Synthesis and Optical Properties of Cubic Gold Nanoframes

Abstract

This paper describes a facile method of preparing cubic Au nanoframes with open structures via the galvanic replacement reaction between Ag nanocubes and AuCl(2) (-). A mechanistic study of the reaction revealed that the formation of Au nanoframes relies on the diffusion of both Au and Ag atoms. The effect of the edge length and ridge thickness of the nanoframes on the localized surface plasmon resonance peak was explored by a combination of discrete dipole approximation calculations and single nanoparticle spectroscopy. With their hollow and open structures, the Au nanoframes represent a novel class of substrates for applications including surface plasmonics and surface-enhanced Raman scattering.

Figure 1

SEM and TEM (insets) images showing different stages of the galvanic replacement reaction where Ag nanocubes were titrated with different volumes of 0.2 mmol /L AuCl₂⁻ in 1 mL increments ranging from (a) 1 mL to (h) 8 mL. The scale bar below the images applies to all SEM images. The scale bar in the inset (a) represents 100 nm and applies to all TEM images

Figure 2

A schematic detailing the mechanism of the galvanic replacement reaction between Ag nanocubes and AuCl₂⁻. The cross-sectional view corresponds to the plane marked by dashed lines. The major steps of the reaction include the following: (a) formation of a pinhole at one of the side faces; (b) continuation of the replacement reaction resulting in a partially hollow structure; (c) development of a seamless nanobox with truncated corners; (d) generation of pores at the corners and side faces by a dealloying process; (e), (f) enlargement of pores at the side faces accompanied by shrinkage of pores at the corners via migration of atoms to the corners; (g) reduction of the ridge thickness; (h) fragmentation of the nanoframes

Figure 3

Electron microscopy characterization of the Au nanoframes prepared by the galvanic replacement reaction between 50 µL of Ag nanocubes and 6 mL of 0.2 mmol / L AuCl₂⁻: (a) SEM image of the Au nanoframes and (b) the same sample tilted by 45°; (c) TEM image of the Au nanoframe; (d) high-resolution TEM image of a corner of the Au nanoframe taken from the region as labeled in the inset. The scale bar in the inset represents 20 nm. The lattice spacing of 2.04 Å can be indexed as the {200} planes of Au. In the FFT pattern (inset), the spots enclosed by the circle and square can be indexed to the {220} and {200} reflections, respectively

Figure 4

(a) UV–vis spectra taken from aqueous suspensions of the structures in Figs. 1(a)–(f), which were synthesized by titrating Ag nanocubes with 1 mL to 6 mL of 0.1 mmol / L AuCl₂⁻. (b) DDA-calculated scattering spectra for nanoframes with an edge length of 57.0, 59.4, 61.8, 64.1, 66.5, and 68.9 nm, having a constant ridge thickness of 19 nm, together with a fixed composition of 89% Au and 11% Ag. The inset shows a drawing of the nanoframe used in these calculations, which has both sharp corners and edges. The red-shifts of the plasmon resonance peak increase with increasing ratio (R) between the outer edge length (l) and the ridge thickness (t). (c) DDA-calculated extinction, scattering, and absorption spectra for a nanoframe with R=3.37. (d) Scattering spectra of individual Au nanoframes and the SEM images of the corresponding nanoframes. For DDA calculations in (b) and (c), the nanoframes were filled and surrounded by air

Figure 5

(a) SERS spectrum of Au nanoframes whose surface had been derivatized with 4-MBT; (b) The ordinary Raman spectrum of 4-MBT (0.1 mol / L in 12 mol / L aqueous NaOH) taken for reference. The measurements were performed in a solution phase with an excitation wavelength of 785 nm