Dr. Daniela Carta received her degree in Chemistry from University of Cagliari, Italy and her PhD in Chemistry from the University of Kent, Canterbury in 2006, working on sol-gel materials for biomedical applications. Since completing her PhD, she has worked as a Postdoctoral Researcher at the Department of Chemistry, University of Cagliari, Italy on sol-gel synthesis and characterization of metals/alloys and functional oxide nanoparticles dispersed in highly porous matrices (xerogels, aerogels, mesoporous silica) for catalytic applications.
In 2013 she became Research Fellow at the NanoGroup at the Nanofabrication Centre, University of Southampton, working on nanodevices for memory applications based on thin film oxides.
In November 2016 she was appointed as a Lecturer in Physical Chemistry in the Department of Chemistry at the University of Surrey.
University roles and responsibilities
- International Student Coordinator for the Department of Chemistry
- Module coordinator for: CHE3045 Topics in Physical Chemsitry; CHEM027 Advanced topics in Physical Chemistry; CHE1039 Fundamentals of forensic science: from crime scene to court; SOC1042 Fundamentals of forensic science for social scientists
Affiliations and memberships
- Nanocomposites/mesostructured materials
- Sol-gel chemistry
- Advanced structural characterization at the atomic- and nano-scale level of nanostructured solids and amorphous materials.
"Bioresorbable 2D and 3D ordered mesoporous phosphate glasses for bone tissue regeneration and drug delivery" (Oct 17 - March 20)
Royal Society (RSG\R1\180191)
"Design of antimicrobial phosphate-based glass fibres for wound healing"
Impact Acceleration Account (EPSRC/University of Surrey/National Physical Laboratory NPL)
"Mesoporous phosphate glasses nanospheres as contrast agents for ultrasound imaging and delivery of therapeutic molecules" (Oct 18 - Sept 19)
EPSRC and University of Surrey Capital Award
"Bioprinting of bioresorbable skin constructs for wound healing" (Oct 18 - March 20)
Dr Richard Martin, Aston Research Centre for Healthy Ageing, Aston University, Birmingham, UK
Dr. Isaac Abrahams, School of Biological and Chemical Sciences, Queen Mary University, UK
Prof. Jonathan Knowles, UCL Eastman Dental Institute, University College London, UK
Prof. Anna Corrias and Dr Gavin Mountjoy, School of Physical Sciences, University of Kent, Canterbury, UK
Prof. Maria F. Casula, School of Chemical Sciences, University of Cagliari, Italy
Fourth State Medicine Ltd. Haselmere, UK
Current Group members
PDRA Dr Farzad Foroutan
Postgraduate research supervision
Current PhD students (Primary Supervisor)
2017 - 2020 Mr Benjamin Alexander Kyffin - "Phosphate-based glasses for tissue regeneration and drug delivery"
2018-2021 Mr Athanasios Nikolaou - "Understanding the molecular and cellular mechanisms that bioactive phosphate glasses and cold plasma regulate to inhibit bacterial growth"
Completed postgraduate research projects I have supervised
- Physical Processes in Chemistry (CHE1043)
- Transferable and Quantitative Skills 2 (CHE1037)
- Fundamentals of Forensic Science (CHE1033)
- Fundamentals of Forensic Science for Social Scientists (SOC1042)
- Intermediate Physical Chemistry (CHE2025)
- Intermediate Inorganic Chemistry (CHE2031)
- Forensic Chemistry (CHE2033)
- Topics in Physical Chemistry (CHE3045)
- Advanced Topics in Physical Chemistry (CHEM027)
- Management, communication and IT skills (CHRM002)
Novel nanocomposite catalysts for single step Water Gas Shift Reaction (WGSR) were prepared by deposition-precipitation and impregnation of Pt-CeO2 nanophases onto an ordered mesoporous silica support featuring a cubic arrangement of mesopores (SBA-16 type). The highly interconnected porosity of the SBA-16 developing in three-dimension (3D) provides a scaffold which is easily accessible to reactants and products by diffusion. The textural and morphological properties of the final catalyst were affected by the procedure utilized for dispersion of the nanophases onto SBA-16. Catalysts prepared by deposition-precipitation present highly dispersed nanocrystalline CeO2 on the surface of SBA-16 and retain high surface area, high thermal stability and high Pt accessibility. Catalysts prepared by impregnation show improved Pt-CeO2 interaction but a more significant decrease of surface area compared to pure SBA-16, due to the confinement of the CeO2 crystallites within the mesoporous matrix. As a result, catalysts prepared by deposition-precipitation are effective for WGSR under working conditions in the high temperature range (around 300-350 °C), whereas catalysts prepared by impregnation are suitable for the process operative at low temperature (LT-WGSR). Our results point out that catalyst preparation procedures can be used to optimise the performance of heterogenous catalysts, by controlling the CeO2 crystallites size and optimizing Pt-CeO2 contact by embedding. Improved thermal and chemical stability was achieved using a mesoporous scaffold.
Printed circuit boards (PCB) technologies are an attractive system for simple sensing and microfluidic systems. Controlling the surface properties of PCB material is an important part of this technology and to date there has been no study on long-term hydrophilisation stability of these materials. In this work, the effect of different oxygen plasma input power and treatment duration times on the wetting properties of FR-4 surfaces was investigated by sessile droplet contact angle measurements. Super and weakly hydrophilic behaviour was achieved and the retention time of these properties was studied, with the hydrophilic nature being retained for at least 26 days. To demonstrate the applicability of this treatment method, a commercially manufactured microfluidic structure made from a multilayer PCB (3-layer FR-4 stack) was exposed to oxygen plasma at the optimum conditions. The structures could be filled with deionised (DI) water under capillary flow unlike the virgin devices.
Resistive random access memory (ReRAM) crossbar arrays have become one of the most promising candidates for next-generation non volatile memories. To become a mature technology, the sneak path current issue must be solved without compromising all the advantages that crossbars offer in terms of electrical performances and fabrication complexity. Here, we present a highly integrable access device based on nickel and sub-stoichiometric amorphous titanium dioxide (TiO2−x), in a metal insulator metal crossbar structure. The high voltage margin of 3 V, amongst the highest reported for monolayer selector devices, and the good current density of 104 A/cm2 make it suitable to sustain ReRAM read and write operations, effectively tackling sneak currents in crossbars without compromising fabrication complexity in a 1 Selector 1 Resistor (1S1R) architecture. Furthermore, the voltage margin is found to be tunable by an annealing step without affecting the device's characteristics.
Glasses in the system 40(P2O5)–x(B2O3)–(60 x)(Na2O) (10 6 x 6 30 mol%) have been prepared by the melt-quenching technique. Thermal properties were studied using differential thermal analysis and the relationship between composition and thermal stability was obtained. Structural characterization was achieved by a combination of experimental data (infrared and Raman spectroscopy, 11B and 31P solid state NMR). In particular, variations in the phosphate network structure upon addition of B2O3 and Na2O were investigated. Analysis of the data indicates that with increasing B2O3 content and decreasing Na2O, the glass network shows increasing levels of cross-linking between phosphate and borate units. Evidence of direct B–O–P bonds was observed. In the compositional range investigated, borate groups contain boron almost exclusively in four-fold coordination.
Metal–insulator–metal (MIM) devices based on titanium dioxide thin films exhibit resistive switching behavior (RS); i.e., they have the ability to switch the electrical resistance between high-resistive states (HRS) and low-resistive states (LRS) by application of an appropriate voltage. This behavior makes titanium dioxide thin films extremely valuable for memory applications. The physical mechanism behind RS remains a controversial subject but it has been suggested that it could be interface-type, without accompanying structural changes of the oxide, or filament-type with formation of reduced titanium oxide phases in the film. In this work, X-ray absorption spectroscopy (XAS) at the Ti K-edge (4966 eV) was used to characterize the atomic-scale structure of a nonstoichiometric TiO2–x thin film before and after annealing and for the first time after inclusion in a MIM device based on a Cr/Pt/TiO2–x/Pt stack developed on an oxidized silicon wafer. The advantage of the XAS technique is that is element-specific. Therefore, by tuning the energy to the Ti K-edge absorption, contributions from the Pt, Cr, and Si in the stack are eliminated. In order to investigate the structure of the film after electrical switching, XAS analysis at the Ti K-edge was again performed for the first time on the Cr/Pt/TiO2–x/Pt stack in its virgin state and after switching to LRS by application of an appropriate bias. X-ray absorption near-edge structure (XANES) was employed to assess local coordination and oxidation state of the Ti and extended X-ray absorption fine structure (EXAFS) was used to assess bond distances, coordination numbers, and Debye–Waller factors. XAS analysis revealed that the as-deposited film is amorphous with a distorted local octahedral arrangement around Ti (average Ti–O distance of 1.95 Å and coordination number of 5.2) and has a majority oxidation state of Ti4+ with a slight content of Ti3+. The film remains amorphous upon insertion into the stack structure and after electrical switching but crystallizes as anatase upon annealing at 600 °C. These results do not give any indication of the appearance of conducting filaments upon switching and are more compatible with homogeneous interface mechanisms.
Mesoporous phosphate-based glasses have great potential as biomedical materials being able to simultaneously induce tissue regeneration and controlled release of therapeutic molecules. In the present study, a series of mesoporous phosphate-based glasses in the P2O5-CaO-Na2O system doped with 1, 3, and 5 mol % of Sr2+ were prepared using the sol-gel method combined with supramolecular templating. A sample without strontium addition was prepared for comparison. The non-ionic triblock copolymer EO20PO70EO20 (P123) was used as a templating agent. SEM images revealed that all synthesized glasses have an extended porous structure. This was confirmed by N2 adsorption-desorption analysis at 77 K that shows a porosity typical of mesoporous materials. 31P magic angle spinning nuclear magnetic resonance (31P MAS-NMR) and Fourier transform infrared (FTIR) spectroscopies have shown that the glasses are mainly formed by Q1 and Q2 phosphate groups. Degradation of the glasses in deionized water assessed over a 7-day period shows that phosphate, Ca2+, Na+ and Sr2+ ions can be released in a controlled matter over time. In particular, a direct correlation between strontium content and degradation rate was observed. This study shows that Sr-doped mesoporous phosphate-based glasses have great potential in bone tissue regeneration as materials for controlled delivery of therapeutic ions.
TiO2 is commonly used as the active switching layer in resistive random access memory. The electrical characteristics of these devices are directly related to the fundamental conditions inside the TiO2 layer and at the interfaces between it and the surrounding electrodes. However, it is complex to disentangle the effects of film “bulk” properties and interface phenomena. The present work uses hard X-ray photoemission spectroscopy (HAXPES) at different excitation energies to distinguish between these regimes. Changes are found to affect the entire thin film, but the most dramatic effects are confined to an interface. These changes are connected to oxygen ions moving and redistributing within the film. Based on the HAXPES results, post-deposition annealing of the TiO2 thin film was investigated as an optimisation pathway in order to reach an ideal compromise between device resistivity and lifetime. The structural and chemical changes upon annealing are investigated using X-ray absorption spectroscopy and are further supported by a range of bulk and surface sensitive characterisation methods. In summary, it is shown that the management of oxygen content and interface quality is intrinsically important to device behavior and that careful annealing procedures are a powerful device optimisation technique.
Lab-on-Chip is a technology that could potentially revolutionize medical Point-of-Care diagnostics. Considerable research effort is focused towards innovating production technologies that will make commercial upscaling financially viable. Printed circuit board manufacturing techniques offer several prospects in this field. Here, we present a novel approach to manufacturing Printed Circuit Board (PCB)-based Ag/AgCl reference electrodes, an essential component of biosensors. Our prototypes were characterized both structurally and electrically. Scanning Electron Microscopy (SEM) and X-Ray Photoelectron Spectroscopy (XPS) were employed to evaluate the electrode surface characteristics. Electrical characterization was performed to determine stability and pH dependency. Finally, we demonstrate utilization along with PCB pH sensors, as a step towards a fully integrated PCB platform, comparing performance with discrete commercial reference electrodes
The structural properties of CoFe2O4–SiO2 highly porous nanocomposite aerogels have been investigated by X-ray Absorption Spectroscopy and Transmission Electron Microscopy techniques. The aerogels are obtained by supercritical drying of composite gels obtained using a two step procedure where fast gelation is achieved using urea in the second step. The formation of CoFe2O4 nanocrystals in the silica matrix begins after calcination at 750 °C of the parent aerogel and is complete after calcination at 900 °C, while the high porosity of the sample is mostly retained.
The structure of the iron oxyhydroxide called feroxyhyte (δ-FeOOH), which shows an elusive X-ray powder diffraction pattern, has been represented so far using models describing a mean structure based on the crystalline network of the iron(III) oxide hematite (α-Fe2O3). In this paper, a novel description of the mean structure of feroxyhyte is presented, which is based on the structure of the thermodynamically stable iron oxyhydroxide goethite. Starting from different local arrangements present in the goethite network, a mean structural model is determined which shows an X-ray powder diffraction pattern almost coincident with previous studies. This outcome enables to integrate the structure of feroxyhyte among those of other well characterized iron oxyhydroxides.
Titanium dioxide thin films have attracted increasing attention due to their potential in next-generation memory devices. Of particular interest are applications in resistive random access memory (RRAM) devices, where such thin films are used as active layers in metal–insulator–metal (MIM) configurations. When these devices receive a bias above a certain threshold voltage, they exhibit resistive switching (RS), that is, the resistance of the oxide thin film can be tuned between a high resistive state (HRS) and a low resistive state (LRS). In the context of this work, we have used conductive atomic force microscopy (C-AFM) to identify the resistive switching thresholds of titanium dioxide thin films deposited on Si/SiO2/Ti/Pt stacks to be used in memory devices. By performing a set of reading/writing voltage scans over pristine areas of the thin films, we have identified the critical thresholds, which define a reversible operation (soft-breakdown, SB) via localized changes in electrical resistance across the film and an irreversible operation (hard-breakdown, HB) that includes both changes in local electrical resistance and thin film topography. We have also assessed the transition from SB to HB when thin films are stimulated repeatedly with potentials below the identified onsets of HB, validating a history dependent behavior. This study is therefore aimed at presenting new insights in RRAM device programmability, reliability, and eventually failure mechanisms.
The FDA approved BioGlass® product is one of very few bone regenerative materials actually in clinical use, having been used in ∼650,000 cases already. The BioGlass family are, in essence, melt quenched calcium silicate glasses with additional alkali and aklaline earth modifiers, of general compositions CaO–Na2O–P2O5–SiO2. We report herein the first high energy x-ray diffraction data on this important material in the hope of providing a more direct experimental insight into the glass structure. The structure of three compositions of melt quenched bioactive glass (45S5 (Bioglass), 55S5, and 60S5) and one composition of an analogous (i.e. it has similar structure and properties but has different elemental composition) sol-gel bioactive glass (77S5) have been investigated. There are significant changes occurring as a function of composition and of preparation method, but these may all be ascribed to the relative concentrations of the sample's constituent elements (e.g. the absence of Na as a network modifier in the sol-gel sample and the counter effect of the presence of –OH, and the overall variation of the Si-associated features as the relative SiO2 fraction alters). Overall, the underlying network structure is seen to be comparable in all samples.
A series of catalysts containing iron and cobalt nanoparticles supported on a highly ordered mesoporous cubic Im3m silica (SBA-16) were prepared by wet impregnation and used for the production of multi-walled carbon nanotubes (MWCNTs) by catalytic chemical vapor deposition (CCVD) of acetylene. The catalysts were characterized by low- and wide-angle X-ray diffraction, N2 physisorption analysis at 77 K and transmission electron microscopy to study the influence of different metal loading and impregnation time on the CCVD process. Quality and morphology of the MWCNTs was assessed by transmission and scanning electron microscopy, whereas thermal analysis was used to estimate the amount of CNTs produced. It was found that the nanocomposites are catalytically active with particular reference to samples with relatively high metal loading, and are stable under the conditions adopted for the CNT production by the CCVD process.
Samples of nickel cobaltite, a mixed oxide occurring in the spinel structure which is currently extensively investigated because of its prospective application as ferromagnetic, electrocatalytic, and cost-effective energy storage material were prepared in the form of nanocrystals stabilized in a highly porous silica aerogel and as unsupported nanoparticles. Nickel cobaltite nanocrystals with average size 4 nm are successfully grown for the first time into the silica aerogel provided that a controlled oxidation of the metal precursor phases is carried out, consisting in a reduction under H2 flow followed by mild oxidation in air. The investigation of the average oxidation state of the cations and of their distribution between the sites within the spinel structure, which is commonly described assuming the Ni cations are only located in the octahedral sites, has been carried out by X-ray Absorption Spectroscopy providing evidence for the first time that the unsupported nickel cobaltite sample has a Ni:Co molar ratio higher than the nominal ratio of 1:2 and a larger than expected average overall oxidation state of the cobalt and nickel cations. This is achieved retaining the spinel structure, which accommodates vacancies to counterbalance the variation in oxidation state.
Copper-based nanoparticles, supported on either a silica aerogel or cubic mesostructured silicas obtained by using two different synthetic protocols, were used as catalysts for the water gas shift reaction. The obtained nanocomposites were thoroughly characterised before and after catalysis through nitrogen adsorption–desorption measurements at −196 °C, TEM, and wide- and low-angle XRD. The samples before catalysis contained nanoparticles of copper oxides (either CuO or Cu2O), whereas the formation of metallic copper nanoparticles, constituting the active catalytic phase, was observed either by using pre-treatment in a reducing atmosphere or directly during the catalytic reaction owing to the presence of carbon monoxide. A key role in determining the catalytic performances of the samples is played by the ability of different matrices to promote a high dispersion of copper metal nanoparticles. The best catalytic performances are obtained with the aerogel sample, which also exhibits constant carbon monoxide conversion values at constant temperature and reproducible behaviour after subsequent catalytic runs. On the other hand, in the catalysts based on cubic mesostructured silica, the detrimental effects related to sintering of copper nanoparticles are avoided only on the silica support, which is able to produce a reasonable dispersion of the copper nanophase.
Porous monoliths of nanocomposites containing Ni (5 wt.%) and FeNi (5 wt.%) nanoparticles dispersed on an SBA-16 type matrix were prepared following a templated-gelation method based on the sol–gel process. The nanocomposites were characterized by energy dispersive X-ray spectroscopy, N2 physisorption at 77 K, X-ray diffraction and transmission and scanning electron microscopy. In particular, N2 physisorption and transmission electron microscopy analysis show that the ordered mesoporous structure and the high surface area of all the samples are preserved after calcination in air at 500 °C and also after reduction in H2 flux at 800 °C, indicating a very high thermal stability of the samples. As a result of the effective dispersion of the nanophase within the porous texture, nanocomposites containing Ni nanocrystals with an average size of 6 nm homogeneously dispersed within the pores of the amorphous silica matrix were obtained.
The effect of O2 and Ar plasma etching on poly(chloro‐p‐xylylene) (Parylene C) is thoroughly studied by atomic force microscopy, X‐ray photoelectron spectroscopy, and static contact angle measurements. Results indicate that O2 plasma changes the topography more drastically than Ar plasma. Furthermore, despite the fact that Ar plasma is expected to be chemically inert, both plasmas introduce O2 to the surface of the Parylene C films, while Ar plasma additionally reduces the amount of Cl present in the polymer. The effect on the viability of cultured cardiomyocytes is also examined, indicating that cells attach and survive both on Ar and O2 treated films in contrast to untreated Parylene. These observations can provide useful insight into the field of material science and tissue engineering.
The structure of aged melt-quenched sodium borophosphate glasses of composition (P2O5)40(B2O3)x(Na2O)60−x (with x in the range 10–40) has been studied by high-energy X-ray diffraction (HEXRD), 31P and 11B magic angle spinning (MAS) NMR. Similar to the fresh samples, both P O P and P O B linkages are found to be present in these glasses. All three techniques show that the cross-linking between borate and phosphate units increases with boron oxide content. Distinctively upon aging, the glass is found to hydrolyze causing the network to degrade. At the same time, crystalline phases are now also observed. XRD and DTA show that the samples have a higher tendency towards crystallization with increasing boron oxide content upon exposed to moisture. 31P and 11B MAS NMR results are in agreement with these findings. TGA data show that samples with higher boron oxide content take up more moisture upon aging, suggesting that crystallization may be associated with glass hydrolysis. HEXRD results also suggest that sodium ions are preferentially associated with borate units with increasing boron oxide content. © 2008 Elsevier B.V. All rights reserved.
Ferrihydrite is a generic term for various poorly ordered Fe(III) oxyhydroxides which are naturally occurring as nanocrystals and are believed to constitute the ferric core of ferritine, the main iron storage protein in biological systems. Unlike other iron oxides, the exact structure and composition of ferrihydrite is still a matter of debate. In this work, we have prepared and characterized the two main forms of ferrihydrite referred to as 2-lines and 6-lines, on the basis of the number of reflections observed in the (X-ray) diffraction pattern. Thermal and textural properties have been studied; structural characterization has been performed by X-ray diffraction, transmission electron microscopy and X-ray absorption spectroscopy (EXAFS and XANES). The structure of the two forms results to be quite similar. The study of the magnetic properties indicates that the small differences between the 2-lines and 6-lines ferrihydrite samples are mainly caused by the different weight of the magnetic spins located on the particle surface, related to the different nanoparticles mean size.
Glasses in the system 40(P2O5)–x(B2O3)–(60 − x)(Na2O) (10 ≤ x ≤ 25 mol%) were prepared by the sol–gel technique. A mixture of mono- and diethylphosphates was used as precursor for P2O5, boric acid and sodium methoxide were used as source compounds for B2O3 and Na2O, respectively. The dried gels obtained were heat treated at 200, 300 and 400 °C. Structural development occurring during heat treatment and changes with composition were investigated using X-ray diffraction, thermal analysis, infrared spectroscopy, 11B and 31P solid state NMR. Systems with x = 20 and x = 25 mol% are amorphous up to 400 °C, whereas systems with lower B2O3 content are partially crystalline. This work extends sol–gel preparation of amorphous borophosphate systems having P2O5 as the main component.
The formation of NiFe2O4 nanoparticles dispersed in an aerogel silica matrix was investigated as a function of calcination temperature by X-ray absorption fine structure and X-ray absorption near edge structure at both the Fe and Ni K-edges. In particular, nanocomposite aerogels containing a relative NiFe2O4 amount of 10 wt % and calcined at 450, 750 (1 h and 20 h), and 900 °C were studied. A quantitative determination of the relative occupancy of iron and nickel cations in the octahedral and tetrahedral sites of the spinel structure was obtained. It has been found that nickel ferrite prepared by sol−gel has the classical inverted spinel structure found in bulk materials with nickel(II) cations fully occupying the octahedra sites and iron(III) equally distributed between octahedra and tetrahedra sites.
Titanium oxide (TiOx) has attracted a lot of attention as an active material for resistive random access memory (RRAM), due to its versatility and variety of possible crystal phases. Although existing RRAM materials have demonstrated impressive characteristics, like ultra-fast switching and high cycling endurance, this technology still encounters challenges like low yields, large variability of switching characteristics, and ultimately device failure. Electroforming has been often considered responsible for introducing irreversible damage to devices, with high switching voltages contributing to device degradation. In this paper, we have employed Al doping for tuning the resistive switching characteristics of titanium oxide RRAM. The resistive switching threshold voltages of undoped and Al-doped TiOx thin films were first assessed by conductive atomic force microscopy. The thin films were then transferred in RRAM devices and tested with voltage pulse sweeping, demonstrating that the Al-doped devices could on average form at lower potentials compared to the undoped ones and could support both analog and binary switching at potentials as low as 0.9 V. This work demonstrates a potential pathway for implementing low-power RRAM systems.
Nanocomposites containing FeCo alloy nanoparticles dispersed in a highly ordered cubic mesoporous silica (SBA-16) matrix were prepared using two different synthetic methods, co-precipitation and impregnation. Extended X-ray Absorption Spectroscopy (EXAFS) technique at both Fe and Co K-edges was used to investigate the structure of FeCo nanoparticles and the presence of additional disordered oxide phases. EXAFS technique gives evidence of differences in the oxidation degree of the FeCo nanoparticles depending on the synthetic method used.
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 silver 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 characterisation 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 Ag2O 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.
The influence of PtRu bimetallic particle size and composition on cinnamaldehyde selective hydrogenation has been investigated for the first time using well-defined catalysts based on carbon nanotubes support. Very high selectivity towards cinnamyl alcohol together with high activity have been obtained provided a high temperature treatment of the catalyst is performed. HRTEM, WAXS and EXAFS analyses permit us to conclude that the remarkable influence of this high temperature treatment on both activity and selectivity arises from different phenomena. First, a particle size and a structural effect have been evidence that permits to increase the selectivity. WAXS and EXAFS point the formation of alloyed PtRu nanoparticles. Second, the heat treatment allows the removal of oxygenated groups from CNT surface. This may increase the cinnamaldehyde adsorption capacity and decrease the activation barrier for diffusion of substrate and product on the CNT surface, thus contributing to an increase in the activity.
The atomic level structure of a series of monodisperse single crystalline nanoparticles with a magnetic core of manganese ferrite was studied using X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) techniques at both the Fe and Mn K-edges, and conventional and high resolution transmission electron microscopy (TEM and HRTEM). In particular, insights on the non-stoichiometry and on the inversion degree of manganese ferrite nanocrystals of different size were obtained by the use of complementary structural and spectroscopic characterization techniques. The inversion degree of the ferrite nanocrystals, i.e. the cation distribution between the octahedral and tetrahedral sites in the spinel structure, was found to be much higher (around 0.6) than the literature values reported for bulk stoichiometric manganese ferrite (around 0.2). The high inversion degree of the nanoparticles is ascribed to the partial oxidation of Mn2+ to Mn3+ which was evidenced by XANES, leading to non-stoichiometric manganese ferrite.
Nanocomposites containing FeCo alloy nanoparticles dispersed in a highly ordered 3D cubic Im3m mesoporous silica (SBA-16) matrix were prepared by a novel, single-step templated-assisted sol–gel technique. Two different approaches were used in the synthesis of nanocomposites; a pure SBA-16 sample was also prepared for comparison. Low-angle X-ray diffraction, transmission electron microscopy and N2 physisorption at 77 K show that after metal loading, calcination at 500 °C and reduction in H2 flux at 800 °C the nanocomposites retain the cubic mesoporous structure with pore size not very different from the pure matrix. X-ray absorption fine structure (EXAFS) analysis at Fe and Co K-edges demonstrates that the FeCo nanoparticles have the typical bcc structure. The final nanocomposites were tested as catalysts for the production of carbon nanotubes by catalytic chemical vapour deposition and high-resolution TEM shows that good quality multi-walled carbon nanotubes are obtained.
A series of phosphosilicates P2O5 (45 mol%)–CaO (20–40 mol%)–Na2O (5–20 mol%)–SiO2 (10–15 mol%) has been synthesised by the sol–gel route using PO(OH)3−x(OC2H5)x (x = 1, 2) as the phosphorus precursor. Evolution ofthe gels with temperature was studied using thermogravimetric analysis and differential thermal analysis. These phosphosilicates show good stability against crystallisationremaining fully amorphous even after calcination at 400 ◦C. The short-range structure of the stabilised sol–gel glasses and the effect of adding modifier oxides to the network structure have been investigated using a combination of techniques: FT-IR spectroscopy, 31P magic angle spinning NMR, high energy XRD and P L-edge XANES.
A series of novel nanocomposites constituted of FeCo nanoparticles dispersed in an ordered cubic Im3m mesoporous silica matrix (SBA-16) have been successfully synthesized using the wet impregnation method. SBA-16, prepared using the non-ionic Pluronic 127 triblock copolymer as a structuredirecting agent, is an excellent support for catalytic nanoparticles because of its peculiar three-dimensional cage-like structure, high surface area, thick walls, and high thermal stability. Low-angle X-ray diffraction, N2 physisorption, and transmission electron microscopy analyses show that after metal loading, calcination at 500 C, and reduction in H2 flux at 800 C, the nanocomposites retain the wellordered structure of the matrix with cubic symmetry of pores. FeCo alloy nanoparticles with spherical shape and narrow size distribution (4–8 nm) are homogeneoulsy distributed throughout the matrix and they seem in a large extent to be allocated inside the pores.
Structural information on a MnFe2O4–SiO2 nanocomposite aerogel and on the pure silica aerogel matrix were obtained by total X-ray scattering experiments. The total pair distribution function of the silica aerogel is in agreement with literature data on melt-quenched silica. The total pair distribution function of the nanocomposite contains the contribution of all the pair correlations of the atomic species making the interpretation more difficult. The difference curve obtained by subtracting the total pair distribution function of the matrix from that of the nanocomposite, allows to selectively study the structural environment of the nanoparticles. © 2011 Elsevier B.V. All rights reserved
Phosphate-based glasses (PBGs) are traditionally prepared using the high-temperature melt-quenching (MQ) route or via the more recent sol–gel (SG) method 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 does not require the use of toxic solvents. Whereas the atomic structure of the MQ and SGPBGs is known, the atomic structure of those prepared via coacervation has yet to be investigated. 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. High-energy X-ray diffraction (HEXRD), 31P and 23Na solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR), and X-ray absorption Spectroscopy (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. 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 characterisation results indicate that the structure of these glasses is akin to that of other silver-doped phosphate glasses prepared using the MQ and SG methods. 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.
The next generation of nonvolatile memory storage may well be based on resistive switching in metal oxides. TiO2 as transition metal oxide has been widely used as active layer for the fabrication of a variety of multistate memory nanostructure devices. However, progress in their technological development has been inhibited by the lack of a thorough understanding of the underlying switching mechanisms. Here, we employed high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) combined with two-dimensional energy dispersive X-ray spectroscopy (2D EDX) to provide a novel, nanoscale view of the mechanisms involved. Our results suggest that the switching mechanism involves redistribution of both Ti and O ions within the active layer combined with an overall loss of oxygen that effectively render conductive filaments. Our study shows evidence of titanium movement in a 10 nm TiO2 thin-film through direct EDX mapping that provides a viable starting point for the improvement of the robustness and lifetime of TiO2-based resistive random access memory (RRAM).
A series of Fe/Co based nanocomposites where the matrix is mesoporous ordered cubic Im3m silica (SBA-16 type) characterized by a three dimensional cage-like structure of pores were obtained by two different approaches: impregnation and gelation. X-ray diffraction and transmission electron microscopy analysis show that after metal loading, calcination at 500 °C and reduction in H2 fl ux at 800 °C the nanocomposites retain the well-ordered structure of the matrix with cubic symmetry of pores. All nanocomposites prepared were tested for the production of carbon nanotubes by catalytic chemical vapour deposition. Transmission electron microscopy points out that good quality multi-walled carbon nanotubes are obtained.
Calcium phosphate glasses are a promising new generation of biomaterials that can simultaneously induce tissue regeneration and controlled release of therapeutic molecules. In this work, novel calcium phosphate glasses containing 0, 2, 4, and 6 mol % Cu2+ were synthesized via room temperature precipitation reaction in aqueous solution. The effect of Cu2+ addition on the glass properties and structure was investigated using thermal analysis, 31P solidstate MAS NMR, Raman spectroscopy, and X-ray diffraction. All glasses crystallize at temperature >500 °C and are mainly formed by Q1 groups. The release of P, Ca, and Cu in solution over time was monitored via inductively coupled plasma-optical emission spectroscopy. It was found that with increasing Cu content, the amount of P and Ca released decreases whereas the amount of Cu released increases. The effect of Cu2+ release on the antibacterial activity against S. aureus, a bacterial strain commonly found in postsurgery infections, has been investigated. The addition of copper has been shown to infer the glasses antibacterial properties. As expected, the antibacterial activity of the glasses increases with increasing Cu2+ content. Cytocompatibility was assessed by seeding human osteoblast-like osteosarcoma cells Saos-2 (HTB85) on the glass particles. A significant increase in cell number was observed in all the glasses investigated. The copper-doped calcium phosphate glasses have proven to be multifunctional, as they combine bone regenerative properties with antibacterial activity. Therefore, they have great potential as antibacterial bioresorbable materials for hard tissue regeneration.
The formation of FeCo alloy nanoparticles embedded in a highly ordered 3D cubic mesoporous silica matrix (SBA-16) was thoroughly studied using several techniques. In particular, the selectivity of Extended X-ray absorption fine structure and X-ray absorption near-edge structure spectroscopy at both the Fe and Co K-edges allowed us to determine that before reduction treatment Fe and Co are present in a poorly crystalline environment, while after reduction treatment FeCo nanoparticles with the typical bcc structure are formed. FeCo alloy nanoparticles are used in several applications: biomedical (magnetic carriers for drug delivery and cell separation), magnetic (data storage) and catalytic. In this work, FeCo nanoparticles formed in situ in the SBA-16 matrix were used for the production of carbon nanotubes by catalytic chemical vapour deposition. Transmission electron microscopy indicates that good quality multi-walled carbon nanotubes are obtained.
A mesoporous ordered cubic Im3m silica (SBA-16) characterized by a three dimensional cage-like structure of pores was used as a host matrix for the preparation of a series of FeCo-SiO2 nanocomposites with different alloy loading and composition by the wet impregnation method. The mesoporous structure of the SBA-16-type support, prepared according to a versatile sol-gel templated synthetic method, which makes use of n-butanol as a co-surfactant, is stable during the treatments necessary to obtain the final nanocomposites, as pointed out by low-angle X-Ray diffraction, transmission electron microscopy, and N2 physisorption at 77 K. Wide-angle X-ray diffraction shows that upon reduction at 800 °C, FeCo nanocrystals (6–7 nm) with the typical bcc structure are formed and energy-dispersive X-ray spectroscopy analysis, performed by scanning transmission electron microscopy on one of the samples, shows that the Fe/Co atomic ratio in the alloy nanoparticles is very close to the expected value of two. Electron tomography was used for the first time to gain evidence on the highly interconnected mesoporous structure of SBA-16 and the arrangement of the nanoparticles within the matrix. It was found that spherical alloy nanocrystals with narrow size distribution are homogeneously distributed throughout the mesoporous matrix and that the resulting FeCo-SiO2 nanocomposite material displays superparamagnetic behavior with high strength dipolar interactions, as expected for particles with a large magnetic moment.
The structural properties of zinc ferrite nanoparticles with spinel structure dispersed in a highly porous SiO2 aerogel matrix were compared with a bulk zinc ferrite sample. In particular, the details of the cation distribution between the octahedral (B) and tetrahedral (A) sites of the spinel structure were determined using X-ray absorption spectroscopy. The analysis of both the X-ray absorption near edge structure and the extended X-ray absorption fine structure indicates that the degree of inversion of the zinc ferrite spinel structures varies with particle size. In particular, in the bulk microcrystalline sample, Zn2+ ions are at the tetrahedral sites and trivalent Fe3+ ions occupy octahedral sites (normal spinel). When particle size decreases, Zn2+ ions are transferred to octahedral sites and the degree of inversion is found to increase as the nanoparticle size decreases. This is the first time that a variation of the degree of inversion with particle size is observed in ferrite nanoparticles grown within an aerogel matrix.
X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) techniques at both Fe and Co K-edges were used to investigate the formation of CoFe2O4 nanoparticles embedded in a silica aerogel matrix as a function of calcination temperature and CoFe2O4 content. In particular, nanocomposite aerogels containing relative CoFe2O4 amounts of 5 and 10 wt % and calcined at 450, 750, and 900 °C were studied. The evolution of the nanophase with calcination temperatures depends on the composition. In the sample containing 10 wt % of nanophase, results indicate that CoFe2O4 nanocrystals were formed after calcination at 750 °C, whereas in the sample containing 5 wt % of nanophase, they were obtained only after calcination at 900 °C. Quantitative determination of the distribution of the iron and cobalt phase in the octahedral and tetrahedral sites of the spinel structure shows that cobalt ferrite prepared by sol−gel has a partially inverted spinel structure with a degree of inversion around 0.70.
In this paper we review an interesting method of PET recycling, i.e. chemical recycling; it is based on the concept of depolymerizing the condensation polymer through solvolytic chain cleavage into low molecular products which can be purified and reused as raw materials for the production of high-quality chemical products. In this work our attention is confined to the hydrolysis (neutral, acid and alkaline) and glycolysis processes of PET chemical recycling; operating conditions and mechanism of each method are reported and described. The neutral hydrolysis has an auto accelerating character; two kinetic models have been proposed: an half-order and a second order kinetic model. The acid hydrolysis could be explained by a modified shrinking core model under chemical reaction control and the alkaline hydrolysis by a first-order model with respect to hydroxide ion concentration. To describe glycolysis, two different kinetic models have been proposed where EG can act or not as internal catalyst. Further experimental and theoretical investigations are required to shed light on the promising processes of PET chemical recycling reviewed in this work.
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 Tg values of the silver containing glasses, while XRD analysis revealed the presence of a NaCa(PO3)3 phase. NMR showed the dominance of Q2 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.
Multinuclear 1 H, 13C, 17O, 29Si MAS and 93Nb static NMR is reported from a series of sol–gel prepared (Nb2O5)x(SiO2)1x materials with x ¼ 0:03; 0.075 or 0.30. 13C NMR shows that by 500 1C the organic precursor fragments have been removed although some residual carbon remains as a separate phase. The 29Si NMR typically shows three Q-species (Q2,3,4) in the initial gels, and that with increasing heat treatment the average n of the Qn -species increases as the organic fragments and hydroxyl groups are removed. 17O shows unequivocally that the x ¼ 0:03 and 0.075 samples are not phase separated, while at the much higher niobia-content of x ¼ 0:30 Nb–O–Nb signals are readily detected, a definite indication of the atomic scale phase separation of Nb2O5. Th e x ¼ 0:03 and 0.075 samples heated to 750 1C are thus representative of amorphous niobium silicates. Comparison is made to other sol–gel prepared metal silicates especially with another Group Va metal tantalum. The effects of tantalum and niobium on the silica network are very different and it is suggested here that most of the niobium is present as NbO4, forming part of the silicate network.
Nanocomposites made out of FeCo alloy nanocrystals supported onto pre-formed mesoporous ordered silica which features a cubic arrangement of pores (SBA-16) were investigated. Information on the effect of the nanocrystals on the mesostructure (i.e. pore arrangement symmetry, pore size, and shape) were deduced by a multitechnique approach including N2 physisorption, low angle X-ray diffraction, and Transmission electron microscopy. It is shown that advanced transmission electron microscopy techniques are required, however, to gain direct evidence on key compositional and textural features of the nanocomposites. In particular, electron tomography and microtomy techniques make clear that the FeCo nanocrystals are located within the pores of the SBA-16 silica, and that the ordered mesostructure of the nanocomposite is retained throughout the observed specimen.
Ternary phosphate-based glasses in the system P2O5–CaO–Na2O were synthesized using the sol–gel approach. Glasses in this system have the potential for use as bioactive materials. A mixture of mono- and dialkyl phosphate PO(OH)3−x(OC2H5)x (x = 1, 2) and alkoxides of sodium and calcium in an ethylene glycol solution were used as precursors. One of the compositions has also been synthesized by sonocatalysis (application of ultrasonic vibration to the sol). The systems synthesized, which remain fully amorphous even after calcination at 400 °C given the appropriate composition, have been characterized using X-ray diffraction (XRD). Thermal properties have been examined by means of thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The structure of the phosphate network has been studied as a function of composition using Fourier transform infrared spectroscopy (FT-IR) and 31P MAS NMR.
Reduction in metal-oxide thin films has been suggested as the key mechanism responsible for forming conductive phases within solid-state memory devices, enabling their resistive switching capacity. The quantitative spatial identification of such conductive regions is a daunting task, particularly for metal-oxides capable of exhibiting multiple phases as in the case of TiOx. Here, we spatially resolve and chemically characterize distinct TiOx phases in localized regions of a TiOx–based memristive device by combining full-field transmission X-ray microscopy with soft X-ray spectroscopic analysis that is performed on lamella samples. We particularly show that electrically pre-switched devices in lowresistive states comprise reduced disordered phases with O/Ti ratios around 1.37 that aggregate in a ~100 nm highly localized region electrically conducting the top and bottom electrodes of the devices. We have also identified crystalline rutile and orthorhombic-like TiO2 phases in the region adjacent to the main reduced area, suggesting that the temperature increases locally up to 1000 K, validating the role of Joule heating in resistive switching. Contrary to previous studies, our approach enables to simultaneously investigate morphological and chemical changes in a quantitative manner without incurring difficulties imposed by interpretation of electron diffraction patterns acquired via conventional electron microscopy techniques.
Phosphate-based glasses are materials of great interest for the regeneration and repair of damaged hard or soft tissues. They have the desirable property of slowly dissolving in the physiological environment, eventually being totally replaced by regenerated tissue. Being bioresorbable, they can simultaneously induce tissue regeneration and deliver therapeutic agents (e.g. antibacterial ions) in a controlled way. In this work, we have synthesised a series of glasses in the P2O5–CaO–Na2O system doped with Ag2O using the coacervation method. The addition of silver is known to provide the glass with antibacterial properties due to the release of Ag+ ions into the body fluid. The coacervation method is a facile, water-based technique which offers significant advantages over the conventional melt-quench route for preparing phosphate-based glasses which requires melting of metal oxide powders at high temperatures (1000–1200 °C). The properties of the initial colloidal polyphosphate systems (coacervates) as a function of the Ag2O content were characterised using rheology and liquid state 31P NMR. The effect of Ag+ addition on the final dried glasses was investigated using thermal analysis, Raman spectroscopy and X-ray diffraction. The antibacterial activity was assessed against Staphylococcus aureus (S. aureus), a bacterial strain commonly found in post-surgery infections. A dose-dependent antimicrobial effect was seen with an increasing silver content.
Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) techniques at both Fe and Mn K-edges were used to investigate the formation of MnFe2O4 nanoparticles embedded in a silica aerogel matrix as a function of calcination temperature (at 450, 750 and 900 1C). Up to 450 1C, two separated highly-disordered phases of iron and manganese are present. With increasing the temperature (to 750 and 900 1C), the structure of aerogel nanoparticles becomes progressively similar to that of the spinel structure MnFe2O4 (jacobsite). Quantitative determination of cations distribution in the spinel structure shows that aerogels calcined at 750 and 900 1C have a degree of inversion i = 0.20. A pure jacobsite sample synthesised by co-precipitation and used as a reference compound shows a much higher degree of inversion (i = 0.70). The different distribution of iron and manganese cations in the octahedral and tetrahedral sites in pure jacobsite and in the aerogels can be ascribed to partial oxidation of Mn2+ to Mn3+ in pure jacobsite, confirmed by XANES analysis, probably due to the synthesis conditions.
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.
A series of phosphate-based sol–gel glasses in the system P2O5–CaO–Na2O–SiO2 were synthesised using PO(OH)32x(OC2H5)x (x = 1, 2) as a phosphorus precursor and alkoxides of sodium, calcium and silicon in an ethylene glycol solution. It has been found that the upper limit for gel formation is about 22 mol% phosphorus and that the gelation time increases with increasing phosphorus content of the sol. X-ray diffraction (XRD) along with X-ray fluorescence chemical analysis (XRF) have been performed on samples containing 45 mol% of P2O5 and 0, 10, 15 and 25 mol% of SiO2 with varying amount of modifier oxides (CaO, Na2O). All the samples are predominantly amorphous up to 400 uC and some of them, depending on the composition, retain their amorphous structure up to 600 and 800 uC. To the knowledge of the authors, this is the first time that phosphate-based glasses having these compositions have successfully been synthesised via the sol–gel method.
Sol-gel derived calcium silicate glasses may be useful for the regeneration of damaged bone. The mechanism of bioactivity is as yet only partially understood but has been strongly linked to calcium dissolution from the glass matrix. In addition to the usual laboratory-based characterisation methods, we have used neutron diffraction with isotopic substitution to gain new insights into the nature of the atomic-scale calcium environment in bioactive sol-gel glasses, and have also used high energy X-ray total diffraction to probe the nature of the processes initiated when bioactive glass is immersed in vitro in simulated body fluid. The data obtained point to a complex calcium environment in which calcium is loosely bound within the glass network and may therefore be regarded as facile. Complex multistage dissolution and mineral growth phases were observed as a function of reaction time between 1 min and 30 days, leading eventually, via octacalcium phosphate, to the formation of a disordered hydroxyapatite (HA) layer on the glass surface. This methodology provides insight into the structure of key sites in these materials and key stages involved in their reactions, and thereby more generally into the behaviour of bone-regenerative materials that may facilitate improvements in tissue engineering applications.
Phosphate based glasses have attracted great interest because of their application as bioresorbable materials. Because of their solubility in body fluid they can be used as degradable temporary implants for hard and soft tissue replacement and augmentation. In the present work, glasses with a composition of 45% P2O5, 35% CaO, 25% Na2O have been synthesised using the sol-gel method. Gels were obtained using OP(OR)x(OH)3−x (x=1, 2) as phosphorus precursor and alcoxides of calcium and sodium in ethylene glycol. Gels remained amorphous up to a calcination temperature of 400°C. At higher temperature, crystalline γ-Ca2P2O7 was identified. A comparison between the structure of the synthesised sol-gel glass and the structure of an analogue glass generated via the conventional melt quenching method has been performed. A broad based characterisation approach combining many techniques (x-ray diffraction, thermal analysis, infrared spectroscopy, 31P solid state NMR spectroscopy, P L-edge XANES) has been used. The structure of sol-gel, and its melt quenched analogue, is mainly formed by chains of (PO4)3− tetrahedral sharing two bridging oxygen (i.e. methaphosphate units).