Light Absorption Properties and Electronic Band Structures of Lead Titanium Oxyfluoride Photocatalysts Pb2Ti4O9F2 and Pb2Ti2O5.4F1.2

Light absorption capability and electronic band structure are both fundamental information for the development of a new photocatalyst. Here, we investigated two oxyfluoride photocatalysts Pb2Ti4O9F2 and Pb2Ti2O5.4F1.2, which were active for H-2 evolution in the presence of a sacrificial reagent, by means of X-ray diffraction, UV-visible diffuse reflectance spectroscopy, electrochemical impedance spectroscopy, and density functional theory calculations. Pb2Ti4O9F2 and Pb2Ti2O5.4F1.2 show absorption edges at around 410 and 510 nm, respectively, corresponding to band gaps of 3.0 and 2.4 eV. The different band gap values of the two materials are mainly due to their valence band maximum (VBM); the VBM of Pb2Ti4O9F2 is positioned at approximately 0.9 V more positive than that of Pb2Ti2O5.4F12. The significantly different VBM positions in these oxyfluorides could be explained in terms of the orbital interaction between Pb 6s/6p and O 2p in the valence band, where the shorter Pb-O bond in Pb2Ti2O5.4F1.2 reinforced the interaction, leading to more elevated VBM and a narrower band gap.