Relative stability of F-covered TiO2 anatase (101) and (001) surfaces from periodic DFT calculations and ab initio atomistic thermodynamics

A density functional theory based computational study is presented of fluorine adsorption on anatase TiO2(101) and TiO2(001) surfaces. A variety of surface coverage situations ranging from 10 to 100% have been considered, and the effect on the electronic structure of the material has been analyzed. The adsorption of F atoms on these surfaces directly affects the band structure with new features in the band gap due to the presence of partially oxidized anions. The effect of F coverage on the relative stability of both surfaces has been studied by applying the ab initio thermodynamic formalism. Interestingly, the effect of fluorine on the surface free energy at 298.15 K and 1 bar of F2 pressure is to stabilize the (001) surface and destabilize the (101), which nicely explains the experimental observation that the presence of F allows growing TiO2 anatase nanoparticles with a higher percentage of the more reactive 001 surface with enhanced photocatalytic activity