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J. Phys. Chem. C 2010, 114, 15916–15923
Wet Chemical Synthesis of Monodisperse Colloidal Silver Nanocrystals Using Digestive Ripening† Ravi Shankar,‡ Bin Bin Wu, and Terry P. Bigioni* Department of Chemistry, UniVersity of Toledo, Toledo, Ohio 43606 ReceiVed: NoVember 29, 2009; ReVised Manuscript ReceiVed: January 27, 2010
Silver metal nanocrystals with passivating ligand shells of dodecanethiol were synthesized by a one-phase wet chemical method. Postsynthesis thermochemical processing, known as digestive ripening, was used to focus the size distribution from a very broad distribution to 5.6 ( 0.4 nm. Chemical analysis of the final Ag nanocrystals showed a metallic core with a densely packed ligand shell. No oxidation was observed despite the entirely aerobic conditions. The size distribution was found to change most dramatically after the two steps proceeding digestive ripening, namely, ligand exchange and precipitation. Imaging also showed that the digestive ripening mechanism of Ag nanocrystals could be significantly different than that for Au, and involved a competitive silver thiolate side product. This synthesis was optimized to produce Ag nanocrystals with little to no silver thiolate materials. Introduction The study of colloidal metal nanoparticles began with the celebrated experiments of Michael Faraday in the mid 1800s.1 It has only been in the last few decades, however, that their synthesis has been extensively studied.2-25 Both aqueous and nonaqueous methods have been developed, primarily focused on Au and Ag. Producing narrow size distributions has been an ongoing challenge; however, there have been some notable successes. The most successful aqueous syntheses are largely based on the work of Turkevich and Frens.2-4 These original experiments used low concentrations of reactive species to promote a brief nucleation burst followed by diffusion-controlled growth kinetics, while suppressing secondary nucleation.26 This led to uniform growth and produced narrow size distributions over a wide range of sizes (16-150 nm). Approaches to synthesizing monodisperse nanoparticles in nonaqueous syntheses have been quite different. Wilcoxon and Brust developed the first methods,5,6 which used a two-phase approach in order to satisfy the different solvation requirements for the reagents and products. These reactions necessarily had heterogeneous reagent distributions and uncontrolled nucleation, and therefore produced polydisperse nanoparticles. However, the Brust method produced very small (
