Electron Transfer Dynamics in Semiconductor–Chromophore–Polyoxometalate Catalyst Photoanodes

Triadic photoanodes have been prepared based on nanoporous films of the metal oxides ZrO 2, TiO 2, and SnO 2, sensitizer [Ru(bpy) 2 (dpbpy)] 2+ (P2), and polyoxometalate water oxidation catalyst [{Ru 4 O 4 (OH) 2 (H 2 O) 4 }(γ-SiW 10 O 36 ) 2 ] 10– ( 1 ) and investigated for their potential utility in water-splitting dye-sensitized photoelectrochemical cells. Transient visible and mid-IR absorption spectroscopic studies were carried out to investigate the charge separation dynamics of these systems, indicating that electron transfer from photoexcited P2 to TiO 2 and SnO 2 is still the main excited state quenching pathway in the presence of 1 . Furthermore, the accelerated recovery of the P2 ground state bleach in the presence of 1 results from ultrafast (nanosecond) electron transfer from catalyst to oxidized dye. Catalyst loading appears to depend largely on the point of zero charge of the supporting oxide and as such is significantly lower on SnO 2 than on TiO 2: nonetheless, the rate of recovery of the ground state bleach is similar in both TiO 2 - P2 -1 and SnO 2 - P2 -1 films. Spectral evidence for the formation of long-lived charged separated states is provided by the observation of signals persisting beyond 0.5 μs which are attributed to Stark effect induced change of the P2 spectrum and/or formation of oxidized 1 . Photoelectrochemical measurements on TiO 2 - P2 and TiO 2 - P2 -1 photoanodes under visible light irradiation indicate a ca. 100% photocurrent enhancement in the presence of 1, suggesting light-driven water oxidation by the TiO 2 - P2 -1 system with an internal quantum efficiency of ca. 0.2%. The fast formation and long lifetime of the photo-oxidized catalyst suggest that photoanodes of this type may reward further optimization through the introduction of faster catalysts and stabilization of the binding of the dye to the electrode.