Article pubs.acs.org/JPCC
Polyhedral CeO2 Nanoparticles: Size-Dependent Geometrical and Electronic Structure Cristina Paun,†,⊥ Olga V. Safonova,‡,⊥ Jakub Szlachetko,‡ Paula M. Abdala,§ Maarten Nachtegaal,‡ Jacinto Sa,†,‡ Evgeny Kleymenov,‡ Antonio Cervellino,‡ Frank Krumeich,∥ and Jeroen A. van Bokhoven*,†,‡ †
ETH Zurich, Institute for Chemical and Bioengineering, 8093 Zurich, Switzerland Paul Scherrer Institut, 5232 Villigen, Switzerland § SNBL at ESRF, 38043 Grenoble, France ∥ ETH Zurich, Laboratory of Inorganic Chemistry, 8093 Zurich, Switzerland ‡
S Supporting Information *
ABSTRACT: Ceria-based materials have many interesting applications including catalysis, fuel cells, and biology. The size- and shape-dependent changes in the catalytic properties of nanoceria are often attributed to stabilization Ce3+ defects on the nanoparticle surface. In this paper, we have performed a systematic analysis of the structure of polyhedral CeO 2 nanoparticles of 2−10 nm, under ambient conditions, using a combination of transmission electron microscopy, X-ray diffraction, and X-ray spectroscopy at Ce K- and L-edges. We reveal that under ambient conditions Ce3+ concentration does not depend on the size; however, the unit cell parameter and the pseudo Debye−Waller factors systematically change due to size-dependent surface contribution. The presence of Ce3+ traces relates to the use of Ce3+ precursors during synthesis. Exposure of nanoparticles to an intense beam of X-ray radiation causes reduction of Ce4+ ions, the extent of which is size-dependent.
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are often reported.13−15 However, whether these parameters are size-dependent and how they correlate to each other is not clear. Many experimental and theoretical studies addressed this topic.16−23 Tsunekawa et al.13 and Hailstone et al.22 determined by electron diffraction that the unit cell parameter of c-CeO2 particles stabilized in sols increased by 2.6% for 2.1 nm and 7% for 1 nm sizes, respectively, suggesting that more than 50% of Ce4+ ions were transformed into Ce3+. For nanoceria prepared by thermal evaporation in vacuum, X-ray photoelectron spectroscopy (XPS) and electron energy-loss spectroscopy (EELS) determined that the majority of Ce4+ was reduced into Ce3+ on the NP surface,18,23 which correlated to a very large CeO2 lattice expansion (3.5% for 2 nm NPs) according to electron diffraction.18 In contrast, Vayssilov et al.21 demonstrated by resonance photoemission spectroscopy (RPES) that 2 nm NPs, prepared by thermal evaporation, contained only 2% of Ce3+. NPs in the form of powders prepared by different wetchemistry methods and annealed in air17,19,24,25 are typically used in catalysis. Baranchikov et al.26 systematically studied the correlation between the size and the unit cell parameter of such NPs by XRD: the unit cell parameter was increasing for small NPs independently of the synthesis method up to 5.45 Å for 2
INTRODUCTION Ceria-based materials are applied in a wide range of fields such as catalysis,1,2 fuel cells,3,4 optics,5 gas sensors,6 and biology.7 As an active component of catalysts, it is, for example, used for abatement of CO, NOx, and hydrocarbons from automobile exhaust,8 soot removal from diesel fuels,9 total or partial oxidation of hydrocarbons in solid oxide fuel cells (SOFCs),10 and CO elimination from hydrogen streams for protonexchange membrane fuel cells (PEMFCs).11 It is generally accepted that the reactivity of ceria is related to the low redox potential of the Ce3+/Ce4+ pair and to the high oxygen storage capacity (OSC) that allow this oxide to store oxygen under oxidizing conditions (Ce4+) and to release it under reducing conditions, thus creating oxygen vacancies and Ce3+ defects.1 Nowadays, many efforts focus on the preparation of welldefined ceria nanoparticles (NPs) in a particular shape and narrow size distribution, growing in a certain crystallographic direction and exposing particular crystallographic surfaces. The ultimate goal of these studies is the development of advanced materials having high OSC, improved catalytic activity, selectivity, and stability.12 One of the main questions that still remain is how does the concentration of defects (Ce3+ and oxygen vacancies) in CeO2 change as a function of the NP size and morphology. For ceria NPs of 2−10 nm, stabilization of Ce3+ defects (bigger radius compared to Ce4+) on the surface and the related lattice expansion of the cubic c-CeO2 structure © 2012 American Chemical Society
Received: January 11, 2012 Revised: February 15, 2012 Published: February 28, 2012 7312
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Table 1. Samples Used in the Study: Preparation Methods, Annealing Conditions, Particle Size (d), Unit Cell Parameter (a), and Concentration of Ce3+ samples
preparation method
precursor
annealing conditions
d, nm
a, Å
Ce3+, %
M_110 M_200 M_300 M_400 M_500 P1_60 P2_50 P2_400 H_60 S_500 CeO2 NIST
Microemulsion
Ce(NO3)3·6H2O
Precipitation_1 Precipitation_2
Ce(NO3)3·6H2O (NH4)2Ce(NO3)6 Ce(NO3)3·6H2O Ce(NO3)3·6H2O -
2.0 3.0 4.1 5.0 7.0 4.9 2.1 8.6 8.9 9.6 380
5.4235 (5) 5.4230 (5) 5.4183 (3) 5.4168 (2) 5.4139 (2) 5.4258 (3) 5.4295 (6) 5.4146 (2) 5.4164 (1) 5.4150 (2) 5.41172 (5)
4.5(4) 4.2(4) 4.4(3) 4.1(3) 4.0(3) 2.0(3) 0.0(6) 0.0(3)
Hydrothermal Solvothermal NIST
110 °C, 24 h 200 °C, 6 h 300 °C, 6 h 400 °C, 6 h 500 °C, 6 h 60 °C, 24 h 50 °C, 24 h 400 °C, 6 h 60 °C, 24 h 500 °C, 6 h -
1.7(2) -
Figure 1. TEM images of polyhedron nanoparticles with sizes between 2 and 10.3 nm, synthesized by different procedures: (a) M_110; (b) P2_50; (c) M_200; (d) M_300; (e) M_500; (f) P2_400; (g) H_60; (h) S_500.
respectively) for 24 h, the samples were annealed at the desired temperature (200, 300, 400, and 500 °C, respectively) for 6 h. Micrograph images of nanoceria were obtained using a transmission electron microscope (TEM Tecnai F30 ST (FEI) with field emission gun) operated at 300 kV. The highresolution XRD patterns were collected at the SNBL beamline (BM01B, ESRF, Grenoble, France).30 We applied the Rietveld method to determine the unit cell parameter of cubic CeO2 structure using the FullProf program package.31 The average isotropic crystallite size was estimated from the broadening of the 111 reflection of CeO2 using the Scherrer’s formula. The strain estimated using the Williamson−Hall’s method32 was below 1.6%. Bulk CeO2 purchased from NIST (unit cell parameter a = 5.4117 Å, average crystallite size d = 380 ± 4 nm) was used as a reference. X-ray absorption spectra at the Ce K-edge were measured at the SNBL beamline30 in transmission mode, in air under ambient conditions. The spectra were acquired between 40 200 and 41 700 eV with a step size of 1 eV. A CeO2 NIST standard was measured simultaneously with each sample for energy calibration.33 No radiation damage of ceria34,35 was observed during the XRD and Ce K-edge XANES measurements. To determine the Ce3+ concentration, the XANES spectra of the NPs were fitted as a linear combination of the spectra of CeO2 NIST and Ce2(CO3)3·xH2O (Aldrich) standards. The EXAFS data were analyzed using the Ifeffit program package.36 The high-energy-resolution fluorescence detected (HERFD) XANES at the Ce L3-edge were measured at the SuperXAS beamline at the Swiss Light Source (PSI,
nm (0.7% expansion) suggesting increased concentration of Ce3+. In contrast, Nachimuthu et al.17 examined by Ce L3-edge X-ray absorption near-edge structure (XANES) spectroscopy the oxidation state in NPs prepared by a similar method and estimated that the Ce3+ concentration was lower than 5% even for 2 nm NPs. These examples clearly demonstrate that the unit cell parameter increases for small NPs; however, it is not clear whether these changes are always related to stabilization of Ce3+ in small NPs. The aim of this work was to determine systematically how the size of well-defined polyhedral ceria NPs affects their electronic and geometric structure. Transmission electron microscopy (TEM), XRD, and extended X-ray absorption fine structure (EXAFS) spectroscopy were used to characterize the morphology and the crystallographic structure of the NPs, while XANES at the Ce K- and L3-edges was applied to probe the electronic properties, in particular the concentration of Ce3+. To allow comparison, all measurements were performed in air under identical experimental conditions, and special care was taken to avoid ceria photoreduction in the X-ray beam.
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EXPERIMENTAL METHODS Ceria NPs of different size (2−10 nm), having polyhedral shapes, were prepared by modified microemulsion (M),24,27 precipitation (P1 and P2),26 solvothermal (S),28 and hydrothermal (H)29 methods (Table 1). Details about preparation and characterization are provided in the Supporting Information. After the initial drying (at 50, 60, and 110 °C, 7313
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Figure 2. (a) X-ray diffraction patterns of samples prepared by the microemulsion method (M). (b) Estimated CeO2 unit cell parameters as a function of particle diameters: ●, M_110, 200, 300, 400, and 500 (2.0, 3.0, 4.1, 5.0, and 7.0 nm); □, P1_60 (4.9 nm); ○, P2_50, 400 (2.1 and 8.6 nm); ◊, H_60 (8.9 nm); Δ, S_500 (9.6).
this theory, lattice expansion takes place in stoichiometric ceria containing no Ce3+ defects. Figure 3 summarizes the results of
Villigen, Switzerland) using a Si(111) double-crystal monochromator, focusing optics, and a high-energy resolution emission spectrometer,37,37 employing one spherically bent Ge(331) crystal analyzer (R = 1 m) suitable for detection of the Lα1 emission line of Ce (4839 eV) with an energy resolution of 1.3 eV. All the measurements were done in air under ambient conditions. During these measurements, the X-ray beam flux on the sample was between 1 and 5 × 1012 ph s−1 mm−2, which was about 3 orders of magnitude higher compared to the flux applied during XRD and Ce K-edge XAS measurements. To avoid radiation damage, spectra were acquired between 5715 and 5750 eV with a step size of 0.5 eV for 1 min, where the samples were only exposed during measurements using a fast shutter. The Ce L3-edge HERFD XANES of 2−11 nm ceria NPs was simulated based on the full multiple scattering theory using the FEFF9 code.38
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RESULTS AND DISCUSSION
Figure 1 shows typical TEM micrographs of the ceria NPs. The NPs were well-crystallized and had polyhedral shapes regardless of the preparation method. The values of the average NP size shown on the figure were estimated by XRD (Table 1). A statistical analysis of the NP size distribution from TEM data was difficult due to agglomeration; however, the sizes of up to 30 well-separated particles were measured, showing good agreement with the XRD data. Figure 2a presents typical XRD patterns of ceria NPs. All synthetic routes lead to particles with the cubic fluorite structure typical of bulk CeO2. Figure 2b shows that the unit cell parameter (a) of CeO2 increased as the NP size decreased; thus, the lattice expansion reached 0.2− 0.3% for the smallest NPs of 2 and 2.1 nm. The unit cell parameter showed significant variation for the NPs prepared by different synthetic methods. Qualitatively, Figure 2b reproduces the trend observed by Baranchikov et al.;26 however, the variations of the unit cell parameters were smaller. The results showed good agreement with the theoretical curve (dashed line in the Figure 2b) proposed by Perebeinos et al.39 who explained the lattice expansion in ceria by effective negative Madelung pressure taking place in ionic crystals. According to
Figure 3. Coordination numbers (N, in black) and the pseudo Debye−Waller factors (DW, in red) of oxygen in the first coordination shell (a) and cerium in the second coordination shell (b) estimated from EXAFS for ceria NPs as a function of the particle size: ●, M_110, 200, 300, 400, and 500 (2.0, 3.0, 4.1, 5.0, and 7.0 nm); □, P1_60 (4.9 nm); ○, P2_50, 400 (2.1 and 8.6 nm); ◊, H_60 (8.9 nm); Δ, S_500 (9.6).
EXAFS analysis. The coordination number of oxygen in all samples was close to the bulk value of 8, while the corresponding pseudo Debye−Waller factor (DWO) increased for small particles (Figure 3a) due to increased structural disorder on their surface and increased surface contribution that was previously observed by neutron diffraction.40 A first oxygen coordination shell in nanocrystalline oxides is typically close to the bulk values because the undercoordinated metal atoms on the surface complete their coordination shell by using 7314
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hydroxyl groups or water molecules.41 Second and further shells are more structure, size, and shape sensitive. Figure 3b shows decreased Ce−Ce coordination and increased DWCe as the NP size decreased. All samples followed this trend; however, the ones prepared by the same synthetic methods showed smaller dispersion. Ce K-edge XANES allowed quantification of the Ce3+ concentration in the NPs by comparing the edge position to that of Ce3+ and Ce4+ standards, which showed a difference of 6.2 eV (Figure 4a).
Figure 5. Normalized Ce L3 HERFD XANES spectra of ceria samples with different particles sizes: 2 nm (M_110), 2.1 nm (P2_50), 4.9 nm (P1_60), 7.0 nm (M_500), 9.6 nm (S_500), 380 nm (CeO2 NIST), and Ce2(CO3)3·xH2O standard. The arrows indicate the changes related to the decrease of the NP size.
peaks in the Figure 5) and 5740−5745 eV (A1 and A2 peaks in the Figure 5), respectively, were size-dependent. These features were broader and showed systematic energy shifts for small NPs indicating that the electronic structure of cerium atoms in nanoparticles was different from that in the bulk CeO2. However, no significant edge shift, characteristic of Ce3+, was observed in any of the spectra. Linear combination analysis for precise determination of the Ce3+ concentration in the NPs was not possible due to significant size-dependent changes in the corresponding spectra. The theoretical simulations of HERFD Ce L3 XANES of octahedral ceria NPs14 of different size and for massive CeO2 are shown in the Supporting Information. The simulations qualitatively reproduced the size-dependent changes in the first doublet feature as the NP size decreased: peaks B and C became larger, the intensity of peak C slightly increased and shifted toward higher energy, while peak B became less intense and shifted toward lower energy. Accordingly, the changes in the shape of features B and C toward smaller size can be explained by an enhanced contribution of cerium atoms situated on the NP surface and having a different local environment compared to the bulk atoms. The A1 and A2 peaks are related to multielectron processes and, therefore, could not be simulated using the single-electron FEFF code. In addition, the broadening of HERFD Ce L3 XANES spectral features relates to enhanced disorder in small NPs as also indicated by EXAFS. Overall, the observed size-dependent changes in the geometric and the electronic structure of CeO2 can explain the enhanced oxygen mobility, reducibility, and OSC of nanoceria reported in the literature. During longer exposure of nanoceria to the X-ray beam, the L3-edge position was progressively and significantly shifting toward lower energy, indicating the photoreduction of Ce 4+ into Ce 3+ (details are given in the Supporting Information). For smaller NPs, the shift was more pronounced, while for bulk ceria it was not observed. Thus, the reduction likely occurred on the NP surface and shows a dependence on particle size.
Figure 4. (a) Ce K-edge XANES spectra of CeO2 NIST (red curve), Ce2(CO3)3·xH2O (blue curve), and M_300 sample (black dash curve). (b) Variation of the Ce3+ concentration estimated from XANES Ce K edge spectra as a function of particle size: ●, M_110, 200, 300, 400, and 500 (2.0, 3.0, 4.1, 5.0, and 7.0 nm); □, P1_60 (4.9 nm); ○, P2_50, 400 (2.1 and 8.6 nm); ◊, H_60 (8.9 nm); Δ, S_500 (9.6).
The spectrum of one nanocrystalline sample is shown for comparison. The spectra of all nanocrystalline samples had the position of the absorption edge and the features above the absorption edge very close to that of bulk CeO2 NIST. The results of the linear combination analysis demonstrated that the concentration of Ce3+ in the NPs was smaller than 5% and independent of the NP size (Table 1, Figure 4). The concentration of Ce3+ was related to the preparation method. Samples that were prepared using a Ce3+ precursor showed traces of Ce3+ in the NPs. The NPs prepared using a Ce4+ precursor (P2 method) showed no Ce3+. Clearly under air Ce4+ does not reduce to Ce3+. Figure 5 compares the Ce L3-edge HERFD XANES42−44 for the ceria NPs of different size to the spectrum of CeO2 NIST and Ce2(CO3)3·xH2O. The spectra of the nanoceria had similar shapes as that of CeO2 NIST and the one reported by Kvashnina et al.43 Compared to the spectra of Ce L3 XANES of ceria NPs reported by Nachimuthu et al.,17 HERFD provided much better resolved spectra that allowed revealing the differences. The complex structure of the Ce L3 main edge arises from transitions from 2p3/2 to 5d5/2 orbitals, while the pre-edge peak (D in the Figure 5) is in addition related to the 2p → 4f process.43,45 For CeO2, the two main edge features have been assigned to screened (B and C in the Figure 5) and unscreened (A1 and A2 in the Figure 5) excited states, and their doublet structure reveals the crystal field splitting of 5d orbitals.The intensity and position of the two doublet features above the edge jump at 5730−5735 (B and C 7315
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CONCLUSIONS Using a combination of structure-sensitive techniques applied under identical experimental conditions (room temperature, air atmosphere), we established the correlation between the geometric structure and the electronic properties of polyhedral ceria NPs prepared by a number of wet-chemical methods and annealed in air. Correlations between the NP size, the unit cell parameter of the cubic CeO2, the structural disorder, and the electronic structure exist. The unit cell parameter increases when the NP size decreases (up to 0.3% for 2 nm NPs) correlating to an effective negative Madelung pressure taking place in ionic crystals. The structural disorder, characterized by pseudo Debye−Waller factors, increases for smaller NPs. Under ambient conditions, the oxidation state of ceria NPs is always close to 4+, and there is no stabilization of Ce3+ in smaller ceria nanoparticles; however, the structure is size-dependent due to the enhanced surface contribution in the smaller particles. Significant reduction to Ce3+ occurs during exposure of nanoceria to intense X-ray beams. Higher concentrations of Ce3+ previously observed on the surface of ceria NPs could be related to employing a reducing atmosphere (including vacuum) and exposure to intense X-ray and electron beams. Stronger structural disorder in ceria NPs compared to the bulk and size-dependent differences in the electronic structure revealed by HERFD XANES at the Ce L3 edge could be the main reasons for enhanced oxygen mobility, reducibility, and OSC of nanoceria reported in the literature.
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ASSOCIATED CONTENT
S Supporting Information *
Full description of synthesis procedures, characterization methods, and details on the theoretical simulations. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +41 44 632 55 42. E-mail: jeroen.vanbokhoven@ chem.ethz.ch. Author Contributions ⊥
These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Erich de Boni (PSI) and Markus Williman (PSI) for the technical help during experiments at SLS. We thank to the SNBL at the ESRF and the superXAS beamline at the SLS for the provision of beamtimes. TEM was performed at the electron microscopy center of ETH Zurich (EMEZ)
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