Three-dimensional pulsed field gradient NMR measurements of self-diffusion in anisotropic materials for energy storage applications.

Anisotropic battery electrodes that allow enhanced diffusion through the thickness of the electrode can be engineered to improve the rate performance, but direct measurement of 3D diffusion in this pore structure is extremely challenging. To address this, we used 1H and 7Li pulsed field gradient (PFG) NMR to measure anisotropic diffusion in a model porous silicon substrate. We show that NMR spectroscopy can resolve solvent molecules and ions (here, in H2O, DMSO, and the battery electrolyte LIPF6:DC:EMC) in and outside of the pores of the Si substrate, allowing the diffusion coefficients of the ion/molecules in the two components to be individually determined. Exchange between ions/molecules inside and outside of the pores is observed with 1H 2D exchange spectroscopy (EXSY). The pore dimensions can extracted from the diffusivity of the in-pore component and the results are in reasonable agreement with the pore dimensions measured with electron microscopy. Better agreement is obtained for pore diameters; for pore length measurements, exchange between the in-pore and ex-pore solvents should be accounted for. These results suggest that PFG-NMR can serve as a non-destructive characterisation method for both in situ and ex situ analyses of materials ranging from complex battery and supercapacitor electrodes to catalyst supports and tissue scaffolds.

Thermally reversible nanoparticle gels with tuneable porosity showing structural colour.

We present colloidal gels formed from dispersions of PEG- and PEG+DNA-coated silica nanoparticles showing structural colour. The PEG- and PEG+DNA-coated silica colloids are functionalized using exclusively covalent bonds in aqueous conditions. Both sets of colloids self-assemble into thermally-reversible colloidal gels with porosity that can be tuned by changing the colloid volume fraction, although the interaction potentials of the colloids in the two systems are different. Confocal microscopy and image analysis tools are used to characteraize the gels' microstructures. Optical reflection spectroscopy is employed to study the underlying gel nanostructure and to characterize the optical response of the gels. X-ray nanotomography is used to visualize the nanoscale phase separation between the colloid-rich gel branches and the colloid-free gel pores. These nanoparticle gels open new routes for creating structural colour where the gel structure is decoupled from the form factor of the individual colloids. This approach can be extended to create unexplored three dimensional macroscale materials with length scales spanning hundreds of nanometers, which has been difficult to achieve using other methods.

Sustainability and in situ monitoring in battery development.

The development of improved rechargeable batteries represents a major technological challenge for this new century, as batteries constitute the limiting components in the shift from petrol (gasoline) powered to electric vehicles, while also enabling the use of more renewable energy on the grid. To minimize the ecological implications associated with their wider use, we must integrate sustainability of battery materials into our research endeavours, choosing chemistries that have a minimum footprint in nature and that are more readily recycled or integrated into a full circular economy. Sustainability and cost concerns require that we greatly increase the battery lifetime and consider second lives for batteries. As part of this, we must monitor the state of health of batteries continuously during operation to minimize their degradation. It is thus important to push the frontiers of operando techniques to monitor increasingly complex processes. In this Review, we will describe key advances in both more sustainable chemistries and operando techniques, along with some of the remaining challenges and possible solutions, as we personally perceive them.

Designing disordered materials using DNA-coated colloids of bacteriophage fd and gold.

DNA has emerged as an exciting binding agent for programmable colloidal self-assembly. Its popularity derives from its unique properties: it provides highly specific short-ranged interactions and at the same time it acts as a steric stabilizer against non-specific van der Waals and Coulomb interactions. Because complementary DNA strands are linked only via hydrogen bonds, DNA-mediated binding is thermally reversible: it provides an effective attraction that can be switched off by raising the temperature only by a few degrees. In this article we introduce a new binary system made of DNA-functionalized filamentous fd viruses of ∼880 nm length with an aspect ratio of ∼100, and 50 nm gold nanoparticles (gold NPs) coated with the complementary DNA strands. When quenching mixtures below the melt temperature Tm, at which the attraction is switched on, we observe aggregation. Conversely, above Tm the system melts into a homogenous particulate 'gas'. We present the aggregation behavior of three different gold NP to virus ratios and compare them to a gel made solely of gold NPs. In particular, we have investigated the aggregate structures as a function of cooling rate and determine how they evolve as function of time for given quench depths, employing fluorescence microscopy. Structural information was extracted in the form of an effective structure factor and chord length distributions. Rapid cooling rates lead to open aggregates, while slower controlled cooling rates closer to equilibrium DNA hybridization lead to more fine-stranded gels. Despite the different structures we find that for both cooling rates the quench into the two-phase region leads to initial spinodal decomposition, which becomes arrested. Surprisingly, although the fine-stranded gel is disordered, the overall structure and the corresponding length scale distributions in the system are remarkably reproducible. Such highly porous systems can be developed into new functional materials.

IR near-field spectroscopy and imaging of single Li(x)FePO4 microcrystals.

This study demonstrates the unique capability of infrared near-field nanoscopy combined with Fourier transform infrared spectroscopy to map phase distributions in microcrystals of Li(x)FePO4, a positive electrode material for Li-ion batteries. Ex situ nanoscale IR imaging provides direct evidence for the coexistence of LiFePO4 and FePO4 phases in partially delithiated single-crystal microparticles. A quantitative three-dimensional tomographic reconstruction of the phase distribution within a single microcrystal provides new insights into the phase transformation and/or relaxation mechanism, revealing a FePO4 shell surrounding a diamond-shaped LiFePO4 inner core, gradually shrinking in size and vanishing upon delithiation of the crystal. The observed phase propagation pattern supports recent functional models of LiFePO4 operation relating electrochemical performance to material design. This work demonstrates the remarkable potential of near-field optical techniques for the characterization of electrochemical materials and interfaces.

Revealing lithium-silicide phase transformations in nano-structured silicon-based lithium ion batteries via in situ NMR spectroscopy.

Nano-structured silicon anodes are attractive alternatives to graphitic carbons in rechargeable Li-ion batteries, owing to their extremely high capacities. Despite their advantages, numerous issues remain to be addressed, the most basic being to understand the complex kinetics and thermodynamics that control the reactions and structural rearrangements. Elucidating this necessitates real-time in situ metrologies, which are highly challenging, if the whole electrode structure is studied at an atomistic level for multiple cycles under realistic cycling conditions. Here we report that Si nanowires grown on a conducting carbon-fibre support provide a robust model battery system that can be studied by (7)Li in situ NMR spectroscopy. The method allows the (de)alloying reactions of the amorphous silicides to be followed in the 2nd cycle and beyond. In combination with density-functional theory calculations, the results provide insight into the amorphous and amorphous-to-crystalline lithium-silicide transformations, particularly those at low voltages, which are highly relevant to practical cycling strategies.

(2)H MAS NMR studies of the manganese dioxide tunnel structures and hydroxides used as cathode materials in primary batteries.

Variable-temperature (2)H MAS NMR spectroscopy was used to investigate the local environments and mobility of deuterons in the manganese dioxide tunnel structures. Five systems were investigated: electrolytic manganese dioxide (EMD), the model compounds groutite and manganite, and deuterium intercalated ramsdellite and pyrolusite. Ruetschi deuterons, located in the cation vacancy sites in EMD, were detected by NMR and give rise to a resonance at 150 ppm at room temperature. These deuterons are rigid on the (2)H MAS NMR time scale (i.e., the correlation time for motion, tau(c), is >10(-3) s) at room temperature, but start to become mobile above 150 degrees C. No Coleman protons (in the so-called 1 x 1 and 1 x 2 tunnels in EMD) were observed. Much larger (2)H NMR hyperfine shifts of approximately 300 and approximately 415 ppm were observed for the deuterons in the tunnel structures of manganite and groutite, which could be explained by considering the different bonding arrangements for deuterons in the 1 x 1 and 1 x 2 tunnels. The smaller shift of the EMD deuterons was primarily ascribed to the smaller number of manganese ions in the deuterium local coordination sphere. Experiments performed as a function of intercalation level for ramsdellite suggest that the 1 x 1 tunnels are more readily intercalated in highly defective structures. The almost identical shifts seen as a function of intercalation level for deuterons in both 1 x 1 and 1 x 2 tunnels are consistent with the localization of the e(g) electrons near the intercalated deuterium atoms. A Curie-Weiss-like temperature dependence for the hyperfine shifts of EMD and groutite was observed with temperature, but very little change in the shift of the manganite deuterons was observed, consistent with the strong antiferromagnetic correlations that exist above the Néel temperature for this compound. These different temperature dependences could be used to identify manganite-like domains within the sample of groutite, which could not be detected by X-ray diffraction.

Kinetics and mechanism of the beta- to alpha-CuAlCl(4) phase transition: a time-resolved (63)Cu MAS NMR and powder X-ray diffraction study.

The beta and alpha phases of CuAlCl(4) have been characterized by solid-state (27)Al and (63)Cu magic angle spinning nuclear magnetic resonance. The very short spin--lattice relaxation times of the copper spins, and the sensitivity of the I = 3/2 (63)Cu nucleus to the small differences in the local structure of Cu in the two phases, allowed (63)Cu spectra to be acquired in very short time periods (1 min), in which the beta and alpha phases were clearly resolved. This time resolution was exploited to follow the phase transition from the pseudohexagonal close-packed beta-CuAlCl(4) into the pseudocubic close-packed alpha-CuAlCl(4), which occurs above 100 degrees C. In situ time-resolved (63)Cu MAS NMR and synchrotron X-ray diffraction experiments were used to measure the kinetics of this phase transition as a function of temperature. The transformation was shown to be a first-order phase transition involving no intermediate phases with an activation energy of 138 kJ/mol. The kinetic data obey a first-order Avrami--Erofe'ev rate law. A one-dimensional growth mechanism is proposed that involves a combination of Cu(+) ion self-diffusion and a translational reorganization of the close-packed anion layers imposed by the periodic rotations of [AlCl(4)](-) tetrahedra. This beta to alpha phase transformation can be induced at ambient temperatures by low partial pressures of ethylene.

In situ X-ray diffraction and solid-state NMR study of the fluorination of gamma-Al(2)O(3) with HCF(2)Cl.

In situ X-ray diffraction (XRD) and NMR methods were used to follow the structural changes that occur during the dismutation reaction of hydrochlorofluorocarbon-22 (CHClF(2)) over gamma-alumina. Use of a flow cell allowed diffraction patterns to be recorded, while the reaction products were simultaneously monitored downstream of the catalyst bed, by gas chromatography. No visible structural changes of gamma-Al(2)O(3) were observed at 300 degrees C, the temperature at which this material becomes active for catalysis. A new phase began to form at 360 degrees C, which by 500 degrees C completely dominated the XRD powder pattern. (19)F/(27)Al cross-polarization (CP) experiments of gamma-Al(2)O(3) activated at 300 degrees C showed that AlF(3) had already begun to form at this temperature. By 400 degrees C, resonances from a phase that resembles alpha-AlF(3) dominate both the (19)F and (27)Al NMR spectra of the used catalyst. In situ XRD experiments of the catalytically inactive alpha-AlF(3) phase were performed to investigate the structural changes of this material, associated with the extent of tilting of the AlF(6) octahedra in this ReO(3)-related structure, as a function of temperature. Structural refinements of this sample, and the catalytically active phase that grows over gamma-Al(2)O(3), demonstrate that the catalyst is structurally similar to the rhombohedral form of alpha-AlF(3). Differences between the two phases are ascribed to defects in the catalyst, which limit the flexibility of the structure; these may also be responsible for the differences in the catalytic behavior of the two materials.

High-order spin diffusion mechanisms in (19)F 2-D NMR of oxyfluorides.

Negative cross-peaks have been observed in the (19)F 2-D magnetization-exchange MAS NMR spectra of Ba(2)MoO(3)F(4) under fast-spinning conditions. The polarization transfer dynamics are studied as a function of the spinning frequency and the frequency separation of the resonances. The results are consistent with a novel mechanism, in which four spins simultaneously exchange Zeeman magnetization with each other, in an energy-conserving process.

Optimizing the 13C-14N REAPDOR NMR experiment: a theoretical and experimental study.

The optimum 14N pulse lengths in the 13C-14N rotational-echo adiabatic-passage double-resonance (REAPDOR) NMR experiment are determined from calculations and from experiments on samples of glycine and L-alanine. The REAPDOR experiment utilizes the adiabatic passages that 14N spins make between the 14N Zeeman energy levels during the application of a single, short 14N radiofrequency pulse. Use of a short 14N irradiation time of less than one-quarter of a rotor period ensures that the number of 14N spins that undergo more than one passage is minimized. This simplifies calculations describing 13C dipolar dephasing and provides better agreement between calculations and experiments. Recovery of the 13C-14N dipolar couplings and 14N quadrupolar coupling constants and asymmetry parameters is described.

Analysis of the anisotropic dimension in the RIACT (II) multiple quantum MAS NMR experiment for I = 3/2 nuclei.

Numerical simulations of the anisotropic dimension of the Rotation-Induced Adiabatic Coherence Transfer (RIACT) MQ experiment have been performed as a function of the asymmetry parameter, eta, for different spacings between the triple quantum (3Q) excitation and 3Q to single quantum (1Q) reconversion pulses. Large distortions of the spectra are observed, in comparison to the spectra obtained with 1D MAS NMR methods. The method is very sensitive to the relative orientation of the quadrupolar tensor and rotor axis, and signal can only be obtained from a maximum of 60% of the powder. The intensity varies with the spacing between the two pulses, t1, reaching a minimum of 30% when t1 is either a multiple of a full rotor period (n tau(r)) or for (n + 1/2) rotor periods, for pulse lengths (tau1SL) of a quarter of a rotor period. The maximum of 60% is obtained when t1 = n tau(r) - tau1SL. Experimental spectra were acquired for anhydrous Na2HPO4. Good fits were obtained between the experimental and simulated spectra, even for the non-rotor synchronized experiment, by choosing fixed values of t1. The simulations allowed the quadrupole coupling constants and asymmetry parameters to be extracted from the experimental data.

14N population transfers in two-dimensional 13C-14N-1H triple-resonance magic-angle spinning nuclear magnetic resonance spectroscopy.

A two-dimensional (2D) experiment has been used to show that 14N irradiation and magic-angle spinning (MAS) results in population transfers between the 14N Zeeman levels. This experiment was applied to a sample of N-acetyl-D,L-valine, a material where asymmetric doublets resulting from 13C-14N dipolar coupling are clearly resolved in the 13C spectrum at a field of 7 T for Carbon atoms directly bonded to the nitrogen atom. The 13C transverse magnetization was allowed to evolve in the F1 and F2 dimensions, and the 14N spins were irradiated during the mixing period. Cross-peaks were observed in the 2D 13C spectrum between the two peaks of the CH asymmetric doublet. Since one peak of the doublet results primarily from coupling to the [formula: see text] state and the other peak from coupling to [formula: see text] states, population changes between the 14N Zeeman levels have occurred during the mixing period. These population transfers are a consequence of the time dependence of the 14N quadrupole splitting Q under MAS conditions and 14N irradiation. Level anti-crossings of the 14N Zeeman levels occur at the zero-crossings of Q, and a continuous and slow change in Q will result in the transfer of 14N populations between the different Zeeman levels. If these passages are adiabatic, then the system returns to its original state after two zero-crossings. This is consistent with the experimental observation that the intensities of the cross-peaks for 14N irradiation are greater for half a rotor period than a full rotor period.

Uranium bioaccumulation by a Citrobacter sp. as a result of enzymically mediated growth of polycrystalline HUO2PO4.

A Citrobacter sp. accumulates heavy deposits of metal phosphate, derived from an enzymically liberated phosphate ligand. The cells are not subject to saturation constraints and can accumulate several times their own weight of precipitated metal. This high capacity is attributable to biomineralization; uranyl phosphate accumulates as polycrystalline HUO2PO4 at the cell surface. The precipitated metal is indistinguishable from crystalline HUO2PO4.4H2O grown by chemical methods.

Simultaneous vision bifocal contact lenses: a comparative assessment of the in vitro optical performance.

We measured the modulation transfer functions (MTF's) of five simultaneous vision bifocal contact lenses: center-near (Alges, University Optical Products), center-distance (BiSoft, CIBA Vision), diffractive rigid gas-permeable (Diffrax, Pilkington Barnes-Hind), diffractive hydrogel (Hydron Echelon, Allergan), and varifocal center-near (PS-45, G. Nissel). MTF's were measured at various aperture sizes (1 to 7 mm) and at a wavelength of 546 nm. The soft diffractive bifocal MTF's were measured at various wavelengths. The results tended to confirm previous theoretical calculations. The optical performance of the concentric designs was highly sensitive to variations in aperture size. The optical performance of both the rigid and soft diffractive lenses was largely (but not entirely) independent of aperture size. The varifocal lens was sensitive to changes in aperture size with respect to both optical quality and optimal focus.

Edge contrast sensitivity in optometric practice: an assessment of its efficacy in detecting visual dysfunction.

The utility of the Melbourne Edge Test (MET) in optometric practice was evaluated by screening 293 consecutive patients who presented for routine eye examination. The MET identified ocular abnormalities in 37 eyes. Of these, 36 also manifested a reduction of visual acuity. The incidence of "false positives" was 1.0%. The MET test failed to detect ocular abnormalities revealed by conventional examination techniques in 29 eyes (4.9%), of which 28 exhibited Snellen acuity losses.

Changes in contrast sensitivity during the first six months of soft lens wear.

Changes in the contrast sensitivity function (CSF) and corneal thickness of six subjects were monitored during the first 6 months of soft lens wear. Contrast sensitivity suffered a temporary reduction during the first two weeks of wear, subsequently recovering rapidly to normal baseline levels. Corneal thickness increased by 4.9% on the first day of wear, thereafter gradually returning to, but never attaining, prefitting levels. No evidence was found to support complaints of poor vision by long-term soft lens patients. The etiology of the contrast decrements revealed in the early (or adaptation) stages of soft lens wear is discussed.

Changes in contrast sensitivity during the first hour of soft lens wear.

Changes in contrast sensitivity were monitored immediately before, during, and after 1 hour's soft lens wear. Soft lenses of three different center thicknesses, 0.03, 0.07, and 0.12 mm, were used. Results demonstrated a gradual reduction in contrast sensitivity during wear, with a faster but not instantaneous recovery after removal. An increase in soft lens center thickness caused a more pronounced loss of contrast sensitivity, with a slower recovery. The implications of this research on the various reasons put forward for the impairment of the contrast sensitivity function by soft lenses are discussed.