Water Affinity and Surface Charging at the z‑Cut and y‑Cut LiNbO₃ Surfaces: An Ambient Pressure X‑ray Photoelectron Spectroscopy Study

Polarization dependence of water adsorption and desorption on LiNbO₃ surfaces was demonstrated using X-ray photoelectron spectroscopy (XPS) carried out in situ under near-ambient conditions. Positive and negative (0001) faces (z-cut) of the same crystal were compared for the same temperature and pressure conditions. Our results indicate a preferential adsorption on the positive face of the crystal with increasing water pressure and also higher desorption temperature of the adsorbed molecular water at the positive face. Adsorption measurements on the (1100) face (y-cut) showed also strong affinity to water, as observed for the z-cut positive surface. We found a direct relation between the capacity of the surface to discharge and/or to screen surface charges and the affinity for water of each face. XPS spectra indicate the presence of OH groups at the surface for all the conditions and surfaces measured

Formation of Subsurface W⁵⁺ Species in Gasochromic Pt/WO₃ Thin Films Exposed to Hydrogen

M/WO₃ (M = Pt, Pd) systems formed by a porous WO₃ thin film decorated by metal nanoparticles are known for their reversible coloring upon exposure to H₂ at room temperature. In this work, this gasochromic behavior is investigated in situ by means of near-ambient photoemission (NAPP). Pt/WO₃ systems formed by very small Pt nanoparticles (10 ± 1 nm average size) incorporated in the pores of nanocolumnar WO₃ thin films prepared by magnetron sputtering at an oblique angle have been exposed to a small pressure of hydrogen at ambient temperature. The recorded UV–vis transmission spectra showed the reversible appearance of a very intense absorption band responsible for the blue coloration of these gasochromic films. In an equivalent experiment carried out in the NAPP spectrometer, W 4f, O 1s, Pt 4f, and valence band photoemission spectra have been recorded at various photon energies to follow the evolution of the reduced tungsten species and hydroxyl groups formed upon film exposure to hydrogen. The obtained results are compared with those of a conventional X-ray photoemission study after hydrogen exposure between 298 and 573 K. As investigated by NAPP, the gasochromic behavior at 298 K is accounted for by a reaction scheme in which hydrogen atoms resulting from the dissociation of H₂ onto the Pt nanoparticles are spilt over to the WO₃ substrate where they form surface OH^–/H₂O species and subsurface W⁵⁺ cations preferentially located in buried layers of the oxide network

Experimental and Computational Insight into the Chemical Bonding and Electronic Structure of Clathrate Compounds in the Sn–In–As–I System

Inorganic clathrate materials are of great fundamental interest and potential practical use for application as thermoelectric materials in freon-free refrigerators, waste-heat converters, direct solar thermal energy converters, and many others. Experimental studies of their electronic structure and bonding have been, however, strongly restricted by (i) the crystal size and (ii) essential difficulties linked with the clean surface preparation. Overcoming these handicaps, we present for the first time a comprehensive picture of the electronic band structure and the chemical bonding for the Sn_{24–<i>x</i>–δ}In_<i>x</i>As_{22–<i>y</i>}I₈ clathrates obtained by means of photoelectron spectroscopy and complementary quantum modeling

Catalytic Oxidation of Carbon Monoxide on a Curved Pd Crystal: Spatial Variation of Active and Poisoning Phases in Stationary Conditions

Understanding nanoparticle catalysis requires novel approaches in which adjoining crystal orientations can be studied under the same reactive conditions. Here we use a curved palladium crystal and near-ambient pressure X-ray photoemission spectroscopy to characterize chemical species during the catalytic oxidation of CO in a whole set of surfaces vicinal to the (111) direction simultaneously. By stabilizing the reaction at fixed temperatures around the ignition point, we observe a strong variation of the catalytic activity across the curved surface. Such spatial modulation of the reaction stage is straightforwardly mapped through the photoemission signal from active oxygen species and poisoning CO, which are shown to coexist in a transient regime that depends on the vicinal angle. Line-shape analysis and direct comparison with ultrahigh vacuum experiments help identifying and quantifying all such surface species, allowing us to reveal the presence of surface oxides during reaction ignition and cooling-off