Synthesis and Characterization of Hydrophobic, Approtically-Dispersible, Silver Nanoparticles in Winsor II Type Microemulsions Abhijit Manna and B. D. Kulkarni* Chemical Engineering Division, National Chemical Laboratory, Pune-411 008, India Krisanu Bandyopadhyay and K. Vijayamohanan Physical Chemistry Division, National Chemical Laboratory, Pune-411 008, India Received May 20, 1997. Revised Manuscript Received October 7, 1997 X Dodacanethiol-capped silver “quantum dot” particles (Q-particles) have been synthesized using a novel biphasic microemulsion (Winsor II type) of diethyl ether/AOT/water. FT-IR investigations and elemental analyses support the encapsulation of silver particles by dodecanethiol while the transmission electron micrograph reveals an average size of 11 nm. The optical band corresponding to a surface plasmon of silver confirms the nanoparticle nature. The evidence from X-ray photoelectron spectroscopy investigation confirms the metallic state of silver (Ag 0 ) and the encapsulation by thiol molecules. The thermogravimetric and differential thermal analysis indicates a heterogeneous interaction between thiol group and silver surfaces. Introduction The possibility of a dramatic change in electronic properties by varying the size of metal and semiconduc- tor particles has emerged recently as an area of impor- tant and fruitful research activity due to its fundamen- tal and technological relevance. For example, synthesis of various particles of “quantum dots” with sizes varying from 1 to 100 nm have found promising applications in microelectronic devices, 1 photocatalysis, 2 electrolumi- nance and electrocatalysis, 3 reprography, 4 etc. When the electrons and holes are confined within the three- dimensional potential well, the continuum of states in the conduction and valance bands is broken down into discrete states with an energy spacing, relative to band edge, which is approximately inversely proportional to the square of the particle size. 5,6 They have a charac- teristic high surface-to-volume ratio, providing sites for the efficient adsorption of the reacting substrates lead- ing to unusual size dependent chemical reactivity. 7-9 Although synthesis of nanodimensional colloids in biphasic system was known earlier, the problems such as their stability and precise control of reactivity have been tackled only recently using different strategies. Size control is often sought either through the attach- ment of appropriate protecting agents, such as gelatins, albumines, and other peptides, and macromolecules, such as polyethylene imine or polyvinylimidazole, on the surface of the cluster or to one another without leading to coalescence which results into the loss of their size- induced electronic properties. Another expedient method involves the use of self-assembled monolayer (SAM) formation with alkanethiols and amines for noble metal surfaces leading to the successful synthesis of stable “quantum dots.” For example, the work recently re- ported by Brust et al. 10 involves this type of a method using sodium borohydride reduction of an aqueous tetrachloroaurate in excess diethyl ether. Tetraoctyl- ammonium bromide was used as a phase-transfer cata- lyst to exchange tetrachloroaurate ions from aqueous to organic phase. Although this is an efficient one-step method for the synthesis of nanoparticles, the transfer- able metal ion should be in the form of anionic com- plexes, and hence, this method cannot be extended to common water soluble salts of the metals such as simple salts of silver and copper. The use of an inorganic phase in reverse micelles/ microemulsions has received considerable attention for preparing semiconductor and metal particles including platinum group 11,12 and the noble metals. 13-16 Micelles/ microemulsions are thermodynamically stable transpar- ent liquid systems consisting of, at least, a ternary mixture of water, a surfactant, or a mixture of surface- active agents and oil. Depending on the proportion of X Abstract published in Advance ACS Abstracts, November 15, 1997. (1) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (2) Hoffman, A. J.; Mills, G.; Yee, H.; Hoffmann, M. R. J. Phys. Chem. 1992, 96, 5546. (3) Brus. L. J. Phys. Chem. 1986, 90, 2555. 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(16) Gan, L. M.; Chew, C. H. Bull. Singapur Nat. Inst. Chem. 1995, 23, 27. 3032 Chem. Mater. 1997, 9, 3032-3036 S0897-4756(97)00374-8 CCC: $14.00 © 1997 American Chemical Society