Simulation of Surface Resonant X-ray Diffraction.

We present an ab initio numerical tool to simulate surface resonant X-ray diffraction experiments. The crystal truncation rods and the spectra around a given X-ray absorption edge are calculated at any position of the reciprocal space. Density functional theory is used to determine the resonant scattering factor of an atom within its local environment and to calculate the diffraction peak intensities for surfaces covered with a thin film or with one or several adsorbed layers. Besides the sample geometry, the collected data also depend on several parameters, such as beam polarization and incidence and exit angles. In order to account for these factors, a numerical diffractometer mimicking the experimental operation modes has been created. Finally two case studies are presented in order to compare our simulations with experimental spectra: (i) a magnetite thin film deposited on a silver substrate and (ii) an electrochemical interface consisting of bromine atoms adsorbed on copper.

Palladium Electrodeposition onto Pt(100): Two-Layer Underpotential Deposition.

Electrodeposition of the first Pd layers onto Pt(100) was investigated using cyclic voltammetry at a low scan rate (0.1 mV·s-1). Ultrathin films were characterized by cyclic voltammetry in 0.1 M H2SO4 solution and with ex situ AFM (atomic force microscopy). For the first time, we evidenced the underpotential character of the deposition of the first two Pd layers, characterized by a two-step mechanism, each step corresponding to the deposition of a complete Pd atomic layer. For thicker deposits, especially above 10 monolayers as equivalent thickness, the electrochemical characterization displays a strong irreversibility and a broadening of the adsorption/desorption peaks, associated with a reduction of long-range ordered flat areas. Ex situ AFM images are in agreement with this description. They show rough thick deposits and the growth of (100)-oriented rectangular shaped islands with their sides aligned with the two [011] and [0-11] perpendicular directions of the (100) Pt surface.

Growth mechanisms of Pd nanofilms electrodeposited onto Au(111): an in situ grazing incidence X-ray diffraction study.

Electrochemical deposition of ultra-thin Pd films onto Au(111) single crystals in a solution containing chloride was studied with in situ surface X-ray diffraction measurements. We report a detailed description of the growth mode, as well as film morphology and lattice parameters as a function of thickness, from 2 up to 10 monolayers (ML) as equivalent thickness. An almost ideal layer-by-layer pseudomorphic growth is observed up to two deposited ML. For higher thicknesses, it is followed by the growth of large 3D Pd bulk-like islands. They are about 20 ± 5 ML high and ∼220 Å in diameter for the Pd4ML film and occupy only about 20% of the surface. Their height increases faster than their size with the Pd deposited amount. We could clearly show that chlorides do not play any role in inhibiting the three-dimensional growth of Pd/Au(111) films. We could also unequivocally correlate the features observed by electrochemical surface characterisation in an acidic medium with the detailed structure obtained by diffraction.

Reversibility of Pt-Skin and Pt-Skeleton Nanostructures in Acidic Media.

Following a well-defined series of acid and heat treatments on a benchmark Pt3Co/C sample, three different nanostructures of interest for the electrocatalysis of the oxygen reduction reaction were tailored. These nanostructures could be sorted into the "Pt-skin" structure, made of one pure Pt overlayer, and the "Pt-skeleton" structure, made of 2-3 Pt overlayers surrounding the Pt-Co alloy core. Using a unique combination of high-resolution aberration-corrected STEM-EELS, XRD, EXAFS, and XANES measurements, we provide atomically resolved pictures of these different nanostructures, including measurement of the Pt-shell thickness forming in acidic media and the resulting changes of the bulk and core chemical composition. It is shown that the Pt-skin is reverted toward the Pt-skeleton upon contact with acid electrolyte. This change in structure causes strong variations of the chemical composition.

Evidence of the substrate effect in hydrogen electroinsertion into palladium atomic layers by means of in situ surface X-ray diffraction.

In this work, we report an in situ surface X-ray diffraction study of the hydrogen electroinsertion in a two-monolayer equivalent palladium electrodeposit on Pt(111). The role of chloride in the deposition solution in favoring layer-by-layer film growth is evidenced. Three Pd layers are necessary to describe the deposit structure correctly, but the third-layer occupancy is quite low, equal to about 0.22. As a major result, resistance to hydriding of the two atomic Pd layers closest to the Pt interface is observed, which is linked to a strong effect of the Pt(111) substrate. As a consequence, we observe the lowering of the total hydride stoichiometry compared to bulk Pd. Our measurements also reveal good reversibility of the deposit structure, at least toward one hydrogen insertion-desorption cycle.

Electrochemical deposition of platinum nanoparticles on carbon: a study by standard and anomalous X-ray diffraction.

This paper is devoted to an alternative method to characterize platinum nanoparticles: X-ray powder diffraction with synchrotron radiation in classical and anomalous dispersion modes. We could straightforwardly determine the mean diameter and the surface concentration of carbon-supported platinum nanoparticles, even down to diameters of 2-3 nm and catalyst amounts of 0.03 mgcm(-2). We could study early stages of the formation of electrochemically prepared platinum nanoparticles from [PtCl4(2-) species preadsorbed on carbon inside a carbon-Nafion layer, to obtain a fuel-cell electrode. Our X-ray diffraction (XRD) results demonstrate that, provided the superficial concentration is not too high, new and smaller particles appear for each current pulse, since there is not any strong nucleation limitation for the high overvoltages obtained. Hydrogen evolution becomes the main electrochemical phenomenon on particles of sufficient size and it explains the noteworthy size limitation. Better yields of Pt metal are obtained for smaller current densities and longer times: the rate-determining step is then not electrochemical, but chemical or related to superficial diffusion.