In situ oxidation study of MgO(1 0 0) supported Pd nanoparticles

We have investigated the oxidation behavior of Pd nanoparticles grown epitaxially on MgO(1 0 0) single crystal substrates. We find that the interaction of oxygen with octahedral Pd nanoparticles at 500 K can be subdivided in three stages: above 10⁻⁶ mbar O₂ pressure, the particles start to flatten; above 10⁻³ mbar, the particles begin to shrink laterally and to be less truncated at the corners. The formation of epitaxial bulk PdO sets in at oxygen pressures above 0.1 mbar, which is accompanied by a continuous shrinkage of the Pd particles. Our results point to a novel nanoparticle oxidation mechanism: the Pd particles act as dissociation centers for O₂ and serve at the same time as source for Pd atoms resulting in epitaxial PdO growth on MgO(1 0 0).     

Influence of CrN surface compound on the initial stages of high temperature oxidation of ferritic stainless steel

Segregation phenomena and formation of surface compounds on Fe-17Cr (ferritic stainless steel) were studied at temperatures up to 800 °C upon annealing and kinetically controlled oxidation by photoelectron spectroscopy and inelastic electron background analysis. The results revealed the formation of a chromium nitride surface compound during annealing in ultrahigh vacuum at temperatures exceeding 527 °C. Surface enrichment of P, As, and other trace elements became more prominent at higher temperatures approaching 800 °C. It was found that nitrogen was buried below the surface oxide layer during oxygen exposure, yet it had little effect on the rate of oxidation. However, the formation of CrN surface compound promoted the selective oxidation of Cr initially, which is of great importance in processing and application environments involving high temperature and controlled atmosphere conditions.     

Controlling metal/oxide interactions in bifunctional nanostructured model catalysts: Pd and BaO on Al2O3/NiAl(1 1 0)

We have investigated the structure and growth of Pd and BaO containing nanoparticles sequentially co-deposited on an ordered Al₂O₃/NiAl(1 1 0) by scanning tunneling microscopy (STM), and their interaction with CO and NO₂ by infrared reflection absorption spectroscopy (IRAS).Ba deposition and subsequent oxidation result in Ba_xAl_2yO_x+3y nanoparticles being formed which are characterized by high particle densities and nucleation both on regular terraces and at oxide defects. In contrast, Pd interaction with the model support is weaker and preferential nucleation occurs mainly at rotational domain boundaries and to a lesser extent at anti-phase domain boundaries. For subsequent co-deposition of Pd on preformed Ba_xAl_2yO_x+3y/Al₂O₃/NiAl(1 1 0), Pd nucleates at the Ba_xAl_2yO_x+3y nanoparticles and covers them. The reverse deposition sequence, i.e. subsequent Ba co-deposition and oxidation on preformed Pd/Al₂O₃/NiAl(1 1 0), leads to formation of small isolated Ba_xAl_2yO_x+3y nanoparticles without contact to Pd, together with large Pd crystallites modified by Ba_xAl_2yO_x+3y. The present results provide control over the degree of interaction between metal nanoparticles and oxide nanoparticles on a well-defined model catalyst and thus allow us to study effects related effects on the reactivity and catalytic behavior.     

Formation of the Si/Ti interface

The formation of the Si/Ti interface during the deposition of silicon on titanium polycrystalline substrates has been studied at room temperature (RT) using X-ray photoelectron spectroscopy (XPS), angle-resolved XPS (ARXPS), ultraviolet photoelectron spectroscopy (UPS) and ion scattering spectroscopy (ISS). The experimental results are consistent with a two-stage mechanism for Si growth: a first stage characterized by the simultaneous formation of a uniform titanium silicide layer, that reaches a limiting thickness of ∼3 monolayer (ML), and pure silicon islands 1 ML thick that grow on top of this layer up to coalescence, followed by a second stage in which pure silicon islands, with an average thickness of 9 ML, grow on top of the uniform titanium silicide layer + pure silicon ML structure formed during the first stage. As a whole, pure silicon species grows according to a Stranski-Krastanov mechanism, where the first ML is formed during the first stage and the islands during the second stage. The comparison of Ti/Si and Si/Ti interfaces shows that the structure and composition of the interface do not depend substantially on the deposition sequence, suggesting that the bulk chemistry of the compound formed at the interface dominates over the surface kinetics and the bulk substrate chemistry in determining the composition and structure of the interface.     

Growth and decay of the Pd(1 1 1)-Pd5O4 surface oxide: Pressure-dependent kinetics and structural aspects

Growth and decomposition of the Pd₅O₄ surface oxide on Pd(1 1 1) were studied at sample temperatures between 573 and 683 K and O₂ gas pressures between 10⁻⁷ and 6 × 10⁻⁵ mbar, by means of an effusive O₂ beam from a capillary array doser, scanning tunnelling microscopy (STM) and thermal desorption spectrometry (TDS). Exposures beyond the p(2 × 2)O adlayer (saturation coverage 0.25) at 683 K (near thermodynamic equilibrium with respect to Pd₅O₄ surface oxide formation) lead to incorporation of additional oxygen into the surface. To initiate the incorporation, a critical pressure beyond the thermodynamic stability limit of the surface oxide is required. This thermodynamic stability limit is near 8.9 × 10⁻⁶ mbar at 683 K, in good agreement with calculations by density functional theory. A controlled kinetic study was feasible by generating nuclei by only a short O₂ pressure pulse and then following further growth kinetics in the lower (10⁻⁶ mbar) pressure range.Growth of the surface oxide layer at a lower temperature (573 K) studied by STM is characterized by a high degree of heterogeneity. Among various metastable local structures, a seam of disordered oxide formed at the step edges is a common structural feature characteristic of initial oxide growth. Further oxide nucleation appears to be favoured along the interface between the p(2 × 2)O structure and these disordered seams. Among the intermediate phases one specifically stable phase was detected both during growth and decomposition of the Pd₅O₄ layer. It is hexagonal with a distance of about 0.62 nm between the protrusions. Its well-ordered form is a superstructure.Isothermal decay of the Pd₅O₄ oxide layer at 693 K involves at first a rearrangement into the structure, indicating its high-temperature stability. This structure can break up into small clusters of uniform size and leaves a free metal surface area covered by a p(2 × 2)O adlayer. The rate of desorption increases autocatalytically with increasing phase boundary metal-oxide. We propose that at close-to-equilibrium conditions (693 K) surface oxide growth and decay occur via this intermediate structure.     

Growth and characterization of model oxide catalysts

Oxide catalysts are frequently used to convert toxic species to environmentally benign molecules, and to prevent the formation of toxic species in the first place. In this paper, growth and characterization of model oxide systems employed in both approaches is discussed. An example of the former approach is the selective catalytic reduction (SCR) of NO emitted from power plants by NH₃, which employs tungsten and vanadium oxides supported on the anatase polymorph of TiO₂. To model SCR catalysts, epitaxial titanium, vanadium and tungsten oxide films were grown using molecular beam epitaxy and magnetron sputtering. Two different anatase orientations were grown on LaAlO₃ substrates and their interactions with vanadia were characterized. On LaAlO₃ (0 0 1), anatase exposed a (4 × 1) reconstructed (0 0 1) surface. Vanadia lifted the reconstruction and at 1 ML a (1 × 1) surface with mostly V⁵⁺ was observed. Continued V₂O₅ growth led to loss of order, but at high temperatures epitaxial VO₂ could be grown; vanadia behaved similarly on anatase films on LaAlO₃ (1 1 0). Results suggested that the monolayer is pseudomorphic with O adsorption oxidizing the surface V to 5+, since the anatase structure cannot accommodate more bulk oxygen, only a monolayer can be pseudomorphic and have only V⁵⁺. Thus the vanadia monolayer has unique structural and chemical properties that can help explain why vanadia monolayers on TiO₂ are much more active than bulk V₂O₅. For WO₃, a series of added row reconstructions were observed as the epitaxial films were reduced. The effect of these structures on surface chemistry was characterized by studying 1-propanol adsorption. The results indicated that the structure of the WO₃ surface did not alter its catalytic function but had a strong effect on reaction kinetics. As an example of a system where catalysts prevent the formation of toxic species, the reactivity of oxidized Pd surfaces used in CH₄ catalytic combustion were studied. An ordered PdO-like monolayer was found to be less reactive towards CO than adsorbed O on Pd. On the other hand, the PdO layer favored a lower activation energy C₃H₆ oxidation pathway. The results indicated that Pd oxidation reduces the sticking coefficient of reactive species but once molecules adsorb, the oxide surface can reduce the activation energy for subsequent reaction.     

Kinetic modelling of the H2---O2 reaction on Pd and of its influence on the hydrogen response of a hydrogen sensitive Pd metal-oxide-semiconductor device

We have established a model for the water forming reaction on Pd in the temperature range 350–475 K. Importantly, the model takes into account the possibility that hydrogen may absorb and adsorb at interface sites on supported Pd catalysts. It is shown that already at modest conditions interface adsorption may significantly affect reaction rates. The model may also be used to quantify the response of a hydrogen sensitive Pd-MOS device during hydrogen sensing in oxygen. In the case of Pd supported on SiO₂, the concentration of interface sites is so low that interface hydrogen adsorption will have only a minor influence on a catalytic reaction. The fact that a Pd-MOS device may be used as a very sensitive hydrogen detector at atmospheric oxygen conditions, despite a steric oxygen blocking of hydrogen dissociation sites, is predicted by the model.     

Study of the state of Ag and Pd in silver-palladium catalysts on a titanium dioxide substrate by IR spectroscopy of adsorbed CO

We have used IR spectroscopy of adsorbed CO to study the effect of the method for preparation of silver-palladium catalysts on a TiO₂ substrate and exposure to an oxidizing or reducing gaseous atmosphere on the state of Ag and Pd centers. We have shown that as a result of the metal-support interaction, the reducibility of the Ag and Pd particles is increased, which leads to a change in their adsorption and catalytic properties. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 1, pp. 16–20, January–February, 2007.     

Formation and characterization of Au/Pd surface alloys on Pd(1 1 1)

The formation of alloys by adsorbing gold on a Pd(1 1 1) single crystal substrate and subsequently annealing to various temperatures is studied in an ultrahigh vacuum by means of Auger and X-ray photoelectron spectroscopy. The nature of the alloy surface is probed by CO chemisorption using temperature-programmed desorption and reflection-absorption infrared spectroscopy. It is found that gold grows in a layer-by-layer fashion on Pd(1 1 1) at 300 K, and starts to diffuse into the bulk after annealing to above ∼600 K. Alloy formation results in a ∼0.5 eV binding energy decrease of the Au 4f XPS signals and a binding energy increase of the Pd 3d features of ∼0.8 eV, consistent with results obtained for the bulk alloy. The experimentally measured CO desorption activation energies and vibrational frequencies do not correlate well with the surface sites expected from the bulk alloy composition but are more consistent with significant preferential segregation of gold to the alloy surface.     

Photoelectron spectroscopy study of the K-covered ZnO(1 0 1̄ 0) surface; annealing-induced changes in the electronic structure and the chemical composition

The electronic structure and the chemical composition of the K-covered ZnO(1 0 1̄ 0) surface at temperatures between 300 and 1200 K are investigated by X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. Adsorption of K on ZnO(1 0 1̄ 0) at room temperature results in the formation of a two-dimensional disordered K overlayer and induces 0.2 eV downward bending of the substrate’s bands going from the bulk to the surface. Upon annealing the K-covered surface, initial downward bending turns to upward bending with maximum bending of 0.5 eV at 700-800 K. The thermally induced migration of the bulk O atoms and the resultant increase in the number of the O atoms on the surface is responsible for upward bending on the annealed surface. The accumulated O atoms interact with the predeposited K atoms on the surface to form non-stoichiometric K-O complexes with the O/K atomic ratio being 1.6-1.8 in the temperature range between 600 and 1000 K.     

A structural study of a C3H3 species coadsorbed with CO on Pd(1 1 1)

The combination of chemical-state-specific C 1s scanned-energy mode photoelectron diffraction (PhD) and O K-edge near-edge X-ray absorption fine structure (NEXAFS) has been used to determine the local adsorption geometry of the coadsorbed C₃H₃ and CO species formed on Pd(1 1 1) by dissociation of molecular furan. CO is found to adopt the same geometry as in the Pd(1 1 1)c(4 × 2)-CO phase, occupying the two inequivalent three-fold coordinated hollow sites with the C–O axis perpendicular to the surface. C₃H₃ is found to lie with its molecular plane almost parallel to the surface, most probably with the two ‘outer’ C atoms in equivalent off-atop sites, although the PhD analysis formally fails to distinguish between two distinct local adsorption sites.     

Methanol adsorption on a CeO2(1 1 1)/Cu(1 1 1) thin film model catalyst

Adsorption and desorption of methanol on a CeO₂(1 1 1)/Cu(1 1 1) thin film surface was investigated by XPS and soft X-ray synchrotron radiation PES. Resonance PES was used to determine the occupancy of the Ce 4f states with high sensitivity. Methanol adsorbed at 110 K formed adsorbate multilayers, which were partially desorbed at 140 K. Low temperature desorption was accompanied by formation of chemisorbed methoxy groups. Methanol strongly reduced cerium oxide by forming hydroxyl groups at first, which with increasing temperature was followed by creation of oxygen vacancies in the topmost cerium oxide layer due to water desorption. Dissociative methanol adsorption and creation of oxygen vacancies was observed as a Ce⁴⁺ → Ce³⁺ transition and an increase of the Ce 4f electronic state occupancy.     

Electron surface states in short-period superlattices: (GaAs)2/(AlAs)2(1 0 0)-c(4 × 4)

The electronic structure of (GaAs)₂/(AlAs)₂(1 0 0)-c(4 × 4) superlattice surfaces was studied by means of angular-resolved photoelectron spectroscopy (ARUPS) in the photon energy range 20-38 eV. Four samples with different surface termination layers were grown and As-capped by molecular beam epitaxy (MBE). ARUPS measurements were performed on decapped samples with perfect c(4 × 4) reconstructed surfaces. An intensive surface state was, for the first time, observed below the top of the valence band. This surface state was found to shift with superlattices’ different surface termination in agreement with theoretical predictions.     

Interaction of O2 with Pd single crystals in the range 1-150 Torr: Surface morphology transformations

The interaction of O₂ with Pd single crystals including Pd(1 1 1), Pd(1 1 0) and Pd(1 0 0) in the pressure range 1-150 Torr was studied using scanning tunneling microscopy (STM). The Pd single crystal surface morphologies were determined by the oxidation conditions: O₂ pressure, exposure time and treatment temperature. Oxygen dissolution into Pd metal followed by the formation of bulk oxide was observed. The dissolution of oxygen resulted in the increase of the inter-planar spacing between the first two layers, 9-14% increase after an exposure of Pd(1 1 1) to 10-25 Torr O₂ at 600 K for 10 min, and 10-20% increase after exposing Pd(1 1 0) and Pd(1 0 0) to 1 Torr O₂ at 600 K for 10 min. Elongated or semi-spherical oxide agglomerates along the steps nucleated and grew on both Pd(1 1 0) and Pd(1 0 0) surfaces after oxidation in 5-25 Torr O₂ at 600 K. When bulk PdO was formed, the single crystal surface was covered with semi-spherical agglomerates 2-4 nm in size, which tended to aggregate to form a “cauliflower-like” structure. The single crystal surface area also increased during oxidation.     

Formation, stability and CO adsorption properties of PdAg/Pd(1 1 1) surface alloys

The formation, stability and CO adsorption properties of PdAg/Pd(1 1 1) surface alloys were investigated by X-ray photoelectron spectroscopy (XPS) and by adsorption of CO probe molecules, which was characterized by temperature-programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). The PdAg/Pd(1 1 1) surface alloys were prepared by annealing (partly) Ag film covered Pd(1 1 1) surfaces, where the Ag films were deposited at room temperature. Surface alloy formation leads to a modification of the electronic properties, evidenced by core-level shifts (CLSs) of both the Pd(3d) and Ag(3d) signal, with the extent of the CLSs depending on both initial Ag coverage and annealing temperature. The role of Ag pre-coverage and annealing temperature on surface alloy formation is elucidated. For a monolayer Ag covered Pd(1 1 1) surface, surface alloy formation starts at ∼450 K, and the resulting surface alloy is stable upon annealing at temperatures between 600 and 800 K. CO TPD and HREELS measurements demonstrate that at 120 K CO is exclusively adsorbed on Pd surface atoms/Pd sites of the bimetallic surfaces, and that the CO adsorption behavior is dominated by geometric ensemble effects, with adsorption on threefold hollow Pd₃ sites being more stable than on Pd₂ bridge sites and finally Pd₁ a-top sites.     

Low temperature CO induced growth of Pd supported on a monolayer silica film

Nucleation, growth and sintering of Pd deposited on an ultra-thin silica film were studied by scanning tunneling microscopy and infrared reflection absorption spectroscopy. No preferential nucleation of Pd on the silica surface was observed both at 90 and 300 K deposition. When adsorbed on Pd clusters formed at 90 K, CO causes a strong sintering effect even at this temperature. The results are rationalized on the basis of a high mobility of Pd carbonyl-like species on the silica film. At a given Pd coverage, the extent of CO induced sintering cannot be achieved by annealing in vacuum. In addition, vacuum sintering, which commences above 700 K, goes simultaneously with interdiffusion of Pd and support.     

Core-level spectroscopy of the Pd/W(1 1 0) interface: Evidence of long-range Pd-island-W interactions at submonolayer Pd coverages

We have measured W 4f_7/2 core-level photoemission spectra from W(1 1 0) in the presence of Pd overlayers for coverages up to ∼1 pseudomorphic monolayer (ML). At coverages close to 0.05 ML a striking change in the W core-level spectrum is observed, which we interpret as indicating a long-range lateral effect of 2D Pd islands upon the W electronic structure in both the first and second W layers. As the coverage increases the long-range effect weakens and finally vanishes near 0.85 ML. Above this coverage the W spectra are typical for a W-based bimetallic interface, with the first-layer W atoms exhibiting a small interfacial core-level shift (−95 ± 5 meV) compared to the bulk atoms.     

Initial oxidation of polycrystalline Permalloy surface

X-ray photoelectron spectroscopy (XPS) and work-function measurements were used in combination to investigate the initial steps of Permalloy (Ni₈₀Fe₂₀) oxidation at room temperature. They showed that, after oxygen saturation, the surface is covered by nickel oxide (NiO), nickel hydroxide (Ni(OH)₂) and iron oxides (Fe_xO_y), and there is no preferential oxidation. Iron oxidation proceeds through the formation of FeO (Fe²⁺) followed with Fe₂O₃ growth (Fe³⁺). The oxidation is governed by a dissociative Langmuir-type oxidation: the sticking coefficient is decreasing over oxygen exposure. Oxidation continues by oxygen dissolution into the first layers to form a nano-oxide of about 8 Å in thickness.     

Defects and Pd growth on the reduced SnO2(1 0 0) surface

Scanning tunneling microscopy experiments on a clean, reduced SnO₂(1 0 0)-(1 × 1) surface reveal surface defects with zero-, one-, and two-dimensions. Point defects consist of missing SnO/SnO₂ units. Line defects are probably crystallographic shear planes that extend to the surface and manifest themselves as rows of atoms, shifted half a unit cell along the [0 1 0] direction. Their ends act as preferential nucleation sites for the formation of Pd clusters upon vapor deposition. Areas of a more reduced surface phase, still with a (1 × 1) structure and a half-unit cell deep, form at [0 0 1]-oriented step edges.     

Quantitative measurements of Ge surface segregation during SiGe alloy growth

Ge segregation during the growth of Si_{1 − x}Ge_x alloys (x = 5, 10, 20, and 40%) was studied using X-ray photoelectron spectroscopy. The alloys were grown in thicknesses up to 20.0 nm at 500°C to measure quantitatively the amount of segregated surface Ge. The length of alloy needed to reach steady-state growth edge was found to decrease with increasing alloy concentration (4.8, 2.8, 2.4, and 2.0 nm, respectively). It was found that each alloy had a complete monolayer of Ge on the surface and an increasing amount of segregated Ge in the second layer (20, 55, 80, and 95%, respectively) during steady-state growth. An increase in the temperature of alloy growth (400–750°C) resulted in an increase in the leading edge of alloy growth but did not change the amount of segregated Ge during steady-state growth. We propose that film stress is responsible for the amount of Ge segregation.