Derrick Mott

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.

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.

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.

Nanocrystal and surface alloy properties of bimetallic Gold-Platinum nanoparticles

We report on the correlation between the nanocrystal and surface alloy properties with the bimetallic composition of gold-platinum(AuPt) nanoparticles. The fundamental understanding of whether the AuPt nanocrystal core is alloyed or phase-segregated and how the surface binding properties are correlated with the nanoscale bimetallic properties is important not only for the exploitation of catalytic activity of the nanoscale bimetallic catalysts, but also to the general exploration of the surface or interfacial reactivities of bimetallic or multimetallic nanoparticles. The AuPt nanoparticles are shown to exhibit not only single-phase alloy character in the nanocrystal, but also bimetallic alloy property on the surface. The nanocrystal and surface alloy properties are directly correlated with the bimetallic composition. The FTIR probing of CO adsorption on the bimetallic nanoparticles supported on silica reveals that the surface binding sites are dependent on the bimetallic composition. The analysis of this dependence further led to the conclusion that the relative Au-atop and Pt-atop sites for the linear CO adsorption on the nanoparticle surface are not only correlated with the bimetallic composition, but also with the electronic effect as a result of the d-band shift of Pt in the bimetallic nanocrystals, which is the first demonstration of the nanoscale core-surface property correlation for the bimetallic nanoparticles over a wide range of bimetallic composition.

Chromium-assisted synthesis of platinum nanocube electrocatalysts

This report demonstrates a novel strategy of chromium-assisted synthesis of platinum nanocubes as electrocatalysts for oxygen reduction reaction with enhanced specific activity.

Rigid, conjugated and shaped arylethynes as mediators for the assembly of gold nanoparticles

The ability to control and tailor the interparticle structures and properties of molecularly-mediated assembly of nanoparticles in terms of molecular size, shape, and structure is essential for exploring the nanostructured multifunctional properties. This report describes the results of an investigation on how such molecular size, shape, and structure are operative in the assembly of gold nanoparticles mediated by rigid methylthio arylethynes (MTAs) of different shapes (e.g., I, V, Y, and X shapes). The goal of this investigation focuses on understanding the correlation between the interparticle molecular structures and the optical and spectroscopic (e.g., surface-enhanced Raman scattering (SERS)) properties in this class of molecularly-mediated assembly of nanoparticles. The surface plasmon resonance bands of the nanoparticles, the electronic transition bands of the mediator molecules, the dynamic light scattering of the assembly processes, and the SERS signatures of the interparticle structures are systematically compared for a series of MTAs. Important insights have been gained into the relative binding strength of the different MTAs in the single- and two-component mediated assembly processes. The results have demonstrated that the interparticle spacing and structures of nanoparticle assemblies can be defined by these rigid and shaped molecules. This type of interparticle structural definition can be controlled to produce well-defined optical and spectroscopic signatures, which is especially important for understanding interparticle optical or spectroscopic characteristics (e.g., “hot-spot” in nanoparticle-based SERS effect). Implications of the findings to enabling the design of interparticle structure and the control of the interparticle properties in nanoparticle assembly systems for potential applications in chemical/bio sensing, spectroscopic signal amplification, and microelectronics are also briefly discussed.

An in situ real-time x-ray diffraction study of phase segregation in Au–Pt nanoparticles

In situ real-time x-ray diffraction was used to study phase segregation and coarsening of Au-Pt nanoparticles supported on silica powder, and porous alumina membranes. Contrary to the expectations from the bulk phase diagram, silica supported Au-Pt nanoparticles have an alloyed structure that is preserved even after extensive annealing at temperatures as high at 700 degrees C. In stark contrast, alumina supported Au-Pt nanoparticles exhibit a rich phase behaviour that is sensitive to alloy composition and the details of the synthesis process. In particular, low-density as-prepared Au(41)Pt(59) nanoparticles exhibit the signature of incipient phase segregation that develops into full phase separation during annealing at high temperature.

Low-temperature phase and morphology transformations in noble metal nanocatalysts

In situ real-time x-ray diffraction was used to study temperature-induced structural changes of 1-5 nm Au, Pt, and AuPt nanocatalysts supported on silicon substrates. Synchrotron-based x-ray diffraction indicates that the as-synthesized Au and Au(64)Pt(36) nanoparticles have a non-crystalline structure, while the Pt nanoparticles have the expected cubic structure. The nanoparticles undergo dramatic structural changes at temperatures as low as 120 °C. During low-temperature annealing, the Au and AuPt nanoparticles first melt and then immediately coalesce to form 4-5 nm crystalline structures. The Pt nanoparticles also aggregate but with limited intermediate melting. The detailed mechanisms of nucleation and growth, though, are quite different for the three types of nanoparticles. Most interestingly, solidification of high-density AuPt nanoparticles involves an unusual transient morphological transformation that affects only the surface of the particles. AuPt nanoparticles on silicon undergo partial phase segregation only upon annealing at extremely high temperatures (800 °C).

Silica-Supported Au and Pt Nanoparticles and CO Adsorption

The understanding of the surface properties of metal nanoparticles is essential for exploiting their unique catalytic properties. This paper reports findings of the preparation of silica-supported Pt and Au nanoparticles and the FTIR characterization of CO adsorption on the supported nanoparticles. The nanoparticles were prepared by both a traditional impregnation method and molecular-capping based synthesis method. By comparing the spectroscopic characteristics of CO adsorption on these catalysts, similarities and differences in CO stretching bands have been identified. The findings are significant because important insights have been gained into the surface binding properties of Au and Pt nanoparticle catalysts.