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A novel approach to metal and metal oxide nanoparticle synthesis: the oil-in-water microemulsion reaction method

  • Margarita Sanchez-Dominguez
  • Magali Boutonnet
  • Conxita Solans
Brief Communication

Abstract

A novel and straightforward approach, based on oil-in-water (o/w) microemulsions, was developed for the synthesis of inorganic nanoparticles at ambient conditions. It implies the use of organometallic precursors dissolved in nanometre-scale oil droplets of o/w microemulsions. Addition of reducing or oxidizing/precipitating agents results in the formation of metallic or metal oxide nanoparticles, respectively. Nonionic o/w microemulsion systems were chosen, and several key compositions were selected for nanoparticle synthesis at 25 °C. High Resolution Electron Microscopy revealed that small nanoparticles of metals (Pt, Pd and Rh) and nanocrystalline metal oxide (cerium (IV) oxide with cubic type crystalline structure confirmed by XRD), of less than 7 nm can be obtained in mild conditions.

Keywords

o/w Microemulsion Nanoreactor Nanoceria Pt-nanoparticles Rh-nanoparticles Pd-nanoparticles Nanomanufacturing 

Introduction

The use of colloidal self-assemblies as confined reaction media for nanoparticle synthesis is an important tool for the development of materials with controlled size and shape. Microemulsions, one kind of colloidal systems used for this matter, are transparent and thermodynamically stable dispersions in which two liquids initially immiscible (typically water and oil) coexist in one phase due to the presence of a monolayer of surfactant molecules (Lindman and Friberg 1999). Depending on the ratio of oil and water and on the hydrophilic–lipophilic balance (HLB) of the surfactant (s), microemulsions can exist as oil-swollen micelles dispersed in water (oil-in-water or o/w microemulsions), or water-swollen inverse micelles dispersed in oil (water-in-oil or w/o microemulsions); at intermediate compositions, microemulsions with both aqueous and oily continuous domains can exist as interconnected sponge-like channels (bicontinuous microemulsions). As a result of their small droplet size (2–100 nm), discrete type microemulsions (o/w and w/o) are subject to continuous Brownian motion; collisions between micelles are hence frequent, leading to the formation of transient dimmers and continuous exchange of the interior of the droplets. These dynamic properties facilitate their use as confined reaction media. In the pioneering research of Boutonnet et al. (1982) w/o microemulsions were used as nano-reactors for the synthesis of metallic nanoparticles. Since then, an increasing interest in this approach has been observed, given the advantages that this method offers such as control of the size, shape, and composition of the nanomaterials, as well as the use of simple equipment and, in general, soft reaction conditions. In addition, the presence of surfactant molecules can protect nanoparticles against agglomeration. Many kinds of nanomaterials, such as metallic, single and mixed metal oxide nanoparticles, quantum dots, and even more complex ceramic materials and metals coated by metal oxide and vice versa have been prepared by this technique (Boutonnet et al. 2008; Cushing et al. 2004; Destrée and Nagy 2006; Eastoe et al. 2006; Holmberg 2004; López-Quintela et al. 2004; Pileni 1997). Materials prepared in w/o microemulsions often exhibit unique surface properties; for example, nano-catalysts prepared by this method have been found to display better performance (activity, selectivity) than those prepared by other methods (Boutonnet et al. 2008).

Given that inorganic salt precursors are generally water soluble, w/o microemulsions are convenient for the preparation of such materials. However, the use of this type of microemulsions requires high amounts of solvent (oil), hindering its applications at the industrial scale (Boutonnet et al. 2008). Hence, from a practical and environmental point of view, the possibility of preparing inorganic nanoparticles using o/w microemulsions could be highly advantageous, since the major (continuous) phase is water. Thus far, o/w microemulsions have been used for the preparation of lipid (Cavalli et al. 1998) and polymeric (Hentze and Kaler 2003) nanoparticles, as well as for organic (Holmberg 2003) and enzymatic (Larsson et al. 1991) reactions, and as formulations (Lawrence and Rees 2000; Dörfler and Große 1996). In this study, a novel approach was explored as proof of concept for the synthesis of metal and metal oxide nanoparticles. The method consists in the use of organometallic precursors, dissolved in nanometer-scale oil droplets of o/w microemulsions (Scheme 1). To the best of our knowledge, this approach has not been previously reported. The results obtained for the first time demonstrate the feasibility of this approach and boost the potential of the microemulsion reaction method for the synthesis of nanoparticles.
Scheme 1

Strategies for the preparation of inorganic nanoparticles by the o/w microemulsion reaction approach: a mixing two microemulsions of identical composition except for reactants A* and B**; b using a microemulsion containing reactant A*, whilst reactant B** is added directly. *organometallic precursor, **precipitating/reducing/oxidizing agent

Experimental section

Materials

Synperonic® 10/5 was a gift from Croda. Butyl-S-lactate (Purasolv BL) was a gift from Purac. Brij® 96V, Bis(1,5-cyclooctadiene) dirhodium (I) dichloride (Rh-COCl), 1,2-hexanediol and hydrogen peroxide (3% and 30%) were purchased from Fluka. Cerium (III) 2-ethylhexanoate (Ce-EH), (1,5-cyclooctadiene)dimethyl platinum (II) (Pt-COD) and palladium (II) acetylacetonate (Pd-AAc) were purchased from Aldrich. Tween® 80, isooctane (Suprasolv, for gas chromatography), ethyl oleate, sodium borohydride (NaBH4), methanol, and ammonia 32% were purchased from Merck. Span 20® was purchased from Roig Farma. Isopropanol was purchased from Carlo Erba.

Preparation of nanoparticles by the o/w microemulsion reaction method

The microemulsion systems studied were: water/Tween® 80/Span® 20/1,2-hexanodiol/ethyl oleate (System A); water/Brij® 96V/butyl-S-lactate (System B) and water/Synperonic® 10/5/isooctane (System C). The organometallic precursors used as defined above were: Pt-COD for Pt nanoparticles, Pd-AAc for Pd nanoparticles, Rh-COCl for Rh nanoparticles and Ce-EH for CeO2 nanoparticles. The procedure for the synthesis of Pt, Pd and Rh nanoparticles, was as follows. The microemulsion containing the corresponding organometallic precursor was prepared by mixing appropriate amounts of surfactant (plus cosurfactant(s) if needed), oily phase (as a solution of the corresponding organometallic precursor) and deionized water. The mixture was stirred (vortex) at 25 °C until a homogeneous, transparent and fluid isotropic phase was obtained. The microemulsion was kept at 25 °C. Next, a small amount of an aqueous solution of sodium borohydride (molar ratio of NaBH4:Metal of 2:1 or higher) was added under vigorous stirring at 25 °C. Formation of metallic nanoparticles was indicated by grey or brown colouration of the reaction mixture and evolution of H2 gas; although the microemulsion became coloured, it remained translucent, and no phase separation was observed. For the preparation of CeO2 nanoparticles, there were two approaches. In the first one, appropriate amounts of surfactant (plus cosurfactant if needed), oily phase (as a solution of Ce-EH) and deionized water were mixed. The mixture was homogenized as explained above, and the microemulsion was left to stabilize at 25 °C. Next, an appropriate amount of H2O2 3% or 30% (in excess, up to 10 moles of H2O2 per mol of Ce) was added under vigorous stirring at 25 °C; a yellow-orange colour appeared in the microemulsion and the particles sediment. In the other approach, instead of adding H2O2, a second microemulsion containing NH3 (but without Ce-EH in the oil phase) was added followed by vigorous stirring at 25 °C; a yellow-orange colour appeared in the microemulsion and the particles sediment. For the synthesis of CeO2, in both approaches, the reaction mixture was kept stirring at 25 °C (24–48 h), followed by centrifugation and washing cycles (methanol) and dried at 70 °C.

Characterisation of nanomaterials

For Transmission Electron Microscopy (TEM) observation, a drop of reaction mixture (dispersion of nanoparticles in microemulsion media) was sonicated in isopropanol (2 mL), and a drop of this dispersion was deposited onto a formvar/carbon copper grid. Observation was carried out using either a TEM 300KV Philips CM30 or a FETEM JEOL 2100 F, HR. Average particle size was determined from TEM by measuring at least 250 nanoparticles per sample (Digital Micrograph 3.4, Gatan Inc.). Dried CeO2 powders were characterized by X-Ray Diffraction (XRD). For this purpose, a Siemens Diffractometer D5000 was employed.

Results and discussion

A key part of the investigation was the formulation of appropriate o/w microemulsions; biodegradability of surfactants and oils was taken into account in their selection, and in some systems, pharmaceutically acceptable oils and surfactants were used. A variety of systems, (Systems A, B and C as defined in the “Experimental section”) based on aliphatic nonionic surfactants (such as ethoxylated fatty alcohols or ethoxylated sorbitan esters) and aliphatic hydrocarbons or esters were studied. The formation of o/w microemulsions, with the corresponding organometallic precursor dissolved in the oil phase was explored by means of phase diagram determinations at 25 °C. O/W microemulsions were formed with surfactant concentrations ranging from 7 to 20 wt%. Although description of phase behaviour is not the object of this communication, it is worth noting that microemulsion region boundaries (limit of monophasic microemulsion zone) were verified in the presence of organometallic precursors. Except for Ce-EH, for which the boundary was somehow dependant on precursor concentration (possibly due to slight decomposition or oxidation of Ce-EH), the microemulsion region was only slightly affected by incorporation of organometallic precursor. Typically, for most systems used in the w/o microemulsion approach, phase behaviour is highly affected by the presence of inorganic precursors in the aqueous phase; usually, the w/o microemulsion zone is reduced, limiting the range of microemulsion compositions useful for nanoparticle synthesis and consequently reducing the possibilities for size control and the yield of the process. This phenomenon is attributed to ‘salting out’ effects from the inorganic precursors affecting the polar groups of surfactant molecules and the water molecules bound to them. In contrast, o/w microemulsion phase behaviour is expected to be less affected by the presence of organometallic precursors dissolved in the oil phase, since the interactions between them and oil would be weaker in nature, although this could depend on the penetration of the oil into the hydrophobic surfactant chains, as well as on the preferred precursor solubilization site, which, in turn, would depend on the degree of amphiphilicity of the precursor. For Ce-EH, this phenomenon could affect phase behaviour. Another advantage of the present approach is that there are several possibilities for the solubilization of precursor in the microemulsion components. Since the precursor is organometallic, it may be possible to dissolve it not only in the oil phase but also in the cosurfactants (typically liquid medium chain alcohols or di-alcohols) and/or in the surfactant (if liquid surfactants such as nonionic surfactants of the fatty alcohol ethoxylated type are employed). In contrast, in the w/o microemulsion method, the precursor is only dissolved in the aqueous phase. These aspects of the o/w microemulsion approach offer possibilities for yield improvement and for the preparation of materials with varied surface properties depending on the solubilization site of the precursor.

Several key microemulsions were selected for nanoparticle synthesis; details on microemulsion composition are shown in Table 1. The microemulsion systems used are denoted as A, B or C as defined in the “Experimental section”. Details on average particle size as determined by TEM are also given in Table 1.
Table 1

Parameters of microemulsion composition used for the synthesis of metal and metal oxide nanoparticles prepared in different systems and average particle size (d TEM)

Sample

Microemulsion system

Precursor solution(s)

Oil phase content (wt%)

S a content (wt%)

Water content (wt%)

Metal loadingd

d TEM (nm)

Pt-A1

A

Pt-COD in oil and Pt-COD in CS2b

5.0

25.0c

70.0

1.75

3.1

Pt-C1

C

Pt-COD in oil

7.5

9.0

83.5

0.75

3.2

Pt-C2

C

Pt-COD in oil

14.0

21.5

64.5

1.40

6.3

Pd-C1

C

Pd-AAc in S a

7.5

9.0

83.5

0.08

6.0

Rh-B2

B

Rh-COCl in oil

20.0

20.0

60.0

0.61

3.9

CeO2-B1

B

Ce-EH in oil

5.0

10.0

85.0

0.50

3.6

CeO2-C2

C

Ce-EH in oil

14.0

21.5

64.5

2.58

CeO2-B3

B

Ce-EH in oil

12.6

21.7

65.7

0.77

aS = surfactant

bCS2 = 1,2-hexanodiol

cS = S/CS1/CS2 (Tween® 80/Span® 20/1,2-hexanediol; weight ratio 42/28/30)

dg metal or metal oxide per Kg of microemulsion

As regards the synthesis of metallic nanoparticles (Pt, Pd or Rh), addition of aqueous solutions of sodium borohydride (NaBH4) to o/w microemulsions containing the corresponding organometallic precursor (synthesis strategy indicated in Scheme 1b) resulted in the rapid formation of metallic nanoparticles, as indicated by the colouring of the microemulsion (from grey to brown depending on the metal, the microemulsion composition, and the concentration of metal). As shown in Fig. 1a–c, small metallic nanoparticles were obtained; average particle size determined by TEM are indicated in Table 1. The results shown in Fig. 1 and Table 1 demonstrate that small nanoparticles with narrow size distribution are obtained, either when the precursor is dissolved in the oil (Rh-B2), in the cosurfactant and the oil (Pt-A1) or in the surfactant Pd-C1).
Fig. 1

TEM micrographs and related particle size distribution histograms of nanoparticles prepared in o/w microemulsions: a Pt-A1, b Pd-C1, c Rh-B2 and d CeO2-B1. Scale bar: 50 nm, except d (10 nm) and inset of d (5 nm)

It should be noted that since the resulting nanoparticle dispersions from the reaction media were too concentrated for direct TEM observation, samples were diluted and dispersed (by sonication) in isopropanol, and as a result deposited nanoparticles may appear agglomerated (e.g. Fig. 1b). Nevertheless, Pt, Pd and Rh nanoparticles remained stable in the reaction media (no sedimentation of the nanoparticles was observed after several weeks). On the other hand, the use of high excess of NaBH4 or concentrations of precursor above certain limit resulted in the partial precipitation of metallic particles from the microemulsion reaction media, possibly due to the formation or larger particles, indicating that optimum reaction and composition conditions are necessary to form stable nanoparticle dispersions. The nanoparticle stabilization depicted in Scheme 1 (nanoparticle C), where the formed nanoparticle remains in the oil droplet, stabilized by a surfactant monolayer is only one of several possibilities. It is also possible that the nanoparticles exit the oil droplets remaining stabilized in the aqueous phase by surfactant molecules. Detailed studies are needed to understand the mechanism of stabilization of nanoparticles prepared by this method.

The feasibility of controlling particle size is shown in Fig. 2. Pt nanoparticles were prepared using System C with two different compositions. For sample Pt-C1 (Fig. 2a), 7.5 wt% isooctane and 9 wt% Synperonic® 10/5 was used; for sample Pt-C2 (Fig. 2b), isooctane concentration was 14 wt% while Synperonic® 10/5 concentration was 21.5 wt%. Even though the [oil]/[surfactant] molar ratios are quite similar, a considerable difference in particle size was obtained (3.2 nm for Pt-C1 and 6.3 nm for Pt-C2), possibly due to overall composition differences (leading to different oil droplet size).
Fig. 2

TEM micrographs and particle size distribution histograms for Pt nanoparticles prepared using System C: a Pt-C1 (scale bar 10 nm) and b Pt-C2 (scale bar 20 nm)

Concerning metal oxide nanoparticles, CeO2 was chosen as example. For the synthesis of CeO2-B1 and CeO2-B3, H2O2 was added to a microemulsion containing Ce-EH in the oil phase. For CeO2-C2, a second microemulsion containing NH3 in the aqueous phase (and no precursor in the oily phase) was added to the microemulsion containing Ce-EH. In contrast to the metallic materials, CeO2 had a tendency to sediment from the microemulsion reaction mixture (except for very dilute systems, in which CeO2 nanoparticles remained dispersed); hence, the nanoparticles appear more agglomerated in TEM micrographs (Fig. 1d). Nevertheless, it is observed that the agglomerates are made up of small nanoparticles with a narrow size distribution. Formation of nanocrystalline ceria for CeO2-B1 was confirmed by High Resolution TEM (HRTEM) as shown in the inset of Fig. 1d. For CeO2-C2 and CeO2-B3 crystallinity was confirmed by XRD as shown in Fig. 3. From the peak broadening (following the Debye–Scherrer equation), small crystallite sizes are obtained (4.1 and 2.2 nm for CeO2-C2 and CeO2-B3, respectively). Hence, it was confirmed that crystalline nano-ceria was obtained directly in the microemulsion at 25 °C, without the need of calcination. For samples precipitated with H2O2, a better crystallinity was obtained in this study on increasing H2O2 concentration (studied up to 10:1 H2O2:CeEH molar ratio for CeO2-B3). In contrast, in the study of Scholes et al. (2007) on the homogeneous precipitation of CeCl3 with NH3 in the presence of H2O2, the formation of amorphous CeO2 was observed upon increasing the concentration of H2O2.
Fig. 3

XRD spectra for (a) CeO2-C2 and (b) CeO2-B3; (c) reference diffraction lines for cerianite (cubic fluorite type structure)

Conclusions

A novel approach to metal and metal oxide nanoparticle synthesis by the microemulsion reaction method has been developed using o/w instead of w/o microemulsions. Small nanoparticles of less than 7 nm, with narrow size distributions and crystalline structures can be synthesized in mild conditions (ambient temperature and pressure) and simple equipment. Although acceptable metal loadings in the microemulsions have been achieved, further studies should be carried out in order to optimize the method. The approach can be extended to the synthesis of other materials, including bimetallic nanoparticles and mixed oxides. The use of an environmentally friendly continuous phase (water) and the choice of biodegradable surfactants, cosurfactants and lower oil content, should improve the potential of the synthesis of nanoparticles by the microemulsion reaction technique. In particular, the o/w microemulsion reaction method reported here could be applied where the w/o microemulsion method fails. Furthermore, the surface properties of the nanoparticles obtained in o/w microemulsions, may be different from those of materials obtained in w/o microemulsions and other methods. This is one of the aspects foreseen for complementary studies. Other perspectives for further research include systematic studies on how the microemulsion parameters such as composition and droplet size affect nanoparticle properties (size, shape, crystallinity, specific surface area, etc.) as well as the mechanistic aspects of the method.

Notes

Acknowledgement

This research was financially supported by Ministerio de Educación y Ciencia (MEC Spain, grant number CTQ2005-09063-CO3-O2 and CTQ2008-01979) and Generalitat de Catalunya (grant number 2005-SGR-00812). M. Sanchez-Dominguez is grateful to CSIC for a JAE-Doc contract and to COST ACTION D43 for a Short Term Scientific Mission.

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Copyright information

© Springer Science+Business Media B.V. 2009

Authors and Affiliations

  • Margarita Sanchez-Dominguez
    • 1
  • Magali Boutonnet
    • 2
  • Conxita Solans
    • 1
  1. 1.Consejo Superior de Investigaciones Científicas (CSIC)Instituto de Química Avanzada de Cataluña (IQAC), CIBER-BBNBarcelonaSpain
  2. 2.Department of Chemical Engineering and Technology, Chemical TechnologyRoyal Institute of Technology (KTH)StockholmSweden

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