Heterogeneous electron transfer at designed semiconductor/liquid interfaces. Rate of reduction of surface-confined ferricenium centers by solution reagents

Abstract

Reduction of surface-confined ferricenium by solution reductants iodide, diindenyliron, (η⁵-C₆H₅)4Fe₄(CO)₄, 1,l'-dimethylferrocene, ferrocene, and phenylferrocene has been studied in EtOH-O.l M [n-Bu4N]C1O₄ and also in H₂O-NaC1O₄ for iodide. The surface-confined ferricenium can be generated on n-type Si by illumination of the electrode at some potential more positive than ~-0.2 V vs. SCE. Owing to the two stimuli (light and potential) response of the derivatized photoelectrode it is possible to directly measure (by linear sweep voltammetry) the time dependence of the surface ferricenium concentration in the dark and in the presence of the various reducing agents mentioned above. At a given concentration of iodide in solution we find the rate of reduction of surface ferricenium to be directly proportional to the surface ferricenium concentration. By measuring rate of ferricenium reduction at various iodide concentrations, the rate law is thus determined to be rate = kₑₜ[Fe(Cp)₂⁺][T] where [Fe(Cp)₂⁺] is the surface ferricenium concentration in mol/cm² and [I⁻] is the solution concentration of iodide in mol/cm3. We find kₑₜ to be (3 ± 1) X 10⁴ cm³/(mol s) in EtOH solvent and only (1 ± 0.5) X 10³ cm³/(mol s) in H₂O. The value in EtOH is somewhat lower than would be estimated from homogeneous solution reaction of ferricenium with iodide under the same conditions. All other reductants mentioned above reduce the surface ferricenium in EtOH solvent with a value of kₑₜ > 6 X 10⁷ cm³/(mol s); that is, the reduction rate is mass transport, not charge transfer, limited under the conditions employed including well-stirred solutions with stationary electrodes or rotated (up to 2000 rpm) disk electrodes. However, the relative ordering of the "fast" reductants has been determined to be diindenyliron > (η⁵-C₅H₅)₄Fe₄(CO)₄ ~ 1,l'-dimethylferrocene > ferrocene ~ phenylferrocene. A large value of kₑₜ is expected based on the fast self-exchange rates of ferrocene and its derivatives. Acetylferrocene is not expected to be able to reduce ferricenium on thermodynamic grounds, and we find that surface ferricenium is inert in its presence. Most of the derivatized surfaces have been prepared from (l,1'-ferrocenediyl)dichlorosilane, but preliminary results with polyvinylferrocene modified and (1,1'-ferrocenediyl) dimethylsilane derivatized surfaces are similar. Very high coverage surfaces from (1,l'-ferrocenediyl)dichlorosilane show some evidence for selective reduction of the more accessible ferricenium centers when a "fast" reductant is used. Steady-state photoanodic current at a given concentration of reductant generally accords well with the measured kₑₜ values, and for the iodide experiments the steady-state photocurrent is directly proportional to surface coverage of electroactive ferrocene. Such studies are relevant to use of derivatized photoelectrodes in energy conversion applications.

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