Photocurrent Spectroscopy in Corrosion and Passivity Studies. A Critical Assessment

Abstract

Photocurrent Spectroscopy (PCS) is currently employed for the characterization of solid-state properties of semiconducting and insulating materials, since the knowledge of their band gap is a prerequisite to any possible application in different fields such as: solar energy conversion (photoelectrochemical and photovoltaic solar cells, photocatalysis) and microelectronics (high-k, high band-gap materials) (1-2). In the last 20-30 years an increasing number of scientists working in the area of corrosion has been attracted by this technique owing to its versatility and ability to scrutinize in situ corrosion layers and passive films having semiconducting or insulating behaviour. In previous works (3-4) we have shown that PCS is able to provide detailed information on characteristic energy levels of passive film/electrolyte junctions (flat band potential: Ufb; internal photoemission threshold: Eth; band gap value: Eg), which control the mechanism of charge transfer (electrons and ions) at the metal/corrosion layer/ electrolyte interface. Further advantages stem out from the fact that PCS does not require particular surface finishing control and can be used both in large area as well as in microscopic region of the electrode, and can reach high sensitivity by using a lock-in amplifier coupled to a mechanical light chopper. According to this, numerous experimental studies have shown how it is possible to scrutinize corrosion layer and passive films of very limited thickness grown on metals and alloys (Fe, Cr, Ni and SS) of relevant industrial interest (3-4 and refs. therein). In an attempt to make more quantitative the characterization of corrosion layer and passive by PCS, in previous works (3-4) we have shown that it is possible to correlate semi-empirically the band gap values of numerous binary oxides to their composition by using the Pauling’s equation for the average bond energy, i.e.: Eg =2[EI(χM -χO)2 +Ξ] (1) where χM and χO are the electronegativities, in the Pauling’s scale, of metal and oxygen respectively, and the remaining terms have been defined in the original work (5). Apart few noticeably exceptions, eq. (1) assumed, for sp-metal and d-metal oxides the following expression: Eg - ΔEam (eV) = 2.17(χM−χO)2 − 2.71 for sp metal (1b) Eg - ΔEam(eV) = 1.35(χM −χO)2 − 1.49 for d metal (1c) where ΔEam = 0 for crystalline oxides, whilst increasing ΔEam values have been suggested with a decreasing crystallinity degree. Moreover in several successive works (see refs. in 3-4), we have been able to show that in the case of ternary oxides by using of an average electronegativity parameter defined as the atomic % average between electronegativities of partner metals, eqs. (1b) and (1c) were able to correlate the band gap values measured by PCS with the mixed oxide composition. In this work, in order to test the validity of eq.1 for ternary oxides, we will carry out a comparison between several experimental Eg values of ternary oxides and the Eg values obtained by using eq.1 with the average electronegativity value for ternary oxide given by: χav = x1χcat1 + x2χcat2 where χ cat1,cat2 and x1,2 are the electronegativity parameter and atomic fraction of cations 1,2 in the ternary oxides, respectively. Moreover, some very recently publishes Eg values of mixed d-d metal oxides, of large interest in corrosion studies, theoretically derived from Density Functional Theory will be compared with PCS measured data and with those estimated according to eq. (1). Possible explanations of the difference in experimental and theoretical data will be presented and discussed.