Bimetallic Nanocrystals: Syntheses, Properties, and Applications

Review pubs.acs.org/CR

Bimetallic Nanocrystals: Syntheses, Properties, and Applications Kyle D. Gilroy,† Aleksey Ruditskiy,‡ Hsin-Chieh Peng,‡ Dong Qin,§ and Younan Xia*,†,‡,∥ †

The Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, Georgia 30332, United States ‡ School of Chemistry and Biochemistry, §School of Materials Science and Engineering, and ∥School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ABSTRACT: Achieving mastery over the synthesis of metal nanocrystals has emerged as one of the foremost scientific endeavors in recent years. This intense interest stems from the fact that the composition, size, and shape of nanocrystals not only define their overall physicochemical properties but also determine their effectiveness in technologically important applications. Our aim is to present a comprehensive review of recent research activities on bimetallic nanocrystals. We begin with a brief introduction to the architectural diversity of bimetallic nanocrystals, followed by discussion of the various synthetic techniques necessary for controlling the elemental ratio and spatial arrangement. We have selected key examples from the literature that exemplify critical concepts and place a special emphasis on mechanistic understanding. We then discuss the composition-dependent properties of bimetallic nanocrystals in terms of catalysis, optics, and magnetism and conclude the Review by highlighting applications that have been enabled and/or enhanced by precisely controlling the synthesis of bimetallic nanocrystals.

CONTENTS 1. Introduction 2. The Possible Architecture of Bimetallic Nanocrystals 2.1. Atomic Ordering 2.1.1. Alloys 2.1.2. Intermetallic Compounds 2.2. Core−Shell Nanocrystals and Their Derivatives 2.2.1. Shells of a Single Atomic Layer Thick 2.2.2. Shells of Multiple (n = 2−6) Atomic Layers Thick 2.2.3. Shells Thicker than Six Atomic Layers 2.2.4. Hollow Derivatives of Core−Shell Nanocrystals: Nanoboxes and Nanocages 2.3. Core-Frame Nanocrystals and Their Derivatives 2.4. Dendritic and Heterostructured Nanocrystals 3. Synthetic Approaches to Bimetallic Nanocrystals 3.1. Coreduction 3.1.1. Importance of Reduction Potentials 3.1.2. Role of Coordination Ligands 3.1.3. Possible Role of Surface Capping Agents 3.2. Thermal Decomposition 3.2.1. Concurrent Decomposition of Two Precursors 3.2.2. Decomposition in Conjunction with Reduction 3.2.3. Decomposition of a Bimetallic Precursor 3.3. Seed-Mediated Growth 3.3.1. The Hallmark of Seed-Mediated Growth © 2016 American Chemical Society

3.3.2. Seeds with Diversified Internal Structures 3.3.3. Growth Modes on Nanoscale Substrates 3.3.4. Thermodynamic versus Kinetic Control 3.3.5. Bimetallic Nanocrystals Enabled by Seed-Mediated Growth 3.4. Galvanic Replacement 3.4.1. The Role of Reduction Potential and the Mechanism of Galvanic Replacement 3.4.2. Bimetallic Nanocrystals Enabled by Galvanic Replacement 3.4.3. One-Pot Synthesis of Hollow Nanocrystals via Temporal Separation 3.4.4. Combining Galvanic Replacement with Coreduction 3.4.5. Elimination of Galvanic Replacement for Achieving Seed-Mediated Growth 3.4.6. Combining Galvanic Replacement with Underpotential Deposition 3.4.7. Combining Galvanic Replacement with the Kirkendall Effect 4. Properties of Bimetallic Nanocrystals 4.1. Catalytic Properties 4.2. Optical Properties 4.2.1. Localized Surface Plasmon Resonance (LSPR)

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Special Issue: Nanoparticle Chemistry Received: April 4, 2016 Published: July 1, 2016 10414

DOI: 10.1021/acs.chemrev.6b00211 Chem. Rev. 2016, 116, 10414−10472

Chemical Reviews 4.2.2. Dependence of LSPR on the Size or Shape of a Nanocrystal 4.2.3. Dependence of LSPR on the Composition of a Nanocrystal 4.2.4. Effect of Shape and Composition on the Sensitivity of LSPR-Based Detection 4.2.5. Effect of Electron Density on LSPR 4.2.6. Hybridization of LSPR with Magnetism 4.3. Photocatalytic Properties 4.4. Magnetic Properties 4.4.1. Finite Size Effects of Magnetic Nanocrystals 4.4.2. Enhancing Magnetic Properties with Bimetallic Composition 5. Applications of Bimetallic Nanocrystals 5.1. Catalytic Applications 5.1.1. Pt−Cu and Au−Cu Nanocrystals 5.1.2. Pt−Ni Nanocrystals 5.1.3. Pd@PtnL Core−Shell Nanocrystals 5.2. Optical and Photocatalytic Applications 5.2.1. Hybridization of Plasmonic and Catalytic Metals 5.2.2. Indirect Plasmonic Sensing 5.3. Magnetic Applications 5.3.1. Bimetallic Magnetic Nanocrystals as Theranostic Agents 5.3.2. Bimetallic Magnetic Nanocrystals for Separation 6. Concluding Remarks Author Information Corresponding Author Notes Biographies Acknowledgments References

Review

properties. An array of novel features will arise when bulk metals are transformed into nanocrystals, including drastically increased specific surface areas, higher densities of lowcoordination atoms on the surface, changes to phase stability and miscibility, quantum-confinement effects, and novel magnetic phenomena such as superparamagnetism. Analogous to the discoveries made back in the Bronze Age, upgrading the simplest monometallic nanocrystals to bimetallic or more complex compositions would greatly enhance their properties, often making them superior to their monometallic counterparts. For example, optimizing some of these characteristics has led to the development of effective heterogeneous catalysts. Since the industrial production of heterogeneous catalysts is roughly a 13 billion dollar per year business,2 even small enhancements can result in large economic savings. Among those most extensively investigated for catalysis are the platinum group metals (PGMs, including Pt, Pd, and Rh; see Figure 1 for a partial list). These metals are able to catalyze a

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1. INTRODUCTION Metals represent more than 75% of the elements in the periodic table. The properties of metals have fascinated humans since the Iron Age, while Sterling Au and Ag were known to man since the Stone Age. It could be argued that one of the most transformative periods of time in ancient history was the Bronze Age, during which humans discovered that simply mixing two metals to form an alloy would result in a hybrid material stronger than either one of the metals involved.1 Remarkably, this discovery was made more than 4000 years ago, and today we still manufacture and use alloys as structural materials in a similar way. Aside from the improvement in mechanical properties for bulk metals (e.g., hardness, ductility, and malleability, among others), alloying also plays an essential role in defining their chemical properties. A classic example can be found in iron, which is prone to surface oxidation (or rusting) when placed in an environment containing water, oxygen, and salt. However, by simply alloying with small amounts of Cr and C, for the formation of stainless steel, the resultant surface is capable of protecting the bulk from oxidation and loss of structural integrity almost forever. In addition to their ubiquitous use as structural materials, metals and their alloys have also found indispensable value in the finely divided state known as nanocrystals. The worth of metal nanocrystals stems not only from the access to tiny structures but more importantly the size- and shape-dependent

Figure 1. A section of the periodic table showing the metals covered in this Review. These metals can be divided into three major groups according to their best known property or application.

wide variety of chemical reactions such as coupling, hydrogenation, and dehydrogenation that are widely used in the pharmaceutical industry, as well as in the production of polymers, pesticides, and dyes.3 Nevertheless, the bulk use of these metals (45%, 78%, and 80% of their global production, respectively) is in the three-way catalytic converters, which contain at least two of the PGMs to convert the hazardous byproducts (NOx, CO, and hydrocarbons) exhausted from the automobiles into less harmful gases such as N2, CO2, and H2O.3,4 As evident from this example, a practical catalytic system often requires the involvement of two or more metals to simultaneously or sequentially handle different types of reactions. Metal nanocrystals are also fascinating for their composition-, size-, and shape-dependent optical properties. The prototypical example is Au, a highly reflective metal that readily absorbs visible light by stimulating localized surface plasmon resonance (LSPR) when reduced to the nanometer size.5,6 This optical response arises from the collective oscillation of conduction electrons in a nanocrystal in response to the oscillating electric 10415


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field of the incident light. When the incident light matches the resonance wavelength of the Au nanocrystal, the photons will be absorbed and a localized surface plasmon will be stimulated. The spectral features of LSPR (e.g., the number of resonant modes and their peak positions) depend critically on the size and shape of the nanocrystal.5,7 The LSPR properties of Au nanocrystals can be further enhanced through alloying with another metal. For example, the electric near-field associated with LSPR can be intensified with the incorporation of Ag. Although Ag is classically known for surface oxidation, alloying with Au results in a hybrid material that can take on the inertness of Au while retaining the remarkable optical properties of Ag. The Au/Ag pair is just one example; a large number of bimetallic systems have been exploited in a similar way for the formation of diversified hybrid materials. The novel optical properties associated with bimetallic nanocrystals have enabled innovative applications, including those related to sensing, imaging, catalysis, as well as plasmonically enhanced chemistry, spectroscopy, and light harvesting. It is inadequate to just use “composition” to describe bimetallic nanocrystals because their expressed physicochemical properties are highly sensitive to the spatial arrangement and atomic ordering of the different types of atoms. Therefore, a great deal of effort has been placed on developing syntheses capable of generating bimetallic nanocrystals with well-defined atomic-scale features (e.g., faceting, atomic ordering, and spatial arrangement, among others). Thanks to the effort from many research groups around the world, a large number of synthetic protocols have been established that are capable of producing a broad range of bimetallic nanocrystals. This Review mainly covers recent advances related to the syntheses, properties, and applications of well-defined bimetallic nanocrystals composed of the metals shown in Figure 1. It is our intention to highlight the most relevant and recent advances by selecting articles that best represent vital concepts. It should be noted that, in the past few years, several related reviews have been published, including those about the architectural design of heterogeneous metallic nanocrystals,8−10 the synthesis and optical properties of alloyed plasmonic nanomaterials,11−17 the synthesis and catalytic applications of nanoalloys,18−33 and the synthesis of alloys for electrochemical energy applications.34,35 In preparing this Review, we have tried to avoid overlap as much as possible. Simply let the large number of review articles serve as a testament to the large activity and great importance of this research field.

Figure 2. Schematic illustration showing the evolution from two types of metal atoms (center) to four types of atomic distributions (middle ring) and then a large number of bimetallic nanocrystals with distinct architectures (outer ring). The two different atoms have a yellow and gray color, while an alloy or intermetallic compound has an orange color.

2.1. Atomic Ordering

In terms of atomic ordering, bimetallic nanocrystals can be divided into two major classes: alloys and intermetallic compounds. Throughout this Review, alloy is reserved for nanocrystals consisting of two metals that are randomly and thoroughly mixed. In contrast, an intermetallic compound refers to a system that has both long-range atomic order and well-defined stoichiometry. This specification is important because alloys and intermetallic compounds tend to have different properties even if they share the same elemental composition and atomic ratio. Thermodynamic considerations (e.g., atomic radii, lattice parameters, and atomic interactions, among others) ultimately prescribe the most favorable positions for atoms both locally (alloyed vs intermetallic) and globally (facet and internal structure). However, as a general theme of nanocrystal growth, the final state of a nanocrystal is seldom defined by thermodynamics alone, but rather in conjunction with kinetics.36−39 2.1.1. Alloys. It is widely accepted that M−N alloys exhibit physicochemical properties that are unique and often superior to nanocrystals comprised of the individual metals.20,28,40 To uncover the role composition plays in the overall effectiveness of a given bimetallic nanocrystal, mastery over the synthesis is essential. Currently, the most commonly used method for synthesizing M−N solid nanocrystals is based on coreduction (see section 3.1) and, to a lesser extent, on concurrent thermal decomposition (see section 3.2). While both methods are particularly effective in controlling the atomic ratio of M to N, there are only a few reports demonstrating simultaneous control over both the composition and the shape. Aside from these two methods, galvanic replacement and utilization of the Kirkendall effect have been explored as effective routes to the production of M−N hollow nanocrystals (see section 3.4).

2. THE POSSIBLE ARCHITECTURE OF BIMETALLIC NANOCRYSTALS While only comprised of two elements, the architecture of bimetallic nanocrystals can still differ in terms of atomic ordering (alloyed or intermetallic), crystal structure, internal structure (with different numbers of twin defects and/or stacking faults), shape or type of facet, as well as configuration (dimeric, dendritic, or core−shell, including concentric/nonconcentric). For simplicity, here we classify bimetallic nanocrystals according to the spatial distribution of their constituent elements. The outermost ring in Figure 2 shows a summary of the major bimetallic architectures typically found in the literature. Although the number of possible architectures is vast, they can be synthesized using a limited number of approaches discussed in section 3. 10416


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another metal (N), typically denoted by [email protected] Its synthesis typically involves a seed-mediated approach, in which preformed M-nanocrystals (often referred to as seeds or templates) act as heterogeneous nucleation sites for the atoms of metal N generated through the chemical reduction or thermal decomposition of an N-containing precursor. This approach was originally developed for templates made of colloidal particles (e.g., polymer latex beads, silica spheres, and metal−oxide particles) and is amendable to virtually any template geometry (e.g., spheres, rods, wires, prisms, and polyhedra).19 Repeating this process sequentially can be used to generate multilayered core−shell structures resembling that of an onion or matryoshka. In recent years, this approach has been successfully extended to templates based on metal nanocrystals in a process commonly known as seed-mediated growth (see section 3.3). Remarkably, the synthesis has now become sophisticated enough to produce core−shell nanocrystals with shell thicknesses controllable down to one atomic layer, which is appealing for a number of reasons including: (i) enhancement of catalytic performance through electronic coupling and/ or the strain effect, (ii) improvement in chemical and thermal stability, (iii) tuning of optical properties, (iv) augmenting of magnetic anisotropy, and (v) serving as a critical intermediate in the formation of other types of structures (e.g., boxes, cages, or frames). Because the physicochemical properties of the shell are extremely sensitive to its thickness, here we further categorize core−shell nanocrystals into three subgroups depending on the shell thickness: (i) a single atomic layer, (ii) 2−6 atomic layers, and (iii) more than 6 atomic layers. 2.2.1. Shells of a Single Atomic Layer Thick. A nanocrystal core encapsulated by a single atomic layer (also referred to as a skin layer) is a unique subtype of M@N core− shell nanocrystal, denoted by M@N 1L. When used as heterogeneous catalysts, this system has the highest possible utilization efficiency and thereby the lowest materials cost for the atoms on the surface. The heteroepitaxial strains imposed on the shell by the core, together with the electronic coupling, also provide an effective means for tuning and maximizing the specific activity of the catalytic system. There are a number of methods for controllably growing a monolayer of one metal on the surface of another, including: (i) seed-mediated growth, where stoichiometric consideration can be used to deposit just one atomic layer of the shell metal on the seed (section 3.3), (ii) formation of a sacrificial monolayer (e.g., Cu) through underpotential deposition (UPD) and then replacing this layer with another metal having a higher reduction potential via galvanic replacement (section 3.4), and (iii) formation of an alloy or intermetallic nanocrystal and then inducing phasesegregation (a method limited to bimetallic compounds that favor this kind of transformation, such as Pt3Ni@Pt or Pt3Cu@ Pt). For most bimetallic combinations, however, the M@N1L structure is a far-from-equilibrium configuration, and the involvement of a long period of time and/or a high temperature will result in a detrimental interdiffusion process. 2.2.2. Shells of Multiple (n = 2−6) Atomic Layers Thick. A nanocrystal core encapsulated by 2−6 atomic layers, represented by M@NnL (where n denotes the number of atomic layers), is another class of core−shell nanocrystal with valuable properties. The range of 2−6 atomic layers covers the characteristic thickness over which strain and electronic coupling can still exist between the core and the shell. Shells with thicknesses greater than six atomic layers are generally considered free from core-induced property modulation and

The mixing of two metals (M and N) will be favored when (i) the M−N bond is stronger than both the N−N and the M− M bonds; (ii) the two metals have similar lattice parameters (e.g., crystal structure and lattice constant); and (iii) the two metals share similar surface energies.9 These parameters only describe some of the attributes of thermodynamically stable alloys, and there is always the possibility of forming a nonequilibrium phase during the synthesis. At a relatively low reaction temperature, for example, the atoms are prevented from reaching their thermodynamically prescribed positions through surface and/or bulk diffusion. As such, it is not uncommon to observe two highly miscible metals (e.g., Au and Ag) forming nanocrystals with a core−shell or dimeric structure rather than an alloy.41 In this case, additional heating is necessary to accelerate the interdiffusion of atoms, promoting the formation of an alloy (see section 3.3.5). 2.1.2. Intermetallic Compounds. For some bimetallic systems (e.g., Au−Cu or Pt−Ni), they might favor atomic ordering that gives rise to a lattice structure distinct from the individual metals, and take on a well-defined stoichiometry, such as Au3Cu or PtNi2, rather than random mixing or alloying (see section 4.4.2). Thermodynamically, the formation of an alloy or intermetallic compound can be predicted by considering the change in Gibbs free-energy upon mixing (ΔGmix), which is defined by42 ΔGmix = ΔHmix − T ΔSmix

(1)

where ΔHmix and ΔSmix are the changes in enthalpy and entropy during mixing, respectively, and T is absolute temperature. In principle, the atomic ordering in a bimetallic system should be determined by the thermodynamic drive to minimize ΔGmix under a given set of experimental conditions.42 In most cases, however, bimetallic nanocrystals that are prescribed by thermodynamics to form intermetallic compounds end up in alloys with disordered atomic arrangements. Postsynthesis annealing (under the protection of an inert gas) is often needed to promote the transformation from an alloy to an intermetallic compound.43 It should be pointed out that eq 1 falls short of being a fully quantitative description as nanoscale systems can have a number of other energetic terms that need to be considered, including strain-, defect-, and surface-free energies. The growing interest in intermetallic compounds, over alloys, stems from the enhanced properties along with the higher degree of control achievable for composition and surface structure. In recent years, intermetallic compounds have emerged as superior nanomaterials, especially in the context of catalysis and magnetics. Several studies reveal that intermetallic compounds outperform structures equivalent in atomic ratio but with a disordered atomic arrangement, in terms of catalytic and magnetic properties.44,45 In the context of electrocatalysis, Pt−Ni-based intermetallic nanocrystals have gained significant attention due to their impressive activity toward the oxygen reduction reaction (ORR) (see section 5.1). In terms of magnetism, intermetallic compounds comprised of one 3d metal (e.g., Fe, Co, Ni) and one 4d or 5d metal (e.g., Pd, Pt) have been identified for future information storage. Overall, nanocrystals made of intermetallic compounds are promising candidates for a variety of niche applications. 2.2. Core−Shell Nanocrystals and Their Derivatives

A core−shell nanocrystal consists of an arbitrarily shaped inner core (metal M) conformally coated with a shell made of 10417


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leave behind a shell or cage made of metal N or, in most cases, M−N alloys; (ii) reacting sacrificial templates comprised of a less noble metal (e.g., Ag, Ni, or Cu) with the ions of a more noble metal (e.g., Au3+, Pt2+, or Pd2+), through a process commonly referred to as galvanic replacement; (iii) making use of the Kirkendall effect; or (iv) through a combination of these approaches.

behave as if completely comprised of the shell metal. Perhaps the greatest advantage of having the ability to controllably derive this subclass of core−shell nanocrystals is to access the thickness-dependent catalytic activity. For example, in a recent study involving Pd@PtnL core−shell icosahedra (n = 0.7, 2.0, 2.7, 4.8), it was determined that the optimal shell thickness for achieving the highest ORR mass activity was the intermediate thickness of n = 2.7.46 This result, among many others, highlights the importance of having synthetic methods capable of depositing material with monolayer accuracy. There are four general methods for controllably generating M@NnL (n = 2−6) nanocrystals, including: (i) seed-mediated growth, where the salt precursor to seed ratio can be controlled to grow the shell to a desired thickness, (ii) sequentially repeating the UPD and galvanic replacement processes, (iii) phase segregation, which is common to the Pt−Ni system, where Pt atoms tend to concentrate at the surface, forming an ultrathin Pt skin,47 and last, (iv) anion coordination, which involves seeds stabilized with ligands containing functionalities capable of binding the metal precursor before becoming reduced.48 2.2.3. Shells Thicker than Six Atomic Layers. A majority of the literature related to the synthesis and application of core−shell nanocrystals pertains to those with shells more than six atomic layers (or 1.5 nm) thick. In these cases, the electronic coupling between the core and outermost layer in the shell is essentially lost, and thus the ability to access the straindependent catalytic activity will be gone. However, there are many advantages to having core−shell nanocrystals with such thick shells, including: (i) tuning the optical properties over a broad range, (ii) enhancing the thermal and chemical stability, and in some cases, (iii) improving the magnetic properties. In the context of optical properties, the position and intensity of the LSPR peaks are extremely sensitive to the shell thickness. However, as the shell reaches a critical thickness, it is generally accepted that the optical properties of the shell metal will dominate the optical response, behaving as if the core is screened by the shell (section 5.2). Therefore, tailoring the thickness of the shell is critical for tuning the optical properties of core−shell nanocrystals. The synthetic approach most commonly used for controllably depositing thick shells is based on seed-mediated growth, where a metal-containing precursor is reduced or thermally decomposed in the presence of preformed seeds. 2.2.4. Hollow Derivatives of Core−Shell Nanocrystals: Nanoboxes and Nanocages. Nanoboxes and nanocages represent two classes of highly open structures with optical resonances tunable across the visible and near-infrared (NIR) region of the electromagnetic spectrum.49,50 The highly open structure and optical tunability, unique to both structures, is a key feature that has enabled a number of advanced applications in biomedicine including drug-delivery, photothermal cancer therapy, sensing, and many others.51,52 In terms of structure, the primary difference between nanoboxes and nanocages is the porosity of the walls. Nanocages have obvious pores in the walls, allowing easy access for solvent and small molecules to enter and exit their internal cavities, ideal for encapsulation, controlled release, and drug delivery.52,53 Access to the cavities also increases the specific surface area, making nanocages catalytically superior to their solid counterparts.54 On the other hand, nanoboxes generally have pore-free walls that effectively isolate the interiors from the external environment. General synthetic approaches to hollow structures include: (i) selectivity etching away the core from M@N core−shell nanocrystals to

2.3. Core-Frame Nanocrystals and Their Derivatives

The core-frame nanocrystal is a bimetallic structure that has a well-defined solid core with a second metal that coats and interconnects the edges, corners, or in some cases, the twin defects. As discussed in section 3.3, encasing a nanocrystal with a frame can help maintain its shape at elevated temperatures. The frame can also introduce a high density of surface atoms that have low coordination numbers, a key feature critical to many catalytic applications. Core-frame nanocrystals can be readily obtained through a kinetically controlled seed-mediated process, in which the second metal is selectively deposited onto sites that have relatively high surface energies (e.g., edges, corners, and twin defects, as well as regions free from capping agents). In addition, UPD and galvanic replacement can also be combined to generate a range of bimetallic nanocrystals, including the core-frame type (see section 3.4). The intimate contact between the frame and core provides the necessary channel for intermixing between the two metals involved. Therefore, forming an alloyed frame is often an inevitable consequence of the process, especially for metals with relatively low melting points that can readily interdiffuse at typical experimental temperatures (23−100 °C). An immediate derivative of the core-frame nanocrystal is nanoframe, characterized by a frame-like structure with feature sizes typically below a few nanometers.55 A common method for generating a nanoframe involves the selective removal of the core from a core-frame nanocrystal through the introduction of a core-specific etchant. The etching process leaves behind a bimetallic nanoframe whose dimensions (e.g., size and number of cross-bars) are directly linked to the geometry of the initial core. Because seeds can be diversified in terms of size, shape, and internal defect structure, there exists a large number of potential nanoframe architectures. Other physical aspects of the frame, such as uniformity and thickness, are mainly controlled by synthetic conditions. Outside from seed-mediated syntheses, several other methods based on galvanic replacement, dealloying, and, most recently, a corrosion-based strategy have all been successfully developed for the formation of nanoframes (see section 3.3.5). In terms of application, nanoframes have emerged as one of the most promising catalytic and plasmonic nanomaterials due to their high surface-to-volume ratio, high density of low coordination surface atoms, and LSPR peak positions that are readily tunable with the dimensions of the nanoframe. 2.4. Dendritic and Heterostructured Nanocrystals

In addition to the aforementioned bimetallic nanocrystals, there are a number of other structural types, including those in the dendritic (e.g., highly branched nanocrystals, multipods, stars, pentacles, sea urchin-like, and flowers, among others) and heterostructured forms (e.g., dimers, dumbbell structures, and Janus particles, among others).41,56−64 A M/N dendritic nanocrystal is characterized by a solid core made of metal M and branches or arms composed of metal N stemming from the core. There are four general routes to the generation of dendritic nanocrystals, including a (i) kinetically controlled, 10418


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parameters such as the reduction potentials of the metal ions involved, the strength of the reducing agent, the nature of coordination ligand, the capping agent, and the reaction temperature. The nucleation and growth of nanocrystals depends critically on the reduction rate of the metal precursor or, equally, the rate at which metal atoms are generated in the solution. On a more fundamental level, this process is governed by the rate at which electrons are transferred from the reductant to the metal precursor. Our understanding of the fundamental physics of electron transfer between molecular species became modernized in 1957 with the introduction of Marcus theory.66 This theory culminated into what is widely known as a familiar relation between the reaction rate constant k and the change in Gibbs free energy (ΔG) of the reaction:

seed-mediated growth, in which the rate of deposition is much greater than the rate of surface diffusion, (ii) galvanic replacement carried out in the presence of bromide or other halide ions, (iii) anisotropic etching of facet-controlled nanocrystals, and (iv) aggregation or oriented attachment, in which small nanocrystals merge with larger nanocrystals along specific crystallographic directions. The appeal for dendritic nanocrystals are their large surface-to-volume ratios and accessible high index surfaces (characteristic of the high curvature at and around the branches and tips); both of them are important in heterogeneous catalysis. The sharp tips also provide sites for concentrating electrons and thus generating intense electric fields upon resonant excitation, critical to optical applications such as surface-enhanced Raman scattering (SERS). Highly branched plasmonic structures are also exceedingly sensitive to small changes in the local refractive index (e.g., change of solvent, ligand exchange, and binding events), making them promising substrates for sensing (see section 4.2). A heterostructured bimetallic nanocrystal (represented by M/N) generally refers to a nanocrystal comprised of two distinct metals joined at one or a few interfaces. While this classification of nanocrystal is a bit vague, these types of bimetallic nanocrystals fall under the concept of symmetry breaking, by which their formation generally relies on a kinetically controlled, seed-mediated process (see section 3.3). As discussed later, the formation of heterostructured nanocrystals will be favored when the rate of surface diffusion is greatly decelerated (via deposition at a relatively low temperature) or when the deposition is nonepitaxial. In terms of application, the unique feature of the M/N bimetallic nanocrystals is the presence of two distinct surfaces that can carry out separate chemical transformations while at the same time sharing an interface for electron transfer, a characteristic key to the operation of bimetallic photocatalysts (see section 5.2).

k = A ·e−ΔG / kBT

(2)

where A depends on the nature of the electron transfer process (e.g., bimolecular or intramolecular), kB is the Boltzmann constant, and T is the absolute temperature. The change in Gibbs free energy, ΔG, can be further defined by ΔG =

2 λ⎡ ΔG° ⎤ + 1 ⎢ ⎥ λ ⎦ 4⎣

(3)

where ΔG° is the standard free energy of the reaction and λ is the reorganization term, which depends on both the solvational and the vibrational changes that occur during any of the molecular reorganization processes. As evident from these fundamental relationships, the kinetics of an electron transfer process and thus the rate of reduction are intimately linked to the thermodynamic stability of the reactants, products, and other interacting species. To this end, the experimental conditions can be purposefully designed to modulate the rate at which atoms are formed in solution, and, consequently, the shape and internal structure taken by the nanocrystals. 3.1.1. Importance of Reduction Potentials. As a zeroorder approximation, metal ions with more positive reduction potentials tend to be reduced at a faster rate as compared to those with lower potentials. As shown in Table 1, metal ions intrinsically differ in reduction potential. Metal ion combinations that have relatively large differences in reduction potential are especially difficult to work with. In general, only metal ions with similar redox potentials, like those next to each other in Table 1, should be selected for generating alloyed nanocrystals via coreduction. For example, Pd2+/Pd and Pt2+/Pt, which have

3. SYNTHETIC APPROACHES TO BIMETALLIC NANOCRYSTALS Thanks to the great work from many research groups, there exists an extensive collection of literature describing protocols for syntheses capable of generating bimetallic nanocrystals with well-controlled properties. In this section, we only focus on the most commonly practiced synthetic approaches. It should be pointed out that some of the methods are capable of generating very similar products due to similarity in terms of fundamental mechanisms or pathways. It is not uncommon to see more complex structures derived from a combination of two or more different synthetic routes.

Table 1. Reduction Potentials of the Metals in This Review reduction reaction −

Au + 3e → Au Pt2+ + 2e− → Pt Ir3+ + 3e− → Ir Pd2+ + 2e− → Pd Ag+ + e− → Ag Rh3+ + 3e− → Rh Ru3+ + 3e− → Ru Cu2+ + 2e− → Cu Ni2+ + 2e− → Ni Co2+ + 2e− → Co Fe2+ + 2e− → Fe 3+

3.1. Coreduction

Coreduction is arguably the most straightforward method for generating alloyed and intermetallic M−N nanocrystals. This method involves the simultaneous reduction of two metalcontaining precursors to zerovalent atoms, M0 and N0, which then nucleate and grow together to generate M−N nanocrystals. In some cases, a subsequent annealing step (typically, in an inert atmosphere) is required to obtain a homogeneous alloy (e.g., Au−Ag or Pd−Pt) or, for some bimetallic combinations, structural ordering for the formation of intermetallic compounds (e.g., Pt3Ni or AuCu3).43,65 Overall, coreduction can lead to a large number of bimetallic nanocrystals, where the final structure can be tailored by varying experimental

Eo (V vs SHE)a 1.50 1.18 1.16 0.95 0.80 0.76 0.45 0.34 −0.25 −0.28 −0.44

Standard conditions: 25 °C and 1 atm. SHE: standard hydrogen electrode.

a

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reduction potentials of +0.9 and +1.18 V, respectively, can be readily coreduced to form Pd−Pt alloyed nanocrystals. To this end, Sun and co-workers demonstrated an oil-phase method for the synthesis of Pd−Pt alloyed polyhedral nanocrystals by coreducing Pd(acac)2 and Pt(acac)2 with morpholine borane (MB) in oleylamine (OLA). 67 They found that the composition of the nanocrystals could be tuned by varying the feeding ratio of the two precursors while their size was being tuned from 3.5−6.5 nm by changing the temperature in the range of 90−180 °C. They also established that the Pd/Pt atomic ratio in the nanocrystals correlated well with the feeding ratio of Pd(acac)2 to Pt(acac)2. As demonstrated with this and other systems, feeding the precursors at different molar ratios serves as one of the simplest and most versatile approaches to controlling the final composition. The coreduction approach will run into a potential problem if the two metal ions differ by a large gap in terms of reduction potential. In this case, the more noble metal will be reduced first while the other lags behind. This is true for the pairs Au3+/ Au and Ag+/Ag, which have reduction potentials of +1.5 and +0.8 V, respectively. However, the rates of reduction of the two precursors can be synchronized by varying the molar ratio of the Au- and Ag-precursors. For example, Sun and co-workers reported a one-pot synthesis of uniform nanocrystals made of AuxAg100−x alloys by coreducing HAuCl4 and AgNO3 in a solution of octadecene (ODE) at 120 °C, with OLA serving as both the reductant and the surfactant.68 They found that a higher concentration of Ag+ (relative to HAuCl4) in the reaction mixture could compensate its slower reduction and lead to a comparative nucleation and growth rate with AuCl4−. They demonstrated that a molar ratio of 1:10 for HAuCl4 and AgNO3 led to a final composition of Au60Ag40. It is noteworthy to point out that Au and Ag atoms are known to quickly interdiffuse at temperatures as low as 100 °C,69 pointing to the likelihood that a reaction temperature of 120 °C will lead to mixing and thus alloying. Evidence for the formation of an Au− Ag alloy was confirmed by energy-dispersive spectroscopy (EDS) along with optical data, which revealed a single LSPR peak with spectral position linearly correlated to the Au:Ag ratio, a result verified by a number of other groups. They also found that Au−Ag alloys did not form at temperatures that were too low or too high (e.g., 65 or 180 °C). It should also be mentioned that the chloride ligands in the Au-precursor (AuCl4−) will also play a major role in governing the relative reduction rates, because AuCl4− and Au3+ have reduction potentials of +1.00 and +1.50 V, respectively (a topic discussed in more detail in section 3.1.2). Varying the relative concentrations of metal precursors for achieving composition control was also recently demonstrated by Yang and co-workers for the synthesis of Au−Cu alloy nanocrystals.70 They demonstrated that Au−Cu single-crystal nanospheres (e.g., Au, Au3Cu, AuCu, AuCu3, and Cu) could be synthesized by coreducing Au(acac)3 and Cu(acac)2 in a solution containing ODE, oleic acid (OA), OLA, and 1,2hexadecanediol under nitrogen protection. The spherical morphology and single-crystal structure of the Au−Cu nanocrystals could be readily confirmed by high-resolution transmission electron microscopy (HRTEM), which revealed that the nanocrystals were comprised of one complete singlecrystal domain free of planar defects (Figure 3a). They also found that the Au−Cu composition could be controlled simply by changing the relative concentrations of the Au and Cu precursors. Furthermore, they demonstrated that the compo-

Figure 3. Morphological and structural characterization of Au−Cu bimetallic nanocrystals prepared via coreduction. (a) TEM image of AuCu3 alloy nanocrystals, with the inset showing a HRTEM image of an individual nanocrystal (scale bar: 5 nm). (b) Extinction spectra and (c) X-ray diffraction patterns recorded from a set of nanocrystals with different compositions. Reprinted with permission from ref 70. Copyright 2014 Nature Publishing Group.

sition could be interpolated from the spectral position of the LSPR as the peak position red-shifted linearly with increasing Cu content (Figure 3b). The composition and crystal structure were verified by X-ray diffraction measurements, which also revealed that the atomic arrangement of the nanocrystals was random instead of ordered (Figure 3c). Despite these successful demonstrations, it has been difficult to generate alloys from two metals that differ vastly in physicochemical properties such as redox potential, crystal structure, surface energy, cohesive energy, and melting point, especially in a system that involves Au and a magnetic metal (Table 2). Adding to the kinetic difficulty is the fact that the product itself is unfavorable thermodynamically under equilibrium conditions because Au is known to be immiscible with magnetic metals such as Fe, Co, and Ni.71−73 As pointed out by Schaak and co-workers, early attempts to form Au3M (M = Co, Ni, and Fe) intermetallic compounds using common solution reduction techniques all ended in failure. Schaak and coworkers were among the first to solve this long-standing problem by devising a synthetic route capable of generating Au3Fe, Au3Co, and Au3Ni intermetallic compounds with the L12-structure at relatively low temperatures.74 Their synthetic strategy involved mixing HAuCl4, OLA, and the appropriate amount of Fe(acac)3, Co(acac)2, or Ni(acac)2 to yield a Au/M molar ratio of 3:1. A given Au/M-mixture then was added to octyl ether (OE) and heated to 80 °C under Ar. Meanwhile, in a second reaction vessel, a solution of n-butyllithium and OE was maintained at 80 °C under Ar. Last, the OE-metal-salt solution was added to the second reaction vessel via syringe, heated to 250 °C, and then allowed to cool to room temperature, after which the desired intermetallic compound was formed. It was also demonstrated that when OLA was eliminated from the reaction mixture, no intermetallic would be formed, suggesting that this surfactant played a dual role as both a stabilizer and a mild reducing agent. At temperatures exceeding 260 °C, however, it was found that the superlattice peaks (associated with the intermetallic phase) disappeared, 10420


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Table 2. Physicochemical Properties of the Metals in This Review metal

crystal lattice (abbr.)

Fe Co

bcc hcp

Ni Cu Ru

fcc fcc hcp

Rh Pd Ag Ir Pt Au

fcc fcc fcc fcc fcc fcc

lattice constant (Å) 2.87 a = 2.51, c = 4.07 3.52 3.61 a = 2.71, c = 4.28 3.80 3.89 4.09 3.84 3.92 4.08

bond dissociationa (kJ mol−1)

melting point (°C)

cohesive energyb (kJ mol−1)

1538 1495

413 424

204 201 193 ± 19.3

1455 1085 2254

428 336 650

125 41

2.01, 2.43, 2.37 1.95, 2.17, 2.24 3.93, 4.24, 4.86

± 0.05

1963 1554 962 2447 1772 1065

554 367 284 670 564 368

364 180 18 524 322 33

2.47, 1.92, 1.17, 2.97, 2.30, 1.28,

118 136 163 361 307 226

± ± ± ±

2.9 68 1.9 0.5

stacking fault energyc (mJ m−2)

surface free energyd (J m−2) 0.98, 1.27, 1.80 2.78, 3.04, 3.79

2.80, 2.33, 1.20, 3.72, 2.73, 1.63,

2.90 2.23 1.24 3.60 2.82 1.70

a

Values taken from ref 553. bValues taken from ref 554. cAll of the values taken from ref 160 with exceptions: stacking fault energies for Rh and Ir were taken from ref 555. dTheoretical values derived from the FCD method; facet order for bcc, (110), (100), (211); fcc, (111), (100), (110); hcp, (0001), (101̅0)A, (101̅0)B; ref 130.

reaction solution, (ii) alters the anisotropy in surface free energy of the nanocrystal, and (iii) forms a physical barrier to subsequent metal deposition. In a recent perspective article, Murphy and co-workers reviewed many examples of anisotropic growth modes induced and/or mediated by halide ions.77 They concluded the article by highlighting the necessity for observing the growth of an individual nanoparticle in real time and with atomic resolution, as this is the most promising means for elucidating the underlying mechanisms. If anything is safe to say regarding mechanism, it is that nearly all of the chemical species present in the reaction solution (e.g., the solvent, trace amounts of impurity, and even dissolved gases) can to some extent interact with the metal ions and thus have an impact on the nucleation and growth process. Even chemical additives that were once believed to serve a singular task, such as cetyltrimethylammonium bromide (CTAB) as a capping agent, have now been identified for having the potential to strongly interact with the metal ions and influence the overall reduction kinetics. For example, Berhault and co-workers demonstrated that adding CTAB to an aqueous solution containing Na2PdCl4 drastically lowered the rate of precursor reduction, caused primarily by the ligand exchange between Cl− and Br− and also the strong interaction between the cationic micelles and the Pd-halide anions.78 It is worth noting that the stability constant of PdBr42− is roughly 4 orders of magnitude greater than that of PdCl42−.79 As later highlighted by Biacchi and Schaak, even the solvent plays a drastic role on the reduction kinetics. They systematically investigated several rhodium precursors dissolved in a number of polyols including ethylene glycol (EG), triethylene glycol (TEG), and tetraethylene glycol (TREG).80 Because each polyol solvent has a different oxidation potential (defining the temperature at which particle formation occurs), even the selection of a solvent serves as an important synthetic lever for controlling nanocrystal growth.80 The multifunctional nature of many other chemical species, in the context of nanocrystal growth, has recently been reviewed by Ortiz and Skrabalak in a feature article.79 Only recently have several groups exploited the bifunctionality of ligands to control the composition, size, and shape of bimetallic nanocrystals. Of central importance is the design of a ligand environment that is capable of orchestrating the reduction rates of the two metal precursors for a desired

implying that a disordered alloy became the more favored product. These results indicate that the ordered intermetallic is a kinetically stabilized product due to the involvement of relatively low temperatures and short heating times.74 The final product was also found to contain MOx (M = Fe, Co, or Ni) as impurities. In a later report, the same group found that changing the solvent from OE (boiling point = 286 °C) to diphenyl ether (boiling point = 259 °C) could overcome this problem.75 The unbalanced reduction rates of the two metal precursors can also be remedied by cotitrating the solutions dropwise with the assistance of a syringe pump. By adjusting the injection rate, one can ensure that both precursors will be completely consumed before the next drops are introduced. In a recent report, Qin and co-workers demonstrated a new strategy for the synthesis of Ag@Ag−Au core-frame nanocubes by cotitrating AgNO3 and HAuCl4 concomitantly into an aqueous suspension of Ag nanocubes in the presence of ascorbic acid (AA) and poly(vinylpyrrolidone) (PVP) at room temperature.76 As the volumes of the cotitrated precursor solutions were increased, they validated that the added AgNO3 and HAuCl4 were completely reduced to Ag and Au atoms, followed by their codeposition onto the edges, corners, and then side faces of the Ag nanocubes. As a result, the dropwise cotitration method offers exquisite control over the relative amounts of Ag and Au atoms being deposited by varying the molar concentrations of the two precursors. This approach is potentially extendible to other bimetallic combinations. 3.1.2. Role of Coordination Ligands. According to the Marcus theory, the addition of chemical additives that strongly coordinate with the metal ions can profoundly affect their stability and reduction potential, and thus their reduction kinetics. For example, adding a halide (e.g., Cl−, Br−, and I−) to a metal precursor solution, such as Au3+, will result in the formation of Au−halide complexes with overall reduction potential dependent on the identity and relative concentration of the halide. Because the reduction potential decreases as [AuCl4]− > [AuBr4]− > [AuI4]−, the reduction rate would follow the same trend. One of the many aspects responsible for the mayhem behind the nucleation and growth of nanocrystals is the dual roles of ligands, as they can also bind preferentially to certain crystallographic facets of the nanocrystal (e.g., Br− to Pd{100}), a process that (i) depletes the halide from the 10421


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importantly, their data suggested that the final atom positions within resulting structures may be controlled by manipulating the chemical structure of precursor species in the reaction prior to nucleation.83 In summary, the presence of ligands in metal precursor solutions can give rise to some highly complex interactions, and as our understanding continues to grow, these interactions can be exploited to better control the nucleation and growth of bimetallic nanocrystals. 3.1.3. Possible Role of Surface Capping Agents. It is of critical importance to have simultaneous control over both the composition and the shape of bimetallic nanocrystals, as the expressed properties strongly depend on both parameters. As established by the extensive literature pertaining to the synthesis of monometallic nanocrystals, shape control is most commonly achieved through the addition of a proper capping agent. The capping agent is added with the purpose of selectively binding to specific facets on the nanocrystal, an event that effectively redefines the anisotropy in surface energy and thus manipulates the subsequent adsorption events. Capping agents encompass a broad range of chemical species, including monatomic-ions (e.g., Ag+, Cu2+, Ru3+, Br−, and I−),84−87 gases (e.g., O2, H2S, CO, and H2),88−90 organic ligands (e.g., inorganic ions, thiols, and amines), surfactants (e.g., CTAB, cetyltrimethylammonium chloride (CTAC), and sodium dodecyl sulfate), polymers (e.g., PVP, poly(acrylamide), or PAM), and biomolecules (e.g., peptides, alga extract, and plant extract).91−95 It is important to note that facet-selective ligands that are effective for the monometallic nanocrystals may not necessarily translate to the bimetallic systems.96 While a large number of capping agents have already been identified for the facet-specific capping of monometallic nanocrystals, more studies are necessary for identifying viable capping agents for the bimetallic systems. Several recent studies have identified effective shape directing ligands for a variety of alloyed and intermetallic nanocrystals. A good starting point for studying the formation of shapecontrolled bimetallic nanocrystals is the case of Pd−Pt, as this combination has similar lattice parameters and redox potentials, and is among the most interesting combination in terms of heterogeneous catalysis.97,98 As discussed previously, Pd−Pt alloys of varying compositions can be prepared by coreducing Pd(acac)2 and Pt(acac)2 at fixed ratios with MB in OLA.67 While the composition was readily tuned, the shape of the final Pd−Pt nanocrystals ranged from spherical to oblate, suggesting that little role was exerted by any of the ions or surfactants to preferentially stabilize a specific facet or that the kinetics involved during the nucleation and growth did not favor the formation of a particular shape either. By taking a different route, Yan and co-workers demonstrated the shape-controlled synthesis of sub-10 nm Pd−Pt alloyed nanocrystals with a tetrahedral shape (enclosed by {111}-facets) or a cubic shape (enclosed by {100}-facets).99 They found that the {111}faceted structures were prevalent when aqueous solutions of K2PtCl4 and Na2PdCl4 were reduced with formaldehyde (HCHO) in the presence of sodium oxalate (Na2C2O4) (Figure 4a). Therefore, they believed that C2O42− played a certain role in the stabilization of the {111}-facets. On the other hand, when the reaction was carried out at relatively high concentrations of Br− and low concentrations of I−, nanocubes were formed, most likely due to the selective capping of the {100}-facets by the halides (Figure 4b). In both cases, the homogeneity of the Pd−Pt alloyed tetrahedra and nanocubes was confirmed with EDS line scans measured with a high-angle

outcome. For example, as demonstrated by Dimitrov and coworkers, Pt−Cu nanocubes could be formed by simultaneously reducing Pt(acac)2 and Cu(acac)2 with 1,2-tetradecanediol (TDD) and ODE in the presence of tetraoctylammonium bromide (TOAB), OLA, and trace amounts of 1-dodecanethiol (DDT).81 The composition of the Pt−Cu nanocubes could be precisely tuned by varying the ratio of metal precursors, the amount of stabilizing/coordination agents, and reaction temperature. Interestingly, they found that the DDT could synchronize the reduction of Pt2+ and Cu2+ ions during a synthesis. Therefore, by maintaining the precursor concentrations while varying the amount of DDT involved from 40 and 60 μmol, the composition could be tuned from Pt54Cu46 to Pt80Cu20, respectively, while preserving the cubic shape. In another example, Skrabalak and co-workers demonstrated the importance of choosing the correct coordination ligand for controlling the spatial distribution of elements in bimetallic nanodendrites.82 They carried out a systematic study that exploited three metal−ligand precursor systems: (i) Pd(acac)2 and Pt(acac)2, (ii) Pd(acac)2 and Pt(hfac)2, and (iii) Pd(acac)2 and H2PtCl6, where all precursors were maintained at a 1:1 ratio and where OLA served as both the solvent and the reducing agent. In the first and second systems, they found that heating the solution to 160 °C led to the formation of Pd@Pt core−shell nanodendrites. The authors attributed the spatial separation of the Pd and Pt to the greater stability of the Pt complexes as compared to the Pd complexes, which permitted the Pd complexes to become reduced first and serve as nucleation centers for the later-reducing Pt complexes. To overcome the large difference in stability among the two precursors, and thus achieve a well-mixed alloy, they instead used a Pt precursor that is quickly reduced the moment a Pd surface is present. As they demonstrated, these requirements were satisfied with a precursor based on H2PtCl6. Upon coreduction of Pd(acac)2 and H2PtCl6 in OLA at 160 °C, Pd− Pt alloy nanodendrites were formed. As a proof of mechanism, the H2PtCl6 precursor was heated under identical conditions but in the absence of Pd(acac)2, where no reduction was observed. However, when Pd-seeds were injected into this solution, nanostructures with a bimetallic composition were obtained. Therefore, the deposition of Pt required the formation of a Pd surface to catalyze the reduction H2PtCl6. In summary, the work by Skrabalak and co-workers highlights the importance of the local ligand environment together with the catalytic nature of the nanocrystals in controlling the spatial distribution of metal atoms in bimetallic nanocrystals. Upon until now, the role of ligands was discussed in terms of their effect on the reduction potential of the metal precursor ions. However, as it turns out, there is still more to the story. As demonstrated by Millstone and co-workers, the addition of certain ligands can have a dramatic impact on the chemical nature of the metal precursor solution. Using a series of characterization techniques including nuclear magnetic resonance spectroscopy (NMR), mass spectrometry (MS), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) techniques, Millstone and co-workers showed that the addition of thiol-based ligands such as poly(ethylene glycol) methyl ether thiol (PEGSH) to an aqueous solution containing two metal precursors (e.g., HAuCl4 and Cu(NO3)2) led to the formation of bimetallic prenucleation species with sizes around 2 nm.83 They ultimately concluded that the prenucleation species were discrete from the molecular precursors, but perhaps most 10422


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Figure 4. TEM images of Pt−Pd (a) tetrahedra and (b) nanocubes that were prepared via coreduction. HAADF-STEM images (scale bar: 20 nm) and EDS line scan profiles recorded from an individual (c) tetrahedron and (d) nanocube. Reprinted with permission from ref 99. Copyright 2011 American Chemical Society.

annular dark-field scanning transmission electron microscope (HAADF-STEM), which showed overlapping Pd- and Ptsignals (Figure 4c and d). Along with Pd−Pt alloys, the synthesis of Pt−M (M = V, Ti, Co, Ni, and Fe) nanocrystals with controlled composition and facets has gained significant attention due to their remarkable activity toward the ORR (see section 5.1.2). In one example, Zou and co-workers demonstrated the synthesis of Pt3Ni nanocubes and octahedra.96 To form octahedra, Pt(acac)2, Ni(acac)2, OLA, and OA were mixed under an argon stream and then heated to 130 °C. With the introduction of W(CO)6 at 230 °C and under vigorous agitation, Pt3Ni octahedra were obtained with a proportion of ∼80% in the product. In this case, they hypothesized that OLA acted as both a reducing agent and a stabilizer for the Pt3Ni{111}. To generate nanocubes, a solution consisting of Pt(acac)2, OLA, and OA was heated to 130 °C, and then W(CO)6 was added. Finally, a Ni-precursor solution, consisting of NiCl2·6H2O, OLA, and OA, was added dropwise within 15 min while the temperature was increased from 130 to 200 °C, achieving a ∼70% yield of Pt3Ni nanocubes. The authors attributed the formation of {100}-facets to a slower injection rate for the NiCl2 solution and also to the fact that the surface of the nanocrystals was rich in Pt. The careful choice of capping agents and relative precursor ratios is also being used to fine-tune the atomic ratio of Pt and Ni in Pt−Ni alloys. To this end, Li and co-workers synthesized PtxNi1−x (0 < x < 1) nanocrystals with shapes ranging from octahedral, truncated octahedral, and cubic by regulating the presence of benzoic acid, aniline, carbon monoxide, and KBr (Figure 5).100 They found that Pt−Ni octahedra could be formed by reducing Pt(acac)2 and Ni(acac)2 in the presence of PVP and benzoic acid in benzylalcohol (Figure 5a and b). By simply replacing the benzoic acid with aniline, the formation of truncated octahedra became favorable (Figure 5c and d).

Figure 5. (a,c,e) TEM and (b,d,f) HAADF-STEM images and the corresponding elemental mapping of Pt (yellow) and Ni (red) for (a,b) PtNi2 octahedra, (c,d) PtNi2 truncated octahedra, and (e,f) PtNi2 nanocubes. Reprinted with permission from ref 100. Copyright 2012 American Chemical Society.

Furthermore, carrying out the reaction in the presence of KBr and CO (both known to selectively adsorb onto Pt{100}) was found to be essential in generating well-defined nanocubes in high yields (Figure 5e and f). The authors found that the composition of the Pt−Ni nanocrystals could be controlled by tuning the ratio of the Pt- and Ni-precursors. In all, a number of similar approaches have been developed for the synthesis of bimetallic nanocrystals with controlled shapes, where careful choice of reaction temperature, relative precursor concentration, capping agent, coordination agent, solvent, and timing all play a critical role in defining the composition, size, and shape of the final product. 3.2. Thermal Decomposition

Another powerful route to the synthesis of bimetallic nanocrystals is based on concurrent thermal decomposition, otherwise referred to as thermolysis. Thermal decomposition has proven effective for the formation of a variety of both mono- and bimetallic nanocrystals, as well as semiconducting and metal−oxide nanocrystals. Traditionally, thermal decomposition has been the favored method when working with metal precursors that have relatively low reduction potentials and cannot be easily reduced chemically, especially metals that are toward the bottom of Table 1 (e.g., Fe, Co, and Ni).101−103 10423


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Figure 6. (a) Schematic illustration showing the formation of FePt nanocrystals via simultaneous thermal decomposition of Fe(CO)5 and chemical reduction of Pt(acac)2. (b) TEM image of a representative sample of the FePt spherical nanocrystals. (c) Correlation between the amount of Fe(CO)5 used and the x in FexPt(100−x) based on the use of 0.5 mmol of Pt(acac)2. (d) TEM image of FePt nanocubes obtained from a synthesis that involved the introduction of oleic acid in the initial stage of the reaction. The schematics in (a) and (c) were reprinted with permission from ref 110. Copyright 2006 Wiley-VCH. The image in (b) was reprinted with permission from ref 111. Copyright American Chemical Society 2004. The image in (d) was reprinted with permission from ref 112. Copyright 2006 American Chemical Society.

slightly below the decomposition temperature of the less stable metal precursor leads to the formation of monometallic nanocrystals, with the second metal precursor still intact in the solution. The newly formed nanocrystals can catalyze the decomposition of the second metal precursor and serve as active sites (or seeds) for heterogeneous nucleation. The second metal can be deposited by simply letting the reaction run for a prolonged period of time or increasing the temperature once the first metal precursor is completely decomposed. This was demonstrated by Hyeon and coworkers, who synthesized Pd@Ni core−shell nanocrystals through the sequential decomposition of Pt and Ni precursors.107 In the first step, Pd(acac)2 and Ni(acac)2 were added into trioctylphosphine (TOP) to form Pd−TOP and Ni−TOP complexes. The precursor solution was then injected into a heated solution of OLA, in which the Ni−TOP complex decomposed at a lower temperature as compared to Pd−TOP. The mixture of Pd−TOP and Ni−TOP was maintained at 205 °C for 30 min to completely decompose Ni−TOP. Finally, the temperature was slowly increased to 235 °C, at which the Pd− TOP complex was decomposed, forming Pd shells on the already formed Ni nanocrystals. One should keep in mind that the use of a high reaction temperature may result in interdiffusion and thus formation of alloys or intermetallic compounds. 3.2.2. Decomposition in Conjunction with Reduction. Bimetallic nanocrystals can also be prepared by combining thermal decomposition with reduction. One successful example was demonstrated by Sun and co-workers, where FePt nanocrystals were formed via the simultaneous reduction of Pt(acac) 2 and decomposition of Fe(CO) 5 .108−110 The fundamental chemistry is shown in Figure 6a, where Fe(CO)5 is decomposed to Fe0 and CO at a high temperature while

Some commonly used organometallic precursors that readily decompose under moderate heating include the acetylacetonates (M(acac)n), carbonyls (Mx(CO)y), and cupferronates.104,105 Similar to coreduction, the ratio of the metal precursors, surfactants, solvent, capping agents, as well as temperature and time, are all key parameters in controlling the composition, size, and shape of the resultant nanocrystals. 3.2.1. Concurrent Decomposition of Two Precursors. Similar to coreduction, the simultaneous thermal decomposition of two metal-containing precursors results in the formation of zerovalent metal atoms that nucleate and grow together to form bimetallic nanocrystals. Also like coreduction, the difference in decomposition rates can be matched by controlling the relative precursor concentrations and/or by introducing additional chemical species (e.g., catalysts or coordination ligands). For example, Bönnemann and coworkers simultaneously decomposed Fe(CO)5 and Co2(CO)8 in tetrahydronaphthalene to generate Fe−Co nanocrystals.106 These two precursors have very different decomposition temperatures: 150 °C for Co2(CO)8 and 200 °C for Fe(CO)5. When the synthesis was carried out at 200 °C, Co nanocrystals were formed together with a small amount of Fe nanocrystals. To promote the formation of an Fe−Co alloy, the authors found that the decomposition rates of the two precursors should match. Therefore, to match the two decomposition rates, a catalyst based on aluminum trialkyl was introduced into the reaction mixture, causing both the Co2(CO)8 and Fe(CO)5 to decompose at 150 °C, thereby producing Fe−Co alloyed nanocrystals with a mean size around 10 nm. However, the large difference in thermal decomposition temperature is not always a negative feature since it enables the opportunity to generate core−shell nanocrystals using a onepot approach. Carrying out a reaction at temperatures at or 10424


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Pt(acac)2 is reduced to Pt0 and acac by the polyol. A combination of Fe and Pt results in FePt spherical nanocrystals with a uniform size (see Figure 6b). The authors found that the overall size and elemental homogeneity were greatly influenced by the nucleation rate, with slower rates generating nanocrystals of larger sizes and nonhomogeneous compositions while faster nucleation rates resulted in smaller, more homogeneous nanocrystals.110 As illustrated in Figure 6c, the composition could be readily controlled by manipulating the Pt(acac)2 to Fe(CO)5 ratio. The same group later demonstrated that the composition, size, and shape of the nanocrystals could all be controlled by varying the synthetic parameters, including the (i) molar ratio of stabilizer to metal precursor, (ii) addition sequence of the stabilizers and metal precursors, (iii) heating profile and temperature, and (iv) duration of synthesis.111 For example, they found that highly faceted FePt nanocubes were formed when the simultaneous decomposition of Fe(CO)5 and reduction of Pt(acac)2 occurred after the reaction temperature had been lowered from 300 to 205 °C and also by adding OA first into the reaction solution (Figure 6d).112 This general synthetic procedure is also amenable to other metal precursors. In fact, prior to the work of Sun and co-workers, Weller and coworkers showed that CoPt3 nanocrystals could be formed by simultaneously reducing and decomposing Pt(acac)2 and Co2(CO)8, respectively.113 3.2.3. Decomposition of a Bimetallic Precursor. It is not always possible to match the rates of reduction or thermal decomposition for the two metal precursors involved. In addition, the introduction of additional chemical compounds (e.g., complexing ligands, capping agents, and catalysts) can drastically complicate the reaction and result in unexpected or irreproducible product. These problems can be solved by using a single bimetallic precursor based on a heteronuclear cluster complex. Because stable metal−metal bonds are already present in the cluster, the composition of the resultant alloy should reflect the composition of the bimetallic precursor.29 In one example, Lukehart and co-workers prepared a bimetallic precursor, Pt3Fe3(CO)15, by reacting Fe(CO)5 with tris(norbornylene)-platinum(0).114 They then thermally decomposed Pt3Fe3(CO)15 in toluene in the presence of OA and OLA, resulting in FePt nanocrystals with an average diameter of 5.8 nm. Later, Thanh and co-workers reported on the synthesis of bimetallic magnetic nanocrystals through thermal decomposition of different types of molecular precursors, including bimetallic carbonyl cluster anions such as [FeCo3(CO)12]1−, [Fe3Pt3(CO)15]1−, [FeNi5(CO)13]2−, and [Fe4Pt(CO)16]2−.115 As shown in Figure 7a−d, they ultimately demonstrated that the composition of the resultant nanocrystals indeed reflected that of the corresponding precursor, that is, FeCo3, FePt, FeNi4, and Fe4Pt, respectively. Altogether, the thermal decomposition of bimetallic precursors drastically simplifies the reaction mechanism and provides a novel route for achieving precise control over the composition.

Figure 7. (a−d) Atomic models of bimetallic carbonyl complexes that have been explored as precursors for the synthesis of bimetallic nanocrystals (red = O, gray = C, yellow = Co, blue = Fe, purple = Pt, and green = Ni). Below each model is a TEM image of the corresponding Fe-based nanocrystals derived from the precursor. The atomic ratios of the bimetallic nanocrystals were nearly identical to that of the corresponding precursor. Reprinted with permission from ref 115. Copyright 2009 American Chemical Society.

synthesis of Au nanorods.116−118 Around the same time, Xia and co-workers demonstrated the synthesis of Ag nanowires from Pt seeds.119 Several years later, Yang and co-workers further expanded the seed-mediated method to other bimetallic systems, showing that faceted Pt nanocubes could serve as seeds for the overgrowth of Pd, thereby successfully generating Pt@Pd core−shell nanocrystals.120 From these seminal examples, a vast body of literature has since emerged with a focus on the use of seed-mediated growth as a means for deriving a broad range of bimetallic core−shell, dendritic, and heterostructured nanocrystals. In addition to the remarkable control over the composition and architecture of resultant bimetallic nanocrystals (e.g., variations in size, shape, composition, and structure), seed-mediated growth can also serve as an instrumental model system for the elucidation of the nucleation and growth mechanisms involved in nanocrystal synthesis. 3.3.1. The Hallmark of Seed-Mediated Growth. The synthesis of metal nanocrystals typically involves the reduction or decomposition of a metal-containing precursor into chargeneutral atoms, which then undergo one of the two types of phase transformations: (i) homogeneous nucleation, when the concentration of atoms reaches a high enough level known as supersaturation, the atoms will cluster together to form stable seeds, or (ii) heterogeneous nucleation, the newly formed atoms are added directly to the surface of preformed seeds. According to the classical nucleation theory, the required concentration of atoms to undergo heterogeneous nucleation (or driving force) is far less than that for homogeneous nucleation. Therefore, heterogeneous nucleation can be carried out in the absence of homogeneous nucleation, provided that the atom concentration is kept below supersaturation, but high enough to overcome the heterogeneous nucleation barrier. When both homogeneous and heterogeneous nucleation occur simultaneously, the products will be generally characterized by polydispersity in terms of shape and internal defect structure (e.g., single-crystal, and singly- or multiply twinned). While several examples have demonstrated that uniform sizes, shapes, and internal structures are possible, the extreme sensitivity of a homogeneous nucleation process to slight variations in temperature, impurity concentration, and stirring rate makes it nearly impossible to reproducibly generate the same number of seeds, with identical internal structures, from

3.3. Seed-Mediated Growth

Seed-mediated growth is the foremost synthetic route to the generation of well-defined bimetallic nanocrystals. In this process, preformed nanocrystals (or seeds) with well-defined characteristics serve as primary sites for the heterogeneous nucleation of newly formed atoms generated from the reduction or thermal decomposition of a metal precursor. This methodology was initially reported in 2001 by Murphy and co-workers, who used Au nanocrystals as seeds for the 10425


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Figure 8. (a) Schematic illustration of fcc and hcp stacking. HAADF-STEM images showing an individual (b) single-crystal cuboctahedron, (c) triangular nanoplate lined with stacking faults, (e) multiply twinned decahedron, and (f) multiply twinned icosahedron. (d) HRTEM image showing an individual singly twinned right bipyramid. All insets correspond to the respective atomic models with red lines highlighting the position of the planar defects. The images in (b,c,f) were reprinted with permission from ref 124. Copyright 2015 American Chemical Society. The image in (d) was reprinted from ref 162. Copyright 2013 American Chemical Society. The image in (e) was reprinted from ref 556. Copyright 2012 American Chemical Society.

different facets. For example, during seed-mediated growth, as long as the deposition is heteroepitaxial, single-crystal seeds can give rise to octahedra, cubes, cuboctahedra, rectangular or octagonal rods; singly twinned seeds can evolve into right bipyramids or nanobeams; multiply twinned seeds will generate decahedra or icosahedra; and last, seeds lined with stacking faults will produce either triangular or hexagonal nanoplates.123 At the atomic scale, the fundamental difference between singlecrystal structures and those that contain planar defects is the atomic packing. Single crystals have a uniform face-centered cubic ( fcc) lattice, while defect-containing seeds have a fcclattice with short segments of symmetry redefining hexagonal close-packed (hcp) lattice (Figure 8a). Only recently have syntheses begun to emerge that have control over the internal defect structure. It has been found that the initial reaction rate during the nucleation stage plays a crucial role in defining the internal structure of the seeds. By taking a quantitative approach, Xia and co-workers identified a clear trend where slow, moderate, and fast initial reduction rates produced Pd plates (lined with stacking faults), icosahedra (multiply twinned), and cuboctahedra (single crystal), respectively.124 While it is still difficult to experimentally uncover the atomistic details involved in the formation of seeds, theoretical calculations can be used to understand the stability of seeds with various internal defect structures over a range of sizes and temperatures. By taking Au as a starting point, Barnard and coworkers used theoretical calculations to generate a quantitative phase diagram that shows the size and temperature ranges over which the various seed structures are stable.125 At room temperature, they found that Au single crystals were favorable at sizes larger than 15 nm, decahedra at intermediate sizes between 3 and 15 nm, and icosahedra at sizes less than 3 nm. Because all metals have a distinctive character, the size (or crossover point), where favorability transitions from one type of internal structure to another (e.g., icosahedron → decahedron → single crystal), is unique for each metal.126,127 For example, icosahedra are predicted to be the most stable when containing

one batch to another and thus achieve credible batch-to-batch reproducibility. This is especially troublesome when researchers from various laboratories attempt to repeat the same protocol, a process that often ends in failure. This characteristic inconsistency and irreproducibility resulting from the homogeneous nucleation process makes seed-mediated growth the most viable method for the synthesis of nanocrystals with wellcontrolled size, shape, composition, and structure. Therefore, as its most important hallmark, seed-mediated growth can provide the means to circumvent self-nucleation by depositing atoms directly onto seeds already existing in the reaction solution. Another great advantage of seed-mediated growth is the ability to be quantitative. Once a seed solution is prepared, the size and shape of seeding particles, as well as their volume and surface area, can all be determined by electron microscopy (EM), while their concentration can be conveniently quantified using inductively coupled plasma mass spectrometry (ICP-MS). Therefore, calculations based on stoichiometry can be carried out such that the exact amount of precursor is added to the reaction vessel necessary for overgrowing the seeds to a desired end point, effectively avoiding the waste of valuable reagents while maintaining the atom-concentration below supersaturation to prevent self-nucleation. This strategy has been demonstrated for (i) size control, isotropically increasing the size of the nanocrystals while maintaining the initial shape,121 (ii) shape control, transforming a nanocrystal from one shape to another (e.g., nanocubes into octahedra or other shapes),122 and (iii) shell thickness control, reducing a predetermined amount of metal precursor to deposit a conformal shell with a precisely controlled thickness on the surface of a seed.46 3.3.2. Seeds with Diversified Internal Structures. Among the many discoveries associated with seed-mediated growth, perhaps the most pivotal was the realization that the internal defect structure of a seed governs the possible shapes taken by the resultant nanocrystal. The internal defect structure of a seed ultimately defines the surface energy landscape, along with the initial symmetry, and the relative proportions of 10426


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3{422} reflections in the electron diffraction pattern along the [111]-orientation, a phenomenon attributed to the presence of stacking faults.132 A stacking fault requires roughly twice the amount of energy to form as compared to the twin defect.133

less than 55 atoms for Pt, 147 atoms for Au, and 309 atoms for Pd, respectively.128 It should be emphasized that these theoretical calculations only provide information about the equilibrium structure under idealized conditions, and that nanocrystals commonly assume shapes and internal structures that differ substantially from these theoretical predictions due to kinetic effects. 3.3.2.1. Single-Crystal Seeds. The single-crystal seed is by far the simplest type, defined by a single-crystal domain with a continuous stacking order free from planar defects (Figure 8b). The symmetry of a single-crystal seed is defined by its lattice structure, and for the metals discussed herein, they crystallize in either an fcc structure (for Ni, Cu, Rh, Pd, Ag, Ir, Pt, Au), an hcp structure (for Co, Ru), or a body-centered cubic (bcc) structure (for Fe). However, the observed lattice structure may also depend on the synthetic conditions. Fan and Zhang recently highlighted a number of studies where the lattice structures of noble metals can be controlled synthetically.129 While the symmetry is defined by both the lattice structure and the presence of planar defects, the expressed shape (or faceting) is strongly influenced by the anisotropy in surface energy landscape of the seed, the experimental conditions, and any surface-interacting chemical additives (e.g., capping agents). The shape of a nanocrystal at thermodynamic equilibrium (and free from capping agents) is defined by the anisotropy in surface energy. The specific surface free energies for the lowindex facets of an fcc metal increase in the following order: γ{111} < γ{100} < γ{110}, with varying degrees of anisotropy between the metals (i.e., γAg{111}/γAg{100} > γPd{111}/ γPd{100}).130 However, all of the fcc metals should share a similar equilibrium shape, which is a truncated octahedron enclosed by eight {111}- and six {100}-facets. For a bcc metal, on the other hand, the specific surface energy increases in the order of γ{110} < γ{111} ≤ γ{100} when ρ ≤ 0.366, and γ{110} < γ{100} < γ{111} when ρ > 0.366 (here ρ is the ratio of the second neighbor bond energy to the nearest neighbor bond energy).131 In the case of an hcp structure, the specific surface energy increases as (0001) < (101̅0)A < (101̅0)B.130 A vast majority of the seed-mediated growth work has been focused on fcc metals due to their chemical stability, synthetic maturity, and relevancy in industrial applications. In the context of wet chemistry, single-crystal seeds of fcc metals can be synthesized (preferentially to those that contain internal planar defects) in two ways: (i) directly from self-nucleation post reduction or decomposition under the proper experimental conditions, typically when the formation of atoms is fast (e.g., strong reducing agent, fast injection, or relatively high temperatures), or (ii) through the use of oxidative etching to selectively remove seeds containing twin defects to leave behind single-crystal species only. Once synthesized, singlecrystal seeds can be used in seed-mediated syntheses to generate a number of bimetallic architectures including core− shell, dendritic, highly asymmetric, core-frame, and nanoframe nanocrystals. 3.3.2.2. Seeds Lined with Stacking Faults. The plate-like seed is unique from the other seed categories since it has a distinctive type of planar defect known as the stacking fault. This defect can be observed directly when viewing a nanoplate from the side, as shown in Figure 8c. Another way of identifying the presence of stacking faults, outside from structural clues (e.g., triangular or hexagonal plate-like shape), is by carrying out electron diffraction measurements, since Au and Ag nanoplates are known to give rise to forbidden 1/

2ΔEhcp − fcc ≅ 2γtwin ≅ γintrinsic ≅ γextrinsic

(4)

where ΔEhcp−fcc is the difference between the structural energies of hcp- and fcc-type lattice structures; γtwin is the twin energy; and γintrinsic and γextrinsic refer to the energies of intrinsically and extrinsically formed stacking faults, respectively. While similar in energy to the extrinsic stacking fault, an intrinsic stacking fault occurs when a (111) layer is missing from the fcc stacking order; for example, if “∥” represents the missing plane, the stacking order becomes AB∥ABC. On the other hand, the extrinsic stacking fault is defined by having an extra (111) layer added; if the added layer is |C|, the stacking order becomes ABCA|C|BC. Interestingly, unlike some of the other types of seeds, the stacking fault-lined seeds are purely a kinetically controlled product, meaning there exists no size range in which the plate-like seed is favored by thermodynamics.125 Despite its seemingly unfavorable formation, stacking fault-lined seeds are commonly observed for the group 11 metals, likely due to their relatively low stacking fault energies (Table 2). In general, robust nanoplate syntheses rely on (i) slow reduction rates124,132,134 and (ii) coupling the reaction to an oxidation process135 or an Ostwald ripening process.136 Several experimental strategies involved lowering the reduction rate through the use of a weak reducing agent such as citrate, PVP, AA (at low pH),137,138 glucose, or plant extract (e.g., lemon grass, aloe vera),139,140 or the introduction of a chemical agent that can coordinate and stabilize metal ions, such as PAM, bis(p-sulfonatophenyl)phenylphosphine dipotassium (BSPP), or citrate.141−143 Nanoplate formation has also been achieved using photo- and seed-mediated methods,142,144 thermal methods,144,145 as well as through nanocrystal ripening or oriented attachment.146 Interestingly, Yin and co-workers found that hydrogen peroxide (H2O2) could be used to transform various Ag sources (e.g., silver salts and metallic silver) into Ag nanoplates with the assistance of a capping agents containing di- and tricarboxylate or hydroxyl groups.147 In terms of finetuning of the structural features, Mirkin and co-workers found that a thermal route could be used to control the edge length and thickness of Ag nanoplates.148 Similarly, Xia and coworkers later reported that the edge length and thickness of Ag nanoplates could be controlled by choosing the appropriate capping agent (e.g., citrate vs PVP).149 Despite the large number of reports regarding the formation of metal nanoplates, an explicit mechanism of formation is yet to be universally accepted. A number of successful synthetic strategies regarding the formation of bimetallic nanoplates have also been reported in the literature. In one study, Peng and co-workers carried out a one-pot synthesis, where Ni(acac)2 and CuCl2 were reduced by OLA in the presence of TOP, a capping agent.150 They found that the magnetic properties of the as-synthesized Ni−Cu alloy nanoplates could be tuned by adjusting the Ni content, a feature that correlated well with reaction temperature. Similar strategies have also been used for the formation of other bimetallic nanoplates, such as Fe−Co and Ni−Co.151,152 Outside from alloys, bimetallic heterostructural nanoplates, core-frame, core−shell, and triangular nanoframe structures, have also been demonstrated by implementing monometallic 10427


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from the single-crystal seed in that it has noncrystallographic symmetry, where if the tetrahedral domains are placed together in the unstrained state, a gap totaling 7.35° would result. However, during the assembly process, the lattice is strained to prevent this gap. As previously pointed out, the decahedral seed is thermodynamically stable at intermediate sizes (e.g., 100− 500 atoms for Au, 100−6500 atoms for Pt and Pd, and 1000− 30 000 atoms for Cu).126 There have been many reports demonstrating the synthesis of monometallic decahedra in high yields, for example, Rh,164 Pd,165,166 Ag,167 and Au.168 The formation of decahedra alloys is also possible through coreduction, as reported for Pd−Pt.169 However, there have been more reports regarding the use of decahedra in seed-mediated syntheses. For example, a number of core−shell decahedra have been obtained from conformal overgrowth (e.g., Au@Ag,170 Au@Cu,171 and Pd@Pt172); nanostars and pentacles derived from twin-targeted deposition (e.g., Ag−Pt173 and Au−Cu174); pentagonal nanorods (e.g., Ag nanorods templated from Au,175−178 Ag,179 and Pd180 decahedral seeds), and high aspect ratio nanorods and nanowires (e.g., Ag−Au−Ag181,182 and Ag/Ni core/sheath183) enabled by one-dimensional (1-D) growth; and even Au/Ag icosahedra derived from Au decahedra.178 3.3.2.5. Icosahedral Seeds. The icosahedron is by far the most complex structure, comprised of 20 single-crystal, tetrahedral subunits joined together by twin planes, giving the structure both 3- and 5-fold symmetry (Figure 8f). The surface is typically enclosed by 20 {111}-facets, and like the decahedron, this crystal type is commonly referred to as a pseudocrystal because great lattice distortion is required to accommodate the large number of twin defects. Another intriguing feature of the icosahedron is that the surface gives rise to a diverse surface energy landscape that is defined by 30 edges, 20 faces, and 12 vertices. Despite the high complexity of this structure, the icosahedron is found by theory to have the most energetically favorable internal structure for seeds at very small sizes. This favorability is rationalized by realizing that the low surface energy of the 20 {111}-facets can more than compensate the increases to free energy associated with lattice strain and twin defects. Although favored at small sizes, kinetic factors can allow icosahedra to retain their structure even at much larger sizes.184 Because each metal is unique in terms of twin energy, surface energy, and bulk free energy, the size at which icosahedra become favorable tends to vary among the metals. Garnered from the limited number of monometallic cases, the formation of icosahedra is generally favorable when the reaction is carried out in (i) a kinetically controlled regime (e.g., slow reduction rate), (ii) an O2-free environment (as to prevent oxidative etching of the twin boundaries), or (iii) in the presence of capping agents that can stabilize the {111}-facets (e.g., CO, C2O42−, and citrate ions). In one example, Yan and co-workers carried out an aqueous phase coreduction by heating a mixture of K2PtCl4, Na2PdCl4, PVP, Na2C2O4, HCHO, and HCl to 180 °C for 2 h.185 They found that Pt−Pd icosahedra formation became favorable upon increasing the amount of Na2C2O4, which likely stabilized the {111}-facets and also slowed the rate of reduction due to the coordination effects of C2O42− with the Pt2+/Pd2+ ions. In a recent report, Yang and co-workers used the “gas-reducing-agent-in-liquidsolution” (GRAILS) method to synthesize a range of bimetallic icosahedra, including Pt−M (M = Au, Ni, and Pd).128 Outside from alloys, the formation of bimetallic core−shell icosahedra is

plate-like seeds in seed-mediated growth or galvanic replacement. For example, a number of core−shell nanoplates can be readily derived from the seed-mediated route, including Au@ Ag,153,154 Ag@Au,155,156 Pd@Pt,157 Au@Pd,158 Pd@Ag,159 and many other combinations.19 3.3.2.3. Singly Twinned Seeds. For fcc metals, the singly twinned seed is characterized by two single-crystal domains conjoined at a single twin plane. This type of planar defect is often referred to as a mirror plane, because the stacking order becomes ABC|A|CBA, where |A| acts as a mirror-type plane separating two single-crystal domains with opposite crystal directions along the ⟨111⟩ direction. Metals that are prone to expressing twin defects are those with relatively low twinboundary energies, which are common to the group 11 metals: Ag, Au, and Cu (9, 16, and 20 mJ/m2, respectively).160 However, singly twinned seeds have also been observed for Pd and Pt, despite their high twin-boundary energies that are roughly 10 and 15 times greater than the group 11 metals (86 and 161 mJ/m2, respectively).160 Similar to single crystals, the singly twinned seeds tend to achieve the lowest possible total surface energy through the expression of both {100}- and {111}-facets, unlike other internally defected nanocrystals (stacking-faulted, decahedra, and icosahedra) that tend to favor the formation of only {111}-facets, a concept discussed in the subsequent sections. Some common singly twinned seeds include the right bipyramid (consisting of two single-crystal pyramids conjoined at a single twin defect enclosed by a mix of {100}- or {110}-facets (see Figure 8d) and bitetrahedra (enclosed by {111}-facets only). The synthesis of singly twinned seeds at high yields seems to be tied closely to the reaction kinetics and thus reaction temperature, precursor concentrations, and presence of complexing ligands such as halides. Most of our knowledge about twin formation in colloidal nanocrystals is based on results from the synthesis of monometallic nanocrystals. The common trend among syntheses having high yields of singly twinned structures is the presence of Br− or I−. For example, in the context of monometallic nanocrystals, Xia and co-workers found that by simply replacing NaCl with NaBr in a high yielding polyol synthesis of Ag nanocubes, right bipyramids were obtained at a purity yield of 80% instead.161 They also demonstrated that Pd right bipyramids, with yields greater than 90%, could be obtained by reducing Na2PdCl4 with EG at 160 °C in the presence of PVP and NaI.162 In a more recent investigation, Wang and co-workers showed that Au right bipyramids could be obtained in purity >90% through a room-temperature synthesis involving the reduction of HAuBr4 with AA in the presence of CTAC.163 Although the mechanism for defect formation is largely unknown, it seems that the presence of Br− and I− is crucial. However, as previously discussed, it is difficult to agree upon a universal mechanism because these species are known to serve as both capping agents (which preferentially bind to specific facets) and complexing ligands (which modulate the reduction rate). 3.3.2.4. Decahedral Seeds. The decahedral seed is composed of five single-crystal, tetrahedral domains joined together at five twin boundaries by sharing one of the edges along a 5-fold axis (Figure 8e). Other names for the decahedral seed include the 5-fold-twinned or the penta-twinned seed. While a single-crystal seed can only have eight {111}-facets, the presence of the symmetry redefining twin boundaries permits the decahedron to have ten. The decahedron is also unique 10428


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also readily achievable by carrying out a seed-mediated growth, where icosahedral seeds serve as templates for a heteroepitaxial formed shell that also conforms to the icosahedral structure (e.g., Pd@Pt46 and Au@Ag186). In another report, Mirkin and co-workers showed that it was possible to form Ag@Au core− shell icosahedra by starting with single-crystal Ag seeds through a plasmon-assisted, seed-mediated synthesis (see section 3.3.5).187 In summary, from the aforementioned examples, it is clear that the formation of bimetallic icosahedra is not limited by a particular synthetic strategy as they can be prepared through either coreduction or seed-mediated growth. The highly intricate and complex nature of bimetallic icosahedra has captured the interest of many theorists who want to know how such structures form and equilibrate. To this end, Ferrando and co-workers used atomistic density functional theory (DFT) calculations to compute the preferred ordering of Ag−M (M = Cu, Ni, and Co) icosahedra for sizes up to about 1400 atoms.188 They found that these structures preferentially assumed a core−shell structure, with Ag dominant in the shell. Interestingly, they found that a central vacancy is favorable in pure Ag clusters but not in binary clusters with cores of smaller sizes. This is similar to the earlier work where molecular dynamics (MD) simulations were carried out to study the growth and formation of Ag−M (M = Pd, Cu, and Ni) nanocrystals.189 In that case, it was found that depending on both the temperature and the morphology of the initial core, the M atoms would either segregate into a layer just beneath the surface, forming a (Ag−M−Ag structure), or diffuse fast toward the cluster center to form a M@Ag core−shell structure. However, while these results are very interesting, it still remains a challenge to systematically investigate these predictions experimentally. 3.3.3. Growth Modes on Nanoscale Substrates. In the context of seed-mediated growth, the deposition and assembly of atoms on the surface of a seed is governed by both kinetics and thermodynamics. Since the deposition of atoms is generally favored at surface sites with high surface energies (e.g., edges, corners, vertices, and uncapped regions), the seed tends to grow continuously away from the equilibrium shape. Therefore, the degree of rearrangement toward the thermodynamically prescribed configuration depends strongly on the rate of atom deposition relative to the surface diffusion rate for the adatoms (see the next section for a detailed discussion). These two kinetic parameters depend strongly on experimental conditions. In general, there are three recognized growth modes (Figure 9): (i) Frank−van der Merwe (FM), adatoms grow in a layerby-layer manner; (ii) Volmer−Weber (VW), adatoms form 3D-islands on the surface of the seed; and (iii) Stranski− Krastanov (SK), adatoms first assemble according to the Frank−van der Merwe mode but later switch to the Volmer− Weber mode.190 These modes were first introduced in 1986 by Bauer and van der Merwe for thin film deposition on macroscopic substrates.191 In 2008, Tian and co-workers connected this same concept to growth modes observed during the seedmediated overgrowth of Au octahedra with Ag, Pd, and Pt.192 They found that a FM growth mode was observed for Ag and Pd, but Pt resulted in the VW growth mode. Typically, the VW growth mode is seen when the two interfacing materials have a relatively large difference in lattice parameters (e.g., lattice constant and crystal structure). However, in this specific case, Pd has a larger lattice-mismatch than Pt (4.7% vs 3.9%, respectively). To rationalize this result, Tian and co-workers

Figure 9. Schematic illustration of the three possible growth modes involved in the deposition of metal atoms onto the surface of a seed. In the literature of surface science, these modes are commonly known as Frank−van der Merwe or layer-by-layer growth (top), Stranski− Krastanov or layer-by-layer and island growth (middle), and Volmer− Weber or island growth (bottom), respectively.

considered the relative bond energies of the metals in their system (Table 2). Because the Pt−Pt bond energy is relatively high, the ability for Pt adatoms to diffuse across the surface becomes drastically hindered, preventing the Pt-layer from reaching the equilibrium FM growth mode. As discussed in section 3.3.4, these two products can be derived through a careful choice of the experimental conditions. Altogether, Tian and co-workers proposed three criteria for achieving conformal overgrowth: (i) a lattice mismatch less than 5%, (ii) the electronegativity of the shell metal should be less than that of the seed metal, and (iii) the bond energies between the core and shell atoms should be larger than those between the shell atoms.192 Experimental observations, for the most part, have obeyed these rules with some rather surprising exceptions. For example, smooth and heteroepitaxial growth modes have been reported for Au@Pt, Pd@Cu, Au@Cu, and Au@Ni, although these systems have lattice mismatches of 3.9%, 7.1%, 11.4%, and 13.6%, respectively.171,173,193,194 Another notable study includes the conformal growth of Rh on Au nanorods despite their large lattice mismatch (6.9%), large difference in bulk cohesive energy (368 and 554 kJ/mol for Au and Rh, respectively), and large difference in surface energy (1.6 vs 2.8 J/m2).195 It has even been observed that, in some cases, an hcp-metal (e.g., Ru) will conform to the lattice parameters of an fcc-core (e.g., Pd), as demonstrated by Xia and co-workers in the formation of fcc-Pd@Ru core-frame structures.196 In a recent report, Millstone and co-workers found that Pt deposited to the surface of Au nanoplates grew according to an island-type mode and where the islands were arranged on the surface in a linear fashion.197 It was hypothesized that the linear arrangement of Pt-islands may have resulted from a supramolecular architecture formed by the adsorbate (e.g., CTAB), which could have acted as a template for the observed growth pattern (as discussed in more detail in section 3.3.5). 3.3.4. Thermodynamic versus Kinetic Control. As explained by Xia and co-workers in a recent featured article, a thermodynamically controlled nanocrystal is one that has global minimum in Gibbs free energy, where the sum of the surface and volume free energies, internal defects, and strain energies are collectively minimized (Figure 10a).37 As previously 10429


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corners of Pd nanocubes were selectively overgrown, resulting in the formation of octapods. When the temperature was increased to 75 °C and thus Vdep < Vdiff, the surface diffusion was promoted and cuboctahedra were formed. Intermediate shapes were also observed by tuning the reaction temperature between these two extremes. In the case of Pt deposition on Pd seeds, three much higher temperatures (160, 180, and 200 °C) had to be used to see a comparable trend. When relatively lower temperatures were used (160 °C), discrete Pt crystallites formed on the surface of the Pd seeds. However, when the deposition was carried out at 180 and 200 °C, Pd@Pt core− shell concave cubes and Pd@Pt core−shell nanocubes formed, respectively. A similar explanation has been extended monometallic platforms,199 and also to other bimetallic systems, such as Pd−Rh.200 In conclusion, the final morphology can be tuned simply by controlling the ratio of the deposition and surface diffusion rates. It is helpful to consider the physical and chemical properties of the various metals as listed in Table 2, as these parameters can be used to assist in selecting the experimental conditions for specific bimetallic combinations. 3.3.5. Bimetallic Nanocrystals Enabled by SeedMediated Growth. The seed-mediated growth of bimetallic nanocrystals is continuously growing in popularity due to the remarkable control offered by this technique. This route is also much more reliable than the traditional one-pot approach, because the highly sensitive and enigmatic self-nucleation process can be avoided. These two aspects have elevated seedmediated growth to a status of becoming the most commonly used method for generating bimetallic nanocrystals with controlled compositions, sizes, shapes, and structures. As demonstrated throughout the upcoming sections, seedmediated growth provides access to a wide range of structures including core−shells, alloys, highly branched or dendritic structures, core-frames, nanoframes, as well as asymmetric nanocrystals. The core−shell architecture, in particular, has found a large number of applications related to targeted drug delivery, catalysis, or, more generally, enhancing the stability of nanocrystals. 3.3.5.1. Core−Shell Nanocrystals. Among the earliest examples was that from Yang and co-workers, who demonstrated the formation of Pt@Pd core−shell nanocrystals of various shapes (e.g., cubes, cuboctahedra, and octahedra) all derived from the overgrowth of cubic Pt seeds, as shown in Figure 11a−f.120 Because NO2 can dissociate on Pd surfaces and leave behind adsorbed NO and atomic oxygen, they posited that the interaction of NO2 with the Pd{111} may lead to partial passivation. To support this assertion, they found that by adding NO2 to the reaction mixture, the growth rates along the ⟨100⟩ and ⟨111⟩ directions could be altered. They also suggested that the selective passivation of the {111}-facets by adsorbed oxygen may possibly alter the interaction between the surfactant tetradecyltrimethylammonium bromide (TTAB) and the Br− counterion.120 Realizing that shape-control provides the unique opportunity to access the facet-activity dependency, the catalytic activity of each structure was tested against the electrochemical oxidation of formic acid. They found that the Pt@Pd cubes showed a peak current 5 times greater than the Pt@Pd octahedra, and an intermediate activity for Pt@Pd cuboctahedra. This early example, among others, set the stage for a rich literature pertaining to shape-controlled heteroepitaxial overgrowth, as well as the structure−activity assessment of bimetallic nanocrystals.201−206

Figure 10. (a) Schematic illustration of two different reaction pathways under kinetic and thermodynamic controls, respectively. The kinetic product represents a structure that has a relatively high free energy, in contrast to a thermodynamic product that has the lowest possible free energy. (b) Schematic illustration showing the shape evolution of a cubic seed with side faces covered by a capping agent under four distinct kinetic conditions. The 2-D atomic models correspond to the cross-section of the 3-D model indicated by a dashed green box. Reprinted with permission from ref 37. Copyright 2015 American Chemical Society.

discussed, a thermodynamic product will be favored when the temperatures are relatively high and the deposition rate is relatively low. In contrast, a kinetically controlled product is one that deviates from the shape prescribed by the thermodynamics. In terms of experimental parameters, the ratio between the rate of deposition (Vdep) and the rate of surface diffusion (Vdiff) ultimately determines which pathway a growing nanocrystal will follow (Figure 10b). When Vdep > Vdiff, the deposited adatoms tend to “hit-and-stick” to the site of deposition, facilitating the site-selective, kinetically controlled overgrowth. On the contrary, if Vdep < Vdiff, the adatoms “hitthen-diffuse” across the surface of a seed, resulting in a conformal growth mode favored by thermodynamics. Xia and co-workers experimentally validated this aforementioned assertion through the seed-mediated growth of Pd or Pt on Pd nanocubes, where the rates of deposition and diffusion were controlled by the injection rate and temperature, respectively.198 In the case of Pd on Pd seeds, the metal precursor was controllably injected using a syringe pump into a growth solution containing Pd nanocubes and AA at a series of temperatures (0, 22, 50, and 75 °C). They found that when the reaction was carried out at 0 °C to achieve Vdep > Vdiff, the 10430


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metal (e.g., Pd) that is more stable at higher temperatures. To demonstrate this, Zheng and co-workers synthesized Pd@Ag nanoplates through a seed-mediated approach, where Pd nanoplates were coated with Ag shells of various thicknesses.159 They found that the LSPR position could be readily tuned across the visible region simply by adjusting the Ag thickness (e.g., with LSPR ranging from 477−971 nm). To make the Pd@Ag nanocrystals comparable to the Ag nanocrystals, the Ag-shell-thickness was adjusted such that the LSPR of the Pd@ Ag was 812 nm (∼59 nm from the LSPR of the Ag nanoplates but closer in resonance to the 808 nm laser). Then, upon irradiating the Pd@Ag nanoplates under the same conditions that would transform the Ag nanoplates, no change was observed in the LSPR peak position or intensity, and no obvious changes in morphology were seen after TEM characterization. Taken together, by combining the enhanced thermal properties of Pd, with the remarkable optical properties of Ag, a superior hybrid structure can be formed that expresses both properties. Although the seed-mediated approach is generally regarded as a two-step process, many groups have reported the synthesis of equivalent structures using a one-step procedure. This method relies on a two-phase synthesis involving (i) a selfnucleation process in which the precursor containing the more noble metal (faster reducing) becomes reduced first to form nanocrystals that then (ii) serve as seeds for heterogeneous nucleation and overgrowth of a less noble metal (slower reducing). In one example, Teranishi and co-workers carried out a systematic study detailing the nucleation and growth of Au@Ag heterogeneous nanorods with highly tunable plasmon resonances.175 By coreducing Au and Ag precursors in EG with poly(diallyldimethylammonium) chloride (PDDA), it was found that Au decahedral seeds formed first, followed by the anisotropic heterogeneous nucleation of Ag. A similar one-pot approach was demonstrated for Au@Co and Au@Ni core− shell nanocrystals. Wang and Li demonstrated this concept by mixing HAuCl4 with either Co(NO3)2 or Ni(NO3)2 in octadecylamine (ODA) at 120 °C. The end result was Au@ Co core−shell nanocrystals or Au@Ni spindly nanocrystals.209 In this case, the authors state that the reducing power of ODA was only strong enough to reduce HAuCl4 but not the Co or Ni precursor. However, once Au nanocrystals were formed, they believe that the Au-adsorbed-ODA could facilitate electron transfer from ODA to the Co or Ni precursor, leading to the deposition of Co or Ni shells. 3.3.5.2. Core−Shell Nanocrystals with Ultrathin Shells. Among the many advantages of seed-mediated growth is the ability to use stoichiometry to deposit, very precisely in terms of thickness, ultrathin shells of another metal onto preformed seeds. Because the size, shape, and number of seeds can be characterized using EM and ICP-MS, respectively, the total surface area of the seeds in the system can be determined and then used to calculate the amount of precursor necessary for growing conformal shells to a desired thickness. In terms of experimental conditions, the deposition of an ultrathin shell onto the surface of a seed requires a faster surface diffusion rate for the adatoms relative to the deposition rate. This level of precision not only provides an ideal route for thickness control down to the monolayer level, but helps avoid wasting valuable reagents while maintaining the atom concentration well below supersaturation, an effective means for inhibiting selfnucleation.

Figure 11. HAADF-STEM images and the corresponding models of Pt@Pd core−shell nanocrystals: (a,b) cubes, (c,d) cuboctahedra, and (e,f) octahedra. In each model, the Pd core was oriented along an axis to clearly show the facets. Reprinted with permission from ref 120. Copyright 2007 Nature Publishing.

Outside from enhancing the catalytic activity, the shell-layer can be used to protect the core-metal from adverse environments. For example, Ag nanocrystals are renowned for their fantastic optical properties; however, their structure tends to be compromised by gaseous oxygen, high and low pH, or increased temperatures. In this case, it is often favorable to protect a less noble nanocrystal with a more noble shell. As an example, Yin and co-workers developed a seed-mediated route for the formation of chemically stable Ag@Au core−shell nanoplates (see section 3.4.5).155 To assess the chemical stability, the samples were reacted with H2O2, an oxidizer that would have otherwise completely dissolved pure Ag nanoplates. To prove that the Ag@Au core−shell nanoplates were chemically resistant to an oxidizer such as H2O2, the LSPR of the Ag@Au nanoplates was monitored over time after the addition of H2O2. The LSPR was maintained even after exposure to H2O2 for 48 days, proving that the Au shell could indeed enhance the chemical stability of Ag nanoplates. The core−shell architecture can also be used for enhancing thermal stability. For example, together with excellent plasmonic properties, the Ag-based nanocrystals are also renowned for their excellent photothermal properties.207 However, excessive heating can lead to drastic shape changes, and thus property changes. This is especially true for hollow or highly anisotropic structures.208 For example, irradiating a solution of Ag nanoplates (LSPR = 753 nm) with a 2 W, 808 nm laser for 30 min will transform the 2-D nanoplates into 3-D irregularly shaped nanocrystals due to the photothermal heating effect. However, the photostability of the Ag nanoplates can be drastically improved through the incorporation of a second 10431


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19 mL of the Na2PtCl6 precursor solution, respectively. This same strategy has also been extended to other systems to generate, for example, Pd@IrnL, Pd@RhnL, and Pd@Pt−NinL core-shell nanocrystals.212−214 3.3.5.3. Alloys Derived from Core−Shell Nanocrystals. Core−shell nanocrystals provide the ideal starting point for the generation of homogeneous alloys through controlled heating. Alloying is a thermally activated physicochemical process involving the interdiffusion and mixing of two metals that share a common interface. This process is favorable for two metals that satisfy the Hume−Rothery rules: (i) the atomic radii differ by less than 15%; (ii) the crystal lattices match; (iii) the valency is the same; and (iv) the electronegativities are similar.215 The alloying process is discussed in detail in section 3.4. One such model system can be found in the Au@Ag core− shell nanocrystals, not only because the Au/Ag pair satisfies the Hume−Rothery rules, but also because the LSPR peak position and intensity is highly sensitive to changes in the spatial distribution of the two elements (e.g., Au@Ag vs Ag@Au or Au−Ag).216 This concept was demonstrated by Tracy and coworkers, who used a seed-mediated synthesis to conformally deposit a controlled thickness of Ag onto the surface of 8 nm Au nanocrystals.217 Once the Au@Ag core−shell structures were formed, they were dispersed in OLA and then heated to 250 °C for 2 h, which led to interdiffusion and mixing of the Au and Ag. The transformation could be transduced optically by monitoring the LSPR peak position and intensity (see section 4.2 for details). To derive atomically homogeneous alloys, heating to much higher temperatures is often necessary. However, nanocrystals suspended in a solution phase cannot be heated to substantially high temperatures because (i) the solvent is susceptible to evaporation, and (ii) the nanocrystals may become unstable and aggregate. To avoid these issues, Yin and co-workers found that coating the Au@Ag core−shell nanocrystals with a 15 nm protective SiO2 shell allowed for the particles to be dispersed on a surface, dried, and then heated to 1000 °C in an inert atmosphere.218 The enhanced mobility of the metal atoms at such an elevated temperature could (i) drive the formation of a homogeneous distribution for both Au and Ag, (ii) eliminate crystallographic defects, and (iii) lead to a narrow LSPR peak.218 In addition to the improvement in optical properties, the transformation from a core−shell structure to an alloy can also be used to enhance magnetic properties, a subject discussed later in section 4.4.2. Furthermore, researchers in the catalysis community have also been exploiting this method to derive alloys for heterogeneous catalysis. A couple of notable examples include the transformation of Ru@Pt core−shell nanocrystals to a Ru−Pt alloy for CO oxidation,219 and Pt@Cu core−shell nanocrystals to a Pt−Cu alloy for the NO reduction.220 The role played by elemental composition in heterogeneous catalysis, in terms of atomic ordering and spatial distribution, is discussed in section 5.1. 3.3.5.4. Dendritic Nanocrystals. As discussed previously, a dendritic nanocrystal consists of a nanocrystal core surrounded by branches or arms that protrude from its surface. This type of nanocrystal is far from an equilibrium structure, and it is appealing for catalytic applications because the branches generally have high curvature and thus a high density of atomic steps together with high-index facets. The formation of nanocrystals with high surface energies, such as the dendritic ones, generally involves a kinetically controlled synthesis where

Several recent examples based on Pd and Pt have emerged, where Xia and co-workers implemented facet-controlled Pd nanocrystals as seeds for the conformal deposition of Pt shells, as schematically illustrated in Figure 12a. They demonstrated

Figure 12. (a) Schematic illustration showing the formation of a Pd@ PtnL nanocube. HAADF-STEM images of Pd@PtnL (n = 1−6) nanocrystals with different shapes: (b) cubic, (c) octahedral, (d) decahedral, and (e) icosahedral. The image in (b) was reprinted with permission from ref 211. Copyright 2014 American Chemical Society. The image in (c) was reprinted with permission from ref 210. Copyright 2015 American Chemical Society. The image in (d) was reprinted with permission from ref 172. Copyright 2015 American Chemical Society. The image in (e) was reprinted with permission from ref 46. Copyright 2015 Nature Publishing Group.

this for Pd@PtnL (n = 1−6) nanocubes, octahedra, decahedra, and icosahedra (Figure 12b−d).46,172,210,211 Because Pt is wellknown for its inability to diffuse on a surface (because of the strong Pt−Pt bond), an island growth mode will prevail if the reaction temperature is not sufficiently high. Therefore, the syntheses typically involve high temperatures (90−200 °C) with relatively slow injection and deposition rates. For example, in a relatively recent study, a solution containing 18 nm Pd nanocubic seeds, KBr as a capping agent, PVP as a colloidal stabilizer, EG as the solvent and reducing agent, and AA as an additional reducing agent was mixed in a flask and heated to 200 °C.211 Next, Pt deposition was initiated by pumping a specified volume of Na2PtCl6 solution into the reaction solution at a rate of 4.0 mL/h. The thickness of the Pt shells could be readily controlled by regulating the amount of Pt precursor introduced into the system. For example, Pd@Pt1L, Pd@Pt4L, and Pd@Pt6L nanocubes resulted from pumping in 3, 12, and 10432


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TEM images of Pd/Pt bimetallic nanocrystals that were obtained by depositing Pt on Pd cubic seeds at 160 °C by injecting Na2PtCl6 into EG that contained Pd seeds, AA, and Br−. Xia and co-workers demonstrated that the dendritic growth mode could be activated simply by maintaining a relativity low temperature to slow the surface diffusion of Pt adatoms. The dendritic-growth mode can also be activated when the rate of deposition is tuned, as demonstrated by Han and co-workers, who synthesized Pd/Pt dendrites by reducing K2PtCl4 with a relatively high concentration of AA (Figure 13d).221 In all, the two-step, seed-mediated approach for the formation of bimetallic nanodendrites is not exclusive to the Pd/Pt system, as this strategy has been demonstrated for the Cu/Pt, Ni/Pt, and Au/Pt systems.222−224 In the aforementioned studies, the spatial position of the nucleated Pt dendrites appeared to be somewhat random and uncontrollable. This lack of control is likely tied to the Pt nucleation process. For example, Xia and co-workers showed that both homogeneous and heterogeneous nucleation of Pt could occur at the very early stages of a synthesis and that the Pt branches likely grew through the oriented attachment of small Pt particles formed through homogeneous nucleation.225 Since this report, a number of HRTEM-based in situ studies readily confirmed the oriented attachment mechanism.226−228 In a more recent study, Millstone and co-workers highlighted the importance of the organic ligand adsorbates in the seedmediated overgrowth of Pt on Au.197 Remarkably, they discovered a unique, highly periodic island growth mode after purified Au nanoplates were overgrown with Pt by reducing aqueous H2PtCl6 by AA. An image of the structures captured by a scanning electron microscope (SEM) is shown in Figure 13e. They hypothesized that the linear arrangement of Pt islands might have resulted from a supramolecular architecture formed by CTAB on the Au seed surface. To test their hypothesis, CTAB was exchanged for 11-amino-1-undecanethiol (AUT) and, in another trial, poly(ethylene glycol) methyl ether thiol (PEGSH, Mn = 900 Da). They found that both AUT- and PEGSH-coated Au nanoplates exhibited Pt deposition patterns that were different from those observed for the CTABfunctionalized nanoplates.197 In summary, the dendritic, island growth mode depends on (i) the interplay between nucleation modes (e.g., homogeneous vs heterogeneous), and (ii) the nature of the seed surface (e.g., adsorbates, surface defects, and surface energy, among others). The seed-mediated growth of bimetallic nanocrystals, like the aforementioned examples, typically requires two processes: (i) the reduction of a metal precursor to form seeds, and then (ii) the reduction of a second metal precursor, whose atoms or nuclei deposit or attach onto the preformed seeds. Recently, some groups have even been able to condense these two steps into a one-pot process. This concept is analogous to the aforementioned one-pot approach for core−shell nanocrystals except that the overgrowth is island-like instead of conformal. In one example, Yamauchi and co-workers synthesized Pt-onPd nanodendrites at room temperature.229 They first prepared a mixture consisting of K2PtCl4, Na2PdCl4, and Pluronic P123 (a triblock copolymer) at a molar ratio of 10:1.4:1. AA then was quickly added under stirring for 30 min, leading first to the formation of Pd seeds and later to the deposition of Pt. The difference in reduction rate between Pd and Pt was attributed to the different complexes that Pd and Pt formed with AA. During the later stages, they stated that the formation of Pt dendrites was supported by the presence of P123. Similar to the

atoms are deposited quickly together with slow surface diffusion (Figure 13a). Xia and co-workers demonstrated a

Figure 13. (a) Schematic illustration showing the deposition of Pt atoms onto the surface of a Pd seed under kinetically controlled conditions, leading to the formation of Pt branches protruding from the surface of the seed. TEM images of Pd/Pt dendrites resulting from the overgrowth of Pt from Pd (b) cuboctahedral and (c) cubic seeds, respectively. (d) TEM image showing Pd nanocubes whose surfaces are covered by a dense array of Pt dendrites. (e) SEM image of Au nanoplates whose surfaces are covered by Pt islands arranged in a periodic array. The image in (b) was reprinted with permission from ref 98. Copyright 2009 AAAS. The image in (c) was reprinted with permission from ref 198. Copyright 2013 National Academy of Sciences, USA. The images in (d) were reprinted with permission from ref 221. Copyright 2012 American Chemical Society. The images in (e) were reprinted with permission from ref 197. Copyright 2014 American Chemical Society.

seed-mediated approach to the formation of Pd/Pt nanodendrites, with Pt branches being supported by a Pd core.98 The Pd-seeds, which were 9 nm Pd truncated octahedra, were first formed by reducing Na2PdCl4 with AA in water. By reducing K2PtCl4 with AA in the presence of the Pd seeds, Pt branches then sprouted from the surface of the seeds (Figure 13b). HRTEM images together with FT-patterns indicated that the Pt overgrowth was epitaxial, a result not too surprising because Pd and Pt have a lattice mismatch of only 0.77%. The individual Pt branches were covered by a number of facets including {111}-, {110}-, and high-index {311}-facets, along with a small number of {100}-facets. The dendritric growth mode is not limited by seed structure nor solvent, because it was later demonstrated that Pt-dendrites could also be seeded by Pd nanocubes using a polyol synthesis.198 Figure 13c shows 10433


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Figure 14. (a) Schematic illustration showing the selective deposition of Rh onto the edges and corners of a Pd nanocube whose side faces are covered by Br− ions. (b) SEM image showing the morphology and structure of the Pd@Rh core-frame nanocubes. (c) HAADF-STEM image of an individual Pd@Rh core-frame nanocube at room temperature (before heating test). The inset shows the corresponding elemental map (yellow, Pd and green, Rh). (d) HAADF-STEM image of the same Pd@Rh core-frame nanocube, after heating to 500 °C for 1 h. Note that the cubic shape was still retained. The inset shows rounded Pd nanocrystals when Pd nanocubes without the Rh frames were subjected to the same heating regime. The images were reprinted with permission from ref 237. Copyright 2013 The Royal Society of Chemistry.

removing the core provides a powerful means for deriving the nanoframe structure, as discussed next. 3.3.5.6. Nanoframes. The primary motivation for generating nanoframes is the enhanced catalytic properties resulting from the high specific surface areas and high densities of lowcoordination surface atoms characteristic of high curvature at the nanoscale.238 This structure could also give rise to very interesting mechanical properties.239 The earliest synthetic route used for generating nanoframe-like bimetallic nanocrystals relied on galvanic replacement, by which sacrificial templates (e.g., Ag nanocubes) were reacted with HAuCl4 for the formation of hollow Au−Ag alloyed nanoboxes. Continued dealloying of Ag from the walls of the nanoboxes resulted in the formation of nanocages and eventually nanoframes (see section 3.4). While galvanic replacement is a convenient method for the formation of hollow, porous, and frame-like structures, a major drawback is that the oxidative etching and atom deposition process are linked through stoichiometry (e.g., three Ag atoms are removed for every one Au atom added). The nanoframe dimensions are predefined by the volume of the sacrificial template, provided that the reaction is carried out to completion. To overcome this limitation, galvanic replacement has been replaced by a two-step process involving site-selective deposition (via seed-mediated growth), followed by the removal of the seed via selective etching (Figure 15a). This route enables the ability to regulate the amount of metal deposited at the edges and corners of seed for the formation of frames with controllable ridge thicknesses. Xia and co-workers demonstrated this concept through the fabrication of Rh nanoframes by templating against Pd nanocubes.200 The synthesis started with the formation of Pd−Rh core-frame structures, which involved the site-selective deposition of Rh atoms at the corners and edges of Pd cubic seeds. This type of spatial confinement was achieved through a combination of kinetically controlled deposition and the selective capping of the Pd{100}-facets by Br− ions. The Pd in the core could be selectively removed by etching the Pd−Rh structures with the

aforementioned Pd/Pt nanodendrites reported by Xia, no obvious grain boundaries were observed between the Pt branches and the Pd cores. This one-pot approach has also been successfully extended to the synthesis of Au/Pd,230 Au/ Pt,231−233 Pd/Rh,234 Pt/Ni,235 and Pt/Cu236 nanodendrites. 3.3.5.5. Core-Frame Nanocrystals. The aforementioned synthetic strategy for the formation of Pd@Pt core−shell nanocrystals relies on high temperatures and slow deposition rates for the smooth and conformal deposition of Pt on the entire surface of a Pd seed. However, it is not always desirable to have conformal overgrowth of the seed. In some cases, the preferred outcome is a site-selective growth where only the high-surface-energy sites (e.g., edges, corners, vertices, and defects) are targeted for deposition. This type of kinetically controlled growth is prevalent in systems where the deposition is fast while the surface diffusion of adatoms is slow, inhibiting the atoms from rearranging into more favorable configurations (Figure 14a). Core-frame nanocrystals have captured the interest of many researchers because this geometry provides a potential means to greatly enhance the thermal stability of the structure. For example, excessive heating of nanocrystals generally leads to the degradation of sharp features such as edges or corners and also the conversion of high-index facets to low index facets. However, the core-frame geometry can protect the structure against this unfavorable thermal restructuring. In one example, Xia and co-workers used Pd nanocubes as seeds for the kinetically controlled overgrowth of Rh (melting point = 1964 °C).237 The thermal stability of the as-prepared Pd−Rh core-frame nanocubes (Figure 14b and c) was then evaluated with in situ HAADF-STEM imaging. The Pd−Rh core-frame nanocube could maintain its cubic shape even after annealing at 500 °C for 1 h, in contrast to Pd nanocubes, whose corners were truncated after heating at 400 °C for only 8.5 min (Figure 14d). Overall, framing nanocrystals with a metal that has a relatively high melting temperature offers a promising strategy for preserving far-from-equilibrium shapes when situated in scenarios having high working temperatures. Last, further modification to the core-frame structure by selectively 10434


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Fe3+/Br− pair, a process that left behind Rh cubic frames (Figure 15b). Recently, Yang and co-workers carried out a similar synthesis albeit with the Pd−Rh core-frame nanocrystals being formed through a one-pot approach and where the Pd cores were selectively removed by HNO3.240 It should be noted that this process can also be extended to other shapes and bimetallic combinations. For example, Park and co-workers recently reported a general synthetic pathway to various types of Au−Pt nanoframes (Figure 15c).241 The synthetic route was comprised of three steps: (i) site-specific (edge and vertex) deposition of Pt, (ii) etching of Au in the core, and (iii) regrowth of Au on the Pt framework. As mentioned previously, the diversity of nanocrystals derived through seed-mediated syntheses can be expanded through the implementation of seeds with different shapes and/ or internal structures (e.g., singly and multiply twinned). This concept can be readily extended to the synthesis of nanoframes. Because kinetically controlled deposition is favored at surface sites with low coordination numbers, atoms tend to be selectively deposited onto planar defects, vertices, corners, and edges. Xue and co-workers demonstrated this concept for the synthesis of sub-2 nm Au triangular nanoframes.242 By starting with Ag nanoplates as seeds, Au was selectively deposited onto the edges by slowly introducing HAuCl4 and hydroxylamine (the reducing agent) simultaneously into a solution containing the Ag nanoplates. The Au thicknesses could be controlled by simply regulating the deposition time. In their case, 15 min corresponded to a Au ridge thickness of 2.5 nm. Once the core-frame structures were formed with the desired ridge thickness, the structures were etched with a Agspecific wet etchant based on H2O2 and NH4OH, leaving behind Au nanoframes with ridge thicknesses as thin as 2 nm (Figure 15d). In another example, Park and co-workers used a three-step seed-mediated process for the formation of circular, triangular, and hexagonal Au−Pt nanoframes.243 Outside from plate-like seeds, multiply twinned structures can also be used for even greater structure diversification. For example, Kitaev and co-workers implemented Ag decahedral seeds for the selective deposition of Au onto the edges and twin defects only. The Ag seeds were then selectively removed by exposing the structures to H2O2, leaving behind ultrathin pentagonal Au nanoframes (Figure 15e).244

Figure 15. (a) Schematic illustration of a synthetic route used for the formation of nanoframes, which involves the formation of core-frame structures, followed by the selective removal of the core. TEM and SEM images of (b) cubic and (c) octahedral nanoframes derived from single crystal cubic and octahedral seeds, respectively. TEM images showing (d) triangular and (e) decahedral nanoframes derived from seeds containing twin defects. The image in (b) was reprinted with permission from ref 200. Copyright 2012 Wiley-VCH. The image in (c) was reprinted with permission from ref 241. Copyright 2015 Wiley-VCH. The image in (d) was reprinted with permission from ref 242. Copyright 2013 Wiley-VCH. The image in (e) was reprinted with permission from ref 244. Copyright 2011 American Chemical Society.

Figure 16. Schematic illustration and the corresponding TEM images of the samples obtained at four representative stages during the evolution from solid polyhedra to nanoframes. This route first requires the formation of (a) solid PtNi3 polyhedra, followed by their transformation to (b) PtNi intermediates, and then to (c) Pt3Ni nanoframes. After being annealed under Ar at temperatures between 370 and 400 °C, the structure was finally transformed into (d) Pt3Ni nanoframes encapsulated with a Pt-skin. Reprinted with permission from ref 245. Copyright 2014 AAAS. 10435


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Figure 17. (a) Schematic illustration of three different types of Pd−Ag bimetallic nanocrystals obtained by simply modulating the injection rate of AgNO3. (b) Schematic illustration of symmetric growth, where atoms are deposited equally among the six faces of a nanocube and asymmetric growth, where atoms are unequally distributed among the six faces. TEM images and EDS mapping of (c) Pd−Ag core−shell nanocrystals obtained by adding AgNO3 in one shot, (d) nonconcentric Pd−Ag nanocrystals derived from a moderate injection rate of 30 mL h−1, and (e) Pd−Ag dimeric nanocrystals obtained at a slow injection rate of 1 mL h−1. Schematics (a) and (b) were modified with permission from ref 37. Copyright 2015 American Chemical Society. TEM images from (c−e) were reprinted with permission from ref 253. Copyright 2012 Wiley-VCH.

catalyze hydrogenation, carbonylation, hydroformylation, and oxidation reactions.248,249 In fact, roughly 80% of the worldwide annual production of Rh is used in the fabrication of three-way catalytic converters in automobiles.248,250 The Pt−Rh bimetallic alloys have also been identified as candidates for the reduction of NO with H2, the oxidation of CO, and the ORR.251 To more effectively utilize Rh, several groups have been working toward the development of Rh-based nanoframes. In addition to the protocol developed by Xia and co-workers,200 Lee and coworkers recently demonstrated a one-pot approach to the formation of Rh−Cu nanoframes.249 The nanoframes were synthesized by heating a solution of Rh(acac)3, Cu(acac)2, stearic acid (SA), and OLA to 250 °C under 1 atm CO. After being cooled, the samples were washed with toluene and ethanol and centrifuged twice. The product consisted of 25 nm Rh−Cu truncated octahedral nanoframes, which were found to have superior electrocatalytic performance toward the oxygen evolution reaction (OER). In another report, Zhang and coworkers extended Rh-based bimetallic nanoframes to those containing Cu, Ni, and Pd−Cu.252 They first synthesized Rh− Cu octahedra via an OLA-mediated solvothermal route. The structures were then loaded onto carbon black and then transformed into Rh−Cu octahedral frames by preferentially etching away the Cu with HCl. 3.3.5.7. Asymmetric Nanocrystals. The exploration of new synthetic methods for the diversification of nanocrystals has also led to new and interesting nanocrystal structures. In particular, one scientific endeavor is having the ability to controllably break symmetry during nanocrystal growth. For single-crystal nanostructures, the total number of possible shapes is fixed by the inherent crystal lattice. The unit cell of an fcc metal, for example, has Oh symmetry, and together with thermodynamics, the number of possible shapes is limited to cubes, cuboctahedra, and octahedra as defined by the relative

Outside from forming nanoframes using galvanic replacement or seed-mediated syntheses, there is another strategy that has only recently emerged. It involves the formation of solid bimetallic nanocrystals, followed by their physical transformation into Pt-based intermetallic nanoframes (e.g., Pt3Ni, Cu3Pt, and Pt3Co).245−247 This method was initially demonstrated by Stamenkovic and co-workers, who first synthesized single-crystal Ni3Pt rhombic dodecahedra of 20 nm in size.245 By simply storing the structures in hexane or chloroform for 2 weeks, the structures underwent both structural and compositional transformation into Pt3Ni nanoframes. Additional heating to 120 °C decreased the waiting time from 2 weeks to 12 h. The final nanoframes consisted of 24 edges (∼2 nm in ridge thickness), exhibiting a geometry consistent with the initial rhombic dodecahedral shape. They also found that subsequent heating in an inert atmosphere such as Ar gas, between 370−400 °C, caused most nanoframes to develop a smooth Pt skin. The transformation processes are illustrated in Figure 16, together with representative TEM images. Using a similar approach, Yang and co-workers demonstrated independently the synthesis of Cu3Pt nanoframes.246 In this case, uniform Cu@Pt core−shell rhombic dodecahedra were first formed via successive reduction of Cu(acac)2 and H2PtCl6 in OLA. Afterward, the Cu@Pt core−shell nanocrystals were left in toluene under ambient conditions for 3 weeks. As a consequence of the Kirkendall effect, the Cu@Pt nanocrystals underwent compositional and structural transformation to form Cu3Pt nanoframes. The large number of reports on Pt-based alloys stems from their expected application as ORR catalysts with enhanced activity. Meanwhile, some work has also been dedicated to nonPt nanoframes for applications related to other important catalytic reactions. A metal that has received some increased attention is Rh, which is known for its ability to effectively 10436


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Figure 18. Models and representative STEM images depicting the proposed growth pathway for a single-crystal Au seed overgrown with Ag in a plasmon-mediated synthesis. (a) Model and the STEM image of the pseudospherical single-crystal Au seeds. Models and STEM images of bimetallic particles with (b) octahedral, (c) truncated tetrahedral, (d) tetrahedral, (e) truncated bitetrahedral, (f) bitetrahedral, (g) truncated decahedral, (h) decahedral, (i) decahedral with an additional tetrahedral growth, and (j) icosahedral morphologies. All particles were observed as products from the same reaction. All scale bars are 25 nm. Reprinted with permission from ref 187. Copyright 2012 AAAS.

growth rates along the ⟨100⟩ and ⟨111⟩ directions. Therefore, one of the most effective means for escaping from this confinement is to implement seeds with diverse internal structures, including nanoplates, decahedra, or icosahedra. By simply changing the initial symmetry of the seed, a whole host of new shapes become possible that would not otherwise be possible with the Oh symmetry of single-crystal seeds. Aside from simply implementing seeds with unique internal structures, recent studies have focused on methods for controllably stimulating highly asymmetric growth modes, including: (i) disproportionate growth among equivalent facets or (ii) carrying out a deposition that favors the formation planar defects (e.g., stacking faults or twinning). If symmetry could be controllably broken during the growth process, the number of possible shapes and structures would greatly increase, and, correspondingly, the tunability of property. While controlled symmetry breaking is largely a new approach to nanocrystal diversification, several trends have already been revealed. In the context of seed-mediated growth, it is commonly observed that highly asymmetric growth modes will kick in when (i) the deposition rate is sufficiently low, (ii) the energy barrier to surface diffusion is sufficiently high, and/ or (iii) the deposition results in the formation of planar defects. With respect to the first case, the overgrowth of Pd cubic seeds with Ag can be directed to selectively occur on one, three, or six of the side faces of a Pd cubic seed simply by modulating the injection rate of AgNO3 (Figure 17a and b).253 Slow, moderate, and fast injection rates resulted in Pd−Ag hybrid bars (or dimers), nonconcentric Pd−Ag nanocubes, and Pd@Ag core− shell nanocubes, respectively (Figure 17c−e). This trend has also been observed for the growth of Ag or Au on Pd

octahedral seeds,254 Au on Pd cubic and octahedral seeds,255 Ag on Au seeds supported on a solid substrate,256 and Cu or Pd on Pd cubic seeds.195,257 The second method for achieving asymmetric growth involves a combination of metals that are prone to slow surface diffusion. In essence, slowing surface diffusion inhibits adatoms from transitioning to more energetically favorable sites, a requisite for achieving a kinetically controlled asymmetric product. One example involves the use of Pd nanocubes as seeds for the overgrowth of Cu.195 Because Cu and Pd have a lattice mismatch of 7.1%, the rate of surface diffusion is likely very low. It was found that at low deposition rates, the deposition location was initiated exclusively on one or two faces of the Pd cubic seed. As the growth continued past 40 min, it was found that the Cu eventually encapsulated the entire Pd seed, forming Pd@Cu core−shell nanocrystals. In summary, the asymmetry in growth during the early stages was likely induced by the low rate for the supply of atoms, coupled with the high energy barrier to Cu surface diffusion. In the aforementioned examples, the overgrowth process is asymmetric yet still heteroepitaxial. In many cases, especially when the lattice mismatch is large, the overgrowth can result in new internal crystal structures. This was demonstrated early on by Yang and co-workers, where Pt nanocubes were used as seeds for Au deposition.120 Instead of forming single-crystal Pt@Au core−shell nanocubes, the Au portion was poly crystalline in nature and penta-twinned Au nanorods resulted. Further investigation with selected area electron diffraction revealed that the Pt nanocubes were epitaxially oriented with one of the five [112]-zones of the Au rod. A similar observation was recently demonstrated by Yin and co-workers, who used single-crystal Au seeds for Cu overgrowth (with 11.4% lattice 10437


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Figure 19. (a) Schematic illustration of the key steps involved in the galvanic replacement between Ag nanocubes and HAuCl4. SEM images captured at different stages of the reaction process, where (b) shows early stage pit formation on the surface of Ag nanocubes, (c) followed by partial cavity formation, then (d) the formation of hollow Au−Ag nanoboxes. Continued galvanic replacement dealloys the Au−Ag walls resulting in the formation of nanocages. The insets in (c) and (d) show TEM images of individual structures that have been microtomed to reveal the hollow interior. The images (b−e) were reprinted with permission from ref 274. Copyright 2010 Elsevier.

mismatch).258 In this case, they showed theoretically and experimentally that Cu deposition induced both twinning and anisotropic growth. While lattice mismatch seems to be a key player in nonepitaxial seed-mediated growth events, there have also been some reports on nonepitaxial growth modes resulting from interfacing two metals with identical lattice parameters. This concept was demonstrated by Mirkin and co-workers, who showed that single-crystal cuboctahedral Au seeds could be transformed into 20-fold twinned Au−Ag icosahedra using a plasmon-driven seed-mediated growth (Figure 18a−j).187 They found that the overgrowth involved a set of complex asymmetric growth modes where, upon careful examination of the products, the incorporation of twin planes were responsible for the asymmetric growth. For example, the transition from octahedron to truncated tetrahedron was a result of twinning along an individual {111}-facet of the octahedron. The authors argued that because the newly formed twin defects contained reactive, self-propagating ledges, Ag should be selectively deposited there to transform the shape from octahedron to truncated tetrahedron. They also proposed that a tetrahedron could then develop a third twin plane, causing a change in shape to a bitetrahedron, and then eventually develop a fourth and fifth twin plane, resulting in the formation of a decahedron. Additional twinning regimes are expected for the transformation from decahedron to icosahedron. In another study, Xia and co-workers discovered that even the deposition of Au atoms onto the surface of Au seeds could result in the formation of nonepitaxial growth modes. In this case, when the single-crystalline spherical or rod-like Au seeds were pretreated with Na2S2O3, twin defects would develop during the overgrowth process to yield twinned nanocrystals.259 In contrast, in the absence of Na2S2O3, homoepitaxial growth was observed and single crystals resulted. It was proposed that the strong Au−S bond might play an important role in transforming the crystallinity during growth. In summary, it is clear that the growth mode depends both on the mutual lattice mismatch between the two metals involved and on the presence of surface capping species.

3.4. Galvanic Replacement

Galvanic replacement provides access to a wide variety of bimetallic hollow nanocrystals that are unattainable through simple coreduction, thermal decomposition, or seed-mediated growth.260 A galvanic replacement reaction is an electrochemical process involving the oxidation of one metal (often referred to as a sacrificial template) by the ions of another metal having a more positive reduction potential.261 If this condition is met, the metal ions will remove electrons from the template, become reduced, and then plate the exterior of the template, while its interior dissolves into the solution. The final structure is characterized by a hollow bimetallic nanostructure that takes on the shape of the initial template. 3.4.1. The Role of Reduction Potential and the Mechanism of Galvanic Replacement. Galvanic replacement is driven by the favorable difference in reduction potential between the two elements under consideration. The standard reduction potentials of all of the metals highlighted herein are presented in Table 1. The change in Gibbs free energy (ΔG) is given by ΔG = − nFE

(5)

where n is the number of moles of electrons transferred in the half reaction, F is Faraday’s constant, and E is the potential difference between the electrochemical half reactions in question. From this equation, it can be deduced that a positive E will result in a negative ΔG, and thus a spontaneous reaction. The potential difference, E, can be evaluated using the Nernst equation: E = E° +

RT ln Q nF

(6)

where E° is the standard potential difference at 25 °C, R is the ideal gas constant, T is the temperature, and Q is the reaction quotient derived from the concentration of the products and reactants at a given time. A classic example of galvanic replacement in action involves the oxidation and dissolution of Ag nanocubes and concurrent reduction of HAuCl4 and deposition of Au (Figure 19). The reaction can be summarized as follows: 10438


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structure similar to a core−shell nanocrystal. Each core−shell structure was then reacted with an aqueous solution of PtCl62+ at 90 °C, with each case resulting in a Pt nanoshell of varying degrees of purity. It was found that when the sacrificial template was made of either Cu, Ni, or Co, the resultant shell was primarily comprised of Pt (with little or no trace of the template metal). On the contrary, the templates made of Ag or Pb resulted in a Pt nanoshell rich in the template metal. These observations support the conclusion that shell-template interdiffusion is promoted when the deposited shell is placed in tension (e.g., Ag and Pb), whereas a template that places the deposited shell in compression (e.g., Cu, Ni, and Co) will inhibit the interdiffusion of atoms between the shell and the template.270 In the latter stages of a galvanic replacement reaction, the atoms that diffused from the template into the shell can also be selectively removed from the shell. This process is referred to as dealloying, and it can transform a M−N nanobox into a highly porous nanocage comprised primarily of metal N. As the atoms of the template are continually removed from the side walls, vacancies will coalesce into voids that continually enlarge until the structure is transformed into a nanocage. If this process is carried out to completion, the structure will fragment into smaller, irregularly shaped pieces.260 Interestingly, either outcome can be achieved (e.g., nanoboxes or fragments) for the same pair of metals, depending on the oxidation state of the precursor used to initiate the replacement reaction. For instance, if a large amount of Au3+ is used in the galvanic replacement reaction with Ag nanocubes, the final product consists of pure Au fragments. However, if Au+ is used instead, the product consists of Au nanoboxes with thicker walls.271 This phenomenon can be explained by the stoichiometric differences between the replacement reactions involving Au3+ and Au+ ions. In the former case, every three Ag atoms only reduces one Au3+ ion, whereas in the latter, only one Ag atom is needed to reduce one Au+ ion. As such, when HAuCl2 is used as the precursor, the thickness of deposited Au will be increased, thereby creating a more stable structure. 3.4.2. Bimetallic Nanocrystals Enabled by Galvanic Replacement. For the most part, Ag nanocrystals with welldefined sizes and shapes have served as the common sacrificial template for galvanic replacement reactions. The choice of Ag is based on the fact that it has a relatively low reduction potential (as compared to Pd2+, Au3+, and Pt2+) and a colloidal chemistry that is rich and well-established.260,272−274 To date, a variety of other template materials have also been used, including other metals such as Ni,275,276 Co,275−279 Pb,275 and Cu;275,280,281 semiconducting materials such as Se282 and Te;283 and even oxide materials such as Mn3O4,284 Co3O4,284 and Cu2O.285,286 Although we have restricted our discussion to the cubic template, a large variety of shapes have been successfully implemented, including spheres, hemispheres, cubes, rods, wires, and bimetallic heterostructures.275,287−299 In most cases, the interdiffusion process is generally favorable at the reaction temperatures used (23−100 °C) and across the characteristically thin walls, making alloy formation intrinsic to the galvanic replacement process.260,300 The sheer number of templates and the broad range of possible bimetallic nanostructures already produced is a testament to the versatility of this synthetic strategy. To add, the internal structure of the nanocrystals, as well as the extent to which the original template is removed, can be tuned by changing the reaction parameters such as the concentration of the added metal ions and/or the reaction time.

anode half reaction: + − Ag(s) → Ag(aq) (+ 0.80 V vs SHE) + e(aq)

cathode half reaction: − − AuCl−4(aq) + 3e(aq) (+ 0.93V vs SHE) → Au(s) + 4Cl(aq)

combined reaction: + − 3Ag(s) + AuCl−4(aq) → 3Ag(aq) + Au(s) + 4Cl(aq)

The sequence of events upon titrating HAuCl4 into a suspension of Ag nanocubes is shown in Figure 19b−e. First, galvanic replacement will be initiated at sites with relatively high surface energies, such as defects, stacking faults, or steps. Upon reaction, Ag atoms will be oxidized and dissolved into solution as Ag+ while Au3+ ions are simultaneously reduced and plated on the exterior of the template. Because of the small lattice mismatch between Au and Ag (4.086 vs 4.078 Å for Ag and Au, respectively), the newly deposited Au atoms tend to grow epitaxially on the Ag template. In the early stages of the reaction, only a thin and incomplete layer of Au forms on the Ag template, just enough to prevent the covered Ag from being directly oxidized. To empty the rest of the structure, a pit is typically formed, and it serves as the primary site for continuous dissolution of Ag from the core as Au3+ is continuously reduced and deposited as Au atoms on the exterior of the structure. Because Au and Ag have similar lattice parameters and of the fact that the Au−Ag alloy is more thermodynamically stable than the phase-segregated structure, an Au−Ag nanobox results.261 The alloying process can be modeled by Fick’s second law of diffusion:260 C = Cs − Cs·erf[x /2(+t )1/2 ]

(7)

where + is the interdiffusion coefficient of the two metals in question, C is the atomic concentration of metal N as a function of time t at a distance of x from the M/N interface, and Cs is the initial concentration of metal N at the M/N interface. It should be noted that + strongly depends on the temperature and also on the energy barrier to diffusion. For example, the value of + for the interdiffusion of Au through an atomic layer of Ag increases 5 orders of magnitude from 10−24 to 10−19 m2 s−1 when the reaction temperature is raised from 25 to 100 °C.262,263 Consequently, increasing the rate of metal interdiffusion increases the rate of alloying, and thus the rate at which the galvanic replacement reaction proceeds. By carefully tuning this process, it is possible to create hollow nanocrystals with solid alloyed shells, such as Au−Ag and Pd−Ag nanoboxes, Au−Ag nanotubes, and a number of other bimetallic nanostructures.264−266 In many cases, the interdiffusion process between the core and shell is an inevitable consequence of bringing two metals into physical contact. However, interdiffusion has been shown both theoretically and experimentally to be strongly influenced by the elastic strains generated at the interface between two metal thin films.267−269 The nanocrystal version of this concept was demonstrated by Neretina and co-workers, who showed that the rate of interdiffusion could be controlled by carefully selecting which pair of metals served as the shell and template.270 To demonstrate this notion, five different template metals were investigated, including Cu, Ni, Co, Pb, and Ag, all supported on a sapphire substrate. For each of the templates, a 3 nm thick layer of Pt was sputtered on the surface, forming a 10439


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More recent endeavors have centered around fine-tuning the structural features for the synthesis of nanoframes. In a recent example, Huang and co-workers carried out an impressive onepot approach for the formation of Pt−Cu nanoframes.301 In a typical synthesis, a mixture of Pt(acac)2, CuCl2, glucose, OLA, and OA was ultrasonicated for 2 h and then heated to 180 °C and maintained for 3 h. The authors found that polyhedral Cu nanocrystals formed first, followed by a galvanic replacement reaction with Pt(acac)2, resulting in 17 nm Pt−Cu rhombic dodecahedral nanoframes. Because the number of metal atoms deposited is proportional to the number of template atoms removed, the ridge thickness is difficult to control. To avoid this constraint, a relatively low concentration of metal precursor (or a relatively high concentration of templates) can be used to partially react the templates. When the precursor to template ratio is relatively low, the deposition generally targets the edges and corners of the template, producing a core-frame structure. After this step, if a template-selective etchant is added, a nanoframe structure is generated.302 This concept was demonstrated by Xue and co-workers, who synthesized triangular rings of Ag−Pd alloys by reacting Ag nanoplates with PdCl42+.303 They found that the degree of galvanic replacement between the Ag and H2PdCl4 could be used to form Ag−Pd porous nanoplates. When treated with H2O2, a Ag-specific etchant, the Ag-rich core could be selectively removed, leaving behind well-defined triangular rings made of Ag−Pd alloys with a ridge thickness of ∼7 nm. Taken altogether, so long as there exists a favorable difference in the reduction potential, galvanic replacement can be employed to generate a wide variety of bimetallic nanocrystals by carefully controlling the reaction parameters. As was already mentioned, the dealloying process is an attractive route to the formation of highly porous bimetallic nanocrystals.304 Most of the delloying work is centered around introducing a chemical agent that selectively removes (leaches) or replaces one of the elemental components. However, the dealloying process can also be performed electrochemically, by first dispersing the alloy or intermetallic nanocrystals on a conductive surface, submerging in an electrolytic solution, and applying an appropriate bias. Strasser and co-workers demonstrated this process for the synthesis of a series of novel Pt-based catalysts for the ORR.305−309 In one particular study, they compared the products obtained through chemical and electrochemical dealloying of Cu3Pt intermetallic structures.310 They found that the electrochemical dealloying method led to the formation an ordered Cu3Pt core structure with an ultrathin (∼1 nm) Pt skin, while the chemical dealloying route gave rise to a “spongy” structure with random atomic ordering. Nevertheless, both materials showed similar enhancements in specific and mass activities toward the ORR, as well as improved durability relative to Pt/C. 3.4.3. One-Pot Synthesis of Hollow Nanocrystals via Temporal Separation. As previously discussed, the most common methodology for generating hollow bimetallic nanocrystals via galvanic replacement involves three steps: (i) the formation of sacrificial templates, (ii) purification through a series of centrifugation and washing steps, and finally (iii) the addition of metal ions that have a reduction potential greater than the templates. Recently, several groups have demonstrated that this multistep process could be combined using a one-pot reaction. In one example, the templates were first formed via reduction, and then, after some time, metal ions were introduced into the same reaction solution without a

purification step. This strategy was demonstrated by Schaak and co-workers for the synthesis of hollow Co−Pt nanospheres (Figure 20a).278 The sacrificial templates consisting of Co

Figure 20. (a) TEM image of Co−Pt hollow nanospheres formed via a one-pot approach. The inset shows a model of the Co−Pt hollow nanosphere. (b) TEM image of Pd−Pt hollow nanocubes formed via a one-pot approach, in which template formation and galvanic replacement occur in the same reaction vessel but in two temporally separated phases. The inset shows a model of the Pd−Pt hollow nanocube. The image in (a) was reprinted with permission from ref 278. Copyright 2005 American Chemical Society. The image in (b) was reprinted with permission from ref 311. Copyright 2009 WileyVCH.

nanospheres were first generated by reducing Co2+ with NaBH4 in the presence of PVP. Afterward, Pt4+ was added. Because of the favorable difference in reduction potential (−0.28 V for Co2+/Co and 0.74 V for PtCl62−/Pt), the Co core was dissolved as the Pt shell was deposited, resulting in the formation of Co− Pt hollow nanospheres.278 If the experimental conditions are suitable, it is even possible to carry out the synthesis of hollow Pt−Pd bimetallic nanocrystals in a one-step one-pot synthesis as shown by Zheng and co-workers.311 In this case, both Pd and Pt precursors were present when the reducing agent was added. As the authors pointed out, the key to a successful one-step one-pot synthesis of hollow bimetallic nanocrystals is the temporal separation of template formation and galvanic replacement reaction. To achieve such separation, they found that the use of both I− ions and acetylacetonate precursors was essential for enabling the reduction of Pd2+ first, for the formation of Pd templates, followed by galvanic replacement with Pt2+ to generate Pd−Pt hollow nanocubes (Figure 20b). 3.4.4. Combining Galvanic Replacement with Coreduction. Because of the nature of a galvanic replacement process, the final nanocrystals will generally take a shape that closely resembles that of the sacrificial template. Nevertheless, additional shape control can also be achieved by coupling galvanic replacement with coreduction. This involves simply introducing a reducing agent into the same reaction solution, which acts concurrently with the galvanic replacement process. This approach was initially used by Xia and co-workers to create Pd−Pt nanocages with enhanced catalytic activities toward the ORR. A typical synthesis involved the use of Br−capped Pd nanocubes as templates, Na2PtCl4 as the precursor, and citric acid as the reducing agent.312 The galvanic replacement reaction began at the side faces of the Pd nanocubes, initially resulting in the formation of Pd concave nanocubes. As the reaction progressed, the citric acid reduced both the Pt2+ and the Pd2+ ions to atoms for their deposition on the template as a Pd−Pt alloy. As the reaction proceeded, the templates were gradually removed through the galvanic replacement process, generating Pd−Pt nanocages. 10440


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for AuCl4−/Au to +0.56 V for AuI4−/Au, which is below the reduction potential of Ag+/Ag at +0.80 V. As such, the reduction of the Au precursor was driven exclusively by the introduced reductant, thereby achieving the formation of Ag@ Au core−shell nanoplates. Most recently, Yin and co-workers further demonstrated that instead of I−, sulfite could also be used to serve the same role.314 They found that, like I−, the sulfite strongly complexes with Au3+ ions and successfully inhibits galvanic replacement while at the same time remaining benign to the Ag surface to avoid any ligand-assisted oxidative etching. However, the introduction of coordination ligands into a reaction system may not always be a viable option because they have the potential to strongly interact with the template (e.g., capping effect, oxidative etching). An alternative method for inhibiting galvanic replacement involves increasing the reduction rate of the metal precursor (e.g., by adding a strong reducing agent to the system). Realizing that the reducing power of AA could be increased with pH, Qin and co-workers demonstrated the formation of Ag@AunL core−shell nanocubes by rapidly reducing AuCl4− with AA at pH = 11.02 with the introduction of NaOH, as shown schematically in Figure 22a.315 HAADF-STEM images revealed a smooth, continuous,

The coreduction method can also be used to achieve good control over the composition in terms of both atomic ratio and elemental distribution. Recently, Qin and co-workers utilized this approach to prepare Au−Ag nanocages with Ag-enriched walls.313 By simply carrying out the standard galvanic replacement reaction in the presence of a reducing agent, the freed Ag+ ions could be immediately reduced and deposited back onto the surface of the template, as shown schematically in Figure 21a. This resulted in an alloyed Au−Ag nanocage with

Figure 21. (a) Schematic illustration showing concurrent galvanic replacement and coreduction on the surface of a Ag nanocube. TEM images of Ag nanocubes after being reacted with (b) 0.2 and (c) 0.6 mL of 0.2 mM HAuCl4, respectively, in the presence of ascorbic acid. (d) HAADF-STEM and EDS mapping of an individual nanocrystal obtained by reacting the Ag nanocubes with 0.6 mL of HAuCl4 in the presence of ascorbic acid. Reprinted with permission from ref 313. Copyright 2014 American Chemical Society.

>80% of the Ag in the initial template being retained in the walls. Figure 21b−d shows TEM images of the Ag nanocubes post reaction with increasing concentrations of HAuCl4. Because additional Ag gives rise to a more intense localized surface plasmon, these structures showed enhanced performance in surface enhanced Raman spectroscopy (SERS), producing signals that were 15- and 33-fold, respectively, stronger than the Ag nanocubes and the Au−Ag nanocages prepared through galvanic replacement only. 3.4.5. Elimination of Galvanic Replacement for Achieving Seed-Mediated Growth. In some reactions, galvanic replacement is actually considered troublesome, especially when trying to deposit a more noble metal on a less noble seed (e.g., Ag@Au or Ag@Pd). Therefore, at times it is useful to inhibit, or entirely eliminate, the galvanic replacement process. As can be inferred from the Nernst equation, reducing the difference in reduction potentials between the metals in question is an effective method for preventing galvanic replacement from occurring. Yin and coworkers demonstrated one such approach for the synthesis of Ag@Au core−shell nanoplates.155 This was achieved by introducing I− into the reaction system, which rapidly displaced Cl− in the AuCl4− complex. As a result of this substitution, the reduction potential of the precursor was reduced from +0.93 V

Figure 22. (a) Schematic illustration showing the transformation of Ag nanocubes to Ag@AunL core−shell nanocubes when the rate of reduction is much greater than the rate of galvanic replacement. (b) HAADF-STEM image of an individual Ag@Au6L core−shell nanocube. (c) HAADF-STEM image taken from the boxed region in (b), revealing a heteroepitaxial deposition of Au atoms on the surface of a Ag nanocube. Reprinted with permission from ref 315. Copyright 2014 American Chemical Society.

and epitaxial deposition of Au onto the Ag template (Figure 22b and c) and, most importantly, with no obvious signs of galvanic replacement (e.g., pitting and hollowing). They found that when the pH was adjusted to 2.63 through the addition of HCl, chemical reduction will fall short to galvanic reduction, and the effects characteristic of galvanic replacement will be evident. It is worth noting that other strong reducing agents such as hydroxylamine or NaBH4 could also be used to replace AA/NaOH for the formation of Ag@AunL nanocrystals.156,316 10441


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replaced with Pt to create a Pd@Pt1L core−shell structure (Figure 23b and c). Subsequent studies by the same group extended this methodology to insert a Au-rich interlayer between the Pd core and the Pt shell.328 By varying the concentration of the Au precursor used for the galvanic replacement, the interlayer composition could be tuned, allowing for a 3.3-fold improvement in ORR mass activity over the Au-free Pd@Pt1L counterpart. They found that the Au interlayer not only improved catalytic activity but also stabilized the catalyst during cycling. It should be noted that this strategy is not restricted to noble metals because other metals such as Co and Ni and even bimetallics such as PtCo and Pd3Fe have also been demonstrated.325,328−330 In addition to enabling the synthesis of core−shell nanocrystals, UPD-coupled galvanic replacement can also be used to produce a variety of complex bimetallic heterostructures, such as those comprised of Au, Ag, Pd, and Pt.331 Figure 24 shows a schematic illustration of the diverse products

Most recently, Qin and co-workers showed that Ag@Pd−Ag core−shell nanocubes could be prepared through coreduction in the absence of galvanic replacement.317 A typical synthesis involved the cotitration of PdCl42− and Ag+ into an aqueous suspension of Ag nanocubes in the presence of AA and NaOH. The AA and Ag+ ions worked together to inhibit the expected galvanic replacement reaction between Ag nanocubes and the PdCl42− ions, producing a Pd−Ag alloy on the surface of the Ag nanocubes. Interestingly, the Pd−Ag alloy appeared to deposit sequentially on edges, corners, and then faces of the Ag nanocubes, as controlled by the relative surface free energies of those surfaces. The Ag@Pd−Ag core−shell nanocubes could serve as a dual catalyst for both reduction (by Pd) and oxidation (by Ag) reactions, while serving as an excellent SERS substrate for spectroscopy fingerprinting. 3.4.6. Combining Galvanic Replacement with Underpotential Deposition. When attempting to engineer nanocrystals composed of scarce elements, such as Pt, it is critical to keep the material loading to a minimum. However, precise control over the number of deposited atomic layers can be difficult when attempting to do so purely through seedmediated growth. An alternative approach is based on the coupling of galvanic replacement with UPD. This process begins by depositing a sacrificial-monolayer at underpotential onto the surface of nanocrystals, followed by the addition of metal ions capable of galvanically replacing the UPDlayer. 318,319 To date, several combinations have been investigated such as Au, Ag, Pd, and Pt substrates for the UPD of metal ions including Cu 2+ , Pb 2+ , Bi 3+ , and Ag+.318,320−324 Once deposited, metal ions with relatively higher reduction potential (but lower than the substrate metal) can be added to selectively replace the newly deposited monolayer, as schematically illustrated in Figure 23a. This general strategy can be employed for monolayer deposition, and repeated sequentially for additional layers. The resultant core-monolayer structures are promising catalysts for reactions such as the ORR.314,325,326 Adzic and co-workers have successfully demonstrated this approach by employing UPD to deposit a monolayer of Cu onto Pd nanocrystals.327 Following this, the Cu layer was galvanically

Figure 24. Schematic illustration of the different types of bimetallic nanostructures that could be obtained by combining galvanic replacement with underpotential deposition. Reprinted with permission from ref 331. Copyright 2013 Nature Publishing Group.

that have been reported by first depositing a UPD monolayer of Ag on the surface of Au seeds, and then reacting them with metal ions that have a higher reduction potential. The authors state that the in situ formed Ag UPD layer then undergoes galvanic replacement reaction with the precursor of the less reactive metal precursor (relative to the Au-core). As the reaction continues, electron-rich regions of the Au seeds (e.g., edges, corners, vertices) are targeted for continuous deposition. Because the UPD-Ag layer is continuously oxidized back into solution and a new Ag-free surface is created, the Ag again forms a UPD layer and thus creates a self-sustaining cycle. They also noted that variations in reaction kinetics could be used to further tune the size and geometry of the deposited structures. 3.4.7. Combining Galvanic Replacement with the Kirkendall Effect. The core−shell nanocrystal can be further transformed into a number of unique derivative structures through additional chemical treatments (i.e., etching, galvanic replacement, additional shell deposition, and corrosion, among others). Other than simply subjecting core−shell nanocrystals to transformative chemical environments, an alternative strategy is based on heating. For many bimetallic combinations, the core−shell architecture represents a far-from-equilibrium state. Therefore, heating will not only change the shape, but also the distribution of elements in the nanocrystals. Interestingly, when the mutual interdiffusion coefficients differ by a considerable amount, a phenomenon known as the

Figure 23. (a) Schematic illustration showing the formation of a monolayer of Cu, through underpotential deposition, on the surface of a Pd seed, followed by the replacement of the Cu monolayer with Pt using a galvanic reaction. (b) HAADF-STEM image and (c) line-scan of a Pd@Pt1L core−shell nanocrystal. Reprinted with permission from ref 327. Copyright 2010 Wiley-VCH. 10442


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Kirkendall effect will prevail.332−334 This unbalanced interdiffusion will lead to the accumulation of vacancies (in the metal with a faster diffusion rate), which become supersaturated and eventually coalescence to create voids (or Kirkendall voids). While this effect has been mainly observed in bimetallic nanocrystal systems for the diversification of structure and property,335,336 it has also been extended to a range of other materials such as semiconductors and insulators.337−341 In one example, Puntes and co-workers demonstrated the synthesis of a variety of bimetallic and multimetallic concentric hollow nanocrystals by coupling the Kirkendall effect with galvanic replacement at room temperature.335 By reacting Ag nanocubes with Au or Pd compounds, bimetallic or multimetallic hollow nanocrystals with various morphologies and compositions could be generated at room temperature. In the early stages of the reaction (for the Au−Ag case), a thin Au shell is deposited on the surface of the Ag template, followed by the formation of pinholes in the walls of the template and formation of a hollow interior. As the reaction ensues, the walls eventually take on a trilayer configuration of Au/Ag/Au. Because the diffusion of Ag into Au is faster than Au into Ag, void bubbles grow between the two Au layers and eventually coalesce to generate a continuous cavity between the Au layers, resulting in the formation of Au−Ag double-walled nanoboxes. The process, from start to finish, is illustrated in Figure 25. In the same report, they also showed that this protocol could be extended to trimetallic nanocrystals comprised of Au, Pd, and Ag.

example, depositing catalytic Pd on the surface of magnetic Ni nanocrystals not only reduces the loading of Pd but also makes the hybrid system magnetically recoverable (as discussed in section 5.3.2). 4.1. Catalytic Properties

Heterogeneous catalysis refers to any chemical transformation involving the adsorption of reactants from a gas or liquid phase onto the surface of a solid substrate, reaction of the adsorbed species, and desorption of products into the gas or liquid phase.342 The correlation between the catalytic performance (i.e., activity, selectively, and durability) and the nature of catalyst, the composition, size, shape (more precisely, the type of facet), and structure of the catalytic particles, has been investigated for nearly a century. Leaps in progress can be attributed to the emergence of ultrahigh vacuum technology combined with single-crystal substrates, as well as HRTEM techniques that enable invaluable correlations between atomicscale features and overall catalytic performance. In this section, we only focus on the fundamentals of heterogeneous catalysis in terms of achievements enabled by bimetallic nanocrystals. It was nearly a century ago when French chemist Paul Sabatier postulated that the rate of a catalytic reaction had a strong correlation with the adsorption energy, now known as the Sabatier principle.343 This principle states that the adsorption energy between the catalytic surface and the reactants should not be too strong, nor too weak, but rather just right to achieve optimal catalytic performance. Later in 1969, Balandin plotted the catalytic activity as a function of the heat of adsorption and revealed a clear trend known as the volcano plot or volcano surface, where the maxima corresponded to the binding energy that was just right.344 Ever since, the volcano plot has become one of the most useful concepts in the design of heterogeneous catalysts. More recently, concepts such as the Brønsted−Evans−Polanyi (BEP) relation have emerged, which correlate the activity of a catalytic surface with the position and width of the d-band defined by the electronic structure of the catalytic surface.345 The combination of these invaluable concepts with powerful computational tools, such as DFT, has made the development of nanocrystal-based catalysts less of a trial and error practice and more of a scientific endeavor centered around selecting the correct combination of metals engineered to some theoretically prescribed composition, size, shape, and structure to optimize the catalytic performance.346 The critical roles played by size and facet in heterogeneous catalysis have a strong correlation with the coordination number of atoms on the surface.347 Taking fcc metals as an example, surface atoms in the {111}-facets have a coordination number of 9 and thus the highest possible stability among all facets. The {100}- and {110}-facets, the next most stable, have coordination numbers of 8 and 7, respectively. Nanosized defects, such as steps or kinks, can have a coordination number of 6. As demonstrated by Bligaard and Nørskov, energetic differences between these different sites on a catalytic surface can vary by up to 1 eV, for the chemisorption energy of CO on Pt.348 Because any variation in adsorption energy will result in an exponential change in the reaction rate, differences in the reaction site will have a substantial influence over activity. In addition to the strong tie between the coordination number and the size and facet, nanocrystals on the order of 2 nm or smaller begin to exhibit quantum size effects. Goodman and coworkers demonstrated this effect by investigating the size-

Figure 25. Optical and morphological evolution of Au−Ag doublewalled nanoboxes together with HAADF-STEM images and EDS maps of individual particles obtained at different stages of the reaction (scale bar: 20 nm). Reprinted with permission from ref 335. Copyright 2011 AAAS.

4. PROPERTIES OF BIMETALLIC NANOCRYSTALS The metals in Figure 1 can be clustered into three major groups according to the property/application they typically represent: Au, Ag, and Cu for plasmonics; Ru, Rh, Pd, Ir, and Pt for catalysis; and Ni, Co, and Fe for magnetism. Perhaps the most important hallmark of bimetallic nanocrystals is their ability to naturally integrate the physicochemical properties of the two metals from which they are comprised. In general, bimetallic nanocrystals can be designed and synthesized to simultaneously express the unique properties of each metal. For example, growing catalytic Pt from the ends of plasmonic Au rods gives a novel class of bimetallic hybrid whose catalytic activity can be enhanced using light (as discussed in section 5.2.1). In another 10443


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Figure 26. (a) Formic acid decomposition rate as catalyzed by M@Pd core−shell structures (M = Ag, Rh, Au, Ru, and Pt). A positive correlation is obtained when plotting the decomposition rate against the work function of the core metal, highlighting the role played by charge transfer in heterogeneous catalysis. (b) Relative specific activities of Pd@PtnL icosahedra at 0.9 V calculated using the PtnL*/Pd(111) icosahedral slab models, denoted as PtnL*/Pd(111)ico, at a tensile strain of 2.4% for the Pd surface. The result for Pt3L*/Pd(111)oct is included for comparison. All values are given relative to the pure Pt(111) surface. The plot in (a) was reprinted with permission from ref 355. Copyright 2011 Nature Publishing Group. The plot in (b) was reprinted with permission from ref 46. Copyright 2015 Nature Publishing Group.

charge transfer in affecting the catalytic activity. On a fundamental level, interfacing dissimilar metals will lead to charge redistribution between them, where the metal with a higher Fermi level will transfer charge to the other (as reflected in the work functions).355 Tsang and co-workers conducted a systematic study and demonstrated that the rate of formic acid decomposition increased linearly with a decreasing work function of the metal used for the core of M@Pd core−shell nanocrystals, where M = Rh, Pt, Ru, Au, and Ag (Figure 26a).355 In this case, as the d-band center gets closer to the Fermi level of the shell metal, the adsorption energy, and thus the decomposition rate, will increase commensurately.355 As noted by the authors, it is often difficult to rule out the geometric effect because the overlayer tends to take the lattice constant of the underlying substrate. In fact, core−shell nanocrystals have also served as a model system for investigating the geometric effect because the shell thickness can be readily controlled down to a single atomic layer. The degree of strain in the shell can be tuned by varying the shell thickness, with high strains being typically observed for n = 1− 3, low strains when 3 > n > 6, and disappearance of coreinduced strains if n > 6. In the case of heteroepitaxy, the degree of strain can be controlled by choosing different metals for the core and shell, with acore < ashell and acore > ashell (a: lattice constant) resulting in compression and tension, respectively, in the shell. The core−shell structure has stimulated the development of a number of electrocatalysts. In one example, Strasser and coworkers synthesized Pt−Cu@Pt core−shell nanocrystals and provided evidence for strain-dependent catalytic activity toward the ORR.351 Because the Pt−Cu core had a smaller lattice constant relative to Pt, the shell was placed under a compressive strain. They demonstrated that the compressive strain in the shell modified the d-band structure of the surface Pt atoms. For the ORR, the d-band shift weakened the adsorption energy of reaction intermediates, making this bimetallic system catalytically superior to that of unstrained Pt catalysts. In a number of relatively recent studies, Xia and co-workers developed a series of Pd@PtnL core−shell nanocrystals whose activity could be manipulated by controlling the number of Pt atomic layers present on the Pd core. In one particular example, where Pd icosahedra served as seeds for the conformal deposition of Pt

dependent catalytic activity of Au nanocrystals supported on titania for the low-temperature oxidation of CO, attributing the unusual size-dependence to quantum size effects.349 As is evident from the aforementioned examples, the size and facet of nanocrystals play a vital role in defining the overall catalytic performance. By introducing a second metal, the already vast property landscape of nanocrystals can be further expanded as bimetallic nanocrystals can give rise to a number of new effects.350 As pointed out by Strasser and co-workers,351 the addition of a second metal will result in three unique effects: (i) ensemble, where specific groupings of surface atoms take on distinct mechanistic functionalities,352,353 (ii) ligand, where a charge transfer between two dissimilar surface atoms alters their electronic structure and activity,354,355 and (iii) geometric, the spatial arrangement of surface atoms affected by strain, geometry, and size.356 However, it should be pointed out that, in most cases, it is difficult to disentangle these effects because ligand and geometric effects357−360 and, in some cases, all three effects353 occur simultaneously.361 In 2001, Magnussen and co-workers investigated the ensemble effect by studying CO adsorption and oxidation on atomically flat electrodes composed of PdAu(111) on Au(111).352 The ratio and distribution of metal atoms on the surface were determined using in situ atomic resolution scanning tunneling microscopy (STM) with elemental contrast. Isolated Pd atoms (monomers) were identified as the smallest ensemble for CO adsorption and oxidation, whereas hydrogen adsorption required at least Pd dimers. Using a similar approach, Goodman and co-workers demonstrated the ensemble effect of Au in Pd−Au alloys during the acetoxylation of ethylene to vinyl acetate.353 They showed that the formation rate of vinyl acetate was significantly enhanced on Pd/Au(100) as compared to Pd/Au(111), implying that the critical reaction site consisted of two noncontiguous, suitably spaced Pd monomers. Although established using bulk electrode surfaces, these concepts can also be extended to bimetallic catalysts based on alloys or intermetallic compounds. The ligand effect is a result of the charge transfer between two dissimilar atoms through d-state hybridization, a process that fundamentally changes their electronic structure and activity.348 The core−shell structure has been extensively explored as a model system to investigate the role played by 10444


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(e.g., Pt, Pd, Ru, Rh, or Ir), effectively upgrading the more simple catalytic nanocrystals to those that can couple with light and thus become photocatalytically active. 4.2.1. Localized Surface Plasmon Resonance (LSPR). The interaction between the incident light and a nanoparticle was pondered early on by Faraday in 1857, when he attributed the beautiful ruby red color displayed by his Au colloids to the “finely divided metallic state”.393 It was not until 1908 when Faraday’s qualitative assertion was mathematically verified by Mie, who solved Maxwell’s equations for a homogeneous sphere irradiated by an electromagnetic plane wave.394,395 In the quasi-static approximation (valid when the size of the spherical particle is much smaller than the wavelength of incident light), the optical interaction only depends on the radius (r) of the particle, the wavelength (λinc) of incident light, the dielectric constant (εm) of the surrounding medium, and the real and imaginary parts of the particle’s dielectric constant, εr and εi, respectively. The extinction cross section is given by396

overlayers (n = 0.7−4.3), the dependence of catalytic activity on the shell thickness was unveiled.46 Remarkably, the Pd@PtnL catalysts showed a volcano-type dependence on the number of Pt atomic layers, with [email protected] exhibiting the highest specific activity, an experimental observation also validated by selfconsistent DFT calculations, as shown in Figure 26b. In these examples, the enhancement in catalytic activity should be attributed to a combination of ligand and strain effects. As illustrated by the aforementioned examples, the spatial distribution of atoms in a bimetallic nanocrystal plays a pivotal role in defining the geometric and electronic structure of the surface, and thus the corresponding catalytic efficacy. Because energy dispersive spectroscopy (EDS) techniques can only provide a rough picture about the elemental makeup of a bimetallic nanocrystal, it is still challenging to elucidate the explicit role played by elemental distribution in nanocatalysis. In an attempt to reconcile composition and activity, another technique, known as atom-probe tomography (APT), has been developed to obtain three-dimensional (3-D) atomic maps and reveal both the position and the identity of the constituent atoms in a nanocrystal at near-atomic resolution.362 Maschmeyer and co-workers used this technique to uncover the relationship between the elemental distribution and surface morphology of a single Au@Ag core−shell nanocrystal.363 The authors also used a statistical method to identify a clear relationship between surface curvature and the amount of Ag present. In all, this advanced characterization tool can provide the detailed atomic-scale information necessary for deriving important conclusions regarding the formation mechanisms of bimetallic nanocrystals and also for resolving the elemental composition and structure of a surface involved in nanocatalysis.

Cext =

24π 2r 3εm3/2 εi λ inc (εr + 2εm)2 + εi2

(8)

From this relation, Cext has a maximum when the denominator is minimized. As evident from eq 8, the physical parameter that ultimately defines the position of LSPR peak is the particle’s dielectric constant, ε(ω), which is defined by ε(ω) = εr(ω) + iεi(ω)

(9)

where εr(ω) and εi(ω) are the real and imaginary parts, respectively. In the context of nanocrystals, the dielectric constant (typically extracted from bulk materials) is generally considered to be invariant to the nanocrystal’s size and shape, with the exception of very small nanocrystals, or those with extremely small feature sizes (see section 4.2.2).397 Therefore, simply knowing the dielectric constant of the given material of interest (for pure metals, see Johnson and Christy398 or Palik399), one only needs to simply locate the wavelength at which εr(λ) = −2εm. According to the dielectric constant provided by Rakić, the LSPR peak of Au nanospheres in an aqueous suspension is located at 518 nm, in good agreement with experimental observation.400 This concept can be readily extended to spherical bimetallic nanocrystals. 4.2.2. Dependence of LSPR on the Size or Shape of a Nanocrystal. The spectral position and profile of the LSPR of a nanocrystal are determined by a number of factors including the size, shape, composition (elemental ratio and spatial distribution), and electron density of the nanocrystal, as well as the dielectric constant of the surrounding environment. As pointed out by Marks and co-workers, for nanocrystals with sizes between 50 and 350 nm, a primary LSPR-defining feature is the oscillation-distance or “plasmon length”.401 In other words, a Au cube of 50 nm in edge length should have an LSPR peak position similar to that of a Au nanosphere of 50 nm in diameter. In addition to the peak position, the intensity and line width are also sensitive to the size. For example, when nanocrystals have sizes smaller than their bulk electron mean free paths (e.g., λ∞ ≈ 42 and 52 nm for Au and Ag, respectively), the increased number of electron−surface collisions will cause the LSPR to dampen and broaden.402 Because the dielectric constant is dependent on the electron mean free path, the dielectric constant from eq 9 will also become size-dependent, with ε(ω) replaced by ε(ω,r).396,397,402

4.2. Optical Properties

Noble-metal nanoparticles have long attracted great interest due to their intense colors. The Roman Lycurgus cup, for example, from the fourth century, and other pottery from the 16th century Renaissance era, are known for their vibrant colors from glass-impregnated colloidal Au, Ag, Cu, and their alloys.11,364 This interesting optical property emanates from a strong interaction between the confined free electrons in a metal nanoparticle and incident light under the resonant condition. Now this optical phenomenon is known as the LSPR, and the resonance peak is typically located in the visible region of the electromagnetic spectrum for metal nanoparticles made of Cu, Ag, and Au, and in the ultraviolet (UV) region for nanoparticles comprised of Ru, Rh, Pd, and Pt.365,366 There are a broad range of applications that utilize the LSPR phenomenon, including SERS,367−369 photocatalysis,370−374 artificial photosynthesis,375,376 solar cells driven by hot electrons,377 biomedicine,52,378 photothermal therapy,379,380 plasmon-mediated nanocrystal growth,381−387 and sensing.388,389 Each of these applications utilizes at least one of the many features associated with LSPR, including light scattering, photothermal conversion, particle−particle plasmonic coupling, plasmon decay into hot electrons (or electron−hole pairs), and two-photon excitation, as well as the high sensitivity of resonance peak position to variations in size, shape, composition, and/or local dielectric environment.11,390−392 Since the establishment of seed-mediated growth, hybrid nanocrystals consisting of multiple elements have attracted interest due to the ability to integrate a visibly plasmonic metal (e.g., Au, Ag, or Cu) with a catalytic metal 10445


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Figure 27. TEM images of Ag−Au−Ag nanorods of (a) 61 ± 9 nm, (b) 130 ± 20 nm, and (c) 660 ± 90 nm in length for the Ag segment. (d) Vis− near-IR spectra recorded during the deposition of Ag. (e) Plot trending the change in LSPR peak position (dipolar mode) with increasing Ag deposition. The spectral position of the initial Au nanorod-cores is indicated by a □ data point. Reprinted with permission from ref 182. Copyright 2015 American Chemical Society.

relatively constant, in contrast to the longitudinal mode, which greatly red-shifted upon increase in length. Interestingly, they found that the presence of a Au segment in the middle of each nanorod only played a minor role in determining the LSPR peak position once the nanorod had grown to a certain length. As shown in Figure 27d and e, the LSPR peak of the Ag−Au− Ag nanorods could be tuned from ∼1100 nm all of the way to the 2250 nm regime by continually adding more Ag along the axial direction. While increasing the aspect ratio (e.g., nanorods and nanowires) is a popular method for achieving plasmonic resonators in the IR-region, the use of nanoshells may be more advantageous due to their compact size. The formation of nanoshells made of metals such as Au, Ag, and Cu has demonstrated the capability to shift the LSPR peak from the visible to the near-IR simply by controlling the thickness or diameter of the nanoshell. In the case of UV-resonant metals such as Pt and Pd, it has been demonstrated that the LSPR peak can be shifted from the UV-region to the visible and nearIR regions.408−410 Similar to the increase in aspect ratio, hollowing also has profound impacts on the position and number of resonance peaks. Because the number of peaks is related to the number of ways a nanocrystal can be polarized, the nanoshell structure can support LSPR modes with frequencies that are sensitive to the inner and outer radius of the shell.411 The finite thickness of the shell allows for interactions between the external sphere- and internal cavityplasmons, resulting in the formation of new resonance modes: a lower energy symmetric or “bonding” plasmon and a higher energy antisymmetric or “antibonding” plasmon.412 The appeal of nanoshells over nanorods/nanowires is that the LSPR peak can be tuned into the near-IR while keeping the overall dimensions very compact, a requisite for applications requiring nanocrystals that are relatively small in size. 4.2.3. Dependence of LSPR on the Composition of a Nanocrystal. Alloying also has a profound impact on the LSPR due to direct modulation to the dielectric constant. Early

This effect has also been observed in bimetallic systems; for example, Cottancin and co-workers found good agreement between experimental and calculated spectra when the dielectric constant was modified to account for shortening of the electron mean free path for Ni−Ag nanocrystals of 2.7 nm in diameter.403 If the size is further reduced to 2 nm or lower, the structures fall into an entirely new regime commonly known as clusters. At such a small size, the electronic structure becomes discretized and the nanocrystals can lose their LSPR and become photoluminescent.404 For example, the emission of Au clusters can be tuned from the NIR to the UV regions by tuning the core-size, with Au31, Au23, Au13, Au8, and Au5 emitting in the near-IR, red, green, blue, and UV regions, respectively.404−406 The emission wavelength can also be tuned through composition, as shown by Millstone and co-workers who demonstrated this for a series of Au−Cu alloyed nanoparticles of 2−3 nm in diameter.407 Noble-metal nanocrystals can also exhibit more than one LSPR peak as the number of peaks directly corresponds to the number of unique ways the nanocrystal can be polarized. For example, a nanorod gives rise to longitudinal modes that are resonant at longer wavelengths (where the free electrons are polarized along the long axis of the nanorod) and transverse modes resonant at shorter wavelengths (where the free electrons are polarized along the short axis of the nanorod). Each of the two modes can be readily tuned through adjustments to the aspect ratio by increasing the nanowire thickness or length. Very recently, Liz-Marzán and co-workers clearly demonstrated this capability by synthesizing Ag−Au−Ag segmented nanorods with precise control over their length (Figure 27a−c).182 Starting with penta-twinned Au nanorods of 210 nm in length and 32 nm in width, seed-mediated growth was used to deposit Ag at both ends of each Au nanorod. They demonstrated that the bimetallic nanorods could be controllably grown to several micrometers in length while their thickness was slightly increased by 4 nm. Because the thickness remained approximately the same, the transverse mode stayed 10446


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work suggested that the dielectric constant of a MαN(1−α) alloy could be approximated by simply taking a weighted average of the two respective dielectric constants, where εMN(α) = αεM + (1 − α)εN, where α represents the mole-fraction ranging between 0 and 1. However, El-Sayed and co-workers noticed discrepancies between experiment and theory dealing with the optical properties of alloyed nanocrystals.413 As a model system, they synthesized Au−Ag nanocrystals with varying Au:Ag mole fractions by coreducing HAuCl4 and AgNO3 with sodium citrate. Because the optical absorption showed only one resonance peak, it was assumed that the Au and Ag were well mixed to form alloys. They found that the spectral position of the LSPR blue-shifted linearly with increasing Ag content while the extinction coefficient decreased exponentially with the increase in Au content (a result also reproduced by others217). However, modeling the optical properties using a weighted combination of the respective dielectric constants of Ag and Au led to very poor agreement with the experimental data. They found that excellent agreement between experiment and theory could be reached by extracting the dielectric constants from experimentally measured Au−Ag alloy films. Therefore, in general, the dielectric constants of alloys and intermetallic compounds should be extracted from the ellipsometric data for use in simulation-based investigations. Although the dielectric constant of many pure metals is well documented, only several alloy combinations and intermetallic compounds have been reported in the literature. The formation of alloys or intermetallic compounds comprised of two plasmonic metals that are resonant in the visible region (e.g., Au−Ag,413−416 Au−Cu,417,418 and Ag− Cu419) gives rise to structures whose LSPR peak position varies linearly with the stoichiometry of the nanocrystal.11 However, alloying with any of the UV-resonant metals (e.g., Pd or Pt) will lead to drastic dampening or quenching of the plasmon. For example, as shown by Sun and co-workers, pure Ag nanocrystals present a strong resonance at 425 nm. Upon alloying the Ag nanocrystals with Pd to a composition of Ag80Pd20, the LSPR almost completely damped.420 A similar dampening was demonstrated by Murray and co-workers, who formed Au−Pt nanocrystals using a seed-mediated approach.421 They showed that the 5 nm Au nanocrystals gave a strong LSPR peak at 515 nm, but when alloying with Pt, the LSPR was drastically dampened, resulting in an extinction spectrum that monotonically decreased from higher to lower energies. The authors go on to posit that this change could be attributed to a different electronic structure that modifies the intrinsic optical properties of the materials and/or to the presence of inhomogeneous doping/alloying. In contrast to alloys and intermetallic compounds, bimetallic heterostructures have a much different and more complex optical behavior. Take the core−shell structure as an example, in general, the LSPR of the shell dominates the optical response of the nanocrystal. To demonstrate this, Xia and co-workers used seed-mediated growth to deposit Ag onto 11 nm spherical Au seeds capped with CTAC for the formation of Au@Ag core−shell nanocrystals. The Ag shell thickness was controlled by increasing the amount of AgNO3 relative to the number of Au seeds involved (Figure 28a−d).422 It was found that as the thickness of the Ag was increased, the LSPR became dominated by the intense resonance of the Ag shell (Figure 28e). They found that complete screening of the Au core required only a Ag shell of 3 nm in thickness. Huang and co-workers later expanded on this work using Au rhombic dodecahedra as seeds

Figure 28. TEM images of individual Au@Ag core−shell nanocubes obtained by injecting different volumes of AgNO3 into growth solutions containing a fixed number of Au seeds. The volumes of AgNO3 were (a) 0.25 mL, (b) 0.5 mL, (c) 1 mL, and (d) 2 mL (scale bar: 8 nm). (e) UV−vis extinction spectra of Au@Ag core−shell nanocubes with different thicknesses “t” for the Ag shells. Reprinted with permission from ref 422. Copyright 2010 American Chemical Society.

for the generation of cubic, truncated cubic, cuboctahedral, truncated octahedral, and octahedral Au@Ag core−shell nanocrystals and detailed the nature of the LSPR shift with respect to increasing Ag thickness.423 Interestingly, when Ag served as the core and Au as the shell instead, the LSPR of the Ag core was less rapidly masked.424 As pointed out by Cortie and co-workers, the shell generally dominates the optical properties as long as it is metallic and has an imaginary part, εi, similar to that of the shell.11,425,426 This argument has been validated for Cu@Ag,427 Pt@Au,428 Pt@ Ag,429 Pd@Ag,430 Ni@Ag,403 and [email protected] Since there exists no optical resonance in the visible region for PGMs (at least, for their solid, spherical form) and the core−shell nanocrystal adopts the optical properties of the shell, Au@Pd and Au@Pt nanocrystals generally show no resonance in the visible region when the shell is sufficiently thick. Many groups have reported that only a thin Pt-layer was necessary for completely dampening the LSPR of Au core.431−435 However, in the case of Au@Pd, Huang and co-workers showed that the LSPR of a Au core can be maintained as long as the Pd shell thickness is just 1 nm at the thinnest points of the Au@Pd structures.436 In conclusion, coating a plasmonic nanocrystal that has a resonance in the visible regime (e.g., Au, Ag, or Cu) with a PGM (e.g., Pd or Pt) will attenuate the plasmon resonance. In the converse case, coating a PGM nanocrystal with a metal plasmonically active in the visible region leads to LSPR very similar to a structure completely comprised of the shell metal. 4.2.4. Effect of Shape and Composition on the Sensitivity of LSPR-Based Detection. In general, both the spectral position and the intensity of the LSPR are sensitive to the dielectric properties of the local environment. This dependency has given rise to a number of LSPR-based sensors that utilize the sensitivity of the LSPR as a transducer for tracking adsorbate binding events.437 The sensitivity of such a sensor is directly related to (i) the composition, size, and shape of the nanocrystal, (ii) polarizability of the adsorbate, and (iii) the adsorbate coverage. In general, the LSPR wavelength redshifts linearly with increasing the refractive index of the medium (note: dielectric constant εm = n2). For example, the LSPR will be red-shifted when displaced from air (n = 1.00), into water (n = 1.33), ethanol (n = 1.36), and toluene (n = 1.50). The sensitivity/slope (dλLSPR/dn) is typically reported in the unit of nm/RIU, where RIU stands for refractive index unit (provided 10447


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where N is the electron density in the uncharged nanocrystal, ε∞ is the high frequency contribution to the metal dielectric constant, εm is the dielectric constant of the medium, ε0 is the vacuum permittivity, and L is the shape factor of the nanocrystal. Interestingly, the same authors found that for a Au nanorod with L = 0.12, approximately 85 000 electrons must be injected to account for a blue shift of 11 nm. This same concept can readily be expanded to bimetallic systems. 4.2.6. Hybridization of LSPR with Magnetism. Hybridizing plasmonic and magnetic metals has recently been utilized to demonstrate new optical phenomena such as the ferroplasmon. As reported by Kalyanaraman and co-workers, Ag/Co dimers (supported on a solid surface) displayed surface plasmons that propagated on the ferromagnetic region of the Ag/Co structure.452 Using electron energy-loss spectroscopy (EELS), they found distinct resonances near the surface region of Co that had large scattering intensities and extremely narrow bandwidths. Such a resonance was absent in the isolated Co or CoFe alloy nanoparticles. Soon after this discovery, Coronado and co-workers followed up with theoretical calculations, demonstrating that this phenomenon can also be achieved by simply placing a plasmonic nanocrystal next to a nanocrystal that is ferromagnetic.453 In general, combining plasmonic and magnetic systems can lead to unexpected phenomena, as well as new applications.

that the nanocrystals maintain their initial shape and composition). It is generally accepted, and well supported by theory and experiment, that highly anisotropic nanocrystals (e.g., nanorods, nanotriangles, and branched structures) exhibit superior refractive index sensitivity.437,438 Taking Au nanocrystals as an example, the sensitivity increases in the order of Au spheres (44 nm/RIU),439 cubes (83 nm/RIU),439 shells (125 nm/RIU),440 rods (150−263 nm/RIU),441 and rattles (285 nm/RIU).441 Much higher sensitivities have been achieved for Au nanocrystals with more complex structures, including stars (660 nm/RIU),442 branches (703 nm/RIU),439 and rings (880 nm/RIU).443 Outside from geometry, the choice of metal also plays an important role in determining the sensitivity. This can be realized simply by differentiating the expression λLSPR with respect to n to reveal the highest value and thus the ideal candidate. As shown by Miller and Lazarides, the quasistatic approximation for sensitivity is given by444 2ε ⎛ ∂ε ⎞ Δλ = = 1⎜ 1⎟ Δn n ⎝ ∂λ ⎠ λ

−1

SQS

(10)

o

Ag is well-known for having a higher sensitivity than Au when compared with similar sizes and shapes. For example, Van Duyne and co-workers demonstrated that Ag triangular nanoplates were more sensitive than Au nanoplates of identical shapes and sizes.445 In a similar study, Wang and co-workers showed that Ag nanocubes were twice as sensitive than Au nanocubes.446 Remarkably, it has been shown that Ag triangular nanoplates can have sensitivities of up to 1096 nm/RIU.447 From the above expression, it is apparent that a nanocrystal having a large ε1 or small dispersion (∂ε1/∂λ) at the LSPR peak for a given refractive index n should have high sensitivity.448 Overall, Ag is generally dominant in terms of sensitivity while Au has superior chemical properties in terms of biocompatibility and oxidation resistance. Recently, bimetallic nanocrystals have gained attention for enhancing the detection sensitivity.448,449 In particular, bimetallic nanocrystals comprised of Au and Pd have emerged as a particularly promising combination. To this end, DeSantis and Skrabalak identified Au−Pd octopods as a potential platform material for LSPR sensing, achieving sensitivities as high as 556 nm/RIU.450 In another report, eq 10 was used to rationalize the enhanced sensitivities measured for Au@Pd core−shell nanocrystals.433 It was demonstrated experimentally and theoretically that the RI-sensitivity of the Au@Pd nanocrystals was much higher than that of other commonly used monometallic substrates (e.g., Au and Ag). As the authors argued, Pd could be a promising candidate for acting as the “third plasmonic sensing material” following Ag and Au. 4.2.5. Effect of Electron Density on LSPR. The electron density of nanocrystals also plays an important role in defining the spectral position and intensity of the LSPR peak. This concept was initially demonstrated by Mulvaney and coworkers, who placed Au nanorods on an ITO electrode and applied potentials ranging from 0−1.6 V to show that the LSPR peak exhibited a clear and reversible shift upon charging and discharging.451 They noted that when the plasmon band shift was significant enough, color changes could be perceived by eye. The change in LSPR peak position, Δλ, is defined by451 Δλ = −

ΔN 2N

4π 2c 2mε0 Ne

2

ε∞ +

⎛1 − L ⎞ ⎜ ⎟ε ⎝ L ⎠ m

4.3. Photocatalytic Properties

There is a strong interest for hybridizing plasmonic nanocrystals (e.g., Au, Ag, or Cu) with highly catalytic metals (e.g., Pd or Pt). In such a system, the catalytic activity can be readily enhanced through coupling with visible light. Outside from simply amplifying classical reactions, new and interesting photochemical transformations have been demonstrated. At resonant plasmonic frequencies, light is absorbed, manifested as localized surface plasmons that relax on the order of tens of femtoseconds,370 and then released as energy radiatively (a photon is re-emitted) or nonradiatively (charge-carriers are excited). In the latter case, transient charge-carriers can populate vacant electronic states of adsorbates to form transient ions or excited states. In other cases, phonon modes in the nanocrystal may couple vibrationally to a reaction adsorbate.370 The mechanisms of phonon- and electron-driven reactions occurring on the surface of metals are schematically illustrated in Figure 29a−d. Only recently have several reports emerged with regard to charge-carrier-mediated chemical transformations occurring on the surface of resonantly excited plasmonic nanocrystals. In particular, Linic and co-workers investigated ethylene epoxidation on resonantly excited 75 nm Ag nanocubes (supported by α-Al2O3) at temperatures ranging from 375−450 K.454 Two important conclusions were drawn: (i) photocatalytic reactions on plasmonic nanostructures are characterized by a positive relationship between quantum efficiency and photon flux, and (ii) the rate and quantum efficiency of photocatalytic reactions increase with operating temperature. Halas and co-workers found that energetic electrons (hot electrons), generated by the resonant excitation of substrate-supported Au nanocrystals, led to the dissociation of H2 adsorbed on the Au surface.455 To support their claim, both H2 and D2 were flowed together while the Au nanocrystals were resonantly excited, a process that resulted in the formation of HD molecules. Remarkably, they found that the same phenomenon is also possible for Al nanocrystals encapsulated in an oxide.456 There have also been

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reduced even further in size, it eventually reaches what is known as the superparamagnetic limit. Below this limit, nanocrystals have a magnetic spin that randomly flips due to thermal fluctuations, leading to a zero net magnetization (or zero coercivity, where no hysteresis is measured in the magnetization curve). In contrast to ferromagnetic nanocrystals, superparamagnetic nanocrystals have a relatively small energy barrier to changing the magnetic spin direction, and the thermal energy, kBT, is sufficient to overcome that barrier. This concept is shown schematically in Figure 30. The transition

Figure 29. Mechanisms of phonon- and electron-driven reactions occurring on the surfaces of metals. (a) Thermal activation for the dissociation of a diatomic molecule where the molecule remains on the ground state for all points along the reaction coordinate. (b) Electrondriven dissociation of a diatomic molecule, an event that promotes the molecule to a new potential energy surface. This charged state then decays, which drops the molecule back down to the ground state while populating a higher vibrational mode. (c) At high photon flux, subsequent electron injections can occur before the molecular vibration has fully dissipated, thus leading to additional vibrational excitation and a superlinear dependence of the rate of reaction on light intensity. (d) Schematics illustrating an electron-driven reaction (top), and a thermally driven reaction (bottom). Reprinted with permission from ref 370. Copyright 2015 Nature Publishing Group.

Figure 30. Schematic illustration showing that the spin-direction is insensitive to thermal fluctuations for ferromagnetic nanocrystals but sensitive for the superparamagnetic type. Potential energy diagram showing the barrier between spin-states for both ferromagnetic (blue line) and superparamagnetic (red line) nanocrystals, and where the dashed line represents the thermal energy, kBT. Reprinted with permission from ref 469. Copyright 2008 American Chemical Society.

reports demonstrating that plasmons could be used to drive chemical transformations such as the reduction of nitroaromatic compounds, hydrogen production, room-temperature esterification, and others.457−459 In section 5.2.1, we remark on examples that instead use bimetallic plasmonic nanocrystals.

temperature, above which ferromagnetic nanocrystals become superparamagnetic, is known as the blocking temperature, Tb, given by

4.4. Magnetic Properties

Bimetallic magnetic nanocrystals have stimulated interest in a broad range of areas such as biomedicine,460−462 ultrahighdensity information storage,463 and a number of others.464−469 Nanocrystals commonly associated with ferromagnetism include the simple monometallics made of Ni, Co, and Fe; their alloys and intermetallic combinations such as CoNi, FePt, and CoPt3; metal oxides such as NiO, CoO, and Fe3O4; spineltype ferromagnets such as MgFe2O4, CoFe2O4, and MnFe2O4; and more recently, core−shell nanocrystals such as CoFe2O4@ MnFe2O4 and [email protected] The most investigated finite-size effects in magnetic nanocrystals are the single-domain limit, the superparamagnetic limit, and the disappearance of coercivity (Hc). 4.4.1. Finite Size Effects of Magnetic Nanocrystals. In contrast to bulk magnets (comprised of multiple magnetic domains), nanocrystals can behave as a single magnetic domain when reduced down to a critical size. This is referred to as the single-domain limit, denoting the maximum size a nanocrystal can have such that magnetization induces unidirectional spin alignment. This limit is sensitive to both composition and crystal structure. For example, spherical nanocrystals comprised of fcc Co, hcp Co, bcc Fe, and bcc Fe−Co have approximate single-domain diameters of 55, 20, 15, and 45 nm, respectively.104,105,471 When a ferromagnetic nanocrystal is

Tb = K uV /25kB

(12)

where Ku is the uniaxial magnetic anisotropy constant, V is the volume of the nanocrystal, and kB is the Boltzmann constant. Therefore, as the size of a nanocrystal is reduced, the transition from ferromagnetic-to-superparamagnetic will occur at progressively lower temperatures. 4.4.2. Enhancing Magnetic Properties with Bimetallic Composition. The superparamagnetic limit is a significant challenge for researchers tasked at fabricating next-generation magnetic recording devices, because reliable recording must satisfy the thermal stability condition of KuV/kBT > 60.472 Therefore, designing nanomaterials with a large Ku is desired to reduce V at room temperature, and a requisite for outcompeting current technologies.110,473,474 Therefore, tuning Ku through composition, shape, and crystal structure has served as a viable means for increasing Tb while maintaining a compact size. The uniaxial magnetic anisotropy constant is defined by475 K u = Ms[Ha + (N2 − N1)Ms]/2

(13)

where Ms is the saturation magnetization of the material in bulk form, Ha is the anisotropy field, and N2 and N1 are the shapedependent demagnetization factors parallel and perpendicular 10449


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nanocrystals ( disordered Cu3Au nanorods > ordered Cu3Au nanocrystals > disordered Cu3Au nanocrystals. While the various aforementioned Au−Cu nanocrystals showed a very strong dependence on the shape and atomic ordering (order vs disordered), further enhancements can be achieved through resonant excitation. For example, Neretina and co-workers showed that sapphire-supported Au−Cu triangular nanocrystals could serve as effective photocatalysts for the same model reaction.418 It was found that resonant excitation at 10 mW/cm2 led to a 32-fold enhancement to the reaction rate constant relative to the result of a control experiment conducted in the dark. The advantages of using Au−Cu in this particular example were found to be 3-fold: (i) introducing Cu into the system decreases the amount of Au necessary for catalysis, (ii) the Au−Cu combination is found to be well-suited for the reduction of 4-NP according to binding energy consideration (the volcano plot), and (iii) the degree of alloying can serve as a means to fine-tune the LSPR peak position. 5.2.2. Indirect Plasmonic Sensing. Plasmonic nanocrystals are well-known for their extreme sensitivity toward changes in their local dielectric environment. Therefore, any object brought into their proximity will drastically alter the spectral position and profile of plasmon resonance. This concept has been adopted by researchers interested in H2 gas detection. Because H2 concentrations exceeding 4% can ignite explosively, there is a driving force to fabricate highly sensitive and inexpensive H2 sensors.509 Palladium is known for its ability to absorb H2 gas reversibly by incorporating atomic hydrogen (H) into its lattice, forming a new phase known as palladium hydride (PdH).510 Because higher partial pressures of H2 lead to increasingly distinct electrical and dielectric properties, the absorption/desorption process can be monitored optically. However, because Pd only has a LSPR in the UV region (in the solid, and spherical form), visible sensing is simply not feasible with only Pd. Instead, coupling Pd with a metal that is plasmonically active in the visible region can be used to address this issue. As a proof of concept, optical transduction (via indirect plasmon sensing) of H2 was demonstrated by Alivisatos and co-workers, who positioned a Pd nanodisc near the tip of a Au nanotriangle (all supported on a surface) and cycled the partial pressure of H2. The transition from Pd to PdH was plasmonically transduced by the neighboring Au nanocrystal, resulting in a reversible peak shift upon increasing and decreasing the H2 pressure.511 The work by Alivisatos and co-workers has motivated a number of studies aimed at synthesizing indirect plasmonic H2 sensors based on the core−shell nanocrystal structure, where a plasmonic core is surrounded by a H2-sensitive Pd-shell. Chiu and Huang, for example, implemented seed-mediated growth for the production of Au@Pd core−shell nanocrystals with a variety of shapes including tetrahexahedra, octahedra, and cubes.512 In this system, the Au core served as a plasmonic

Figure 37. (a) Schematic illustration showing the reversible transformation from Au@Pd to Au@PdH upon exposure to H2. (b) Absorbance spectra of Au@Pd octahedra before (gray) and after (blue) exposure to H2. (c) Plot showing the change in LSPR peak position upon H2 cycling, a trend demonstrating that the transformation from Pd to PdH is reversible. Reprinted with permission from ref 512. Copyright 2013 Wiley-VCH.

with thinner Pd shells resulted in the ability to visibly detect H2 absorption. They demonstrated selectivity of the Pd−Au system toward H2-gas by observing negligible changes when exposing the system to O2 and CO. Their H2-cycling tests demonstrated that the H2-absorption and desorption process could be monitored optically (Figure 37b and c). The selectivity and reversibility of the Au@Pd core−shell nanocrystals make them a valuable H2 sensing platform for various applications. A similar system was recently reported by Neretina and co-workers, where Au−Ag@Pd nanotriangles supported on Al2O3 could rapidly sense changes in H2 partial pressure.513 5.3. Magnetic Applications

Bimetallic magnetic nanocrystals have found a bulk of their use in biomedical applications as the theranostic agents.514 In the context of pharmaceutical and biomedical realms, it is favorable to have magnetic particles with both small size and narrow size distributions together with high magnetization values. To this end, the precision of a synthesis is key to controlling the physicochemical aspects of magnetic nanocrystals, especially over surface functionalization, stability, and behavior in biological settings. Because magnetic nanocrystals are generally not biocompatible, a surface coating is typically required. In general, a multistep approach must be taken to obtain bimetallic magnetic nanocrystals suitable for theranostic applications. 5.3.1. Bimetallic Magnetic Nanocrystals as Theranostic Agents. As was previously mentioned, magnetic nanocrystals have many potential biomedical applications, especially as contrast agents for magnetic resonance imaging (MRI). The ideal properties of an MRI contrast agent include high magnetic moments, thermal and chemical stability, biocompatibility, and 10454


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small sizes for navigating through the body. Ferrite nanocrystals, including magnetite (Fe3O4) and maghemite (γ-Fe2O3), have been actively explored as promising MRI contrast agents.515 Their primary shortcoming is that a portion of their magnetic spins cancels each other. On the other hand, mono- and bimetallic nanocrystals (e.g., Fe, Co, FeCo, FePt, and CoPt) offer higher magnetization because there spins do not cancel. For example, FePt have a magnetic moment on the order of ∼1000 emu/cc, comparable to Co (∼1400 emu/cc) and Fe (∼1700 emu/cc) and much higher than commonly used iron oxides (300−400 emu/cc).110 However, the major drawback associated with bimetallic magnetic nanocrystals, with regard to biomedical applications, is their potential oxidation and toxicity.516 Therefore, coating magnetic nanocrystals with shells composed of silica, noble metals, or carbon species is necessary. In one successful example, demonstrated by Dai and coworkers, Fe−Co nanocrystals encapsulated with a single graphitic shell (Fe−Co@GC) were investigated as prospective MRI contrast agents.516 The Fe−Co@GC nanocrystals were produced by (i) heating silica powders loaded with Fe and Co salts to 800 °C under H2, (ii) deposition of carbon by methane chemical vapor deposition, and (iii) dissolving the silica with HF. The resultant structures showed remarkable chemical stability, capable of withstanding HF and resistance to oxidation by air, maintaining a constant Ms over a one-month period. After functionalizing with phospholipid-poly(ethylene glycol), the Fe−Co@GC nanocrystals were injected into a rabbit to demonstrate long-lasting positive contrast (Figure 38a and b). The structures remained stable in circulation for more than 20 min, much longer than conventional Gadolinium-based agents. This same platform was later used by the same group as a multimodal-nanocrystal for drug delivery, MRI imaging, and near-IR-induced hyperthermia (Figure 38c and d).517 The authors identified the graphitic shell as a means for loading the anticancer drug doxorubicin (DOX) via π-stacking. When loaded with DOX and delivered into MCF-7 cells (a human breast cancer line), the Fe−Co@GC-DOX conjugates were found to be slightly less toxic than free DOX. However, when combined with 20 min of near-IR photothermal heating to 43 °C, a drastic increase in toxicity was observed for cancer cells treated with the Fe−Co@GC-DOX. Taken together, the Fe− Co@GC platform serves as a promising multimodal theranostic agent. 5.3.2. Bimetallic Magnetic Nanocrystals for Separation. Perhaps the most thrilling aspect of magnetic nanocrystals is the resultant force generated upon the application of an external magnetic field. This unique handle opens a whole host of potential applications related to external activation, detection, and separation. One of the hallmarks of magnetic nanocrystals is the ability to be manipulated over relatively large distances. In terms of biomedicine, as mentioned previously, the magnetic nanocrystals can be manipulated or activated from outside the body. In the context of catalysis, hybridizing catalytic metals with magnetic metals provides the opportunity to separate and collect the catalyst from the reaction solution upon completion. As was mentioned previously, hybrid nanocrystals containing a magnetic core and a catalytic surface are attractive because the loading of precious metal can be drastically reduced while at the same time promoting a synergy between the surface geometry and electronic structure of the shell. As a third advantage, magnetic@catalytic core−shell nanocrystals can be readily

Figure 38. (a) Schematic illustration of a single FeCo/GC nanoparticle functionalized with phospholipid−poly(ethylene glycol). (b) Magnetic resonance images taken from a rabbit before (left) and 30 min after (right) the injection of a solution containing the 4 nm FeCo/GC nanocrystals, revealing significantly enhanced signal in the aorta, as well as the medulla and cortex of the kidney. (c) Temperature measurements acquired during a 10 min laser irradiation, showing the corresponding temperature rise for free DOX, FeCo/GC-DOX, and FeCo/GC nanoparticles. (d) Bar graph comparing the viability of cells for purely DOX, FeCo/GC-DOX, and a control sample. The drawings and images in (a) and (b) were reprinted with permission from ref 516. Copyright 2006 Nature Publishing Group. The plots in (c) and (d) were reprinted with permission from ref 517. Copyright 2011 American Chemical Society.

forced to the side of the reaction vessel via an external magnetic field by simply placing a magnet next to the reaction vessel (Figure 39a−c). This action provides the opportunity to force the hybrid catalysts to the side-wall of the reaction vessel while the reaction products are extracted. This is especially advantageous for air-sensitive catalytic systems, because the reaction solution can stay in an inert atmosphere during the isolation and recycling process.518 This method also prevents potential contamination issues while elongating the lifetime of the catalyst. Traditional methods, which include centrifugation or filtration, are time-consuming, inefficient, and can potentially influence the activity of the catalyst due to changes in shape or aggregation. Many successful examples have been demonstrated for magnetic metal−oxide-based systems, where precious metal nanocrystals are loaded on magnetic@metal−oxide nanocrystals.519−521 A major concern in the catalyst community is aggregation-induced changes to the shape of nanocrystals. When nanocrystals are brought into close contact with one another, it is often the case that neighboring nanocrystals fuse together, rendering the catalysts either less active or completely inactive. To prevent this from happening, Ma and co-workers prepared Ni@Pd core−shell nanocrystals immobilized on fibrous nanosilica.522 They demonstrated that the nanocrystals could serve as highly efficient catalysts for the reduction of 4NP and the hydrodechlorination of 4-chlorophenol (4-CP) under ecofriendly conditions. Most importantly, the Ni-cores served the dual role of reducing the loading of the precious Pd 10455


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achievements, there is still much work to be done in terms of attaining precisely controlled and reproducible syntheses in parallel with establishing the fundamental science involved in the nucleation and growth processes. The transition from what is now an empirical science to one based on theory and solid understanding requires that experimentalists, theoreticians, and instrumentalists all work together in concert. Among the many hallmarks of nanomaterial synthesis, perhaps the most unfortunate is the well-known issue of irreproducibility. Because of the extreme sensitivity of the nucleation and growth processes to experimental conditions, it is a daunting challenge, or at times even impossible, to draw conclusions from experimental observations. This difficulty has led to conclusions that may be incomplete, ambiguous, or just simply incorrect. For example, early reports related to the mechanistic understanding of nanocrystal nucleation and growth have relied primarily on qualitative assessments involving one-pot approaches, where one or two experimental parameters are varied while others are fixed. While this approach has served as the primary format for investigating nanocrystal nucleation and growth, it often results in purely speculative conclusions. As in any scientific research, moving away from qualitative assessment to those that are more quantitative will ultimately provide the most useful and reliable insights regarding the nucleation and growth of nanocrystals. The focus of this Review has been placed on bimetallic nanocrystals synthesized in the solution phase because it allows for exquisite control over their size, shape, composition, and structure. It is worth pointing out that similar nanocrystals may also be prepared using other approaches. For example, the heterogeneous catalysts (including both mono- and bimetallic) currently used in the industry are most commonly prepared through impregnation of porous supports with metal precursors, followed by reduction/decomposition in the gas phase.525−529 While this approach allows for a good control over the size and elemental composition, it is still very difficult to maneuver the shape, crystallinity, and internal structure of the nanocrystals. Vapor-phase deposition, on the other hand, has been actively explored for generating metal nanocrystals on a solid support, through a mask, or into the cavities of a porous template.530−535 Again, the products are typically characterized by a polycrystalline structure unless extensive annealing is applied, which will cause changes to shape, structure, and elemental distribution due to the acceleration in diffusion. Furthermore, filling the channels or cavities in a porous template through vapor- or solution-phase deposition has been extensively explored by Martin, Mirkin, Stein, and many others for the fabrication of mono-, bi-, and multimetallic nanostructures with a variety of structures and compositions.16,536−539 The major restrictions include the need of a sacrificial template, the polycrystallinity of the as-obtained structure, the limited availability of shape/morphology, and the small quantity of production. Because industrial applications will ultimately require large quantities of the nanocrystals, it is also essential to develop methods for scaling up production. Currently, the most common experimental setting for the synthesis of bimetallic nanocrystals involves the stirring of solution hosted in a vial, beaker, or flask with volumes ranging from 10−100 mL, and the subsequent injection of a metal-containing precursor. While this method may be ideal for making batches of samples on the milligram scale, it can never meet the demand from industriallevel applications that typically require nanocrystals on the

Figure 39. (a) Photograph of a cuvette containing well-dispersed fibrous nanosilica-based Ni@Pd core−shell nanocrystals. (b) TEM image of the Ni@Pd nanocrystals. (c) Photograph of the cuvette containing the nanosilica-based Ni@Pd nanocrystals after placing a magnet adjacent to one of the side walls, demonstrating that the Ni@ Pd nanocrystals could be effectively separated from the solution by an external magnetic field. The black arrow points to the nanomaterials deposited on the wall after the application of an external magnet. (d) Reusability of the Ni@Pd nanocatalysts for the reduction of 4nitrophenol and the hydrodechlorination of 4-chlorophenol. Reprinted with permission from ref 522. Copyright 2015 Elsevier.

while acting as the magnetic component necessary for a convenient recovery and catalyst recycling (Figure 39d). Outside from catalytic recovery, bimetallic magnetic nanocrystals can also readily be functionalized with ligands that can selectively target bacteria. To this end, Xu and co-workers attached vacomycin (Van), an antibiotic, to the surface of FePt nanocrystals.523 This ligand is capable of binding to the terminal peptide sequence, D-Ala-D-Ala, which is present on the surface of Gram-positive bacteria such as Escherichia coli (E. coli). After mixing FePt-Van nanocrystals with a solution containing E. coli, and shaking for 20 min, a 3000 G magnetic field was applied to the solution. They found that the magnetic field induced an irreversible aggregation event, during which a conglomerate of FePt-Van nanocrystals and bacteria could be quickly isolated for analysis. The same group later expanded this platform to other Gram-positive strains including Staphylococcus aureus, S. epidermidis, and a coagulase negative staphylococci, with sensitivity comparable to the polymerase chain reaction (PCR).524

6. CONCLUDING REMARKS Great progress has been made in recent years with regard to the synthesis, characterization, and application of bimetallic nanocrystals. The research has been mainly fueled by the remarkable properties exhibited by bimetallic nanocrystals that can be optimized in terms of composition, in addition to size, shape, and structure. Thanks to the efforts from many groups, we now have access to a vast set of bimetallic nanocrystals well-suited for a broad range of applications. Despite the many 10456


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kilogram level, if not larger.540 Simply increasing the size of the reaction vessel, while maintaining the molar ratio between all reagents, will drastically alter the growth kinetics to a point where the products are no longer comparable. To overcome this limitation, continuous flow and droplet-based systems have recently been introduced.541 Advantages over the conventional batch methods include improved product control and reproducibility, as well as the potential for automation.542 Also imaginable is the use of external stimuli (e.g., radiation, heat, and ultrasound) as a means to carry out a uniform reduction where intensity and frequency can be used as two knobs for kinetic control. Although tremendous achievements have been made on diversifying the shape and composition of nanocrystals, another unfortunate shortcoming of some bimetallic structures is their chemical and thermal instability. Metastable nanocrystals can undergo detrimental composition and shape changes during storage or when used in an application, two types of transformation that drastically change their overall performance. In terms of thermal stability, excessive heating together with long times can transform a nanocrystal from the its original shape or architecture to another that is more thermodynamically favorable. Transformation, while not always unfavorable, generally results from one of the modes of diffusion, including surface diffusion (responsible for changes to shape and facet expression), interdiffusion (responsible for changes to composition and structure), and the Kirkendall effect (responsible for changes to both composition and structure). Resistance to structural reconfiguration can be resolved through the incorporation of metals with high melting points (e.g., Rh, Ir, and Ru) or through the use of nanocrystals whose composition and shape are already at or near thermodynamic equilibrium. As for chemical stability, the more reactive element in bimetallic nanocrystals tends to dissolve or leach into the solution over time. For example, it is well-known that Pt−M (M = Ni, Co, and Fe) bimetallic nanocrystals are prone to compositional changes due to leaching and corrosion during catalytic operation. This is particularly true for cathode electrocatalysts that are exposed to potentials up to +1.1 V vs reversible hydrogen electrode (RHE) during the electroreduction of oxygen.543 In terms of Pt, its solubility is highly dependent on the potential, potential dynamics, temperature, and pH, with clear mechanistic details still unknown.544 While Pt itself is thermodynamically stable toward dissolution in a broad pH and potential window, it is susceptible to dissolution at potentials higher than 0.85 V vs RHE and at pH values lower than 2 (at 25 °C), which are actually typical conditions for PEM fuel cell catalysts.544 Therefore, the key design components for Pt-based nanocatalysts should also include resistance to degradation. While the study of bimetallic systems serves as the obvious next step after the mastery of monometallic nanocrystals, a number of groups have already pushed forward to multimetallic nanocrystals, making use of the additional scope that three or more elements have to offer. The advantage here is the even wider property landscape and potential for unique physicochemical properties. However, the impending challenge is to sort out the fundamental mechanisms involved in their formation and also the how and why each component adds to, or takes away from, the overall performance. It should be noted that perfect mechanistic clarity is hardly established for even the simpler monometallic nanocrystals, and, therefore, adding two or more components will greatly increase the

complexity of both the synthesis and also the role that the various metals play in the respective application. Despite the challenge, several groups have recently reported some very promising results. It seems that adding a third component to already successful bimetallic catalytic platforms, for example, the Pt3Ni system, is an effective method for achieving even greater enhancements. By decorating Pt3Ni nanocrystals with Au, Li and co-workers demonstrated that [email protected] could serve as a superior catalyst to Pt3Ni, Pt3Ni@ Au12, and Pt3Ni@Au8 for the reduction of nitrobenzene.545 In another example, Sun and co-workers demonstrated the synthesis of FePtAu nanocrystals through a combination of coreduction and thermal decomposition.546 They discovered that the presence of Au in FePtAu facilitates FePt structural transformation from fcc to fct, effectively promoting the activity toward FAO. In another application, as compared to the bimetallic FePt nanocatalysts, the fct FePtAu nanocrystals (i) showed higher CO poisoning resistance, (ii) achieved a mass activity of 2809.9 mA/mg Pt, (iii) retained 92.5% of this activity after a 13 h durability test, and (iv) are among the most active and durable catalysts ever reported for FAO. Several groups have even reported trimetallic nanoframes. For example, Li and co-workers demonstrated the synthesis of a Pt−Ni nanoframe structure with an overall truncated octahedral shape where Au occupied the corner sites.547 The Pt−Ni−Au nanoframes showed greatly enhanced catalytic activity toward the electrooxidation of methanol. From these and many other examples,548−552 there is an unlimited number of directions possible for enhancing the properties of metal nanocrystals. Literally, the scope of research on bi- and multimetallic nanocrystals will only be limited by our imagination.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Kyle D. Gilroy received his B.S. degree in biomedical physics from the College of New Jersey in 2011 and Ph.D. degree in engineering from Temple University in 2015. He joined the Xia group as a postdoctoral fellow in August 2015. His research interests include the development of synthetic protocols for producing novel inorganic nanomaterials for applications in catalysis, renewable energy, and sensing. Aleksey Ruditskiy is a Ph.D. candidate in chemistry at Georgia Tech, and has been supported by a National Science Foundation Fellowship since February 2013. He received his B.E. in chemical engineering from the City College of New York in 2012, where he worked with Professor Ilona Kretzschmar on the electromagnetic assembly of Janus particles. His research interests focus on the design and synthesis of novel inorganic nanomaterials for application in photonics and electronics. Hsin-Chieh Peng received his B.S. and M.S. degrees in chemistry from National Taiwan University in 2006 and 2008, respectively. He received his Ph.D. degree in chemistry from Georgia Tech in December 2015 and currently continues his research as a postdoctoral fellow in the Xia group. His research focuses on the synthesis of noblemetal nanocrystals for exploration of their shape-dependent catalytic properties. 10457


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Dong Qin was born and raised in Shanghai, China. Her academic records include a B.S. degree in chemistry from Fudan University (1990), a Ph.D. in physical chemistry from University of Pennsylvania (1996) with Professor Hai-Lung Dai, a postdoctoral stint in materials chemistry at Harvard University with Professor George M. Whitesides, and an MBA from the University of Washington (Seattle) in 2002. She works in research fronts that connect the traditional fields with an engineering approach to study peculiar properties and phenomena emerging from materials and systems at the nanoscale. She is the site director and principal investigator of NSF-supported National Nanotechnology Infrastructure Network (NNIN) at Washington University in St. Louis, with a goal to pursue national leadership in areas of public health, environment, renewable energy, and sustainability for the future. Younan Xia studied at the University of Science and Technology of China (B.S., 1987) and University of Pennsylvania (M.S., 1993), and received his Ph.D. from Harvard University in 1996 (with George M. Whitesides). He started as an assistant professor of chemistry at the University of Washington (Seattle) in 1997 and joined the department of biomedical engineering at Washington University in St. Louis in 2007 as the James M. McKelvey Professor. Since 2012, he holds the position of Brock Family Chair and GRA Eminent Scholar in Nanomedicine at Georgia Tech.

ACKNOWLEDGMENTS The work from the Xia group was supported in part by the NSF (DMR-0804088, DMR-1215034, DMR-1506018, and CHE1505441), NCI (R01 CA138527), an NIH Director’s Pioneer Award, and startup funds from the Georgia Institute of Technology and Washington University in St. Louis. The work from the Qin group was supported in part by the NSF (CHE-1412006), start-up funds from the Georgia Institute of Technology, and a 3M Nontenured Faculty Award. We are extremely grateful to many collaborators for their stimulating discussions and valuable contributions related to this research topic over the past decade. REFERENCES (1) Reardon, A. C. Metallurgy for the Non-Metallurgist, 2nd ed.; ASM International: OH, 2011; p 75. (2) Busca, G. Heterogeneous Catalytic Materials: Solid State Chemistry, Surface Chemistry and Catalytic Behaviour; Elsevier B. V.: Amsterdam, The Netherlands, 2014. (3) Zereini, F.; Wiseman, C. L. S. Platinum Metals in the Environment; Springer-Verlag: Berlin, Heidelberg, 2015. (4) http://www.platinum.matthey.com/documents/market-datatables/platinum/pdf-2004-to-2013.pdf. (5) Xia, Y.; Xiong, Y. J.; Lim, B.; Skrabalak, S. E. Shape-Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics. Angew. Chem., Int. Ed. 2009, 48, 60−103. (6) Hutter, E.; Fendler, J. H. Exploitation of Localized Surface Plasmon Resonance. Adv. Mater. 2004, 16, 1685−1706. (7) Mayer, K. M.; Hafner, J. H. Localized Surface Plasmon Resonance Sensors. Chem. Rev. 2011, 111, 3828−3857. (8) Yu, Y.; Zhang, Q.; Yao, Q.; Xie, J.; Lee, J. Y. Architectural Design of Heterogeneous Metallic Nanocrystals Principles and Processes. Acc. Chem. Res. 2014, 47, 3530−3540. (9) Ferrando, R.; Jellinek, J.; Johnston, R. L. Nanoalloys: From Theory to Applications of Alloy Clusters and Nanoparticles. Chem. Rev. 2008, 108, 845−910. (10) Carbonea, L.; Cozzoli, P. D. Colloidal Heterostructured Nanocrystals: Synthesis and Growth Mechanisms. Nano Today 2010, 5, 449−493. 10458


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