Optimized Sequence-controlled Synthesis of Fe3O4 -Ag-rGO Core-Shell Nanocatalysts for Enhanced 4-Nitrophenol Reduction and Magnetic Recovery

Abstract

The catalytic reduction of 4-nitrophenol (4-NP), a major industrial pollutant, is of urgent environmental significance. Here, we report the synthesis of Fe3O4-Ag-rGO core-shell ternary nanocomposites (CSTNs) via a time-controlled reflux method, designed to achieve high-performance reduction of 4-NP to 4-aminophenol (4-AP). We systematically investigated how precursor sequence and pre-stirring duration prior to silver (Ag) precursor addition govern nanostructure formation and catalytic efficiency. Among the synthesized variants, FaAG-60, prepared with a 60-minute pre-stirring step, exhibited a well-defined reverse core-shell architecture with a magnetic Fe3O4 core, uniform Ag shell, and conductive reduce graphene oxide (rGO) substrate. Structural analyses, High-Resolution Transmission Electron Microscopy (HRTEM), Raman Spectroscopy and Vibrating Sample Magnetometer (VSM) confirmed its coherent morphology and strong magnetic recoverability. Kinetic evaluation revealed that reagent sequencing critically determines catalytic performance: pre-adsorbing 4-NP onto the catalyst before sodium borohydride (NaBH4) addition (Sequence II) yielded the highest rate constant (k = 0.0862 s-1), a seven-fold enhancement over the non-optimized system. This synergy of ternary integration, precise synthesis control, and strategic reagent sequencing enabled rapid hydride transfer, efficient electron mobility, and facile catalyst recovery. These findings establish a robust framework for designing magnetically recoverable and potentially recyclable, high-efficiency nanocatalysts, offering scalable solutions for wastewater treatment and environmental remediation.