ARTICLE pubs.acs.org/JPCC
Photoinduced Electron Transfer from Nitrogen-Doped Tantalum Oxide to Adsorbed Ruthenium Complex Ken-ichi Yamanaka,* Shunsuke Sato, Masayo Iwaki, Tsutomu Kajino, and Takeshi Morikawa Toyota Central R&D Labs., Inc., Nagakute, Aichi 480-1192, Japan
bS Supporting Information ABSTRACT: 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 (NTa2O5) and that adsorbed with Ru complexes (Ru/NTa2O5) were investigated. Upon adsorption of the Ru complex ([Ru(dcbpy)2(CO)2]2+) on NTa2O5 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: 4,40 -dicarboxy-2,20 -bipyridine). A detailed analysis of time-resolved emission measurements after excitation at 400 nm (Ta 4f r 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 NTa2O5 powder. In Ru/NTa2O5, ultrafast electron transfer from the shallow defect sites of NTa2O5 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 s1 and 0.50, respectively. No electron injection from the Ru complex to NTa2O5 was observed upon selective excitation of the Ru complex. The primary photochemical process for the CO2 reduction photocatalyst Ru/NTa2O5 was explained based on competition between ultrafast electron transfer from NTa2O5 to [Ru(dcbpy)2(CO)(COOH)]+, and the carrier trapping processes in NTa2O5.
’ INTRODUCTION Photoinduced electron transfer is one of the most important processes in photochemistry, and plays a central role in many areas of science and technology, such as biological photosynthesis,1 solar cells,2 and photocatalysts.3 Metal complexes adsorbed onto semiconductors are particularly widely used, as they are superior in terms of reaction quantum yield and industrial strength. Almost all primary processes in these systems consist of photoinduced charge injections from the metal complex, which acts as an electron donor, to the semiconductor, which acts as an electron acceptor. By contrast, there have been few reports, to our knowledge, on photoinduced electron transfer from a semiconductor, as an electron donor, to a metal complex as an electron acceptor,4 whereas interfacial charge transfer from semiconductor quantum dots to surface-adsorbed electron acceptors has been extensively studied.5 We previously reported visible-light-induced reduction of CO2 to HCOOH using a combination of a p-type semiconductor photosensitizer, nitrogen-doped Ta2O5 (NTa2O5), and a reducing catalyst, a Ru complex in acetonitrile containing triethanolamine.4e The system consisting of NTa2O5 linked with [Ru(dcbpy)2(CO)2]2+ (dcbpy: 4,40 -dicarboxy-2,20 -bipyridine), designated as Ru/NTa2O5, showed good performance, with greater than 75% selectivity for HCOOH and quantum efficiency of 1.9% under irradiation at 405 nm. It was suggested that electron transfer from the conduction band (CB) of NTa2O5 r 2011 American Chemical Society
to the lowest unoccupied molecular orbital (LUMO) of the Ru complex was involved; however, the details of the dynamics of the primary photophysical and photochemical processes have not yet been interpreted. Time-resolved spectroscopic studies should facilitate discussion of the reaction mechanism and a deeper understanding of photoinduced electron transfer between semiconductors and Ru complexes, although spectroscopic study of the NTa2O5 powder is difficult due to its opaqueness and the weakness of the emission quantum yield (
