Etching and growth: an intertwined pathway to silver nanocrystals with exotic shapes

Abstract

Two-faced nanocrystals: Rapid addition of a second aliquot of silver nitrate during a polyol synthesis led to the formation of anisotropically truncated octahedrons as a result of oxidative etching and overgrowth of silver nanocubes. Three adjacent faces of the nanocube grew more rapidly than the three other faces, generating a non-centrosymmetric structure (see picture).

Figure 1

When a second aliquot of AgNO₃ was introduced at the end of a sulfide-mediated polyol synthesis, the Ag nanocubes evolved into a new anisotropic structure rapidly: a, b) SEM images with TEM insets of the product (a) before and (b) 10 min after introduction of additional AgNO₃ (scale bars in the insets: 50 nm). The arrows indicate the corners of nanocubes that had been truncated due to oxidative etching. c) A proposed mechanism for this transformation. Silver ions reduced more rapidly on three {100} faces adjacent to the truncated corner, leading to the formation of an anisotropically-truncated octahedron. For comparison, an octahedron is also shown with three of the corners removed. Note that the third detached corner is not visible at this orientation. White and grey denote {111} and {100} facets, respectively. d) The two most common orientations of the anisotropically-truncated octahedron on a flat substrate, as viewed from above.

Figure 2

a) A high-magnification SEM image of the anisotropically-truncated octahedrons of Ag with well-developed facets. b) TEM images taken from a single anisotropically-truncated octahedron at three different tilting angles and images of a model which has been tilted the same amount (the direction is indicated by the circular arrows). White and grey signify {111} and {100} facets, respectively.

Figure 3

High-resolution TEM analysis of the two orientations typically observed, with the anisotropically-truncated octahedron sitting on a (a, b) triangular, {111} face and (c, d) square, {100} face. Insets for (a) and (c) are models of the anisotropically-truncated octahedron at that orientation, and for (b) and (d) are fast Fourier transform (FFT) patterns of the high-resolution TEM images, respectively.

Figure 4

a) Normalized UV-vis spectra of aqueous suspensions of the Ag nanocubes (dashed) and the corresponding product of anisotropically-truncated octahedrons (solid). b) DDA calculations of the extinction (solid), absorption (dashed), and scattering (dotted) coefficients for an anisotropically-truncated octahedron suspended in water with random orientations. The sharp corners were snipped by 11.7 nm to reflect the slightly truncated nature to provide a better fit with the experimental data. c) Solution phase SERS spectrum of 1,4-BDT adsorbed onto the surface of the anisotropically-truncated octahedrons. The overlapping peaks at 1067 cm⁻¹ and 1085 cm⁻¹ are attributed to the fundamental benzene ring breathing mode 1, the peak at 1182 cm⁻¹ is attributed to the 9a ring breathing vibration, and the peak at 1563 cm⁻¹ is attributed to the 8a ring breathing vibration. d) Single particle SERS spectrum of 1,4-BDT adsorbed on the anisotropically-truncated octahedron shown in the inset. In addition to the peaks seen in c, a broad peak just below 1000 cm⁻¹ is visible, which can be ascribed to the silicon substrate. The polarization direction is indicated by the white arrow.