Journal of Colloid and Interface Science 316 (2007) 189–195 www.elsevier.com/locate/jcis Disclosure of the imidazolium cation coordination and stabilization mode in ionic liquid stabilized gold(0) nanoparticles Henri S. Schrekker a,b,∗ , Marcos A. Gelesky a , Marcelo P. Stracke a , Clarissa M.L. Schrekker a , Giovanna Machado a , Sergio R. Teixeira c , Joel C. Rubim d,∗ , Jairton Dupont a,∗ a Laboratory of Molecular Catalysis, Institute of Chemistry, UFRGS, Av. Bento Gonçalves 9500, P.O. Box 15003, CEP: 91501-970, Porto Alegre, RS, Brazil b Laboratory of Technological Processes and Catalysis, Institute of Chemistry, UFRGS, Av. Bento Gonçalves 9500, P.O. Box 15003, CEP: 91501-970, Porto Alegre, RS, Brazil c Institute of Physics, UFRGS, Av. Bento Gonçalves 9500, P.O. Box 15051, CEP: 91501-970, Porto Alegre, RS, Brazil d Laboratory of Materials and Fuels, Institute of Chemistry, Universidade de Brasília, P.O. Box 04478, CEP: 70904-970, Brasília, DF, Brazil Received 26 June 2007; accepted 9 August 2007 Available online 14 August 2007 Abstract A surface-enhanced Raman spectroscopy (SERS) study of imidazolium ionic liquid stabilized gold(0) nanoparticles (GNPs) furnished previ- ously unknown knowledge about the coordination and stabilization mode of the imidazolium cation. GNPs were prepared by hydrazine reduction of a chloroauric acid solution in 1-triethylene glycol monomethyl ether-3-methylimidazolium methanesulfonate 2 as ether-functionalized room- temperature ionic liquid (RTIL). UV–vis spectroscopy showed the presence of GNP aggregates as absorptions extended to the NIR region. A parallel coordination mode for the imidazolium cation of RTIL 2 on the GNP surface was observed by SERS, which occurred without the simultaneous coordination of the 1-triethylene glycol monomethyl ether-functionality. Instead of this, the ether-functionality was directed away from the GNP surface and acted as steric barrier between the GNPs/GNP aggregates, thus preventing further aggregation. These new insights suggest that the imidazolium cation is responsible for electrosteric stabilization.  2007 Elsevier Inc. All rights reserved. Keywords: SERS; Transition-metal nanoparticles; Gold; Electrosteric stabilization; Interface; Ether-functionalized imidazolium ionic liquids 1. Introduction Imidazolium ionic liquids (ILs) have proven to be a valu- able medium for organometallic catalysis [1–4] and prepara- tion of catalytically active transition-metal nanoparticles (NPs) [5–8]. For instance, platinum, rhodium and iridium NPs are prepared in 1-butyl-3-methyl-imidazolium ILs with control of size, near-monodispersity, shape and stabilization, and applied in catalysis without the need of an activation procedure [6]. Excellent catalytic activities are achieved in the hydrogena- tion of olefins and benzene under both solventless and multi- phasic conditions. This suggests that the IL does not occupy * Corresponding authors. Faxes: +55 51 3308 7304 (H.S. Schrekker, J. Dupont), +55 61 273 4149 (J.C. Rubim). E-mail addresses: schrekker@iq.ufrgs.br (H.S. Schrekker), jocrubim@unb.br (J.C. Rubim), dupont@iq.ufrgs.br (J. Dupont). all the coordination sites on the metal surface or that sub- strates are able to remove and exchange with the weakly co- ordinated IL. In spite of this, little is known about the actual mode of stabilization for transition-metal NPs in ILs [6,8]. Transmission electron microscopy (TEM) and small-angle X- ray scattering (SAXS) studies offered strong evidence for the presence of an IL protective layer surrounding the transition- metal NPs [9–11]. Interactions between IL anions, including PF − 6 and BF − 4 , and transition-metal NP surfaces were detected by X-ray photoelectron spectroscopy (XPS) and X-ray absorp- tion spectroscopy (EXAFS) measurements [11–14]. As a con- sequence, an anion electrostatic/Derjaugin–Landau–Verwey– Overbeek (DLVO)-type stabilization was assumed to be the principal factor for imidazolium RTIL stabilized transition- metal NPs [15,16]. However, no concrete information was available until now about a possible stabilization role of imi- dazolium cations [5,6,8]. 0021-9797/$ – see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.08.018