Reactivity of the Defective Rutile TiO2 (110) Surfaces with Two Bridging-Oxygen Vacancies: Water Molecule as a Probe

Defective rutile TiO2 (110) surfaces with one bridging-oxygen vacancy pair (OW) and two next nearest neighbored bridging-oxygen vacancies belonging to the same row (NNN-OVs, i.e., two bridging-oxygen vacancies separated by a single oxygen atom) were studied using density functional theory (DFT) calculations. The results of a perfect surface and a defective surface with single bridging-oxygen vacancy (OV) were also shown. The reactivity of these surfaces was investigated by studying their interaction with a water molecule. Results show the NNN-OVs site is the most favorable site for water adsorption of two modes, molecular and dissociated adsorption, especially for dissociated adsorption. Upon dissociated adsorption on the NNN-OVs site, the whole system would release energy of 2.07 eV, much more than the energy released in any other site. It indicates the high reactivity of NNN-OVs as the best trap center. The 5-fold Ti sites show similar behaviors despite the existence of different defects. Adsorption on this site is the least stable, and molecular adsorption is favored. A water molecule needs to overcome energy barriers of 0.25-0.27 eV to dissociate on 5-fold Ti atoms. However, the recombination barrier is even lower, and the fragments would recombine and exist stably in the molecular mode. Slightly higher barriers are observed on the defective sites.