Synthesis of tetrahexahedral platinum nanocrystals with high-index facets and high electro-oxidation activity

Facile synthesis of highly faceted multioctahedral Pt nanocrystals through controlled overgrowth

Highly faceted Pt nanocrystals with a large number of interconnected arms in a quasi-octahedral shape were synthesized simply by reducing H2PtCl6 precursor with poly(vinyl pyrrolidone) in aqueous solutions containing a trace amount of FeCl3. The iron species (Fe(3+) or Fe(2+)) play a key role in inducing the formation of the multioctahedral structure by decreasing the concentration of Pt atoms and keeping a low concentration for the Pt seeds during the reaction. This condition favors the overgrowth of Pt seeds along their corners and thus the formation of multiarmed nanocrystals. Electron microscopy studies revealed that the multioctahedral Pt nanocrystals exhibit a large number of edge, corner, and surface step atoms. The size of the multioctahedral Pt nanocrystals can be controlled by varying the concentration of FeCl3 added to the reaction and/or the reaction temperature. These multioctahedral Pt nanocrystals were tested as electrocatalysts for the oxygen reduction reaction in a proton exchange membrane fuel cell and exhibited improved specific activity and durability compared to commercial Pt/C catalyst.

Synthesis of heterogeneous catalysts with well shaped platinum particles to control reaction selectivity

Colloidal and sol-gel procedures have been used to prepare heterogeneous catalysts consisting of platinum metal particles with narrow size distributions and well defined shapes dispersed on high-surface-area silica supports. The overall procedure was developed in three stages. First, tetrahedral and cubic colloidal metal particles were prepared in solution by using a procedure derived from that reported by El-Sayed and coworkers [Ahmadi TS, Wang ZL, Green TC, Henglein A, El-Sayed MA (1996) Science 272:1924-1926]. This method allowed size and shape to be controlled independently. Next, the colloidal particles were dispersed onto high-surface-area solids. Three approaches were attempted: (i) in situ reduction of the colloidal mixture in the presence of the support, (ii) in situ sol-gel synthesis of the support in the presence of the colloidal particles, and (iii) direct impregnation of the particles onto the support. Finally, the resulting catalysts were activated and tested for the promotion of carbon-carbon double-bond cis-trans isomerization reactions in olefins. Our results indicate that the selectivity of the reaction may be controlled by using supported catalysts with appropriate metal particle shapes.

Growth of tetrahedral silver nanocrystals in aqueous solution and their SERS enhancement

Silver nanocrystals with tetrahedral shapes and {111} faces have been synthesized by the light-driven growth method in an aqueous solution. The nanocrystals of T(d) symmetry were formed under the effect of tartrate and citrate as the structural-directing reagents at the appropriate stages of reaction. Further, the nanocrystals may be assembled through electrostatic interaction to develop large-scale particle surfaces with sharp vertexes, which can generate strong localized electromagnetic field for surface-enhanced Raman scattering (SERS) studies. Benzenethiol was used as the probe to evaluate their SERS enhancement, and enhancement factors of up to 10(6) are reached. As a kind of promising material, these novel nanocrystals will be applied in surface enhanced spectroscopy and plasmonics field.

Mechanistic study of the synthesis of Au nanotadpoles, nanokites, and microplates by reducing aqueous HAuCl4 with poly(vinyl pyrrolidone)

This article describes a simple approach to anisotropic Au nanostructures with various shapes by reducing HAuCl 4 with poly(vinyl pyrrolidone) (PVP) in aqueous solutions without the use of any additional capping agent or reductant. In this approach, the commercially available PVP servers as a mild reducing agent thanks to its hydroxyl (-OH) end groups, enabling kinetic control over both nucleation and growth. As the volume of HAuCl 4 solution added to the reaction was increased, the morphology of Au nanostructures evolved from nanotadpoles to nanokites and then triangular and hexagonal microplates. The slow reduction rate associated with the mild reducing power of PVP plays a critical role in forming nanoplates during nucleation as well as their growth into highly anisotropic nanostructures. Electron microscopy studies reveal that the nanotadpoles and nanokites are formed through the linear fusion of small Au particles (<10 nm) to the initially formed nanoplates, whereas the microplates result from the continuous addition of Au atoms to the side faces of nanoplates. Through this morphological control, the localized surface plasmon resonance peaks of these Au nanostructures can be tuned in the visible and near-IR regions.

Organometallic clusters as precursors for metallic nanoparticles: effect of cluster size, ligand set, and decomposition method

The pyrolysis of organometallic clusters containing osmium and/or ruthenium on a silica support leads to metallic nanoparticles in the 1 to 10 nm size range. The particle size obtained is correlated to the ligand set, and there are also indications that the shape of the cluster may have some effect on the shape of the nanoparticles obtained. In comparison, the size of nanoparticles obtained through thermolysis in a coordinating solvent are fairly independent of the nature of the organometallic precursor and tends to lead to aggregation of smaller, spherical nanoparticles into larger particles.

Interfacial and solvent effects govern the formation of tris(dibenzylidenacetone)dipalladium(0) microstructures

Organometallic palladium adducts have application as catalysts and as precursors for nanoparticle synthesis. Herein, we study the spontaneous formation of molecular crystals of the organometallic reagent tris(dibenzylidenacetone)dipalladium(0) (Pd(2)(DBA)(3)) in THF/H(2)O binary solvent systems. We report structural and chemical characterization of the resulting diverse structures with shapes including hexagonal platelets, rods, cubes, and stars. Optical microscopy, transmission electron microscopy, scanning electron microscopy, and energy-dispersive spectroscopy were used to determine representative structures and corresponding compositions when formed either in a binary solution or upon evaporation on a surface. The difference in Pd(2)(DBA)(3) particle morphology was attributed to differences in the surface tension of growing crystalline faces. The formation of a majority of rods or hexagonal platelets in solution was shown to be determined by the ratio of THF to H(2)O in the solvent, whereas supersaturation effects and interfacial surface tension played a major role in creating the shape of particles formed upon evaporation of Pd(2)(DBA)(3) droplets on a surface.

Gold nanocones fabricated by nanotransfer printing and their application for field emission

Arrays of gold nanocones have been fabricated by the nanotransfer printing (nTP) method and we have utilized these nanocones for field emission. By the nature of the printing, any shape of metal structure can be fabricated only at desired locations and a step-and-repeat process, which enables large-area fabrication, is possible. We demonstrate step-and-repeat printing with gold nanocone patterns occupying an area of 9 mm × 8 mm.

FTIR and XPS study of Pt nanoparticle functionalization and interaction with alumina

Platinum nanoparticles with a mean size of 1.7 nm were synthesized by reduction in sodium acetate solution in 1,2-ethanediol. The particles were then functionalized with dodecylamine, dodecanethiol, and omega-mercapto-undecanoic acid (MUDA). Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) showed important variations of the particle surface state with functionalization whereas their structure differs only slightly. Platinum-to-sulfur charge transfer inferred from XPS of thiol-coated particles enabled the identification of the formation of Pt (delta+)-S (delta-) bonds. The native carbon monoxide (CO) at the surface of the particles was a very efficient probe for following the functionalization of the particles by FTIR. The red shift of nu(CO) accounts for the nature of the ligands at the surface of the particles and also for their degree of functionalization. Immobilization on alumina substrates of particles functionalized with MUDA was realized by immersion in colloidal solutions. Free molecules, isolated particles, and aggregates of particles interconnected by hydrogen bonds at the surface of alumina were evidenced by FTIR. With successive washings, the energy variation of the CO stretch of carbon monoxide and of carboxylic acid groups and the relative intensity nu(CH2)/nu(CO) showed that the free molecules are eliminated first, followed by aggregates and less-functionalized particles. Particles presenting a high degree of functionalization by MUDA remain and interact strongly with alumina.

Comparative study of the shape-dependent electrocatalytic activity of platinum multipods, discs, and hexagons: applications for fuel cells

We here demonstrate a remarkable potential-dependent morphological evolution of platinum mesostructures in the form of multipods, discs, and hexagons using a porous anodic alumina membrane (PAAM). These structures prepared potentiostatically at -0.7, -0.5 and -0.3 V, respectively, reveal unique shape-dependent electrocatalytic activity toward both formic acid and ethanol oxidation reactions. A comparison of the electrooxidation kinetics of these structures illustrates that hexagons show better performance toward formic acid oxidation whereas, for ethanol oxidation, multipods show significantly enhanced activity. Interestingly, the enhancement factor (R) for these mesostructures with respect to that of commercial platinized carbon toward formic acid oxidation ranges up to 2000% for hexagons whereas for multipods and disc they are about 700% and 300%, respectively. Similarly, for ethanol oxidation, the calculated value of R varies up to 600% for multipods while for disc and hexagons these values are 500% and 200%, respectively. These shape-dependent electrocatalytic activity of Pt mesostructures have been further correlated with XRD results. Thus, the present results demonstrate the importance of precise control of morphology by an electric field and their potential benefits especially for fuel cell applications since designing a better electrocatalyst for many fuel cell reactions continues to be an important challenge.

Morphological control of synthesis and anomalous magnetic properties of 3-D branched Pt nanoparticles

Morphology-controllable platinum nanostructures could be obtained by modulating the growth kinetics in oleylamine. The nanostructures evolve from spherical particles to branched networks with decreasing reaction temperature, and the complexity of the branched-network nanostructures increases with the extended reaction period. Size-dependent magnetic properties and enhanced ferromagnetism in dodecanethiol-capped Pt branched nanostructures indicate that the permanent magnetic moments are probably introduced by broken symmetry and charge transfer because charge transfers more effectively from dodecanethiol than from oleylamine.

Composition effects of FePt alloy nanoparticles on the electro-oxidation of formic acid

The catalytic activities of FexPt100-x alloy nanoparticles at different compositions (x=10, 15, 42, 54, 58, and 63) in the electro-oxidation of formic acid have been investigated by using cyclic voltammetry (CV), chronoamperometry, and electrochemical impedance spectroscopy (EIS). It was observed that the electrocatalytic performance was strongly dependent on the FePt particle composition. In chronoamperometric measurements, the alloy particles at x approximately 50 showed the highest steady-state current density among the catalysts under study and maintained the best long-term stability. In addition, on the basis of the anodic peak current density, onset potentials, and the ratios of the anodic peak current density to the cathodic peak current density in CV studies, the catalytic activity for HCOOH oxidation was found to decrease in the order of Fe42Pt58>Fe54Pt46 approximately Fe58Pt42>Fe15Pt85>Fe10Pt90>Fe63Pt37. That is, within the present experimental context, the alloy nanoparticles at x approximately 50 appeared to exhibit the maximum electrocatalytic activity and stability with optimal tolerance to CO poisoning. Consistent responses were also observed in electrochemical impedance spectroscopic measurements. For the alloy nanoparticles that showed excellent tolerance to CO poisoning, the impedance in the Nyquist plots was found to change sign from positive to negative with increasing electrode potential, suggesting that the electron-transfer kinetics evolved from resistive to pseudoinductive and then to inductive characters. However, for the nanoparticles that were heavily poisoned by adsorbed CO species during formic acid oxidation, the impedance was found to be confined to the first quadrant at all electrode potentials. The present work highlights the influence of the molecular composition of Pt-based alloy electrocatalysts on the performance of formic acid electro-oxidation, an important aspect in the design of bimetal electrocatalysts in fuel cell applications.

Catalytic growth of silica nanoparticles in controlled shapes at planar liquid/liquid interfaces

The shape of silica nanoparticles is controlled when they are synthesized at liquid/liquid interfaces; the combination of organic and aqueous phases that form the interface can change the shape of silica into a triangle, cube, or rod.