Miquel Salmeron

Reaction-Driven Restructuring of Rh-Pd and Pt-Pd Core-Shell Nanoparticles

Heterogeneous catalysts that contain bimetallic nanoparticles may undergo segregation of the metals, driven by oxidizing and reducing environments. The structure and composition of core-shell Rh0.5Pd0.5 and Pt0.5Pd0.5 nanoparticle catalysts were studied in situ, during oxidizing, reducing, and catalytic reactions involving NO, O2, CO, and H2 by x-ray photoelectron spectroscopy at near-ambient pressure. The Rh0.5Pd0.5 nanoparticles underwent dramatic and reversible changes in composition and chemical state in response to oxidizing or reducing conditions. In contrast, no substantial segregation of Pd or Pt atoms was found in Pt0.5Pd0.5 nanoparticles. The different behaviors in restructuring and chemical response of Rh0.5Pd0.5 and Pt0.5Pd0.5 nanoparticle catalysts under the same reaction conditions illustrates the flexibility and tunability of the structure of bimetallic nanoparticle catalysts during catalytic reactions.

Electron Spectroscopy of Aqueous Solution Interfaces Reveals Surface Enhancement of Halides

It has been suggested that enhanced anion concentrations at the liquid/vapor interface of airborne saline droplets are important to aerosol reactions in the atmosphere. We report ionic concentrations in the surface of such solutions. Using x-ray photoelectron spectroscopy operating at near ambient pressure, we have measured the composition of the liquid/vapor interface for deliquesced samples of potassium bromide and potassium iodide. In both cases, the surface composition of the saturated solution is enhanced in the halide anion compared with the bulk of the solution. The enhancement of anion concentration is more dramatic for the larger, more polarizable iodide anion. By varying photoelectron kinetic energies, we have obtained depth profiles of the liquid/vapor interface. Our results are in good qualitative agreement with classical molecular dynamics simulations. Quantitative comparison between the experiments and the simulations indicates that the experimental results exhibit more interface enhancement than predicted theoretically.

A differentially pumped electrostatic lens system for photoemission studies in the millibar range

A high pressure photoemission system is described that combines differential pumping with an electrostatic lens system. This approach allows optimized differential pumping without loss of signal, thereby increasing the high-pressure performance by at least 2 orders of magnitude compared to passive differential pumping systems. A general analysis of aperture-based high-pressure photoemission is presented, followed by a description of the prototype system which has operated at pressures up to 7 mbar on a synchrotron beamline. Using this approach, photoemission experiments should be possible up to 100 mbar. Example data are presented for dielectric samples in gas atmospheres, for a copper catalyst under reaction conditions, and for liquid water in equilibrium with its vapor.

Imaging the Condensation and Evaporation of Molecularly Thin Films of Water with Nanometer Resolution

The polarization force between an electrically charged atomic force microscope tip and a substrate has been used to follow the processes of condensation and evaporation of a monolayer of water on mica at room temperature. Condensation proceeds in two distinct structural phases. Up to about 25 percent humidity, the water film grows by forming two-dimensional clusters of less than a few 1000 angstroms in diameter. Above about 25 percent humidity, a second phase grows, forming large two-dimensional islands with geometrical shapes in epitaxial relation with the underlaying mica lattice. The growth of this second water phase is completed when the humidity reaches about 45 percent. The reverse process of evaporation has also been imaged.

Break-Up of Stepped Platinum Catalyst Surfaces by High CO Coverage

From Steps to Clusters When a flat surface of a single crystal is formed by cutting or cleavage, the atoms may move little from their bulk positions, or the surface may reconstruct as the atoms move to more energetically favorable positions. The adsorption of molecules can also change the energetic landscape and cause reconstruction. Tao et al. (p. 850; see the Perspective by Altman) examined “stepped” platinum surfaces, the (557) and (332) surfaces in which flat terraces are connected by atomic steps. Scanning tunneling microscopy and x-ray photoelectron spectroscopy revealed a reversible breakup into nanometer-scale clusters when CO surface coverages were very high. Density functional theory calculations suggest that this new morphology increases the number of edge sites for adsorption and relieves unfavorable CO-CO repulsions. Stepped platinum surfaces break up into nanometer-scale clusters at high carbon monoxide surface coverages. Stepped single-crystal surfaces are viewed as models of real catalysts, which consist of small metal particles exposing a large number of low-coordination sites. We found that stepped platinum (Pt) surfaces can undergo extensive and reversible restructuring when exposed to carbon monoxide (CO) at pressures above 0.1 torr. Scanning tunneling microscopy and photoelectron spectroscopy studies under gaseous environments near ambient pressure at room temperature revealed that as the CO surface coverage approaches 100%, the originally flat terraces of (557) and (332) oriented Pt crystals break up into nanometer-sized clusters and revert to the initial morphology after pumping out the CO gas. Density functional theory calculations provide a rationale for the observations whereby the creation of increased concentrations of low-coordination Pt edge sites in the formed nanoclusters relieves the strong CO-CO repulsion in the highly compressed adsorbate film. This restructuring phenomenon has important implications for heterogeneous catalytic reactions.

Evolution of Structure and Chemistry of Bimetallic Nanoparticle Catalysts under Reaction Conditions

Three series of bimetallic nanoparticle catalysts (Rh(x)Pd(1-x), Rh(x)Pt(1-x), and Pd(x)Pt(1-x), x = 0.2, 0.5, 0.8) were synthesized using one-step colloidal chemistry. X-ray photoelectron spectroscopy (XPS) depth profiles using different X-ray energies and scanning transmission electron microscopy showed that the as-synthesized Rh(x)Pd(1-x) and Pd(x)Pt(1-x) nanoparticles have a core-shell structure whereas the Rh(x)Pt(1-x) alloys are more homogeneous in structure. The evolution of their structures and chemistry under oxidizing and reducing conditions was studied with ambient-pressure XPS (AP-XPS) in the Torr pressure range. The Rh(x)Pd(1-x) and Rh(x)Pt(1-x) nanoparticles undergo reversible changes of surface composition and chemical state when the reactant gases change from oxidizing (NO or O(2) at 300 degrees C) to reducing (H(2) or CO at 300 degrees C) or catalytic (mixture of NO and CO at 300 degrees C). In contrast, no significant change in the distribution of the Pd and Pt atoms in the Pd(x)Pt(1-x) nanoparticles was observed. The difference in restructuring behavior under these reaction conditions in the three series of bimetallic nanoparticle catalysts is correlated with the surface free energy of the metals and the heat of formation of the metallic oxides. The observation of structural evolution of bimetallic nanoparticles under different reaction conditions suggests the importance of in situ studies of surface structures of nanoparticle catalysts.

Hydroxylation of the surface of PbS nanocrystals passivated with oleic acid

Nanoparticle lattices and surfaces The challenge of resolving the details of the surfaces or assemblies of colloidal semiconductor nanoparticles can be overcome if several characterization methods are used (see the Perspective by Boles and Talapin). Boneschanscher et al. examined honeycomb superlattices of lead selenide nanocrystals formed by the bonding of crystal faces using several methods, including high-resolution electron microscopy and tomography. The structure had octahedral symmetry with the nanocrystals distorted through “necking”: the expansion of the contact points between the nanocrystals. Zherebetskyy et al. used a combination of theoretical calculations and spectroscopic methods to study the surface layer of lead sulfide nanocrystals synthesized in water. In addition to the oleic acid groups that capped the nanocrystals, hydroxyl groups were present as well. Science, this issue p. 1377, p. 1380; see also p. 1340 The surfaces of lead sulfide nanocrystals capped with an organic acid can also bear hydroxyl groups. [Also see Perspective by Boles and Talapin] Controlling the structure of colloidal nanocrystals (NCs) is key to the generation of their complex functionality. This requires an understanding of the NC surface at the atomic level. The structure of colloidal PbS NCs passivated with oleic acid has been studied theoretically and experimentally. We show the existence of surface OH– groups, which play a key role in stabilizing the PbS(111) facets, consistent with x-ray photoelectron spectroscopy as well as other spectroscopic and chemical experiments. The role of water in the synthesis process is also revealed. Our model, along with existing observations of NC surface termination and passivation by ligands, helps to explain and predict the properties of NCs and their assemblies.

Scanning polarization force microscopy: A technique for imaging liquids and weakly adsorbed layers

The atomic force microscope is used to measure dielectric polarization forces on surfaces induced by a charged tip. On insulators, the major contribution to the surface polarizability at low frequencies is from surface ions. The mobility of these ions depends strongly on the humidity. Using polarization forces we have been able to image liquid films, droplets, and other weakly adsorbed material.

Friction Anisotropy–Driven Domain Imaging on Exfoliated Monolayer Graphene

Otherwise identical regions of supported graphene can be distinguished by changes in friction with sliding direction. Graphene produced by exfoliation has not been able to provide an ideal graphene with performance comparable to that predicted by theory, and structural and/or electronic defects have been proposed as one cause of reduced performance. We report the observation of domains on exfoliated monolayer graphene that differ by their friction characteristics, as measured by friction force microscopy. Angle-dependent scanning revealed friction anisotropy with a periodicity of 180° on each friction domain. The friction anisotropy decreased as the applied load increased. We propose that the domains arise from ripple distortions that give rise to anisotropic friction in each domain as a result of the anisotropic puckering of the graphene.

Molecular beam study of the H2–D2 exchange reaction on stepped platinum crystal surfaces: Dependence on reactant angle of incidence

The exchange reaction H2+D2→2HD was studied as a function of the angle of incidence (polar and azimuthal) of the beam of reactants on two stepped platinum surfaces, the Pt(S) ‐[6(111) × (111)] and Pt(S) ‐[5(111) × (111)]. The reaction probability was found to depend markedly on the direction of approach of the reactants to the step structure. The production of HD is highest when the reactants strike the open side of the step structure, decreasing by approximately a factor of 2 when the inner corner of the step is shadowed. These variations are intimately related to the geometry of the stepped surfaces and cannot be attributed to an activation energy barrier for adsorption. The observed dependence can be accounted for in a simple model, assigning different activities for H–H bond breaking to the step and terrace sites. For the best fit with the experimental data we find that the activity of a step site is seven times that of a terrace site at normal incidence.