Synthesis and Characterization of Au@Co and Au@Ni Core−Shell Nanoparticles and Their Applications in Surface-Enhanced Raman Spectroscopy

Abstract

Au@Co and Au@Ni core−shell nanoparticles with controllable shell thicknesses were prepared by reduction of Co2+ and Ni2+ salts with hydrazine hydrate in ethanol over preformed Au seeds. Both cyclic voltammetric and surface-enhanced Raman studies using CO as the probe molecule confirmed the core−shell structure of the synthesized nanoparticles. The Au@Co and Au@Ni nanoparticles dispersed on a glassy carbon electrode surface exhibit high surface-enhanced Raman scattering (SERS) effect for the adsorbed pyridine. High-quality SERS spectra of CO absorbed on Co and Ni have been obtained through the high enhancement of the core−shell nanoparticles. The originally low surface enhancement of the Co and Ni can be substantially improved giving total enhancement factors up to 103-104. Such a SERS-active substrate can be used as an alternative substrate for investigating adsorption and reactions occurring on the Co and Ni metal surfaces.

Coating matters: the influence of coating materials on the optical properties of gold nanoparticles

“…Besides the group 11 metals (coinage metals) such as copper and silver, the group 10 elements such as nickel, palladium and platinum have also been successfully employed ( Figure 9 ). Other unusual metal coatings comprising single metal coatings such as cobalt Au/Co [187] and tin (Au@Sn) [188] or multishell coatings such as Ag/Hg [189] or Pb/Cd [190] coatings have been reported too. Apart from the elemental metal coatings, also metal oxides of titanium (Au@TiO 2 ) [191] , iron (Au@ Fe 2 O 3 ) [192] and nickel (NiO) [164] , metal-semiconductors consisting of metal halogenides (Au@CuI) [193] , metal sulfi des (Au@CdS) [193] and metal selenides (Au@CdSe) [194] have been successfully coated on gold nanoparticles in coreshell structure.…”

“…For example, Sanchez-Iglesias et al have shown that the plasmon resonance of an Au nanospheres was still evident after coating with up to 8 nm of Ni and NiO [164] . Other metals, different to coinage or platinum group metals, such as lead [190] or cobalt [187,218] have also been deposited on gold nanoparticles. Lead deposited on gold nanoparticles also caused blue-shift of the Au plasmon band and three monolayers of Pb were suffi cient to produce the plasmon band of pure lead [190] .…”

“…Lead deposited on gold nanoparticles also caused blue-shift of the Au plasmon band and three monolayers of Pb were suffi cient to produce the plasmon band of pure lead [190] . On the other hand, little is known about the effect of cobalt on the optical properties of gold, due to the dispersion instability as a result of magnetic properties of cobalt [187] . Multishell coatings of gold nanoparticles with more than one type of metal have also been reported, and the outcome strongly depends on the nature of employed metals, and the type of structure they build on the gold cores.…”

First-principles modeling of magnetic misfit interfaces

“…Exciting properties (e.g., optical, electrical, and catalytic) of systems built of magnetic and nonmagnetic materials are well known from nanoparticles. [4][5][6][7] Despite their potential for applications, the physical properties of magnetic/nonmagnetic interfaces, such as the origin of the perpendicular magnetic anisotropy, are not well understood, mainly because they depend on external factors such as the interface quality and the film thickness in thin-film geometries. A change of structural details can strongly alter the electronic states, which in turn influences the magnetic properties.…”

Structure and composition of Au/Co magneto-plasmonic nanoparticles

“…[16] There exist several studies on the synthesis and properties of Au-Ni and Au-Co NPs. [17][18][19][20][21][22][23] However, thiolate protected Au/Co NPs have not been reported so far. In general terms, segregation of Au toward the surface is predicted, considering only the metal-metal interaction, i.e., neglecting the effect of the surfactant molecules.…”