Dynamic Nuclear Polarization NMR of Low-Î³ Nuclei - ACS Publications

Letter pubs.acs.org/JPCL

Dynamic Nuclear Polarization NMR of Low‑γ Nuclei: Structural Insights into Hydrated Yttrium-Doped BaZrO3 Frédéric Blanc,*,† Luke Sperrin,‡ Daniel Lee,§ Rıza Dervişoğlu,‡,∥ Yoshihiro Yamazaki,⊥,# Sossina M. Haile,# Gael̈ De Paep̈ e,§ and Clare P. Grey*,‡,∥ †

Department of Chemistry and Stephenson Institute for Renewable Energy, University of Liverpool, Crown Street, Liverpool L69 7ZD, United Kingdom ‡ Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom § Laboratoire de Chimie Inorganique et Biologique, UMR-E 3 (CEA/UJF) and CNRS, Institut Nanosciences et Cryogénie, CEA, 38054 Grenoble, France ∥ Department of Chemistry, Stony Brook University, Stony Brook, New York 11794-3400, United States ⊥ Japan Science and Technology Agency, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan # Materials Science, California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, United States S Supporting Information *

ABSTRACT: We demonstrate that solid-state NMR spectra of challenging nuclei with a low gyromagnetic ratio such as yttrium-89 can be acquired quickly with indirect dynamic nuclear polarization (DNP) methods. Proton to 89Y cross polarization (CP) magic angle spinning (MAS) spectra of Y3+ in a frozen aqueous solution were acquired in minutes using the AMUPol biradical as a polarizing agent. Subsequently, the detection of the 89Y and 1H NMR signals from technologically important hydrated yttrium-doped zirconate ceramics, in combination with DFT calculations, allows the local yttrium and proton environments present in these protonic conductors to be detected and assigned to different hydrogenbonded environments.

SECTION: Spectroscopy, Photochemistry, and Excited States

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radicals are predominantly used as the unpaired electron sources, and excitation saturation of the electron paramagnetic resonance (EPR) transitions of these radicals results in hyperpolarization enhancement of the magnetization of the (usually) abundant 1H spins, and these nuclei themselves directly (direct DNP). In the former case, the magnetisation is subsequently transferred to nuclei with lower γ such as 13C or 15 N by CP (indirect DNP), both approaches allowing structural investigations of a very large range of biomolecules and materials.8,9,11−23 Although DNP enhanced ssNMR spectroscopy of a large range of nuclei has been recently performed, the applicability of this promising technique to greatly facilitate the detection of very low sensitivity nuclei such as those with very low γ has thus far not been demonstrated. We24,25 and others2,26,27 have been interested in one particular low-γ nucleus, 89Y (γ = −13.2 × 106 rad·s−1 T−1), for structural elucidation of inorganic materials. Hyperpolarization in liquid-state 13C and 1H NMR has recently been

olid-state nuclear magnetic resonance (ssNMR) spectroscopy is a widely used analytical technique. It is often employed in conjunction with diffraction-based techniques and/or first-principles calculation methods for atomic structure elucidation of an ever-increasing number of systems of chemical and biological interest.1 Due to the low nuclear spin polarization at room temperature, ssNMR suffers from a lack of sensitivity, preventing even wider use of the technique, despite the introduction and widespread implementation of cross polarization (CP), magic angle spinning (MAS), and high magnetic fields. The challenge of low sensitivity in NMR is exacerbated further for nuclei with low gyromagnetic ratio, γ (arbitrarily classified as nuclei with a γ lower than that of 15N (γ = 27.1 × 106 rad·s−1 T−1)), because the intrinsic sensitivity of a nuclear spin is proportional to γ2.5. Therefore, low-γ nuclei are particularly difficult to detect, despite being present in many materials used in a large range of applications.2 Dynamic nuclear polarization (DNP) enhanced ssNMR spectroscopy3−10 is becoming a very powerful approach for enhancing the signal by multiple orders of magnitude, making use of a microwave-induced transfer of polarization from electron to nuclear spins at cryogenic temperatures.5,10 Stable © 2014 American Chemical Society

Received: April 16, 2014 Accepted: June 18, 2014 Published: June 18, 2014 2431

dx.doi.org/10.1021/jz5007669 | J. Phys. Chem. Lett. 2014, 5, 2431−2436

The Journal of Physical Chemistry Letters

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Figure 1. 1H 89Y CP MAS spectra of 0.5 M Y(NO3)3 with (a) 12 mM AMUPol in 60/40 (v/v) d6-DMSO/H2O and (b) 12 mM AMUPol in 60/30/ 10 (v/v) d8-glycerol/D2O/H2O recorded with (green) and without (red) microwave irradiation at ν0(e−) = 263.7 GHz. The 1H build-up polarization curves were fitted with a biexponential function, and the time constants, TB, were 0.4 and 3.1 s in (a) and 0.5 and 4.4 s in (b); 128 scans were recorded at ∼105 K with ν0(1H) = 400.260 MHz and ν0(89Y) = 19.613 MHz under νrot = 5 kHz, a CP contact time of 8 ms, and SPINAL-64 1H heteronuclear decoupling with ν1(1H) = 100 kHz (exptl. time = 32 min).

Figure 2. 1H 89Y CP MAS spectra of (a) BZY20 and (b) BZY30 recorded with (green) and without (red) microwave irradiation at ν0(e−) = 263.7 GHz. The 1H build-up polarization curves were fitted with a biexponential function, and two time constants, TB, were 4 and 31.3 s in (a) and 2.4 and 13.4 s in (b). All of the spectra were recorded under conditions similar to the ones given in Figure 1, except the CP contact time, which was set to 20 ms and with 64 scans accumulated (exptl. time = 22 min). All samples were wet with 12 mM AMUPol in 60/40 (v/v) d6-DMSO/H2O. ε(1H) and ε(89Y CP) were determined by comparing the spectra with and without microwave irradiation (calculated as ε = ADNP/ABoltzmann, where ADNP and ABoltzmann are the signal areas with and without microwave irradiation, respectively) and were 10 and 3 for BZY20 and 9 and 2 for BZY30, respectively.

achieved by dissolution DNP28,29 and subsequently applied to 89 Y to detect yttrium complexes used as magnetic resonance imaging (MRI) contrast agents.30−33 There, frozen samples (at ∼1 K) are hyperpolarized before being rapidly warmed up to room temperature to detect the yttrium-containing molecules in the liquid state while maintaining (most of) the solid-state polarization. This approach does not lend itself directly to the study of ceramics and solid-state compounds in general. Here, we demonstrate that the poor sensitivity of low-γ nuclei, such as yttrium-89, can be overcome for solid-state MAS NMR spectroscopy studies at 9.4 T by the signal enhancement gained from indirect DNP and appropriate choice of particle size. In principle, direct DNP can be used for detection of low-γ nuclei; however, the material investigated here has protons, which enables indirect DNP. Note that the long T1 relaxation times typical of 89Y nuclei render direct DNP less attractive when indirect transfer is possible. Furthermore, indirect DNP using protons as the excitation species provides (as shown

below) the desired insight into the yttrium−proton interactions. We first show that solid-state 1H 89Y CP MAS34 combined with DNP NMR enables the 89Y NMR signal of frozen aqueous solutions of Y3+, at ∼105 K containing the cryoprotectant d6DMSO (or d8-glycerol) and the very efficient, water-soluble AMUPol biradical polarizing agent,35 to be detected rapidly (see the Supporting Information (SI) for details of all samples preparations). We then show that indirect 89Y DNP can be used to observe the 89Y signal in hydrated yttrium-doped barium zirconate.36−38 This material shows both high proton conductivity and chemical stability, making it a very promising electrolyte for intermediate-temperature solid oxide fuel cells and electrolyzers.39 We have recently investigated the defect chemistry and transport mechanism in this material using thermogravimetry, conductivity, diffusivity, and 1H hightemperature solid-state NMR spectroscopy and proposed that, at temperatures relevant for operation, a large fraction 2432



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Figure 3. Contour plot of the two-dimensional 1H 89Y CP MAS FSLG HETCOR62 spectrum of BZY20 recorded under microwave irradiation at ν0(e−) = 263.7 GHz. All of the other acquisition parameters and sample details are identical to those of Figure 2. A total of 37 t1 increments of 36 μs with 72 scans and a 20 s recycle delay were acquired (exptl. time = 15 h). (Top) 1H 89Y CP MAS spectra (280 scans accumulated, exptl. time = 1.5 h). (Left) 1H MAS spectrum of the sample obtained under νrot = 8 kHz at ∼105 K without polarizing agents or solvents. The experimental and DFTcalculated 1H and 89Y isotropic chemical shifts are indicated on the spectra. Inset figures show the optimized local structures about yttrium atoms (purple). Zirconium, oxygen, and hydrogen atoms are in blue, red, and white, respectively. Barium atoms have been omitted for clarity.

The CP experiments require long CP contact times (>20 ms) (Figure S2, SI) due to the weak 1H−89Y dipolar interactions.34 Thus, effective magnetization transfer can only occur for protons with long rotating frame longitudinal relaxation times T1ρ in a rigid network, which can only be generally achieved at low temperature for these classes of compounds.45 More specifically, for these systems, we find that T1ρ varies noticeably with temperature from ∼5 ms at 295 K, B0 = 11.7 T, to ∼35 ms at ∼195 K, B0 = 11.7 T, to ∼60 ms at ∼105 K, B0 = 9.4 T (when using a radio frequency spin-locking field amplitude of ν1(1H) ≈ 50 kHz). These 1H 89Y CP experiments are challenging on the NMR hardware due to the requirement of long CP contact times and are therefore further improved by the better heat dissipation in a low-temperature probe operating at 105 K allowing the implementation of these long times. Therefore, the low temperatures enable safe and efficient 1H 89Y CP transfer, further increasing the sensitivity and, combined with the aforementioned DNP enhancements, permitting the fast acquisition (minutes only) of previously extremely challenging experiments, which would take many hours at room temperature without DNP. We note that, surprisingly, smaller ε(89Y CP) values of ∼1− 1.5 were obtained for BZY30 impregnated under other robust DNP sample preparation conditions (12 mM AMUPol35 solution in d8-glycerol/D2O/H2O or 10 mM bCTbK46,47 in 1,1,2,2-tetrabromoethane,48 Figures S3−S5 (SI)). These conditions were found to be very efficient for other systems, and despite large 1H signal DNP enhancement factors ε(1H) of the solvent, only small ε(89Y CP) values could be obtained here. The behavior will be subject to further work. The modest 89Y signal DNP enhancement factors obtained can be explained by a number of factors. First, the particle sizes of the BZY20 and BZY30 samples used in these experiments are rather large, up to 45 μm in diameter.38,49 The signal enhancement, ε, which depends on the transfer of magnet-

of the protons in this material are trapped in defect sites near the yttrium dopant.38 In addition to one-dimensional 89Y CP MAS DNP NMR spectra, the 1H 89Y two-dimensional spectra reported here enabled, in combination with DFT calculations, the determination of the yttrium coordination environments and, notably, further insight into the existence of trapped proton sites in this protonic conductor.38 Figure 1 shows the 89Y CP MAS NMR spectra of Y3+ in a d6DMSO/H2O and d8-glycerol/D2O/H2O frozen glass, both containing the AMUPol polarizing agent.35 While no 89Y signal is detected without microwave irradiation, strong signals are observed with microwave irradiation, demonstrating that the substantial polarization enhancement observed on 1H (see Figure S1, SI) is successfully transferred to the low-γ 89Y by CP. On the basis of previous studies,40,41 we tentatively assign the peak at −20 ppm to free Y(aq)3+ ions and the signals at 23 and 34 ppm to [Y(DMSO)]3+ and [Y(glycerol)]3+ complexes, respectively. The 89Y CP MAS NMR spectra of hydrated yttrium-doped barium zirconate samples containing 20 and 30% yttrium as a dopant and water contents of ∼7 and ∼9%,42 respectively, (subsequently labeled BZY20 and BZY30) and impregnated with a 12 mM AMUPol solution in d6-DMSO/H2O are given in Figure 2 (see also Figure 3 and the SI for spectra recorded with more transients). While the 89Y signal is barely visible without microwave irradiation, two 89Y signals at 413−416 and 351−352 ppm are readily observed with microwave irradiation and can be assigned to yttrium in six-fold coordination25,43 (see below for further spectral assignments). The 89Y CP signal indirect DNP enhancement factors are clearly modest (ε(89Y CP) = 3 and 2 for BZY20 and BZY30, respectively). However, the sensitivity is further increased beyond these values by the larger Boltzmann factor at ∼105 K (∼3.6 including reduced thermal noise44). 2433



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(2.3 versus 3.9 Å) for this site and thus the more efficient 1H to Y magnetization transfer. The favorable interaction between the hydroxyl group and the dopant has been suggested in cubic perovskite systems in experimental38,52−55 and theoretical51,56−61 studies. Our recent conductivity and this current NMR study provide compelling evidence for proton trapping near the defect.38 In conclusion, we have shown that the solid-state NMR of low-γ nuclei such as 89Y can be significantly enhanced by indirect DNP. The applicability of the approach was demonstrated on aqueous frozen glass of Y3+, where an 89Y NMR signal is observed in only minutes with indirect DNP while no signal could be observed without DNP. The method enabled the detection of the 89Y NMR spectra of hydrated yttrium-doped barium zirconates, potential proton conductors for use in an intermediate-temperature solid oxide fuel cell.38 These experiments were performed on micron-sized particles, suggesting that increased signal enhancements could be achieved by tailoring the particle size. This work provides a powerful alternative strategy for acquiring solid-state NMR spectra of very insensitive nuclei, allowing structural characterization of an ever-increasing range of materials.

ization from protons on or near the surface of the particles, will thus be small for small surface to volume ratios (i.e., for large particle sizes).17,23 In support of this, negligible 89Y signal indirect DNP enhancement is detected for a sample of BZY40 having even larger particle sizes ranging from 45 to 75 μm (see Figure S6, SI), despite the material being appropriately hydrated (see Figure S7, SI). Furthermore, the 1H content in these samples is low (∼2−3 mmol·cm−3),38,42 preventing efficient 1H−1H spin diffusion and transfer of hyperpolarization over micrometer length scales.17 A 1H−1H spin diffusion model,17,23,50 taking into account the short 1H build-up polarizing times TB measured here (Table S2, SI), suggests that the small observed ε values are to be expected. The direct 1 H DNP enhancement largely measures the enhancement of the protons in the solvent, and the surface sorbed protons should have a similar enhancement to the solvent protons. The fact that the indirect DNP enhancement is smaller than the direct 1H DNP enhancement (Table S2, SI) shows that the protons mainly responsible for CP are bulk protons, rather than the less technologically interesting surface sorbed protons. Despite the moderate gains in sensitivity, the reduction in experimental time obtained by DNP at low temperatures is however sufficient to allow 1H 89Y CP HETCOR spectra of BZY20 (Figure 3) and BZY30 (Figure S8, SI) with good signalto-noise ratio to be recorded in a reasonable time (15 h rather than in days without DNP). Furthermore, the low temperatures substantially reduce the mobility of the protons, allowing the different 1H local environments to be resolved. Two sets of 1H 89 Y correlations are observed at 1H: 5.4 and 89Y: 416 ppm and at 1H: 6.8 and 89Y: 351 ppm. DFT and GIPAW51 NMR parameter calculations27 of a series of possible proton local environments, within the Ysubstituted BaZrO3 2 × 2 × 2 supercells of composition Ba8Zr7YO24H, show good agreement with the CP HETCOR spectra. This enables the different chemical shift to be assigned to specific configurations, Figure 3. The main NMR correlation, 1 H: 5.4 and 89Y: 416 ppm, is assigned to a proton bound to a yttrium atom (“near” configuration, Figure 3), the first direct observation of the proton “trapped” site proposed in our earlier publication.38 This configuration is one of the two lowest energies in the possible configurations explored, with an energy that is ∼24 kJ molH−1 lower than the “distant” configuration, a proton far from the yttrium dopant (Table S3 and Figures S9− S11, SI). This is in quantitative agreement with the measured association energy of 29 kJ molH−1, the energy difference between the trapping and trap-free sites.38 The DNP experiment combined with DFT calculations further revealed another possible trapping site (1H: 6.8 and 89Y: 351 ppm), which is assigned to the next-nearest oxygen to yttrium with the hydroxyl group orientated within the plane containing the yttrium (the ground-state “planar” configuration). This configuration is the most stable arrangement within the current calculations, 1.4 kJ molH−1 lower than that of the near configuration, consistent with previous DFT studies on yttrium-doped barium zirconate but within typical DFT errors.49,51 This suggests that the proton trapping sites are not only limited to the nearest neighbor of yttrium dopant but also expanded a little further to the next-nearest neighbor depending on the hydroxyl group orientation (Figure S11, SI). Note that a more intense 1H 89Y correlation is observed for the near configuration than for the in-plane configuration, consistent with the much shorter proton−yttrium distance

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ASSOCIATED CONTENT

S Supporting Information *

Samples preparation, details and discussions of the calculations, and additional figures, including the 1H MAS spectra of Y(NO3)3, BZY20, BZY30, and BZY40 and 89Y CP MAS spectra of BZY20, BZY30, and BZY40 under various conditions, and contour plots of the 1H 89Y CP MAS HECTOR spectra of BZY30. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (F.B.). *E-mail: [email protected] (C.P.G.). Funding

We acknowledge funding from the University of Liverpool (F.B.), EPSRC (L.S.), the Japan Science Technology Agency PRESTO (Y.Y.), the Gordon and Betty Moore Foundation (S.M.H.), the French National Research Agency through the “programme blanc” (ANR-12-BS08-0016-01) and the Labex ARCANE (ANR-11-LABX-0003-01) (G.D.P.), and the ERC for an Advanced Fellowship (C.P.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Prof. Paul Tordo and Dr. Olivier Ouari (AixMarseille Université) for providing the AMUPol radical. We thank Dr. Lucienne Buannic for fruitful discussions concerning 89 Y NMR and for performing the T1ρ measurements at 11.7 T.

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REFERENCES

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Dynamic Nuclear Polarization NMR of Low-Î³ Nuclei - ACS Publications

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