Daniel Hennessy

Shape-Dependent Activity of Platinum Array Catalyst

We produced millions of morphologically identical platinum catalyst nanoparticles in the form of ordered arrays epitaxially grown on (111), (100), and (110) strontium titanate substrates using electron beam lithography. The ability to design, produce, and characterize the catalyst nanoparticles allowed us to relate microscopic morphologies with macroscopic catalytic reactivities. We evaluated the activity of three different arrays containing different ratios of (111) and (100) facets for an oxygen-reduction reaction, the most important reaction for fuel cells. Increased catalytic activity of the arrays points to a possible cooperative interplay between facets with different affinities to oxygen. We suggest that the surface area of (100) facets is one of the key factors governing catalyst performance in the electrochemical reduction of oxygen molecules.

Surface X-Ray Speckles: Coherent Surface Diffraction from Au(001)

We present coherent speckled x-ray diffraction patterns obtained from a monolayer of surface atoms. We measured both the specular anti-Bragg reflection and the off-specular hexagonal reconstruction peak for the Au(001) surface reconstruction. We observed fluctuations of the speckle patterns even when the integrated intensity appears static. By autocorrelating the speckle patterns, we were able to identify two qualitatively different surface dynamic behaviors of the hex reconstruction depending on the sample temperature.

Persistent oscillations of x-ray speckles: Pt (001) step flow

We observed well-defined oscillations of speckle intensities from Pt (001) surfaces at high temperatures, persisting for tens of minutes. We used a model of hex-reconstructed terraces to show that the coherent x-rays reflected from the terraces retain their phases relative to the illumination boundary and the observed oscillations come from surface dynamics due to “step-flow” motion. Our results demonstrate a possibility that x-ray speckles can be applied to monitor the real-time evolution of surfaces.

Epitaxial oxide bilayer on Pt (001) nanofacets

We observed an epitaxial, air-stable, partially registered (2 × 1) oxide bilayer on Pt (001) nanofacets [V. Komanicky, A. Menzel, K.-C. Chang, and H. You, J. Phys. Chem. 109, 23543 (2005)]. The bilayer is made of two half Pt layers; the top layer has four oxygen bonds and the second layer two. The positions and oxidation states of the Pt atoms are determined by analyzing crystal truncation rods and resonance scattering data. The positions of oxygen atoms are determined by density functional theory (DFT) calculations. Partial registry on the nanofacets and the absence of such registry on the extended Pt (001) surface prepared similarly are explained in DFT calculations by strain relief that can be accommodated only by nanoscale facets.

Fabrication of platinum nano-array model catalysts

We fabricated and characterized one- and two- dimensional nanoscale arrays of platinum for study of model catalysts. One-dimensional arrays of nanoscale facets were fabricated by annealing a high-index plan of platinum single crystals. The high-index plane forms rows of alternating two low-index facets, (111) and (100), widths of which are ~10 nanometers. Two-dimensional arrays were fabricated lithographically from the epitaxial films of platinum grown on SrTiO3 substrates. Electron beam lithography was used to create precisely registered square arrays of millions of identical platinum nanocrystals with ~30 nm in diameter.