Galvanic Replacement-Driven Transformations of Atomically Intermixed Bimetallic Colloidal Nanocrystals: Effects of Compositional Stoichiometry and Structural Ordering

Abstract

Galvanic replacement reactions dictated by deliberately designed nanoparticulate templates have emerged as a robust and versatile approach that controllably transforms solid monometallic nanocrystals into a diverse set of architecturally more sophisticated multimetallic hollow nanostructures. The galvanic atomic exchange at the nanoparticle/liquid interfaces induces a series of intriguing structure-transforming processes that interplay over multiple time and length scales. Using colloidal Au-Cu alloy and intermetallic nanoparticles as structurally and compositionally fine-tunable bimetallic sacrificial templates, we show that atomically intermixed bimetallic nanocrystals undergo galvanic replacement-driven structural transformations remarkably more complicated than those of their monometallic counterparts. We interpret the versatile structure-transforming behaviors of the bimetallic nanocrystals in the context of a unified mechanistic picture that rigorously interprets the interplay of three key structure-evolutionary pathways: dealloying, Kirkendall diffusion, and Ostwald ripening. By deliberately tuning the compositional stoichiometry and atomic-level structural ordering of the Au-Cu bimetallic nanocrystals, we have been able to fine-maneuver the relative rates of dealloying and Kirkendall diffusion with respect to that of Ostwald ripening through which an entire family of architecturally distinct complex nanostructures are created in a selective and controllable manner upon galvanic replacement reactions. The insights gained from our systematic comparative studies form a central knowledge framework that allows us to fully understand how multiple classic effects and processes interplay within the confinement by a colloidal nanocrystal to synergistically guide the structural transformations of complex nanostructures at both the atomic and nanoparticulate levels.

Transforming Noble‐Metal Nanocrystals into Complex Nanostructures through Facet‐Selective Etching and Deposition

“…In this review, we have mainly focused on transformations involving monometallic seeds with simple geometry in order to reveal the insights behind facet selectivity. It should be pointed out that there is also active research in progress regarding the transformation of bimetallic nanocrystals, in which several additional nanoscale processes such as solid‐state diffusion, Kirkendall diffusion, and dealloying other than galvanic replacement could kick in to generate nanocrystals with even more complex shapes, morphologies, and compositions. From all these endeavors, we will surely gain further insights into the facet‐selective etching and deposition mechanisms while significantly increase the diversity and functionality of metal nanocrystals.…”

Dealloyed Nanoporous Gold Catalysts: From Macroscopic Foams to Nanoparticulate Architectures

“…Fourth and most importantly, alloy nanoparticles exhibit drastically enhanced structural diversity and tunability compared to their bulk counterparts, creating unique opportunities for us to fine‐tailor a series of geometric and compositional parameters. Taking Au−Cu bimetallic nanoparticles as an example, recent advances in colloidal syntheses allow one to fine‐tailor not only the size, shape, and composition but also intraparticle atomic configurations (disordered alloys, ordered intermetallic phases, intraparticle compositional gradient, and phase segregation),, all of which are crucial factors profoundly influencing the structural transformations of the nanopartilces during the percolation dealloying. By coupling the percolation dealloying with other chemical reactions, such as galvanic replacement reactions and electrochemical atomic layer deposition,,, it becomes possible to incorporate other catalytically active materials into spongy Au nanoparticles in a highly controllable manner, enbaling us to further fine‐tune the catalytic properties of the dealloyed nanoparticles at a level of detail and precision unachievable on those dealloyed bulk materials.…”

“…We started from Au@Cu 2 O core‐shell nanoparticles whose core and shell dimensions could be precisely tuned over a broad size range using a seed‐mediated growth method . The Au@Cu 2 O core‐shell nanoparticles first transformed into Au−Cu bimetallic heteronanostructures through chemical reduction, and then underwent intraparticle alloying to form Au−Cu alloy nanoparticles upon thermal treatment in either a reducing atmosphere, such as H 2 , or in a high boiling point‐polyol solvent, such as tetraethylene glycol. The sizes and Cu/Au stoichiometric ratios of the alloy nanoparticles were essentially predetermined by the core and shell dimensions of their parental Au@Cu 2 O core‐shell nanoparticles and thereby could be systematically tuned over a broad range.…”

Complementing Nanoscale Galvanic Exchange with Redox Manipulation toward Architectural Control of Multimetallic Hollow Nanostructures

“…Au-Cu bimetallic nanoparticles constitute a unique structurally and compositionally tunable materials system that enables us to pinpoint the effects of compositional stoichiometry and structural ordering on the relative rates of dealloying, Kirkendall diffusion, and Ostwald ripening during nanoscale galvanic exchange. [28] The bulk phase diagram of the Au-Cu binary system (Figure 8A) shows that Au and Cu form fcc alloys spanning the entire stoichiometric range. An fcc AuCu3 intermetallic phase (AuCu3-I, Figure 8B) and a face-centered tetragonal (fct) AuCu intermetallic phase (AuCu-I, Figure 8C) are thermodynamically favoured in the temperature range below ∼400 °C.…”

“…Guided by this phase diagram, we designed and developed a multistep synthetic approach to Au-Cu alloy and intermetallic nanoparticles with fine-controlled sizes and compositional stoichiometries. [28] When dispersed in tetraethylene glycol (TEG) at 300 ⁰C, Au@Cu2O core-shell nanoparticles evolved into Au-Cu alloy nanoparticles through a stepwise Cu reduction and Au-Cu alloying process (Figure 8D). The Au/Cu stoichiometric ratios of the alloy nanoparticles were predetermined by their parental Au@Cu2O core-shell nanoparticles, whose core and shell dimensions could be precisely tuned through seed-mediated growth.…”