Structure and dynamics of CO₂ on rutile TiO₂ (110)-1×1

Adsorption, binding, and diffusion of CO₂ molecules on model rutile TiO₂ (110)-1×1 surfaces were investigated experimentally using scanning tunneling microscopy, infrared reflection adsorption spectroscopy (IRAS), molecular beam scattering, and temperature programmed desorption and theoretically via dispersion corrected density functional theory and ab initio molecular dynamics. In accord with previous studies, bridging oxygen (Ob) vacancies (V_O's) are found to be the most stable binding sites. Additional CO₂ adsorbs on 5-coordinated Ti sites (Ti_5c) with the initial small fraction stabilized by CO₂ adsorbed on VO sites. The Ti_5c-bound CO₂ is found to be highly mobile at 50 K at coverages of up to 1/2 monolayer (ML). Theoretical studies show that the CO₂ diffusion on Ti_5c rows proceeds via a rotation-tumbling mechanism with extremely low barrier of 0.06 eV. The Ti_5c-bound CO₂ molecules are found to bind preferentially to a single Ti_5c with the O=C=O axis tilted away from the surface normal. The binding energy of tilted CO₂ molecules changes only slightly with changes in the azimuth of the CO₂ tilt angle. At 2/3 ML, CO₂ diffusion is hindered and at 1 ML an ordered (2×2) overlayer with a zigzag arrangement of tilted CO₂ molecules develops along the Ti_5c rows. Out of phase arrangement of the zigzag chains is observed across the rows. An additional 0.5 ML of CO₂ can be adsorbed at O_b sites with a binding energy only slightly lower than that on Ti_5c sites presumably due to quadrupole-quadrupole interactions with the Ti_5c-bound CO₂ molecules.