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

Abstract

In recent years magic angle spinning-dynamic nuclear polarization (MAS-DNP) has 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 0.038%. 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.

Recent developments in MAS DNP-NMR of materials

“…In hot-melt extrusion, the solid constituents are mechanically mixed at elevated temperatures. Paramagnetic metal ions have been incorporated into the inorganic solids using co-crystallization [71] or via solid-state reactions [72,73].…”

“…DNP has also been applied to enhance the NMR signals of the core region of inorganic microparticles. Such enhancement is possible, when the core of the microparticles contains PAs [71][72][73]. DNP has particularly been used to detect the distinct 17 O sites of crystalline inorganic microparticles doped with Mn(II) ions, including battery anode materials Li4Ti5O12 and Li2ZnTi3O8, as well as the materials NaCaPO4 and MgAl2O4 (see Figure 25) [73].…”

Probing the Surface Structure of Semiconductor Nanoparticles by DNP SENS with Dielectric Support Materials

“…The theoretical maximum DNP signal enhancement (ε) is proportional to the gyromagnetic ratio of the electrons and a given nucleus (maximum εH of 658 for protons). 59,61 DNP solid-state NMR has previously been applied to a variety of systems including biomolecules, [62][63] pharmaceuticals, 64 polymers, 65 heterogeneous catalysts, [66][67][68] bulk inorganic materials, [69][70][71] battery materials, 72 and nanoparticles. 51-52, 55, 66, 73-80 DNP surface enhanced NMR spectroscopy (SENS) has been demonstrated as a general method to enhance NMR signals from interfaces/surfaces of inorganic materials and heterogeneous catalysts.…”

Blind spheres of paramagnetic dopants in solid state NMR

“…If a spherical regime around a paramagnetic centre is assumed, from which no NMR signals can be detected, it is possible to relate the signal loss to the size of this sphere of influence which is known under the name blind-sphere 38,39 or wipe-out radius. [40][41][42] Such radii are important in solution NMR 38,39 where paramagnetic constraints are used for structure solution, and in dynamic nuclear polarization (DNP) to estimate the zone which cannot be accessed by NMR [43][44][45] and for luminescent materials to study doping homogeneity. 21 Spectroscopically the blind sphere can be explained by line broadening which leads to undetectable signal by standard experiments, 39 or signal shift 46 that puts the signal outside of spectral window.…”