Tunable synthesis of platinum nanoparticles by EtOH reduction in presence of poly (vinylpyrrolidone)

Available online at www.banglajol.info Bangladesh J. Sci. Ind. Res. 50(2), 87-92, 2015 Tunable synthesis of platinum nanoparticles by EtOH reduction in presence of poly (vinylpyrrolidone) M. S. Rahman1, S. Akhter2, K. N. Ahmed1, M. S. Rahman2, R. K. Saha2 and M. J. Hossain1,2* 1 Industrial Botany Research Division, BCSIR Laboratories Chittagong, Chittagong 4220, Bangladesh. Forest Chemistry Division, Bangladesh Forest Research Institute, Chittagong 4211, Bangladesh. 2 Abstract Monodispersed platinum nanoparticles (Pt-NPs) with an average diameter dTEM 3.1±1.0 nm were synthesized by EtOH reduction method in the presence of an organic polymer poly (vinylpyrrolidone) (PVP) with an average molecular weight of 40,000. Using this particles as seed, size tunable Pt-NPs of dTEM 3.1±1.0 nm to 5.7±1.6 nm with sufficient monodispersity were synthesized by multiple step seeding growth. Formation of Pt-NPs was confirmed by the UV-visible absorption spectra. Transmission electron micrographs (TEM) and powder X-ray diffraction (XRD) patterns confirmed that the particles were single crystalline with fcc crystal geometry. Key words: Platinum; EtOH; Nanoparticles; PVP; Synthesis Introduction Colloidal noble metal particles have recently attracted considerable attention in many areas of research due to their novel physical and chemical properties. Research on nanoscience has been growing steadily due to their unique position as a bridge between atoms and bulk solids and are of fundamental interest to both homogeneous and heterogeneous catalytic applications (Boudart, 1985; McLeod and Gladden, 1998). The electron confinement effect of nanocrystal is responsible for manipulating the electronic, optical, and magnetic properties of solid materials. Both catalytic activity (Rioux et al., 2005) and selectivity (Eppler et al, 2000) are known to be influenced by the size and the shape of the nanoparticles (Narayanan and El-Sayed, 2004; Tsung et al. 2009; Kuhn et al., 2008) and therefore the synthesis of well-controlled sizes and shapes of colloidal particles could be critical for this purpose. Due to very tiny size, the size controlled synthesis is a big challenge for nano-research. Metal NPs can be prepared by physical and chemical methods. The physical methods like vapour deposition in principle subdivide bulk precursor to NPs. Chemical procedures involve reduction of metal ions to metal atoms, followed by controlled aggregation of atoms. Utilization of simple laboratory equipments and easy procedure to obtain small and uniform metal NPs made chemical reduction method unparallel for NPs synthesis. Pt-NPs are prepared in general, via the chemical reduction of *Corresponding author: E-mail: Pt ions by borohydride (Van Rheenen et al., 1987; Knecht et al., 2008), hydrogen (Ahmadi et al. 1996), alcohols (Toshima et al., 1991; Teranishi et al., 1999), glycol or ethylene glycol (Herricks et al., 2004) in the presence of a stabilizer or on a solid support. Despite a large choice of synthetic protocols, an accurate control of the particle size, simple and economical strategies for synthetic method is most important to investigate their novel physical and chemical properties. Typically nanoparticles provide highly active centers which are not in a thermodynamically stable state due to their high surface energies (Schmid et al., 1996; Doyle et al., 2003). Soluble organic polymers are widely employed supports for synthesizing metal NPs because of their availability and enhanced stabilization properties (Toshima et al., 1991). PVP is the most studied polymer that can not only control the size of the particles but also dictates the shape as well. PVP molecules interact strongly through their carbonyl group with the Pt surface for their enhance stabilization (Teranishi et al., 1999). In the typical mechanism of PVP protected metal nanoparticles formation involve three steps: (I) coordination of PVP and metal ions, (II) PVP-promoted nucleation that produces smaller particles, and (III) stabilization of formed particles through physical and chemical bonding with PVP by steric shielding and also minimizing the particles surface energies which inhibited particle–particle contact and, thus, the agglomeration of particles. 88 Platinum nanoparticles by EtOH reduction Chemical syntheses offer a versatile route allowing tailoring of the properties of materials by assembling atoms and particles from the atomic or molecular state to the macroscopic scale (Elechiguerra et al., 2006). The characteristics of the crystals can be controlled by the thermodynamics and kinetics of the synthesis (Goia, 1998) and the great progress has been made. However, there is still need for development of chemical methods that can tailor the morphology of Pt crystals at different scales. This article describes a simple preparation and size control of small Pt-NPs stabilized by PVP with narrow size distribution by EtOH reduction method. The nanoparticles were characterized by UV-Visible optical spectroscopy, transmission electron microscopy (TEM) and powder x-ray diffractometry (XRD). Materials and methods Chemicals and materials Hydrogen hexachloroplatinate(IV) hexahydrate (H2PtCl6.6H2O) (Sigma-Aldrich), ethanol (C2H6O) (Merck), Poly(vinylpyrrolidone) (PVP, MW 40,000) (TCI), were of analytical grade and used as received. Double distilled water was used to prepare aqueous solutions. All of the glasswares along with teflon coated magnetic stirrer were cleaned with aqua regia for fine cleaning followed by rinsing with double distilled water. Preparation of Pt:PVP NPs As reported earlier (Hossain et al., 2012, Teranishi et al., 1999), a mixture of ethanol/water solution (9:1, v/v, 100 mL; 90% EtOH) containing H2PtCl6 (30 mM, 3.34 mL) and PVP (444 mg, PVP-monomer/Pt = 40:1) was refluxed at 90 °C in a 150 mL round bottom flask for 3 h using water bath. After cooling to room temperature, the produced Pt:PVP was purified using a dialysis membrane of cut off molecular weight 10 kDa for the removal of inorganic ions and ethanol. Finally, the purified sample was stored in an air tight desiccators after freeze drying. Larger Pt:PVP by seed mediated growth To form larger Pt-NPs, smaller Pt:PVP NPs were used as seed particles. In a typical procedure, a mixture of total 25 mL aqueous solution of H2PtCl6 (10 mM, 2.5 mL) and ethanol/water solution (9:1, v/v, 22.5 mL) were added to the 25 mL of Pt:PVP seed solution of 1.0 mM concentration. The final concentration of Pt in the resultant 50 mL mixed solution was 1.0 mM including both seeds and ions (0.5 mM from precursor solution and 0.5 mM from seed particles). 50(2) 2015 PVP concentration was adjusted to PVP-monomer to Pt-atom ratio 40 by addition of 222 mg extra PVP to the solution before refluxing. To reduce, the mixed solution was refluxed at 90 °C in a 100 mL round bottom flask for 3 h in air using water-bath. Initially prepared Pt:PVP NPs sample (step-1) was used as seed particles for larg (...truncated)