Electronic barriers in the iron oxide film govern its passivity and redox behavior: Effect of electrode potential and solution pH

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Abstract

We have measured in situ the electronic conductance spectra of the passive film formed on an Fe electrode immersed in a borate buffer solution using electrochemical tunneling spectroscopy (ECTS) and electrochemical impedance spectroscopy (EIS) techniques, and we have followed their changes as the electrode is electrochemically oxidized and reduced. We demonstrate that pre-passive Fe(II) oxide and the passive Fe(II)/Fe(III) film, behave as p- and n-type semiconductors, respectively and that their reversible inter-conversion is mediated by the availability of free charge carriers on the electrode surface. ECTS spectra have been also modeled to obtain the main electrochemical kinetic parameters of the electron transfer through both p-Fe(II) and n-Fe(III) oxides at different sample potentials and pHs values. We find that the electronic energy barrier in the oxide and its dependence with electrode potential and solution pH, determine the reactivity and passivity of iron.

Introduction

The phenomenon of passivity is based on the spontaneous formation of a surface oxide layer that prevents a metallic electrode from corroding [1]. However, the detailed mechanism of the passive film growth and the electronic effects responsible for the electrode protection, are still poorly understood. Fundamental work on passive films on metallic electrodes has been based on conventional electrochemical techniques, typically in neutral and alkaline media. The chemical composition and structure of passivated surface electrodes have been recently investigated by ex situ structural methods like microscopy and spectroscopy in vacuum [2], [3]. In situ techniques have been the key to understand the chemical composition and structure of passive film on metals and steels in solution [4], [5], [6], [7]. The introduction of scanning tunneling microscopy (STM) and its application to electrochemistry (ECSTM) [8] has allowed further insights into the structure and electronic properties of the passive film in the solution of interest [9], [10], including the recent applications of ECTS [11], [12], [13], [14].

The passive film on iron and other transition metals present a semiconducting character [15]. The complete model proposed for the Fe passive film structure in most of the experimental conditions (static hydrodynamic conditions and potential scanning) consists on a structure that resembles a spinel Fe3O4 or a defective γ-Fe2O3, being Fe1.9±0.2O3 the closest measured stoichiometry (mostly Fe(III)) [4]. The outer interphase of the passive layer (the oxide/electrolyte side) is suggested to have a higher concentration of cation interstices (Fe(III) form is predominant) [4], eventually terminated by a hydrated Fe(III) hydroxide film [10], [15], while at the inner metal/oxide interphase an excess of Fe(II) vacancies is expected. The presence of this particular outer interphase confers a fairly ideal n-type behavior to the Fe passive film that is named Fe(III) passive film for simplification. In general, the Fe passive film presents an n-type behavior with the Fe(II) centers acting as electron donors [15], [16]. The excess of vacancies close to the metal/oxide interphase may serve as an ohmic contact between the metal and the n-Fe(III) outer interphase, as suggested by the increase in conductance when the outer film is dissolved [17]. This is analogous to heavily doped metal/n++/n and metal/p++/p ohmic contacts used in microelectronics. On the other hand, the properties of the Fe pre-passive oxide that forms at low electrochemical potentials (⩽−0.3 V vs. reference electrode and Fe(II) called oxide again for simplification) [10], [18], [19] have not been well defined, although they can strongly influence the properties of the final passive film. As relevant information of this Fe(II) pre-passive layer, its partial solubility in nearly neutral media has been demonstrated in some instances [10], [20]. Although several general models of the Fe passive film were proposed 15 years ago [21], [22], the implications of its electronic properties on the electrode redox behavior are not yet completely understood.

In this study, we show reproducible local in situ electrochemical tunneling spectra of the iron electrode/borate buffer interface while it is reversibly oxidized and reduced under electrochemical control. The experimental ECTS spectra are presented in the form of conductograms [23] that allow to analyze the observed transitions in the electronic properties of the electrode during the electrochemical processes. Our EIS results confirm the n-type properties of Fe(III) and reveal the p-type character of Fe(II). The electrochemical oxidation and reduction of Fe to form and dissolve its surface passive film can be then explained by the interconversion of p-Fe(II) to n-Fe(III) oxides and vice versa, at the electrode surface. The availability of charge carriers on both semiconducting surfaces gives rise to the oxidation of p-Fe(II) and the reduction of n-Fe(III) when the Fermi level (EF) approaches the corresponding Flatband values. The electronic properties of Fe(II) and Fe(III) oxides (Flatband potential, doping type and concentration) are combined with ECTS data to elaborate a band diagram model of the electrochemical processes. The electrochemical kinetic parameters of the electron transfer through the Fe passive film at different sample potentials and pH values are quantitatively obtained by fitting the ECTS curves to a double exponential Butler–Volmer relation. It is found that electrode reactivity and passivity are coupled to the availability of free charge carriers in the semiconducting Fe passive film, as given by the electrode potential and solution pH.

Section snippets

Sample and electrolyte

Mechanically polished iron polycrystalline disks were used as working electrodes and prepared as previously described [10], [18]. The working electrolyte consists on a pH 7.5 borate buffer solution prepared from 0.3 M H3BO3 and 0.0375 M Na2B4O7 · 10H2O. In all cases pro-analysis purity grade chemicals from Merck (Darmstadt, Germany) and Milli-Q water of 18  cm, were used.

Electrochemical measurements

Cyclic voltammetry and capacitance measurements were performed at room temperature using a Solartron Instruments electrochemical

Capacitance measurements

EIS was employed to obtain capacitance data of the Fe passive film [15], [16], [17], [18] at the different potential regions during anodic oxidation in borate buffer pH 7.5. In order to grow thick and well-controlled Fe oxide films, we follow a 2-step electrochemical procedure as in previous works [10], [18], being the final US value located within the upper limit of the corresponding Fe(II) and Fe(III) ranges, respectively. Once the electrode is passivated (transient constant current IS < 0.5 μA)

Conclusions

We have measured in situ the electron energy spectrum of an Fe electrode during its reversible electrochemical oxidation and reduction in borate buffer medium at different pHs by using EIS and ECTS techniques. While Fe(III) displays an n-type behavior in agreement with previous reports, Fe(II) is a highly doped, narrow bandgap, p-type semiconductor. We find that the main Fe redox transitions on the iron electrode surface (p-Fe(II)  n-Fe(III) and n-Fe(III)  p-Fe(II)) result from the availability

Acknowledgements

This work was supported by a grant of MEC (Ministry of Education and Culture) under Project CTQ2004-08046-C02-01. P.G. acknowledges financial support of the MEC through the program Ramón y Cajal. We thank P. Allongue for useful discussions and the Scientific–Technical Services of the University of Barcelona for use of their facilities.

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