Category Archives: Material Science

Paramagnetic NMR in solution and the solid state

Pell, Andrew J., Guido Pintacuda, and Clare P. Grey. “Paramagnetic NMR in Solution and the Solid State.” Progress in Nuclear Magnetic Resonance Spectroscopy 111 (April 2019): 1–271.

https://doi.org/10.1016/j.pnmrs.2018.05.001

The field of paramagnetic NMR has expanded considerably in recent years, both in solution and the solid state. This review addresses both the theoretical description of paramagnetic NMR, and the way in which it is currently practised. We provide a review of the theory of the NMR parameters of systems in both solution and the solid state. Here we unify the different languages used by the NMR, EPR, quantum chemistry/DFT, and magnetism communities to provide a comprehensive and coherent theoretical description. We cover the theory of the paramagnetic shift and shift anisotropy in solution both in the traditional formalism in terms of the magnetic susceptibility tensor, and using a more modern formalism employing the relevant EPR parameters, such as are used in first-principles calculations. In addition we examine the theory first in the simple non-relativistic picture, and then in the presence of spin-orbit coupling. These ideas are then extended to a description of the paramagnetic shift in periodic solids, where it is necessary to include the bulk magnetic properties, such as magnetic ordering at low temperatures. The description of the paramagnetic shift is completed by describing the current understanding of such shifts due to lanthanide and actinide ions. We then examine the paramagnetic relaxation enhancement, using a simple model employing a phenomenological picture of the electronic relaxation, and again using a more complex state-of-the-art theory which incorporates electronic relaxation explicitly. An additional important consideration in the solid state is the impact of bulk magnetic susceptibility effects on the form of the spectrum, where we include some ideas from the field of classical electrodynamics. We then continue by describing in detail the solution and solid-state NMR methods that have been deployed in the study of paramagnetic systems in chemistry, biology, and the materials sciences. Finally we describe a number of case studies in paramagnetic NMR that have been specifically chosen to highlight how the theory in part one, and the methods in part two, can be used in practice. The systems chosen include small organometallic complexes in solution, solid battery electrode materials, metalloproteins in both solution and the solid state, systems containing lanthanide ions, and multi-component materials used in pharmaceutical controlled-release formulations that have been doped with paramagnetic species to measure the component domain sizes.

Room temperature CO oxidation catalysed by supported Pt nanoparticles revealed by solid-state NMR and DNP spectroscopy #DNPNMR

Klimavicius, Vytautas, Sarah Neumann, Sebastian Kunz, Torsten Gutmann, and Gerd Buntkowsky. “Room Temperature CO Oxidation Catalysed by Supported Pt Nanoparticles Revealed by Solid-State NMR and DNP Spectroscopy.” Catalysis Science & Technology 9, no. 14 (2019): 3743–52.

https://doi.org/10.1039/C9CY00684B.

A series of 1 and 2 nm sized platinum nanoparticles (Pt-NPs) deposited on different support materials, namely, γ-alumina (γ-Al2O3), titanium dioxide (TiO2), silicon dioxide (SiO2) and fumed silica are investigated by solid-state NMR and dynamic nuclear polarization enhanced NMR spectroscopy (DNP). DNP signal enhancement factors up to 170 enable gaining deeper insight into the surface chemistry of Pt-NPs. Carbon monoxide is used as a probe molecule to analyze the adsorption process and the surface chemistry on the supported Pt-NPs. The studied systems show significant catalytic activity in carbon monoxide oxidation on their surface at room temperature. The underlying catalytic mechanism is the water–gas shift reaction. In the case of alumina as the support the produced CO2 reacts with the surface to form carbonate, which is revealed by solid-state NMR. A similar carbonate formation is also observed when physical mixtures of neat alumina with silica, fumed silica and titania supported Pt-NPs are studied.

Detection of the Surface of Crystalline Y2O3 Using Direct 89Y Dynamic Nuclear Polarization #DNPNMR

Brownbill, Nick J., Daniel Lee, Gaël De Paëpe, and Frédéric Blanc. “Detection of the Surface of Crystalline Y2O3 Using Direct 89Y Dynamic Nuclear Polarization.” The Journal of Physical Chemistry Letters 10, no. 12 (June 20, 2019): 3501–8.

https://doi.org/10.1021/acs.jpclett.9b01185

Nuclei with low gyromagnetic ratio (γ) present a serious sensitivity challenge for nulear magnetic resonance (NMR) spectroscopy. Recently, dynamic nuclear polarization (DNP) has shown great promise in overcoming this hurdle by indirect hyperpolarization (via 1H) of these low-γ nuclei. Here we show that at a magnetic field of 9.4 T and cryogenic temperature of about 110 K direct DNP of 89Y in a frozen solution of Y(NO3)3 can offer signal enhancements greater than 80 times using exogeneous trityl OX063 monoradical by satisfying the cross effect magic angle spinning (MAS) DNP mechanism. The large signal enhancement achieved permits 89Y NMR spectra of Y2O3 and Gd2O3-added Y2O3 materials to be obtained quickly (∼30 min), revealing a range of surface yttrium hydroxyl groups in addition to the two octahedral yttrium signals of the core. The results open up promises for the observation of low gyromagnetic ratio nuclei and the detection of corresponding surface and (sub-)surface sites.

Recent developments in MAS DNP-NMR of materials #DNPNMR

Rankin, Andrew G.M., Julien Trébosc, Frédérique Pourpoint, Jean-Paul Amoureux, and Olivier Lafon. “Recent Developments in MAS DNP-NMR of Materials.” Solid State Nuclear Magnetic Resonance 101 (September 2019): 116–43.

https://doi.org/10.1016/j.ssnmr.2019.05.009

Solid-state NMR spectroscopy is a powerful technique for the characterization of the atomic-level structure and dynamics of materials. Nevertheless, the use of this technique is often limited by its lack of sensitivity, which can prevent the observation of surfaces, defects or insensitive isotopes. Dynamic Nuclear Polarization (DNP) has been shown to improve by one to three orders of magnitude the sensitivity of NMR experiments on materials under Magic-Angle Spinning (MAS), at static magnetic field B0 ! 5 T, conditions allowing for the acquisition of highresolution spectra. The field of DNP-NMR spectroscopy of materials has undergone a rapid development in the last ten years, spurred notably by the availability of commercial DNP-NMR systems. We provide here an in-depth overview of MAS DNP-NMR studies of materials at high B0 field. After a historical perspective of DNP of materials, we describe the DNP transfers under MAS, the transport of polarization by spin diffusion and the various contributions to the overall sensitivity of DNP-NMR experiments. We discuss the design of tailored polarizing agents and the sample preparation in the case of materials. We present the DNP-NMR hardware and the influence of key experimental parameters, such as microwave power, magnetic field, temperature and MAS frequency. We give an overview of the isotopes that have been detected by this technique, and the NMR methods that have been combined with DNP. Finally, we show how MAS DNP-NMR has been applied to gain new insights into the structure of organic, hybrid and inorganic materials with applications in fields, such as health, energy, catalysis, optoelectronics etc.

Mapping the dynamics of methanol and xenon co-adsorption in SWNTs by in-situ continuous-flow hyperpolarized 129Xe NMR

Xu, Shutao, Xin Li, Cheng Sun, Anmin Zheng, Weiping Zhang, Xiuwen Han, Xianchun Liu, and Xinhe Bao. “Mapping the Dynamics of Methanol and Xenon Co-Adsorption in SWNTs by in-Situ Continuous-Flow Hyperpolarized 129Xe NMR.” Physical Chemistry Chemical Physics 21, no. 6 (2019): 3287–93.

https://doi.org/10.1039/C8CP07238H.

A comparative study of the adsorption and desorption processes of methanol in two kinds of nanochannels (i.e. MCM-41 and SWNTs) is performed by in situ continuous-flow laser-hyperpolarized 129Xe NMR. The high sensitivity and short acquisition time of hyperpolarized 129Xe allow for probing the molecular dynamics in a confined geometry under real working conditions. Hyperpolarized 129Xe NMR spectra indicate that the methanol adsorption behavior in nanochannels is determined by the characters of adsorption sites and that the methanol adsorption rate in the nanochannels of SWNTs is faster than in MCM-41. The experimental data shown in this work also indicate that there is a change in gas phase 129Xe NMR signal intensity during the adsorption and desorption of methanol in SWNTs. This may be because there is a strong depolarization of hyperpolarized 129Xe in SWNTs.

The effects of sample conductivity on the efficacy of dynamic nuclear polarization for sensitivity enhancement in solid state NMR spectroscopy #DNPNMR

Svirinovsky-Arbeli, Asya, Dina Rosenberg, Daniel Krotkov, Ran Damari, Krishnendu Kundu, Akiva Feintuch, Lothar Houben, Sharly Fleischer, and Michal Leskes. “The Effects of Sample Conductivity on the Efficacy of Dynamic Nuclear Polarization for Sensitivity Enhancement in Solid State NMR Spectroscopy.” Solid State Nuclear Magnetic Resonance 99 (July 2019): 7–14.

https://doi.org/10.1016/j.ssnmr.2019.02.003

In recent years dynamic nuclear polarization (DNP) has greatly expanded the range of materials systems that can be studied by solid state NMR spectroscopy. To date, the majority of systems studied by DNP were insulating materials including organic and inorganic solids. However, many technologically-relevant materials used in energy conversion and storage systems are electrically conductive to some extent or are employed as composites containing conductive additives. Such materials introduce challenges in their study by DNP-NMR which include microwave absorption and sample heating that were not thoroughly investigated so far.

Effect of nitroxide spin probes on the transport properties of Nafion membranes #DNPNMR #ODNP

Überrück, Till, Oliver Neudert, Klaus-Dieter Kreuer, Bernhard Blümich, Josef Granwehr, Siegfried Stapf, and Songi Han. “Effect of Nitroxide Spin Probes on the Transport Properties of Nafion Membranes.” Physical Chemistry Chemical Physics 20, no. 41 (2018): 26660–74.

https://doi.org/10.1039/C8CP04607G.

Nafion is the most common material used as proton exchange membrane in fuel cells. Yet, details of the transport pathways for protons and water in the inner membrane are still debated. Overhauser Dynamic Nuclear Polarization (ODNP) has proven a useful tool for probing hydration dynamics and interactions within 5–8 Å of protein and soft material surfaces. Recently it was suggested that ODNP can also be applied to analyze surface water dynamics along Nafion’s inner membrane. Here we interrogate the viability of this method for Nafion by carrying out a series of measurements relying on 1H nuclear magnetic resonance (NMR) relaxometry and diffusometry experiments with and without ODNP hyperpolarization, accompanied by other complementary characterization methods including small angle X-ray scattering (SAXS), thermal gravimetric analysis (TGA) of hydration, and proton conductivity by AC impedance spectroscopy. Our comprehensive study shows that the commonly used paramagnetic spin probes—here, stable nitroxide radicals—for ODNP, as well as their diamagnetic analogues, reduce the inner membrane surface hydrophilicity, depending on the location and concentration of the spin probe. This heavily reduces the hydration of Nafion, hence increases the tortuosity of the inner membrane morphology and/or increases the activiation barrier for water transport, and consequently impedes water diffusion, transport, and proton conductivity.

Endogenous Dynamic Nuclear Polarization for Natural Abundance 17O and Lithium NMR in the Bulk of Inorganic Solids #DNPNMR

Wolf, Tamar, Sandeep Kumar, Harishchandra Singh, Tanmoy Chakrabarty, Fabien Aussenac, Anatoly I Frenkel, Dan Thomas Major, and Michal Leskes. “Endogenous Dynamic Nuclear Polarization for Natural Abundance 17O and Lithium NMR in the Bulk of Inorganic Solids.” Journal of the American Chemical Society, n.d., 23. 

https://pubs.acs.org/doi/abs/10.1021/jacs.8b11015

In recent years magic angle spinning – dynamic nuclear polarization (MAS-DNP) developed as an excellent approach for boosting the sensitivity of solid state NMR (ssNMR) spectroscopy, thereby enabling the characterization of challenging systems in biology and chemistry. Most commonly, MAS-DNP is based on the use of nitroxide biradicals as polarizing agents. In materials science, since the use of nitroxides often limits the signal enhancement to the materials’ surface and subsurface layers, there is need for hyperpolarization approaches which will provide sensitivity in the bulk of micron sized particles. Recently, an alternative in the form of paramagnetic metal ions has emerged. 

Here we demonstrate the remarkable efficacy of Mn(II) dopants, used as endogenous polarization agents for MAS-DNP, in enabling the detection of 17O at a natural abundance of only 0.037%. Distinct oxygen sites are identified in the bulk of micron-sized crystals, including battery anode materials Li4Ti5O12 (LTO) and Li2ZnTi3O8, as well as the phosphor materials NaCaPO4 and MgAl2O4, all doped with Mn(II) ions. Density functional theory calculations are used to assign the resonances to specific oxygen environments in these phases. Depending on the Mn(II) dopant concentration, we obtain significant signal enhancement factors, 142 and 24, for 6Li and 7Li nuclei in LTO, respectively. We furthermore follow the changes in the 6,7Li LTO resonances and determine their enhancement factors as a function of Mn(II) concentration. The results presented show that MAS-DNP from paramagnetic metal ion dopants provides an efficient approach for probing informative nuclei such as 17O, despite their low gyromagnetic ratio and negligible abundance, without isotope enrichment.

Surface chemical heterogeneity modulates silica surface hydration #DNPNMR #ODNP

Schrader, Alex M., Jacob I. Monroe, Ryan Sheil, Howard A. Dobbs, Timothy J. Keller, Yuanxin Li, Sheetal Jain, M. Scott Shell, Jacob N. Israelachvili, and Songi Han. “Surface Chemical Heterogeneity Modulates Silica Surface Hydration.” Proceedings of the National Academy of Sciences 115, no. 12 (March 20, 2018): 2890–95.

https://doi.org/10.1073/pnas.1722263115.

An in-depth knowledge of the interaction of water with amorphous silica is critical to fundamental studies of interfacial hydration water, as well as to industrial processes such as catalysis, nanofabrication, and chromatography. Silica has a tunable surface comprising hydrophilic silanol groups and moderately hydrophobic siloxane groups that can be interchanged through thermal and chemical treatments. Despite extensive studies of silica surfaces, the influence of surface hydrophilicity and chemical topology on the molecular properties of interfacial water is not well understood. In this work, we controllably altered the surface silanol density, and measured surface water diffusivity using Overhauser dynamic nuclear polarization (ODNP) and complementary silica–silica interaction forces acrosswater using a surface forces apparatus (SFA). The results show that increased silanol density generally leads to slower water diffusivity and stronger silica– silica repulsion at short aqueous separations (less than ∼4 nm). Both techniques show sharp changes in hydration properties at intermediate silanol densities (2.0–2.9 nm−2). Molecular dynamics simulations of model silica–water interfaces corroborate the increase in water diffusivity with silanol density, and furthermore show that even on a smooth and crystalline surface at a fixed silanol density, adjusting the spatial distribution of silanols results in a range of surface water diffusivities spanning ∼10%. We speculate that a critical silanol cluster size or connectivity parameter could explain the sharp transition in our results, and can modulate wettability, colloidal interactions, and surface reactions, and thus is a phenomenon worth further investigation on silica and chemically heterogeneous surfaces.

NMR study of optically hyperpolarized phosphorus donor nuclei in silicon #DNPNMR

Gumann, P., H. Haas, S. Sheldon, L. Zhu, R. Deshpande, T. Alexander, M. L. W. Thewalt, D. G. Cory, and C. Ramanathan. “NMR Study of Optically Hyperpolarized Phosphorus Donor Nuclei in Silicon.” Physical Review B 98, no. 18 (November 16, 2018). 

https://doi.org/10.1103/PhysRevB.98.180405

We use above-band-gap optical excitation, via a 1047-nm laser, to hyperpolarize the 31P spins in low-doped (ND = 6×10^15 cm−3) natural abundance silicon at 4.2 K and 6.7 T, and inductively detect the resulting NMR signal. The 30-kHz spectral linewidth observed is dramatically larger than the 600-Hz linewidth observed from a 28Si-enriched silicon crystal. We show that the broadening is consistent with previous electron-nuclear double-resonance results showing discrete isotope mass effect contributions to the donor hyperfine coupling. A secondary source of broadening is likely due to variations in the local strain, induced by the random distribution of different isotopes in natural silicon. The nuclear spin T1 and the buildup time for the optically induced 31P hyperpolarization in the natural abundance silicon sample were observed to be 178 +/- 47 and 69 +/- 6 s, respectively, significantly shorter than the values previously measured in 28Si-enriched samples under the same conditions. We measured the T1 and hyperpolarization buildup time for the 31P signal in natural abundance silicon at 9.4 T to be 54 +/- 31 and 13 +/- 2 s, respectively. The shorter buildup and nuclear spin T1 times at high field are likely due to the shorter electron spin T1, which drives nuclear spin relaxation via nonsecular hyperfine interactions. At 6.7 T, the phosphorus nuclear spin T2 was 16.7 +/- 1.6 ms at 4.2 K, a factor of 4 shorter than in 28Si-enriched crystals. This was observed to shorten to 1.9 +/- 0.4 ms in the presence of the infrared laser.

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