Abstract:

Metal oxides anchored to oxide supports often exhibit greater catalytic activity as monolayers than as thicker films. Understanding this phenomenon requires a chemically sensitive, atomic-scale view of the interfacial processes. We use in situ X-ray standing wave (XSW) 3D atomic imaging [1-3] combined with ex situ X-ray photoelectron spectroscopy (XPS) to follow the redox-induced surface site exchange of cations on a single crystal oxide support as well as the concurrent changes in the oxidation states of the supported cations. This is then compared to density functional theory. As an example, we follow the reversible changes during the redox cycle of a 1/3 ML WO_X / α-Fe₂O₃ (0001) interface grown by atomic layer deposition.[4] The XSW measured W atomic maps and XP spectra show dramatic changes for the as-deposited, oxidized and reduced interfaces, which are explained by models that account for W incorporation at Fe sites with various coordination schemes. The 3D W atomic map for each condition is measured by the summation of the XSW measured hkl Fourier components for the XRF selected W distribution. This strategy was then also applied to redox-induced structural and chemical changes for the sub-ML and 2 ML VO_X /αTiO₂ (110) interfaces.[5]

The above measurements used XSW facilities at the Advanced Photon Source and XPS facilities at Northwestern University. We recently used the unique capabilities of ESRF ID32 to make a combined XSW - XPS study of the chemically sensitive interfacial structure of epitaxial graphene grown on SiC(0001) by thermal processing.[6]  There are distinct C 1s chemically shifted XPS peaks for C in the graphene layers, 6√3 interface layer and SiC substrate that allow us to determine the multiple C positions and compare to X-ray reflectivity measurements [7,8] and theoretical predictions [8].

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[2] J. S. Okasinski, C.-Y. Kim, D. A. Walko, M. J. Bedzyk, Phys. Rev. B 69, 041401(R) (2004).

[3] Z. Zhang, P. Fenter, L. Cheng, N. C. Sturchio, M. J. Bedzyk, M. L. Machesky, D. J. Wesolowski, Surf. Sci. Lett. 554, L95 (2004).

[4] Z. Feng, C.-Y. Kim, J. W. Elam, Q. Ma, Z. Zhang, M. J. Bedzyk, J. Am. Chem. Soc. 131, 18200 (2009).

[5] C.-Y. Kim, J. W. Elam, P. C. Stair, M. J. Bedzyk, J. Phys. Chem C 114, 19723 (2010).

[6] J. D. Emery, B. Detlefs, H. Karmel, M. C. Hersam, D. K. Gaskill, J. Zegenhagen, M. J. Bedzyk, in preparation

[7] J. Hass, J. E. Millan-Otoya, P. N. First, E. H. Conrad, Phys. Rev. B 78, 205424 (2008).

[8] J. D. Emery, Q. H. Wang, M. Zarrouati, P. Fenter, M. C. Hersam, M. J. Bedzyk, Surf. Sci. 605, 1685 (2011).

[9] F. Varchon, P. Mallet, J.-Y. Veuillen, and L. Magaud. Phys. Rev. B 77, 235412 (2008).