Synthesis of Self-Assembled Multifunctional Nanocomposite Catalysts with Highly Stabilized Reactivity and Magnetic Recyclability

Abstract

In this paper, a multifunctional Fe3O4@SiO2@PEI-Au/Ag@PDA nanocomposite catalyst with highly stabilized reactivity and magnetic recyclability was synthesized by a self-assembled method. The magnetic Fe3O4 nanoparticles were coated with a thin layer of the SiO2 to obtain a negatively charged surface. Then positively charged poly(ethyleneimine) polymer (PEI) was self-assembled onto the Fe3O4@SiO2 by electrostatic interaction. Next, negatively charged glutathione capped gold nanoparticles (GSH-AuNPs) were electrostatically self-assembled onto the Fe3O4@SiO2@PEI. After that, silver was grown on the surface of the nanocomposite due to the reduction of the dopamine in the alkaline solution. An about 5 nm thick layer of polydopamine (PDA) was observed to form the Fe3O4@SiO2@PEI-Au/Ag@PDA nanocomposite. The Fe3O4@SiO2@PEI-Au/Ag@PDA nanocomposite was carefully characterized by the SEM, TEM, FT-IR, XRD and so on. The Fe3O4@SiO2@PEI-Au/Ag@PDA nanocomposite shows a high saturation magnetization (Ms) of 48.9 emu/g, which allows it to be attracted rapidly to a magnet. The Fe3O4@SiO2@PEI-Au/Ag@PDA nanocomposite was used to catalyze the reduction of p-nitrophenol (4-NP) to p-aminophenol (4-AP) as a model system. The reaction kinetic constant k was measured to be about 0.56 min(-1) (R(2) = 0.974). Furthermore, the as-prepared catalyst can be easily recovered and reused for 8 times, which didn't show much decrease of the catalytic capability.

Figure 1. Scheme of synthesis of the Fe₃O₄@SiO₂@PEI–Au/Ag@PDA nanocomposite.

Figure 2

SEM images of the Fe₃O₄ NPs (a,b) low and high magnification of the Fe₃O₄ NPs; Fe₃O₄@SiO₂ (c) Fe₃O₄@SiO₂ @PEI (d) Fe₃O₄@SiO₂@PEI-Au/Ag@PDA (e,f) low and high magnification of the Fe₃O₄@SiO₂@PEI-Au/Ag@PDA nanocomposite.

Figure 3

Zeta potentials of the GSH-AuNPs (A), Fe₃O₄@SiO₂ (B), Fe₃O₄@SiO₂@PEI (C), Fe₃O₄@SiO₂@AuNPs (D), Fe₃O₄@SiO₂@Au-SH-PEG (E) and the Fe₃O₄@SiO₂@PEI-Au/Ag@PDA nanocomposite (F) respectively.

Figure 4. FT-IR characterization of the Fe₃O₄, Fe₃O4@SiO₂ and Fe₃O₄@SiO₂@PEI-Au/Ag@PDA nanocomposite.

Figure 5. HAADF STEM images of the Fe₃O₄@SiO₂@PEI-Au/Ag@PDA nanocomposite.

(a,b) low and high magnification, respectively; TEM images of the Fe₃O₄@SiO₂@PEI-Au/Ag@PDA (c,d) low and high magnification of the Fe₃O₄@SiO₂@PEI-Au/Ag@PDA nanocomposite, respectively.

Figure 6. XRD patterns of the Fe₃O₄ NPs (black) and the Fe₃O₄@SiO₂@PEI-Au/Ag@PDA nanocomposite (red) (The new diffraction peaks on the Fe₃O₄@SiO₂@PEI-Au/Ag@PDA nanocomposite were labeled with stars (*)).

Figure 7

(a) HAADF SEM mapping of the elements of the Fe₃O₄@SiO₂@PEI-Au/Ag@PDA nanocomposite; (b) The EDX spectrum of the Fe₃O₄@SiO₂@PEI-Au/Ag@PDA nanocomposite. The Cu element comes from the substrate of the Cu grid.

Figure 8. Magnetic hysteresis loops of the Fe₃O₄ (black), Fe₃O₄@SiO₂ (red) and the Fe₃O₄@SiO₂@PEI-Au/Ag@PDA nanocomposite (blue) at 300 K.

Inset: the photographs showing magnetic Fe₃O₄@SiO₂@PEI-Au/Ag@PDA nanocomposite separated by a magnet.

Figure 9. Catalytic performance of Fe₃O₄@SiO₂@PEI-Au/Ag@PDA nanocomposite to convert 4-NP to 4-AP.

(a) UV–vis spectra of 4-NP (a) before and (b) after the addition of NaBH₄; (b) Time-dependent UV–vis spectra of the reaction solution in the presence of the Fe₃O₄@SiO₂@PEI-Au/Ag@PDA nanocomposite catalyst; (c) Plot of ln (C_t/C₀) against the reaction time; (d) The recyclability of the Fe₃O₄@SiO₂@PEI-Au/Ag@PDA nanocomposite as the catalyst for the reduction of 4-NP with NaBH₄.