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

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Short communication

Room temperature synthesis of platinum nanoparticles in water-in-oil microemulsion Angshuman Pal a, Sunil Shah a, Serguei Belochapkine b, David Tanner b,c, Edmond Magner b, Surekha Devi a,∗ a b c

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

a r t i c l e

i n f o

Article history: Received 2 October 2008 Received in revised form 20 November 2008 Accepted 22 November 2008 Available online 3 December 2008

a b s t r a c t Platinum nanoparticles of less than 5 nm size have been synthesized by the reduction of H2 PtCl6 using sodium borohydride in water-in-oil (w/o) microemulsions of water/TritonX-100/cyclohexene/1-hexanol at 25 ± 2 ◦ C. Size and shape of the particles are determined through HRTEM images. © 2008 Elsevier B.V. All rights reserved.

Keywords: Nanoparticles Platinum Microemulsion

1. Introduction Enhancements in the catalytic properties of metal particles are associated with changes in surface area and reactivity relative to bulk metal samples [1]. The catalytic activity of silver, gold, palladium and platinum nanoparticles has been described in detail [2]. A range of approaches have been used for the preparation of metallic nanoparticles; co-precipitation [3] of the appropriate metals, sol–gel encapsulation, solvothermal [4], sputtering [5], sonochemical [6], and UV-irradiation [7] microemulsion [8] methods. The use of water-in-oil (w/o) microemulsions is potentially a very useful technique for the preparation of metallic nanoparticles [8]. Water-in-oil microemulsion consists of a single phase, transparent isotropic liquid medium with nanosized water droplets dispersed in a continuous oil phase and stabilized by surfactant molecules at the water/oil interface. The water droplets offer a unique microenvironment for the formation of highly monodisperse nanoparticles. The growth of the particles is controlled by the size of the microemulsion droplets, particularly in anionic microemulsion systems [9]. This template-based synthesis of nanoparticles suffers from the disadvantage that removal of the nanoparticles from the template can be difficult. Platinum nanoparticles catalyse a range of reactions, including the evolution of hydrogen, reduction of oxygen, oxidation of hydrogen and

methanol and hydrogenation reactions [10]. Nanoparticles require the addition of a capping agent (e.g. polymer) in order to prevent coagulation and precipitation of the particles. A fraction of the added polymer performs a protective function through adsorption onto the metal nanoparticles with the remainder dissolved in the suspension. The relative amounts of the polymer adsorbed on the surface of metal nanoparticles and in solution are important for applications of the metal nanoparticles as adsorption of the polymer can decrease the catalytic activity of the particles. In the present work we have synthesized monodispersed Pt nanoparticles in w/o microemulsions. Using this approach, highly monodispersed Pt nanoparticles can be synthesized at room temperature (25 ± 2 ◦ C) in less than 1 min. 2. Experimental 2.1. Materials Chloroplatinic acid (H2 PtCl6 , 8% wt aqueous solution), sodium borohydride (granular 99.99% metal basis) and 1-hexanol were purchased from Aldrich, Steinheim, Germany. TritonX-100 and cyclohexane were purchased from Sigma–Aldrich, Steinheim, Germany. 2.2. Synthesis of platinum nanoparticles

∗ Corresponding author. Tel.: +91 2652795552. E-mail address: surekha [email protected] (S. Devi). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.11.044

The microemulsion system was composed of TritonX-100, cyclohexane, 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

Fig. 1. TEM images of Pt nanoparticles, (B) expanded section from (A).

in a typical reaction for Wo (water to surfactant molar ratio) of 3 are 8% TritonX-100, water and 1-hexanol at 0.027% (v/v) each with cyclohexane as the remainder. The chloroplatinic acid solution was reduced using sodium borohydride. Water-in-oil microemulsion systems containing a reducing agent and the appropriate metal solution were mixed under constant stirring at 25 ± 2 ◦ C. Instantaneous formation of the particles was observed. 0.1 M metal ion concentration and 2% (w/v) sodium borohydride were used throughout the work. The molar ratio of water to surfactant was varied from 3 to 7. The concentrations of metal ions for Wo = 3, 5 and 7 are 2.7 × 10−4 M, 4.5 × 10−4 M and 6.3 × 10−4 M respectively. Free metal nanoparticles could be isolated from the microemulsion system by the addition of a short chain alcohol followed by centrifugation. 2.3. Measurement Size, shape and particle size distributions were determined using a JEOL JEM-2011 transmission electron microscope operated at an accelerating voltage of 200 kV. Images were recorded using a Gatan DualVision 600t CCD camera attached to the microscope and were analyzed using Gatan Digital Micrograph Version 3.11.1. The TEM was calibrated for diffraction and imaging mode using standard samples. Energy dispersive X-ray analysis was undertaken with a Princeton Gamma Tech Prism 1G system with a 10 m2 silicon detector attached to the TEM and the peaks were analysed with Imix 10.594 software. The resolution of the system was calibrated with manganese (Mn). Samples were prepared for TEM analysis by placing a drop of the solution on a carbon coated copper grid and drying in air. UV–visible spectra were obtained on a PerkinElmer Lambda 35 UV–vis spectrophotometer.

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

composed of four to six nanoparticles. On increasing Wo from 3 to 5, the size of the circular clusters increased (Fig. 4) with the clusters containing six to eight nanoparticles (based on the analysis of 20 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

3. Results and discussion Observed changes in the UV–visible absorption spectra (Fig. S1) can be taken as some evidence of reduction of Pt4+ ions and subsequent formation of Pt nanoparticles. TEM images show that Pt nanoparticles of 3 ± 1 nm in size were formed (Fig. 1A). Fig. 1B is the expanded section from Fig. 1A that shows a single cluster composed of six nanoparticles. The high-resolution image (Fig. 2) shows the oriented and ordered lattice fringes for Pt nanoparticles, the “d” spacing value of 2.27 Å coincides with that of cubic Pt d(1 1 1) [11]. Energy-dispersive X-ray (EDX) spectroscopy (Fig. 3) also confirms the presence of Pt. The TEM images in Fig. 4 indicate that the Pt nanoparticles were arranged in a circular manner (from analysis of a series of five separate images containing a total of 90 clusters). Each cluster is

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

Fig. 4. TEM image of Pt nanoparticles showing cluster formation at (A) Wo = 3 and (B) 5. The inset in (B) shows a cluster formed at Wo = 7.

21 nm, 23 nm and 27 nm. The HRTEM images show that each circular cluster is made of a mixture of spherical and non-spherical particles. However, the calculated “d” spacing values indicate cubic arrangement of atoms. 4. Conclusion Non-spherical Pt nanoparticles were synthesized at room temperature through water-in-oil microemulsion. High-resolution image shows the formation of 3 nm diameter nanocrystal of platinum nanoparticles. Formation of Pt nanoparticles was confirmed through EDX. Observed “d” spacing value indicate cubic arrangement of atoms in nanoparticles. Acknowledgements The authors are thankful to GUJCOST (Gandhinagar, Gujarat) for the financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.colsurfa.2008.11.044.

References [1] P.V. Kamat, D. Meisel, Studies in surface science and catalysis Semiconductor Nanoclusters – Physical, Chemical, and Catalytic Aspects, vol. 103, Elsevier, Amsterdam, 1997. [2] (a) V.I. Bukhtiyarov, M.G. Slinklo, Russ. Chem. Rev. 70 (2001) 147; (b) Y. Li, X.M. Hong, D.M. Collard, M.A. El-Sayed, Org. Lett. 2 (2000) 2385; (c) M. Adlim, M.A. Bakar, K.Y. Liew, J. Ismail, J. Mol. Catal. A 212 (2004) 141; (d) M. Murkarian, M. Harakeh, L.I. Halaoui, J. Phys. Chem. B 109 (2005) 11616. [3] B.L. Cushing, V.L. Kolesnichenko, C.J. O’Connor, Chem. Rev. 104 (2004) 3893. [4] C. Burda, X. Chen, R. Narayanan, M.A. El-Sayed, Chem. Rev. 105 (2005) 1025. [5] D. Kabiraj, S.R. Abhilash, L. Vanmarcke, N. Cinausero, J.C. Pivin, D.K. Avasthi, Nuc. Instr. Meth. B 244 (2006) 100. [6] J. Park, M. Atobe, T. Fuchigami, Electrochim. Acta 51 (2005) 848. [7] K. Malik, M. Mandal, N. Pradhan, T. Pal, Nano. Lett. 1 (2001) 319. [8] (a) C. Stubenrauch, T. Wielputz, T. Sottmann, C. Roychowdhury, F.J. DiSalvo, Colloids Surf. A: Physicochem. Eng. Aspects 317 (2008) 328; (b) A. Pal, S. Shah, S. Devi, Colloids and Surfaces A: Physicochem. Eng. Aspects 302 (2007) 483. [9] M.G. Spirin, S.B. Brichkin, V.F. Razumov, J. Photochem. Photobiol. A: Chem. 196 (2008) 174. [10] (a) N.M. Markovicˇı, V. Radmilovic, P.N. Ross Jr., in: A. Wieckowski, E.R. Savinova, C.G. Vayenas (Eds.), Catalysis and Electrocatalysis at Nanoparticle Surfaces, Marcel Dekker, New York, 2003 [Chapter 9]; (b) Z. Liu, X.Y. Ling, X. Su, J.Y. Lee, J. Phys. Chem. B 108 (2004) 8234; (c) J.-W. Yoo, D. Hathcock, M.A. El-Sayed, J. Phys. Chem. A 106 (2002) 2049; ´ H.A. Gasteiger, P.N. Ross Jr., J. Phys. Chem. 99 (1995) 3411. (d) N.M. Markovic, [11] X. Fu, Y. Wang, N. Wu, L. Gui, Y. Tang, J. Mater. Chem. 13 (2003) 1192.