Multifunctional nanocomposites of Fe₃O₄-graphene-Au for repeated use in simultaneous adsorption, in situ SERS detection and catalytic reduction of 4-nitrophenol in water

This work is directed towards the synthesis of a ternary nanocomposite of Fe₃O₄-graphene-Au, i.e. Fe₃O₄ nanoparticles (∼300 nm in size) and Au nanoparticles (∼50 nm in size) loaded on the carbon basal planes of reduced graphene oxide, aimed for repeated use in simultaneous adsorption, in situ SERS detection and catalytic reduction of 4-nitrophenol (4-NP) in water, and also for recovering the useful reduction product of 4-aminophenol (4-AP). The results indicate that the amount of 4-NP and 4-AP absorbed to the prepared Fe₃O₄-graphene-Au nanocomposite can reach 170 mg g⁻¹ and 447 mg g⁻¹, respectively. The reduction reaction of 4-NP to 4-AP by NaBH₄ with the Fe₃O₄-graphene-Au nanocomposite as a catalyst follows first-order kinetics with a rate constant (k) of about 0.4964 min⁻¹, remarkably superior to the 0.1199 min⁻¹ for the reduction reaction with the bare Au nanoparticles under the same conditions. In addition, in situ SERS can also be carried out to detect 4-NP and to monitor the reduction reaction with Fe₃O₄-graphene-Au as the substrate. Recycling of the composite can be achieved by simply applying an external magnetic field and the results demonstrate that it can be reused at least eight times with almost unaffected catalytic efficiency. (paper)