Study of Excited States and Electron Transfer of Semiconductor‐Metal‐Complex Hybrid Photocatalysts for CO₂ Reduction by Using Picosecond Time‐Resolved Spectroscopies

Abstract

A semiconductor‐metal‐complex hybrid photocatalyst was previously reported for CO2 reduction; this photocatalyst is composed of nitrogen‐doped Ta2O5 as a semiconductor photosensitizer and a Ru complex as a CO2 reduction catalyst, operating under visible light (>400 nm), with high selectivity for HCOOH formation of more than 75 %. The electron transfer from a photoactive semiconductor to the metal‐complex catalyst is a key process for photocatalytic CO2 reduction with hybrid photocatalysts. Herein, the excited‐state dynamics of several hybrid photocatalysts are described by using time‐resolved emission and infrared absorption spectroscopies to understand the mechanism of electron transfer from a semiconductor to the metal‐complex catalyst. The results show that electron transfer from the semiconductor to the metal‐complex catalyst does not occur directly upon photoexcitation, but that the photoexcited electron transfers to a new excited state. On the basis of the present results and previous reports, it is suggested that the excited state is a charge‐transfer state located between shallow defects of the semiconductor and the metal‐complex catalyst.

Carbon dioxide reduction mechanism on Ru-based electrocatalysts [Ru(bpy)2(CO)2]2+: insights from first-principles theory

“…5,6 In a hybrid system comprising a semiconductor as a light harvesting unit and a metal-complex working as an electrocatalyst, the electron transfer rate is then expected to be determined by the thermodynamic driving force, 18,20 density of accepting states, reorganization energy and the photo-electrocatalyst electron coupling. 1,28,29 In 2010, Sato et al reported the development of a hybrid system consisting of an N-doped Ta 2 O 5 p-type semiconductor and Rucomplex, namely [Ru(bpy) 2 (CO) 2 ] 2+ , [Ru(dcbpy)(bpy)(CO) 2 ] 2+ and [Ru(dcbpy) 2 (CO) 2 ] 2+ , where bpy ¼ 2,2 0 bipyridine, for carbon dioxide conversion in an acetonitrile/triethanolamine mixture. In these systems, the electron transfer has been enhanced by the combined effect of the photo-electrocatalyst thermodynamic driving force and the linkage with Ru-complex carboxyl groups.…”

“…45,46 However, recent TR-IR spectroscopy measurements have demonstrated that anchoring groups linking N-Ta 2 O 5 and Ru-complex photo-electrocatalysts induce the formation of non-radiative charge-transfer states that concentrate the photoelectrons immediately aer photoexcitation, thus implying a non-direct charge transfer process that is enhanced alongside the electron coupling. 29 At this point, it is crucial that the rst unoccupied molecular orbital (LUMO) energy is lower than that of the photoelectron, which is conned in the conduction band maximum or the chargetransfer state. 19 By a more accurate denition, 47 the Rucomplex energy level is assessed through the reduction potential f, a quantity that accounts for the internal relaxation that is expected to occur upon electron injection.…”

Solar-Driven CO2 Reduction Using a Semiconductor/Molecule Hybrid Photosystem: From Photocatalysts to a Monolithic Artificial Leaf

“…Recently, time-resolved infrared absorption spectroscopy suggested that photoexcited electron transfer from N-Ta 2 O 5 to the [Ru(dcbpy) 2 (CO) 2 ] 2+ catalyst does not occur directly upon photoexcitation, but occurs within 1 ps through a new nonradiative charge transfer (CT) state located near the N-Ta 2 O 5 /[Ru(dcbpy) 2 (CO) 2 ] 2+ interface. 20 The overall mechanism of the visible-light-driven CO 2 RR over N-Ta 2 O 5 /[Ru(dcbpy) 2 (CO) 2 ] 2+ is proposed (Figure 3c). Nonadiabatic molecular dynamics simulations also indicated that the electron transfer to the Ru-complex with COOH anchors (7.5 ps) is faster than that with PO 3 H 2 groups (56.7 ps), owing to greater nonadiabatic coupling, 33 which is in agreement with experimental results.…”

“…Nonadiabatic molecular dynamics simulations also indicated that the electron transfer to the Ru-complex with COOH anchors (7.5 ps) is faster than that with PO 3 H 2 groups (56.7 ps), owing to greater nonadiabatic coupling, 33 which is in agreement with experimental results. 20 Hence, the selection of the anchoring groups is also important for improving the photoinduced CO 2 RR activity.…”

“…In contrast, the CO 2 RR rate was negligibly small when a representative visible-light-responsive semiconductor, N-doped TiO 2 , was linked with [Ru­(dcbpy) 2 (CO) 2 ] 2+ , because the E CBM (−0.2 V) is at a more positive position than that of the LUMO . However, by satisfying the primary importance of the sufficiently negative E CBM , the concept of hybrid photocatalysts is reported to applicable to various semiconductor/metal-complex combinations. − …”