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 (Vos) 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+.