While the optical and spectroscopic properties of gold nanoparticles are widely used for chemical, bioanalytical, and biomedical applications, the study of the size correlation with these properties for nanoparticles in solutions is rather limited. This paper describes the results of a systematic study of such a correlation for gold nanoparticles with diameters ranging from 10 to 100 nm in aqueous solutions. The high monodispersity of these nanoparticles permitted a meaningful correlation of the particle size with the surface plasmon (SP) resonance band properties and the surface-enhanced Raman scattering (SERS) spectroscopic properties. This correlation is compared to the results from the simulation based on Mie theory. The close agreement between the experimental and the theoretical results provides insight into the validity of determining the wavelength of the SP resonance band as a measure of the particle size. The size correlation with the SERS intensity from the adsorption of 4-mercaptobenzoic acid on the nanoparticles in aqueous solutions reveals the existence of a critical size of the nanoparticles in the solution beyond which the particle−particle interaction is operative and responsible for the SERS effect. These findings serve as the basis of size correlations for exploiting the optical and spectroscopic properties of gold nanoparticles of different sizes in aqueous solutions in analytical or bioanalytical applications.

The synthesis of stable, monodisperse, shaped copper nanoparticles has been difficult, partially because of copper's propensity for oxidation. This article reports the findings of an investigation of a synthetic route for the synthesis of size-controllable and potentially shape-controllable molecularly capped copper nanoparticles. The approach involved the manipulation of reaction temperature for the synthesis of copper nanoparticles in organic solvents in the presence of amine and acid capping agents. By manipulating the reaction temperature, this route has been demonstrated for the production of copper nanoparticles ranging from 5 to 25 nm. The size dependence of the melting temperature of copper nanoparticles, especially for surface melting, is believed to play an important role in interparticle coalescence, leading to size growth as the reaction temperature is increased. Control of the reaction temperature and capping molecules has also been demonstrated to produce copper nanoparticles with different shapes such as rods and cubes. The previously proposed combination of the selective formation of a seed precursor and a selective growth direction due to the preferential adsorption of capping agents on certain nanocrystal facets is believed to be responsible for shape formation by kinetically controlling the growth rates of crystal facets. The nanoparticles are characterized using TEM, XRD, and UV-visible techniques. A mechanistic consideration of the size control and shape formation is also discussed.

The ability to control the composition and phase properties of bimetallic nanoparticles is critical in exploring catalytic properties. In this paper we present results from a study aimed at determining those properties for carbon-supported gold−platinum (AuPt) catalysts with different bimetallic compositions. The bimetallic nanoparticle catalysts are prepared by a two-phase synthesis protocol employing organic monolayer encapsulation on bimetallic AuPt cores (∼2 nm). The size-controlled nanoparticles are assembled on carbon black support materials with controllable dispersion and metal loading and are further treated by calcination under controlled temperature and atmosphere. The core composition of the bimetallic nanoparticles is determined by direct current plasma-atomic emission spectroscopy. Structural characterization is carried out by X-ray diffraction. The bimetallic nanoparticles were shown to display alloy properties, which is in sharp contrast to the bimetallic miscibility gap known for the bulk counterpart of the bimetallic metals. This finding demonstrates the difference of the physical and chemical properties for nanoscale materials from the bulk crystalline state, revealing important details of the phase properties of the bimetallic nanoparticle catalysts and new information for the correlation between the composition and the phase properties at the nanoscale. Implications of our findings to the design and manipulation of the bimetallic nanoparticles for catalytic applications are also discussed.

The design of active and robust bimetallic nanoparticle catalysts requires the control of the nanoscale alloying and phase-segregation structures and the correlation between the nanoscale phase structures and the catalytic properties. Here we describe new findings of a detailed investigation of such nanoscale phase structures and their structure-catalytic activity correlation for gold-platinum nanoparticles prepared with controllable sizes and compositions. The nanoscale alloying and phase-segregation were probed as a function of composition, size, and thermal treatment conditions using X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, electrochemical characterization, and density functional theory modeling. The results have provided the experimental evidence in support of the theoretically simulated dependence of alloying and phase segregation on particle size and temperature. More importantly, new insights have been gained into the control of the nanoscale phase properties of this bimetallic system among alloyed, partially alloyed, or partially phase segregated structures. In contrast to the largely alloyed character for the catalysts treated at 300-400°C, the higher-temperature treated catalysts (e.g., 800°C) are shown to consist of a Pt-rich alloy core and a Au shell or a phase-segregated Au domains enriched on the surface. This conclusion is further supported by the electrochemical and electrocatalytic data revealing that the catalytic activity is highly dependent on the nanoscale evolution of alloying and phase segregation. The thermal control of the nanoscale alloying, phase-segregation, and core-shell evolution of the nanoscale bimetallic catalysts provided the first example for establishing the correlation between the nanoscale phase structures and the electrocatalytic activity for oxygen reduction reaction correlation, which has profound implications to the design and nanoengineering of a wide variety of bimetallic or multimetallic nanostructures for advanced catalysts.

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In view of the recent finding that the bimetallic AuPt nanoparticles prepared by molecular-capping-based colloidal synthesis and subsequent assembly on carbon black support and thermal activation treatment exhibit alloy properties, which is in sharp contrast to the bimetallic miscibility gap known for the bulk counterparts in a wide composition range, there is a clear need to assess the electrocatalytic properties of the catalysts prepared with different bimetallic composition and different thermal treatment temperatures. This paper reports recent results of such an investigation of the electrocatalytic methanol oxidation reaction (MOR) activities of the carbon-supported AuPt nanoparticle catalysts with different bimetallic composition and thermal treatment temperatures. Au m Pt 100-m nanoparticles of 2-3 nm core sizes with different atomic compositions ranging from 10% to 90% Au (m ) 10∼90) have been synthesized by controlling the feeding of the metal precursors used in the synthesis. The electrocatalytic MOR activities of the carbon-supported AuPt bimetallic catalysts were characterized in alkaline electrolytes. The catalysts with 65% to 85% Au and treated at 500 °C were found to exhibit maximum electrocatalytic activities in the alkaline electrolytes. The findings, together with a comparison with some well-documented catalysts as well as recent experimental and theoretical modeling results, have revealed important insights into the participation of CO ad and OH ad on Au sites in the catalytic reaction of Pt in the AuPt alloys with ∼75% Au. The insights are useful for understanding the correlation of the bifunctional electrocatalytic activity of the bimetallic nanoparticle catalysts with the bimetallic composition and the thermal treatment temperatures.

The ability to nanoengineer catalysts in terms of size, composition, shape, and phase properties is essential in exploiting the catalytic properties. This paper reports the results of an investigation of the structural and electrocatalytic properties of PtM (M = Co and Ni) nanoparticles and their carbon-supported electrocatalysts for an oxygen reduction reaction (ORR). Examples are focused on PtCo and PtNi nanoparticles in the range of 2−9 nm and in the composition range of 50−75% Pt. A sharp contrast in size dependence of the activity was revealed between PtCo/C and PtNi/C catalysts, showing a clear trend of decrease in activity with increasing particle size for PtCo/C and a subtle increase in activity for PtNi/C. The size−activity correlation also depends on the bimetallic composition. The detailed analyses of the structures of the catalysts by XAS technique revealed important information for assessing the electrocatalytic properties in relation to the relative amount of oxygenated species and the relative change in interatomic bond distance in the bimetallic nanoparticles, which suggest that a combination of structural parameters such as the change in Pt−Pt bond distance, the segregation of metal phases, and the formation of surface oxygenated species is operative for the size dependence of the enhanced electrocatalytic activity.

Nanoscale catalysis depends on spatial and surface arrangements of atoms. The synthesis of core/shell nanoparticles by synthetic approaches, the characterization of the core/shell nanostructures in terms of the relative core/shell compositions, and the demonstration of the electrocatalytic activities for fuel cell reactions (see figure), as reported in this Communication, have important implications to the design of desired nanoscale catalysts.