“Green” and Size-Controlled Synthesis of Pd Nanoparticles
Juncheng Liu, Department of Chemical Engineering, Auburn University, 230 Ross Hall, Department of Chemical Engineering, Auburn, AL 36849 and Christopher B. Roberts, Chemical Engineering Department, Auburn University, 230 Ross Hall, Auburn, AL 36849.
Central to the concept of seed-mediated growth of nanoparticles is that small nanoparticle seeds serve as nucleation centers to precisely grow nanoparticles to a desired size. Herein we present an environmentally benign and size-controlled seed-mediated growth method to yield monodisperse Pd nanoparticles using either polysugar or traditional alkane-thiol capping ligands. Monodisperse Pd nanoparticles (ca. 3.4 nm, standard deviation: 0.6 nm) were first synthesized and stabilized in aqueous solution at 95 oC using non-toxic ascorbic acid and sodium carboxymethyl cellulose (CMC) as reducing agent and capping agent, respectively. The monodisperse Pd nanoparticles were subsequently employed as seeds on the surface of which fresh Pd 2+ ions were reduced by the weak reducing agent ascorbic acid. Optimal conditions were determined that favored controlled growth on the surface of nanoparticle seeds rather than formation of new nucleation sites, thereby yielding improved control over particle size with very narrow size distribution. Using different amounts of Pd salt in this seed-mediated growth process, different sized Pd nanoparticles were controllably and stoichiometrically synthesized. The favorable characteristics of the capping agent CMC including its molecular structure allows sequential addition of Pd onto the nanoparticle seeds. The underpinning mechanisms that govern this seed-mediated growth process were investigated and will be discussed. One advantage of the seed-mediated growth method used in this study is that the sizes of the Pd nanoparticles were precisely controlled (from 3.4 to 7.6 nm) by simply varying the ratio of the seed to Pd salt. Furthermore, the use of environmentally friendly reagents, such as water (solvent), CMC (capping agent) and ascorbic acid (reducing agent) in both the seed synthesis and subsequent seed-mediated growth process provides both "green" and economic attributes to this process. Moreover, these Pd nanoparticles were efficiently extracted from the aqueous phase into an organic phase (i.e. hexane) by using dodecanethiol as extractant along with the assistance of some HCl. The ability to effectively transfer the Pd nanoparticles into an organic phase opens opportunities for these monodisperse nanoparticles to be used in a number of non-aqueous applications. The assembly of these particles into ordered arrays via organic solvent evaporation will be presented as an example.