PHOTOSENSITIZATION OF NANOCRYSTALLINE SEMICONDUCTOR-FILMS - MODULATION OF ELECTRON-TRANSFER BETWEEN EXCITED RUTHENIUM COMPLEX AND SNO2 NANOCRYSTALLITES WITH AN EXTERNALLY APPLIED BIAS

In situ Spectroelectrochemical measurements have been carried out to p robe the charge injection from excited Ru(bpy)(2)(dcbpy)(2+), Ru(II), into the SnO2 nanocrystallites. The dependence of luminescence yield a nd lifetime at various applied potentials suggests that the heterogene ous electron transfer from excited sensitizer into the semiconductor c an be controlled by the externally applied electrochemical bias. The m aximum quenching is seen at positive potentials while an increase in t he luminescence yield and lifetime is seen at negative potentials. Las er flash photolysis of Ru(II)-modified SnO2 nanocrystalline film has b een carried out to record the transient absorption spectra at differen t applied potentials, The yield of electron transfer product, Ru(III), decreases as the applied bias is switched to negative potentials. At an applied bias of -0.7 V the only observable transient is the excited Ru(II) complex (Ru(II)). The maximum apparent electron transfer rate constant, k(et) (similar to 4 x 10(8) s(-1)), observed at positive bi as agrees with the previously determined electron transfer rate consta nts from emission lifetime and microwave conductivity experiments. The apparent rate constant for heterogeneous electron transfer is depende nt on the applied bias, and it decreases as the difference between the pseudo-Fermi level of SnO2 and oxidation potential of Ru(II) decreas es, These results suggest that the decreased rate of charge injection is responsible for lower IPCE (incident photon-to-photocurrent efficie ncy) observed in photoelectrochemical cells under negative bias, No si gnificant change in the rate of reverse electron transfer was observed at potentials greater than -0.4 V.