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 I-3(-)/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 I-3(-)/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.