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Complementing Nanoscale Galvanic Exchange with RedoxManipulation toward Architectural Control of Multimetallic HollowNanostructuresGuangfang Grace Li, Zixin Wang, and Hui Wang*[a]

MinireviewDOI: 10.1002/cnma.202000139

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Abstract: Galvanic exchange occurring within the confine-ment by nanoparticulate templates provides a unique path-way to controllably transform solid metallic nanoparticlesinto architecturally more sophisticated multimetallic hollownanostructures under facile reaction conditions. In thisMinireview, we elaborately discuss how the nanoparticle-templated galvanic exchange can be deliberately coupledwith redox manipulation to synthesize a large library of

complex multimetallic hollow nanostructures exhibiting pre-cisely tailored architectures and compositions, with a partic-ular emphasis on important mechanistic insights concerningthe dynamic interplay among multiple structure-remodelingprocesses that synergistically dictate the versatile nano-particle transforming behaviors during kinetically and regio-selectively modulated galvanic exchange reactions.

1. Introduction

The physical and chemical properties of a nanoparticle can bedrastically modified by excavating the particle interior to createa hollow nanostructure. In comparison to their solid counter-parts, hollow nanostructures with deliberately tailored interiorarchitectures exhibit greatly enhanced tunability in terms ofboth structures and compositions, enabling us to rationally fine-tune their optical, electronic, and catalytic properties forspecifically targeted applications.[1] The past two decades havewitnessed rapid advancement in structure- and composition-controlled synthesis of hollow nanostructures,[1–2] which has laida solid foundation for detailed, quantitative investigations ofthe intriguing structure-property relationships. Within thiscontext, galvanic exchange templated by metallic nanoparticleshas emerged as a unique approach that transforms solidmonometallic nanoparticles of simple geometries into multi-metallic hollow nanostructures adopting sophisticatedarchitectures.[2]

Galvanic exchange is a classic electrochemical redox processthermodynamically driven by the difference in the redoxpotentials of a pair of metals. During galvanic exchange, twohalf reactions occur concurrently at the solid-electrolyte inter-faces, the oxidation of the less-noble metal at the anode andthe reduction of the ionic species of the more-noble metal atthe cathode. When galvanic exchange takes place within theconfinement by a nanoparticulate template, both the anodicand the cathodic reactions occur on the surfaces of thestructurally evolving template, giving rise to the formation ofmultimetallic hollow nanostructures that are typically unrealiz-able through other synthetic pathways.[2a] The structures andcompositions of the resulting hollow nanoparticles are pro-foundly influenced by the structural characteristics of thetemplates and detailed experimental conditions under whichthe galvanic exchange reactions occur. Further complementingnanoscale galvanic exchange with deliberate redox manipula-tion enables architectural control of hollow nanostructures at aremarkably higher level of precision and versatility.

This Minireview highlights the latest progress on thestructure-controlled synthesis of multimetallic hollow nano-structures through galvanic exchange coupled with redoxmanipulation and epitomizes recently gained mechanistic in-sights regarding the dynamic interplay of multiple galvanicexchange-induced structure-rearranging processes. Thesemechanistic insights are distilled and extracted from ampleexperimental observations through systematic case studies ofseveral representative nanoparticle systems, which are dis-cussed in the order of increasing complexity in terms ofnanoparticle structures and synthetic conditions. We start withrelatively simple cases, in which monometallic or heterostruc-tured bimetallic nanocrystals are converted into hollow nano-particles with varying degree of architectural complexity upondirect galvanic exchange with a more-noble metal. We thendiscuss how to synthesize even more complex hollow nano-structures by coupling galvanic exchange with other redoxprocesses, such as seed-mediated reduction, oxidative etching,and regioselective surface passivation. Employing composition-ally tunable Au� Cu alloy and intermetallic nanocrystals asmodel sacrificial templates, we further show that atomicallyintermixed multimetallic nanoparticles undergo structural trans-formations remarkably more complicated than those of theirmonometallic and phase-segregated heterostructured counter-parts as galvanic exchange becomes further entangled withseveral other processes, such as percolation dealloying, seed-mediated deposition, Ostwald ripening, and Kirkendall diffusion.Finally, we briefly comment on how to apply classic effects andparadigms to intriguing nanoparticle systems to generate ageneric conceptual framework guiding rational design of newsynthetic approaches to multimetallic hollow nanostructureswith unprecedented tunability of structures, compositions, andproperties.

2. Galvanic Exchange: A Versatile Pathway toHollow Nanostructures

2.1. From Solid Ag Nanocrystals to Au-Rich HollowNanostructures: Model Systems for Mechanistic Studies

A prototypical materials system fully demonstrating theversatility of galvanic exchange-driven nanoparticle transforma-tions comprises Au� Ag bimetallic nanoboxes, nanocages, andnanoframes, all of which are derived from the galvanic

[a] Dr. G. G. Li, Z. Wang, Prof. H. WangDepartment of Chemistry and BiochemistryUniversity of South Carolina631 Sumter Street, Columbia, South Carolina 29208 (United States)E-mail: wang344@mailbox.sc.eduThis manuscript is part of a special collection on Hollow Nanostructures.Click here to see the Table of Contents of the special collection.

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exchange between colloidal Ag nanocubes and ionic Auprecursors in aqueous solutions.[1d,2c,3] Galvanic exchange occursspontaneously when exposing Ag nanocrystals to AuCl4

� ,substituting every 3 Ag atoms with 1 Au atom. The mostconvenient way to resolve the structural evolution at variousreaction stages is to titrate colloidal Ag nanocrystals withvarying amount of AuCl4

� followed by ex situ structuralcharacterizations of the nanoparticles separated from thereaction mixtures. Figures 1A–1E clearly show the morpholog-ical changes of single-crystalline Ag nanocubes upon titrationwith incremental amount of AuCl4

� .[3a] Galvanic exchange wasinitiated locally at a high-energy surface site, such as surfaceatomic steps or point defects, rather than non-specifically over

the entire nanocube surface. Upon the initiation of Agdissolution, a surface pinhole was generated, which served asthe anode site for continuous Ag dissolution. Meanwhile, thereduction of AuCl4

� resulted in epitaxial deposition of metallicAu on Ag surfaces. The deposited Au then became alloyed withAg through atomic interdiffusion, resulting in Au� Ag alloynanoboxes. At sufficiently high AuCl4

� concentrations, Ag wasselectively etched from the Au� Ag alloy walls through deal-loying, eventually giving rise to the formation of Au-richnanocages, a hollow nanostructure with nanopores randomlydistributed on the walls.

Introduction of corner truncations to the nanocubes drasti-cally modified the structural evolution during galvanic ex-change (Figures 1F–1J).[3b] Because polyvinylpyrrolidone (PVP)served as the surface-capping ligands selectively passivatingthe {100} facets, the dissolution of Ag preferentially occurred onthe {111} facets at the truncated corners, while Au was primarilydeposited on the {100} side facets, leading to the formation ofnanocages with pores exclusively located at the particle corners.The detailed mechanisms of such regioselective etching anddeposition will be further discussed later in this Minireview.

When switching the Au precursor from AuCl4� to AuCl2

� , Agand Au underwent galvanic exchange at an atomic ratio of 1 : 1,and an altered structure-transforming process was observed. Agnanocubes gradually evolved into nanoboxes, nanocages, andcubic nanoframes when titrated with increasing amount ofAuCl2

� (Figures 1K–1P).[3c] The strikingly different nanoparticletransforming behaviors for the Au(III) and Au(I) systems wereessentially caused by the difference in redox potentials of theAu precursors, the alloying/dealloying kinetics, and the stoichio-metric ratios of Au� Ag atomic exchange.

During galvanic exchange, both alloying and dealloyinginvolved the atomic interdiffusion of Au and Ag within eachnanoparticle. Because the outward diffusion of Ag atoms wasfaster than the inward diffusion of Au atoms, cavities werecreated in the particle interior and gradually expanded involume as the galvanic exchange proceeded, a classic effectknown as the Kirkendall effect.[4] Therefore, the versatile trans-forming behaviors of Ag nanocubes observed during galvanicexchange were intimately tied to three key intertwiningprocesses, alloying, dealloying, and Kirkendall diffusion.[2a,4d,5]

2.2. Architecturally Complex Hollow Nanostructures Derivedfrom Galvanic Exchange

Although structurally different, the nanoboxes, nanocages, andnanoframes generated upon galvanic exchange all inherit thesame cubic shape from the Ag nanocubes, implying that theoverall shapes of the resulting hollow nanostructures are pre-defined to certain extent by the parental sacrificial templates.This shape-defining rule, which has been further verified byseveral other model nanoparticle geometries,[4d,6] serves as ageneric design principle guiding the development of galvanicexchange-based synthetic approaches to hollow nanostructureswith more sophisticated interior architectures. Here we selectseveral representative examples to highlight the structure-

Guangfang Grace Li received her B.S.degree in Chemistry from Wuhan Insti-tute of Technology in China in 2009and her Ph.D. degree in PhysicalChemistry from the University of SouthCarolina in 2018. Her Ph.D. work,supervised by Hui Wang, was focusedon the structure-controlled synthesisand electrocatalytic properties of multi-metallic nanostructures. After a postdocstint with John C. Flake at LouisianaState University, she joined the facultyof Chemistry and Chemical Engineeringat Huazhong University of Science andTechnology, Wuhan, China, as an Asso-ciate Professor in October 2019.

Zixin Wang received her B.E. degreefrom South China University of Technol-ogy in 2015 and her M.S. degree inPolymer Engineering from University ofAkron in 2017. She is currently agraduate student majoring in PhysicalChemistry under the supervision of HuiWang at the University of South Caro-lina. Her current research focuses on thecolloidal synthesis and catalytic proper-ties of metallic nanoparticles.

Hui Wang received his B.S. in Chemistrywith honors (advisor: Jun-Jie Zhu) fromNanjing University in China in 2001 andPh.D. in Physical Chemistry (advisor:Naomi J. Halas) from Rice University in2007. After postdoctoral training underthe tutelage of Paul F. Barbara at theUniversity of Texas at Austin, he joinedthe faculty of Chemistry and Biochemis-try at the University of South Carolinaas a tenure-track Assistant Professor in2010 and was promoted to AssociateProfessor with tenure in 2016. Hisindependent research has been focusingon the structure-property relationshipsof complex nanostructures and catalyticand photocatalytic molecular transfor-mations on nanoparticle surfaces.

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directing role of the nanoparticulate templates during galvanicexchange reactions.

As schematically illustrated in Figure 2A, matryoshka-likeshell-in-shell nanostructures could be synthesized through amultiple-step process involving sequential galvanic exchangeand metal deposition.[7] Single-walled Au� Ag alloy nanoshellswere first synthesized by reacting spherical Ag nanoparticleswith AuCl4

� under standard galvanic exchange reaction con-ditions. Then a conformal Ag layer was deposited on the outersurface of the alloy nanoshells to form heterostructured nano-shells, which underwent a second-round galvanic exchangeupon exposure to AuCl4

� , eventually evolving into double-

walled nanoshells. Similarly, double-walled nanotubes weresynthesized following this procedure using Ag nanowires as theinitial templates (Figure 2B). Repeating this sequential cycle ofgalvanic exchange/metal deposition resulted in multi-walledhollow nanostructures (Figures 2C and 2D).

Substituting the monometallic templates with heterostruc-tured bimetallic core-shell nanoparticles enabled synthesis ofcomplex hollow heteronanostructures containing solid coresconfined within hollow structures.[8] An interesting examplereported by Neretina and coworkers was the “Wulff-in-frame”nanostructure derived from substrate-supported core-shellnanoparticles (Figure 2E).[9] This synthetic procedure started

Figure 1. (A) Scheme illustrating the structural evolution of an Ag nanocube through galvanic exchange with various amounts of AuCl4� . Scanning electron

microscopy (SEM) images of (B) Ag nanocubes and Au� Ag bimetallic hollow nanostructures obtained by titrating Ag nanocubes with (C) 0.3, (D) 0.5, and (E)2.25 mL of 1 mM HAuCl4. The insets show the transmission electron microscopy (TEM) images of thesamples. Reprinted with permission from ref [3a].Copyright 2004, American Chemical Society. (F) Scheme illustrating the structural evolution of an Ag nanocuboctahedron (nanocube with truncated corners)through galvanic exchange with various amounts of AuCl4

� . SEM and (insets) TEM images of (G) Ag nanocuboctahedra and Au� Ag bimetallic hollownanostructures obtained by titrating Ag nanocuboctahedra with (H) 0.6, (I) 1.6, and (J) 3.0 mL of 0.1 mM HAuCl4. Reprinted with permission from ref [3b].Copyright 2006, American Chemical Society. (K) Scheme illustrating the structural evolution of an Ag nanocube through galvanic exchange with variousamounts of AuCl2

� . SEM and (insets) TEM images of Au� Ag bimetallic hollow nanostructures obtained by titrating Ag nanocubes with (L) 1, (M) 3, (N) 4, (O) 5,and (P) 6 mL of 0.2 mM AuCl2

� . Reprinted with permission from ref [3c]. Copyright 2008, Springer.

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with Wulff-shaped (cuboctahedral) nanoparticles of Au, Pt, orPd synthesized through dewetting of ultrathin metallic films. Agwas then deposited epitaxially on the Wulff-shaped nano-particles to selectively form Wulff- or cube-shaped core-shellnanoparticles. Through galvanic exchange of Ag with Au, thecore-shell nanoparticles evolved into a series of heteronanos-trucrtures in which a Wulff-shaped core was confined within ananoshell, nanocage, or nanoframe. As shown in Figures 2F and2G, the Wulff structure of the core, the shape of the Ag shells,and the epitaxial relationship between the nanoparticles andthe substrates were all well-preserved during galvanic ex-change.

Monometallic Ag nanocrystals may evolve into a series ofcomplex multimetallic hollow nanostructures through simulta-neous or sequential action of galvanic exchange and Kirkendalldiffusion. As demonstrated by Puntes and coworkers,[10] Agnanocubes transformed into Au� Ag double-walled nanoboxes(Figure 2H) through simultaneous action of galvanic exchangeand the Kirkendall effect. During galvanic exchange, a cavitywas developed in the interior of the Ag template with a layer ofAu deposited on the cavity surfaces, forming a thin Ag layersandwiched between two Au layers. The compositional gradientacross the wall induced the interdiffusion of Au and Ag atoms,creating Kirkendall voids within the nanocrystal walls. The voidsthen grew and coalesced to form a continuous cavity parallel to

Figure 2. Schemes illustrating the synthesis of double-walled (A) nanoshells and (B) nanotubes through sequential galvanic exchange and metal deposition.(C) TEM images of Au� Ag alloy multi-walled nanoshells. (D) SEM images of Au� Ag alloy multi-walled nanotubes. Reprinted with permission from ref [7].Copyright 2004, American Chemical Society. (E) Scheme illustrating the synthesis of Wulff-in-frame nanostructures. (left) Geometric models and (right) SEMimages of individual nanoparticles composed of (F) a Wuff-shaped Pt core in Wuff-shaped Au frame and (G) a Wuff-shaped Pt core in cube-shaped Au frameviewed along different zone axis as labled in each panel. Reprinted with permission from ref [9]. Copyright 2016, American Chemical Society. (H) Schematicillustration (top row), high-angle annular dark field (HAADF) STEM (Z-contrast) images (middle row), and energy dispersive spectroscopy (EDS) elemetnalmapping reuslts (bottom row) revealing the evolution of an Ag nanocube into a double-walled nanobox through Ag� Au galvanic exchange. (I) TEM imageand EDS elemental maps of a trimetallic Pd� Au� Ag nanobox containing multiple interior chambers. Reprinted with permission from ref [10]. Copyright 2011,American Association for the Advancement of Science.

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the nanocube surface, resulting in the formation of Au� Agdouble-walled nanoboxes. Galvanic exchange could also becoupled with Kirkendall diffusion sequentially by exposing Agnanocubes to Pd(II) and Au(III) precursors in a stepwise manner.Galvanic exchange between Ag and Pd first converted thenanocubes to Ag� Pd bimetallic hollow nanoparticles, whichfurther reacted with Au(III) to generate Au� Pd� Ag trimetallicnanoboxes with multiple interior compartments (figure 2I)through a process dominated by the Kirkendall effect.

2.3. Mechanistic Insights Gained from in situ Spectroscopiesand Microscopies

The conventional way to study the structure-evolving kineticsduring galvanic exchange is to stop the reactions at variousstages followed by ex situ structural characterizations. However,it remains difficult to fully capture all the intermediatestructures, especially those short-lived ones, using such“quench-and-look” approaches. In addition, the particle-to-particle deviation and heterogeneous structural dynamics areeasily smeared out when ex situ characterizations are performedat the ensemble level. In situ spectroscopic and microscopicapproaches have unique capabilities of resolving structuralevolution at the single-particle level in real time during galvanicexchange reactions,[11] providing mechanistic insights at a levelof detail and comprehensiveness not achievable through ex situ“quench-and-look” measurements.

Jain and co-workers used in situ single-particle scatteringspectroscopy to monitor the transformation of individual Agnanoparticles in real time during galvanic exchange withAuCl4

� .[11a] The light scattering features of individual metallicnanoparticles essentially reflected the characteristics of plasmonresonances, serving as the spectral signatures of the particlestructures and compositions. The transition from brightlyscattering solid Ag nanoparticles to dimly scattering Au� Agbimetallic nanocages could be monitored in situ one particle ata time based on the temporal evolution of scattering intensitiesin the dark-field microscopic images and the spectral shift ofthe plasmon resonance in the single-particle scattering spectra(Figures 3A and 3B). As shown in Figure 3C, the scatteringtrajectory of the ensemble (black trace) was featured by agradual conversion, whereas each singe-particle scatteringtrajectory (colored traces) showed a drastically distinct, abruptconversion after an induction or “waiting” time. The onset ofthe sharp transitions corresponded to the formation of a criticalvoid containing approximately 20 atomic vacancies at theparticle surface. Once a critical void formed, the transition tonanocages occurred much more rapidly in a highly cooperativemanner, kinetically limited solely by the mass transport ofdepositing Au and dissolving Ag. The rate of critical-voidformation varied significantly from particle to particle, resultingin a broad distribution of single-particle waiting times. There-fore, the gradual transition observed in the ensemble trajectorywas essentially an outcome averaged over many sharp trans-formation events of individual nanoparticles and the ensemblereaction rate was kinetically limited by the stochastic formation

of critical voids. The rate of critical void formation dependedsensitively on the concentration of Au(III) ions. As the Au(III)concentration increased, the average waiting time prior tocritical void formation was shortened and the dispersion insingle-particle waiting times became narrower (Figure 3D),giving rise to faster transition kinetics at the ensemble-scale. Instriking contrast, the intrinsic transforming rates of singlenanoparticles following the critical void formation appearedindependent of the Au(III) concentration (Figure 3E), suggestinga mass-transport-limited hollowing process. When sufficient Cl�

was added into the reaction mixtures, the nanoparticle trans-formations switched from a critical step-limited, cooperative“particle-to-particle” process to a noncooperative, “atom-by-atom” process (Figure 3F).[11d] The inhibition of cooperativitywithin a nanoparticle could be interpreted as the consequenceof an AgCl layer generated in situ on the template surfaces,which hindered the cavity growth by suppressing the Kirkendallinterdiffusion of Ag and Au atoms.

Using liquid cell transmission electron microscopy (TEM) asan in situ imaging tool enables direct observation of richstructural dynamics during galvanic exchange reactions.[11g–i]

Although the state-of-the-art in situ TEM offers sub-nm spatialresolution and sub-100 ms time resolution for imaging nano-scale objects in liquid cells, the effects of electron beamirradiation may result in structural transformations distinct fromthose under real synthetic conditions.[11g–i] Mirsaidov and co-workers[11i] investigated how individual Ag nanocubes evolvedinto Ag� Au bimetallic hollow nanoparticles during galvanicexchange through carefully designed in situ liquid cell TEMmeasurements. To minimize the perturbation caused by theelectron beam, the samples were imaged at an acquisition rateof 25 frames per second using a fast and sensitive CMOScamera with an incident electron flux <30 e� Å� 2 s� 1. The use ofa temperature-controlled liquid cell enabled systematic compar-ison across different reaction regimes by varying the reactiontemperature and the Au precursors.

Figure 4A shows the in situ TEM results revealing themorphological evolution of an Ag nanocube upon exposure toHAuCl4 at 23 °C. At the early stage, a thin layer composed of Auand AgCl was rapidly deposited on the Ag nanocubes surface,followed by the formation of a void gap between Ag and theouter layer. Due to the Kirkendall effect, the outer layercontinuously expanded outwards and became rougher, result-ing in a corrugated shell encompassing multiple voids. Asignificant portion of the original Ag nanocube still remainedwith an internal gap isolating the Ag core from the outer shell.The remaining Ag then underwent a second-round galvanicexchange till the outer shell eventually collapsed. Increasing thereaction temperature significantly accelerated the nucleationand propagation of the voids during galvanic exchange. At90 °C (Figure 4B), multiple voids quickly nucleated within thesame nanocube at the early stage of galvanic exchange. Theinitial voids immediately merged into a larger void, whichsubsequently grew and propagated across the nanocube toform an Au� Ag alloy nanobox. Dealloying of the nanobox walleventually led to the formation of a nanocage. Figure 4C showsa series of in situ TEM images revealing a different transforming

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process when an Ag nabocube underwent galvanic exchangewith AuCl2

� at 90 °C. The Ag nanocube rapidly transformed intoan Au� Ag alloy nanobox at the early stage of the reaction.Dealloying of the nanobox occurred immediately and voidswere nucleated at the shell surface. The voids subsequentlyexpanded and merged into a single channel to form a double-walled nanobox through a Kirkendall diffusion-driven process.The in situ TEM observations across different reaction regimesprovided direct experimental evidence verifying the stronginterplay between Kirkendall diffusion and galvanic exchangeduring the nanoparticle hollowing processes.

3. Coupling Galvanic Exchange with RedoxManipulation

3.1. Coupling Galvanic Exchange with Seed-MediatedReduction

Bedzyk and coworkers[11e] recently found that the hollownanostructures generated at various reaction stages weresignificantly more Au-rich than expected when citrate-cappedAg nanospheres underwent galvanic exchange with AuCl4

� . Thesurface-capping citrate served as a mild reducing agent causingthe deposition of additional Au on the nanoparticle surfaces.Therefore, the deviation from the expected atomic ratio of 3 : 1for Ag� Au exchange was essentially caused by the coupling ofgalvanic exchange with seed-mediated reduction.

Using galvanic exchange between Ag nanocubes andH2PdCl4 as a model system, we demonstrated that the relative

Figure 3. (A) Snapshot dark-field scattering microscopy images of Ag nanoparticles at various reaciton times during galvanic exchange with AuCl4� . (B) Time

trajectory of the scattering intensity of a single nanoparticle during galvanic exchange. Single-particle scattering spectra at four time points along thetrajectory are shown as the insets. (C) The time trajectory of scattering intensity for an ensemble of nanoparticles (black trace) and trajectories of singlenanoparticles (colored traces). The distribution of single-nanoparticle (D) waiting times and (E) time constants for the abrupt solid-to-hollow transitions atvarious AuCl4

� concentrations as labeled in each panel. Reprinted with permission from ref [11a]. Copyright 2014, Wiley-VCH. (F) Ensemble (upper panels) andsingle-particle (lower panels) time-trajectories of scattering intensity during galvanic exchange with 5 mM AuCl4

� in the absence of NaCl (left panels) and inthe presence of 10 mM NaCl (right panels). Reprinted with permission from ref [11d]. Copyright 2017, Royal Society of Chemistry.

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rate of seed-mediated reduction with respect to that of galvanicexchange could be manipulated with the aid of mild reducingagents, such as ascorbic acid (AA) and formaldehyde (HCHO).[12]

In a reducing agent-free environment, Ag nanocubes graduallyevolved into Ag� Pd bimetallic nanoboxes and nanocagesthrough a transforming process dominated by alloying, deal-loying, and Kirkendall diffusion,[12–13] analogous to thosetypically observed in the Ag� Au systems. In the presence ofmild reducing agents, the coupled action of seed-mediatedreduction and galvanic exchange resulted in a variety of Ag� Pdbimetallic hollow nanostructures with complex interior andsurface architectures (Figure 5A). The seed-mediated depositionof Pd on the template surfaces created additional surface sites

with local compositional gradient, which triggered the Ag� Pdatomic interdiffusion and facilitated the nanoparticle hollowingprocess. With the same amount of H2PdCl4, the hollow nano-structures obtained in the presence of a mild reducing agentwere compositionally more Pd-rich than their counterpartssynthesized under reducing agent-free conditions. Besides theAg� Pd bimetallic system, complex hollow nanostructures ofother bimetallic and trimetallic systems, such as Ag� Au,[14]

Pt� Pd,[15] Pd� Ir,[16] and Ag� Au� Pd,[17] have also been successfullysynthesized through reducing agent-mediated galvanic ex-change. While a mildly reducing environment promoted thenanoparticle hollowing process, introduction of strong reducingagents inhibited the galvanic exchange, resulting in epitaxial

Figure 4. Time series of in situ TEM images and corresponding schematic illustrations showing the galvanic exchange-driven morphological evolution of an Agnanocube upon exposure to (A) AuCl4

� at 23 °C, (B) AuCl4� at 90 °C, and (C) AuCl2

� at 90 °C. The arrows in the TEM images indicate the locations for thenucleation and propagation of Kirkendall voids. Reprinted with permission from ref [11i]. Copyright 2017, Nature Publishing Groups.

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growth of the more-noble metal on the less-noble templates toform core-shell heteronanostructures.[18]

Employing bimetallic heteronanostructures as the initialtemplates, hollow nanoparticles with further increased struc-tural complexity could be synthesized through galvanicexchange coupled with seed-mediated reduction. As demon-strated by Liz-Marzán and coworkers,[19] Au@Ag core-shellnanocuboids gradually evolved into octahedral nanorattleswhen exposed to AuCl4

� for galvanic exchange in the presenceof AA (Figures 5B–5G). Each octahedral nanorattle was com-posed of an Au nanorod core and a bimetallic heterostructuredshell (an inner Au� Ag alloy shell covered by a monometallic Auouter shell) separated by a nanoscale intraparticle gap, as

revealed by quantitative X-ray energy-dispersive spectroscopytomography (Figure 5H). This study suggested that the shape ofthe hollow nanoparticles could be controlled beyond theirtemplate morphology when coupling galvanic exchange withseed-mediated reduction, resulting in the formation of nano-particles of exotic geometries with complex elemental distribu-tions.

3.2. Coupling Galvanic Exchange with Oxidative Etching

As exemplified by the Ag� Au and Ag� Pd galvanic exchangereactions discussed above, the hollow nanostructures obtained

Figure 5. (A) TEM images of Ag� Pd bimetallic hollow nanostructures synthesized through galvanic exchange of Ag nanocubes with various amounts ofH2PdCl4 in the absence of reducing agents (top row) and in the presence of AA (middle row) and HCHO (bottom row). The cartoons in the insets illustrate thecross-sectional views of the nanostructures. All the TEM images share the same scale bar. Reprinted with permission from ref [12]. Copyright 2015, AmericanChemical Society. TEM images of Au@Ag core-shell nanocuboids (B) before and (C� G) after galvanic exchange with increasing amounts of 0.5 mM HAuCl4 inthe presence of AA. The corresponding HAADF-STEM images (i) and EDS-elemental maps (ii) (green, Ag; red, Au) are included as the insets in panels C–G.Scale bars for the TEM and HAADF-STEM images are 100 and 40 nm, respectively. (H) Quantitative EDS tomography images showing the elementaldistributions in individual nanoparticles obtained at various reaction stages (from i to v, the amount of HAuCl4 increases). The top, middle, and bottom rowsshow the reconstructed 3D structures, the inner view of the 3D reconstructions, and reconstruction slices (the color scale reflects the percentage of Au),respectively. Reprinted with permission from ref [19]. Copyright 2016, American Chemical Society.

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at various reaction stages were all bimetallic in nature,containing significant fractions of the less-noble Ag, whicheither existed as monometallic domains or formed alloys withthe more-noble metals. By exposing these bimetallic hollownanostructures to appropriate oxidants, the less-noble compo-nents could be selectively etched while the more-noblecomponents underwent structural remodeling to form hollownanostructures unrealizable solely from a galvanic exchangereaction. A hollow nanostructure of particular interest to thecatalysis communities is the ultrathin nanoframes of noblemetals, which could be controllably synthesized throughsequential action of galvanic exchange and oxidative etching.[20]

As shown in Figures 6A–6C, Au� Ag nanoboxes derived fromgalvanic exchange between Ag nanocubes and AuCl4

� furtherevolved into nanocages and nanoframes when exposed to an

oxidative etchant, such as Fe(NO3)3 and NH4OH.[20a] The resulting

nanoframes were significantly thinner than those obtainedthrough direct galvanic exchange between Ag nanocubes andAuCl2

� (Figure 1P).[3c] Because the more-noble metals werepreferentially deposited at high energy surface sites, such asparticle edges and vertices, at the early stage of galvanicexchange, the nanoframes obtained after oxidative etchingtypically inherited the shapes defined by the parentaltemplates.[20b–d] Figures 6D–6G show the TEM images of sub-5 nm ultrathin nanoframes adopting several representativeframework shapes, which were derived from Ag triangularnanoprisms, decahedral nanocrystals, and truncated nanocubes,respectively, through sequential action of galvanic exchangeand oxidative etching.

Figure 6. (A) Schematic illustration of the galvanic exchange-driven transformation of an Ag nanocube into an Au� Ag nanobox, which further evolves into anAu-rich nanoframe through etching by Fe(NO3)3 or NH4OH. TEM and (insets) SEM images of (B) nanocages and (C) nanoframes produced by etching thenanoboxes with 15 and 20 μL of 50 mM aqueous Fe(NO3)3 solution. Reprinted with permission from ref [20a]. Copyright 2007, American Chemical Society. TEMimages of ultrathin Au nanoframes synthesized through sequential action of galvanic exchagne and oxidative ethcing with (D, E) triangular, (F) decahedral,and (G) cuboctahederal shapes. Reprinted with permission from refs [20b–d]. Copyright 2013, Wiley-VCH. Copyright 2011 and 2015, American ChemicalSociety. (H) Schematic illustration of the structural transformations of a quasi-spherical Ag nanoparticle into an Au� Ag octahedral nanoframe through a one-pot synthetic process involving simutanueous action of galvanic exchange and oxidative ethcing. (I) TEM images of Au� Ag octahedral nanoframes. Reprintedwith permission from ref [21]. Copyright 2012, American Chemical Society. (J) Schematic illustration of the structural evolution from a truncated octahedralPtNi3 alloy nanoparticle to a trimetallic Pt3Ni nanoframe decorated with Au islands exclusively located at the nanoframe corners during galvanic exchangecoupled with simutaneous etching. (K, L) TEM images and (M) EDS-elemental maps of an individual trimetallic Au-Pt3Ni nanoframe. Reprinted with permissionfrom ref [22]. Copyright 2014, American Chemical Society.

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Oxidative etching could also work simultaneously withgalvanic exchange to form complex multimetallic hollow nano-structures. As reported by Li and coworkers, quasi-spherical Agnanocrystals transformed into Au� Ag octahedral nanoframesthrough a facile one-pot synthetic process involving simulta-neous action of galvanic exchange and oxidative etching(Figures 6H and 6I).[21] They further demonstrated that galvanicexchange could be coupled with oxidative etching in aregioselective manner to create complex heterostructurednanoframes, as exemplified by the Pt� Ni bimetallic nanoframesdecorated with Au islands exclusively located at the nanoframecorners (Figures 6J–6M).[22]

3.3. Coupling Galvanic Exchange with Regioselective SurfacePassivation

Single-crystalline nanoparticles enclosed by one specific type offacets, such as the {100}-faceting Ag nanocubes and {111}-faceting Ag nanooctahedra, provide well-defined materialssystems for us to study the site-specific, regioselectivechemistry on nanocrystal surfaces. When more than one type offacets are present on the surface of a nanoparticulate template,oxidative etching of the template materials starts selectively onthe facets with the highest surface free energy, whereasdeposition of the more-noble metals occurs on the remainingfacets with lower surface free energies. The surface freeenergies of low-index facets of a face-centered-cubic (fcc) noblemetal decrease in the order of {110}> {100}> {111}. For chemi-

cally synthesized nanocrystals with well-defined shapes, how-ever, their surfaces are capped with molecular ligands, whichare intentionally introduced to control the shape evolutionduring the nanoparticle synthesis. These surface-cappingligands can alter the surface free energies of various facets andeven reverse the order of their thermodynamic stability. Forinstance, PVP preferentially adsorbs to the Ag {100} facets,making the {100} facets even more stable than the {111}facets.[23] Therefore, PVP-capped truncated Ag nanocubes trans-formed into nanocages with well-defined pores at the cornersthrough galvanic exchange with AuCl4

� (Figures 1F–1J) becauseAg dissolution occurred selectively on the {111} facets while Audeposition took place on {100} facets.[3b]

Selective passivation of certain facets during galvanicexchange provides a unique approach to architectural controlof hollow nanostructures.[24] Qin and coworkers[25] recentlydemonstrated that Au could be orthogonally deposited on the{100} and {111} facets of Ag cuboctahedral nanocrystals tocreate nanoboxes with complementary surfaces by couplinggalvanic exchange with regioselective surface passivation (Fig-ure 7A). With PVP serving as the surface-capping ligands, theoxidation of Ag was initiated on the {111} facets while thedeposition of Au occurred preferentially on the {100} facets ofthe nanocuboctahedra. The dissolved Ag+ reacted with NaOHto form an Ag2O layer on the {111} facets,[24a] which locallyinhibited further galvanic exchange reaction. Meanwhile, Auatoms were continuously deposited on the {100} facets throughthe reduction of the AuCl4

� by ascorbate monoanions (HAsc� ),resulting in Ag@Au cuboctahedra with Ag2O-covered {111}

Figure 7. (A) Schematic illustration of the two orthogonal pathways for the synthesis of cuboctahedral nanoboxes with complementary facets. (B) TEM and (C)SEM images of the Ag–Au{100} cuboctahedral nanoboxes. (D) TEM and (E) SEM images of the Ag–Au{111} cuboctahedral nanoboxes. Reprinted with permissionfrom ref [25]. Copyright2020, Royal Society of Chemistry.

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facets, which further evolved into cuboctahedral nanoboxesenclosed by {100} facets (Figures 7B and 7C) after etching theAg cores and the Ag2O layers by H2O2 and a weak acid,respectively. When cetyltrimethylammonium chloride (CTAC)was used as the surface-capping ligands, the galvanic exchangestarted regioselectively on the {100} facets and the in situgenerated Ag+ reacted with the Cl� ions derived from CTAC toproduce soluble AgCl2

� ions. Both AgCl2� and AuCl4

� werereduced by HAsc� and co-deposited onto the {111} facets,leading to the formation of concave Ag@Au nanocuboctahedra.After the removal of Ag from the core by H2O2 etching,cuboctahedral nanoboxes with well-refined {111} facets wereobtained (Figures 7D and 7E). This work provided an excellentexample fully demonstrating the beauty of a deliberatelydesigned synthetic approach, which couples nanoscale galvanicexchange with regioselective confinement of the redox reactionsites.

4. Multiple Structural Remodeling Processes inAction: Galvanic Exchange-DrivenTransformations of Atomically IntermixedBimetallic Nanocrystals

4.1. Effects of Compositional Stoichiometry and StructuralOrdering: A Case Study of Au� Cu Alloy and IntermetallicNanoparticles

When serving as the sacrificial templates for galvanic exchange,bimetallic nanocrystals composed of atomically intermixed alloy(disordered solid solutions) and intermetallic (atomically or-dered compounds) phases undergo structural transformationsdictated by percolation dealloying,[26] Kirkendall diffusion,[4a] andOstwald ripening,[27] all of which are classic effects that havebeen known for decades and intensively investigated in bulkmaterials systems. However, how they work synergisticallywithin the same nanoparticulate template to guide the galvanicexchange-driven nanoparticle transformations still remainspoorly understood. Au� Cu bimetallic nanoparticles constitute aunique structurally and compositionally tunable materialssystem that enables us to pinpoint the effects of compositionalstoichiometry and structural ordering on the relative rates ofdealloying, Kirkendall diffusion, and Ostwald ripening duringnanoscale galvanic exchange.[28]

The bulk phase diagram of the Au� Cu binary system(Figure 8A) shows that Au and Cu form fcc alloys spanning theentire stoichiometric range. An fcc AuCu3 intermetallic phase(AuCu3� I, Figure 8B) and a face-centered tetragonal (fct) AuCuintermetallic phase (AuCu� I, Figure 8C) are thermodynamicallyfavored in the temperature range below ~400 °C. Guided bythis phase diagram, we designed and developed a multistepsynthetic approach to Au� Cu alloy and intermetallic nano-particles with fine-controlled sizes and compositionalstoichiometries.[28] When dispersed in tetraethylene glycol (TEG)at 300 °C, Au@Cu2O core-shell nanoparticles evolved intoAu� Cu alloy nanoparticles through a stepwise Cu reduction and

Au� Cu alloying process (Figure 8D). The Au/Cu stoichiometricratios of the alloy nanoparticles were predetermined by theirparental Au@Cu2O core-shell nanoparticles, whose core andshell dimensions could be precisely tuned through seed-mediated growth.[29] The AuCu3 and AuCu alloy nanoparticles,denoted as AuCu3� A and AuCu� A respectively, represented thekinetically trapped metastable structures, which further evolvedinto intermetallic phases upon atomic ordering when dispersedin TEG at 300 °C over longer reaction times.

Dealloying occurred continuously throughout the entiregalvanic exchange reaction when Au� Cu alloy or intermetallicnanoparticles were exposed to AuCl4

� . The Cu atoms in thealloy matrices became less mobile and more resistive againstdealloying, which was manifested by the positively shiftedonset potentials for Cu dissolution, when the alloy nanoparticlesbecame more Au-rich (Figure 8E). Intermetallic nanoparticlesexhibited more positive onset potentials for Cu dissolution thanalloy nanoparticles with the same compositional stoichiometries(Figure 8E) because ordered atomic arrangements increased theenergy barriers for the atomic migration.

The diffusion rate of atoms within each nanoparticle wasfound to be a key factor affecting the nucleation and growth ofKirkendall voids. Dealloying and Kirkendall diffusion werefurther entangled with Ostwald ripening, which occurred notonly among different nanoparticles (thermodynamic drivingforce for size increase and size focusing of colloids) but alsoamong discrete crystalline domains within the same nano-particle, a process also known as domain coarsening. Duringgalvanic exchange, the excessive HAuCl4 triggered the intra-particle Ostwald ripening of Au domains through a reversedisproportionation reaction, in which Au(III) reacted with Au(0)to form soluble Au(I) species.[30] By varying the amount ofHAuCl4 in the reaction mixtures, the rates of Ostwald ripeningcould be further maneuvered relative to those of dealloyingand Kirkendall diffusion, greatly influencing the nanoparticletransformations.

Due to fast interdiffusion of Au and Cu atoms, the Cu-richAuCu3 alloy nanoparticles quickly evolved into sponge-likenanoparticles within a few minutes upon exposure to HAuCl4.Each spongy nanoparticle consisted of a bicontinuous networkof Au-rich nanoligaments (Figure 8F and 8G), structurallyresembling the nanoporous particles derived from percolationdealloying of alloys.[31] In contrast to AuCu3 alloy nanoparticles,AuCu3 intermetallic nanoparticles evolved into fragmented,irregularly-shaped ligaments (Figure 8H and 8I) due to muchslower atomic diffusion in the intermetallic matrices than in thealloys. For AuCu3 alloy and intermetallic nanoparticles, galvanicexchange was kinetically distinguishable from Ostwald ripeningbecause it was much faster than Ostwald ripening. However, inthe case of AuCu alloy and intermetallic nanoparticles, bothgalvanic exchange and Ostwald ripening occurred on the sametime-scale. Through galvanic exchange with HAuCl4, AuCu alloynanoparticles gradually evolved into yolk-shell nanoparticlesand eventually into nanocups, each of which was composed ofa cavity enclosed by a semi-open Au-rich shell (Figure 8J and8 K) over a timescale of hours. Because of the Ostwald ripening-driven domain coarsening, continuous Au-rich shells were

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produced instead of forming spongy nanoligaments. Thegalvanic exchange between AuCu intermetallic nanoparticlesand HAuCl4 led to the formation of quasi-spherical solidnanoparticles rather than hollow nanostructures because ofsuppressed Kirkendall atomic interdiffusion. Each of the result-ing solid nanoparticle was composed of an AuCu intermetalliccore surrounded by a monometallic Au shell (Figure 8L and8M).

This work shed light on the dynamic interplay of dealloying,Kirkendall diffusion, and Ostwald ripening during galvanicexchange-driven structural evolution of atomically intermixedbimetallic nanocrystals. The relative rates of the three intertwin-ing processes could be maneuvered by tuning the composi-tional stoichiometry and the atomic-level structural ordering ofthe bimetallic nanocrystals. These rules also explain the trans-forming behaviors of anisotropically structured Au� Cu alloynanorods during galvanic exchange with HAuCl4. Alloy nano-rods with intrinsic local compositional gradient underwent anasymmetric hollowing process due to composition-dependentdealloying and Kirkendall diffusion.[32] In the presence ofexcessive HAuCl4, an Au� Cu alloy nanorod gradually evolvedinto a dumbbell-shaped hollow structure before eventuallysplitting into two Au-rich nanospheroids through an Ostwaldripening-driven process.[33]

4.2. Kinetic Manipulation of Galvanic Exchange, OxidativeEtching, and Seed-Mediated Deposition: A Case Study ofGalvanic Exchange in Polyols

The galvanic exchange-based synthetic approaches to multi-metallic hollow nanostructures have been developed primarilybased on reactions in aqueous solutions. Switching the reactionmedium from water to polyols represents an interestingparadigm-shift, allowing us to couple the galvanic exchangewith other kinetically controlled redox processes, such asoxidative etching and seed-mediated deposition, without theneed to introduce any additional oxidizing or reducingagents.[34] When taking place in polyol solvents, galvanicexchange reactions were entangled with seed-mediated reduc-tion due to the intrinsic reducing capabilities of polyols.[35]

Meanwhile, the dissolved O2 in the solvents and the halide ionsderived from the metal precursors served as a pair of mildoxidants[36] that etched the nanoparticles in situ during thegalvanic exchange reactions. As illustrated in Figure 9A, throughgalvanic exchange with H2PtCl6 in ethylene glycol (EG) at 140 °C,AuCu3 alloy nanoparticles evolved into spongy nanoparticles(denoted as S-EG) composed of Au� Cu alloy nanoligamentscovered with an Au� Pt alloy skin (Figures 9B–9D). Simplyswitching the polyol solvent from EG to TEG while keeping all

Figure 8. (A) Phase diagram of the bulk Au� Cu binary system. Unit cell structures of (B) fcc AuCu3 and (C) fct AuCu intermetallic compounds, denoted asAuCu3-I and AuCu� I, respectively. The red and yellow spheres represent Cu and Au atoms, respectively. (D) Scheme illustrating the transformations ofAu@Cu2O core-shell NPs into the Au� Cu alloy and intermetallic nanoparticles. (E) Linear sweep voltammetry curves of Cu, AuCu3-A, AuCu3-I, AuCu� A, andAuCu� A nanoparticles supported on glassy carbon electrodes in 0.5 M H2SO4 electrolyte at a potential sweep rate of 50 mVs

� 1. (F, H, J, L) SEM and (G, I. K, M)TEM images of nanoparticles obtained after exposing (F, G) AuCu3-A, (H, I) AuCu3-I, (J, K) AuCu� A, and (L, M) AuCu� I nanoparicles to HAuCl4 for galvanicexchange. Reprinted with permission from ref [28]. Copyright 2018, American Chemical Society.

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the other reaction conditions the same led to formation ofAu� Cu� Pt trimetallic hollow nanoparticles (denoted as S-TEG),each of which was enclosed by a triple-layered shell consistingof an Au inner surface, a dendritic Pt outer surface, and a thinlayer of Au� Cu alloys sandwiched between the Au and Pt layers(Figures 9E–9G).

It was essentially the dynamic interplay among Cu� Ptgalvanic exchange, oxidative etching of Cu, and seed-mediateddeposition of Pt that dictated the versatile transformations ofAu� Cu alloy nanoparticles. The striking morphological differ-ences between S-EG and S-TEG could be interpreted as theconsequence of drastically different Pt deposition rates indifferent polyol solvents. In EG at 140 °C, the deposition ofadditional Pt (excluding the galvanically exchanged Pt) on thealloy surfaces was much slower than the oxidative etching ofCu due to the moderate reducing capability of EG under thisreaction condition. The fast etching of Cu led to the formationof spongy nanostructures following a process analogous to thenanoporosity-evolving percolation dealloying.[26] Meanwhile, thein situ deposition of Pt atoms on the alloy nanoparticle surfacescreated local compositional gradient, kinetically boosting the

atomic interdiffusion within individual nanoparticles. In EG, thePt deposition occurred at a sufficiently slow rate with respect tothat of Au� Pt atomic interdiffusion, enabling the deposited Ptto get fully alloyed with Au to form an Au� Pt alloy skin thatprotected the Au� Cu ligament cores from further dealloying. InTEG at 140 °C, however, the Pt deposition became significantlyfaster than Au� Pt interdiffusion because TEG displayed a muchstronger reducing capability than EG, leading to the formationof dendritic monometallic Pt on the surfaces of Au� Cu alloys.The deposition of Pt on Au� Cu alloys also triggered theKirkendall diffusion of Cu and Au, resulting in a thin layer ofmonometallic Au covering the inner surfaces of the hollownanoparticles. The relative rates of oxidative etching, seed-mediated reduction, and galvanic exchange could be systemati-cally modulated by tuning several key synthetic parameters,such as the choice of polyol solvents, reaction temperature,abundance of O2 in the solvents, and the structures of thestarting alloy nanoparticles.

Figure 9. (A) Schemes illustrating the galvanic exchenage-driven structural transformations of AuCu3 alloy nanoparticles when reacting with H2PtCl6 in EG andTEG at 140 °C. (B,C) HAADF-STEM image of Au� Pt alloy skin-covered, spongy Au� Cu� Pt trimetallic nanoparticles synthesized in EG (S-EG). (D) EDS-intensityprofiles (left panel) of Au Lα, Pt Lα, and Cu Kα along the yellow line highlighted in the right panel. (E,F) HAADF-STEM image of hollow nanoparticles withdendritic Pt surfaces synthesized in TEG (S-TEG). (G) EDS-intensity profiles (left panel) of Au Lα, Pt Lα, and Cu Kα along the yellow line highlighted in the rightpanel. Reprinted with permission from ref [34]. Copyright 2019, American Chemical Society.

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5. Summary

The galvanic exchange-driven transformations of solid metallicnanoparticles into architecturally sophisticated multimetallichollow nanostructures are mechanistically far more complicatedthan a simple atomic exchange process, involving multipleintertwining structural remodelling processes that occur eithersimultaneously or sequentially over multiple time- and length-scales. Development of detailed mechanistic understanding ofthe nanoparticle transforming behaviors not only requiressuccess in structure-controlled nanoparticle synthesis, but alsorelies crucially on our capability to fine-resolve the detailednanoparticle structural evolution in real time using the state-of-the-art in situ spectroscopic and microscopic techniques. At firstglance, the transforming behaviors of nanoparticles duringgalvanic exchange appear extremely diverse and may varydrastically even when subtle modifications of the nanoparticlestructures or the synthetic conditions are introduced. However,when scrutinizing the detailed transforming behaviors ofjudiciously tailored nanoparticle systems under deliberatelydesigned galvanic exchange reaction conditions, coherentdefining rules start to emerge, which help us elucidate howmultiple classic effects and processes, such as alloying, deal-loying, Kirkendall diffusion, Ostwald ripening, oxidative etching,and seed-mediated reduction, dynamically interplay within theconfinement by a nanoparticulate template to drive structuralrearrangements at both the atomic and the nanoparticulatelevels. Several key defining rules are summarized as follows.(1) Transformations of monometallic nanocrystals or monome-

tallic domains in heteronanostructures during galvanicexchange are synergistically dictated by alloying, deal-loying, and Kirkendall diffusion. The nanoparticle hollowingprocess requires the formation of a critical void containingabout 20 atomic vacancies in each nanoparticle prior to anabrupt, cooperative transition into hollow nanostructures.

(2) Coupling galvanic exchange with seed-mediated reduction,oxidative etching, and regioselective surface passivationwith the aid of deliberately selected reducing agents,etchants, and surface-passivating species remarkably en-hances our capabilities to further fine-tune the architecturesand compositions of multimetallic hollow nanostructures.

(3) Bimetallic nanocrystals composed of atomically intermixedalloy and intermetallic phases undergo structural trans-formations substantially more sophisticated than those oftheir monometallic and phase-segregated heterostructuredcounterparts, because galvanic exchange is further en-tangled with percolation dealloying, Kirkendall diffusion,and Ostwald ripening.

(4) Switching the reaction medium from water to organicsolvents with intrinsic oxidizing or reducing capabilitiesallows galvanic exchange to interplay with oxidativeetching and seed-mediated deposition even in the absenceof any additional oxidizing or reducing agents. Kineticmodulation of the oxidative etching and seed-mediatedreduction with respect to galvanic exchange enablesarchitectural control of multimetallic hollow heteronanos-

tructures at a level of sophistication typically not accom-plishable through reactions in aqueous environments.The key message delivered by this Minireview is that

nanoparticle-templated galvanic exchange, when furthercoupled with redox manipulation, provides a unique pathwayto fine-tailor the architectures and compositions, and therebyfine-tune the plasmonic and catalytic properties of multimetallichollow nanoparticles with great precision and versatility. Themechanistic insights summarized in this minireview serve asgeneric design principles that guide the development ofgalvanic exchange-based synthetic approaches to specificallytargeted multimetallic hollow nanostructures with optimizedproperties for applications in catalysis, photocatalysis, biomedi-cine, molecular sensing, and energy conversion.

Acknowledgements

The authors thank the United States National Science Founda-tion (DMR-1253231), the United States Department of Energy(DE-SC0016574), and the University of South Carolina (StartupFunds, an ASPIRE� I Track I Award, and an ASPIRE� I Track IVAward) for funding support.

Conflict of Interest

The authors declare no conflict of interest.

Keywords: Hollow nanostructures · galvanic exchange ·Kirkendall effect · dealloying · alloying

[1] a) X. J. Wang, J. Feng, Y. C. Bai, Q. Zhang, Y. D. Yin, Chem. Rev. 2016, 116,10983–11060; b) K. An, T. Hyeon, Nano Today 2009, 4, 359–373; c) X. W.Lou, L. A. Archer, Z. C. Yang, Adv. Mater. 2008, 20, 3987–4019; d) S. E.Skrabalak, J. Y. Chen, Y. G. Sun, X. M. Lu, L. Au, C. M. Cobley, Y. N. Xia,Acc. Chem. Res. 2008, 41, 1587–1595; e) Y. N. Xia, W. Y. Li, C. M. Cobley,J. Y. Chen, X. H. Xia, Q. Zhang, M. X. Yang, E. C. Cho, P. K. Brown, Acc.Chem. Res. 2011, 44, 914–924.

[2] a) X. H. Xia, Y. Wang, A. Ruditskiy, Y. N. Xia, Adv. Mater. 2013, 25, 6313–6333; b) Q. Zhang, W. S. Wang, J. Goebl, Y. D. Yin, Nano Today 2009, 4,494–507; c) C. M. Cobley, Y. N. Xia, Mater. Sci. Eng. R 2010, 70, 44–62.

[3] a) Y. Sun, Y. Xia, J. Am. Chem. Soc. 2004, 126, 3892–3901; b) J. Chen,J. M. McLellan, A. Siekkinen, Y. Xiong, Z.-Y. Li, Y. Xia, J. Am. Chem. Soc.2006, 128, 14776–14777; c) L. Au, Y. C. Chen, F. Zhou, P. H. C. Camargo,B. Lim, Z. Y. Li, D. S. Ginger, Y. N. Xia, Nano Res. 2008, 1, 441–449; d) J.Zeng, Q. Zhang, J. Chen, Y. Xia, Nano Lett. 2010, 10, 30–35; e) Y. G. Sun,Y. N. Xia, Science 2002, 298, 2176–2179.

[4] a) A. D. Smigelskas, E. O. Kirkendall, Trans. Am. Inst. Min. Metall. Pet. Eng.1947, 171, 130–142; b) H. J. Fan, M. Knez, R. Scholz, K. Nielsch, E. Pippel,D. Hesse, M. Zacharias, U. Gosele, Nat. Mater. 2006, 5, 627–631; c) Y. D.Yin, R. M. Rioux, C. K. Erdonmez, S. Hughes, G. A. Somorjai, A. P.Alivisatos, Science 2004, 304, 711–714; d) H. J. Fan, U. Gosele, M.Zacharias, Small 2007, 3, 1660–1671.

[5] Y. G. Sun, Y. N. Xia, Nano Lett. 2003, 3, 1569–1572.[6] a) G. S. Metraux, Y. C. Cao, R. C. Jin, C. A. Mirkin, Nano Lett. 2003, 3, 519–

522; b) Y. Wang, D. Wan, S. Xie, X. Xia, C. Z. Huang, Y. Xia, ACS Nano2013, 7, 4586–4594.

[7] Y. Sun, B. Wiley, Z.-Y. Li, Y. Xia, J. Am. Chem. Soc. 2004, 126, 9399–9406.[8] a) E. C. Cho, P. H. C. Camargo, Y. N. Xia, Adv. Mater. 2010, 22, 744–748;

b) S. Hong, J. A. I. Acapulco, H. Y. Jang, S. Park, Chem. Mater. 2014, 26,3618–3623; c) Y. Khalavka, J. Becker, C. Sonnichsen, J. Am. Chem. Soc.2009, 131, 1871–1875; d) K. K. Liu, S. Tadepalli, L. M. Tian, S. Singamane-

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2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

ni, Chem. Mater. 2015, 27, 5261–5270; e) S. Neretina, R. A. Hughes, K. D.Gilroy, M. Hajfathalian, Acc. Chem. Res. 2016, 49, 2243–2250; f) S. Lee, J.Kim, H. Yang, E. Cortes, S. Kang, S. W. Han, Angew. Chem. Int. Ed. 2019,58, 15890–15894.

[9] M. Hajfathalian, K. D. Gilroy, S. D. Golze, A. Yaghoubzade, E. Menumerov,R. A. Hughes, S. Neretina, ACS Nano 2016, 10, 6354–6362.

[10] E. Gonzalez, J. Arbiol, V. F. Puntes, Science 2011, 334, 1377–1380.[11] a) J. G. Smith, Q. Yang, P. K. Jain, Angew. Chem. Int. Ed. 2014, 53, 2867–

2872; Angew. Chem. 2014, 126, 2911–2916; b) J. G. Smith, I. Chakraborty,P. K. Jain, Angew. Chem. Int. Ed. 2016, 55, 9979–9983; Angew. Chem.2016, 128, 10133–10137; c) J. G. Smith, P. K. Jain, J. Am. Chem. Soc.2016, 138, 6765–6773; d) J. G. Smith, X. Q. Zhang, P. K. Jain, J. Mater.Chem. A 2017, 5, 11940–11948; e) L. M. Moreau, C. A. Schurman, S.Kewalramani, M. M. Shahjamali, C. A. Mirkin, M. J. Bedzyk, J. Am. Chem.Soc. 2017, 139, 12291–12298; f) Y. G. Sun, Y. X. Wang, Nano Lett. 2011,11, 4386–4392; g) S. F. Tan, G. H. Lin, M. Bosman, U. Mirsaidov, C. A.Nijhuis, ACS Nano 2016, 10, 7689–7695; h) E. Sutter, K. Jungjohann, S.Bliznakov, A. Courty, E. Maisonhaute, S. Tenney, P. Sutter, Nat. Commun.2014, 5, 4946; i) S. W. Chee, S. F. Tan, Z. Baraissov, M. Bosman, U.Mirsaidov, Nat. Commun. 2017, 8, 1224.

[12] H. Jing, H. Wang, Chem. Mater. 2015, 27, 2172–2180.[13] J. Y. Chen, B. Wiley, J. McLellan, Y. J. Xiong, Z. Y. Li, Y. N. Xia, Nano Lett.

2005, 5, 2058–2062.[14] Y. Yang, Q. Zhang, Z. W. Fu, D. Qin, ACS Appl. Mater. Interfaces 2014, 6,

3750–3757.[15] H. Zhang, M. Jin, H. Liu, J. Wang, M. J. Kim, D. Yang, Z. Xie, J. Liu, Y. Xia,

ACS Nano 2011, 5, 8212–8222.[16] M. C. Liu, Y. Q. Zheng, S. F. Xie, N. X. Li, N. Lu, J. G. Wang, M. J. Kim, L. J.

Guo, Y. N. Xia, Phys. Chem. Chem. Phys. 2013, 15, 11822–11829.[17] R. G. Weiner, A. F. Smith, S. E. Skrabalak, Chem. Commun. 2015, 51,

8872–8875.[18] a) Y. Yang, J. Liu, Z.-W. Fu, D. Qin, J. Am. Chem. Soc. 2014, 136, 8153–

8156; b) M. M. Shahjamali, M. Bosman, S. W. Cao, X. Huang, S. Saadat, E.Martinsson, D. Aili, Y. Y. Tay, B. Liedberg, S. C. J. Loo, H. Zhang, F. Boey,C. Xue, Adv. Funct. Mater. 2012, 22, 849–854.

[19] L. Polavarapu, D. Zanaga, T. Altantzis, S. Rodal-Cedeira, I. Pastoriza-Santos, J. Pérez-Juste, S. Bals, L. M. Liz-Marzán, J. Am. Chem. Soc. 2016,138, 11453–11456.

[20] a) X. Lu, L. Au, J. McLellan, Z.-Y. Li, M. Marquez, Y. Xia, Nano Lett. 2007,7, 1764–1769; b) M. M. Shahjamali, M. Bosman, S. W. Cao, X. Huang,X. H. Cao, H. Zhang, S. S. Pramana, C. Xue, Small 2013, 9, 2880–2886;c) M. McEachran, D. Keogh, B. Pietrobon, N. Cathcart, I. Gourevich, N.

Coombs, V. Kitaev, J. Am. Chem. Soc. 2011, 133, 8066–8069; d) J. Li, J.Liu, Y. Yang, D. Qin, J. Am. Chem. Soc. 2015, 137, 7039–7042.

[21] X. Hong, D. Wang, S. Cai, H. Rong, Y. Li, J. Am. Chem. Soc. 2012, 134,18165–18168.

[22] Y. Wu, D. Wang, G. Zhou, R. Yu, C. Chen, Y. Li, J. Am. Chem. Soc. 2014,136, 11594–11597.

[23] J. Ahn, D. Wang, Y. Ding, J. Zhang, D. Qin, ACS Nano 2018, 12, 298–307.[24] a) X. Sun, J. Kim, K. D. Gilroy, J. Liu, T. A. F. König, D. Qin, ACS Nano

2016, 10, 8019–8025; b) J. Ahn, D. Qin, Chem. Mater. 2019, 31, 9179–9187; c) J. Ahn, L. Zhang, D. Qin, ChemNanoMat 2020, 6, 5–14.

[25] J. Ahn, J. Kim, D. Qin, Nanoscale 2020, 12, 372–379.[26] a) J. Erlebacher, M. J. Aziz, A. Karma, N. Dimitrov, K. Sieradzki, Nature

2001, 410, 450–453; b) A. Wittstock, V. Zielasek, J. Biener, C. M. Friend,M. Bäumer, Science 2010, 327, 319–322.

[27] W. Ostwald, Lehrbuch der Allgemeinen Chemie, vol. 2, part 1,, Leipzig,Germany, 1896.

[28] G. G. Li, M. Q. Sun, E. Villarreal, S. Pandey, S. R. Phillpot, H. Wang,Langmuir 2018, 34, 4340–4350.

[29] L. Zhang, H. Jing, G. Boisvert, J. Z. He, H. Wang, ACS Nano 2012, 6,3514–3527.

[30] C. H. Gammons, Y. M. Yu, A. E. Wiliams Jones, Geochim. Cosmochim. Acta1997, 61, 1971–1983.

[31] a) G. G. Li, Y. Lin, H. Wang, Nano Lett. 2016, 16, 7248–7253; b) G. G. Li, E.Villarreal, Q. F. Zhang, T. T. Zheng, J. J. Zhu, H. Wang, ACS Appl. Mater.Interfaces 2016, 8, 23920–23931; c) G. G. Li, H. Wang, ChemNanoMat2018, 4, 897–908.

[32] S. Thota, S. T. Chen, J. Zhao, Chem. Commun. 2016, 52, 5593–5596.[33] S. Thota, Y. D. Zhou, S. T. Chen, S. L. Zou, J. Zhao, Nanoscale 2017, 9,

6128–6135.[34] G. G. Li, Z. X. Wang, D. A. Blom, H. Wang, ACS Appl. Mater. Interfaces

2019, 11, 23482–23494.[35] H. Dong, Y. C. Chen, C. Feldmann, Green Chem. 2015, 17, 4107–4132.[36] a) A. Ruditsicy, M. Vara, H. Huang, Y. N. Xia, Chem. Mater. 2017, 29,

5394–5400; b) Y. Q. Zheng, J. Zeng, A. Ruditskiy, M. C. Liu, Y. N. Xia,Chem. Mater. 2014, 26, 22–33; c) E. Villarreal, G. G. Li, H. Wang,Nanoscale 2018, 10, 18457–18462.

Manuscript received: February 27, 2020Accepted manuscript online: March 27, 2020Version of record online: April 14, 2020

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