Interfacial charge transfer in a gallium oxide/covalent organic framework heterojunction for enhanced photocatalytic degradation of perfluorooctanoic acid.
Abstract
Per- and polyfluoroalkyl substances (PFAS), particularly perfluorooctanoic acid (PFOA), are highly persistent pollutants owing to their strong CF bonds and exceptional chemical stability. Herein, a sp2-conjugated covalent organic framework (COF)/gallium oxide (Ga2O3) heterojunction was constructed through an in situ growth strategy to integrate PFOA enrichment with photocatalytic transformation. The optimized 36% Ga2O3/COF composite achieved approximately 97% PFOA removal and 47% defluorination under the investigated conditions. Comprehensive characterization using ultraviolet-visible spectroscopy, photoluminescence analysis, transient photocurrent measurements, electrochemical impedance spectroscopy (EIS), Kelvin probe force microscopy (KPFM), and in situ X-ray photoelectron spectroscopy (XPS) revealed enhanced light harvesting, accelerated interfacial charge transfer, and suppressed charge recombination. Density functional theory (DFT) calculations demonstrated that the carboxyl terminal of PFOA is the preferential site for initial oxidative activation. Radical-scavenging experiments and electron paramagnetic resonance (EPR) measurements identified superoxide radicals (·O2-) as the dominant oxygen-derived reactive species, whereas photogenerated holes mainly participate in carboxyl-terminal oxidation and decarboxylation. Liquid chromatography-mass spectrometry (LC-MS) analysis suggested that PFOA undergoes sequential transformation involving decarboxylation, H/F exchange, oxygenation, CF bond cleavage, CC bond cleavage, and progressive chain shortening. The enhanced photocatalytic activity originates from the synergistic integration of interfacial PFOA enrichment, spatial charge separation, oxygen activation, and stepwise defluorination. This work provides an organic-inorganic heterointerface engineering strategy for the destructive transformation of persistent PFAS pollutants.