Photoinduced Electron Transfer from Nitrogen-Doped Tantalum Oxide to Adsorbed Ruthenium Complex

To understand the photoinduced electron-transfer process and the reaction mechanism of semiconductor/metal-complex hybrid CO2-reducing photocatalysts, the excited-state dynamics of nitrogen-doped Ta2O5 (N-Ta2O5) and that adsorbed with Ru cornplexes (Ru/N-Ta2O5) were investigated. Upon adsorption of the Ru complex ([Ru(dcbpy)(2)(CO)(2)](2+)) on N-Ta2O5 powder, one of the CO ligands was exchanged to COOH ([Ru(dcbpy)(2)(CO)(COOH)](+)), which resulted in absorption spectral changes in UV/visible and infrared regions (dcbpy: 4,4'-dicarboxy-2,2'-bipyridine). A detailed analysis of time-resolved emission measurements after excitation at 400 nm (Ta 4f <- N 2p transition) revealed a fast trapping process from shallow defect sites to deep defect sites with a time constant of 24 +/- 1 Ps in N-Ta2O5 powder. In Ru/N-Ta2Os, ultrafast electron transfer from the shallow defect sites of N-Ta2O5 to the adsorbed Ru complex occurred with a time constant of 12 +/- 1 ps. The values of rate constant (k(et)) and quantum yield (Phi et) of the electron transfer process were estimated to be 4.2 x 10(10) s(-1) and 0.50, respectively. No electron injection from the Ru complex to N-Ta2O5 was observed upon selective excitation of the Ru complex. The primary photochemical process for the CO2 reduction photocatalyst Ru/N-Ta2O5 was explained based on competition between ultrafast electron transfer from N-Ta2O5 to [Ru(dcbpy)(2)(CO)(COOH)](+), and the carrier trapping processes in N-Ta2O5.