The Atomic-Scale Structure of Amorphous Hydrogenated Carbon

The structure of a common batch of amorphous hydrogenated carbon samples (a-C:H) has been studied in detail using time of flight neutron diffraction, inelastic neutron scattering, NMR and molecular dynamics (MD) simulation. Supplementary work has included differential scanning calorimetry (DSC), infrared (1R) spectroscopy and combustion analysis. A summary of the results is presented as evidence for a new structural model for a-C:H

An atomic scale comparison of the reaction of Bioglass® in two types of simulated body fluid

A class of melt quenched silicate glasses, containing calcium, phosphorus and alkali metals, and having the ability to promote bone regeneration and to fuse to living bone, is produced commercially as Bioglass. The changes in structure associated with reacting the bioglass with a body fluid simulant (a buffered Tris(hydroxymethyl)aminomethane growth medium solution or a blood plasma-like salt simulated body fluid) at 37°C have been studied using both high energy and grazing incidence x-ray diffraction. This has corroborated the generic conclusions of earlier studies based on the use of calcia–silica sol-gel glasses whilst highlighting the important differences associated with glass composition; the results also reveal the more subtle effects on reaction rates of the choice of body fluid simulant. The results also indicate the presence of tricalcium phosphate crystallites deposited onto the surface of the glass as a precursor to the growth of hydroxyapatite, and indicates that there is some preferred orientation to their growth

The structure of phosphate glass biomaterials from neutron diffraction and 31P nuclear magnetic resonance data

Neutron diffraction and 31P nuclear magnetic resonance spectroscopy were used to probe the structure of phosphate glass biomaterials of general composition (CaO)0.5?x(Na2O)x(P2O5)0.5 (x = 0, 0.1 and 0.5). The results suggest that all three glasses have structures based on chains of Q2 phosphate groups. Clear structural differences are observed between the glasses containing Na2O and CaO. The P–O bonds to bridging and non-bridging oxygens are less well resolved in the neutron data from the samples containing CaO, suggesting a change in the nature of the bonding as the field strength of the cation increases Na+ ? Ca2+. In the (CaO)0.5(P2O5)0.5 glass most of the Ca2+ ions are present in isolated CaOx polyhedra whereas in the (Na2O)0.5(P2O5)0.5 glass the NaOx polyhedra share edges leading to a Na–Na correlation. The results of the structural study are related to the properties of the (CaO)0.4(Na2O)0.1(P2O5)0.5 biomaterial

The structure of calcium metaphosphate glass obtained from x-ray and neutron diffraction and reverse Monte Carlo modelling

The short range structure of (CaO)(0.5)(P2O5)(0.5) glass has been studied using x-ray and neutron diffraction and modelled using the reverse Monte Carlo method. Using this combination of techniques has allowed six interatomic correlations to be distinguished and fitted to obtain a set of bond lengths and coordination numbers that describe the structure of the glass. The glass consists of metaphosphate chains of phosphate tetrahedra and each phosphate unit has two non-bridging oxygen atoms available for coordination with Ca. The Ca-O correlation was fitted with two peaks at 2.35 and 2.86 angstrom, representing a broad distribution of bond lengths. The total Ca-O coordination is 6.9 and is consistent with distorted polyhedral units such as capped octahedra or capped trigonal prisms. It is found that most non-bridging oxygen atoms are bonded to two calcium atoms. All of these observations are consistent with Hoppe's model for phosphate glasses. Furthermore, the medium range order is revealed to consist of phosphate chains intertwined with apparently elongated clusters of Ca ions, and the Ca-O and Ca-P correlations contributed significantly to the first sharp diffraction peak in x-ray diffraction

Probing vibrational modes in silica glass using inelastic neutron scattering with mass contrast

The effective vibrational density of states (VDOS) has been derived from inelastic neutron-scattering data, for isotopically substituted Si O 18 2 and Si O 16 2 glasses, to gain information about the relative contribution to the Si and O partial VDOS. This is a necessary point of comparison for vibrational mode analyses of molecular-dynamics models. The mass contrast has led to a measurable shift between vibrational mode frequencies in the effective VDOS of Si O 18 2 and Si O 16 2, which is well reproduced in an ab initio simulation. The vibrational band centered at 100.2 meV is confirmed to have significantly lower contribution from the oxygen partial VDOS, than the higher (150.3 and 135.8 meV) and lower energy bands (53.3 meV)

An EXAFS study of rare-earth phosphate glasses in the vicinity of the metaphosphate composition

A study of rare-earth phosphate glasses, in the vicinity of the metaphosphate composition, has been undertaken at different temperatures using the extended X-ray absorption fine structure (EXAFS) facility at the Synchrotron Radiation Source, Daresbury Laboratory, UK. The metaphosphate-like glasses examined contained the rare-earth elements La, Sm. Eu and Gd as R3+ ions. The experiments were carried out at room temperature, 145 and 79 K. The data show that the first shell surrounding rare-earth ions contains only oxygen atoms at a mean distance of similar to 2.2-2.4 Angstrom. The 'Iantfianide contraction' is clearly observed, i.e., the R-O distance decreases with increasing atomic number. The observed R(-O) coordination numbers are in the range 5-7. A second correlation shell was found associated with phosphorus atoms around the central rare-earth atom (similar to 3.5 Angstrom), and another rare-earth:oxygen correlation was also identified (similar to 4 Angstrom). Static disorder dominates the Debye-Waller term, but thermal disorder is not negligible. Within the accuracy of the EXAFS data, no significant structural variations were observed over the temperature range studied. We conclude that the anomalous bulk properties are associated with either subtle structural features or primarily due to rare-earth ion Interactions

Multiple rare-earth ion environments in amorphous (Gd2O3)(0.230)(P2O5)(0.770) revealed by gadolinium K-edge anomalous x-ray scattering

A Gd K-edge anomalous x-ray scattering (AXS) study is performed on the rare-earth (R) phosphate glass, (Gd2O3)0.230(P2O5)0.770, in order to determine Gd⋯Gd separations in its local structure. The minimum rare-earth separation is of particular interest given that the optical properties of these glasses can quench when rare-earth ions become too close to each other. To this end, a weak Gd⋯Gd pairwise correlation is located at 4.2(1)Å, which is representative of a metaphosphate R⋯R separation. More intense first-neighbor Gd⋯Gd pairwise correlations are found at the larger radial distributions, 4.8(1), 5.1(1), and 5.4(1)Å. These reflect a mixed ultraphosphate and metaphosphate structural character, respectively. A second-neighbor Gd⋯Gd pairwise correlation lies at 6.6(1)Å which is indicative of metaphosphate structures. Meta- and ultraphosphate classifications are made by comparing the R⋯R separations against those of rare-earth phosphate crystal structures, R(PO3)3 and RP5O14, respectively, or difference pair-distribution function (ΔPDF) features determined on similar glasses using difference neutron-scattering methods. The local structure of this glass is therefore found to display multiple rare-earth ion environments, presumably because its composition lies between these two stoichiometric formulae. These Gd⋯Gd separations are well-resolved in ΔPDFs that represent the AXS signal. Indeed, the spatial resolution is so good that it also enables the identification of R⋯X(X=R, P, O) pairwise correlations up to r∼9Å; their average separations lie at r∼7.1(1), 7.6(1), 7.9(1), 8.4(1), and 8.7(1)Å. This is a report of a Gd K-edge AXS study on an amorphous material. Its demonstrated ability to characterize the local structure of a glass up to such a long range of r heralds exciting prospects for AXS studies on other ternary noncrystalline materials. However, the technical challenge of such an experiment should not be underestimated, as is highlighted in this work where probing AXS signal near the Gd K edge is found to produce inelastic x-ray scattering that precludes the normal AXS methods of data processing. Nonetheless, it is shown that AXS results are not only tractable but they also reveal local structure of rare-earth phosphate glasses that is important from a materials-centered perspective and which could not be obtained by other materials characterization methods.J.M.C. is grateful to the Royal Commission of the Great Exhibition 1851 for a 2014 Design Fellowship hosted by Argonne National Laboratory (ANL) where work done was supported by the U.S. Department of Energy (DOE) Office of Science, Office of Basic Energy Sciences, and X-ray 1-BM beam line of the Advanced Photon Source, which is a DOE Office of Science User Facility, all under Contract No. DE-AC02-06CH11357. J.M.C. and R.J.N. are also indebted to the Engineering and Physical Sciences Research Council Grant No. GR/L41035 for funding

Effect of silver content on the structure and antibacterial activity of silver-doped phosphate-based glasses

Staphylococcus aureus can cause a range of diseases, such as osteomyelitis, as well as colonize implanted medical devices. In most instances the organism forms biofilms that not only are resistant to the body's defense mechanisms but also display decreased susceptibilities to antibiotics. In the present study, we have examined the effect of increasing silver contents in phosphate-based glasses to prevent the formation of S. aureus biofilms. Silver was found to be an effective bactericidal agent against S. aureus biofilms, and the rate of silver ion release (0.42 to 1.22 µg·mm–2·h–1) from phosphate-based glass was found to account for the variation in its bactericidal effect. Analysis of biofilms by confocal microscopy indicated that they consisted of an upper layer of viable bacteria together with a layer (20 µm) of nonviable cells on the glass surface. Our results showed that regardless of the silver contents in these glasses (10, 15, or 20 mol%) the silver exists in its +1 oxidation state, which is known to be a highly effective bactericidal agent compared to that of silver in other oxidation states (+2 or +3). Analysis of the glasses by 31P nuclear magnetic resonance imaging and high-energy X-ray diffraction showed that it is the structural rearrangement of the phosphate network that is responsible for the variation in silver ion release and the associated bactericidal effectiveness. Thus, an understanding of the glass structure is important in interpreting the in vitro data and also has important clinical implications for the potential use of the phosphate-based glasses in orthopedic applications to deliver silver ions to combat S. aureus biofilm infections

Synthesis, characterisation and performance of (TiO2)(0.18)(SiO2)(0.82) xerogel catalysts

The synthesis of high surface area xerogels has been achieved using the sol-gel route. Heptane washing was used during the stages of drying to minimise capillary pressures and hence preserve pore structure and maximise the surface area. SAXS data have identified that heptane washing during drying, in general, results in a preservation of the pore structure and surface areas of up to 450 m(2) g(-1). O-17 NMR showed that Ti is fully mixed into the silica network in all of the samples. XANES data confirm that reversible 4-fold Ti sites are more prevalent in samples with high surface areas, as expected. The calcined xerogels were tested for their catalytic activity using the epoxidation of cyclohexene with tert-butyl hydroperoxide (TBHP) as a test reaction, with excellent selectivities and reasonable percentage conversions. FT-IR spectroscopy has revealed that the catalytic activity is correlated with the intensity of the Si-O-Ti signal, after accounting for variations in Si-OH and Si-O-Si. The most effective catalyst was produced with heptane washing, a calcination temperature of 500 degreesC, and a heating rate of 5 degreesC min(-1)