Use of Chemically Derivatized n-Type Silicon Photoelectrodes in Aqueous Media. Photooxidation of Iodide, Hexacyanoiron(II), and Hexaammineruthenium(II) at Ferrocene-Derivatized Photoanodes

n-Type Si can be derivatized using (1,1'-ferrocenediyl)dichlorosilane yielding a photoanode that can be used in aqueous electrolyte solutions under conditions where the naked (nonderivatized) n-type Si undergoes photoanodic corrosion yielding an insulating SiOx surface layer. Derivatized electrodes in aqueous electrolyte solution exhibit chemically reversible oxidation of the surface-confined, ferrocene-centered redox reagent when the electrode is illuminated. Oxidation is not detectable in the dark, consistent with the fact that n-type semiconductors are blocking to oxidation in the dark. At sufficiently high light intensity, the surface-confined material can be oxidized at more negative potentials than when the same material is confined to the surface of a reversible electrode material such as Pt or Au. Light intensity of ~40 mW/cm2 at 632.8 nm is typically sufficient to effect oxidation 200-400 mV thermodynamically uphill for electrodes exhibiting coverages of 10-10-10-9 mol/cm2 of electrochemically active material. Such electrodes can be used to effect the persistent oxidation of I-, [FefCNJe]4-, and [RufNHsls]2+. The current efficiency for 3I- → I3- and [Fe(CN)e]4- →[Fe(CN)6]3- is measured to be >90% and these oxidations can be effected thermodynamically uphill, but with low efficiency. The formal potential of [Ru(NH3)6]3+/ [Ru(NH3)6]2+ is sufficiently negative, -0.2 V vs. SCE, that oxidation cannot be effected thermodynamically uphill with ntype Si. Oxidation of H2O using derivatized n-type Si does not occur under any conditions used. The oxidation of the three reductants used occurs by (1) photogeneration of electron-hole pairs by absorption of band-gap irradiation by n-type Si (band gap =1.1 eV); (2) oxidation of the surface-confined ferrocene-centered redox reagent by the photogenerated holes; (3) oxidation of the solution reductant (I-, [Fe(CN)6]4-, [Ru(NH3)6]2+) by the surface-confined ferricenium oxidizing reagent. This mechanism is established by cyclic voltammetry. Current-time plots show that ferrocene-ferricenium turnover numbers exceed 105 and constant (within 10%) photocurrents can be obtained for >5 h, whereas naked electrodes give photocurrents which decay by >90% within 5 min. Energy conversion efficiency for [Fe(CN)6]4- oxidation or I- oxidation at 632.8 nm is of the order of 1%.