Chemically Derivatized n-Type Semiconducting Gallium Arsenide Photoelectrodes. Thermodynamically Uphill Oxidation of Surf ace-Attached Ferrocene Centers

n-Type semiconducting GaAs surfaces can be derivatized using (1,1’-ferrocenediyl)dichlorosilane (1), resulting in surface-attached, electroactive, ferrocene centers. Coverage of electroactive material significantly exceeds monolayer coverage in most experiments. The surface-attached ferrocene material can be oxidized in an uphill sense by irradiating the surface with light of energy greater than 1.4 cV, the band gap of GaAs. Ferrocene can be photooxidized to 50% conversion at a potential of ~-0.25 V vs. SCE, whereas the 50% conversion occurs at a potential of +0.40 V at a reversible electrode. The ferrocene, then, can be driven uphill by greater than 0.6 V. Surface-attached, photogenerated ferricenium can in turn oxidize solution reductants such as ferrocene itself, I^-, or N,N,N’,N’-tetramethyl-p-phenylenediamine as determined by cyclic voltammetry in CH₃CN/0.1 M [n-Bu₄N]CIO₄ solutions. Derivatized electrodes are fairly rugged and result in modestly improved optical to electrical energy conversion compared to a naked GaAs photoelectrode-based cell employing a CH₃CN/0.1 M [n-Bu₄N]CIO₄ solution of ferrocene and ferricenium.