Artificial photosynthesis by using chloroplasts from spinach adsorbed on a nanocrystalline TiO₂ electrode for photovoltaic conversion

Abstract

Photovoltaic conversion has been achieved by use of chloroplasts (photosynthetic organs) from spinach adsorbed on a nanocrystalline TiO₂ film on an indium tin oxide (ITO) glass electrode (chloroplast/TiO₂ electrode). The shape of the absorption spectrum of the chloroplast/TiO₂ electrode is almost the same that of a dispersion of the chloroplasts. Absorption maxima of the chloroplast/TiO₂ electrode observed at 430, 475, and 670 nm were attributed to carotenoid and chlorophyll molecules, suggesting that chloroplasts have been adsorbed by the nanocrystalline TiO₂ film on the ITO electrode. The photocurrent responses of chloroplast/TiO₂ electrodes were measured by using a solution of 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile as redox electrolyte in the presence or absence of water and 100 mW cm⁻² irradiation. The photocurrent of the chloroplast/TiO₂ electrode was increased by adding water to the redox electrolyte. The photocurrent responses of chloroplast/TiO₂ electrodes irradiated with monochromatic light (680 nm, the absorption band of photosystem II complexed with evolved oxygen) were measured by use of a solution of 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile as redox electrolyte in the presence or absence of water. A chloroplast/TiO₂ electrode photocurrent was observed only when the redox electrolyte containing water was used, indicating that the oxygen evolved from water by photosystem II in chloroplasts adsorbed by a nanocrystalline TiO₂ film on an ITO electrode irradiated at 680 nm is reduced to water by the catalytic activity of the platinum electrode. The maximum incident photon-to-current conversion efficiency (IPCE) was 0.8 % on irradiation at 670 nm.