Synergizing Mild Reducing Agents for Geometry-Controlled Nanocrystal Growth: A Case Study of Gold Nanooctahedra

Colloidal metal nanocrystals with precisely tailored geometric shapes have garnered immense research interest owing to their fascinating shape-dependent physicochemical properties. In comparison to one-pot seedless nanocrystal synthesis, seed-mediated nanocrystal growth offers a substantially higher level of precision and versatility for nanoparticle geometry control. Success in precision synthesis of metal nanocrystals with targeted geometric shapes relies critically on the structure-templating effects governed by preformed seeds, selective stabilization of targeted crystallographic facets by surface-capping agents, and rigorous kinetic control of relevant redox reactions by mild reducing agents. Here, we have observed a critical nanocrystal shape-directing effect originating from the synergism between different mild reducing agents, which has so far been overlooked in previous studies. When combining a primary reducing agent, such as ascorbic acid, with an even milder secondary reducing agent, such as citrate or tartrate, at appropriate molar ratios, the emergent synergistic effects can be exploited as a fine-adjustable synthetic tool for geometry control of colloidal Au nanocrystals at a precision level unachievable with a single reducing agent. Employing the {111}-faceting Au nanooctahedra as a model system for detailed mechanistic studies, we demonstrate that the interplay between the primary and secondary reducing agents can be deliberately tweaked to not only shift the redox equilibria but also fine-regulate the nanocrystal growth rates, selectively promoting the development of the {111} facets while effectively suppressing the intraparticle crystal twinning during seed-mediated nanocrystal growth in a {111} facet-favoring environment. As a proof-of-concept, we further demonstrate that such a synthetic strategy leveraging the synergism between dual-reducing agents also excels at nanocrystal shape control under {100} facet-favoring reaction conditions, enabling spherically shaped Au seeds to transform selectively into single-crystalline Au nanocubes enclosed by {100} facets.