An EXAFS Study of Nanocrystalline Yttrium Stabilized Cubic Zirconia

Lotus Effect in Engineered Zirconia. Fereydoon Namavar , Chin Li ... Jing Zhang, and Tao Liu. The Journal of Physical Chemistry B 2003 107 (27), 6511-...
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J. Phys. Chem. B 2000, 104, 9597-9606

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An EXAFS Study of Nanocrystalline Yttrium Stabilized Cubic Zirconia Films and Pure Zirconia Powders Georgina E. Rush and Alan V. Chadwick* Centre for Materials Research, School of Physical Sciences, UniVersity of Kent, Canterbury, Kent CT2 7NR, U.K.

Igor Kosacki and Harlan U. Anderson Electronic Materials Applied Research Center, UniVersity of MissourisRolla, 303 MRC, Rolla, Missouri 65401 ReceiVed: March 23, 2000; In Final Form: August 8, 2000

Detailed EXAFS (extended X-ray absorption fine structure spectroscopy) measurements have been collected for two nanocrystalline forms of zirconia, namely, dense films of yttria-stabilized cubic zirconia (YSZ) and tetragonal phase powders of pure ZrO2. Zr and Y K edge EXAFS spectra for the YSZ films with grain sizes of 6, 15, and 240 nm showed no major differences with the corresponding spectra of the bulk counterpart. This is clear proof that these nanocrystalline films exhibit similar levels of disorder to that of large crystals. In particular, there is no support for the view that the intergrain regions are highly disordered, and the present work is consistent with recent EXAFS studies of other nanocrystalline oxides (SnO2 and ZnO) and metals (Cu). The pure nanocrystalline ZrO2 powders were produced by calcining zirconium hydroxide, a widely used method of synthesising ZrO2. The Zr K edge EXAFS of the powders, with grain sizes of 10 and 20 nm, yielded spectra in which the signal was strongly attenuated in comparison to the EXAFS bulk of ZrO2. A significant feature is the dramatically reduced amplitude of the second peak in the Fourier transform, which is due to the Zr-Zr correlation. This feature is often interpreted as evidence of high levels of disorder in nanocrystalline materials. However, using the results from other techniques, notably, NMR measurements, it is argued that the samples contained amorphous material due to an incomplete conversion of the hydroxide precursor. Overall, the studies of the two types of nanocrystalline zirconia emphasize the need for careful characterization of the materials prior to the application of techniques such as EXAFS, which provide an average picture of the local structure.

1. Introduction There is a growing interest in nanocrystalline materials because their properties can be markedly different from the parent bulk solids.1-6 Reasons for the interest in nanocrystalline oxides include the possibilities of producing superhard and superplastic ceramics and catalysts with enhanced activities. Clearly, the increased surface area-to-volume ratio afforded by preparing a catalyst in nanocrystalline form will increase activity; however, recent work on simple binary oxides7-9 suggests other factors, i.e., surface morphology and surface chemistry, are advantageously different for nanocrystals. In addition, nanocrystalline materials will dissolve higher concentrations of impurities than their bulk counterparts and this offers further scope for tailoring new catalytic materials. A thorough understanding of the chemistry of these new materials is reliant on a detailed characterization and particularly on the role of the preparative route in the nature of the microstructure of the nanocrystal. Several methods have been developed to prepare nanocrystalline oxides which include the calcination of hydroxides,10,11 sol-gel syntheses,12-14 oxidation of nanocrystalline metals,15,16 chemical vapor phase deposition,17,18 polymeric precursor spin coating,19,20 radio frequency sputtering,21,22 and high-energy ball milling.23-25 It is far from clear that the * To whom correspondence should be addressed.

different routes produce the same microstructure. In addition, there is a more general question that needs to be resolved concerning the level of structural disorder in nanocrystalline oxides. It is often assumed, based on the work on metals and general intuitive reasoning, that in a nanocrystalline oxide, due to the large fraction of the atoms being in the surface of the crystals, these surfaces will be highly defective and there will necessarily be highly disordered interfacial regions in a ceramic sample. However, reliable evidence for this assumption is lacking and for the specific cases of tin oxide, SnO2,26-29 and zinc oxide, ZnO,30-32 there is sound evidence the nanocrystals are not highly disordered; recent work29 on 3 nm nanocrystals of SnO2 shows that the nanocrystals have a similar level of static and dynamic disorder to that found in bulk material. In this paper, we report new structural information on nanocrystalline zirconia, ZrO2 and focus on films of yttrium-doped ZrO2 stabilized in the cubic phase (YSZ). The major technique used in the current work was extended X-ray absorption fine structure (EXAFS) spectroscopy, which is a local structural probe that enables the structure around a specific atom to be determined accurately.33-35 EXAFS is the oscillations in the X-ray absorption coefficient that occur above the absorption edge required for the emission of a photoelectron from a core (K or L) shell. The oscillations arise from the interference of the outgoing photoelectron wave with that part

10.1021/jp001105r CCC: $19.00 © 2000 American Chemical Society Published on Web 09/23/2000

9598 J. Phys. Chem. B, Vol. 104, No. 41, 2000 of itself that is backscattered by atoms surrounding the excited atom. The oscillations contain information on the nature of the backscattering atoms, including their number and distance from the target atom. A simplified plane wave approach to the EXAFS spectrum means that the normalized absorption, χ(k) as a function of the wave vector of the photoelectron, k, can be described by the heuristic equation

χ(k) )

∑i NiSi(k) exp(-2Ri/λ) exp(-2σi2k2) × sin(2kRi + 2δ + φi) (1)

The summation is over the i shells of coordinating atoms, Ni and Si(k) are the number of backscattering atoms and their amplitude factor of the atoms in the shell, respectively, Ri is the distance between the excited atom and the shell, δ is the phase shift induced in the electron wave as it propagates through the excited atom potential, φi is the phase shift induced by the backscattering atom potential, λ is the photoelectron mean free path for inelastic scattering, and σi is a Debye-Waller term which accounts for static and dynamic disorder in the structure. EXAFS is atom specific and, unlike Bragg diffraction, does not rely on long-range order and can be used to study local structure in liquids and amorphous and crystalline solids. However, it is important to stress that EXAFS monitors the aVerage local environment of the target atom and that a detailed interpretation will be difficult if the target atom is in more than one environment, for example, in different crystallographic sites in a solid. A number of nanocrystalline structures have been characterized by EXAFS including high nuclearity Pd clusters,36 semiconducting CdX (X ) S,37-39 Se,37-40 and Te37), binary metal oxides such as ZnO,30-32 ZrO2,23,41-48 and SnO2,23-26 and small supported catalyst particles.49,50 The type of information that has been deduced has been particle sizes, bond lengths, dynamics within the particles, and the location of dopants. Zirconia is an exceptionally important technological ceramic with a variety of applications including use as a tough engineering material,51-53 a highly durable catalyst54,55 and catalyst support,56-58 and an oxygen conducting solid electrolyte membrane, used in fuel cells,59,60 oxygen separators,61 and oxygen sensors.62,63 As a result there have been extensive studies of its chemical and physical properties. The phase behavior of zirconia is complex, and the key crystallographic parameters of zirconia are summarized in Table 1. The normal phase at room temperature is monoclinic, which converts at elevated temperatures to a tetragonal and then a cubic, fluorite-structured phase. The tetragonal phase can be stabilized at room temperature by the addition of small amounts (less than ∼16 mole %) of aliovalent cation impurities such as Ca2+, Mg2+, or Y3+. Higher concentrations of these dopants will stabilize the cubic form at room temperature, and it is this material, particularly the yttrium-stabilized cubic zirconia, YSZ, that is used as a solid electrolyte.59-62 The tetragonal phase is also stabilized in small crystallites (less than ∼30 nm) of pure ZrO2.68,69 The common synthetic route to zirconia is the calcination of zirconium hydroxide, a complex, amorphous material which is not simply Zr(OH)4 but a hydroxylated zirconyl cluster whose composition depends on the preparation conditions. The calcination conditions can be used to control the product, via the annealing temperature and time, to a prescribed particle size and phase. For example, heating for 1 h at 500 °C will produce tetragonal crystallites with an average size of 10 nm.70 However, the structural changes in the conversion from the hydroxide are complex,11,71 and very recent work combining EXAFS and nuclear magnetic resonance measurements (NMR) has empha-

Rush et al. TABLE 1: Crystallographic Data for Zirconia composition ZrO2

phase tetragonal

atom

O O Zr Zr ZrO2 + Y tetragonal O (6 atom %) O Zr(Y) Zr(Y) ZrO2 cubic O Zr ZrO2 + Y cubic O (26 atom %) O Zr(Y) ZrO2 monoclinic O O O O O O O Zr Zr Zr Zr Zr Zr Zr Zr

CN RD/Å 4 4 4 8 4 4 4 8 8 12 3 4 12 1 1 1 1 1 1 1 1 2 2 1 1 2 2 1

2.065 2.455 3.64 3.68 2.08 2.38 3.61 3.63 2.28 3.72 2.04 2.23 3.64 2.051 2.057 2.151 2.163 2.189 2.222 2.285 3.341 3.433 3.461 3.463 3.588 3.923 4.030 4.540

comment/ [ref] 1250 °C [64]

room temp [65]

2400 °C [66] room temp [67] room temp [68]

sized the multiphase nature of ZrO2 produced by low-temperature calcination routes.47,48 Calcination temperatures in excess of 700 °C are necessary to remove traces of hydrogen and amorphous material from the sample. As a result, the final material has relatively large crystallites, average size of ∼25 nm, and is a mixture of tetragonal and monoclinic phases. There have been extensive EXAFS studies of ZrO2 to determine the local structure72-74 and location of chargecompensating defects created dopants,35,75-82 the latter topic being one of considerable debate. The effect of adding the lower valent substitutional cationic impurities to ZrO2 is the creation of oxygen ion vacancies. Intuitively, on the basis of electrostatic arguments, these vacancies would be expected to be on nearest neighbor sites adjacent to the dopant. However, early EXAFS studies of YSZ gave clear evidence that the oxygen vacancy was adjacent to the Zr4+ ion rather than the Y3+ ion. This is rationalized by the preference of Zr4+ to adopt a 7-fold coordination in its crystal chemistry rather than the 8-fold coordination in its perfect fluorite structure. Other work has confirmed this picture of the local structure of YSZ, particularly more recent detailed and extensive EXAFS measurements.73 EXAFS measurements have also been widely used to study nanocrystalline ZrO2 produced by calcining the hydroxide,11,42,44,47,48,71 mechanical attrition of bulk ZrO2,23 or oxidation of the nanocrystalline metal41,43 and an unspecified chemical method.45 However, given the possible phase complexity of the nanocrystalline ZrO2 samples, which in the case of material produced from the hydroxide has been definitively shown to contain some amorphous phase,47,48 it is uncertain how much reliable information can be deduced about the structure of the nanocrystals since EXAFS will monitor the average environment of the target atom in the various phases. The uncertainties of the phase structure of nanocrystalline ZrO2 samples have focused the present EXAFS measurements on materials that have been thoroughly characterized by other techniques. These samples are YSZ films that have been specially produced on monocrystalline sapphire substrates20,83

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and that have been extensively characterized by a variety of techniques.20,84-89 The preparation technique employed, spin coating of a polymeric precursor followed by low-temperature calcination, produces high-density nanocrystalline films with a narrow range of crystallite size, and the crystallites only show rapid grain growth when heated to 1000 °C. As a result, the current EXAFS measurements are the first to truly reflect the nature of the ZrO2 nanocrystallites and enable meaningful conclusions to be drawn about the levels of disorder at the atomic level.

the following relationship can be applied:

2. Experimental Section

This equation can be used to determine strain and grain size from a plot of β cos θ vs 4 sin θ. 2.3. EXAFS Measurements. Measurements were made on station 9.2 at the CLRC Daresbury Synchrotron Radiation Source. The synchrotron has an electron energy of 2 GeV, and the average current during the measurements was 150 mA. Station 9.2 is equipped with a high-stability, double Si(220) crystal monochromator that can be offset from the Bragg angle to reject harmonic contaminants from the monochromatic beam. In the present work the harmonic rejection was set at 50%. Zr and Y K edge EXAFS spectra for the powdered materials were collected in conventional transmission mode using gas-filled ion chamber detectors. The samples were prepared by thoroughly mixing the ground material with fumed silica or powdered polythene diluents and then pressing the sample into pellets in a 13 mm IR press. Spectra were typically collected to k ) 14 Å-1 and several scans were taken to improve the signal-to-noise ratio. For these measurements, the amount of sample in the pellet was adjusted to give an adsorption, µd, approximately equal to 1. The presence of the sapphire substrate precluded the collection of EXAFS spectra in transmission mode for the YSZ films. Y and Zr K edge spectra for these samples were collected in fluorescence mode. The samples were mounted at a 45° angle to the incident X-ray beam, and the fluorescence signal was monitored with a 32-element Canberra solid state detector, which was normal to the beam. Again, spectra were typically collected to k ) 14 Å-1 and several scans were taken. For a concentrated sample (i.e., greater than 1 atom % of the target element), it is necessary to correct the fluorescence EXAFS for the attenuation of the signal due to self-absorption by the sample. In the present case, the correction is less than 1% for both the Y and Zr K edge data91 and can be regarded as negligible. The data were processed in the conventional manner using the Daresbury suite of EXAFS programs: EXCALIB, EXBACK (or EXBROOK), and EXCURV98.92 Phase shifts were derived from ab initio calculations within EXCURV98. This code also includes routines to treat multiple scattering effects in highly symmetric structures. For each spectrum, a theoretical fit was obtained by adding shells of atoms around the central excited atom and least-squares iterating the radial distances, RD, and the Debye-Waller type factors, A ()2σ2). This latter factor will contain contributions from both thermal disorder and static variations in RD. In some cases the coordination number, CN, was also iterated. The quality of the fit is measured by an R factor, given as a percentage92 and the errors in RD are approximately (0.02 Å and approximately (20% in A and CN.

2.1. Materials Preparation. Dense nanocrystalline YSZ thin films (containing 16 atom % Y) with a thickness of 0.5 µm were prepared on sapphire single-crystal substrates (Kyocera Ceramics Corp.) using a polymer precursor spin-coating technique. The procedure has been described in detail in other publications;19,20,83,84 briefly described, it involves the preparation of an aqueous solution of ZrOCl2‚8H2O and Y(NO3)3 of the appropriate composition and adjusting the viscosity by the addition of ethylene glycol and glycine. This precursor is spincoated on the substrate and subsequently converted to the oxide by heat treatment at relatively low temperatures. The crystallinity,83,86,87 integrity,86,87 phase purity,83,86 electrical conductivity,83,89 and band gap energy85 of these YSZ films have been thoroughly characterized. The microstructure of the films depends strongly on sintering temperature, and the grain size can be varied from 1 to 330 nm over the temperature range 300-1500 °C. Due to the nucleation of the material occurring at the molecular level in liquid solution, the films are dense and have a very uniform microstructure; more than 80% of the particles have the average grain size. The YSZ films used in the present work had grain sizes of 6, 15, and 240 nm. Some other zirconia samples were also used for comparative purposes. Commercial samples of YSZ (16 atom % Y, from the Tosoh Corp., Japan) and monoclinic ZrO2 (Magnesium Elektron Ltd.) were used as the EXAFS reference standards for the bulk material. Tetragonal nanocrystals of ZrO2 were prepared by the standard synthetic route from the hydroxide. This involved the precipitation of zirconium hydroxide from an aqueous solution of ZrOCl2‚8H2O with aqueous ammonia and washing and drying the hydroxide, followed by calcination in air. Two samples of tetragonal ZrO2 were produced by heating the hydroxide at 500 and 600 °C for 1 h with average grain sizes expected to be of the order of 10 nm.42,70 2.2. Laboratory X-ray Powder Diffraction. X-ray powder diffraction patterns of the films and powdered samples were collected on a conventional SCINTAC X-ray diffractometer. For the powders, the particle sizes, dg, were determined from the Debye-Scherrer equation,90 i.e.,

dg )

Kλ β cos θ

(2)

where K is a constant (0.89), β is the full-width-at-halfmaximum-height of a diffraction peak at angle θ, and λ is the X-ray wavelength (in the present work Cu KR radiation was employed). For the films with smaller grain sizes, the Debye-Scherrer equation is not appropriate as it neglects the influence of strain on the peak broadening. In this case, both the grain size, dg, and the strain,  ()∆d/d, where d is the lattice spacing), of the films were determined from an analysis of the shape of the peak. Assuming that the broadening of the peak is the sum of the contributions attributed to the grain size (β′) and the strain (β′′),

β ) β′ + β′′ )

λ + 4 tan θ dg cos θ

(3)

This relation can be rearranged to give the following relationship between broadening, strain, and size

β cos θ ) 4 sin θ +

λ dg

(4)

3. Results 3.1. X-ray Diffraction Studies. The crystallization and grain growth of the YSZ thin films when annealed for 4 h at different sintering temperatures, Ts, was monitored by X-ray powder diffraction, and the results are shown in Figure 1. As expected, the lines sharpened as the sintering temperature was increased.

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Figure 3. The laboratory XRPD pattern for calcined samples of zirconium hydroxide. The patterns were collected with Cu KR radiation. The standard patterns for monoclinic and tetragonal zirconia are shown for comparison.

Figure 1. The laboratory XRPD patterns for the YSZ films using different sintering temperatures, Ts. Samples were annealed for 4 h at each sintering temperature, and the patterns were collected with Cu KR radiation. The standard patterns for sapphire and bulk YSZ are shown for comparison.

Figure 2. A plot of the strain versus grain size for the YSZ films. The plot was obtained by fitting the data shown in Figure 1 to eq 4.

An important feature of the results is that the relative intensities of the peaks correlate well with those for the bulk YSZ standard, showing that the films are growing without a preferred crystallographic orientation. The diffraction peaks for the films were fitted to eq 4, and the plot of strain, , versus grain size, dg, is shown in Figure 2. The strain only becomes significant when dg is less than 50 nm and then increases rapidly with decreasing dg. Previous work86 on these films has shown that for thicknesses greater than 200 nm, the strain is due to the microstructure of

the film and is not influenced by the substrate or substrate/film interface. The origin of the higher strain in the smaller crystallites is probably attributable to the influence of the increased surface area-to-volume ratio of the crystallites or due to different surface morphologies of the crystallites. Both of these factors would increase the strain without changing the average lattice constant. It is worth noting that the magnitude of the strain, even in a film with 2 nm crystallites, when converted to a change in the EXAFS Debye-Waller factor, would not be experimentally detectable in an EXAFS experiment. The morphology and particle sizes of samples of zirconia powder prepared by the calcination of the hydroxide depend strongly on heating temperature and time. The X-ray powder diffraction patterns for the particular samples used in the present work are displayed in Figure 3. The sample produced after calcining at 500 °C shows only diffraction peaks of the tetragonal phase, and the peak widths indicate an average particle size of 10 nm. Calcining at 600 °C yielded tetragonal phase particles with an average size of 20 nm and evidence from the diffraction peaks of about 10% monoclinic phase in the sample. 3.2. EXAFS Standards for Bulk Zirconia Polymorphs. The EXAFS spectra were collected for the commercial samples of monoclinic zirconia and YSZ. The results were used as standards to calibrate the procedures and as references for the results on the nanocrystalline films. The Zr K edge normalized EXAFS and the corresponding Fourier transform for monoclinic zirconia collected at room temperature are shown in Figure 4i. For the YSZ sample, both Zr K and Y K edge transmission EXAFS were collected and the normalized EXAFS are shown in parts ii and iii of Figure 4, respectively. The best-fit parameters to the spectra are listed in Table 2. The crystallographic data for the polymorphs, listed in Table 1, show a large spread of Zr-O and Zr-cation distances, which are too complex to apply directly to the EXAFS data without some simplification into a model with a smaller number of subshells. The starting models employed in the present work were those developed and verified in the extremely thorough EXAFS studies of zirconia by Li et al.72-74 In the case of monoclinic zirconia, the diffuse Zr-O shell is reduced to a single shell of 7 O atoms at 2.16 Å and the

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Figure 4. EXAFS spectra (k3-weighted) (a) and corresponding Fourier transforms (b) for bulk ZrO2 samples. The corresponding Fourier transform is corrected with the phase shift of the first shell. The experimental data are represented by the solid line and the best fit is the dotted line: (i) Zr K edge EXAFS of monoclinic ZrO2, (ii) Zr K edge EXAFS of YSZ, (iii) Y K edge EXAFS of YSZ.

Zr-Zr shell is fitted by 3 subshells of 7, 4, and 1 Zr atoms at 3.46, 4.01, and 4.55 Å, respectively. In the case of YSZ the Zr atom is assumed to be surrounded by a shell of 7 O atoms at

2.15 Å and 12 Zr(Y) atoms at 3.58 Å. Thus the model assumes the O vacancy is adjacent to the Zr atom. It should be noted that Zr and Y are equivalent backscatters and will be indistin-

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Figure 5. EXAFS spectra (k3-weighted) (a) and corresponding Fourier transforms (b) for a 6 nm grain size YSZ film. The corresponding Fourier transform is corrected with the phase shift of the first shell. The experimental data are represented by the solid line and the best fit is the dotted line: (i) Zr K edge EXAFS, (ii) Y K edge EXAFS.

TABLE 2: EXAFS Results for Bulk Zirconia Standards Zr K edge EXAFS

Y K edge EXAFS

phase

atom

CN

RD/Å

A/Å2

ZrO2

monoclinic

ZrO2

monoclinic

YSZ (16 atom % Y)

cubic

YSZ (20 atom % Y)

cubic

O Zr Zr Zr O Zr Zr Zr O Zr(Y) O Zr(Y)

7 7 4 1 7 7 4 1 7 12 7 12

2.15 3.46 4.03 4.55 2.16 3.46 4.01 4.55 2.13 3.58 2.15 3.58

0.020 0.015 0.017 0.002 0.020 0.007 0.012 0.005 0.022 0.023 0.019 0.022

composition

guishable. The local environment of Y atoms in YSZ in this model consists of 8 O atoms at 2.32 Å and 12 Zr(Y) atoms at 3.62 Å. The parameters listed in Table 2 were obtained by fitting with the values of CN held fixed at the values used by Li et al.72,73 The data of Li et al72,73 are also listed in Table 2 for comparative purposes. It can be seen that for monoclinic ZrO2, the agreement between the present and previous work is good. For this system Li et al collected their EXAFS at 10 K, so the Debye-Waller factors are expected to be lower, as was observed. The agreement in the case of YSZ is excellent for both the Zr and Y data. Although the compositions used in the two studies were slightly different, the previous work has shown that there is not a strong dependence of the parameters on the Y content. 3.3. EXAFS for YSZ Films. Zr and Y K edge EXAFS were collected at room temperature for the YSZ films with grain sizes of 6, 15, and 240 nm. The normalized EXAFS and the corresponding Fourier transforms for the 6 nm film are shown

atom

CN

RD/Å

A/Å2

comment/[ref] room temp, this work

data collected at 10 K [72]

O Zr(Y) O Zr(Y)

8 12 8 12

2.31 3.61 2.33 3.63

0.018 0.020 0.018 0.016

room temp, this work room temperature [73]

in parts i and ii of Figure 5 for the Zr and Y K edges, respectively. The data are noisier than the transmission data collected for the bulk samples; however, the Fourier transforms for both edges show two clearly resolved peaks at ∼2.2 and 3.6 Å. By analogy with the bulk YSZ, these peaks arise from the nearest neighbor shells of O and Zr atoms, respectively. The results for the two films with the larger grain sizes were very similar to those shown in Figure 5. Therefore, it is necessary only to report the Fourier transforms, and these are displayed in Figure 6. All the spectra were subjected to a detailed fitting using the model for YSZ outlined in the previous section, and the results are given in Table 3 along with the parameters for the bulk YSZ sample. It can be seen that, within the experimental errors, the parameters are the same for all the samples. The fitting for each sample was repeated while the coordination numbers were allowed to vary during the iterations. This resulted in marginal improvements in the fitting error, and the parameters were within

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Figure 7. The Fourier transforms of the Zr K edge EXAFS for the nanocrystalline ZrO2 produced by calcining zirconium hydroxide: (a) sample heated for 1 h at 500 °C; (b) sample heated for 1 h at 600 °C. For clarity, the successive plots have been shifted by 20 units on the y-axis.

Figure 6. The Fourier transforms of the EXAFS for the nanocrystalline YSZ films: (a) Zr K edge EXAFS and (b) Y K edge EXAFS. The grain sizes are listed beside each plot. For clarity, successive the plots have been shifted by 10 and 15 units on the y-axis for the Zr and Y data, respectively.

the errors for those listed in Table 3, i.e., (0.002 Å in RD and (20% in CN and A. 3.4. EXAFS for Nanocrystalline Zirconia Produced by Calcining Hydroxide. Zr K edge transmission EXAFS were collected at room temperature for the samples with 10 and 20 nm grain sizes and the parent hydroxide. The Fourier transforms of the EXAFS are shown in Figure 7. The parameters obtained from the fitting of these EXAFS spectra are listed in Table 4. 4. Discussion 4.1. EXAFS of Nanocrystalline Materials. The comparison between the present EXAFS for bulk zirconia samples and the data of Li et al.72,73 was performed to test the experimental and analysis procedures. The agreement between the two sets of experiments, as shown in Table 2, is good and provides the necessary confidence in the procedures when the nanocrystalline materials are discussed. There are three possible effects of using nanocrystalline oxides in EXAFS measurements. These will be treated separately, although it must be borne in mind that they are interrelated. The first is the relatively simple matter of the reduction in the coordination numbers due to crystallite size. EXAFS is an aVerage technique in that it monitors the local environment of the average atom. Hence the coordination number will be reduced if a large fraction of the total number of atoms are on surface sites. For the EXAFS of the cations, this is not expected to be evident in the O atom shells if the oxides are studied in air because the surface will be covered by a layer of adsorbed O or OH. The effect will be seen in the coordination numbers of the cation shells and will be more marked the greater the distance of the shell from the target cation. The Debye-Waller

factors will be unchanged from the values in bulk samples. This effect can be clearly seen in the Sn K edge EXAFS of 3 nm SnO2 nanocrystals29 where the second peak in the Fourier transform is markedly attenuated. The effect is easily estimated by a simple geometrical calculation. For 6 nm YSZ particles and assuming a cubic particle geometry, the average CN of the first Zr-cation and Y-cation shells is 10.8. Similar calculations for 5 nm spherical particles of cubic ZrO2 yielded a value of CN for the Zr-Zr shell of 10.6.44 These values are so close to the value of 12 for bulk samples that the difference is well within the measurement error of (20% and could not be reliably determined experimentally. The second possible influence of nanocrystallinity on the EXAFS is that of disorder, either static or dynamic. In this case, the estimation of the magnitudes of the effects is speculative. Unusually high levels of point defects, either vacancies or interstitial atoms, in the bulk or at the surface of the crystallites, might be expected due to the crystallite size being comparable to the space-charge distance.93,94 This would lead to considerable relaxation of the lattice. In turn, this could potentially affect the all the EXAFS parameters, i.e., coordination numbers, radial distances, and Debye-Waller factors. However, it is difficult to make any meaningful quantitative predictions, although defect levels of several percent in crystallites of 10 nm diameter would probably be required to produce observable differences from bulk samples. Similarly, a distortion of the bonds at the surface of the crystallites, referred to as surface rumpling,95 would have to be very large to perturb the EXAFS. A third possible effect is due to the fact that in nanocrystals a large fraction of the atoms are at surface or interfacial regions (about 25% and 50% in 5 and 2 nm diameter particles, respectively1). These intergrain regions could be amorphous in nature with a range of radial distances. This would clearly produce an attenuation of the EXAFS signal and a reduction of the peak amplitudes in the Fourier transform. In the EXAFS analysis, this would result in reduced coordination numbers and larger Debye-Waller factors. However, the qualitative effect is the same as that produced by the simple reduction of crystallite size discussed above, and care has to be taken in the EXAFS analysis to separate the two effects. It is worth noting that in the case of nanocrystalline metals, there has been considerable controversy over the level of disorder in the intergrain regions. However, a recent EXAFS study of nanocrystalline copper, in which particular care was taken over the sample preparation and measurement procedures, showed these regions are similar

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TABLE 3: EXAFS Results for YSZ Films Zr K edge EXAFS composition

grain size

YSZ

6 nm

YSZ

15 nm

YSZ

240 nm

YSZ

bulk

Y K edge EXAFS

atom

CN

RD/Å

A/Å2

atom

CN

RD/Å

A/Å2

comment/[reference]

O Zr(Y) O Zr(Y) O Zr(Y) O Zr(Y)

7 12 7 12 7 12 7 12

2.15 3.56 2.17 3.61 2.17 3.60 2.13 3.58

0.020 0.029 0.025 0.025 0.018 0.026 0.022 0.023

O Zr(Y) O Zr(Y) O Zr(Y) O Zr(Y)

8 12 8 12 8 12 8 12

2.31 3.61 2.30 3.62 2.31 3.61 2.31 3.61

0.020 0.019 0.017 0.020 0.018 0.020 0.018 0.020

room temperature, this work room temperature, this work room temperature, this work room temperature [73]

TABLE 4: EXAFS Results for Nanocrystalline Zirconia Powders and Precursors Zr K edge EXAFS composition

grain size

atom

CN

RD/Å

A/Å2

comment/[reference]

O Zr O O (or OH/H2O) Zr Zr O Zr O Zr O Zr(Y)

7 4 2.0 ( 0.5 5 ( 0.5 1.7 ( 1.0 2.0 ( 1.0 4 12 7 6 5 12

2.13 3.37 2.08 2.16 3.27 3.41 2.13 3.41 2.13 3.44 2.13 3.62

0.018 0.028 0.01 0.022 0.023 0.023 0.018 0.079 0.020 0.016 0.013 0.017

room temperature, this work

phase

zirconium hydroxide

amorphous

zirconium hydroxide

amorphous

ZrO2

10 nm

tetragonal

ZrO2

20 nm

tetragonal + 10% monoclinic

ZrO2+Y (6 atom %)

bulk

tetragonal

to the grain boundaries in conventional polycrystalline copper and are not highly disordered.96 4.2. EXAFS of YSZ Films. The outstanding feature of the present EXAFS results, for both the Zr and Y K edges, for the nanocrystalline YSZ films is the very close agreement with those for the bulk material. It can be seen in Table 3 that even for the films with 6 nm particles, both radial distances and DebyeWaller factors are almost identical to those for bulk YSZ and certainly within the expected experimental uncertainties. The key parameters are those for the second coordination shell, which will be the most sensitive to the effects of nanocrystallinity. It has already been seen that the physical size of the crystallites is not expected to have any marked effect on CN. The unchanged radial distances and Debye-Waller factors indicate there is no relaxation around the cations. The obvious conclusion of these results is that the YSZ nanocrystals are not highly disordered, either statically or dynamically, and that there is no evidence for massively disordered intergrain regions. In this respect, YSZ is similar to SnO2 and ZnO in exhibiting similar levels of disorder in both bulk and nanocrystalline samples. The results are also consistent with the EXAFS study of copper96 in providing no evidence for massively disordered intergrain regions. The probable explanation in the case of oxides is that the strong ionic forces in these systems minimize any tendency to disorder, even in the surface and intergrain regions. In contrast to the present results for YSZ, there are several EXAFS studies that have claimed evidence for high levels of disorder in nanocrystalline ZrO2. However, we believe that the explanation lies in the method of sample preparation. Hence the present results for the nanocrystalline powders of pure ZrO2 will be discussed prior to any wider comparison with the literature data. 4.3. EXAFS of ZrO2 Powders. The Zr K edge EXAFS results for the two nanocrystalline powders are shown in Figure 7, and the fitted parameters are listed in Table 4. The Fourier transforms show a very small second shell. In the fitting, a CN of 12 was used throughout for the Zr-Zr shell, and the reduction

room temperature [11]

room temperature, this work room temperature, this work room temperature [73]

in the peak magnitude is reflected in the Debye-Waller factors. In the absence of other evidence, the reduced Zr-Zr peaks might be interpreted as evidence for massively disordered surface regions. However, the similarity to the EXAFS of the original hydroxide cautions against this interpretation. In addition, it should be borne in mind that the hydroxide is amorphous and, therefore, is not detected in the powder diffraction. Hence an alternative explanation is that samples are a mixture of nanocrystalline material and unconverted hydroxide. Support for this viewpoint is the fact that the EXAFS of calcined samples is very dependent on the heat treatment. Very slow crystallization of the oxide at low temperatures (∼400 °C) will produce samples of 10 nm particle size with EXAFS data very close to those for bulk tetragonal ZrO2; in particular, the EXAFS is not strongly attenuated and CN for the Zr-Zr shell is 12.47,48 More direct evidence for the multiphase nature of nanocrystalline ZrO2 samples produced by calcining the hydroxide is available from NMR measurements.47,48 17O NMR measurements give clear evidence that after the formation of tetragonal nanocrystals a considerable fraction of the O is in an amorphous environment. 1H NMR indicates the presence of a significant hydroxyl presence even after heating at 500 °C, and this is only eliminated by heating at 700 °C. Further evidence for the presence of significant quantities of hydroxyls in tetragonal nanocrystals of ZrO2 produced from the hydroxide has recently been reported in the Reitveld analysis of X-ray powder diffraction data.10 The overall conclusion must be that the preparation of nanocrystalline ZrO2 powders by heating of the hydroxide has the tendency to produce samples containing some amorphous material, which must be quantified if they are to be used in the study of nanocrystalline phenomena. 4.4. Comparison with Published EXAFS Studies of Nanocrystalline ZrO2. Some of the published EXAFS studies of nanocrystalline ZrO2 report spectra that are distinctly different from the spectra of bulk samples. Yuren et al.44 studied the Zr K edge EXAFS of ZrO2 powders containing 14 atom % Y with grain sizes of 5, 8, and 11 nm. These were obtained by calcining the hydroxide, and the temperature was used to control the

EXAFS Study of Zirconia particle size; however, no details were given in the publication. The main qualitative feature of the data is a dramatic reduction of the second peak in the Fourier transform with decreasing particle size and corresponding decreases in the Zr-Zr(Y) shell CN: 11.1 ( 0.5, 9.7 ( 0.5, and 4.5 ( 1.0 for the 11, 8, and 5 nm crystallites, respectively. The authors state that the values for the smallest particles are not consistent with spherical particles and that lattice relaxation may be significant. However, the data show the same qualitative features of the present results for the nanocrystalline ZrO2 powders in which there is clear evidence of amorphous material. In view of the lack of detail on the preparation method and better characterization of the nature of the samples, the results of Yuren et al.44 have to be treated with caution. Deng et al.45 reported Zr K edge EXAFS studies of nanocrystalline ZrO2 containing Y2O3. The particle size was reported as 14 nm, but the composition was not recorded and the preparation method was given as a chemical method with no detail. The data for loosely compacted and compacted powders, 50 and 73% theoretical density, respectively, yielded an attenuated second peak in the Fourier transform and very low values of CN for the Zr-Zr(Y) shell, 7.2 and 5.9, respectively, compared to 12 for bulk material. This was claimed as evidence for unusual disorder in the intergrain regions. This claim must also be treated with caution in the absence of more details on the nature of the sample preparation. The Zr K edge EXAFS of nanocrystalline zirconia produced by inert gas condensation of evaporated ZrO yields spectra different from those of bulk material.41,43 For a 30 nm powder, the Fourier transform yielded a second peak, the Zr-Zr shell, which was considerably reduced in amplitude and shifted to a longer radial distance. In the original work,41 the data were not subjected to a quantitative analysis, as the sample was a mixture of tetragonal and monoclinic polymorphs. The same workers43 in a later EXAFS study of nanocrystalline zirconia with grain sizes of 5 and 8 nm made a detailed study of the data for the Zr-O shell. They concluded the broad asymmetric nature of the peak in the Fourier transform was due to polymorphism and developed a method to analyze this situation. It was concluded that there was no evidence of substantial disorder in the grain boundaries, in agreement with the present work. Finally, we consider the Zr K edge EXAFS studies of nanocrystalline (10-20 nm) YSZ, stabilized in both the cubic and tetragonal phases produced by high-energy ball milling. For both samples, there was a dramatic reduction of the Zr-cation peak in the Fourier transform, although it was not subjected to a quantitative analysis. It was concluded that this was due to nearly perfect nanocrystals being surrounded by poorly crystalline and amorphous regions, which was consistent with HRTEM (high resolution transmission electron microscopy) observations of the sample.97 However, this disordered material may not be intrinsic to nanocrystalline samples but could be metastable material produced during ball milling. Therefore, we do not consider these data as contradicting the present work for the YSZ films. 5. Conclusions The Zr and Y K edge EXAFS spectra of the dense, wellcharacterized nanocrystalline YSZ films show no major differences to the corresponding EXAFS spectra of the bulk counterpart. The significant conclusion is that the levels of dynamic and static disorder in these films, even when the grain size is only 6 nm, are similar to those in large crystals. This is important information in the debate concerning the nature of the intergrain

J. Phys. Chem. B, Vol. 104, No. 41, 2000 9605 region in nanocrystalline solids.96 The present results for the YSZ films do not support the view that these intergrain regions are highly disordered.1 On the contrary, the results indicate there is no real difference between these regions and conventional grain boundaries and, thus, are consistent with recent EXAFS studies of nanocrystalline oxides (SnO229 and ZnO30) and metals (Cu96). The Zr K edge EXAFS spectra for the nanocrystalline powders of pure ZrO2, produced by calcining zirconium hydroxide, are significantly different from those of the bulk counterparts. In particular, the EXAFS is attenuated and the second peak in the Fourier transform, due to Zr-Zr correlations, is dramatically reduced. Other EXAFS studies of nanocrystalline zirconia44,45 have yielded similar spectra to those reported here, and the effects have been attributed to high levels of disorder in the materials. However, other studies using NMR methods47,48 have shown that nanocrystalline ZrO2, produced by heating the hydroxide, can contain significant amounts of an amorphous phase which is a residue of the starting material. This residue is only completely removed by a high-temperature treatment, which in turn will cause grain growth and the loss of the nanocrystals. The mixed phase nature of the nanocrystalline ZrO2 powder provides the most reasonable interpretation of the current EXAFS spectra since the technique averages over the local environments of all the Zr atoms. EXAFS measurements on this type of sample will not reflect the intrinsic structure of nanocrystals. In summary, the present work emphasizes the need for the careful characterization of nanocrystalline samples prior to any EXAFS studies, otherwise conclusions on disorder in the materials can be misleading. In the case of the present work on the YSZ films, where this precaution was observed, the results show levels of disorder in nanocrystals that are similar to those in the bulk counterpart. Acknowledgment. We wish to acknowledge the EPSRC (Grant GR/K74876) and CCLRC for providing support for the use of the Daresbury facilities in the present work and the Missouri Department of Economic Development for partial support of the thin films making facility at EMARC. We also thank the Daresbury Laboratory staff, particularly Drs. N. Chinnery, I. Harvey, and F. Mosselmans, for assistance with the synchrotron experiments and Dr. A. J. Dent for advice on the data analysis. Dr. G. Sankar (Royal Institution, London) was particularly helpful in the reduction of the raw data. We are grateful to Mr. G. Rafeletos for the preparation of the zirconium hydroxide and the nanocrystals of pure ZrO2. References and Notes (1) Gleiter, H. AdV. Mater. (Weinheim, Ger.) 1992, 4, 474. (2) Henglein, A. Chem. ReV. 1989, 89, 1061. (3) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41. (4) Seigel, R. W.; Fougere, G. E. Nanostruct. Mater. 1995, 6, 205. (5) See, for example, papers in the special issue of: Chem. Mater. 1996, 8, No. 8. (6) Kosacki, I.; Anderson, H. U. Grain Boundary Effects in Nanocrystalline Oxide Thin Films. In Encyclopedia of Materials; Pergamon Press: New York, in press. (7) Stark, J. V.; Park, D. G.; Lagadic, I.; Klabunde, K. J. Chem. Mater. 1996, 8, 1904. (8) Stark, J. V.; Klabunde, K. J. Chem. Mater. 1996, 8, 1913. (9) Koper, O.; Lagadic, I.; Klabunde, K. J. Chem. Mater. 1997, 9, 838. (10) Bokhimi, X.; Morales, A.; Portilla, M.; Gracia-Ruiz, A. Nanostruct. Mater. 1999, 12, 589. (11) Turillas, X.; Barnes, P.; Dent, A. J.; Jones, S. L.; Norman, C. J. J. Mater. Chem. 1993, 3, 583. (12) Schneider, M.; Baiker, A. Catal. ReV.-Sci. Eng. 1995, 37, 515. (13) Pascual, R.; Sayer, M.; Kumar, C. V. R. V.; Zou, L. J. Appl. Phys. 1991, 70, 2348.

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