Pedro H. C. Camargo

Pd-Pt Bimetallic Nanodendrites with High Activity for Oxygen Reduction

Extending Platinum Catalysts Platinum performs extremely well as a catalyst for the oxygen-reduction reaction that runs under highly acidic conditions in proton-exchange membrane fuel cells, but is expensive. One strategy for reducing costs is to increase the surface area of the platinum. Lim et al. (p. 1302, published online 14 May) describe a simple chemical route, in which Pt ions in solution are reduced onto Pd seed crystals, which creates faceted Pt nanocrystals with a high area owing to their dendritic architecture. On a Pt mass basis, these catalysts are several times more active than conventional Pt catalysts. The catalytic activity of platinum is enhanced through a growth process that creates nanocrystals with high surface area. Controlling the morphology of Pt nanostructures can provide a great opportunity to improve their catalytic properties and increase their activity on a mass basis. We synthesized Pd-Pt bimetallic nanodendrites consisting of a dense array of Pt branches on a Pd core by reducing K2PtCl4 with L-ascorbic acid in the presence of uniform Pd nanocrystal seeds in an aqueous solution. The Pt branches supported on faceted Pd nanocrystals exhibited relatively large surface areas and particularly active facets toward the oxygen reduction reaction (ORR), the rate-determining step in a proton-exchange membrane fuel cell. The Pd-Pt nanodendrites were two and a half times more active on the basis of equivalent Pt mass for the ORR than the state-of-the-art Pt/C catalyst and five times more active than the first-generation supportless Pt-black catalyst.

Dimers of silver nanospheres: facile synthesis and their use as hot spots for surface-enhanced Raman scattering.

This paper describes a simple, one-pot method that generates dimers of silver nanospheres in one step without any additional assembly steps. The dimers are consisted of single-crystal silver nanospheres approximately 30 nm in diameter and separated by a gap of 1.8 nm wide. The key to the success of this method lies in the control of colloidal stability and oxidative etching by optimizing the amount of chloride added to a polyol synthesis. The dimers provide a well-defined system for studying the hot spot phenomenon (hot spot: the gap region of a pair of strongly coupled silver or gold nanoparticles), an extremely important but poorly understood subject in surface-enhanced Raman scattering (SERS). Because of the relatively small size of the silver nanospheres, only those molecules trapped in the hot spot region are expected to contribute to the detected SERS signals. By correlating SERS measurements with SEM imaging, we found that the SERS enhancement factor within the hot spot region of such a dimer was on the order of 2 x 10(7).

Nanocomposites: synthesis, structure, properties and new application opportunities

Nanocomposites, a high performance material exhibit unusual property combinations and unique design possibilities. With an estimated annual growth rate of about 25% and fastest demand to be in engineering plastics and elastomers, their potential is so striking that they are useful in several areas ranging from packaging to biomedical applications. In this unified overview the three types of matrix nanocomposites are presented underlining the need for these materials, their processing methods and some recent results on structure, properties and potential applications, perspectives including need for such materials in future space mission and other interesting applications together with market and safety aspects. Possible uses of natural materials such as clay based minerals, chrysotile and lignocellulosic fibers are highlighted. Being environmentally friendly, applications of nanocomposites offer new technology and business opportunities for several sectors of the aerospace, automotive, electronics and biotechnology industries.

Synthesis of Pd-Pt Bimetallic Nanocrystals with a Concave Structure through a Bromide-Induced Galvanic Replacement Reaction

This article describes a systematic study of the galvanic replacement reaction between PtCl(6)(2-) ions and Pd nanocrystals with different shapes, including cubes, cuboctahedrons, and octahedrons. It was found that Br(-) ions played an important role in initiating, facilitating, and directing the replacement reaction. The presence of Br(-) ions led to the selective initiation of galvanic replacement from the {100} facets of Pd nanocrystals, likely due to the preferential adsorption of Br(-) ions on this crystallographic plane. The site-selective galvanic replacement resulted in the formation of Pd-Pt bimetallic nanocrystals with a concave structure owing to simultaneous dissolution of Pd atoms from the {100} facets and deposition of the resultant Pt atoms on the {111} facets. The Pd-Pt concave nanocubes with different weight percentages of Pt at 3.4, 10.4, 19.9, and 34.4 were also evaluated as electrocatalysts for the oxygen reduction reaction (ORR). Significantly, the sample with a 3.4 wt.% of Pt exhibited the largest specific electrochemical surface area and was found to be four times as active as the commercial Pt/C catalyst for the ORR in terms of equivalent Pt mass.

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.

Facile Synthesis of Bimetallic Nanoplates Consisting of Pd Cores and Pt Shells through Seeded Epitaxial Growth

Pd-Pt core-shell nanoplates with hexagonal and triangular shapes were synthesized through the heterogeneous, epitaxial growth of Pt on Pd nanoplates. The Pd nanoplates were synthesized by reducing Na2PdCl4 precursor with PVP as a reducing agent, which then served as seeds for the nucleation of Pt atoms formed by reducing H2PtCl6 with citric acid. Characterization of the as-prepared Pd-Pt nanoplates by scanning transmission electron microscopy and high-resolution transmission electron microscopy reveals that a thin, uniform Pt shell was formed around the Pd nanoplate, demonstrating the layer-by-layer epitaxial growth of Pt on Pd surface in this approach. The close lattice match between Pd and Pt (lattice mismatch of only 0.77%) and the slow reduction rate associated with the mild reducing power of citric acid play key roles in achieving the epitaxial growth of Pt shells on Pd nanoplates.

Synthesis and Characterization of Noble-Metal Nanostructures Containing Gold Nanorods in the Center

2010 WILEY-VCH Verlag Gm Figure 1. A) A schematic image showing the formation of Au@Ag core–shell nanocrystals. B,C) TEM images of (B) the Au nanorods Noble-metal nanocrystals have received increasing interest because they display unique catalytic and optical properties sought for applications such as catalysis, diagnosis, plasmonics, and surface-enhanced Raman spectroscopy (SERS). The catalytic and optical properties of a noble-metal nanocrystal can be tailored by controlling its size, shape, elemental composition, as well as the internal and surface structures. Most recently, special attention has been paid to core–shell, bimetallic nanocrystals because they provide a new system with tunable catalytic and optical properties. In this case, the core–shell structure can be achieved through deposition of a metal on the surface of core made of another metal or via a galvanic replacement reaction between the core and a salt precursor to the second metal. In both cases, the metal nanocrystal serving as the core can have a specific geometric shape and this shape might be able to sustain during the coating or galvanic process. In general, it is a combination of the facets expressed on the core nanocrystal and the degree of lattice mismatch between the two metals that dictates the shape or morphology of the final product. Many attempts have been made to explore Au nanorods as the starting material to construct nanocrystals with a core–shell structure. For example, the ends and the side surface of Au nanorods have been coated with other metals such as Pd, Pt, Ag, Ni, and Au to add new functions to the Au nanorods. Besides coating, other strategies have been reported to generate Au nanocrystals with new geometric shapes by templating against Au nanorods. Specifically, Au nanorods have been transformed into various shapes via overgrowth on the entire surface or the side surface of Au nanorods. However, this strategy has not been widely extended to noble metals other than Au. Only recent studies by Xiang et al. and Becker et al. showed a system, where Ag could grow on specific sides of Au nanorods to generate nanocrystals with a semicircular or triangular morphology. Overall, it remains a challenge to find a simple way to control the overgrowth of other metals on Au nanorods in an effort to generate bimetallic nanocrystals with a core–shell structure and the desired properties. In this Communication, we present a facile method for the synthesis of Ag nanocrystals containing Au nanorods in the center, which will be denoted as Au@Ag for simplicity. Figure 1A shows schematically how the core–shell bimetallic nanocrystals are formed. The final products are mostly octahedrons with a few in other shapes (e.g., decahedrons). Since we used Au nanorods with a uniform size distribution, the core–shell nanocrystals were fairly uniform in terms of both size and shape. In general, the dimensions of the core–shell nanocrystals could be easily tuned by using Au nanorods with different aspect ratios and/or by controlling the amount of AgNO3 added into the reaction system. In a subsequent step, the Au@Ag nanocrystals can also be converted into hollow nanostructures made of Au, Pt, and Pd via galvanic replacement reactions, with Au nanorods encapsulated in the center.

Isolating and probing the hot spot formed between two silver nanocubes.

Out of the frying pan: Hot spots can greatly increase the sensitivity of surface-enhanced Raman scattering, but they remain poorly understood. A new strategy based on plasma etching (see picture) can be used to isolate and exclusively probe the SERS-active molecules adsorbed in the hot-spot region between two silver nanocubes.

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.

Etching and Dimerization: A Simple and Versatile Route to Dimers of Silver Nanospheres with a Range of Sizes

This paper describes a facile method that generates dimers of Ag nanospheres by etching Ag nanocubes with Fe(NO3)3 in ethanol with the assistance of poly(vinyl pyrrolidone) (PVP). During the etching process, the corners and edges of the Ag nanocubes were truncated off to generate spherical particles, accompanied by dimerization as a result of reduction in colloidal stability due to the addition of ionic species. Both ethanol and PVP play an important role in the etching and dimerization processes. By starting with Ag nanocubes of different sizes, we obtained well-defined dimers of Ag spheres 40, 63, and 80 nm in diameter with percentages of dimerization >60%. Since this approach can be used to fabricate dimers of Ag nanospheres with a range of different sizes, it allows for a systematic study of the hot-spot phenomenon in SERS. By correlating with SEM imaging, we measured the SERS enhancement factors for individual dimers from the three different samples, and an average value of 3.9×107, 9.3×107, and 1.7×108 was obtained, respectively.