Homogeneously-Alloyed Gold–Silver Nanoparticles as per Feeding Moles

Abstract

Wet-chemical methods involving the coreduction of HAuCl 4 and AgNO 3 have been proven particularly suitable for producing stable Au−Ag alloy NPs with controllable structure−property relationship. However, very poor-solubility of AgCl in aqueous medium and intrinsically different surface energies of Au and Ag remained detrimental-factors in synthesizing so-called "alloy" NPs above the solubility-product of AgCl. Here, we report a robust coreduction procedure for producing citrate-stabilized "homogeneously alloyed" Au−Ag NPs of average size sub-10 nm at room-temperature upon simultaneously overcoming the detrimental factors by a simple reagent NH 4 OH. The alloy NPs revealed a high-degree of crystallinity, composition-tunable surface plasmon resonance (SPR) behavior, controlled-catalysis, biocompatibility, surface enhance Raman scattering (SERS) activity and high-chemical stability. The alloy NPs could withstand corrosive chemical environment and be easily transferred from aqueous medium to various organic media. Fusion of NPs under high-energy electron-beam suggested an inertial coalescence. Our method may lead to the developments of metallic and bimetallic alloy NPs in the fulfillment of various applications in the future.

Fabricating a Homogeneously Alloyed AuAg Shell on Au Nanorods to Achieve Strong, Stable, and Tunable Surface Plasmon Resonances

“…In the other study, Yin's group fabricated fully alloyed AuAg nanospheres with good compositional homogeneity through the high temperature (≈1000 °C) annealing of SiO 2 ‐coated Au–Ag core–shell nanopolyhedrons. [20b] This method requires complex procedures (e.g., coating and removal of SiO 2 ) and harsh annealing conditions, but more importantly, it cannot produce alloyed nanocrystals with special shapes (e.g., rods, cubes) other than spheres because regardless of their original shape, at such high annealing temperatures, nanocrystals will fuse into spherical particles (see Figure S2, Supporting Information). Therefore, shape‐dependent SPR properties are yet unavailable for homogeneously alloyed nanocrystals synthesized by the existing methods, and their SPRs are confined within the characteristic wavelengths of the Au and Ag spheres (i.e., ≈380–520 nm) …”

“…However, these efforts have paid little attention to achieving compositional homogeneity in the crystals produced because they did not consider the chemical stability of Ag. Two recent studies indicated that uniform mixing of Au and Ag atoms (compositional homogeneity) in the nanocrystal is required to achieve the “stabilizing” effect and that Ag remains susceptible to oxidation if the two elements are segregated into large domains (results were verified in this work (vide infra)) . These two studies are currently the only successful examples of the preservation of both plasmonic activity and chemical stability by synthesizing homogeneously alloyed AuAg nanocrystals; however, their synthetic methods were limited to the production of isotropic (nearly spherical) nanocrystals.…”

“…These two studies are currently the only successful examples of the preservation of both plasmonic activity and chemical stability by synthesizing homogeneously alloyed AuAg nanocrystals; however, their synthetic methods were limited to the production of isotropic (nearly spherical) nanocrystals. In one of these studies,[20a] the combined use of NH 4 OH and NaBH 4 facilitated a uniform mixing of Au and Ag during coreduction, but the strong reducing power of NaBH 4 and the consequential fast reduction kinetics resulted in the formation of small round crystals (<10 nm) with a poor size distribution. In the other study, Yin's group fabricated fully alloyed AuAg nanospheres with good compositional homogeneity through the high temperature (≈1000 °C) annealing of SiO 2 ‐coated Au–Ag core–shell nanopolyhedrons.…”

Alloy Materials for Plasmonic Refractive Index Sensing

“…Chemical synthesis is just one example of a particularly rapid way of creating various plasmonic nanoparticles. (1)(2)(3)(4)(5)(6)(7) When estimating the optical properties of alloys, weighted averaging of the constants associated with the constituent pure metals is often attempted; however, the results are often inconsistent with observations. We first demonstrated the importance of experimental determinations of the optical constants/permittivity of metals (n = n + ik, ε = ε 1 + iε 2 = n 2 − k 2 + 2ink).…”

A halogen-free synthesis of gold nanoparticles using gold(III) oxide

“…The presence of halides in a reaction mixture can also influence the final composition of NPs, e.g. Au–Ag alloys (Rajendra et al 2015 ). Hence, we wondered whether gold oxide could be an alternative to gold chloride.…”