A Process-controlled Synthesis of Corrosion-resistant Fe₃O₄/Au Nanoparticles as a Precursor for Stable Magnetic Fluids in Harsh Environments

Abstract

Background: Conventional direct reduction synthesis approaches encounter several significant challenges during the fabrication process. The acidic nature of chloroauric acid can induce etching of the Fe₃O₄ core, thereby compromising the structural integrity of the nanoparticles. In addition, rapid reduction kinetics frequently promote homogeneous nucleation of gold nanoparticles rather than controlled deposition onto the Fe₃O₄ surface. Furthermore, fluctuations in pH during the reaction process can result in uneven and non-uniform coating formation, ultimately affecting the stability, morphology, and reproducibility of the final nanocomposite material. Aims: To develop a process-controlled synthesis of Fe₃O₄/Au nanocomposites with uniform Au decoration, high corrosion resistance, and superparamagnetism, serving as a stable precursor for magnetic fluids operable in harsh acidic environments. Research Design: Experimental synthesis and characterization of Fe₃O₄/Au nanoparticles using a dynamic pH-buffered reduction strategy; evaluation of structural, magnetic, and anti-corrosion properties. Methods: Fe₃O₄ nanoparticles (~12 nm) were prepared by coprecipitation. Fe₃O₄/Au nanocomposites were synthesized using hexamethylenetetramine (HMT) as an in-situ pH buffer, with real-time pH feedback and ultra-slow addition of ascorbic acid. Morphology, structure, composition, optical properties, magnetic properties, and corrosion resistance (0.1 mol/L HCl, 0.5 mol/L NaOH, 60 °C, 24 h) were characterized by TEM, SEM, XRD, FT-IR, UV-Vis, VSM, and TGA. Results: Uniform Au nanoparticles (~50 nm) were anchored on Fe₃O₄ clusters (30–100 nm). The composite exhibited superparamagnetism with saturation magnetization 33 emu/g. After 24 h in 0.1 mol/L HCl, magnetization remained 31.4 emu/g with negligible iron leaching. Alkaline corrosion caused no observable degradation. TGA showed thermal stability below 250 °C. Oleic acid modification enabled oil-phase dispersibility. Conclusion: The dynamic pH-buffered epitaxial strategy effectively suppresses Fe₃O₄ core dissolution and Au self-nucleation, yielding Fe₃O₄/Au nanocomposites with excellent acid resistance and magnetic integrity. This material is a promising precursor for stable magnetic fluids in harsh environments.