Effects of Bridging Ligands on the Current−Potential Behavior and Interfacial Kinetics of Ruthenium-Sensitized Nanocrystalline TiO_2 Photoelectrodes

Abstract

We have shown that Ru^(II)(bpy)_2(bpy-4-(xylyl)_x-≡-phenyl-COOH)(PF_6)_2 (abbreviated Rux, where x = 0, 1 or 2 xylyl groups; bpy = 2,2‘-bipyridine) dyes can act as sensitizers for nanocrystalline TiO_2 in functional photoelectrochemical cells under simulated solar illumination, albeit with low efficiencies. Both the short-circuit photocurrent density and the open-circuit voltage decreased as x was increased. Electron injection (10^6−10^8 s^(-1)) was slightly faster for the x = 0 dye, but both recombination (10^(-15)−10^(-13) cm^3 s^(-1)) and regeneration (10^4−10^6 s^(-1) for 10 mM I^-) were slightly faster for the x = 2 dye. We suggest that the lack of distance dependence is due to the flexible one-carboxyl attachment to the surface resulting in the Ru−TiO_2 electron-tunneling distance being very similar for x = 0, 1, and 2. For all of the Rux sensitizers, a relatively small potential was needed for generation of current in the dark, indicating that the reaction between electrons in TiO_2 and the I_3^-/I^- electrolyte solution is as favorable for the Rux sensitizers as for unmodified TiO_2 electrodes.