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Chem. Mater. 2005, 17, 1952-1958
Detecting Different Oxygen-Ion Jump Pathways in Bi2WO6 with 1and 2-Dimensional 17O MAS NMR Spectroscopy Namjun Kim,†,§ Rose-Noelle Vannier,‡ and Clare P. Grey*,† Department of Chemistry, State UniVersity of New York at Stony Brook, Stony Brook, New York 11794-3400, and Laboratoire de Cristallochimie et Physicochimie du Solide, UniVersite´ des Sciences et Technologies de Lille, CNRS UMR 8012, ENSCL, B.P. 108, 59652 VilleneuVe d’Ascq Cedex, France ReceiVed September 16, 2004. ReVised Manuscript ReceiVed February 7, 2005
Bismuth tungsten oxide (Bi2WO6) is an m ) 1 Aurivillius phase and some of its doped compounds show moderately high oxygen ion conductivity. These materials are structurally related to the more disordered bismuth vanadium oxide Aurivillius phases (or BIMEVOXes), which exhibit extremely high conductivities. We demonstrate that a combination of one- and two-dimensional, variable-temperature, 17 O MAS NMR spectroscopy may be used to probe ionic motion in the Nb5+-doped bismuth tungstates and resolve different oxide-ion conduction mechanisms that occur over a very wide range of different time scales (10-1 to 10-4 s). The use of 17O two-dimensional exchange spectroscopy dramatically increases the range of oxide-ion conductors that can be investigated by MAS NMR. Motion commences in the (WO4)n2n- perovskite layers, with slow exchange between the Bi2O22+ and (WO4)n2n- layers occurring at higher temperatures, as detected by 2D NMR. Simulation of the 1D 17O NMR spectra of Bi2W1-xNbxO6 with x ) 0.05 and 0.1 shows an increase in ionic conductivity from x ) 0.05 to 0.1 consistent with the anionic conductivity of these materials measured by ac impedance spectroscopy.
Introduction Oxide ion conductors are an extremely important class of materials which can be used in various applications such as electrolytes in solid oxide fuel cells (SOFCs), oxygen separation membranes, and oxygen sensors.1,2 Although it is extremely important to understand the mechanism of anionic conduction to develop improved or optimized materials for these devices, this information is extremely difficult to obtain experimentally. Motion may sometimes be inferred on the basis of an analysis of the thermal parameters obtained in diffraction experiments, but in a disordered system, motion is not generally the only source of large thermal parameters. Solid state MAS NMR is an excellent probe of both local structure and dynamics. In practice, however, it has not been widely used to study oxideion motion, as very high temperatures are required to detect the relatively slow motion of oxide ions. Notable exceptions to this are the high-temperature static 17O NMR work of Adler et al.3,4 and our more recent 17O lower temperature work on the BIMEVOXes and doped pyrochlores.5,6 Here we show that 17O variable-temperature, two-dimensional (2D) * To whom correspondence should be addressed. Tel: 1-631-632-9548. Fax: 1-631-632-5731. E-mail:
[email protected]. † State University of New York at Stony Brook. ‡ Universite ´ des Sciences et Technologies de Lille. § Present address: Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 93305.
(1) Boivin, J. C.; Pirovano, C.; Nowogrocki, G.; Mairesse, G.; Labrune, P.; Lagrange, G. Solid State Ionics 1998, 113-115, 639. (2) Boivin, J. C.; Mairesse, G. Chem. Mater. 1998, 10, 2870. (3) Adler, S. B.; Reimer, J. A.; Baltisberger, J.; Werner, U. J. Am. Chem. Soc. 1994, 116, 675. (4) Adler, S. B.; Smith, J. W.; Reimer, J. A. J. Chem. Phys. 1993, 98, 7613. (5) Kim, N.; Grey, C. P. Science 2002, 297, 1317. (6) Kim, N.; Grey, C. P. Dalton Trans. 2004, 3048.
exchange NMR spectroscopy7-9 can be used to resolve different jump mechanisms for the oxygen ions involving different sites in the lattice. We demonstrate the approach for doped-Bi2WO6 systems, since these compounds represent examples of oxides showing only moderate conductivity at ambient temperatures. Much effort has been expended to develop materials with higher oxygen-ion conductivity to lower the operating temperature of SOFCs and separation membranes. To this end, the BIMEVOX materials10,11 have been widely studied because they show extremely high oxide ion conductivity even at moderate temperatures (300-600 °C). They derive from the parent compound Bi4V2O11, and are obtained by partial substitution for vanadium with a metal (Bi4V2xMexO11-y; Me ) metal). Their structure is closely related to that of the m ) 1 member of the Aurivillius family12 with general formula (Bi2O2)(Am-1BmO3m+1) and comprise (Bi2O2)n2+ layers spaced with oxygen deficient (VO3.5)n2perovskite-like slabs in which the oxygen diffusion takes place. At room temperature, ordering of the oxygen sublattice is observed for the R-Bi4V2O11 polymorph and at least three different vanadium sites are seen by diffraction,13 making this structure considerably more complicated than that of the m ) 1 Aurivillius members. Bismuth tungstate (Bi2WO6) (7) Wang, F.; Grey, C. P. J. Am. Chem. Soc. 1998, 120, 970. (8) Xu, Z.; Stebbins, J. F. Science 1995, 270, 1332. (9) Chaudhuri, S.; Wang, F.; Grey, C. P. J. Am. Chem. Soc. 2002, 124, 11746. (10) Aurivillius, B. ArkiV. Kemi. 1949, 1, 463. (11) Yan, J.; Greenblatt, M. Solid State Ionics 1995, 81, 225. (12) Abraham, F.; Debreuille-Gresse, M. F.; Mairesse, G.; Nowogrocki, G. Solid State Ionics 1988, 28-30, 529. (13) Mairesse, G.; Roussel, P.; Vannier, R. N.; Anne, M.; Nowogrocki, G. Solid State Sci. 2003, 5, 861.
10.1021/cm048388a CCC: $30.25 © 2005 American Chemical Society Published on Web 03/25/2005
Oxygen-Ion Jump Pathways in Bi2WO6
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Figure 2. 17O One-pulse MAS NMR spectrum of Bi2W0.9Nb0.1O5.95, acquired with a short rf pulse and a pulse delay of 5 s at a spinning speed of 15 kHz. Isotropic resonances in this and all subsequent 1D spectra are marked with their respective chemical shifts.
Figure 1. Structure of Bi2WO6 showing the WO42- and Bi2O22+ layers.
represents an ordered m ) 1 Aurivillius phase, also consisting of alternating (Bi2O2)n2n+ layers and perovskite-like (WO4)n2nlayers (Figure 1).14,15 Doping the W6+ sites with Nb5+ or Ta5+ increases the concentration of anion vacancies, raising the conductivity from ∼10-5 S‚cm to 10-3-10-4 S‚cm-1 at 500 °C16,17 These materials serve as good models for the more disordered BIMEVOXes. Previous studies on Bi2WO6 and its doped compounds, utilizing conductivity measurements,16 X-ray diffraction studies, and atomistic simulations,17 have determined the structure of the parent and doped compounds, and the effect of the dopant cations on the conductivities of different compounds. The energies for different possible conduction pathways were calculated, and the lowest energy pathway for oxygen vacancy migration was proposed to involve a zigzag type mechanism between adjacent apical and equatorial oxygen sites of the WO6 octahedra.16 The predicted conduction pathway has yet to be tested experimentally. We show in this paper that 17O MAS NMR provides an ideal method for monitoring the different jump processes that occur in these materials. The results are compared with bulk conductivities obtained for this system. Experimental Section Undoped and Nb doped bismuth tungsten oxide (Bi2WO6) powders were prepared as described earlier.16 They were enriched with 17O by heating the sample with 17O2 gas (Isotec, min. 40 atom (14) Knight, K. S. Mineral. Magn. 1992, 56, 399. (15) Rae, A. D.; Thompson, J. G.; Withers, R. L. Acta Crystallogr. 1991, B47, 870. (16) Baux, N.; Vannier, R. N.; Mairesse, G.; Nowogrocki, G. Solid State Ionics 1996, 91, 243. (17) Islam, M. S.; Lazure, S.; Vannier, R.-N.; Nowogrocki, G.; Mairesse, G. J. Mater. Chem. 1998, 8, 655.
%) at 600 °C for 12 h. The maximum level of enrichment was estimated to be no more than approximately 15% based on the volume of gas used. 17O MAS NMR experiments were carried out at 48.84 MHz, corresponding to a magnetic field strength of 8.45 T, with a Chemagnetics-360 spectrometer and a 4-mm MAS probe. Some experiments were performed with a rotor synchronized Hahn-echo pulse sequence (π/6 - τ - π/3 - τ) to avoid probe ringing with pulse widths of typically ∼2 µs () π/6) at 8.4 T and pulse delays of 1-5 s and τ ) 1/Vr where Vr, the spinning speed, is typically 15 kHz. One-pulse experiments were also performed with very short pulses (0.5 µs) to ensure quantitative excitation of all the resonances. The T1 and T2 relaxation times were obtained by varying the pulse delay, and τ in the Hahn-echo sequence, respectively. 2D exchange spectra were obtained with a magnetization-exchange pulse sequence (π/6 - t1 - π/6 - tm - π/6 - t2), where t1 and t2 are first and second time variables, respectively, and tm is mixing time. Time proportional phase incrementation (TPPI) mixing times of 1100 ms and pulse delays of 1 s were used to acquire the 2D data. π/6 (and π/3) were used instead of π/2 (and π) pulses in the 1 and 2D experiments, to account for the faster nutation frequency of the quadrupolar (I ) 5/2) 17O nucleus. 17O multiple-quantum (MQ) MAS experiments were carried out at 14.1 T (on a Bruker DMX-600 instrument and a 4-mm MAS probe), by using a threepulse MQ-MAS sequence with a z-filter18,19 (two hard pulses followed by a soft pulse). Conductivity measurements were carried out in air using impedance spectroscopy in the 1-106 Hz frequency range (SI 1255 Schlumberger). Measurements were performed on dense pellets, 5 mm in diameter and about 2 mm thick, from 200 to 860 °C with 20 °C steps and 1 h stabilization prior to measurement. A gold paste (A-1644, Engelhard-CLAL) was used for the electrodes, which was sintered for 1 h at 650 °C prior to the experiment.
Results and Discussion 1-D NMR. The 17O one-pulse NMR spectrum of Bi2W0.9Nb0.1O5.95 is shown in Figure 2. The spectrum shows three resonances at around 242, 400, and 462 ppm with an intensity ratio of approximately 1: 1: 1. Bi2WO6 contains six distinct crystallographic oxygen sites (Figure 1), which can be divided into three groups: oxygen atoms in the bismuth oxide (18) Frydman, L.; Harwood: J. S. J. Am. Chem. Soc. 1995, 117, 5367. (19) Amoureux, J. P.; Fernandez, C.; Steuernagel, S. J. Magn. Reson. A. 1996, 116.
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Table 1. Room Temperature Quadrupole Coupling Constants (QCC, in MHz) of Three 17O Resonances in Bi2W0.9Nb0.1O5.95, Extracted by Simulation of the Spinning Sideband Patterns 242 ppm
400 ppm
462 ppm
1.8
1.2
1.5
layer, and equatorial oxygen and axial oxygen atoms in the tungsten oxide perovskite-like layer. The resonance at 242 ppm can be assigned to oxygen in the bismuth oxide layer based on previous NMR studies of Bi4V2O11 (δ ) 265 ppm)5 and Bi2O3 (δ ) 195 ppm).20 The equatorial oxygen atom is coordinated to two tungsten atoms, while the axial oxygen is coordinated to one tungsten atom and four bismuth atoms. Oxygen atoms that bridge two tungsten atoms in, for example, tungsten polyoxometalates resonate at higher frequencies (between 389 and 431 ppm21) than the bismuth oxide ions. On that basis, it appears reasonable to assign the resonance at 462 ppm to equatorial oxygens in the tungsten oxide layer and the one at 400 ppm to the axial oxygen, i.e., oxygen between the two layers, connected to both tungsten and bismuth. The quadrupole coupling constants for these sites are listed in Table 1. No significant improvement in resolution was obtained in the spectrum acquired at 16.4 T or in the MQ-MAS spectrum of Bi2W1.9Nb0.1O5.95 obtained at the same field (data not shown), indicating that a major contribution to the line width comes from a distribution of chemical shifts, and not from second-order quadrupolar broadening. The signal-to-noise ratio of the spectrum is not sufficiently good to determine whether the oxygen-ion vacancies preferentially occupy one out of the three oxygen sites, since only 0.05 out of 6 oxygen sites (0.83%) per formula unit are occupied by vacancies. Furthermore, very accurate quantification of signals from quadrupolar nuclei is difficult: first, the isotropic resonance contains contributions from not only the central transition (|+1/2> - |-1/2>) but also the satellite transitions, and second, some of the central transition resonance may reside in the spinning sidebands. Both of these effects need to be accounted for, and can result in errors in quantification, particularly for sites with different quadrupolar coupling constants.22 The 17O Hahn-echo MAS NMR spectra of Bi2WO6 and Bi2W1.9Nb0.1O5.95 are compared in Figure 3. The two spectra do not show significant differences in chemical shift and line shape. The intensity of the Bi2W1.9Nb0.1O5.95 spectrum is much larger than that of the poorer anion conductor Bi2WO6, indicating a higher 17O enrichment level for Bi2W1.9Nb0.1O5.95. This indicates that the level of enrichment depends strongly on the anionic conductivity of the bulk phase at least for short enrichment times, and demonstrates that heating the sample in 17O gas is a reasonably straightforward method for enriching materials with moderately high conductivities. The relative intensities for the 3 sites differ from those shown in Figure 2, most likely due to the longer pulses used in the Hahn-echo sequence. Despite the nonuniform (20) Yang, S.; Park, K. D.; Oldfield, E. J. Am. Chem. Soc. 1989, 111, 7278. (21) Filowitz, M.; Ho, R. K. C.; Klemperer, W. G.; Shum, W. Inorg. Chem. 1979, 18, 93. (22) Massiot, D.; Bessada, C.; Coutures, J. P.; Taulelle, F. J. Magn. Reson. 1990, 90, 231.
Figure 3. 17O MAS NMR echo spectra of (a) Bi2WO6 and (b) Bi2W0.9Nb0.1O5.95 acquired using a Hahn echo sequence with a spinning speed of 15 kHz and τ value of one rotor period. An enlargement of the Bi2WO6 spectrum is shown.
Figure 4. Comparison of WO42- 17O resonances in the variable temperature 17O MAS spectra of (a) Bi W 2 0.95Nb0.05O5.975 and (b) Bi2W0.9Nb0.1O5.95.
excitation of the three sites obtained with the echo-sequence, these spectra were included to illustrate the effect of anion conductivity on the level of enrichment achieved with our procedure. Coalescence of the 400 and 462 ppm resonances due to the axial and the equatorial oxygen atoms, respectively, is seen at and above 200 °C (Figure 4) in the one-dimensional (1D) variable temperature (VT) 17O MAS spectra of Bi2W1.9Nb0.1O5.95. Coalescence is observed when the jump frequency between two sites with resonances separated by a frequency ν is higher than πν/x2. Thus, the anions jump between axial and equatorial sites with frequencies higher than 6.7 kHz (∼62 ppm) at 250 °C in Bi2W1.9Nb0.1O5.95, corresponding to a correlation time, τc, of 2 2 0.5 0.1 0.033
2.5-135
2D
12-22
the 10% doped compound contains twice the number of vacancies. Further, it appears that the effect of vacancydopant clustering on the conductivity is similar in both compounds; this clustering was suggested to be the cause of the reduced conductivity in the 10%-doped compound in the previous study.16 Analysis of the Bulk and Local Measures of Conductivity. Simulations of the line shapes using the equation for an uncoupled two-site exchange24 were used to extract more accurate values for the exchange rates between the axial and equatorial oxygen ions (Figure 7). Jump frequencies of 30 and 5 kHz at 250 °C for the 10% and the 5% doped Bi2WO6, respectively, are estimated. The different time scales for motion estimated from the 1D coalescence exchange experiments are summarized in Table 3. The correlation times measured by NMR can be related to the bulk conductivity via an expression of the form: σ)
nq2r2 4kTτc
(1)
where we have assumed that ionic conduction is twodimensional and thus a value for the geometric factor, k, of 4 (instead of 6 for 3D conductivity) has been used; n is the number of charge carriers per unit volume, q is the net charge of the charge carrier, r is the jump distance, k is Boltzmann constant, T is temperature in Kelvin, and τc is the correlation time. The value of r was assumed to equal the average distance between equatorial and axial oxygen atoms (2.7 Å) and τc was estimated from the jump frequency estimated from the simulations of the 1D 17O NMR line shapes. The variable (24) Sandstrom, J. Dynamic NMR Spectroscopy; Academic Press: London, 1982.
Oxygen-Ion Jump Pathways in Bi2WO6
n for oxygen anion conductors generally refers to the oxygen ion vacancies. However, NMR spectroscopy only detects the motion of these vacancies indirectly via the motion of the oxygen anions themselves. Thus, n is given by the total number of oxygen atoms participating in the motion (i.e., the oxygen atoms in tungsten oxide layer). The calculated conductivity at 250 °C with a jump frequency k (1/τc) of 30 kHz for Bi2W0.9Nb0.1O5.95 is approximately 2.5 × 10-6 Scm-1. The calculated conductivity is about 1 order of magnitude lower than the measured bulk conductivity of 1.9 × 10-5 Scm-1 at this temperature. This is probably because the bulk conduction does not involve all the oxygen ions in the tungsten oxide layer, while the coalescence occurs in the 17O NMR when all the oxygen ions in the tungsten oxide layers are involved in the exchange process. This may indicate that there are fast and slow moving oxygen ions associated with different correlation times. The fast moving ions appear to be responsible for most of the ionic conduction, while the exchange rate measured from 1D NMR involves all the oxygen atoms in the tungsten oxide layer. A similar observation was reported earlier in NMR relaxation studies of Li+ ion conducting glasses.25-27 The conductivities calculated using correlation times from NMR experiments were about 2 orders of magnitude lower than the bulk conductivities of Li+ ion conducting glasses and the discrepancy appears to be due to a broad distribution of activation energies and correlation times in glasses. Our previous study of Bi4V2O11 showed that the exchange processes involved only a subset of sites in the vanadium oxide layers at lower temperatures, with exchange between all the sites in the vanadium oxide layers only commencing at a higher temperature.5 In the current system, Nb5+ doping will introduce local distortions in the lattice, and hence, a distribution in the activation energies for the jump processes. Furthermore, vacancy-dopant trapping also leads to a distribution of correlation times, which is pronounced at lower temperatures, as seen in our earlier 19F studies of fluorideion conductors.7,28 It should also be noted that these expressions were not derived for motion in the distorted, WO422D lattice, also resulting in errors in the calculation of σ. The activation energy was estimated for the 10% doped Bi2WO6 using the exchange rates in Table 3. An Arrhenius plot of the exchange rate (lnk) (1/τc) versus the inverse temperature (1/T) leads to an activation energy (Ea) of 1.1 eV. This is slightly higher than the activation energy in the low-temperature regime, extracted from the conductivity measurements (Figure 6) of 0.75 eV.17 This lower number is consistent with the suggestion that 1D NMR is also measuring processes with higher activation energies, and that the major contribution to the bulk conductivity occurs through lower activation energy paths. The cross-peak intensities from 2D NMR spectra acquired at 150 °C clearly indicate that a distribution of correlation times is present (Figure 8). For two-site exchange between (25) Svare, I.; Borsa, F.; Torgeson, D. R.; Martin, S. W. Phys. ReV. B 1993, 48, 9336. (26) Kim, K. H.; Torgeson, D. R.; Borsa, F.; Cho, J.; Martin, S. W.; Svare, I. Solid State Ionics 1996, 91, 7. (27) Svare, I.; Martin, S. W.; Borsa, F. Phys. ReV. B 2000, 61, 228. (28) Wang, F.; Grey, C. P. Chem. Mater. 1997, 9, 1068.
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Figure 8. Plot of pA and pB for the WO42- oxygen atoms as a function of mixing time for magnetization exchange data collected at 150 °C, where pA ) (IAA - IAB)/(IAA+ IAB); IAA and IAB are the intensities of the diagonal peaks and cross-peaks, respectively. The data have been fit with the function pA/B ) e-2t/τ. τ ) 135 and 77.5 ms for A (the 462 ppm resonance) and B (400 ppm resonance), respectively.
two sites A and B with the same T1, p ) e-2t/τ, where p ) (IAA - IAB)/(IAA + IAB); IAA and IAB are the intensities of the diagonal peaks and cross-peaks, respectively.29 Slightly different values for p are obtained for cross-peaks originating from the resonances at 400 and 462 ppm. This is ascribed to the small differences in the T1s of the two sites, as described in an earlier publication.9 The data at both temperatures cannot be fitted with a single-exponential decay function: the data at longer mixing times at 150 °C can be fitted with correlation times of 75-135 ms (shown in Figure 8), while the shorter data are consistent with a correlation time of 2.5-5 ms. More detailed analysis of the cross-peak intensities is in progress to describe the distributions of correlation times more accurately and to incorporate the effect of different T1s into the analysis. The distributions in correlation times seen by 2D NMR spectroscopy, however, are consistent with the explanation used above to explain the differences in conductivities as measured by 1D NMR and ac conductivity measurements. Environments with shorter correlation times must provide a larger contribution to the conductivities as seen by ac spectroscopy. Interestingly, preliminary analysis of data for the exchange between the two layers shows a much smaller distribution of correlation times, with the values at short and long mixing times differing by only 40-50% (7-9 and 12-22 ms at short and long times, respectively, at 250 °C). Although there is a difference between the estimated conductivity from NMR data and the measured bulk conductivity, we can still conclude that the major contribution to the oxygen conduction at intermediate temperature range (up to 250 °C) is the jump between two different sites in the tungsten oxide layer. Conclusions We have shown that variable temperature 17O NMR 1D and 2D 17O NMR spectroscopy can be used to study the oxygen motion in solid oxide electrolytes. Atomic-scale (29) Jeener, J.; Meier, B. H.; Bachmann, P.; Ernst, R. R. J. Chem. Phys. 1979, 71, 4546.
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information becomes available via these experiments: exchange between anions in the tungsten oxide occurs first, with exchange between the two different layers occurring only at higher temperatures. Doping with Nb5+ increases the conductivity dramatically, as seen by both NMR and ac impedance spectroscopy methods. 2D NMR exchange spectroscopy can be used to study systems with much slower motions than those accessible by using 1D NMR methods alone. This allows the anionic
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conductivity of a greater range of oxide ion conductors to be investigated by NMR, particularly those of interest for use in solid oxide fuel cells, which typically have conductivities that fall into the range probed in this study. Acknowledgment. C.P.G. thanks the NSF for financial support via grant DMR 0211353. CM048388A
