Platinum nanoparticles are extremely small clusters of Pt atoms, usually 1-50 nanometers (nm) in width, homogeneously dispersed in a colloidal solution. Pt nanoparticles have excellent catalytic strength, even more so than bulk platinum. This is due to nanoparticles’ high surface area to volume ratio. Shape also plays a large role in particle functionality.1

Pt nanoparticles by themselves have a high tendency to agglomerate, which hinders their effectiveness. For this reason, molecules called ligands are used to bind to the surface of the particles, stabilizing them and preventing agglomeration.

Despite platinum’s high cost, it is still by far the best catalyst for these fuel cell reactions. Therefore, it is important to develop methods of Pt nanoparticle synthesis in a way that is simple and more cost-effective than the methods currently in use3.

“Top-down” Synthesis Methods:• Breaking down bulk platinum until it is at the nanoscopic level.

“Bottom-up” Synthesis Methods:A. Adding a reducing agent to a solution containing a platinum salt (ex: K2PtCl6),

reducing Pt ions to neutral-charge Pt0 atoms.4

B. Applying DC voltage to a Pt electrode and some other electrode partially submerged in a solution containing both an electrolyte and a stabilizing ligand, with oxidation occurring at the Pt electrode, and reduction at the other electrode, releasing Pt0 atoms into the solution.5

C. Our AVINS method involves applying an alternating current to two Pt wires dipped into an electrolyte/ligand solution. Pt is oxidized and reduced rapidly to release Pt0 atoms into the solution.

Objectives:• To develop optimal pararmeters for our AVINS method of nanoparticle synthesis.• To be able to control the particle size between 1-15 nm.• To be successful in ligand exchange with the particles and transferring them from

their original aqueous solution into a nonpolar solvent such as toluene or hexane.

4. Summary and Outlook:• AVINS can be used to synthesize 3-5 nm Pt particles.• K2SO4 solutions seemed to be the most effective in avoiding large secondary

precipiates. • K2SO4 was tested at concentrations of ~0.50 M and ~0.01M. • For ~0.5M K2SO4 , 4.5-4.75V was sufficient for creating particles.• ~0.01M K2SO4, solution required 19-20V and yielded slightly different color. • Aqueous solutions of PVP, CTAB, CTAC, and sodium citrate were all capable

of producing stabilized nanoparticles.• PVP and CTAB were most successful stabilizing particles in aqueous

solution.• Hope to further explore the effects of parameters such as temperature,

wave shape/frequency, and pH value to control value and shape.• Develop more effective method of phase transfer.

5. Acknowledgements:Kyle Snow thank sthe REMRSEC REU program and Yongan Yang thank his CSM professional development start-up fund.

6. References:1N. Tian, Z. Y. Zhou, S. G. Sun, Y. Ding, and Z. L. Wang, Science, 2007, 316, 732.2http://en.wikipedia.org/wiki/Fuel_cell3H. A. Gasteiger, S. S. Kocha, B. Sompalli, and F. T. Wagner, Applied Catalysis B, 2005, 56, 9.4S. Y. Zhao, S. H.Chen, S. Y. Wang, D. G. Li, and H. Y. Ma, Langmuir, 2002, 18, 3315.5H. Bönnemann, R. M. Richards, European Journal of Inorganic Chemistry, 2001, 2001, 2455.

Example Procedure:• Prepared solution of ~0.5M K2SO4 & 30 mg/mL PVP.• Applied a voltage of 4.7 V for 10 minutes with transformer (60 Hz) while

stirring on stir plate.

2.2 Ligand ExchangeLigand exchange involves transferring the particles from their original

aqueous phase to a nonpolar, organic solvent such as toluene. However, ligands like PVP and CTAB are not highly soluble in these organic solvents. In order for the Pt to transfer, the aqueous ligand must be removed and replaced by a nonpolar-soluble ligand, such as 1-Dodecanethiol (DDT).

Example Procedure:• Placed small amount of sample (roughly 2-3 mL) into a 20 mL vial• Added about 2-3 mL of 1M 1-Dodecanethiol (DDT) in Toluene to the vial,

yielding an approx. 1:1 ratio between the aqueous and toluene phases.• Added small stir bar and stirred solution on hot plate at 85⁰C and 1200 rpm

for approximately 24 hours.

A FACILE METHOD FOR MAKING MONODISPERSED COLLOIDAL PLATINUM NANOCRYSTALS

Kyle Snow1,2, Kevin McCann1, Yongan Yang1

1Department of Chemistry/Geochemistry, Colorado School of Mines, 2Department of Chemical Engineering, Kansas State University

1. Introduction2.1 Synthesis

3. Results

KCl NaNO3 K2SO4 KNO3 LiCl LiBr NaCl NaBrYes Yes Yes Yes No No No No

10-15 minutes

Pt

AC

Pt

Vinyl-pyrrolidone Polyvinylpyrrolidone (PVP)

Cetyl trimethylammonium Bromide (CTAB)

PolymerMonomer

1-Dodecanethiol (DDT)

Figure 3: Before reaction, solution is clear. After reaction, solution turns a dark brown color.

Pt Ligand exchangePVP

(water-soluble) Pt

DDT(toluene-soluble)

Parameters:·Aqueous solution·Organic Solvent·Organic-soluble ligand· Aqueous to organic volume ratio· Temperature

Figure 5: Under heating and vigorous stirring, DDT molecules replace PVP.

Figure 4: Structures of some possible stabilizing ligands.

Pt- +DC

PtX62-

Figure 6: Before and after ligand exchange. Pt particles begin in aqueous phase, PVP is replaced with DDT and become soluble in toluene, transferring to organic phase. Aqueous phase becomes clear/cloudy white, organic phase becomes colored.

Fig. 7: Time-evolution Ultraviolet-Visible (UV-Vis) spectra during synthesis.

Fig. 10: UV-Vis of sample made in 0.01M K2SO4 & 30 mg/mL PVP, before & after ligand exchange. Includes a TEM image of the sample. The low-concentration salt remained a golden-brown color after ligand exchange.

Fig. 9: UV-Vis of sample made in 0.5M K2SO4 & 10 mg/mL CTAB, before & after ligand exchange, and a Transmission Electron Microscope (TEM) image. With the high-concentration salt, the solution became a pale, pinkish color after ligand exchange.

Figure 2: A. Pt salt reduction method. B. Electrochemical deposition by direct current. C. Our method, “Alternating Voltage Induced Nanoparticle Synthesis” (AVINS).

Table 1: These salts were either successful or not successful as electrolytes in producing Pt particles.

20-24 hours

Fig. 8: Energy-dispersive X-ray spectroscopy (EDX) spectrum & TEM of a sample made from sat. NaNO3.

Pt0 + byproduct

Pt0 Pt0

2H2O

Figure 1: Due to platinum’s catalytic strength, it is commonly used in fuel cells, catalyzing hydrogen oxidation at the anode and oxygen reduction at the cathode. These fuel cells offer a potential means to efficient, clean, and sustainable energy for use in automobiles and homes.2

Pt

A. B. C.

O2 + 4H+ + 4e-

PtCl62- + NaBH4

2. Methods

3-5 nm

3-5 nm