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Inorganic–Organic Composites
Abstract
IntroductionLength Scales of Inorganic-Organic NanomaterialsMolecular Components of Nanoscale MaterialsOrganization of the Building Blocks in One, Two, and Three DimensionsConcluding Remarks and Future OutlookReferencesProblemsAnswers
... Unfortunately, naked nanoparticles are prone to agglomeration, which hinders their effective use [3,4]. This obstacle may be overcome by deposition of metal nanoparticles on an appropriate support, and/or by stabilizing their dispersion with use of various capping agents [3][4][5][6][7]. To this end, the use of polymers containing functional groups that provide attractive interaction with nanoparticles is of particular interest [8]. ...
The effect of synthesis parameters on the physicochemical properties of clay/ polydiallyldimethylammonium (PDDA)/Ru composites and their applicability in hydrogenation of 2-butanone under very mild conditions (room temperature, atmospheric pressure, and aqueous solution) was studied. Three synthetic procedures were employed, differing in the order of addition of components and the stage at which metallic Ru species were generated. The materials were characterized with XRD (X-ray diffraction), XRF (X-ray fluorescence), EDS (energy-dispersive spectroscopy), AFM (atomic force microscopy), TEM/HRTEM (transmission electron microscopy/high resolution transmission electron microscopy), and TG/DSC (thermal gravimetry/differential scanning microscopy techniques. The study revealed that the method of composite preparation affects its structural and thermal properties, and controls the distribution and size of Ru particles. All catalysts are active in hydrogenation of 2-butanone. For best catalytic performance (100% conversion within 30 min) both the size of Ru particles and the load of polymer had to be optimized. Superior catalytic properties were obtained over the composite with intermediate crystal size and intermediate PDDA load, prepared by generation of metallic Ru species in the polymer solution prior to intercalation. This method offers an easy way of controlling the crystal size by modification of Ru/PDDA ratio.
Polymer nanocomposites (PNCs)–that is, nanopar ticles (spheres, rods, plates) dispersed in a polymer matrix–have garnered substantial academic and industrial in terest since their inception, circa 1990. This is due in large part to the incredible promise demonstrated by these early efforts: PNCs will not only expand the per form ance space of traditional filled polymers, but introduce completely new combinations of properties and thus enable new applications for plastics. Low volume additions (1–5%) of nanopar -ticles, such as layered silicates or carbon nanotubes, provide property enhancements with respect to the neat resin that are comparable to those achieved by conventional loadings (15–40%) of traditional fillers. The lower loadings facilitate proc essing and re duce component weight. Most important, though, is the unique value - added properties not normally possible with traditional fillers, such as reduced permeability, optical clarity, self - passivation, and increased re sis tance to oxidation and ablation. These characteristics have been transformed into numerous commercial suc cesses, including automotive parts, coatings, and flame retardants. This issue of the MRS Bulletin provides a snapshot of these exemplary successes, future opportunities, and the critical scientific challenges still to be addressed for these nanoscale multiphase systems. In addition, these ar ticles provide a perspective on the current status and future directions of polymer nanocomposite science and technology and their potential to move beyond additive concepts to designed ma te rials and devices with prescribed nanoscale composition and morphology.
A simple route to the production of high-quality CdE (E = S, Se, Te) semiconductor nanocrystallites is presented. Crystallites from approximately 12 angstrom to approximately 115 angstrom in diameter with consistent crystal structure, surface derivatization, and a high degree of monodispersity are prepared in a single reaction. The synthesis is based on the pyrolysis of organometallic reagents by injection into a hot coordinating solvent. This provides temporally discrete nucleation and permits controlled growth of macroscopic quantities of nanocrystallites. Size selective precipitation of crystallites from portions of the growth solution isolates samples with narrow size distributions (<5% rms in diameter). High sample quality results in sharp absorption features and strong ''band-edge' emission which is tunable with particle size and choice of material. Transmission electron microscopy and X-ray powder diffraction in combination with computer simulations indicate the presence of bulk structural properties in crystallites as small as 20 angstrom in diameter.
Macroporous metals with strong diffractive properties at visible wavelengths can be synthesized from colloidal crystal templates. The synthesis, characterization, and potential applications of macroporous metals created in this manner are summarized in this article. The Figure shows a macroporous copper film, illustrating the long-range order of the porous structure (see also cover).
This article presents an overview of current research activities that center on monodispersed colloidal spheres whose diameter falls anywhere in the range of 10 nm to 1 μm. It is organized into three parts: The first part briefly discusses several useful methods that have been developed for producing monodispersed colloidal spheres with tightly controlled sizes and well-defined properties (both surface and bulk). The second part surveys some techniques that have been demonstrated for organizing these colloidal spheres into two- and three-dimensionally ordered lattices. The third part highlights a number of unique applications of these crystalline assemblies, such as their uses as photonic bandgap (PBG) crystals; as removable templates to fabricate macroporous materials with highly ordered and three-dimensionally interconnected porous structures; as physical masks in lithographic patterning; and as diffractive elements to fabricate new types of optical sensors. Finally, we conclude with some personal perspectives on the directions towards which future research in this area might be directed.
The process of drying of a porous material can be divided into several stages. At first, the body shrinks by an amount equal to the volume of liquid that evaporates and the liquid-vapor interface remains at the exterior surface of the body. The second stage begins when the body becomes too stiff to shrink and the liquid recedes into the interior, leaving air-filled pores near the surface. Even as air invades the pores, a continuous liquid film supports flow to the exterior, so evaporation continues to occur from the surface of the body. Eventually, the liquid becomes isolated into pockets and drying can proceed only by evaporation of the liquid within the body and diffusion of the vapor to the outside. This chapter discusses the transport processes acting during these stages. It also discusses the factors affecting stress development and various strategies for avoiding warping and cracking.
A comprehensive reference on nanoscale materials chemistry-now revised and updated. This extensive text provides twenty-two revised chapters on the preparations, applications, and characterization as well as the environmental and toxicological aspects of nanoscale materials, with an emphasis on the chemistry component. This Second Edition contains core topics including: New synthetic methods for nanomaterials. Nanostructured solids. Organized two- and three-dimensional nanocrystals. Nanotubes, ribbons, and sheets. Nanocatalysts, sorbents, and energy applications. Unique physical properties of nanomaterials. Photochemistry of nanomaterials. Biological and environmental aspects of nanomaterials. With input from top experts in the field, such as Bruce Dunn, Vicki Grassian, Warren Ford, and Chris Sorensen, among others, Nanoscale Materials in Chemistry presents a balanced survey of different topics in basic nanoparticle science, and includes helpful end-of-chapter questions and answers. Significantly expanded, the Second Edition remains a key text for understanding the fundamentals of nanoscale materials chemistry and a reliable resource for scientists and researchers.
In-line for promotion: A study of self-assembled monolayers of sulfanylalkanoic acids containing odd and even numbers of methylene groups supported on gold and silver has demonstrated that the crystallographic direction of nucleated calcite crystals is controlled by the orientation of the functional groups in the templating surface (see picture).
Titania nanoshells with an external diameter of 10–30 nm and a wall thickness of 3–5 nm were prepared by dissolving the silver cores of Ag@TiO2 nanoparticles in a concentrated solution of ammonium hydroxide. The nanoshells were assembled layer-by-layer (LBL), with negatively charged poly(acrylic acid) (PAA) to produce coatings with a network of voids and channels in the interior of the film. The diameter of the channels in the titania shells was comparable to the thickness of the electrical double layer in porous matter (0.3–30 nm). The prepared nanoparticulate films demonstrated strong ion-sieving properties due to the exclusion of some ions from the diffuse region of the electrical double layer. The permeation of ions could be tuned effectively by the pH and ionic strength of a solution between “open” and “closed” states. The ion-separation effect was utilized for the selective determination of one of the most important neurotransmitters, dopamine, on a background of ascorbic acid. Under physiological conditions, the negative charge on the surface of TiO2 facilitated the permeation of positively charged dopamine through the LBL film to the electrode, preventing the access of the negatively charged ascorbic acid. The deposition of the nanoshell/polyelectrolyte film resulted in a significant improvement to the selectivity of dopamine determination. The prepared nanoshell films were also found to be compatible with nervous tissue secreting dopamine. Although the obtained data demonstrated the potential of TiO2 LBL films for implantable biomedical devices for nerve tissue monitoring, the problem of electrode poisoning by the by-products of dopamine reduction has yet to be resolved.
A brief review of the historical development of photonic bandgap (PBG) materials is provided and the fabrication methods employed are discussed with emphasis on self-assembly processes. The factors influencing the generation of a complete bandgap, from both an experimental and a calculational standpoint are then presented and discussed. The Figure shows a diamond-like 3D periodic structure.
Colloidal crystals are three-dimensional periodic structures formed from
small particles suspended in solution. They have important technological
uses as optical filters1-3, switches4 and
materials with photonic band gaps5,6, and they also provide
convenient model systems for fundamental studies of crystallization and
melting7-10. Unfortunately, applications of colloidal
crystals are greatly restricted by practical difficulties encountered in
synthesizing large single crystals with adjustable crystal
orientation11. Here we show that the slow sedimentation of
colloidal particles onto a patterned substrate (or template) can direct
the crystallization of bulk colloidal crystals, and so permit tailoring
of the lattice structure, orientation and size of the resulting
crystals: we refer to this process as 'colloidal epitaxy'. We also show
that, by using silica spheres synthesized with a fluorescent
core12,13, the defect structures in the colloidal crystals
that result from an intentional lattice mismatch of the template can be
studied by confocal microscopy14. We suggest that colloidal
epitaxy will open new ways to design and fabricate materials based on
colloidal crystals and also allow quantitative studies of heterogeneous
crystallization in real space.
Carbon nanotubes (CNTs) are one-dimensional nanostructures with unique properties. This article discusses why CNTs provide an ideal basis for a future carbonbased nanoelectronic technology, focusing specifically on single-carbon-nanotube fieldeffect transistors (CNT-FETs). Results of transport experiments and theoretical modeling will be used to address such issues as the nature of the switching mechanism, the role of the metal contacts, the role of the environment, the FET scaling properties, and the use of these findings to produce high-performance p-type, n-type, and ambipolar CNT-FETs and simple intra-nanotube circuits. CNTs are also direct-gap nanostructures that show promise in the field of optoelectronics. This article briefly reviews their optical behavior and presents results that show that ambipolar CNT-FETs can be used to produce electrically controlled light sources based on radiative electron–hole recombination. The reverse process—that is, the generation of photocurrents by the irradiation of single CNT—FETs—and photoconductivity spectra of individual CNTs are also demonstrated.
Technical Feature
From Opals to Optics: Colloidal Photonic Crystals
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Vicki L. Colvin
Over a decade ago, theorists predicted that photonic crystals active at visible and near-infrared wavelengths would possess a variety of exciting optical properties. Only in the last several years, however, have experimentalists begun to build materials that realize this potential in the laboratory. This lag between experiment and theory is primarily due to the to the challenges associated with fabricating these unique materials. As the term “crystal” suggests, these samples must consist of highly perfect ordered arrays of solids. However, unlike conventional crystals, which exhibit order on the angstrom length scale, photonic crystals must have order on the submicrometer length scale. In addition, many of the most valuable properties of photonic crystals are only realized when samples possess a “full” photonic bandgap. For such systems, large dielectric contrasts and particular crystal symmetries create a range of frequencies over which light cannot propagate. Realizing the nanoscopic architectures required to form such systems is a challenge for experimentalists. As a result, fabrication schemes that rely on lithographic techniques or spontaneous assembly have been a focus in the development of the field.
Layered smectite nanoclays are being developed for incorporation into a variety of host polymer systems. Nanoscopic phase distribution can impart enhanced stiffness at low addition levels and improve barrier and flame-retardant properties. When combined with other inorganic and organic modifiers, nanoclays can provide synergies to generate the desired formulation properties and cost/per form ance characteristics. Developments with existing nanoclay products using conventional amine chemistries are described for thermoplastic, thermoset, and rubber formulations. Nanoclays are demonstrating unique, multidimensional per form ance and proc essing capabilities. Commercial applications are emerging in a variety of diverse markets ranging from automotive to packaging.
▪ Abstract Soft lithography represents a non-photolithographic strategy based on selfassembly and replica molding for carrying out micro- and nanofabrication. It provides a convenient, effective, and low-cost method for the formation and manufacturing of micro- and nanostructures. In soft lithography, an elastomeric stamp with patterned relief structures on its surface is used to generate patterns and structures with feature sizes ranging from 30 nm to 100 μm. Five techniques have been demonstrated: microcontact printing (μCP), replica molding (REM), microtransfer molding (μTM), micromolding in capillaries (MIMIC), and solvent-assisted micromolding (SAMIM). In this chapter we discuss the procedures for these techniques and their applications in micro- and nanofabrication, surface chemistry, materials science, optics, MEMS, and microelectronics.
Starburst dendrimers are three-dimensional, highly ordered oligomeric and polymeric compounds formed by reiterative reaction sequences starting from smaller molecules—“initiator cores” such as ammonia or pentaerythritol. Protecting group strategies are crucial in these syntheses, which proceed via discrete “Aufbau” stages referred to as generations. Critical molecular design parameters (CMDPs) such as size, shape, and surface chemistry may be controlled by the reactions and synthetic building blocks used. Starburst dendrimers can mimic certain properties of micelles and liposomes and even those of biomolecules and the still more complicated, but highly organized, building blocks of biological systems. Numerous applications of these compounds are conceivable, particularly in mimicking the functions of large biomolecules as drug carriers and immunogens. This new branch of “supramolecular chemistry” should spark new developments in both organic and macromolecular chemistry.
Polymer science, an interdisciplinary science well-rooted in organic chemistry and in materials science, encompasses an inordinate number and diversity of substance classes and thus has far-reaching applications. Interestingly, polymers also represent a great challenge to the theoreticians, since their theoretical treatment often necessitates appropriate extensions of the classical methods from solid state physics and from statistical physics. Thus, new concepts often have to be invoked when considering the special properties of polymers. In this review we concentrate on one of the modern concepts in the theory of polymers, namely on scaling. Scaling is closely associated with new developments in the field of fractals and of hierarchical structures. Such concepts are invaluable for the modeling of complex geometries and for describing dynamical processes in polymeric materials. Here, we focus on a presentation of these ideas and we outline examples of recent research in which these concepts have been successfully applied.
An unusual sample of gem opal has been found to contain silica spheres of two different sizes, mixed in various proportions. Electron micrographs show extensive areas of long-range order. The structures of two different ordered phases, corresponding to compounds AB2 and AB13, have been deduced from the micrographs. Their calculated densities are found to be greater than that of the two separate components in a close-packed regular arrangement.
Research News: The ability to manipulate colloidal crystallization is a challenging task, a consequence of the size and Brownian motion of the colloid particles. As a direct result, temperature is not a suitable experimental parameter, even though it is important for tuning molecular crystal growth. Large single colloidal crystals with well-defined crystal orientations can, however, be grown through combined use of a polymer template and gravity. This and other methods that have been employed in an attempt to control colloid crystal growth, such as use of external fields, are described.
A procedure for generating robust polymeric composites which exhibit photonic bandgap (PBG) and mechanochromatic properties is introduced. With this procedure, the optical qualities of a hydrogel based PCCA are retained in a highly durable and water-free composite. Furthermore, the procedure allows for the tailoring of the mechanical characteristics of the composite to suit an end-use criteria.
Although the science and technology of sol-gel processing has experienced enormous growth in the last decade, few books have been available to help researchers cope with the flood of results published in various journals and conference proceedings. This book presents and understanding of sol-gel processing. Following the sol-gel processing sequence from beginning to end, it includes discussions on the chemistry of hydrolysis and condensation of metalorganics and inorganic salts, the growth of polymeric or particulate species in sols, gelation, aging of gels, drying, structure of gels, and sintering. In addition, it compares the properties of gel-derived and conventionally prepared ceramics, examines films in detail, and presents a variety of applications.
Multilayer films of organic compounds on solid surfaces have been studied for more than 60 years because they allow fabrication
of multicomposite molecular assemblies of tailored architecture. However, both the Langmuir-Blodgett technique and chemisorption
from solution can be used only with certain classes of molecules. An alternative approach—fabrication of multilayers by consecutive
adsorption of polyanions and polycations—is far more general and has been extended to other materials such as proteins or
colloids. Because polymers are typically flexible molecules, the resulting superlattice architectures are somewhat fuzzy structures,
but the absence of crystallinity in these films is expected to be beneficial for many potential applications.
Reinforcement of polymers with a second phase, whether inorganic or organic, to produce a polymer composite is common in the production of modern plastics. Polymer nanocomposites (PNCs) represent a radical alternative to these conventional polymer composites.
High density arrays of chromium (Cr) and layered gold/chromium (Au/Cr) nanodots and nanoholes in metal films were fabricated by evaporation onto nanoporous templates produced by the self-assembly of poly(styrene-b-methyl methacrylate) (P(S-b-MMA)) diblock copolymers. The cylindrical microdomains of the asymmetric block copolymer were oriented normal to the surface by balancing interfacial interactions of the blocks with the substrate. By selectively removing either the minor or major component, nanoporous films of PS or nanoscopic posts of PS could be produced. Thus, a template, comprising an array of hexagonally packed pores in a PS matrix or PS posts, was easily fabricated. Evaporation of Cr and Au onto the template, followed by sonication and UV degradation of the PS, left Cr or Au/Cr nanodots or a nanoporous metallic film.
The self-assembly of thin films of organometallic cylinder-forming amorphous polystyrene-block-polyferrocenylsilane diblock copolymers is described. By varying film thickness and/or the conditions of toluene during evaporation annealing, well-ordered arrays of hexagonally packed iron-rich cylindrical microdomains oriented either parallel to or normal to the substrate were produced. In the latter case, when the film thickness was very small (12−15 nm), well-defined nanoporous cylindrical domains were found. This unique morphology persists in UV ozone etched films where inorganic, ring-like cylindrical domains were produced. By varying the rate of solvent evaporation from solvent-swollen films, control over the orientation, order, and size of the cylindrical microdomains was achieved and variation of the fundamental morphology was observed.
Microphase-separated block copolymers were introduced as melts into nanoscopic cylindrical pores in alumina membranes via capillary action. The geometric confinement of both lamellar and cylindrical microdomain morphologies of styrene/butadiene block copolymers, PS-b-PBD, was investigated by transmission electron microscopy. Well-developed microphase-separated structures were formed within the resulting nanorods. Polymers that exhibit cylindrical microdomains in the bulk orient with cylindrical microdomains along the nanorod axis due to the preferential segregation of the PBD block to the walls of the pores. The period and packing of the microdomains differ from those observed in the bulk due to an incommensurability between the pore geometry and the natural period and hexagonal packing of the copolymer microdomains. With polymers exhibiting bulk lamellar morphology, confinement forces the formation of concentric cylinders oriented along the nanorod axis. The number of concentric cylinders depends on the ratio of the nanorod diameter to the equilibrium period of the copolymer. Because of the preferential segregation of PBD at the alumina surface, either PBD or PS can form the central core. These results indicate a method by which copolymer microdomains can be manipulated in a simple manner for the fabrication of isolated nanostructures.
Charged poly(amidoamine) (PAMAM) dendrimers are used to create organic−inorganic hybrid colloids in aqueous solution. The formation of gold colloids upon reduction of a gold salt precursor serves as a model reaction to study the influence of reaction conditions and dendrimer generation on the resulting nanostructures. Characterization by transmission electron microscopy (TEM), small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS) show that the gold particles are formed inside the dendrimer and located offset from the center. Although lower generation dendrimers aggregate when stabilizing the metal particles formed, dendrimers of generation 6−9 can template one gold colloid per dendrimer molecule, the size of which is well-controlled by the number of gold atoms added per dendrimer. For generation 10, multiple smaller gold particles per dendrimer are observed. The effectiveness of PAMAM dendrimers as templates in the host−guest nanoscale synthesis is confirmed for different chemical reactions.
Nanocrystals of MgO and CaO have been prepared by a modified aerogel/hypercritical drying/dehydration method. For nanocrystalline MgO (AP-MgO) surface areas ranged from 250 to 500 m2/g, whereas for AP-CaO 100−160 m2/g. These materials have been compared with more conventional (CP) microcrystalline samples of lower surface area with regard to (1) morphology (AP-samples (autoclave preparation) are tiny polyhedral crystallites, while CP-samples (conventional preparation) are larger, hexagonal platelets and cubes); (2) residual surface OH (AP-samples have less acidic OH, which are more isolated from each other; (3) acid gas adsorption (AP-samples adsorb more SO2 and CO2 at low pressures and room temperature and prefer monodentate rather than bidentate adsorption modes, but at higher pressures CP-samples adsorb more SO2 and HCl apparently due to the formation of more well ordered multilayers); (4) destructive adsorption of organophosphorus compounds and chlorocarbons (AP-samples are superior due to higher surface areas and higher surface reactivities), and (5) very thin layers of transition metal oxides on the MgO and CaO nanocrystals that significantly enhance destructive adsorption capacities to the point where [MxOy]AP-MgO and [MxOy]AP-CaO become stoichiometric in reaction with CCl4. The data are conclusive that the nanocrystals are more reactive than the microcrystals, and this is mainly attributed to morphological differences, including defects. However, intrinsic electronic effects due purely to “smallness” cannot be ruled out.
The fabrication of polymeric materials with ordered submicron-sized void structures is potentially valuable for many separation technologies as well as for emerging optical applications. This paper reports the preparation of macroporous polymer membranes with regular voids and the characterization of their diffractive optical properties. These materials are made using a colloidal crystal template of silica microspheres; the air between the spheres can be replaced by monomers that can be subsequently polymerized. The use of silica microspheres as templates makes it possible to employ chemical rather than thermal methods for template removal. For this reason, polymers as diverse as polyurethane and polystyrene can be used to create free-standing macroporous films, with thickness ranging from 0.5 to 50 μm. Scanning electron microscopy of these samples indicates a well-formed porous structure consisting of voids ranging in diameter from 200 to 400 nm. These large cavities are not isolated, but rather interconnected by a network of monodisperse smaller pores (d = 50−130 nm) whose size can be controlled by varying the polymerization temperature. These membranes exhibit striking optical properties due to the periodic arrangement of air spheres in the polymer medium. Normal-incidence transmission measurements of these samples are compared to a theoretical model based on a scalar wave approximation. This model assumes an ordered structure of close-packed, three-dimensional air spheres. The good agreement between theory and experiment provides additional evidence of the long-range order of these samples.
The temporal shape evolution of CdSe quantum confined nanorods (quantum rods) in nonaqueous solvents with organometallic precursors was studied quantitatively and systematically. The experimental results revealed three distinguishable stages in the shape evolution. At high monomer concentrations, nanocrystals grow exclusively along the c-axis of the wurtzite structure, making this axis the long axis of the rods. At intermediate concentrations, nanocrystals grow simultaneously in three dimensions. At low monomer concentrations, the aspect ratio decreases in a process controlled by intraparticle diffusion on the surface of the nanocrystals. This intraparticle ripening stage is different from normal Ostwald ripening, which occurs at lower monomer concentrations and is by monomer migration from small to larger ones. Addition of hexylphosphonic acid or tetradecylphosphonic acid, strong cadmium ligands, is important mainly because it enables the high monomer concentrations needed for the growth of quantum rods. A simple model is proposed to explain the growth of faceted CdSe nanocrystals on the basis of diffusion control.
Long-chain alkanethiols, HS(CH2)nX, adsorb from solution onto gold surfaces and form ordered, oriented monolayer films. The properties of the interfaces between the films and liquids are largely independent of chain length when n > 10; in particular, wetting is not directly influenced by the proximity of the underlying gold substrate. The specific interaction of gold with sulfur and other "soft" nucleophiles and its low reactivity toward most "hard" acids and bases make it possible to vary the structure of the terminal group, X, widely and thus permit the introduction of a great range of functional groups into a surface. Studies of the wettability of these monolayers, and of their composition using X-ray photoelectron spectroscopy (XPS), indicate that the monolayers are oriented with the tail group, X, exposed at the monolayer-air or monolayer-liquid interface. The adsorption of simple n-alkanethiols generates hydrophobic surfaces whose free energy (19 mJ/m2) is the lowest of any hydrocarbon surface studied to date. In contrast, alcohol and carboxylic acid terminated thiols generate hydrophilic surfaces that are wet by water. Measurement of contact angles is a useful tool for studying the structure and chemistry of the outermost few angstroms of a surface. This work used contact angles and optical ellipsometry to study the kinetics of adsorption of monolayer films and to examine the experimental conditions necessary for the formation of high-quality films. Monolayers of thiols on gold appear to be stable indefinitely at room temperature but their constituents desorb when heated to 80°C in hexadecane. Long-chain thiols form films that are thermally more stable than films formed from short-chain thiols.
Monolayers of a series of terminally substituted alkyl thiols, X(CH2)15SH (X = CH3, CH2OH, CO2H, CO2CH3, and CONH2), have been prepared by adsorption from solution onto evaporated gold substrates. The structures have been characterized by infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), ellipsometry, and temperature-programmed desorption (TPD). The IR data shows the monolayer films to be densely packed, crystalline-like structures with all-trans conformation alkyl chains exhibiting average tilt angles of the chain axis in a range of 28-40° from the surface normal and an approximate 55° twist of the chain axis away from a configuration with the CCC plane perpendicular to the surface plane. TPD results set a lower limit of interaction between the CH2 groups in the films of ∼0.8 kcal/mol. We find that the terminal groups are exposed at the ambient interface, and, based on an analysis of the IR data, an assessment of the conformations and molecular environments of these groups is made. The CO2H terminal groups appear to exist in short hydrogen-bonded (perhaps dimeric) sequences. Detailed examination of the IR data shows some unexplained abnormalities with respect to the quantitative treatment of the terminal functional group spectra. These effects can be traced to the perturbations which occur when a group is placed at an interface relative to the environment experienced in a crystalline solid. The major feature of this study is the finding that the reported series of derivatives form a structurally self-consistent set of well-defined reagent surfaces suitable for use in further studies of the surface properties of organic materials.
Long-chain alkanethiols, HS(CH2)nCH3, adsorb from solution onto the surfaces of gold, silver, and copper and form monolayers. Reflection infrared spectroscopy indicates that monolayers on silver and on copper (when carefully prepared) have the chains in well-defined molecular orientations and in crystalline-like phase states, as has been observed on gold. Monolayers on silver are structurally related to those formed by adsorption on gold, but different in details of orientation. The monolayers formed on copper are structurally more complex and show a pronounced sensitivity to the details of the sample preparation. Quantitative analysis of the IR data using numerical simulations based on an average single chain model suggests that the alkyl chains in monolayers on silver are all-trans zig-zag and canted by approximately 12-degrees from the normal to the surface. The analysis also suggests a twist of the plane containing the carbon backbone of approximately 45-degrees from the plane defined by the tilt and surface normal vectors. For comparison, the monolayers that form on adsorption of alkanethiols on gold surfaces, as judged by their vibrational spectra, are also trans zig-zag extended but, when interpreted in the context of the same single chain model, have a cant angle of approximately 27-degrees and a twist of the plane of the carbon backbone of approximately 53-degrees. The monolayers formed on copper (when they are obtained in high quality) exhibit infrared spectra effectively indistinguishable from those on silver and thus appear to have the same structure. Films on copper are also commonly obtained that are structurally ill-defined and appear to contain significant densities of gauche conformations. These spectroscopically based interpretations are compatible with inferences from wetting and XPS measurements. The structure of the substrate-sulfur interface appears to control molecular orientations of the alkyl groups in these films. An improved structural model, incorporating a two-chain unit cell and allowing for the temperature-dependent population of gauche conformations, is presented and applied to the specific case of the structures formed on gold.
This review covers catalysis by metal clusters and colloids. Any system that is a continuous phase is considered. Specifically not covered will be supported catalysts or heterogeneous catalysts except as they serve as comparative examples. Polymer-stabilized clusters and colloids are considered if they are soluble. The review will roughly cover the period from 1980 to mid-1992. Essential to this review is the establishment of definitions for the terms cluster and colloid. There are certain intuitive definitions for cluster and colloid. Recent work discussed below shows that the terms cluster and colloid are not clearly distinguishable. This review uses cluster and colloid interchangeably. However, several studies describe methods for distinguishing homogeneous from heterogeneous catalysts where heterogeneous catalysts may include colloids. For purposes of this review, a heterogeneous catalyst is defined as one where the catalyst is a separate phase, usually a filterable solid from a liquid phase. The distinction between mononuclear or low nuclearity clusters and large clusters or colloids does not necessarily distinguish homogeneous from heterogeneous catalysts. Methods for distinguishing whether a reaction is catalyzed by a mononuclear species or one where metal-metal bonds are present are described. The paper discusses the following: structure of large clusters and small colloids; onset to metallic properties; mononuclear vs. cluster catalysis; hydrogenation; CO activation; hydroformylation; H-H and C-H activation; hydrosilylation; isomerization; oligomerization; redox reactions; photocatalysis; water activation; oxidation; electrochemistry and electroless metal deposition; and ammonia synthesis and related nitrogen chemistry. 402 refs.
Uniform inorganic and hybrid inorganic−organic hollow microspheres have been produced by coating colloidal core templates with alternating layers of oppositely charged nanoparticles and polymer, and thereafter removing the core either by heating or chemical treatment. The multilayers were constructed by consecutively depositing the nanoparticles and polymer onto the colloidal templates, utilizing the electrostatic attraction between the particles and polymer for layer build-up. Hollow silica spheres were obtained by calcining polymer latex spheres coated with multilayers of silica nanoparticles (SiO2) bridged by polymer [poly(diallyldimethylammonium chloride) (PDADMAC)]. Exposure of SiO2/PDADMAC multilayer-coated polymer latex colloids to acidic solutions, dimethyl sulfoxide, or tetrahydrofuran yielded hollow composite silica−polymer spheres. The fabricated hollow spheres have been characterized using scanning and transmission electron microscopy. The approach employed is well suited to generating hollow spheres of controlled diameter, wall thickness, and composition.
Composites of ordered colloidal crystals of amorphous monodisperse silica particles in amorphous poly(methyl acrylate) (PMA) films selectively Bragg diffract visible light. The 153 nm diameter colloidal silica particles coated with 3-(trimethoxysilyl)propyl methacrylate form colloidal crystals in methyl acrylate (MA) monomer, and the crystal order is locked into place by polymerization of the MA. Bragg diffraction from the films is detected spectrophotometrically as narrow peaks of low percent transmission of visible light normal to the film plane. The diffraction wavelength is tuned by varying the d spacing of the crystal lattice and by varying the Bragg angle. Variation in the lattice spacing is achieved via the particle size or particle concentration, uniaxial stretching of the composite, and swelling the composite with monomers such as styrene or MA followed by photopolymerization of the imbibed monomers. Films that diffract from 434 to 632 nm have been prepared.
Composites of single-walled carbon nanotubes (SWNT) and polystyrene have been prepared using three different types of SWNT: HiPco, CoMoCat, and pulsed laser vaporization (PLV). Nanotubes were incorporated into the polystyrene matrix by two methods: (1) evaporation of chloroform solutions of SWNT noncovalently functionalized with poly[(m-phenylenevinylene)-co-(2,5-dioctoxy-p-phenylenevinylene)] (PmPV) and polystyrene; (2) coagulation in water of DMF solutions containing polystyrene and nitric acid oxidized SWNT. From measurements of the electrical conductivities of the composites over a range of concentration from 0.1 to 6 wt % SWNT, the percolation threshold of conductivity was 0.17−0.3% SWNT for the PmPV-coated materials and 0.4–0.5% for those made by coagulation. Of the three types of SWNT, composites made with HiPco tubes had the highest conductivity.
Several ligands, such as alkylthiols, -amines, -silanes, -phosphines, -halides, and simple alkanes, were employed for digestive ripening, a process in which a colloidal suspension in a solvent is refluxed at the solvent boiling temperature in the presence of a surface-active ligand to convert a highly polydisperse colloid into a nearly monodisperse one. Apart from thiols, which are the only established digestive-ripening ligands, amines, silanes, and phosphines were found to be similarly efficient for this purpose. The important steps involved in the digestive ripening were identified to be (1) breaking the polydisperse colloid into smaller size particles upon addition of the ligand, (2) isolating this colloid from the reaction side products, and finally (3) heating this isolated colloid in the presence of the ligand to form a nearly monodisperse colloid. The successful ligands could be differentiated from the others based on their effectiveness to perform the different tasks in each step. Namely, they broke the bigger nanoparticles into smaller ones in the first step, formed a stable redispersable colloid in toluene after the second step, and at the end of the third step lead to a nearly monodisperse colloid. The ability of the different ligands to break the bigger, prismatic as-prepared particles in the first step varied as RSH ≈ RNH2 ≈ R3P ≈ RSiH3 > RI > ROH ≈ RBr and simple alkanes completely failed to induce any changes in the size and shape of the as-prepared colloid. Ligands such as RI, RBr, and ROH failed in the second step, possibly because of the poor ligand−gold interaction. The ligand−gold interaction trends observed here could be rationalized semiqualitatively by invoking the hard and soft acid and base theory, which suggests that a soft acid-like gold likes to interact with softer bases such as RSH and R3P rather than hard bases such as ROH. After the third step, the sizes of the nearly monodisperse particles depended on the ligand used for digestive ripening and correlated well with the ligand−gold interaction trends.
A polyisoprene-block-poly(2-cinnamoylethyl methacrylate)-block-poly(tert-butyl acrylate), PI-b-PCEMA-b-PtBA, sample with 370 isoprene, 420 CEMA, and 550 tBA repeat units forms spherical micelles in tetrahydrofuran (THF)/hexanes (HX) with 65% volume fraction of HX. The micelles consist of a PI corona, a solvent-insoluble PCEMA shell, and PtBA core. Their structure is locked in by photo-cross-linking the PCEMA shell to yield nanospheres. The nanospheres were made water-dispersible by hydroxylating the PI double bonds. The core was made compatible with inorganic species by removing the tert-butyl groups of PtBA. The feasibility of using such nanospheres as templates for inorganic nanoparticle preparation was demonstrated by incorporating iron oxide magnetic particles into the cores.
Block copolymers are all around us, found in such products as upholstery foam,adhesive tape and asphalt additives. This class of macromolecules is produced by joining two or more chemically distinct polymer blocks, each a linear series of identical monomers, that may be thermodynamically incompatible (like oil and vinegar). Segregation of these blocks on the molecular scale (5–100 nm) can produce astonishingly complex nanostructures, such as the “knitting pattern” shown on the cover of this issue of PHYSICS TODAY. This striking pattern, discovered by Reimund Stadler and his coworkers, reflects a delicate free‐energy minimization that is common to all block copolymer materials. Advances in synthetic chemistry and statistical theory provide unparalleled control over molecular scale morphology in this class of macromolecules.
Nanoporous film generation from diblock copolymers has been used to direct the assembly of ligand-stabilized CdSe nanoparticles (see Figure). The number of particles forced into each pore is dependent on the concentration of the nanoparticles in solution. Further, the photoluminescence of the particles is maintained in the assembly process.
Arrays of functioning nanobatteries were constructed by using nanoporous aluminum oxide membranes and sol–gel technology. These battery arrays had performance benefits derived from the nanoscale assembly and nanoscale structure of the various components. V2O5 ambigel used to make the nanobatteries was characterized by X-ray diffraction and found to be slightly crystalline in the bulk state, but it was completely amorphous when confined in the pores of aluminum oxide filter membranes. The gel confined in the pores served as cathodes for individual nanobatteries. PEO wax electrolyte was also confined in the pores and then coupled with a lithium metal anode. An a.c. impedance analysis indicated that there was little or no unstable passivation of the lithium anode in contact with the PEO wax electrolyte. This was attributed to a self-assembly process of a hydrocarbon layer at the surface of the wax preventing unwanted chemical reactions of the lithium with the electrolyte. Individual nanobatteries in the arrays were then characterized by charge/discharge tests using the cantilever tip of an atomic force microscope to make electrical contact with the 200 nm cathodes of the nanobatteries. Average volumetric capacities of these cells were found to be in the range of 23–30 μA h/cm2 μm, which is higher than similar systems found in the literature and can be attributed to the nanostructure of these systems.
A mechanism for distributing excess polymer surface charge is used to model the growth of multilayers of strongly charged polyelectrolytes. Two parameters are required for the semiempirical analysis: the surface, or unrestricted, charge overcompensation level, phi, which is assumed to decrease exponentially from the film surface to bulk, and the characteristic length for this decay, l(cp), which is termed the charge penetration length. Modeling of the data reveals that only modest levels of polymer charge overcompensation are required to account for large increments in polymer thickness, realized at high salt concentration, since the excess charge is distributed over several "layers". Experimentally, phi appears to be roughly independent of salt concentration. The thickness increment is primarily controlled by l(cp), which is about 2.5 nominal layers for the system studied. Whereas the growth conditions and polyelectrolyte type lead to the formation of intrinsically compensated multilayers in this work, conditions for obtaining extrinsic compensation are also discussed. Kinetic vs thermodynamic limitations for polymer addition during a deposition cycle are contrasted.
The procedure of separating single wall carbon nanotubes (SWNT) from catalyst and excess dispersant for the formation of SWNT with grafted poly(sodium 4-styrenesulfonate) (PSS) was discussed. The excess PSS was removed by ultrafiltration and ultracentrifugation and 68mg of SWNT was produced in the sonification having a mixture of pristine HiPco SWNT and sodium 4-styrene sulfonete (Nass). The diameter length of SWNT was measured by atomic force microscopy in which the diameter came out to be 0.6 to 1.3nm 18. Analysis shows that the disorder band (D) band was at 1315cm -1 with a radial breathing band at 180 to 260 cm -1.
Polystyrene-coated gold nanoparticles (molecular weight (M n) of PS ∼1300 g/mol, average particle diameter d (core + shell) ∼ 9.2 (2.2 nm, corresponding to a high areal chain density of PS) are incorporated into symmetric poly(styrene-b-2 vinylpyridine) diblock copolymers with total M n ∼ 60 000-380 000 g/mol at volume fractions of 0.07-0.32. For all M n of diblock copolymers and particle filling fractions examined, particles are centralized in the PS domains with Gaussian distribution profiles. The distribution width generally decreases as the PS domain size decreases or as the particle weight fraction increases. These observations are discussed in terms of the relative entropic contributions of the polymer chains and particles.