F: , .":'," ','. w!LP. ":. , • • ELSEVIER Journal of Electroanalytical Chemistry 379 (1994) 173-180 JOURNAL OF Characterisation of titanium passivation films by in situ ac impedance measurements and XPS analysis Carlos da Fonseca, Stephane Boudin, Manuel da Cunha Belo Centre d'ff,tudes de Chimie Mitalurgique (C.N.R.S.), 15 Rue Georges Urbain, 94407 Vitry-Seine, France Received 10 December 1993; in revised form 27 April 1994 Abstract Capacitance-potential measurements by ac impedance methods have shown that for polarisations of 0.5 to 0.7 V from the fiat band potential, titanium passivation films formed between 0.2 and 2 V (thicknesses ranging from 15 to 100 .~) display Mott-Schottky behaviour, with frequency dependent slopes. This study develops a theoretical approach which takes account of the frequency dispersion and allows the determination of fiat band potentials, donor densities, and Helmholtz capacitances for the films. The Mott-Schottky behaviour was attributed to the presence of Ti 3÷ as the doping element. Experimental evidence for this was obtained from XPS spectra. For films formed above 2 V the linear Mott-Schottky region was difficult to define, and capacitance vs. potential plots resembled the behaviour of amorphous semiconductors. Keywords: Titanium passivation; ac impedance; APS analysis 1. Introduction It is well known that the high corrosion resistance of titanium is due to the presence of a thin stable passive film which forms on the metal surface. This and the relative simplicity of the system have justified the large number of studies performed on the TiO 2 thin film[ electrolyte interface in order to understand the elec- tronic properties of these passive layers [1-3]. Among the experimental techniques which have been used, impedance spectroscopy, and more particularly capaci- tance measurements, have played an important role. In semiconductor (sc) electrochemistry, capacitance studies, often based on Mott-Schottky (M-S) plots, are a classical method for determining key parameters related to the electronic structure of semiconductor electrodes [4,5]. In recent years the M-S method has been extended to the study of passive films, which can be regarded in many cases as thin oxide layers with semiconducting character. However, results were often ambiguous due to the small observed M-S region and the large frequency dispersion of the M-S plots [6,7]. For these reasons, the interpretation of passive film properties in terms of a M-S approach is still a matter of discussion [8]. In our work we have used a modified M-S expression which takes into account the frequency dispersion of the M-S plots and allows the calculation 0022-0728/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSDI 0022-0728(94)03537-D of coherent values for the Helmholtz capacitances and the flat band potentials of the passive films. Frequency independent donor densities can also be estimated from the M-S slopes. The identification of the structural defects giving rise to semiconductivity is important for the under- standing of the physico-chemical properties of passive films, namely ionic and electronic transport properties. Defects also play an essential role in many models of the formation of the passivation layer [9]. In the case of futile, published TiO 2 single crystal data suggest that oxygen vacancies and Ti 3÷ interstitials may be among the important defects in the system [10], being respon- sible for the semiconducting behaviour of the crystals. However, for passive films, the problem of the identifi- cation of the structural defects has received little atten- tion. In this work we relate the M-S behaviour with the presence of titanium(Ill) in the films. Experimental evidence for this was obtained from a parallel M-S and XPS study of the passive films. 2. Experimental Titanium oxide films were prepared from 99.99% pure metallic rods 9.5 mm in diameter purchased from Goodfellow Metals, Cambridge. The samples and elec-