Star formation triggered by non-head-on cloud-cloud collisions, and clouds with pre-collision sub-structure

In an earlier paper, we used smoothed particle hydrodynamics (SPH) simulations to explore star formation triggered by head-on collisions between uniform-density 500 M clouds, and showed that there is a critical collision velocity, vCRIT. At collision velocities below vCRIT, a hub-and-spoke mode operates and delivers a monolithic cluster with a broad mass function, including massive stars (M 10 M) formed by competitive accretion. At collision velocities above vCRIT, a spider’s-web mode operates and delivers a loose distribution of small sub-clusters with a relatively narrow mass function and no massive stars. Here we show that,if the head-on assumption is relaxed, vCRIT is reduced. However, if the uniform-density assumption is also relaxed, the collision velocity becomes somewhat less critical: a low collision velocity is still needed to produce a global hub-and-spoke system and a monolithic cluster, but, even at high velocities, large cores – capable of supporting competitive accretion and thereby producing massive stars – can be produced. We conclude that cloud–cloud collisions may be a viable mechanism for forming massive stars – and we show that this might even be the major channel for forming massive stars in the Galaxy

Etudes électroanalytiques de processus de transfert de matière et de charge au sein de silices mésoporeuses organiquement modifiées

The first part of this study deals with the interactions between Hg(II) and mesoporous silicas displaying an organized structure and functionalized by thiol groups. The influence of the structure type (hexagonal or cubic), the way of functionalization (in situ co-condensation or post-synthesis grafting), the functionalization level and the pore size on the sorption capacities and Hg(II) transport rates were examined. The use of various composition media allows the development of some environmental applications, in solid-liquid extraction for instance, or in new amperometric sensors for Hg(II). In a second part, the electrochemical behaviour of mesoporous silicas functionalized by ferrocene groups was studied. The electron transfer reactions inside such systems imply probably a mechanism by electron hopping.La première partie de cette étude traite des interactions entre Hg(II) et des silices mésoporeuses à structure organisée, fonctionnalisées par des groupements thiol. L'influence du type de structure (hexagonale ou cubique), du mode de fonctionnalisation (co-condensation in situ ou greffage post-synthèse), du taux de fonctionnalisation et de la taille des pores sur la capacité de fixation des matériaux et la diffusion de Hg(II) dans les mésopores ont été examinés. L'utilisation de milieux de compositions variées (pH, cations métalliques potentiellement interférents) permet d'envisager des applications environnementales en extraction solide-liquide, ou bien dans le développement de capteurs ampérométriques de Hg(II). Dans une seconde partie, le comportement électrochimique de silices mésoporeuses fonctionnalisées par des groupements ferrocène a été étudié, les réactions de transfert électronique au sein de ces systèmes impliquant vraisemblablement un mécanisme par saut d'électrons

Analyse électrochimique des processus de transferts de matière au sein de silices mésoporeuses organiquement modifiées

Journée : du 02.07.2007 au 06.07.200

Voltammetric response of ferrocene-grafted mesoporous silica

International audienceThe electrochemical behavior of ferrocene moieties immobilized by covalent grafting on ordered mesoporous silica samples has been studied by cyclic voltammetry in aqueous medium using carbon paste electrodes modified with these solids. The functionalized silica samples were obtained using (3-(ferrocenylamide)propyl)triethoxysilane as the grafting agent. The results have been discussed in relation to the ferrocene content in the materials and compared to those obtained with non-ordered ferrocene-grafted silica gels. Well-defined voltammetric signals have been observed in spite of the insulating character of the mesoporous silica matrix and they were found to increase as a function of the ferrocene groups content in the material. They displayed a better stability upon continuous cycling potentials in comparison to those recorded with amorphous gels, suggesting a beneficial effect of the long-range structural order on the electron transfer processes in such confined media

Factors affecting the reactivity of thiol-functionalized mesoporous silica adsorbents toward mercury(II)

International audienceNumerous mercaptopropyl-functionalized silica spheres have been prepared by either post-synthesis grafting of MCM-41 and MCM-48 or self-assembly co-condensation of mercaptopropyltrimethoxysilane (MPTMS) or mercaptopropyltriethoxysilane (MPTES) and tetraethoxysilane (TEOS) precursors in hydroalcoholic medium in the presence of a cationic surfactant as templating agent and ammonia as catalyst. These materials of approximately the same particle size and morphology featured different functionalization levels, various degrees of structural order, and variable distribution of thiol groups in the mesopores. Their reactivity in solution has been studied using Hg(II) as model analyte. Total accessibility (on a 1:1 S:Hg stoichiometry basis) was demonstrated and quantified for well-ordered materials whereas less open and less organized structures with high degrees of functionalization were subject to less-than-complete loadings. Capacities measured at pH 2 were lower than at pH 4 because of distinct mercury-binding mechanisms. Kinetics associated to the uptake process were studied by in situ electrochemical monitoring of Hg(II) consumption from aqueous suspensions containing the various adsorbents. They indicate only little difference between materials of the MCM-41 and MCM-48 series at similar functionalization levels, fast mass transport in well-ordered mesostructures in comparison to the poorly or non-ordered ones (except at pH 2 where charge formation induced some restriction in materials characterized by long-range structural order), and even faster processes in the wormlike frameworks (characterized by shorter range structural order). Hg(II) binding to thiol-functionalized materials obtained by post-synthesis grafting was found to occur more rapidly in the early beginning of the uptake process as a result of a higher concentration of binding sites at the pore entrance in comparison to the more homogeneous distribution of these groups in the mesochannels of materials obtained by co-condensation

Dealloying of gold-copper alloy nanowires: From hillocks to ring-shaped nanopores.

We report on a novel fabrication approach of metal nanowires with complex surface. Taking advantage of nodular growth triggered by the presence of surface defects created intentionally on the substrate as well as the high tilt angle between the magnetron source axis and the normal to the substrate, metal nanowires containing hillocks emerging out of the surface can be created. The approach is demonstrated for several metals and alloys including gold, copper, silver, gold-copper and gold-silver. We demonstrate that applying an electrochemical dealloying process to the gold-copper alloy nanowire arrays allows for transforming the hillocks into ring-like shaped nanopores. The resulting porous gold nanowires exhibit a very high roughness and high specific surface making of them a promising candidate for the development of SERS-based sensors

Dealloying of gold–copper alloy nanowires: From hillocks to ring-shaped nanopores

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