Symmetry-Breaking Synthesis of Multicomponent Nanoparticles

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Symmetry-Breaking Synthesis of Multicomponent Nanoparticles Zhiqi Huang,‡ Jinlong Gong,*,‡ and Zhihong Nie*,† †

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State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai 200438, P.R. China ‡ Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University; Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China CONSPECTUS: Multicomponent nanoparticles (MCNs) composed of disparate inorganic colloidal components have attracted great attention from researchers in both the academic and industrial community, because of their unique properties and diverse applications in energy conversion and storage; heterogeneous catalysis; optics and electronics; and biomedical imaging, diagnosis, and therapy. Compared with singlecomponent nanoparticles (NPs), new or advanced properties of MCNs arise from the synergistic effect between their constituent components and the presence of nanoscale interfaces between distinct materials within the particles. Consequently, the spatial arrangement of nanoscale domains of MCNs becomes equally important in property or function control of MCNs as their size, shape, and composition, if not more. In particular, compositionally asymmetric MCNs may outperform their symmetric counterparts in many of their applications. To this end, the seed-mediated growth (SMG) method, which involves depositing a second material onto seed NPs, has been considered as the most common strategy for the synthesis of asymmetric MCNs with desired complexity. In this approach, the control of symmetry breaking during MCN growth is usually achieved by manipulating the growth kinetics or using seed NPs with asymmetric shapes or surfaces. Although great progress has been made in the past decade, there remains a challenge to control the shape, orientation and organization of colloidal components of MCNs with a high yield and reproducibility. Recently, several unconventional methods have been developed as an important addition to the synthetic toolbox for the production of complex MCNs that otherwise may not be readily attainable. This Account summarizes recent advancements on the development of unconventional synthetic strategies for breaking the growth symmetry in the synthesis of asymmetric MCNs. We start with a brief discussion of the achievements and limitations of the conventional strategies for symmetry breaking synthesis. In the subsequent section, we present three unconventional approaches toward symmetry-breaking synthesis of asymmetric MCNs, namely, surface-protected growth, interface-guided growth, and welding-induced synthesis. First, we discuss how commonly used soft agents (e.g., collapsed polymer) and hard agents (e.g., silica) can be asymmetrically coated on seed NPs to template the asymmetric growth of secondary material, generating a broad range of MCNs with complex architectures. The unique features and key factors of this surface-protected synthesis are discussed from the viewpoints of the surface chemistry of seed NPs. We further discuss the use of a solid/liquid or liquid/liquid interface to guide the synthesis of Janus or more complex MCNs through two general mechanisms; that is, selective blocking or impeding the access of precursors to one side of seed NPs and interfacial reaction-enabled generation of asymmetric seeds for further growth. Finally, we discuss a symmetry-breaking method beyond the SMG mechanism, directed welding of as-synthesized single-component NPs. Moreover, we discuss how the unique structural symmetry and compositional arrangement of these MCNs significantly alter the physical and chemical properties of MCNs, thus facilitating their performance in exemplary applications of photocatalysis and electrocatalysis. We finally conclude this Account with a summary of recent progress and our future perspective on the future challenges.

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INTRODUCTION Multicomponent nanoparticles (MCNs) have multiple nanoscale domains of distinct materials within one particle. The configurations of MCNs can be symmetric (e.g., core−shell and yolk−shell MSNs) or asymmetric (e.g., dumbbell, oligomer, and coaxial MCNs). The conjugation or close placement of different components at nanoscale results in a strong synergistic coupling effect that can significantly alter the intrinsic physical and chemical properties of the constituent materials.1−3 In particular, this coupling effect (e.g., electronic, magnetic and plasmonic coupling) may generate new © XXXX American Chemical Society

intriguing properties that may not be attainable by any single-component nanoparticles (NPs). Thus, MCNs have shown emerging applications in solar energy harvesting, batteries, catalysis, optical and electronic devices, and biomedical imaging and delivering.4 The properties of MCNs are strongly dependent on not only the size, shape, and composition of individual components but also the relative spatial arrangement of these constituent domains. Moreover, Received: January 20, 2019

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DOI: 10.1021/acs.accounts.9b00038 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

reaction kinetics is essential to the formation of high-quality MCNs, which poses a synthetic challenge.

recent research has shown that MCNs with asymmetric spatial distribution of constituent domains may outperform their symmetric counterparts in some applications.5−8 For instance, asymmetric TiO2-tipped Au nanorods showed dramatically enhanced photocatalytic hydrogen evolution activity, compared with symmetric core−shell TiO2-covered Au nanorods.6 Over the decade, great progress has been achieved in the synthesis of asymmetric MCNs with desired properties.8−10 Conventional strategies for breaking symmetry in the growth of one material on another can be largely summarized into two categories, (1) kinetically controlled manipulation11 and (2) anisotropic NP seed-guided growth.12 Despite their initial success, these approaches have intrinsic limitations, such as being restricted to specific types of materials, being sensitive to small variations in synthetic conditions, and poor reproducibility in scale-up. More recently, we and others have developed several nonconventional approaches (e.g., surface-protected growth, interface-guided synthesis, and reaction or weldinginduced synthesis) to the symmetry−breaking synthesis of MNCs with asymmetric arrangement of multiple components.13−17 Given the growing interest in the synthesis of asymmetric MCNs and the application of them in diverse fields, this Account summarizes recent developed strategies/concepts for the symmetry-breaking synthesis of asymmetric MCNs and succinctly reviews the emerging applications of these asymmetric MCNs in nanocatalysis (e.g., photocatalysis and electrocatalysis). We aim to highlight the pros and cons of these strategies over conventional methods, as well as point out the major challenges associated with them. We hope this Account can provide insights into the rational design and synthesis of MCNs for practical applications and inspire the development of new synthetic strategies.

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SYMMETRY BREAKING IN SYNTHESIZING ASYMMETRIC MCNS In the past six years, several unconventional concepts have been demonstrated for breaking symmetry in the synthesis of MCNs. Protection of certain surfaces of seed NPs can induce selective deposition of a secondary material on the seeds, leading to the generation of various MCNs with unique morphologies (Scheme 1a). Alternatively, a solid/liquid or Scheme 1. Schematic Illustration of (a) Surface-Protected Growth, (b) Interface-Guided Growth, and (c) WeldingInduced Symmetry Breaking in the Synthesis of Asymmetric MCNs

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liquid/liquid interface can be used for the synthesis of asymmetric MCNs, owing to the breaking of the symmetry of NP seeds in the vicinity of the interface (Scheme 1b). Beyond the SMG mechanism, welding-induced synthesis has also been proven as a promising approach in the generation of asymmetric MCNs (Scheme 1c).

CONVENTIONAL STRATEGIES FOR SYMMETRY BREAKING The wet chemical synthesis of MCNs has mostly relied on the seed-mediated growth (SMG) method, which can be understood by the classical nucleation theory3,18 from a thermodynamics point-of-view. Typically, when the lattice mismatch (and associated interfacial free energy) between the seed and secondary materials is sufficiently high, asymmetric growth of the secondary material preferentially takes place on the seed; otherwise, symmetric growth is adopted to yield symmetric core−shell-type MCNs. To break the thermodynamic limitation, a kinetic-control concept has been widely used to promote the growth of nanocrystals into nonsymmetric modes.11 This strategy is applicable to the synthesis of MCNs from a broad range of materials, including metals, chalcogenides, oxides, and their hybrids. However, nowadays control over kinetic parameters (e.g., precursor concentration and reaction time) is still largely based on empirical or qualitative principles, which may cause batch-to-batch variations in synthetic products. Anisotropic NP seed-guided growth represents another scenario of symmetry-breaking synthesis of MCNs. Shapeanisotropic NPs constitute distinctive crystal facets with a difference in surface energy and ligand packing density caused by surface curvature. The shape anisotropy of the seeds can drive the growth of a second material selectively on facets with high surface energy.6,12 Since such a surface energy difference is often small, fine-tuning of both the shape of seeds and

Surface-Protected Growth

Heterogeneous nucleation and growth of a secondary material selectively on certain surfaces of existing seeds are essential for breaking the symmetry in the SMG of asymmetric MCNs. To achieve this selectivity, one simple yet straightforward philosophy is to cover part of the surface of the seed NPs with “protecting agents”.14 As such, the access of a precursor in the growth solution to protected surfaces is selectively blocked, and the deposition of a secondary material is only allowed on an unprotected area. The key to symmetry breaking of this approach is the anisotropic coating of the seed NPs with proper “protecting agents”. Commonly used protecting agents can be classified into two categories, soft agents (e.g., polymers14,17,19) and hard agents (e.g., silica20,21). Collapsed Polymer-Protected Synthesis. Polymers have been proven to be effective in protecting seed surfaces for the anisotropic growth of MCNs. Upon reducing the quality of solvent, polymeric ligands change their conformation and collapse selectively on NP surfaces to form a dense protective layer.22 After the nucleation and growth of inorganic components on exposed seed surfaces, polymers can be readily dissolved or removed in good solvents. Importantly, the collapse of polymers can be asymmetrically patched on the B


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good solvents for PS (Figure 1c). This general method is applicable to seed NPs with isotropic shape. When Au nanospheres were used as seeds, laterally phase-separated PS and CTAB ligands on NP seeds promoted the asymmetric deposition of metals or metal oxides to produce Saturn-like MCNs with an Au core and a ring-like secondary material shell (Figure 1d,e). Other types of polymers have also been used as protecting agents in the synthesis of MCNs.17,19 For example, thiolterminated poly(ethylene oxide)-b-polystyrene (PEO-b-PS) was used as a protecting agent in the synthesis of Au−Pd dimers (Figure 1f) and Au−Ag alloy nanobowls (Figure 1g).19 The phase separation of PEO-b-PS chains and citrate residual on Au nanospheres led to the protection of one side of the seed surface by collapsed PS domains in the subsequent SMG or galvanic replacement in aqueous solutions. Polystyrene-bpoly(acrylic acid) (PS-b-PAA) was coated on Au nanorods, triangular nanoprisms, and bipyramids as a protecting agent for selective metal (e.g., Pd and Ag) deposition on the tips of the nanorods, nanoprisms, and bupyramids (Figure 1h−j).17 The tips of the three shaped Au NPs were exposed after the collapse of the PS block of PS-b-PAA upon heating in a DMF/ water mixture solution, allowing for the secondary metal (e.g., Pd and Ag) to deposit on these sites. In this case, the phase separation of mixed hydrophobic and hydrophilic small molecular ligands (e.g., CTAB and 1-dodecanethiol mixture) capped on Au NPs allowed for the asymmetric coating of the PS block of the PS-b-PAA chains on hydrophobic ligand covered surfaces. The formation of collapsed polymer domains is essential for the protective growth. Although brush-like polymer ligands are effective in promoting asymmetry growth of some particular materials (e.g., silica21), they are often not sufficiently robust to block the access and nucleation of most inorganic materials onto the surfaces of seed NPs.24 Silica-Protected Synthesis. Silica can be asymmetrically coated on NPs due to the shape anisotropy of seed NPs or ligand segregation on the NP surface. The silica blocks act as a rigid, robust protecting layer for asymmetric secondary deposition and growth.20,21,25 Wang et al. demonstrated controlled deposition of metals on either the sides or ends of Au nanorods by varying the location of silica coating on the ends or sides of nanorod seeds.21 Silica was first asymmetrically coated on the ends of Au nanorods due to the loose ligand packing in this area. Subsequent metal deposition occurred exclusively on the uncoated sides of Au/end-silica nanorods, leading to the formation of anisotropic Au/end-silica/side-M (M = Au, Ag, Pd, and Pt) nanorods (Figure 2a,b). When Au nanorods end-functionalized with PEO were used as the starting material, silica was selectively deposited on the side of nanorods, as the PEO chains hindered the condensation of silica on the ends. In this case, silica-protected metal deposition led to the formation of Au/side-silica/end-M nanorods (Figure 2c,d). Furthermore, silica can be coated asymmetrically on isotropic seed NPs to guide the anisotropic metal growth.20,25 For instance, Chen et al. coated silica on one-half of 4mercaptophenylacetic acid (4-MPAA) and PAA-capped Au nanospheres using the segregation of 4-MPAA and PAA on NP surfaces (Figure 2e). The Janus Au-silica NPs were used as seeds for the selective growth of Ag only on the exposed Au surface to produce ternary Ag−Au−SiO2 nanostructures (Figure 2f).20

seeds due to the phase separation of polymers with bare NP surfaces or other ligands on NP surfaces (Figure 1a).

Figure 1. Collapsed polymer-directed synthesis of asymmetric MCNs. (a) Schematic illustration of polymer-directed growth of second material on seed NPs with different shapes. (b, c) TEM images of coaxial-like Au−Pd MCNs with Au core and Pd shell in linear chains (b) and as an individual (c). (d, e) TEM images of Saturn-like Au−Pd MCNs with an Au core and Pd belt from (d) side-view and (e) topview. (f) TEM image of Au−Pd heterodimers synthesized by depositing Pd on patched Au/polymer NP seeds. (g) TEM image of Au−Ag-alloyed nanobowls by galvanic reaction of Ag/polymer NPs and Au ions. (h−j) TEM images of Au@PSPAA nanorods deposited with Pd/Ag at two terminals (h), Au@PSPAA nanoprisms deposited with Ag at three tips, and Au@PSPAA bipyramids deposited with Ag at two tips (j). Insets in (c−e) and (g−j) are the corresponding structural models. Panels b−e were reproduced with permission from ref 14. Copyright 2016 Springer Nature. Panels f and g were reproduced with permission from ref 19. Copyright 2017 Wiley. Panels h−j were reproduced with permission from ref 17. Copyright 2018 Springer Nature.

The regio-selectivity of polymer layers can be originated from differential affinity of polymer ligands to a certain crystal facet or the phase separation of surface ligands (e.g., polymers and other small molecules).23 In 2007, Kumacheva et al. first demonstrated the selective modification of thiol-terminated polystyrene (PS) on both ends of Au nanorods through ligand exchange in proper solvents, such as dimethylformamide (DMF). Subsequent addition of bad solvents (e.g., water) for PS caused the collapse of the polymers at both ends of the nanorods. This triggered the end-to-end association of nanorods into linear chains, while leaving the sides of Au nanorods covered by only hydrophilic cetyl trimethylammonium bromide (CTAB). Inspired by this work, Nie et al. utilized collapsed-polymers to direct the deposition of secondary material on seed NPs to prepare a variety of coaxial-like and Saturn-like MCNs (Figure 1b−e).14 In the synthesis, collapsed PS domains selectively protected the ends of Au nanorods to guide the deposition of metals (e.g., Pd, Pt, Ag, and Ni) or metal oxides (e.g., cuprous oxide and ceria) exclusively on the sides of nanorods (Figure 1b). Subsequently, the nanorod chains were dissociated to produce individual coaxial-like MCNs with different shell compositions by adding C


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prepare Pt@TiO2@In2O3@MnOx mesoporous hollow spheres (PTIM-MSs) composed of a thin TiO2−In2O3 bilayer heterojunction with selective decoration of Pt clusters on the inner surface and MnOx clusters on the outer surface of the hollow spheres (Figure 2h,i).28 The surface-protected growth is not prone to variation in kinetic factors (e.g., temperature, solvents, reactant amount, deposition rate, etc.), compared with conventional kinetics controlled- and anisotropic seed guided-symmetry breaking. In general, this is a fairly robust and scalable approach for symmetry breaking synthesis and is applicable to the growth of a broad range of inorganic materials. Moreover, polymer or silica coating can be readily removed after synthesis, thus facilitating their utilization in various applications. Nevertheless, a significant challenge still lies in the site-specific coating of protecting agents on the seed NPs, especially for isotropic NPs. Future efforts should be devoted to a deeper understanding and precise control of phase separation of organic or polymeric molecules on the surfaces of seed NPs.

Figure 2. Silica-protected synthesis of asymmetric MCNs. (a−d) Schematic illustration (a, c) and TEM images (b, d) of the synthesis of Au/end-silica/side-M (a, b) and Au/side-silica/end-M nanorods (c, d). (e, f) TEM image of Janus Au-silica NPs (e) and ternary Ag− Au-silica NPs (f). (g) Schematic illustration of a Pt-TiO2−MnOx hollow sphere. (h) TEM image of PTIM-MSs and corresponding models. (i) The magnified image of the area highlighted by a dotted box in (h). Panels a−f were reproduced with permission from ref 21. Copyright 2013 Wiley. Panel g was reproduced with permission from ref 27. Copyright 2016 Wiley. Panel h and i were reproduced with permission from ref 28. Copyright 2016 Wiley.

Interface-Guided Synthesis

The interfaces between distinct phases of matter (e.g., liquid/ solid, liquid/air, and liquid/liquid) offer an ideal platform for the synthesis of asymmetric MCNs. The symmetry breaking in growth is originated from the separation of precursors in distinct phases or the blocking or impeding of the diffusion of precursors toward some parts of the seeds.29 Solid/Liquid Interface. NP seeds can be immobilized on a substrate (e.g., silicon wafer or polymers) and immersed in a solution containing precursors of the secondary material for further growth.15,30−32 The solid substrate limits the diffusion of precursors toward the bottom side of the seed NPs, promoting the nucleation and growth of secondary material selectively on the top surfaces of seeds. For instance, Neretina et al. demonstrated the synthesis of asymmetric Au−Ag MCNs with different configurations.32 The Au NP array on a sapphire substrate was first prepared by templated dewetting of periodic Au films and then immersed in an AgNO3 growth solution. At a slow reaction rate, Ag was deposited solely on the top surface of Au seeds to produce Janus Au−Ag dimers (Figure 3a,b). When the reaction rate was fast, more Ag atoms could go

Silica can be used as both a template and protecting agent to prepare an asymmetric hollow structure.26−28 Gong et al. recently demonstrated the construction of hollow spheres with inner and outer surfaces asymmetrically decorated with different materials using silica nanospheres as a sacrificial template.27 Sequential deposition of Pt NPs, TiO2 shells, and SiO2 protective layers on silica spheres and subsequent calcination and removal of silica produced Pt-TiO2 hollow spheres with Pt decorated on the inter surfaces. MnOx were subsequently deposited exclusively onto the outer surface due to the limited penetration of precursors into the interior of the hollow spheres (Figure 2g). This approach was extended to

Figure 3. Solid/liquid interface-guided synthesis of asymmetric MCNs by using NP seeds immobilized on substrates. (a) SEM image and (b) HAADF-STEM image of asymmetric Ag−Au MCNs. (c, d) SEM images of core-half satellite Au−Ag (c) and Au@void@half Au frame (d) MCNs obtained at different reaction conditions. (e) Top-view SEM image of 2D array of Janus cuboctahedral Au-cubic Pd MCNs. (f) Side-view TEM image of an individual asymmetrical Au−Pd polyhedral MCN highlighting the asymmetric Pd growth. (g, h) SEM images of Au@Pd (g) and Au@ Pd cubic shell (h) MCNs collected at different reaction times. Panels a−c were reproduced with permission from ref 32. Copyright 2014 American Chemical Society. Panel d was reproduced with permission from ref 31. Copyright 2018 American Chemical Society. Panels e−h were reproduced with permission from ref 15. Copyright 2018 American Chemical Society. D


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NPs.37 When ultrasound was applied, reaction at the liquid/ liquid/gas ternary interface yielded high-quality conical-shaped Au-PEOA MCNs.36 The polymer domain in these Au-polymer MCNs can be easily removed by organic solvents to produce Au nanocups and nanocones. The use of a liquid/liquid interface offers abundant interfacial area for scalable production of asymmetric Januslike MCNs, compared with solid/liquid interface-based synthesis. However, the deformation of interface or tumbling of NPs at the interface may affect the precision in the nucleation sites and growth directions. Moreover, for small NPs at the interface, thermal energy becomes comparable to or higher than interfacial energy, and interfacial thermal fluctuations cause the migration of NPs away from the interface, resulting in undesired interference to the growth of inorganic MCNs.38 The solidification of one liquid (e.g., made from phase changing material) may offer a stable trapping of seeds at the interface for subsequent growth.39

through the electric double layer (consisting of silver and nitrate ions) surrounding the Au seeds, and homogeneous coating of Ag to the exposed surface of the Au seed led to the formation of asymmetric core-half satellite MCNs (Figure 3c) or core-half shell MCNs (where the bottom half of the shell was cut due to the blocking effect of the substrate). The same group further prepared Au−Ag core-half shell and Au@void@ half Au frame nanostructures using a similar Au NP array (Figure 3d).31 The combination of a collapsed-polymer-protected growth and interface-assisted strategy can be used to engineer the orientation and symmetry of 2D MCN arrays on substrates.15 PS-tethered Au NPs (e.g., spheres, cubes) were immobilized on a silicon wafer, and oxygen plasma etching was used to selectively expose the top surface of Au seeds. The remaining collapsed-PS at the bottom surfaces of Au seeds fixed the NPs on substrates and confined the secondary deposition solely on the top surfaces of seeds to yield Janus cuboctahedral Au-cubic Pd MCNs (Figure 3e,f). This unique shape could be attributed to the preferential orientation of the cubioctahedral seeds along the direction. Moreover, by tuning the reaction time and reactant concentration, the shape of the Pd shell can be tuned to produce Au@Pd (with Pd coating on the (111) facet of Au NPs) and Au@Pd cubic shells (with Pd coating on all exposed facets of Au NPs) MCNs (Figure 3g,h). The solid substrate naturally blocks one side of the seed NPs and typically leads to the formation of Janus-type MCNs upon secondary growth. The immobilization of seed NPs on solid substrates brings additional advantages in synthesis, such as ease in removal of surfactants (or no need of surfactants), increased stability of NPs, convenience in integrating structures on devices, etc. However, the intrinsic challenge for scale-up synthesis largely limits the development of this method. Liquid/Liquid Interface. Driven by the minimization of interfacial energy, NPs tend to assemble at the liquid/liquid interfaces, with the opposite NP surfaces being exposed to different liquids.33 This phenomenon has been widely used for selective surface modification of NPs (or micro/submicrometer particles) to produce Janus-like particles. The selective exposure of an NP surface to two liquids can be utilized to guide the selective growth of a secondary material to produce Janus-like or dumbbell-like MCNs (e.g., asymmetric dimer-like Fe 3 O 4 −Ag MCNs, FePt-Ag, and Au−Ag Janus-like MCNs).34,35 Furthermore, interfacial reactions can also be used to prepare hybrid Janus MCNs. Recently, Nie et al. demonstrated a simultaneous polymerization-nucleation approach for the synthesis of a series of asymmetric metal−polymer Janus MCNs.11 As shown in Scheme 1b, a solution of monomer (e.g., aniline) in organic solvent and an aqueous solution of inorganic precursors (e.g., HAuCl4) formed a water/oil interface in a glass vial. The interfacial redox reaction initiated the polymerization of aniline to generate polyaniline (PANi) nanoparticles and the reduction of HAuCl4 to form Au NPs on the surface of PANi NPs. By fine-tuning the concentration of organic and inorganic precursors, asymmetric Au-PANi MCNs with lollipop-, dumbbell-, and frog-egg-like shapes were synthesized at high yield. This method can be extended to synthesize other asymmetric metal−polymer structures.36,37 For example, the combined interfacial reaction and subsequent Au growth led to the generation of Au-poly-2-ethoxyaniline (PEOA) patchy

Welding-Induced Symmetry Breaking

Recently, some new concepts beyond the SMG have been developed to achieve symmetry breaking in growth. One representative approach is based on a partial cation exchange reaction, which is capable of generating a gallery of asymmetric chalcogenide MCNs by reacting single-component chalcogenide NPs with proper cations.40−42 This approach is, however, largely limited for the synthesis of chalcogenide-based MCNs. Direct welding of as-synthesized single-component NPs is another scenario to break the original symmetry. Early research mostly focused on the welding of small NPs via an oriented attachment mechanism to produce various complex nanostructures (e.g., one-dimensional nanowires, two-dimensional nanosheets, etc.).43,44 However, this mechanism only allows for welding of NPs of the same kind, due to the requirement of lattice matching. Recently, a nonoriented attachment mechanism has been utilized by several groups for the welding of the same or dissimilar NPs.45,46 For instance, welding of Au and Ag NPs was observed upon conformal contact in a vacuum condition.45 Nevertheless, the welding of as-synthesized dissimilar NPs in dispersion has been rarely explored. By taking the combination of Au NPs and chalcogenide NPs as an example, Nie and coworkers made an effort to develop the directed welding of dissimilar NPs in dispersions as a new strategy for breaking symmetry in the synthesis of asymmetric MCNs (Figure 4a).13 When polyethylene glycol (PEG)/citrate comodified Au nanospheres (negatively charged) were mixed with bovine serum albumin (BSA)-modified chalcogenide (e.g., Ag2S, positively charged) nanospheres in dispersion, the dissimilar NPs preferentially associated into clusters due to electrostatic repulsion between the same NPs and attraction between dissimilar NPs. The subsequent diffusion of atoms between Au and Ag2S NPs welded them to yield asymmetric oligomer-like Au−Ag2S MCNs (Figure 4b,c). The number of Au and Ag2S domains within the welded oligomer-like MCNs could be roughly tuned by varying the feeding ratio of dissimilar NPs. This welding process was mainly driven by (i) the ligand desorption-induced Van der Waals attraction between NPs and (ii) the reaction of Au and Ag2S to form AuAg3S2 compounds upon attachment. This general method was applicable for the welding of NPs with different shapes (e.g., nanorods, nanowires) to produce more complex nanostructures, such as asymmetric matchstick-like and tree burl-like Au−Ag2S MCNs E



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ASYMMETRIC MCNS FOR CATALYSIS The asymmetric arrangement of constituent components in MCNs can benefit their performances in catalytic reactions (e.g., photocatalysis and electrocatalysis).4 First, the spatial arrangement of constituent components dictates the electron transfer pathway, thus impacting the catalytic efficiency of MCN-based catalysts. Second, the spatial separation of domains with different functionalities prevents the undesired interruption and reverse reaction, thus dramatically enhancing the overall reaction rate. Finally, a synergistic effect may arise at the interfaces of different domains, which is of paramount importance in tandem or cooperative catalysis. In photocatalysis, the spatial separation of photogenerated charge carriers is crucial to prevent the recombination and increase the lifetime of these charge carriers, thus leading to dramatically enhanced catalytic performances.14,27,28,48−50 Compared with symmetric MCNs, asymmetric MCNs with spatially separated oxidation and reduction cocatalysts can promote the separation of hot electrons and holes. For instance, in a system of Pt@TiO2@In2O3@MnOx mesoporous hollow spheres (PTIM-MSs) with a heterojunction TiO2− In2O3 layer and inside−outside asymmetrically decorated reduction cocatalyst Pt and oxidation cocatalyst MnOx(Figure 5a), photogenerated electrons and holes flow inward and outward to accumulate on the corresponding cocatalysts and participate in redox reactions, respectively (Figure 5b). As a result, the PTIM-MSs displayed a 466.6 μmol g−1 h−1 of oxygen evolution in a photocatalytic water oxidation reaction. In contrast, the oxygen evolution rate for nanostructures with random cocatalyst distribution (TI/P/M-MSs) and nanostructures without heterojunction (PTM-MSs) was around 280 and 320 μmol g−1 h−1, respectively (Figure 5c).28 The critical role of asymmetric arrangement of cocatalysts has also been demonstrated by other groups.48,50 As an example, Moskovits et al. showed that symmetric coating of the Pt-based reduction catalyst and co-based oxidation catalyst on core−shell Au@ TiO2 nanorods was essential for their overall photocatalytic performance in water splitting.50 Similarly, Li et al. found that

Figure 4. Welding-induced synthesis of asymmetric MCNs. (a) Schematic illustration of assembly assisted welding of Au NPs and Ag2S NPs. (b) TEM image of the welded Au−Ag2S oligomers (gray, Ag2S; dark, Au). (c) HAADF-TEM image of an individual welded Au−Ag2S oligomer. (d, e) TEM image of welded matchstick-like (d) and tree burl-like (e) Au−Ag2S MCNs. Reproduced with permission from ref 13. Copyright 2019 Springer Nature.

(Figure 4d,e). It is worth noting, that the welding occurred exclusively on the Au nanorod tips with loose ligand packing. Recent advances in the synthesis of single-component NPs with controlled sizes and shapes have provided a huge library of NP building blocks that can be used for programmable construction of MCNs via welding.47 This welding method potentially allows for the preservation of the original shapes of the building blocks and control over the orientation of constituent components, which represents a major challenge in the conventional SMG method. The welding of NPs as a synthetic method is still in its infant stage. The next phase of research will be to refine the process to achieve a higher yield (or purity), as well as expand the horizon of materials for welding. The initial success suggests two potential avenues toward rational construction of MCNs by controlled welding, (1) controlling the feeding ratio of building blocks and (2) using shaped NPs with asymmetric surface ligand packing to induce site-specific welding.

Figure 5. Asymmetric MCNs for photocatalysis. (a) The PTIM-MS structure and the mechanism for photocatalytic oxidation. A represents an electron acceptor, NaIO3. (b) Simplified band structure of PTIM-MS. The heterojunction shell and spatially separated cocatalysts favor the opposite flow direction of photo-generated hot electrons and holes. (c) The activity of various photocatalysts in water oxidation under simulated sunlight irradiation. (d) Benzaldehyde generation rate over coaxial Au-CeO2 MCNs, core−shell Au-CeO2 MCNs, and CeO2 NP catalysts under visible light irradiation. (e, f) Proposed photo-oxidation mechanisms for coaxial-like Au-CeO2 MCNs (e) and core−shell type Au-CeO2 MCNs (f). Panels a−c were reproduced with permission from ref 14. Copyright 2016 Wiley. Panels d−f were reproduced with permission from ref 28. Copyright 2016 Springer Nature. F


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impact on the further advances in MCN synthesis. For instance, liquid-cell TEM allows for in situ observation of the growth of nanomaterials with atomic resolution, which will significantly improve our understanding of the growth mechanism.54 Finally, reproducibility and scale-up in synthesis have already hindered the practical applications of asymmetric MCNs, in spite of the proven merits of these MCNs. Further advancement requires better understanding of the synthetic mechanism and roles of different influencing factors. For instance, kinetics is important in the symmetry-breaking step, but in most cases, our control over kinetics is largely empirical, which imposes a challenge in optimization.

asymmetric deposition of reduction cocatalysts Pt and oxidation cocatalysts MnOx on monoclinic BiVO4 single crystals led to a high photocatalytic oxygen evolution rate of approximately 650 μmol g−1 h−1; whereas the crystals with random coverage of reduction and oxidation cocatalysts showed negligible activity.48 In another example, Pt-tipped Au nanorods outperformed Pt-thoroughly covered Au nanorods in a photocatalytic hydrogen evolution reaction, thanks to the structurally asymmetry-promoted charge separation.49 The symmetry of MCNs can also affect the catalytic reaction pathways. For instance, asymmetric coaxial-like Au-CeO2 MCNs exhibited two times higher catalytic activity than symmetric core−shell MCNs in selective photo-oxidation of benzyl alcohol to benzaldehyde by altering the reaction pathways (Figure 5d).14 As illustrated in Figure 5e, two reaction routes occurred over the coaxial-like catalysts, (i) the photoreduced Ce(III) species-mediated generation of superoxide species oxidized by benzyl alcohol to benzaldehyde, and (ii) the photogenerated positive charges that accumulated on Au cores directly participated in the oxidation reaction on the exposed Au nanorod tips. Both charge carriers contributed to the benzaldehyde generation using coaxial MCNs, while the CeO2 shell in core−shell Au-CeO2 MCNs completely blocked the function of the second reaction route (Figure 5f). In addition to photocatalysis, the symmetry of MCNs is also crucial to their activity and selectivity in electrocatalysis. As an example, Pd−Cu MCNs with different symmetry (ordered, disordered, and phase separated) showed distinct electrochemical CO2 reduction selectivity. The ordered structures exhibited high hydrogen selectivity, while the asymmetric structures favored the formation of the C−C bond and generated a large amount of C2 products (e.g., ethylene and ethanol).51

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jinlong Gong: 0000-0001-7263-318X Zhihong Nie: 0000-0001-9639-905X Notes

The authors declare no competing financial interest. Biographies Zhiqi Huang obtained his B.S. and Ph.D. degree in Chemical Engineering from Tianjin University in 2011 and 2017, respectively, under the tutelage of Professor Jinlong Gong. From 2012 to 2015, he was a joint Ph.D. student in Professor Zhihong Nie’s group at the University of Maryland, College Park (UMCP). He is currently a Postdoctoral Fellow at Nanyang Technological University with Professor Hua Zhang. His research focuses on the synthesis and photo/electrocatalytic applications of multicomponent nanomaterials.

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PERSPECTIVES The demand for asymmetric MCNs has been on the rise, which is the driving force for better synthetic methodology. Recent advances have offered several effective aforementioned routes to breaking the symmetry in the synthesis of asymmetric MCNs, especially for the system following a layer-by-layer growth mode.13−15,21 These strategies are a good addition to the existing synthetic toolboxes for the preparation of structurally unique MCNs and development of new or advanced applications of these nanostructures. Despite the initial success in symmetry-breaking synthesis of MCNs, a set of challenges are still confronting this field. First, existing synthetic approaches do not offer sufficient control over the site and shape of constituent components with high precision. This has become a barrier for establishing a systematic structure−property correlation of MCNs. In this regard, the direct welding of as-synthesized NPs with welldefined sizes and shapes may be a promising solution to tackle this issue, as the synthesis of single component NPs with desired size and shape is quite mature.52 Second, our understanding of synthetic mechanisms is still limited and largely built on hypothesis and nondirect evidence, mostly due to the lack of relevant capable characterization tools. For instance, surface chemistry of NPs (e.g., ligand distribution and orientation) is often key to the symmetry-breaking synthesis,23,53 but current characterization techniques usually do not allow for in situ monitoring of the surface chemistry changes during synthesis. The development or utilization of advanced characterization techniques would have a positive

Jinlong Gong studied chemical engineering and obtained his B.S. and M.S. degrees from Tianjin University and his Ph.D. degree from the University of Texas at Austin under the guidance of C.B. Mullins. After a stint with Professor George M. Whitesides as a postdoctoral fellow at Harvard University, he joined the faculty of Tianjin University, where he currently holds a Cheung Kong Chair Professorship in chemical engineering. His research interests in surface science and catalysis include catalytic conversions of green energy, novel utilizations of carbon oxides, and synthesis and applications of nanostructured materials. He is an elected Fellow of the Royal Society of Chemistry. Zhihong Nie received his Ph.D. degree in chemistry from the University of Toronto and worked as a NSERC Postdoctoral Fellow with Professor George M. Whitesides at Harvard University. He is currently a Professor in the Department of Macromolecular Science at Fudan University. Before that, he was an Associate Professor with tenure in the Department of Chemistry and Biochemistry at UMCP. He is the recipient of the 3M Nontenured Faculty Award, ACS PRF Doctoral New Investigator Award, NSF CAREER Award, etc. His research interests include molecular and nanoparticle self-assembly, biomedical imaging and delivery, programmable soft materials, and microfluidics.

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ACKNOWLEDGMENTS Z.N. gratefully acknowledges the financial support of the Startup Fund from Fudan University. J.G. thanks National Key Research and Development Program of China (2016YFB0600901), the National Science Foundation of G


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China (21525626), and the Program of Introducing Talents of Discipline to Universities (B06006).

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Symmetry-Breaking Synthesis of Multicomponent Nanoparticles

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