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, Pb₂Ti₂O_5.4F_1.2, having a band gap of ca. 2.4 eV. Pb₂Ti₂O_5.4F_1.2 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 Pb₂Ti₂O_5.4F_1.2 (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 CoO_x. Nevertheless, loading CoO_x onto the TiO₂/Pb₂Ti₂O_5.4F_1.2/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 Pb₂Ti₂O_5.4F_1.2 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.