Effects of Ta2O5 Surface Modification by NH3 on the Electronic Structure of a Ru-Complex/N–Ta2O5 Hybrid Photocatalyst for Selective CO2 Reduction

This work examined a Ru-complex/N–Ta2O5 (N–Ta2O5: nitrogen-doped Ta2O5) hybrid photocatalyst for CO2 reduction. In this material, electrons are transferred from the N–Ta2O5 to the Ru-complex in response to visible light irradiation, after which CO2 reduction occurs on the complex. N-doping is believed to produce an upward shift in the conduction band minimum (CBM) of the Ta2O5, thus allowing more efficient electron transfer, although the associated mechanism has not yet been fully understood. In the present study, the effects of NH3 adsorption (the most likely surface modification following nitrification) were examined using a combined experimental and theoretical approach. X-ray photoelectron spectroscopy data suggest that NH3 molecules are adsorbed on the N–Ta2O5 surface, and it is also evident that the photocatalytic activity of the Ru-complex/N–Ta2O5 is decreased by the removal of this adsorbed NH3. Calculations show that both the occupied and unoccupied orbital levels of Ta16O40(NH3) x clusters (x = 4, 8, 12, or 16) are shifted upward as x is increased. Theoretical analyses of Ru-complex/cluster hybrids demonstrate that the gap between the lowest unoccupied molecular orbital of the Ta16O40 moiety and the unoccupied orbitals of the Ru-complex in Ru-complex/Ta16O40(NH3)12 is much smaller than that in Ru-complex/Ta16O40. The highest occupied molecular orbital of [Ru-complex/Ta16O40]− is evidently localized on the Ta16O40 moiety, whereas that of [Ru-complex/Ta16O40(NH3)12]− is spread over both the Ta16O40 and Ru-complex. These results indicate that the NH3 adsorption associated with N-doping can result in an upward shift of the CBM of Ta2O5. Additional calculations for Ta16O40–y (NH) y (y = 2, 4, 6, 8, or 10) suggest that the substitution of NH groups for oxygen atoms on the Ta2O5 surface may be responsible for the red shift in the adsorption band edge of the oxide but makes only a minor contribution to the upward shift of the CBM.