Effect of Li ions doping into p -type semiconductor NiO as a hole injection/transfer medium in the CO 2 reduction sensitized/catalyzed by Zn-porphyrin/Re-complex upon visible light irradiation

The molecular catalyst sensitized system (MCSS) composed of a sensitizer and a molecular catalyst co-adsorbed on semiconductor should be one of the promising candidates of an artificial photosynthetic system that enables CO2 fixation with water as the electron donor. Here, monocarboxyphenyltrimesitylporphyrinatezinc(II) as the sensitizer and Re(dicarboxybipyridine)(CO)3(CH3CN) as the catalyst co-adsorbed on p-type semiconductor, NiO, were adopted for the MCSS catalyzed photoreduction of CO2 into CO. NiO was prepared by sol/gel method, and Li ions were doped to promote the hole carrier mobility with the amount of [Li]/[Ni] = 0, 3, 6, 9%, respectively. The conductivity of NiO increased linearly with the doping amount of Li ions, while the photoreduction of CO2 in the MCSS was enhanced under the 3, 6% doping of Li ions but was retarded under the higher doping amount of 9%. The apparently contradicting enhancement and the retardation were both successfully explained by the promotive effect of Li ions doping on the hole carrier mobility within NiO to enhance both the charge separation from the radical ion pair and charge recombination with the free hole carrier within NiO.