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Moving from surfactant-stabilized aqueous rhodium (0) colloidal suspension to heterogeneous magnetite-supported rhodium nanocatalysts: Synthesis, characterization and catalytic performance in hydrogenation reactions

Pelisson, Carl-Hugo; Vono, Lucas L. R.; Hubert, Claudie; Denicourt-Nowicki, Audrey; Rossi, Liane M.; Roucoux, Alain
Fonte: ELSEVIER SCIENCE BV; AMSTERDAM Publicador: ELSEVIER SCIENCE BV; AMSTERDAM
Tipo: Artigo de Revista Científica
ENG
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Wet impregnation of pre-synthesized surfactant-stabilized aqueous rhodium (0) colloidal suspension on silica was employed in order to prepare supported Rh-0 nanoparticles of well-defined composition, morphology and size. A magnetic core-shell support of silica (Fe(3)O4@SiO2) was used to increase the handling properties of the obtained nanoheterogeneous catalyst. The nanocomposite catalyst Fe3O4@SiO2-Rh-0 NPs was highly active in the solventless hydrogenation of model olefins and aromatic substrates under mild conditions with turnover frequencies up to 143,000 h(-1). The catalyst was characterized by various transmission electron microscopy techniques showing well-dispersed rhodium nanoparticles (similar to 3 nm) mainly located at the periphery of the silica coating. The heterogeneous magnetite-supported nanocatalyst was investigated in the hydrogenation of cyclohexene and compared to the previous surfactant-stabilized aqueous Rh-0 colloidal suspension and various silica-supported Rh-0 nanoparticles. Finally, the composite catalyst could be reused in several runs after magnetic separation. (C) 2011 Elsevier B. V. All rights reserved.; Region Bretagne [MAGFOCAT program, PARTICAT program]; Region Bretagne; Universite Europeenne de Bretagne; Universite Europeenne de Bretagne; International Cooperation Programs PICS BrazilFrance; International Cooperation Programs PICS Brazil-France [490678/2008-4...

Synthesis of supported metal nanoparticle catalysts using ligand assisted methods

Costa, Natália J. S.; Rossi, Liane M.
Fonte: ROYAL SOC CHEMISTRY; CAMBRIDGE Publicador: ROYAL SOC CHEMISTRY; CAMBRIDGE
Tipo: Artigo de Revista Científica
ENG
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The synthesis and characterization methods of metal nanoparticles (NPs) have advanced greatly in the last few decades, allowing an increasing understanding of structure-property-performance relationships. However, the role played by the ligands used as stabilizers for metal NPs synthesis or for NPs immobilization on solid supports has been underestimated. Here, we highlight some recent progress in the preparation of supported metal NPs with the assistance of ligands in solution or grafted on solid supports, a modified deposition-reduction method, with special attention to the effects on NPs size, metal-support interactions and, more importantly, catalytic activities. After presenting the general strategies in metal NP synthesis assisted by ligands grafted on solid supports, we highlight some recent progress in the deposition of pre-formed colloidal NPs on functionalized solids. Another important aspect that will be reviewed is related to the separation and recovery of NPs. Finally, we will outline our personal understanding and perspectives on the use of supported metal NPs prepared through ligand-assisted methods.; FAPESP; CAPES; CNPq; INCT-Catalise

Singularidades associadas ao mecanismo de estabilização eletroestérico e seus efeitos sobre o comportamento reológico de suspensões concentradas.; Singularities related to the electrosteric stabilization mechanism and their effects over the rheological behaviour of concentrated suspensions.

Guimarães, Kleber Lanigra
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 13/11/2008 PT
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O presente projeto de trabalho buscou avaliar e estabelecer relações de causa e efeito que justificassem a ocorrência de singularidades no comportamento reológico de formulações complexas contendo elevada carga de sólidos em suspensão incorporada. O mecanismo de estabilização coloidal empregado para preparação das formulações e posterior análise foi o mecanismo de estabilização eletroestérico. O caso particular estudado correspondeu à dispersão de cargas inorgânicas (óxido de alumínio) em meio aquoso auxiliada pela incorporação de conteúdo orgânico com ação dispersante (poliacrilato de amônio) e ligante (emulsão acrílica) respectivamente. A implementação de técnicas analíticas complementares permitiu o estudo detalhado de uma singularidade associada ao comportamento reológico do sistema, exclusivamente evidenciada em função da elevada carga de sólidos, oferecendo, portanto, subsídios para elaboração de um modelo supramolecular fundamentado em possíveis alterações de configuração das cadeias poliméricas para justificativa do fenômeno. A contribuição referente ao pleno entendimento do fenômeno, considerando-se suas causas e efeitos, reside na possibilidade de se estabelecer diretrizes de formulação que propiciem a otimização das quantidades de aditivos incorporados e o respectivo controle do estado de dispersão e estabilidade do sistema. Considerou-se...

Caracterização ácido-base de polieletrólitos e suspensões coloidais de ocorrência em sistemas aquáticos; Acid base characterization of the polyeletrolytes and colloidal suspensions of occurrence in aquatic systems

Lima, Elizabete Campos de
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 11/03/1999 PT
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Foram estudadas as propriedades ácido-base de alguns polieletrólitos de interesse ambiental; no sentido de se avaliar a adequabilidade do modelo de sítios discretos na determinação e caracterização dos grupos ionizáveis presentes nestas macromoléculas. Estudou-se os seguintes sistemas de polieletrólitos: ácido húmico comercial (Aldrich) e extraído de vermicomposto (Humus de minhoca); microalga Spirulina (cianobactéria), ácido algínico e ácido poliacrílico de massas molares 2000 e 250000g/mol. Foram realizadas titulações potenciométricas alcalimétricas em diferentes meios iônicos à 25°C com os compostos acima citados. Os dados obtidos foram tratados por linearização (Funções de Gran Modificadas) e regressão não-linear. Os resultados obtidos pelo ácido húmico extraído de vermicomposto (humus de minhoca) e da microalga Spirulina mostraram-se indepentes da força iônica; indicando que o modelo de sítios discretos representa adequadamente os dados experimentais. Para as demais amostras, a estequiometria dos grupos determinados mostrou-se independente do meio iônico, porém os valores de pKa foram bastante afetados, devido à forte influência das interações eletrostáticas decorrentes da acumulação de cargas sobre as macromoléculas durante as titulações. Comparativamente os resultados obtidos tanto por regressão não linear como pelas Funções de Gran modificadas foram similares.; Acid base properties of some polyeletrolytes and colloidal suspensions were studied to evaluate the suitability of discrete site distribution model to fit potentiometric data for determination and characterization of ionizable groups presents in these materials. The polyelectrolytes studied were: commercial humic acid from Aldrich...

Energy landscapes of planar colloidal clusters

Morgan, John W. R.; Wales, David J.
Fonte: Royal Society of Chemistry Publicador: Royal Society of Chemistry
Tipo: Artigo de Revista Científica
EN
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A short-ranged pairwise Morse potential is used to model colloidal clusters with planar morphologies. Low-lying potential energy minima, free energy minima and rearrangement mechanisms states are predicted.

Novel synthetic routes to high-quality II-VI colloidal nanocrystals : controlled growth using mild precursors in the presence of selected ligands

Stott, Nathan E., 1973-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 175 p.; 17725357 bytes; 49583769 bytes; application/pdf; application/pdf
EN_US
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by Nathan E. Stott.; Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2004.; This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.; Includes bibliographical references.

Slow dynamics in supercooled liquids : matrix formalism, mode coupling and glass transition

Wu, Jianlan, 1976-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 169, [1] p.; 5109951 bytes; 5131120 bytes; application/pdf; application/pdf
EN_US
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In this thesis, slow dynamics of supercooled liquids are investigated in the framework of the mode-coupling theory (MCT). Following the real-time generalized Langevin equation in Newtonian liquids, the dynamic Gaussian factorization scheme leads to mode-coupling (MC) closures. As an alternative approach to the projection operator technique, the matrix formalism based on the complete basis set is developed for studying dynamics in many-particle systems. In a dissipative system, the MC closures have to be obtained from irreducible memory kernels instead of standard memory kernels. The matrix formalism provides a new explanation of this preference by comparing linear and nonlinear relaxation time scales, and generalizes the irreducible memory kernel to higher orders. A simple kinetic spin model, the East model, is used to test the matrix formalism and the mode-coupling closures, where the dynamic Gaussian factorization scheme is replaced by a linear approximation due to the kinetic constraint. Next, slow dynamics in Brownian liquids is studied and generalization of MC closures is derived for both coherent and incoherent intermediate scattering functions. Predictions of nonergodic parameters for these two correlation functions in a hard-sphere colloidal suspension improve as the order of the MC closure increases. New glass-transition phenomena are revealed by applying the standard MC closure to a two-Yukawa colloidal suspension.; by Jianlan Wu.; Thesis (Ph. D.)--Massachusetts Institute of Technology...

Colloidal semiconductor nanocrystals as nanoscale emissive probes in light emitting diodes and cell biology; Colloidal semiconductor NCs as nanoscale emissive probes in LEDs and cell biology

Huang, Hao, Ph. D. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 178 p.
ENG
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This thesis employs colloidal semiconductor nanocrystals (NCs) as nanoscale emissive probes to investigate the physics of light emitting diodes (LEDs), as well as to unveil properties of cells that conventional imaging techniques cannot reveal. On the LED side, in particular, Chapter 2 utilizes individual NCs to alter layered organic LED structures at nanometer scale, resulting in spectrally resolved electroluminescence from single colloidal CdSe/ZnS (core/shell) NCs at room temperature. Chapter 3 takes NCs as emissive probes in layered organic LEDs, and shows that the photoluminescence of single NCs is bias dependent which helps elucidate the interactions between NCs and organic semiconductors, knowledge useful for designing efficient NC organic optoelectronics. Instead of using a planar LED geometry, Chapter 4 presents a technique for making nanoscale gap LEDs which allow the spectrally coincidental photoluminescence and electroluminescence from NCs. The work investigates the interactions between NCs and different metal gaps, and suggests electromigrating leads made of different metals as a promising route to fabricating nanoscale gaps with workfunction offsets for optoelectronic devices. On the cell biology side, we develop a three-dimensional sub-diffraction limited single fluorophore imaging method for proteins labeled with NCs. Chapter 5 applies the method to measure the endothelial glycocalyx thickness in vitro for the first time...

Synthesis and characterization of II-VI and IV-VI colloidal semiconductor quantum dots

Guo, Li (1980 - ); Krauss, Todd D.
Fonte: University of Rochester. Publicador: University of Rochester.
Tipo: Tese de Doutorado Formato: Number of Pages:xix, 155 leaves
ENG
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Thesis (Ph. D.)--University of Rochester. Dept. of Chemistry, 2008.; The synthesis and optical characterization of colloidal CdSe core and CdSe/ZnS core/shell quantum dots (QDs) is presented in this thesis. Scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS) was used to determine the chemical distribution of semiconductor shell material around colloidal core-shell CdSe/ZnS QDs. Our studies have shown that although growth of a simple shell of ZnS on the surface of a CdSe core QD does improve the optical properties considerably, EELS signals from positions around the QD indicate a well-defined shell of ZnS surrounding the CdSe core, but the distribution of the shell material is highly anisotropic.
We have reported the formation of magic sized PbSe nanoclusters (MSCs) together with regular sized PbSe QDs. A method of preparing pure PbSe MSCs at low reaction temperature under ambient conditions is also presented. In comparison to other semiconductor QDs, these small sized MSCs show high fluorescence efficiency. The optical properties of PbSe MSCs provides them with enormous potential as superior fluorophores for biological imaging applications at NIR wavelengths between 700 and 1000 nm. We also report the two-stage preparation of core/shell PbSe/SrSe colloidal quantum dots (QDs) and their characterization. Absorption...

Chemical processing of colloidal cadmium selenide nanoparticles: New approaches to dimensional and morphological control

Asokan, Subashini
Fonte: Universidade Rice Publicador: Universidade Rice
ENG
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Cadmium selenide (CdSe) quantum dots (QDs) are colloidal semiconductor nanoparticles (NPs) that are nanometer sized fragments of the corresponding bulk crystals. They are being probed as a very interesting system for their applications in LEDs, solar cells and biomedical labeling because of their rich photophysics arising from their size dependent optical and electronic properties and flexible processing chemistry. 1-Octadecene was the only non-coordinating solvent used for the synthesis of CdSe NPs. It was imperative to understand the chemistry of synthesis of CdSe NPs using other non-coordinating solvents. Also, there is a constant search for the greener, cheaper, reproducible and scalable methods for the synthesis of CdSe NPs while not compromising on their quality. Towards the above mentioned goals, the use of heat transfer fluids was successfully demonstrated as cost-effective alternative solvents for quantum dot synthesis. Heat transfer fluids (HTF) are a class of organic liquids commonly used in chemical process industries to transport heat between unit operations. The solvents were found suitable for the hot injection synthesis of QDs while reducing the cost of the raw materials. These solvents were found to slow the growth kinetics of the CdSe NPs...

A colloidal singularity reveals the crucial role of colloidal stability for nanomaterials in-vitro toxicity testing: nZVI-microalgae colloidal system as a case study

Gonzalo, Soledad; Pulido-Reyes, Gerardo; Fernández-Piñas, Francisca; Bonzongo, Jean Claude; Leganés, Francisco; Rosal, Roberto; García-Calvo, Eloy; Rodea-Palomares, Ismael
Fonte: Public Library of Science Publicador: Public Library of Science
Tipo: Artigo de Revista Científica
ENG
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Aggregation raises attention in Nanotoxicology due to its methodological implications. Aggregation is a physical symptom of a more general physicochemical condition of colloidal particles, namely, colloidal stability. Colloidal stability is a global indicator of the tendency of a system to reduce its net surface energy, which may be achieved by homo-aggregation or hetero-aggregation, including location at bio-interfaces. However, the role of colloidal stability as a driver of ENM bioactivity has received little consideration thus far. In the present work, which focuses on the toxicity of nanoscaled Fe° nanoparticles (nZVI) towards a model microalga, we demonstrate that colloidal stability is a fundamental driver of ENM bioactivity, comprehensively accounting for otherwise inexplicable differential biological effects. The present work throws light on basic aspects of Nanotoxicology, and reveals a key factor which may reconcile contradictory results on the influence of aggregation in bioactivity of ENMs

FGM Stainless steel-Ti(CN) cermets through colloidal processing

Escribano Quintana, Juan Antonio; García, J.L.; Alvaredo Olmos, Paula; Ferrari, Begoña; Gordo Odériz, Elena; Sánchez Herencia, Antonio Javier
Fonte: Elsevier Publicador: Elsevier
Tipo: info:eu-repo/semantics/acceptedVersion; info:eu-repo/semantics/article; info:eu-repo/semantics/conferenceObject
Publicado em /03/2014 ENG
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In this work, the colloidal approach was used to promote a gradation in the composition of a cermet, based on the mixture of stainless steel and Ti(C,N) during powder consolidation into a bulk part. The colloidal processing ofnon-oxides in aqueous media requires an elevated control over the evolution of the surface chemistry of the powders, in order to obtain stable and high concentrated slurries of the mixture of metal/non-oxide ceramic. The advantage of those methods lies on the fabrication of complex micro-architectures where the second phases are intimately and homogeneously dispersed in the microstructure of the composite. Moreover, those techniquesallow the processing of fine particles with low compressibility and flowability which difficult conventional powder metallurgical processing. The results shown in this work evidence the feasibility of obtaining continuous functionally graded materials (FGM) through slip casting in porous molds, as well as the relevance of the rheological properties of the composite slurries on the final characteristics of the material.; The authors acknowledge the support from Spanish Government through the project MAT2012-38650-C02. J. A. Escribano acknowledges MINECO through the grant FPI-2010.; This proceeding at: 10th International Conference on the Science of Hard Materials (ICSHM10)...

Colloidal crystal approach to the fabrication of photonic band gap materials

Jiang, Peng
Fonte: Universidade Rice Publicador: Universidade Rice
Tipo: Thesis; Text Formato: 208 p.; application/pdf
ENG
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389.27703%
Planar single-crystalline colloidal crystals are fabricated by exploiting the spontaneous crystallization of monodisperse silica spheres into close-packed arrays using a convective self-assembly method. Film thicknesses ranging from single monolayers to over 200 layers can be precisely controlled through varying solution concentration and colloid diameter. These high quality periodic arrays exhibit partial photonic band gaps, whose positions and band width depends on the sphere diameters and the number of layers. Their optical transmission is well described by the scalar wave approximation to Maxwell's equations. The thickness dependence of the photonic band gap has also been studied for the first time. Superlattice colloidal crystals comprised of alternating layers of different sphere sizes can also be formed by the convective self-assembly method. The resulting photonic crystal structures exhibit optical properties which resemble the superposition of the properties of each individual component, with additional structure that suggests the onset of superlattice-type miniband formation. These superlattice structures thus provide a new way to couple light into and out of photonic crystals. These planar colloidal films are then used as scaffolds to make macroporous materials with crystalline arrays of voids. Macroporous polymers are formed by filling the interstitial area with monomer which is subsequently polymerized. The silica templates can be removed by etching with hydrofluoric acid. The large voids defined by the silica colloids are not isolated...

Hofmeister effects on the colloidal stability of poly(ethylene glycol)-decorated nanoparticles

Blachechen, Leandro S.; Silva, Jaqueline Oliveira; Barbosa, Leandro Ramos Souza; Itri, Rosangela; Petri, Denise Freitas Siqueira
Fonte: SPRINGER; NEW YORK Publicador: SPRINGER; NEW YORK
Tipo: Artigo de Revista Científica
ENG
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The colloidal stability of poly(ethylene glycol)-decorated poly(methyl methacrylate), PMMA/Tween-20, particles was investigated by means of phase separation measurements, in the presence of sodium fluoride (NaF), sodium chloride, sodium bromide, sodium nitrate, or sodium thiocyanate (NaSCN) at 1.0 mol L-1. Following Hofmeister's series, the dispersions of PMMA/Tween-20 destabilized faster in the presence of NaF than with NaSCN. After the phase separation, the systems were homogenized and except for the dispersions in NaF, re-dispersed particles took longer to destabilize, indicating that anions adsorbed on the particles, creating a new surface. Except for F- ions, the adsorption of anions on the polar outmost shell was evidenced by means of tensiometry and small-angle X-ray scattering measurements. Fluoride ions induced the dehydration of the polar shell, without affecting the polar shell electron density, and the formation of very large aggregates. A model was proposed to explain the colloidal behavior in the presence of Hofmeister ions.; CNPq; CNPq; CAPES; CAPES; FAPESP [2010/51219-4]; FAPESP; ProReitoria de Graduacao da Universidade de Sao Paulo; Pro-Reitoria de Graduacao da Universidade de Sao Paulo

Photodetectors based on colloidal quantum dots; Photodetectors based on colloidal QDs

Oertel, David C. (David Charles)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 215 p.
ENG
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Inspired by recent work demonstrating photocurrent enhancement in quantum-dot (QD) solids via post-deposition chemical annealing and by recent successes incorporating single monolayers of QDs in light-emitting devices (QD-LEDs), we set out to develop thin-film, layered photodetectors in which the active layer is a chemically annealed QD solid. This thesis reports initial steps in this development. Chapters 1 and 2 contain introductory material. In Chapter 3, we demonstrate a layered QD photodetector (QD-PD) in which the active layer is a 200-nm thick film of CdSe QDs annealed with n-butylamine. These "thick-slab" devices, active in the visible spectrum, represent a minimal transformation from the transverse channel test structures employed for more fundamental studies to a layered, sandwich geometry. The thick-slab design exhibits a number of desirable characteristics, including near-unity internal quantum efficiency and a 50-kHz bandwidth. More complex QD-PD designs, allowing the successful incorporation of much thinner QD solids, are discussed in Chapter 4, and these devices provide insight into the zero-bias operation of thick-slab QD-PDs.; (cont.) The final three chapters of the thesis focus on PbSe QDs, this group's initial material of choice for accessing the short-wavelength-IR spectral window. Initial study of PbSe QDs as a material for QD-PDs is described in Chapter 5. Chapter 6 discusses an 31P-based NMR investigation of the mechanism of PbSe monomer formation...

The synthesis of inorganic semiconductor nanocrystalline materials for the purpose of creating hybrid organic/inorganic light-emitting devices; Synthesis of inorganic semiconductor NC materials for the purpose of creating hybrid organic/inorganic LEDs

Steckel, Jonathan S. (Jonathan Stephen)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 213 p.
ENG
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Colloidal semiconductor nanocrystals (NCs) or quantum dots (QDs) can be synthesized to efficiently emit light from the ultraviolet, across the entire visible spectrum, and into the near infrared. This is now possible due to the continual development of new core and core-shell NC structures to meet specific color needs in areas as diverse as optoelectronic devices to biological imaging. Core-shell semiconductor NCs are unique light emitters. They are more stable overtime to photobleaching compared to organic dyes. Their emission is efficient and their spectral full width at half maximum remains highly narrow as their size is synthetically changed to provide desired peak wavelengths of emission to within plus or minus a couple of nanometers. They can be purified and manipulated in solution and their chemical interaction with the environment is the same for all sizes and can be modified using chemical techniques. These unique properties make semiconductor NCs ideal for use in light emitting devices (QD-LEDs). This work shows how electroluminescence can be extended into the near infrared region of the spectrum by employing infrared emitting NCs as well as into the blue region of the spectrum by designing and synthesizing NCs specifically for this application.; (cont.) Once efficient and color saturated electroluminescence at the visible spectrum's extremes had been realized it was a natural extension to begin exploring the potential of QD-LED devices to satisfy the technological requirements of flat panel displays and imaging applications. This led to the synthesis of a new green-emitting core-shell NC material to meet the specific color needs for flat panel display applications. At the same time we developed a new QD-LED device fabrication method to allow the patterning of the NC monolayer in our devices. Micro-contact printing the NC monolayer instead of using phase separation provided efficient and highly color saturated QD--LEDs in the red...

Photoredox Chemistry in Oxide Clusters. Photochromic and Redox Properties of Polyoxometalates in Connection with Analog Solid State Colloidal Systems

Gómez-Romero, P.; Casañ Pastor, Nieves
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Artículo Formato: 579917 bytes; image/jpeg
ENG
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Data on the electrochemical and photoredox chemistry of polyoxometalates of formula H4[SiW12O40] and H3[PMo12O40] are reported and analyzed in relation to those of related extended solids (WO3 and colloidal WO3 particles). These soluble multinuclear anions show reversible single-electron and multielectron reduction processes to yield blue mixed valent species. Similar reduced species can be obtained photochemically, with a concomitant oxidation of organic substrates. The reported photochromic behavior is similar to that of colloidal WO3 particles. On the other hand, electrochemical potentials differ substantially from the flat band potential of WO3 colloids. Finally, the title compounds are more stable and present the added advantage of having well-characterized monodisperse and controllable structures and tunable electrochemical properties; We thank the DGICYT (PB93-0122) (Spain) for financial support.; Peer reviewed

Comportamiento coloidal de un caolín deshidratado; Colloidal behavior of a dehydrated kaolin

Ferrari, B.; Moreno Botella, Rodrigo María; Lange, F. F.
Fonte: Sociedad Española de Cerámica y Vidrio Publicador: Sociedad Española de Cerámica y Vidrio
Tipo: Artículo Formato: 843154 bytes; application/pdf
SPA
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[ES] El control coloidal de suspensiones permite desarrollar fuerzas atractivas de corto alcance entre partículas, a partir de las cuales se puede aumentar la viscosidad de una suspensión estable hasta proporcionarle la plasticidad deseada. Las suspensiones de caolín poseen elevada plasticidad, pero en aplicaciones que requieren una elevada densidad y control dimensional, la extracción del agua durante el secado y deshidroxilación puede originar grietas y defectos como consecuencia de la elevada contracción. En aplicaciones de este tipo se prefiere utilizar un caolín deshidratado, lo que supone la pérdida de plasticidad. En este trabajo se parte de un caolín deshidratado, con el cual se estudian las condiciones de defloculación utilizando tanto un defloculante (HTMA) que genera interacciones de tipo electrostático entre partículas, como un polielectrolito (PAA) que genera repulsiones electroestéricas entre las mismas. Se estudian las propiedades reológicas de suspensiones del caolín calcinado molido y sin moler en función de la concentración de defloculante y del contenido en sólidos. Una vez seleccionadas las condiciones óptimas de defloculación, se procede a la adición de diferentes concentraciones de sales con el fín de promover la formación de una estructura a través de fuerzas atractivas de corto alcance. Se han realizado ensayos previos de conformado por filtración con presión de las distintas suspensiones...

Fabrication of Planar Colloidal Clusters with Template-Assisted Interfacial Assembly

Wirth, Christopher L.; De Volder, Michael; Vermant, Jan
Fonte: ACS Publicador: ACS
Tipo: Article; accepted version
EN
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This is the accepted manuscript. The final version is available from ACS at http://pubs.acs.org/doi/abs/10.1021/la504383m.; The synthesis of nanoparticle clusters, also referred to as colloidal clusters or colloidal molecules, is being studied intensively as a model system for small molecule interactions as well as for the directed self-assembly of advanced materials. This paper describes a technique for the interfacial assembly of planar colloidal clusters using a combination of top-down lithographic surface modification and bottom-up Langmuir?Blodgett deposition. Micrometer sized polystyrene latex particles were deposited onto a chemically modified substrate from a decane?water interface with Langmuir?Blodgett deposition. The surface of the substrate contained hydrophilic domains of various size, spacing, and shape, while the remainder of the substrate was hydrophobic. Particles selectively deposited onto hydrophilic regions from the decane?water interface. The number of deposited particles depended on the size of each patch, thereby demonstrating that tuning cluster size is possible by engineering patch geometry. Following deposition, the clusters were permanently bonded with temperature annealing and then removed from the substrate via sonication. The permanently bonded planar colloidal clusters were stable in an aqueous environment and at a decane?water interface laden with isotropic colloidal particles. The method is a simple and fast way to synthesize colloidal clusters with few limitations on particle chemistry...

Colloidal iron dispersions prepared via the polymer-catalyzed decomposition of iron pentacarbonyl

Smith, Thomas; Wychick, Darlene
Fonte: American Chemical Society: Journal of Physical Chemistry Publicador: American Chemical Society: Journal of Physical Chemistry
Tipo: Abstract Formato: 31371 bytes; application/pdf
EN_US
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Stable colloidal dispersions (50-150 A) of zero-valent iron have been prepared by the thermolysis of Fe(CO)5 in dilute solutions of “functional” polymers. The formation of these colloidal dispersions has been rationalized in terms of a locus control formalism for particle nucleation and growth. The functional polymer serves as a catalyst for the decomposition of the metal carbonyl and induces particle nucleation in its domain. Subsequent to particle nucleation, the reaction proceeds in a zero-order fashion indicative of disproportionation of Fe(CO)5 on the surface of the nucleated particles. The thermal decomposition of Fe(CO)5 was studied in solvent alone as well as in the presence of “active” polymers, which react directly with Fe(CO)5, and “passive” polymers, which react with intermediates such as [Fe(CO)4].