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

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.