Published: March 23, 2011 r2011 American Chemical Society 5224 dx.doi.org/10.1021/ja200650w | J. Am. Chem. Soc. 2011, 133, 5224–5227 COMMUNICATION pubs.acs.org/JACS Layer-by-Layer Processing and Optical Properties of Core/Alloy Nanostructures Peter N. Njoki, † Wenjie Wu, † Hui Zhao, ‡ Lukas Hutter, † Eric A. Schiff, ‡ and Mathew M. Maye* ,† † Department of Chemistry and ‡ Department of Physics, Syracuse University, Syracuse, New York 13244, United States b S Supporting Information ABSTRACT: A novel hydrothermal layer-by-layer proces- sing method for the fabrication of core/alloy nanoparticles with highly tunable surface plasmon resonance is described. For a model system of Au/Au x Ag 1Àx , the processing tem- perature, alloy composition, and alloy thickness resulted in unique and tailorable plasmonic signatures. The discrete dipole approximation and selective alloy etching were used to correlate this optical response with the particle morphol- ogy and alloy phase ultrastructure. P ostsynthetic processing of nanomaterials may allow research- ers to obtain specific properties, morphologies, or phase regimes that are not accessible by simple synthesis alone. 1À10 Emerging techniques take advantage of the diffusion or inter- diffusion of impurities such as dopants, defects, or atoms at nanointerfaces, which even at modest temperatures have pro- found effects for confined nanosystems. 1,2 These effects and the resulting changes to the microstructure, lattice type, and spacing are emerging examples of the Kirkendall effect, where, in addition to redox potential, atomic and defect diffusion at the nanoparticle (NP) interface and interior is key. 2À6 Examples of this approach to alter the catalytic, optical, or morphological properties have recently been demonstrated. For example, the use of galvanic reactions at the NP interface has proven to be especially interes- ting. 7À10 Through the use of sacrificial palladium or silver nano- cubes, hollow gold shells or cubic gold cages can be fabricated, and this process can be followed in situ by monitoring the rich plasmonic behavior. 6À10 Another galvanic development is the ability for researchers to alter the optical and catalytic properties via reversible ion-exchange reactions in quantum-dot and quan- tum-rod superlattices. 3À5 A number of theoretical descriptions have also begun to investigate the role that nanoscale interfacial energies may play in alloy formation. 2,12 For example, alloy interdiffusion is expected to be thermodynamically and kinetically favorable, 2a and nanoalloys also may be highly stable, largely as a result of relieved interfacial energies. 2b In the present system, we explored the use of high-tempera- ture processing to promote alloy-shell formation on a presynthe- sized NP. The use of high temperature allows for alloy inter- diffusion into the NP core, which results in fine control of the shell thickness and alloy composition. For temperature control in an aqueous system, we explored a novel hydrothermal layer-by- layer processing method 11 to fabricate a binary Au/Au x Ag 1Àx core/alloy system. The Au x Ag 1Àx system was chosen as a model to explore this approach in large part because of the consti- tuents' miscible binary phase diagram and rich plasmonic behavior. 2À6,9,10,12,14À19 Figure 1a shows an illustration of our approach. The pre- synthesized AuNPs were combined with known feed ratios of the alloying components, which in this proof-of-principle study were [AuBr 4 ] À and AgNO 3 . The [AuBr 4 ] À complex was chosen in order to have similar redox potentials between the precursors and the Au 0 NP interface (E° ≈ 0.858 V). The feed ratio, r = ([Ag þ ] þ [AuBr 4 ] À )/[AuNP], was limited to that required for growth of a shell with a thickness (t S ) of only 0.25À0.5 nm. This process was then repeated for n layers. To promote reduction as well as alloy annealing, we employed a novel hydrothermal temperature (T H ) processing method exploiting automated microwave irradiation (MWI) for rapid and controllable dielectric heating. 11,13,14aÀ14c The use of a synthetic MW reactor for dynamic MWI facilitates fine control of the heating rate, cooling rates, and processing temperature as well as in situ monitoring of the reaction tem- perature (Figure 1b) and pressure (Figure 1c), as reported by Strouse, 13 El-Shall, 14aÀc and our previous work. 11 We began with an aqueous dispersion of AuNP cores (core diameter (d C ) = 15.4 ( 0.7 nm) and deposited shells of Au x- Ag 1Àx alloys with x = 0.0, 0.15, 0.50, 0.85, and 1.0 and controlled t S . Processing at T H = 120 or 160 °C produced a highly control- lable and unique optical surface plasmon resonance (SPR) as a function of x, t S , and T H . The SPR of a core/shell nanomaterial is derived from the size, shape, structure, composition, and sur- rounding medium of the nanostructure, 15À19 thus making Au/ Au x Ag 1Àx an attractive test case. To follow these optical and morphology changes, we employed UVÀvis spectrophotometry (UVÀvis) and transmission electron microscopy (TEM). Fig- ure 2 shows a representative set of UVÀvis and TEM results for the Au/Au x Ag 1Àx system at T H = 120 °C and x = (a) 0.15, (b) 0.50, and (c) 0.85. For example, at x = 0.15 (Figure 2a), we observed a systematic blue shift in λ SPR from the value for AuNPs (520 nm) to 490 nm at n = 3 and a broad SPR further shifted to 405 nm at n = 7. For UVÀvis of NPs with n =1À10 layers, see Figures S1 and S2 in the Supporting Information. Such a blue shift of the SPR is indicative of an increasingly Ag-rich nanostructure. 15,20 The corresponding TEM results for n =3 and 7 are shown in Figure 2a. Two observations can be made. First, the overall morphology of the core/alloy NPs remained largely unchanged (e.g., spherical), and second, an increase in the core þ shell diameter (d CþS = 18.1 ( 1.4 and 21.2 ( 2.0 nm) was observed. Received: January 22, 2011