(Invited) New Mixed-Anion Photocatalysts for Water Splitting and CO2 Fixation

Abstract

Water splitting and CO2 fixation using semiconductor photocatalysts are important reactions from the viewpoint of solar-to-fuel energy conversion toward artificial photosynthesis. Mixed-anion compounds containing more than one anionic species in a single-phase have attracted attention as visible-light-driven photocatalysts, since as compared to oxygen 2p orbital, p orbitals of less electronegative anion can form a valence band that possesses more negative potential. In this talk, recent progress on the development of new mixed-anion photocatalysts that are applicable to water splitting and CO2 fixation will be given. We developed Ta and N-codoped rutile TiO2 for visible-light-driven water oxidation. TiO2:Ta,N powders modified with a RuO2 cocatalyst were active under visible light up to 540 nm for water oxidation to produce O2 in the presence of reversible electron acceptors (IO3 – or Fe3+), while TiO2:N exhibited negligible activity. Results of time-resolved infrared absorption spectroscopy revealed that the codoping Ta with N into TiO2 prolonged the lifetime of photogenerated free electrons, leading to high photocatalytic activity. Overall water splitting into H2 and O2 was achieved using TiO2:Ta,N, in combination with a H2-evolution photocatalyst of SrTiO3:Rh and an Fe3+/Fe2+ or IO3 –/I– redox couple even under simulated sunlight (AM1.5G). A similar result was obtained with rutile TiO2 codoped with N and F. TiO2:Ta,N also served as a very stable photoanode to oxidize water under visible light, and could be coupled to not only a H2 evolution cathode but also a molecular photocathode for CO2 reduction. A photoelectrochemical cell constructed with RuO2/TiO2:Ta,N photoanode and Ru(II)–Re(I) supramolecular photocathode was capable of reducing CO2 into CO using water as the electron source upon visible light, with stoichiometric production of CO and O2. Another interesting example of mixed-anion photocatalyst is an oxyfluoride pyrochlore Pb2Ti2O5.4F1.2. It had been believed that oxyfluorides are unsuitable as visible-light-responsive photocatalysts because of the highest electronegativity of fluorine. Surprisingly, Pb2Ti2O5.4F1.2 had an unprecedented small band gap of ca. 2.4 eV, and worked as a stable photocatalyst for visible-light H2 evolution and CO2 reduction when modified with suitable promoters. Density functional theory calculations indicated that the unprecedented visible-light-response of Pb2Ti2O5.4F1.2 originates from strong interaction between Pb-6s and O-2p orbitals, which is enabled by a short Pb–O bond in the pyrochlore lattice due to the fluorine substitution.