Explosive enantiospecific decomposition of aspartic acid on Cu surfaces.

Aspartic acid adsorbed on Cu surfaces is doubly deprotonated. On chiral Cu(643)R&S its enantiomers undergo enantiospecific decomposition via an autocatalytic explosion. Once initiated, the decomposition mechanism proceeds via sequential cleavage of the C3-C4 and C1-C2 bonds each yielding CO2, followed by conversion of the remaining species into N[triple bond, length as m-dash]CCH3.

Chiral nanoscale pores created during the surface explosion of tartaric acid on Cu(111).

The autocatalytic decomposition of tartaric acid on Cu(111) exhibits unique kinetics, which are linked to a hexagonal surface structure adopted at high coverage. The chirality imposed on the surface by tartaric acid throughout the explosion process is presented, and the hexagonal structure shows promise as a chiral template for enantiospecific surface chemistry.

Influence of step faceting on the enantiospecific decomposition of aspartic acid on chiral Cu surfaces vicinal to Cu{111}.

On surfaces vicinal to Cu{111}, l-aspartic acid (l-Asp) adsorption causes steps to facet enantiospecifically into {310}R and {320}S steps. l-Asp has its highest heat of adsortion on surfaces that naturally expose the {310}R or {320}S steps but decomposes preferentially on the {310}R steps.

Fe2o3 shell growth on Pt nanoparticles.

Fe2O3 shells have been synthesized around Pt cores to create Pt@Fe2O3 core-shell nanoparticles. The synthesis conditions allow control of the shell shape and allow the preparation of both hexagonal shells and spherical shells. 2D cross-sectional TEM images show that the cores are not positioned at the centers of the shells. By rotating the nanoparticles and monitoring the apparent motions of the cores in the 2D cross-sectional images, it is possible to determine quantitatively the radial position of the Pt core with respect to the center of the Fe2O3 shell. The distribution of core positions within the core-shell structures is bimodal. These observations suggest that the Fe2O3 shells grow on the Pt cores by a nucleation process, rather than layer-by-layer growth.

Tailoring the shapes of Fe(x)Pt(100-x) nanoparticles.

Fe(x)Pt(100-x) nanoparticles of varying composition have been synthesized with various shapes and sizes using a high pressure synthesis method which allows control of synthesis conditions, in particular the reaction temperature. Tailoring the shapes and sizes of Fe(x)Pt(1-x) nanoparticles allows one to control a variety of properties that are relevant to the many potential applications of metallic nanoparticles. Shape and composition can be used to control catalytic activity and to achieve high packing density in self-assembled films. Variation of both nanoparticle size and shape has been achieved by using various different solvents. The solvents used in the nanoparticle synthesis can influence the product because they can play a role as surfactants. Using solvents of various types it has been possible to synthesize Fe(x)Pt(100-x) nanoparticles with a variety of shapes including spherical, rod-like, cubic, hexagonal and high aspect ratio wires. Control of nanoparticle shape opens the door to their being used in various technological applications for which spherical nanoparticles are ineffective.

Enantioselective surface chemistry of R-2-bromobutane on Cu(643)R&S and Cu(531)R&S.

The enantioselective surface chemistry of chiral R-2-bromobutane was studied on the naturally chiral Cu(643)R&S and Cu(531)R&S surfaces by comparing relative product yields during temperature-programmed reaction spectroscopy. Molecularly adsorbed R-2-bromobutane can desorb molecularly or debrominate to form R-2-butyl groups on the surfaces. The R-2-butyl groups react further by beta-hydride elimination to form 1- or 2-butene or by hydrogenation to form butane. Temperature-programmed reaction spectroscopy was used to quantify the relative yields of the various reaction products. At low coverages of R-2-bromobutane on Cu(643)R&S and Cu(531)R&S, the surface chemistry is not enantioselective. At monolayer coverage, however, the product yields indicate that the R-2-bromobutane decomposition reaction rates are sensitive to the handedness of the two chiral surfaces. The impact of surface structure on enantioselectivity was examined by studying the chemistry of R-2-bromobutane on both Cu(643)R&S and Cu(531)R&S. The selectivity of R-2-bromobutane desorption versus debromination is enantiospecific and differs significantly from Cu(643) to Cu(531). The selectivity of the R-2-butyl reaction by beta-hydride elimination versus hydrogenation is only weakly enantiospecific and is similar on both the Cu(643) and Cu(531) surfaces. These results represent the first quantitative observations of enantioselectivity in reactions with well-known mechanisms probed using a simple adsorbate on naturally chiral metal surfaces.

Oxidative and tribological properties of amorphous and quasicrystalline approximant Al-Cu-Fe thin films.

The origins of the tribological properties and corrosion resistance of amorphous and quasicrystalline approximant alloys have been studied by comparing their properties in thin Al-Cu-Fe alloy films with compositions lying near the quasicrystalline region of the ternary compositional phase diagram. Six sputtered thin films of an Al-Cu-Fe alloy were studied using X-ray diffraction, X-ray photoemission spectroscopy (XPS), and an in situ ultrahigh vacuum (UHV) tribometer. The films were annealed in UHV to induce the formation of orthorhombic, rhombohedral, and amorphous bulk structures. The properties of these thin films were then determined in the same UHV apparatus without exposing the films to air. The rates of surface oxidation by H2O and O2 were measured using XPS. Although the oxidation rates and oxide thicknesses were dependent on the oxidant, they were not sensitive to the structures of the films. Friction was measured between identical samples in sliding contact. The friction coefficients (micros = 0.36 +/- 0.11 to 0.56 +/- 0.08) were comparable to those observed in other experiments using quasicrystals and approximants in UHV; however, there was no strong correlation between the friction coefficients and either the film structure or the degree of surface oxidation. These results suggest that the tribological and corrosion resistance properties of these quasicrystalline approximant alloys are not directly connected to crystalline structure.

Kinetics and energetics of oligomer desorption from surfaces.

The dynamics of oligomer desorption from surfaces has been studied by measuring the desorption kinetics of a set of straight chain alkanes [ H(CH2)(n)H, with n = 5 to 60] from the surface of single crystalline graphite. Desorption is observed to be a first-order process and the preexponent of the desorption rate constant has a value nu = 10(19.6+/-0.5) sec(-1) and is independent of the oligomer chain length. More interestingly, we find that the barrier to desorption has a nonlinear dependence on chain length and takes the form DeltaE(double dagger)(des) = A+Bn(alpha), with the exponent alpha = 0.50+/-0.01.

Transition states for surface-catalyzed chemistry.

Fluorine substituent effects have been used to probe the nature of the transition states for the several elementary reaction steps occurring on metal surfaces. The reactions described include beta-hydrogen elimination in adsorbed alkoxide and alkyl groups, coupling of alkyl groups, and dehalogenation of alkyl chloride and iodides. The substituent effect method can provide a connection between heterogeneous catalysis, surface science, and computational molecular simulation of surface reactions.