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

Despite the importance of CO2/TiO2 model systems in investigating the mechanism of photocatalytic reduction of CO2, the adsorption configuration of 5-fold coordinated Ti (Ti5c) bound CO2 on TiO2 is controversial and the experimental interfacial electronic structure is lacking. Herein, using ultraviolet photoelectron spectroscopy (UPS), four highest occupied molecular orbitals (HOMOs) of Ti5c bound CO2, namely, 1 pi g, 1 pi u, 3 sigma u, and 4 sigma g, have been identified on rutile TiO2(110). Similar energy differences between the orbitals of adsorbed CO2 to those in the gas phase suggest a weak adsorbate-substrate interaction and CO2 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 Ti5c bound CO2 lies at similar to 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 CO2. 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 CO2-TiO2 interaction and CO2 reduction from the electronic structure point of view.