Stability and dissolution of single-crystalline iron oxide thin films in electrochemical environments

The stability of single-crystalline monolayer FeO(111) and 10 nm thin Fe₃O₄(111) films on Pt(111) upon exposure to environments of increasing chemical complexity has been studied with X-ray photoelectron spectroscopy, temperature-programmed desorption, in-situ scanning tunneling microscopy, and cyclic voltammetry. The well-defined oxide films, which were prepared under ultrahigh-vacuum conditions, were exposed to aqueous solutions of different pH and electrochemical cycling in pure and catechol-containing electrolyte. The films are stable in neutral (pH 7) and alkaline (pH 13) solutions both at open circuit conditions and during electrochemical cycling within the limits of hydrogen and oxygen evolution potentials. Also in strongly acidic (pH 1) perchlorate solution the films remain intact under open circuit conditions, but they quickly dissolve on application of electrochemical potential. Especially for the ultrathin FeO(111) films, catechol enhances the dissolution at neutral pH during electrochemical cycling. A comparison of Pt(111), FeO(111) and Fe₃O₄(111) substrates in the electrochemical catechol oxidation reaction reveals enhanced and sustained activity of FeO in alkaline environment, while strong deactivation occurs on Pt(111) and Fe₃O₄(111). This is explained by the weaker interaction between catechol and FeO(111) compared to the other substrates, which hampers the formation of a barrier layer on the electrode surface.