Crystallographic Orientation Dependence of Surface Segregation and Alloying on PdCu Catalysts for CO₂ Hydrogenation

The influence of the crystallographic orientation on surface segregation and alloy formation in model PdCu methanol synthesis catalysts was investigated in situ using near-ambient pressure X-ray photoelectron spectroscopy under CO2 hydrogenation conditions. Combined with scanning tunneling microscopy and density functional theory calculations, the study showed that submonolayers of Pd undergo spontaneous alloy formation on Cu(110) and Cu(100) surfaces in vacuum, whereas they do not form an alloy on Cu(111). Upon heating in H2, inward diffusion of Pd into the Cu lattice is favored, facilitating alloying on all Cu surfaces. Under CO2 hydrogenation reaction conditions, the alloying trend becomes stronger, promoted by the reaction intermediate HCOO*, especially on Pd/Cu(110). This work demonstrates that surface alloying may be a key factor in the enhancement of the catalytic activity of PdCu catalysts as compared to their monometallic counterparts. Furthermore, it sheds light on the hydrogen activation mechanism during catalytic hydrogenation on copper-based catalysts

Characterization of free standing InAs quantum membranes by standing wave hard x-ray photoemission spectroscopy

Free-standing nanoribbons of InAs quantum membranes (QMs) transferred onto a (Si/Mo) multilayer mirror substrate are characterized by hard x-ray photoemission spectroscopy (HXPS), and by standing-wave HXPS (SW-HXPS). Information on the chemical composition and on the chemical states of the elements within the nanoribbons was obtained by HXPS and on the quantitative depth profiles by SW-HXPS. By comparing the experimental SW-HXPS rocking curves to x-ray optical calculations, the chemical depth profile of the InAs(QM) and its interfaces were quantitatively derived with angstrom precision. We determined that: i) the exposure to air induced the formation of an InAsO$_4$ layer on top of the stoichiometric InAs(QM); ii) the top interface between the air-side InAsO$_4$ and the InAs(QM) is not sharp, indicating that interdiffusion occurs between these two layers; iii) the bottom interface between the InAs(QM) and the native oxide SiO$_2$ on top of the (Si/Mo) substrate is abrupt. In addition, the valence band offset (VBO) between the InAs(QM) and the SiO$_2$/(Si/Mo) substrate was determined by HXPS. The value of $VBO = 0.2 \pm 0.04$ eV is in good agreement with literature results obtained by electrical characterization, giving a clear indication of the formation of a well-defined and abrupt InAs/SiO$_2$ heterojunction. We have demonstrated that HXPS and SW-HXPS are non-destructive, powerful methods for characterizing interfaces and for providing chemical depth profiles of nanostructures, quantum membranes, and 2D layered materials.Comment: three figure

X-Ray Spectroscopic Characterization of BaO, Ba(OH)2, BaCO3, and Ba(NO3)2

Simple inorganic barium compounds are difficult to study spectroscopically in the surface sensitive soft X-ray regime due to significant surface contamination that can dominate the spectra. Here we present a near-edge X-ray absorption (NEXAFS) and X-ray photoelectron spectroscopic (XPS) study of four barium compounds: BaO, Ba(OH)2, BaCO3, and Ba(NO3)2. Using an ambient-pressure XPS instrument we prepared thin film samples in situ, which provided a high degree of control over the surface chemistry and significantly reduced the amount of contamination. The O K-edge spectrum for BaO presented here is in excellent agreement with the latest calculations in the literature, and indicates that experimental spectra in the literature for this compound may have suffered from carbonate contamination

X-Ray Spectroscopic Characterization of BaO, Ba(OH)2, BaCO3, and Ba(NO3)2

Simple inorganic barium compounds are difficult to study spectroscopically in the surface sensitive soft X-ray regime due to significant surface contamination that can dominate the spectra. Here we present a near-edge X-ray absorption (NEXAFS) and X-ray photoelectron spectroscopic (XPS) study of four barium compounds: BaO, Ba(OH)2, BaCO3, and Ba(NO3)2. Using an ambient-pressure XPS instrument we prepared thin film samples in situ, which provided a high degree of control over the surface chemistry and significantly reduced the amount of contamination. The O K-edge spectrum for BaO presented here is in excellent agreement with the latest calculations in the literature, and indicates that experimental spectra in the literature for this compound may have suffered from carbonate contamination

An Efficient Algorithm for Automatic Structure Optimization in X-ray Standing-Wave Experiments

X-ray standing-wave photoemission experiments involving multilayered samples are emerging as unique probes of the buried interfaces that are ubiquitous in current device and materials research. Such data require for their analysis a structure optimization process comparing experiment to theory that is not straightforward. In this work, we present a new computer program for optimizing the analysis of standing-wave data, called SWOPT, that automates this trial-and-error optimization process. The program includes an algorithm that has been developed for computationally expensive problems: so-called black-box simulation optimizations. It also includes a more efficient version of the Yang X-ray Optics Program (YXRO) [Yang, S.-H., Gray, A.X., Kaiser, A.M., Mun, B.S., Sell, B.C., Kortright, J.B., Fadley, C.S., J. Appl. Phys. 113, 1 (2013)] which is about an order of magnitude faster than the original version. Human interaction is not required during optimization. We tested our optimization algorithm on real and hypothetical problems and show that it finds better solutions significantly faster than a random search approach. The total optimization time ranges, depending on the sample structure, from minutes to a few hours on a modern computer, and can be up to 100x faster than a corresponding manual optimization. These speeds make the SWOPT program a valuable tool for real-time analyses of data during synchrotron experiments

NO₂ Interactions with MoO₃ and CuO at Atmospherically Relevant Pressures

NOx concentrations in some geographic regions are harmful to human health. Gas filters to trap NOx and other toxic chemicals contain metal oxides, including MoO3 and CuO. These materials are also being investigated for NOx gas sensors. In a step to understand the fundamental adsorption mechanism in sensors and the effect on binding site availability in gas filters, ambient-pressure X-ray photoelectron spectroscopy (APXPS) was used to study the interaction of NO2 with polycrystalline MoO3 and CuO surfaces under pressures up to 0.01 Torr (14 parts per million volume (ppmv)). Density functional theory-based computational modeling was performed to reveal the mechanisms of NO2 interactions with the MoO3(010) and CuO(111) surfaces to aid interpretation of the experimental results. With pressure dependence, NO2 interacts with reduced Mo5+ atoms generated by oxygen vacancies and abstracts hydrogen atoms from hydroxyl groups on MoO3 without accumulating N-containing species on the surface; vacancy-induced electronic states in the band gap are also removed, hinting toward an increase in the resistivity of the material. N-containing species begin accumulating on the CuO surface at atmospherically relevant pressures of 140 ppbv. NO2 only decomposes at oxygen vacancy sites of CuO. The nitrogen species leave the CuO surface upon evacuation, highlighting the importance of in situ surface characterization when studying gas sensing and adsorption mechanisms. These results imply that NO2 removes hydroxyl and Ovac binding sties on these materials when used in gas filtration and sensing applications. Furthermore, the results show the key role of Ovac sites in the gas sensing mechanism of MoO3 and highlight the potential of APXPS for further studies of gas sensors

In situ characterization of the initial effect of water on molecular interactions at the interface of organic/inorganic hybrid systems

Probing initial interactions at the interface of hybrid systems under humid conditions has the potential to reveal the local chemical environment at solid/solid interfaces under real-world, technologically relevant conditions. Here, we show that ambient pressure X-ray photoelectron spectroscopy (APXPS) with a conventional X-ray source can be used to study the effects of water exposure on the interaction of a nanometer-thin polyacrylic acid (PAA) layer with a native aluminum oxide surface. The formation of a carboxylate ionic bond at the interface is characterized both with APXPS and in situ attenuated total reflectance Fourier transform infrared spectroscopy in the Kretschmann geometry (ATR-FTIR Kretschmann). When water is dosed in the APXPS chamber up to 5 Torr (∼28% relative humidity), an increase in the amount of ionic bonds at the interface is observed. To confirm our APXPS interpretation, complementary ATR-FTIR Kretschmann experiments on a similar model system, which is exposed to an aqueous electrolyte, are conducted. These spectra demonstrate that water leads to an increased wet adhesion through increased ionic bond formation.(OLD) MSE-