Oxyfluoride Pb₂Ti₄O₉F₂ as a Stable Anode Material for Photoelectrochemical Water Oxidation

Abstract

An oxyfluoride, Pb2Ti4O9F2, was examined as an electrode material for water splitting, and the electrochemical band edge potentials were compared with those of an analogous oxide, Bi2Ti4O11. Pb2Ti4O9F2 with a band gap of 3.0 eV, which is slightly smaller than that of Bi2Ti4O11 (3.1 eV), worked as an n-type semiconductor anode for water oxidation upon band-gap photoexcitation (λ < 420 nm) in the presence of a CoOx cocatalyst to produce O2 without noticeable degradation. Mott–Schottky analysis and photocurrent onset potential measurement showed that the flat-band potential of Pb2Ti4O9F2 was positioned at around −0.5 V versus NHE at pH 7.0, which was 0.2 V more negative than that of Bi2Ti4O11. Results of physicochemical analyses, together with previous first-principles calculations, suggest that the relatively negative flat-band potential of Pb2Ti4O9F2 is due to the stronger orbital interaction between Pb-6s and O-2p in the valence band than that between Bi-6s and O-2p in Bi2Ti4O11 because the difference in the orbital energy between metal-6s and O-2p is smaller in the former than in the latter. This stronger Pb-6s/O-2p interaction increases the valence band maximum and conduction band minimum, which causes Pb2Ti4O9F2 to be operable as a photoanode at lower applied potential. The O-2p orbitals that constitute the valence band maximum of Pb2Ti4O9F2 also appear to be essential for the realization of stable water oxidation to form O2.