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 probe the charge injection from excited Ru(bpy)2(dcbpy)2+, Ru(II), into the SnO2 nanocrystallites. The dependence of luminescence yield and lifetime at various applied potentials suggests that the heterogeneous electron transfer from excited sensitizer into the semiconductor can be controlled by the externally applied electrochemical bias. The maximum quenching is seen at positive potentials while an increase in the luminescence yield and lifetime is seen at negative potentials. Laser flash photolysis of Ru(II)-modified SnO2 nanocrystalline film has been carried out to record the transient absorption spectra at different 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 (∼4 × 108 s-1), observed at positive bias agrees with the previously determined electron transfer rate constants from emission lifetime and microwave conductivity experiments. The apparent rate constant for heterogeneous electron transfer is dependent on the applied bias, and it decreases as the difference between the pseudo-Fermi level of SnO2 and oxidation potential of Ru(II)* decreases. These results suggest that the decreased rate of charge injection is responsible for lower IPCE (incident photon-to-photocurrent efficiency) observed in photoelectrochemical cells under negative bias. No significant change in the rate of reverse electron transfer was observed at potentials greater than −0.4 V.