Search

Increased demand for lithium products for use in lithium-ion batteries has led to a search for new lithium resources in recent years to meet projected future consumption. One potential lithium resource is low lithium bearing brines that are discharged from hydraulically fractured oil and gas wells as flowback and produced water (FPW). In this way, hydraulic fracturing presents an opportunity to turn what is normally considered wastewater into a lithium resource. In this research, two manganese-based lithium-selective adsorbents were prepared using a co-precipitation method and were employed for lithium recovery from FPW. At optimized conditions, lithium uptake reached 18 mg g−1, with a > 80% lithium recovery within 30 min. The recovered lithium was isolated and concentrated to 15 mM in an acidic final product. The degree of sorbent loss during acid desorption of lithium was significantly higher for sorbents used in the FPW as compared to recovery from a synthetic lithium-bearing brine (4.5% versus 0.8%). Thus, we propose that organic molecules present in the FPW reduce manganese in the sorbent structure during lithium sorption, leading to increased sorbent loss through reductive dissolution. Systematic characterization including wet chemical manganese valence measurements, along with EXAFS, XPS, and TEM-EELS show that exposure to FPW causes tetravalent manganese in the bulk sorbent structure to be reduced during lithium sorption, which subsequently dissolves during acid desorption. Partial removal of these organic molecules by nanofiltration leads to decreased sorbent dissolution in acid. In this way, we show that dissolved organic molecules represent a critical control on the reductive dissolution of manganese-based lithium ion exchange sorbents. This research provides promising results on the use of manganese-based lithium sorbents in FPW.

Phosphate-based glasses (PBGs) are traditionally prepared using the high temperature melt quenching route (MQ) or via the more recent sol-gel method (SG) that requires the use of organic solvents. The coacervation method represents an excellent inexpensive and green alternative to MQ and SG, being performed in aqueous solution and at room temperature. Coacervation is particularly applicable for the production of PBGs designed for biomedical applications, because it allows for the inclusion of temperature sensitive molecules and it does not require the use of toxic solvents. Whereas the atomic structure of the melt quenched and sol-gel PBGs is known, the atomic structure of those prepared via coacervation has yet to be investigated.\ud In this study, a comprehensive advanced structural characterization has been performed on phosphate-based glasses in the system P2O5–CaO–Na2O–Ag2O (Ag2O mol% = 0, 1, 3, 5, 9 and 14) prepared via the coacervation method. Glasses within this system should find application as bioresorbable biomaterials thanks to their ability to release bioactive ions in a controlled manner. In particular, they possess antibacterial properties, inferred by the release of Ag+ over time. \ud High energy X-ray diffraction (HEXRD), 31P and 23Na solid state magic angle spinning nuclear magnetic resonance (MAS NMR) and X-ray Absorption (XAS) at the Ag K-edge were used to probe the atomic structure of the glasses after drying in vacuum and after calcination at 300 °C. \ud The length of the polyphosphate chains in the solid state appears to be independent of silver concentration, however, significant degradation of these chains is seen after calcination at 300 °C. Atomic-scale structure results indicate that the structure of these glasses is akin to that of other silver doped phosphate glasses prepared using the MQ and SG method. This suggests that phosphate-based glasses prepared using milder and greener conditions may have similar chemical and physical properties such as solubility, biocompatibility, and antibacterial properties.

EJK would like to thank the Alistore ERI for the award of a studentship. This work was supported by the Faraday Institution (grant number FIRG018). Additional capacity delivered by oxygen redox activity may in principle represent a means of enhancing the electrochemical performance of layered sodium transition metal oxides. However, irreversible structural changes occurring during cycling typically cause significant capacity fade with limited reversibility of oxygen redox processes. Here, P3-structure Na0.67Co0.2Mn0.8O2 was synthesised under two different reaction conditions. Both materials exhibit very stable cycling performance in the voltage range 1.8-3.8 V where the redox couples of transition metals entirely dominate the electrochemical reaction. For the compound prepared under more oxidising conditions, anion redox activity is triggered in the wider voltage window 1.8-4.4 V in a reversible manner with exceptionally small voltage hysteresis (20 mV). The presence of vacancies in the transition metal layers is shown to play a critical role not only in generating unpaired O 2p states but also in stabilising the crystal structure in the high voltage region. Postprint Postprint

EJK would like to thank the Alistore ERI for the award of a studentship. This work was supported by the Faraday Institution (grant number FIRG018). Lithium-rich layered oxides and sodium layered oxides represent attractive positive electrode materials exhibiting excess capacity delivered by additional oxygen redox activity. However, structural degradation in the bulk and detrimental reactions with the electrolyte on the surface often occur, leading to limited reversibility of oxygen redox processes. Here we present the properties of P3-type Na0.67Mg0.2Mn0.8O2 synthesized under both air and oxygen. Both materials exhibit stable cycling performance in the voltage range 1.8-3.8 V where the Mn3+/Mn4+ redox couple entirely dominates the electrochemical reaction. Oxygen redox activity is triggered for both compounds in the wider voltage window 1.8-4.3 V with typical large voltage hysteresis from non-bonding O 2p states generated by substituted Mg. Interestingly, for the compound prepared under oxygen, an additional reversible oxygen redox activity is shown with exceptionally small voltage hysteresis (20 mV). The presence of vacancies in the transition metal layers is shown to play a critical role not only in forming unpaired O 2p states independent of substituted elements but also in stabilising the P3 structure during charge with reduced structural transformation to the O’3 phase at the end of discharge. This study reveals the important role of vacancies in P3-type sodium layered oxides to increase energy density using both cationic and anionic redox processes. Postprint Postprint

The effects of mechanochemical synthesis and postsynthetic grinding on the spin crossover material [Fe(atrz)3](BF4)2 was examined in detail using a combination of X-ray diffraction, magnetometry, EXAFS and TEM. Mechanochemical synthesis yielded a different polymorph (&beta, phase) to the solution synthesised sample (&alpha, phase), with a lower temperature spin crossover. Milling duration did not significantly affect this temperature but did result in the production of smaller nanoparticles with a narrower size distribution. It is also possible to convert from &alpha, to the &beta, phase via postsynthetic grinding.

EJK would like to thank the Alistore ERI for the award of a studentship and Ok Sung Jeon at Yonsei University for ICP-OES measurement. The authors are grateful for the provision of beam time and assistance from instrument scientists at beamlines B18 at the Diamond Light source (as part of the Energy Materials Block Allocation Group SP14239), BL27SU at Spring 8 and GEM diffractometer at ISIS at the Rutherford Appleton Laboratory. Increasing dependence on rechargeable batteries for energy storage calls for the improvement of energy density of batteries. Toward this goal, introduction of positive electrode materials with high voltage and/or high capacity is in high demand. The use of oxygen chemistry in lithium and sodium layered oxides has been of interest to achieve high capacity. Nevertheless, a complete understanding of oxygen-based redox processes remains elusive especially in sodium ion batteries. Herein, a novel P3-type Na0.67Ni0.2Mn0.8O2, synthesized at low temperature, exhibits oxygen redox activity in high potentials. Characterization using a range of spectroscopic techniques reveals the anionic redox activity is stabilized by the reduction of Ni, because of the strong Ni 3d–O 2p hybridization states created during charge. This observation suggests that different route of oxygen redox processes occur in P3 structure materials, which can lead to the exploration of oxygen redox chemistry for further development in rechargeable batteries. Postprint

The search for improved energy-storage materials has revealed Li- and Na-rich intercalation compounds as promising high-capacity cathodes. They exhibit capacities in excess of what would be expected from alkali-ion removal/reinsertion and charge compensation by transition-metal (TM) ions. The additional capacity is provided through charge compensation by oxygen redox chemistry and some oxygen loss. It has been reported previously that oxygen redox occurs in O 2\(p\) orbitals that interact with alkali ions in the TM and alkali-ion layers (that is, oxygen redox occurs in compounds containing Li\(^+\)–O(2\(p\))–Li\(^+\) interactions). Na\(_{2/3}\)[Mg\(_{0.28}\)Mn\(_{0.72}\)]O\(_2\) exhibits an excess capacity and here we show that this is caused by oxygen redox, even though Mg\(^{2+}\) resides in the TM layers rather than alkali-metal (AM) ions, which demonstrates that excess AM ions are not required to activate oxygen redox. We also show that, unlike the alkali-rich compounds, Na\(_{2/3}\)[Mg\(_{0.28}\)Mn\(_{0.72}\)]O\(_2\) does not lose oxygen. The extraction of alkali ions from the alkali and TM layers in the alkali-rich compounds results in severely underbonded oxygen, which promotes oxygen loss, whereas Mg\(^{2+}\) remains in Na\(_{2/3}\)[Mg\(_{0.28}\)Mn\(_{0.72}\)]O\(_2\), which stabilizes oxygen.

Bioactive sol-gel calcia-silica glasses can regenerate damaged or diseased bones due to their ability to stimulate bone growth. This capability is related to the formation of a hydroxyapatite layer on the glass surface, which bonds with bone, and the release of soluble silica and calcium ions in the body fluid which accelerates bone growth. The addition of s ilver ions imbues the glass with antibacterial properties due to the release of antibacterial Ag + ion. The antibacterial activity is therefore closely dependent on the dissolution properties of the glasses which in turn are related to their atomic-level structure. Structural characterization of the glasses at the atomic level is therefore essential in order to investigate and control the antibacterial properties of the glass. We have used neutron diffraction to investigate the structure of silver-containing calcia-silica sol-gel bioactive glasses with different Ag 2 O loading (0, 2, 4, 6 mol%). The presence of the silver had little effect on the host glass structure, although some silver metal nanoparticles were present. Results agreed with previous computer simulations.

Sol-gel-synthesized bioactive glasses may be formed via a hydrolysis condensation reaction, silica being introduced in the form of tetraethyl orthosilicate (TEOS), and calcium is typically added in the form of calcium nitrate. The synthesis reaction proceeds in an aqueous environment; the resultant gel is dried, before stabilization by heat treatment. These materials, being amorphous, are complex at the level of their atomic-scale structure, but their bulk properties may only be properly understood on the basis of that structural insight. Thus, a full understanding of their structure-property relationship may only be achieved through the application of a coherent suite of leading-edge experimental probes, coupled with the cogent use of advanced computer simulation methods. Using as an exemplar a calcia-silica sol-gel glass of the kind developed by Larry Hench, in the memory of whom this paper is dedicated, we illustrate the successful use of high-energy X-ray and neutron scattering (diffraction) methods, magic-angle spinning solid-state NMR, and molecular dynamics simulation as components to a powerful methodology for the study of amorphous materials.

Despite the numerous studies of bone mineral, there are still many questions regarding the exact structure and composition of the mineral phase, and how the mineral crystals become organised with respect to each other and the collagen matrix. Bone mineral is commonly formulated as hydroxyapatite, albeit with numerous substitutions, and has previously been studied by (31)P and (1)H NMR, which has given considerable insight into the complexity of the mineral structure. However, to date, there has been no report of an NMR investigation of the other major component of bone mineral, calcium, nor of common minority cations like sodium. Here, direct analysis of the local environment of calcium in two biological apatites, equine bone (HB) and bovine tooth (CT), was carried out using both (43)Ca solid state NMR and Ca K-edge X-ray absorption spectroscopy, revealing important structural information about the calcium coordination shell. The (43)Ca delta(iso) in HB and CT is found to correlate with the average Ca-O bond distance measured by Ca K-edge EXAFS, and the (43)Ca NMR linewidths show that there is a greater distribution in chemical bonding around calcium in HB and CT, compared to synthetic apatites. In the case of sodium, (23)Na MAS NMR, high resolution 3Q-MAS NMR, as well as (23)Na{(31)P} REDOR and (1)H{(23)Na} R(3)-HMQC correlation experiments give the first direct evidence that some sodium is located inside the apatite phase in HB and CT, but with a greater distribution of environments compared to a synthetic sodium substituted apatite (Na-HA).

This study investigates the effect of doping a high calcium oxide containing metaphosphate glass series (CaO)(40)(Na2O)(10)(P2O5)(50) with TiO2 (1, 3, and 5 mol%). TiO2 incorporation increased the density and glass transition temperature while reduced the degradation rate (5 mol% in particular) by twofold compared with (CaO)30 system reported previously. This has been confirmed by ion release and the minimal pH changes. TiP2O7, NaCa(PO3)(3) and CaP2O6 phases were detected for all TiO2-containing ceramics. XPS showed that the surface is composed of Ca, h, and Ti. Ti was recognized mainly as TiO2, but its total amount was lower than theoretical values. P-31 magic angle spinning (MAS) NMR showed a downfield shift of the P-31 lineshape with increasing TiO2, interpreted as an effect of the titanium cation rather than an increase in the phosphate network connectivity. FTIR showed that incorporation of TiO2 increased the strength of the phosphate chains, and the O/P ratio while introducing more Q(1) units into the structure at the expense of the Q(2) units. There were no differences, however, in surface topography roughness and free energies between these glasses. These results suggested that TiO2 and CaO were acting synergistically in producing glasses with controllable bulk and structural properties.

A sol-gel preparation of Ga-doped phosphate-based glass with potential application in antimicrobial devices has been developed. Samples of composition (CaO)(0.30)(Na2O)(0.20-x) (Ga2O3) (x) (P2O5)(0.50) where x = 0 and 0.03 were prepared, and the structure and properties of the gallium-doped sample compared with those of the sample containing no gallium. Analysis of the P-31 MAS NMR data demonstrated that addition of gallium to the sol-gel reaction increases the connectivity of the phosphate network at the expense of hydroxyl groups. This premise is supported by the results of the elemental analysis, which showed that the gallium-free sample contains significantly more hydrogen and by FTIR spectroscopy, which revealed a higher concentration of -OH groups in that sample. Ga K-edge extended X-ray absorption fine structure and X-ray absorption near-edge structure data revealed that the gallium ions are coordinated by six oxygen atoms. In agreement with the X-ray absorption data, the high-energy XRD results also suggest that the Ga3+ ions are octahedrally coordinated with respect to oxygen. Antimicrobial studies demonstrated that the sample containing Ga3+ ions had significant activity against Staphylococcus aureus compared to the control.

The general trend in biomaterials is to use and employ materials that play an active role in tissue regeneration rather than passive and inert materials. Therefore, understanding how a material interacts with the surrounding environments, including cells and tissue fluid, allows material design to be tailored so that implants can be constructed to promote a specific biological response, helping them better perform their function. This class of materials has been described as the "Third Generation" of biomaterials. Phosphate based glasses fall into this category and it has been shown that the properties of these glasses can be tuned via their composition according to the desired end application. These glasses can be prepared as melt-quenched or sol-gel bulk form suitable for potential hard tissue engineering applications and as vehicles for antimicrobial agents. They can also be prepared as fibres suitable for soft tissue engineering applications such as those involving muscle, ligaments, and tendon, where, like the fibres, the tissue has a high degree of anisotropy.

Neutron diffraction with isotopic substitution has been used to probe the local environment of silver in a melt-quench derived bioresorbable phosphate glass. Samples enriched with Ag-107 and Ag-109 were prepared and neutron diffraction data collected. A first-order difference was taken between the data sets to give detailed information about the silver environment in the glass matrix. The measured Ag-O correlation has three components in the first coordination shell at 2.28 angstrom, 2.51 angstrom, and 2.58 angstrom with coordination numbers of 2.1,23, and 1.1, respectively, consistent with silver occupying a distorted octahedral environment. These results have been correlated with those from Ag K-edge X-ray absorption near-edge spectroscopy measurements. The results suggest that the addition of silver to the (CaO)(0.3)(Na2O)(0.2)(P2O5)(0.5) system has a significant effect on the host phosphate network, with shorter and more branched chains replacing the long chains and rings normally associated with the metaphosphate composition.

The HfO2-SiO2 system is attracting interest as a possible new dielectric material in semiconductor devices. Knowledge of the location of hafnium within the silica network and the effect hafnium has on the structure will be central to the successful use of this material system in this application. Here, sol-gel techniques have been used to manufacture (HfO2)(x)(SiO2)(1-x) samples (x = 0.1, 0.2 and 0.4, each heat treated at 250, 500 and 750 degrees C) and these have been characterised by magic angle spinning (MAS) NMR (H-1, C-13, O-17, Si-29), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis. 29Si MAS NMR showed that increasing the hafnia content decreases the connectivity of the silicate network, i.e. increases the range of differently connected SiO4 (Q(n)) units with more having increased numbers of non-bridging oxygens (i.e. lower n). FTIR and O-17 MAS NMR showed unequivocally that the x = 0.4 sample phase-separated at higher temperatures, while in the x = 0.1 sample the hafnium was homogeneously mixed into the SiO2 phase without any phase separation. (C) 2007 Elsevier Inc. All rights reserved.

An improved method for the sol-gel synthesis of binary (TiO2)(0.5)(P2O5)(0.5) glasses has been developed. Elemental analysis of the products showed that the loss of phosphorous upon drying and heat treatment is low. The structure of the heat-treated glasses was studied using neutron diffraction and high-energy X-ray diffraction, analysis of which revealed a structure consisting Of PO4 tetrahedra and TiO6 octahedra sharing corners in a three-dimensional amorphous network. The Reverse Monte Carlo method was used to produce a structural model which illustrated that the structure of the glass is, at the near-neighbour level, closely analogous to the superstructure of crystalline TiP2O7. No significant atomic-scale structural differences were observed between glasses prepared by acid- or base-catalysed sol-gel reactions. (c) 2007 Elsevier Ltd. All rights reserved.

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 31 P 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.

An undoped and two silver-doped (0, 3 and 5 mol% Ag) phosphate glass compositions were investigated for their structure and properties. These compositions had in a previous study been investigated for their antimicrobial properties, and were found to be extremely potent at inhibiting the micro-organisms tested. Thermal, X-ray diffraction (XRD), nuclear magnetic resonance (NMR) and X-ray absorption Near Edge Structure (XANES) studies were used to elucidate the structure of the compositions investigated, whilst degradation and ion release studies were conducted to investigate their properties. No significant differences were found between the T (g) values of the silver containing glasses, while XRD analysis revealed the presence of a NaCa(PO3)(3) phase. NMR showed the dominance of Q(2) species, and XANES studies revealed the oxidation state of silver to be in the +1 form. No correlation was seen between the degradation and cation release profiles observed, and the P3O93 anion was the highest released anionic species, which correlated well with the XRD and NMR studies. Overall, it was ascertained that using Ag2SO4 as a precursor, and producing compositions containing 3 and 5 mol% Ag, the levels of silver ions released were within the acceptable cyto/biocompatible range.

Neutron diffraction and P-31 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 Q(2) 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.

Phosphate-based glasses have recently attracted much interest as a new generation of biomaterials because of their ability to react and dissolve in the physiological environment and eventually to be replaced by regenerated hard or soft tissue. A series of phosphate-based glasses containing 45 mol% P2O5 and various amounts of CaO and Na2O were synthesized by sol–gel and melt-quenching techniques. A comparison between the structure of the sol–gel glass and the structure of the analogous melt-quenched glasses has been undertaken. A broad-based characterization approach combining different techniques has been used to investigate the short-range structure of the glasses and the effect of adding modifier oxides to the network structure (conventional and high energy X-ray diffraction, infra-red spectroscopy, 31P solid state magic angle spinning NMR spectroscopy). Sol–gel and melt-quenched glasses appear to have a similar structure, showing similar Qn distributions and atomic correlations.

Bioactive sol-gel derived glass scaffolds bond to bone and their dissolution products stimulate new bone growth in vitro and in vivo; they may therefore be used to regenerate diseased or damaged bone to its original state and function in bone tissue engineering applications. We seek herein to cast light upon these reaction mechanisms by attempting to quantify changes in the atomic-scale structure of the glass scaffold as a result of in vitro reaction with simulated body fluid (SBF). We report the results of a study using neutron diffraction with isotopic substitution (NDIS) to gain new insights into the nature of the atomic scale calcium environment in bioactive sol-gel glasses. This is augmented by high-energy X-ray total diffraction. We have thereby begun to explore the nature of the principal stages to the generation of hydroxyapatite (i.e. the mineral 'building block' of bone) on the bioactive glass surface. The data are examined in light of our complementary solid-state NMR and computer modelling studies. The results reveal that the Ca-O environment in an SBF exposed (CaO)(0.3)(SiO2)(0.7) sol-gel glass, which initially comprises three distinct but partially overlapping correlation shells centered at 2.3 angstrom, 2.5 angstrom and 2.75 angstrom, preferentially loses the shortest length correlation. A (CaH)-H-... correlation appears at 2.95 angstrom. The surface deposited (CaP)-P-... environment consists of three partially overlapping, but nonetheless distinct, correlation shells, at 3.15 angstrom, 3.40 angstrom and 3.70 angstrom. (c) 2007 Elsevier B.V. All rights reserved.

Phosphate-based glasses of the general formula Na2O-CaO-P2O5 are degradable in an aqueous environment, and therefore can act as antibacterial materials through the inclusion of ions such as copper. In this study, CuO and Cu2O were added to Na2O-CaO-P2O5 glasses (1-20 mol% Cu) and X-ray absorption spectroscopy (XAS) and high-energy X-ray diffraction (HEXRD) used to probe the local environment of the copper ions. Copper K-edge X-ray absorption near-edge structure (XANES) spectra confirm the oxidation state of copper to be predominantly 2+ in all samples regardless of which copper oxide was used in the preparation. The XANES results suggest the structural environment of copper to be octahedral with respect to oxygen in all samples. The HEXRD results yield a Cu-O nearest-neighbour distance of 1.98 angstrom and associated coordination number of approximately six, both consistent with octahedral coordination. Analysis of the extended X-ray absorption fine structure (EXAFS) data also yields structural parameters consistent with copper in an octahedral environment. The HEXRD and EXAFS results reveal a Cu-P distance of 3.13 angstrom, which confirms that the copper ions are coordinated within the phosphate glass network and not phase-separated in domains of copper oxide. (c) 2006 Elsevier B.V. All rights reserved.

The formation of a carbonate-containing hydroxyapatite, HCAp, layer on bioactive calcium silicate sol-gel glass of the formula (CaO)0.3(SiO2)0.7 has been studied in-vitro in Simulated Body Fluid (SBF). Extended X-ray Absorption Fine Structure (EXAFS), X-ray Absorption Near Edge Structure (XANES), X-ray diffraction (XRD), and solid state nuclear magnetic resonance (NMR) measurements have been performed with results showing the formation of a significantly amorphous HCAp layer after less than 5 hours in solution.

A range of nitrogen-doped amorphous hydrogenated carbon samples (a-C:Nx:H, where x = 3 at.% and x = 7 at.%) have been studied using neutron diffraction, inelastic neutron scattering (INS) and Fourier transform infra-red (FTIR) spectroscopy to obtain detailed information about their atomic-scale structure, particularly the bonding environment of the hydrogen. The results show that the overall atomic scale network structure of the two samples is very similar; however, the hydrogen-bonding sites alter subtly as the nitrogen content of the samples is increased.

Neutron diffraction with 46Ti and 48Ti stable isotopes and isotope-enriched 17O and 49Ti MAS NMR have been used to characterize the structure of (TiO2)x(SiO2)1-x sol−gel glass as a function of composition (x = 0.08, 0.18, and 0.41) and calcination temperature (T = 250, 500, and 750 °C). The results reveal the first direct observation of two Ti−O distances in a homogeneous (TiO2)0.18(SiO2)0.82 sol−gel derived glass. In the sample heat treated at 250 °C, the Ti occupies a distorted octahedral environment similar to that found in the mineral ramsayite with four Ti−O bond lengths of around 1.89 A and two close to 2.11 A. After heating to 750 °C, two shorter bond distances are observed: a short distance at 1.81 A due to tetrahedrally coordinated Ti and a longer distance of 1.94 A due to a minority species of octahedrally coordinated Ti. The (TiO2)0.08(SiO2)0.92 sample exhibits similar behavior. After heating to 250 °C, two Ti−O distances are observed at 1.84 and 2.10 A consistent with the presence of both tet...

Amorphous (TiO2) x (SiO2)1−x (x = 0.08, 0.18, and 0.41) sol-gel formed materials have been characterised by a combination of X-ray and neutron diffraction, infra-red and 29Si and 17O magic angle spinning NMR spectroscopy. This combination allows new insight into the fundamental structural role titanium additions play in silica, entering the network at x = 0.08 but largely phase separating at x = 0.41. At an intermediate value of x = 0.18 there is complex coordination behaviour with initially some Ti—O—Ti linkages forming and both TiO4 and TiO6 coordinations present. It appears that at 500°C for the x = 0.18 sample all titanium is present in Ti—O—Si linkages but that this situation is unstable on further calcination. The new information presented here is amalgamated with that from our previous EXAFS, XANES and SAXS on the same samples to produce the most complete view to date of the local and mesoscopic structural behaviour of the (TiO2) x (SiO2)1−x system produced by the sol-gel method.

Extended X-ray absorption fine structure spectroscopy and X-ray absorption near edge structure, X-ray fluorescence spectroscopy, and X-ray powder diffraction have been used to study the local calcium environment in four sol–gel-derived bioactive calcium silicate glasses of the general formula (CaO)x(SiO2)1−x. The formation of a hydroxyapatite layer on the composition with the highest bioactivity (x = 0.3) with time has been studied, in an in vitro environment, by immersion in simulated body fluid (SBF) at 37°C. The calcium oxygen environment in the four compositions has been shown to be six-coordinate in character. Both the extended X-ray absorption fine structure spectroscopy and X-ray absorption near edge structure show a gradual increase in coordination number and CaO bond distance with longer exposure to SBF. X-ray fluorescence show that calcium is quickly lost from the samples on exposure to SBF and the calcium concentration then recovers with time. There is clear evidence that the recovery of calcium content is due to the formation of a CaO-P2O5-rich layer. Annealing of samples at 650°C shows the presence of what, on the length scales probed by X-ray diffraction, appears to be noncrystalline calcium phosphate after 1 h of exposure to an SBF solution, which becomes more crystalline on longer exposure. © 2004 Wiley Periodicals, Inc. J Biomed Mater Res 70A: 354–360, 2004

The X-ray absorption near edge structure (XANES) at the calcium K-edge is rich in information, but complex and difficult to interpret fully. We present here a systematic study of a range of calcium/oxygen containing compounds and minerals and show that the XANES may be used to obtain qualitative information on the calcium coordination environment.

The effects of different heat treatment schemes (i.e. successively or directly heated to particular temperatures) and atmospheres (air or nitrogen) on the solid-state NMR spectra obtained from (TiO2)(0.15)(ZrO2)(0.05)(SiO2)(0.80) sol-gel materials are investigated. A combination of H-1, C-13, O-17 and Si-29 NMR is used. Si-29 MAS NMR indicates that the extent of condensation of the silica-based network strongly depends on the maximum temperature the sample has experienced, but the condensation is largely independent of the details of the heat treatment scheme and atmosphere used. For sol-gel produced silicate-based materials the results show that the equilibrium structure at each temperature is reached rapidly compared to the time (2 h) spent at that temperature. The O-17 NMR results confirm that a nitrogen atmosphere does significantly reduce loss of O-17 from the structure but care must be taken since there could be differential loss of O-17 from the regions having different local structural characteristics. (C) 2003 Elsevier Science (USA). All rights reserved.

A combination of Si-29 and O-17 MAS NMR, EXAFS and FT-IR spectroscopy has been used to study the atomic structure of (Ta2O5)(x)(SiO2)(1 - x) (x = 0.05, 0.11, 0.18 and 0.25) xerogels prepared by reacting partially-hydrolysed tetraethyl orthosilicate with tantalum(V) ethoxide. Amorphous tantala, a-Ta2O5, xerogels have also been prepared and their structures studied in detail for the first time. Results have shown that in all these materials, Ta adopts predominantly 5-fold coordination with respect to oxygen. For the mixed oxide xerogels, partial phase separation of the two component oxides occurs for x > 0.11.

The height and position of the pre-edge peak in Ti K-edge X-ray absorption near-edge structure (XANES) is a sensitive indicator of the coordination of Ti. This method is used in situ to investigate Ti coordination in titania–silica xerogels with low TiO 2 content. Unheated xerogels contain Ti with isolated, distorted 6-fold coordination ( [6] Ti ). Initial heating causes [6] Ti to be rapidly converted into 4-fold coordinated Ti( [4] Ti ), which upon cooling reverts to [6] Ti . Increased heat treatment creates more stable [4] Ti , which remains after cooling. Thus, the coordination of Ti depends on ambient conditions in addition to heat treatment. In-situ XANES is important for distinguishing different kinds of [4] Ti , and hence for understanding catalytic properties.

A combination of Si-29 and O-17 MAS NMR, EXAFS and FTIR spectroscopy has been used to study the atomic structure of(ZrO2)(x)(SiO2)(1-x) (x = 0.1, 0.2, 0.3 and 0.4) xerogels prepared by reacting partially hydrolysed tetraethyl orthosilicate with zirconium(rv) propoxide. Results from (ZrO2)(0.1)(SiO2)(0.9) samples reveal the oxides to be atomically mixed with no evidence of phase separation. In these samples, the nearest neighbour environment of zirconium is similar to that found in cubic zirconia. In the (ZrO2)(0.4)(SiO2)(0.6) samples, phase separation occurs with a significant proportion of the zirconium present as amorphous ZrO2 with a local structure similar to that of monoclinic zirconia. O-17 MAS NMR and EXAFS have proven valuable techniques for gauging the level of atomic mixing in these materials.

Sol-gel produced mixed oxide materials have been extensively studied using conventional, ex situ structural techniques. Because the structure of these materials is complex and dependent on preparation conditions, there is much to be gained from in situ techniques: the high brightness of synchrotron x-ray sources makes it possible to probe atomic structure on a short timescale, and hence in situ. Here we report recent results for mixed titania- (and some zirconia-) silica gels and xerogels. Titania contents were in the range 8–18 mol%, and heat treatments up to 500°C were applied. The results have been obtained from intrinsically rapid synchrotron x-ray experiments: i) time-resolved small angle scattering, using a quadrant detector, to follow the initial stages of aggregation between the sol and the gel; ii) the use of a curved image plate detector in diffraction, which allowed the simultaneous collection of data across a wide range of scattering at high count rate, to study heat treatments; and iii) x-ray absorption spectroscopy to explore the effects of ambient moisture on transition metal sites.