Surface-dominated photocatalytic degradation of polystyrene microplastics in TiO2/Fe3O4 thin films under UV irradiation

Polystyrene (PS) microplastics are persistent environmental contaminants whose degradation under realistic solid-state conditions remains poorly understood. In this study, the ultraviolet (UV-A, 365 nm, 143.5 h)-induced degradation of PS microplastics is systematically investigated using Fe 3 O 4 and Fe 3 O 4 /TiO 2 composite thin films under dry (air) conditions. This approach provides a controlled model for surface-exposed plastic aging beyond conventional aqueous systems. Structural, chemical, optical, and surface modifications were comprehensively analyzed using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), UV–visible spectroscopy (UV–vis), and X-ray photoelectron spectroscopy (XPS). XRD analysis reveals enhanced structural disorder of the PS matrix in the presence of Fe 3 O 4 /TiO 2 compared with Fe 3 O 4 alone. However, the crystalline phases of the inorganic components remain unchanged. Under identical UV irradiation conditions, FTIR carbonyl index quantification and UV–vis absorbance measurements indicate that Fe 3 O 4 alone induces only limited bulk photo-oxidation and partial aromatic degradation compared with the Fe 3 O 4 /TiO 2 system. Meanwhile, the incorporation of TiO 2 suppresses extensive bulk macromolecular chain fragmentation despite prolonged UV exposure. In contrast, XPS reveals pronounced surface oxidation for the Fe 3 O 4 /TiO 2 system, evidenced by a significant increase in oxygen-containing carbon species in the C 1s spectra. FTIR carbonyl index measurements and UV–vis absorbance changes indicate that bulk chemical and optical modifications remain modest under the applied UV conditions. Together, these results reveal that Fe 3 O 4 /TiO 2 preferentially promotes the surface-localized photo-oxidation of PS microplastics rather than uniform bulk macromolecular chain fragmentation. Based on these findings, we propose a solid-state degradation mechanism in which UV-excited TiO 2 generates charge carriers that may undergo interfacial charge interactions with Fe 3 O 4 . This mechanism facilitates oxygen-mediated surface oxidation in the absence of water. Accordingly, this work provides new mechanistic insight into the spatial nature of microplastic degradation under dry conditions. The work also highlights why surface-localized photo-oxidation should be distinguished from macromolecular chain fragmentation in environmental aging processes.