Nanorods, nanowires and nanocages from seeds

Growth from seeds in six panels: seed-mediated gold nanorods step by step and one rod with CTAB and silver on its sides, a penta-twinned silver nanowire capped by PVP, and galvanic replacement turning silver nanocubes into nanoboxes, nanocages and nanoframes. Each caption quotes the cited papers.

1. Seeds: HAuCl₄ in CTAB reduced by NaBH₄ freshly made with cold water, under vigorous stirring; the yellow solution turned brownish yellow and was aged 30 min at 30 °C (Wei 2021). 2. Growth: CTAB with sodium oleate, AgNO₃ and HAuCl₄ (colourless after 90 min of stirring), then HCl, ascorbic acid and the seeds; left undisturbed at 30 °C for 12 h. More seeds gave shorter, thinner rods with a higher aspect ratio (Wei 2021). Seeds of about 4 nm grow into rods in CTAB with ascorbic acid, and AgNO₃ strongly enhanced rod formation (Iqbal & Tae 2006). 3. Why rods: Each rod is a single fcc crystal with the ⟨001⟩ direction along its long axis; Ag⁺ is believed to bind selectively to the {110} facets and slow their growth, and CTAB binds mainly along the middle of the rods (Wei 2021). With one recipe, CTAB from 3 of 10 suppliers gave only spheres: an impurity in the CTAB matters (Smith & Korgel 2008). 4. Penta-twinned silver nanowires: Silver nanowires grow from penta-twinned seeds, and bromide drives the seeds to evolve into wires (Liu 2024). With Br⁻ and high-molecular-weight PVP capping the {100} side faces and AgNO₃ added slowly by syringe pump, the wires grow along their length only: under 20 nm wide, aspect ratios over 1000 (da Silva 2016). 5. Nanoboxes and nanocages: AuCl₄⁻/Au (0.99 V vs SHE) lies above AgCl/Ag (0.22 V), so HAuCl₄, added like a titration to boiling cubes, oxidises the silver: one Au atom deposits for every three Ag atoms removed. The reaction starts at a pinhole on one face; gold grows epitaxially on the faces and the cube becomes a hollow Au–Ag nanobox; more HAuCl₄ dealloys the walls, truncating the corners and opening pores (Skrabalak 2008). 6. Corners and frames: PVP binds {100} most strongly, so on cubes with rounded corners silver dissolves at all the poorly passivated {111} corners while gold deposits on the faces. Fe(NO₃)₃ only dealloys: nanoframes of almost pure gold form when nearly all the silver is gone (Skrabalak 2008); a gold layer deposited first (HAuCl₄ reduced by ascorbic acid) keeps the frame from fragmenting (Wan 2013).

Starting materials: HAuCl₄, NaBH₄, CTAB, sodium oleate, AgNO₃, HCl and ascorbic acid; AgNO₃, PVP and a halide in ethylene glycol; silver nanocubes with HAuCl₄ and Fe(NO₃)₃. Product: Gold nanorods, penta-twinned silver nanowires, Au–Ag nanoboxes and nanocages, gold nanoframes.

Wei et al., “Seed-mediated synthesis of gold nanorods at low concentrations of CTAB”, ACS Omega 2021 (doi:10.1021/acsomega.1c00510); Iqbal & Tae, “Unstable reshaping of gold nanorods prepared by a wet chemical method in the presence of silver nitrate”, J. Nanosci. Nanotechnol. 2006 (doi:10.1166/jnn.2006.009); Smith & Korgel, “The importance of the CTAB surfactant on the colloidal seed-mediated synthesis of gold nanorods”, Langmuir 2008 (doi:10.1021/la703625a); da Silva et al., “Facile synthesis of sub-20 nm silver nanowires through a bromide-mediated polyol method”, ACS Nano 2016 (doi:10.1021/acsnano.6b03806); Liu et al., “Biobased polymers enabling the synthesis of ultralong silver nanowires and other nanostructures”, Nano Lett. 2024 (doi:10.1021/acs.nanolett.4c04130); Skrabalak et al., “Gold nanocages: synthesis, properties, and applications”, Acc. Chem. Res. 2008 (doi:10.1021/ar800018v); Wan et al., “Robust synthesis of gold cubic nanoframes through a combination of galvanic replacement, gold deposition, and silver dealloying”, Small 2013 (doi:10.1002/smll.201203233); Nikoobakht & El-Sayed, “Preparation and growth mechanism of gold nanorods (NRs) using seed-mediated growth method”, Chem. Mater. 2003 (doi:10.1021/cm020732l); Naz et al., “High-yield synthesis of silver nanowires for transparent conducting PET films”, Beilstein J. Nanotechnol. 2021 (doi:10.3762/bjnano.12.51).

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