Fast Electron Transfer Across Semiconductor-Molecule Interfaces: GaAs/Co(Cp)2 +/0

The kinetics of majority electron transfer in the dark from n-GaAs electrodes to cobaltocenium (Co(Cp)₂⁺) acceptors in acetonitrile has been studied in detail, both experimentally and theoretically. The experimental results were obtained from electrochemical impedance spectroscopy, quartz crystal microbalance (QCM and EQCM) studies, and current-potential characteristics. The theoretical work involved calculating the adsorption energy and molecular configuration of the cobaltocenium acceptors at the GaAs surface using high level density functional theory (B3LYP and variations thereof) as well as semiempirical methods. The QCM experiments showed that both Co(Cp)₂⁺ and Co(Cp)₂⁰ are physisorbed at GaAs surfaces, with adsorption energies of about 0.2 and 0.4 eV, respectively. The theoretical results are consistent with these experimental results. They indicate that adsorption of the Co(Cp)₂^+/0 redox system occurs on GaAs, with Co(Cp)₂⁰ somewhat more strongly adsorbed than Co(Cp)₂⁺; the Co(Cp)₂^+/0 molecules were found to adsorb with the cyclopentadienyl rings parallel to the GaAs surface. A model for the overall electron-transfer process was developed that incorporates Co(Cp)₂⁺ adsorption. Analysis of the detailed impedance spectra over the range of 1 Hz to 600 kHz showed that the sequential electron-transfer steps in the model (i.e., electron transfer from the GaAs conduction band to adsorbed Co(Cp)₂⁺, followed by electron transfer from the adsorbed Co(Cp)₂⁰ to free Co(Cp)₂⁺ in solution) are very fast and that the observed overall rate of electron transfer is limited by the rate of thermionic emission from the GaAs bulk region to the surface. The implications of these results for the theory of electron transfer at semiconductor-liquid interfaces, and the associated controversies surrounding theory and various experimental results for GaAs-metallocenium systems, are discussed.