Sensitization and Stabilization of TiO2 Photoanodes with Electropolymerized Overlayer Films of Ruthenium and Zinc Polypyridyl Complexes: A Stable Aqueous Photoelectrochemical Cell

Overlayer thin films of vinylbipyridine (vbpy)-containing Ru and Zn complexes have been formed on top of ruthenium dye complexes adsorbed to TiO2 by reductive electropolymerization. The goal was to create an efficient, water-stable photoelectrode or electrodes. An adsorbed-[Ru(vbpy)2(dcb)](PF6)2/poly-[Ru(vbpy)3](PF6)2 surface composite displays excellent stability toward dissolution in water, but the added overlayer film greatly decreases incident photon-to-current conversion efficiencies (IPCE) in propylene carbonate with I3-/I- as the carrier couple. An ads-[Ru(vbpy)2(dcb)](PF6)2/poly-[Zn(vbpy)3](PF6)2 composite displays no loss in IPCE compared to ads-[Ru(vbpy)2(dcb)](PF6)2 but is susceptible to film breakdown in the presence of water by solvolysis and loss of the cross-linking Zn2+ ions. Success was attained with an ads-[Ru(vbpy)2(dcb)](PF6)2/poly-[Ru(vbpy)2(dppe)](PF6)2 composite. In this case the electropolymerized layer is transparent in the visible. The composite electrode is stable in water, the IPCE in propylene carbonate with I3-/I- is comparable to the adsorbed complex, and a significant IPCE is observed in water with the quinone/hydroquinone carrier couple. The assembly [(bpy)2(CN)Ru(CN)Ru(vbpy)2(NC)Ru(CN)(bpy)2](PF6)2 ([Ru(CN)Ru(NC)Ru](PF6)2) adsorbs spontaneously on TiO2, and electropolymerization of thin layers of the assembly to give ads-[Ru(CN)Ru(NC)Ru](PF6)2/poly-[Ru(CN)Ru(NC)Ru](PF6)2 enhances IPCE and has no deleterious effect on the IPCE/Ru.