A Quantitative Analysis of the Reduction Kinetics Involved in the Synthesis of Au@Pd Concave Nanocubes

Surface capping has been shown to play a pivotal role in controlling the evolution of metal nanocrystals into different shapes or morphologies. With the synthesis of Au@Pd concave nanocubes as an example, here we demonstrate that the capping agent can also impact the reduction kinetics of a precursor, and thereby its reduction pathway, for the formation of metal nanocrystals with distinct morphologies. A typical synthesis involves the reduction of a Pd^II precursor by ascorbic acid at room temperature in the presence of Au nanospheres as seeds, together with the use of hexadecyltrimethylammonium chloride (CTAC) or hexadecyltrimethylammonium bromide (CTAB) as the capping agent. In the case of CTAC, the Pd^II precursor prevails as PdCl₄²⁻, leading to the formation of Au@Pd concave nanocubes with a rough surface because of the fast reduction kinetics and thus the dominance of solution reduction pathway. When switched to CTAB, the Pd^II precursor changes to PdBr₄²⁻ that features slow reduction kinetics and surface reduction pathway. Accordingly, the Au@Pd concave nanocubes take a smooth surface. This work demonstrates that both reduction kinetics and surface capping play important roles in controlling the morphology of metal nanocrystals and these two roles are often coupled to each other.     

Transition metal nanoparticles protected by amphiphilic block copolymers as tailored catalyst systems

 Several stable palladium, platinum, silver, and gold colloids were prepared by reducing the corresponding metal precursors in the presence of protective amphiphilic block copolymers. Some palladium and platinum precursors with different hydrophobicities, namely palladium chloride PdCl₂, palladium acetate Pd (CH₃COO)₂, hexachloroplatinic acid H₂PtCl₆, and platinum acetylacetonate Pt (CH₃COCH=C(O–)CH₃)₂, have been used in order to investigate differences in their catalytic activity. The polymers investigated for their ability to stabilize such transition metal colloids were polystyrene-b-poly(ethylene oxide) and polystyrene-b-poly(methacrylic acid). The metal particle sizes and morphologies were determined by transmission electron microscopy and found to be in the M28.8nnanometer range. The catalytic activity of the palladium and platinum colloids was tested by the hydrogenation of cyclohexene as a model reaction. The protected palladium and platinum nanoparticles were found to be catalytically active, and final conversions up to 100% cyclohexane could be obtained. Depending on the choice of polymer block types and lengths, the precursor type, and the reduction method, different nanoparticle morphologies and catalytic activities could be obtained. These novel catalytically active metal–polymer systems are thus promising candidates for the development of tailored catalyst systems. Received: 10 June 1996 Accepted: 30 October 1996     

Effects of protective colloids on the preparation of poly(l-lactide)/poly(butylene succinate) microcapsules

Poly(l-lactide)/poly(butylene succinate) microcapsules containing an aqueous solution of sodium(+)-tartrate dihydrate were prepared by the interfacial precipitation method through solvent evaporation from (w/o)/w emulsion. The effects of poly(vinyl alcohol) used as a protective colloid in the microencapsulation were investigated regarding thermal properties, particle size distributions, surface morphologies, and release behaviors of the biodegradable microcapsules. It was concluded that encapsulation efficiency, surface morphologies, thermal properties, and releasing speed were closely related to the particle size distributions of microcapsules under different conditions of the protective colloid.     

Polymer-Protected Palladium Nanoparticles and Their Use in Catalysis

Abstract

Stable palladium colloids were prepared by the in-situ reduction of palladium chloride (PdCl₂) in the presence of protective water-soluble polymers and cationic polyelectrolytes. The particle sizes, morphologies, and particle-size distributions were determined by transmission electron microscopy and found to be in the nanometer size range. The catalytic activity of these colloidal metal-polymer systems was tested by the hydrogenation of cyclohexene as a model reaction. Most of the polymer-protected palladium nanoparticles were found to be catalytically active, and final conversions up to 100% were obtained in many cases.     

Probing nucleation pathways for morphological manipulation of platinum nanocrystals

Understanding the formation process in the controlled synthesis of nanocrystals will lead to the effective manipulation of the morphologies and properties of nanomaterials. Here, in-situ UV-vis and X-ray absorption spectroscopies are combined to monitor the tracks of the nucleation pathways in the solution synthesis of platinum nanocrystals. We find experimentally that the control over nucleation pathways through changing the strength of reductants can be efficiently used to manipulate the resultant nanocrystal shapes. The in-situ measurements show that two different nucleation events involving the formation of one-dimensional "Pt(n)Cl(x)" complexes from the polymerization of linear "Cl(3)Pt-PtCl(3)" dimers and spherical "Pt(n)(0)" clusters from the aggregation of Pt(0) atoms occur for the cases of weak and strong reductants; and the resultant morphologies are nanowires and nanospheres, respectively. This study provides a crucial insight into the correlation between the particle shapes and nucleation pathways of nanomaterials.     

Study of the mechanism of the emulsifier-free emulsion polymerization of the styrene/4-vinylpyridine system

For the emulsifier-free emulsion copolymerization of styrene with the water-soluble comonomer 4-vinylpyrindine (4VP), and using ammonium persulfate as an initiator, the kinetics, the content of 4VP, the molecular weight and the molecular weight distributions of the polymer at various conversions, and particle morphologies have been investigated. Based on the results obtained, the particle nucleation mechanism was discussed. Received: 9 September 1998 Accepted in revised form: 23 February 1999     

Direct Conversion of Bulk Metals to Size‐Tailored,Monodisperse Spherical Non‐Coinage‐Metal Nanocrystals

Monodisperse non‐noble metal nanocrystals (NCs) that are highly uniform in shapes and particle size are much desired in various advanced applications, and are commonly prepared by either thermal decomposition or reduction, where reactive organometallic precursors or/and strong reducing agents are mandatory; however, these are usually toxic, costly, or suffer a lack of availability. Bulk Group 12 metals can now be converted into ligand‐protected, highly crystalline, monodisperse spherical metal NCs with precisely controlled sizes without using any precursors and reducers. The method is based on low‐power NIR‐laser‐induced size‐selective layer‐by‐layer surface vaporization. The monodisperse Cd NCs show pronounced deep‐UV (DUV) localized surface plasmon resonance making them highly competitive DUV‐plasmonic materials. This approach will promote appreciably the emergence of a wide range of monodisperse technically important non‐coinage metal NCs with compelling functionalities.     

Direct Conversion of Bulk Metals to Size‐Tailored,Monodisperse Spherical Non‐Coinage‐Metal Nanocrystals

Monodisperse non‐noble metal nanocrystals (NCs) that are highly uniform in shapes and particle size are much desired in various advanced applications, and are commonly prepared by either thermal decomposition or reduction, where reactive organometallic precursors or/and strong reducing agents are mandatory; however, these are usually toxic, costly, or suffer a lack of availability. Bulk Group 12 metals can now be converted into ligand‐protected, highly crystalline, monodisperse spherical metal NCs with precisely controlled sizes without using any precursors and reducers. The method is based on low‐power NIR‐laser‐induced size‐selective layer‐by‐layer surface vaporization. The monodisperse Cd NCs show pronounced deep‐UV (DUV) localized surface plasmon resonance making them highly competitive DUV‐plasmonic materials. This approach will promote appreciably the emergence of a wide range of monodisperse technically important non‐coinage metal NCs with compelling functionalities.     

Designing the Hotspots Distribution by Anisotropic Growth

Changing the morphology of noble metal nanoparticles and polarization dependence of nanoparticles with different morphologies is an important part of further research on surface plasma enhancement. Therefore, we used the method based on Matlab simulation to provide a simple and effective method for preparing the morphologies of Au nanoparticles with different morphologies, and prepared the structure of Au nanoparticles with good uniformity and different morphologies by oblique angle deposition (OAD) technology. The change of the surface morphology of nanoparticles from spherical to square to diamond can be effectively controlled by changing the deposition angle. The finite difference time domain (FDTD) method was used to simulate the electromagnetic fields of Au nanoparticles with different morphologies to explore the polarization dependence of nanoparticles with different shapes, which was in good agreement with Raman spectrum.     

Dependence of the localized surface plasmon resonance of noble metal quasispherical nanoparticles on their crystallinity-related morphologies

The absorption spectra of 5 nm noble metal nanoparticles (Ag, Au, and Cu) with typical morphologies of multiply twinned particles (MTPs) and single crystals are calculated by using the discrete dipole approximation method. Among the considered morphologies, it is found that icosahedral, cuboctahedral and truncated octahedral particles behave like quasispherical particles whereas the optical response of the decahedral particles significantly differs from the others. This result, which originates from the shape anisotropy of the decahedron, points out the capacity to discriminate decahedral MTPs from a population of particles with mixed crystallinities and related quasispherical shapes.     

A strategy for synthesis of polymer-protective bimetallic colloids

A strategy for the synthesis of polymer-protective bimetallic sols is proposed which has the advantage of the strong protective effect of the polymer agent on one of the composites to stabilize the bimetallic colloidal particles. This strategy is certified to be useful to the predication of the formation of stable bimetallic colloids protected by polymer and to the control of the particle size to a certain extent. A series of new PVP-protective bimetallic colloidal dispersions containing noble metal element (Pt, Rh, Pd), light transition metal element (Co, Fe), and boron are obtained from the corresponding salts by the methods of reduction by stage and coreduction, using the NaBH4 as reductant. The TEM, XRD and EPMA measurements indicate the formation of bimetallic colloids. An interesting kind of coil aggregation is observed in the systems of PVP-Pt-Co and PVP-Rh-Co prepared by the method of reduction by stage.     

Protecting structure analyses of organic molecule-protected nanoscopic noble metal clusters

The stabilizing structure of cationic surfactant-protected platinum clusters in water and tertiary amine-protected rhodium clusters in chloroform, prepared by photo- and hydrogen-reduction, respectively, was investigated. These nanoscopic noble metal clusters present a narrow size distribution and are stable. The structural information of protective organic molecules on the surface of metal clusters was studied by transmission electron microscopy and hydrodynamic radius measurements according to the Taylor dispersion method. The size of the entire cluster with the protective layer surrounding the metal surface, obtained as Stokes' radii by the Taylor dispersion method, is considered to be fairly consistent with the sum of the naked particle size, obtained by transmission electron micrographs, and the size of the adsorbed protective layer, supporting the conformational information.     

Optical encoding with shaped DAM-1 molecular sieve particles

This work introduces a new high-throughput screening particle - a Dallas Amphorous Material No. 1 (DAM-1) molecular sieve particle. In contrast to porous silica microspheres, the 2-8-microm sized DAM-1 molecular sieve particles are available in a variety of shapes and morphologies including spheres, hexagons, rods, gyroids, and discoids. The advantage of using DAM-1 molecular sieve particles is the ability to encode an array by particle shape, which in turn permits the repeated use of luminescent reporter dyes. In this technical note, we demonstrate optical decoding of fluorescein- and Texas Red-modified shaped molecular sieve particles using reflectance and fluorescence microscopies.     

Comparisons between cationic polyelectrolytes and nonionic polymers for the protection of palladium and platinum nanocatalysts

Several palladium and platinum nanocatalysts protected by cationic polyelectrolytes were prepared by the in-situ reduction of palladium chloride, PdCl₂, and dihydrogen hexachloroplatinate, H₂PtCl₆. The particle sizes and size distributions were determined by transmission electron microscopy, and the colloids were further characterized by UV-vis spectroscopy. The catalytic activity of these nanoparticles was qualitatively investigated by the hydrogenation and conversion of cyclohexene as a model reaction and compared to palladium and platinum colloids protected by a selection of water-soluble, nonionic polymers. The results show that the catalytic activity is strongly influenced by the protective polymer chosen, as well as particle size and morphology. The use of cationic polyelectrolytes decreases the catalytic activities significantly, in comparison to several water-soluble, nonionic polymers investigated. The effects depend strongly on the particular metal, as illustrated in this case by differences observed between palladium and platinum. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3151–3160, 1997     

Effects of preparation conditions on the synthesis of nano-sized Ag metal particles by the wet-process using 3-mercapto-propionic acid

By the addition of sodium borohydride as a reducing agent into an aqueous solution of AgNO₃ mixed with 3-mercapto-propionic acid as a protective agent, nano-sized Ag metal particles could be synthesized. Using this advanced wet-process the synthesis of an aqueous Ag colloid system with a high density became possible, because the surfaces of the synthesized nano-sized Ag metal particles were covered and protected by the adsorbed 3-mercapto-propionic acid. By changing the mix ratio of AgNO₃ to 3-mercapto-propionic acid, the particle size of synthesized nano-sized Ag metal particles could be controlled; a higher 3-mercapto-propionic acid/AgNO₃ ratio was preferable in the synthesis of smaller Ag metal particles. From the relationship between the Ag metal particle size and the residual S content adsorbed on the metal particles, the mechanism of dispersion of Ag particles can be proposed as that the 3-mercapto-propionic acid having a thiol group adsorbs on Ag particles by forming Ag-S bonds as a protective agent.     

Polyhedral Cu2O Crystals for Diverse Aryl Alkyne Hydroboration Reactions

Cu₂O cubes, octahedra, and rhombic dodecahedra have been used to examine facet-dependent catalytic activity in aryl alkyne hydroboration reactions. Although the reaction can proceed by using ethanol or other alcohols as solvent, the use of 1,4-dioxane gave the best product yield. All particle shapes gave exclusively the E-product, but the rhombic dodecahedra exposing {110} surfaces were consistently far more reactive than the other particle morphologies. A product yield of 99 % was achieved by using Cu₂O rhombic dodecahedra to catalyze the hydroboration of phenylacetylene at 60 °C for 5 h. The rhombic dodecahedra have been shown to catalyze a variety of substituted aryl alkynes, which demonstrates their potential as a versatile catalyst.     

Polarization-Dependent Sum-Frequency-Generation Spectroscopy for In Situ Tracking of Nanoparticle Morphology

The surface structure of oxide-supported metal nanoparticles can be determined via characteristic vibrations of adsorbed probe molecules such as CO. Usually, spectroscopic studies focus on peak position and intensity, which are related to binding geometries and number of adsorption sites, respectively. Employing two differently prepared model catalysts, it is demonstrated that polarization-dependent sum-frequency-generation (SFG) spectroscopy reveals the average surface structure and shape of the nanoparticles. SFG results for different particle sizes and morphologies are compared to direct real-space structure analysis by TEM and STM. The described feature of SFG could be used to monitor particle restructuring in situ and may be a valuable tool for operando catalysis.     

Covalently Immobilized Colloidal Metal Particles as Selective Catalysts for Olefin Hydrogenation

Abstract

Homogeneous colloidal dispersions of ultrafine noble metal particles have been prepared by the reduction of the corresponding metal ions in the presence of protective polymers. These colloidal metal particles show effective and selective catalyses in hydrogenation of olefins’. However, separation of these homogeneous catalysts from reaction mixtures for the repeated use is difficult. Thus, immobilization of these colloidal metal particles to supports is required. This paper reports immobilization of colloidal rhodium particles onto polymer support by use of covalent bonding between the protective polymer and the support. Activities and selectivities of the resultant immobilized catalysts for hydrogenation of olefins are shown.     

铁基超细粒子催化剂结构与还原行为的Mssbauer谱研究

铁基超细粒子催化剂具有优良的F-T反应性能。本工作利用原位Mssbauer谱辅以XRD技术研究了含有钾助剂的F-T合成铁基超细粒子催化剂的结构与还原行为,考察了催化剂组成和第二金属组分(Mn,Zn,Mg)的影响。“纯”铁超细粒子催化剂在氢气中很容易经Fe_3O_4还原为零价铁。第二金属组分的引入,一方面阻碍了催化剂的还原,另一方面稳定了Fe~(2+)的存在而使其成为还原的主要中间相,这有利于反应条件下形成类尖晶石结构的活性相。对于Fe-Mn催化剂,上述效应随锰含量的增加而更趋显著。铁基超细粒子催化剂在氢气中的还原能力依“纯”Fe>Fe-Zn>Fe-Mn>Fe-Mg的顺序递减,这在一定程度上反映了铁与第二金属组分之间相互作用的强弱。     

Structure and properties of PBO–PEO diblock copolymer modified epoxy

Amphiphilic poly(n‐butylene oxide)‐b‐poly(ethylene oxide) (PBO–PEO) diblock copolymers of various compositions were synthesized and studied as modifiers for epoxy resins. In blends of PBO–PEO, epoxy resin, and curing agent, the copolymers formed well‐defined microstructures that persisted upon curing of the epoxy. The resulting morphologies were vesicles, worm‐like micelles, and spherical micelles (in order of increasing size of PEO block), as well as transitional morphologies. Addition of 5% by weight of these block copolymers improved the fracture toughness of the epoxy by as much as 19 times with relatively small reduction in the elastic modulus. The highest level of toughness was measured in a system containing branched worm‐like micelles. Close examination of the fracture surfaces of these compositions suggests that although all the dispersed morphologies played a similar role to inclusions in particle‐toughened thermosets, crack deflection toughening contributed to the significantly higher levels of toughness in the worm‐like micelle systems. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Chem 43: 1950–1965, 2005