Controlling Configurational Isomerism in Three-Component Colloidal

May 18, 2017 - Department of Chemistry and Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, Unite...
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Controlling Configurational Isomerism in Three-Component Colloidal Hybrid Nanoparticles James M. Hodges and Raymond E. Schaak* Department of Chemistry and Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States CONSPECTUS: Colloidal hybrid nanoparticles are solutiondispersible constructs that join together multiple distinct nanoscale materials through direct solid−solid interfaces. Given their multifunctionality and synergistic properties that emerge from interfacial coupling, hybrid nanoparticles are of interest for applications in biomedical imaging, solar energy conversion, heterogeneous catalysis, nanophotonics, and beyond. High-order hybrid nanoparticles, which incorporate three or more nanocrystal domains, offer greater tunability and functional diversity relative to one or two-component nanoparticles. The multiple heterojunctions within these structures can facilitate complex electromagnetic coupling as well as cooperative surface processes. Additionally, these materials can be used as model systems for studying fundamental structure−property relationships at the nanoscale that arise from particle coupling and interfacial exchanges. Limiting these advances is the inability to synthesize hybrid nanoparticles with precise morphologies and geometries. High-order hybrid nanoparticles can adopt more than one configuration, and each unique arrangement will have different heterointerfaces and, accordingly, different functions. Seeded-growth methods are among the most effective methods for producing high-quality hybrid nanoparticles. Engineering complex heterostructures using these stepwise reactions is in some ways conceptually analogous to the total synthesis of large organic molecules. However, unlike in molecular synthesis, the rules and guidelines that underpin the formation of hybrid nanoparticles are less understood. For example, when a third domain is added to a two-component heterodimer nanoparticle seed, several distinct types of hybrid nanoparticle products are possible, but only one is typically observed due to preferred growth at specific locations. The three-component heterotrimer products that preferentially form are not necessarily those that have the domain configurations and heterojunctions required to facilitate a targeted application. Different arrangements of the three nanoparticles that comprise a heterotrimer lead to distinct configurational isomers. Accordingly, understanding and controlling configurational isomerism in nanoparticle heterotrimers is foundational for engineering high-order hybrid nanostructures with targeted heterointerfaces, properties, and functionalities. This Account highlights recent insights into the pathways by which three-component nanoparticle heterotrimers form and how their configurations can be controlled and modified. In-depth microscopic investigations into the formation of heterotrimers by growing a third nanoparticle domain on a two-component heterodimer seed have revealed that in some cases indiscriminate nucleation first occurs on all exposed surfaces followed by surface-mediated migration and coalescence to the preferred interface. This insight helps to rationalize observed site-specific, chemoselective growth phenomena. Additionally, new approaches for directing growth in heterotrimer synthesis, such as protection−deprotection schemes inspired by organic chemistry, are becoming effective tools for constructing hybrid nanoparticles having nonpreferred domain configurations. Alternatives to traditional seeded-growth approaches are also emerging, including insertion reactions driven by saturation−precipitation processes and orthogonal transformations of preformed hybrid constructs using ion exchange. These and other recent advances are providing a powerful suite of synthetic tools that are anticipated to enable function-driven design of high-order hybrid nanoparticles having targeted properties and applications. Ag and Au nanoparticles.3 Similarly, FePt−Fe3O4 heterodimers exhibit single-phase-like magnetic behavior that is attributed to exchange-coupling processes that occur through the particle− particle interface.4 Such interparticle coupling has been likened to the electronic exchanges that occur between atoms bonded together in molecules, making colloidal hybrid constructs an emerging class of “artificial molecule” nanoparticles.5−7 Because of their synergistic properties, multifunctionality, and tunability,

1. INTRODUCTION Colloidal nanoparticles are well-known to exhibit properties that depend on their sizes, shapes, and surfaces, as well as their collective interactions with one another in assemblies. Hybrid nanoparticles that contain multiple distinct materials connected directly to one another offer an even broader scope of nanostructure-dependent synergistic properties (Figure 1).1,2 For example, two-component Ag−Au hybrid nanoparticle “heterodimers” have unique optical properties defined by charge-transfer plasmons that oscillate between the Ag and Au domains, a phenomenon not observed in physical mixtures of © 2017 American Chemical Society

Received: March 1, 2017 Published: May 18, 2017 1433

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heterodimer nanoparticles used in biomedical applications can be enhanced by growing a conformal shell of SiO2 onto the MnO domain to form Au−MnO@SiO2 (Figure 1), which provides an ideal surface for attaching antibodies and proteins used in drug delivery while also improving the biocompatibility of the colloidal substrate.18 An almost limitless palette of multifunctional particles can be envisioned by creating hybrids from various combinations of materials, morphologies, and interfaces. However, only a small number of systems have been made in high yield, limited in part by synthetic bottlenecks that must be overcome in order to access a targeted construct. Most colloidal hybrid nanoparticles are synthesized using heterogeneous seeded growth methods, where a preformed nanocrystal serves as a seed for growing an additional domain.7,23 To synthesize an A−B heterodimer, B is grown on the surface of A under conditions that favor heterogeneous (surface-seeded) growth rather than homogeneous growth in solution. Such heterogeneous growth can result in either a core−shell (A@B) or heterodimer (A−B) morphology depending on the reaction conditions and the energetics of the interface between the two domains (Figure 2).23,24 If an A−B heterodimer then serves as a seed for adding a third domain, C, several growth modes are possible and configurational ambiguities arise, so multiple products can form: A−B−C, C−A−B, C−A−B−C, (A−B)−Cx, (A−B)@C, etc. (Figure 2). These possible products can be thought of as distinct configurational isomers, nanoparticle analogues of molecular isomers, that contain the same components but

Figure 1. (top) Cartoon representations of hybrid nanoparticle heterodimers, heterotrimers, and heterotetramers. (bottom) TEM images and cartoon representations of functional hybrid nanoparticles: (a) Au−In2O3, (b) PbS−CdS−Pt, (c) Au−MnO@SiO2, (d) Ag−Au, (e) FePt−MnO, (f) Ru−CdSe@CdS−Pt, (g) a hybrid construct for overall light-driven water splitting. Panel a adapted with permission from ref 17. Copyright 2014 American Chemical Society. Panel b adapted with permission from ref 2. Copyright 2016 American Chemical Society. Panel c adapted with permission from ref 18. Copyright 2014 American Chemical Society. Panel d adapted with permission from ref 3. Copyright 2013 American Chemical Society. Panel e adapted with permission from ref 15. Copyright 2012 American Chemical Society. Panel f adapted with permission from ref 1. Copyright 2015 WILEY-VCH Verlag Gmbh & KGaA Weinheim. Panel g adapted with permission from ref 11. Copyright 2015 American Chemical Society.

hybrid nanoparticles are of interest in diverse applications that include solar energy,8−12 heterogeneous catalysis,13,14 magnetism,4,15 nanophotonics,9,16,17 and biomedicine.18−20 Most existing colloidal hybrid nanoparticles are twocomponent heterodimer systems, but higher-order systems containing three or more components are increasingly targeted because of their expanded functionality and synergy. For example, a three-component nanoparticle heterotrimer can be designed to facilitate overall photocatalytic water splitting by placing a hydrogen evolution catalyst and an oxygen evolution catalyst at opposite ends of a light-absorbing semiconductor.21,22 Heterotrimer nanoparticles also offer more material surfaces for binding molecules, which can be used to further modulate physical properties or introduce new chemical functionality. For instance, the properties of Au−MnO

Figure 2. Different A,B heterodimer and A,B,C heterotrimer configurations that are possible using sequential seeded growth reactions. 1434

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Several heterotrimer systems made by growing a third nanoparticle onto a heterodimer seed are known to exhibit chemoselective behavior, including (PbS,PbSe)−Au-Fe3O4,26 (Au,Ag,Pd,Ni)−Pt-Fe3O4,6 and Ag−Au-(Fe3O4,SiO2).27,28 We sought to identify additional examples of chemoselectivity in heterotrimer formation by studying the growth of various metal chalcogenides on Au−Pt heterodimer seeds (Figure 3).29 For example, PbS grows on both Pt and Au nanoparticles to form Pt−PbS and Au−PbS, respectively. Under the same reaction conditions, we found that PbS grows exclusively on the Au domain of Pt−Au heterodimers to form Pt−Au−PbS. Similarly, amorphous copper sulfide (CuxSy) grows on both Pt and Au to form Pt−CuxSy and Au−CuxSy, respectively, but only on the Au domain of Pt−Au to form Pt−Au−CuxSy.29 CdS, which forms at higher temperatures than PbS and CuxSy, also grows exclusively on the Au domain of Pt−Au, although the highertemperature reaction led to the transformation of the Pt−Au heterodimers into Pt@Au core−shell particles having an exposed Au shell.29 In all of these cases, the metal sulfides grew exclusively on the Au domains of the Pt−Au particles. This observation is instructive, because it allows us to understand the various competing factors that are known or hypothesized to lead to the formation and stability of heterointerfaces in hybrid nanoparticles, including lattice matching, surface energies, electronic effects, and interfacial bonding strengths. The three studied metal sulfides, PbS, CuxSy, and CdS, span a range of synthetic conditions, crystal structures with different lattice constants, and crystalline vs amorphous forms. At the same time, the Au and Pt domains of the Au−Pt heterodimer seeds have similar lattice constants but different surface chemistries and bonding preferences with the various metal sulfides. All three metal sulfides grow on Au rather than Pt, suggesting that the formation of strong interfacial Au−S bonds is a key driving force for the observed chemoselectivity. The other factors, which vary significantly among the systems despite identical preferred growth locations, play an insignificant role in these systems. This result is intuitive (metal sulfides are expected to prefer the Au surface rather than Pt given the greater affinity of Au for sulfur) but at the same time interesting, because of the knowledge that the corresponding heterodimers of platinum and each of the metal sulfides also are stable and form under the same conditions. A deeper understanding of the pathway by which a third nanoparticle grows chemoselectively on a heterodimer seed is needed.

arranged in different ways. Because each configurational isomer has distinct arrangements and interfaces, and therefore distinct properties, it is important to identify synthetic pathways capable of controlling the arrangements of the nanoparticle domains.21 For example, a hypothetical Pt−TiO 2 −IrO 2 heterotrimer system is expected to be active for overall photocatalytic water splitting, but different configurational isomers, IrO2−Pt−TiO2 or TiO2−IrO2−Pt, lack the requisite interfaces and therefore do not exhibit the desired properties.25 In this Account, we highlight recent empirical and mechanistic studies that help to rationalize and predict which heterotrimer configurations are most favorable and also summarize emerging strategies for synthesizing new classes of three-component hybrid nanoparticles. The pathways used to synthesize such high-order hybrid nanoparticles are conceptually analogous to the total synthesis methods used by organic chemists to construct large molecules in a rational, stepwise, and retrosynthetic manner.

2. IDENTIFYING SYSTEMS EXHIBITING CHEMOSELECTIVE GROWTH The first step toward understanding how to achieve precise A− B−C vs C−A−B configurations in a heterotrimer system is to identify the preferred domain(s) on a heterodimer seed particle where a third nanoparticle will grow. The Ag−Pt−Fe3O4 system provides an illustrative example (Figure 3).6 Under

Figure 3. TEM images showing that (a−c) Ag grows on both Fe3O4 and Pt nanoparticles but only on the Pt domain of Pt−Fe3O4 and (d− f) PbS grows on both Au and Pt nanoparticles but only on the Au domain of Au−Pt. Panels a−c adapted with permission from ref 6. Copyright 2011 Macmillan Publishers Limited. Panels d−f adapted with permission from ref 29. Copyright 2015 American Chemical Society.

3. MICROSCOPIC INSIGHTS INTO Ag−Pt−Fe3O4 HETEROTRIMER FORMATION Microscopic insights into the formation of the Ag−Pt−Fe3O4 model system help us to better understand how seeded-growth syntheses of heterotrimers proceed and can be controlled (Figure 4).28 Aliquots taken during the growth of Ag on Pt− Fe3O4 heterodimer seeds were analyzed using a suite of electron microscopy tools. Early in the synthesis, small Ag nanocrystals begin to appear on both the Pt and Fe3O4 nanoparticle surfaces, which is consistent with the expected indiscriminate growth observed for the Ag−Pt and Ag−Fe3O4 controls but contrasts with the observed chemoselective growth of Ag on Pt−Fe3O4 to form only Ag−Pt−Fe3O4. Analysis of subsequent aliquots indicates that the Ag clusters grow over the course of the reaction and also appear to migrate onto the Pt domain, ultimately yielding the expected Ag−Pt−Fe 3O4 heterotrimer after several hours. The observed chemoselective

identical reaction conditions, Ag grows on Pt to form Ag−Pt and on Fe3O4 to form Ag−Fe3O4. When a physical mixture of Pt and Fe3O4 nanoparticles is present, Ag grows indiscriminately on both. In contrast, when using Pt−Fe3O4 heterodimers as seeds, Ag grows exclusively on the Pt domain to form Ag− Pt−Fe3O4; no Ag is observed on the Fe3O4 domain. The growth of Ag on Pt−Fe3O4 can therefore be considered as chemoselective, as it grows on only one surface (Pt) when multiple (both Pt and Fe3O4) are exposed, accessible, and appropriately reactive. 1435

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Figure 4. Cartoon schematics and corresponding STEM-EDS element maps characterizing the pathway by which Ag grows selectively on the Pt domain of Pt−Fe3O4 heterodimers. Adapted with permission from ref 28. Copyright 2015 American Chemical Society.

despite the fact that PbSe instead forms a conformal shell on Au particles.26 The insights gained from the model Ag−Pt− Fe3O4 system will guide future efforts to tune and control subtle morphological features of hybrid nanoparticles that can lead to unique or enhanced properties, including sizedependent optical and catalytic properties, interfacial communication for enhanced functional synergy, and secondary and tertiary particle−particle interactions.

addition of Ag to Pt−Fe3O4 therefore does not occur through domain-selective nucleation but rather through indiscriminate nucleation on both Pt and Fe3O4 followed by ripening and migration to Pt. Further evidence to support the surface migration pathway was provided by studying the growth of Ag off of distinct types of Pt−Fe3O4 heterodimers having different Fe3O4 domain sizes but identical Pt sizes. Ag grown on Pt−Fe3O4 heterodimers having larger Fe3O4 domains was larger than Ag grown on Pt− Fe3O4 heterodimers having smaller Fe3O4 domains. The direct correlation between the size of the Fe3O4 domain in the Pt− Fe3O4 seed and the size of the Ag domain in the Ag−Pt−Fe3O4 product suggests that Ag nucleates everywhere on Pt−Fe3O4. The Pt−Fe3O4 sample having the larger Fe3O4 domain and therefore the larger surface area can nucleate more Ag and ultimately produce a larger Ag domain after migration and coalescence. The observation that the most stable configuration has Ag on Pt and not Fe3O4 is consistent with the formation of stronger Ag−Pt interactions due to interfacial epitaxy and alloying. Also, the size of the Ag domain can be tuned by modifying the size of the Fe3O4 domain, a domain to which it is not directly attached, which provides important insights into how domain morphology can be tuned and controlled. Additional electron microscopy studies provided a detailed understanding of how the surface of the Pt−Fe3O4 seeds influences the deposition and growth of Ag to form Ag−Pt− Fe3O4. When Ag grows on Pt, a conformal shell having a large Ag−Pt interface forms, but Ag grows on the Pt domain of Pt− Fe3O4 as a distinct particle with a small interface. As shown in Figure 5, small iron oxide deposits decorate the surface of the Pt domain of Pt−Fe3O4, and the Ag appears to attach to a region where there is a gap in the iron oxide shell. The iron oxide deposits, which are not readily detected by routine electron microscopy, apparently form during the reaction of Pt seeds with iron-based reagents to form Pt−Fe3O4. The previously undetected iron oxide deposits limit how much of the Pt surface is available for Ag to attach, ultimately preventing the formation of a conformal shell and forcing outward growth that results in a Ag−Pt−Fe3O4 heterotrimer. Subtle morphological features introduced during various stages of hybrid nanoparticle synthesis, which often go unnoticed, can strongly influence the pathway by which higher-order hybrid nanoparticles grow and therefore motivate further microscopic investigations into hybrid nanoparticle growth and nanostructural features. Such insights also may help to rationalize previous observations in other hybrid nanoparticle systems. For example, PbSe was observed to grow on Au−Fe3O4 seeds to form PbSe−Au−Fe3O4 heterotrimers,

4. PROTECTING GROUP STRATEGIES FOR ACCESSING ALTERNATE HETEROTRIMER CONFIGURATIONS The pathway by which Ag−Pt−Fe3O4 forms involves nonspecific nucleation of Ag on all surfaces of the Pt−Fe3O4 seed, followed by coalescence and migration to the Pt domain. Ag− Pt−Fe3O4 is therefore the preferred configuration for a heterotrimer comprised of Ag, Pt, and Fe3O4 nanoparticles. In addition to understanding how preferred configurations form, it is also important to develop strategies to access alternate nonpreferred configurations. Such capabilities will help to facilitate the rational design of higher-order hybrid constructs with targeted functions that require precise domain arrangements and particle−particle interfaces. Ag nucleates first on both Pt and Fe3O4, and it is also known that Ag−Pt and Ag−Fe3O4 hybrids form and are stable under analogous reaction conditions. However, given the competition between Ag on Pt vs Fe3O4, Pt is preferred. This observation suggests that if the Pt surface of the Pt−Fe3O4 heterodimer seed is blocked prior to Ag deposition, Ag will only nucleate and grow on Fe 3 O 4 , and an alternate Pt−Fe 3 O 4 −Ag heterotrimer isomer may be accessible. Blocking one reactive surface while another on the same heterodimer remains accessible is conceptually analogous to installing a protecting group on a molecule containing two reactive and accessible functional groups. Here, the molecular protecting group selectively blocks one functional group that otherwise would be reactive, so that a subsequent chemical reaction is localized to the desired functional group that remains accessible. Indeed, upon installing a protecting group on the Pt domain of Pt− Fe3O4 by overgrowing a shell of amorphous iron oxide, the Pt surface is blocked, and subsequent deposition of Ag is localized to the Fe3O4 domain (Figure 6).30 The resulting Pt−Fe3O4−Ag heterotrimer is a configurational isomer of the product that would otherwise be preferred, Ag−Pt−Fe3O4. In molecular systems, the protecting group would then be removed so that the functional group it blocks could be reexposed. The iron-oxide protecting group that facilitated the formation of Ag−Pt−Fe3O4 cannot be removed, unfortunately, because the chemistry that would remove it (e.g., by 1436

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Figure 6. (a) Schematic and TEM images showing that a FeOx shell serves as protecting group to form the Ag−Fe3O4−Pt vs Fe3O4−Pt− Ag configurational isomers. (b) TEM images corresponding to a ZnO protecting group that leads to the formation of Au−Fe3O4−Ag vs Ag− Au−Fe3O4 configurational isomers after deprotecting. Panel a reproduced with permission from ref 30. Copyright 2014 American Chemical Society.

which may be driven by the formation of a favorable ZnO−Au interface. Upon reaction with silver(I) acetate [Ag(C2H3O2)] in the presence of oleylamine and toluene, Ag deposits exclusively on the Fe3O4 domain to form ZnO−Au−Fe3O4− Ag, which is likely facilitated by intraparticle ripening and formation of the stable Fe3O4−Ag interface.27 When the ZnO− Au−Fe3O4−Ag nanoparticles are dissolved in a solution of oleic acid and hexanes (1:4) and sonicated for 5 min, the ZnO protecting group is removed. The resulting heterotrimer, Au− Fe3O4−Ag, is a configurational isomer of Ag−Au−Fe3O4, which would form when Ag is deposited directly onto Au− Fe3O4 without the ZnO protecting group. Other protecting group strategies have also been reported for helping to direct the formation of a targeted heterotrimer configuration. For example, Ag would typically encapsulate Au to form Au@Ag core−shell particles, but upon first installing a hemispherical SiO2 particle that blocks half of the Au surface, subsequent deposition of Ag forms Ag−Au−SiO2 heterotrimers. 27 Similarly, Pd can be grown off of Au−SiO 2 heterodimers to form Pd−Au−SiO2 heterotrimers, followed by removal of the SiO2 protecting group to form Pd−Au heterodimers (Figure 7).31 Molecular protecting groups may also show promise for directing heterotrimer configurations. For example, Wu et al. showed that SiO2 could be selectively deposited onto the Fe3O4 domain of Au−Fe3O4 heterodimers to form Au−Fe3O4@SiO2 when the heterodimer seeds were treated with a thiol surfactant.32 Interestingly, when the thiol ligands were not added to the two-component seeds, the SiO2

Figure 5. (top) Cartoon schematic showing the two distinct products that form by seeded growth of Ag on Pt. (bottom) STEM-EDS element maps and TEM images for (a) Pt seeds and corresponding (b,c) Pt@Ag core−shell particles and (d) Pt@FeOx seeds and corresponding (e,f) Pt−Ag heterodimers. Panels a−f adapted with permission from ref 28. Copyright 2015 American Chemical Society.

dissolution) would also remove the Fe3O4 domain. As a first step toward identifying solid-state protecting groups capable of deprotection, we considered ZnO, as it can be dissolved readily in dilute acid that does not attack other nanoparticles typically incorporated into heterotrimer constructs (Figure 6). Preliminary data shows that when zinc oleate is reacted with dodecanol in the presence of Au−Fe3O4 heterodimer seeds, ZnO grows selectively on the Au domain to produce ZnO−Au−Fe3O4, 1437

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Figure 7. Multistep synthesis of Au−Pd heterodimers using a SiO2 domain as a mask that facilitates protection/deprotection. Adapted with permission from ref 31. Copyright 2014 American Chemical Society.

formed a shell surrounding the entire particle [SiO2@(Au− Fe3O4)], which indicates that the strong Au−thiol bonds were inhibiting nucleation or growth of SiO2, forcing growth on the Fe3O4 domain.

5. OTHER PATHWAYS TO NANOPARTICLE HETEROTRIMERS Seeded-growth pathways continue to expand in scope as we learn more about how high-order hybrid constructs form, including how particle surfaces and reaction parameters help facilitate chemoselective deposition. However, other pathways that do not rely on surface-driven nucleation and growth processes have the potential to offer new synthetic levers for controlling important aspects of morphology and composition while providing access to systems that otherwise would remain elusive.

Figure 8. Cartoon schematic and TEM images characterizing the insertion of Ge between the (a) Au−Fe3O4 heterodimer interface to form (b) Au−Ge−Fe3O4 heterotrimers. STEM-EDS element maps of (c) Fe3O4−Au and the corresponding (d) Fe3O4−Au−Ge addition product and (e) Fe3O4−Ge−Au insertion product. HAADF-STEM images of the (f) addition and (g) insertion products. Adapted with permission from ref 34. Copyright 2015 American Chemical Society.

5.1. Insertion vs Addition Reactions

Colloidal reactions that exploit supersaturation−precipitation can circumvent challenges in seeded growth processes and provide access to hybrid nanoparticles that contain synthetically challenging components. For example, colloidally synthesized Au and Ag nanocrystals react with a germanium-hexamethyldisilazane (Ge-HMDS) complex to form Au−Ge and Ag−Ge heterodimers through a process that first supersaturates the Au and Ag with Ge and then precipitates the Ge, which grows outward.33 Such processes, which are well-known in the vapor− liquid−solid (VLS) and solution−liquid−solid (SLS) growth of semiconductor nanowires, are portable to colloidal Ge systems to produce hybrid constructs. Interestingly, when the same reaction used to transform Au nanoparticle seeds into Au−Ge hybrids is instead applied to Au−Fe3O4 heterodimer seeds, the Ge precipitates between the Au−Fe3O4 domains, resulting in the insertion of Ge between Au and Fe3O4 to transform Au− Fe3O4 into Au−Ge−Fe3O4 (Figure 8).34 When using small amounts of the Ge-HMDS reagent, Ge deposits on the outer Au nanoparticle surface to form the Ge−Au−Fe3O4 addition product, which contrasts with the insertion product formed using larger amounts of Ge-HMDS. Au−Ge−Fe3O4 and Ge− Au−Fe3O4 can be considered as heterotrimer isomers and are selectively produced through complementary insertion vs addition pathways. Similar reactivity was observed for the Au−In2O3 system, producing Au−Ge−In2O3 through insertion of Ge between the Au and In2O3 domains. This new nanoparticle insertion reaction conceptually parallels the molecular insertion reactions that are used in organometallic

synthesis and, similarly, provides access to isomeric structures having otherwise inaccessible connectivities. 5.2. Chemical Transformation Reactions

In addition to offering rules and guidelines for adding and linking nanocrystals at precise locations within hybrid architectures, the anisotropic reactivity of individual nanocrystals within hybrids can be exploited to accomplish domainselective transformations. For example, Murray and co-workers showed that indium-doped cadmium oxide (ICO) could be grown from metallic nanocrystal seeds (M = Au, Pt, Pd, FePt), and the ICO domain of the resulting M−ICO heterodimers could then be selectively transformed into either CdS or CdSe using an anion exchange process.35 Recently, Ouyang and coworkers demonstrated that such approaches can be extended to higher-order systems, which allowed them to generate interesting three-component hybrids.36 For example, the authors showed that Pt−Ag−CdSe could be converted to Pt−Ag−Ag2Se or Pt−Ag0.21Au0.79−CdSe through cation exchange and alloying, respectively. Importantly, these apporaches can deliver high-order hybrid nanoparticles with material combinations and material interfaces that are synthetically inaccessible using traditional seeded-growth methods. 1438

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6. CONCLUSIONS AND OUTLOOK Colloidal hybrid nanoparticles integrate multiple nanomaterials into a single construct that can have unique properties not found in single-component systems. Among these emerging materials, high-order hybrid nanoparticles that contain three or more domains can have highly sophisticated functionalities and can serve as discrete models of complex device architectures. Although such materials offer synthetic control over nanoscale phenomena, configurational ambiguities and other complexities associated with producing high-order nanostructures present unique challenges for colloidal nanochemists. In recent years, new strategies have emerged for controlling nanoparticle arrangements in high-order systems that leverage the conceptual similarities that colloidal hybrids share with complex molecules. Such strategies, including chemoselective addition, insertion reactions, and protection−deprotection capabilities, provide rational approaches for chemists to synthesize targeted materials with predetermined geometries and properties. However, in many cases, the current arsenal of synthetic tools has been only demonstrated for a limited scope of material systems; opportunities exist for extending these capabilities to a more diverse scope of nanoparticle materials that better map onto the range of desired functions and anticipated applications. Continued methodology development to overcome these synthetic bottlenecks is therefore needed to provide access to high-order hybrid constructs with greater compositional and morphological diversity. Fundamental knowledge regarding the physical and chemical pathways that underpin the formation of hybrid nanoparticles is critically important as we move toward the goal of being able to synthesize hybrids with arbitrary compositions and configurations. Mechanistic insights, such as the indiscriminate nucleation and ripening processes described above for the formation of Ag−Pt−Fe3O4 heterotrimers, provide important information that can be used to develop synthetic protocols for generating challenging nanoparticle isomers, although each system can exhibit unique growth processes. Insights into the underlying principles that dictate which formation pathway(s) is(are) operable in a given synthesis will be invaluable for guiding the development of reaction protocols prior to running laborious experiments. Additionally, the manipulation of such processes using the tools of colloidal chemistry is anticipated to play an increasingly important role as material requirements become simultaneously more stringent and more sophisticated. For example, surface functionalization can be envisioned as an important synthetic lever for controlling the dynamics of nucleation and surface migration of nanoparticles across hybrid seeds, although limited knowledge exists. Likewise, siteselective chemical transformations of preformed hybrids, which exploit well-understood colloidal nanochemistry and can be applied rationally, are offering a new perspective for controlling configurational nanoparticle isomerism and are expected to deliver the next generation of high-order hybrid nanoparticles that will facilitate new fundamental studies and advanced applications.

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The authors declare no competing financial interest. Biographies James M. Hodges received his B.S. in chemistry from Rider University in 2012. He earned his Ph.D. at the Pennsylvania State University in 2016 studying the synthesis and applications of inorganic nanostructures under the supervision of Raymond E. Schaak. Currently, he is a postdoctoral fellow with Mercouri G. Kanatzidis at Northwestern University, where his research focuses on the design of new metal chalcogenide systems for thermoelectric applications. Raymond E. Schaak received his B.S. in chemistry from Lebanon Valley College in 1998 and his Ph.D. in chemistry from the Pennsylvania State University in 2001 under the direction of Thomas E. Mallouk. He then did postdoctoral work at Princeton University with Robert J. Cava. He is currently the DuPont Professor of Materials Chemistry at the Pennsylvania State University. His research interests include synthetic inorganic nanochemistry and new materials discovery.

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ACKNOWLEDGMENTS This work was supported by the U.S. National Science Foundation under Grant CHE-1410061 (J.M.H., R.E.S.). TEM imaging was performed in the Penn State Microscopy and Cytometry facility.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Website: https://sites.psu.edu/ rayschaak/. ORCID

Raymond E. Schaak: 0000-0002-7468-8181 1439

DOI: 10.1021/acs.accounts.7b00105 Acc. Chem. Res. 2017, 50, 1433−1440

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Accounts of Chemical Research

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DOI: 10.1021/acs.accounts.7b00105 Acc. Chem. Res. 2017, 50, 1433−1440