BF3 adsorption on stoichiometric and oxygen-deficient SnO2(110) surfaces

BF3 has been used as a probe of the basicity of surface oxygen anions on SnO2(110) surfaces. BF3 interacts directly with surface lattice oxygen "base" sites on SnO2(110) and acts as a reasonable molecular chemisorption probe for the basicity of thermally stable three-coordinate O2- anions by forming a Lewis acid/base adduct with these particular surface oxygen anions. However, BF3 reacts irreversibly with the more labile two-coordinate bridging oxygen on the stoichiometric surface, and no distinctive BF3 desorption feature is observed in thermal desorption that can be used to provide a measure of the basicity of bridging oxygen anions. These results are indicative of an inherent limitation in the application of BF3 thermal desorption measurements as a simple probe for the basicity of surface oxygen species of limited thermal stability. The presence of bridging oxygen on the stoichiometric surface does gives rise to gas-phase reaction products (HF and F-2) in thermal desorption that are not seen on the more oxygen-deficient surfaces. For all surfaces, some BF3 dissociation is observed, which results in the slow build up of surface boron and fluoride during consecutive thermal desorption runs.