Ulrike Werner-Zwanziger

A silica sol–gel design strategy for nanostructured metallic materials

Batteries, fuel cells and solar cells, among many other high-current-density devices, could benefit from the precise meso- to macroscopic structure control afforded by the silica sol-gel process. The porous materials made by silica sol-gel chemistry are typically insulators, however, which has restricted their application. Here we present a simple, yet highly versatile silica sol-gel process built around a multifunctional sol-gel precursor that is derived from the following: amino acids, hydroxy acids or peptides; a silicon alkoxide; and a metal acetate. This approach allows a wide range of biological functionalities and metals--including noble metals--to be combined into a library of sol-gel materials with a high degree of control over composition and structure. We demonstrate that the sol-gel process based on these precursors is compatible with block-copolymer self-assembly, colloidal crystal templating and the Stöber process. As a result of the exceptionally high metal content, these materials can be thermally processed to make porous nanocomposites with metallic percolation networks that have an electrical conductivity of over 1,000 S cm(-1). This improves the electrical conductivity of porous silica sol-gel nanocomposites by three orders of magnitude over existing approaches, opening applications to high-current-density devices.

Solid polymer electrolytes which contain tricoordinate boron for enhanced conductivity and transference numbers

Here we report syntheses and study of composite solid polymer electrolytes (SPEs) based on a poly(ethylene glycol)-in-Li triflate material that contains an organic-inorganic composite (OIC) in which boron species are incorporated into a silica network. The structure and properties of the SPEs synthesized were characterized by scanning transmission electron microscopy (STEM), 29Si, 11B and 13C solid state NMR, differential scanning calorimetry, and impedance spectroscopy. STEM allowed assessment of OIC particles in their native environment without removal of an organic component. The Lewis acid tricoordinate boron sites formed in OIC are proposed to have a stronger interaction with triflate anions than silica sites, which results in enhanced lithium ion conductivity and Li transference numbers at optimal boron concentrations. The optimum triethyl borate (TEB) concentration also leads to formation of smaller (higher surface area) OIC particles, which expose more boron sites to triflate anions. The SPE sample prepared with 10 mol% TEB exhibited a conductivity of 4.3 × 10−5 S cm−1 and a Li transference number of 0.89, which represents nearly single-ion conductor behaviour for the salt-in-polymer–borosilicate composite.

A simple complex on the verge of breakdown: Isolation of the elusive cyanoformate ion

Cyanide Hitches a Ride Cyanide is a by-product of the biosynthesis of ethylene in plants and it has been somewhat puzzling how the ion is safely removed before it can shut down enzymatic pathways by coordination to catalytic iron centers. A proposed mechanism has implicated the cyanoformate ion—essentially, a weak adduct of cyanide and carbon dioxide—as the initial product, although its lifetime was uncertain. Murphy et al. (p. 75; see the Perspective by Alabugin and Mohamed) crystallized this previously elusive adduct and found that its solution-phase stability varies inversely with the dielectric properties of the medium. The results bolster a picture in which the adduct shuttles the cyanide away from the hydrophobic confines of the enzyme before releasing the cyanide into the more polar aqueous surroundings. Characterization of a cyanide–carbon dioxide adduct bolsters its possible role in protecting a plant enzyme from cyanide inhibition. [Also see Perspective by Alabugin and Mohamed] Why does cyanide not react destructively with the proximal iron center at the active site of 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase, an enzyme central to the biosynthesis of ethylene in plants? It has long been postulated that the cyanoformate anion, [NCCO2]–, forms and then decomposes to carbon dioxide and cyanide during this process. We have now isolated and crystallographically characterized this elusive anion as its tetraphenylphosphonium salt. Theoretical calculations show that cyanoformate has a very weak C–C bond and that it is thermodynamically stable only in low dielectric media. Solution stability studies have substantiated the latter result. We propose that cyanoformate shuttles the potentially toxic cyanide away from the low dielectric active site of ACC oxidase before breaking down in the higher dielectric medium of the cell.

Residual internal stress in partially crystallized photothermorefractive glass: Evaluation by nuclear magnetic resonance spectroscopy and first principles calculations

In some circumstances, the mechanical and optical properties of multiphase brittle materials strongly depend on the level of residual micromechanical stresses that arise upon cooling due to thermal and elastic mismatch between the constituent phases. Here we study the residual internal stress in a partially crystallized oxyfluoride glass, best known as photothermorefractive (PTR) glass. This material is composed of a glass matrix with embedded nanosize sodium fluoride (NaF) crystals. Using both the Selsing model and solid-state nuclear magnetic resonance in combination with first principles calculations we found that the crystals are under a tensile stress field of approximately 610–800MPa. For this stress level the estimated critical crystal diameter for spontaneous cracking is about 2300–1900nm, which greatly exceeds the observed diameters of 7–35nm. Hence no spontaneous cracking is expected for the PTR glasses. First principles calculations indicate that the stress induced change of the refractive inde...

The ring structure of boron trioxide glass

Abstract A new experiment, sensitive to the dihedral angle distribution of neighboring structural units, was applied to boron trioxide glass. The experiment consisted of correlating the NMR frequencies of neighboring boron nuclei. The frequencies are identical if the boron oxide polyhedra containing the two borons are co-planar, as found in ring systems, but differ for non-co-planar geometries. Through the rate of growth of signal intensity arising from the non-co-planar case, as compared to that obtained in a boron trioxide crystal of known structure, the fraction of boron contained in rings in the glass was determined to be 0.70±0.08 .

A triazine–resorcinol based porous polymer with polar pores and exceptional surface hydrophobicity showing CO2 uptake under humid conditions

Several applications including post-combustion carbon capture require capturing CO2 under humid conditions. To obtain a material capable of interacting more strongly with CO2 than water, surface hydrophobicity and polarizing pores have been incorporated simultaneously into an ultra-microporous Bakelite-type polymer comprising of triazine–triresorcinol building units. Being built from C–C bonds, it exhibits exceptional chemical stability (survives conc. HNO3(g) + SO3(g) without losing any porosity). Triazine–phenol lined channels enable adsorption of CO2 (2.8 mmol g−1 with a good selectivity of 120 : 1 (85% N2 : 15% CO2) at 303 K, 1 bar) and the inherent surface hydrophobicity amply minimizes the affinity for H2O. When the adsorption was carried out using a humid CO2 stream (∼50% RH) the material loses only about 5% of its capacity. In a steam-conditioning experiment, the sample was exposed to high humidity (∼75% RH) for a day, and without any further activation, was tested for CO2 adsorption. It retains more than 85% of its CO2 capacity. And this capacity was intact even after 48 h of steam conditioning. The role of phenol in contributing to the surface hydrophobicity is exemplified by the fact that a ∼17% lithiation of the phenolic sites nearly removes all of the surface hydrophobicity. The local structure of the polymer has been modeled using tight-binding DFT methods (Accelrys) and three low energy conformers were identified. Only the CO2 isotherm simulated using the lowest energy conformer matches the experimental isotherm quite well. The triazine–phenol polymer presented here has good hydrophilic–hydrophobic balance, where the basic triazine units and the phenol groups seem to co-operatively assist the CO2 capture under humid conditions. These properties along with its excellent acid stability make the material a suitable candidate for post-combustion CO2 capture. Also, the study presents a new approach for simultaneously introducing polarizing character and surface hydrophobicity into a porous material.

Comprehensive Chemical Characterization of Complexes Involving Lead-Amino Acid Interactions

Complexes of lead with L-phenylalanine, L-isoleucine, L-valine, or L-arginine have been isolated from reaction mixtures containing lead nitrate and the respective amino acid in acidic aqueous solution. The compounds have been comprehensively characterized using X-ray crystallography, solid state NMR spectroscopy and solution state NMR spectroscopy, IR and Raman spectroscopies, and electrospray ionization mass-spectrometry. The solid state structures of lead-phenylalanine, lead-valine, and lead-valine-isoleucine complexes show a lead center coordinated by two amino acid ligands, while the lead-arginine complex is a cluster involving two lead centers and three arginine molecules. The structural, spectroscopic, and spectrometric characterization of the complexes provides a basis to establish a fundamental understanding of heavy metal-amino acid interactions.

Stimuli-Responsive Shapeshifting Mesoporous Silica Nanoparticles

Stimuli-responsive materials have attracted great interest in catalysis, sensing, and drug delivery applications and are typically constituted by soft components. We present a one-pot synthetic method for a type of inorganic silica-based shape change material that is responsive to water vapor exposure. After the wetting treatment, the cross-sectional shape of aminated mesoporous silica nanoparticles (MSNs) with hexagonal pore lattice changed from hexagonal to six-angle-star, accompanied by the loss of periodic mesostructural order. Nitrogen sorption measurements suggested that the wetting treatment induced a shrinkage of mesopores resulting in a broad size distribution and decreased mesopore volume. Solid-state (29)Si nuclear magnetic resonance (NMR) spectroscopy of samples after wetting treatment displayed a higher degree of silica condensation, indicating that the shape change was associated with the formation of more siloxane bonds within the silica matrix. On the basis of material characterization results, a mechanism for the observed anisotropic shrinkage is suggested based on a buckling deformation induced by capillary forces in the presence of a threshold amount of water vapor available beyond a humidity of about 50%. The work presented here may open a path toward novel stimuli-responsive materials based on inorganic components.

Preparation and comprehensive characterization of [Hg6(alanine)4(NO3)4].H2O.

A new mercury-alanine complex has been isolated from reaction mixtures of mercurous nitrate dihydrate and alanine (L and D enantiomers). The solid-state structure contains mercury(I) and mercury(II) associated by alanine ligands in a polymeric array. The disproportionation of mercury(I) to mercury(II) and mercury(0) was facilitated by alanine and is evidenced by the appearance of mercury(0) in reactions of mercury(I) with the 20 common amino acids. This complex is the first mercury(I)-amino acid complex characterized in the solid state. The compounds have been comprehensively characterized using X-ray crystallography, solid-state and solution-state nuclear magnetic resonance spectroscopy, vibrational spectroscopies, and electrospray ionization mass spectrometry.

Deuterium NMR studies of guest motions in urea inclusion compounds of 1,6-dibromohexane with analytical evaluation of spectra in the fast motion limit

Abstract2H NMR (nuclear magnetic resonance) spectroscopy, in conjunction with X-ray diffraction experiments, was used to characterize the guest motions of 1,6-dibromohexane in its urea inclusion compound. These motions are characterized by alkyl chain jumps between two conformations, each approximately gauche to the terminal bromines, which remain stationary. In this distorted urea channel, one conformer is heavily preferred, but thermally activated population of the unfavorable conformer leads to reversible, temperature-dependent changes in the unit cell parameters. Although rapid motions of the guest chain give rise to ambiguities in the interpretation of the2H NMR spectra, fortuitous temperature-independent spectral features of guests containing deuterium at the α, β and γ positions indicate that the guest motion resembles a two-site jump with unequal probabilities. Analytical lineshape calculations on the three sets of2H NMR spectra indicate a single jump mechanism in which the range of jump angles is narrowly prescribed. This NMR model provided a starting point for successful solution and refinement of the crystal structures at 213 and 298 K, which had been complicated by motional disorder.