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.

Synthesis of a Layered Niobium Oxynitride, Rb2NdNb2O6N·H2O, Showing Visible-Light Photocatalytic Activity for H2 Evolution.

Two-dimensional (2D) layered oxynitrides are promising candidates as visible-light-driven photocatalysts, but the actual examples are rare because of the difficulty in synthesizing the 2D oxynitrides. Here a phase-pure layered perovskite, Rb2NdNb2O6N·H2O, that belongs to a tetragonal P4/ mmm space group was successfully synthesized by thermal ammonolysis of a mixture of layered RbNdNb2O7 and Rb2CO3, as revealed by synchrotron X-ray diffraction, elemental analyses, and atomic-scale electron microscopy observation. The synthesized Rb2NdNb2O6N·H2O had an absorption edge at around 500 nm and a sufficiently high conduction-band potential to allow for proton reduction. With modification by a platinum cocatalyst, Rb2NdNb2O6N·H2O became photocatalytically active for H2 evolution in the presence of triethanolamine as an electron donor under visible light (λ > 400 nm).