CO₂ adsorption on anatase TiO₂(101) surfaces: a combination of UHV-FTIRS and first-principles studies

The CO₂ adsorption and dynamic behaviors on single crystal anatase TiO₂(101) surfaces were investigated by UHV-FTIRS and first-principles calculations. The IRRAS results at 90 K show that the ν₃(OCO) asymmetric stretching vibration of adsorbed CO₂ exhibits band splitting at rather low CO₂ coverage in p-polarized IR spectra for the IR beam incident along the [101[combining macron]] direction. Co-adsorbed CO can prevent such band splitting. Ab initio molecular dynamics (AIMD) simulations revealed that the adsorbed CO₂ at finite temperature does not keep a stationary adsorption state but keeps a certain swing motion: one end of the linear CO₂ molecule binds to surface Ti_5c sites and the other end swings within the (010) plane with a tilted angle distribution ranging from 10° to 60° relative to the [101[combining macron]] direction. By suggesting a statistical model, we confirmed that it is the swing motion that results in the band splitting phenomenon of CO₂ vibration in IR spectra. The co-adsorbed CO decreases the swing angle distribution ranging from 10° to 45° through the intermolecular interaction between CO and CO₂, leading to the disappearance of CO₂ band splitting.