Surface Science

1. Cysteine and cystine adsorption on FeS2(100)

   Publication date: August 2018
   Source:Surface Science, Volume 674
   
   Author(s): Teppei Suzuki, Taka-aki Yano, Masahiko Hara, Toshikazu Ebisuzaki
   
   Iron pyrite (FeS2) is the most abundant metal sulfide on Earth. Owing to its reactivity and catalytic activity, pyrite has been studied in various research fields such as surface science, geochemistry, and prebiotic chemistry. Importantly, native iron–sulfur clusters are typically coordinated by cysteinyl ligands of iron–sulfur proteins. In the present paper, we study the adsorption of l-cysteine and its oxidized dimer, l-cystine, on the FeS2 surface, using electronic structure calculations based density functional theory and Raman spectroscopy measurements. Our calculations suggest that sulfur-deficient surfaces play an important role in the adsorption of cysteine and cystine. In the thiol headgroup adsorption on the sulfur-vacancy site, dissociative adsorption is found to be energetically favorable compared with molecular adsorption. In addition, the calculations indicate that, in the cystine adsorption on the defective surface under vacuum conditions, the formation of the S–Fe bond is energetically favorable compared with molecular adsorption. Raman spectroscopic measurements suggest the formation of cystine molecules through the S–S bond on the pyrite surface in aqueous solution. Our results might have implications for chemical evolution at mineral surfaces on the early Earth and the origin of iron–sulfur proteins, which are believed to be one of the most ancient families of proteins.
   

   Graphical abstract

   
   
   
   
   
   

2. An evaluation of the kinematic approximation in helium atom scattering using wavepacket calculations

   Publication date: Available online 27 April 2018
   Source:Surface Science
   
   Author(s): Andrew R. Alderwick, Andrew P. Jardine, William Allison, John Ellis
   
   We use 2-D wavepacket calculations to examine the scattering of helium atoms from dynamic assemblies of surface adsorbates, and in particular to explore the validity of the widely used kinematic scattering approximation. The wavepacket calculations give exact results for quasi-elastic scattering that are closely analogous to time-of-flight (TOF) experiments and they are analysed as such. A scattering potential is chosen to represent 8 meV helium atoms scattering from sodium atoms adsorbed on a Cu(001) surface and the adsorbates in the model move according to an independent Langevin equation. The energy broadening in the quasi-elastic scattering is obtained as a function of parallel momentum transfer and compared with the corresponding results using the kinematic scattering approximation. Under most circumstances the kinematic approximation and the more accurate wavepacket method are in good agreement; however, there are cases where the two methods give different results. We relate these differences to pathological features in the scattering form-factor.
   

   Graphical abstract

   
   
   
   
   
   

3. Growth morphologies of dihydro-tetraaza-acenes on c-plane sapphire

   Publication date: Available online 13 March 2018
   Source:Surface Science
   
   Author(s): Aleksandar Matković, Aydan Çiçek, Markus Kratzer, Benjamin Kaufmann, Anthony Thomas, Zhongrui Chen, Olivier Siri, Conrad Becker, Christian Teichert
   
   Dihydro-tetraaza-acenes are promising candidates for future applications in organic electronics, since these molecules form crystals through an interplay between H-bonding, dipolar, and van der Waals interactions. As a result, densely packed $\pi -\pi$ structures – favorable for charge transport – are obtained, with exceptional stability under ambient conditions. This study investigates growth morphologies of dihydro-tetraaza-pentacene and dihydro-tetraaza-heptacene on vicinal c-plane sapphire. Submonolayers and thin films are grown using hot wall epitaxy, and the structures are investigated ex-situ by atomic force microscopy. Molecular arrangement, nucleation densities, sizes, shapes, and stability of the crystallites are analyzed as a function of the substrate temperature. The two molecular species were found to assume a different orientation of the molecules with respect to the substrate. An activation energy of (1.23  ±  0.12) eV was found for the nucleation of dihydro-tetraaza-heptacene islands (composed of upright standing molecules), while (1.16  ±  0.25) eV was obtained for dihydro-tetraaza-pentacene needles (composed of lying molecules). The observed disparity in the temperature dependent nucleation densities of the two molecular species is attributed to the different thermalization pathways of the impinging molecules.
   

   Graphical abstract

   
   
   
   
   
   

4. A structural investigation of the interaction of oxalic acid with Cu(110)

   Publication date: February 2018
   Source:Surface Science, Volume 668
   
   Author(s): T.W. White, D.A. Duncan, S. Fortuna, Y.-L. Wang, B. Moreton, T.-L. Lee, P. Blowey, G. Costantini, D.P. Woodruff
   
   The interaction of oxalic acid with the Cu(110) surface has been investigated by a combination of scanning tunnelling microscopy (STM), low energy electron diffraction (LEED), soft X-ray photoelectron spectroscopy (SXPS), near-edge X-ray absorption fine structure (NEXAFS) and scanned-energy mode photoelectron diffraction (PhD), and density functional theory (DFT). O 1s SXPS and O K-edge NEXAFS show that at high coverages a singly deprotonated monooxalate is formed with its molecular plane perpendicular to the surface and lying in the $\left[1\overline{1}0\right]$ azimuth, while at low coverage a doubly-deprotonated dioxalate is formed with its molecular plane parallel to the surface. STM, LEED and SXPS show the dioxalate to form a (3 × 2) ordered phase with a coverage of 1/6 ML. O 1s PhD modulation spectra for the monooxalate phase are found to be simulated by a geometry in which the carboxylate O atoms occupy near-atop sites on nearest-neighbour surface Cu atoms in $\left[1\overline{1}0\right]$ rows, with a CuO bondlength of 2.00 ± 0.04 Å. STM images of the (3 × 2) phase show some centred molecules attributed to adsorption on second-layer Cu atoms below missing [001] rows of surface Cu atoms, while DFT calculations show adsorption on a (3 × 2) missing row surface (with every third [001] Cu surface row removed) is favoured over adsorption on the unreconstructed surface. O 1s PhD data from dioxalate is best fitted by a structure similar to that found by DFT to have the lowest energy, although there are some significant differences in intramolecular bondlengths.
   

   Graphical abstract

   
   
   
   
   
   

5. Role of step edges on the structure formation of α-6T on Ag(441)

   Publication date: January 2018
   Source:Surface Science, Volume 667
   
   Author(s): Thorsten Wagner, Daniel Roman Fritz, Zdena Rudolfová, Peter Zeppenfeld
   
   Controlling the orientation of organic molecules on surfaces is important in order to tune the physical properties of the organic thin films and, thereby, increase the performance of organic thin film devices. Here, we present a scanning tunneling microscopy (STM) and photoelectron emission microscopy (PEEM) study of the deposition of the organic dye pigment α-sexithiophene (α-6T) on the vicinal Ag(441) surface. In the presence of the steps on the Ag(441) surface, the α-6T molecules exclusively align parallel to the step edges oriented along the [1$\overline{1}$0]-direction of the substrate. The STM results further reveal that the adsorption of the α-6T molecules is accompanied by various restructuring of the substrate surface: Initially, the molecules prefer the Ag(551) building blocks of the Ag(441) surface. The Ag(551) termination of the terraces is then changed to a predominately Ag(331) one upon completion of the first α-6T monolayer. When closing the two layer thick wetting layer, the original ratio of Ag(331) and Ag(551) building blocks ( ≈ 1:1) is recovered, but a phase separation into microfacets, which are composed either of Ag(331) or of Ag(551) building blocks, is found.
   

   Graphical abstract

   
   
   
   
   
   

6. On the microscopic structure of a nominal Ag(441) surface

   Publication date: July 2017
   Source:Surface Science, Volume 661
   
   Author(s): Thorsten Wagner, Daniel Roman Fritz, Robert Zimmerleiter, Peter Zeppenfeld
   
   Regularly stepped (vicinal) surfaces provide a convenient means to control the number of defects of a surface. They can easily be prepared by a slight miscut of a low index surface. In the case of an fcc($nn1$) surface with small integer n, it is even expected that the large number of steps will dominate the surface properties. We are the first to study the Ag(441) surface with a combination of scanning tunneling microscopy (STM) and high resolution electron diffraction (SPA-LEED). The surface is found to consist of different building blocks, which can be either (331) or (551) microfacets. To unravel the actual morphology, we carried out simulations of the reciprocal space maps (RSMs) in the framework of the simple kinematic approximation.
   

   Graphical abstract

   Highlights

   fx1

   
   
   
   
   
   

7. Pd segregation to the surface of Au on Pd(111) and on Pd/TiO2(110)

   Publication date: February 2017
   Source:Surface Science, Volume 656
   
   Author(s): Ryan Sharpe, Jon Counsell, Michael Bowker
   
   The interaction of Au and Pd in bimetallic systems is important in a number of areas of technology, especially catalysis. In order to investigate the segregation behaviour in such systems, the interaction of Pd and Au was investigated by surface science methods. In two separate sets of experiments, Au was deposited onto a Pd(111) single crystal, and Pd and Au were sequentially deposited onto TiO2(110), all in ultra-high vacuum using metal vapour deposition. Heating Au on Pd/TiO2(110) to 773K resulted in the loss of the Au signal in the LEIS, whilst still remaining present in the XPS, due to segregation of Pd to the surface and the formation of a Au-Pd core-shell structure. It is likely that this is due to alloying of Au with the Pd and surface dominance of that alloy by Pd. The Au:Pd XPS peak area ratio is found to substantially decrease on annealing Au/Pd(111) above 773K, corresponding with a large increase in the CO sticking probability to that for clean Pd(111). This further indicates that Au diffuses into the bulk of Pd on annealing to temperatures above 773K. It therefore appears that Au prefers to be in the bulk in these systems, reflecting the exothermicity of alloy formation.
   

   Graphical abstract

   
   
   
   
   
   

8. Adsorption of formate species on Cu(h,k,l) low index surfaces

   Publication date: November 2016
   Source:Surface Science, Volume 653
   
   Author(s): Arunabhiram Chutia, Ian P. Silverwood, Matthew R. Farrow, David O. Scanlon, Peter P. Wells, Michael Bowker, Stewart F. Parker, C. Richard A. Catlow
   
   We report a density functional theory study on the relative stability of formate species on Cu(h,k,l) low index surfaces using a range of exchange-correlation functionals. We find that these functionals predict similar geometries for the formate molecule adsorbed on the Cu surface. A comparison of the calculated vibrational transition energies of a perpendicular configuration of formate on Cu surface shows an excellent agreement with the experimental spectrum obtained from inelastic neutron spectroscopy. From the calculations on adsorption energy we find that formate is most stable on the Cu(110) surface as compared to Cu(111) and Cu(100) surfaces. Bader analysis shows that this feature could be related to the higher charge transfer from the Cu(110) surface and optimum charge density at the interfacial region due to bidirectional electron transfer between the formate and the Cu surface. Analysis of the partial density of states finds that in the –5.5eV to –4.0eV region, hybridization between O p and the non-axial Cu d yz and d xz orbitals takes place on the Cu(110) surface, which is energetically more favourable than on the other surfaces.
   

   Graphical abstract

   
   
   
   
   
   

9. STM study of the preparation of clean Ta(110) and the subsequent growth of two-dimensional Fe islands

   Publication date: November 2016
   Source:Surface Science, Volume 653
   
   Author(s): T. Eelbo, V.I. Zdravkov, R. Wiesendanger
   
   This report deals with the preparation of a clean Ta(110) surface, investigated by means of scanning tunneling microscopy/spectroscopy as well as by low-energy electron diffraction and Auger electron spectroscopy. The surface initially exhibits a surface reconstruction induced by oxygen contamination. This reconstruction can be removed by annealing at high temperatures under ultrahigh vacuum conditions. The reconstruction-free surface reveals a surface resonance at a bias voltage of about −500mV. The stages of the transformation are presented and discussed. In a next step, Fe islands were grown on top of Ta(110) and investigated subsequently. An intermixing regime was identified for annealing temperatures of (550–590)K.
   

   Graphical abstract

   
   
   
   
   
   

10. Density functional theory study of the interaction of H2O, CO2 and CO with the ZrO2 (111), Ni/ZrO2 (111), YSZ (111) and Ni/YSZ (111) surfaces

    Publication date: November 2016
    Source:Surface Science, Volume 653
    
    Author(s): Abdelaziz Cadi-Essadek, Alberto Roldan, Nora H. de Leeuw
    
    The triple phase boundary (TPB), where the gas phase, Ni particles and the yttria-stabilised zirconia (YSZ) surface meet, plays a significant role in the performance of solid oxide fuel cells (SOFC). Indeed, the key reactions take place at the TPB, where molecules such as H2O, CO2 and CO interact and react. We have systematically studied the interaction of H2O, CO2 and CO with the dominant surfaces of four materials that are relevant to SOFC, i.e. ZrO2(111), Ni/ZrO2(111), YSZ(111) and Ni/YSZ(111) of cubic ZrO2 stabilized with 9% of yttria (Y2O3). The study employed spin polarized density functional theory (DFT), taking into account the long-range dispersion forces. We have investigated up to five initial adsorption sites for the three molecules and have identified the geometries and electronic structures of the most stable adsorption configurations. We have also analysed the vibrational modes of the three molecules in the gas phase and compared them with the adsorbed molecules. A decrease of the wavenumbers of the vibrational modes for the three adsorbed molecules was observed, confirming the influence of the surface on the molecules' intra-molecular bonds. These results are in line with the important role of Ni in this system, in particular for the CO adsorption and activation.
    

    Graphical abstract

    
    
    
    
    
    

11. Coverage-dependent two-photon photoexcitation at the H2O/TiO2 interface

    Publication date: October 2016
    Source:Surface Science, Volume 652
    
    Author(s): D.T. Payne, Y. Zhang, C.L. Pang, H.H. Fielding, G. Thornton
    
    Excited electrons and holes are crucial for redox reactions on metal oxide surfaces. However, precise details of this charge transfer process are not known. We report two-photon photoemission (hν =3.23eV) measurements of rutile TiO2(110) as a function of exposure to water below room temperature. The two-photon resonance associated with bridging hydroxyls is enhanced following water exposure, reaching a maximum at a nominal coverage of one monolayer. Higher coverages attenuate the observed resonance. Ultraviolet photoemission spectroscopy (hν =21.22eV) of the initial, band gap states shows little change up to one monolayer water coverage. It is likely that the enhancement arises from dissociation within the adsorbed water monolayer, although other mechanisms cannot be excluded.
    

    Graphical abstract

    
    
    
    
    
    

12. Changing interfaces: Photoluminescent ZnO nanoparticle powders in different aqueous environments

    Publication date: October 2016
    Source:Surface Science, Volume 652
    
    Author(s): Krisztina Kocsis, Matthias Niedermaier, Johannes Bernardi, Thomas Berger, Oliver Diwald
    
    We transformed vapor phase grown ZnO nanoparticle powders into aqueous ZnO nanoparticle dispersions and studied the impact of associated microstructure and interface property changes on their spectroscopic properties. With photoluminescence (PL) spectroscopy, we probed oxygen interstitials Oi ²⁻ in the near surface region and tracked their specific PL emission response at hvEM =2.1eV during the controlled conversion of the solid–vacuum into the solid–liquid interface. While oxygen adsorption via the gas phase does affect the intensity of the PL emission bands, the O2 contact with ZnO nanoparticles across the solid–liquid interface does not. Moreover, we found that the near band edge emission feature at hvEM =3.2eV gains relative intensity with regard to the PL emission features in the visible light region. Searching for potential PL indicators that are specific to early stages of particle dissolution, we addressed for aqueous ZnO nanoparticle dispersions the effect of formic acid adsorption. In the absence of related spectroscopic features, we were able to consistently track ZnO nanoparticle dissolution and the concomitant formation of solvated Zinc formate species by means of PL and FT-IR spectroscopy, dynamic light scattering, and zeta potential measurements. For a more consistent and robust assessment of nanoparticle properties in different continuous phases, we discuss characterization challenges and potential pitfalls that arise upon replacing the solid–gas with the solid–liquid interface.
    

    Graphical abstract

    
    
    
    
    
    

13. On the nucleation and initial film growth of rod-like organic molecules

    Publication date: October 2016
    Source:Surface Science, Volume 652
    
    Author(s): Adolf Winkler
    
    In this article, some fundamental topics related to the initial steps of organic film growth are reviewed. General conclusions will be drawn based on experimental results obtained for the film formation of oligophenylene and pentacene molecules on gold and mica substrates. Thin films were prepared via physical vapor deposition under ultrahigh-vacuum conditions and characterized in-situ mainly by thermal desorption spectroscopy, and ex-situ by X-ray diffraction and atomic force microscopy. In this short review article the following topics will be discussed: What are the necessary conditions to form island-like films which are either composed of flat-lying or of standing molecules? Does a wetting layer exist below and in between the islands? What is the reason behind the occasionally observed bimodal island size distribution? Can one describe the nucleation process with the diffusion-limited aggregation model? Do the impinging molecules directly adsorb on the surface or rather via a hot-precursor state? Finally, it will be described how the critical island size can be determined by an independent measurement of the deposition rate dependence of the island density and the capture-zone distribution via a universal relationship.
    

    Graphical abstract

    
    
    
    
    
    

14. Adjusting island density and morphology of the SrTiO3(110)-(4×1) surface: Pulsed laser deposition combined with scanning tunneling microscopy

    Publication date: September 2016
    Source:Surface Science, Volume 651
    
    Author(s): Stefan Gerhold, Michele Riva, Bilge Yildiz, Michael Schmid, Ulrike Diebold
    
    The first stages of homoepitaxial growth of the (4×1) reconstructed surface of SrTiO3(110) are probed by a combination of pulsed laser deposition (PLD) with in-situ reflection high energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). Considerations of interfacing high-pressure PLD growth with ultra-high-vacuum surface characterization methods are discussed, and the experimental setup and procedures are described in detail. The relation between RHEED intensity oscillations and ideal layer-by-layer growth is confirmed by analysis of STM images acquired after deposition of sub-monolayer amounts of SrTiO3. For a quantitative agreement between RHEED and STM results one has to take into account two interfaces: the steps at the circumference of islands, as well as the borders between two different reconstruction phases on the islands themselves. Analysis of STM images acquired after one single laser shot reveals an exponential decrease of the island density with increasing substrate temperature. This behavior is also directly visible from the temperature dependence of the relaxation times of the RHEED intensity. Moreover, the aspect ratio of islands changes considerably with temperature. The growth mode depends on the laser pulse repetition rate, and can be tuned from predominantly layer-by-layer to the step-flow growth regime.
    

    Graphical abstract

    
    
    
    
    
    

15. Adsorption of isophorone and trimethyl-cyclohexanone on Pd(111): A combination of infrared reflection absorption spectroscopy and density functional theory studies

    Publication date: August 2016
    Source:Surface Science, Volume 650
    
    Author(s): Karl-Heinz Dostert, Casey P. O'Brien, Wei Liu, Wiebke Riedel, Aditya Savara, Alexandre Tkatchenko, Swetlana Schauermann, Hans-Joachim Freund
    
    Understanding the interaction of α,β-unsaturated carbonyl compounds with late transition metals is a key prerequisite for rational design of new catalysts with desired selectivity towards C=C or C=O bond hydrogenation. The interaction of the α,β-unsaturated ketone isophorone and the saturated ketone TMCH (3,3,5-trimethylcyclohexanone) with Pd(111) was investigated in this study as a prototypical system. Infrared reflection–absorption spectroscopy (IRAS) and density functional theory calculations including van der Waals interactions (DFT+vdW^surf) were combined to form detailed assignments of IR vibrational modes in the range from 3000cm⁻¹ to 1000cm⁻¹ in order to obtain information on the binding of isophorone and TMCH to Pd(111) as well as to study the effect of co-adsorbed hydrogen. IRAS measurements were performed with deuterium-labeled (d 5-) isophorone, in addition to unlabeled isophorone and unlabeled TMCH. Experimentally observed IR absorption features and calculated vibrational frequencies indicate that isophorone and TMCH molecules in multilayers have a mostly unperturbed structure with random orientation. At sub-monolayer coverages, strong perturbation and preferred orientations of the adsorbates were found. At low coverage, isophorone interacts strongly with Pd(111) and adsorbs in a flat-lying geometry with the C=C and C=O bonds parallel, and a CH3 group perpendicular, to the surface. At intermediate sub-monolayer coverage, the C=C bond is strongly tilted, while the C=O bond remains flat-lying, which indicates a prominent perturbation of the conjugated π system. Pre-adsorbed hydrogen leads to significant changes in the adsorption geometry of isophorone, which suggests a weakening of its binding to Pd(111). At low coverage, the structure of the CH3 groups seems to be mostly unperturbed on the hydrogen pre-covered surface. With increasing coverage, a conservation of the in-plane geometry of the conjugated π system was observed in the presence of hydrogen. In contrast to isophorone, TMCH adsorbs in a strongly tilted geometry independent of the surface coverage. At low coverage, an adsorbate with a strongly distorted C=O bond is formed. With increasing exposure, species with a less perturbed C=O group appear.
    

    Graphical abstract

    
    
    
    
    
    

16. XPS and STM studies of the oxidation of hydrogen chloride at Cu(100) surfaces

    Publication date: August 2016
    Source:Surface Science, Volume 650
    
    Author(s): Hatem Altass, Albert F. Carley, Philip R. Davies, Robert J. Davies
    
    The dissociative chemisorption of HCl on clean and oxidized Cu(100) surfaces has been investigated using x-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Whereas the dissociation of HCl at the clean surface is limited to the formation of a (√2×√2)-R45° Cl(a) monolayer, the presence of surface oxygen removes this barrier, leading to chlorine coverages up to twice that obtained at the clean surface. Additional features in the STM images that appear at these coverages are tentatively assigned to the nucleation of CuCl islands. The rate of reaction of the HCl was slightly higher on the oxidized surface but unaffected by the initial oxygen concentration or the availability of clean copper sites. Of the two distinct domains of adsorbed oxygen identified at room temperature on the Cu(100) surfaces, the (√2×√2)-R45° structure reacts slightly faster with HCl than the missing row (√2×2√2)-R45° O(a) structure. The results address the first stages in the formation of a copper chloride and present an interesting comparison with the HCl/O(a) reaction at Cu(110) surfaces, where oxygen also increased the extent of HCl reactions. The results emphasize the importance of the exothermic reaction to form water in the HCl/O(a) reaction on copper.
    

    Graphical abstract

    
    
    
    
    
    

17. Atmospheric corrosion of Cu, Zn, and Cu–Zn alloys protected by self-assembled monolayers of alkanethiols

    Publication date: June 2016
    Source:Surface Science, Volume 648
    
    Author(s): Saman Hosseinpour, Mattias Forslund, C. Magnus Johnson, Jinshan Pan, Christofer Leygraf
    
    In this article results from earlier studies have been compiled in order to compare the protection efficiency of self-assembled monolayers (SAM) of alkanethiols for copper, zinc, and copper–zinc alloys exposed to accelerated indoor atmospheric corrosion conditions. The results are based on a combination of surface spectroscopy and microscopy techniques. The protection efficiency of investigated SAMs increases with chain length which is attributed to transport hindrance of the corrosion stimulators in the atmospheric environment, water, oxygen and formic acid, towards the copper surface. The transport hindrance is selective and results in different corrosion products on bare and on protected copper. Initially the molecular structure of SAMs on copper is well ordered, but the ordering is reduced with exposure time. Octadecanethiol (ODT), the longest alkanethiol investigated, protects copper significantly better than zinc, which may be attributed to the higher bond strength of Cu–S than of Zn–S. Despite these differences, the corrosion protection efficiency of ODT for the single phase Cu20Zn brass alloy is equally efficient as for copper, but significantly less for the heterogeneous double phase Cu40Zn brass alloy.
    

    Graphical abstract

    
    
    
    
    
    

18. An ex situ study of the adsorption of calcium phosphate from solution onto TiO2(110) and Al2O3(0001)

    Publication date: April 2016
    Source:Surface Science, Volume 646
    
    Author(s): M. Murphy, M.S. Walczak, H. Hussain, M.J. Acres, C.A. Muryn, A.G. Thomas, N. Silikas, R. Lindsay
    
    Ex situ atomic force microscopy and x-ray photoelectron spectroscopy are employed to characterise the adsorption of calcium phosphate from an aqueous solution of CaCl2.H2O and KH2PO4 onto rutile-TiO2(110) and α-Al2O3(0001). Prior to immersion, the substrates underwent wet chemical preparation to produce well-defined surfaces. Calcium phosphate adsorption is observed on both rutile-TiO2(110) and α-Al2O3(0001), with atomic force microscopy images indicating island-type growth. In contrast to other studies on less well-defined TiO2 and Al2O3 substrates, the induction period for calcium phosphate nucleation appears to be comparable on these two surfaces.
    

    Graphical abstract

    
    
    
    
    
    

19. A versatile instrument for ambient pressure x-ray photoelectron spectroscopy: The Lund cell approach

    Publication date: April 2016
    Source:Surface Science, Volume 646
    
    Author(s): Jan Knudsen, Jesper N. Andersen, Joachim Schnadt
    
    During the past one and a half decades ambient pressure x-ray photoelectron spectroscopy (APXPS) has grown to become a mature technique for the real-time investigation of both solid and liquid surfaces in the presence of a gas or vapour phase. APXPS has been or is being implemented at most major synchrotron radiation facilities and in quite a large number of home laboratories. While most APXPS instruments operate using a standard vacuum chamber as the sample environment, more recently new instruments have been developed which focus on the possibility of custom-designed sample environments with exchangeable ambient pressure cells (AP cells). A particular kind of AP cell solution has been driven by the development of the APXPS instrument for the SPECIES beamline of the MAX IV Laboratory: the solution makes use of a moveable AP cell which for APXPS measurements is docked to the electron energy analyser inside the ultrahigh vacuum instrument. Only the inner volume of the AP cell is filled with gas, while the surrounding vacuum chamber remains under vacuum conditions. The design enables the direct connection of UHV experiments to APXPS experiments, and the swift exchange of AP cells allows different custom-designed sample environments. Moreover, the AP cell design allows the gas-filled inner volume to remain small, which is highly beneficial for experiments in which fast gas exchange is required. Here we report on the design of several AP cells and use a number of cases to exemplify the utility of our approach.
    

    Graphical abstract

    
    
    
    
    
    

20. Local reaction kinetics by imaging

    Publication date: January 2016
    Source:Surface Science, Volume 643
    
    Author(s): Yuri Suchorski, Günther Rupprechter
    
    In the present contribution we present an overview of our recent studies using the “kinetics by imaging” approach for CO oxidation on heterogeneous model systems. The method is based on the correlation of the PEEM image intensity with catalytic activity: scaled down to the μm-sized surface regions, such correlation allows simultaneous local kinetic measurements on differently oriented individual domains of a polycrystalline metal-foil, including the construction of local kinetic phase diagrams. This allows spatially- and component-resolved kinetic studies and, e.g., a direct comparison of inherent catalytic properties of Pt(hkl)- and Pd(hkl)-domains or supported μm-sized Pd-powder agglomerates, studies of the local catalytic ignition and the role of defects and grain boundaries in the local reaction kinetics.
    

    Graphical abstract