In a liquid-phase system containing an added reducing agent, the galvanic replacement reaction (GRR) between the gold precursor HAuCl₄ and Ag nanocuboids can proceed concurrently with the reduction reaction (RE) of the gold precursor, providing an effective strategy for constructing multifunctional nanostructures with complex morphologies. The resulting Au-Ag alloy nanoparticles synthesized through this strategy have shown broad application potential in optics, nanomedicine, sensing, and catalysis. However, previous studies that regulate the morphology of template particles through the coupled action of GRR and RE, Ag nanocubes have been predominantly used as templates, whereas Au@Ag core-shell nanocuboids, which possess distinct structural and performance advantages, remain insufficiently explored. In addition, most studies have isolated single factors (e.g., pH, temperature, or surface ligands), with the synergistic regulation among multiple reaction parameters largely overlooked. To address these issues, this work employs Au@Ag nanocuboids as template in a reaction system where GRR and RE coexist. By varying the initial pH of the solution, the surface ligand of the Au@Ag nanocuboids, and the injection mode of HAuCl₄, precise control over the particle morphology is achieved, and the synergistic mechanism among pH, ligand, and injection rate is systematically analyzed.
Under one-shot injection of HAuCl₄, an increase in pH reduces the reduction potential of the Au precursor species and enhances the reducing ability of L-ascorbic acid (AA), thereby driving a transition in the reaction pathway from GRR-dominated to RE-dominated. During this process, CTAC not only maintains colloidal stability, but also induces selective atomic deposition at the corners of the Au@Ag nanocuboids at pH ≈ 10.28 and pH ≈ 11.08 through its facet-selective adsorption on 100 facets, leading to the formation of protruded corner structures. However, at pH ≈ 3.16 and pH ≈ 12.10, this directing effect becomes less pronounced because of the excessively vigorous GRR or RE process. In contrast, owing to its pronounced steric hindrance, PVP consistently acts as a surface diffusion inhibitor and regulates the morphology in synergy with pH: it leads to the accumulation of Au-Ag particles at pH ≈ 3.35 (GRR-dominated); induces island-like growth at pH ≈ 10.00 or 10.79 (GRR-RE competitive); and results in a non-smooth Au layer at pH ≈ 12.06 (RE-dominated). Furthermore, regardless of the ligand type, switching the HAuCl₄ injection method from one-shot injection to dropwise titration significantly decreases the deposition rate, causing the particle morphology to evolve from rough to smooth and from disordered to ordered.
Furthermore, in the PVP-capped system with one-shot injection of HAuCl₄, Au@Ag-Au nanoparticles with rough surface layers of different thicknesses can be obtained by adjusting the pH, namely Thick-NPs with a rough layer thickness of ~17 nm at pH ≈ 3.35 and Thin-NPs with a rough layer thickness of ~5 nm at pH ≈ 10.00. Thick-NPs and Thin-NPs were then assembled into monolayer films through a three-phase interfacial self-assembly method, and their surface-enhanced Raman scattering (SERS) responses on Si wafers and Au films were comparatively investigated. The results show that the presence of the Au film significantly enhances the detection sensitivity. More unexpectedly, regardless of the substrate used, the SERS enhancement of the monolayer films does not increase with the rough layer thickness. Based on electromagnetic simulations and electron microscopy characterizations, the difference in SERS performance is attributed mainly to two factors: the surface curvature of the particles and the openness of the nanogaps. Specifically, for Thin-NPs, the surface protrusions have well-defined boundaries and relatively large local curvature, giving rise to strong near-field enhancement in the nanocavities formed between the tips of opposing surface particles (tip-tip) and between the surface particles and the Au film (tip-Au film). Moreover, the open nanocavities facilitate molecular diffusion and adsorption. In contrast, the outermost surface profile of Thick-NPs tends to be smoother and more rounded, resulting in much lower maximum near-field enhancement in both the tip-tip and tip-Au film regions than that of Thin-NPs. Meanwhile, these hotspot regions are relatively closed, making them difficult to access and thus less efficiently converted into actual SERS signal enhancement.
This work provides a useful guidance for the morphological regulation of noble-metal alloy nanoparticles and for the rational design and optimization of SERS substrates.