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Abstract: Core/shell nanoparticles of M–B (M = Fe, Co) with amorphous and mixed amorphous + bcc-Fe core have been synthesized by chemical reduction of metallic salts by sodium borohydride. The XANES spectra show the existence of a surface layer consisting of metallic hydroxides in a percentage around 15 at.%. The EXAFS reveals the existence of short-range order in the amorphous core with a contribution from the M–O pairs of the shell and a variation of the coordination numbers with respect to bulk alloys. The existence of Fe-bcc in the mixed alloys is observed in the Fourier transform of the EXAFS data.
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Apr 2007
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Abstract: This study investigated proton adsorption to an extracellular polymeric substance (EPS) producing bacterial strain, Bacillus licheniformis S-86, in order to characterise and quantify the contribution made by EPS to cell surface reactivity. Potentiometric titrations were conducted using both untreated cells and cells from which the EPS layer had been extracted. Surface-complexation modelling indicated the presence of four different functional groups in both untreated and EPS-free cells. These sites are assigned to phosphodiester, (pKa 3.3–3.4), carboxylic (pKa 5.3–5.4), phosphoryl/ (pKa 7.4–7.5) and hydroxyl/amine (pKa 9.9–10.1) type groups. The pKa values for the four groups were very similar for untreated and EPS-free cells, indicating no qualitative difference in composition, but site concentrations in the untreated cells were statistically found to be significantly higher than those in the EPS-free cells for the pKa 3.3–3.4 and pKa 9.9–10.1 sites. Infrared analysis provided supporting evidence that site 2 is carboxylic in nature but did not reveal any difference in IR absorption between the native and EPS-free cells. Dissolved organic carbon (DOC) analysis conducted during this study indicated that DOC release by cells is significant, and that the EPS layer is the major contributor.
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Jan 2008
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Abstract: AC electrochemical processes have found applications in controlled surface roughening of aluminium (AC electrograining), fine-tip sharpening for field ion microscopy (AC machining) and thin film anodising (AC anodising). The formation of a surface layer and copious amounts of hydrogen gas are inherent in these AC processes. The presence of a resistance is observed in these processes but it is the source of the resistance that is important to the understanding of ionic transport through the surface film.The AC electrograining process is chosen here, as the annual worldwide production of aluminium plate for high quality lithographic printing and for energy storage super-capacitors is in excess of 800 km(2). In this study, a method to detect gas in the surface layer (smut) in-situ with Small Angle X-ray Scattering (SAXS) is proposed. The total scattering from the in-situ SAXS is used with knowledge of the total volume of smut to explain how a gas fraction can be determined by comparing two samples. Results suggest that a gas fraction can be retained in smut during AC electrograining, the degree to which varies with smut properties. (C) 2010 Elsevier B.V. All rights reserved.
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Mar 2010
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Abstract: We have determined the structure of a complex rhodium carbonyl chloride [Rh(CO)2Cl] molecule adsorbed on the TiO2(110) surface by the normal incidence x-ray standing wave technique. The data show that the technique is applicable to reducible oxide systems and that the dominant adsorbed species is undissociated with Rh binding atop bridging oxygen and to the Cl found close to the fivefold coordinated Ti ions in the surface. A minority geminal dicarbonyl species, where Rh-Cl bond scission has occurred, is found bridging the bridging oxygen ions forming a high-symmetry site.
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Feb 2007
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Abstract: In this paper seventeenth century gilded paintings from the crypt of Sant Benet de Bages, a medieval monastery in the Catalonia region of Spain, near Barcelona, have been studied. Cross sections from two different gilded decorations were studied by means of optical microscopy and electron microscopy and EDS to determine the statigraphy and elemental composition, and by means of FTIR coupled to a microscope to determine the binding media associated to each layer. These preliminary results demonstrated that gilded decorations were made by the application of a gold foil on a mordant substrate on a gypsum base, while the mouldings of the vaults seem to be gilded on a bol with a glaze on top of the gold leaf. It is interesting to notice that the first remained unaltered, while the gilded vault mouldings look almost black, due to the darkening of the organic material. To elucidate the causes involved in the darkening of the sample from the vaults a set of synchrotron ìXRD and ìFTIR experiments have been carried out on these samples at the ESRF (ID18F and ID21, respectively). High brightness and small spot working conditions revealed the development and distribution of calcium oxalates in the binding media, which seem to be responsible for the darkening. Results point out the fact that weddellite (CaC2O4·2H2O) is the phase formed in those layers where organic material has also been identified or at least it would be supposed to be by bibliographic sources and not necessarily those superficial as it would have been suggested due to the similarities with patinas formation.
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Jan 2008
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Diamond Proposal Number(s):
[316]
Abstract: Three new structures of Escherichia coli succinate-quinone oxidoreductase (SQR) have been solved. One with the specific quinone-binding site (Q-site) inhibitor carboxin present has been solved at 2.4 Å resolution and reveals how carboxin inhibits the Q-site. The other new structures are with the Q-site inhibitor pentachlorophenol and with an empty Q-site. These structures reveal important details unresolved in earlier structures. Comparison of the new SQR structures shows how subtle rearrangements of the quinone-binding site accommodate the different inhibitors. The position of conserved water molecules near the quinone binding pocket leads to a reassessment of possible water-mediated proton uptake networks that complete reduction of ubiquinone. The dicarboxylate-binding site in the soluble domain of SQR is highly similar to that seen in high resolution structures of avian SQR (PDB 2H88) and soluble flavocytochrome c (PDB 1QJD) showing mechanistically significant structural features conserved across prokaryotic and eukaryotic SQRs.
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Aug 2009
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Abstract: The group 13 monoalkoxometallanes [Me_{2}Ga(OC(CH_{3})_{2}CH_{2}OMe)]_{2} (1) and [Me_{2}In(OCH(CH_{3})CH_{2}OMe)]_{2} (2), incorporating donor functionalised alkoxides, were synthesised from the reaction of GaMe_{3} or InMe_{3} with ROH (R=C(CH_{3})_{2}CH_{2}OMe (1); R=CH(CH_{3})CH_{2}OMe (2)) in toluene. X-ray crystallography showed that both compounds adopt dimeric structures with a planar M_{2}O_{2} ring, and each group 13 atom is coordinated in a distorted trigonal bipyramidal geometry. The AACVD reaction of GaMe_{3} and ROH (R =C(CH_{3})_{2}CH_{2}OMe, CH_{2}CH_{2}OMe, CH_{2}CH_{2}NMe_{2}) resulted in the formation of thin films of Ga_{2}O_{3} on glass substrates at 450°C. The gallium oxide films were analyzed by scanning electron microscopy, X-ray powder diffraction, energy dispersive analysis of X-rays, wavelength dispersive analysis of X-rays and X-ray photoelectron spectroscopy. This CVD technique offers a rapid, convenient route to Ga_{2}O_{3}, which involves the in situ formation of dimethylgallium alkoxides, of the type [Me_{2}Ga(?-OR)]_{2} similar to compound 1.
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Mar 2010
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Abstract: Controlling a phase of matter by coherently manipulating specific vibrational modes has long been an attractive (yet elusive) goal for ultrafast science. Solids with strongly correlated electrons, in which even subtle crystallographic distortions can result in colossal changes of the electronic and magnetic properties, could be directed between competing phases by such selective vibrational excitation. In this way, the dynamics of the electronic ground state of the system become accessible, and new insight into the underlying physics might be gained. Here we report the ultrafast switching of the electronic phase of a magnetoresistive manganite via direct excitation of a phonon mode at 17-THz. A prompt, five-order-of-magnitude drop in resistivity is observed, associated with a non-equilibrium transition from the stable insulating phase to a metastable metallic phase. In contrast with light-induced1, 2, 3 and current-driven4 phase transitions, the vibrationally driven bandgap collapse observed here is not related to hot-carrier injection and is uniquely attributed to a large-amplitude Mn-O distortion. This corresponds to a perturbation of the perovskite-structure tolerance factor, which in turn controls the electronic bandwidth via inter-site orbital overlap5, 6. Phase control by coherent manipulation of selected metal-oxygen phonons should find extensive application in other complex solids-notably in copper oxide superconductors, in which the role of Cu-O vibrations on the electronic properties is currently controversial.
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Sep 2007
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Diamond Proposal Number(s):
[316]
Open Access
Abstract: The mechanism by which nucleotide-binding domains (NBDs) of ABC transporters power the transport of substrates across cell membranes is currently unclear. Here we report the crystal structure of an NBD, FbpC, from the Neisseria gonorrhoeae ferric iron uptake transporter with an unusual and substantial domain swap in the C-terminal regulatory domain. This entanglement suggests that FbpC is unable to open to the same extent as the homologous protein MalK. Using molecular dynamics we demonstrate that this is not the case: both NBDs open rapidly once ATP is removed. We conclude from this result that the closed structures of FbpC and MalK have higher free energies than their respective open states. This result has important implications for our understanding of the mechanism of power generation in ABC transporters, because the unwinding of this free energy ensures that the opening of these two NBDs is also powered.
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Sep 2009
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Abstract: This editorial serves as the preface to a special issue of New Journal of Physics, which collects together solicited papers on a common subject, x-ray beams with high coherence. We summarize the issue's content, and explain why there is so much current interest both in the sources themselves and in the applications to the study of the structure of matter and its fluctuations (both spontaneous and driven). As this collection demonstrates, the field brings together accelerator physics in the design of new sources, particle physics in the design of detectors, and chemical and materials scientists who make use of the coherent beams produced.
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Mar 2010
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