Ultraviolet Photoelectron Spectroscopy Study of the Adsorption and Electronic Structure at the CO₂/TiO₂(110) Interface

Abstract

Despite the importance of CO 2 /TiO 2 model systems in investigating the mechanism of photocatalytic reduction of CO 2 , the adsorption configuration of 5-fold coordinated Ti (Ti 5c ) bound CO 2 on TiO 2 is controversial and the experimental interfacial electronic structure is lacking. Herein, using ultraviolet photoelectron spectroscopy (UPS), four highest occupied molecular orbitals (HOMOs) of Ti 5c bound CO 2 , namely, 1π g , 1π u , 3σ u , and 4σ g , have been identified on rutile TiO 2 (110). Similar energy differences between the orbitals of adsorbed CO 2 to those in the gas phase suggest a weak adsorbate−substrate interaction and CO 2 likely adsorbs as a neutral linear molecule. Tilted adsorption configuration at coverages below 1 monolayer has been confirmed by comparing the measured polarization-and azimuth-resolved UPS spectra and prediction according to symmetry selection rules. HOMO of Ti 5c bound CO 2 lies at ∼8.19 eV below the Fermi level (E F ), locating the lowest unoccupied molecular orbital (LUMO) at 0.13−2.81 eV above E F given the reported 8.30−11.00 eV HOMO−LUMO gap of gas-phase CO 2 . Interfacial electron transfer pathways are discussed by taking into account the substrate electronic structure and the interfacial level alignment. This work provides knowledge about the CO 2 −TiO 2 interaction and CO 2 reduction from the electronic structure point of view.