(5cv) Preparation of Heterogeneous Nanostructures for Catalytic and Magnetic Applications

The unique catalytic, electronic, and magnetic properties of nanostructured noble metals and their alloys have assured their widespread usage. The performance of these nanomaterials is strongly related to their size, shape, chemical composition, interaction with support, and, to a great extent, to their structural diversity. Although significant advancements were made in controlling the size and size-distribution of metal and alloy nanomaterials, it is no trivial to prepare these materials with well-controlled morphology, chemical composition, and heterogeneous structures. The design, characterization, and applications of these materials will continue to be the focus of vigorous, multi-disciplinary investigations in the future.

My research focuses were on the morphology control of noble-metal-containing nanomaterials such as Pt, FePt, AuPt, Pd, AuPd. The novel catalytic and magnetic performance of these nanostructured materials may afford a solution to present and future energy-related problems. The three distinct research objectives described below spearhead this effort.

?« Synthesis of nanoalloys with controlled distribution of atoms via a wet chemistry approach.

Detailed studies of various two-metal structures, such as intermetalic alloy, random alloy, non-random alloy, or phase segregation, are essential for their applications. Recently, non-random alloys have shown promise in designing catalysts. Our research demonstrated the successful synthesis of a Pd/Au non-random alloy with an Au-rich core and Pd-rich shell by galvanic replacement between Pd nanostructures and Au cations in solution at room temperature, offering great advantages over conventional methods of synthesis that employ the complex UHV system.

?« Designing nanostructures with improved catalytic properties in Direct Methanol Fuel Cell (DMFC) reactions, and the water gas shift (WGS) reaction.

Nanostructures with controlled morphology and chemical composition demonstrated improved catalytic properties. Beyond the conventional synthesis of monoshapes with some basic morphology defined by the crystal's configuration, our established methodology provided a new growth model based on single crystal seeds or multiple twinned crystal seeds that was applicable to the design of, but not limited to, various noble metals nanostructures such as Pt multipods, and Pt and PtRu porous dendrites. In particular, these as-made Pt and PtRu dendrites had a high surface area (14-20 m2/g), and showed improved catalytic activities towards DMFC reactions.

?« Studying ferromagnetic properties of one-dimensional 4d and 5d metals and alloys nanowires.

Magnetic material at the nanoscale presents surprising behavior that does not appear in its bulk form. In confined nanoscale systems, such as a few atomic layers, ultrathin nanowires, or nanoclusters, the reduced coordination and bonding favor more localized electronic states, as well as narrower bands and larger densities of states, bringing very exotic features to the magnetism. Two-dimensional atomic layers and zero-dimensional nanoclusters in several 4d and 5d metals display ferromagnetism, such as palladium, rhodium, ruthenium, and gold, in contrast to diamagnetism or paramagnetism in their bulk counterparts.

Our data from Pd and Pt nanowires revealed the appearance of strong "localized" ferromagnetism in these conventional paramagnetic materials. The unique features of one-dimensionality and localization of electronic states also are highly anticipated in other 4d and 5d metals; such materials likely will carry great potential for designing nano-spintronics devices.