LIMITS ON THE CORROSION RATE OF SI SURFACES IN CONTACT WITH CH3OH-FERROCENE(+ 0) AND CH3OH-1,1'-DIMETHYLFERROCENE(+/0) SOLUTIONS/

Although Si/CH3OH contacts have been extensively investigated and repo rted to provide highly efficient photoelectrochemical energy conversio n devices, a recent study using the scanning electrochemical microscop e (SECM) has claimed that, in CH3OH solutions, Si surfaces in contact with 4.57 mM ferrocenium (Fc(+)) were etched in the dark at a mass-tra nsport-limited rate. The reported etching rate constant of >0.37 cm s( -1) at 4.57 mM ferrocenium corresponds to an equivalent corrosion curr ent density of >240 mA cm(-2) and to a Si etch rate of >75 nm s(-1). T he presence of such severe corrosion was inferred from an unexpectedly large feedback current in an SECM experiment. The present work descri bes a search for corrosion of Si in contact with CH3OH-ferrocene(+/0) and CH3OH-dimethylferrocene(Me(2)Fc)(+/0) solutions through the use of very sensitive electrochemical, chemical, and physical methods. For C H3OH-1.0 M LiClO4-100 mM Me(2)Fc-80 mM Me(2)Fc(+) solutions, an upper limit on the etch rate of 6.6 x 10(-6) nm s(-1) has been established t hrough direct experimental measurements; thus, a 400 mu m thick Si pho toelectrode in contact with the CH3OH-Me(2)-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 exi stence of an inversion layer at the Si/liquid contact, is presented an d shown to be consistent with the available data.