Formation mechanism of Y-junctions in arrays of multi-walled carbon nanotubes

in Colloids and Surfaces A : Physicochemical and Engineering Aspects (2008), 327(1-3), 140-143

Transmission electron microscopy of arrays of multi-walled carbon nanotubes reveals the presence of numerous Y-junctions. The diameter distributions of the samples are found to be bimodal, with one mode ... [more ▼]

Transmission electron microscopy of arrays of multi-walled carbon nanotubes reveals the presence of numerous Y-junctions. The diameter distributions of the samples are found to be bimodal, with one mode corresponding to the branches and the other mode to the trunk of the Y-junctions. The analysis of the micrographs shows that the junctions form via the merger of metal particles during the tip-growth of the tubes. (C) 2008 Elsevier B.V. All rights reserved. [less ▲]

Critical opalescence points to thermodynamic instability:relevance to small-angle X-ray scattering of resorcinol–formaldehyde gel formation at low pH

in Journal of Applied Crystallography (2008), 41(4), 663-668

During the formation at low pH of resorcinol–formaldehyde gels with a structure in the micrometre range, small-angle X-ray scattering exhibits a nonmonotonic intensity variation as a function of reaction ... [more ▼]

During the formation at low pH of resorcinol–formaldehyde gels with a structure in the micrometre range, small-angle X-ray scattering exhibits a nonmonotonic intensity variation as a function of reaction time. The data are analyzed in terms of scattering by statistical fluctuations of polymer concentration, the amplitude of which is maximal close to the critical point for phase separation between polymer and solvent. The data do not carry any morphological information, but they unambiguously show that the driving force of the gel formation is a thermodynamic instability of the polymerizing solution. [less ▲]

Relevance of Spinodal Decomposition for Support Formation and Metal Dispersion in Cogelled Pd/SiO2 Catalysts

in Journal of Physical Chemistry C (2007), 111

The structure formation of cogelled Pd/SiO2 catalysts is followed in situ at the nanometer scale using timeresolved small-angle X-ray scattering (SAXS). The SAXS patterns are analyzed in terms of a ... [more ▼]

The structure formation of cogelled Pd/SiO2 catalysts is followed in situ at the nanometer scale using timeresolved small-angle X-ray scattering (SAXS). The SAXS patterns are analyzed in terms of a reaction-induced spinodal decomposition that is responsible for the formation of the silica support and most likely for the metal dispersion as well. The results are discussed in the light of recent electron tomography characterizations of the catalysts. The latter technique reveals regularity in the metal dispersion, the origin of which is likely to be the very occurrence of a spinodal phase separation. [less ▲]

Characterization of gels via solvent desorption measurements

in Adsorption (2007), 13(5-6), 533-540

The present paper shows how a standard volumetric adsorption device can be used to measure solvent desorption isotherms on gels. As gels are very soft materials, they shrink significantly during the ... [more ▼]

The present paper shows how a standard volumetric adsorption device can be used to measure solvent desorption isotherms on gels. As gels are very soft materials, they shrink significantly during the measurement, and the data have to be analyzed in terms of the mechanical properties of the gel's skeleton. Methanol desorption isotherms are measured on a series of silica gels, and the results are compared with independent characterizations, notably beam bending. [less ▲]

Positive curvature effects and interparticle capillary condensation during nitrogen adsorption in particulate porous materials

in Journal of Colloid and Interface Science (2007), 314

The adsorption of nitrogen in a collection of spheres that touch or merge in a sintering-like manner is modeled using a Derjaguin–Broeckhof–de Boer approach. The proposed model accounts for both positive ... [more ▼]

The adsorption of nitrogen in a collection of spheres that touch or merge in a sintering-like manner is modeled using a Derjaguin–Broeckhof–de Boer approach. The proposed model accounts for both positive curvature effects and for capillary condensation at the contact between two spheres. A methodology is proposed to fit the P/P0 > 0.4 adsorption region with the coordination number of the spheres as the only adjustable parameter. The use of the model is illustrated on a series of silica aerogels. The suitability of various standard isotherms needed for the modeling is also discussed. [less ▲]

Time-resolved SAXS study of the formation of resorcinol-formaldehyde aqueous gels

Poster (2006, August 06)

Structure of Silica Xerogels Synthesized with Organoalkoxysilane Co-reactants Hints at Multiple Phase Separation

in Journal of Physical Chemistry B (2006), 110

The microstructure of hybrid silica xerogels synthesized by the base-catalyzed polymerization of tetraethoxysilane (TEOS) in ethanol in the presence of 3-aminopropyltriethoxysilane (AES) and of 3-(2 ... [more ▼]

The microstructure of hybrid silica xerogels synthesized by the base-catalyzed polymerization of tetraethoxysilane (TEOS) in ethanol in the presence of 3-aminopropyltriethoxysilane (AES) and of 3-(2-aminoethylamino)- propyltrimethoxysilane (EDAS) as co-reactants, and dried in subcritical conditions, is analyzed. A thorough structural characterization of the samples is performed combining nitrogen adsorption, small-angle X-ray scattering (SAXS), and transmission electron microscopy coupled with digital image analysis. The use of these methods shows that, for both co-reactants, the xerogels are made of macropores supported by filaments, with each filament being formed of smaller structures. The quantitative impact of the additive on each structural level is assessed. The data are compared with a previous time-resolved SAXS study conducted during the formation of the gels (J. Phys. Chem. B 2004, 108, 8983-8991). The results are analyzed in the framework of a double phase separation model. [less ▲]

Quantitative morphological studies of mesoporous catalysts at nanometer scale resolution

in Studies in Surface Science and Catalysis (2005), 158

We have combined electron tomography with posterior image processing for a quantitative morphological study of mesoporous catalysts at nanometer scale resolution. As a first example for the possibilities ... [more ▼]

We have combined electron tomography with posterior image processing for a quantitative morphological study of mesoporous catalysts at nanometer scale resolution. As a first example for the possibilities of the approach, pore size distributions of single particles of commercial USY and XVUSY zeolites were derived. [less ▲]

Qualitative assessment of the purity of multi-walled carbon nanotube samples using krypton adsorption

in Studies in Surface Science and Catalysis (2005), 160

Krypton is a subcritical vapour at the nitrogen boiling temperature. As such, its adsorption on crystalline surfaces leads to condensation steps, typical of type VI isotherms according to IUPAC, while its ... [more ▼]

Krypton is a subcritical vapour at the nitrogen boiling temperature. As such, its adsorption on crystalline surfaces leads to condensation steps, typical of type VI isotherms according to IUPAC, while its adsorption on rough surfaces is BET-like. Based on this property of krypton adsorption at 77 K, a methodology is proposed to determine the purity of carbon nanotubes samples. The method is tested on model samples obtained by mixing mechanically purified multi-walled carbon nanotubes with various amounts of the same catalyst as used for their synthesis. [less ▲]

Assessment of the 3D Localization of Metallic Nanoparticles in Pd/SiO2 Cogelled Catalysts by Electron Tomography

in Langmuir (2005), 21

The purpose of this study is to analyze the localization of palladium nanoparticles within their silica support, in two heterogeneous catalysts synthesized by the sol-gel process, with different metal ... [more ▼]

The purpose of this study is to analyze the localization of palladium nanoparticles within their silica support, in two heterogeneous catalysts synthesized by the sol-gel process, with different metal loadings. Electron tomography demonstrates that the palladium particles are localized deep inside the silica skeleton. The use of digital image analysis further shows that the dispersion of palladium is optimal in the sample with the lowest loading. The particles are regularly spaced in the middle of the silica skeleton, with a distance between them comparable to the diameter of the struts of silica. [less ▲]