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The ability to control the size and composition of metal nanoparticles is essential in preparing binary or ternary catalysts. This paper reports the results of an investigation of a core−shell synthesis protocol for the preparation of monolayer-capped ternary platinum−vanadium−iron (PtVFe) nanoparticles with a few nanometers in core sizes. Our approach involves the use of metal precursors, capping agents, and reducing agents in controllable ratios for the reaction in a single organic phase. By manipulating the relative concentration ratios of the metal precursors such as platinum acetylacetonate, vanadyl acetylacetonate, and iron pentacarbonyl, and capping agents such as oleic amine and oleic acids, the core−shell type nanoparticles in which the nanocrystal PtVFe cores are encapsulated with a shell of mixed amine/acid monolayer have been successfully synthesized. The average diameters of the nanocrystalline cores are well controlled between 1.4 and 3.5 nm with high monodispersity (±0.3−0.5 nm). The chemical composition of the nanocrystalline cores is shown to be effectively controllable in the range of Pt (20−40%), V (10−40%), and Fe (40−70%) by manipulating the feeding ratios of the metal precursors. The results of characterizations using TEM, DCP-AES, XRD, FTIR, and TGA techniques are discussed, along with their implications to the exploration of the nanoparticles as fuel cell catalysts.

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 ability to control composition and size in the synthesis of bimetallic nanoparticles is important for the exploitation of the bimetallic catalytic properties. This paper reports findings of an investigation of a new approach to the synthesis of gold−platinum (AuPt) bimetallic nanoparticles in aqueous solution via one-phase reduction of AuCl4^- and PtCl4^2- using a combination of reducing and capping agents. Hydrogen served as a reducing agent for the reduction of Pt(II), whereas acrylate was used as a reducing agent for the reduction of Au(III). The latter reaction was found to be catalyzed by the formation of Pt as a result of the reduction of Pt(II). Acrylate also functioned as capping agent on the resulting nanocrystals. By controlling the feed ratios of AuCl4^- and PtCl4^2- and the relative concentrations of acrylate, an effective route for the preparation of AuPt nanoparticles with bimetallic compositions ranging from ~4 to 90% Au and particle sizes ranging from 2 to 8 nm has been demonstrated. The composition, size, and shell properties were characterized using transmission electron microscopy, direct current plasma−atomic emission spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffraction. Implications of the results to the exploration of bifunctional catalysts are also briefly discussed.

This paper reports findings of an investigation of the synthesis of monolayer-capped binary gold−silver (AuAg) bimetallic nanoparticles that is aimed at understanding the control factors governing the formation of the bimetallic compositions. The synthesis of alkanethiolate-capped AuAg nanoparticles was carried out using two related synthetic protocols using aqueous sodium borohydride as a reducing agent. One involves a two-phase reduction of AuCl4^-, which is dissolved in organic solution, and Ag⁺, which is dissolved in aqueous solution. The other protocol involves a two-phase reduction of AuCl4^- and AgBr2^-, both of which are dissolved in the same organic solution. AuAg nanoparticles of 2−3 nm core sizes with different compositions in the range of 0−100% Au have been synthesized. The two synthetic routes were compared in terms of bimetallic composition and size properties. Our new findings have allowed us to establish the correlation between synthetic feeding of metals and metal compositions in the bimetallic nanoparticles, which have important implications to the exploration of gold-based bimetallic nanoparticles for constructing sensing and catalytic nanomaterials.

The preparation of nanocomposite materials from carbon nanotubes (CNTs) and metal or metal oxide nanoparticles has important implications to the development of advanced catalytic and sensory materials. This paper reports findings of an investigation of the preparation of nanoparticle-coated carbon nanotube composite materials. Our approach involves molecularly mediated assembly of monolayer-capped nanoparticles on multiwalled CNTs via a combination of hydrophobic and hydrogen-bonding interactions between the capping/mediating shell and the CNT surface. The advantage of this route is that it does not require tedious surface modification of CNTs. We have demonstrated its simplicity and effectiveness for assembling alkanethiolate-capped gold nanoparticles of 2−5 nm core sizes onto CNTs with controllable coverage and spatially isolated character. The loading and distribution of the nanoparticles on CNTs depend on the relative concentrations of gold nanoparticles, CNTs, and mediating or linking agents. The composite nanomaterials can be dispersed in organic solvent, and the capping/linking shells can be removed by thermal treatment to produce controllable nanocrystals on the CNT surfaces. The nanocomposite materials are characterized using transmission electron microscopy and Fourier transform infrared spectroscopy techniques. The results will be discussed in terms of developing advanced catalytic and sensory nanomaterials.

The understanding of surface properties of core−shell type nanoparticles is important for exploiting the unique nanostructured catalytic properties. We report herein findings of a spectroscopic investigation of the thermal treatment of such nanoparticle assemblies. We have studied assemblies of gold nanocrystals of ~2 nm core sizes that are capped by alkanethiolate shells and are assembled by covalent or hydrogen-bonding linkages on a substrate as a model system. The structural evolution of the nanoparticle assemblies treated at different temperatures was probed by several spectroscopic techniques, including UV−visible, Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS). The results show that the capping/linking shell molecules can be effectively removed to produce controllable surface and optical properties. The data further revealed that the thermally induced evolution of the surface plasmon resonance property of gold nanoparticles is dependent on the chemical nature of the linker molecule. The spectral evolution is discussed in terms of changes in particle size, interparticle distance, and dielectric medium properties, which has important implications for controlled preparation and thermal processing of core−shell nanostructured metal catalysts.

Results of an investigation of hydrogen-bonding mediated interparticle assembling and spatial properties from extremely dilute concentrations of the mediation agent are reported. Nanoparticles consisting of highly monodispersed decanethiolate-capped gold nanocrystal cores and α,ω-mercapto-alkanoic acids are highlighted as a model system. The formation of a stable ordered array is demonstrated via manipulating the constituents in solution. Infrared reflection spectroscopy, spectrophotometry, transmission electron microscopy, and atomic force microscopy have been utilized to probe the interparticle structural and spatial properties. A “squeezed” interparticle spatial model involving both hydrogen-bonding at the carboxylic acid groups and cohesive van der Waals interaction through interdigitation of the capping decanethiolate molecules is concluded to be responsible for the interparticle assembling forces. This interparticle spatial property depends on the relative amount of the hydrogen-bonding mediation agent, nanocrystal core size, and size monodispersity. Implications of these findings to the creation of well-defined nanostructures from nanoscale building blocks toward functional nanomaterials are also discussed.

This paper reports the results of a study on the activation of core−shell assembled gold nanoparticle catalysts using X-ray photoelectron spectroscopy (XPS). The goal is to determine the surface reconstitution of the nanostructured catalysts upon electrochemical activation for the electrocatalytic oxidation of methanol. The decanethiolate-capped gold nanoparticles of 2~5 nm core sizes were assembled as catalyst thin films on electrode surfaces using 1,9-nonanedithiol and 11-mercaptoundecanoic acid as model molecular linkers. The XPS results have provided two important insights into the surface reconstitution of the activated nanostructure. First, the capping/linking thiolates or dithiolates are partially removed to produce the catalytic access, with the degree of removal being dependent on the nature of the molecular linker. Second, oxygenated species are detected on the activated gold nanocrystals, demonstrating the formation of surface gold oxide and its participation in the electrocatalytic oxidation of methanol. The findings are also correlated with results from studies of surface microscopic morphology and interfacial mass flux and provide further insights into issues related to the design and preparation of highly active nanostructured gold catalysts.

Thin films derived from nanocrystal cores and functionalized linkers provide a large surface area-to-volume ratio and three-dimensional ligand framework. This paper describes the results of an investigation of the interfacial mass flux and binding properties of such thin films using an electrochemical quartz crystal nanobalance technique. The hydrogen-bonding assembly from gold nanocrystals and 11-mercaptoundecanoic acid was studied as a model system. The results reveal four distinctive mass response characteristics upon pH tuning or metal ion binding. First, the protonation−deprotonation characteristic of the carboxylic acid groups in the nanostructured framework is dependent on particle core size and film thickness. Second, the pH-tunable cationic redox reaction across the electrode|film|electrolyte interface is accompanied by a large cationic electrolyte mass flux. Third, the spontaneous complexation to copper ions by the nanostructured carboxylate framework is reflected by a mass increase of the film. Fourth, the redox reaction of copper loaded in the nanostructured film is accompanied by fluxes of electrolyte cations across the electrode|film|electrolyte interface which compensate electrostatically the fixed negative charges. On the basis of the mass change detected in the presence of a series of electrolyte cations, a linear relationship was determined between the mass increase and the atomic mass of the cation, and a concurrent flux of solvent molecules was also revealed. Implications of the findings to the delineation of the design parameters of the nanostructured ligand framework for controlled release and environmental monitoring or removal of metals are also discussed.

This paper reports novel findings of an investigation of pH-tuned morphological changes in a core−shell gold nanoparticle assembly derived from hydrogen-bonding at the encapsulating shells. Our strategy couples the selective assembly of such hydrogen-bonding-linked nanoparticles at patterned substrates with the capability of atomic force microscopy to detect the nanostructural changes in the pH-tuned interfacial process. The selective assembly on a monolayer-patterned substrate provided an ideal imaging platform with an internal standard from the unassembled areas. The detection of significant and reversible changes of both friction and height in the nanoparticle-assembled areas provided new insights into understanding of ionic fluxes and solvation effect within the nanoporous structure, which have important implications to the design of nanoparticle-based interfaces for nanostructured catalysis and molecular recognition.

This paper reports novel findings of a study of interfacial chemical derivatization reactivities at the interconnecting linkages of core−shell nanoparticle network films. Starting from thin film assemblies of gold nanoparticles linked by head-to-head hydrogen-bonded carboxylic acid terminals, we have demonstrated the conversion of the hydrogen-bonded linkages to anhydride linkages via dehydration reaction. The anhydride was further shown to serve as a new platform for immobilizing amine-functionalized molecular species. We present infrared reflection spectroscopic data to characterize the interfacial structures derived from each of these interfacial reactivities. The unique symmetric and asymmetric stretching bands of the anhydride groups allowed facial identification and assessment of the interfacial reactivities. The potential application of the derivatization strategy for immobilizing biological molecules onto the derivatized nanoparticle assemblies was also explored.

This paper reports findings of an investigation of the electrocatalytic oxidation of carbon monoxide (CO) that occurs at nanocrystal gold cores with thiolate monolayer encapsulation and within a core−shell network assembly. The core−shell and network combinations allow the manipulation of core size properties and enhance the stability of nanosized catalysts against the propensity of aggregation while being catalytically active. Using alkanedithiolate-linked thin films assembled from two different gold core sizes (2 and 5 nm), we have demonstrated that the capped nanosites are both electrochemically accessible and catalytically active to CO oxidation upon electrochemical activation. Cyclic voltammetric data are presented for assessing the electrocatalytic properties. The results have important implications for the design and tailoring of nanosized gold catalysts via manipulating core−shell chemistry.

A heating treatment strategy for inducing size and shape change of composite nanoparticles in solutions is described. The composite nanoparticles are ~2 nm gold cores encapsulated with alkanethiolate monolayers. The development of abilities in size and shape controls constitutes the motivation of this work. We demonstrated a remarkable evolution of the preformed particles in solutions toward monodispersed larger core sizes with well-defined and highly faceted morphologies. The particles thus evolved were encapsulated with the thiolate shells, and exhibited striking propensities of forming long-range ordered arrays. The morphological and structural evolutions were characterized using transmission electron microscopy, X-ray diffraction, UV−vis and infrared spectroscopies. Although temperature-driven crystal growth is known for nonencapsulated particles, the evolution of the thiolate-encapsulated nanoparticles in solutions into well-defined morphologies represents an intriguing example of temperature manipulations in size monodispersity and shape control.

This paper presents the results from a reexamination of the adsorption of organosulfides on gold films. Our original goal was to explore the adsorption of different organosulfur precursors (e.g., sulfides) as a basis for addressing details related to gold−sulfur interactions in thiol-derived monolayers. The results of our previous work (J. Am. Chem. Soc. 1994, 116, 11616) were, however, complicated by the purity of most organosulfides as well as by the competitive adsorption of thiol and disulfide impurities. By using preparative gas chromatographic purification, we were able to reduce significantly the levels of both types of impurities from the organosulfide precursors, as confirmed by mass spectrometric analyses. Spectroscopic (infrared and X-ray photoelectron) and electrochemical characterizations of adlayers formed on gold from these highly purified organosulfides have led us to reach new conclusions regarding our earlier work. First, the amount of adsorbed sulfide corresponds to only a small fraction of a monolayer. Second, the chemical identity of sulfur for the adsorbed sulfide remains unchanged. These results not only correct our earlier assessment but also illustrate the importance of purifying these precursors to unexpectedly high levels when probing issues related to the formation and structure of this adsorbate−substrate combination. The implications of these results, particularly those developed from the XPS data, to the nature of the interactions of the sulfur from thiols and disulfides with gold are discussed.

The mean size of the gold (Au) core in the synthesis of dodecanethiolate-stabilized Au cluster compounds can be finely adjusted by choice of the Au:dodecanethiolate ratio and the temperature and rate at which the reduction is conducted. The Au clusters have been examined with a large number of independent analytical tools, producing a remarkably consistent picture of these materials. Average cluster and core dimensions, as ascertained by ¹H NMR line broadening, high-resolution transmission electron microscopy, small-angle X-ray scattering, and thermogravimetric analysis, vary between diameters of 1.5 and 5.2 nm (~110−4800 Au atoms/core). The electronic properties of the Au core were examined by UV/vis and X-ray photoelectron spectroscopy; the core appears to remain largely metallic in nature even at the smallest core sizes examined. The alkanethiolate monolayer stabilizing the Au core ranges with core size from ~53 to nearly 520 ligands/core, and was probed by Fourier transform infrared spectroscopy, differential scanning calorimetry, contact-angle measurements, and thermal desorption mass spectrometry. The dodecanethiolate monolayer on small and large core clusters exhibits discernable differences; the line dividing “3-dimensional” monolayers and those resembling self-assembled monolayers on flat Au (2-dimensional monolayers) occurs at clusters with ~4.4 nm core diameters.