NANO LETTERS
Precise Control of the Pt Nanoparticle Size by Seeded Growth Using EO13PO30EO13 Triblock Copolymers as Protective Agents
2005 Vol. 5, No. 11 2238-2240
Krisztian Niesz, Michael Grass, and Gabor A. Somorjai* Department of Chemistry, UniVersity of California, Berkeley, Berkeley, California 94720-1460, and the Chemical and Materials Science DiVisions, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720 Received August 8, 2005
ABSTRACT Here, we report an efficient way to produce homogeneous Pt nanoparticles within a well-defined size range (3.5−6.6 nm) as a result of the seeded growth procedure using Pluronic L64 polymer capping agent. First, small seeds (3.5 nm) were prepared by the reduction of H2PtCl6‚ 6H2O in water with NaBH4 in the presence of the capping poly(ethylene oxide)13−poly(propylene oxide)30−poly(ethylene oxide)13 triblock copolymer at room temperature. Additional anionic Pt salt was then introduced under flowing H2 to obtain larger nanoparticles.
The ultimate goal in industrial heterogeneous catalytic processes is to perform reactions with high selectivity toward the desired product molecules without deactivating the catalyst particles. To reach this aim, one of the challenging tasks is to design and synthesize active metal nanoparticles with well-defined and finely tunable size, surface structure, and composition. This has attracted the attention of the current nanochemistry research and led to several discoveries of how to control the size and shape of nanoparticles. Monodispersed inorganic nanocrystals with various shapes1-12 such as spherical, rod, cubic, octahedral, prism, tetrapod, and other branched structures in the 1-10 nm range were successfully synthesized and characterized, and their optical, electronic, magnetic, and catalytic properties were examined.1,3,11-20 A convenient way to produce noble metal nanoparticles is the reduction of metal halides or anionic metal chloride precursors with alcohols, NaBH4, or H2 in the presence of surface stabilizers such as poly(vinylpyrrolidone) (PVP), polyacrylate, and others. The role of the capping agents is not only to protect the nanoparticles against aggregation but also to influence the final shape and size of the particles. El-Sayed and co-workers have demonstrated that the concentration of the surfactant molecules influences the final shape of Pt nanocrystals, by determining the relative growth rate in the (111) and (100) directions by blocking specific surface sites and slowing down or preventing their further *
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© 2005 American Chemical Society
growth.19,21 It also has been shown that particle size is precisely tunable and strongly depends on the concentration of surface stabilizing molecules relative to the metal ion precursors. Seeded growth methods were developed to control the size of transition metal nanoparticles.22-29 In this procedure, small nanoparticles are synthesized first and used as seeds to form larger size particles. For seeded growth, the rate of addition of metal ions and their concentration are key parameters in controlling the size distribution of the final material. As long as the consumption of the metal ion reactants by the growth of the particles is not exceeded by the rate of the precursor addition to the solution, no new nuclei form.1 Using the seeded growth method in the synthesis of Rh nanoparticles, our group has recently shown that the size and shape of the final nanocrystals is strongly influenced by the temperature of the reaction. At lower temperatures, anisotropic growth occurred, leading to the formation of different branched structures such as Rh multipods.30 Finally, to provide good catalytic performance, the protective agent has to be easily removable from the nanoparticle surface to avoid blocking and poisoning of the active surface sites. Poly(ethylene oxide)x-poly(propylene oxide)y-poly(ethylene oxide)x triblock copolymer, because it does not have a pure carbon-carbon backbone, decomposes under milder conditions than PVP and therefore is an ideal candidate as a metal nanoparticle stabilizer for the purpose of catalysis. They are commercially available, inexpensive materials. They have also been used for drug delivery,31 as structure-directing agents in the synthesis of
Table 1. Synthesis Parameters and the Final Size of the Pt Nanoparticles Prepared by Seeded Growth Method sample name
[Pluronic L64]a [mM]
[Pt]b [mM]
[Pluronic L64]/[Pt]
reducing agent
XRDc [nm]
TEMd [nm]
Pt (A) Pt (B) Pt (C) Pt (D) Pt (E)
6 5.7 5.45 5 6
0.6 0.86 1.1 1.5 1.2
10 6.6 5 3.3 5
1 mg NaBH4 1 mg NaBH4 + H2 1 mg NaBH4 + H2 1 mg NaBH4 + H2 2 mg NaBH4
3.4 5.2 5.6 6.3 3.8
3.5 ( 0.38 5.4 ( 0.77 6.0 ( 0.98 6.6 ( 0.0.67 3.9 ( 0.33
a The final concentration of the Pluronic L64 in the solutions. The cmc of Pluronic L64 at room temperature ) 26.31 mM.37 b The final concentration of the platinum in the solutions. c The size of the nanoparticles determined by XRD using the Debye-Scherrer equation: d ) Aλ/∆θ cos θ, where d is the particle diameter, A is a constant (generally ) 1), λ is the X-ray wavelength (Co-KR ) 1.79 Å), ∆θ is the full width at half-maximum of the (111) peak in radians, and θ is half the Bragg angle of the peak. d The size of the particles determined by TEM measurements by counting 300 particles.
mesoporous materials,32-34 and as templates for nanoparticle synthesis.35,36 Alexandridis et al. recently reported gold nanoparticle synthesis where they used PEO-PPO-PEO block copolymers both as protective agents and as reductants in a single-step process.37,38 The platinum nanoparticles reported here were synthesized using the seeded growth method. First, homogeneous Pt seeds were produced by reducing 1 mL of 6 mM H2PtCl6‚6H2O (6 µmol) in the presence of 174 mg Pluronic L64 (EO13PO30EO13) triblock copolymer (60 µmol) dissolved in 9 mL of H2O. NaBH4 (1 mg) was used as the reducing agent, and the reaction was performed at room temperature in a closed vial. The fast reduction of the Pt precursor salt and the high excess of the block copolymer capping agent resulted in a homogeneous solution of small Pt seeds (3.5 nm, Pt (A)), which was used as the starting solution toward synthesizing bigger particles. A subsequent addition of 0.5, 1, or 2 mL of 6 mM hexachloroplatinic acid-water solution over time (7, 14, 28 min) to the Pt seed sol while H2 was flowing through the reaction vessel led to larger nanoparticles (referred to as Pt (B), Pt (C), and Pt (D), respectively). After the second addition of platinum precursor was completed, H2 was bubbled for an additional 5 min, and the reaction mixture was stirred overnight before taking samples for TEM analysis. The nanoparticles were stable in water for several weeks. For XRD measurements, the particles were precipitated from solution by the addition of a 2-propanol/hexane mixture and placed onto the sample holder. The synthesis parameters and the final size of the particles are listed in Table 1. As shown, the concentration of the Pluronic L64 triblock copolymer in the solutions in each case was below the critical micellization concentration (cmc),37 which allows us to assume that no micelle formation took place during the reaction and the copolymer bonded to the metal surface as a single macromolecule. This was checked and proven by SAXS measurements where no characteristic scattering due to the formation of micelles was observed (not shown here). TEM and XRD were used as independent techniques to measure and calculate the final Pt nanocrystal size. From the transmission electron micrographs shown in Figure 1, the particle sizes and size distributions of the samples were calculated. The results, included in Table 1, show good monodispersity of the 3.5, 5.4, 6.0, and 6.6 nm particles. The XRD-based nanoparticle sizes (Table 1) were determined by the line broadening of the (111) reflection peaks (Figure 2). The differences between the sizes determined by the two ways can be due to the particles being coated with triNano Lett., Vol. 5, No. 11, 2005
Figure 1. EO13PO30EO13-coated Pt nanoparticles grown on seeds (A) with the subsequent addition of 0.5 mL (B), 1 mL (C), and 2 mL (D) of 6mM H2PtCl6‚6H2O . The scale bars represent 50 nm. The insets show the corresponding particle size distributions.
Figure 2. XRD diffractograms of the EO13PO30EO13-coated Pt nanoparticles prepared by seeded growth method. The (111) reflection peaks are normalized to the highest-intensity value to demonstrate the line broadening related to the nanoparticle size. The Pt (A-D) sample names refer to the Pt seeds and the larger particles prepared from the seeds by the subsequent addition of 0.5, 1, and 2 mL of 6 mM hexachloroplatinic acid solution, respectively.
block copolymers during the TEM analysis. Interestingly, if [Pluronic L64]/[Pt ion] ) 5 was used in the starting solution instead of a ratio of 10, then after the NaBH4 reduction the sizes of the metal particles were much smaller (3.9 nm, Pt (E)) than after seeded growth with the same ratio ()5) in the final solution (6.0 nm, Pt (C)). This demonstrates the much faster reduction of the Pt ions leading to small particles if borohydride was used as the reducing agent and that there is no nucleation during the hydrogen reduction process. To summarize, Pt particles were prepared under ambient conditions using Pluronic L64 triblock copolymer as a 2239
surface protective agent. Hexachloroplatinic acid was reduced by NaBH4 in the presence of the capping agent in tenfold excess to produce small Pt seeds with 3.5 nm diameter. Additional amounts of the precursor salt were added slowly to this seed solution while H2 was flowing through the vessel in order to produce larger particles and fine-tuning their diameters up to 6.6 nm. The samples were characterized by XRD and TEM. The results show that, using the seeded growth method and combining NaBH4 and H2 reduction techniques at room temperature, we are able to precisely control the size of Pt nanocrystals. Acknowledgment. This work was supported by the National Science Foundation under contract no. DMR0244146 and the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division, of the U. S. Department of Energy under contract no. DEAC02-05CH11231. The authors thank the UC Berkeley Electron Microscope Lab for the use of TEM and A. Paul Alivisatos for the use of the powder XRD. References (1) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545. (2) Cheon, J.; Jun, Y. W.; Lee, S. M. Architecture of Nanocrystal Building Blocks. In Nanoparticles Building Blocks for Nanotechnology; Rotello, V., Ed.; Nanostructure Science and Technology Series; Lockwood, D. J., Ed.; Kluwer Academic/Plenum Publisher: New York, 2004. (3) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (4) Cushing, B. L.; Kolesnichenko, V. L.; O’Connor, C. J. Chem. ReV. 2004, 104, 3893. (5) Song, H.; Kim, F.; Connor, S.; Somorjai, G. A.; Yang, P. J. Phys. Chem. B 2005, 109, 188. (6) Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Nat. Mater. 2003, 2, 382. (7) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8684. (8) Johnson, C. J.; Dujardin, E.; Davis, S. A.; Murphy, C. J.; Mann, S. J. Mater. Chem. 2002, 12, 1765. (9) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Chem. Commun. 1994, 801.
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NL051561X
Nano Lett., Vol. 5, No. 11, 2005
