Wednesday, March 04, 2009

J Phys Chem C, vol 113, Issues 8 and 9

Structure−Property Relations in Mixed-Network Glasses: Multinuclear Solid State NMR Investigations of the System xAl2O3:(30 − x)P2O5:70SiO2

Bruce G. Aitken* and Randall E. Youngman, Rashmi R. Deshpande and Hellmut Eckert*

J. Phys. Chem. C, 2009, 113 (8), pp 3322–3331
DOI: 10.1021/jp809208m
Abstract: New mixed-network glasses along the composition line xAl2O3−(30 − x)P2O5−70SiO2 have been prepared and characterized in terms of their density, thermal expansion coefficient, refractive index, and characteristic temperatures. The compositional changes in these macroscopic properties have been correlated with structural information, obtained via Raman spectroscopy and state-of-the-art solid state NMR techniques, including 27Al, 29Si, and 31P magic-angle spinning (MAS) NMR, 27Al triple quantum MAS NMR, as well as static 31P spin echo decay spectroscopy. In addition, the extent of P−O−Al connectivity has been quantified on the basis of 27Al{31P} rotational echo double resonance (REDOR) and 31P{27Al} rotational echo adiabatic passage double resonance (REAPDOR) measurements. Both the macroscopic and the structural properties show nonlinear dependences on x, including abrupt changes at a nominal Al/P ratio of 1 (x = 0.15), where no glasses can be formed by melt quenching under the conditions used in this study. The structure of phosphorus-rich glasses (Al/P <> 1), where the alumina component is involved in Al−O−P, Al−O−Si, and possibly also Al−O−Al linkages. All of these results indicate that the structure of these glasses is dominated by the strong mutual affinity of the phosphorus oxide and alumina components. To quantify this affinity, the experimental REDOR and REAPDOR results have been compared with a cluster model assuming that both components react completely under formation of aluminum phosphate-like domains, thereby maximizing the number of Al−O−P linkages. Both the REDOR and the REAPDOR results show, however, clear deviations from such a structural scenario, supporting a more homogeneous glass structure with a certain degree of connectivity randomization.

Preferential Adsorption of Lower-Charge Glutamate Ions on Layered Double Hydroxides: An NMR Investigation

Marc X. Reinholdt*§, Panakkattu K. Babu and R. James Kirkpatrick
J. Phys. Chem. C, 2009, 113 (9), pp 3378–3381

Abstract: 13C MAS NMR spectroscopy of isotopically enriched samples of the layered double hydroxide hydrotalcite (HT) [(Mg2Al)(OH)6A−,nH2O, where A− is a counteranion that may bear different charges] exchanged with glutamate (Glu) shows an unexpected preferential adsorption of the lower-charged species (Glu1−) relative to the higher-charged species (Glu2−) by a layered double hydroxide (LDH) compound. At pH 11.0, the Glu1−/Glu2− ratio is about 0.44, an order of magnitude greater than expected in solution. Previous studies of phosphate and carbonate exchange onto LDH compounds (refs 25 and 26) show a strong preference for the higher-charged anion. The preference for Glu1−, in which the amine site is protonated, may be due in part to −NH3+ allowing for an energetically more favorable H-bonding network among the anions, the metal hydroxide substrate, and the interlayer and surface water molecules compared to −NH2. Changes in the pH and the pKa of Glu near the HT surface and due to nanoconfinement may also play important roles. These results suggest that the interactions dominating the exchange of amino acids and proteins onto LDH compounds may be quite different from those that control the exchange of small inorganic anions.

Homogeneously-Alloyed CdTeSe Single-Sized Nanocrystals with Bandgap Photoluminescence
Ruibing Wang
, Olivier Calvignanello, Christopher I. Ratcliffe, Xiaohua Wu, Donald M. Leek, Md. Badruz Zaman, David Kingston§, John A. Ripmeester and Kui Yu*

J. Phys. Chem. C, 2009, 113 (9), pp 3402–3408

Abstract: Homogeneously alloyed ternary CdTeSe magic-sized nanocrystals (MSNs) with bandgap emission were synthesized in 1-octadecene (ODE) via a noninjection one-pot approach featuring synthetic reproducibility and large-scale capability. The noninjection approach used cadmium acetate dihydrate (Cd(OAc)2·2H2O), elemental selenium, and elemental tellurium as Cd, Se, and Te sources, respectively. The growth of the CdTeSe nanocrystals was carried out at temperatures from 120 to 200 °C for several hours in a reaction flask containing the reactants together with a long-chain fatty acid as capping ligands and ODE as the reaction medium. During synthesis, the CdTeSe nanocrystals exhibited persistent bandgap and did not grow in size anymore after their formation, either with longer growth periods or higher reaction temperature. Also, they exhibit an absorption doublet with 288 meV energy difference between the two peaks; the first excitonic transition peak is at 520 nm and bandgap photoemission peak at 524 nm with full width at half-maximum (fwhm) of ca. 20 nm. Both the growth pattern and the optical properties suggest that they are magic-sized and thus termed as single-sized. Nuclear magnetic resonance (NMR), with sensitivity to local environment, provided valuable information regarding the structure and composition of the nanocrystals. Solid-state 13C cross polarization/magic angle spinning (CP/MAS) NMR spectra showed that the carboxylate capping ligand is firmly attached to the crystal, and 1H decoupled 113Cd MAS spectra, with and without CP, distinguished between the surface Cd species and the Cd inside a nanocrystal. The 113Cd NMR results also confirmed, unambiguously, that the nanocrystals are homogeneously alloyed ternary CdTeSe, with 113Cd resonances located between those of CdTe and CdSe nanocrystals indicating a stoichiometry of approximately 1Se:1Te ratio throughout the whole nanocrystal. XRD supported that they are ternary-alloyed CdTeSe rather than binary CdTe or CdSe, with a wurtzite crystal structure. In addition, both energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) supported an approximate stoichiometric ratio of 1Se:1Te of the CdTeSe nanocrystals.

27Al NMR Chemical Shifts in Oxide Crystals: A First-Principles Study
Minseok Choi
*, Katsuyuki Matsunaga, Fumiyasu Oba and Isao Tanaka*

J. Phys. Chem. C, 2009, 113 (9), pp 3869–3873
DOI: 10.1021/jp810484j
Abstract: In the framework of density functional theory using periodic boundary conditions, 27Al nuclear magnetic resonance (NMR) parameters of 20 aluminum sites in 12 aluminum-containing crystalline oxides, i.e., Al2O3, LiAlO2, SiAl2O5, MgAl2O4, YAlO3, AlVO4, and their polymorphs, are investigated. The present method excellently reproduces the 27Al NMR parameters and the well-known empirical correlation of the isotropic chemical shifts, δiso, of 27Al nuclei with its number of coordinating oxygen atoms, i.e., coordination number. Through systematic calculations, we demonstrate that the mean Al−O bond order, QAl−O, can be a better parameter to correlate with 27Al δiso than merely averaged bond length or the coordination number. The relationship between δiso and QAl−O is also found to be valid for α-Al2O3 under hydrostatic pressures in which the coordination number is unchanged from six.

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