High-Resolution 89Y and 45Sc NMR Spectroscopic Study of Short

Mar 30, 2009 - ... Vittorio Foglietti , Piero Torelli , Anton V. Ievlev , Giorgio Rossi , Silvia Licoccia , Giuseppe Balestrino , Sergei V. Kalinin , ...
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J. Phys. Chem. C 2009, 113, 6553–6560

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High-Resolution 89Y and 45Sc NMR Spectroscopic Study of Short-Range Structural Order in Nanocrystalline Y- and Sc-doped CeO2 and ZrO2 Pragati Jain, Hugo J. Avila-Paredes, Christine Gapuz, Sabyasachi Sen,* and Sangtae Kim Department of Chemical Engineering and Materials Science, UniVersity of California, DaVis, DaVis, California 95616 ReceiVed: January 14, 2009; ReVised Manuscript ReceiVed: March 2, 2009

The effect of crystallite size on cation coordination environments and oxygen vacancy ordering has been investigated in micro- and nanocrystalline Y- and Sc-doped ZrO2 and CeO2 by using high-resolution 89Y and 45 Sc magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Our results indicate that irrespective of crystallite size the vacancies are preferentially associated with the host cation (i.e., Zr) in Y-doped ZrO2 while they display a preference for the dopant cation (i.e., Sc) in Sc-doped ZrO2. On the other hand, vacancies prefer to be associated with the dopant cation in both Y- and Sc-doped CeO2. However, the reduction of crystallite size to a few nanometers shows an unexpected and remarkable effect of increasing randomness in the vacancy distribution in all materials. Such an effect is hypothesized to result from a higher degree of short-range structural disorder in the cation coordination environments in nanocrystals compared to that in their microcrystalline counterparts that controls the energetics of vacancy ordering via a complex balance between electrostatic and strain energy terms. Finally, a clear connection is established between vacancy ordering, oxygen ion transport, and electrical conductivity in microcrystalline Y-doped CeO2 and its possible implications on ionic transport in nanocrystalline materials are discussed. 1. Introduction Fuel cells are electrochemical energy conversion devices that directly convert the energy released by fuel oxidation to electricity. Most common types of fuel cells are designed around gas-impermeable solid barriers, i.e., electrolytes that separate the H2-rich fuel gas from the O2-rich oxidant but allow rapid transport of either H+ or O2- ions through them.1-11 One of the most important classes of materials used for this purpose is the O2- ion conducting solid electrolytes such as Y2O3-stabilized ZrO2 (YSZ). The O2- ion conduction in solid electrolytes involves hopping-activated diffusion of O2- ions through the crystal lattice at a rate that depends primarily on concentration, distribution, and mobility of oxygen vacancies in the atomic structure. Such vacancies are typically introduced by creating solid solutions via “doping” with cations having formal valences that are lower than that of the cation of the host phase.7-9 For example doping of oxides of four-valent metals such as those of ZrO2 and CeO2 with 1 mole of the oxide of a three-valent metal such as the rare earth elements, Y and Sc produces 1 mole of oxygen vacancies. These vacancies provide suitable lattice sites that O2- ions can hop into, resulting in tremendous enhancement in the ionic conductivity. Conventional solid oxide fuel cells (SOFCs) must be operated at high temperatures (∼1300 K) where ionic conductivity of solid electrolytes such as those based on ZrO2 and CeO2 is sufficient to reduce resistive energy losses to acceptable values.1-8 Besides cost and materials issues associated with the high operating temperature, it is desirable for advanced fuel cell applications in the areas of transportation and portable electronic devices such as batteries and environmental sensors to operate SOFCs at low temperatures of ∼900 K or below.9,12,13 Hence, demands for new solid electrolytes that are capable of functioning at temperatures substantially lower than those attainable with currently available * To whom correspondence should be addressed.

materials are growing rapidly. Nanostructured conducting solids such as dense polycrystalline thin films and bulk ceramics with the size of the crystallites or grains being e50 nm represent a promising new class of materials for such applications, where reduction of grain size has recently been suggested to be an effective strategy for enhancing ionic and electronic transport properties.14-16 “Size effects” on transport in polycrystalline ionic conductors17-23 can be expected to arise generally as consequences of interface-controlled mechanisms, associated with, e.g., the formation of extended strain fields, composition gradients, and space-charge layers.19-26 Nanocrystals are characterized by large surface-to-volume ratios and the atomic structure and especially vacancy distribution can be very different in the interior of the crystals with respect to that at or near the surface. Moreover, such structural aspects can be strongly dependent on the size of the crystal, thus offering possible tunability of electrical properties as a function of processing conditions. Therefore, the size effects may arise in nanocrystalline ceramics where interfacial regions, i.e., the grain boundaries, become responsible for the conduction, altering defect densities and associated migration pathways.19-26 Understanding ionic transport in nanostructured materials requires knowledge of the atomic structure and spatial distribution of defects, i.e., the charge carriers such as oxygen-ion vacancies in the crystal interior and at the grain boundaries, and the manners in which they govern transport mechanisms and rates. While significant advances in synthesis and electrical characterization of nanostructured ionic materials have been made in the past decade,14-33 detailed atomic-scale understanding of transport properties in these materials is not at hand. We have recently investigated the dopant cation coordination environments in microcrystalline and nanocrystalline (particle size ∼10 nm) Y-doped CeO2 (YDC) using high-resolution 89Y magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy.34 Our preliminary results have indicated the

10.1021/jp900398x CCC: $40.75  2009 American Chemical Society Published on Web 03/30/2009

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Figure 1. XRD patterns of representative microcrystalline (denoted as “bulk”) and nanocrystalline YSZ, YDC, SSZ, and SDC samples.

existence of large differences in Y-coordination environments and hence in the oxygen-vacancy ordering between micro- and nanocrystalline YDC materials of the same composition. Specifically, nanocrystalline YDC was shown to be characterized by a higher ratio of eight-coordinated to seven-coordinated Y atoms (i.e., Y[8]:Y[7]) compared to that in their microcrystalline counterparts with identical chemical composition. Since Y[8] and Y[7] are characterized by zero and one oxygen vacancy, respectively, in their nearest-neighbor coordination environments, such a result implies a higher preference for vacancies to be associated with Y atoms in microcrystalline YDC compared to that in the nanocrystals. These preliminary results indicate an unexpected and remarkable effect of reduction of particle size to nanometer scale on vacancy ordering in YDC. The extent and type of such ordering, if controllable, can have major effects on oxygen/vacancy mobility and therefore on conductivity of nanostructured solid electrolytes. Such an important implication of these preliminary results has prompted us to systematically investigate the effect of crystallite size on the atomic structure of solid oxide electrolytes. Here we report the results of a detailed comparative study of cation coordination environments and vacancy ordering in nano- and microcrystalline YSZ and YDC over a wide composition range using highresolution 89Y MAS NMR spectroscopy. Supporting evidence has been drawn from similar structural studies on nano- and microcrystalline Sc-stabilized ZrO2 (SSZ) and Sc-doped CeO2 (SDC) with 45Sc MAS NMR spectroscopy. The implications of these results on the energetics of vacancy ordering and ionic transport are discussed. 2. Experimental Section 2.1. Sample Synthesis and Preliminary Characterization. All nanocrystalline YSZ, YDC, SSZ, and SSC samples were synthesized via a coprecipitation method where an aqueous solution of NH4OH was added dropwise to an aqueous solution containing the stoichiometric amounts of constituent metal nitrates. The precipitated oxides were collected by centrifugation and were washed with water, a 50% v/v ethanol-water solution, and subsequently with pure ethanol. The resulting nanopowders were then dried at 120 °C for 12 h, ground, and finally annealed under air at temperatures ranging between 450 and 1000 °C for 1 to 2 h (see Table 1 for details) to obtain crystallites ranging

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Figure 2. 89Y MAS NMR spectra of microcrystalline (left) and nanocrystalline (right) Y-doped ZrO2 samples. The Y content for each sample is indicated alongside the corresponding spectrum. The peaks corresponding to 6-, 7-, and 8-coordinated Y sites are denoted by Y[6], Y[7], and Y[8], respectively.

between ∼4 and 12 nm in diameter. Gd was incorporated in the solution phase into the YDC and YSZ samples at a level of ∼1000 ppm to shorten the spin-lattice relaxation time of 89Y nuclides and hence to reduce the collection time of 89Y NMR spectra. The incorporation of Gd was not necessary for 45Sc NMR spectroscopy of the Sc-doped ceria and zirconia samples as the quadrupolar nature of the 45Sc nuclides ensures short spin-lattice relaxation times. Samples with larger grain size (100 to 600 nm in diameter) were obtained from the nanocrystalline powders that were pelletized via cold isostatic pressing at 276 MPa and subsequently sintered at temperatures ranging between 1000 and 1600 °C for 10 to 12 h (see Table 1 for details). In spite of the submicron size of the crystallites, these samples are referred to as “microcrystalline” in the subsequent discussion. Powder X-ray diffraction (XRD) measurements (Scintag XDS-2000) indicated that all micro- and nanocrystalline doped ceria and zirconia samples studied here were characterized by the cubic fluorite crystal structure (Figure 1). The crystallite size of the nanocrystalline samples was estimated from widths of the 4 strongest peaks in the XRD patterns using the Williamson-Hall analysis. The particle size of the microcrystalline samples was determined from field-emission scanning electron microscopy (SEM) images (microscope FEI XL30SFEG). The Y contents of the YSZ and YDC samples and the Sc contents of the SSZ and SSC samples were analyzed with energy dispersive X-ray spectroscopy in an SEM (FEI XL30-SFEG microscope operated at 10 kV of accelerating voltage) and were found to be within (1 cation % of the nominal composition in all cases. Hence, nominal sample compositions are used in the subsequent discussion (see Table 1). 2.2. NMR Spectroscopy. All 89Y and 45Sc magic-anglespinning (MAS) NMR spectra were collected with a Bruker Avance 500 spectrometer and Bruker magnet (11.7 T) operating at Larmor frequencies of 24.5 and 121.5 MHz for 89Y and 45Sc, respectively. A low-γ Bruker 4 mm MAS probe was used to collect the 89Y MAS NMR data, crushed samples were spun at 10 kHz, and free induction decays (FID) were collected with a π/2 rf pulse (6 µs) and a recycle delay of 5 s. The probe ringdown time of ∼75 µs was short enough such that significant distortion of the 89Y NMR line shape could be avoided.

Nanocrystalline Y- and Sc-doped CeO2 and ZrO2

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TABLE 1: Nominal Compositions, Annealing Conditions, and Crystallite Sizes of YSZ, YDC, SSZ, and SDC Samples Investigated in This Study nanocrystalline samples nominal dopant content (cation %)

nicrocrystalline samples

Annealing conditions

Crystallite size (nm)

650 °C, 1 h 800 °C, 1 h 450 °C, 2 h 800 °C, 1 h 1000 °C, 1 h 450 °C, 2 h 450 °C, 2 h

9.5 ( 0.1 9.0 ( 0.2 7.9 ( 0.2 7.7 ( 0.2 7.9 ( 0.2 4.7 ( 0.4 4.1 ( 0.4

450 °C, 2 h 650 °C, 1 h 450 °C, 2 h 650 °C, 1 h

YSZ 17 22 30 41 49 59 64 YDC 5 10 15 25 SSZ 16 SDC 5 [8]

[7]

Annealing conditions

Particle size (nm)

1400 °C, 10 h not prepared 1400 °C, 10 h 1400 °C, 10 h 1400 °C, 10 h 1400 °C, 10 h 1400 °C, 10 h

350

9.3 ( 0.2 12.7 ( 0.2 9.8 ( 0.1 11.9 ( 0.1

1600 °C, 10 h 1400 °C, 12 h 1600 °C, 10 h 1400 °C, 12 h

200 600 300 600

450 °C, 2 h

8.7 ( 0.3

1600 °C, 10 h

100

450 °C, 2 h

9.5 ( 0.1

1000 °C, 12 h

200

[6]

TABLE 2: Relative Fractions of Y , Y , and Y

400 400 300 300 300

Sites (within (3%) in Micro- and Nanocrystalline YSZ Samples

nanocrystalline samples

microcrystalline samples

Y content (cation %)

% Y[8]

% Y[7]

% Y[6]

% Y[8]

% Y[7]

% Y[6]

17 22 30 41 49 59 64

90 86 70 61 43 37 31

10 14 30 39 53 61 66

0 0 0 0 4 2 3

98

2 not prepared 9 32 50 62 61

0

91 68 47 25 15

0 0 3 13 24

TABLE 3: Relative Fractions of Y[8], Y[7], and Y[6] sites (within (5%) in Micro- and Nanocrystalline YDC Samples nanocrystalline samples Y content (cation %) 5 10 15 25

[8]

[7]

%Y

%Y

32 28 19 15

68 72 81 85

Approximately 15 000 to 55 000 FIDs were averaged and Fourier-transformed to obtain each 89Y MAS NMR spectrum. 89 Y NMR chemical shifts were referenced to that of crystallineY2Sn2O7 (δiso ) 150 ppm).35,36 45 Sc NMR spectra were collected with a Bruker tripleresonance 4 mm MAS probe, crushed samples were spun at 10 kHz, and FIDs were collected with a π/24 rf pulse (0.12 µs) and a recycle delay of 0.2 s. Approximately 15 000 to 20 000 FIDs were averaged and Fourier-transformed to obtain each 45Sc MAS NMR spectrum. 45Sc NMR chemical shifts were referenced to that of crystalline LiScO2 (δiso ) 148 ppm).37 3. Results 3.1. YSZ. The 89Y MAS NMR spectra of microcrystalline YSZ samples and those of the corresponding, chemically identical, nanocrystalline samples are shown in Figure 2. The spectra of microcrystalline YSZ samples (Figure 2, left) are characterized by two or three main resonances depending on the Y content. At low Y content of e41 cation % Y these spectra display two main resonances at ∼190 and 86 ppm that can be assigned to seven- and eight-coordinated Y atoms, Y[7] and Y[8], respectively.38 At high Y content (g49 cation % Y), the 89Y MAS NMR spectra display an additional resonance at ∼295 ppm that can be assigned to six-coordinated Y, i.e., Y[6] atoms.38

microcrystalline samples [6]

%Y 0 0 0 0

[8]

%Y 23 16 13 8

% Y[7]

% Y[6]

77 84 87 91

0 0 0 1

It may be noted here that Y[8], Y[7], and Y[6] sites correspond to Y atoms with zero, one, and two oxygen vacancies, respectively, in their nearest-neighbor coordination environment. As the Y content increases, the relative intensity of the resonance corresponding to Y[8] sites decreases and those corresponding to the Y[6] and Y[7] sites increase (Figure 2). The 89Y MAS NMR spectra of the nanocrystalline YSZ samples show the same resonances and similar compositional trends as those of their microcrystalline counterparts (Figure 2). However, the peaks in the 89Y MAS NMR spectra of the nanocrystalline samples are significantly broader than those in the case of the microcrystalline samples. The relative concentrations of Y[8], Y[7], and Y[6] sites in these samples are directly proportional to the areas under the corresponding resonances in the 89Y MAS NMR spectra. The relative fractions of these three Y sites have therefore been determined by quantitative simulation of the 89Y MAS NMR spectral line shapes by using Gaussian peaks and are listed in Table 2. The compositional variation of the relative fractions of Y[8], Y[7], and Y[6] sites are plotted in Figure 3 for micro- and nanocrystalline YSZ samples. The results for the microcrystalline samples agree well with those reported previously in the literature.38 Nanocrystalline samples of nominally identical composition show significant difference in the Y speciation with lower (higher) ratio of Y[7]:Y[8] above (below)

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Figure 3. Relative fractions of Y[8], Y[7], and Y[6] sites in microcrystalline (open symbols) and nanocrystalline (filled symbols) YSZ samples. Y[8], Y[7], and Y[6] fractions are shown with circles, triangles, and squares, respectively. Lines through the data points are guides for the eye only.

Figure 4. Average coordination numbers of Y (CY, red squares) and Zr (CZr, blue circles) in microcrystalline (open symbols) and nanocrystalline (filled symbols) YSZ samples. Lines through the data points are guides for the eye only. The straight, dashed line shows the compositional dependence of the ideal cation coordination number, Cideal. The error bars are within the size of the symbols.

∼50% Y content, compared to their microcrystalline counterparts (Figure 3). At high Y contents of g50% Y, Y[6] sites begin to appear in the structure of microcrystalline samples, and increase rapidly in concentration with increasing Y content. On the other hand, the relative fraction of Y[6] sites in the corresponding nanocrystalline YSZ samples remains below ∼4% even at the highest Y content (Figure 3). The average coordination number of Y atoms CY can be calculated from the relative fractions of Y[8], Y[7], and Y[6] sites (I8, I7, and I6, respectively) as obtained from the 89Y MAS NMR spectra, using the following relation: CY ) 8I8 + 7I7 + 6I6. The average coordination number of the Zr atoms in the lattice is then given by the following relation: CZr ) [Cideal - xCY] /[1 - x], where x denotes the atomic fraction of Y and Cideal ) 8 - 2x is the “ideal” coordination number of Y and Zr atoms corresponding to a random distribution of vacancies without any preference for any particular cation.38,39 The average and ideal coordination numbers of Y and Zr in micro- and nanocrystalline YSZ samples are plotted as a function of composition in Figure 4. The experimentally determined CY values in micro- and nanocrystalline YSZ samples are clearly higher than the ideal coordination number over the entire composition range. This result implies a stronger

Jain et al.

Figure 5. 89Y MAS NMR spectra of microcrystalline (left) and nanocrystalline (right) Y-doped CeO2 samples. The Y content for each sample is indicated alongside the corresponding spectra. The peaks corresponding to 7- and 8-coordinated Y sites are denoted by Y[7] and Y[8], respectively.

Figure 6. Relative fractions of Y[8], Y[7], and Y[6] sites in microcrystalline (open symbols) and nanocrystalline (filled symbols) YDC samples. Y[8], Y[7], and Y[6] fractions are shown with circles, triangles, and squares, respectively. Lines through the data points are guides for the eye only.

preference of vacancies to be associated with Zr rather than with Y in YSZ. The CY and CZr values are closer to Cideal in the nanocrystalline YSZ compared to that in the case of their microcrystalline counterparts, at Y concentrations below ∼50 cation %, while this trend is reversed at higher Y contents (Figure 4). 3.2. YDC. The 89Y MAS NMR spectra of micro- and nanocrystalline YDC samples are shown in Figure 5. These spectra are characterized by two main resonances at ∼120 and -5 ppm, corresponding to Y[7] and Y[8] sites, respectively.39 Similar to YSZ, the 89Y MAS NMR resonances for the nanocrystalline YDC samples are significantly broader than those of their microcrystalline counterparts (Figure 5). Moreover, similar to YSZ, the intensity of the resonance corresponding to Y[8] sites decreases and that corresponding to the Y[7] sites increases with increasing Y content in both micro- and nanocrystalline samples. The compositional variation of the relative fractions of Y[8], Y[7], and Y[6] sites are listed in Table 3 and are plotted in Figure 6 for the YDC samples. The results for the microcrystalline samples agree well with those reported previously in the literature.39 Nanocrystalline samples show

Nanocrystalline Y- and Sc-doped CeO2 and ZrO2

Figure 7. Average coordination numbers of Y (CY, red squares) and Ce (CCe, blue circles) in microcrystalline (open symbols) and nanocrystalline (filled symbols) YDC samples. Lines through the data points are guides for the eye only. The straight, dashed line shows the compositional dependence of the ideal cation coordination number, Cideal. The error bars are within the size of the symbols.

Figure 8. 45Sc MAS NMR spectra of microcrystalline (solid line) and nanocrystalline (dashed line) 16% Sc-stabilized ZrO2. The asterisk denotes the spinning sideband.

significant difference in the Y speciation with higher Y[8]:Y[7] ratio compared to their microcrystalline counterparts with nominally identical composition, over the entire composition range studied here (Figure 6). The experimentally determined average and ideal coordination numbers of Y and Ce in microand nanocrystalline YDC samples are plotted as a function of composition in Figure 7. The CY values in micro- and nanocrystalline YDC samples are remarkably lower than Cideal, over the entire composition range. This result implies a stronger preference of vacancies to be associated with Y rather than with Ce in YDC. The CY and CCe values are closer to Cideal in the nanocrystalline YDC compared to that in the case of their microcrystalline counterparts over the entire composition range (Figure 7). 3.3. SSZ and SDC. The 45Sc MAS NMR spectra of microand nanocrystalline SSZ with 16% Sc are shown in Figure 8. These spectra show a single broad asymmetric line shape centered at 32 ppm, corresponding to seven-coordinated Sc, i.e., Sc[7] sites.37 It may be noted that the 45Sc MAS NMR line shapes in Figure 8 are quite broad with long asymmetric tails to lower frequency. Such line shapes are often typical for quadrupolar nuclides located at lattice sites with a distribution of quadrupolar coupling constants. On the other hand, although the chemical shift of Sc[8] sites in SSZ is not known a priori, it is expected

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6557 to be located at a lower frequency (more shielded) compared to that for Sc[7] sites.37 Hence, it is possible that a small concentration of Sc[8] sites is present in these SSZ samples and the corresponding signal overlaps with that of the Sc[7] sites. However, even if the latter scenario is true, the average coordination number of Sc, CSc, is expected to be ∼7 in these SSZ samples, at least to a first approximation. This value of CSc is significantly lower than the ideal coordination number of Cideal ) 7.66, a result that implies a stronger preference of vacancies to be associated with Sc rather than with Zr in SSZ. The 45Sc MAS NMR spectra of micro- and nanocrystalline SDC with 5% Sc are shown in Figure 9. These spectra show two relatively sharp resonances at 25 and -32 ppm, corresponding to Sc[7] and Sc[8] sites, respectively.37 The average coordination numbers of Sc and Ce in SDC are compared with those of Y and Ce in YDC in Figure 10 and the average coordination numbers of Sc and Zr in SSZ are compared with those of Y and Zr in YSZ in Figure 11. It is clear that in SDC the value of CSc (∼7.25) is significantly less than the ideal coordination number of Cideal ) 7.9, indicating that, similar to SSZ, the vacancies prefer to be associated with Sc rather than with Ce in SDC. The Sc speciation does not display any significant dependence on grain size in SSZ while the CSc value is closer to Cideal in the nanocrystalline SDC sample compared to that in the case of its microcrystalline counterpart (Figures 10 and 11). 4. Discussion 4.1. Energetics of Vacancy Ordering. When taken together, the compositional dependence of Y speciation as shown in Figures 3, 4, 6, and 7 can be used to compare the role of Y as a dopant in ZrO2 and CeO2. It is clear that irrespective of crystallite size and dopant concentration the average coordination number of yttrium CY in YSZ is higher than the ideal average coordination number Cideal while the exact opposite is true in YDC. This result implies that oxygen vacancies prefer to be associated with Zr in YSZ while the preference changes for Y in YDC. Theoretical calculations in previous studies based on classical and ab initio force fields have shown that the vacancy-cation interaction energy can be described primarily as a sum of the electrostatic interaction between the cation and oxygen vacancy and elastic or strain energy associated with incorporation of vacancies in the nearest-neighbor coordination environment of a cation.40-46 The effective charges associated with trivalent dopant cations such as Y3+ and Sc3+ and with the oxygen vacancies in ZrO2 or CeO2 structure are -1 and +2, respectively, resulting in a Coulombic attraction between the oxygen vacancies and Y3+ or Sc3+ cations. This Coulombic attraction would imply a preferential association of the oxygen vacancies with trivalent dopant cations rather than with the Ce4+ or Zr4+ ions, the latter being characterized by an effective charge of zero. On the other hand, the strain energy term depends on the size of the cations around the vacancy and is expected to increase with increasing cation size. The Shannon-Prewitt ionic radii of Y3+, Zr4+, and Ce4+ cations are 1.01, 0.84, and 0.97 Å, respectively. Since the Zr4+ cation is significantly smaller than the Y3+ cation, the strain energy term will favor preferential association of oxygen vacancy with Zr4+in YSZ. Therefore, the experimental observation of preferential association of vacancies with Zr4+ cations in YSZ implies that the strain energy term is larger than the electrostatic interaction term, at least in this material. On the other hand, the strain energy term will not exert an important influence on vacancy-cation association in YDC due to the similar size of Y3+ and Ce4+ cations. Hence, the Coulombic attraction term will dominate and result in preferential association of vacancies with Y3+ cations, as is indeed

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Figure 9. 45Sc MAS NMR spectra of microcrystalline (solid line) and nanocrystalline (dashed line) 5% Sc-doped CeO2. Peaks corresponding to 7- and 8-coordinated Sc ions are denoted by [7] and Sc[8], respectively.

Figure 10. The average coordination numbers of Sc (CSc) and Ce (CCe) in 5% Sc-doped CeO2 are shown with green diamonds and circles, respectively. Open and filled symbols correspond to microcrystalline and nanocrystalline samples, respectively. Other symbols are the same as those in Figure 7.

Figure 11. The average coordination numbers of Sc (CSc) and Zr (CZr) in 16% Sc-doped ZrO2 are shown with green diamonds and circles, respectively. Note that the average cation coordination numbers in microcrystalline and nanocrystalline SSZ samples are equal. Other symbols are the same as those in Figure 4.

observed experimentally in YDC. Besides its lower charge, the Shannon-Prewitt ionic radius of the Sc3+ cation (0.87 Å) is very similar to that of Zr4+ and is substantially smaller than that of Ce4+ cations. Hence, both electrostatic and strain energy terms will be important in SDC while the former term will dominate in SSZ and in both cases one would predict a preferential association of oxygen vacancies with Sc3+. This prediction is indeed experimentally borne out in both SSZ and SDC compositions studied here, where the values of CSc (∼7.0 and 7.25, respectively) are significantly smaller than the values expected from a random distribution of vacancies (Cideal ≈ 7.66 and 7.9, respectively). 4.2. Effects of Crystallite Size on Vacancy Ordering. Perhaps the most intriguing result that has been observed in this study is the significant differences in Y-speciation and in vacancy ordering as a function of crystallite size in samples of nominally identical composition for both SSZ and SDC (Figures 2-7). A quantitative comparison of the 89Y MAS NMR spectra indicate that irrespective of Y concentration the nanocrystalline YDC is characterized by a higher ratio of 8-coordinated to

7-coordinated yttrium (Y[8]:Y[7]) with respect to that in the microcrystalline material (Figures 5-7). The Y[8] and Y[7] sites have zero and one oxygen vacancy in the nearest neighbor shell, respectively. Therefore, the relatively larger concentration of Y[8] in the nanocrystalline YDC implies higher preference of vacancies for Ce, relative to the “bulk”, i.e., the microcrystalline material. A similar trend has also been observed in the 45Sc NMR spectra of the SDC samples where the nanocrystalline SDC is characterized by a higher Sc[8]:Sc[7] ratio compared to that in microcrystalline SDC (Figures 9 and 10). This result again implies higher preference of vacancies for Ce in nanocrystalline SDC compared to that in the microcrystalline. On the other hand, in strong contrast with YDC and SDC, our results on YSZ display the opposite effect of crystallite size on oxygen vacancy distribution, at least for compositions with up to ∼50% Y (Figures 2-4). In this composition range the nanocrystalline YSZ samples are characterized by a lower Y[8]:Y[7] ratio compared to their microcrystalline counterparts. This result indicates higher preference of vacancies for Y in nanocrystalline material compared to the “bulk” in the case of YSZ. This

Nanocrystalline Y- and Sc-doped CeO2 and ZrO2 situation reverses for samples with very high Y contents (>50% Y). This composition range is also characterized by the nearly complete avoidance of the formation of Y[6] sites in the nanocrystalline YSZ and, therefore, by the avoidance of clustering of vacancies around Y atoms. Perhaps a more unifying picture of the effects of crystallite size on Y-speciation and vacancy distribution emerges when one considers the composition dependence of the average coordination numbers of Y, Sc, Zr, and Ce atoms in all three systems, i.e., in YSZ with up to 50% Y, YDC, and SDC as shown in Figures 4, 7, and 10. From such a consideration it becomes evident that the vacancy distribution becomes more randomized, i.e., CY, CZr, CCe, and CSc values become closer to Cideal in nanocrystals compared to their microcrystalline counterparts in all three systems. Besides an entropic stabilization of a more random spatial distribution of vacancies, such a decrease in the preference for vacancies to be associated with one cation over the other may arise from a concomitant reduction in the energetic differences for such association in nanocrystalline solid electrolytes. It is important to note in this regard that both 89Y and 45Sc MAS NMR line shapes for all Y and Sc coordination environments in nanocrystalline YSZ, YDC, and SDC are significantly broader than those corresponding to their microcrystalline counterparts. This result implies a greater degree of short-range structural disorder in nanocrystals compared to that in microcrystalline material. Increasing short-range structural disorder in the nearest-neighbor coordination environments of cations in nanocrystalline materials would result in a strain energy component of the vacancy-cation interaction energy that would be progressively less discriminatory between cations of different sizes. This scenario is thus consistent with the observed increase in the randomization of vacancy distribution in nanocrystalline YSZ, YDC, and SDC. 4.3. Vacancy Ordering and Ionic Transport. A close inspection of the 89Y MAS NMR spectra of microcrystalline YDC samples (Figure 5) indicates that the resonance corresponding to Y[7] sites consists in fact of two components, one near 120 ppm and the other near 80 ppm. The latter component becomes progressively weaker as the Y content increases in these YDC samples. The main Y[7] resonance near 120 ppm has previously been assigned to Y[7] sites with one Y nextnearest neighbors, i.e., Y-V-Y type environments while the shoulder near 80 ppm is believed to represent Y[7] sites with only Ce next-nearest neighbors and Y[7] sites with more than one Y next-nearest neighbors are expected to have resonances near ∼160 ppm.39 A simulation of this region of the 89Y MAS NMR line shape corresponding to the Y[7] sites indicates that the total number of Y[7] sites with only Ce next-nearest neighbors increases with increasing Y content in microcrystalline YDC samples and goes through a maximum near 10% Y beyond which the number of such sites rapidly decreases with further increase in Y content (Figure 12). On the other hand, the total number of Y[7] sites with only Ce next-nearest neighbors increases rapidly with increasing Y content up to 10% Y and remains nearly constant thereafter, at higher Y concentrations in the case of nanocrystalline YDC. Moreover, the concentrations of such sites are higher in the nanocrystalline samples compared to those in their microcrystalline counterparts, over the entire composition range (Figure 12). Such clear and pronounced size-induced structural differences in vacancy ordering between nano- and microcrystalline YDC may play a profound role in controlling the ionic transport properties in these materials. For example, previous computational studies have indicated that oxygen vacancies shared by two or more Y

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Figure 12. Compositional dependence of the number of Y[7] sites with only Ce as next-nearest neighbors, per formula unit of (YO1.5)x(CeO2)100-x in microcrystalline (open symbols) and nanocrystalline (filled symbols) YDC.

atoms would be more strongly bound than those connected to one Y atom and a higher concentration of the latter would imply higher electrical conductivity.40-43 Therefore, one can expect that YDC with 10% Y that is characterized by the highest concentration of Y[7] sites with only Ce next-nearest neighbors would also have the highest electrical conductivity and lowest activation energy. This hypothesis is completely consistent with previous reports of electrical conductivity measurements in “bulk” YDC that have shown that the ionic conductivity increases with increasing Y content until it goes through a maximum near ∼10% Y, beyond which further increase in Y content results in a progressive decline in the conductivity.47 The corresponding activation energy shows the opposite behavior, i.e., it goes through a minimum near 10% Y and then increases with increasing Y content. By the same token, the nanocrystalline YDC samples that are characterized by a higher concentration of Y[7] sites with only Ce next-nearest neighbors are also expected to have higher intrinsic electrical conductivities compared to their microcrystalline counterparts. 5. Conclusions Analyses of the high-resolution 89Y and 45Sc MAS NMR spectra have allowed the identification and quantification of Y and Sc sites with zero, one, and two nearest-neighbor oxygen vacancies and consequently the estimation of average coordination numbers of Y3+, Sc3+, Zr4+, and Ce4+ cations in microand nanocrystalline YSZ, YDC, SSZ, and SDC. A comparison of these average coordination numbers with those expected from a random distribution of oxygen vacancies indicates that oxygen vacancies are preferentially associated with Zr4+ cations in YSZ. In contrast with YSZ, the oxygen vacancies are preferentially associated with Y3+ or Sc3+ cations in YDC, SSZ, and SDC. Such preference for oxygen vacancy to be associated with any particular cation is significantly reduced in nanocrystalline samples which exhibit a more randomized distribution of oxygen vacancies and a higher degree of short-range structural disorder compared to their microcrystalline counterparts. The cation-vacancy association appears to be controlled by a balance between the electrostatic interaction between cations and oxygen vacancies and the strain energy introduced by the presence of vacancies in the nearest-neighbor coordination sphere of a cation. Increased short-range structural disorder may result in strain energy terms that become less discriminatory between different cation-vacancy associations in nanocrystal-

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