[show abstract][hide abstract] ABSTRACT: The growth of vapour-deposited silver nanoparticles on α-Al₂O₃ was studied in situ from 190 to 675 K by surface differential reflectivity spectroscopy in the UV-visible range. Changes in size, shape and density were derived from the plasmonic response modelled in the framework of interface susceptibilities by assuming that supported clusters were in the form of truncated spheres. The sticking coefficient of silver on alumina is close to one up to T ≃ 575 K before entering a regime of incomplete condensation. The Arrhenius dependence of the saturation density indicates a nucleation on defects at low temperature (T ≤ 300 K) and detrapping above. The particle size D evolution follows temporal power laws, independent of temperature and flux, which characterize the growth (D ∼ t(0.31)) and coalescence (D ∼ t(0.55)) of the film. These are indicative of the growth of isolated particles at constant density and dynamic coalescence, respectively. The wetting angle of the silver clusters is shown to increase during the growth regime, which is assigned to a combination of surface stress and mismatch-induced strain, and to decrease upon coalescence, which is attributed to plastic relaxation. For particles larger than 10 nm in size, the values of contact angle and adhesion energy level off with asymptotic limits (θ(c) = 127.5° ± 1° and 0.48 ± 0.02 J m⁻²) that nicely agree with tabulated data. This work highlights the ability of nanoplasmics to monitor in situ the growth kinetics of thin supported films.
[show abstract][hide abstract] ABSTRACT: The gap state that appears upon reduction of TiO2 plays a key role in many of titania's interesting properties but its origin and spatial localization have remained unclear. In the present work, the TiO2(110) surface is reduced in a chemically controlled way by sodium adsorption. By means of resonant photoelectron diffraction, excess electrons are shown to be distributed mainly on subsurface Ti sites strikingly similar to the defective TiO2(110) surface, while any significant contribution from interstitial Ti ions is discarded. In agreement with first principles calculations, these findings demonstrate that the distribution of the band gap charge is an intrinsic property of TiO2(110), independent of the way excess electrons are produced.
[show abstract][hide abstract] ABSTRACT: The influence of oxygen (O2) and carbon monoxide (CO) on Au nanoparticles supported on TiO2(1 1 0) in the size range of 2–3 nm has been studied using X-ray photoelectron spectroscopy (XPS) and in situ (high pressure) XPS at 300 K for O2 and/or CO pressures of 0.1–1 mbar. These experiments were aimed at revisiting Au 4f core level shifts as reported in the literature and most importantly, to establish the dependence of the core-level shifts on the knowledge that there exists a maximum in reactivity for CO oxidation. Two samples were prepared with a coverage corresponding to that maximum (Au coverage 0.14–0.2 ML, particle size estimated to ∼2–2.5 nm) while a third sample was expected to be less reactive (Au coverage 0.4 ML, particle size estimated to ∼3.3 nm). At elevated O2 pressures, a new Au 4f component at higher binding energy (2.4–2.6 eV relative to the Au(0) bulk signal) evolved at all particle sizes. Its appearance was attributed to a radiation-induced activation of oxygen and simultaneous oxidation of gold. The activation was much more efficient on the ∼2–2.5 nm particles. The relative intensity of the oxide component depended strongly on O2 pressure and, thus, on the equilibrium coverage of O2. While not present in 0.1 mbar O2 regardless of exposure time and particle size, it dominated the Au 4f spectrum of particles ∼2–2.5 nm in size at 1 mbar oxygen pressure. This pressure-dependent formation reconciles previously conflicting XPS data. Finally, the activated oxygen species were very reactive toward CO as manifested by the rapid disappearance of the new Au 4f component in a 1:1 mixture of CO and O2. The rates of evolution and consumption of this component were found to depend on gold coverage (and thus, particle size) and were highest for the smaller particles.
[show abstract][hide abstract] ABSTRACT: Supported platinum nanoparticles in the presence of carbon monoxide in the 10–6–103 Pa CO pressure range were studied by grazing incidence small-angle X-ray scattering. Disruption and aggregation of platinum nanoparticles upon CO exposure are shown to occur in parallel. Particles smaller than a critical size of 1 nm undergo scavenging after disruption in the form of carbonyls at CO pressure as low as 10–1 Pa. Mobile carbonyls then agglomerate to larger clusters. Upon annealing, a CO-driven diffusion and agglomeration of clusters of size ranging between the critical size and 2 nm is observed. These phenomena are discussed in relation to the CO–metal, metal–metal, and metal–substrate relative bond strengths. Similar mechanisms are suggested to hold for other supported metal catalysts.
The Journal of Physical Chemistry C 01/2012; 116:23362-23370. · 4.81 Impact Factor
[show abstract][hide abstract] ABSTRACT: Plasmon excitation appears to be a powerful and flexible tool for probing in situ and in real time the growth of supported conducting metal nanoparticles. However, although models exist for analysing optical profiles, limitations arise in the realistic modelling of particle shape from the lack of knowledge of temperature effects and of broadening sources. This paper reports on the growth of silver on alumina at 190-675 K monitored by surface differential reflectivity spectroscopy in the UV-visible range. In the framework of plasmonic response analysis, particles are modelled by truncated spheres. Their polarizabilities are computed within the quasi-static approximation and used as an input to the interface susceptibilities model in order to determine the Fresnel reflection coefficient. The pivotal importance of the thermal variation of the metal dielectric constant is demonstrated. Finite-size effects are accounted for. As size distribution fluctuations contribute marginally to the lineshape compared to the aspect ratio (diameter/height) distribution, a convolution method for representing the experimental broadening is introduced. Effects of disorder on the lineshape are discussed. It is highlighted that beside the quality of the fit (not a proof by itself!), physical meaning of the parameters related to the sticking probability, growth and wetting is crucially required for validating models. The proposed modelling opens interesting perspectives for the quantitative study of growth via plasmonics, in particular in the case of noble metals.
[show abstract][hide abstract] ABSTRACT: Here, we show that the stoichiometry and, consequently, the chemical activity toward hydroxylation of MgO(100) films grown by reactive deposition on Ag(100) strongly depend on the O(2) partial pressure during film growth. Oxygen-deficient films undergo dramatic relative oxygen uptake either by exposure to a partial pressure of water vapor or by aging in vacuum for a sufficiently long time. Conversely, on stoichiometric monolayer MgO islands, photoemission analysis of the O 1s level and scanning tunneling microscopy images are consistent with the prediction that dissociative adsorption of water occurs only at the borders of the islands.
The Journal of Physical Chemistry A 06/2011; 115(25):7161-8. · 2.77 Impact Factor
[show abstract][hide abstract] ABSTRACT: The origin of the catalytic activity of gold nanoparticles remains debated despite extensive studies. This in operando work investigates the relationship between catalytic activity and size/shape of gold nanoparticles supported on TiO2(110) during CO oxidation. The nanoparticles were synthesized by vapor deposition in ultrahigh vacuum. Their geometry was monitored in the presence of O2, Ar, or a mixture of O2 + CO and of Ar + CO by grazing incidence small-angle X-ray scattering simultaneously with the catalytic activity. The occurrence of CO oxidation induces a sintering directly correlated to the reaction rate. The catalytic activity is optimum for a nanoparticle’s diameter of 2.1 ± 0.3 nm and a height of about six atomic layers. Below this size, the activity drop corresponds to a height decrease. Rescaling of activities obtained in different experimental conditions shows consistency of these results with published data using both “model” and “real” catalysts.
[show abstract][hide abstract] ABSTRACT: The redox profiles of tin, iron and sulfur at the float glass surface were determined on purposely cut samples by Electron Probe Micro Analysis (EPMA), X-ray Fluorescence mapping and X-ray Absorption Spectroscopy (XAS) at the micron scale. Going inward from the surface to the bulk, it was observed that (features do not depend on the glass thickness (holding time) though they extend over depths that vary from ca. 25 to 50μm): (i) after a diffusion-driven decrease and prior to vanishing, the tin concentration passes through a local maximum (the tin hump), where stannous ions, which dominate the shallow layers, switch to stannic ions; (ii) the iron concentration decreases, passes through a minimum at the tin hump where iron is in the more reduced form (lowest Fe3+/Fe2+ ratio); it then increases and, after a hump which appears as a chemical echo of the tin hump, it reaches the bulk value; (iii) the concentration of sulfur increases up to reach the bulk concentration beyond the tin hump region. In a mixed S6+/S2− form at the surface, sulfur is only in sulfate form in the bulk.In the case under study, the iron concentration is much too low to balance the redox reaction Sn2+→Sn4+ that occurs at the tin hump. Sulfur is shown to play the role usually attributed to iron, according to the reaction4Sn2++S6+→4Sn4++S2−The occurrence of that reaction is supported by the appearance of sulfide S2− in the tin hump region with an appropriate concentration profile of a much stronger S2−/S6+ ratio on the tin side than on the atmosphere side of the float glass. The conclusions drawn herein likely apply to the many cases in which the glass composition is similar as that encountered herein.
Journal of Non-Crystalline Solids 01/2011; 357(16):3200-3206. · 1.60 Impact Factor
[show abstract][hide abstract] ABSTRACT: X-ray diffraction stress analyses have been performed on two different thin films deposited onto silicon substrate: ZnO and ZnO encapsulated into Si3N4 layers. We showed that both as-deposited ZnO films are in a high compressive stress state. In situ X-ray diffraction measurements inside a furnace revealed a relaxation of the as-grown stresses at temperatures which vary with the atmosphere in the furnace and change with Si3N4 encapsulation. The observations show that Si3N4 films lying on both sides of the ZnO film play an important role in the mechanisms responsible for the stress relaxation during heat treatment. The different temperatures observed for relaxation in ambient and argon atmospheres suggest that the thermally activated stress relaxation may be attributed to a variation of the stoichiometry of the ZnO films. The present observations pave the way to fine tuning of the residual stresses through thermal treatment parameters.
Thin Solid Films 12/2010; 519(5):1563-1567. · 1.60 Impact Factor
[show abstract][hide abstract] ABSTRACT: Residual stresses in sputtered ZnO films on Si are determined and discussed. By means of X-ray diffraction, we show that as-deposited ZnO films are highly compressively stressed. Moreover, a transition of stress is observed as a function of the post-deposition annealing temperature. After an 800 degrees C annealing, ZnO films are tensily stressed while ZnO films encapsulated by Si3N4 are stress-free. With the aid of in-situ X-ray diffraction under ambient and argon atmosphere, we argue that this thermally activated stress relaxation may be attributed to a variation of the stoichiometry of the ZnO films.
[show abstract][hide abstract] ABSTRACT: The infrared spectra of MgO smoke exposed to water are compared to high-resolution electron energy loss spectra (HREELS) of hydroxylated ultrathin MgO(1 0 0)/Ag(1 0 0) films. Very similar bands are observed at 3458–3480 cm−1 and 3710–3714 cm−1. On the basis of first principle calculations, these bands are interpreted as the stretching modes of the two distinct OH groups that are formed at monatomic steps parallel to 〈1 0 0〉 crystallographic directions. The lower frequency band is due to H adsorbed on O at the step edge, while the higher frequency one originates from OH groups that are twofold coordinated with Mg. Consistently, scanning tunnelling microscopy images of MgO films, prepared in similar conditions as during the HREELS experiments, show that the MgO/Ag(1 0 0) island edges are mainly non-polar, i.e. oriented along the 〈1 0 0〉 direction. In spite of that, a minor contribution to the OH-stretch intensity from hydroxylated polar 〈1 1 0〉 steps cannot be excluded; for such geometry density functional theory predicts indeed a single OH species with a vibrational frequency very close to the high-frequency band of OH adsorbed at 〈1 0 0〉 steps.
[show abstract][hide abstract] ABSTRACT: As recently established (J. Am. Chem. Soc. 2008, 130, 12552), ethene adsorption on Cu(410) occurs both molecularly and dissociatively, the latter resulting in carbon contamination of the surface. Here we report on the coverage-dependent dynamics of C2H4 adsorption on clean and carbon-contaminated Cu(410). For the bare surface, the initial sticking probability has a very weak dependence on kinetic energy and is almost independent of angle of incidence. Molecular adsorption is in both cases precursor-mediated and nonactivated. Ethene dissociation takes place during adsorption as well as upon annealing. Both paths proceed via a molecular precursor. The former is translationally activated, while the latter depends strongly on the heating rate. The presence of preadsorbed carbon, resulting from previous uptakes, affects both the sticking probability and the attained saturation coverage. The latter quantity is shown to be a sensitive probe of carbon precoverage. A scheme of the complicated potential energy surface of this system is derived and discussed.
Journal of Physical Chemistry C - J PHYS CHEM C. 12/2009; 113(49).
[show abstract][hide abstract] ABSTRACT: Sulfur is an important element of glasses, not because of its amount, always very low (less than 0.4 % in weight of SO3), but because of its role since it actively participates to the refinement process and, combined to other elements, it can be responsible for the coloration of the glass. Iron is also of a major importance in most of the glasses. In the case of the float glass, the two faces, because of the fabrication process, are different in terms of composition (presence of Sn for one face) and also in terms of oxidation state of these minority elements (Fe, Sn, S). There should be a subttle interplay between the concentrations and the oxidation states of these different minority elements, and anyway these variations occur over a thickness of the order of few micrometers below the surface. Using the high intensity and the focusing properties (3 × 3 μm2) of the x-ray beam from the Lucia beamline, we have therefore studied the speciation of iron and sulfur near the face of a float glass in relation with the behavior of tin. This has been obtained by combining elemental x-ray fluorescence cartography and x-ray micro-absorption at the different K-edges.
Journal of Physics Conference Series 11/2009; 190(1):012079.
[show abstract][hide abstract] ABSTRACT: At the onset of dissolution in water, cubic MgO smoke crystals present (110) cuts of the edges of the cubes. Next, (111) facets progressively dominate the shape of the crystallites, which finally transform into truncated octahedra. The morphology of the crystallites that are derived from surface energies computed within the density functional theory (DFT), only involve (100) and (111) facets. We explain the unexpected (110) cuts via a "constrained" Wulff equilibrium shape that arises from a slower kinetics of formation of (111) facets than (110) ones. Experiment and theory fully agree on the hierarchy of hydroxylated surface energies: Gamma(111) < Gamma(100) < Gamma(110), both supporting the partial dissociation of water on MgO(100). Finally, from low to high P(H2O) (high to low T), DFT-based calculations predict a switch from Wulff shapes involving dry (100) facets, in which the (100)/(111) area ratio decreases upon increasing P(H2O), to shapes involving hydroxylated (100) surfaces, in which the above ratio increases with P(H2O).
Physical Chemistry Chemical Physics 05/2009; 11(13):2228-33. · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: The growth of Ag/MgO(001) was investigated at three temperatures, 300, 540, and 640 K, in situ, in ultrahigh vacuum, by grazing incidence small angle x-ray scattering (GISAXS). The samples prepared at 300 and 540 K were also examined ex situ by transmission electron microscopy (TEM). Nucleation, growth, and coalescence are studied via the evolution of the average particle diameter, height, interparticle distance, and size distributions. Power laws are given. Size-spacing correlations are evidenced by TEM and GISAXS. A heterogeneous nucleation of silver on the defects [(7.5±1.5)×1012 defects cm−2] of the MgO surface is evidenced. Above a silver coverage of approximately 2 nm, a dynamic coalescence regime takes place involving diffusion of the islands. A value of 0.76±0.1 J m−2 for the Ag/MgO(001) adhesion energy is derived by using a Wulff-Kaishew construction from the particle shape obtained by GISAXS. The diffusion energy of Ag on MgO(001) is estimated to be 0.05±0.02 eV/atom via kinetic Monte Carlo simulations of the growth stage.
[show abstract][hide abstract] ABSTRACT: The etching of MgO smoke in wet conditions – an archetype for aqueous dissolution of oxide crystals – is explored on the basis of transmission electron microscope (TEM) analyses. In pure water with neutral pH, in acid solutions and in moist air, the MgO cubic crystallites show first (110) truncations of the 〈110〉 edges and then (111) cuts at the corners. Criteria are given to characterize (110) and (111) cuts on the TEM images. It is shown that (110) truncations appear independently of the formation of the (111) cuts and are associated with a stationary state. The picture is fully consistent with the previous suggestion of a “constrained Wulff profile” which explains the formation of (110) cuts by excluding (111) facets.
Journal of Crystal Growth 01/2009; 311(21):4598-4604. · 1.55 Impact Factor
[show abstract][hide abstract] ABSTRACT: This paper compares the capability of surface differential reflectivity spectroscopy (SDRS) and grazing incidence small angle x-ray scattering (GISAXS) to characterize in situ, “at a glance,” a collection of nanoclusters by statistically defining the geometry of an average particle (diameter, height, and interparticle distance). SDRS and GISAXS are run simultaneously on Ag/MgO(001) films during their growth. They are shown to lead to consistent values of the morphological parameters, with a particular good agreement about the aspect ratio (height/diameter) which, thanks to the basic physics underlying the two measurements, is determined in both cases. By modeling silver particles by truncated spheres, close estimates of the wetting angle and of the adhesion energy are derived (0.75±0.08 J m−2 for SDRS and 0.85±0.20 J m−2 for GISAXS). This demonstrates that the very flexible laboratory SDRS can be used to study the growth of a film in a similar way as the well-documented small angle x-ray scattering provided it is analyzed in an appropriate framework.
[show abstract][hide abstract] ABSTRACT: The dissociative adsorption of water vapor on MgO smokes is studied by infrared spectroscopy and first-principles calculations. The MgO smokes are synthesized in controlled conditions so to display a particularly high concentration of defect-free step edges (4-fold coordinated “4C” sites). Exposure to water vapor results in a set of twinned infrared bands at νA = 3480 and νB = 3710 cm−1the former having never been observed beforewith an intensity ratio I(νA)/I(νB) ≈ 2. Simulated configurations include water adsorption, for various coverages, on terraces of distinct orientations, at step edges and corners. It is shown that OH stretching frequencies can be robustly correlated with OH bond lengths. The twinned (νA,ανB) bands are assigned the hydrogen-bonded (Os4C−H, Mgs4C−OH) moiety adsorbed at fully decorated monatomic <100> step edges on (100) terraces. The coordination number of adsorbed OH species is shown not to be sufficient to characterize the physicochemical properties of wet MgO smokes and it is suggested that the hydrated surface can be better described in terms of (OH, MgO) complexes.
Journal of Physical Chemistry C - J PHYS CHEM C. 08/2008; 112(34).
[show abstract][hide abstract] ABSTRACT: Polar oxide surfaces constitute a class of surfaces long considered as unstable because of their theoretically predicted diverging electrostatic surface potential, although such crystal orientations form spontaneously in nature. Because of the intrinsic hindrances (charge build-up, electron correlation effects) a reliable description of such surfaces necessitates a combined theory–experiment approach. Experimentally, detailed investigations became possible in recent years because of the availability of charge build-up insensitive techniques such as surface x-ray diffraction, which is thoroughly used in the presented work. We consider here prototypical polar oxide surfaces of different crystalline structures and show that their stability regime strongly depends on the external conditions applied to the surface in terms of oxygen chemical potential. In order to understand the structure and stability of polar oxide surfaces, their chemical environment must be included.