Ultra-thin films of In on Pd(111) characterized by X-ray photoelectron diffraction (original) (raw)
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Journal of Molecular Catalysis A: Chemical, 2008
We report studies of ultra-thin films of Pd and Ni deposited on Ni(1 1 1) and Pd(1 1 1) surfaces, respectively, using X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and X-ray photoelectron diffraction (XPD). For a 1.5 ML Pd film deposited on Ni(1 1 1) at room temperature, XPS indicates that Pd grows in a layer-by-layer fashion and does not diffuse on the Ni substrate, whereas LEED exhibits a reconstructed pattern, which can be attributed to a distribution of bi-dimensional islands on the surface with a lateral lattice parameter different from that of . Annealing the film at 650 • C produces a (1 × 1) LEED pattern, which suggests Pd diffusion and alloy formation. By using a systematic XPD analysis, we were able to determine that Pd diffused at least to the fourth layer into the Ni(1 1 1) substrate in low concentrations (10-20%), and that 75% of the surface remained covered by Pd bi-dimensional islands. The complementary system, Ni on Pd(1 1 1), presented similar LEED and XPS results. The comparison between experimental and theoretical XPD results indicated that the surface was partially covered by Ni islands (50-60%), and the other part was formed by random Ni x Pd 100−x surface alloy.
Surface Science, 2019
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Highlights The formation of surface alloys Pd 2 Sn was characterized by XPS, LEED and XPD. XPD results indicated the formation of a corrugated bi-dimensional Pd2Sn surface alloy. Annealing the surface alloy at at 600 K produced a (3 3x)R30 o LEED pattern.
Surface composition and structure of palladium ultra-thin films deposited on Ni(111)
Surface Science, 2006
Ultra-thin palladium films deposited on the Ni(1 1 1) surface were characterized by X-ray photoelectron spectroscopy (XPS), lowenergy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD). For low coverage, LEED shows a (1 · 1) pattern similar to that of the substrate. For intermediate coverage, the LEED pattern displays extra spots around the main (1 · 1) spots, resembling a Moiré coincidence pattern, probably associated with the formation of Pd bi-dimensional islands oriented in different directions on the Ni(1 1 1) surface. The results obtained by XPS and XPD corroborate this finding. The LEED pattern displays this structure up to 500°C. Annealing at 650°C brings back the (1 · 1) pattern, which is associated with a Pd island coalescence and alloy formation by Pd diffusion in the first atomic layers of the Ni(1 1 1). In this paper we present a detailed study of this surface structure via a comparison between XPD experiment and theory.
Structure of ultrathin Pd films determined by low-energy electron microscopy and diffraction
New Journal of Physics, 2010
Palladium (Pd) films have been grown and characterized in situ by low-energy electron diffraction (LEED) and microscopy in two different regimes: ultrathin films 2-6 monolayers (ML) thick on Ru(0001), and ∼20 ML thick films on both Ru(0001) and W(110). The thinner films are grown at elevated temperature (750 K) and are lattice matched to the Ru(0001) substrate. The thicker films, deposited at room temperature and annealed to 880 K, have a relaxed in-plane lattice spacing. All the films present an fcc stacking sequence as determined by LEED intensity versus energy analysis. In all the films, there is hardly any expansion in the surface-layer interlayer spacing. Two types of twin-related stacking sequences of the Pd layers are found on each substrate. On W(110) the two fcc twin types can occur on a single substrate terrace. On Ru(0001) each substrate terrace has a single twin type and the twin boundaries replicate the substrate steps.
Surface composition and structure of nickel ultra-thin films deposited on Pd(111)
Journal of Electron Spectroscopy and Related Phenomena, 2007
Ultra-thin nickel films deposited on the Pd(111) surface were characterized by X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and X-ray photoelectron diffraction (XPD). Up to 3 ML coverage, a LEED (1 × 1) pattern with a diffuse background due to a random distribution of Ni atoms on the surface is observed. Annealing at 600 • C reduced the background drastically and sharp (1 × 1) spots appeared on the screen, but XPS showed no presence of nickel on the surface, indicating diffusion into the bulk. Annealing at 300 • C for 30 min yielded also a sharp (1 × 1) LEED pattern, and the XPS Ni/Pd intensity ratio decreased with annealing time. The comparison between experimental and theoretical XPD results indicated that the surface was covered partially by Ni islands and partially by a random Ni-Pd surface alloy.
Surface Science, 2002
The structure of electrochemically deposited Pd thin films on the Pt(1 1 1) electrode surface has been studied by a combination of cyclic voltammetry (CV) and in situ synchrotron surface X-ray scattering. The films were examined at successively higher levels of thickness, ranging from the sub-monolayer regime to the n ML (n > 2) regime. We found that on top of a pseudomorphic monolayer film, Pd forms three-dimensional islands which, in the n ML (n > 2) regime are described using a half-Lorentzian distribution of layer occupation. These islands provide the large number of Pd step sites that give rise to the peak in the CV at 0.3 V. The deposition of Pd onto Pt(1 1 1), therefore, proceeds via a pseudomorphic Stranski-Krastanov growth mode.
Single crystal growth of the intermetallic compound InPd
Journal of Crystal Growth, 2014
Quite recently intermetallic compounds have been investigated as catalysts for heterogeneous catalysis as they can be highly active regarding a specific reaction and often show advantages in selectivity and long-term stability. The intermetallic phase InPd (CsCl structure type) is considered to be a possible catalyst for methanol steam reforming. Single crystals are needed to study the basic processes of catalysis. Using the Czochralski technique, InPd single crystals were grown from In-rich solutions as to reduce the vapor pressure of In. The crystals show some unusual rough surface morphology and gradients concerning the main components composition. By adjusting the growth parameters like growth temperature and growth rate we succeeded to get inclusion-free single crystals.
Physical properties of the InPd intermetallic catalyst
The intermetallic phase InPd is a candidate material for the use as a catalyst in the methanol steam reforming process. To study the connection between the catalytic properties of the surface and the structural and electronic properties of the bulk, we have grown single crystals of the InPd phase by the Czochralski method and determined their electronic, thermal, magnetic and hydrogen-absorption properties. By growing crystals from a high-temperature solution, we could crystallize a slightly offstoichiometric In-rich composition In 1.04 Pd 0.96 , which contained a significant amount of constitutional defects in the lattice (Pd vacancies on the Pd sublattice) to retain the CsCl-type structure. The strongly inhomogeneously broadened 115 In NMR spectrum and the high residual (T / 0) electrical resistivity confirmed the presence of constitutional defects. Single crystals of InPd do not absorb hydrogen, as requested for a good hydrogenation catalyst material. Calculated electronic density of states (DOS) shows large contribution of Pd(d) states at the Fermi level. Application of the electron localizability indicator reveals ionic and multi-centre InePd interactions stabilizing the crystal structure. The electrical and thermal conductivities of InPd show metallic character, whereas the thermoelectric power and the Hall coefficient both show positive sign, revealing that InPd is a predominant hole-type conductor. The calculated electronic DOS at the Fermi energy is in a good agreement with the experimental value determined from the low-temperature specific heat. Magnetic measurements have shown that InPd is a diamagnet. All results are compared to the chemically related intermetallic compound GaPd. The active esite-isolation concept for increased catalytic selectivity is discussed in relation to the InPd and GaPd structures.
Characterization of Ultra-Thin Films of Pd Deposited on Au(111)
Topics in Catalysis, 2011
Ultra-thin films (1 and 3 monolayers) of Pd were deposited on the Au(111) surface and then characterized by X-ray photoelectron spectroscopy (XPS), X-ray excited Auger spectroscopy (XAES), low-energy electron diffraction (LEED), and X-ray photoelectron diffraction (XPD). For the 1 ML Pd film annealed at 450°C, XPS and XAES results indicated that Pd had diffused into the Au substrate. For the 3 ML Pd film deposited at room temperature, the comparison between experimental and theoretical XPD results indicated approximately 30% of the surface was formed by 2 ML Au layers, and 70% of the surface, by 1 ML Au layers.
Electronic properties of Sn/Pd intermetallic compounds on Pd(110)
Surface Science, 2005
We have studied the Sn/Pd(1 1 0) adsorption system by synchrotron radiation photoelectron spectroscopy and lowenergy electron diffraction (LEED). For room temperature evaporation, two surface reconstructions were observed: c(2 · 2) and (3 · 1), corresponding to about 0.5 ML and 0.75 ML of Sn adlayer coverage. The Pd 3d and Sn 4d core levels as well as valence band spectra indicate a strong chemical interaction between Sn and Pd, and the formation of an intermetallic interface. Structural models are proposed for both of these phases based on the photoemission and CO adsorption results. We show that at coverage higher than 0.7 ML, tin is alloyed with the Pd crystal forming a subsurface layer of Pd-Sn intermetallic compound of stoichiometry which varies with tin coverage. CO adsorption occurs only at low temperature (120 K) and depends on the Sn coverage and reconstruction of the Pd(1 1 0) surface. We estimate the CO adsorption energy for the c(2 · 2)-and (3 · 1)-Sn/Pd(1 1 0) surfaces to be reduced by 40% compared to the clean palladium (1 1 0) surface.