Showing papers by "Florence Babonneau published in 2000"

The true structure of hexagonal mesophase-templated silica films as revealed by X-ray scattering : Effects of thermal treatments and of nanoparticle seeding

Abstract: This work describes the detailed structural investigation of mesophase-templated mesoporous silica films by 1D and 2D X-ray scattering techniques and transmission electron microscopy. The films are prepared by sol−gel dip coating with 2D hexagonal templating mesophases, yielding 2D mesoporous structures consisting of cylindrical pores whose axes are aligned parallel to the surface. It is shown that drying and thermal treatments induce an unidirectional shrinkage of the layers in the direction of the normal of the film. The true rectangular symmetry is only evidenced by 2D X-ray scattering in two different scattering geometries. 1D diffraction gives only an apparent hexagonal symmetry. It is furthermore shown that although the cylinder axes are randomly orientated within the plane parallel to the surface, there are large domains with well aligned 2D planar unit cells perpendicular to the surface. It is demonstrated that this preferential ordering is destroyed by nanoparticle seeding with amorphous silica o...

Highly oriented 3D-hexagonal silica thinfilms produced with cetyltrimethylammonium bromide

Abstract: Mesoporous silica thin films have been produced by sol–gel chemistry in the presence of cetyltrimethylammonium bromide (CTAB) template. The films were deposited on silicon or glass substrates by dip-coating and underwent different treatments to eliminate the CTAB and create porosity. As-prepared and treated coatings exhibit good optical quality. Their structures were fully characterised by transmission electron microscopy (TEM) performed on film cross-sections and by X-ray diffraction (XRD) in θ–2θ scan mode, as well as in transmission mode using two different scattering geometries. The films exhibit large and homogeneous domains organised in a 3D-hexagonal (P63/mmc) structure with the c axis normal to the surface throughout their whole thickness. Numerical analysis of the TEM pictures confirms the space group deduced from the XRD measurements. To our knowledge, these are the first reported thin films obtained by dip-coating in the presence of CTAB which show such extended and highly mono-oriented 3D-hexagonal (P63/mmc) domains. The film thickness, porosity and refractive index were evaluated by ellipsometry for the various treated films.

Competitive Polymerization between Organic and Inorganic Networks in Hybrid Materials

Abstract: Hybrid organic−inorganic materials derived from 3-glycidoxypropyltrimethoxysilane have been synthesized via sol−gel using BF3 etherate as a catalyst of organic polymerization. Different amounts of BF3 etherate have been used, from B/Si = 0.01 up to B/Si = 0.1, to prepare powder samples. The materials have been characterized by Fourier transform infrared spectroscopy (FTIR) and multinuclear solid-state NMR spectroscopy. FTIR and NMR spectra have shown that with a B/Si = 0.03 molar ratio a complete opening of epoxy in 3-glycidoxypropyltrimethoxysilane is achieved. The different amounts of BF3 etherate employed have been found to affect the degree of inorganic cross-linking and the length of the ethylene oxide chains. Larger amounts of BF3 enhance the inorganic network cross-linking and give oligo(ethylene oxide) derivatives. Boron has been found to remain in the matrix as a network former of the inorganic side. The formation of Si−O−B bonds has been observed by the FTIR spectra.

Hybrid RSiO1.5/B2O3 gels from modified silicon alkoxides and boric acid

Abstract: A new sol-gel route, without any external water addition, for the preparation of hybrid RSiO1.5/B2O3 gels (R = Me, Et, Vi) is reported. The gels are easily synthesized by reacting B(OH)3 with the liquid silicon alkoxide. The gels have been characterized mainly by FT-IR and 11B MAS NMR spectroscopy. The results show that this new sol-gel process allows the formation of a homogeneous borosilicate gel in which trigonal BO3 units are incorporated into the siloxane network via B—O—Si bonds.

Neutral alkoxysilanes from silica

Abstract: Silica (SiO2) is found to react readily with ethylene glycol (EGH2) to form neutral glycoxysilanes in the presence of catalytic amounts of high-boiling organic amines, such as triethylenetetramine (TETA), trishydroxymethyleneaminomethane [H2NC(CH2OH)3, THAMH3], and triethanolamine [N(CH2CH2OH)3, TEAH3]. Kinetic studies show that these amines offer similar catalytic efficiencies although their pKb values differ by 3 orders of magnitude. In addition, silica dissolution is found to be pseudo-zero order in silica. These kinetic data can be explained by a rate-limiting step involving release of free base from an intermediate pentacoordinated silicate coincident with the formation of a tetraalkoxysilane. The products from these reactions were characterized by 1H, 13C, and 29Si solution and solid-state NMR, thermal gravimetric analysis, and mass spectroscopy. Depending on the type and amount of base used, different products form: either neutral tetraalkoxysilanes, such as Si(OCH2CH2OH)4 and its soluble oligomer...

Phenyl-functionalized silicate mesophases with hexagonal or cubic symmetries: influence of synthesis parameters.

Abstract: Organically modified porous silicates have been prepared under acidic conditions, by direct reaction of a mixture of phenyltriethoxysilane (PTES) and tetraethoxysilane (TEOS), and an aqueous solution of cetyltrimethylammonium bromide (C.TAB). For a 1:4 molar ratio between PTES and TEOS, the hexagonal (2d, p6m) phase, but also a cubic phase analogous to the already reported SBA-1 phase (Pm3n), can be prepared. The surfactant can then be efficiently removed by calcination at 350°C, leading to phenyl-functionalized microporous silicates with two types of architecture. The influence of several parameters (PTES/TEOS ratio; ethanol content) that affect the organization of the samples, will be discussed.

Preparation and properties of silica hybrid gels containing phytic acid

Abstract: Silica–phosphate mixed gels have been prepared by the sol–gel process from tetramethoxysilane and phytic acid (IP6H12). They contain inositol phosphate ester units anchored by covalent P–O–X bonds (X = P or Si) as isolated polyphytate clusters either embedded or bound in the amorphous network. 31P NMR of the liquid phases at low temperature was used to monitor the initial steps of hydrolysis–condensation reactions at the sol stage. Dried gels have been characterized by 13C, 29Si and 31P MAS NMR, powder X-ray diffraction and FTIR. The ability of these gels to sequestrate metallic ions, such as La3+ and Zn2+, on the open phosphate sites has been investigated. The resulting samples undergo structural modifications during the heat treatment which leads to the formation of silica–lanthanum phosphate and silica–zinc pyrophosphate mixtures. The specific influence of the inserted cation on the structure of the final silica–phosphate obtained at 600 and 900 °C is discussed.

Newly Applied Two-Dimensional Solid-State NMR Correlation Techniques for the Characterization of Organically Modified Silicates

Abstract: This paper highlights the use of two-dimensional (2D) solid-state NMR correlation techniques to probe the chemical homogeneity of organically modified silicate networks. Specifically, 29Si{1H} heteronuclear correlation (HETCOR) NMR experiments have revealed the spatial proximity of the two types of Si units present in a gel obtained from co-hydrolysis of methyldiethoxysilane and triethoxysilane. Similar information has also been obtained by using 2D 1H homonuclear correlation NMR spectroscopy. Such experiments were only possible by combining the use of high magnetic field (14.10 T) with fast MAS spinning rate (30 kHz).

Time-resolved in-situ X-ray diffraction study of the formation of 3D-hexagonal mesoporous silica films

Abstract: Highly oriented 3D-hexagonal silica thin films have been produced on silicon substrates by dip-coating technique, using cetyltrimethylammonium (CTAB) bromide as structuring agent. For the first time, time-resolved in situ X-ray diffraction experiments have been used to investigate the formation of such mesostructured films. Interestingly, the data shows that an intermediate mesophase appears just after film deposition (t<500 ms), characterized by one diffraction peak centered at a d spacing of 65 A. Then after 20 s, another mesophase appears with a shorter d spacing (44 A), which corresponds to the 3D-hexagonal structure found for the final dried film.

Structural analysis of hexagonal mesoporous silica films produced from triblock-copolymer-structuring sol-gel

Abstract: The increasing interest in mesoporous materials in various fields of application has motivated the investigation of novel high-technology thin films with pore size, structure and organisation controlled by the presence of directing agents. The design of optical coatings made by this method is described in this work. Silica thin films with Id-hexagonal mesoporous structure were deposited on silicon and glass substrates from a silicate precursor solution, containing a non-ionic triblock copolymer (EO106PO70EO106 : F127) as structuring agent. 70 to 700 nm thick films, with an excellent optical quality, were prepared by single dip-coating at a constant withdrawal rate. The surfactant was removed from the coatings by various treatments (i.e. thermal treatment, washing, ultra-sonic bath or sohxlet extraction). XRD investigations showed that shrinkage or collapsing of the film network can take place upon removal of the surfactant. Profiles of the relative amounts of surfactant, present in the pores, were measured by Rutherford Back Scattering. Refractive index, thickness and porosity of untreated and treated coatings were deduced from ellipsometry measurements. Each treated film exhibits an uni-axial anisotropy. The highest porosity (46%) and the lowest structural degradation are obtained for samples pre-treated at 400 K before extraction of the surfactant by sohxlet technique, as a result of the rigidification of the silica matrix.

Study of Amorphous Compounds Obtained by Ball Milling in the Si–C–O System

Abstract: Generally, the Si–C–O system is composed of SiO2, SiC, and pure C as crystalline phases. In the present study, we focus on the preparation of ternary silicon oxycarbide compound. For this purpose, different mixtures of quartz, silicon, and graphite were ball milled to cover the following range of composition: (1) SiC x O2(1 − x) + excess of C; (2) SiC x O2(1 − x) stoichiometric; (3) SiC x O2(1 − x) + excess of Si. The course of the reaction is followed by X-ray diffraction, by measuring the relative intensity change of the Bragg's peaks as a function of the mechanical treatment. The Rietveld method is applied to the patterns for quantitative analysis and determination of crystallite size and microstrain. Finally, the behavior of each phase is reported across the three starting compositions examined here and the presence of silicon oxycarbide compounds induced by ball milling is assessed by [29Si-MAS]NMR.