Gérald Lelong

Physical and optical properties of sol-gel nano-silver doped silica film on glass substrate as a function of heat-treatment temperature

Nano-silver doped silica films were deposited on glass slides using a sol-gel process and heat-treated at different temperatures. The films were characterized by ultraviolet-visible spectroscopy, x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), Rutherford backscattering spectrometry (RBS), and transmission electron microscopy for their optical, chemical, and structural properties. The absorption peak of silver colloids (wavelength from 400 to 460 nm) was present and a blueshift and intensity reduction of the absorption peak was observed during heat-treatment. Particle size reduction and surface morphology changes in the films were observed by AFM as a function of varying heat treatment temperatures. Silver nanoparticles were formed through spontaneous reduction of silver ions. The oxidation of silver occurs during heat-treatment, causing a reduction of absorption intensity. An interdiffusion between the Ag in the film and Na in the substrate glass was observed by XPS and RBS. Sodium ...

Recent progress in the synthesis and selected applications of MCM-41: a short review

Recent progress in the synthesis and applications of MCM-41 based mesoporous materials is reviewed. Since the independent discovery in the early 1990s by groups in the Japan and USA of the formation of mesostructured silica using surfactants as structure directing agents, a variety of alternative synthesis routes have been proposed. These include the use of ionic (both cationic and anionic) surfactants, neutral surfactants based on block and star diblock copolymers, non-surfactant organic compounds and the Stöber process for synthesizing silica spheres. The unique properties of MCM-41 based silica materials make them attractive candidates for applications in catalysis, production of novel materials by encapsulating metals, semiconductors and biofluids. Particular attention is given to the use of these composites in biotechnology including biosensors, biocatalysis and drug delivery.

Molecular dynamics and neutron scattering study of glucose solutions confined in MCM-41.

Glucose aqueous solutions confined in MCM-41 cylindrical pores of diameter 3.2 nm have been studied by molecular dynamics (MD) simulations and quasielastic neutron scattering (QENS). MD simulations reveal a strong preferential interaction of glucose molecules with the silica walls, which induces significant concentration gradients within the pore. The influence of glucose on the structural and dynamical properties of water strongly depends on the region of the pore considered. The distortion of the hydrogen bond network (HBN) and of the tetrahedral organization of interfacial water molecules induced by silica is much stronger than that induced by glucose molecules. The interfacial glucose molecules diffuse about 1 order of magnitude slower than those in the core region. Differences in affinities for silica of the different species in confined hydrogen-bonded mixtures induce significant structural and dynamical heterogeneities not present in bulk solutions.

Translational and rotational dynamics of monosaccharide solutions

Molecular dynamics computer simulations have been carried out on aqueous solutions of glucose at concentrations bracketing those previously measured with quasi-elastic neutron scattering (QENS), in order to investigate the motions and interactions of the sugar and water molecules. In addition, QENS measurements have been carried out on fructose solutions to determine whether the effects previously observed for glucose apply to monosaccharide solutions. The simulations indicate a dynamical analogy between higher solute concentration and lower temperature that could provide a key explanation of the bioprotective phenomena observed in many living organisms. The experimental results on fructose solutions show qualitatively similar behavior to the glucose solutions. The dynamics of the water molecules are essentially the same, while the translational diffusion of the sugar molecules is slightly faster in the fructose solutions.

Crystal Structures of Li6B4O9 and Li3B11O18 and Application of the Dimensional Reduction Formalism to Lithium Borates

The crystal structures of two members of the Li2O-B2O3 phase diagram, Li6B4O9 and Li3B11O18, have been solved from single-crystal X-ray diffraction, and their structure has been further confirmed by Rietveld refinement on powder samples. Li6B4O9 crystallizes in the P2(1)/n space group with a = 3.31913(15) Å, b = 23.361(2) Å, c = 9.1582(4) Å, and β = 92.650(4)°. It is the only lithium borate being built upon clusters made of four BO3 triangular units linked by vertices. Li3B11O18 adopts also a monoclinic symmetry with a = 17.7607(8) Å, b = 7.7737(4) Å, c = 9.6731(4) Å, and β = 100.906(4)° (space group P2(1)/c); it contains 73% BO3 triangular units and 27% BO4 tetrahedra, linked by vertices such that it forms a 3D network containing B3O7 and B5O10 rings. These two new structures and their specificities are discussed in the framework of the dimensional reduction formalism together with other reported lithium borates and may serve as a crystalline reference to study borate glasses.

Dynamics of trehalose molecules in confined solutions.

The dynamics of trehalose molecules in aqueous solutions confined in silica gel have been studied by quasielastic neutron scattering (QENS). Small-angle neutron scattering measurements confirmed the absence of both sugar clustering and matrix deformation of the gels, indicating that the results obtained are representative of homogeneous trehalose solutions confined in a uniform matrix. The pore size in the gel is estimated to be 18 nm, comparable to the distances in cell membranes. For the QENS measurements, the gel was prepared from D2O in order to accentuate the scattering from the trehalose. Values for the translational diffusion constant and effective jump distance were derived from model fits to the scattering function. Comparison with QENS and NMR results in the literature for bulk trehalose shows that confinement on a length scale of 18 nm has no significant effect on the translational diffusion of trehalose molecules.

Molecular dynamics of confined glucose solutions

Silica gels containing solutions of glucose in heavy water at different concentrations have been prepared by a sol-gel method. Dynamical studies with quasielastic neutron scattering, compared with previous results on bulk solutions, show that the dynamics of the glucose molecules are not appreciably affected by the confinement, even though the gels behave macroscopically as solid materials. Small-angle neutron-scattering spectra on the same systems, fitted with a fractal model, yield a correlation length that decreases from 20 to 2.5 nm with increasing glucose concentration, suggesting a clustering of glucose molecules in concentrated solutions that is consistent with the dynamical measurements. These two sets of results imply that 20 nm is an upper limit for the scale at which the dynamics of glucose molecules in solution are affected by confinement.

Detecting Non-bridging Oxygens: Non-Resonant Inelastic X-ray Scattering in Crystalline Lithium Borates

Probing the local environment of low-Z elements, such as oxygen, is of great interest for understanding the atomic-scale behavior in materials, but it requires experimental techniques allowing it to work with versatile sample environments. In this paper, the local environment of lithium borate crystals is investigated using non-resonant inelastic X-ray scattering (NRIXS) at energy losses corresponding to the oxygen K-edge. Large variations of the spectral features are observed close to the edge onset in the 535-540 eV energy range when varying the Li2O content. Calculations allow identification of contributions associated with bridging oxygen (BO) and non-bridging oxygen (NBO) atoms. The main result resides in the observed core-level shift of about 1.7 eV in the spectral signatures of the BO and NBO. The clear signature at 535 eV in the O K-edge NRXIS spectrum is thus an original way to probe the presence of NBOs in borates, with the great advantage of making possible the use of complex environments such as a high-pressure cell or high-temperature device for in situ measurements.

Color-center production and recovery in electron-irradiated magnesium aluminate spinel and ceria

Single crystals of magnesium aluminate spinel (MgAl2O4) with (1 0 0) or (1 1 0) orientations and cerium dioxide or ceria (CeO2) were irradiated by 1.0 MeV and 2.5 MeV electrons in a high-fluence range. Point-defect production was studied by off-line UV-visible optical spectroscopy after irradiation. For spinel, regardless of both crystal orientation and electron energy, two characteristic broad bands centered at photon energies of 5.4 eV and 4.9 eV were assigned to F and F(+) centers (neutral and singly ionized oxygen vacancies), respectively, on the basis of available literature data. No clear differences in color-center formation were observed for the two crystal orientations. Using calculations from displacement cross sections by elastic collisions, these results are consistent with a very large threshold displacement energy (200 eV) for oxygen atoms at room temperature. A third very broad band centered at 3.7 eV might be attributed either to an oxygen hole center (V-type center) or an F2 dimer center (oxygen di-vacancy). The onset of recovery of these color centers took place at 200 °C with almost full bleaching at 600 °C. Activation energies (~0.3-0.4 eV) for defect recovery were deduced from the isochronal annealing data by using a first-order kinetics analysis. For ceria, a sub-band-gap absorption feature, which peaked at ~3.1 eV, was recorded for 2.5 MeV electron irradiation only. Assuming a ballistic process, we suggest that the latter defect might result from cerium atom displacement on the basis of computed cross sections.

Assessment of Transition Element Speciation in Glasses Using a Portable Transmission Ultraviolet–Visible–Near-Infrared (UV-Vis-NIR) Spectrometer

A new low-cost experimental setup based on two compact dispersive optical spectrometers has been developed to measure optical absorption transmission spectra over the 350–2500 nm energy range. We demonstrate how near-infrared (NIR) data are essential to identify the coloring species in addition to ultraviolet visible data. After calibration with reference glasses, the use of an original sample stage that maintains the window panel in the vertical position enables the comparison of ancient and modern glasses embedded in a panel from the Sainte-Chapelle of Paris, without any sampling. The spectral resolution enables to observe fine resonances arising in the absorption bands of Cr3+, and the complementary information obtained in the NIR enables to determine the contribution of Fe2+, a key indicator of glassmaking conditions.