A Molecular Z-Scheme Artificial Photosynthetic System Under the Bias-Free Condition for CO2 Reduction Coupled with Two-electron Water Oxidation: Photocatalytic Production of CO/HCOOH and H2 O2.

Bio-inspired molecular-engineered systems have been extensively investigated for the half-reactions of H2 O oxidation or CO2 reduction with sacrificial electron donors/acceptors. However, there has yet to be reported a device for dye-sensitized molecular photoanodes coupled with molecular photocathodes in an aqueous solution without the use of sacrificial reagents. Herein, we will report the integration of SnIV - or AlIII -tetrapyridylporphyrin (SnTPyP or AlTPyP) decorated tin oxide particles (SnTPyP/SnO2 or AlTPyP/SnO2 ) photoanode with the dye-sensitized molecular photocathode on nickel oxide particles containing [Ru(diimine)3 ]2+ as the light-harvesting unit and [Ru(diimine)(CO)2 Cl2 ] as the catalyst unit covalently connected and fixed within poly-pyrrole layer (RuCAT-RuC2 -PolyPyr-PRu/NiO). The simultaneous irradiation of the two photoelectrodes with visible light resulted in H2 O2 on the anode and CO, HCOOH, and H2 on the cathode with high Faradaic efficiencies in purely aqueous conditions without any applied bias is the first example of artificial photosynthesis with only two-electron redox reactions.

Solar-Driven Photoelectrochemical Water Oxidation over an n-Type Lead-Titanium Oxyfluoride Anode.

Mixed-anion compounds (e.g., oxynitrides and oxysulfides) are potential candidates as photoanodes for visible-light water oxidation, but most of them suffer from oxidative degradation by photogenerated holes, leading to low stability. Here we show an exceptional example of a stable, mixed-anion water-oxidation photoanode that consists of an oxyfluoride, Pb2Ti2O5.4F1.2, having a band gap of ca. 2.4 eV. Pb2Ti2O5.4F1.2 particles, which were coated on a transparent conductive glass (FTO) support and were subject to postdeposition of a TiO2 overlayer, generated an anodic photocurrent upon band gap photoexcitation of Pb2Ti2O5.4F1.2 (λ <520 nm) with a rather negative photocurrent onset potential of ca. -0.6 V vs NHE, which was independent of the pH of the electrolyte solution. Stable photoanodic current was observed even without loading a water oxidation promoter such as CoOx. Nevertheless, loading CoOx onto the TiO2/Pb2Ti2O5.4F1.2/FTO electrode further improved the anodic photoresponse by a factor of 2-3. Under AM1.5G simulated sunlight (100 mW cm-2), stable water oxidation to form O2 was achieved using the optimized Pb2Ti2O5.4F1.2 photoanode in the presence of an applied potential smaller than 1.23 V, giving a Faradaic efficiency of 93% and almost no sign of deactivation during 4 h of operation. This study presents the first example of photoelectrochemical water splitting driven by visible-light excitation of an oxyfluoride that stably works, even without a water oxidation promoter, which is distinct from ordinary mixed-anion photoanodes that usually require a water oxidation promoter.