Lee, Jong-Sook

Maier, Joachim

Merkle, Rotraut

Yu, Ji-Haeng

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) 33  © J.-S. Lee Novel Features of Water Incorporation in Oxides Revealed by in situ Local Absorption Spectroscopy Jong-Sook Lee,*,1 Ji-Haeng Yu,2 Rotraut Merkle,3 Joachim Maier3  1 School of Materials Science and Engineering, Chonnam National University, Gwangju 500-757, Korea  2 Korea Institute of Energy Research, Daejon 305-343, Korea 3 MPI for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany  E-Mail: jongsook@jnu.ac.kr  Presented at the Bunsen Colloquium: Spectroscopic Methods in Solid State Diffusion and Reactions September 24th − 25th, 2009, Leibniz University Hannover, Germany   Fe-doped SrTiO3 is a model mixed conducting perovskite offering the perspective to develop an accurate description of thermodynamics and kinetics of water incorporation, since thermodynamic parameters in oxygen incorporation and intrinsic redox reactions are well established [1]. Water incorporation into the oxygen vacancies of Fe-doped SrTiO3 was found to occur to a small but nonzero degree [2]. In acceptor-doped oxides with predominant oxygen vacancies water incorporation fills the vacancies with protonic defects OHO. in a pure acid-base reaction (Fig. 1a). The purpose of the present study is to investigate the kinetics of water incorporation in Fe-doped SrTiO3 involving redox as well as acid-base processes owing to the presence of three charge carriers, VO.., OHO., h. (Fig. 1b).    a)                  b)          Fig. 1: Scheme of water incorporation into SrTiO3: a) pure acid-base mechanism, b) combined acid-base and redox process  In order to resolve the complex kinetics with contributions from surface reaction as well as bulk diffusion, space-resolved optical absorption measurements were performed on a 6 × 6 × 1 mm3 sized Fe-doped SrTiO3 single crystal [3]. In these experiments the large crystal faces are sealed, thus the in-diffusion of the different charge carriers from the edges can be followed by space-resolved measurements of the FeTix absorption which serves as an indicator also for all other point defect concentrations. An example is shown in Fig. 2.  J.-S. Lee et al., Nonmonotonic water incorporation in oxides  diffusion-fundamentals    www.diffusion-online.org  Diffusion Fundamentals 12 (2010) 34   © J.-S. Lee    Fig. 2  Insets: time-dependent and space-resolved optical absorption in Fe-doped SrTiO3 after a p(H2O) increase from 4 to 20 mbar in oxygen at  500 °C ; false-color representation. The left and right edges of the sample are coated with a porous Pt layer facilitating the water dissociation reaction H2O   2 H+ + 1/2 O2 + 2 e−. Colored symbols: FeTix concentration in selected regions, black squares: averaged over whole sample  Directly after an increase in p(H2O), a zone of lower FeTix concentration starts to extend into the sample from the left and right edges. After about 1 hour the whole sample is in an "over-reduced" state due to the fast incorporation of protons and electrons emerging from the dissociation of water at the surface according to H2O   2 H+ + 1/2 O2 + 2 e−. Because of the much slower oxygen diffusion in SrTiO3, the following oxygenation 1/2 O2 + VO..   OOx + 2 h. occurs on a significantly longer time scale. According to the defect-chemical model [2], the complete water incorporation process leads to a slight overall decrease of the FeTix concentration.  The in situ optical absorption spectroscopy not only unambiguously indicates that the oxide was strongly reduced first and then re-oxidized to the final equilibrium. It also reveals more subtle features of water incorporation such as predominant diffusion control (i.e., fast surface reaction) for the hydrogen incorporation at the Pt coated edges, while the bare surface exhibits much slower kinetics for this combined acid-base and redox reaction.  References [1] I. Denk, W. Münch, J. Maier, J. Am. Ceram. Soc. 78 (1995) 3265; R. Merkle, J. Maier, Angew. Chemie Int. Ed. 47 (2008) 3874; and references therein. [2] J.-H. Yu, J. S. Lee, J. Maier, Phys. Chem. Chem. Phys. 7 (2005) 3560. [3] J.-H. Yu, J. S. Lee, J. Maier, Angew. Chemie Int. Ed. 46 (2007) 8992. 

Novel features of water incorporation in oxides revealed by in situ local absorption spectroscopy

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Novel features of water incorporation in oxides revealed by in situ local absorption spectroscopy

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