Synthesis of Au(Core)/Ag(Shell) nanoparticles and their conversion to AuAg alloy nanoparticles.

Metal nanoparticles (NPs) are of great interest due to their special optical, [ 1–3 ] electronic, [ 4–8 ] and catalytic [ 9,10 ] properties. [ 11 ] Among metal NPs, Au NPs have been investigated most extensively because of their facile preparation, resistance to oxidation, and surface plasmon resonance (SPR) band that can absorb and scatter visible light. [ 3 ] Core/ shell and alloy bimetallic NPs are especially interesting because they provide opportunities to tune the NPs’ optical and catalytic properties [ 12–15 ] and are potentially useful as taggants for security applications. [ 2 ] The AuAg system is of particular interest because the SPR band is tunable between ∼ 520 nm for Au [ 11 ] and ∼ 410 nm for Ag. [ 16 ] Several syntheses for AuAg alloy, [ 15 , 17–37 ] Au(core)/Ag(shell), [ 22 , 25 , 31 , 33 , 35–45 ] and Ag(core)/Au(shell) NPs [ 28 , 31 , 33–35 , 42 , 46–48 ] have already been reported. Here, we report a facile, stoichiometrically controlled synthesis of Au(core)/Ag(shell) and AuAg alloy NPs through digestive ripening, [ 49–51 ] which is a potentially general method for synthesizing alloy NPs. [ 37 , 52,53 ] Au(core)/Ag(shell) NPs were synthesized and annealed to form AuAg alloy NPs, followed by elemental analysis and structural and optical characterization. Methods utilizing Au rather than Ag NPs as the seed particles are advantageous: obtaining monodisperse Ag NPs is signifi cantly more challenging [ 16 ] because it is harder to control the nucleation of Ag NPs and to avoid oxidation. Huang and co-workers recently demonstrated the conversion of Au(core)/Ag(shell) to AuAg alloy NPs as a part of a larger study showing the generality of digestive ripening for synthesizing alloy NPs, but very limited data without quantitative elemental analysis or optical characterization of the AuAg alloy NPs was provided. [ 37 ] In this study, we compare a twostep synthesis of Au(core)/Ag(shell) NPs and their conversion to AuAg alloy NPs through annealing with a one-step, direct conversion of Au NPs to AuAg alloy NPs. The products of