PHOIBOS

In this article, we present the results of an investigation into chemical processes which take place at the surface of SnO₂-based chemiresistor in various atmospheres (1 mbar of argon, 1 mbar of oxygen, 0.1 mbar of ethanol, 1 mbar of oxygen + 0.1 mbar of ethanol mixture) at common working temperatures (450 and 573 K). The key method for nanoscale analysis was the Near Ambient Pressure X-ray Photoelectron Spectroscopy. In parallel the resistance and DC-responses of SnO₂ layer were in-situ monitored providing information about macroscale processes during gas sensing. The change in the sensor resistance after exposure to the ethanol-containing atmospheres together with the disappearance of the band bending effect and observation of different carbonaceous groups including ethoxy groups and acetaldehyde molecules on the sensor surface in the XPS spectra supported the theory of chemical interaction of ethanol with the chemisorbed oxygen. The NAP-XPS spectra also showed that the nanostructured tin oxide is partially reduced even after being exposed to pure oxygen at 573 K. Exposing this surface to the mixture of O₂/EtOH did not significantly increase the surface reduction probably due to slow kinetics of the ethanol reduction process and fast kinetics of the oxygen re-oxidation process. However, it was demonstrated that the surface of sensor is slowly getting contaminated by carbon.

M. Vorokhta, I. Khalakhan, M. Vondráček, D. Tomeček, M. Vorokhta, E. Marešová, J. Nováková, J. Vlček, P. Fitl, M. Novotný, P. Hozák, J. Lančok, M. Vrňata, I. Matolínová, and V. Matolín
Surface Science, Volume 677, November 2018, Pages 284-290

Hybrid plasmonic devices involve a nanostructured metal supporting localized surface plasmons to amplify light–matter interaction, and a non-plasmonic material to functionalize charge excitations. Application-relevant epitaxial heterostructures, however, give rise to ballistic ultrafast dynamics that challenge the conventional semiclassical understanding of unidirectional nanometal-to-substrate energy transfer. Epitaxial Au nanoislands are studied on WSe₂ with time- and angle-resolved photoemission spectroscopy and femtosecond electron diffraction: this combination of techniques resolves material, energy, and momentum of charge-carriers and phonons excited in the heterostructure. A strong non-linear plasmon–exciton interaction that transfers the energy of sub-bandgap photons very efficiently to the semiconductor is observed, leaving the metal cold until non-radiative exciton recombination heats the nanoparticles on hundreds of femtoseconds timescales. The results resolve a multi-directional energy exchange on timescales shorter than the electronic thermalization of the nanometal. Electron–phonon coupling and diffusive charge-transfer determine the subsequent energy flow. This complex dynamics opens perspectives for optoelectronic and photocatalytic applications, while providing a constraining experimental testbed for state-of-the-art modelling.

Tommaso Pincelli, Thomas Vasileiadis, Shuo Dong, Samuel Beaulieu, Maciej Dendzik,
Daniela Zahn, Sang-Eun Lee, Hélène Seiler, Yingpeng Qi, R. Patrick Xian, Julian Maklar,
Emerson Coy, Niclas S. Mueller, Yu Okamura, Stephanie Reich, Martin Wolf,
Laurenz
Advanced Materials 2023, 2209100

Angle-resolved photoelectron spectroscopy is an extremely powerful probe of materials to access the occupied electronic structure with energy and momentum resolution. However, it remains blind to those dynamic states above the Fermi level that determine technologically relevant transport properties. In this work we extend band structure mapping into the unoccupied states and across the entire Brillouin zone by using a state-of-the-art high repetition rate, extreme ultraviolet femtosecond light source to probe optically excited samples. The wide-ranging applicability and power of this approach are demonstrated by measurements on the two-dimensional semiconductor WSe2, where the energy-momentum dispersion of valence and conduction bands are observed in a single experiment. This provides a direct momentum-resolved view, not only on the complete out-of-equilibrium band gap but also on its renormalization induced by electronic screening. Our work establishes a benchmark for measuring the band structure of materials, with direct access to the energy-momentum dispersion of the excited-state spectral function.

M. Puppin, C. W. Nicholson, C. Monney, Y. Deng, R. P. Xian, J. Feldl, S. Dong, A. Dominguez, H. Hübener, A. Rubio, M. Wolf, L. Rettig, and R. Ernstorfer
Phys. Rev. B 105, 075417

The exciton, a bound state of an electron and a hole, is a fundamental quasiparticle induced by coherent light–matter interactions in semiconductors. When the electrons and holes are in distinct spatial locations, spatially indirect excitons are formed with a much longer lifetime and a higher condensation temperature. One of the ultimate frontiers in this field is to create long-lived excitonic topological quasiparticles by driving exciton states with topological properties, to simultaneously leverage both topological effects and correlation. Here we reveal the existence of a transient excitonic topological surface state (TSS) in a topological insulator, Bi₂Te₃. By using time-, spin- and angle-resolved photoemission spectroscopy, we directly follow the formation of a long-lived exciton state as revealed by an intensity buildup below the bulk-TSS mixing point and an anomalous band renormalization of the continuously connected TSS in the momentum space. Such a state inherits the spin-polarization of the TSS and is spatially indirect along the z axis, as it couples photoinduced surface electrons and bulk holes in the same momentum range, which ultimately leads to an excitonic state of the TSS. These results establish Bi₂Te₃ as a possible candidate for the excitonic condensation of TSSs and, in general, opens up a new paradigm for exploring the momentum space emergence of other spatially indirect excitons, such as moiré and quantum well excitons, and for the study of non-equilibrium many-body topological physics.

Ryo Mori, Samuel Ciocys, Kazuaki Takasan, Ping Ai, Kayla Currier,
Takahiro Morimoto, Joel E. Moore, Alessandra Lanzara
Nature 614, 249–255 (2023)

n situ x-ray spectroscopies offer a powerful way to understand the electronic structure of the electrode–electrolyte interface under operating conditions. However, most x-ray techniques require vacuum, making it necessary to design spectro-electrochemical cells with a delicate interface to the wet electrochemical environment. The design of the cell often dictates what measurements can be done and which electrochemical processes can be studied. Hence, it is important to pick the right spectro-electrochemical cell for the process of interest. To facilitate this choice, and to highlight the challenges in cell design, we critically review four recent, successful cell designs. Using several case studies, we investigate the opportunities and limitations that arise in practical experiments.

J.-J. Velasco-Vélez, L. J. Falling, D. Bernsmeier, M. J Sear, P. C. J. Clark, T.-S. Chan, E. Stotz, M. Hävecker, R. Kraehnert, and A. Knop-Gericke
Juan-Jesús Velasco-Vélez et al 2021 J. Phys. D: Appl. Phys. 54 124003

In this topical review we catagorise all ambient pressure x-ray photoelectron spectroscopy
publications that have appeared between the 1970s and the end of 2018 according to their
scientific field. We find that catalysis, surface science and materials science are predominant,
while, for example, electrocatalysis and thin film growth are emerging. All catalysis
publications that we could identify are cited, and selected case stories with increasing
complexity in terms of surface structure or chemical reaction are discussed. For thin film
growth we discuss recent examples from chemical vapour deposition and atomic layer
deposition. Finally, we also discuss current frontiers of ambient pressure x-ray photoelectron
spectroscopy research, indicating some directions of future development of the field.
Keywords: ambient pressure x-ray photoelectron spectroscopy, synchrotron radiation,
catalysis, atomic layer deposition, chemical vapour deposition, operando

J. Schnadt, J. Knudsen, and N. Johansson
J. Phys.: Condens. Matter 32 (2020) 413003 (29pp)

Liquid jet X-ray photoelectron spectroscopy is used under near ambient pressure conditions to characterize Fe²⁺ aqueous solutions. Counter ions, such as Cl⁻ and Br⁻ ions, added to the solution lead to changes in the first solvation sphere of the Fe-aqua complex in solution. Binding energy shifts of 0.4 eV to lower binding energy are observed in the Cl 2p spectra, 2 eV to higher binding energy in the Fe 2p spectra and no shifts are observed in the Br 3d spectra. Depletion of the Cl⁻ species is observed at the interface,
caused by coordination with Fe². Depletion of Cl⁻ at the liquid/vapor interface may have significant impacts on oxidative chemistry at the interface of atmospheric aerosols that contain both chloride and iron. The Cl⁻ complexation with the Fe²⁺ ions will also affect the Fenton chemistry that is dependent on this metal ion.

J. P. Bruce and J. C. Hemminger
J. Phys. Chem. B 2019, 123, 8285−8290

The promotion of Ga on SiO₂ supported Cu in the hydrogenation of CO₂ to methanol at 800 kPa and 200–280 °C was investigated. Cu/SiO₂ and CuGa/SiO₂ catalysts were prepared by a water-in-oil microemulsion technique resulting in Cu clusters of 4–6.5 nm. It was found that Ga addition increased the methanol formation rate by an order of magnitude without significantly changing that for reverse water gas shift (RWGS). This trend is also evidenced by the decrease in the apparent activation barrier for methanol formation from 78 (for Cu/SiO₂) to 26–39 kJ mol⁻¹ when Ga was added, but not for RWGS (107–132 kJ mol⁻¹). Kinetic and in situ DRIFTS analyses revealed that formate intermediates are adsorbed on both Cu and Ga₂O₃ and that methoxy hydrogenation could be the rate determining step of methanol synthesis. In the case of RWGS, a zero order of CO formation with respect to H₂ concentration was consistent with a redox mechanism and with the reaction occurring predominantly on Cu sites. The results suggest that Ga promotes Cu increasing methanol selectivity, likely by creating new active sites for methanol formation without modifying its oxidation state, which under reaction conditions remains mostly metallic.

J. C. Medina, M. Figueroa, R. Manrique, J. R. Pereira, P. D. Srinivasan, J. J. Bravo-Suárez, V. G. Baldovino Medrano, R. Jiméneza and A. Karelovic
Catal. Sci. Technol., 2017,7, 3375-3387

The hydrogenation of CO₂ to methanol is a viable alternative for reducing greenhouse gases net emissions as well as a route for hydrogen storage and transportation. In this context, the synthesis of active and selective catalysts is a relevant objective. In this work, we study the promotion of Pd with Ga and Zn in the hydrogenation of CO₂ to methanol at 800 kPa and 220–280 °C. Mono and intermetallic catalysts (Pd/SiO₂, PdGa/SiO₂ and Pd-Zn/SiO₂) were synthesized by incipient wetness impregnation with the aid of triethanolamine as an organic additive, obtaining similar average metal particle sizes (between 9 and 12 nm). Kinetic analysis reveals that the addition of Ga and Zn increases the turnover frequency for methanol formation by an order of magnitude without significant changes in the reaction rate of the reverse water-gas shift (r-WGS) which is a parallel undesired reaction. The selectivity to methanol (at 220 °C) thus increases from 3% for Pd/SiO₂ to 12% for Pd-Ga/SiO₂ and 30% for Pd-Zn/SiO2. XPS studies, Infrared analysis of CO adsorption, and XRD analyses show the presence of intermetallic phases Pd₂Ga and PdZn on the surface. The results suggest that Ga and Zn promote Pd, increasing its activity towards the synthesis of methanol, by creating more active sites for this reaction. These sites are likely formed by intermetallic compounds such as Pd₂Ga and PdZn.

R. Manrique, R. Jiménez, J. Rodríguez-Pereira, V. G. Baldovino-Medrano, and A. Karelovic
International Journal of Hydrogen Energy, Volume 44, Issue 31, 21 June 2019, Pages 16526-16536

An apparatus for sub-nanosecond time-resolved ambient-pressure X-ray photoelectron spectroscopy
studies with pulsed and constant wave X-ray light sources is presented. A differentially pumped
hemispherical electron analyzer is equipped with a delay-line detector that simultaneously records
the position and arrival time of every single electron at the exit aperture of the hemisphere with
∼0.1 mm spatial resolution and ∼150 ps temporal accuracy. The kinetic energies of the photoelectrons
are encoded in the hit positions along the dispersive axis of the two-dimensional detector. Pumpprobe
time-delays are provided by the electron arrival times relative to the pump pulse timing. An
average time-resolution of (780 ± 20) ps (FWHM) is demonstrated for a hemisphere pass energy Ep
= 150 eV and an electron kinetic energy range KE = 503–508 eV. The time-resolution of the setup
is limited by the electron time-of-flight (TOF) spread related to the electron trajectory distribution
within the analyzer hemisphere and within the electrostatic lens system that images the interaction
volume onto the hemisphere entrance slit. The TOF spread for electrons with KE = 430 eV varies
between ∼9 ns at a pass energy of 50 eV and ∼1 ns at pass energies between 200 eV and 400 eV. The
correlation between the retarding ratio and the TOF spread is evaluated by means of both analytical
descriptions of the electron trajectories within the analyzer hemisphere and computer simulations of
the entire trajectories including the electrostatic lens system. In agreement with previous studies, we
find that the by far dominant contribution to the TOF spread is acquired within the hemisphere. However,
both experiment and computer simulations show that the lens system indirectly affects the time
resolution of the setup to a significant extent by inducing a strong dependence of the angular spread
of electron trajectories entering the hemisphere on the retarding ratio. The scaling of the angular
spread with the retarding ratio can be well approximated by applying Liouville’s theorem of constant
emittance to the electron trajectories inside the lens system. The performance of the setup is demonstrated
by characterizing the laser fluence-dependent transient surface photovoltage response of a
laser-excited Si(100) sample. © 2014 AIP Publishing LLC.

A. Shavorskiy, S. Neppl, D. S. Slaughter, J. P. Cryan, K. R. Siefermann, F. Weise,
Mf Lin, C. Bacellar, M. P. Ziemkiewicz, I. Zegkinoglou, M. W. Fraund, C.
Khurmi, M. P. Hertlein, T. W. Wright, N. Huse, R. W. Schoenlein, T. Tyliszczak, G.
Coslovich, J.
AIP Review of Scientific Instruments 85, 093102 (2014)

Progress in the development of plasmon-enabled light-harvesting technologies requires a
better understanding of their fundamental operating principles and current limitations. Here, we employ
picosecond time-resolved X-ray photoemission spectroscopy to investigate photoinduced electron transfer in
a plasmonic model system composed of 20 nm sized gold nanoparticles (NP_s) attached to a nanoporous film
of TiO₂. The measurement provides direct, quantitative access to transient local charge distributions from
the perspectives of the electron donor (AuNP) and the electron acceptor (TiO₂). On average, approximately
two electrons are injected per NP, corresponding to an electron injection yield per absorbed photon of 0.1%.
Back electron transfer from the perspective of the electron donor is dominated by a fast recombination
channel proceeding on a time scale of 60 ± 10 ps and a minor contribution that is completed after ∼1 ns.
The findings provide a detailed picture of photoinduced charge carrier generation in this NP−semiconductor
junction, with important implications for understanding achievable overall photon-to-charge conversion
efficiencies.

M. Borgwardt, J. Mahl, F.Roth, L. Wenthaus, F. Brauße, M.Blum,
K. Schwarzburg, G. Liu, F.M. Toma, and O.Gessner
The Journal of Physical Chemistry Letters. 2020, 11, 5476−5481

The success of many emerging molecular electronics concepts hinges on an atomistic understanding of
the underlying electronic dynamics. We employ picosecond time-resolved x-ray photoemission spectroscopy
(tr-XPS) to elucidate the roles of singlet and triplet excitons for photoinduced charge generation at a copperphthalocyanine–
C₆₀ heterojunction. Contrary to common belief, fast intersystem crossing to triplet excitons
after photoexcitation is not a loss channel but contributes to a significantly larger extent to the time-integrated
interfacial charge generation than the initially excited singlet excitons. The tr-XPS data provide direct access to
the diffusivity of the triplet excitons D_CuPc = (1.8 ± 1.2) × 10⁻⁵ cm²/s (where CuPc is copper-phthalocyanine)
and their diffusion length L_diff = (8 ± 3) nm.

F. Roth, S.Neppl, A. Shavorskiy, T. Arion, J.Mahl, H. O. Seo,
H. Bluhm, Z.Hussain, O. Gessner, and W. Eberhardt.
PHYSICAL REVIEW B 99, 020303(R) (2019)

Methanol is a promising chemical for the safe and efficient storage of hydrogen, where methanol conversion reactions can
generate a hydrogen‐containing gas mixture. Understanding the chemical state of the catalyst over which these reactions
occur and the interplay with the adsorbed species present is key to the design of improved catalysts and process conditions.
Here we study polycrystalline Cu foils using ambient pressure X‐ray spectroscopies to reveal the Cu oxidation state and
identify the adsorbed species during partial oxidation (CH₃OH + O₂), steam reforming (CH₃OH + H₂O), and autothermal
reforming (CH₃OH + O₂ + H₂O) of methanol at 200 °C surface temperature and in the mbar pressure range. We find that the
Cu surface remains highly metallic throughout partial oxidation and steam reforming reactions, even for oxygen‐rich
conditions. However, for autothermal reforming the Cu surface shows significant oxidation towards Cu₂O. We rationalise
this behaviour on the basis of the shift in equilibrium of the CH₃OH^* + O^* ⇌ CH₃O^* + OH^* caused by the addition of H₂O.

B. Eren, C. G. Sole, J. S. Lacasa, D. Grinter, F. Venturini, G. Held,
C. S. Esconjauregui, and R. S. Weatherup
Physical Chemistry Chemical Physics, 2020, 00, 1‐8

Interfacially confined microenvironments have recently gained attention in catalysis, as they can be used to modulate reaction chemistry. The emergence of a 2D nanospace at the interface between a 2D material and its support can promote varying kinetic and energetic schemes based on molecular level confinement effects imposed in this reduced volume. We report on the use of a 2D oxide cover, bilayer silica, on catalytically active Pd(111) undergoing the CO oxidation reaction. We “uncover” mechanistic insights about the structure‐activity relationship with and without a 2D silica overlayer using in situ IR and X‐ray spectroscopy and mass spectrometry methods. We find that the CO oxidation reaction on Pd(111) benefits from confinement effects imposed on surface adsorbates under 2D silica. This interaction results in a lower and more dispersed coverage of CO adsorbates with restricted CO adsorption geometries, which promote oxygen adsorption and lay the foundation for the formation of a reactive surface oxide that produces higher CO 2 formation rates than Pd alone.

C. Eads, J. A. Boscoboinik, A. R Head, A. Hunt, I. Waluyo, D. J. Stacchiola, and S. A Tenney
Wiley, 18.01.2021

Interfacially confined microenvironments have recently gained attention in catalysis, as they can be used to modulate reaction chemistry. The emergence of a 2D nanospace at the interface between a 2D material and its support can promote varying kinetic and energetic schemes based on molecular level confinement effects imposed in this reduced volume. We report on the use of a 2D oxide cover, bilayer silica, on catalytically active Pd(111) undergoing the CO oxidation reaction. We “uncover” mechanistic insights about the structure‐activity relationship with and without a 2D silica overlayer using in situ IR and X‐ray spectroscopy and mass spectrometry methods. We find that the CO oxidation reaction on Pd(111) benefits from confinement effects imposed on surface adsorbates under 2D silica. This interaction results in a lower and more dispersed coverage of CO adsorbates with restricted CO adsorption geometries, which promote oxygen adsorption and lay the foundation for the formation of a reactive surface oxide that produces higher CO 2 formation rates than Pd alone.

Calley Eads, J Anibal Boscoboinik, Ashley R Head, Adrian Hunt, Iradwikanari Waluyo, Dario J Stacchiola, Samuel A Tenney
Angewandte Chemie is a journal of the German Chemical Society (GDCh).

Three-dimensional (3D) electronic band structure is fundamental for understanding a vast diversity of physical phenomena in solid-state systems, including topological phases, interlayer interactions in van der Waals materials, dimensionality-driven phase transitions, etc.  Interpretation of ARPES data in terms of 3D electron dispersions is commonly based on the free-electron approximation for the  photoemission final states. Our soft-X-ray ARPES data on Ag metal reveals, however, that even at high excitation energies the final states can be a way more complex, incorporating several Bloch waves with different out-of-plane momenta. Such multiband final states manifest themselves as a complex structure and added broadening of the spectral peaks from 3D electron states. We analyse the origins of this phenomenon, and trace
it to other materials such as Si and GaN. Our findings are essential for accurate determination of the 3D band structure over a wide range of materials and excitation energies in the ARPES experiment.

V. N. Strocov, L. L. Lev, F. Alarab, P. Constantinou, X. Wang, T. Schmitt, T. J. Z. Stock, L. Nicolaï, J. Očenášek, J. Minár
Nature Communications, Volume 14, Article number: 4827 (2023)

Methanol formation over Cu/ZnO catalysts is linked with a catalytically active phase created by contact between Cu nanoparticles and Zn species whose chemical and structural state depends on reaction conditions. Herein, we use variable-temperature scanning tunneling microscopy at elevated pressure conditions combined with X-ray photoelectron spectroscopy measurements to investigate the surface structures and chemical states that evolve when a CuZn/Cu(111) surface alloy is exposed to reaction gas mixtures. In CO₂ hydrogenation conditions, Zn stays embedded in the CuZn surface, but once CO gas is added to the mixture, the Zn segregates onto the Cu surface. The Zn segregation is CO-induced, and establishes a new dynamic state of the catalyst surface where Zn is continually exchanged at the Cu surface. Candidates for the migrating few-atom Zn clusters are further identified in time-resolved imaging series. The findings point to a significant role of CO affecting the distribution of Zn in the multiphasic ZnO/CuZn/Cu catalysts.

Sigmund Jensen, Mathias H. R. Mammen, Martin Hedevang, Zheshen Li, Lutz Lammich & Jeppe V. Lauritsen
Nature Communications volume 15, Article number: 3865 (2024)

X-ray photoelectron spectroscopy (XPS) is one of the most powerful techniques to quantitatively ana-lyze the chemical composition and electronic structure of surfaces and interfaces in a non-destructivefashion. Extending this technique into the time domain has the exciting potential to shed new lighton electronic and chemical dynamics at surfaces by revealing transient charge configurations withelement- and site-specificity. Here, we describe prospects and challenges that are associated with theimplementation of picosecond and femtosecond time-resolved X-ray photoelectron spectroscopy atthird-generation synchrotrons and X-ray free-electron lasers, respectively. In particular, we discuss aseries of laser-pump/X-ray-probe photoemission experiments performed on semiconductor surfaces,molecule-semiconductor interfaces, and films of semiconductor nanoparticles that demonstrate the highsensitivity of time-resolved XPS to light-induced charge carrier generation, diffusion and recombinationwithin the space charge layers of these materials. Employing the showcase example of photo-inducedelectronic dynamics in a dye-sensitized semiconductor system, we highlight the unique possibility toprobe heterogeneous charge transfer dynamics from both sides of an interface, i.e., from the perspectiveof the molecular electron donor and the semiconductor acceptor, simultaneously. Such capabilities willbe crucial to improve our microscopic understanding of interfacial charge redistribution and associatedchemical dynamics, which are at the heart of emerging energy conversion, solar fuel generation, andenergy storage technologies.

S. Neppl, O. Gessner
Journal of Electron Spectroscopy and Related Phenomena 200 (2015) 64–77

Understanding interfacial charge-transfer processes on the atomic level is
crucial to support the rational design of energy-challenge relevant systems such as solar cells,
batteries, and photocatalysts. A femtosecond time-resolved core-level photoelectron spectroscopy
study is performed that probes the electronic structure of the interface between
ruthenium-based N3 dye molecules and ZnO nanocrystals within the first picosecond after
photoexcitation and from the unique perspective of the Ru reporter atom at the center of the
dye. A transient chemical shift of the Ru 3d inner-shell photolines by (2.3 ± 0.2) eV to higher
binding energies is observed 500 fs after photoexcitation of the dye. The experimental results
are interpreted with the aid of ab initio calculations using constrained density functional
theory. Strong indications for the formation of an interfacial charge-transfer state are presented,
providing direct insight into a transient electronic configuration that may limit the
efficiency of photoinduced free charge-carrier generation.

K.R. Siefermann, C. D. Pemmaraju, S. Neppl, A. Shavorskiy,
A. A. Cordones, J. Vura-Weis, D. S. Slaughter, F. P. Sturm, F. Weise,
H. Bluhm, M. L. Strader, H. Cho, MF Lin, C. Bacellar,
C. Khurmi, J. Guo, G. Coslovich, J. S. Robinson, R. A. Kaindl,
R. W.
The Journal of Physical Chemistry Letters. 2014, 5, 2753−2759

Nitrogen fixation in a simulated natural environment (i.e., near ambient pressure, room temperature, pure water and incident light) would provide a desirable approach to future nitrogen conversion. As N≡N triple bond has a thermodynamically high cleavage energy, nitrogen reduction under such mild conditions typically undergoes associative alternating or distal pathways rather than follows a dissociative mechanism. Here we report that surface plasmon can supply sufficient energy to activate N₂ through a dissociative mechanism in the presence of water and incident light, as evidenced by in-situ synchrotron radiation-based infrared spectroscopy and near ambient pressure X-ray photoelectron spectroscopy. Theoretical simulation indicates that the electric field enhanced by surface plasmon, together with plasmonic hot electrons and interfacial hybridization, may play a critical role in N≡N dissociation. Specifically, AuRu core-antenna nanostructures with broaden light adsorption cross section and active sites achieve an ammonia production rate of 101.4 μmol·g^-1·h^-1 without any sacrificial agent at room temperature and 2-atm pressure. This work highlights the significance of
surface plasmon to activation of inert molecules, serving as a promising platform for developing novel catalytic systems.

C. Hu, X. Chen, J. Jin, Y. Han, S. Chen, H. Ju, J. Cai, Y. Qiu, C. Gao, C. Wang, Z. Qi, R. Long, L. Song, Z. Liu, Y. Xiong
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 30 Apr 2019

We show that it is possible to use a multichannel electron detector in a zone plate based photoemission spectromicroscopy in a snap shot mode to reduce the total acquisition time for a given counting time by 50% relative to the standard scanning mode while preserving the feature of the spectra. We describe the result of tests performed at Elettra using its microbeam (150 nm) together with a 48-channel detector designed for the PHOIBOS 100 analyzer optimized for extremely small x-ray sources. We also give a short summary of the technical features of the detector and describe one possible calibration procedure for its use in the snap shot mode. We show initial results from using this device to perform chemical maps of surfaces at a resolution of 150 nm.

L. Gregoratti, A. Barinov, E. Benfatto, G. Cautero, C. Fava, P. Lacovig, D. Lonza, M. Kiskinova, R. Tommasini, S. Mähl, W. Heichler
Rev. Sci. Instr. 75 (1), pp. 64-68

Investigations on how to replace the toxic KCN etching for the removal of Cu–S phases during the preparation of CuInS2 (CIS) absorber layers by electrochemical procedures are presented. Starting from a simple anodic treatment in V2+/V3+ electrolyte, a more complex photoelectrochemical technique is developed which consists of different consecutive etching steps for the dissolution of the predominant CuS and for the removal of remaining Cu2S and a small sacrificial layer of CuInS2. This new method also offers the possibility of in situ quality control of the CIS in a photoelectrochemical solar cell (PECS) setup. Further examination of the treated films is carried out using X-ray emission spectroscopy, X-ray photoelectron spectroscopy (XPS) and by further processing of the samples to create solid state solar cells.

T. Wilhelm, B. Berenguier, M. Aggour, K. Skorupska, M. Kanis, M. Winkelnkemper, J. Klaer, C. Kelch, H. J. Lewerenz
Thin Solid Films 480-481, pp. 24-28

The oxidation state of cobalt in various forms (single crystal, nanopowder, deposited on CeO2 and ZnO supports) was investigated in situ under O2, H2 and ethanol steam reforming (ESR) conditions (C2H6O:H2O = 1:3) by using ambient pressure X-ray photoelectron and absorption spectroscopies. In O2 atmosphere single crystalline, nanopowder and CeO2-supported cobalt is readily oxidized into a spinel Co3O4 phase, while over the ZnO a mixed Zn1−xCoxO oxide is formed. In H2 and ESR atmospheres, Co3O4 oxide reduces to the metallic state, with an evident effect of the gas atmosphere and the sample type in this process. On the other hand the Zn1−xCoxO oxide proved to be extremely stable in reductive conditions. Finally, under the ESR atmosphere the amount of carbon deposition and the type of the adsorbed oxygen species considerably varies between supported and single crystalline/nanopowder cobalt catalysts.

S. Turczyniak, W. Luo, V. Papaefthimiou, N. S. Ramgir, M. Haevecker, A. Machocki, S. Zafeiratos
Topics in Catalysis 59 (5-7), pp. 532-542

A "scattering pattern matrix" method is proposed here to overcome the difficulties presented by photoelectron holography, such as forward-scattering and multi-energy problems. This method makes it possible to reconstruct a three-dimensional atomic arrangement from a single-energy hologram. We have utilized the "scattering pattern matrix" that includes the angular variation of the scattered object waves, and we have adopted a special average process and the gradient projection method for minimizing the mean-squared error. The reconstruction of the Si bulk structure is demonstrated by using an experimental Si(111) 2s single-energy hologram. The holographic reconstruction provided a three-dimensional image with a size of 10 Å along both the vertical and horizontal axes.

T. Matsushita, A. Agui, A. Yoshigoe
Urophys. Lett. 65 (2), pp. 207-213

We report on an investigation of spin-polarized secondary electron emission from the chemically inert system: graphene/Ni(111). An ordered passivation graphene layer (monolayer of graphite) was formed on Ni(111) surface via cracking of propylene gas. The spin polarization of secondary electrons obtained from this system upon photoemission is only slightly lower than the one from the clean Ni surface but does not change upon large oxygen exposure. These results suggest to use such passivated Ni(111) surface as a source of spin-polarized electrons stable against adsorption of reactive gases.

Yu. S. Dedkov, M. Fonin, C. Laubschat
Appl. Phys. Lett. 92, 052506

Deactivation of the iron catalyst Fe-Si-Cr-K-O was investigated with the help of X-ray photoelectron spectroscopy and checked in reaction of phenol alkylation with methanol performed in gas phase at the atmospheric pressure. It was indicated that the catalytic action of the alkylation catalyst (active–inactive) could be correlated with the relative contribution of the oxidation states of iron (+2) and (+3). The intensity ratio of the Fe 3p individual components was found to be a good measure of the contribution. The catalyst as-prepared has the (+2)/(+3) ratio 1.7, active in reaction—1.9, calcined at 1000 °C—1.4 and deactivated during the reaction—2.7. The process accompanied other reasons for deactivation as carbon deposition on the surface and/or sintering of catalyst crystallites.

H. Grabowska, R. Klimkiewicz, W. Tylus and P. J. Godowski
Applied Catalysis A: General 240 (1-2), pp .111-117

Zr–V alloy getter films were prepared on stainless steel substrates by magnetron sputtering. The thermal activation behavior of these getters was investigated by synchrotron radiation photoelectron spectroscopy using photon excitation energies of 600, 250 and 73 eV. Depth resolved results were compared to the results of the SIMS profiling. The measurements confirmed the disappearance of the superficial oxide layer covering the air-exposed Zr–V surfaces via its progressive reduction during the thermal activation. The depth sensitive results showed that the activated getter surface is covered by a residual zirconium sub-oxide.

V. Matolín, V. Dudr, S. Fabík, V. Cháb, K. Mašek, I. Matolínová, K.C. Prince, T. Skála, F.Šutara, N. Tsud, K. Veltruská
Applied Surface Science 243 (1-4), pp. 106-112

The oxidation states and reducibility of γ-alumina-supported vanadium oxide catalysts with V loadings between 1.7 and 15.7 wt% were studied by means of temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) for fresh as well as used catalysts. As additional experimental techniques, X-ray diffraction (XRD), diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS), and temperature-programmed desorption (TPD) were applied. For V loadings up to 6.1 wt%, the surface was found to be covered only by vanadate, while at higher loadings the formation of V2O5 crystallites was observed. The XPS and TPR data showed that under moderate oxidizing conditions, only of V in the vanadate catalysts was in the oxidation state +5, while in the catalysts containing V2O5 crystallites it was about 80%, with the remainder being present as V(IV) in both cases. In the ambient pressure TPR experiments, all catalysts were completely reduced by hydrogen to V(III), although the vanadate catalysts were found to be more easily reducible than those containing V2O5. The V dispersion changed during redox cycles, as they appeared under working conditions. These changes were different for the exposure to ethane or hydrogen, but did not influence the maximum oxidation states. In the catalytic oxidation of ethane used as a model reaction, all catalysts were found to be in their oxidized state under steady-state conditions. Significant catalyst reduction occurred only if the surrounding gas-phase oxygen was completely consumed, leading to the loss of catalytic activity due to the disappearance of V(V) species. A novel structure for γ-alumina-supported vanadia catalysts is proposed on the basis of the experimental data.

F. Klose, T. Wolff, H. Lorenz, A. Seidel-Morgenstern, Y. Suchorski, M. Piorkovska, H. Weiss
J. of catalysis 247; pp. 176-193

The Front Cover shows a representation of the ALBA synchrotron, where near ambient pressure photoelectron spectroscopy was performed to study the surface/subsurface of ceria and ceria‐zirconia catalysts for removing soot from combustion engines. In their Communication, L. Soler et al. identified two cooperative routes, one occurring at the ceria–soot interface with formation of O vacancies and CeIII, and another at the surface of soot mediated by superoxide species resulting from the reaction between O2 gas and O vacancies (drawing by Javier Sánchez Ríos, javier. sanchezrios.1978@ieee.org). More information can be found in the Communication by L. Soler et al. on page 2748 in Issue 17, 2016 (DOI: 10.1002/cctc.201600615).

L. Soler, A. Casanovas, C. Escudero, V. Pérez-Dieste, E. Aneggi, A. Trovarelli, J. Llorca
ChemCatChem 8 (17), pp. 2733-2733

Over the past several decades, ambient pressure x-ray photoelectron spectroscopy (APXPS) has emerged as a powerful technique for in situ and operando investigations of chemical reactions under relevant ambient atmospheres far from ultra-high vacuum conditions. This review focuses on exemplary cases of APXPS experiments, giving special consideration to experimental techniques, challenges, and limitations specific to distinct condensed matter interfaces. We discuss APXPS experiments on solid/vapor interfaces, including the special case of 2D films of graphene and hexagonal boron nitride on metal substrates with intercalated gas molecules, liquid/vapor interfaces, and liquid/solid interfaces, which are a relatively new class of interfaces being probed by APXPS. We also provide a critical evaluation of the persistent limitations and challenges of APXPS, as well as the current experimental frontiers.

L. Trotochaud, A. R. Head, O. Karslıoğlu, L. Kyhl, H. Bluhm
J. Phys.: Condens. Matter 29 (5)

A new system for spin resolved photoelectron spectroscopy and bremsstrahlung isochromat spectroscopy has been built and commissioned at Lawrence Livermore National Laboratory for the investigation of the electronic structure of the actinides. Actinide materials are very toxic and radioactive and therefore cannot be brought to most general user facilities for spectroscopic studies. The technical details of the new system and preliminary data obtained therein will be presented and discussed.

S.-W. Yu, J. G. Tobin, B. W. Chung
Review of Scientific Instruments 82, 093903

An experimental setup for time- and angle-resolved photoemission spectroscopy using a femtosecond 1 kHz high harmonic light source and a two-dimensional electron analyzer for parallel energy and momentum detection is presented. A selection of the 27th harmonic (41.85 eV) from the harmonic spectrum of the light source is achieved with a multilayer Mo∕Si double mirror monochromator. The extinction efficiency of the monochromator in selecting this harmonic is shown to be better than 7:1, while the transmitted bandwidth of the selected harmonic is capable of supporting temporal pulse widths as short as 3fs. The recorded E(k) photoelectron spectrum from a Cu(111) surface demonstrates an angular resolution of better than 0.6° (=0.03 Å−1 at Ekin,e=36 eV). Used in a pump-probe configuration, the described experimental setup represents a powerful experimental tool for studying the femtosecond dynamics of ultrafast surface processes in real time.

S. Mathias, L. Miaja-Avila, M. M. Murnane, H. Kapteyn, M. Aeschlimann, M. Bauer
Review of Scientific Instruments 78, 083105

The structure of the solid-liquid interface often defines the function and performance of materials in applications. To study this interface at the atomic scale, we extended an ultrahigh vacuum (UHV) surface-science chamber with an apparatus that allows bringing a surface in contact with ultrapure liquid water without exposure to air. In this process, a sample, typically a single crystal prepared and characterized in UHV, is transferred into a separate, small chamber. This chamber already contains a volume of ultrapure water ice. The ice is at cryogenic temperature, which reduces its vapor pressure to the UHV range. Upon warming, the ice melts and forms a liquid droplet, which is deposited on the sample. In test experiments, a rutile TiO2(110) single crystal exposed to liquid water showed unprecedented surface purity, as established by X-ray photoelectron spectroscopy and scanning tunneling microscopy. These results enabled us to separate the effect of pure water from the effect of low-level impurities present in the air. Other possible uses of the setup are discussed.

J. Balajka, J. Pavelec, M. Komora, M. Schmid, U. Diebold
Review of Scientific Instruments 89, 083906

Calcium doped CeO2 nanoparticles with doping concentrations between 0 and 50 mol% were synthesized by a co-precipitation method for ultraviolet filtration application. Below 20 mol% doping concentration, the samples were single-phase. From 30 mol%, CaCO3 appears as a secondary phase. The calculated CeO2 mean crystallite size was 9.3 nm for the pure and 5.7 nm for the 50 mol% Ca-doped sample. Between 250 and 330 nm, the absorbance increased for the 10, 30, and 40 mol% Ca-doped samples compared to the pure one. The band-gap was found to be 3.20 eV for the undoped, and between 3.36 and 3.51 eV for the doped samples. The blue shifts are attributed to the quantum confinement effect. X-ray photoelectron spectroscopy showed that the Ce3+ atomic concentration in the pure sample was higher than that of the 20 mol% Ca-doped sample.

L. Truffoult, M.-T. Ta, T. Devers, K. Konstantinov, V. Harel, C. Simmonard, C. Andreazza, I. P. Nevirkovets, A. Pineau, O. Veron, J. P. Blondeau
Material Research Bulletin 45, pp. 527-535

We describe a new in operando approach for the investigation of heterogeneous processes at solid/liquid interfaces with elemental and chemical specificity which combines the preparation of thin liquid films using the meniscus method with standing wave ambient pressure X-ray photoelectron spectroscopy [Nemšák et al., Nat. Commun., 5, 5441 (2014)]. This technique provides information about the chemical composition across liquid/solid interfaces with sub-nanometer depth resolution and under realistic conditions of solution composition and concentration, pH, as well as electrical bias. In this article, we discuss the basics of the technique and present the first results of measurements on KOH/Ni interfaces.

O. Karslıoğlu, S. Nemšák, I. Zegkinoglou, A. Shavorskiy, M. Hartl, F. Salmassi, E. M. Gullikson, M. L. Ng, Ch. Rameshan, B. Rude, D. Bianculli, A. A. Cordones, S. Axnanda, E. J. Crumlin, P. N. Ross, C. M. Schneider, Z. Hussain, Z. Liu, C. S. Fadley, H. Bl
Faraday Discuss. 180, pp. 35-53

Gold nanoparticles (Au NPs) on oxygen-free supports were examined using near ambient pressure X-ray photoelectron spectroscopy under CO oxidation conditions, and ex situ using scanning electron microscopy and transmission electron microscopy. Our observations demonstrate that Au NPs supported on carbon materials are inactive, regardless of the preparation method. Ozone (O3) treatment of carbon supports leads oxygen-functionalization of the supports. When subsequently exposed to a CO feed, CO is oxidized by the functionalized sites of the carbon support via a stoichiometric pathway. Microscopy reveals that the reaction with CO does not change the morphology of the Au NPs. In situ XPS reveals that the O3 treatment gives rise to additional Au 4f and O 1s peaks at binding energies of 85.25–85.6 and 529.4–530 eV, respectively, which are assigned to the presence of Au oxide. A surface oxide phase is formed during the activation of Au NPs supported on Au foil by O3 treatment. However, this phase decomposes in vacuum and the remaining low-coordinative atoms do not have sufficient catalytic properties to oxidize CO, so the size reduction of Au NPs and/or oxidation of Au NPs is not sufficient to activate Au.

A. Y. Klyushin, R. Arrigo, Y. Youngmi, Z. Xie, M. Hävecker, A. V. Bukhtiyarov, I. P. Prosvirin, V. I. Bukhtiyarov, A. Knop-Gericke, R. Schlögl
Topics in Catalysis 59 (5-7), pp. 469-477

Two fundamental manifestations of Al conduction electron response to Ar atom core hole in the final state of photoemission have been studied in implanted Ar bubbles in Al(111). Ar 2p binding energy and the Doniach-Šunjić asymmetry of the core-level line shape vary systematically as functions of Ar+ implantation energy and number of ions bombarded (fluence). The observations are explained by relating the strength of Al conduction electron screening to the size of the Ar nanobubbles.

C. Biswas, A. K. Shukla, S. Banik, S. R. Barman, A. Chakrabarti
Phys. Rev. Lett. 92 (11), pp. 115506-1 - 115506-4

One of the main goals in catalysis is the characterization of solid/gas interfaces in a reaction environment. The electronic structure and chemical composition of surfaces become heavily influenced by the surrounding environment. However, the lack of surface sensitive techniques that are able to monitor these modifications under high pressure conditions hinders the understanding of such processes. This limitation is known throughout the community as the “pressure gap.” We have developed a novel experimental setup that provides chemical information on a molecular level under atmospheric pressure and in presence of reactive gases and at elevated temperatures. This approach is based on separating the vacuum environment from the high-pressure environment by a silicon nitride grid—that contains an array of micrometer-sized holes—coated with a bilayer of graphene. Using this configuration, we have investigated the local electronic structure of catalysts by means of photoelectron spectroscopy and in presence of gases at 1 atm. The reaction products were monitored online by mass spectrometry and gas chromatography. The successful operation of this setup was demonstrated with three different examples: the oxidation/reduction reaction of iridium (noble metal) and copper (transition metal) nanoparticles and with the hydrogenation of propyne on Pd black catalyst (powder).

J. J. Velasco-Vélez, V. Pfeifer, M. Hävecker, R. Wang, A. Centeno, A. Zurutuza, G. Algara-Siller, E. Stotz, K. Skorupska, D. Teschner, P. Kube, P. Braeuninger-Weimer, S. Hofmann, R. Schlögl, A. Knop-Gericke
Review of Scientific Instruments 87, 053121

We show that atomically flat single SrO-terminated SrTiO3(001) substrates can be obtained through simple high-temperature treatment. Amplitude-modulation atomic force microscopy with phase-lag analysis and x-ray photoelectron spectroscopy, have been used to demonstrate that the ratio between the two chemical terminations can be tailored by choosing the annealing time. Moreover, the progressive SrO surface enrichment (up to 100%) is accompanied by a self-assembly process which results in the spatial separation at the nanoscale of both chemical terminations. We further demonstrate that this opens a interesting avenue for selective chemical reaction and growth of oxide nanostructures.

R. Bachelet, F. Sánchez, F. J. Palomares, C. Ocal, J. Fontcuberta
Applied Physics Letters 95, 141915

The band line-up at the 4H-SiC/Ni interface was determined using X-ray photoemission spectroscopy (XPS). Ni was deposited in 14 steps on an ex situ cleaned n-type 4H-SiC substrate starting with an initial deposition of 0.5 Å up to a final thickness of 110 Å. The sample surface was characterized in situ by XPS before the growth sequence, and after each Ni deposition step. Analysis of the Ni 3d and O 1s core level peaks indicates that a thin Ni oxide layer was formed at the interface due to a chemical reaction with oxidizing agents on the sample surface due to the ex situ substrate preparation before a metallic Ni film could be grown. The substrate core level spectra exhibit a significant shift to lower binding energy due to the development of an inversion condition in the semiconductor at the interface. This resulted in the formation of an electron injection barrier of 3.26 eV at the interface.

M. M. Beerbom, Z. Bednarova, R. Gargagliano, Y.N . Emirov, R. Schlaf
Applied Surface Science 236 (1-4), pp. 208-216

Because of stability issues, carbyne, a one-dimensional chain of carbon atoms, has been much less investigated than other recent carbon allotropes such as graphene. Beyond that, metalation of such a linear carbon nanostructure with regularly distributed metal atoms is even more challenging. Here we report a successful on-surface synthesis of metalated carbyne chains by dehydrogenative coupling of ethyne molecules and copper atoms on a Cu(110) surface under ultrahigh-vacuum conditions. The length of the fabricated metalated carbyne chains was found to extend to the submicron scale (with the longest ones up to ∼120 nm). We expect that the herein-developed on-surface synthesis strategy for the efficient synthesis of organometallic carbon-based nanostructures will inspire more extensive experimental investigations of their physicochemical properties and explorations of their potential with respect to technological applications.

Q. Sun, L. Cai, S. Wang, R. Widmer, H. Ju, J. Zhu, L. Li∥, Y. He, P. Ruffieux, R. Fasel, W. Xu
J. Am. Chem. Soc. 138 (4), pp. 1106–1109

We have measured the spin polarization of the 4d and 4f core-level photoelectrons from Pt(001) by using unpolarized laboratory x-ray sources under a highly bulk sensitive condition. The 4d and 4f photoelectrons are highly spin polarized perpendicular to the reaction plane as defined by the incident photons and the outgoing electrons. The measured spin polarization and a close look at the core-level photoemission process demonstrate that the bulk core-level photoemission with unpolarized light contributes to the measured spin polarization. This result is in contrast to the valence band photoemission from nonmagnetic solids, wherein the bulk cannot contribute to the measured spin polarization due to the existence of spatial inversion symmetry. Thus, the argument based on spatial inversion symmetry does not apply to the core-level photoemission. The measured spin polarization is in good agreement with an atomic model.

S.-W. Yu, J. G. Tobin
Phys. Rev. B 77, 193409

In the first part of this work, the electrical conductivity of vanadium phosphorous oxide (VPO) catalyst was investigated by means of the 2-probe EIS method. The VPO showed an extremely low conductivity at low oxygen partial pressure, which is the prevailing condition in the anodic compartment in an electrochemical membrane reactor (EMR). In the second part of this study, fresh as well as VPO catalyst already used in an EMR were characterised with XRD, XPS and temperature programmed oxidation (TPO). The XRD measurements revealed an unchanged bulk phase structure after operation in the EMR. Significant differences in the average oxidation states of vanadium in the catalyst layer in the EMR were determined via XPS, where the catalyst surface facing the electrolyte membrane was more oxidised than the surface facing the anodic gas compartment. The lowered uptake and release of oxygen was observed in TPO experiments for the catalyst used in the EMR.

L. K. Rihko-Struckmann, Y. Ye, L. Chalakov, Y. Suchorski, H. Weiss, K. Sundmacher
Catalysis Letters 109, pp. 89-96

This work reports about bulk-sensitive, high energy photoelectron spectroscopy from the valence band of CoTiSb excited by photons from 1.2 to 5 keV energy. The high energy photoelectron spectra were taken at the KMC-1 high energy beamline of BESSY II employing the recently developed Phoibos 225 HV analyser. The measurements show a good agreement to calculations of the electronic structure using the LDA scheme. It is shown that the high energy spectra reveal the bulk electronic structure better compared to low energy XPS spectra.

G. H. Fecher, A. Gloskowsky, K. Kroth, J. Barth, B. Balke, C. Felser, F. Schäfers, M. Mertin, S. Maehl, O. Schaff
Journal of Electron Spectroscopy and Related Phenomena 156-158, pp. 97-101

Clean and stable surface modifications of an iridium (100) single crystal, i.e., the (1 × 1) phase, the (5 × 1) reconstruction, and the oxygen-terminated (2 × 1)-O surface, were prepared and characterized by low energy electron diffraction (LEED), temperature-programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS) and polarization modulation IRAS (PM-IRAS). The adsorption of CO in UHV and at elevated (mbar) pressure/temperature was followed both ex situ and in situ on all three surface modifications, with a focus on mbar pressures of CO. The Ir(1 × 1) surface exhibited c(4 × 2)/c(2 × 2) and c(6 × 2) CO structures under low pressure conditions, and remained stable up to 100 mbar and 700 K. For the (2 × 1)-O reconstruction CO adsorption induced a structural change from (2 × 1)-O to (1 × 1), as confirmed by LEED, TPD, and IR. For Ir (2 × 1)-O TPD indicated that CO reacted with surface oxygen forming CO2. The (5 × 1) reconstruction featured a reversible and dynamic behavior upon CO adsorption, with a local lifting of the reconstruction to (1 × 1). After CO desorption, the (5 × 1) structure was restored. All three reconstructions exhibited CO adsorption with on-top geometry, as evidenced by IR. With increasing CO exposure the resonances shifted to higher wavenumber, due to adsorbate–adsorbate and adsorbate–substrate interactions. The largest wavenumber shift (from 2057 to 2100 cm–1) was observed for Ir(5 × 1) upon CO dosing from 1 L to 100 mbar.

K. Anic, A.V. Bukhtiyarov, H. Li, C. Rameshan, G. Rupprechter
J. Phys. Chem. C 120 (20), pp. 10838–10848

The adsorption of CO on Pd(100) was investigated using simultaneous ambient pressure X-ray photoelectron spectroscopy (APXPS) and infrared reflection absorption infrared spectroscopy (IRRAS). The measurements were performed as a function of CO partial pressures from ultra-high vacuum to 0.5 Torr. Total CO coverages estimated from the complementary APXPS and IRRAS measurements are in good agreement. A signal for atop CO, which is uncommon for Pd(100), was observed in the IRRAS data and was used to identify the C 1 s binding energy of this species. Discerning this binding configuration of CO on the Pd(100) surface at elevated pressures has significance for catalytic reactions involving CO, where bridging CO is often the only configuration considered. We also detail the combined APXPS/IRRAS instrumentation and discuss ways to improve these multimodal measurements, which should have wide applicability across many areas of surface and interface science.

A. R. Head, O. Karslıoǧlu, T. Gerber, Y. Yu, L. Trotochaud, J. Raso, P. Kerger, H. Bluhm
Surface Science 665, pp. 51-55

A catalytic site typically consists of one or more atoms of a catalyst surface that arrange into a configuration offering a specific electronic structure for adsorbing or dissociating reactant molecules. The catalytic activity of adjacent bimetallic sites of metallic nanoparticles has been studied previously. An isolated bimetallic site supported on a non-metallic surface could exhibit a distinctly different catalytic performance owing to the cationic state of the singly dispersed bimetallic site and the minimized choices of binding configurations of a reactant molecule compared with continuously packed bimetallic sites. Here we report that isolated Rh1Co3 bimetallic sites exhibit a distinctly different catalytic performance in reduction of nitric oxide with carbon monoxide at low temperature, resulting from strong adsorption of two nitric oxide molecules and a nitrous oxide intermediate on Rh1Co3 sites and following a low-barrier pathway dissociation to dinitrogen and an oxygen atom. This observation suggests a method to develop catalysts with high selectivity.

S. Zhang, L. Nguyen, J.-X. Liang, J. Shan, J. Liu, A.I. Frenkel, A. Patlolla, W. Huang, J. Li, F. Tao
Nature Communications 6, Article number: 7938

The adsorption of Pb on Pd ( 1 1 0)and co-adsorption with CO have been studied by photoemission spectroscopy and low energy electron diffraction. Two ordered structures are formed at sub-monolayer coverages of Pb with symmetry c(2×2) and (3×1). The valence band and Pd 3d core level spectra indicate a strong chemical interaction between Pb and Pd, and the formation of intermetallic bonds with both the first and second layer of Pd. The heat of CO adsorption on the c(2×2) phase is reduced to about half of the value on clean Pd, and even less on the (3×1) structure so that CO does not adsorb at a temperature of 120 K. Structural models are proposed for both of these phases based on the photoemission and co-adsorption results. The results confirm that the de-activation by Pb of Pd as a CO oxidation catalyst is due to the formation of an intermetallic compound, which drastically lowers the adsorption energy. However even the formation of a sub-monolayer phase is sufficient to radically alter the catalytic properties, and the formation of a bulk intermetallic is not necessary.

N. Tsud, V. Dudr, S. Fabík, C. Brun, V. Cháb, V. Matolín , K. C. Prince
Surface Science 560 (1-3), pp. 259-268

An apparatus for fabrication, surface analysis in ultrahigh vacuum, and testing of the catalytic activity of model metal alloy catalysts is described. Arrays of model catalysts are produced by electron-beam deposition of up to four metals simultaneously onto a substrate. The surface analysis techniques available are scanning electron microscopy, x-ray photoemission spectroscopy, ion scattering spectroscopy, Auger electron spectroscopy, sputter profiling, and temperature programmed desorption. The catalytic activity of the model catalysts is tested individually by scanning a combined gas delivery and gas sampling device over the sample surface. The gas sampled is analyzed with mass spectrometry. Experiments can be made at pressures up to 1 bar and temperatures up to 500 °C. It is shown that the lateral resolution is better than 0.2 mm and that up to 20 circular spots, 1 mm in diameter, can be studied on a substrate 10 mm in diameter. A high pressure cell with an all-metal sealed ultrahigh vacuum lock is also described as part of the work.

M. Johansson, J. Hoffmann Jørgensen, I. Chorkendorff
Review of Scientific Instruments 75, 2082

Using a multi-technical approach, we studied the oxidation of anatase TiO2(1 0 1)-supported vanadium (V) clusters at room temperature. We found by ex situ XPS that the highest oxidation state is +4 at sub-monolayer coverage regardless of the O2 pressure, and STM studies revealed that the initial oxidation proceeds through oxygen-induced disintegration of V clusters into monomeric VO2 species. By contrast, for ∼2 monolayer V coverage, a partial oxidation to V5+ is achieved. By in situ APXPS measurements, we found that V can be maintained in the V5+ oxidation state irrespective of the coverage; however, in the sub-monolayer range, an O2 pressure of at least ∼1 × 10−5 mbar is needed. Our results suggest an enhanced reducibility of V in direct contact with the TiO2 support compared to V in the 2nd layer, which is in line with the observed optimum V2O5 loading in catalytic applications just slightly below a full monolayer.

S. Koust, B. N. Reinecke, K. C. Adamsen, I. Beinik, K. Handrup, Z. Li, P. G. Moses, J. Schnadt, J. V. Lauritsen, S. Wendt
Journal of Catalysis 360, pp. 118-126

Surface islands formed by grain-boundary diffusion has been studied in Ni/Cu nanolayers by in-situ low energy ion scattering spectroscopy, X-ray photoelectron spectroscopy, scanning probe microscopy and ex-situ depth profiling based on ion sputtering. In this paper a new experimental approach of measurement of grain-boundary diffusion coefficients is presented. Appearing time of copper atoms diffused through a few nanometer thick nickel layer has been detected by low energy ion scattering spectroscopy with high sensitivity. The grain-boundary diffusion coefficient can be directly calculated from this appearing time without using segregation factors in calculations. The temperature range of 423–463 K insures the pure C-type diffusion kinetic regime. The most important result is that surface coverage of Ni layer by Cu atoms reaches a maximum during annealing and stays constant if the annealing procedure is continued. Scanning probe microscopy measurements show a Volmer-Weber type layer growth of Cu layer on the Ni surface in the form of Cu atomic islands. Depth distribution of Cu in Ni layer has been determined by depth profile analysis.

V. Takáts, A. Csik, J. Hakl, K. Vad
Applied Surface Science 440, pp. 275-281

We present a versatile apparatus for the study of ferromagnetic surfaces, which combines spin-polarized photoemission and inverse photoemission spectroscopies. Samples can be grown by molecular beam epitaxy and analyzed in situ. Spin-resolved photoemission spectroscopy analysis is done with a hemispherical electron analyzer coupled to a 25 kV-Mott detector. Inverse photoemission spectroscopy experiments are performed with GaAs crystals as spin-polarized electron sources and a UV bandpass photon detector. As an example, measurements on the oxygen passivated Fe(100)-p(1×1)O surface are presented.

G. Berti, A. Calloni, A. Brambilla, G. Bussetti, L. Duò, F. Ciccacci
Review of Scientific Instruments 85, 073901

Studies were performed on the synthesis of spherical silica particles in an emulsion route, and its surface modification using N-2-(aminoethyl)-3-aminopopyltrimethoxysilane. Onto the aminosilane-treated silica surface, C.I. Acid Red 18 dye was adsorbed. The ensuing silica and its derivatives were subjected to comprehensive physicochemical and dispersion analysis. Both the size and shape of the particles were estimated and the surface specific area (BET) and porosity were characterised using adsorption/desorption curves. Moreover, chemical and surface compositions were determined using elemental analysis and XPS. A mechanism was suggested for the interaction betweeen the aminosilane-modified silica and an organic dye. The obtained pigment was tested in an acrylic paint system; the precipitated monodisperse silica was found to exhibit optimum properties as a carrier of organic dye, particularly after modification of the former with aminosilane, thus acting as a valuable filler and pigment in modern paint systems.

T. Jesionowski, M. Pokora, W. Tylus, A. Dec, A. Krysztafkiewicz
Dyes and Pigments 57 (1), pp. 29-41

The LaCoO3 perovskite-type catalysts in this study were monoliths resting on supports made of heat-resisting foil and washcoated with Al2O3. The La0.9Ag0.1CoO3 or La0.92Pd0.08CoO3 perovskite was used as the active phase. Partial substitution of lanthanum in the LaCoO3 perovskite with palladium or silver enhances the activity of the monolithic catalysts in the combustion of methane. XPS and XRD analyses show that during approximately 500 h on stream (at 750 and 704 °C, respectively) the surface composition of the La0.9Ag0.1CoO3 catalyst and that of the La0.92Pd0.08CoO3 catalyst undergoes considerable changes. In both the catalysts, cobalt and aluminum oxide segregate to the surface while the amount of carbonate groups decreases. In the La0.92Pd0.08CoO3 catalyst, the amount of palladium and the average oxidation state of Pd on the surface increase. On the surface of the La0.9Ag0.1CoO3 catalyst, the amount of silver decreases, and Ag in the metallic state oxidizes to Ag+, which probably becomes built-in the perovskite structure. In spite of these changes, the catalysts display a high activity and a good stability during above 500 h of methane combustion.

B. Kucharczyk, W. Tylus
Catalysis Today 90 (1-2), pp. 121-126

In recent years, various doping methods for epitaxial graphene have been demonstrated through atom substitution and adsorption. Here we observe by angle-resolved photoemission spectroscopy (ARPES) a coupling-induced Dirac cone renormalization when depositing small amounts of Ti onto epitaxial graphene on SiC. We obtain a remarkably high doping efficiency and a readily tunable carrier velocity simply by changing the amount of deposited Ti. First-principles theoretical calculations show that a strong lateral (non-vertical) orbital coupling leads to an efficient doping of graphene by hybridizing the 2pz orbital of graphene and the 3d orbitals of the Ti adsorbate, which attached on graphene without creating any trap/scattering states. This Ti-induced hybridization is adsorbate-specific and has major consequences for efficient doping as well as applications towards adsorbate-induced modification of carrier transport in graphene.

J.-W. Chen, H.-C. Huang, D. Convertino, C. Coletti, L.-Y. Chang, H.-W. Shiu, C.-M. Cheng, M.-F. Lin, S. Heun, F. S.-S. Chien, Y.-C. Chen, C.-H. Chen, C.-L. Wu
Carbon 109, pp. 300-305

Thin silicon layers containing about 20% carbon and 20% oxygen were deposited on copper substrates by potentiostatic electroreduction from a 1 M SiCl4 1-butyl-1-methyl-pyrrolidinium bis (trifluoromethyl) sulfonylimide [BMP][TFSI] electrolyte. The electrodeposition process was investigated by means of voltammetric techniques, coupled with in-situ microgravimetry (quartz crystal microbalance, QCM). The electrochemical and QCM data suggest a possible contribution of a partial Si4+ to Si2+ reduction and/or a restructuring of the metallic substrate. Considerable impact of side reactions parallel to the deposition process was indicated by QCM measurements performed under potentiostatic and potentiodynamic conditions. The deposition of silicon-based films was confirmed by energy dispersive X-ray analysis (EDX). Analysis of the chemical composition of the deposit and its elemental distribution were achieved by depth profiling X-ray photoelectron spectroscopy (XPS). The electrodeposited silicon containing layers showed stable lithiation and delithiation with capacity values of about 1200 mAhg−1 and 80% capacity retention after 300 cycles in standard EC/DMC electrolytes. In ionic liquid (IL) the material displayed lower capacity of ca. 500 mAhg−1, which can be attributed to the higher viscosity of this electrolyte and deposition of IL decomposition products during lithiation.

C. A. Vlaic, S. Ivanov, R. Peipmann, A. Eisenhardt, M. Himmerlich, S. Krischok, A. Bund
Electochimica Acta 168, pp. 403-413

It has been suggested that enhanced anion concentrations at the liquid/vapor interface of airborne saline droplets are important to aerosol reactions in the atmosphere. We report ionic concentrations in the surface of such solutions. Using x-ray photoelectron spectroscopy operating at near ambient pressure, we have measured the composition of the liquid/vapor interface for deliquesced samples of potassium bromide and potassium iodide. In both cases, the surface composition of the saturated solution is enhanced in the halide anion compared with the bulk of the solution. The enhancement of anion concentration is more dramatic for the larger, more polarizable iodide anion. By varying photoelectron kinetic energies, we have obtained depth profiles of the liquid/vapor interface. Our results are in good qualitative agreement with classical molecular dynamics simulations. Quantitative comparison between the experiments and the simulations indicates that the experimental results exhibit more interface enhancement than predicted theoretically.

S. Ghosal, J. C. Hemminger, H. Bluhm, B. S. Mun, E. L. D. Hebenstreit, G. Ketteler, D. F. Ogletree, F. G. Requejo, M. Salmeron
Science 307, pp. 563 - 566

Two-dimensional quantum well states in ultrathin metal films generally exhibit a dispersion relation of s-p-derived states that can be described through an effective mass of the corresponding bulk band. By contrast, the effective masses in Pb quantum well states on Si(111), measured through angle-resolved photoemission, are up to an order of magnitude larger than those from the bulk states or predicted by slab calculations, while similar anomalies are not observed in the related In∕Si(111) system. We interpret these data in terms of an enhanced electron localization, and use them to interpret recent scanning tunneling microscopy results.

J. H. Dil, J. W. Kim, Th. Kampen, K. Horn, A. R. H. F. Ettema
Physical Review B 73, 161308 (R)

We present a study of the electronic properties of the interface between the well-established molecular organic semiconductor copper phthalocyanine (CuPc) and the fullerite C60 using photoelectron spectroscopy and the Kelvin-probe (KP) method. Upon deposition of CuPc on C60, we found interfacial shifts of the vacuum level indicating the formation of a dipole layer, while band bending is found to be negligible. The interface dipole of 0.5 eV measured with KP is close to the difference between the work functions of bulk CuPc and C60. No evidence for a chemical interaction at the interface is concluded from the absence of additional features in the core-level spectra at the earliest stages of deposition. The energy-level alignment diagram at the CuPc/C60 interface is derived.

O. V. Molodtsova, T. Schwieger, M. Knupfer
Applied Surface Science 252 (1), pp. 143-147

We have investigated the electronic states of clean and BF3 adsorbed Si(1 0 0) surfaces at low temperature by means of high resolution Si 2p photoelectron spectroscopy. The peak intensities of upper atom and lower atom of the asymmetric dimer in Si 2p spectra do not change even at 30 K compared with those at higher temperature up to 300 K, indicating that the dimer is asymmetric in the ground state. In order to investigate chemical reactivity of asymmetric dimer on Si(1 0 0), a typical Lewis acid molecule BF3 is adsorbed on Si(1 0 0). We have found that BF3 molecules are dissociated into BF2 and F on Si(1 0 0) and dissociated species (BF2 and F) are adsorbed predominantly on the up dimer atoms of the asymmetric dimers.

S. Machida, M. Nagao, Y. Yamamoto, Y. Kakefuda, K. Mukai, Y. Yamashita, J. Yoshinobu
Surface Science 532-535, pp. 716-720

The electronic properties of LiCoO2 have been studied by theoretical band-structure calculations (using density functional theory) and experimental methods (photoemission). Synchrotron-induced photoelectron spectroscopy, resonant photoemission spectroscopy (ResPES), and soft x-ray absorption (XAS) have been applied to investigate the electronic structure of both occupied and unoccupied states. High-quality PES spectra were obtained from stoichiometric and highly crystalline LiCoO2 thin films deposited “in situ” by rf magnetron sputtering. An experimental approach of separating oxygen- and cobalt-derived (final) states by ResPES in the valence-band region is presented. The procedure takes advantage of an antiresonant behavior of cobalt-derived states at the 3p−3d excitation threshold. Information about the unoccupied density of states has been obtained by O K XAS. The structure of the Co L absorption edge is compared to semiempirical charge-transfer multiplet calculations. The experimental results are furthermore compared with band-structure calculations considering three different exchange potentials [generalized gradient approximation (GGA), using a nonlocal Hubbard U (GGA+U) and using a hybrid functional (Becke, three-parameter, Lee-Yang-Parr [B3LYP])]. For these different approaches total density of states and partial valence-band density of states have been investigated. The best qualitative agreement with experimental results has been obtained by using a GGA+U functional with U=2.9 eV.

D. Ensling, A. Thissen, S. Laubach, P.C. Schmidt, W. Jaegermann
Phys. Rev. B 82, 195431

Semiconductor device properties based on electrolyte contacts or modified by electrochemical reactions are dominated by the electronic structure of the interface. Electron spectroscopy as e.g. photoemission is the most appropriate surface science techniques to investigate elementary processes at semiconductor/electrolyte interfaces. For such investigations a specific experimental set-up (SoLiAS) has been built-up which allows performing model experiments as well as surface analysis after emersion under different experimental conditions. The experimental approach is presented by a number of experiments performed during the last years with GaAs as substrate material. Model experiments by adsorption and coadsorption of electrolyte species give information on fundamental aspects of semiconductor/electrolyte interactions. Emersion experiments give information on a final composition and the related electronic structure of electrodes after electrochemical reactions. The use of frozen electrolytes will help to bridge the gap between these two approaches. With the combination of the experimental procedures one may expect a detailed analysis of electrolyte (modified) interfaces covering chemical composition, electronic structure of surfaces/interfaces as well as surface/interface potentials.

Th. Mayer, M. Lebedev, R. Hunger, W. Jaegermann
Applied Surface Scinece 252, pp. 31-42

In a new approach, we introduce a combination of chemical (CBE) and molecular beam epitaxy (MBE) of CuInS2 (CIS) on hydrogen-terminated Si(111) using di-tert-butyl disulfide (TBDS) as sulphur precursor. The films are analysed in situ with photoelectron spectroscopy and low-energy electron diffraction (LEED). Ex situ, the samples are investigated with X-ray diffraction (XRD) and scanning electron microscopy (SEM). We find that, at growth temperatures of 300 °C, no carbon is incorporated into the deposited film. Furthermore, on the In-rich side of the CuInS2 preparation, we additionally observe Cu2In. However, the valence band structure remains that of a typical CuInS2 film. During the growth in the Cu-rich regime, segregation of Cu2S occurs which can be identified by the shifting of the valence band edge towards the Fermi level. Epitaxial growth of CuInS2 is assumed for both regimes. LEED patterns and XRD data support the epitaxial relation Si{111}∣∣CuInS2{112}.

W. Calvet, C. Lehmann, T. Plake, C. Pettenkofer
Thin Solid Films 480-481, pp. 347-351

The cerium oxidation state in novel calixarene-supported cerium(IV) β-diketonate complexes [p-tBu-calix[4](OMe)2(O)2]Ce(acac)2 (1) and [p-tBu-calix[4](OMe)2(O)2]Ce(hfac)2 (2), which are a new class of potential precursors for homogeneous oxidative transformations, has been determined using X-ray photoelectron spectroscopy (XPS). Cerium oxidation states between 3.6 and 3.65 were detected, distinctly different from their nominal value of +4. An X-ray induced photoreduction of these compounds was detected. Because of the observed stability of the X-ray modified oxidation state under ambient conditions this effect might be used for a long-standing fine tuning of the Ce oxidation state in cerium calixarenes.

Y. Suchorski, J. Gottfriedsen, R. Wrobel B. Strzelczyk, H. Weiss
Solid State Phenomena 128; pp. 115-120

The spin-resolved electronic structure of thin Cr overlayers on top of the Fe(110) surface was investigated by means of spin- and angle-resolved photoelectron spectroscopy. The initial fast drop of photoelectron spin-polarization at the Fermi level, followed by weak oscillatory behavior with the period of about 2 ML, can give an evidence for the first time spectroscopic observation of the short period oscillations in (110)-oriented thin Cr films.

Yu. S. Dedkov
Eur. Phys. J. B 57, pp. 15-19

The stability of different vanadium-based catalysts for the selective oxidation of small hydrocarbons under the ultra-high vacuum (UHV) conditions of standard X-ray photoelectron spectroscopy (XPS) was studied by using a multi-purpose surface analysis apparatus which allows time spans of only a few minutes between the sample transfer into vacuum and the first photoelectron spectrum. For vanadium phosphorus oxide catalysts a significant dependence of the average vanadium oxidation state on the time of exposure to the UHV was observed, with a substantial decrease of the V+5/V+4 ratio within only a few minutes. A much less pronounced reduction was found for alumina-supported VOx catalysts. The observed changes are predominantly due to the vacuum environment with a rather minor (if at all) contribution of the X-ray excitation.

Y. Suchorski, L. Rihko-Strckmann, F. Klose, Y. Ye, M. Alandjiyska, K. Sundmacher, H. Weiss,
Applied Surface Science 249 (1-4), pp. 231-237

The adsorption of CO on Pt nanoclusters grown in a regular array on a template provided by the graphene/Ir(111) Moiré was investigated by means of infrared-visible sum frequency generation vibronic spectroscopy, scanning tunneling microscopy, X-ray photoelectron spectroscopy from ultrahigh vacuum to near-ambient pressure, and ab initio simulations. Both terminally and bridge bonded CO species populate nonequivalent sites of the clusters, spanning from first to second-layer terraces to borders and edges, depending on the particle size and morphology and on the adsorption conditions. By combining experimental information and the results of the simulations, we observe a significant restructuring of the clusters. Additionally, above room temperature and at 0.1 mbar, Pt clusters catalyze the spillover of CO to the underlying graphene/Ir(111) interface.

N. Podda, M. Corva, F. Mohamed, Z. Feng, C. Dri, F. Dvorák, V. Matolin, G. Comelli, M. Peressi, E. Vesselli
ACS Nano 11 (1), pp. 1041–1053

Image-potential states have been investigated widely by time-resolved two-photon photoemission (2PPE) as a model system to study electron dynamics at metal surfaces. Rare-gas layers, which represent a prototype for dielectric overlayers, can modify the coupling of these states to the bulk metal in a controlled way. It will be shown that binding energies and lifetimes for inelastic decay depend in a straightforward way on the electron affinity of the rare gas and the layer thickness. Many of the experimental results can be accounted for in a simple one-dimensional description with a potential that only varies perpendicular to the surface. For rare-gas layers of sufficient thickness new electronic states arise from the screened image-potential of the metal within the adlayer. These states have similar properties as the image-potential states on clean surfaces, but are spatially located at the dielectric/metal interface and can have energies above the vacuum level. We discuss the origin and the basic properties of these previously unexplored interface states for the system Ar/Cu(1 0 0). We will show how time-resolved 2PPE can be used to study their decay by resonant tunneling through the layer into vacuum and by electron–hole-pair decay into the metal.

J. Güdde, U. Höfer
Progress in Surface Science 80, pp. 49-91

We report on a magnetic analysis by means of spin-resolved photoelectron spectroscopy of an atomically flat heteromagnetic rare-earth interface of 1 ML Eu∕Gd(0001). The measurements reveal a high net Eu magnetization at low temperatures reflected by a spin polarization ∼15% of the Eu 4f state. This magnetic Eu configuration is due to a strong ferromagnetic interlayer exchange coupling across the Eu∕Gd interface which overcomes a weak negative intralayer coupling between Eu spins in the hexagonal two-dimensional lattice.

Yu. S. Dedkov, Th. Kleissner, E. N. Voloshina, S. Danzenbächer, S. L. Molodtsov, C. Laubschat
Physical Review B 73, 012402

Graphite vaporization provides an uncontrolled yet efficient means of producing fullerene molecules. However, some fullerene derivatives or unusual fullerene species might only be accessible through rational and controlled synthesis methods. Recently, such an approach has been used to produce isolable amounts of the fullerene C60 from commercially available starting materials. But the overall process required 11 steps to generate a suitable polycyclic aromatic precursor molecule, which was then dehydrogenated in the gas phase with a yield of only about one per cent. Here we report the formation of C60 and the triazafullerene C57N3 from aromatic precursors using a highly efficient surface-catalysed cyclodehydrogenation process. We find that after deposition onto a platinum (111) surface and heating to 750 K, the precursors are transformed into the corresponding fullerene and triazafullerene molecules with about 100 per cent yield. We expect that this approach will allow the production of a range of other fullerenes and heterofullerenes, once suitable precursors are available. Also, if the process is carried out in an atmosphere containing guest species, it might even allow the encapsulation of atoms or small molecules to form endohedral fullerenes.

G. Otero, G. Biddau, C. Sánchez-Sánchez, R. Caillard, M.F. López, C. Rogero, F. J. Palomares, N. Cabello, M. A. Basanta, J. Ortega, J. Mendez, A. M. Echavarren, R. Pérez, B. Gómez-Lor, J. A. Martín-Gago
Nature 454, pp. 865-869

Atmospheric pressure X-ray photoelectron spectroscopy (XPS) is demonstrated using single-layer graphene membranes as photoelectron-transparent barriers that sustain pressure differences in excess of 6 orders of magnitude. The graphene serves as a support for catalyst nanoparticles under atmospheric pressure reaction conditions (up to 1.5 bar), where XPS allows the oxidation state of Cu nanoparticles and gas phase species to be simultaneously probed. We thereby observe that the Cu2+ oxidation state is stable in O2 (1 bar) but is spontaneously reduced under vacuum. We further demonstrate the detection of various gas-phase species (Ar, CO, CO2, N2, O2) in the pressure range 10–1500 mbar including species with low photoionization cross sections (He, H2). Pressure-dependent changes in the apparent binding energies of gas-phase species are observed, attributable to changes in work function of the metal-coated grids supporting the graphene. We expect atmospheric pressure XPS based on this graphene membrane approach to be a valuable tool for studying nanoparticle catalysis.

R. S. Weatherup, B. Eren, Y. Hao, H. Bluhm, M. B. Salmeron
J. Phys. Chem. Lett. 7 (9), pp. 1622–1627

Here we report a photoemission study of the Fe intercalation underneath a graphene layer on Ni(111). The process of intercalation was monitored by means of x-ray photoemission of corresponding core levels as well as ultraviolet photoemission of the graphene-derived π states in the valence band. Thin fcc Fe layers (2–5 ML thickness) at the interface between a graphene capping layer and Ni(111) form epitaxial films passivated from the reactive environment.

Yu. S. Dedkov, M. Fonin, U. Rüdiger, C. Laubschat
Appl. Phys. Lett. 93, 022509

A Hard X-ray Photoelectron Spectroscopy (HAXPES) characterisation of the passivation layers formed by electrochemical polarisation of Al–Cr–Fe complex metallic alloys is presented. By employing X-ray excitation energies from 2.3 to 10.0 keV, the depth distributions of Al- and Cr-oxide and hydroxide species in the (Al,Cr)-containing passive layers could be determined. Simultaneous analyses of the shallow Al 2s and deep Al 1s core level lines (respectively, more bulk- and surface-sensitive) provided complementary information to effectively determine the depth-resolved contributions of hydroxide and oxide species within the passivation layer. A Cr threshold concentration of 18 (at.%) was found for effective passivation at pH 1.

A. Beni, N. Ott, M. Pawelkiewicz, M. Wardé, K. Young, B. Bauer, P. Rajput, B. Detlefs, J. Zegenhagen, R. McGrath, M.-G. Barthés-Labrousse, L. P. H. Jeurgens, P. Schmutz
Electrochemistry Communications 46, pp. 13-17

In light of accumulating evidence highlighting the major effect of operational conditions (gas composition, pressure, temperature) on the surface/bulk structure of catalytic materials, their characterization should involve more and more in situ methods. We constructed a synchrotron-based high-pressure X-ray photoelectron spectroscopic (XPS) instrument, allowing us to investigate the surface and near-surface state of a catalyst in the mbar pressure range. We discuss here the surface characteristics of palladium samples as a function of gas phase (hydrogen, oxygen) and temperature. We demonstrate that the surface region of catalytic materials behaves dynamically in its composition, always reflecting its environment. For example, surface oxide can be formed on Pd(111) in oxygen, which decomposes rapidly when the gas supply is switched off. The chemical nature of carbonaceous deposits also depends strongly on the operational conditions (gas-phase hydrogen, temperature). This is the first time that an XPS investigation of palladium β-hydride was performed at RT. The possible drawbacks of using a non-UHV setup (e.g., fast carbon accumulation) are also discussed.

D. Teschner, A. Pestryakov, E. Kleimenov, M. Hävecker, H. Bluhm, H. Sauer, A. Knop-Gericke, R. Schlögl
Journal of Catalysis 230, pp. 186 - 194

Probing initial interactions at the interface of hybrid systems under humid conditions has the potential to reveal the local chemical environment at solid/solid interfaces under real-world, technologically relevant conditions. Here, we show that ambient pressure X-ray photoelectron spectroscopy (APXPS) with a conventional X-ray source can be used to study the effects of water exposure on the interaction of a nanometer-thin polyacrylic acid (PAA) layer with a native aluminum oxide surface. The formation of a carboxylate ionic bond at the interface is characterized both with APXPS and in situ attenuated total reflectance Fourier transform infrared spectroscopy in the Kretschmann geometry (ATR-FTIR Kretschmann). When water is dosed in the APXPS chamber up to 5 Torr (~28% relative humidity), an increase in the amount of ionic bonds at the interface is observed. To confirm our APXPS interpretation, complementary ATR-FTIR Kretschmann experiments on a similar model system, which is exposed to an aqueous electrolyte, are conducted. These spectra demonstrate that water leads to an increased wet adhesion through increased ionic bond formation.

S. Pletincx, L. Trotochaud, L.-L. Fockaert, J. M. C. Mol, A.R. Head, O. Karslıoğlu, H. Bluhm, H. Terryn, T. Hauffman
Scientific Reports 7, Article number: 45123

Using a photoemission spectroscometer that operates close to ambient conditions of pressure and temperature we have determined the Pd−O phase diagram and the kinetic parameters of phase transformations. We found that on the (111) surface oxidation proceeds by formation of stable and metastable structures. As the chemical potential of O2 increases chemisorbed oxygen forms followed by a thin surface oxide. Bulk oxidation is a two-step process that starts with the metastable growth of the surface oxide into the bulk, followed by a first-order transformation to PdO.

G. Ketteler, D. F. Ogletree, H. Bluhm, H. Liu, E. L. D. Hebenstreit, M. Salmeron
J. Am. Chem. Soc. 127 (51), pp. 18269–18273

Methanol decomposition and oxidation on Pd(111) at millibar pressure were studied by in situ polarization−modulation infrared reflection absorption spectroscopy (PM-IRAS), on-line gas chromatography and pre- and postreaction X-ray photoelectron spectroscopy (XPS). Various dehydrogenation products such as methoxy CH3O, formaldehyde CH2O, formyl CHO, and CO could be spectroscopically identified. Methanol oxidation proceeds via dehydrogenation to formaldehyde CH2O, which either desorbs or is further dehydrogenated to CO, which is subsequently oxidized to CO2. Carbonaceous overlayers that are present during the reaction may favorably affect the selectivity toward CH2O. The reaction takes place on metallic Pd, and no indications of an involvement of Pd surface oxide were observed.

M. Borasio , O. Rodríguez de la Fuente, G. Rupprechter, H.-J. Freund
J. Phys. Chem. B 109 (38), pp. 17791–17794

The development of efficient energy conversion systems requires precise engineering of electrochemical interfaces and thus asks for in situ techniques to probe the structure and the composition of the dynamic electrode/electrolyte interfacial region. This work demonstrates the potential of the near ambient pressure X-ray photoelectron spectroscopy (NAPXPS) for in situ studies of processes occurring at the interface between a metal electrode and a liquid electrolyte. By using a model membrane-electrode assembly of a high temperature phosphoric acid-imbibed proton exchange membrane fuel cell, and combining NAPXPS measurements with the density functional theory, it was possible to monitor such fundamental processes as dissociation and migration of the phosphoric acid within a nanostructured Pt electrode under polarization.

Y. T. Law, S. Zafeiratos, S. G. Neophytides, A. Orfanidi, D. Costa, T. Dintzer, R. Arrigo, A. Knop-Gericke, R. Schlögl, E. R. Savinova
Chem. Sci. 6 (10) , pp. 5635-5642

Photoelectron spectroscopy is known for the chemical analysis of surfaces with element specificity. When a material is excited with ultraviolet or X-ray photons, photoelectrons and secondary photons are emitted which carry a kinetic energy and a momentum which reflect the electronic and molecular structure information of the material. The limited mean free path of electrons in matter requires ultrahigh vacuum (UHV) technology warrants that only electrons at or underneath the surface are detected by the instrument. The term surface has different perception and reception in different scientific communities. To the experimental condensed matter physicist, the surface is a two-dimensional system which forms as a result of the cleavage splitting of a three-dimensional body. To a theoretical condensed matter physicist a body may have infinite dimensions, when necessary.The two half bodies respond to such splitting with surface reconstruction, which makes that the crystallographic and electronic structure of a surface differs from the volume. In such case, electronic surface states may evolve which influence the interaction of the body with its environment, be it other solids or fluids. This encyclopedia article features in situ XPS experiments beyond the conventional UHV conditions.

A. Braun
Chemistry, Molecular Sciences and Chemical Engineering; Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry, pp. 264-279

Catalysts used for heterogeneous processes are usually composed of metal nanoparticles dispersed over a high–surface-area support. In recent years, near-ambient pressure techniques have allowed catalyst characterization under operating conditions, overcoming the pressure gap effect. However, the use of model systems may not truly represent the changes that occur in real catalysts (the so-called material gap effect). Supports can play an important role in the catalytic process by providing new active sites and may strongly affect both the physical and chemical properties of metal nanoparticles. We used near-ambient pressure x-ray photoelectron spectroscopy to show that the surface rearrangement of bimetallic (rhodium-palladium) nanoparticles under working conditions for ethanol steam reforming with real catalysts is strongly influenced by the presence of a reducible ceria support.

N. J. Divins, I. Angurell, C. Escudero, V. Pérez-Dieste, J. Llorca
Science 346 (6209), pp. 620-623

In this article we provide a summary of the recent development of ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and its application to catalytic surface chemistry. The methodology as well as significant advantages and challenges associated with this novel technique are described. Details about specific examples of using AP-XPS to probe surface chemistry under working reaction conditions for a number of reactions are explained: CO oxidation, water-gas shift (WGS), CO2 hydrogenation, dry reforming of methane (DRM) and ethanol steam reforming (ESR). Finally, we discuss insights into the future development of the AP-XPS technique and its applications.

R. M. Palomino, R. Hamlyn, Z. Liu, D. C. Grinter, I. Waluyo, J. A. Rodriguez, S. D. Senanayake
Journal of Electron Spectroscopy and Related Phenomena 221, pp. 28-43

Controlling the charge transfer between a semiconducting catalyst carrier and the supported transition metal active phase represents an elite strategy for fine turning the electronic structure of the catalytic centers, hence their activity and selectivity. These phenomena have been theoretically and experimentally elucidated for oxide supports but remain poorly understood for carbons due to their complex nanoscale structure. Here, we combine advanced spectroscopy and microscopy on model Pd/C samples to decouple the electronic and surface chemistry effects on catalytic performance. Our investigations reveal trends between the charge distribution at the palladium–carbon interface and the metal’s selectivity for hydrogenation of multifunctional chemicals. These electronic effects are strong enough to affect the performance of large (~5 nm) Pd particles. Our results also demonstrate how simple thermal treatments can be used to tune the interfacial charge distribution, hereby providing a strategy to rationally design carbon-supported catalysts.

R. G. Rao, R. Blume, T. W. Hansen, E. Fuentes, K. Dreyer, S. Moldovan, O. Ersen, D. D. Hibbitts, Y. J. Chabal, R. Schlögl, J.-P. Tessonnier
Nature Communications, 8, Article number: 340

We have examined the Pb/Ag(1 1 0) adsorption system using low energy electron diffraction (LEED) and synchrotron radiation X-ray photoemission spectroscopy (SRXPS). At a temperature of 120 K, layer-by-layer growth occurs and no superstructure is observed by LEED. On heating a two monolayer film to 230 K, or on deposition of two monolayers at 300 K temperature, a c(2 × 4) superstructure is observed. We interpret this as a reconstruction involving a (1 × 2) Ag substrate, and a buckled hexagonal overlayer of Pb. The reconstruction is unusual as it is induced at a solid–solid interface, rather than by an adsorbate at the solid–vacuum interface. Core level shifts of the Pb 5d and 4f and Ag 3d levels are also reported.

N. Tsud, S. Fabík, V. Dudr, M. Vondrácek, V. Cháb, V. Matolín, K. C. Prince
Surface Science 542 (1-2), pp. 112-119

Heterogeneous chemical reactions at vapor/solid interfaces play an important role in many processes in the environment and technology. Ambient pressure X-ray photoelectron spectroscopy (APXPS) is a valuable tool to investigate the elemental composition and chemical specificity of surfaces and adsorbates on the molecular scale at pressures of up to 130 mbar. In this review we summarize the historical development of APXPS since its introduction over forty years ago, discuss different approaches to minimize scattering of electrons by gas molecules, and give a comprehensive overview about the experimental systems (vapor/solid interfaces) that have been studied so far. We also present several examples for the application of APXPS to environmental science, heterogeneous catalysis, and electrochemistry.

D. E. Starr, Z. Liu, M. Hävecker, A. Knop-Gerickec, H. Bluhm
Chem. Soc. Rev. 42, pp. 5833-5857

Nickel/doped‐ceria composites are promising electrocatalysts for solid‐oxide fuel and electrolysis cells. Very often steam is present in the feedstock of the cells, frequently mixed with other gases, such as hydrogen or CO2. An increase in the steam concentration in the feed mixture is considered accountable for the electrode oxidation and the deactivation of the device. However, direct experimental evidence of the steam interaction with nickel/doped‐ceria composites, with adequate surface specificity, are lacking. Herein we explore in situ the surface state of nickel/gadolinium‐doped ceria (NiGDC) under O2, H2, and H2O environments by using near‐ambient‐pressure X‐ray photoelectron and absorption spectroscopies. Changes in the surface oxidation state and composition of NiGDC in response to the ambient gas are observed. It is revealed that, in the mbar pressure regime and at intermediate temperature conditions (500–700 °C), steam acts as an oxidant for nickel but has a dual oxidant/reductant function for doped ceria.

V. Papaefthimiou, D. K. Niakolas, F. Paloukis, T. Dintzer, S. Zafeiratos
ChemPhysChem 18 (1), pp. 164-170

Chemical interactions which occur at a heterogeneous interface between a gas and substrate are critical in many technological and natural processes. Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) is a powerful spectroscopy tool that is inherently surface sensitive, elemental and chemical specific, with the ability to probe sample surfaces in the presence of a gas phase. In this review, we discuss the evolution of lab-based AP-XPS instruments, from the first development by Siegbahn and coworkers up through modern day systems. A comprehensive overview is given of heterogeneous experiments investigated to date via lab-based AP-XPS along with the different instrumental metrics that affect the quality of sample probing. We conclude with a discussion of future directions for lab-based AP-XPS, highlighting the efficacy for this in-demand instrument to continue to expand in its ability to significantly advance our understanding of surface chemical processes under in situ conditions in a technologically multidisciplinary setting.

C. Arble, M. Jia, J. T. Newberg
Surface Science Reports, 72(2), pp. 37-57

Titania has attracted significant interest due to its broad catalytic applications, many of which involve titania nanoparticles in contact with aqueous electrolyte solutions. Understanding the titania nanoparticle/electrolyte interface is critical for the rational development of such systems. Here, we have employed liquid-jet ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to investigate the solid/electrolyte interface of 20 nm diameter TiO2 nanoparticles in 0.1 M aqueous nitric acid solution. The Ti 2p line shape and absolute binding energy reflect a fully oxidized stoichiometric titania lattice. Further, by increasing the X-ray excitation energy, the difference in O 1s binding energies between that of liquid water (O 1sliq) and the titania lattice (O 1slat) oxygen was measured as a function of probe depth into the particles. The titania lattice, O 1slat, binding energy decreases by 250 meV when probing from the particle surface into the bulk. This is interpreted as downward band bending at the interface.

M. J. Makowski, R. P. Galhenage, J. Langford, J. C. Hemminger
J. Phys. Chem. 7 (9), pp. 1732–1735

Within Mo5O14-like structural type for mixed V–Mo–Nb oxides, the effect of vanadium and niobium content and treatment temperature on the phase composition, structure and catalytic properties of the samples in oxidative dehydrogenation of ethane has been studied. The limits of V and Nb content in the ternary Mo5O14-like oxide are determined. V–Mo–Nb oxides are shown to be the substitution solid solutions of composition (Mo0.6VzNb0.4−z)5O14 (0 < z < 0.2) based on binary oxide (Mo0.6Nb0.4)5O14. The structure of crystalline V–Mo–Nb oxides is refined. Crystallization of Mo5O14-like structure occurs at temperatures ≥550 °C. Below 550 °C, V–Mo–Nb oxides are nanocrystalline materials having a disordered layered structure with the interlayer distance of ca. 4 Å. The structural model of nanocrystalline oxides is discussed. The role of Nb in the formation of ternary V–Mo–Nb oxide compounds is revealed, and the surface state of these oxides at different temperatures is analyzed. The low-temperature V–Mo–Nb oxides are the most active catalysts in the reaction of ethane oxidative dehydrogenation. They are characterized by the ratio V/Nb ≈ 1 in the Mo5O14-like structure.

T. Yu. Kardash, L. M. Plyasova, V. M. Bondareva, T. V. Andrushkevich, L. S. Dovlitova, A. I. Ischenko, A. I. Nizovskii, A. V. Kalinkin
Applied Catalysis A: General 375 (1) , pp. 26-36

We report on an application of a simultaneous perturbation stochastic approximation (SPSA) algorithm to filtering systematic noise (SN) with nonzero mean value in photoemission data. In our analysis, we have used a series of 50 single-scan photoemission spectra of W(110) surface where different SNs were added. It was found that the SPSA-evaluated spectrum is in good agreement with the spectrum measured without SN. On the basis of our results, a wide application of SPSA algorithm for evaluation of experimental data is anticipated.

D. S. Fedin, O. N. Granichin, Yu. S. Dedkov, S. L. Molodtsov
Rev. Sci. Instrum. 79, 036103

The presented paper is a peer-review of plasma action on porous polymer membranes. Considering the plasma medium the discussion is split in two parts: i) the first shows some effects caused by action of non-polymerizing gases, i.e. polymer etching and/or alteration of surface character, and, ii) the second covers action of plasma reagents resulted in the deposition of polymer layers. The authors' attention is focused mainly on alteration of the membranepore structure and surfaces character. Both kinds of plasma can turn porous membrane to new porous species with different surface character and pore size. It is possible also to prepare solid membranes where deposited polymer completely plugs the pores. The presented discussion is illustrated by the results of the authors' own research on preparation of brand-new polymer membranes.

M. Bryjak, G. Pozniak, I. Gancarz, W. Tylus
Desalination 163 (1-3), pp. 231-238

The effect of NH3 and NH3/Ar plasma on ultrafiltration polysulfone membranes have been studied. Results of contact angle, FTIR–ATR and X-ray photoelectron spectroscopy experiments clearly showed that both plasmas introduced hydrophilic, nitrogen- and oxygen-containing moieties on the polymer surface and that NH3/Ar plasma was more efficient. That plasma was also more aggressive––signs of strong etching could be seen on the SEM pictures. Redeposition of etched material seemed to take place inside the pores. On the contrary, ammonia plasma was soft and caused cleaning the surface and pores enlargement. Performance of ammonia plasma modified membranes was greatly improved and independent on solution pH. The last observation proved amphoteric character of the surface. NH3/Ar plasma treatment gave membranes of acidic surface and filtration indices not so good as for ammonia plasma.

M. Bryjak, I. Gancarz, G. Pozniak, W. Tylus
European Polymer Journal 38 (4), pp. 717-726

This paper describes some properties of microwave plasma polymers of n-butylamine and allylamine deposited on the surface of polysulfone substrate. Contact angle evaluation, ATR-FTIR spectroscopy, X-ray photoelectron spectroscopy analysis and estimation of pore size distribution of ultrafiltration polysulfone membranes were used. It was found that addition of Ar to the amine vapor significantly stabilized the plasma and converted it to the ablation mode. The surface became more hydrophilic and the surface groups were enriched in oxygen. Both amines gave deposits of various compositions: the n-butylamine polymer was not as enriched in amines as the polymer formed from allylamine. However, the amounts of nitrogen in both deposits indicated allylamine to be the precursor for the preparation of membranes with weakly basic functionalities. When porous membranes are modified, the ultrafilters obtained may be named `fouling protected' as they do not foul so intensively with proteins as their unmodified analogues. To a lesser extent, similar behavior was shown by membranes modified by deposition of plasma-polymerized n-butylamine.

I. Gancarz, G. Pozniak, M. Bryjak, W. Tylus,
European Polymer Journal 38 (10), pp. 1937-1946

The electron dynamics of buried Ar/Cu(1 0 0) image-potential states was investigated by time-resolved two-photon photoemission (2PPE) as a function of parallel momentum. The first interface state shows a parabolic dispersion with an effective mass of 0.6. Its lifetime of 110 fs at the -point decreases with increasing parallel momentum. The momentum dependence of the decay can be understood by intra- and inter-band decay processes mediated by Cu electrons, just as the decay of image-potential states on the clean Cu(1 0 0) surface.

M. Rohleder, K. Duncker, W. Berthold, J. Güdde, U. Höfer
New Journal of Physics 7, pp. 1367-2630

Understanding the physics of surface plasmons and related phenomena requires knowledge of the spatial, temporal, and spectral distributions of the total electromagnetic field excited within nanostructures and their interfaces, which reflects the electromagnetic mode excitation, confinement, propagation, and damping. We present a microscopic and spectroscopic study of the plasmonic response in single-crystalline Ag wires grown in situ on Si(001) substrates. Excitation of the plasmonic modes with broadly tunable (UV–IR) femtosecond laser pulses excites ultrafast multiphoton photoemission, whose spatial distribution is imaged with an aberration-corrected photoemission electron microscope, thereby providing a time-integrated map of the locally enhanced electromagnetic fields. We show by tuning the wavelength, polarization, and k-vector of the incident laser light that for a few micrometers long wires we can selectively excite either the propagating surface plasmon polariton modes or high-order multipolar resonances of the Ag/vacuum and Ag/Si interfaces. Moreover, upon tuning the excitation wavelength from the UV to the near-IR spectral regions, we find that the resonant plasmonic modes shift from the top of the wires to selvedge at the Ag/Si interface. Our results, supported by numerical simulations, provide a better understanding of the optical response of metal/semiconductor structures and guidance toward the design of polaritonic and nanophotonic devices with enhanced properties, as well as suggest mechanisms for plasmonically enhanced photocatalysis.

M. Da̧browski, Y. Dai, A. Argondizzo, Q. Zou, X. Cui, H. Petek
ACS Photonics 3 (9), pp. 1704–1713

Some properties of ultrafiltration polysulfone membranes modified with n-BuNH2 and n-BuNH2/Ar plasmas were studied. Contact angle measurements, XPS and SEM were used to characterize the surface changes. Pore size estimation, water transport and filtration parameters helped to describe changes in filtration performance. Presence of argon in plasma environment stabilized plasma but made it more aggressive; pore size increased and surface etching took place. This plasma grafted a smaller number of nitrogen features — most of them in the form of amine. Excellent filtration performance was observed for BuNH2/Ar plasma-modified membranes in an acidic environment.

G. Pozniak, I. Gancarz, M. Bryjak, W. Tylus,
Desalination 146 (1-3), pp. 293-299

The atomic layer deposition (ALD) of TiO2 on a RuO2(110) surface from tetrakis(dimethylamido) titanium and water at 110 °C was investigated using near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) at precursor pressures up to 0.1 mbar. In addition to the expected cyclic surface species, evidence for side reactions was found. Dimethylamine adsorbs on the surface during the TDMAT half-cycle, and a second species, likely methyl methylenimine, also forms. The removal of the amide ligand and the formation of an alkyammonium species during the water half-cycle were found to be pressure dependent. The O 1s, Ru 3d, and Ti 2p spectra show the formation of the Ru–O–Ti interface, and the binding energies are consistent with formation of TiO2 after one full ALD cycle. Dosing TDMAT on the RuO2(110) surface at room temperature promotes a multilayer formation that begins to desorb at 40 °C. The imine species is not seen until 60 °C. These insights into the ALD mechanism and precursor pressure dependence on reactivity highlight the utility of NAP-XPS in studying ALD processes and interface formation.

A. R. Head, S. Chaudhary, G. Olivieri, F. Bournel, J. N. Andersen, F. Rochet, J.-J. Gallet, J. Schnadt
J. Phys. Chem. C, 120 (1), pp. 243–251

We have measured the photoelectron spin polarization emitted by unpolarized UV radiation from the valence-bands of the well ordered Pt(0 0 1)-(5 × 1) surface and the disordered surface destroyed by Ar ions bombardment. Almost identical spin polarizations have been observed in both cases. This observation suggests that the electron spin polarization in photoemission caused by unpolarized light is determined by a short-range order of atoms. This finding has an obvious implication that the electron spin polarization in photoemission caused by unpolarized light can be used to study the bulk electronic structure of the nonmagnetic materials.

S.-W. Yu, J. G. Tobin
Surface Science 601, pp. L127–L131

Oxides on the surface of Pt electrodes are largely responsible for the loss of their electrocatalytic activity in the oxygen reduction and oxygen evolution reactions. In this work we apply near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to study in operando the electrooxidation of a nanoparticulated Pt electrode integrated in a membrane-electrode assembly of a high temperature proton-exchange membrane under water and water/oxygen ambient. Three types of surface oxides/hydroxides gradually develop on the Pt surface depending on the applied potential at +0.9, + 2.5, and +3.7 eV relative to the 4f peak of metal Pt and were attributed to the formation of adsorbed O/OH, PtO, and PtO2, respectively. The presence of O2 in the gas-phase results in the increase of the extent of surface oxidation, and in the growth of the contribution of the PtO2 oxide. Depth profiling studies, in conjunction with quantitative simulations, allowed us to propose a tentative mechanism of the Pt oxidation at high anodic polarization, consisting of adsorption of O/OH followed by nucleation of PtO/PtO2 oxides and their subsequent three-dimensional growth.

V. A. Saveleva, V. Papaefthimiou, M. K. Daletou, W. H. Doh, C. Ulhaq-Bouillet, M. Diebold, S. Zafeiratos, E. R. Savinova
J. Phys. Chem. C, 120 (29), pp. 15930–15940

We present the development and performance of a new, reactor‐like, in‐house ambient pressure X‐ray photoelectron spectrometer (AP‐XPS) using Al Kα, which can study material surfaces, particularly surfaces of catalysts during catalysis and under reaction conditions. This ambient pressure X‐ray photoelectron spectroscopy technique uses an affordable bench‐top X‐ray source, monochromated Al Kα, making it available for research groups on any campus. A unique feature of this in‐house AP‐XPS is the integration of a micro ambient pressure reaction cell working in either flowing or batch mode into a monochromatic Al Kα source and an ambient pressure energy analyzer. This integration allows XPS studying surfaces of materials while they functionalize in a flowing gaseous environment. Another feature is the integration of characterization of surfaces of catalysts into simultaneous measurements of catalytic performance by using on‐line, quadrupole mass spectrometry and gas chromatography. Performance tests of this in‐house AP‐XPS have demonstrated that it can study material surfaces at temperatures up to 500–550 °C in a gaseous environment with pressures up to 25–50 Torr. This design has successfully brought the synchrotron‐based AP‐XPS technique to research groups on campuses.

F. Tao
ChemCatChem 4 (5), pp. 583-590

Chemistry of a catalyst surface during catalysis is crucial for a fundamental understanding of mechanism of a catalytic reaction performed on the catalyst in the gas or liquid phase. Due to the pressure- or molecular density-dependent entropy contribution of gas or liquid phase of the reactants and the potential formation of a catalyst surface during catalysis different from that observed in an ex situ condition, the characterization of the surface of a catalyst under reaction conditions and during catalysis can be significant and even necessary for understanding the catalytic mechanism at a molecular level. Electron-based analytical techniques are challenging for studying catalyst nanoparticles in the gas or liquid phase although they are necessary techniques to employ. Instrumentation and further development of these electron-based techniques have now made in situ/operando studies of catalysts possible. New insights into the chemistry and structure of catalyst nanoparticles have been uncovered over the last decades. Herein, the origin of the differences between ex situ and in situ/operando studies of catalysts, and the technical challenges faced as well as the corresponding instrumentation and innovations utilized for characterizing catalysts under reaction conditions and during catalysis, are discussed. The restructuring of catalyst surfaces driven by the pressure of reactant(s) around a catalyst, restructuring in reactant(s) driven by reaction temperature and restructuring during catalysis are also reviewed herein. The remaining challenges and possible solutions are briefly discussed.

J. Dou, Z. Sun, A. A. Opalade, N. Wang, W. Fu, F. Tao
Chem. Soc. Rev. 46, pp. 2001-2027

The factors determining carrier type in intrinsic organic semiconductors are clarified by measuring field‐effect transistor (FET) characteristics and electronic states for an identical specimen, which is transferred between the film growth/FET measurement and electron spectroscopy chambers in situ (see figure). The FET mobility is found to decrease exponentially with the precisely evaluated charge injection barrier both for electrons and holes.

T. Kaji, S Enatni, S. Ikeda, K. Saiki
Adv. Mater. 20 (11), pp. 2084-2089

The interaction of sintered low-loaded Re/γ-Al2O3 catalysts with oxygen has been studied over a wide temperature range, 20–800 °C. The structure of the catalyst was characterized using different techniques: H2 chemisorption, O2 uptake, BET, TEM, and also by Raman and XPS spectroscopy. In the catalyst reduced in H2 at 800 °C, Re is present as a nonuniform phase consisting of metallic particles with sizes of 1–4 nm and a certain amount of very small clusters (below 1 nm) undetectable by TEM. Oxidation at room temperature causes dissociative chemisorption of oxygen on rhenium with possible formation of a superficial oxide on the Re particles and complete oxidation of the clusters to Re4+–Re7+ species. After oxidation at 150 °C only 2% of total Re remains in the reduced state which implies high affinity of highly dispersed Re to oxygen. At 200–300 °C, oxidation of Re hastens giving a mixture of Re4+, Re6+, and Re7+ ions, while at 500 °C, complete oxidation to Re2O7 occurs. At elevated temperatures, Re2O7 sublimates and instantaneously adsorbs on γ-alumina surface as monomeric ReO4 species. At 500–800 °C, these species are strongly bound to alumina forming a kind of a surface compound with an Al–O–ReO3 structure. This accounts for the low loss of Re observed even at 800 °C. Redispersion of rhenium was observed after a mild H2 treatment of the oxidized rhenium catalyst.

J. Okal, L. Kepinski, L. Krajczyk, W. Tylus
Journal of Catalysis 219 (2), pp. 362-371

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.

A. Naitabdi, R. Fagiewicz, A. Boucly, G. Olivieri, F. Bournel, H. Tissot, Y. Xu, R. Benbalagh, M. G. Silly, F. Sirotti, J.-J. Gallet, F. Rochet
Topics in Catalysis 59 (5-7), pp. 550-563

Using high resolution and ambient pressure X-ray photoelectron spectroscopy we show that the catalytically active FeO2 trilayer films grown on Pt(111) are very active for water dissociation, in contrast to inert FeO(111) bilayer films. The FeO2 trilayer is so active for water dissociation that it becomes hydroxylated upon formation, regardless of the applied preparation method. FeO2 trilayers were grown by oxidation of FeO(111) bilayer films either with molecular oxygen in the mbar regime, or by NO2 and atomic oxygen exposures, respectively, in the ultrahigh vacuum regime. Because it was impossible to prepare clean FeO2 without any hydroxyls we propose that catalytically highly active FeO2 trilayer films are generally hydroxylated. In addition, we provide spectroscopic fingerprints both for Pt(111)-supported FeO(111) and FeO2 films that can serve as reference for future in situ studies.

N. Johansson, L. R. Merte, E. Graånäs, S. Wendt, J. N. Andersen, J. Schnadt, J. Knudsen
Topics in Catalysis 59 (5-7), pp. 506-515

Pd-PVP colloid (stabilized with polyvinylpyrrolidone) with a diameter of 19.8 nm in [Bu4N]Br medium catalyzes Heck coupling of bromobenzene with butyl acrylate and methoxycarbonylation of iodobenzene reactions. Oxidative addition of PhI or PhBr to Pd-PVP as the first step of a catalytic reaction was confirmed by TEM and XPS measurements. TEM studies showed significant reduction of Pd nanoparticle size after their reaction with PhX (X = I, Br) and [Bu4N]X (X = Cl, Br, I). The biggest shift of the center of nanoparticle size distribution, from 19.8 nm to 7.6 nm, was found when Pd-PVP reacted with PhI and [Bu4N]Br. The formation of [Bu4N]2[Pd(Ph)Br3]- and [Bu4N]2[PdBr4]-type complexes in that system was evidenced by XPS and UV–vis spectra.

A. Gniewek, A. M. Trzeciak, J.J. Ziolkowski, L. Kepinski, J. Wrzyszcz, W. Tylus
Journal of Catalysis 229 (2), pp. 332-343

Monolithic Pd-based catalysts on heat-resisting foil supports, with different washcoats, were investigated for catalytic activity in the combustion of methane. Palladium was deposited either onto pure aluminium oxide (Pd/Al2O3) or onto Al2O3 modified with 9 wt.% of SiO2 and 0.8 wt.% of La2O3 (Pd/Al2O3-SiO2) or with ZrSiO4 (Pd/Al2O3-ZrSiO4). The modification of the Al2O3 washcoats was found to increase the conversion of methane and to enhance the thermal resistance of the catalysts. XPS, XRD and TEM examinations have shown that PdO well dispersed on the washcoat surfaces is the active form of palladium in the Pd/Al2O3, Pd/Al2O3-SiO2 and Pd/Al2O3-ZrSiO4 catalysts. XPS examinations of the same catalysts after 20-h run have revealed an over twofold increase in surface Pd concentration measured in terms of the Pd/Al ratio, which substantiates an activation of the catalysts. Heating to 1060 °C deactivated all the catalysts due to the agglomeration of crystallites on their surfaces and decomposition of PdO to metallic Pd. The incorporation of additives into the pure Al2O3 washcoat noticeably reduced crystallite growth at high temperatures. As a result, the deactivation of the Pd/Al2O3-ZrSiO4 and Pd/Al2O3-SiO2 catalysts was much lower than that of the Pd/Al2O3 catalyst.

B. Kucharczyk , W. Tylus, L. Kepinski
Applied Catalysis B: Environmental 49 (1), pp. 27-37

A performance test of near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) at Korean Basic Science Institute (KBSI) in Daejeon, Korea is carried out. The NAP-XPS at KBSI is equipped with SPECS PHOIBOS 150 analyzer and 2-dimensional-delay line detector, which provides improved detection efficiency with 1-dimensional chemical imaging mode. The in-situ gas-pressure cell is designed to operate at pressure up to 25 mbar with differential pumping scheme. A micro-focused AlKα X-ray source is utilized as photon excitation source. The NAP-XPS is composed of two separate chambers, e.g. a main preparation chamber and a measurement chamber, which hold a low energy electron diffraction, a high pressure cell, an ion sputter gun, a multi-cell E-beam evaporators, and a four-axis manipulator with heating and cooling capability. As a test run of NAP-XPS, a CO oxidation experiment is carried out on Pt3Ti polycrystalline surface. During the active phase of CO oxidation, a clear formation of Ti oxide is found on surface, revealing the intriguing role of surface metal oxide in surface chemical reaction.

C. Jeong, H. Yun, H. Lee, S. Muller, J. Lee, B. S. Mun
Current Applied Physics 16 (1), pp. 73-78

Nanocrystalline TiO2 (anatase) films were prepared using either colloidal suspensions or a sol–gel route. The electronic structure of these films was analyzed using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Apart from pristine films, films containing defects introduced by annealing under ultra-high vacuum conditions or by ion bombardment were investigated. Generally, annealing in the temperature range up to 720 K results in no significant changes in the XPS and UPS spectra as compared to the pristine state, indicating that the amount of defect formation is too low to be observable by these techniques. On the other hand, ion irradiation causes the appearance of distinct defect states; these could be identified in agreement with previous data from photoemission studies on rutile and anatase single crystals. From UPS, a valence-band width of ∼4.6 eV was determined for the nanocrystalline anatase films.

A. Orendorz, J. Wüsten, C. Ziegler, H. Gnaser
Applied Surface Science 252, pp. 85-88

An experimental photoelectron spectroscopy study is presented highlighting several aspects of importance for the study of deposited metal clusters and particles with photoemission. It is shown that the Fermi level is the correct energy reference for the core level binding energies. The choice of different deposition conditions, well within the range of soft landing, has a strong impact on the outcome of the spectroscopic experiments. Single adatoms as well as clusters deposited with some excess energy display relatively narrow core level spectra at much lower binding energies than previously reported, even when atomic mass selection is not performed. In contrast, single sized Pt19 clusters, deposited onto a thin Ar film before being exposed to the graphite surface show spectral broadening and shifts to higher binding energies. We discuss our results in terms of the cluster substrate interaction and the influence of deposition conditions on the metal adsorbate structure.

K. Fauth, N. Schneider, M. Heßler, G. Schütz
Eur. Phys. J., D 29, pp. 57-61

We have studied the low coverage, reconstructed phases of the Pb/Si(1 1 1) adsorption system by photoelectron spectroscopy. With decreasing coverage from 1.3 ML, the top of the valence band shows a transition from a metallic surface to a semiconducting surface, which is stable below a coverage of 0.2 ML. The Pb 5d core level was fitted by one symmetric peak for coverage from 0.1 to 0.2 ML without any observable shift. This state corresponds to Pb adatoms in a two dimensional surface alloy with Si where both atoms are located in the T4 position, and the surface reconstruction is (√3×√3)R30°. The peak energy and profile are independent of the relative Pb and Si adatom concentrations in the surface. For higher coverages we observe a varying width of the Pb 5d core level due to the appearance of an extra state shifted by about 0.4 eV; the results are consistent with information from STM images. Core level shifts of the Si 2p level are also reported.

V. Dudr, N. Tsud, S. Fabík, B. Ressel, M. Vondrácek, K. C. Prince, V. Matolín, V. Cháb
Surface Science 566-568, Part 2, pp. 804-809

Modified silicas were obtained by precipitation from aqueous solutions of sodium metasilicate and sulphuric acid. For the modification, silane coupling agents with various functional groups were applied, including aminosilane (AEAPTS), glycidoxysilane (GPTS), mercaptosilane (McPTS), and vinylsilane (VTMES). Extensive physicochemical evaluation of the obtained modified silicas was conducted, using FTIR, CP MAS NMR and XPS techniques. The silica surface modification was proven to depend upon chemical reactions and to show intensity increasing with a rising concentration of a given modifier. A surface charge of the formed silica dispersions was also examined, by determining their zeta potential. Moreover, surface morphology, dispersion and particle size of the obtained silicas were evaluated, employing TEM electron microscopy and the technique of a dynamic light scattering (DLS). The studies demonstrated that application of hydrophobic type silanes for surface modification of the hydrated silicas restricted the intense tendency for agglomeration in the formed precipitated silicas.

T. Jesionowski, J. Zurawska, A. Krysztafkiewicz, M. Pokora, D. Waszak, W. Tylus
Applied Surface Science 205 (1-4), pp. 212-224

The atoms at the surfaces of materials represent the frontier separating the bulk from the surrounding medium. Over the last decades, scientists have intensely studied the structure and properties of surfaces with the goal of understanding and improving the electronic and chemical properties of materials. The surface–medium interaction determines wetting, friction, chemical, biological, and electronic properties. The activity of catalysts, phenomena occurring in water droplets and particles in the atmosphere, and the electronic properties of semiconductor devices are direct consequences of surface-environment interactions. While the need to pursue studies in the normal environment that surrounds a material has always been recognized, the techniques used in the past have only partially fulfilled this need, as most of them work best under high vacuum conditions. My research over the last 10 years has focused on discovering the structure of a surface and its dynamics in real life—in everyday environments. This required the development of new techniques and methods. I present some of the new tools developed in my laboratory and new properties that were discovered by their application in the areas of environmental science, surface chemistry, and catalysis.

M. Salmeron
Surface-enhanced Raman spectroscopy: Substrates and materials 38 (8), pp. 650-657

Polysulfone films were modified by ammonia, n-butylamine and allylamine remote plasma using various sample-to-plasma distances. Contact angle measurements, FTIR-ATR and XPS spectroscopy proved the presence of polar, including amine, groups on the modified surface. Presence of argon in the plasma environment made the plasma more stable and in most cases left the surface more hydrophilic but with a lower amount of nitrogen moieties on it. Glucose isomerase was successfully immobilized on the plasma-treated samples. Its activity correlates well with the concentration of C–N bonds on the surface. The highest enzyme activity was achieved for samples treated with allylamine/Ar plasma close to the plasma edge.

I. Gancarz, J. Bryjak, G. Pozniak, W. Tylus
European Polymer Journal 39 (11), pp. 2217-2224

The paper describes deposition of plasma polymerized allyl alcohol on polysulfone film. It is shown that film surface becomes more hydrophilic after plasma treatment independently on presence of argon in a reaction mixture. The chemistry of the new surface layer was established by FTIR-ATR and ESCA spectroscopy. The substrate placed close to the plasma edge was the most hydrophilic but the amount of hydoxyl groups was not the highest there. Presence of argon stabilized the plasma but the deposited layer contained relatively less oxygen-bearing functionalities. The plasma treated polymer was subjected to xylose isomerase immobilization. For this purpose the divinylsulfone method was adapted. The studies revealed no correlation between the surface hydrophilicity and efficiency of immobilization.

I. Gancarz, J. Bryjak, M. Bryjak, G. Poźniak, W. Tylus
European Polymer Journal 39 (8), pp. 1615-1622

The line shape and intensity of surface and bulk plasmon excitations in Al 2s and 2p core-level spectra of Al(111) have been studied as functions of the photoelectron emission angle (θ). For both surface and bulk plasmons, an asymmetric line shape is observed in normal emission, which becomes more symmetric in grazing emission. The asymmetric line shape is in good agreement with theory. The relative contributions of the intrinsic, extrinsic, and interference processes to the surface plasmon intensity are determined from its variation with θ and from theoretical line-shape calculations. We show the importance of the interference process in determining the intensity and line shape of the plasmons. From the intensity variation of multiple (n=1–6) bulk plasmons (nωp) with n, the intrinsic and extrinsic bulk plasmon probabilities are determined.

C. Biswas, A. K. Shukla, S. Banik, V. K. Ahire, and S. R. Barman
Physical Review B 67, 165416

The paper describes a method for preparation of polymer support suitable for covalent invertase immobilization. Modification of poly(phenylene oxide) films by plasma polymerization of allylamine has been applied to introduce amine functionality on the polymer surface. It has been observed that the polymer surface became covered in plasma by a loosely fixed, moderately hydrophilic layer that should be removed before the immobilization process. The chemical character of the stable sub-layer has been related to several modification parameters: geometry of reactor, mode of plasma action and composition of gaseous mixture. Methods for determination of surface concentration of amine groups have also been presented and discussed from the immobilization point of view.

I. Gancarz, J. Bryjak, M. Bryjak, G. Poźniak, W. Tylus
European Polymer Journal 42, pp. 2430-2440

The decomposition of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) molecules during evaporation of unpurified raw material in ultra high vacuum was studied. The fragments were identified by mass spectrometry and the influence of these fragments and further contaminations of the raw material on the electronic structure of PTCDA thin films was measured by photoemission spectroscopy. Annealing of contaminated PTCDA films was tested as cheap and easy to perform method for (partial) post deposition purification of the contaminated films.

J. Wüsten, Th. Ertl, S. Lach, Ch. Ziegler
Applied Surface Science 252, pp. 104-107

We have investigated surface composition, structure, and termination of SrTiO3(001) samples (STO) following typical steps of standard ex situ and in situ preparation procedures by performing x-ray photoelectron spectroscopy (XPS) and x-ray photoelectron diffraction (XPD) experiments, and multiple scattering cluster (MSC) calculations. XPS has confirmed the presence of Sr2+, Ti4+, and O2−related to STO. Well-developed diffraction features have been observed in all XPD polar scans demonstrating the good structural ordering of the investigated surfaces. In order to permit a detailed comparison of XPD experiment and theory, we have especially taken care of effects due to the angular dependent instrument function by applying a new ratio procedure. Thus, we achieved a very good agreement between results of MSC calculations and experimental angular distributions. A fingerprint region has been identified in Sr3d polar scans, which has exhibited both evident dependencies on surface preparation steps in the experiments and clear sensitivity to surface termination in the theoretical modeling. In this manner, we have been able to describe as-received samples and in situ-treated samples as being TiO2- and SrO-terminated, respectively. Standard buffered hydrofluoric acid-treated samples turned out to be mostly TiO2-terminated but with a non-negligible admixture of different, likely SrO-type termination.

C. Raisch, T. Chassé, Ch. Langheinrich, A. Chassé
Journal of Applied Physics 112, 073505

We report a detailed investigation of elementary catalytic decomposition of ammonia on the Pt–Ni–Pt(111) bimetallic surface using in situ near ambient pressure X-ray photoelectron spectroscopy. Under the near ambient pressure (0.6 mbar) reaction conditions, a different dehydrogenation pathway with a reduced activation energy barrier for recombinative nitrogen desorption on the Pt–Ni–Pt(111) bimetallic surface is observed. The unique surface catalytic activity is correlated with the downward shift of the Pt 5d band states induced by the Ni subsurface atoms via charge redistribution of the topmost Pt layer. Our results provide a practical understanding of the unique chemistry of bimetallic catalysts for facile ammonia decomposition under realistic reaction conditions.

J.-Q. Zhong, X. Zhou, K. Yuan, C. A. Wright, A. Tadich, D. Qi, H. X. Li, K. Wu, G. Q. Xua, W. Chen
Nanoscale 9, pp. 666-672

The electron-phonon interaction in thin Ag nanofilms epitaxially grown on Cu(111) is investigated by temperature-dependent and angle-resolved photoemission from silver quantum-well states. Clear oscillations in the electron-phonon coupling parameter as a function of the silver film thickness are observed. Different from other thin film systems where quantum oscillations are related to the Fermi-level crossing of quantum-well states, we can identify a new mechanism behind these oscillations, based on the wave-function localization of the quantum-well states in the film.

S. Mathias, M. Wiesenmayer, M. Aeschlimann, M. Bauer
Phys. Rev. Lett. 97, 236809

We report on angle-resolved photoemission studies of the electronic π states of high-quality epitaxial graphene layers on a Ni(111) surface. In this system the electron binding energy of the π states shows a strong dependence on the magnetization reversal of the Ni film. The observed extraordinarily large energy shift up to 225 meV of the graphene-derived π band peak position for opposite magnetization directions is attributed to a manifestation of the Rashba interaction between spin-polarized electrons in the π band and the large effective electric field at the graphene/Ni interface. Our findings show that an electron spin in the graphene layer can be manipulated in a controlled way and have important implications for graphene-based spintronic devices.

Yu. S. Dedkov, M. Fonin, U. Rüdiger, C. Laubschat
Phys. Rev. Lett. 100, 107602

Valence band photoemission has been measured in chromium silicide as a function of the photon energy near the Cr 2p3/2 absorption threshold. Evidence of resonant photoemission is observed for the 3d valence band and the two-hole satellite. The threshold for normal Auger regime is 2.8 eV below the absorption peak and 0.8 eV below the Cr 2p3/2 binding energy, even lower than in pure Cr metal where it is already at extreme levels. The requirement for good resolution in photon energy relative to absorption width for the resonant Raman Auger to be observed is found to be less restrictive than expected.

L. Galán, M. García, J. M. Ripalda, I. Montero, E. Román, D. R. Batchelor, P. R. Bressler
Appl. Phys. Lett. 84, 4433

A novel type of temporal and spatial self-organization in a heterogeneous catalytic reaction is described for the first time. Using in situ x-ray photoelectron spectroscopy, gas chromatography, and mass spectrometry, we show that, under certain conditions, self-sustained reaction-rate oscillations arise in the oxidation of propane over Ni foil because of reversible bulk oxidation of Ni to NiO, which can be observed even with the naked eye as chemical waves propagating over the catalyst surface.

V. V. Kaichev, A. A. Saraev, A. Yu. Gladky, I. P. Prosvirin, R. Blume, D. Teschner, M. Hävecker, A. Knop-Gericke, R. Schlögl, V. I. Bukhtiyarov
Phys. Rev. Lett. 119, 026001

Electron confinement in thin films of Pb on Cu(111) leads to the formation of quantum well states, formed out of the upper valence band of Pb. Their evolution as a function of film thickness is characterized in angle-resolved photoemission and can be interpreted in terms of a straightforward quantum well model. This permits an identification of film growth mode at low temperatures. Bringing the films into thermal equilibrium by annealing induces strong changes in the spectra. Their interpretation demonstrates that specific “magic” layers are preferred because of total energy minimization induced by the arrangement of quantum well states with respect to the Fermi level.

J. H. Dil, J. W. Kim, S. Gokhale, M. Tallarida, K. Horn
Phys. Rev. B 70, pp. 045405-1 - 045405-5

We present examples of the application of synchrotron-based spectroscopies and microscopies to environmentally relevant samples. The experiments were performed at the molecular environmental science beamline (11.0.2) at the Advanced Light Source, Lawrence Berkeley National Laboratory. Examples range from the study of water monolayers on Pt(1 1 1) single crystal surfaces using X-ray emission spectroscopy and the examination of alkali halide solution/water vapor interfaces using ambient pressure photoemission spectroscopy, to the investigation of actinides, river water biofilms, Al-containing colloids and mineral–bacteria suspensions using scanning transmission X-ray spectromicroscopy. The results of our experiments show that spectroscopy and microscopy in the soft X-ray energy range are excellent tools for the investigation of environmentally relevant samples under realistic conditions, i.e., with water or water vapor present at ambient temperature.

H. Bluhm, K. Andersson, T. Araki, K. Benzerara, G. E. Brown, J. J. Dynes, S. Ghosal, M. K. Gilles, H.-Ch. Hansen, J. C. Hemmingerf, A. P. Hitchcock, G. Ketteler, A. L. D. Kilcoyne , E. Kneedler , J.R . Lawrence , G. G. Leppard , J. Majzlam , B. S. Munl, S
Journal of Electron Spectroscopy and Related Phenomena 150, pp. 86-104

The guanidine ligand attached to vinylbenzyl chloride matrix as well as its sorption ability to tetrachloroaurate and dicyanoaurate ions were investigated upon using X-ray photoelectron spectroscopy. The form of metallic gold, Au(0), was not observed on gold loaded resin surface when solutions of tetrachloroaurate or potassium dicyanoaurate were used. Complete elution of gold was achieved for both regeneration media: thiourea solution and mixture of sodium hydroxide and benzoate. The results demonstrate that XPS can also be a powerful technique for the analysis of the oxidation state of sorbed metal and can be a suitable method for the confirmation of functional groups incorporated in the polymer.

D. Jermakowicz-Bartkowiak, B. N. Kolarz, W. Tylus
Polymer 44 (19), pp. 5797-5802

It is now well established by numerous experimental and computational studies that the adsorption propensities of inorganic anions conform to the Hofmeister series. The adsorption propensities of inorganic cations, such as the alkali metal cations, have received relatively little attention. Here we use a combination of liquid-jet X-ray photoelectron experiments and molecular dynamics simulations to investigate the behavior of K+ and Li+ ions near the interfaces of their aqueous solutions with halide ions. Both the experiments and the simulations show that Li+ adsorbs to the aqueous solution−vapor interface, while K+ does not. Thus, we provide experimental validation of the “surfactant-like” behavior of Li+ predicted by previous simulation studies. Furthermore, we use our simulations to trace the difference in the adsorption of K+ and Li+ ions to a difference in the resilience of their hydration shells.

K. A. Perrine, K. M. Parry, A. C. Stern, M. H. C. Van Spyk, M. J. Makowski, J. A. Freites, B. Winter, D. J. Tobias, J. C. Hemminger
PNAS 114 (51) 13363-13368

A series of alkaline uranyl carbonates, M[UO2(CO3)3]·nH2O (M=Mg2, Ca2, Sr2, Ba2, Na2Ca, and CaMg) was synthesized and characterized by inductively coupled plasma mass spectrometry (ICP-MS) and atomic absorption spectrometry (AAS) after nitric acid digestion, X-ray powder diffraction (XRD), and thermal analysis (TGA/DTA). The molecular structure of these compounds was characterized by extended X-ray absorption fine-structure (EXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). Crystalline Ba2[UO2(CO3)3]·6H2O was obtained for the first time. The EXAFS analysis showed that this compound consists of (UO2)(CO3)3 clusters similar to the other alkaline earth uranyl carbonates. The average U–Ba distance is 3.90±0.02 Å.Fluorescence wavelengths and life times were measured using time-resolved laser-induced fluorescence spectroscopy (TRLFS). The U–O bond distances determined by EXAFS, TRLFS, XPS, and Raman spectroscopy agree within the experimental uncertainties. The spectroscopic signatures observed could be useful for identifying uranyl carbonate species adsorbed on mineral surfaces.

S. Amayri, T. Reich, T. Arnold, G. Geipel, G. Bernhard
Journal of Solid State Chemistry 178 (2), pp. 567-577

Spin- and angle-resolved resonant (Ce 4d→4f) photoemission spectra of a monolayer Ce on Fe(110) reveal spin-dependent changes of the Fermi-level peak intensities. That indicates a spin dependence of 4f hybridization and, thus, of 4f occupancy and local moment. The phenomenon is described in the framework of the periodic Anderson model by 4f electron hopping into the exchange split Fe 3d derived bands that form a spin-gap at the Fermi energy around the ¯Γ point of the surface Brillouin zone.

Yu. S. Dedkov, M. Fonin, Yu. Kucherenko, S. L. Molodtsov, U. Rüdiger, C. Laubschat
Phys. Rev. B 76, 073104

Controlling the interaction between organic semiconductors and ferromagnetic surfaces is one of the key issues for designing metal–organic hybrid interfaces for spintronic applications. The strong chemical interaction across such hybrid interfaces results in the formation of new spin-polarized hybrid interface states which determine all device-relevant properties. Here, we revisit the hybrid interface formed between the prototypical molecule Alq3 and the Co surface using spin- and angle-resolved photoemission. We reveal a significant change of the spectroscopic lineshape of the cobalt 3d bands by the adsorption of Alq3. The hole-like minority and the electron-like majority bands of the bare Co surface are replaced by an energetically very broad band with neglectable band dispersion along the Γ̅–X̅ direction. Moreover, the magnitude and shape of the spin polarization of the Alq3/Co valence band structure are also significantly modified by the adsorption of Alq3 and become completely independent of the momentum space positions along the Γ̅–X̅ direction. Our findings are attributed to an elastic scattering of the Co photoelectrons at the disordered Alq3 overlayer, leading to a redistribution of the spin-dependent spectral intensity in momentum space. A careful analysis of our data shows that such elastic scattering takes place without significant spin-flip scattering processes and that the spectral feature of the highest occupied molecular orbital of Alq3 is fully unpolarized.

J. Stöckl, A. Jurenkow, N. Großmann, M. Cinchetti, B. Stadtmüller, M. Aeschlimann
J. Phys. Chem. C 122 (12), pp. 6585–6592

We study the specific impact of defects such as step edges at the graphite surface on the electronic configuration of adsorbed Pt atoms and Pt8 clusters. Surface diffusion is strongly reduced by depositing Pt and Pt8 into a thin rare gas layer. In this configuration a very narrow adatom Pt 4f spectrum is found at an exceptionally small binding energy, similar to Pt surfaces. Both, adatom and cluster spectra are strongly shifted towards higher binding energy when allowed to diffuse towards defects like step edges. The strong shifts are indicative of a chemical reaction at the step edges and are conjectured to be part of the particle size dependent binding energy shifts typically observed for transition metal clusters grown on the surface of graphite.

K. Fauth, M. Heßler, D. Batchelor, G. Schütz
Surface Science 529 (3), pp. 397-402

An apparatus for sub-nanosecond time-resolved ambient-pressure X-ray photoelectron spectroscopy studies with pulsed and constant wave X-ray light sources is presented. A differentially pumped hemispherical electron analyzer is equipped with a delay-line detector that simultaneously records the position and arrival time of every single electron at the exit aperture of the hemisphere with ∼0.1 mm spatial resolution and ∼150 ps temporal accuracy. The kinetic energies of the photoelectrons are encoded in the hit positions along the dispersive axis of the two-dimensional detector. Pump-probe time-delays are provided by the electron arrival times relative to the pump pulse timing. An average time-resolution of (780 ± 20) ps (FWHM) is demonstrated for a hemisphere pass energy Ep = 150 eV and an electron kinetic energy range KE = 503–508 eV. The time-resolution of the setup is limited by the electron time-of-flight (TOF) spread related to the electron trajectory distribution within the analyzer hemisphere and within the electrostatic lens system that images the interaction volume onto the hemisphere entrance slit. The TOF spread for electrons with KE = 430 eV varies between ∼9 ns at a pass energy of 50 eV and ∼1 ns at pass energies between 200 eV and 400 eV. The correlation between the retarding ratio and the TOF spread is evaluated by means of both analytical descriptions of the electron trajectories within the analyzer hemisphere and computer simulations of the entire trajectories including the electrostatic lens system. In agreement with previous studies, we find that the by far dominant contribution to the TOF spread is acquired within the hemisphere. However, both experiment and computer simulations show that the lens system indirectly affects the time resolution of the setup to a significant extent by inducing a strong dependence of the angular spread of electron trajectories entering the hemisphere on the retarding ratio. The scaling of the angular spread with the retarding ratio can be well approximated by applying Liouville's theorem of constant emittance to the electron trajectories inside the lens system. The performance of the setup is demonstrated by characterizing the laser fluence-dependent transient surface photovoltage response of a laser-excited Si(100) sample.

A. Shavorskiy, S. Neppl, D.S. Slaughter, J. P. Cryan, K. R. Siefermann, F. Weise, M.-F. Lin, C. Bacellar, M. P. Ziemkiewicz, I. Zegkinoglou, M. W. Fraund, C. Khurmi, M. P. Hertlein, T. W. Wright, N. Huse, R. W. Schoenlein, T. Tyliszczak, G. Coslovich, J.
Review of Scientific Instruments 85, 093102

Any substantial move of energy sources from fossil fuels to renewable resources requires large scale storage of excess energy, for example, via power to fuel processes. In this respect electrochemical reduction of CO2 may become very important, since it offers a method of sustainable CO production, which is a crucial prerequisite for synthesis of sustainable fuels. Carbon dioxide reduction in solid oxide electrolysis cells (SOECs) is particularly promising owing to the high operating temperature, which leads to both improved thermodynamics and fast kinetics. Additionally, compared to purely chemical CO formation on oxide catalysts, SOECs have the outstanding advantage that the catalytically active oxygen vacancies are continuously formed at the counter electrode, and move to the working electrode where they reactivate the oxide surface without the need of a preceding chemical (e.g., by H2) or thermal reduction step. In the present work, the surface chemistry of (La,Sr)FeO3−δ and (La,Sr)CrO3−δ based perovskite-type electrodes was studied during electrochemical CO2 reduction by means of near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) at SOEC operating temperatures. These measurements revealed the formation of a carbonate intermediate, which develops on the oxide surface only upon cathodic polarization (i.e., under sufficiently reducing conditions). The amount of this adsorbate increases with increasing oxygen vacancy concentration of the electrode material, thus suggesting vacant oxygen lattice sites as the predominant adsorption sites for carbon dioxide. The correlation of carbonate coverage and cathodic polarization indicates that an electron transfer is required to form the carbonate and thus to activate CO2 on the oxide surface. The results also suggest that acceptor doped oxides with high electron concentration and high oxygen vacancy concentration may be particularly suited for CO2 reduction. In contrast to water splitting, the CO2 electrolysis reaction was not significantly affected by metallic particles, which were exsolved from the perovskite electrodes upon cathodic polarization. Carbon formation on the electrode surface was only observed under very strong cathodic conditions, and the carbon could be easily removed by retracting the applied voltage without damaging the electrode, which is particularly promising from an application point of view.

A. K. Opitz, A. Nenning, C. Rameshan, M. Kubicek, T. Götsch, R. Blume, M. Hävecker, A. Knop-Gericke, G. Rupprechter, B. Klötzer, J. Fleig
ACS Appl. Mater. Interfaces 9 (41), pp. 35847–35860

In situ exposure of FePd chemically disordered alloys to a N plasma prior to capping layer deposition produces a nitridation localized on the topmost 20 Å alloy region. In this region, a N–Fe bond prevails over the N–Pd. From the structural point of view, this nitridation does not induce relevant modifications on the lattice parameters compared with non-nitrided samples of equivalent composition, being only slightly modified in the alloys with higher Fe concentration. From the magnetic point of view, nitrogen incorporation induces a reduction or even a loss of the ferromagnetism of the nitrided FePd region, giving rise to a global decrease in the polar Magneto-Optical Kerr response in a wide spectral range due to the increase of the effective thickness of the non-magnetic capping layer.

M. S. Martin-González, Y. Huttel, A. Cebollada, G. Armelles, F. Briones
Surface Science 571 (1-3), pp. 63-73

Although chalcopyrites have been used as absorber in thin film solar cells for nearly 30 years, not much is known about their surface structure, which might be an important information towards growth and interface optimization. Surface reconstructions, a general feature of semiconductor surfaces, have not been reported on Cu-III–VI2 (001) surfaces so far. We have thus studied the chalcopyrite CuGaSe2 (001) surface preparation by Ar+ ion bombardment and annealing. The samples were grown by MOCVD on a (001) GaAs substrate on axis with a near-stoichiometric ratio Cu/Ga. The preparation was optimized so that we finally obtained a flat, well-ordered surface. The surface composition and structure was analyzed with AES, XPS, and LEED. On uncontaminated surfaces, a (1×4) reconstruction was observed with LEED which we tentatively interpret as a Se-terminated surface. XPS characterization shows a surface core level shift of −0.8 eV for the selenium surface atoms of the (1×4) reconstruction. Further investigations were carried out via AFM to characterize the surface morphology.

Th. Deniozou, N. Esser, S. Siebentritt, P. Vogt, R. Hunger
Thin Solid Films 480-481, pp. 382-387

Synchrotron-induced photoelectron spectroscopy was used to investigate the native-oxide-covered GaAs(1 0 0) surface and changes induced by etching with aqueous ammonia solution and by annealing in vacuum. The etching step removes arsenic and gallium oxides from the surface and the surface gets covered by elemental arsenic and tiny amounts of gallium suboxide. The surface oxygen content is reduced by an order of magnitude after etching, whereas the surface carbon content is somewhat increased. Annealing of this surface at 450 °C results in the disappearance of elemental arsenic and a considerable decrease in surface carbon and oxygen contents. The valence band spectra exhibit clear features typical for As-terminated GaAs(1 0 0) surfaces, as also obtained after As decapping.

M. V. Lebedev, D. Ensling, R. Hunger, T. Mayer, W. Jaegermann
Applied Surface Science 229 (1-4), pp. 226-232

In2O3 thin films with preferred (4 0 0) orientation prepared by the spray pyrolysis method were studied by synchrotron radiation photoemission and ion scattering spectroscopes. O 1s, O 2s, In 4d core level and valence band spectra were monitored at photon energies 660, 245, 150, and 73 eV to see their evolution with UHV treatments (heating, sputtering and exposure of oxygen). Reduction of the surface layer to nearly metallic indium was found with thermal treatment at T ≥ 300 °C. This surface demonstrates high reactivity to reversible oxidation/reduction processes. This was evidenced by evolution of the O 2s core level peak and of the band gap emission intensity. In spite of such surface reduction it was found that within a probing depth of ≤10 Å the material displays spectral features characteristic of stoichiometric In2O3. We tentatively explain such behavior in terms of the In2O3 crystallographic structure and some conclusions relating to gas-sensing properties were made.

V. Brinzari, G. Korotcenkov, V. Matolin
Applied Surface Science 243 (1-4), pp. 335-344

Solid/vapor and liquid/vapor interfaces govern many processes in the environment and atmosphere, energy generation, and electrochemical devices as well as heterogeneous catalysis. Examples include catalytic converters in automobiles , solid oxide fuel cells, cloud droplet nucleation on atmospheric aerosol particles, as well as the interaction of trace gases with polar snow packs. A fundamental understanding of the molecular processes at these interfaces requires experimental methods that allow the investigation of samples under as close to operating conditions as possible. This kind of investigation has become increasingly important over the last few decades, leading to the development of a number of surface-sensitive, in-situ spectroscopies and microscopies, including infrared spectroscopy (IR); vibrational sum-frequency generation (VSFG); X-ray emission spectroscopy (XES); surface X-ray diffraction (SXRD); scanning force microscopy (SFM) in both contact and non-contact modes; scanning tunneling microscopy (STM); as well as transmission electron microscopy and scanning electron microscopy.

A. Shavorskiy, O. Karslioglu, I. Zegkinoglou, H. Bluhm
Synchrotron Radiation News 27 (2), pp. 14-23

Bimetallic nanoparticle (NP) catalysts are interesting for the development of selective catalysts in reactions such as the reduction of CO2 by H2 to form hydrocarbons. Here the synthesis of Ni–Co NPs is studied, and the morphological and structural changes resulting from their activation (via oxidation/reduction cycles), and from their operation under reaction conditions, are presented. Using ambient‐pressure X‐ray photoelectron spectroscopy, X‐ray absorption spectroscopy, and transmission electron microscopy, it is found that the initial core–shell structure evolves to form a surface alloy due to nickel migration from the core. Interestingly, the core consists of a Ni‐rich single crystal and a void with sharp interfaces. Residual phosphorous species, coming from the ligands used for synthesis, are found initially concentrated in the NP core, which later diffuse to the surface.

S. Carenco, C.‐H. Wu, A. Shavorskiy, S. Alayoglu, G. A. Somorjai, H. Bluhm, M. Salmeron
Small 11 (25), pp. 3045-3053

The equimolar reaction of Ce(hfac)4 (1) (hfac = 1,1,1,5,5,5‐hexafluoropentanedionato) with p‐tBu‐calix[4](OMe)2(OH)2 in toluene gave the new cerium(IV) calix[4]arene complex {p‐tBu‐calix[4](OMe)2(O)2}Ce(hfac)2 (2). The single‐crystal X‐ray structure shows the cone geometry of the calixarene ligand with the methoxy groups coordinated to the cerium; it shows slightly longer cerium–oxygen (acetylacetonate ligand) bond lengths than the corresponding bonds in the analogous nonfluorinated complex {p‐tBu‐calix[4](OMe)2(O)2}Ce(acac)2 (3). The bromination reaction of 3 gave the bisbrominated complex {p‐tBu‐calix[4](OMe)2(O)2}Ce(Br‐acac)2 (4). 1H NMR spectroscopic studies and a single‐crystal X‐ray structure of 4 revealed that the bromination took place in the 3‐position of the acac ligand. Furthermore, the first X‐ray photoelectron spectroscopy (XPS) evaluation of the Ce oxidation state in cerium calix[4]arene complexes 2 and 3 is presented, and X‐ray induced changes of Ceox in these complexes are detected. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

J. Gottfriedsen, R. Hagner, M. Spoida, Y. Suchorski
Eur. J. Inorg. Chem. 2007 (16) pp. 2288–2295

The near ambient pressure X-ray photoelectron spectroscopy set up installed recently at SOLEIL synchrotron facility is used to study the electronic structure of NaCl and NaI saturated solutions formed on a gold substrate. The binding energies of the solution constituents are measured with respect to the Fermi level of the gold substrate. The C1s binding energy of the aliphatic contaminant floating at the surface of the solution is an evidence that the Fermi level in the metal and in the solution are aligned. The use of the Fermi level common energy reference is an added value with respect to previous works realized with micro-jets that were calibrated in energy with respect to vacuum level. We observe that the water valence molecular levels binding energies, and hence the Fermi positioning in the gap of the liquid, the Na+ 2s binding energy and even the work function are independent of the nature of the anions. The secondary electron energy distribution curves show that the work functions of the two solutions are equal within experimental uncertainty. We discuss this point considering the different ion distributions at the surface (related to the different size and polarizability of the anions), and the possible contribution of carbon contaminants. We compare the WF values extracted from the secondary electron edges to alternative measurements using the binding energy of the gas phase O1s or 1b1 spectra (referenced to the gold Fermi level). The ionization energies (referenced to the vacuum level), that we obtain by adding the work function to the measured binding energies, are in good accord with previously published works using micro-jets, obtained, however, at much lower solute concentration. Finally we discuss the origin of the Fermi level pinning in the liquid band gap and consider the possibility that the H+/H2 redox level is aligned with the metal Fermi level.

H. Tissot, J.-J. Gallet, F. Bournel, G. Olivieri, M. G. Silly, F. Sirotti, A. Boucly, F. Rochet
Topics in Catalysis 59 (5-7), pp. 605-620

Atomic hydrogen on the surface of a metal with high hydrogen solubility is of particular interest for the hydrogenation of carbon dioxide. In a mixture of hydrogen and carbon dioxide, methane was markedly formed on the metal hydride ZrCoHx in the course of the hydrogen desorption and not on the pristine intermetallic. The surface analysis was performed by means of time‐of‐flight secondary ion mass spectroscopy and near‐ambient pressure X‐ray photoelectron spectroscopy, for the in situ analysis. The aim was to elucidate the origin of the catalytic activity of the metal hydride. Since at the initial stage the dissociation of impinging hydrogen molecules is hindered by a high activation barrier of the oxidised surface, the atomic hydrogen flux from the metal hydride is crucial for the reduction of carbon dioxide and surface oxides at interfacial sites.

S. Kato, S. K. Matam, P. Kerger, L. Bernard, C. Battaglia, D. Vogel, M. Rohwerder, A. Züttel
Angew. Chem. Int. Ed. 55 (20), 6028-6032

SPECIES is an undulator-based soft X-ray beamline that replaced the old I511 beamline at the MAX II storage ring. SPECIES is aimed at high-resolution ambient-pressure X-ray photoelectron spectroscopy (APXPS), near-edge X-ray absorption fine-structure (NEXAFS), X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) experiments. The beamline has two branches that use a common elliptically polarizing undulator and monochromator. The beam is switched between the two branches by changing the focusing optics after the monochromator. Both branches have separate exit slits, refocusing optics and dedicated permanent endstations. This allows very fast switching between two types of experiments and offers a unique combination of the surface-sensitive XPS and bulk-sensitive RIXS techniques both in UHV and at elevated ambient-pressure conditions on a single beamline. Another unique property of the beamline is that it reaches energies down to approximately 27 eV, which is not obtainable on other current APXPS beamlines. This allows, for instance, valence band studies under ambient-pressure conditions. In this article the main properties and performance of the beamline are presented, together with selected showcase experiments performed on the new setup.

S. Urpelainen, C. Såthe, W. Grizolli, M. Agåker, A. R. Head, M. Andersson, S.-W. Huang, B. N. Jensen, E. Wallén, H. Tarawneh, R. Sankari, R. Nyholm, M. Lindberg, P. Sjöblom, N. Johansson, B. N. Reinecke, M. A. Arman, L. R. Merte, J. Knudsen, J. Schnadt, J
J. Synchrotron Rad. 24, pp. 344-353

Methanol adsorption/desorption and its time- and temperature-dependent decomposition on well-annealed and defect-rich (ion-bombarded) Pd(1 1 1) were examined by X-ray photoelectron spectroscopy (XPS) and polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS). Annealing CH3OH multilayers from 100 to 700 K mainly resulted in CH3OH desorption. Dehydrogenation to CO was a minor path and only trace amounts of carbon or carbonaceous species (CHx; x=0–3) were produced, i.e. C–O bond scission was very limited. By contrast, an exposure of 5 × 10−7 mbar CH3OH at 300 K produced CHx (∼0.3 ML) on both surfaces but the rate of formation was not considerably enhanced by surface defects. On well-annealed Pd(1 1 1) isolated carbon atoms were identified by XPS in the early stages of carbon deposition, with carbon diffusion leading to the growth of carbon clusters in the later stages. Since carbon(aceous) species may either originate from C–O bond scission within methanol (or CHxO) or from a consecutive dissociation of the dehydrogenation product CO, analogous experiments were also carried out with CO. PM-IRAS spectra up to 170 mbar CO, acquired using a UHV-high-pressure cell, did not show any indications of CO dissociation, excluding CO as source of carbonaceous deposits.

O. Rodríguez de la Fuente, M. Borasio, P. Galletto, G. Rupprechter, H.-J. Freund
Surface Science 566-568, Part 2, pp. 740-745

The new instrument for near-ambient-pressure X-ray photoelectron spectroscopy which has been installed at the MAX II ring of the Swedish synchrotron radiation facility MAX IV Laboratory in Lund is presented. The new instrument, which is based on a SPECS PHOIBOS 150 NAP analyser, is the first to feature the use of retractable and exchangeable high-pressure cells. This implies that clean vacuum conditions are retained in the instrument's analysis chamber and that it is possible to swiftly change between near-ambient and ultrahigh-vacuum conditions. In this way the instrument implements a direct link between ultrahigh-vacuum and in situ studies, and the entire pressure range from ultrahigh-vacuum to near-ambient conditions is available to the user. Measurements at pressures up to 10-5 mbar are carried out in the ultrahigh-vacuum analysis chamber, while measurements at higher pressures are performed in the high-pressure cell. The installation of a mass spectrometer on the exhaust line of the reaction cell offers the users the additional dimension of simultaneous reaction data monitoring. Moreover, the chosen design approach allows the use of dedicated cells for different sample environments, rendering the Swedish ambient-pressure X-ray photoelectron spectroscopy instrument a highly versatile and flexible tool.

J. Schnadt, J. Knudsen, J. N. Andersen, H. Siegbahn, A. Pietzsch, F. Hennies, N. Johansson, N. Mårtensson, G. Öhrwall, S. Bahr, S. Mähl, O. Schaff
J. Synchrotron Rad. 19, pp. 701-704

The routine study of the solid-water interface by XPS is potentially revolutionary as this development opens up whole new areas of study for photoelectron spectroscopy. To date this has been realised by only a few groups worldwide and current techniques have significant restrictions on the type of samples which can be studied. Here we present a novel and uniquely flexible approach to the problem. By introducing a thin capillary into the NAP-XPS, a small droplet can be injected onto the sample surface, offset from the analysis area by several mm. By careful control of the droplet size a water layer of controllable thickness can be established in the analysis area—continuous with the bulk droplet. We present results from the solid-water interface on a vacuum prepared TiO2(110) single crystal and demonstrate that the solid/liquid interface is addressable.

S. G. Booth, A. M. Tripathi, I. Strashnov, R. A. W. Dryfe, A. S. Walton
J. Phys.: Condens. Matter 29, 45

In this work the electronic structure of V2O5, reduced V2O5−x (V16O39) and sodium intercalated NaV2O5 has been studied by both theoretical and experimental methods. Theoretical band structure calculations have been performed using density functional methods (DFT). We have investigated the electron density distribution of the valence states, the total density of states (total DOS) and the partial valence band density of states (PVBDOS). Experimentally, amorphous V2O5 thin films have been prepared by physical vapour deposition (PVD) on freshly cleaved highly oriented pyrolytic graphite (HOPG) substrates at room temperature with an initial oxygen understoichiometry of about 4%, resulting in a net stoichiometry of V2O4.8. These films have been intercalated by sodium using vacuum deposition with subsequent spontaneous intercalation (NaV2O5) at room temperature. Resonant V3p–V3d photoelectron spectroscopy (ResPES) experiments have been performed to determine the PVBDOS focusing on the calculation of occupation numbers and the determination of effective oxidation state, reflecting ionicity and covalency of the V–O bonds. Using X-ray absorption near edge spectra (XANES) an attempt is made to visualize the changes in the unoccupied DOS due to sodium intercalation. For comparison measurements on nearly stoichiometric V2O5 single crystals have been performed. The experimental data for the freshly cleaved and only marginally reduced V2O5 single crystals and the NaV2O5 results are in good agreement with the calculated values. The ResPES results for V2O4.8 agree in principle with the calculations, but the trends in the change of the ionicity differ between experiment and theory. Experimentally we find partly occupied V 3d states above the oxygen 2p-like states and a band gap between these and the unoccupied states. In theory one finds this occupation scheme assuming oxygen vacancies in V2O5 and by performing a spin-polarized calculation of an antiferromagnetic ordered NaV2O5.

S. Laubach, P. C. Schmidt, A. Thissen, F. J. Fernandez-Madrigal, Q. H. Wu, W. Jaegermann, M. Klemm, S. Horn
Phys. Chem. Chem. Phys. 9, pp. 2564-2576

An impressive gravimetric capacitance of 1300 F g−1 (surface capacitance ∼3.3 mF cm−2) reported by Choi et al., 2006 for nanosized vanadium nitride has stimulated considerable interest in vanadium nitride as a potential electrode material for energy storing systems – supercapacitors. The postulated mechanism of charge storage in vanadium nitride materials involves redox reactions in the thin surface layer of vanadium oxide while the core vanadium nitride serves exclusively as a conducting platform. In this study we have synthesized pure oxygen-free vanadium nitride films and have found that they are capable of delivering a surface capacitance of up to ∼3 mF cm−2 at a potential scan rate of 3 mV s−1 and ∼2 mF cm−2 at a potential scan rate of 1 V s−1 in aqueous electrolytes. Combining electrochemical testing with X-ray photoelectron spectroscopy characterization has revealed that redox reactions play no or little role in the electrochemical response of pure VN, in contrast to the common wisdom stemming from the electrochemical response of oxygen-containing films. An alternative charge storage mechanism – space charge accumulation in a subsurface layer of ∼100 nm – was put forward to explain the experimentally observed capacitance of VN films in aqueous electrolytes.

O. Bondarchuk, A. Morel, D. Bèlanger, E. Goikolea, T. Brousse, R. Mysyk
Journal of Power Sources 324, pp. 439-446

We propose a new algorithm to achieve a very precise investigation of the three-dimensional atomic nucleus position in a material by the electron holography technique [T. Matsushita, A. Agui, Y. Yoshigoe, Europhys. Lett. 65 (2004) 207]. The algorithm is based on the “scattering pattern matrix” and an iterative gradient method, and it enables three-dimensional atomic-image reconstruction from a single-energy hologram. We measured a single-energy Si(0 0 1) photoelectron hologram of Si 2s emission using an Al Kα light source, and reconstructed the bulk structure with the use of the algorithm. We succeeded in locating as many as 29 atoms. The accuracy of the position for most atoms is within 0.05 nm.

T. Matsushita, A. Agui, A. Yoshigoe
Journal of Electron Spectroscopy and Related Phenomena 144-147, pp. 1175-1177

Studies of current dynamics in solids have been hindered by insufficiently brief trigger signals and electronic detection speeds. By combining a coherent control scheme with photoelectron spectroscopy, we generated and detected lateral electron currents at a metal surface on a femtosecond time scale with a contact-free experimental setup. We used coherent optical excitation at the light frequencies ωa and ωa/2 to induce the current, whose direction was controlled by the relative phase between the phase-locked laser excitation pulses. Time- and angle-resolved photoelectron spectroscopy afforded a direct image of the momentum distribution of the excited electrons as a function of time. For the first (n = 1) image-potential state of Cu(100), we found a decay time of 10 femtoseconds, attributable to electron scattering with steps and surface defects.

J. Güdde, M. Rohleder, T. Meier, S. W. Koch, U. Höfer
Science 318 (5854), pp. 1287-1291

We demonstrate the existence of buried image-potential states at the interface between thick Ar films and a Cu(100) substrate. The electron dynamics of these solid-solid interface states, energetically located above the vacuum level in the band gaps of both materials, could be investigated with time-resolved two-photon photoemission for an Ar layer thickness up to 200 Å. Relaxation on time scales between 40 and 200 fs occurs via two distinct channels, resonant tunneling through the insulating layer into the vacuum and electron-hole pair decay in the metal.

M. Rohleder, W. Berthold, J. Güdde, U. Höfer,
Phys. Rev. Lett. 94, 017401

We investigate the electronic structure and photoexcitation dynamics of alkali atoms (Rb and Cs) chemisorbed on transition-metal Ru(0001) single-crystal surface by angle- and time-resolved multiphoton photoemission. Three- and four-photon photoemission (3PP and 4PP) spectroscopic features due to the σ and π resonances arising from the ns and np states of free alkali atoms are observed from ∼2 eV below the vacuum level in the zero-coverage limit. As the alkali coverage is increased to a maximum of 0.02 monolayers, the resonances are stabilized by formation of a surface dipole layer, but in contrast to alkali chemisorption on noble metals, both resonances form dispersive bands with nearly free-electron mass. Density functional theory calculations attribute the band formation to substrate-mediated interaction involving hybridization with the unoccupied d bands of the substrate. Time-resolved measurements quantify the phase and population relaxation times in the three-photon photoemission (3PP) process via the σ and π resonances. Differences between alkali-atom chemisorption on noble and transition metals are discussed.

S. Zhang, C. Wang, X. Cui, Y. Wang, A. Argondizzo, J. Zhao, H. Petek
Phys. Rev. B 93, 045401

Excitons, electron–hole pairs bound by the Coulomb potential, are the fundamental quasiparticles of coherent light–matter interaction relevant for processes such as photosynthesis and optoelectronics. The existence of excitons in semiconductors is well established. For metals, however, although implied by the quantum theory of the optical response, experimental manifestations of excitons are tenuous owing to screening of the Coulomb interaction taking place on timescales of a few femtoseconds. Here we present direct evidence for the dominant transient excitonic response at a Ag(111) surface, which precedes the full onset of screening of the Coulomb interaction, in the course of a three-photon photoemission process with 15 fs laser pulses. During this transient regime, electron–hole pair Coulomb interactions introduce coherent quasiparticle correlations beyond the single-particle description of the optics of metals that dominate the multi-photon photoemission process on the timescale of screening at a Ag(111) surface.

X. Cui, C. Wang, A. Argondizzo, S. Garrett-Roe, B. Gumhalter, H. Petek
Nature Physics 10, pp. 505–509

It is crucial to develop a catalyst made of earth-abundant elements highly active for a complete oxidation of methane at a relatively low temperature. NiCo2O4 consisting of earth-abundant elements which can completely oxidize methane in the temperature range of 350–550 °C. Being a cost-effective catalyst, NiCo2O4 exhibits activity higher than precious-metal-based catalysts. Here we report that the higher catalytic activity at the relatively low temperature results from the integration of nickel cations, cobalt cations and surface lattice oxygen atoms/oxygen vacancies at the atomic scale. In situ studies of complete oxidation of methane on NiCo2O4 and theoretical simulations show that methane dissociates to methyl on nickel cations and then couple with surface lattice oxygen atoms to form –CH3O with a following dehydrogenation to −CH2O; a following oxidative dehydrogenation forms CHO; CHO is transformed to product molecules through two different sub-pathways including dehydrogenation of OCHO and CO oxidation.

F. F. Tao, J.-J. Shan, L. Nguyen, Z. Wang, S. Zhang, L. Zhang, Z. Wu, W. Huang, S. Zeng, P. Hu
Nature Communications 6, Article number: 7798

Recently, the feasibility of butane oxidation in an electrochemical membrane reactor (EMR) using a vanadium phosphorus oxide (VPO) catalyst layer on a tubular anodic electrode has been reported. This novel application of EMR gives rise to questions about the vanadium oxidation state (Vox) under working conditions and about its spatial distribution in the catalyst layer. It has now been determined by means of position-resolved XPS measurements. In addition, model calculations on the spatial Vox distribution have been performed for the first time. The simulations reveal a non-uniform 3D distribution of Vox due to the relative rate of reduction and re-oxidation processes in the catalyst layer, in good agreement with the experimental XPS data.

Y. Suchorski, B. Munder, S. Becker, L. Rihko-Struckmann, K. Sundmacher, H. Weiss
Applied Surface Scinece 253, pp. 5904-5909

We have investigated the C 1s photoelectron spectra of ethylene on the Si(1 0 0)(2 × 1) surface, using high resolution photoelectron spectroscopy. The vibrational structure has been observed in the C 1s spectra where the C–H stretching mode is dominant. The vibrational splittings of C 1s ionized state are very similar to those in the ground state. It is found that linear coupling model is applicable in this system and this excitation process is a Frank–Condon process. The curvature of potential energy surface of C–H and C–D bond is almost unchanged upon core ionization.

Y. Yamashita, S. Machida, M. Nagao, S. Yamamoto, K. Mukai, J. Yoshinobu
Chemical Physics Letters 374 (5-6), pp. 476-481

Non-monochromatic MgKα and monochromatic AgLα irradiations were used to measure Pt4f and Pt3d5/2 X-ray photoelectron spectra from platinum black and from several platinum compounds with different Pt oxidation states. The Pt3d5/2 core level binding energies from platinum compounds were measured for the first time. Potential of these data to XPS study of Pt/Al2O3 catalysts is demonstrated.

A. V. Kalinkin, M. Yu. Smirnov, A. I. Nizovskii, V. I. Bukhtiyarov.
Journal of Electron Spectroscopy and Related Phenomena 177, pp. 15-18

The surface of vanadium phosphorus oxide (VPO) catalysts was investigated by (in situ) X-ray photoelectron spectroscopy (XPS) under reaction conditions. Two differently prepared VPO samples with similar catalytic activities showed different spectral behaviour while the catalytic conditions were changed. The vanadium surface oxidation state of both catalysts was found to have the same value close to 4 under reaction conditions, while the oxidation state of vanadium in deeper layers differed significantly. The experimental results suggest that in VPO the catalytically active species located in the topmost surface layers (up to 1 nm depth) are only weakly related to the structure of deeper layers. Based on our results we suggest that the deeper layers act as a substrate material only and can be different from the surface.

E. Kleimenov, H. Bluhm, M. Hävecker, A. Knop-Gericke, A. Pestryakov, D. Teschner, J. A. Lopez-Sanchez, J. K Bartley, G. J. Hutchings, R. Schlögl
Surface Science 575 (1-2), pp. 181-188

The oxidation of Re/γ-Al2O3 catalysts, containing 1 and 10 wt% of rhenium, sintered in hydrogen was examined in the temperature range of 20–800 °C. The structures of the catalysts were investigated by XPS spectroscopy, TEM, and O2 uptake measurements. The low-loaded catalyst comprises metallic particles with sizes of 1–4 nm (dav=2.1 nm), while the high-loaded catalyst comprises particles with sizes of 1–9 nm (dav=4.9 nm). Even short exposure to air at room temperature causes complete oxidation of small clusters of metallic Re, while larger particles are covered with very thin ReOx skin (undetected by TEM). XPS shows that the high-loaded catalyst still contains 94.5% of metallic Re, while the low-loaded catalyst contains only 60.5%. The remaining part of the Re is oxidized to Re4+, Re6+, and Re7+ species. Oxidation at 150 °C causes enhanced formation of Re4+–Re7+ species and the amount of metallic Re quickly decreases to 33 and 2% for high- and low-loaded catalysts, respectively. This indicates a high affinity of the highly dispersed Re to oxygen. At this temperature, the Re/Al atomic ratio increases 2–4 times, indicating a large spreading of the oxide species on the support surface. Simultaneously, the average size of Re particles decreased as determined by TEM. At 300 °C, whole Re was oxidized mainly to Re2O7, though some amount to Re4+ and Re6+ species remained. The O2 uptake measurements confirm oxidation of rhenium particles. For the high-loaded catalyst O2 uptake attained a maximum level (O/Re=3.3) already at 300 °C, while for the low-loaded catalyst even at 500 °C the uptake (O/Re=2.98) is below the maximum level. XPS data showed, however, that at 500 °C, oxidation of rhenium to Re2O7 occurs for both catalysts. The Re/Al atomic ratio remains nearly constant after oxidation of both catalysts at 300–800 °C, indicating that Re7+ species are firmly bonded to alumina surfaces even at 800 °C. A detailed mechanism of oxidation of Re particles with different sizes is proposed based on a quantitative analysis of the XPS, O2 uptake, TEM, and previous Raman results.

J. Okal, W. Tylus, L. Kepinski
Journal of Catalysis 225 (2), pp. 498-509

Here we report on results of a spin-resolved photoelectron spectroscopic (SRPES) study of YCo2 thin films (150 Å-thick) grown on a W(110) substrate. The films were prepared by co-deposition of stoichiometric amounts of Y and Co onto a clean W surface followed by thermal annealing leading to (2×2) overstructure with respect to W(110) in the low-energy electron diffraction pattern indicated formation of a structurally ordered YCo2(111) surface. While no clear spin asymmetry was observed for bulk-sensitive SRPES data taken at hν=1253.6  eV, the more surface-sensitive SRPES data obtained at hν=21.2  eV photon energy revealed a clear spin-asymmetry probing the validity of the recent theoretical prediction.

Yu. S. Dedkov, C. Laubschat, S. Khmelevskyi, J. Redinger, P.Mohn, M.Weinert
Phys. Rev. Lett. 99, 047204