Facile synthesis of branched au nanostructures by templating against a self-destructive lattice of magnetic fe nanoparticles

Abstract

This paper reports the demonstration of a reactive, self-destructive template for the facile synthesis of branched Au nanostructures. The template is a three-dimensionally porous lattice of uniform, magnetic Fe nanoparticles self-assembled in situ on the surface of a magnetic stir bar. Upon introduction of AuCl, Au atoms are formed in the voids among Fe nanoparticles due to the galvanic replacement reaction between Fe and Au⁺. The Au atoms then nucleate and grow into branched nanostructures under the confinement of Fe nanoparticles. As the replacement proceeds, Fe is consumed to gradually reduce the sizes and magnetic moments of the Fe nanoparticles. At a certain stage of the reaction, the template starts to fall apart to automatically release the branched Au nanostructures. We can routinely obtain Au multipods as pure samples via selective dissolution of the remaining Fe nanoparticles with sulfuric acid. The as-prepared Au multipods show strong absorption in the near infrared region and exhibit distinctive oxidative etching behaviors in different acidic solutions due to the presence of crystal defects and lattice distortions.

Figure 1

Schematic detailing four major steps involved in the formation of Au multipods by templating against a self-destructive lattice of Fe nanoparticles: a) Au nucleates in the voids of aggregated Fe nanoparticles through a replacement reaction between Fe and Au⁺; b) the Au evolves into multipods under the confinement of Fe nanoparticles as the reaction proceeds; c) the lattice of Fe nanoparticles falls apart at a certain point as the volume of Au structures expand and the magnetic moments of individual Fe nanoparticles are reduced; and d) Au multipods are harvested and purified by dissolving the remaining Fe nanoparticles with H₂SO₄.

Figure 2

a, b) TEM images of the as-prepared Au multipods at two different magnifications; c) high-resolution TEM image of a typical Au multipod (see Figure S1 for details); and d) XRD pattern taken from the Au multipods.

Figure 3

TEM images of samples obtained at different stages of the replacement reaction: a) Fe nanoparticles before the reaction that were released from the stir bar by sonication; b) a black chuck initially disassembled from the stir bar after adding AuCl(OLA) and stirring for 2 min; c) completely disassembled Fe nanoparticles and Au mutipods after continuous stirring for 20 min; and d) Au multipods purified by washing the sample with 1 M H₂SO₄/C₂H₅OH solution to selectively dissolve the remaining Fe nanoparticles.

Figure 4

a, b) TEM images of the Au multipods after washing with 1 M HNO₃/C₂H₅OH and 1 M H₂SO₄/C₂H₅OH, respectively. c, d) Extinction spectra and digital photos of Au multipods dispersed in hexane after washing with 1 M H₂SO₄/C₂H₅OH, 1 M HNO₃/C₂H₅OH and 1 M HCl/C₂H₅OH. Note that the spectra in Figure 4c were vertically shifted for display purpose, so the peek height did not match with the absorbance.