Julien Cambedouzou

Insights into the interplay between molecular structure and diffusional motion in 1-alkyl-3-methylimidazolium ionic liquids: a combined PFG NMR and X-ray scattering study

We report on how the local structure and the diffusional motion change upon increasing the alkyl chain length in 1-alkyl-3-methylimidazolium cation ionic liquids. This study has been performed by combining pulse field gradient (PFG) nuclear magnetic resonance (NMR) spectroscopy and small angle X-ray scattering (SAXS) experiments. The cationic side chain length varies from ethyl (n = 2) to hexadodecyl (n = 16), while the anion is always bis(trifluoromethanesulfonyl)imide (TFSI). We find that the self-diffusivity of the individual ionic species is correlated to the local structure in the corresponding ionic liquid, namely the nano-segregation into polar and non-polar domains. In agreement with previous results, we observe that for relatively short alkyl chains the cations diffuse faster than the anions; however we also note that this difference becomes less evident for longer alkyl chains and a cross-over is identified at n ≈ 8 with the anions diffusing faster than the cations. Our results indicate that this controversial behavior can be rationalized in terms of different types of cation-cation and anion-anion orderings, as revealed by a detailed analysis of the correlation lengths and their dispersion curves obtained from SAXS data. We also discuss the validity of the Stokes-Einstein relation for these ionic liquids and the evolution of the extrapolated cationic radius that was found to depend non-strictly linearly on n, in agreement with the cation-cation correlation lengths.

Stability of mesoporous silica under acidic conditions

The stability of various structured mesoporous materials (SBA-1, SBA-3, SBA-15, MCM-41, MCM-48) was tested under acidic stress in order to understand the mechanism of their alteration. The native materials and the materials resulting from acidic attack under various conditions were analysed by small angle X-ray scattering coupled with nitrogen adsorption and solid 29Si NMR. Among three acid media (HCl, H2SO4, H3PO4) at similar ionic strength, we focused particularly on phosphoric acid, which proved to have the strongest impact on the starting materials. The results are discussed by correlating the wall thickness and pore diameter values with regard to stability. In particular, the alteration kinetics were interpreted for the case of SBA-15 in the following scenario: the micropores were first damaged by the acidic stress, and then the mesopores were partially collapsed or partially blocked. We show that a materials resistance against acidic media is linked to a critical wall thickness–pore diameter threshold.

Self-Assembly of Condensable “Bola-Amphiphiles” in Water/Tetraethoxysilane Mixtures for the Elaboration of Mesostructured Hybrid Materials

The self-assembly of condensable amphiphile molecules in water is an attractive approach for the synthesis of mesostructured hybrid materials. In this article, we focus on aminoundecyltriethoxysilane (AUT), a condensable bola-amphiphile, i.e., an amphiphilic molecule possessing two polar heads on both sides of an aliphatic chain. In the present case, one side is a condensable triethoxysilane, and the other side is an amino group. We report on the self-assembly of AUT in mixtures of water and tetraethoxysilane (TEOS). In situ small-angle X-ray scattering (SAXS) measurements allowed us to follow the evolution of the structure from the liquid state up to the solid material formed upon catalytic polycondensation. Depending on the medium composition, hexagonal or lamellar structures can be observed in the final material. These observations allowed us to propose a model for the self-assembly of AUT in water/TEOS mixtures that we were able to validate by simulations of the SAXS profiles. By taking advantage of the modularity of such a system, it proves possible to prepare in a simple way various structured hybrid materials possessing a high number of available organic functions without using sacrificial surfactant molecules.

Quantitative small-angle scattering on mesoporous silica powders: from morphological features to specific surface estimation

A comprehensive method allowing experimentalists to perform a quantitative analysis of small-angle scattering patterns of powdered mesoporous materials of hexagonal symmetry is presented in this paper. Thanks to the rigorous processing of experimental data, and to a straightforward model for the small-angle scattering data simulation, the direct comparison of experimental and calculated patterns is made without any artificial background correction or ad hoc function. This allows the specific surface of mesopores to be estimated without resorting to other methods such as adsorption methods. This approach and its precision are discussed on the basis of the analysis of two real samples.

Tuning the Nanostructure of Highly Functionalized Silica Using Amphiphilic Organosilanes: Curvature Agent Effects

The self-assembly of amino-undecyl-triethoxysilane (AUT) as micelles in water is considered. The behavior of acid/AUT systems is governed by a complete proton transfer from the acid to the amine, leading to the formation of an ammonium headgroup. This moiety is responsible for the bending of the interface between the organic core of the micelles and the surrounding water. By playing with the size of the acid used as curvature agent, the amphiphilic behavior of the organosilane molecule may be adjusted. We follow the aggregation as the curvature agent size increases. This approach constitutes an efficient and original method in order to tune the nanostructure of highly functionalized silica at the early stage of the elaboration. Small-angle X-ray scattering, wet scanning transmission electron microscopy, dynamic light scattering, and complementary characterization techniques indicate that hybrid organic-inorganic planar objects and vesicles are obtained for smaller curvature agents. Increasing the size of the curvature agent results in a transition of the aggregation geometry from vesicles to cylindrical direct micelles, finally leading to nanofibers organized in a 2D hexagonal network resembling a reverse MCM-41 structure. A geometrical molecular self-assembly model is finally proposed, considering the dimensions of the surfactant tail and those of the head groups.

Structural study of hybrid silica bilayers from “bola-amphiphile” organosilane precursors: catalytic and thermal effects

Hybrid materials are obtained upon acid or base-catalyzed hydrolytic condensation of bola-amphiphile organosilanes such as amino-undecyl-triethoxysilane and amino-oxy-undecyl-triethoxysilane. The self-assembling process of the amphiphile organosilanes is described in the light of several experimental techniques involving small angle X-ray scattering, light scattering, Raman, solid-state NMR and thermal analysis. The structural features of the nanostructured organic/inorganic hybrids are discussed as a function of catalytic and thermal effects. Lamellar assemblies were systematically observed, albeit with different structural evolutions during the densification mechanism depending on the catalytic conditions.

Elaborating ordered silicon carbide nanorods by preceramic polymer nanocasting

Meso- and micro-porous silicon carbide (SiC) ceramics have been successfully synthesized via a nanocasting process. These materials exhibit very high specific surface areas (from 240 to 760 m2 g−1) and present morphologies of highly ordered SiC nanorods. Liquid allylhydropolycarbosilane or poly-1,3,5-trisilacyclohexane (pTSCH) were used as starting pre-ceramic polymers and were casted in a meso-porous silica template (SBA-15) which has undergone densification treatment. After thermal conversion, silica hard templates were subsequently removed by an aqueous hydrofluoric acid treatment leading to SiC inverse replicas of the template. The prepared meso-porous SiC products were thoroughly analyzed using small and wide angle X-ray scattering, electronic microscopies and nitrogen physisorption experiments. From our observations, the preparation procedure preserves the nanoscale structure of the silica templates and leads to highly porous SiC materials with tunable morphologies and specific surface areas.

Organosilica-metallic sandwich materials as precursors for palladium and platinum nanoparticle synthesis

A promising strategy to design highly active catalysts is to use multicomponent catalysts including active metals (such as platinoids) and organosilica as a substrate. In this article, we develop stable OrganoSilica Sandwiches (OSS) consisting of sandwiched metallic layers between organosilica layers. The formation of such a sandwich-like organosilica@platinoid@organosilica material was obtained from bis-silica-alkylammonium carbamate dimers, acting as a template for the immobilization of platinoid salt complexes or metallic nanoparticles in the interlayer space after a reduction treatment. The layered templates and the sandwich structures were investigated by small angle X-ray scattering, coupled thermogravimetric analysis and mass spectrometry, and Raman spectroscopy. It was demonstrated that the meso-structure of the initial template material is not affected by the chemical steps involved in the synthesis of the material and does not depend on the incorporated metal.

Copper hexacyanoferrate functionalized single-walled carbon nanotubes for selective cesium extraction

Single-walled carbon nanotubes (SWCNTs) are functionalized with copper hexacyanoferrate (CuHCF) nanoparticles and therefore constitutes promising solid substrates for the sorption of Cs+ ions from liquid effluents. The high mechanical resistance and large electrical conductivity of SWCNTs are associated to the ability of CuHCF nanoparticles to selectively complex Cs+ ions in order to achieve membrane-like buckypapers presenting high loading capacity of cesium. The materials are thoroughly characterized using electron microscopy, Raman scattering, X-ray photoelectron spectroscopy and thermogravimetric analyses. Cs sorption isotherms are plotted after liquid phase ionic chromatography of the Cs solutions before and after exposure to the materials. It is found that the total sorption capacity of the material reaches 230 mg.g-1, and that one third of the sorbed Cs (80 mg.g-1) is selectively complexed in the CuHCF nanoparticles grafted on SWCNTs. These high values open interesting perspectives in the integration of such materials in electrically driven devices for the controlled sorption and desorption of these ions.

Tuning the morphology of functionalized silica using amphiphilic organosilanes

The co-condensation of tetraethoxysilane and an amphiphilic organosilane precursor containing an ammonium part is investigated in presence of different counter-ions. We highlight the morphological versatility offered by such “one pot” synthesis, resulting of the combinatory effects of the addition of tetraethoxysilane and of the nature of the counter-ion involved in the sol-gel process. Indeed, the interactions at the interface governed by the ammonium species affect the shape of the aggregates, making of the counter-ion a critical morphological parameter. The morphology of the particles is also closely linked to the homogeneity of the system. Thereby, using the immiscibility of the tetraethoxysilane/water mixture, we showed that it is possible to get macroporous, blackberry-like or cerasome materials. Oppositely, the use of tetrahydrofuran able to solubilize all the systems leads to monodisperse nanoparticles whose size can be modulated as a function of the involved counter ion.Graphical Abstract