Chuan-Jian Zhong

Nanostructured catalysts in fuel cells

One of the most important challenges for the ultimate commercialization of fuel cells is the preparation of active, robust, and low-cost catalysts. This review highlights some findings of our investigations in the last few years in developing advanced approaches to nanostructured catalysts that address this challenge. Emphasis is placed on nanoengineering-based fabrication, processing, and characterization of multimetallic nanoparticles with controllable size (1-10 nm), shape, composition (e.g. Ml(n)M2(100-n), M1(n)M2(m)M3(100-n-m), M1@M2, where M (1 or 2) = Pt, Co, Ni, V, Fe, Cu, Pd, W, Ag, Au etc) and morphology (e.g. alloy, core@shell etc). In addition to an overview of the fundamental issues and the recent progress in fuel cell catalysts, results from evaluations of the electrocatalytic performance of nanoengineered catalysts in fuel cell reactions are discussed. This approach differs from other traditional approaches to the preparation of supported catalysts in the ability to control the particle size, composition, phase, and surface properties. An understanding of how the nanoscale properties of the multimetallic nanoparticles differ from their bulk-scale counterparts, and how the interaction between the nanoparticles and the support materials relates to the size sintering or evolution in the thermal activation process, is also discussed. The fact that the bimetallic gold-platinum nanoparticle system displays a single-phase character different from the miscibility gap known for its bulk-scale counterpart serves as an important indication of the nanoscale manipulation of the structural properties, which is useful for refining the design and preparation of the bimetallic catalysts. The insight gained from probing how nanoparticle-nanoparticle and nanoparticle-substrate interactions relate to the size evolution in the activation process of nanoparticles on planar substrates serves as an important guiding principle in the control of nanoparticle sintering on different support materials. The fact that some of the trimetallic nanoparticle catalysts (e.g. PtVFe or PtNiFe) exhibit electrocatalytic activities in fuel cell reactions which are four-five times higher than in pure Pt catalysts constitutes the basis for further exploration of a variety of multimetallic combinations. The fundamental insights into the control of nanoscale alloy, composition, and core-shell structures have important implications in identifying nanostructured fuel cell catalysts with an optimized balance of catalytic activity and stability.

Molecularly mediated processing and assembly of nanoparticles: exploring the interparticle interactions and structures.

The harnessing of the nanoscale properties of nanoparticles in most technological applications requires the abilities of controlled processing and assembly, which has been an important challenge because of the difficulty in manipulating interparticle properties. Molecularly mediated processing and assembly of nanoparticles have emerged as an important strategy for addressing this challenge. The capability of this strategy in manipulating size, shape, composition, and interparticle properties has significant implications for designing sensing, biosensing, nanoprobing, and many other functional nanostructures. This Account highlights some of the important findings in investigating both interparticle and collective properties as a forum for discussing new opportunities in exploiting nanoparticle-based designs and applications. The concept of mediator-template assembly of nanoparticles explores the combination of the forces from a mediator and a templating molecule for designing and controlling the interparticle interactions. The manipulation of the interparticle interaction properties and the detection of the molecular signatures are two of the key elements in this concept. A series of well-defined molecular mediators ranging from inorganic, organic, supramolecular, to biological molecules have been explored to ascertain how these two elements can be achieved in nanoparticle assemblies. The emphasis is the fundamental understanding of interparticle molecular interactions, such as covalent, electrostatic, hydrogen bonding, multidentate coordination, pi-pi interactions, etc. Each of these molecular interactions has been examined using specific molecules, such as multifunctional ligands, tunable sizes, shapes, or charges, well-defined molecular rigidity and chirality, or spectroscopic signatures, such as fluorescence and Raman scattering. Examples included thiols, thioethers, carboxylic acids, fullerenes, dyes, homocysteines, cysteines, glutathiones, proteins, and DNAs as molecular mediators for the assembly of gold, alloy, and magnetic nanoparticles. The understanding of these systems provided insights into how the unique electrical, optical, magnetic, and spectroscopic properties of the nanoparticle assemblies can be exploited for potential applications. This Account also highlights a few examples in chemical sensing and bioprobing to illustrate the importance of interparticle interactions and structures in exploiting these properties. One example involves thin-film assemblies of metal nanoparticles as biomimetic ion channels or chemiresistor sensing arrays by exploiting the nanostructured ligand framework interactions. Other examples explore the surface-enhanced Raman scattering signature as nanoprobes for the detection of protein binding or the enzyme-based cutting of interparticle DNAs. The detailed understanding of the design and control parameters in these and other systems should have a profound impact on the exploration of nanoparticles in a wide range of technological applications.

Core@shell nanomaterials: gold-coated magnetic oxide nanoparticles

The study of core@shell magnetic nanoparticles has a wide range of applications because of the unique combination of the nanoscale magnetic core and the functional shell. This report highlights some of the recent findings in the investigation of the synthesis, characterization and application of one important class of core@shell magnetic nanoparticles, i.e., magnetic nanoparticles coated with a gold shell (MNP@Au). The gold shell imparts the magnetic nanoparticles with many intriguing functional properties. Areas specifically highlighted in this report include strategies for the synthesis of MNP@Au nanoparticles, characterization of the core@shell nanostructures, and exploration of potential applications of the core@shell nanomaterials in terms of biological and catalytic interfacial reactivities. Implications of some of the research findings to address both fundamental and practical issues are also briefly discussed in an effort to further broaden the rapidly-emerging field of core@shell magnetic nanoparticles.

Fuel cell technology: nano-engineered multimetallic catalysts

Fuel cells represent an attractive technology for tomorrows energy vector because hydrogen is an efficient fuel and environmentally clean, but one of the important challenges for fuel cell commercialization is the preparation of active, robust and low-cost catalysts. The synthesis and processing of molecularly-capped multimetallic nanoparticles, as described in this report, serves as an intriguing way to address this challenge. Such nanoparticles are exploited as building blocks for engineering the nanoscale catalytic materials by taking advantage of diverse attributes, including monodispersity, processability, solubility, stability, capability in terms of size, shape, composition and surface properties. This article discusses recent findings of our investigations of the synthesis and processing of nanostructured catalysts with controlled size, composition, and surface properties by highlighting a few examples of bimetallic/trimetallic nanoparticles and supported catalysts for electrocatalytic oxygen reduction.

Fine structure in the voltammetric desorption curves of alkanethiolate monolayers chemisorbed at gold

This paper presents the preliminary results from a study of the chain length dependence of the reductive desorption and oxidative re-deposition processes of alkanethiolate monolayers on Au(111) electrodes. In contrast to the more extensively studied short chain length monolayers, we report a previously undetected fine structure in the voltammetric waves for monolayers composed of alkyl chains of more than 10 carbons. The short chain systems in general exhibit a single voltammetric wave for the desorption and re-deposition processes. In contrast, multiple voltammetric waves separated by 20 to 100 mV are found for both the desorption and the re-deposition of the long chain systems. The origin of these results is examined in terms of possible differences in the adlayer domain structures and in the binding modes between sulfur and gold.

Interparticle interactions in glutathione mediated assembly of gold nanoparticles.

The understanding of the detailed molecular interactions between (GSH) glutathione molecules in the assembly of metal nanoparticles is important for the exploitation of the biological reactivity. We report herein results of an investigation of the assembly of gold nanoparticles mediated by glutathione and the disassembly under controlled conditions. The interparticle interactions and reactivities were characterized by monitoring the evolution of the surface plasmon resonance band using the spectrophotometric method and the hydrodynamic sizes of the nanoparticle assemblies using the dynamic light scattering technique. The interparticle reactivity of glutathiones adsorbed on gold nanoparticles depends on the particle sizes and the ionic strength of the solution. Larger-sized particles were found to exhibit a higher degree of interparticle assembly than smaller-sized particles. The assembly-disassembly reversibility is shown to be highly dependent on pH and additives in the solution. The interactions of the negatively charged citrates surrounding the GSH monolayer on the particle surface were believed to produce more effective interparticle spatial and electrostatic isolation than the case of OH (-) groups surrounding the GSH monolayer. The results have provided new insights into the hydrogen-bonding character of the interparticle molecular interaction of glutathiones bound on gold nanoparticles. The fact that the interparticle hydrogen-bonding interactions in the assembly and disassembly processes can be finely tuned by pH and chemical means has implications to the exploitation of the glutathione-nanoparticle system in biological detection and biosensors.

Polypyrrole-based electrode coatings switchable electrochemically between the anion- and cation-exchanger states

Dodecylsulphate was incorporated into a polypyrrole matrix during anodic polymerization of pyrrole. The concentration was adjusted such that in the oxidized film about one half of the polypyrrole cationic groups was charge compensated by the sulphate groups from the incorporated dodecylsulphate, the second half by anions from the electrolyte. Upon electrochemical oxidation/reduction, the film is reversibly “switched” between a cation-exchanger (at negative electrode potentials), and an anion exchanger (at more positive potentials) state. The different states of the film are characterized by electrochemical, EDAX, and in particular by Volta-potential measurements. The Donnan potentials determined with the latter techniques as a function of electrode potential and electrolyte concentration were in good agreement with calculated values.

Gold-platinum nanoparticles: alloying and phase segregation

The ability to control nanoscale alloying and phase segregation properties is important for the exploration of multimetallic nanoparticles for the design of advanced functional materials and catalysts. This report highlights recent insights into the nanoscale phase properties of gold-platinum (AuPt) nanoparticles, which serves as an example to shine a light on the importance of changes in physical and chemical properties in which nanoscale multimetallic materials may differ from their bulk counterparts. In contrast to the wide miscibility gap well known for the bulk gold-platinum system, the bimetallic nanoparticles have been demonstrated to exist in phases ranging from alloy, partial alloy, to phase segregation depending on the preparation conditions, the bimetallic composition, and the supporting materials. For AuPt nanoparticles supported on carbon materials, the nanoscale alloying or phase segregation is shown to be controllable by thermal treatment temperatures, which is not only evidenced by detailed analysis of the phase and surface properties, but also supported by theoretical modeling based on thermodynamic and density function theory. The understanding of the nanoscale phase properties can be correlated with the electrocatalytic activities for fuel cell reactions such as methanol oxidation reaction and oxygen reduction reaction. Implications of the new insights to designing and nanoengineering the phase properties of multimetallic nanoparticles and catalysts are also briefly discussed.

Voltammetric reductive desorption characteristics of alkanethiolate monolayers at single crystal Au(111) and (110) electrode surfaces

Abstract This note describes the results of a comparative study of the reductive desorption characteristics of alkanethiolate monolayers at single crystal Au(111) and Au(110) electrodes. The voltammetric data reveal that the reductive desorption potential is dependent on the surface crystallinity of the underlying gold substrate. The observed difference in potential between these two substrates is indicative of a difference in binding strength of the monolayer at different binding sites in which the gold-sulfur bonding is stronger at the Au(110) than at the Au(111) single crystal. Spectroscopic data (X-ray photoelectron and infrared reflection spectroscopies) provide additional comparisons for the monolayer structures at the two substrates. These findings substantiate our recent study on the voltammetric differences between the atomically smooth surfaces of annealed Au(111) films and the micro-topographically rougher as-evaporated Au(111) films.

Ternary alloy nanoparticles with controllable sizes and composition and electrocatalytic activity

The ability to tailor the size and composition of metal particles at the nanoscale could lead to improved or new catalytic properties. This paper reports the results of an investigation of the preparation and the characterization of platinum–nickel–iron (PtNiFe) ternary alloy nanoparticles. The synthesis of monolayer-capped ternary PtNiFe nanoparticles involved controlling the ratios of metal precursors, capping agents, and reducing agents in a single organic phase. The average diameters of the resulting nanocrystalline cores were well controlled between 1.4 and 1.8 nm with high monodispersity (±0.2–0.4 nm). The catalysts were prepared by loading the as-synthesized PtNiFe nanoparticles onto a high surface area carbon support and subsequent thermal treatment optimized for achieving effective shell removal and alloy formation, as well as controllable size, composition and metal loading. The catalysts were characterized using TEM, DCP-AES, FTIR, TGA, and XRD techniques. The catalysts were also examined for their intrinsic kinetic activities for the electrochemical oxygen reduction reaction. It is shown that the ternary catalysts are highly active towards molecular oxygen electrocatalytic reduction. Implications of our findings for the exploration of the new nanostructured catalyst materials for applications in fuel cell catalysis are also discussed.