Unraveling Pathways of Photocatalytic Oxygen Evolution on Fluorinated Anatase TiO2

Recent experiments reported that the efficiency of the oxygen evolution reaction (OER) was greatly improved in fluorinated TiO2 by forming the surface hydrogen bonds, but the mechanism remains ambiguous. Herein, we systematically investigated dual OER pathways on the fluorinated anatase TiO2(101) surface by using the first-principles calculations. The dual H-bonding water is significantly activated by trapping a first hole in the system and facilitates its proton transfer in pathway I. The inactive Ti-coordinated water also transfers a proton to a bridge O2– ion with a similar barrier and simultaneously traps the first hole in pathway II. Both ·OH radicals separately transfer a proton to the F– ion and bridge O2– ion very quickly and trap the second hole to produce the ·O radical. Consequently, the ·O radical directly couples with the bridge O2– ions with a high barrier of about 0.72 eV to produce the flat peroxo dimer (O22–) in pathway I. The Ti-coordinated ·O radical undergoes two steps of the proton transfer and O–O coupling with both high barriers of about 0.75 eV to produce the oblique O22– species in pathway II. Finally, the O22– spontaneously and successively traps the third and fourth holes to quickly form the oxygen molecule. Additionally, the fluorine atoms are further demonstrated to accelerate the proton transfer and the O–O coupling steps by comparing the pathways on the fluorinated and pure TiO2 surfaces. These results may provide new insights into the OER mechanism in fluorinated TiO2 and roles of the dopant in improving OER efficiency of the TiO2 photocatalysts.