Growth characteristics and optical properties of core/alloy nanoparticles fabricated via the layer-by-layer hydrothermal route

The layer-by-layer formation of core/alloy nanoparticles is described. Using presynthesized gold nanoparticle cores, Au_xAg_1-x alloy shells were deposited and annealed with subnanometer precision using a microwave irradiation (MWI) mediated hydrothermal processing method. The alloy composition, thickness, and nanoparticle morphology governed the surface plasmon resonance characteristics of the particles, as well as growth characteristics. The mechanism for alloy deposition, annealing, and interdiffusion was explored using two gold precursors, [AuBr₄]^- and [AuCl ₄]^-, and two hydrothermal temperatures (120, 160 C). Findings indicate that use of [AuCl₄]^- results in significant galvanic displacement, leading to nonuniform alloy formation and phase segregation at low annealing temperatures, which leads to loss of morphology control at intermediate compositions (x ≈ 0.25-0.75). In contrast, use of [AuBr₄]^- results in alloy shells with low galvanic interactions, leading to optimum alloy distribution and high fidelity control of alloy-shell thickness that, in combination with higher hydrothermal processing temperatures, leads to uniform and monodisperse core/alloy microstructure across all compositions. The alloy deposition and core/alloy nanoparticle growth was followed in situ by monitoring the change in surface plasmon resonance (SPR) signatures by UV-vis, which were unique to alloy shell thickness, as well as composition, and morphology. The interfacial alloy composition was probed by modeling the SPR with discrete dipole approximation, the results of which suggest the final alloy shells are Au-rich compared to the feed ratios, owing in large part to both galvanic displacements as well as core-to-shell alloy interdiffusion.