Evidence for Seed-Mediated Nucleation in the Chemical Reduction of Gold Salts to Gold Nanoparticles Nikhil R. Jana,* Latha Gearheart, and Catherine J. Murphy* Department of Chemistry and Biochemistry, Graduate Science Research Center, University of South Carolina, Columbia, South Carolina 29208 Received August 17, 2000. Revised Manuscript Received May 5, 2001 Central to the concept of seed-mediated growth of nanoparticles is that small nanoparticle seeds serve as nucleation centers to grow nanoparticles to a desired size. We have examined this common assumption in a model system, the wet chemical synthesis of gold nanoparticles via reduction of a gold salt, by transmission electron microscopy and electronic absorption spectroscopy. We find that changing the seed concentration does affect the size of the product nanoparticles, but the method of reagent addition drastically affects the outcome even more. For fast addition of reducing agent, the presence of seeds appears to promote the formation of more seeds instead of growth. The observed nucleations are drastically enhanced (99%) compared to particle growth. For slow addition of reducing agent, the seeds do grow, but the product nanoparticle’s degree of homogeneity in shape is compromised. For higher concentrations of seeds, nanoparticle growth is better controlled for slow addition of reducing agent compared to fast addition of reducing agent. We propose a mechanistic step to explain the commonly observed size distribution. Introduction Metal nanoparticles are of great current interest due to their functions as chemical catalysts, adsorbents, biological stains, and elements of novel nanometer- scale optical, electronic, and magnetic devices. 1-13 As the size of the particle decreases to the 1-100 nm range, it is well-known that the electronic, optical, cata- lytic, and thermodynamic properties of metal particles deviate from bulk properties. 2,3,9-11 Size effects are also observed in surface-enhanced Raman scattering (SERS) experiments, where gold and silver nanopar- ticles are frequently used as substrates for signal enhancements. 14-16 Much work has been devoted to the synthesis of metal nanoparticles, especially the relatively easy solution- phase chemical reduction methods. 17 There is still a significant challenge, however, in understanding and predicting nanoparticle size and shape from a given set of synthetic conditions. 18-21 The use of ligand stabilizers, including dendrimers, is one key element that allows for a degree of control in nanoparticle synthesis. 4,5,17,22-24 The general mechanistic steps frequently assumed, but less often measured, in nanoparticle synthesis are nucleation and growth stages; 5,9,10,25-29 ideally, nucle- ation and growth would be separated in time to achieve * To whom correspondence should be addressed. 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Mater. 2001, 13, 2313-2322 10.1021/cm000662n CCC: $20.00 © 2001 American Chemical Society Published on Web 06/22/2001