Embed Size (px)
Citation preview
Ceria-Zirconia Nanoparticles Doped with La or Gd: Effect of the Doping
Cation on the Real Structure
Vladislav A. Sadykov 1,a, Vladimir V. Kriventsov 1,a, Ella M. Moroz 1,a , Yulia V. Borchert 2,b, Dmitrii A. Zyuzin 1,a, Vera P. Kol’ko 1,a,
Tatyana G. Kuznetsova 1,a, Vyacheslav P. Ivanov 1,a, Sergei Trukhan 1,a, Andrei I. Boronin 1,a, Egor M. Pazhetnov 1,a, Natalya V. Mezentseva 1,a,
Elena B. Burgina 1,a and Julian R. H. Ross 3,c
1 Boreskov Institute of Catalysis (BIC), pr. Akad. Lavrentieva, 5, Novosibirsk, 630090, Russia
2 University of Bremen, Otto-Hahn-Allee, Bremen 28359, Germany
3 University of Limerick (UL), Plassey Technological Park, Limerick 02098, Ireland
e-mail:
Keywords: doped zirconia-ceria, nanoparticles, real structure, surface composition, XRD, EXAFS, Raman, XPS, SIMS.
Abstract. The real structure of nanocrystalline CeO2-ZrO2 (Ce:Zr=1:1) systems prepared via the polymerized polyester precursor (Pechini) route and doped with La3+ or Gd3+ cations, up to 30 at.%, was studied by X-ray powder diffraction, EXAFS and Raman spectroscopy and the surface features characterized by XPS and SIMS. Undoped CeO2-ZrO2 system revealed nanoscale heterogeneity, perhaps due to the co-existence of Zr- or Ce-enriched domains. With large La3+ dopant the system remains bi-phasic within the studied ranges of composition, incorporation of the smaller Gd3+ cation stabilizes the single-phase solid solution. For both systems, the increase of dopant content was accompanied by a decline of domain size and an increase of the average lattice parameter of fluorite-like phases. Depletion of the surface layer by smaller Zr4+ cations was observed, while the surface content of a doping cation is either, close to that in the bulk (La) or below it (Gd). Such a spatial distribution of components results in some ordering of cations within the lattice. It is reflected in different modes of rearrangement of oxygen coordination polyhedra with the Gd or La content (distances and coordination numbers by EXAFS), and specificity of XRD patterns not conforming to a simple model with statistical distribution of oxygen vacancies.
Introduction
CeO2-ZrO2 solid solutions possessing a high oxygen mobility and reactivity are broadly used as components in catalysts for redox reactions, cathodes and anodes in solid oxide fuel cells and oxygen conducting membranes [1-3]. Their structural stability is improved by incorporating low-charge cations in the lattice [2]. However, for nanocrystalline doped ceria-zirconia systems specificity of their real structure is still a subject of debates [1]. This is because of the strong dependence of the spatial distribution of components within nanoparticles (surface/domain segregation), on their chemical composition and the synthesis route. Moreover, for nanoparticles, traditional X-ray diffraction analysis (XRD) does not enable reliable characterization of their structural features, and it should be combined with methods sensitive to the local structure. In this work, a polyester citric acid-ethylene glycol complex precursor (Pechini) method [4] was applied for synthesis of La- or Gd- doped ceria-zirconia nanocrystalline systems. The method was selected to ensure a high uniformity of the spatial distribution of components in particles of complex oxides. Combination of XRD with spectral methods sensitive to the local coordination environment of cations (EXAFS, Raman) and the chemical composition of the surface layers (XPS, SIMS) was applied for clearly characterising their actual structural arrangement, which is of primary importance for their transport and catalytic properties.
Solid State Phenomena Vol. 128 (2007) pp 81-88Online available since 2007/Oct/15 at www.scientific.net© (2007) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/SSP.128.81
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 128.226.37.5, State University of New York at Binghamton, Binghamton, United States of America-17/04/13,06:04:00)
Experimental The mole ratios of citric acid (CA), ethylene glycol (EG) and ethylene diamine (ED) to the total metal ions in the mixed nitrate solution (1.2 M, pH~1) were 3.75:11.25:3.75:1 respectively. During constant stirring, the solution of CA in EG was added dropwise at room temperature to the mixed nitrate solution followed by the addition of ED. The viscous liquid was evaporated on a hot plate at 100 oC for 5 h followed by heating at 150 oC for the next 5 h. The solid resin thus formed was calcined in air at 500 oC. All reagents used for synthesis were graded “chemical pure”. XRD investigations were carried out on a HZG-4C diffractometer (Cu Kα radiation and a flat graphite monochromator). The measurements were made in the 2θ angle range 20°-105° with steps of 0.1°. The structural parameters (unit cell parameters, site occupancies, etc.) were refined by the Rietveld full-profile method using the Fullprof programme package. Synchrotron radiation with λ 0.703Å at the Siberian Synchrotron Radiation Center (SSRC) was applied for measurements in the 2θ range 10-70o with a step of 0.1o for Ce0.5Zr0.5O2 mixed oxide. All EXAFS spectra of the Zr-K, Gd-L3, Ce-L3 edges were recorded at SSRC and analyzed using procedures described earlier [5, 6]. The Raman scattering measurements were carried out using a 100/S-Bruker Raman Fourier Spectrometer (the 1.06 mm line of an Nd-YAG laser was used for excitation). XPS spectra were acquired using an ES-300 spectrometer (Kratos Analytical, UK) equipped with a hemispherical electron analyzer and two anodes (AlKα, 1486.6 eV and MgKα , 1253.6 eV). All spectra were recorded with constant analyzer pass energy and a resolution of about 1.5 eV. The powdered samples were fixed on a holder by double-side scotch tape. The XPS analysis was performed at room temperature and pressures typically below 10-7 mbar. The spectra were calibrated by C1s line (284.8 eV). To prevent reduction of Ce4+ in the surface layer by X-ray radiation, anodes were operated at 50W instead of the standard 200 W. The sample preparation for SIMS included rubbing of a powder into a high-purity indium substrate. An Ar+ ion beam with energy of 3 keV and current density of 5 µA/cm2 was used for the sputtering of the samples. A MC-7201 monopole mass spectrometer was used for the detection of the secondary ions. The thickness of the etched layer was calculated from the sputtering time [7].
Results and discussion
XRD. The diffraction patterns of both series of samples only contain reflections corresponding to fluorite-like solid solutions which can be indexed in the Fm3m space group (Fig. 1), the most intense (111), (200), (220) and (311) peaks being situated at 2θ ~28o, ~32o, ~ 48o and ~ 58o, respectively [1, 8]. However, for the undoped ceria-zirconia and the La-doped samples, a pronounced asymmetry of diffraction peaks is observed which implies the presence of two phases having close lattice parameters. This asymmetry is clearly visible in diffraction patterns recorded using synchrotron radiation (Fig. 2). The average value of the lattice parameter for Ce0.5Zr0.5O2 was 5.285Å, while in the case of homogeneous solid solution with uniform distribution of Ce and Zr cations it should be 5.251Å. This agreed with the recent results of neutronographic studies by Mamontov et al [8] on the nanoscale heterogeneity of a Ce0.5Zr0.5O2 sample composed of zirconia-enriched domains in a ceria-enriched matrix which could not be distinguished by conventional X-ray diffraction methods. With increasing La content, the asymmetry of the diffraction peaks increases (Fig. 1), so at xLa >0.1, co-existence of two phases (perhaps, differing in La content as well) is apparent. The lattice parameter of both phases increases with La content reflecting the increase in concentration of this large cation (rLa3+ = 1.18 Å [9]) in the lattice.
82 Doped Nanopowders
20 40 60 80 100 120
0
50
100
150
200
250
43
2
1
Intensity
2θ 0
1. Ce0.35
Zr0.35
La0.3O
x
2. Ce0.45
Zr0.45
La0.1O
x
3. Ce0.4Zr
0.45La
0.2O
x
4. Ce0.5Zr
0.5O
x
2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0
0
2 0 0
4 0 0In te n s ity , a .u .
1 - C e0 .4 5
Z r0 .4 5
G d0 .1
Ox
2 - C e0 .4
Z r0 .4
G d0 .2
Ox
3 - C e0 .3 5
Z r0 .3 5
G d0 .3
Ox
3
2
1
2 θ
Fig. 1. XRD patterns of ceria-zirconia samples doped with La (a) or Gd (b). Cu Kα radiation For samples doped with the smaller (r = 1.053 Å [9]) Gd cation, the diffraction peaks are more symmetrical, so data can be described by a model of an homogeneous solid solution with the lattice parameter increasing with Gd content (Fig. 3).
2 8 3 0 3 2 3 4 3 6 3 8 4 0 4 20
1 0 0
1 0 2 0 3 0 4 0 5 0 6 0 7 00
5 0 0
2 θ
In t e n s i t y , a .u .
2 θ
In t e n s i ty , a .u .
0.00 0.05 0.10 0.15 0.20 0.25 0.30
0.528
0.530
0.532
0.534
0.536
0.538
0.540
0.542
3
2
1
Lattice parameter, nm
Dopant content, x
Fig. 2. XRD pattern of Ce0.5Zr0.5O2 at λ=0.703Å
Fig. 3. Lattice parameters versus La (1,2) or Gd (3) content in ceria-zirconia solid solutions
For both systems, the X-ray particle (domain) size estimated by the Scherrer equation from the half-width of the (220) peak decreases with the dopant content from ~ 60 to ~ 30-40 Å, thus reflecting overall disordering. The same trend was earlier observed for Gd-doped ceria [10] and was explained by dopant segregation at domain boundaries hampering sintering. For all samples, particles of platelet morphology (thickness up to 20-50 nm, lengths up to 100-200 nm) are formed by stacking of these nanodomains [10-12]. From Table 1, the number of oxygen vacancies obtained by the Rietveld analysis is greater than calculated in the frame of a simple substitution model. A great number of oxygen vacancies can be explained by oxygen displacements from their own sites (8c) in the fluorite-like structure. For Gd- doped mixed oxides, the increase of x from 0.2 to 0.3 decreases the overall thermal parameter. This implies that statistic displacement of oxygen ions from fluorite sites (8c) strongly decreases at high Gd content due to some structural rearrangement.
a b
Solid State Phenomena Vol. 128 83
Refinement with isotropic overall thermal parameter
Bov* [Å-2] 0.415 0.391 0.178
O occ.n** 0.854 (0.975) 0.854 (0.950) 0.824 (0.925) Rp
Rwp 2.50 3.24
2.10 2.70
2.31 2.94
Refinement with isotropic atomic thermal parameters B [Å-2 ] Me
O 0.453 1.10
0.471 1.680
0.176 0.164
O occ.n** 0.886 0.914 0.823
Rp
Rwp 2.50 3.23
2.09 2.69
2.31 2.94
*Bov, B – isotropic overall and atomic thermal parameters; **occ.n – occupation number. Calculated oxygen
occupation numbers given in brackets take into account Gd3+ content
Table 1. Structural parameters for Gdx(Ce0.5Zr0.5)1-xOy system by Rietveld analysis
EXAFS. Typical radial distribution function (RDF) curves for coordination spheres of Zr, Ce and Gd cations in samples of (Ce0.5Zr0.5)1-xGdxOy are shown in Figs. 4-6. Peaks at ~ 2 Å correspond to Me-O distances, while those in the range of 3.5-4.0 Å – to Me-Me distances (Table 2). For the initial nanocrystalline ceria-zirconia solid solution pronounced distortion of both the Ce and Zr first coordination sphere is observed (Table 2), which could be important for the lattice oxygen mobility [1]. With increasing Gd content, oxygen polyhedra around these cations become more symmetrical, while amplitudes of corresponding peaks increase (Fig. 4-7). This reflects increasing Me-O coordination numbers (Table 2) and decreasing distortion of the coordination sphere for a given cation [6]. For the Gd-O sphere, the respective peak also becomes more symmetrical (Fig. 6), while its amplitude declines with Gd content (Fig. 7).
Fig. 4. RDF curves describing Ce local arrangement for studied (Ce0.5Zr0.5)1-xGdxOy samples. Gd content 0.3 (a), 0.2 (b), 0.1 (c) and 0.05 (d)
Fig. 5. RDF curves describing Zr local arrangement for studied (Ce0.5Zr0.5)1-xGdxOy samples. Gd content 0.3 (a), 0.2 (b), 0.1 (c) and 0.05 (d)
84 Doped Nanopowders
Fig. 6. RDF curves describing Gd local arrangement for the (Ce0.5Zr0.5)1-xGdxOy samples. Gd content 0.05 (a), 0.1 (b), 0.2 (c) and 0.3 (d).
0.0 0.1 0.2 0.3-30
-20
-10
0
10
20
30
Zr-O
100x(A-Ao)/Ao, %
Dopant content, X
Zr-O1
2
3
4
Ce-O
Gd-O
Fig. 7. Relative amplitude of Me-O peak versus dopant content in (Ce0.5Zr0.5)1-xGdxOy (1-3) and (Ce0.5Zr0.5)1-xLaxOy (4) systems. Ao- experimental amplitude of Zr-O/Ce-O peaks for the parent Ce-Zr-O or CeO2 samples prepared in identical conditions. For Gd-O peak, Ao was obtained by extrapolation of experimental amplitudes for (Ce0.5Zr0.5)1-xGdxOy samples to xGd=0.
This implies that anion vacancies are mainly located in the coordination sphere of the Gd cation with its progressive rearrangement into a more dense and symmetrical one with the doping level. This phenomenon apparently explains the decline of the lattice oxygen mobility with Gd content in this system [11, 12]. For the La-doped system, this trend is less evident, shown by smaller values of Zr-O coordination numbers, Table 2, which agrees with the higher lattice oxygen mobility. [11, 12].
Sample Distance R [Å] Coordination number CeO2 [13] Ce-O
Ce-O Ce-Ce
2.24 2.39 3.84-3.86
3.0 5.7 8.0-8.5
Ce0.5Zr0.5 Ce-O Ce-O Zr-O Zr-O Ce-Me (Ce, Zr) Zr-Me (Ce, Zr)
2.20 2.40 2.10 2.26 ~3.7-3.8 ~ 3.6-3.8
2.8 4.7 4.1 2.6 - -
Ce0.35Zr0.35 La0.3 Zr-O Zr-O Zr-Zr Zr-Me (Ce, La)
2.12 2.24 3.70 3.83
4.6 2.1 1.3 1.2
Ce0.35Zr0.35 Gd0.3 Zr-O Zr-O Zr-Zr Zr-Me (Ce, Gd)
2.13-2.16 2.27-2.32 3.54 3.83
4.5-4.9 3.0-3.2 0.7 0.6
Table 2. EXAFS parameters for samples of doped Ce-Zr-O system
Solid State Phenomena Vol. 128 85
Overlapping of Ce and La L-edges does not permit to reliable estimation of the characteristics of the Ce-O coordination sphere for the La-doped series. However, some interesting conclusions can be obtained by analysis of the Zr-O coordination sphere. The most important feature is, that despite bigger lattice parameters for La-doped samples, Zr-O distances are shorter than for the Gd-doped system. This can be explained by a local rearrangement of coordination polyhedra around Zr cations caused by the neighbouring bulky La cation. Usually, a larger Me-O coordination number is associated with a longer Me-O bond, which is observed in this case (cf. data for Gd and La-doped samples, Table 2). Apparently, the larger La cation causes stronger contraction of neighbouring Zr-O coordination polyhedra thus favouring smaller Zr-O coordination numbers.
Raman spectra. For the initial ceria-zirconia solid solution (Fig. 8), along with a main peak at ~ 470 cm-1, a broad shoulder situated between 500-600 cm-1 is observed. This agrees with a strong tetragonal distortion of the oxygen coordination polyhedra revealed by EXAFS (see above). The more symmetrical shape of this peak at low dopant content (Fig. 8, 9) suggests a decrease of this distortion. At intermediate dopant content (10% for Gd and 20% for La) the peak intensity strongly declines suggesting a strong statistical disordering of the structure of the solid solutions. At a high doping level, the peak intensity increases again, being accompanied by pronounced peak splitting for the Gd-doped system, suggesting a pronounced local rearrangement of the structure. Similar features were also observed earlier for Sm-doped nanocrystalline ceria samples [13].
400 500 600
0,002
0,004
Intensity, a.u.
Raman shift (cm-1)
1
2
3
4
200 300 400 500 6000.0001
0.0002
0.0003
0.0004
0.0005
0.0006
0.0007
0.0008
Intensity, a.u
.
Wavenumber, cm-1
1
2
3
4
Fig. 8. Raman spectra for La-doped ceria-zirconia solid solution at La content 0 (1), 10 (2), 20 (3) and 30 (4) at. %
Fig. 9. Raman spectra for Gd-doped ceria-zirconia solid solution at Gd content 5 (1), 10 (2), 20 (3) and 30 (4) at. %
SIMS. In the analysis of the surface layers enrichment/depletion by doping cations in complex ceria-based oxides using SIMS data, variation of the ratio of their ionic currents to that of Ce+ is usually considered [14]. For both types of dopants, some depletion of the surface layer by zirconium is observed (Table 3). This could be explained by the size effect, namely, the relative enrichment of the less densely packed surface layer by bigger Ce4+ and Ln3+ cations. Within the sensitivity of SIMS and using applied segregation criteria, both doping cations –La and Gd appear to be distributed uniformly between the surface layer and the bulk particles. Since SIMS is a destructive method, some specific effects, namely, variation of the Me-O bonding strength and coordination numbers with the sputtering depth could complicate the analysis of the dopant spatial distribution [14]. Moreover, for randomly oriented nanocrystallites of a platelet morphology, typical for ceria-zirconia samples prepared via the Pechini route [11-13], during bombardment, some averaging of the surface and bulk composition could occur. Hence, non-destructive XPS analysis is required to verify these conclusions.
86 Doped Nanopowders
Zr+/Ce+ (La,Gd)+/Ce+ Sample Surface* Bulk** Surface* Bulk**
Ce0.5Zr0.5O2-y 0.12 0.11 - - Ce0.4Zr0.4 La0.2 0.15 0.25 0.55 0.55 Ce0.35Zr0.35 La0.3 0.20 0.35 0.9 0.9 Ce0.4Zr0.4 Gd0.2 0.25 0.30 0.08 0.08 Ce0.35Zr0.35 Gd0.3 0.15 0.25 0.10 0.10
* Within depth of 0.5-1 nm; **-after sputtering ~5-10 nm.
Table 3. SIMS ratio of ion currents for doped ceria-zirconia samples XPS. Analysis of XPS spectra of the Gd-doped system obtained with an Al anode (probing depth up to 1-2 nm) with due regard to the atomic sensitivity factors of the elements [15], revealed enrichment of the surface layer by Ce and its respective depletion by Zr. The relative depletion of the surface layer by Gd appears to increase with its content (Fig. 10). In general, this conclusion agrees with SIMS data. However, additional results obtained with a variation of the excitation energy, and, hence, the escape depth of photoelectrons allowed for further precision. Thus, for the Gd3d5/2 level, for a Mg anode (kinetic energy of emitted electrons 160 eV, free path λ 0.3 nm), the intensity of the corresponding line is much lower than that for the Al anode (λ = 1 nm) (Fig. 11). This implies that Gd is nearly absent in the uppermost layer of particles. For Gd 4d level, due to much higher kinetic energy of emitted electrons (Fig. 11), and, hence, an identical depth of analysis, the intensity of Gd 4d lines for both anodes is practically the same.
0.04 0.08 0.12 0.16
0.04
0.08
0.12
0.16
0.20
0.24
Surface concentration (at.%)
Bulk concentration (at.%)
Ce
Zr
Gd
100 120 140 160 180
1210eV
1443eV
Kinetic energy
Mg exitation
Al exitation
Gd4d
Ce4d
1170 1180 1190 1200
Gd3d
Binding energy eV
160eV
393 eV
Kinetic energy
Al exitation
Mg exitation
Fig. 10. Surface concentration of components versus bulk by XPS. AlKα radiation
Fig. 11. Effect of exitation energy on Gd XPS spectra for sample doped with 20% Gd.
Note, that for Gd-doped ceria [10] Gd enrichment of the surface layer was observed. This implies that for ceria-zirconia system, a spatial distribution of Gd is controlled by its preferential location in the vicinity of smaller Zr4+ cations, while the surface layer is mainly comprised of Ce4+ cations which have an intermediate size. This spatial arrangement, with some ordering of cations, favours a fluorite-like structure rearrangement at a high doping level as revealed by Rietveld analysis and Raman (see above). Though similar factors could operate for the La-doped system they are less clearly expressed due to the coexistence of two phases, and, hence, the presence of much disordered inter-phase boundaries, probably enriched by La.
Solid State Phenomena Vol. 128 87
Summary
The real structure of nanocrystalline La-or Gd-doped ceria-zirconia prepared via the polymerized precursor (Pechini) route is controlled by both the content and type of dopant. Incorporation of smaller Gd3+ cation stabilizes a single-phase fluorite-like solid oxide solution. The preferential location of Gd cations in the vicinity of smaller Zr cations in the bulk of nanoparticles takes place. Such an ordering seems to favour the location of anion vacancies (if any) in the coordination sphere of the Gd cations, while the coordination environment of Zr and Ce cations becomes more symmetrical and approaches the 8-fold arrangement with increasing Gd content. For the larger La3+ cation the system remains bi-phasic within all concentration ranges studied, which makes a detailed structural characterization more difficult. However, for the Zr-O coordination sphere a similar trend of increasing symmetry and coordination number with the addition of La was observed. This trend was less strongly expressed than for the Gd-doped system, which can be attributed to greater overall disordering of La-doped bi-phasic ceria-zirconia system.
Acknowledgements. This work is in part supported by INTAS 05-1000005-7663, RFBR-CNRS 05-03-34761 Projects and Integration Projects 95 and 4.15 SB RAS.
References
[1] J. Kašpar, P. Fornasiero, in: Catalysis by Ceria and Related Materials; edited by A. Trovarelli, Ed.; Catal. Sci. Ser.; Vol. 2, pp 217-241, Imperial Coll Press, London, U (2002).
[2] H. He, H.X. Dai, L.H. Ng, K.W. Wong and C.T. Au, J. Catal. Vol. 206 (2002), p. 1. [3] M. Mogensen, N. M. Sammes and G. A. Tompsett, Solid State Ionics Vol. 129 (2000), p.63 [4] M. P. Pechini, U.S. Patent 3,330,697 (1967). [5] V. A. Sadykov, Yu. V. Frolova, et al, Mater. Res. Soc. Symp. Proc. Vol. 835 (2005), p. 199. [6] D. I. Kochubey, EXAFS spectroscopy of catalysts, Nauka, Novosibirsk, 1992. [7] O. M. Ilinitsch, L. V. Nosova, V. V. Gorodetskii, V. P. Ivanov, S. N. Trukhan, E. N. Gribov,
S. V. Bogdanov, F. P. Cuperus, J. Mol. Catal. A Vol. 158 (2000), p. 237. [8] E. Mamontov, R. Brezny, M. Koranne, and T. Egami, J. Phys. Chem. B. Vol. 107 (2003),
p. 13007. [9] R. D. Shannon, Acta Cryst. A Vol. 32 (1976), p. 751 [10] H. Borchert, Yu. Borchert, V. V. Kaichev, I. P. Prosvirin, G. M. Alikina, A. I. Lukashevich,
V. I. Zaikovskii, E. M. Moroz, E. A. Paukshtis, V. I. Bukhtiyarov, and V. A. Sadykov, J. Phys. Chem. B. Vol. 109 (2005), p. 20077.
[11] V. A. Sadykov, Yu. Borchert, N. V. Mezentseva, et al, Mater. Res. Soc. Symp. Proc.Vol. 900E (2006) O10.04.
[12] V.A. Sadykov, T.G. Kuznetsova, Yu.V. Frolova-Borchert, G.M. Alikina, A.I. Lukashevich, V.A. Rogov, V.S. Muzykantov, L.G. Pinaeva, E.M. Sadovskaya, Yu.A. Ivanova, E.A. Paukshtis, N.V. Mezentseva, L.Ch. Batuev, V.N. Parmon, S. Neophytides, E. Kemnitz, K. Scheurell, C. Mirodatos, A.C. van Veen, Catal. Today Vol. 117 (2006), p. 475.
[13] V.A. Sadykov, T.G. Kuznetsova, G.M. Alikina, Yu.V. Frolova, A.I. Lukashevich, et al Catal. Today Vol. 93-95 (2004), p. 45
[14] V.A. Sadykov, T.G. Kuznetsova, S.A. Veniaminov et al, React. Kinet. Catal. Lett. Vol. 76 (2002), p. 83.
[15] Handbook of X-ray photoelectron spectroscopy, edited by J.F. Moulder, W.F. Stickle, P.E.Sobol et al., (Perkin-Elmer Corporation, Physical Electronics Division, Eden Prairie Minnesota, 1992).
88 Doped Nanopowders
Doped Nanopowders 10.4028/www.scientific.net/SSP.128 Ceria-Zirconia Nanoparticles Doped with La or Gd: Effect of the Doping Cation on the Real Structure 10.4028/www.scientific.net/SSP.128.81
DOI References
[3] M. Mogensen, N. M. Sammes and G. A. Tompsett, Solid State Ionics Vol. 129 (2000), p.63
doi:10.1002/chin.200028248 [13] V.A. Sadykov, T.G. Kuznetsova, G.M. Alikina, Yu.V. Frolova, A.I. Lukashevich, et al atal. Today Vol.
93-95 (2004), p. 45
doi:10.1016/j.cattod.2004.05.016
