A general phase transfer protocol for synthesizing alkylamine-stabilized nanoparticles of noble metals

https://doi.org/10.1016/j.aca.2007.01.061Get rights and content

Abstract

The ethanol-mediated phase transfer protocol was extended herein to prepare alkylamine-stabilized nanoparticles of several noble metals by transferring them from aqueous environment into toluene. This method relies on the use of ethanol as a mediator to provide and maintain adequate contact between dodecylamine and metal nanoparticles during the transfer process and involves first mixing the metal hydrosols and an ethanol solution of dodecylamine and then extracting the dodecylamine-stabilized metal nanoparticles into toluene. Alkylamine-stabilized Ag, Pd, Rh, Ir and Os nanoparticles with 7.09, 3.45, 6.89, 2.42 and 4.52 nm in diameter, respectively, could be prepared this way. The self-assembly of dodecylamine-stabilized Ag and Rh nanoparticles was also detected by transmission electron microscopy (TEM).

Introduction

Metal nanoparticles have also attracted considerable interest, in fields such as optics and catalysis, because of their size- and shape-dependent physicochemical properties [1], [2], [3], [4], [5]. Various procedures have been developed for preparing nanoparticles with controlled sizes and shapes [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Among such procedures, those that require an aqueous medium are the most convenient to develop because of water's ability to solubilize a variety of ions and stabilizing molecules. On the other hand, nanoparticles prepared in organic media are interesting for, among other things, applications to catalytic processes and for further surface modifications with organic functional groups to fine-tune their properties [4], [12], [13], [14].

Nanoparticles of different sizes and shapes can be obtained in organic media either by preparing them directly in those media [11] or by transferring nanoparticles from aqueous phases to organic phases [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34]. The latter has the advantage that there are many well-developed procedures available for the preparation of well-defined metal nanoparticles in aqueous media [35], [36]. Phase transfer of metal nanoparticles from an aqueous phase to an organic phase can be achieved by capping the nanoparticles with thiols and amines [15] or by using phase-transfer reagents, such as surfactants, to improve the solubility of nanoparticles in organic media [16].

We have developed a phase transfer method with high efficiency to transfer metal (Au, Pt and Ru) nanoparticles initially stabilized by citrate from aqueous environment to a hydrocarbon layer [37]. Briefly, this method involved first mixing the metal hydrosols and an ethanol solution of dodecylamine and then extracting the dodecylamine-stabilized metal nanoparticles into toluene, in which the ethanol, a water miscible and good solvent for dodecylamine, was used as an intermediate solvent to improve the interfacial contact between citrate-stabilized metal nanoparticles and alkylamine. The extraction of dodecylamine-stabilized metal nanoparticles to the toluene layer, indicated by a vivid transfer of color from the aqueous phase to toluene, would then proceed quickly and completely to leave a colorless aqueous solution behind. Herein, we extend this phase transfer method to prepare the alkylamine-stabilized nanoparticles of other noble metals including Ag, Pd, Rh, Ir and Os, of which reports on the preparation of Ir and Os nanopaticles still remain scarce in the literature [38], [39].

Section snippets

Materials and methods

AgNO3, PdCl2, RhCl3, IrCl4, OsCl3 and dodecylamine (98%) purchased from Aldrich, sodium borohydride (NaBH4, 98%) from Fluka, tri-sodium citrate (98%) and ethanol (99%) from Merck and toluene from J.T. Baker, Inc., were used as received. Deionized water was distilled by a Milli-Q water purification system. All glassware and Teflon-coated magnetic stir bars were cleaned with aqua regia, followed by copious rinsing with distilled water before drying in an oven.

For the preparation of

Results and discussion

Similar to the results we reported in our previous work [37], citrate-stabilized metal nanoparticles (Ag, Pd, Rh, Ir and Os) could not be transferred directly to toluene by mixing the metal hydrosol together with a toluene solution of dodecylamine. Prolonged stirring only produced a milky mixture of metal hydrosol and toluene, but no particle transfer took place after the mixture was settled down into two immiscible layers in a separating funnel. The metal particles would stay at the interface

Conclusions

The ethanol-mediated phase transfer method we developed previously was extended here to prepare alkylamine-stabilized Ag, Pd, Rh, Ir and Os nanoparticles by transferring them from aqueous environment into toluene. This method relies on the use of ethanol as a mediator to provide and maintain adequate contact between dodecylamine and metal nanoparticles during the transfer process. The particle size distributions of dodecylamine-stabilized Ag, Pd, Rh, Ir and Os nanoparticles thus obtained were

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