Oxidative Etching and Metal Overgrowth of Gold Nanorods within Mesoporous Silica Shells

Abstract

Composite noble metal-based nanorods for which the surface plasmon resonances can be tuned by composition and geometry are highly interesting for applications in biotechnology, imaging, sensing, optoelectronics, photovoltaics, and catalysis. Here, we present an approach for the oxidative etching and subsequent metal overgrowth of gold nanorods, all taking place while the nanorods are embedded in mesoporous SiO 2 shells (AuNRs@meso-SiO 2 ). Heating of the AuNRs@meso-SiO 2 in methanol with HCl resulted in reproducible oxidation of the AuNRs by dissolved O 2 , specifically at the rod ends, enabling precise control over the aspect ratio of the rods. The etchedAuNRs@meso-SiO 2 were used as a template for the overgrowth of a second metal (Ag, Pd, and Pt), yielding bimetallic, core−shell structured nanorods. By varying the reaction rates of the metal deposition both smooth core−shell structures or gold nanorods covered with a dendritic overlayer could be made. This control over the morphology, including metal composition, and thus the plasmonic properties of the composite rods were measured experimentally and also confirmed by Finite-Difference Time-Domain (FDTD) calculations. The presented synthesis method gives great control over tuning over both plasmonic properties and the particle stability/affinity for specific applications.

In Situ Observation of Atomic Redistribution in Alloying Gold–Silver Nanorods

“…To this end, the Ag-shell growth as described by Deng et al, comprising the reduction of Ag + ions on the Au nanorods by ascorbic acid, was performed in an acidified, instead of neutral, aqueous solution. 9 The presence of H + ions slowed the Ag-shell growth down considerably (from seconds to minutes), resulting in sufficiently long mixing times for the reagents and homogeneous Ag-shell growth. To limit the variation in particle volume when changing the Au-to-Ag ratios of the particles, both the core and the shell size of the Au core and Ag shell were varied.…”

“…The third step, oxidative etching of the Au@SiO 2 NRs, was performed by following the procedure described by Deng et al 9 but with H 2 O 2 as an oxidant instead of O 2 from air. Different core sizes were obtained by varying the etching time.…”

“…We employed colloidally synthesized Au-core, Ag-shell nanorods of which the composition, size, and shape was tuned precisely. 9 By the coating of the metal nanorods with a protective mesoporous silica coating, 32 preservation of the particle shape during atomic redistribution was ensured. 8 We specifically chose a Au–Ag-based system because alloy formation is thermodynamically favorable at all compositions and the lattice spacings of Au and Ag closely match.…”

From a precursor to an etchant: spontaneous inversion of the role of Au(iii) chloride for one-pot synthesis of smooth and spherical gold nanoparticles

“…In general, researchers have used various types of etchants to control the shape and size of AuNPs. [23][24][25] In this study, we used a AuCl 4 À -cetyltrimethylammonium chloride (CTAC) complex for the oxidative etching of AuNPs, which was previously reported by Halas et al 26 and studied in detail by Liz-Marzán et al 27 In our study, gold sources can act as a gold precursor during the initial AuNP synthesis process and as an etchant during the subsequent etching process. The exchange of the role of AuCl 4 À was achieved by controlling the molar ratio between the reducing agent and AuCl 4 À .…”

Unlocking Synergy in Bimetallic Catalysts by Core-Shell Design

“…However, also the arrangement in which the atoms are assembled is crucial; gas-and thermally-induced metal redistribution can have a large impact on the catalytic performance [12,13,14]. Thanks to recent advances in ma-terial science, it is now possible to synthesize bimetallic nanoparticles with precisely defined atomic arrangements such as single-atom alloys [15,16], intermetallic structures [17] and core-shell materials [18,19]. Yet, the number of systematic studies linking the metal distribution to the performance of bimetallic catalysts is limited [12,13,14].…”