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 complexes (Ru/N–Ta2O5) were investigated. Upon adsorption of the Ru complex (Ru(dcbpy)2(CO)22+) 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–Ta2O5, 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 (ket) and quantum yield (Φet) of the electron transfer process were estimated to be 4.2 × 1010 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.