Category Archives: Material Science

DNP in Materials Science: Touching the Surface #DNPNMR

Berruyer, Pierrick, Lyndon Emsley, and Anne Lesage. “DNP in Materials Science: Touching the Surface,” 7:12, 2018.

https://doi.org/10.1002/9780470034590.emrstm1554.

Dynamic nuclear polarization (DNP)-enhanced solid-state NMR spectroscopy under magic-angle spinning has recently emerged as a unique analytical method to probe surfaces at atomic resolution. In this article, we first describe the basic principles of dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS). The article continues with a large review of recent literature that illustrates the versatility of this technique and its incredible potential to reveal new structural features at surfaces with details at an unprecedented level. The most recent developments, such as the application of DNP SENS to highly reactive surface sites, are finally covered.

Endogenous Dynamic Nuclear Polarization for Sensitivity Enhancement in Solid-State NMR of Electrode Materials #DNPNMR

Harchol, Adi, Guy Reuveni, Vitalii Ri, Brijith Thomas, Raanan Carmieli, Rolfe H. Herber, Chunjoong Kim, and Michal Leskes. “Endogenous Dynamic Nuclear Polarization for Sensitivity Enhancement in Solid-State NMR of Electrode Materials.” The Journal of Physical Chemistry C 124, no. 13 (April 2, 2020): 7082–90.

https://doi.org/10.1021/acs.jpcc.0c00858

Rational design of materials for energy storage systems relies on our ability to probe these materials at various length scales. Solid state NMR spectroscopy is a powerful approach for gaining chemical and structural insight at the atomic/molecular level, but its low detection sensitivity often limits applicability. This limitation can be overcome by transferring the high polarization of electron spins to the sample of interest in a process called dynamic nuclear polarization (DNP). Here we employ for the first time, metal ion-based DNP to probe pristine and cycled composite battery electrodes. A new and efficient DNP agent, Fe(III), is introduced, yielding lithium signal enhancement up to 180 when substituted in the anode material Li4Ti5O12. In addition to being DNP active, Fe(III) improves the anode performance. Reduction of Fe(III) to Fe(II) upon cycling can be monitored in the loss of DNP activity. We show that the dopant can be reactivated (return to Fe(III)) for DNP by increasing the cycling potential window. Furthermore, we demonstrate that the deleterious effect of carbon additives on the DNP process can be eliminated by using carbon free electrodes, doped with Fe(III) and Mn(II), which provide good electrochemical performance as well as sensitivity in DNP-NMR. We expect the approach presented here will expand the applicability of DNP for studying materials for frontier challenges in materials chemistry associated with energy and sustainability.

Probing Functionalities and Acidity of Calcined Phenylene-Bridged Periodic Mesoporous Organosilicates Using (DNP)-NMR, DRIFTS and XPS #DNPNMR

Pirez, Cyril, Hiroki Nagashima, Franck Dumeignil, and Olivier Lafon. “Probing Functionalities and Acidity of Calcined Phenylene-Bridged Periodic Mesoporous Organosilicates Using (DNP)-NMR, DRIFTS and XPS.” The Journal of Physical Chemistry C, February 19, 2020, acs.jpcc.9b11223.

https://doi.org/10.1021/acs.jpcc.9b11223

Owing to their high surface area, their high stability and their hydrophobicity, periodic mesoporous organosilica (PMO) materials represent promising catalytic support for environmental-friendly chemical processes in water. We investigate here how the calcination of PMO material with benzene linkers (PMOB) allows its functionalization. Conventional and Dynamic Nuclear Polarization (DNP)-enhanced NMR spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy and Xray photoelectron spectroscopy prove that calcination at 450°C results in the oxidation of phenylene bridges into (poly)phenols but also in carboxylic acids. Ketone, aldehyde as well as allyl and aliphatic alcohol functionalities are also observed but their amount is much lower than that of carboxylic acids. The calcination also cleaves the Si-C bonds. Nevertheless, N2 adsorption-desorption measurements, powder X-ray diffraction and transmission electron microscopy indicate that the PMOB materials calcined up to 600°C still exhibit ordered mesopores. We show that the phenol and carboxylic acid functionalities of PMOB calcined at 450°C protonate the NH2 group of 1-(3- aminopropyl)imidazole (API) in water at room temperature but no formation of covalent bond between API and the calcined PMOB functionalities has been detected.

Disclosing Interfaces of ZnO Nanocrystals Using Dynamic Nuclear Polarization: Sol-Gel versus Organometallic Approach #DNPNMR

Lee, Daniel, Małgorzata Wolska-Pietkiewicz, Saumya Badoni, Agnieszka Grala, Janusz Lewiński, and Gaël De Paëpe. “Disclosing Interfaces of ZnO Nanocrystals Using Dynamic Nuclear Polarization: Sol-Gel versus Organometallic Approach.” Angewandte Chemie International Edition 58, no. 48 (November 25, 2019): 17163–68.

https://doi.org/10.1002/anie.201906726.

The unambiguous characterization of the coordination chemistry of nanocrystal surfaces produced by wetchemical synthesis presently remains highly challenging. Here, zinc oxide nanocrystals (ZnO NCs) coated by monoanionic diphenyl phosphate (DPP) ligands were derived by a sol-gel process and a one-pot self-supporting organometallic (OSSOM) procedure. Atomic-scale characterization through dynamic nuclear polarization (DNP-)enhanced solid-state NMR (ssNMR) spectroscopy has notably enabled resolving their vastly different surface-ligand interfaces. For the OSSOM-derived NCs, DPP moieties form stable and strongly-anchored m2- and m3-bridging-ligand pairs that are resistant to competitive ligand exchange. The sol-gel-derived NCs contain a wide variety of coordination modes of DPP ligands and a ligand exchange process takes place between DPP and glycerol molecules. This highlights the power of DNP-enhanced ssNMR for detailed NC surface analysis and of the OSSOM approach for the preparation of ZnO NCs.

The Detection and Reactivity of Silanols and Silanes Using Hyperpolarized 29Si Nuclear Magnetic Resonance

Duckett, Simon, Peter Rayner, and Peter Richardson. “The Detection and Reactivity of Silanols and Silanes Using Hyperpolarized 29Si Nuclear Magnetic Resonance.” Angewandte Chemie, December 13, 2019, ange.201915098.

https://doi.org/10.1002/ange.201915098

Silanols and silanes are key precursors and intermediates for the synthesis of silicon-based materials. While their characterization and quantification using 29Si NMR has received significant attention, it is a technique that is limited by the low natural abundance of 29Si and its low sensitivity. Here, we describe a method using p-H2 to hyperpolarize 29Si. The observed signal enhancements, approaching 3000-fold at 11.7 T, would take many days of measurement for comparable results under Boltzmann conditions. The resulting signals are exploited to monitor the rapid reaction of tris(tert-butoxy)silanol with triflic anhydride in a T1 corrected process that allows for rapid quantification. These results demonstrate a novel route to quantify dynamic processes and intermediates in the synthesis of silicon materials.

Elucidation of Oxygen Chemisorption Sites on Activated Carbons by 1 H DNP for Insight into Oxygen Reduction Reactions #DNPNMR

Liu, Xiaoyang, Juan Gu, James Wightman, and Harry C. Dorn. “Elucidation of Oxygen Chemisorption Sites on Activated Carbons by 1 H DNP for Insight into Oxygen Reduction Reactions.” ACS Applied Nano Materials, November 25, 2019, acsanm.9b01308.

https://doi.org/10.1021/acsanm.9b01308

Activated carbons (ACs) are widely used in many industrial and medical adsorbent applications because of their distinct ability to adsorb numerous gaseous and/or liquid analytes. More recently, ACs have been actively explored as an inexpensive alternative to metal catalysts (Pt) for numerous oxygen reduction reactions including microbial fuel cells (MFCs) for wastewater treatment. Although it is well established that O2 is chemisorbed on ACs, the actual chemical site has not been elucidated. In this study, we characterize adsorption of benzene on the surface of ACs in the presence and absence of O2. The AC samples have been heat treated and cover the range of 350−600 °C. The flowing benzene is monitored by solid/liquid intermolecular transfer (SLIT) 1H dynamic nuclear polarization (DNP). We find that the introduction of benzene (N2 atmosphere) flowing over an AC interface leads to a scalar (positive) 1H Overhauser effect in high temperature activated carbons (550−600 °C), whereas this nanoscale close-in Fermi interaction is completely suppressed upon introduction of oxygen (air) to the flowing benzene/ activated carbon interface. We propose these results are consistent with a benzene/delocalized singlet−triplet radical carbene or diradical interaction at the zigzag sites edges of disordered graphene motifs. These unique radical sites chemically react with O2 to form quenched diamagnetic sites. In contrast, a solid-state 1H DNP effect is observed at lower heat treatment temperatures representing different radical sites (e.g., aromatic heteroatom radical sites) in ACs.

Improved Structural Elucidation of Synthetic Polymers by Dynamic Nuclear Polarization Solid-State NMR Spectroscopy #DNPNMR

Ouari, Olivier, Trang Phan, Fabio Ziarelli, Gilles Casano, Fabien Aussenac, Pierre Thureau, Didier Gigmes, Paul Tordo, and Stéphane Viel. “Improved Structural Elucidation of Synthetic Polymers by Dynamic Nuclear Polarization Solid-State NMR Spectroscopy.” ACS Macro Letters 2, no. 8 (August 20, 2013): 715–19.

https://doi.org/10.1021/mz4003003

Dynamic nuclear polarization (DNP) is shown to greatly improve the solid-state nuclear magnetic resonance (SSNMR) analysis of synthetic polymers by allowing structural assignment of intrinsically diluted NMR signals, which are typically not detected in conventional SSNMR. Specifically, SSNMR and DNP SSNMR were comparatively used to study functional polymers for which precise structural elucidation of chain ends is essential to control their reactivity and to eventually obtain advanced polymeric materials of complex architecture. Results show that the polymer chain-end signals, while hardly observable in conventional SSNMR, could be clearly identified in the DNP SSNMR spectrum owing to the increase in sensitivity afforded by the DNP setup (a factor ∼10 was achieved here), hence providing access to detailed structural characterization within realistic experimental times. This sizable gain in sensitivity opens new avenues for the characterization of “smart” functional polymeric materials and new analytical perspectives in polymer science.

Solid-state NMR of nanocrystals #DNPNMR

Gutmann, Torsten, Pedro B. Groszewicz, and Gerd Buntkowsky. “Solid-State NMR of Nanocrystals.” In Annual Reports on NMR Spectroscopy, 97:1–82. Elsevier, 2019.

https://doi.org/10.1016/bs.arnmr.2018.12.001

Recent advances in solid-state nuclear magnetic resonance (NMR) spectroscopy and dynamic nuclear polarization (DNP) of nanostructured materials are reviewed. A first group of materials is based on crystalline nanocellulose (CNC) or microcrystalline cellulose (MCC), which are used as carrier materials for dye molecules, catalysts or in combination with heterocyclic molecules as ion conducting membranes. These materials have widespread applications in sensorics, optics, catalysis or fuel cell research. A second group are metal oxides such as V-Mo-W oxides, which are of enormous importance in the manufacturing process of basic chemicals. The third group are catalytically active nanocrystalline metal nanoparticles, coated with protectants or embedded in polymers. The last group includes of lead-free perovskite materials, which are employed as environmentally benign substitution materials for conventional lead-based electronics materials. These materials are discussed in terms of their application and physicochemical characterization by solid-state NMR techniques, combined with gas-phase NMR and quantum-chemical modelling on the density functional theory (DFT) level. The application of multinuclear 1H, 2H, 13C, 15N and 23Na solid state NMR techniques under static or MAS conditions for the characterization of these materials, their surfaces and processes on their surfaces is discussed. Moreover, the analytic power of the combination of these techniques with DNP for the identification of low-concentrated carbon and nitrogen containing surface species in natural abundance is reviewed. Finally, approaches for sensitivity enhancement by DNP of quadrupolar nuclei such as 17O and 51V are presented that enable the identification of catalytic sites in metal oxide catalysts

Carbon-13 dynamic nuclear polarization in diamond via a microwave-free integrated cross effect #DNPNMR

Henshaw, Jacob, Daniela Pagliero, Pablo R. Zangara, María B. Franzoni, Ashok Ajoy, Rodolfo H. Acosta, Jeffrey A. Reimer, Alexander Pines, and Carlos A. Meriles. “Carbon-13 Dynamic Nuclear Polarization in Diamond via a Microwave-Free Integrated Cross Effect.” Proceedings of the National Academy of Sciences 116, no. 37 (September 10, 2019): 18334–40.

https://doi.org/10.1073/pnas.1908780116

Color-center–hosting semiconductors are emerging as promising source materials for low-field dynamic nuclear polarization (DNP) at or near room temperature, but hyperfine broadening, susceptibility to magnetic field heterogeneity, and nuclear spin relaxation induced by other paramagnetic defects set practical constraints difficult to circumvent. Here, we explore an alternate route to color-center–assisted DNP using nitrogen-vacancy (NV) centers in diamond coupled to substitutional nitrogen impurities, the so-called P1 centers. Working near the level anticrossing condition—where the P1 Zeeman splitting matches one of the NV spin transitions—we demonstrate efficient microwave-free 13C DNP through the use of consecutive magnetic field sweeps and continuous optical excitation. The amplitude and sign of the polarization can be controlled by adjusting the low-to-high and high-to-low magnetic field sweep rates in each cycle so that one is much faster than the other. By comparing the 13C DNP response for different crystal orientations, we show that the process is robust to magnetic field/NV misalignment, a feature that makes the present technique suitable to diamond powders and settings where the field is heterogeneous. Applications to shallow NVs could capitalize on the greater physical proximity between surface paramagnetic defects and outer nuclei to efficiently polarize target samples in contact with the diamond crystal.

Leroy, César, Fabien Aussenac, Laure Bonhomme-Coury, Akiyoshi Osaka, Satoshi Hayakawa, Florence Babonneau, Cristina Coelho-Diogo, and Christian Bonhomme. “Hydroxyapatites: Key Structural Questions and Answers from Dynamic Nuclear Polarization.” Analytical Chemistry 89, no. 19 (October 3, 2017): 10201–7.

https://doi.org/10.1021/acs.analchem.7b01332.

We demonstrate that NMR/DNP (Dynamic Nuclear Polarization) allows an unprecedented description of carbonate substituted hydroxyapatite (CHAp). Key structural questions related to order/disorder and clustering of carbonates are tackled by using distance sensitive DNP experiments using 13C-13C recoupling. Such experiments are easily implemented due to unprecedented DNP gain (orders of magnitude). DNP is efficiently mediated by quasi one-dimensional spin diffusion through the hydroxyl columns present in the CHAp structure (thought as \”highways\” for spin diffusion). For spherical nanoparticles and ∅ < 100 nm, it is numerically shown that spin diffusion allows their study as a whole. Most importantly, we demonstrate also that the DNP study at 100 K leads to data which are comparable to data obtained at room temperature (in terms of spin dynamics and lineshape resolution). Finally, all 2D DNP experiments can be interpreted in terms of domains exhibiting well identified types of substitution: local order and carbonate clustering are clearly favored.

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