Mechanism insights into the facet-dependent photocatalytic degradation of perfluorooctanoic acid on BiOCl nanosheets

Abstract

Photocatalytic degradation of per- and polyfluoroalkyl substances (PFAS) remains insufficient. It is vital to investigate the structure–activity relationship between exposed facets and photocatalytic activity of PFAS. In this study, BiOCl nanosheets with predominantly exposed {010} or {001} facets (i.e. {010}-BiOCl or {001}-BiOCl) were synthesized via the hydrothermal method and applied for the photodegradation of perfluorooctanoic acid (PFOA). We observed that the degradation rate constant of PFOA via the {010}-BiOCl (0.0954 min−1) was 2.64-fold better than that of the {001}-BiOCl (0.0361 min−1) in photocatalytic activity. Density functional theory calculation results revealed that the higher catalytic activity of {010}-BiOCl was attributed to the in-situ generated surface oxygen vacancies (OVs) and the strong interaction with PFOA molecules via the bidentate adsorption configuration. {010}-BiOCl nanosheets with OVs exhibited the highest adsorption capacity (55.6 mg g−1h−1) and lowest adsorption energy (-0.399 eV) for PFOA molecules. The electron spin resonance spectroscopy and radical scavenging experiments showed that both photogenerated holes and surface hydroxyl radicals were generated on {010} facets that contributed to the degradation of PFOA; while only photogenerated holes were formed on {001} facets. Interestingly, F− ions as the by-products were prone to attaching to the BiOCl facets to block the catalytic sites. To recover the catalytic performance, we adopted a simple chemical precipitation approach to precipitate the F− by using Ca2+ ions. Our findings provide fundamental insights into the interface reactions during the photocatalytic degradation of PFOA.