Invited Paper SS2-MoA6
2009 AVS Peter Mark Memorial Award Lecture - Towards the Understanding of Catalytic and Vibrational Properties of Metal Nanostructures

Monday, November 9, 2009, 3:40 pm, Room N

Metallic nanostructures are of great interest in many scientific fields due to their novel size-dependent physical and chemical properties. The origin of the enhanced catalytic reactivity and selectivity displayed by small metal nanoparticles is yet to be explained. However, it is generally agreed that the design of the next generation of nanocatalysts requires detailed knowledge of the correlation between their reactivity and their physical properties such as morphology, electronic structure, chemical composition, and interactions with their support. Intriguing effects including phonon confinement and phonon localization at interfaces have also been observed in low dimensional systems . Such modifications of the vibrational density of states of nanoscale materials are of scientific and technological relevance, because they profoundly affect their thermodynamic properties. In addition, their understanding could help further our insight in the field of catalysis, since phonons might play a decisive role in certain chemical processes. To systematically study these effects, homogeneous and size-selected nanostructures are needed. Diblock-copolymers can aid the synthesis of such well-defined nanoscale systems.

The first part of my talk will illustrate the formation, stability, as well as the electronic and catalytic properties of size- and shape-selected Au, AuFe, Pt, and Pt-M (M = Au, Fe, Ru and Pd) nanoparticles synthesized by micelle encapsulation. CO oxidation and alcohol decomposition and oxidation have been used as model reactions. Emphasis will be given to the role of the nanoparticle support and the oxidation state of the active catalytic species in their reactivity.

In addition to changes in reactivity, nanostructuring materials also affects their vibrational properties. The second part of my talk will discuss the size- and composition-dependent vibrational dynamics of ⁵⁷Fe, ⁵⁷FePt, and ⁵⁷FeAu clusters as well as of nanostructured metal multilayers prepared by molecular beam epitaxy in UHV. The phonon density of states of those systems will be extracted from nuclear resonant inelastic X-ray scattering measurements. An enhancement of the density of low- and high-energy phonon modes as well as non-Debye-like behavior was observed on ⁵⁷Fe clusters. The latter effects were found to depend on the chemical nature of the surface shell on the nanoclusters. Thickness-dependent phonon confinement and interface localization effects were detected on nanoscale ⁵⁷Fe/M multilayers (M = Cu, Pd or Ag). All these effects show the unique physical properties of metal nanoparticles, and their promise in technological applications.

Website: http://www.physics.ucf.edu/~roldan