Endogenous ¹⁷O Dynamic Nuclear Polarization of Gd-Doped CeO₂ from 100 to 370 K

Abstract

17O NMR is an invaluable tool to study the structure and dynamics of oxide materials but remains challenging to apply in many systems. Even with isotopic enrichment, studies of samples with low masses and/or concentrations of the active species, such as thin films or interfaces, are limited by low sensitivity. Here, we show how endogenous dynamic nuclear polarization (DNP) can dramatically improve the sensitivity in the oxide-ion conductor Gd-doped CeO2, with a 17O enhancement factor of 652 at 100 K. This is the highest enhancement observed so far by endogenous DNP or Gd3+ DNP, which is explained in terms of the electron paramagnetic resonance characteristics. The DNP properties are studied as a function of Gd concentration for both enriched and natural-abundance samples, and the buildup behavior shows that spin diffusion in 17O-enriched samples improves sensitivity by relaying hyperpolarization throughout the sample. Notably, efficient hyperpolarization could still be achieved at elevated temperatures, with enhancement factors of 320 at room temperature and 150 at 370 K, paving the way for the characterization of materials under operational conditions. Finally, the application of endogenous Gd3+ DNP is illustrated with the study of interfaces in vertically aligned nanocomposite thin films composed of Gd-CeO2 nanopillars embedded in a SrTiO3 matrix, where DNP affords selective enhancement of the different phases and enables a previously infeasible two-dimensional correlation experiment to be performed, showing spin diffusion between Gd-CeO2 and the solid–solid interface.

Design Principles for the Development of Gd(III) Polarizing Agents for Magic Angle Spinning Dynamic Nuclear Polarization

“…30 Endogenous Gd(III) dopants have been used to hyperpolarize nuclei in inorganic solids such as metal oxides, 31,32 glasses, 33 and battery electrode materials; 34−36 for example, recently, Gd(III) has been shown to provide large 17 O solid effect DNP enhancements of 652 at 100 K and 320 at room temperature in 17 enriched ceria. 32 These high enhancements were attributed to the cubic environment of the metal in the ceria lattice and to the relatively small magnitude of the higher-order electrostatic multipoles, that in turn lead to relatively narrow EPR linewidths which can be easily saturated under DNP conditions. On the other hand, Griffin and co-workers demonstrated that the molecular Gd complexes [Gd(dota) (H 2 O)] − and [Gd(dtpa) (H 2 O)] 2− , hereafter abbreviated as Gd(dota) and Gd(dtpa), respectively (Figure 1A), could be used as exogenous polarizing agents in MAS DNP, 27 where Gd(dota) exhibited a relatively good solid effect 1 H DNP enhancement.…”

“…Gd­(III) complexes also possess favorable EPR properties such as a quenched spin–orbit coupling and weak hyperfine couplings with 155/157 Gd, in comparison to other paramagnetic metal ions . Endogenous Gd­(III) dopants have been used to hyperpolarize nuclei in inorganic solids such as metal oxides, , glasses, and battery electrode materials; − for example, recently, Gd­(III) has been shown to provide large 17 O solid effect DNP enhancements of 652 at 100 K and 320 at room temperature in 17 O-enriched ceria . These high enhancements were attributed to the cubic environment of the metal in the ceria lattice and to the relatively small magnitude of the higher-order electrostatic multipoles, that in turn lead to relatively narrow EPR linewidths which can be easily saturated under DNP conditions.…”

Supercritical CO2 impregnation process applied to polymer samples preparation for dynamic nuclear polarization solid‐state NMR

“…[1][2][3][4][5][6][7][8][9][10][11] DNP enhances nuclear magnetization through the microwave (MW)-driven transfer at cryogenic temperatures of electron spin polarization to nuclei via paramagnetic centers, which are typically referred to as polarizing agents (PAs). In principle, whereas PAs can either be contained in (endogenous) or added to (exogenous) the material under investigation, the best DNP sensitivity enhancements obtained so far have been observed when using exogenous PAs, although there are some notable exceptions, [12][13][14][15] including recent work where endogenous radicals were generated in the sample by γ-ray irradiation. [16,17] As a matter of fact, the design of efficient PAs has been the subject of extensive research over the last few years, which has led to the synthesis of a wide range of biradicals exhibiting increasingly complex molecular structures (e.g., TOTAPOL, [18] bTbK, [19] TEKPol, [20] AMUPol, [21] HyTEK2, [22] cAsymPol-POK, [23] and nitroxyl-trityl biradicals [24] ).…”

Off-the-Shelf Gd(NO3)3 as an Efficient High-Spin Metal Ion Polarizing Agent for Magic Angle Spinning Dynamic Nuclear Polarization

“…Gd 3+ is of particular interest, as it offers large 1 H NMR signal enhancements ϵ of ∼19 for [Gd­(dota)­(H 2 O)] − and ∼37 for [Gd­(tpcatcn)] complexes at 9.4 T . High-spin metal ions also allow for endogenous hyperpolarization where intrinsic ions are contained in the samples as dopants (in inorganic solids) such as Mn 2+ -doped Li 4 Ti 5 O 12 or (in oxide glasses) such as Gd 3+ -doped lithium silicate, lithium borate, and zinc phosphate for bulk signal enhancements, in Gd 3+ -doped CeO 2 nanocomposite thin films on SrTiO 3 for detailed study of material interfaces, and in spin-labels as chelator tags …”

“…More recently, inspired by paramagnetic transition metal and lanthanide complexes used for in vivo magnetic resonance imaging (MRI) applications, 38,39 high-spin metal ions have shown potential 40−44 as alternatives to tailored nitroxide-based biradical PAs for specific applications, 1 H DNP, or the direct DNP of lower gamma nuclear spins. Gd 3+ is of particular interest, as it offers large 1 H NMR signal enhancements ϵ of ∼19 for [Gd(dota)(H 2 O)] − and ∼37 for [Gd(tpcatcn)] complexes at 9.4 T. 43 High-spin metal ions also allow for endogenous hyperpolarization where intrinsic ions are contained in the samples as dopants (in inorganic solids) such as Mn 2+ -doped Li 4 Ti 5 O 12 45 or (in oxide glasses) such as Gd 3+ -doped lithium silicate, lithium borate, and zinc phosphate 46 for bulk signal enhancements, in Gd 3+ -doped CeO 2 nanocomposite thin films on SrTiO 3 for detailed study of material interfaces, 47 and in spin-labels as chelator tags. 41 An exciting opportunity for MAS DNP consists of utilizing an "off-the-shelf" paramagnetic metal ion species as a PA that is easily affordable and does not require any chemical synthesis.…”