Limits on the Corrosion Rate of Si Surfaces in Contact with CH₃OH-Ferrocene^(+/0) and CH₃OH-1,1'-Dimethylferrocene^(+/0) Solutions

Abstract

Although Si/CH₃OH contacts have been extensively investigated and reported to provide highly efficient photoelectrochemical energy conversion devices, a recent study using the scanning electrochemical microscope (SECM) has claimed that, in CH_3OH solutions, Si surfaces in contact with 4.57 mM ferrocenium (Fc⁺) were etched in the dark at a mass-transport-limited rate. The reported etching rate constant of > 0.37 cm s⁻¹) at 4.57 mM ferrocenium corresponds to an equivalent corrosion current density of > 240 mA cm⁻² and to a Si etch rate of > 75 nm s⁻¹. The presence of such severe corrosion was inferred from an unexpectedly large feedback current in an SECM experiment. The present work describes a search for corrosion of Si in contact with CH₃OH-ferrocene^+/0) and CH₃OH-dimethylferrocene(Me₂Fc)^(+/0) solutions through the use of very sensitive electrochemical, chemical, and physical methods. For CH₃OH - 1.0 M LiClO₄ - 100 mM Me₂Fc- 80 mM Me₂Fc⁺ solutions, an upper limit on the etch rate of 6.6 x10⁻⁶ nm s⁻¹) has been established through direct experimental measurements; thus, a 400 pm thick Si photoelectrode in contact with the CH₃OH-Me₂-Fc^(+/0) electrolyte would require over 1500 years to corrode completely at room temperature. An alternative explanation for the SECM data, based on the documented existence of an inversion layer at the Si/liquid contact, is presented and shown to be consistent with the available data.

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