, and hydrothermally treated in a stainless steel reactor vessel with Teflon insert (total free volume 12 mL). Typically, 200 mg of cerium(IV) hydroxide was suspended in 10 mL of deionized water and heated for 3 h under autogenous pressure at different temperatures. After cooling, the solid residue was recovered, washed with deionized water, dehydrated with ethanol and acetone, and dried overnight in a chemical fume hood. The resulting dry powders were analyzed by XRD with a Rigaku Miniflex 600 diffractometer. The crystallite size of the nanopowders was estimated from the XRD pattern using the Scherrer equation and averaging the results of 8 selected peaks in the 2θ range between 25 and 80°. Nanoparticle samples obtained by hydrothermal condensation at 150, 180, and 200 °C were labeled as Ce_2, Ce_4.2, and Ce_5.6, giving their respective crystallite size in nanometer.

Nanoparticles Obtained by Thermal Treatment in Dry Conditions

In order to obtain samples with larger crystallites size, the dry powder samples obtained by hydrothermal treatment were calcined for 1 h at temperatures ranging from 350 to 950 °C in an open furnace. The so obtained NP samples were analyzed by XRD and labeled on the basis of the respective estimated crystallite size. The nomenclature and the synthesis route of the different NP samples are reported in detail in Table 1.
Table 1. Synthesis Route, Particle Size, and Lattice Parametera
sample namesynthesis routecrystallite size (nm)blattice parameter (Å)bstrain (%)b
Ce_2hydrothermal treatment of Ce(IV) hydroxide (3h at 150 °C under autogenic pressure)2.0 ± 0.15.456(3)0.041
Ce_5.6hydrothermal treatment of Ce(IV) hydroxide (3h at 200 °C under autogenic pressure)5.6 ± 0.45.415(1)0.015
Ce_8.4Ce_2 heated 1h at 350 °C in air8.4 ± 0.95.413(2)0.011
Ce_12.2Ce_4.2c heated 1h at 500 °C in air12.2 ± 1.55.412(1)0.008
Ce_18.2Ce_4.2c heated 1h at 700 °C in air18.2 ± 1.45.412(1)0.006
Ce_20.8Ce_4.2c heated 1h at 800 °C in air20.8 ± 0.75.412(1)0.005
Ce_34Ce_4.2c heated 1h at 900 °C in air34 ± 35.412(1)0.004
Ce_59Ce_4.2c heated 1h at 925 °C in air59 ± 55.411(1)0.002
Ce_91Ce_4.2c heated 1h at 950 °C in air91 ± 125.411(1)0.001
a

Note that Ce_2 was measured by HERFD-XANES, but the results are not presented here due to the beam damage.

b

Derived from the refinement of the XRD patterns.

c

Ce_4.2 nanopowder was synthesized by hydrothermal treatment of Ce(IV) hydroxide (3 h at 180 °C under autogenic pressure). The particle size of Ce_4.2 was estimated to be 4.2 ± 0.3 nm from the XRD pattern.

Considering that the sample Ce_2 experienced damages due to the exposure to the beam during the HERFD-XANES measurement, this compound has been discarded from this discussion on the oxidation state determination and on the electronic structure. Further details on this phenomenon are provided in the Supporting Information Figure S2.

Transmission Electron Microscopy

TEM studies were performed using an aberration (image) corrected FEI Titan 80–300 operated at 300 kV providing a nominal information limit of 0.8 Å in TEM mode and a resolution of 1.4 Å in STEM mode. TEM micrographs have been recorded using a Gatan US1000 slowscan CCD camera, while STEM images have been recorded using a Fischione high-angle annular dark-field (HAADF) detector with a camera length of 195 mm. The samples for analysis have been prepared by dropping coating with a suspension of the NPs in ultrapure water on carbon-coated copper grids.

Fourier Transform Infrared Spectroscopy

Dehydrated ceria NPs were analyzed by FTIR in attenuated total reflectance mode with an Alpha Platinum Bruker spectrometer equipped with ZnSe crystal. FTIR spectra were obtained at room temperature in the wavenumber range from 600 to 4000 cm–1 with a resolution of 4 cm–1.

Thermogravimetric Analysis

The thermal behavior of ceria NPs was investigated using a Netzsch STA 449C DTA/TG using an alumina crucible and in air atmosphere. The temperature was controlled by a Pt-PtRh (10%) thermocouple. Measurements were carried out at constant heating and cooling rates of 10 °C/min between 40 and 700 °C.

Raman

Raman measurements of nanocrystalline samples were performed at room temperature with a Horiba Jobin-Yvon T64000 spectrometer using a Kr+ laser with excitation wavelength of 647 nm. A 50× objective was used to irradiate powder samples and collect the backscattered light. The analyses were performed with an incident laser power in the 4–10 mW; no effect of laser power was observed for the resulting spectra in this range.

X-ray Photoelectron Spectroscopy

XPS measurements were performed with a Physical Electronics Quantera Scanning X-ray Microprobe. This system uses a focused monochromatic Al Kα X-ray (1486.7 eV) source for excitation and a spherical section analyzer. The instrument has a 32 element multichannel detection system. The X-ray beam is incident normal to the sample, and the photoelectron detector is at 45° off-normal. High-energy resolution spectra were collected using a pass energy of 69.0 eV with a step size of 0.125 eV. For the Ag 3d5/2 line, these conditions produced a full width at half-maximum (fwhm) of 0.92 eV ± 0.05 eV. The binding energy (BE) scale is calibrated using the Cu 2p3/2 feature at 932.62 ± 0.05 eV and Au 4f7/2 at 83.96 ± 0.05 eV. The sample experienced variable degrees of charging. Low-energy electrons at ∼1 eV, 20 μA and low-energy Ar+ ions were used to minimize this charging. The binding energy scale was charge corrected referencing the Ce 3d3/24f0 (u′′′) line at 916.7 eV. (16,17) The quantification and peak fitting were performed using PHI MultiPak version 9.6.1.7 software, where the experimental results were fitted to 10 peaks, defined as v0, v, v′, v′′, v′′′, u0, u, u′, u′′, and u′′′. (16) In this scheme, v, v′′, v′′′, u, u′′, and u′′′ were associated with CeIV, while v0, v′, u0, and u′ were associated with CeIII. The relative areas of the fitted peaks were measured in order to determine the relative concentration in the surface.

High-Energy Resolution Fluorescence-Detected X-ray Absorption Near Edge Structure

HERDF-XANES measurements were conducted at the CAT-ACT beamline (ACT station) of the KIT synchrotron light source (Karlsruhe Institute of Technology, Karlsruhe, Germany). (18,19) The incident energy was selected using the (111) reflection of a double Si crystal monochromator. The X-ray beam was focused to 500 × 500 μm onto the sample. XANES spectra were measured in HERFD mode using an X-ray emission spectrometer. (12,19) The sample, analyzer crystal, and a single diode VITUS Silicon Drift Detector (KETEK, Germany) were arranged in a vertical Rowland geometry. The Ce HERFD-XAS spectra at the LIII edge were obtained by recording the maximum intensity of the Ce Lα1 emission line (4839 eV) as a function of the incident energy. The emission energy was selected using the ⟨331⟩ reflection of four spherically bent Ge crystal analyzer (with a bending radius R = 1 m) aligned at a 80.7° Bragg angle. The experimental energy resolution was 1.15 eV obtained by measuring the fwhm of the elastically scattered incident beam with an energy of 4.8404 keV. During the measurements, a slit with the dimensions 500 × 500 μm was used in front of the sample, cutting of tails in the profile of the incident beam. This potentially led to a slight improvement of the experimental resolution, that is, 1 eV (it is an estimation). The experimental resolution measured at ID26, ESRF using the same analyzer crystals was 0.9 eV. (20) The sample, crystals, and detector were confined in a box filled with He, and a constant He flow was maintained in order to minimize the loss of intensity due to absorption and scattering of the X-rays. The data were not corrected for self-absorption effects. The sample exposure to the beam was minimized to account for possible beam damage and checked by first collecting short XANES scans (∼10 s) to look for irradiation effect.

Theoretical Calculations

The spectra of bulk CeO2 and CeO2 in 2 nm were performed in a manner described in refs (21−24) using the FEFF 9.6. code. Similar to the work of Li et al. and Plakhova et al., we show here only part of the absorption spectra, which corresponds to the 2p–5d transitions, and omit multielectron excitations from the experimental data, which appear at higher incident energy.

Results and Discussion

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Ce Valence in the As-Synthesized CeO2 NPs

Our XRD and TEM data (Supporting Information Figures S3 and S4) show that the as-synthesized CeO2 NPs crystallize in the Fm3̅m fluorite structure (space group 225). Depending on the experimental conditions (and particularly the annealing temperature), the average crystallite diameters vary from 2.0 ± 0.1 to 91 ± 12 nm. These XRD-refined parameters are gathered in the Table 1 and will be more thoroughly discussed in a following section.
The oxidation state and electronic structure of Ce was assessed using Ce LIII edge HERFD-XANES and XPS, which corresponding spectra are respectively given in Figures 1 and 2.

Figure 1

Figure 1. (Left) Ce L3HERFD-XANES spectra of CeO2 NPs with different sizes compared to bulk CeO2 (>100 nm). (Right) Pre-edge region of the Ce L3HERFD-XANES spectra.

Figure 2

Figure 2. High-energy resolution photoemission spectra of the Ce 3d spectral region. The spectra confirm the presence of the dominating CeIV.

The HERFD-XANES spectra of all the investigated CeO2 NPs exhibit a single pre-edge peak aligned with that of the bulk-CeO2 reference spectrum. This pre-edge peak (noted A in Figure 1) originates from the 2p transition to a mixed 5d–4f valence state (22,25) and is characteristic of the Ce valence of the probed sample. Indeed, a single peak is observed in the pre-edge region of a pure CeIV compound, since the photo absorption process excites an electron to the 4f level, formally empty in the initial state. However, in the case of a CeIII ion, the interaction between the 4f electron in the initial state and the second electron excited by the photon leads to a splitting of the pre-edge feature into two groups of transition, whose energy position is related to the electron–electron interactions in the 4f level. (20,26,27) Consequently, the pre-edge structure of the NPs reveals solely the presence of Ce in the IV oxidation state. Additionally, since XANES is extremely sensitive to the local structure, those similarities corroborate our XRD findings that CeO2 NPs all crystallize in the same space group. (8)
Regarding our XPS results, Figure 2 provides the Ce 3d3/2,5/2 XPS spectra collected for the CeO2 NPs. The spectra of CeO2 are composed of 6 peaks corresponding 3 doublets belonging to 3d3/2 and 3d5/2 core holes spin–orbit splitting. The highest energy peaks, u′′′ at 916.7 eV and v′′′ at 898.2 eV, are the doublet from the ejection with the Ce 3d3/24f0 final state. The next doublet, u′′ at 907.4 eV and v′′ at 888.8 eV, are the result of Ce 3d5/24f1, followed by u at 900.8 eV and v at 882.4 eV, corresponding to the result of a Ce 3d5/24f2 final state. All of these multiplets are associated with CeIV, which are consistent with previously published CeO2 reference XPS data. On the other hand, the Ce 3d XPS spectrum of CeIIIPO4 ref (28) has two distinct sets of doublets: v0 at 880.6 eV and u0 at 897.9 eV as a result of Ce 3d5/24f2 ejection, and v′ at 884.5 eV and u′ at 900.8 eV as a result of the Ce 3d5/24f1 final state, neither of which register a meaningful peak area in this data. Furthermore, from the peak fitting performed (Supporting Information Figure S5), no characteristic band feature of CeIII was found for samples across all crystallite sizes, corroborating the XANES findings that only tetravalent cerium is present.

Size Dependence of the Lattice Parameter

The unit cell parameter variation as a function of the NPs’ size has been reported for several oxide NPs, including ThO2, CeO2, MgO, Co3O4, Fe3O4, TiO2, etc., but remains a subject of discussion. (2,29−35)
Figure 3 presents the size dependence of the lattice parameters of the as-synthesized CeO2 NPs. When the particle size changes from 2.0 to 91 nm, the unit cell value varies from 5.456 (3) to 5.411 (1) Å. Note that the latter value corresponds to bulk CeO2. In our work and as already reported for CeO2-based NPs, (32,36,2,31) noticeable deviation of the lattice parameter can be observed for particles which sizes are smaller than ca. 5 nm. The collected data can be adequately fitted using a power-law relation proposed by Baranchikov et al. (31)

Figure 3

Figure 3. Decrease of both lattice parameter (left) and strain (right) with the particle size. The lattice parameter, the strain, and the particle size have been calculated from the Rietveld refinement of the corresponding XRD patterns. All the values are provided in Table 1 of the Supporting Information. The gray dotted line corresponds to the power-law relation proposed by Baranchikov et al. (31) The goodness of fit was 0.963.

According to the paper of Diehm et al., (37) which gathers a large body of experimental data and theoretical calculation, two main models have been proposed to explain this lattice parameter variation. Tsunekawa et al. argued that the unit cell is affected by the formation of oxygen vacancies in the smaller NPs, which leads to a change in the oxidation state of the constituting cation. The second model, which is most commonly admitted, attributes the variation to the surface stress resulting from the difference of coordination between atoms on the surface and in the bulk. (9,10) This effect becomes more pronounced as the particle size reduces, that is, as the contribution of the surface atoms to the structural characteristics increases. From our HERFD-XANES and XPS findings, the presence of CeIII and hence of the oxygen vacancy has been discarded, which means that the observed lattice expansion might then be only due to surface stress. To corroborate this assumption, the formation of species present at the NPs surface has been studied by FTIR and TGA measurements. Each FTIR spectrum (Figure 4) shows an absorption band at ca. 450 cm–1 characteristic of the CeO2 stretching vibration. Some bands are also visible in the 1300–1700 cm–1 region and may be assigned to C–H bending and stretching of C–O bond. The latter may be caused by absorbed CO2 or contamination with ethanol during the synthesis procedure. The signal at 1630 cm–1 is associated with the bending frequency of molecular H2O (H–O–H). The broad adsorption band at ca. 3400 cm–1 corresponds to hydrated and physically adsorbed water in the sample. (9,38,39) Recently, TGA coupled with mass spectrometry measurement, performed on ThO2 NPs, has shown that H2O and CO2 were adsorbed on the NPs surfaces. The corresponding molar fractions of H2O and CO2 were inversely proportional to the NPs size. (40) Similar observation was obtained from our TGA data presented in the Figure 4. Ce_2 and Ce_59 have similar thermal decompositions as Ce(OH)4 and CeO2, respectively, while Ce_5.6 and Ce_12.2 exhibit intermediates weight losses. The thermal decompositions of these later are clearly composed of three-well-defined steps. The first decomposition step (step I) is observed at 25–180 °C. The second weight loss (step II) occurs at 180–350 °C. The third decomposition step (step III) occurs at 350–700 °C. Our TGA data shows that the weight loss is proportional to the surface to volume ratio, suggesting a higher concentration of adsorbed species on the smaller NPs.

Figure 4

Figure 4. Determination of the adsorbed surface species. (Left) FTIR spectra of CeO2 NPs compared with Ce(OH)4 and bulk-CeO2 references. (Right) TGA data of CeO2 NPs of different sizes compared with Ce(OH)4 and bulk-CeO2.

The Raman spectrum of bulk CeO2 shows one Raman active fundamental mode at 464 cm–1 (Figure 5). This band corresponds to the triply degenerate T2g Raman active mode of the Oh point group and is due to the stretching of the O–O bond. Comparison with the CeO2 NPs reveals a red-shift and an asymmetrical broadening if the T2g mode with decreasing NPs size. Both T2g position and width were fitted and plotted in Figure 5. It can be seen that an increase of the NPs size induces an increase of the T2g position and a decrease of the T2g width. Similar observations have already been reported for CeO2 and other nanocrystals. (41−43) Several factors can contribute to the changes in the Raman peak position and line width of the T2g peak with NP size. These include phonon confinement, strain, broadening associated with the size distribution, defects, and surface effect. (41−46) In most studies, the red shift is majorly attributed to the lattice expansion and associated strain that occurs when oxygen vacancies are created, which leads to the reduction of CeIV (ionic radius 0.970 Å) (47) in CeIII (ionic radius 1.143 Å). (47) However, the creation of O vacancies and the reduction of CeIV in CeIII induce a local symmetry distortion, and new Raman bands located around 550 and 595 cm–1 are observed. (48) The absence of both new Raman bands and detection of CeIII from our HERFD-XANES measurements allows us to discard this hypothesis for the T2g red-shift. One possible explanation could be that the stress generated by the presence of the adsorbed species at the NPs surface would enhance the downward shift and broadening of the T2g.

Figure 5

Figure 5. (Left) Raman spectra. (Right) T2g position and width as a function of the particle size.

Size Dependence of the Electronic Structure

The HERFD-XANES spectrum of bulk CeO2 exhibits three characteristic features A, B, and C with a doublet structure for B and C leading to a total of five bands indicated by A, B1, B2, C1, and C2 in Figure 1. The pre-edge peak (noted A) originates from the dipole-forbidden 2p transition to a mixed 5d–4f valence state, while the B and C arises from transitions from 2p3/2 → 5d5/2 states. The splitting of B1 and C1 into a doublet structure is due to the crystal field splitting of 5d states. (49) These edge features have been assigned to screened (B1 and B2) and unscreened (C1 and C2) excited states. (22,50) B1 and B2 features described the 2p → 5d transition with 4f1L configuration, while C1 and C2 are representatives of the 2p → 5d transition from 4f0L configuration, where L corresponds to the orbital angular momentum. (23,51) In the CeO2 fluorite structure, each Ce atom is surrounded by 8 oxygen atoms located at the corners of a cube creating a cubic crystal field belonging to the Oh point symmetry group. Due to this cubic crystal field, the CeIV 5d0 configuration is split into the eg and t2g bands corresponding to B1, C2 and B2, C1, respectively. (23,52) The CeIV valence corresponds to a 5d0 configuration, implying that in the HERFD-XANES process, t2g is first filled with electrons, while the eg is empty as the transferred energy is not sufficient. Here the experimental crystal-field energy splitting of Ce 5d in bulk CeO2 is ca. 4 eV. This value of energy gap between eg and t2g is in good agreement with previously published values. (53−55)
Now, looking at the experimental HERFD-XANES spectra of CeO2 NPs, one can observe that all the A, B1, B2, C1, and C2 features are presented at the same energy position as in bulk CeO2. A value of ca. 4 eV is found for the experimental crystal-field energy splitting of Ce 5d in bulk CeO2 NPs, indicating that this energy gap is not affected by the particle size. However, one can note that the eg feature intensity remains constant for both NPs and bulk CeO2, while the t2g intensity is proportional to the particle size. This experimental observation is corroborated by our theoretical calculations, showing that the simulated HERFD-XANES spectra of 2 nm CeO2 exhibit a t2g intensity smaller than that of bulk CeO2. The stability of the eg feature is also well reproduced. One possible explanation lies in the presence of adsorbed species at the NPs surfaces. In the case of bulk CeO2, the corresponding HERFD-XANES spectra are a direct measurement of the electronic structure of bulk Ce atoms. However, in the case of our NPs, the measured spectra correspond to the average electronic structure of both Ce atoms in the bulk and at the surface. This implies that the observed t2g variation originates from the electronic structure of surface Ce atoms. We showed in the previous section that Ce remains in the IV oxidation state and that the fraction of adsorbed species is inversely proportional to the particle size. In other words, their content is increasing with the surface Ce atoms to bulk Ce atoms ratio. Considering that the eg level is empty in Ce 5d0, the bonding between the surface Ce atoms and the adsorbed species requires the delocalization of the t2g electrons, hence explaining the observed decrease of the t2g intensity on the Ce LIII HERFD-XANES spectra. One can also assume that this bonding affects the crystal field by creating new d levels. This larger degeneracy of the t2g level is clearly observable for the sample Ce_5.6 which exhibits a broader t2g feature (Figure 6).

Figure 6

Figure 6. Comparison of experimental and calculated Ce LIII HERFD-XANES spectra of both 2 nm CeO2 and bulk CeO2. Calculated data shown here are reproduced from Plakhova et al. (21) Theoretical calculations predict a lower t2g intensity for the 2 nm CeO2.

The effect of the NPs size and the presence of absorbed surface groups on the shape of the Ce L3 HERFD-XANES spectrum has been previously discussed by Plakhova et al. (21) The CeO2 NPs investigated here were made by hydrothermal treatment at a temperature above 500 °C, which limits the availability of the absorbed species at the surface and allows showing the effect of the particle size on the crystal field splitting of the Ce 5d states, which, to our knowledge, is shown here for the first time.

Conclusion

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In this work, we have synthesized fluorite CeIVO2 NPs and studied the effect of the NP size on both local and electronic structure. By analogy with other metal oxides, we have shown that the lattice parameter expands with decreasing particle size. The presence of mainly CeIV, demonstrated by XPS and HERFD-XANES, indicates that the unit cell size dependence is not linked to the Ce valence, but to surface stress. Indeed, our TGA and FTIR data confirm the presence of surface hydroxyl and carbonate groups that have a tensile effect on the crystalline lattice. Additionally, the size dependence of the electronic structure, and especially of the t2g feature in the Ce LIII XANES spectrum, has been experimentally evidenced and confirmed with theoretical calculations.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00506.

  • Additional information includes sample characterizations by HEFRD-XANES, XRD, radiation damage study and TEM (PDF)

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Author Information

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  • Corresponding Authors
    • Damien Prieur - Helmholtz Zentrum Dresden-Rossendorf (HZDR), Institute of Resource Ecology, PO Box 510119, 01314 Dresden, GermanyThe Rossendorf Beamline at ESRF−The European Synchrotron, CS40220, 38043 Grenoble Cedex 9 Francehttp://orcid.org/0000-0001-5087-0133 Email: [email protected]
    • Walter Bonani - European Commission, Joint Research Centre, P.O. Box 2340, D-76125 Karlsruhe, Germany Email: [email protected]
  • Authors
    • Karin Popa - European Commission, Joint Research Centre, P.O. Box 2340, D-76125 Karlsruhe, Germany
    • Olaf Walter - European Commission, Joint Research Centre, P.O. Box 2340, D-76125 Karlsruhe, Germanyhttp://orcid.org/0000-0002-2679-1715
    • Kyle W. Kriegsman - Department of Chemistry and Alexandra Navrotsky Institute for Experimental Thermodynamics, Washington State University, Pullman, Washington 99164, United States
    • Mark H. Engelhard - Environmental Molecular Sciences Laboratory, Pacific Northwest National Lab, Richland, Washington 99352, United Stateshttp://orcid.org/0000-0002-5543-0812
    • Xiaofeng Guo - Department of Chemistry and Alexandra Navrotsky Institute for Experimental Thermodynamics, Washington State University, Pullman, Washington 99164, United Stateshttp://orcid.org/0000-0003-3129-493X
    • Rachel Eloirdi - European Commission, Joint Research Centre, P.O. Box 2340, D-76125 Karlsruhe, Germany
    • Thomas Gouder - European Commission, Joint Research Centre, P.O. Box 2340, D-76125 Karlsruhe, Germany
    • Aaron Beck - Institute for Nuclear Waste Disposal, Karlsruhe Institute of Technology, P.O. Box 3640, 76021 Karlsruhe, Germany
    • Tonya Vitova - Institute for Nuclear Waste Disposal, Karlsruhe Institute of Technology, P.O. Box 3640, 76021 Karlsruhe, Germanyhttp://orcid.org/0000-0002-3117-7701
    • Andreas C. Scheinost - Helmholtz Zentrum Dresden-Rossendorf (HZDR), Institute of Resource Ecology, PO Box 510119, 01314 Dresden, GermanyThe Rossendorf Beamline at ESRF−The European Synchrotron, CS40220, 38043 Grenoble Cedex 9 Francehttp://orcid.org/0000-0002-6608-5428
    • Kristina Kvashnina - Helmholtz Zentrum Dresden-Rossendorf (HZDR), Institute of Resource Ecology, PO Box 510119, 01314 Dresden, GermanyThe Rossendorf Beamline at ESRF−The European Synchrotron, CS40220, 38043 Grenoble Cedex 9 France
    • Philippe Martin - CEA, DEN, DMRC, SFMA, LCC, F30207 Bagnols sur Cèze cedex, France
  • Author Contributions

    K.P. and O.W. carried out the NPs synthesis work. W.B., K.P., and O.W. performed characterizations of the samples using in-house laboratory equipment. D.P. and P.M. performed synchrotrons measurements and associated data treatment. A.B. and T.V. set up the spectrometer for U M4 measurements. K.W.K., M.H.E., X.G., R.E., and T.G. did the XPS measurements. K.O.K. did the theoretical simulation of the experimental spectra. D.P., W.B., K.P., and P.M. cowrote the paper. D.P., W.B., K.P., P.M., O.W., and A.C.S. were involved in planning and supervision of the work. All authors discussed the results and contributed to the final manuscript.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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D.P., K.P., and W.B. acknowledge the KIT light source for provision of instruments at their beamlines and the Institute for Beam Physics and Technology (IBPT) for the operation of the storage ring, the Karlsruhe Research Accelerator (KARA). D.P., K.P., and W.B. are thankful to Dr. Kathy Dardenne and Dr. Joerg Rothe for their help during the beamtime. D.P. is also indebted to the ID26 team that lent us the crystals for XAS measurements. In addition, K.P., W.B., and O.W. acknowledge Sarah Stohr and Herwin Hein for the technical support and TGA and Dr. Heike Störmer from the Laboratory for Electron Microscopy of the Karlsruhe Institute of Technology (KIT) for TEM images. X.G. and K.W.K. acknowledge the institutional funds from the Department of Chemistry at Washington State University (WSU) and the support by the U.S. Department of Energy, Office of Nuclear Energy, grant DE-NE0008582. A portion of this research was performed using EMSL (grid.436923.9), a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research. K.O.K. acknowledges support from European Research Council (grant no. 75969).

References

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  16. Bradley Russell‐Webster, Javi Lopez‐Nieto, Khalil A. Abboud, George Christou. Truly Monodisperse Molecular Nanoparticles of Cerium Dioxide of 2.4 nm dimensions: A {Ce 100 O 167 } Cluster. Angewandte Chemie International Edition 2021, 60 (22) , 12591-12596. https://doi.org/10.1002/anie.202103110
  17. Bradley Russell‐Webster, Javi Lopez‐Nieto, Khalil A. Abboud, George Christou. Truly Monodisperse Molecular Nanoparticles of Cerium Dioxide of 2.4 nm dimensions: A {Ce 100 O 167 } Cluster. Angewandte Chemie 2021, 133 (22) , 12699-12704. https://doi.org/10.1002/ange.202103110
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  • Abstract

    Figure 1

    Figure 1. (Left) Ce L3HERFD-XANES spectra of CeO2 NPs with different sizes compared to bulk CeO2 (>100 nm). (Right) Pre-edge region of the Ce L3HERFD-XANES spectra.

    Figure 2

    Figure 2. High-energy resolution photoemission spectra of the Ce 3d spectral region. The spectra confirm the presence of the dominating CeIV.

    Figure 3

    Figure 3. Decrease of both lattice parameter (left) and strain (right) with the particle size. The lattice parameter, the strain, and the particle size have been calculated from the Rietveld refinement of the corresponding XRD patterns. All the values are provided in Table 1 of the Supporting Information. The gray dotted line corresponds to the power-law relation proposed by Baranchikov et al. (31) The goodness of fit was 0.963.

    Figure 4

    Figure 4. Determination of the adsorbed surface species. (Left) FTIR spectra of CeO2 NPs compared with Ce(OH)4 and bulk-CeO2 references. (Right) TGA data of CeO2 NPs of different sizes compared with Ce(OH)4 and bulk-CeO2.

    Figure 5

    Figure 5. (Left) Raman spectra. (Right) T2g position and width as a function of the particle size.

    Figure 6

    Figure 6. Comparison of experimental and calculated Ce LIII HERFD-XANES spectra of both 2 nm CeO2 and bulk CeO2. Calculated data shown here are reproduced from Plakhova et al. (21) Theoretical calculations predict a lower t2g intensity for the 2 nm CeO2.

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