Unusually efficient photosensitization of nanocrystalline TiO2 films by pomegranate pigments in aqueous medium

Abstract

Unusually high photocurrents, with quantum yields up to Φ = 0.74, are obtained using TiO₂ photosensitized by adsorbed natural pigment(s) from pomegranate liquid, with aqueous iodide/iodine acting as charge carriers. An open circuit photovoltage up to 0.45 V is observed. The quantum yield of the photocurrent, the photovoltage and the stability are unusually high for aqueous solutions. In contrast to the intensively investigated Ru(dcbpy)₂(SCN)₂ system, increasing the TiO₂ nanocrystallite's diameter from 5 to 18 nm considerably decreases the photocurrent. Apparently, the TiO₂ porous layer functions as a nano-filter for the pigment mixture. The effects of changing charge carrier concentrations and the photocurrent spectrum and yields under various conditions are reported. Ethanol used as solvent for the charge carriers yields comparable results (tested at pH 2.8). Our main observations suggest that it may be possible to construct Graetzel-type cells with no special care to eliminate traces of water.