Electro- and Photoinduced Interfacial Charge Transfers in Nanocrystalline Mesoporous TiO 2 and TiO 2 /Iron Porphyrin Sensitized Films under CO 2 Reduction Catalysis.

Electro- and photochemical CO 2 reduction (CO 2 R) is the quintessence of modern-day sustainable research. We report our studies on the electro- and photoinduced interfacial charge transfer occurring in a nanocrystalline mesoporous TiO 2 film and two TiO 2 /iron porphyrin hybrid films ( meso -aryl- and β-pyrrole-substituted porphyrins, respectively) under CO 2 R conditions. We used transient absorption spectroscopy (TAS) to demonstrate that, under 355 nm laser excitation and an applied voltage bias (0 to -0.8 V vs Ag/AgCl), the TiO 2 film exhibited a diminution in the transient absorption (at -0.5 V by 35%), as well as a reduction of the lifetime of the photogenerated electrons (at -0.5 V by 50%) when the experiments were conducted under a CO 2 atmosphere changing from inert N 2 . The TiO 2 /iron porphyrin films showed faster charge recombination kinetics, featuring 100-fold faster transient signal decays than that of the TiO 2 film. The electro-, photo-, and photoelectrochemical CO 2 R performance of the TiO 2 and TiO 2 /iron porphyrin films are evaluated within the bias range of -0.5 to -1.8 V vs Ag/AgCl. The bare TiO 2 film produced CO and CH 4 as well as H 2 , depending on the applied voltage bias. In contrast, the TiO 2 /iron porphyrin films showed the exclusive formation of CO (100% selectivity) under identical conditions. During the CO 2 R, a gain in the overpotential values is obtained under light irradiation conditions. This finding was indicative of a direct transfer of the photogenerated electrons from the film to absorbed CO 2 molecules and an observed decrease in the decay of the TAS signals. In the TiO 2 /iron porphyrin films, we identified the interfacial charge recombination processes between the oxidized iron porphyrin and the electrons of the TiO 2 conduction band. These competitive processes are considered to be responsible for the diminution of direct charge transfer between the film and the adsorbed CO 2 molecules, explaining the moderate performances of the hybrid films for the CO 2 R.