Photoelectron spectroscopy characterization of Pt film gradient over Nb-doped TiO2 support

MIGUEL D. SÁNCHEZ; BASTIAN MEI; WEI XIA; MARTIN MUHLER

Karlsruhe

Conferencia; First Internacional Conference on Materials for Energy; 2010

Gesellschaft für Chemische Technik und Biotechnologie e.V.

Despite considerable advances in fuel cell research, there are still several economical and technical barriers in developing suitable electrocatalysts. As it is well known, the commercially used catalyst is carbon supported platinum which provides high electric conductivity and well-dispersed, small Pt particles [1]. Nevertheless, this material exhibits a decreasing performance in long-term stability tests due to the electrochemical oxidation of carbon and the dissolution of Pt from carbon support. In recent years, several studies on Nb-doped TiO2 as conductive support for platinum catalysts to be used as fuel cell, battery or solar cell have been reported [1-3]. In the present work we characterized a Pt film gradient deposited on Nb-doped TiO2 support by means of XPS and UPS. The sample was prepared by sputtering deposition on a silicon wafer in UHV. The substrate deposition rates were calibrated to obtain a Nb gradient across the wafer from ~23 to ~3.5 at.%) followed by oxidation at 450 °C in ambient air. The Pt film stripes were deposited by sputtering reaching a nominal thickness gradient of 0 to 5 nm, perpendicular to the Nb ones. XPS and UPS measurements were carried out in an ultra-high vacuum set-up equipped with a Gammadata-Scienta SES 2002 analyzer using a monochromatic Al Ka source (1486.6 eV; 14.5 kV, 45 mA) and He II ultraviolet light source (40.8 eV; 1.5 kV, 100 mA) as incident radiation, respectively. The spectra were taken in a spatial resolution mode with a detection area of 0.015 x 1.250 mm. An elemental composition profile analysis based on XPS survey spectra as a function of the position on the Pt stripe showed a noticeable step-like feature at the thinner film part. The decrease in the Pt atomic concentration was accompanied by an increase in the Ti, O and Nb atomic concentration. The UPS (He II) spectra, taken around this point, showed a remarkable decrease at the Fermi Edge accounting for a drop in the electrical conductivity of the film. The depth profile analysis, based on the XPS Pt 3d peak shape and the two-parameter universal cross-section background [4], showed a linear decrease in Pt film thickness as a function of the position on the stripe, in good agreement with the nominal thickness estimation. The overall results were consistent with the electrochemical measurements [5] and could be related to the loss of the Pt film continuity. References [1]    M. Koninck, P. Manseau, and B.J. Marsan, J. Electroanal. Chem. 611(1-2), 67 (2007). [2]    K. Park, and K. Seol, Electrochem. Commun. 9(9), 2256 (2007). [3]    G. Chen, S. Bare, and T. Mallouk, J. Electrochem. Soc. 149(8), A1092 (2002). [4]    S. Tougaard, Surf. Interface Anal. 25, 137 (1997). [5]    To be published.