Symmetry Breaking by Surface Blocking: Synthesis of Bimorphic Silver Nanoparticles, Nanoscale Fishes and Apples

Abstract

A powerful approach to augment the diversity of well-defined metal nanoparticle (MNP) morphologies, essential for MNP advanced applications, is symmetry breaking combined with seeded growth. Utilizing this approach enabled the formation of bimorphic silver nanoparticles (bi-AgNPs) consisting of two shapes linked by one regrowth point. Bi-AgNPs were formed by using an adsorbing polymer, poly(acrylic acid), PAA, to block the surface of a decahedral AgNP seed and restricting growth of new silver to a single nucleation point. First, we have realized 2-D growth of platelets attached to decahedra producing nanoscale shapes reminiscent of apples, fishes, mushrooms and kites. 1-D bimorphic growth of rods (with chloride) and 3-D bimorphic growth of cubes and bipyramids (with bromide) were achieved by using halides to induce preferential (100) stabilization over (111) of platelets. Furthermore, the universality of the formation of bimorphic nanoparticles was demonstrated by using different seeds. Bi-AgNPs exhibit strong SERS enhancement due to regular cavities at the necks. Overall, the reported approach to symmetry breaking and bimorphic nanoparticle growth offers a powerful methodology for nanoscale shape design.

Figure 1

(a) Schematics of different pathways of bi-AgNP formation illustrating 1-D, 2-D and 3-D growth in the system. (b–i) transmission electron microscopy (TEM) images of (b–d) representative 2-D bi-AgNP morphologies, (b,c) optimal preparation: 0.13 mM PAA 450 K, 0.04 mM AgDeNP seeds, 0.27 mM ascorbic acid, and 0.08 mM AgNO₃, (c) low Ag, and (e) uniformly 3-D enlarged decahedra (high PAA). (f–i) representative bi-AgNPs prepared in presence of halides: (f,g) 3-D bi-AgNPs, (f) 75:1 Ag/KBr; (g) 38:1 Ag/KBr; (h) pentagonal rods with 1:12 Ag/KCl, (i) 1-D bi-AgNPs with 1:5 Ag/HCl. All ratios are molar. All scale bars are 50 nm. For detailed description of samples-see Supplementary Table S1.

Figure 2. Development of 2-D bi-AgNP morphologies with different amounts of added silver (the percentage is given in brackets relative to silver present in decahedral seeds).

(a) Representative TEM images; (b) UV-vis spectra; and (c) optical photographs of samples: ➊ 0.008 mM (20%), ➋ 0.016 mM (40%), ➌ 0.04 mM (100%), ➍ 0.05 mM (120%), ➎ 0.08 mM (200%) and ➏ 0.13 mM (320%). All scale bars are 50 nm.

Figure 3. TEM images demonstrating diversity of attainable bi-AgNP shapes.

The scale can be inferred from the largest dimension of the decahedral part of bi-AgNPs (a diamond shape in TEM side projection) being 41.5 ± 1.5 nm, reproducibly prepared by photochemical synthesis.