An Electrochemical, Microtopographical and Ambient Pressure X-Ray Photoelectron Spectroscopic Investigation of Si/TiO_2/Ni/Electrolyte Interfaces

The electrical and spectroscopic properties of the TiO_2/Ni protection layer system, which enables stabilization of otherwise corroding photoanodes, have been investigated in contact with electrolyte solutions by scanning-probe microscopy, electrochemistry and in-situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS). Specifically, the energy-band relations of the p+-Si/ALD-TiO_2/Ni interface have been determined for a selected range of Ni thicknesses. AP-XPS measurements using tender X-rays were performed in a three-electrode electrochemical arrangement under potentiostatic control to obtain information from the semiconductor near-surface region, the electrochemical double layer (ECDL) and the electrolyte beyond the ECDL. The degree of conductivity depended on the chemical state of the Ni on the TiO2surface. At low loadings of Ni, the Ni was present primarily as an oxide layer and the samples were not conductive, although the TiO_2 XPS core levels nonetheless displayed behavior indicative of a metal-electrolyte junction. In contrast, as the Ni thickness increased, the Ni phase was primarily metallic and the electrochemical behavior became highly conductive, with the AP-XPS data indicative of a metal-electrolyte junction. Electrochemical and microtopographical methods have been employed to better define the nature of the TiO_2/Ni electrodes and to contextualize the AP-XPS results

Direct observation of the energetics at a semiconductor/liquid junction by operando X-ray photoelectron spectroscopy

Photoelectrochemical (PEC) cells based on semiconductor/liquid interfaces provide a method of converting solar energy to electricity or fuels. Currently, the understanding of semiconductor/liquid interfaces is inferred from experiments and models. Operando ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) has been used herein to directly characterize the semiconductor/liquid junction at room temperature under real-time electrochemical control. X-ray synchrotron radiation in conjunction with AP-XPS has enabled simultaneous monitoring of the solid surface, the solid/electrolyte interface, and the bulk electrolyte of a PEC cell as a function of the applied potential, U. The observed shifts in binding energy with respect to the applied potential have directly revealed ohmic and rectifying junction behavior on metallized and semiconducting samples, respectively. Additionally, the non-linear response of the core level binding energies to changes in the applied electrode potential has revealed the influence of defect-derived electronic states on the Galvani potential across the complete cell

Investigation of the Si/TiO_2/Electrolyte Interface Using Operando Tender X-ray Photoelectron Spectroscopy

Semiconductor-electrolyte interfaces allow for the creation of photoactive semiconductor systems that have band bending and other characteristics analogous to semiconductor-metal junctions (Schottky junctions). We demonstrate herein that XPS measurements can be obtained on a full three-electrode electrochemical system under potentiostatic control by use of tender X-rays to provide photoelectrons with sufficient kinetic energy to penetrate through a thin electrolyte overlayer on a portion of the working electrode. The response of the photoelectron binding energies to variations in applied voltage demonstrates that the XPS investigation works in an operando manner to elucidate the energetics of such interfaces

Highly active surfaces for CO oxidation on rh, pd, and pt

Studies show that the rate of CO oxidation on Pt-group metals at temperatures between 450 and 600 K and pressures between 1 and 300 Torr increases markedly with an increase in the O-2/CO ratio above 0.5. The catalytic surfaces, formed at discrete O-2/CO ratios >0.5, exhibit rates 2-3 orders of magnitude greater than those rates observed for stoichiometric reaction conditions and similar reactant pressures or previously in ultrahigh vacuum studies at any reactant conditions and extrapolate to the collision limit of CO in the absence of mass transfer limitations. The O-2/CO ratios required to achieve these so-called "hyperactive" states (where the reaction probabilities of CO are thought to approach unity) for Rh, Pd, and Pt relate directly to the adsorption energies of oxygen, the heats of formation of the bulk oxides, and the metal particle sizes. Auger spectroscopy and X-ray photoemission spectroscopy reveal that the hyperactive surfaces consist of approximate 1 ML of surface oxygen. In situ polarization modulation reflectance absorption infrared spectroscopy measurements coupled with no detectable adsorbed CO. In contrast, under stoichiometric O-2/CO conditions and similar temperatures and pressures, Rh, Pd, and Pt are essentially saturated with chemisorbed CO and exhibit far less activity for CO oxidation. (C) 2007 Elsevier B.V. All rights reserved

Vinyl acetate synthesis over model Pd-Sn bimetallic catalysts

Pd-Sn bimetallic model catalysts were prepared as alloy films on a Rh(100) substrate via physical vapor deposition. The surface composition, structure, and chemisorption properties were studied by low energy ion scattering spectroscopy (LEIS), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), infrared reflection adsorption spectroscopy (IRAS), and temperature programmed desorption (TPD). The ordered surface alloy of c(2 x 2) was formed after annealing the Pd-Sn mixtures to 700 K as evidenced by LEED and LEIS showing a 50% surface concentration of Pd. This ordered surface arrangement was further confirmed by IRAS, LEED, and TPD studies using CO as a probe molecule, in which the surface Pd atoms are completely isolated by Sn atoms. The surface Pd composition was determined to be 0.5 monolayers (ML). The catalytic properties of this Pd-Sn surface were tested with respect to vinyl acetate (VA) synthesis by ethylene acetoxylation that showed a maximum in the VA formation rate at a surface Pd coverage of 0.5 ML, i.e., a c(2 x 2) surface arrangement. This is consistent with our previous proposal that a pair of suitably spaced, isolated Pd monomers is the more efficient site for VA synthesis

Facile NOx interconversion over preoxidized Ag(111)

X-ray photoelectron spectroscopy and density functional theory calculations are used to investigate NO adsorption at low (100 K) and room temperature (RT) over preoxidized Ag(111). At 100 K, the data indicates presence of NO and N2O2, with little or no nitrite/nitrate formation. This is consistent with the calculated surface core level shifts and the pronounced barrier for nitrite formation. At RT, the recorded spectra indicate a complex interconversion between adsorbed species with an initial formation of a p(4 x 4) nitrate overlayer. With increasing NO pressure, the experimental results are best rationalized by partial nitrate decomposition into nitrites and subsequent NO physisorption, which leads to the formation of N2O3-like species

Hydrogenation of CO2 to Methanol on CeOx/Cu(111) and ZnO/Cu(111) Catalysts: Role of the Metal-Oxide Interface and Importance of Ce3+ Sites

The role of the interface between a metal and oxide (CeOx-Cu and ZnO-Cu) is critical to the production of methanol through the hydrogenation of CO2 (CO2 + 3H2 → CH3OH + H2O). The deposition of nanoparticles of CeOx or ZnO on Cu(111),oxi < 0.3 monolayer, produces highly active catalysts for methanol synthesis. The catalytic activity of these systems increases in the sequence: Cu(111) < ZnO/Cu(111) < CeOx/Cu(111). The apparent activation energy for the CO2 → CH3OH conversion decreases from 25 kcal/mol on Cu(111) to 16 kcal/mol on ZnO/Cu(111) and 13 kcal/mol on CeOx/Cu(111). The surface chemistry of the highly active CeOx-Cu(111) interface was investigated using ambient pressure X-ray photoemission spectroscopy (AP-XPS) and infrared reflection absorption spectroscopy (AP-IRRAS). Both techniques point to the formation of formates (HCOO-) and carboxylates (CO2δ-) during the reaction. Our results show an active state of the catalyst rich in Ce3+ sites which stabilize a CO2δ- species that is an essential intermediate for the production of methanol. The inverse oxide/metal configuration favors strong metal-oxide interactions and makes possible reaction channels not seen in conventional metal/oxide catalysts