Asymmetric and symmetric absorption peaks observed in infrared spectra of CO₂ adsorbed on TiO₂ nanotubes

Infrared spectra of CO₂ physisorbed on titania nanotubes (TiNTs), predominantly in the anatase polymorph, were measured at 81 K. Asymmetric and symmetric absorption peaks due to the antisymmetric stretch vibration (ν₃) of CO₂ were observed at 2340 cm^-1 and 2350 cm^-1, respectively. On the basis of the exposure- and time-dependence of the spectrum, the 2340 cm^-1 peak was attributed to CO₂ at the defective sites related to subsurface O vacancies (V_os) while the 2350 cm^-1 peak was assigned to that at the fivefold coordinated Ti⁴⁺ sites. It was found that the generalized Fano line shape was well fitted to the 2340 cm^-1 peak. We also observed an absorption peak at 2372 cm^-1, which was attributed to the combination band of ν₃ and the external mode of CO₂ at Ti⁴⁺.