Synthesis and Characterization of Rectangular Palladium Nanoparticles.

The optical, magnetic and catalytic and electronic properties of nanoparticles are affected not only by their size, but also by their shape. We have used a mild reducing agent, ascorbic acid, in the presence of sodium citrate and a surfactant, cetyltrimethylammonium bromide, at ambient conditions to prepare colloidal rectangular palladium nanoparticles, including nanocubes and nanorods. Our procedure requires no seed-mediated growth or nanoporous template so that it is more practical for large-scale synthesis. Upon changing the concentration of sodium citrate from 0.2 $\times $ 10$^{-3}$ M to 1.0 $\times $ 10$^{-3}$ M, TEM measurements indicate that the average size of the nanocubes decreases slightly from 31.8 $\pm $ 3.2 nm to 29.4 $\pm $ 4.2 nm and the aspect ratio of the nanorods increases from 2.48 $\pm $ 1.42 to 3.94 $\pm $ 2.50. HRTEM images and diffraction patterns indicate that the particles are highly crystalline and have strong (100) faceting. Being exposed to air for $\sim $100 days, the particles are oxidized to form 2$\sim $4 nm-thick amorphous shells.