Magnetically Recyclable Fe3O4/Au Nanocomposites for Efficient Degradation of Organic Dyes via Tribocatalysis

Abstract

Tribocatalysis, which converts mechanical energy into chemical energy, offers a green route for water treatment, yet efficient, recoverable, and durable tribocatalysts remain scarce. Herein, a magnetic Fe₃O₄/Au nanocomposite was developed for organic dye degradation. Although Fe₃O₄/Au has been widely studied in photocatalysis and Fenton-like systems, its tribocatalytic application is largely unexplored. The composite was synthesized by a solvothermal method using polyethylenimine as a scaffold, followed by grafting dithiocarbamate and self-assembling AuNPs (5–20 nm) onto Fe₃O₄ hollow microspheres (∼200 nm). Characterization (XRD, TEM, SEM, XPS, BET, VSM) confirmed a stable heterostructure with mesoporous features (21.3 m²/g) and strong superparamagnetism (66.84 emu/g), enabling magnetic recovery within 15 s. XPS revealed a +0.07 eV shift of Fe 2p_3/2 upon Au loading, evidencing interfacial electron transfer from Fe₃O₄ to Au and formation of a Schottky barrier that promotes charge separation. Under optimized stirring (500 rpm, 5 h), Fe₃O₄/Au achieved 100% degradation of (RhB) (5 mg/L, 50 mL) with a rate constant of 0.7751 h^–1, outperforming pure Fe₃O₄ and AuNPs. The near-neutral pH condition (pH ≈ 7) was found to be optimal for the tribocatalytic performance. Radical scavenger and EPR experiments identified ^•OH as the primary active species and ^•O₂^– as secondary. TOC analysis showed 71% mineralization. The catalyst retained 97.47% efficiency after five cycles with negligible Fe (<0.006 ppm) and Au (<0.001 ppm) leaching, and preserved morphology. RhB degraded faster than Methylene Blue, Crystal Violet, Acid Orange 7, and Methyl Orange, correlating with molecular bond strengths. This work demonstrates a novel mechanical-to-chemical energy conversion pathway for sustainable water treatment.