Fleig, Jürgen

Hlavathy, Zoltan

Hävecker, Michael

Januschewsky, Judith

Joo, Jong-Hoon

Knop-Gericke, Axel

Kubicek, Markus

Maier, Joachim

Merkle, Rotraut

Oestereich, Andreas

Schlögl, Robert

English

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diffusion-fundamentals   The Open-Access Journal for the Basic Principles of Diffusion Theory, Experiment and Application www.diffusion-fundamentals.org, ISSN 1862-4138; © 2005-2010   Diffusion Fundamentals 12 (2010) 67  © J.-H. Hoo XPS on Mixed Conducting Perovskites at Elevated Temperatures and Close-to-Ambient Gas Pressure Jong-Hoon Joo,*,1 Rotraut Merkle,1 Joachim Maier,1 Markus Kubicek,2  Judith Januschewsky,2 Jürgen Fleig,2 Andreas Oestereich,3 Zoltan Hlavathy,3  Michael Hävecker,3 Axel Knop-Gericke,3 Robert Schlögl3 1 MPI for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany 2 Vienna University of Technology, Getreidemarkt 9/164EC, 1060 Vienna, Austria 3 Fritz-Haber-Institut der MPG, Faradayweg 4-6, 14195 Berlin, Germany  E-Mail: JH.Joo@fkf.mpg.de   Presented at the Bunsen Colloquium: Spectroscopic Methods in Solid State Diffusion and Reactions September 24th − 25th, 2009, Leibniz University Hannover, Germany   XPS is a valuable technique for surface-sensitive materials characterization. The usual lab XPS setups have the severe drawback to operate only under UHV conditions, which is, e.g., far from the relevant conditions of solid oxide fuel cell (SOFC) cathode materials. These disadvantages can to a large extent be overcome with a “high pressure” XPS setup installed at the BESSY II synchrotron [1]. It allows sample temperatures up to 700 °C, depth profiling over 2 nm without sputtering (i.e. without sputter-ing artifacts) by variation of the excitation energy, and most importantly measurements under a resi-dual gas pressure in the mbar range. Thus in situ measurements close to actual SOFC operation condi-tions become possible. A positive side effect of measuring samples at elevated T and in mbar O2 pres-sure is that the surface becomes “self-cleaning”, i.e., organic contaminations and even carbonate groups decompose.  Here we report on SOFC cathode model systems, thin films (100 nm) of nominally identical La0.6Sr0.4CoO3−δ composition which were prepared by PLD on single-crystalline YSZ substrates under different deposition conditions. Electrochemical investigations prove that the different films indeed have a different electrocatalytic activity for the oxygen incorporation reaction [2], which raises the question of the actual surface composition.   Fig. 1 shows the O1s spectra under p(O2) = 0.1 mbar, T = 300 °C for La0.6Sr0.4CoO3−δ films depo-sited at substrate temperatures of 450 °C (M01) and 650 °C (M02). The peak at highest binding energy (red) corresponds to a surface-related oxygen species; for M02 also the blue peak decreases for higher depths. Fig. 2 demonstrates that also the relative contributions of different La species (green and blue doublets) close to the surface depend on PLD deposition conditions, while they become more similar at higher sampling depth. The different surface composition is also reflected in the different changes of the O1s spectra upon exposure to ≈ 0.05 mbar H2O (Fig. 3). The detailed assignment of the different peaks to certain environments as well as a quantitative depth-resolved analysis of elemental composi-tion is under way.    Joo et al., XPS on mixed conducting perovskites   diffusion-fundamentals    www.diffusion-online.org  Diffusion Fundamentals 12 (2010) 68  © J.-H. Hoo                 Fig. 1 O1s spectra of samples M01 and M02 recorded at kinetic energies of 200 eV (sampling depth ≈ 0.7 nm) and 1000 eV (≈ 2 nm).                 Fig. 2  La4d spectra of samples M01 and M02 recorded at kinetic energies of 200 eV and 1000 eV.           Figure 3: O1s spectra (Ekin = 200 eV) for M01 and M02 samples during exposure to 0.05 mbar H2O.  References [1] H. Bluhm, M. Hävecker, A. Knop-Gericke, E. Kleimenov, R. Schlögl, D. Teschner, V. I. Bukhtiyarov, D. F. Ogletree, M. Salmeron, J. Phys. Chem. B 108 (2004) 14340. A. Knop-Gericke et al., Adv. Catal. 52 (2009) 213. [2] J. Januschewsky, M. Ahrens, A. Opitz, F. Kubel, J. Fleig, Adv. Funct. Mater. 19 (2009) 3151. binding energy / eV524526528530532534536binding energy / eV524526528530532534536T = 300°C     pO2 = 0.1 mbarEkin = 200 eV  sample M01Ekin = 1000 eV  sample M01  sample M02  sample M02binding energy / eV9698100102104106108110112114binding energy / eV9698100102104106108110112114T = 300°C     pO2 = 0.1 mbarEkin = 200 eV  sample M01Ekin = 1000 eV  sample M01  sample M02  sample M02binding energy / eV524526528530532534536binding energy / eV524526528530532534536T = 300°C  pO2 = 0.05 mbar   pH2O = 0.05 mbarsample M01 sample M02t = 0 mint = 30 mint = 60 mingas phase     H2Ot = 0 mint = 25 mint = 45 min

XPS on mixed conducting perovskites at elevated temperatures and close-to-ambient gas pressure

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XPS on mixed conducting perovskites at elevated temperatures and close-to-ambient gas pressure

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