Upward Shift in Conduction Band of Ta₂O₅ Due to Surface Dipoles Induced by N-Doping

Density functional theory calculations were executed to clarify the mechanism of the experimentally observed upward shift in conduction band minimum (CBM) and valence band maximum (VBM) of N-doped Ta₂O₅, which is used as a photosensitizer in CO₂ reduction. Calculations reproduce well the experimental energy levels (with respect to vacuum) of nondoped Ta₂O₅ and N-doped Ta₂O₅. Detailed analyses indicate that N-doping induces formations of defects of oxygenated species, such as oxygen atom and surface hydroxyl group, in the Ta₂O₅, and the defect formations induce charge redistributions to generate excess negative charges near the doped nitrogen atoms and excess positive charges near the defect sites. When the concentration of the doped nitrogen atoms at the surface is not high enough to compensate positive charges induced at the surface defects, the remaining positive charges are compensated by the nitrogen atoms in inner layers. Dipole moments normal to the surface generated in this situation raise the CBM and VBM of Ta₂O₅, allowing photogenerated electrons to transfer from N-doped Ta₂O₅ to the catalytic active sites for CO₂ reduction as realized with Ru complex on the surface in experiment.