Enhanced Solid-State NMR Correlation Spectroscopy of Quadrupolar Nuclei Using Dynamic Nuclear Polarization

By means of a true sensitivity enhancement for a solid-state NMR spectroscopy (SSNMR) experiment performed under dynamic nuclear polarization (DNP) conditions, corresponding to 4–5 orders of magnitude of time savings compared with a conventional SSNMR experiment, it is shown that it is possible to record interface-selective ²⁷Al–²⁷Al two-dimensional dipolar correlation spectra on mesoporous alumina, an advanced material with potential industrial applications. The low efficiency of cross-polarization and dipolar recoupling for quadrupolar nuclei is completely negated using this technique. The important presence of pentacoordinated Al has not only been observed, but its role in bridging interfacial tetra- and hexacoordinated Al has been determined. Such structural information, collected at low temperature (∼103 K) and 9.4 T with the use of DNP, would have been impossible to obtain under standard conditions, even using a higher magnetic field. However, here it is demonstrated that this information can be obtained in only 4 h. This work clearly opens a new avenue for the application of SSNMR to quadrupolar nuclei and notably the atomic-scale structure determination of catalysis materials such as mesoporous alumina

Untangling the Condensation Network of Organosiloxanes on Nanoparticles using 2D ²⁹Si–²⁹Si Solid-State NMR Enhanced by Dynamic Nuclear Polarization

Silica (SiO₂) nanoparticles (NPs) were functionalized by silanization to produce a surface covered with organosiloxanes. Information about the surface coverage and the nature, if any, of organosiloxane polymerization, whether parallel or perpendicular to the surface, is highly desired. To this extent, two-dimensional homonuclear ²⁹Si solid-state NMR could be employed. However, owing to the sensitivity limitations associated with the low natural abundance (4.7%) of ²⁹Si and the difficulty and expense of isotopic labeling here, this technique would usually be deemed impracticable. Nevertheless, we show that recent developments in the field of dynamic nuclear polarization under magic angle spinning (MAS-DNP) could be used to dramatically increase the sensitivity of the NMR experiments, resulting in a timesaving factor of ∼625 compared to conventional solid-state NMR. This allowed the acquisition of previously infeasible data. Using both through-space and through-bond 2D ²⁹Si–²⁹Si correlation experiments, it is shown that the required reaction conditions favor lateral polymerization and domain growth. Moreover, the natural abundance correlation experiments permitted the estimation of ²<i>J</i>^Si–O–Si-couplings (13.8 ± 1.4 Hz for surface silica) and interatomic distances (3.04 ± 0.08 Å for surface silica) since complications associated with many-spin systems and also sensitivity were avoided. The work detailed herein not only demonstrates the possibility of using MAS-DNP to greatly facilitate the acquisition of 2D ²⁹Si–²⁹Si correlation spectra but also shows that this technique can be used in a routine fashion to characterize surface grafting networks and gain structural constraints, which can be related to a system’s chemical and physical properties

Li-Rich Mn/Ni Layered Oxide as Electrode Material for Lithium Batteries: A ⁷Li MAS NMR Study Revealing Segregation into (Nanoscale) Domains with Highly Different Electrochemical Behaviors

We present a ⁷Li MAS NMR study carried out before (pristine material) and during the first cycle of charge/discharge of Li­[Li_0.2Mn_0.61Ni_0.18Mg_0.01]­O₂ layered oxide, a promising active material for positive electrode in Li-ion batteries. For the pristine material, at least five NMR signals were observed. To analyze these results, we developed an 18 cation local model (first and second spheres) aiming at identifying very precise cationic (Li⁺, Mn⁴⁺/Ni²⁺) configurations compatible with all our NMR data while satisfying local electroneutrality constraints (the key ingredient of our approach). Our results strongly suggest that the material presents two types of coexisting nanoscale domains. The first type is highly ordered and consists of pure Li₂MnO₃ cores (volume ∼58%), while the second more disordered type concentrates most of the Ni and is labeled LiMO₂-like (volume ∼20%) where M = Mn_1/2Ni_1/2. Finally, at the interphase of these two Ni-free and Ni-rich domains, there are slightly Ni-contaminated Li₂MnO₃-like regions, most probably surrounding the Li₂MnO₃ domains and thus labeled “Ni-poor boundaries” (volume ∼21%). This partition is confirmed by the behavior of the NMR signals during the first electrochemical cycle. At the initial state of charge (≤4.3 V), Li-ion extraction occurs mainly from the (Ni-rich) Li_1–<i>x</i>MO₂-like domains via Ni²⁺ oxidation. At higher states of charge (≥4.5 V), the Li₂MnO₃-like domains become highly involved via oxygen-based (ir)­reversible oxidation processes, leading to significant structural transformations. During discharge, only ∼60% of the initial lithium is reinserted into the structure. The (Ni-rich) LiMO₂-like domains are fully refilled (via reversible Ni⁴⁺ reduction into Ni²⁺), while the ordered Li₂MnO₃-like domains experience a significant size decrease after the first cycle of charge/discharge. The originality of the present approach consists of analyzing NMR data with a new model that includes at its heart local electroneutrality constraints. This model allowed us to shed light on the processes occurring in the Li-rich Mn/Ni layered oxide compound during the first electrochemical cycle on the microscopic level

Quenching Dynamics in CdSe Nanoparticles: Surface-Induced Defects upon Dilution.

We have analyzed the decays of the fluorescence of colloidal CdSe quantum dots (QDs) suspensions during dilution and titration by the ligands. A ligand shell made of a combination of trioctylphosphine (TOP), oleylamine (OA), and stearic acid (SA) stabilizes the as-synthesized QDs. The composition of the shell was analyzed and quantified using high resolution liquid state 1H nuclear magnetic resonance (NMR) spectroscopy. A quenching of the fluorescence of the QDs is observed upon removal of the ligands by diluting the stock solution of the QDs. The fluorescence is restored by the addition of TOP. We analyze the results by assuming a binomial distribution of quenchers among the QDs and predict a linear trend in the time-resolved fluorescence decays. We have used a nonparametric analysis to show that for our QDs, 3.0 ± 0.1 quenching sites per QD on average are revealed by the removal of TOP. We moreover show that the quenching rates of the quenching sites add up. The decay per quenching site can be compared with the decay at saturation of the dilution effect. This provides a value of 2.88 ± 0.02 for the number of quenchers per QD. We extract the quenching dynamics of one site. It appears to be a process with a distribution of rates that does not involve the ligands

New Solid Electrolyte Na₉Al(MoO₄)₆: Structure and Na⁺ Ion Conductivity

Solid electrolytes are important materials with a wide range of technological applications. This work reports the crystal structure and electrical properties of a new solid electrolyte Na₉Al­(MoO₄)₆. The monoclinic Na₉Al­(MoO₄)₆ consists of isolated polyhedral [Al­(MoO₄)₆]^9– clusters composed of a central AlO₆ octahedron sharing vertices with six MoO₄ tetrahedra to form a three-dimensional framework. The AlO₆ octahedron also shares edges with one Na1O₆ octahedron and two Na2O₆ octahedra. Na3–Na5 atoms are located in the framework cavities. The structure is related to that of sodium ion conductor II-Na₃Fe₂(AsO₄)₃. High-temperature conductivity measurements revealed that the conductivity (σ) of Na₉Al­(MoO₄)₆ at 803 K equals 1.63 × 10^–2 S cm^–1. The temperature behavior of the ²³Na and ²⁷Al nuclear magnetic resonance spectra and the spin-lattice relaxation rates of the ²³Na nuclei indicate the presence of fast Na⁺ ion diffusion in the studied compound. At <i>T</i><490 K, diffusion occurs by means of Na⁺ ion jumps exclusively through the sublattice of Na3–Na5 positions, whereas Na1 and Na2 become involved in the diffusion processes (through chemical exchange with the Na3–Na5 sublattice) only at higher temperatures