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 (VO's) are found to be the most stable binding sites. Additional CO2 adsorbs on 5-coordinated Ti sites (Ti5c) with the initial small fraction stabilized by CO2 adsorbed on VO sites. The Ti5c-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 Ti5c rows proceeds via a rotation-tumbling mechanism with extremely low barrier of 0.06 eV. The Ti5c-bound CO2 molecules are found to bind preferentially to a single Ti5c 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 Ti5c 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 Ob sites with a binding energy only slightly lower than that on Ti5c sites presumably due to quadrupole-quadrupole interactions with the Ti5c-bound CO2 molecules.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films