Local structures of polar wurtzites Zn_{1-x}Mg_{x}O studied by Raman and {67}Zn/{25}Mg NMR spectroscopies and by total neutron scattering

Local compositions and structures of Zn_{1-x}Mg_{x}O alloys have been investigated by Raman and solid-state {67}Zn/{25}Mg nuclear magnetic resonance (NMR) spectroscopies, and by neutron pair-distribution-function (PDF) analyses. The E2(low) and E2(high) Raman modes of Zn_{1-x}Mg_{x}O display Gaussian- and Lorentzian-type profiles, respectively. At higher Mg substitutions, both modes become broader, while their peak positions shift in opposite directions. The evolution of Raman spectra from Zn_{1-x}Mg_{x}O solid solutions are discussed in terms of lattice deformation associated with the distinct coordination preferences of Zn and Mg. Solid-state magic-angle-spinning (MAS) NMR studies suggest that the local electronic environments of {67}Zn in ZnO are only weakly modified by the 15% substitution of Mg for Zn. {25}Mg MAS spectra of Zn_{0.85}Mg_{0.15}O show an unusual upfield shift, demonstrating the prominent shielding ability of Zn in the nearby oxidic coordination sphere. Neutron PDF analyses of Zn_{0.875}Mg_{0.125}O using a 2x2x1 supercell corresponding to Zn_{7}MgO_{8} suggest that the mean local geometry of MgO_{4} fragments concurs with previous density functional theory (DFT)-based structural relaxations of hexagonal wurtzite MgO. MgO_{4} tetrahedra are markedly compressed along their c-axes and are smaller in volume than ZnO_{4} units by ~6%. Mg atoms in Zn_{1-x}Mg_{x}O have a shorter bond to the $c$-axial oxygen atom than to the three lateral oxygen atoms, which is distinct from the coordination of Zn. The precise structure, both local and average, of Zn_{0.875}Mg_{0.125}O obtained from time-of-flight total neutron scattering supports the view that Mg-substitution in ZnO results in increased total spontaneous polarization.Comment: 12 pages, 14 figures, 2 table

Molecular Insights into Carbon Dioxide Sorption in Hydrazone-Based Covalent Organic Frameworks with Tertiary Amine Moieties

Tailorable sorption properties at the molecular level are key for efficient carbon capture and storage and a hallmark of covalent organic frameworks (COFs). Although amine functional groups are known to facilitate CO2 uptake, atomistic insights into CO2 sorption by COFs modified with amine-bearing functional groups are scarce. Herein, we present a detailed study of the interactions of carbon dioxide and water with two isostructural hydrazone-linked COFs with different polarities based on the 2,5-diethoxyterephthalohydrazide linker. Varying amounts of tertiary amines were introduced in the COF backbones by means of a copolymerization approach using 2,5-bis(2-(dimethylamino)ethoxy)terephthalohydrazide in different amounts ranging from 25 to 100% substitution of the original DETH linker. The interactions of the frameworks with CO2 and H2O were comprehensively studied by means of sorption analysis, solid-state NMR spectroscopy, and quantum-chemical calculations. We show that the addition of the tertiary amine linker increases the overall CO2 sorption capacity normalized by the surface area and of the heat of adsorption, whereas surface areas and pore size diameters decrease. The formation of ammonium bicarbonate species in the COF pores is shown to occur, revealing the contributing role of water for CO2 uptake by amine-modified porous frameworks

Selective NMR measurements of homonuclear scalar couplings in isotopically enriched solids

Scalar (J) couplings in solid-state NMR spectroscopy are sensitive to covalent through-bond interactions that make them informative structural probes for a wide range of complex materials. Until now, however, they have been generally unsuitable for use in isotopically enriched solids, such as proteins or many inorganic solids, because of the complications presented by multiple coupled but nonisolated spins. Such difficulties are overcome by incorporating a z-filter that results in a robust method for measuring pure J-coupling modulations between selected pairs of nuclei in an isotopically enriched spin system. The reliability of the new experimental approach is established by using numerical simulations and tested on fully (13)C-labeled polycrystalline L-alanine. It is furthermore shown to be applicable to partially enriched systems, when used in combination with a selective double-quantum (DQ) filter, as demonstrated for the measurement of (2)J((29)Si-O-(29)Si) couplings in a 50% (29)Si-enriched surfactant-templated layered silicate lacking long-range 3D crystallinity. J-coupling constants are obtained with sufficient accuracy to distinguish between different (29)Si-O-(29)Si pairs, shedding insight on the local structure of the silicate framework. The new experiment is appropriate for fully or partially enriched liquid or solid samples

Nanoscale surface compositions and structures influence boron adsorption properties of anion exchange resins

International audienceBoron adsorption properties of poly(styrene-co-divinylbenzene) (PSDVB)-based anion-exchange resins with surface-grafted N-methyl-d-glucamine (NMDG) depend strongly on their local surface compositions, structures, and interfacial interactions. Distinct boron adsorption sites have been identified and quantified, and interactions between borate anions and hydroxyl groups of NMDG surface moieties have been established. A combination of X-ray photoelectron spectroscopy (XPS), solid-state nuclear magnetic resonance (NMR), and Fourier-transform infrared (FT-IR) spectroscopy were used to characterize the atomic-level compositions and structures that directly influence the adsorption of borate anions on the NMDG-functionalized resin surface. Surface-enhanced dynamic-nuclear-polarization (DNP)-NMR enabled dilute (3 atom % N) tertiary alkyl amines and quaternary ammonium ions of the NMDG groups to be detected and distinguished with unprecedented sensitivity and resolution at natural abundance 15N (0.4%). Two-dimensional (2D) solid-state 11B{1H}, 13C{1H}, and 11B{11B} NMR analyses provide direct atomic-scale evidence for interactions of borate anions with the NMDG moieties on the resin surfaces, which form stable mono- and bischelate complexes. FT-IR spectra reveal displacements in the stretching vibrational frequencies associated with the O-H and N-H bonds of NMDG groups that corroborate the formation of chelate complexes on the resin surfaces. The atomic-level compositions and structures are related to boron adsorption properties of resin materials synthesized under different conditions, which have important remediation applications

Scaling analyses for hyperpolarization transfer across a spin-diffusion barrier and into bulk solid media

By analogy to heat and mass transfer film theory, a general approach is introduced for determining hyperpolarization transfer rates between dilute electron spins and a surrounding nuclear ensemble. These analyses provide new quantitative relationships for understanding, predicting, and optimizing the effectiveness of hyperpolarization protocols, such as Dynamic Nuclear Polarization (DNP) under magic-angle spinning conditions. An empirical DNP polarization-transfer coefficient is measured as a function of the bulk matrix H-1 spin density and indicates the presence of two distinct kinetic regimes associated with different rate-limiting polarization transfer phenomena. Dimensional property relationships are derived and used to evaluate the competitive rates of spin polarization generation, propagation, and dissipation that govern hyperpolarization transfer between large coupled spin ensembles. The quantitative analyses agree closely with experimental measurements for the accumulation, propagation, and dissipation of hyperpolarization in solids and provide evidence for kinetically-limited transfer associated with a spin-diffusion barrier. The results and classical approach yield general design criteria for analyzing and optimizing polarization transfer processes involving complex interfaces and composite media for applications in materials science, physical chemistry and nuclear spintronics

Meso/macroporous inorganic oxide monoliths from polymer foams

6 pages, 5 figures.-- Printed version published Apr 2003.Low-density bimodal inorganic oxide monoliths (see Figure) have been prepared via a two-step procedure using macroporous polystyrene foams and amphiphilic block copolymer structure-directing agents. The resulting meso/macroporous inorganic framework structures could be obtained with different 3D bulk shapes and for a range of inorganic oxides, as demonstrated here for silica, zirconia, and titania.This work was supported by the US National Science Foundation through grant CTS-9871970 and a joint research project of the US--Spain Science and Technology Program 2000. Travel assistance was provided by the US NSF Division of International Programs through grant INT-9726744. We thank BASF for the donation of the F127 and P123 triblock copolymer species.Peer reviewe