Abstract
The mechanism of the Brust-Schiffrin gold nanoparticle synthesis has been investigated through the use of ion transfer voltammetry at the water/1,2-dichloroethane (DCE) solution interface, combined with X-ray absorption fine structure (XAFS) of the reaction between [AuCl4](-) and thiol (RSH) in homogeneous toluene (TL) solution. Ion transfer calculations indicate the formation of [AuCl2](-) at RSH/Au ratios from 0.2-2 with a time-dependent variation observed over several days. At RSH/Au ratios above 2 and after time periods greater than 24 h, the formation of Au(I)SR is also observed. The relative concentrations of reaction products observed at the liquid/liquid interface are in excellent agreement with those observed by XAFS for the corresponding reaction in a single homogeneous phase. BH4(-) ion transfer reactions between water and DCE indicate that the reduction of [AuCl4](-) or [AuCl2](-) to Au nanoparticles by BH4(-) proceeds in the bulk organic phase. On the other hand, BH4(-) was unable to reduce the insoluble [Au(I)SR]n species to Au nanoparticles. The number and size of the nanoparticles formed was dependent on the concentration ratio of RSH/Au, as well as the experimental duration because of the competing formation of the [Au(I)SR]n precipitate. Higher concentrations of nanoparticles, with diameters of 1.0-1.5 nm, were formed at RSH/Au ratios from 1 to 2.
Highlights
The Brust−Schiffrin method is the earliest reported phase transfer approach to prepare thiol-stabilized metal nanoparticles.[1]. In this two-step approach, the gold ions from an aqueous solution are first extracted to a hydrocarbon
Subsequent reduction reactions in the organic solution, using a second aqueous solution of NaBH4 in the presence of an alkanethiol added as a capping agent, yield
In early studies of the Brust−Schiffrin process, when the precursor question was addressed, researchers generally assumed the formation of Au(I) thiolate polymers[9,10] in both the one- and two-phase reactions, with evidence that the thiol species functioned as reducing agents, forming Au(I) from Au(III).[3]
Summary
The Brust−Schiffrin method is the earliest reported phase transfer approach to prepare thiol-stabilized metal nanoparticles.[1]. Subsequent reduction reactions in the organic solution, using a second aqueous solution of NaBH4 in the presence of an alkanethiol added as a capping agent, yield. One of the specific questions has been the identification of the precursor species present in solution prior to reduction with NaBH4. In early studies of the Brust−Schiffrin process, when the precursor question was addressed, researchers generally assumed the formation of Au(I) thiolate polymers[9,10] in both the one- and two-phase reactions, with evidence that the thiol species functioned as reducing agents, forming Au(I) from Au(III).[3] Recently, a revised view of Brust−Schiffrin nanoparticle syntheses has been pioneered by Goulet and Lennox,[11] which indicates that the thiol (RSH) behaves solely as a reductant for [AuX4]− before the addition of NaBH4, with no Au−thiol bonding observed. Dodecanethiol was shown to be precursors of one-phase reactions conducted in organic solvents (usually TL) as in eq 1: TOA+[AuX4]− + 2RSH
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