Disclosure of the imidazolium cation coordination and stabilization mode in ionic liquid stabilized gold(0) nanoparticles

Journal of Colloid and Interface Science 316 (2007) 189–195www.elsevier.com/locate/jcis

Disclosure of the imidazolium cation coordination and stabilization modein 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, Brazilb Laboratory of Technological Processes and Catalysis, Institute of Chemistry, UFRGS, Av. Bento Gonçalves 9500, P.O. Box 15003, CEP: 91501-970,

Porto Alegre, RS, Brazilc 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 reductionof 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 thesimultaneous coordination of the 1-triethylene glycol monomethyl ether-functionality. Instead of this, the ether-functionality was directed awayfrom the GNP surface and acted as steric barrier between the GNPs/GNP aggregates, thus preventing further aggregation. These new insightssuggest 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 areprepared in 1-butyl-3-methyl-imidazolium ILs with control ofsize, near-monodispersity, shape and stabilization, and appliedin 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: [email protected] (H.S. Schrekker),[email protected] (J.C. Rubim), [email protected] (J. Dupont).

0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2007.08.018

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 actualmode 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 thepresence of an IL protective layer surrounding the transition-metal NPs [9–11]. Interactions between IL anions, includingPF−

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 theprincipal factor for imidazolium RTIL stabilized transition-metal NPs [15,16]. However, no concrete information wasavailable until now about a possible stabilization role of imi-dazolium cations [5,6,8].

190 H.S. Schrekker et al. / Journal of Colloid and Interface Science 316 (2007) 189–195

Gold(0) nanoparticles (GNPs) have found many importantapplications in the research areas of catalysis, biology, andnanotechnology [17,18]. Catalytically active GNPs are receiv-ing special attention due to their ability to oxidize carbonmonoxide, alkanes, olefins, and alcohols with molecular oxy-gen as cheap and environmental friendly oxidant at low tem-peratures [19–22]. The direct preparation of GNPs in a cat-alytically active form, without the need of an activation pro-cedure [23–25], requires agglomeration- and poison-resistanceat the same time [26,27]. In general, transition-metal NPs arekinetically unstable and require stabilization, but GNPs arecharacterized by higher agglomeration tendencies. The stabi-lization strength of the standard 1-butyl-3-methyl-imidazoliumILs with noncoordinating anions seems to be insufficient toprevent agglomeration of GNPs [28]. However, attractive of im-idazolium ILs is that task specific ILs are easily accessible byfacile structural modifications [29–31]. GNPs are successfullyprepared in diethylene glycol dimethyl ether; however, inhibi-tion of agglomeration is not achieved [32]. This suggested tous that the balance between stability and catalytic activity ofGNPs in imidazolium ILs could be tuned by the introductionof ether-functionalities [33]. In this study, ether-functionalized[34] imidazolium IL 2 (Scheme 1) was synthesized and ap-plied in the preparation of GNPs. The surface-enhanced Ramanscattering (SERS) is a powerful tool in the characterization ofspecies adsorbed on nanostructured silver and gold surfaces[35,36], providing information of specific sites of the adsor-bate that are interacting with the SERS-active surface. A SERSstudy of the GNPs prepared in IL 2 disclosed both the coor-dination mode of the imidazolium cation and its stabilizationrole.

2. Materials and methods

2.1. Materials

Acetone was purchased from VETEC Química Fina LTDAand used without further purification. Hydrazine monohydrateand chloroauric acid trihydrate were used as purchased fromSigma–Aldrich.

Scheme 1. Synthesis of 1-triethylene glycol monomethyl ether-3-methylimid-azolium methanesulfonate.

2.2. Synthetic procedures

Gold(0) nanoparticlesAll reactions were carried out under an argon atmosphere

in dried glassware using standard Schlenk, syringe, and septatechniques. Chloroauric acid trihydrate (40 mg HAuCl4·3H2O/

mL 2) was dissolved in 2 under vacuum for 5 min at 25 ◦C.Hydrazine monohydrate (3.0 or 12.0 equiv.) was added to thesolution under an argon atmosphere at 25 ◦C, and the formationof GNPs and/or GNP aggregates occurred instantaneously. Thereaction mixture was stirred at 25 ◦C for the time mentioned andthe dispersions thus obtained were used for UV–vis and SERSmeasurements. Acetone (10 mL) was added to the reaction mix-ture in case the GNPs and/or GNP aggregates were isolated.The gold(0) was sedimentated by centrifugation (3500 rpm) for3 min and the liquid phase was removed. The GNPs and/or GNPaggregates were washed with acetone (2 × 10 mL) and vacuumdried at 25 ◦C for 4 h. The isolated gold(0) thus obtained wasused for XRD and SERS measurements.

2.3. Characterization

2.3.1. UV–visUV–visible absorption spectra were recorded on a Varian

Cary 100 UV–visible spectrophotometer. Sample preparation ofthe GNP dispersion involved its dilution in 2.

2.3.2. X-ray diffraction (XRD)The phase structures of GNPs prepared in 2 were charac-

terized by XRD. For the XRD analysis, the nanoparticles wereisolated as a powder and placed in the sample holder. The XRDexperiments were carried out on a SIEMENS D500 diffrac-tometer equipped with a curved graphite crystal using CuKα

radiation (λ = 1.5406 Å). The diffraction data were collectedat room temperature in a Bragg–Brentano θ–2θ geometry us-ing a curved graphite crystal as monochromator. The equipmentwas operated at 40 kV and 17.5 mA with a scan range between20◦ and 80◦. The diffractograms were obtained with a constantstep, �2θ = 0.05. The indexation of Bragg reflections was ob-tained by a pseudo-Voigt profile fitting using the FULLPROFcode [37].

2.3.3. Transmission electron microscopy (TEM)TEM of GNPs was performed on a JEOL JEM-2010 mi-

croscope operating at accelerating voltages of 200 kV with anominal resolution of 0.25 nm. Sample preparation included thedilution of two drops of GNP dispersion in 2.0 mL isopropanol,followed by deposition on a holey-carbon grid at room temper-ature.

2.3.4. Raman and SERSThe Raman and SERS spectra were excited with a 1064 nm

line from a Bruker Nd:YAG laser operated at 150 mW (120 mWfor the isolated GNPs) and the backscattered radiation was col-lected by an objective coupled to an Equinox 55 Bruker spec-trometer equipped with a FT-Raman accessory using a liquidN2 cooled Ge detector. The final Raman spectrum of 2 and the

H.S. Schrekker et al. / Journal of Colloid and Interface Science 316 (2007) 189–195 191

Table 1Preparation of GNPs in 2 by the hydrazine reduction of chloroauric acid at25 ◦Ca

Entry H2N–NH2 (equiv.)b Time (h) Yield (%)c

1 3.0 1.0 352 3.0 72.0 403 12.0 1.0 98

a HAuCl4·3H2O (40 mg/mL 2), and ±2.0 mL 2.b Equivalents.c Isolated yield; under the assumption that the isolated GNPs contain only

gold(0).

SER spectrum of the isolated GNPs correspond to the averageof 1280 scans, while the SER spectrum of the GNP dispersion 2was the result of 2560 scans. In all cases the nominal resolutionwas set to 4 cm−1.

3. Results and discussion

Room temperature ionic liquid 2 (Scheme 1) was synthe-sized in a straightforward manner and high yield, using a simpleand practical method for the preparation of halide-free ionic liq-uids [38]. Alkylating agent 1 was obtained by the treatmentof triethylene glycol monomethyl ether with methanesulfonylchloride. The reaction of 1 with 1-methylimidazole resulted inthe formation of 2.

Ether-functionalized IL 2 was applied as medium for thepreparation of GNPs [39]. A solution of chloroauric acid (40mg/mL 2) in 2 was treated with hydrazine as reducing agentat 25 ◦C (Table 1). The reaction time and amount of hydrazinewere varied.

A moderate yield of 35% was obtained after a reaction of1 h with 3.0 equivalents hydrazine (entry 1). Almost no furtherreaction took place after 1 h as a similar yield was obtainedafter 72 h (entry 2). UV–vis suggested an incomplete conver-sion of chloroauric acid as the typical plasmon surface band ofGNPs was not observed in 2 after isolation of the GNPs [18].Indeed, a dramatic increase of the yield to 98% was observedwhen chloroauric acid was reduced with 12.0 equivalents hy-drazine (entry 3). The yellow chloroauric acid solutions in 2turned immediately to homogeneous deeply bluish to blackGNP dispersions after the addition of hydrazine. Sedimenta-tion of the GNPs occurred slowly after 24 h. Interestingly, theGNPs were easily redispersed, which signified that no fully ag-glomerated bulk gold(0) was formed upon sedimentation. Fur-thermore, this demonstrated the improved stabilization capacityof RTIL 2 compared to the 1-butyl-3-methylimidazolium basedRTILs [28].

The X-ray diffraction (XRD) pattern of isolated GNPs (Ta-ble 1, entry 3) showed the formation of pure gold(0) (Fig. 1).The peaks of the scattering angles 2θ at 38.2◦, 44.4◦, 64.6◦,and 77.6◦, correspond to the gold(0) (111), (200), (220), and(311) planes, respectively. The most representative reflectionsof gold(0) were indexed as face-centered cubic (fcc). A meandiameter of 7.5 nm was estimated from the XRD pattern bymeans of the Debye–Scherrer equation calculated from full

Fig. 1. X-ray diffraction pattern of isolated GNPs (Table 1, entry 3).

Fig. 2. UV–vis absorption spectrum of GNPs/GNP aggregates in 2 before iso-lation.

width at half-maximum (fwhm) of the (111), (200), (220), and(311) planes obtained after Rietveld’s refinement.

In general, GNPs with mean diameters of 5–20 nm havea visible absorption maximum around 520 nm caused by thesurface plasmon band [18]. In this case, a very broad surfaceplasmon band was observed in the UV–vis spectrum, whichindicated the formation of GNP aggregates (Fig. 2) [18]. Fur-thermore, the absorption band showed a tail that extended tothe near infrared (NIR) region, which suggested the presenceof larger aggregates that usually display SERS-activity in theNIR [40]. Note that the mentioned GNP aggregates are notthe same as fully agglomerated bulk gold(0). Apparently, ether-functionalized RTIL 2 is capable of stabilizing these GNP ag-gregates.

Transmission electron microscopy experiments were per-formed in an attempt to obtain a representative picture of theGNP stabilization in RTIL 2. Unfortunately, these attemptswere unsuccessful due to the following reasons: (1) RTIL 2is highly viscous, which complicated the formation of a thinfilm that is necessary to obtain good quality TEM micrographs.(2) The ether-functionalized RTIL 2 stabilized GNPs are im-

192 H.S. Schrekker et al. / Journal of Colloid and Interface Science 316 (2007) 189–195

miscible with apolar organic solvents. (3) Organic solvents witha higher polarity allowed the initial dispersion of the GNPs, butsedimentation due to agglomeration followed rapidly. Substitu-tion of RTIL 2 by polar solvent molecules on the GNP surfacesprovokes the formation of bulk gold(0). Interestingly, isolatedregions with stabilized GNPs and GNP aggregates were ob-served by TEM (see Fig. S8 of Supplementary material) andthe diameters of the individual GNPs are in close proximityof the XRD value of 7.5 nm. Although the exact nature of theformed GNPs (individual GNPs and/or GNP aggregates) is notconclusive, it is clear that RTIL 2 has an improved stabilizationstrength compared to the 1-butyl-3-methylimidazolium basedRTILs [28], which made it possible to study the imidazoliumcation coordination and stabilization mode of the RTIL 2 stabi-lized GNPs and/or GNP aggregates by SERS.

The Raman spectrum of RTIL 2 is presented in Fig. 3 (a), andthe SERS spectra of a GNP dispersion in 2 and that of isolatedGNPs prepared in 2 are displayed in Fig. 3 (b) and (c), respec-tively. Table 2 presents the observed Raman wavenumbers, theirrelative intensities and a tentative vibritional assignment. Sincethere is no available vibrational assignment for RTIL 2, the pre-sented tentative assignment was based on the vibrational assign-ments for parent species like the 1-butyl-3-methylimidazoliumcation [41–44]. Literature data were used to assign the vibra-tions associated with the methanesulfonate anion [45,46]. Forcomparison purposes Table 2 also presents the SERS signalsfor the 1-butyl-3-methylimidazolium cation adsorbed on a sil-ver electrode at the potential of −1.6 V vs a Pt quasi referenceelectrode [41].

The relative intensities of the spectra presented in Fig. 3(a) and (b) were adjusted for comparison reasons in order tohave the ν(C–S) mode of the anion at 769 cm−1 with equalintensity [45]. A remarkable strong and broad signal centeredat 276 cm−1 was observed in the SERS spectrum of the GNPdispersion, together with a shoulder near 291 cm−1 Fig. 3 (b).The SERS spectra of nitrogen containing species adsorbed onSERS-active gold surfaces are characterized by a broad Ra-man signal in the 255–290 cm−1 region that is assigned to theν(Au–N) stretching mode [47,48]. Therefore, the broad fea-ture near 276 cm−1 was assigned to the ν(Au–N) stretchingmode, characterizing the interaction of the Au surface with at

least one of the two nitrogen atoms in the imidazolium ring.Furthermore, vibrational modes related to motions of atoms inthe imidazolium ring or directly attached to it, e.g., the imida-zolium CH stretchings at 3099 and 3162 cm−1, the νring at 1387cm−1, and the ν(N–CH3) stretching at 1565 cm−1, had their in-tensities enhanced relative to the reference 769 cm−1 signal.Note that this occurred with a simultaneous shift of these vibra-tional modes to lower wavenumbers as compared to the Ramanwavenumbers of RTIL 2. These results suggest that the imi-dazolium ring interacted with the GNP surface via a parallelcoordination mode (Fig. 4). In contrast to this, the intensities ofthe vibrational modes associated with the MeSO−

3 anion wereabsent or weak in the SERS spectra of the GNPs. Furthermore,even when observed these modes were not shifted. This sug-gests that there was almost no interaction of the MeSO−

3 anionwith the GNP surface. These results are consistent with a SERSstudy of the 1-butyl-3-methylimidazolium cation adsorbed on asilver electrode; (1) imidazolium vibrational modes were alsopreferentially enhanced as well as shifted to lower wavenum-bers; (2) the enhancement of the PF−

6 vibrational modes wasminimal and without a shift on its wavenumber position [41].

A precise determination of the stabilization mode of transi-tion-metal NPs requires a careful evaluation of all possibilities[6,8]. The presence of an oxide layer as source of stabilitywas ruled out due to the obtained SERS results as gold oxidescause the passivation of the SERS-active sites, which was notthe case [49]. Another excluded possibility was the GNP sta-bilization by N-heterocyclic carbenes as SERS did not showevidence for such species. However, the disclosed parallel co-ordination mode of the imidazolium cation seems to providea plausible explanation why H/D and D/H exchange reactionsoccurred at the three possible C-2, C-4 and C-5 imidazoliumpositions and not just one C position, which proved the involve-ment of surface-ligand-coordinated N-heterocyclic carbenes foriridium(0) NPs prepared in imidazolium ionic liquids [6,50].

Measurements of the zeta potential for gold colloids suggestthat GNPs prepared by chemical reduction processes, includ-ing hydrazine as in the present case, have negatively chargedsurfaces [51]. Such a negative charge then could promotethe interaction of the imidazolium cation with the gold sur-face, which could suggest a cation-only mode of stabilization

Fig. 3. (a) Raman spectrum of 2; (b) and (c) SERS spectra of a GNP/GNP aggregate dispersion in 2 and of isolated GNPs/GNP aggregates prepared in 2, respectively.

H.S. Schrekker et al. / Journal of Colloid and Interface Science 316 (2007) 189–195 193

Table 2Raman wavenumbers and relative intensities as observed in the Raman spectrum of RTIL 2 and in the SERS spectra of GNPs/GNP aggregates dispersed in 2 andisolated with the corresponding tentative vibrational assignmenta

Raman RTIL 2 SERS GNP dispersion GNP isolated SERS b BMIPF6/Ag at −1.6 V Raman MeSO−3

c Tentative assignment

180 (w) ν(Au–Au)223 (m, br) ν(Ag–Ag) and/or ν(Ag–N)

276 (vvs, br) 276 (m, br) ν(Au–N)291 (sh)

605 (w) 602 (w) 606 (w) CH3(N) str, CH2(N) str, ring op bendd

769 (m) 769 (m) Not observed 779 (m) ν(C–S)c

1023 (m) 1023 (m) 1023 (sh) 1026 (ms) δ ringd

1041 (vs) 1041 (s) 1039 (w) 1057 (vs) ν(S–O)c

1340 (w) 1352 (w) 1352 (m) 1348 (s) ν ringd

1389 (w) 1387 (s) 1387 (vs) 1384 (ms) ν ringd

1421 (m) 1416 (s) 1423 (s) 1420 (s) δ(CH3)e, δsym(CH3)d

1452 (w) 1453 (w) 1454 (sh)1475 (w) 1478 (w)1569 (vw) 1565 (s) 1565 (s) 1564 (ms) CH3(N) str, CH2(N) str, ring ip asym strf

2831 (m) 2827 (m)2876 (sh) 2878 (s) νsym(N–CH3)g

2886 (m) 2904 (vs) νasym(N–CH3)g

2939 (vvs) 2935 (vs) 2932 (s) 2935 (vs) 2941 ν(C–H)e

2964 (vs) 2967 (vs) ν(CH2)—ethylene glycol3009 (m) 3021 ν(C–H)e—MeSO−

33107 (vw) 3099 (vw) 3099 (w) 3107 (vw) νasym(HCCH)f—imidazolium3167 (vw) 3162 (vw) 3162 (m) 3174 (w) νsym(HCCH)d—imidazolium

a Abbreviations: vw = very weak; w = weak; m = medium; ms = medium strong; s = strong; vs = very strong; vvs = very very strong; br = broad; sh =shoulder; str = stretching; op = out of plane; ip = in plane; sym = symmetric; asym = asymmetric.

b Ref. [41].c Ref. [45].d Ref. [42].e Ref. [46].f Ref. [43].g Ref. [44].

Fig. 4. Imidazolium cation coordination and stabilization mode.

due to the absence of an interaction between the MeSO−3 an-

ion and GNP/GNP aggregate surface [8]. However, an anionelectrostatic/Derjaugin–Landau–Verwey–Overbeek (DLVO)-type stabilization by Cl− anions could not be ruled out aschloroauric acid was used as GNP precursor [8]. The broadSERS feature near 276 cm−1 may also contain a contributionfrom the ν(Au–Cl) stretching mode and could also account forthe adsorption of Cl− anions on the GNP/GNP aggregate sur-face [47,48]. Two possible stabilization features that could notbe excluded are: (1) The involvement of hydrazine or hydrazinerelated species, and (2) the role of possible water impurities thatcould originate from the IL. Independent of these possibilities,the interaction between the imidazolium cation and GNP/GNPaggregate surface is only indirectly responsible for the stabiliza-tion of the same in RTIL 2. The neutralization of the negativelycharged GNPs would favor the van der Waals attractive forces,

causing the nanoparticles to aggregate [52]. Further analysesof the SERS spectra provided previously unknown informa-tion about the mode of transition-metal NP stabilization byionic liquids. Note that the very strong Raman feature observedat 2964 cm−1 and assigned to the ν(CH2) stretching modeof the 1-triethylene glycol monomethyl ether-functionality ofthe imidazolium cation was not surface-enhanced, suggest-ing that the long ether tail was far away from the GNP/GNPaggregate surface as compared to the imidazolium ring. Fur-thermore, no Raman signal characteristic for the ether tail wasobserved in the SERS spectra (Fig. 3 (b) and (c)). This pro-vided a strong evidence that the ether tail was directed awayfrom the GNP/GNP aggregate surface (Fig. 4), which is inagreement with the previously observed arrangement of imida-zolium ILs at an electrode surface [41,53]. As a consequence,this increases the mean distance between the GNPs/GNP ag-gregates, resulting in a strongly diminished attractive van derWaals forces as these decay exponentially with the distance be-tween the NPs [52]. As a consequence, the GNP aggregatesthat gave origin to the observed SERS effect remained stablein RTIL 2 due to this steric stabilization component of the im-idazolium cation. This arrangement of the tail could also berelated to the reduced mean diameter, narrowed size distribu-tion, and increased shape regularity, of nickel(0) NPs preparedin 1-alkyl-3-methylimidazolium ILs upon an increase in lengthof the alkyl-functionality [6].

194 H.S. Schrekker et al. / Journal of Colloid and Interface Science 316 (2007) 189–195

Obviously, the imidazolium cation is a decisive stabilizationfactor for the GNPs/GNP aggregates prepared in 2, which isresponsible for two stabilization modes: (1) The steric stabiliza-tion component due to the arrangement of the tail attached to theimidazolium ring. (2) The cation stabilization component viathe parallel coordinated imidazolium ring. Further studies needto be performed to elucidate the possibility of a cation-only sta-bilization or if for instance the present Cl− anions are responsi-ble for an anion electrostatic/DLVO-type stabilization compo-nent. However, independent of this an electrosteric stabilizationmode seems to be the correct classification for the GNPs/GNPaggregates stabilized by imidazolium RTIL 2 [5,6,8].

4. Conclusions

GNP aggregates were prepared in ether-functionalized imi-dazolium RTIL 2 as characterized by absorptions extending tothe NIR region. This made it possible to study the interactionsbetween RTIL 2 and the GNP surface by SERS, which sug-gested the presence of an ionic liquid protecting layer aroundthe GNPs/GNP aggregates. Interpretation of the SERS resultsshowed the interaction of the imidazolium cation with the GNPsurface via a parallel coordination as the imidazolium ring vi-brational modes were selectively enhanced and shifted to lowerwavenumbers. In contrast to this, almost no interaction of theMeSO−

3 anion with the GNP surface was observed. The stabil-ity of these GNP aggregates in RTIL 2 is attributed to the wayRTIL 2 adsorbs on the GNP surface, i.e., with the 1-triethyleneglycol monomethyl ether tail projected away from the surface.As such, the tail acts as a barrier increasing the mean distancebetween the GNP aggregates, thus preventing further aggrega-tion. As a consequence, an electrosteric stabilization mode isproposed for imidazolium RTIL 2 stabilized GNPs/GNP aggre-gates. In general, these new insights show the importance tostudy the role of the imidazolium cation in order to get a fullunderstanding of the stabilization nature of imidazolium RTILstabilized transition-metal NPs.

Acknowledgments

The authors thank the CNPq and CAPES for financial sup-port. H.S.S. thanks the CNPq for a visiting scientist fellowship.

Supplementary material

The online version of this article contains additional sup-plementary material: Synthesis, characterization, 1H NMR and13C NMR of 1 and 2; DSC and TGA of 2; Rietveld’s refinement(XRD) and TEM micrograph of GNPs.

Please visit DOI: 10.1016/j.jcis.2007.08.018.

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