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The investigation of nanocatalysts under ambient pressure by X-ray photoelectron spectroscopy gives access to a wealth of information on their chemical state under reaction conditions. Considering the paradigmatic CO oxidation reaction, a strong synergistic effect on CO catalytic oxidation was recently observed on a partly dewetted ZnO(0001)/Pt(111) single crystal surface. In order to bridge the material gap, we have examined whether this inverse metal/oxide catalytic effect could be transposed on supported ZnPt nanocatalysts deposited on rutile TiO2(110). Synchrotron radiation near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) operated at 1 mbar of O2 : CO mixture (4 : 1) was used at a temperature range between room temperature and 450 K. To tackle the complexity of the problem, we have also studied the catalytic activity of nanoparticles (NPs) of the same size, consisting of pure Pt and Zn nanoparticles (NPs), for which, moreover, NAP-XPS studies are a novelty. The comparative approach shows that the CO oxidation process is markedly different for the pure Pt and pure Zn NPs. For pure Pt NPs, CO poisoned the metallic surfaces at low temperature at the onset of CO2 evolution. In contrast, the pure Zn NPs first oxidize into ZnO, and trap carbonates at low temperature. Then they start to release CO2 in the gas phase, at a critical temperature, while continuously producing it. The pure Zn NPs are also immune to support encapsulation. The bimetallic nanoparticle borrows some of its characteristics from its two parent metals. In fact, the ZnPt NP, although produced by the sequential deposition of platinum and zinc, is platinum-terminated below the temperature onset of CO oxidation and poisoned by CO. Above the CO oxidation onset, the nanoparticle becomes Zn-rich with a ZnO shell. Pure Pt and ZnPt NPs present a very similar activity towards CO oxidation, in contrast with what is reported in a single crystal study. The present study demonstrates the effectiveness of NAP-XPS in the study of complex catalytic processes at work on nanocatalysts under near-ambient pressures, and highlights once more the difficulty of transposing single crystal surface observations to the case of nanoobjects.

International audience; The investigation of nanocatalysts under their working conditions of pressures and temperatures represents a real strategy toward a realistic understanding of their chemical reactivity and related issues. Additionally, the reduction of Pt load in the catalysts while maintaining their optimum performances is essential to large scale practical applications. Here, we show that small PtZn bimetallic nanoparticles (NPs) supported on the rutile and reduced TiO2(110)-(1 × 1) surface can be prepared by a two step consecutive deposition process where Pt was deposited first and followed by Zn. In situ synchrotron-based near ambient pressure photoemission spectroscopy experiments are used to monitor the evolution of the oxidation states and surface elemental composition of pure Pt and PtZn NPs under high exposure to O2 pressure. The formation of stable Pt surface oxide was evidenced for both pure and PtZn NPs. While a sizeable encapsulation of pure Pt NPs by TiOx was seen after annealing at 440 K under 1 mbar of O2, no such effect was noticed for PtZn NPs. The formation of a zinc oxide layer on PtZn NPs enhances the stability of the NPs and induces a partial reduction of the TiO2(110) surface. Spontaneous formation of a Pt–Zn alloy phase at room temperature was seen in PtZn NPs.

International audience; Organic layers chemically grafted on silicon offer excellent interfaces that may open up the way for new organic–inorganic hybrid nanoelectronic devices. However, technological achievements rely on the precise electronic characterization of such organic layers. We have prepared ordered grafted organic monolayers (GOMs) on Si(111), sometimes termed self-assembled monolayers (SAMs), by a hydrosilylation reaction with either a 7-carbon or an 11-carbon alkyl chain, with further modification to obtain amine-terminated surfaces. X-ray photoelectron spectroscopy (XPS) is used to determine the band bending (∼0.3 eV), and ultraviolet photoelectron spectroscopy (UPS) to measure the work function (∼3.4 eV) and the HOMO edge. Scanning tunneling microscopy (STM) confirms that the GOM surface is clean and smooth. Finally, conductive AFM is used to measure electron transport through the monolayer and to identify transition between the tunneling and the field emission regimes. These organic monolayers offer a promising alternative to silicon dioxide thin films for fabricating metal–insulator–semiconductor (MIS) junctions. We show that gold nanoparticles can be covalently attached to mimic metallic nano-electrodes and that the electrical quality of the GOMs is completely preserved in the process.

Etched and coated multilayer gratings and simple multilayers are investigated by means of x-ray scattering (XRS) and grazing x-ray reflectivity (XRR). Two types of gratings are studied: one lamellar grating coated by a La/B4C stack and two lamellar gratings etched in a Mo/B4C multilayer structure with different cyclic ratios. XRS measurements are presented as reciprocal space mappings or Q-mapping and the origin of the different structures observed in the Q-mapping are discussed in terms of typical coherent and diffuse scattering processes (Bragg spots, grating-Bragg spots, Bragg sheets, grating diffuse sheets, Yoneda wings, etc). From the XRS and XRR measurements, some structural parameters of the mirrors and gratings such as periods and roughness characteristics are estimated.

Artificially fabricated multilayers that exploit the effect of Bragg diffraction at long wavelengths are used as X-ray optical components. Periodic ultrathin bilayer stacks of, alternatingly, a metallic reflection layer and a non-metallic spacer layer prepared as @mm-scaled surface grating promise particularly high performance in applications as X-ray filters for high-resolution spectroscopy. Such gratings can be prepared by multilayer deposition onto Si(001) gratings or by coating flat Si(011) substrate surfaces with a multilayer, followed by subsequent etching of a grating structure. The structural quality of ultrathin multilayer gratings of both types has been characterized quantitatively combining TEM bright-field imaging of specimen cross-sections and applying a geometric phase method. The geometric phase method has been originally developed for the analysis of local displacement fields from HREM images and allows to obtain the relevant structure parameters. The application of this method to the characterization of Mo/B"4C and La/B"4C multilayer systems shows that the functionally decisive structure parameters, such as layer perfection, layer periodicity, and layer orientation, can be obtained with high precision from evaluation of TEM bright-field images. The essential role of such data analyses for a quantitative characterization of multilayer systems and for optimising the layer deposition techniques in the fabrication of X-ray optical layer systems will be demonstrated and discussed.

We investigate on a Mo/Si multilayer interferential mirror (MIM) as a function of annealing at temperatures up to 600 °C. The physicochemical state of the silicon atoms within the MIM is characterized by X-ray emission spectroscopy (XES). A thin silicide layer is detected at the interfaces of the as-prepared system, whose thickness increases upon annealing. Measurements of the reflecting power at 1.33 nm show that both the reflectivity and the period markedly decrease as the annealing temperature increases, thus stressing on the interplay between the interface physico-chemical state and the optical properties. Our results shed light on the highest temperature regime, 600 °C, where the MIM loses its periodic structure as the majority of the silicon atoms are involved in the MoSi 2 compound. The diffusion coefficient is estimated from the evolution of the reflected intensity and the period. An activation energy of the diffusion process of 0.52 eV is deduced.

An etched multilayer, a 2D structure fabricated by etching a periodic multilayer according to the pattern of a laminar grating, is applied in the soft X-ray range to improve the spectral resolution of wavelength dispersive spectrometers. The present article gathers all the successive stages of the development of such a device optimized to analyze the characteristic emission of light elements: design, structural and optical characterization and applications to X-ray spectroscopy. The evolution of the shape of the C Kα emission band of highly oriented pyrolytic graphite (HOPG), as a function of the angle between the emission direction and the (0 0 0 1) planes, is measured. These results, compared to those with a grating, demonstrate that the achieved spectral resolution enables disentangling σ → 1s and π → 1s transitions within the C K emission band.

International audience; An x-ray multilayer monochromator with improved resolution and a low specular background is presented. The monochromator consists of a lamellar multilayer amplitude grating with appropriate parameters used at the zeroth diffraction order. The device is fabricated by means of combining deposition of thin films on a nanometer scale, UV lithography, and reactive ion etching. The performance of this new monochromator at photon energies near 1500 eV is shown.