J. Electroanal. Chem., 78 (1977) 55--69 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands 55 XPS SPECTROSCOPIC STUDY OF POTENTIOSTATIC AND GALVANOSTATIC OXIDATION OF Pt ELECTRODES IN H:SO4 AND HC104 J.S. HAMMOND and NICHOLAS WINOGRAD * Department of Chemistry, Purdue University, West Lafayette, Ind. 47907 (U.S.A.) (Received 1st June 1976) ABSTRACT The surface oxides produced from potentiostatic and galvanostatic oxidation of Pt elec- trodes in HC104 and H2SO 4 are examined using X-ray photoelectron spectroscopy. The oxide I species produced as the initial oxidation product by successively more anodic potentiostatic oxidation in 0.2 M HCIO4 is found to have a Pt 2+ oxidation state, a binding energy character- istic of neither PtO, Pt(OH)2 or PtO2, and a limiting thickness of 8 ~. Galvanostatic oxidation in HCIO4 and H2SO 4 is found to produce PtO 2 • H20 as an unlimiting growth oxide or a limiting growth oxide layer depending on the concentration of the acid electrolyte. The incor- poration of the acid electrolyte anion in the surface layer is shown to have an effect on which type of oxide layer is produced. X-ray decomposition and chemical modification by Ar + stripping are shown to produce chemical artifacts complicating any interpretation of a Pt oxide surface layer. Surface oxides prepared by anodization of Pt electrodes in acidic media re- main a research area of controversy with numerous recent studies [1--20]. A ma- jor thrust of this research has been the identification of the chemical nature of the limiting growth oxide I versus the properties of multilayer oxide II [7,9--12, 14--16,19]. Each oxide can be identified by its characteristic potential region of voltammetric reduction [16]. To attempt to clarify the chemical composition of these distinctive oxide layers, ellipsometry and reflection spectroscopy [3,8,21-- 28], electron diffraction [29], field-ion microscopy [30], Auger spectroscopy [31], infrared spectroscopy [32], and internal reflection spectroscopy [33], have been used to characterize the electrode surfaces. The results from ellipsometry, external and internal reflection experiment [ 3,8,21--28,33] have produced con- sistent thickness measurements for the limiting growth oxide I of approximately 8 A. However, the chemical identification of the Pt oxidation state and the oxide layer composition has not been obtained directly from these measurements but rather inferred from correlation of thickness and coulometric data to be two layers of a "PtO like" species for the oxide I phase. The electron diffraction [28], field ion microscopy [30] and infrared spectroscopy [32] measurements produced ambiguous results relative to the question of the chemical composition of oxide I. From intensity ratios Johnson and Heldt assigned a Pt electrode sur- face oxide structure to a "PRO" stoichiometry [31]. X-ray photoelectron spectroscopy (XPS or ESCA) has been established as a spectroscopic technique capable of analyzing electrode surfaces [34--36]. Kim et al. [34] carried out a Preliminary investigation of Pt electrode oxidation in