L. Malier

Sol-gel transition in CdS colloids

We report the preparation of transparent gels from concentrated sols of CdS nanocrystals. The principles of our method can be applied to other systems, enlarging the domain of the inorganic sol–gel process which so far has been restricted mainly to oxides.

Sulfide Gels and Films: Products of Non-Oxide Gelation

The preparation of concentrated sols and transparent stiff gels of II-VI semiconductors nanocrystals is reported. A two-step process for the production of cadmium sulfide is reported. Sol stabilization and gelation control are achieved through successive passivation and depassivation of the surface of the nanocrystals which are complexed with thiols. The mechanisms driving the aggregation and the gelation are explained on the basis of NMR and SAXS experiments. Thin films as well as monoliths can be produced. The general principles of the method presented are not restricted to chalcogenide systems and thus enlarge the domain of application of the inorganic sol-gel process.

CdS nanoparticles and the sol-gel process

We present two approaches for the sol-gel synthesis of transparent solids in which CdS nanoparticles are homogeneously dispersed. In any case CdS nanocrystals are obtained by precipitation either in reverse micelles or in the presence of a complexing thiol. The grafting of the 4- fluorothiophenol at the surface of the particles leads to highly concentrated colloids in acetone. In a first approach, CdS/silica nanocomposites are prepared introducing this concentrated CdS colloid in a silica sol which contains 3- mercaptopropyltrimethoxysilane to ensure the homogeneity of the CdS dispersion. The luminescence properties of the materials are improved by using methyltriethoxysilane instead of tetraethoxysilane as the alkoxide precursor of the silica matrix. Further increase of the luminescence efficiency is also observed if a single Mn2+ ion is present inside the CdS particle. In a second approach, pure CdS nanocrystalline materials (bulk or thin films) are obtained from the sol-gel processing of the concentrated colloids stabilized by 4- fluorothiophenol. The mechanisms driving the aggregation of the particles and the gelation of the system are explicited. Thin films as well as monoliths can be produced. The general principles of this method are not restricted to chalcogenide materials, and should therefore enlarge the applications of the sol-gel proces to other non-oxide materials.

29Si NMR and viscosity study of the sol-gel transition and evolution after the gel time

Abstract In order to study the sol to gel transition and the early stage of aging, 29 Si NMR and viscosity measurements were performed on acid-catalyzed TEOS in excess of water, where evaporation is hindered. NMR spectra evolution was followed over an interval running from 0.5 t g to 4 t g ( t g is gelation time). The difference between static spectra and magic angle spinning (MAS) spectra, analyzed in terms of anisotropy of the chemical shift, allowed evaluation of the gel weight-fraction. Its evolution appeared to be quite slow. A kinetic model of post-gelation aggregation accounts for this time dependance. NMR measurements were not very accurate close to the transition. Rheology experiments over a frequency range from 10 −2 Hz to 100 Hz were performed in this region. Scalar percolation transition was observed within a very narrow interval.

HYBRID XEROGEL STRUCTURE AND MOLECULAR GUEST DYNAMICS

A new experimental approach to study the porosity of hybrid organic-inorganic glasses is reported. It relies upon nuclear magnetic resonance observation of molecular guest dynamics. It is shown that in MTEOS xerogels, there exists an interconnected organic-rich network in which organic guests are trapped and can eventually diffuse. The interpore translational diffusion is strongly activated and its spatial extension gives information on the pore connectivity, whose decay is characterized by the interpore jump probability given by the theoretical model.

Quenched molecular reorientation and angular velocity in nanopores

A theoretical treatment shows how the orientational dependent and spin rotation relaxation rates of a confined nonpolar liquid depend on the average pore size. Experimental nuclear relaxation data on carbon disulfide and cyclohexane in a set of calibrated porous glasses support the theory.

Surface Rotational Dynamics of Liquids Confined in Nanopores

2 H NMR relaxation times of selectively deuterated polar molecules confined to a set of calibrated nanoporous silica glasses are reported. These experiments, combined with the consideration of different time scales in the theory of surface relaxation, show how confinement effects can provide detailed information on the rotational dynamics of temporarily adsorbed liquid layers in presence of biphasic fast exchange.

Nuclear spin relaxation in alumino-silicate aerogels☆

Spin relaxation allows one to probe lattice dynamics at a local level. Measurements in aerogels and xerogels reveal an identical behaviour as compared with glasses obtained by thermal quenching. While all thermal variations can be accounted for by a large number of activated local vibrations, these cannot explain the observed density dependence. When the density is lowered without modification of the local structure, relaxation is accelerated.

Nuclear Spin Relaxation in Aerogels and Porous Glasses

It has been known for two decades that most of the physical properties are drastically different in amorphous materials as compared to their crystalline counterparts, specially when examined at low temperatures1,2. These specific behaviors have been accounted for by the phenomenological two-level-system (TLS) model3,4 or its more recent generalization through soft modes5. One of the properties specific to the glassy state concerns the nuclear magnetic relaxation, which presents common features over numerous inorganic glasses6,7,8. Though different mechanisms have been debated, all of them require the TLS model to account for the weak temperature-dependance of the relaxation. The lack of structural model for these TLS prevents precise estimation of their coupling with the spins and then an absolute evaluation of the relaxation. In the case of electron spin resonance in bio-polymers, relaxation involving fractal-structure vibrations (fractons) have been considered9. Primarily introduced by S.Alexander and R. Orbach10 on a theoretical basis, fractons have been experimentally tracked during these last years, with peculiar attention paid on aerogel dynamics11-14. Aerogels are materials whose fractal structure has been evidenced by small angle scattering15,16 in the reciprocal space and by nuclear spin relaxation in the direct space17. Their density of state, deduced from inelastic light and neutron scattering, presents a weak energy dependance in part of the giga-Hertz range (typically 3 to 100 GHz). Nevertheless, this density cannot be described by a single spectral dimension as initially proposed, and the relevance of scalar or tensorial models is still a subject of debate. Moreover, the characteristic structural length determined from small angle scattering and the one deduced from the dynamics measurements present disrepancies which are not yet understood.