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Colloids and Surfaces A: Physicochem. Eng. Aspects 337 (2009) 205–207

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l homepage: www.e lsev ier .com/ locate /co lsur fa

hort communication

oom temperature synthesis of platinum nanoparticles in water-in-oilicroemulsion

ngshuman Pala, Sunil Shaha, Serguei Belochapkineb, David Tannerb,c, Edmond Magnerb, Surekha Devia,∗

Department of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara 390002, Gujarat, IndiaMaterials & Surface Science Institute, University of limerick, Limerick, IrelandManufacturing and Operations Engineering, University of limerick, Limerick, Ireland

r t i c l e i n f o

rticle history:eceived 2 October 2008eceived in revised form

a b s t r a c t

Platinum nanoparticles of less than 5 nm size have been synthesized by the reduction of H2PtCl6 usingsodium borohydride in water-in-oil (w/o) microemulsions of water/TritonX-100/cyclohexene/1-hexanolat 25 ± 2 ◦C. Size and shape of the particles are determined through HRTEM images.

0 November 2008ccepted 22 November 2008vailable online 3 December 2008

eywords:

© 2008 Elsevier B.V. All rights reserved.

anoparticleslatinumicroemulsion

. Introduction

Enhancements in the catalytic properties of metal particlesre associated with changes in surface area and reactivity relativeo bulk metal samples [1]. The catalytic activity of silver, gold,alladium and platinum nanoparticles has been described in detail2]. A range of approaches have been used for the preparationf metallic nanoparticles; co-precipitation [3] of the appropri-te metals, sol–gel encapsulation, solvothermal [4], sputtering5], sonochemical [6], and UV-irradiation [7] microemulsion8] methods. The use of water-in-oil (w/o) microemulsions isotentially a very useful technique for the preparation of metallicanoparticles [8]. Water-in-oil microemulsion consists of a singlehase, transparent isotropic liquid medium with nanosized waterroplets dispersed in a continuous oil phase and stabilized byurfactant molecules at the water/oil interface. The water dropletsffer a unique microenvironment for the formation of highlyonodisperse nanoparticles. The growth of the particles is con-

rolled by the size of the microemulsion droplets, particularly innionic microemulsion systems [9]. This template-based synthesis

f nanoparticles suffers from the disadvantage that removal ofhe nanoparticles from the template can be difficult. Platinumanoparticles catalyse a range of reactions, including the evolutionf hydrogen, reduction of oxygen, oxidation of hydrogen and

∗ Corresponding author. Tel.: +91 2652795552.E-mail address: surekha devi@yahoo.com (S. Devi).

927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2008.11.044

methanol and hydrogenation reactions [10]. Nanoparticles requirethe addition of a capping agent (e.g. polymer) in order to preventcoagulation and precipitation of the particles. A fraction of theadded polymer performs a protective function through adsorptiononto the metal nanoparticles with the remainder dissolved in thesuspension. The relative amounts of the polymer adsorbed onthe surface of metal nanoparticles and in solution are importantfor applications of the metal nanoparticles as adsorption of thepolymer can decrease the catalytic activity of the particles.

In the present work we have synthesized monodispersed Ptnanoparticles in w/o microemulsions. Using this approach, highlymonodispersed Pt nanoparticles can be synthesized at room tem-perature (25 ± 2 ◦C) in less than 1 min.

2. Experimental

2.1. Materials

Chloroplatinic acid (H2PtCl6, 8% wt aqueous solution), sodiumborohydride (granular 99.99% metal basis) and 1-hexanol werepurchased from Aldrich, Steinheim, Germany. TritonX-100 andcyclohexane were purchased from Sigma–Aldrich, Steinheim, Ger-many.

2.2. Synthesis of platinum nanoparticles

The microemulsion system was composed of TritonX-100, cyclo-hexane, water and 1-hexanol. The amounts of each component used

206 A. Pal et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 337 (2009) 205–207

rticles, (B) expanded section from (A).

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composed of four to six nanoparticles. On increasing Wo from 3 to5, the size of the circular clusters increased (Fig. 4) with the clus-ters containing six to eight nanoparticles (based on the analysis of20 clusters). On increasing Wo to 7, larger clusters were observed(Fig. 4B). The average size of the clusters for Wo = 3, 5 and 7 are

Fig. 1. TEM images of Pt nanopa

n a typical reaction for Wo (water to surfactant molar ratio) ofare 8% TritonX-100, water and 1-hexanol at 0.027% (v/v) eachith cyclohexane as the remainder. The chloroplatinic acid solutionas reduced using sodium borohydride. Water-in-oil microemul-

ion systems containing a reducing agent and the appropriateetal solution were mixed under constant stirring at 25 ± 2 ◦C.

nstantaneous formation of the particles was observed. 0.1 M metalon concentration and 2% (w/v) sodium borohydride were usedhroughout the work. The molar ratio of water to surfactant wasaried from 3 to 7. The concentrations of metal ions for Wo = 3, 5nd 7 are 2.7 × 10−4 M, 4.5 × 10−4 M and 6.3 × 10−4 M respectively.ree metal nanoparticles could be isolated from the microemul-ion system by the addition of a short chain alcohol followed byentrifugation.

.3. Measurement

Size, shape and particle size distributions were determinedsing a JEOL JEM-2011 transmission electron microscope operatedt an accelerating voltage of 200 kV. Images were recorded usingGatan DualVision 600t CCD camera attached to the microscope

nd were analyzed using Gatan Digital Micrograph Version 3.11.1.he TEM was calibrated for diffraction and imaging mode usingtandard samples. Energy dispersive X-ray analysis was undertakenith a Princeton Gamma Tech Prism 1G system with a 10 m2 sili-

on detector attached to the TEM and the peaks were analysed withmix 10.594 software. The resolution of the system was calibrated

ith manganese (Mn). Samples were prepared for TEM analysis bylacing a drop of the solution on a carbon coated copper grid andrying in air. UV–visible spectra were obtained on a PerkinElmerambda 35 UV–vis spectrophotometer.

. Results and discussion

Observed changes in the UV–visible absorption spectra (Fig. S1)an be taken as some evidence of reduction of Pt4+ ions and sub-equent formation of Pt nanoparticles. TEM images show that Ptanoparticles of 3 ± 1 nm in size were formed (Fig. 1A). Fig. 1B ishe expanded section from Fig. 1A that shows a single cluster com-osed of six nanoparticles. The high-resolution image (Fig. 2) showshe oriented and ordered lattice fringes for Pt nanoparticles, the “d”pacing value of 2.27 Å coincides with that of cubic Pt d(1 1 1) [11].

nergy-dispersive X-ray (EDX) spectroscopy (Fig. 3) also confirmshe presence of Pt.

The TEM images in Fig. 4 indicate that the Pt nanoparticles wererranged in a circular manner (from analysis of a series of fiveeparate images containing a total of 90 clusters). Each cluster is

Fig. 2. HRTEM images of Pt nanoparticles and (inset) image at higher magnification.

Fig. 3. Energy-dispersive X-ray spectra of Pt nanoparticles.

A. Pal et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 337 (2009) 205–207 207

t (A) W

2cpa

4

tiitm

A

t

A

t

[C.G. Vayenas (Eds.), Catalysis and Electrocatalysis at Nanoparticle Surfaces, Mar-cel Dekker, New York, 2003 [Chapter 9];

Fig. 4. TEM image of Pt nanoparticles showing cluster formation a

1 nm, 23 nm and 27 nm. The HRTEM images show that each cir-ular cluster is made of a mixture of spherical and non-sphericalarticles. However, the calculated “d” spacing values indicate cubicrrangement of atoms.

. Conclusion

Non-spherical Pt nanoparticles were synthesized at roomemperature through water-in-oil microemulsion. High-resolutionmage shows the formation of 3 nm diameter nanocrystal of plat-num nanoparticles. Formation of Pt nanoparticles was confirmedhrough EDX. Observed “d” spacing value indicate cubic arrange-

ent of atoms in nanoparticles.

cknowledgements

The authors are thankful to GUJCOST (Gandhinagar, Gujarat) forhe financial support.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.colsurfa.2008.11.044.

[

o = 3 and (B) 5. The inset in (B) shows a cluster formed at Wo = 7.

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