Photochemical Fluorination of TiO2(110) Produces an Atomically Thin Passivating Layer

Fluorine has been widely reported to improve the photoreactivity of TiO2 nanocrystals, but surface science studies of this enhancement have been stymied by the lack of well controlled fluorination chemistries. Fluorine-terminated rutile (110) surfaces were produced by the photochemical degradation of solution-prepared trimethyl acetate monolayers in the presence of XeF2 (g) at room temperature. X-ray photoemission spectroscopy showed that the fluorinated surfaces were terminated by 0.8 monolayers of F bound to initially undersaturated Ti atoms. The fluorinated surface remained notably contamination free, even after immersion in solution and exposure to air for tens of minutes. The contamination-resistant properties of the fluorinated surface were attributed to the random blocking of 80% of the undersaturated Ti atoms, which simulations showed would block 93% of bidentate binding sites. This fluorine termination was very robust. Immersing the fluorinated surface in boiling H2O for 60 min or boiling base for 10 min removed only similar to 60% of the F atoms. Although the initial TiO2(110) surface was atomically flat, scanning tunneling microscopy images showed that the fluorinated surface was rough on an atomic scale, displaying short, atomically straight rows parallel to the [001] direction. The roughened morphology was indication of a relatively isotropic etching reaction. This isotropy was attributed to both the destabilization of the surface by F as well as the unusual reaction dynamics of XeF2. The increased reactivity of the fluorinated surface toward etching can be rationalized in terms of disrupted charge balance in the surface layer. Consistent with this, density functional theory simulations showed that the removal of bridging O atoms from the fully fluorinated surface to produce O-2 would be exoergic.