Recent progress in intermetallic nanocrystals for electrocatalysis: From binary to ternary to high-entropy intermetallics

Abstract

Developing sustainable and clean energy-conversion techniques is one of the strategies to simultaneously meet the global energy demand, save fossil fuels and protect the environment, in which nanocatalysts with high activity, selectivity and durability are of great importance. Intermetallic nanocrystals, featuring their ordered atomic arrangements and predictable electronic structures, have been recognized as a type of active and durable catalysts in energy-related applications. In this minireview, the very recent progress in the syntheses and electrocatalytic applications of noble metal-based intermetallic nanocrystals is summarized. Various synthetic strategies, including the conventional thermal annealing approach and its diverse modifications, as well as the wet-chemical synthesis, for the construction of binary, ternary and high-entropy intermetallic nanocrystals have been discussed with representative examples, highlighting their strengths and limitations. Then, their electrocatalytic applications toward oxygen reduction reaction, small molecule oxidation reactions, hydrogen evolution reaction, CO₂/CO reduction reactions, and nitrogen reduction reaction are discussed, with the emphasis on how the ordered intermetallic structures contribute to the enhanced performance. We conclude the minireview by addressing the current challenges and opportunities of intermetallic nanocrystals in terms of syntheses and electrocatalytic applications.

INTRODUCTION

Concerning the ever-increasing global energy demand, the limited fossil fuel reserves, and the environmental pollution issues such as greenhouse gas emissions caused by the excessive usage of fossil fuels, it is highly imperative to develop sustainable and clean energy-conversion techniques. For example, fuel cells, powered by fuels such as hydrogen, methanol and ethanol, can produce clean electricity with high energy-conversion efficiency, high energy density and environmental friendliness.¹ Electrolyzers enable the production of high-purity H₂ under mild conditions through water splitting with renewable electricity.² For these clean energy devices, electrochemical hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR) and small molecule oxidation reactions are of significant importance. Furthermore, greenhouse gas CO₂ and/or earth-abundant N₂ can be converted into value-added chemicals such as methane, formate, ethylene, ammonia and urea through electrochemical CO₂ reduction and/or N₂ reduction under ambient conditions.^3–8 In order for all these electrochemical reactions to take place with satisfying kinetics, electrocatalysts with high activity, selectivity and stability are normally required. For most of these reactions, noble metals, especially the platinum-group metals, are the most efficient catalysts among other elements. However, considering the scarcity, high cost and the ambition to further boost the catalytic performance, great research efforts have been devoted to exploring high-performance metal-based or even non-metal-based nanocatalysts.^9–11

The catalytic performance of metal-based nanocatalysts can be modulated by adjusting their electronic structures, which can be achieved via tuning their size, composition, shape, exposed facet, crystal phase, defect, etc.^12–15 In particular, for binary or even multinary metallic systems, manipulating the degree of ordering of their constituent elements has been proven as an effective approach to tune the catalytic performance.^16–18 For disordered alloys, the constituent elements can randomly occupy any crystallographic site without local preference. The probability of occupancy by an element is determined by the stoichiometric ratio of the alloy. In contrast, for ordered intermetallics, every crystallographic site is expected to be occupied by a definite element, showing atomic ordering. Thus, the crystal structure of the intermetallic materials, the orbital bonding of metals, and the stoichiometry of each constituent element are explicit, enabling the predictable control of the electronic structures for optimizing their catalytic performance.¹⁹

To date, intermetallic nanocrystals with long-range atomic ordering have been shown to outperform their disordered counterparts toward many reactions,^17,18 which could be ascribed to several mechanisms: (i) Upon transforming disordered alloys to their intermetallic counterparts, bond length and coordination environment could be altered, leading to change of the electronic structures and thus the position of the d-band center, which could directly affect the adsorption and desorption abilities of reactants and/or intermediates on the catalyst surface, thereby affecting the catalytic performance.^20,21 (ii) Intermetallic structures have a more negative enthalpy of formation and thus stronger interatomic bonding compared to their disordered counterparts with similar composition and morphology, leading to enhanced resistance to corrosion and excellent catalytic durability in harsh chemical environments. In contrast, non-noble metals in disordered alloys could undergo severe oxidation and dissolution after long-term operation, especially in acidic environment, leading to the destruction of catalytic active sites and thus loss of catalytic activity.^19,22 For example, the L1₂ Pt₃Zr nanoparticles exhibited tremendously enhanced stability than the fcc counterparts, showing a minimal Cr leaching (13.5%) after 4 weeks immersion in acidic electrolyte and a minimal loss of ORR activity (14.7%) after 5000 cycles, both of which were smaller than those of the fcc Pt₃Zr nanoparticles (22.3% and 23.4%, respectively).²³ (iii) For both ordered intermetallics and disordered alloys, leaching of the less-noble metal from the near-surface region results in the formation of a core-shell structure, in which a thin shell composed of the more noble metal is coated over the intermetallic/alloy core. The lattice mismatch between core and shell, induced by the difference in crystal structure or lattice parameter, could cause compressive or tensile strain, thereby affecting the position of the d-band center and the binding energy between the catalyst surface and the reaction species.^24,25 Due to their structural difference, ordered intermetallics and disordered alloys may exhibit distinct degrees of metal leaching, as well as different underlayer and surface atomic structures, and therefore dissimilar strain effects. For example, when ordered and disordered PtCu₃ nanoparticles were subject to acid leaching in 0.1 mol/L HClO₄, after dealloying for 12 h, in the ordered sample, the coordination number of Cu around Pt was 15%–30% higher than that in the disordered counterpart, resulting in more pronounced ligand and/or strain effects and 23%–37% higher ORR activities than the disordered one.²⁶ (iv) Intermetallic nanocrystals with tunable stoichiometries and crystal structures enable the creation of isolated active sites resembling single-atom catalysts, as well as active-site ensembles that are composed of more than one catalytic active metal. Such concept has been widely used in selective heterogeneous catalysis^27–30 and electrocatalysis.^31,32 For example, by constructing Pd-In intermetallic structures, the contiguous Pd sites could be isolated into single-Pd sites or Pd ensembles.³⁰ It was theoretically and experimentally shown that on the (110) facet of the intermetallic PdIn, where Pd atoms were present as single-atom sites, the selectivity for acetylene hydrogenation to ethylene was higher (92%) than that on the (111) facet of the intermetallic Pd₃In, where Pd atoms were present as trimer sites (21%).

Although several review papers on intermetallic nanocatalysts have been reported,^{16–19,33–35} a timely review summarizing the very recent progress in this field is still of high significance, considering its fast development. In this minireview, very recent progress in the syntheses and electrocatalytic applications of noble metal-based binary, ternary, and high-entropy intermetallic (HEI) nanocrystals is summarized (Scheme 1). Diverse synthetic strategies are first introduced, including the conventional thermal annealing approach and its various modifications, as well as the wet-chemical synthetic method, with highlight on their strengths and limitations. Then, the electrocatalytic applications of intermetallic nanocrystals toward ORR, small molecule oxidation reactions, HER, CO₂/CO reduction reactions, and nitrogen reduction reaction are summarized. Finally, current challenges and future opportunities in this research field are discussed.

Scheme 1. Overview of this review.

SYNTHESIS OF BINARY INTERMETALLIC NANOCRYSTALS

The crystal structures of intermetallic nanocrystals are mainly derived from three types of monometallic structures, i.e., face-centered cubic (fcc), body-centered cubic (bcc), and hexagonal close-packed (hcp). As shown in Figure 1, typical fcc-derived intermetallic structures include L1₀ (CuAu-type, space group P4/mmm, e.g., PtFe,³⁶ PtCo,³⁷ PtZn,³⁸ PdZn³⁹), L1₁ (PtCu-type, space group R$$\bar{3}{\rm{m}}$$, e.g., PtCu⁴⁰), and L1₂ (Cu₃Au-type, space group Pm$$\bar{3}$$m, e.g., Pt₃Co,^31,41 Pt₃Zn,⁴² Pt₃Sn,⁴³ Pd₃Pb,^44,45 Au₃M (M = Fe, Co, Ni)⁴⁶). A typical bcc-derived intermetallic structure is B2 (CsCl-type, space group Pm$$\bar{3}{\rm{m}}$$, e.g., PdCu,⁴⁷ FeRh⁴⁸). A typical hcp-derived intermetallic structure is B8₁ (NiAs-type, space group P6₃/mmc, e.g., PtBi,^49,50 PtPb^25,51). For the reported noble metal-based intermetallic nanocrystals, L1₀ and L1₂ are the most commonly observed structures. The L1₀ structure can be viewed as the tetragonal distortion of the fcc structure, in which the atomic layers of two elements with a stoichiometric ratio of 1:1 are stacked alternately along the c-axis. In the L1₂ structure, two elements with a stoichiometric ratio of 3:1 are located in the sites of an fcc lattice, in which the element with a higher atomic ratio is located at the face-centered sites, while the other is located at vertices. These intermetallic structures are identified mainly by two techniques, i.e., X-ray diffraction (XRD) and atomic-resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). XRD identifies the phase of crystalline materials, in which the characteristic ordering peaks at relatively small angles are normally present for the ordered intermetallic structures, while absent for the disordered counterparts. In HAADF-STEM images, the local ordered arrangement of the constituent elements in an intermetallic structure along a specific zone axis can be directly visualized based on their different atomic numbers which give different Z contrasts. Element with a larger atomic number appears as the brighter spot, while the one with a smaller atomic number appears as the darker spot. Nevertheless, the Z contrasts may become ill-defined when the atomic numbers of both elements are quite close, necessitating other characterizations to confirm the ordered atomic arrangement.

Figure 1. Typical intermetallic structures, i.e., L10, L11, and L12 derived from fcc, B2 derived from bcc, B81 derived from hcp. fcc, face-centered cubic; hcp, hexagonal close-packed.

Conventional approach—thermal annealing

The past decades have witnessed tremendous development in synthesizing binary intermetallic nanocrystals with controllable degree of ordering, composition, size, morphology, uniformity and architecture via diverse synthetic methods.^{16–19,33–35} Among them, thermal annealing at high temperatures to induce the formation of ordered intermetallic structures is the conventional approach. Briefly, metal precursors are first impregnated onto a substrate. After drying, the powders are placed in a furnace for thermal annealing at a specific temperature for a specific period of time.^52–55 For example, [Fe(bpy)₃]²⁺[PtCl₆]²⁻ (bpy = bipyridine) precursors were first prepared and impregnated on graphene oxide (GO).⁵⁶ After drying, the mixture was annealed at 700°C under Ar, leading to the formation of intermetallic L1₀ PtFe nanoparticles loaded on reduced graphene oxide (rGO), whose ordered structure was visualized in the HAADF-STEM image, showing alternating Pt and Fe atomic columns with bright and dark intensities, respectively (Figure 2A). Meanwhile, a thin N-doped carbon layer, derived from the bipyridine ligand, was formed over the nanoparticle surface during annealing, serving as a protective shell to prevent particle aggregation. Alternatively, metal precursors are first reduced at relatively lower temperatures to form disordered alloys, which are then loaded on the substrate and subjected to high-temperature thermal annealing to induce disorder-to-order transition.^31,57,58 For example, disordered PtM₃ (M = Co, Fe) nanoparticles were first prepared by reducing Pt(acac)₂ and Co(acac)₂/Fe(acac)₃ precursors in organic solvents, which were then supported on carbon black.⁵⁹ After drying, the supported nanoparticles were annealed at 180°C in air to remove the capping ligands, and then 400°C under H₂/Ar. The disordered-to-ordered transition was induced by further annealing at 600°C under H₂/Ar, resulting in the formation of intermetallic L1₂ PtCo₃ and PtFe₃ nanoparticles, whose ordered structures were confirmed by the appearance of the (100) and (110) ordering peaks in the XRD patterns (Figure 2B).

Figure 2. Intermetallic nanocrystals prepared by the conventional thermal annealing method. (A) Scheme showing the synthesis of intermetallic PtFe nanoparticles supported on rGO (upper panel), and the HAADF-STEM images (lower panel). Reproduced with permission,56 copyright 2020 American Chemical Society. (B) Scheme showing the synthesis of intermetallic PtM3 (M = Co, Fe) nanoparticles supported on carbon black (upper panel), and the XRD patterns (lower panel). Reproduced with permission,59 copyright 2019 Wiley-VCH. rGO, reduced graphene oxide; XRD, X-ray diffraction.

In this method, three parameters are highly important, including the substrate, annealing temperature and annealing period. High-surface-area substrates can enlarge the interparticle distance and provide a support for the migration and diffusion of metal atoms, such that particle aggregation can be mitigated to some extent during annealing. Various two-dimensional (2D) substrates have been used for this purpose, such as carbon black,^54,59 GO,^56,60,61 and metal oxides.⁶² The annealing temperature and period are determinant parameters for manipulating the degree of ordering of intermetallic nanocrystals. Normally, higher temperature and longer annealing period could overcome the kinetic energy barrier and promote atom diffusion, resulting in the formation of intermetallic nanocrystals with higher degree of ordering. The annealing temperature is set at or above the disorder-to-order transition temperature, which is different for different bimetallic systems. To highlight, due to the extremely reduced size of nanoparticles compared to bulk crystals, quantum-size effect arises, which can influence the thermodynamic and kinetic stabilities of many intermetallic materials.⁶³ Besides, nanoparticles with a high surface-area-to-volume ratio can facilitate atom diffusion and promote the ordering transformation.¹⁹ Therefore, the disorder-to-order transition temperatures shown in the binary phase diagrams of bulk crystals can only be taken as a guidance for the nano-systems, while the real transition temperatures are typically lower. For example, the disorder-to-order transition temperature for Pt-Co nanoparticles with size of 2.4–3 nm was 175°C–325°C, while it increased to approximately 825°C for the bulk counterpart.⁶⁴ In addition to size, the shape of nanocrystals^64,65 and the presence of vacancy^66,67 also affect the disorder-to-order transition temperature. It is generally believed that a higher density of vacancies promotes the atom diffusion process.⁶⁶

Despite the effectiveness and widespread applications of the thermal annealing method for synthesizing intermetallic nanocrystals, the disadvantages of this method are also evident.¹⁸ First, high temperature may lead to particle aggregation and sintering, resulting in the formation of large particles with broad size distributions. Second, high temperature disables control over the shape of intermetallic nanocrystals, because particles with anisotropic shapes and high-index facets are prone to undergo shape reconstructions in an effort to minimize the total surface energy.¹⁹ All these lead to the reduction of active sites and surface areas of the nanocatalysts, and therefore the catalytic performance deteriorates. Besides, high temperature also disables the synthesis of intermetallic nanocrystals whose structures are only stable at lower temperatures.

Modifications to the thermal annealing method

To solve the above-mentioned problems, various strategies have been developed as modifications to the conventional thermal annealing method. First, protective shells, such as silica,^38,40 MgO,^22,36 and carbon,⁶⁸ can be coated over the disordered alloy nanoparticles before the disorder-to-order transition, functioning to protect the nanoparticles from aggregation during annealing at high temperatures. For example, silica-protected intermetallic L1₀ PtZn nanoparticles loaded on multiwalled carbon nanotubes exhibited size of approximately 3.2 nm, while those without the silica protection had larger size of approximately 27 nm, demonstrating the protective function of silica in constraining the particle size during annealing.³⁸ Polydopamine, as a polymer, was coated over the caron-supported disordered fcc PtFe nanoparticles.⁶⁸ After thermal annealing at 700°C, intermetallic L1₀ PtFe nanoparticles coated with thin layers of N-doped carbon shell derived from polydopamine were formed (Figure 3A). The resultant intermetallic nanoparticles had smaller size of only approximately 6.5 nm, which was similar to the size of disordered nanoparticles before annealing, thanks to the protective effect of the carbon shell. However, these protective shells may limit the atom mobility to some extent, thus impeding the complete disorder-to-order transition. Besides, most of the protective shells could block the catalytic active sites and limit the mass transport of reactant molecules. Thus, an addition step is normally required to remove these protective shells before catalytic testing, which may add complexity, increase cost, and limit the practical applications. Nevertheless, thin and porous protective shells that are permeable to the reactant molecules do not have such problem, such as the in-situ formed N-doped carbon shell with thickness smaller than 1 nm that is permeable to the O₂ molecule during the ORR test,⁶⁸ and thus shell removal is unnecessary.

Figure 3. Intermetallic nanocrystals prepared by the modified thermal annealing methods. (A) Scheme showing the synthesis of carbon-supported and N-doped carbon-coated intermetallic PtFe nanoparticles (left panel), and the HAADF-STEM image of a PtFe nanoparticle (right panel). Reproduced with permission,68 copyright 2015 American Chemical Society. (B) Scheme showing the synthesis of intermetallic PtM nanoparticles through high-temperature S-anchoring approach (left panel), and the S L-edge XANES spectra (right panel). Reproduced with permission,69 copyright 2021 AAAS. (C) Scheme showing the synthesis of intermetallic RhZn nanoparticles within the mesoporous silica SBA-15 framework. Reproduced with permission,70 copyright 2021 American Chemical Society. (D) Scheme showing the synthesis of intermetallic Pt3Fe nanoparticles within the in-situ formed KCl matrix. Reproduced with permission,71 copyright 2012 American Chemical Society. XANES, X-ray absorption near edge structure.

Second, substrates having a strong interaction with the supported nanoparticles can be used. Such a strong metal-support interaction can effectively immobilize the particles and prevent them from aggregation during high-temperature annealing, enabling the synthesis of size-controlled intermetallic nanoparticles.⁷² S- or N-containing substrates are normally used for this purpose, due to the strong interactions between metals and S/N elements.^69,73 For example, S-doped carbon substrate was first prepared, onto which H₂PtCl₆ and other metal salts were wet-impregnated.⁶⁹ Upon annealing under H₂ at 800°C–1100°C, a library of intermetallic Pt-based nanoparticles with size of sub-5 nm were prepared, including Pt alloyed with the early transition metals (Pt₃Sc, Pt₃Ti, Pt₃V, Pt₃Cr, and Pt₃Zr), late transition metals (Pt₃Mn, Pt₃Fe, PtFe, Pt₃Co, PtCo, PtNi, PtCu, PtCu₃, Pt₃Zn, and PtZn), and p-block metals (Pt₃Al, Pt₃Ga, Pt₃Ge, Pt₃In, and Pt₃Sn). Size control was achieved due to the strong chemical interaction between Pt and S, as demonstrated by the presence of Pt-S bond in the S L-edge X-ray absorption near edge structure spectra (Figure 3B). As another example, the strong interaction between Mo and C was utilized to synthesize carbon-supported intermetallic twinned Pt₂Mo nanoparticles with size of 3–5 nm.⁷⁴ It was experimentally shown that H₂PtCl₆ was preferentially reduced to Pt nanoparticles at 800°C, followed by the diffusion of Mo into Pt nuclei at 1000°C. The key to the formation of intermetallic structure was the slow diffusion rate of Mo into Pt, which was induced by the strong Mo-C interaction. If the carbon substrate was replaced with MgO, due to the weak interaction between MgO and Mo, Mo tended to undergo self-nucleation, forming the undesired Mo particles. Furthermore, some substrates can also react with the impregnated metal precursors for the formation of intermetallic nanoparticles. For example, through impregnating Pt on two types of MXenes, i.e., Ti₃C₂T_x and Nb₂CT_x, followed by reduction at 550°C under H₂, intermetallic L1₂ Pt₃Ti and Pt₃Nb nanoparticles loaded on MXenes were formed, respectively, via the reaction between Pt and the surface Ti (or Nb) atoms.²¹ The reaction was driven by the relatively weak metal-carbon bonds in MXenes and the thermodynamic stability of the Pt₃M (M = Ti, Nb) compounds.

Third, instead of using 2D sheets as substrates, three-dimensional (3D) frameworks can be used to confine the growth of nanoparticles inside the pores, thereby preventing particle aggregation and enabling size control. Mesoporous carbon,^24,75 silica,^70,76–80 zeolite,⁸¹ and salt matrix^23,71,82,83 have been reported as the 3D frameworks. For example, mesoporous silica (SBA-15) was used as the substrate, onto which Rh and Zn precursors were impregnated.⁷⁰ Upon thermal annealing under H₂ at 500°C, intermetallic RhZn nanoparticles with size of approximately 9.6 nm were formed, which were evenly distributed within the pores of SBA-15 frameworks (Figure 3C). During the synthesis of Pt₃Fe nanoparticles, the by-product KCl, in situ formed from the metal chloride precursors and the reducing agent KEt₃BH, served as a protective matrix to confine the nanoparticles and prevent them from aggregation during the subsequent thermal annealing process at approximately 600°C, leading to the formation of intermetallic Pt₃Fe nanoparticles with size of approximately 4 nm (Figure 3D).⁷¹ The as-obtained Pt₃Fe nanoparticles were then released from the KCl matrix and supported on carbon black by dissolving KCl in the mixture of ethylene glycol and water.

Furthermore, metal-containing 3D nanostructures can also be used as sacrificial templates, onto which another metal is impregnated. Upon thermal annealing, metals from the 3D nanostructures will form chemical bond with the loaded metals, leading to the formation of intermetallic nanoparticles.^84,85 For example, ZnO nanorods coated with polydopamine were used as the sacrificial template, onto which the Pt precursor was loaded.⁸⁵ Upon reduction under H₂ at 800°C, intermetallic L1₀ PtZn nanoparticles supported on hollow N-doped carbon nanotubes were formed, where Zn came from the sacrificial template ZnO, and the N-doped carbon nanotubes came from the decomposition of polydopamine, functioning to restrict the migration of Pt atoms during high-temperature annealing. The remaining ZnO template was subsequently removed through acid wash (Figure 4A). Metal-organic frameworks, due to the presence of metal cations and cavities in their structures, have also been widely used as the sacrificial templates.^86,87 For example, ZIF-8, with Zn being its skeleton metal, was used as the sacrificial template, onto which metals such as Pt⁸⁶ or Pd⁸⁷ were incorporated. Upon annealing, Zn diffused inside the Pt or Pd nanoparticles, leading to the formation of intermetallic L1₀ PtZn nanoparticles with size of approximately 3.2 nm (Figure 4B) and PdZn nanoparticles with size of sub-2 nm, respectively.

Figure 4. Intermetallic nanocrystals prepared by the modified thermal annealing methods. (A) Scheme showing the synthesis of intermetallic PtZn nanoparticles loaded on N-doped carbon nanotubes by using ZnO nanorods as the sacrificial template. Reproduced with permission,85 copyright 2019 Nature Publishing Group. (B) Scheme showing the synthesis of intermetallic PtZn nanoparticles by using ZIF-8 as the sacrificial template (left panel), and the HAADF-STEM image of a PtZn nanoparticle (right panel). Reproduced with permission,86 copyright 2022 Wiley-VCH. (C) Scheme showing the synthesis of intermetallic Pd3Pb nanoparticles with smaller and larger sizes via rapid Joule heating and traditional annealing approaches, respectively (left panel), and the HAADF-STEM image of a Pd3Pb nanoparticle (right panel). Reproduced with permission,88 copyright 2022 American Chemical Society. (D) Scheme showing the synthesis of orthorhombic PdSn and monoclinic Pd3Sn2 via rapid quenching and natural cooling processes, respectively (upper panel), and the HAADF-STEM image of a PdSn nanoparticle (lower panel). Reproduced with permission,89 copyright 2021 Wiley-VCH.

Forth, instead of directly conducting thermal treatment on substrate-supported metal precursors or disordered alloys, heterostructures, such as core-shell^90–94 and dimer⁹⁵ structures, can be prepared first via wet-chemical methods, which are subsequently transformed into intermetallic nanoparticles via thermal annealing. Metal oxides are usually synthesized as the shell, such that during thermal annealing in the reductive environment, rich oxygen vacancies can be generated, promoting the inter-diffusion between core and shell.^91,92,95 For example, intermetallic L1₀ PdFe nanoparticles were prepared by synthesizing Pd@FeO_x core-shell structures first, followed by annealing under H₂ at 600°C.⁹¹ In the reductive environment, the FeO_x shell was reduced to metallic Fe, with the simultaneous generation of rich oxygen vacancies. The vacancies facilitated the inter-diffusion between Pd core and Fe shell. As another example, intermetallic L1₀ PtFe nanoparticles were prepared by annealing the pre-synthesized dumbbell-like PtFe-Fe₃O₄ nanoparticles under H₂ at 700°C.⁹⁵ The oxygen vacancies created during the Fe₃O₄ reduction process could facilitate the inter-diffusion between Fe and Pt.

Other modifications to the conventional thermal annealing method have also been developed to controllably synthesize intermetallic nanoparticles with desired size and structure. For example, instead of the conventional oven drying process, a hydrogel freeze drying approach was developed before thermal annealing, for the synthesis of a series of intermetallic Pt₃M (M = Mn, Cr, Fe, Co, etc.) nanoparticles supported on rGO.⁹⁶ The freeze drying process could retain the 3D porous structure of the GO hydrogel, such that metal precursors could be confined and immobilized on GO, enabling the formation of intermetallic nanoparticles with small size of approximately 3 nm. Besides, the heating and cooling rates in the thermal annealing process can be controlled. Rapid heating rate (on the order of 10⁵ K/s) achieved by Joule heating has been demonstrated to promote the generation of rich vacancies in the initially formed disordered structures, thereby facilitating atomic ordering and the formation of carbon nanofiber-supported L1₂ Pd₃Pb nanoparticles with small size of approximately 6 nm within 60 s.⁸⁸ In contrast, traditional furnace heating with a slower heating rate and longer annealing time (3 h) resulted in large intermetallic nanoparticles with size of approximately 85 nm (Figure 4C). On the other hand, rapid quenching process has been shown to enable the capture of metastable phases, such as PdSn intermetallic nanoparticles with a metastable FeAs-type orthorhombic phase, whereas the thermodynamically stable monoclinic Pd₃Sn₂ phase was obtained through the natural cooling process (Figure 4D).⁸⁹ Very recently, a general small molecule-assisted impregnation strategy was reported to prepare 18 intermetallic Pt-based nanoparticles supported on carbon black.⁹⁷ These small molecules contain heteroatoms, i.e., O, N, or S, such that they were coordinated with Pt during the impregnation process and thermally converted into heteroatom-doped graphene layers coated over nanoparticles during high-temperature annealing, thereby suppressing particle aggregations. By utilizing the sodium thioglycolate additive, several intermetallic nanoparticles with small sizes of approximately 5 nm were obtained, while those without sodium thioglycolate exhibited larger sizes of 8.2–12.2 nm.

Wet-chemical synthesis

Alternative to the conventional thermal annealing method, wet-chemical synthesis, featuring its strong ability to prepare nanocrystals with highly controlled size, shape, composition, phase, etc., has been widely explored to directly synthesize intermetallic nanocrystals in solution.³⁵ For wet-chemical synthesis of noble metal-based intermetallic nanocrystals, those containing low-melting-point metals (such as Bi-271°C, Pb-327°C, Sn-232°C, and Zn-419°C) are more accessible, as compared to those containing high-meting-point metals (such as Fe-1538°C, Co-1495°C, and Cu-1083°C).⁹⁸ This is partially because the mobility of metallic atoms is generally high at temperatures close to their melting points. As a result, when low-melting-point metals are incorporated, atomic diffusion is sufficient to induce ordering transformation at the reaction temperatures. In contrast, for high-melting-point metals, atomic diffusion may be limited by the reaction temperatures that are severely restricted by the solvent boiling points, thereby impeding the formation of intermetallic nanocrystals with high degree of ordering.⁹⁸ Besides, differences in crystals structure between noble metals (fcc or hcp) and metals such as Bi (rhombohedral), Sn (body-centered tetragonal), Sb (rhombohedral) suppress the formation of solid-solutions, while enable the direct synthesis of ordered structures according to the phase diagrams.⁹⁸

Wet-chemical synthesis of intermetallic nanocrystals can be categorized into two types, i.e., co-reduction of metal precursors and seed-mediated growth. Co-reduction refers to the reduction of the constituent metal precursors in a one-pot reaction.^99,100 Since each metal has its own standard reduction potential, their reduction rates will be different. If the reduction potentials of metal ions are similar or a strong reducing agent is used, they will be reduced simultaneously. For example, NaBH₄, as a strong reducing agent, has been applied to synthesize several intermetallic nanocrystals via co-reduction, such as Pd-M (M = Cu, Zn, Ga, Ge, Sn, Pb, Cd, and In),¹⁰¹ AuCu,¹⁰² and PtBi₂.¹⁰³ Despite its effectiveness, the usage of strong reducing agents hinders the shape control of intermetallic nanocrystals, due to the rapid nucleation and growth rates. On the other hand, if the difference between their reduction potentials is large, and a relatively mild reducing agent is used, metal ions with a more positive reduction potential will be reduced first, serving as the seeds, onto which the slow-reducing metals are reduced, followed by their inter-diffusion to form intermetallic nanocrystals. For example, L1₂ crenel-like Pt₃Co hierarchical nanowires with high-index facets and Pt-rich surfaces were synthesized by co-reducing Pt(acac)₂ and Co(acac)₃, where Pt nanowires were formed first followed by inter-diffusion between the newly formed Co atoms and Pt.⁴¹ In addition, the reduction rates of the constituent metal ions can be further manipulated by a wide choice of solvents, capping agents, halide ions, and additives. For example, with the assistance of Br⁻ ions, three types of noble metal-based intermetallic nanocrystals, including hcp PtBi nanoplates, fcc Pd₃Pb nanocubes and hcp Pd_2.5Bi_1.5 nanoparticles, were prepared through a one-pot co-reduction method (Figure 5A).¹⁰⁴ Here, Br⁻ ions could not only slow down the reaction rate by coordinating with the metal ions, but also induce oxidative etching with the presence of oxygen to facilitate atom rearrangement, contributing to the formation of intermetallic nanocrystals.

Figure 5. Intermetallic nanocrystals prepared by wet-chemical synthesis. (A) Scheme showing the one-pot synthesis of intermetallic nanocrystals with the assistance of Br− ions (upper panel), and HAADF-STEM images of PtBi and Pd2.5Bi1.5 nanoparticles (lower panel). Reproduced with permission,104 copyright 2020 Wiley-VCH. (B) Scheme showing the phase-controlled synthesis of ordered L12 and disordered fcc Pd3Sn nanorods (left panel), and their XRD patterns (right panel). Reproduced with permission,105 copyright 2022 Wiley-VCH.

Alternatively, seed-mediated growth is another effective approach to synthesize intermetallic nanocrystals in solution. Briefly, monometallic nanocrystals are prepared first, which are then used as seeds for the diffusion of the second metal to form intermetallics.^106–108 Normally, seeds with small size, ultrathin thickness and rich defects can promote the diffusion process. Seed-mediated growth enables control over size and shape of intermetallic nanocrystals, because the overgrowth of secondary metals would inherit the structural features of seeds. For example, monodisperse Au nanoparticles were used as seeds. Upon reduction and diffusion of Cu, intermetallic AuCu and AuCu₃ nanoparticles with narrow size distributions were formed.¹⁰⁶ Ultrathin Pd nanosheets were used as seeds for the formation of Pd-M (M = Pb, Sn, and Cd) ultrathin porous intermetallic nanosheets.¹⁰⁸ As another example, a general seeded diffusion method was developed to prepare a library of intermetallic nanocrystals consisting of transition metals (i.e., Au, Ag, Cu, Pd, and Ni) and low-melting-point metals (i.e., Ga, In, and Zn).¹⁰⁹ Briefly, transition metal nanocrystals were prepared first, followed by the injection of low-melting-point metal precursors. Upon annealing in oleylamine at temperatures >260°C for 10 min, the low-melting-point metals were diffused into the transition metal seeds, leading to the formation of intermetallic nanocrystals with precisely controlled size and composition.

In addition to monometallic seeds, bimetallic nanocrystals can also function as seeds. Compared to the transformation from monometallic seeds, the chemical conversion between bimetallic nanocrystals can be carried out more easily in solution owing to the shorter average diffusion length and the relatively lower diffusion barrier. For example, intermetallic PtSn nanoparticles were converted into intermetallic PtSn₂ and Pt₃Sn nanoparticles by reacting with SnCl₂ and K₂PtCl₆, respectively, at temperature of approximately 280°C.¹¹⁰ Disordered PdCu nanoparticles were converted into intermetallic B2 PdCu nanoparticles by reacting with Pd and Cu precursors in oleylamine at 270°C.⁴⁷

To highlight, a significant strength of wet-chemical synthesis is its ability to prepare intermetallic nanocrystals with highly controlled size, composition, shape and crystal phase, thanks to its great flexibility in tunning a wide range of parameters such as the precursor type, solvent, temperature, capping agent, additive and seed. In this regard, phase-controlled synthesis of bimetallic nanocrystals with either disordered alloy or ordered intermetallic structures has been achieved. For example, by using different types of Sn precursors and solvents, i.e., Sn(acac)₂ in oleylamine and SnCl₂ in oleylamine/octadecene, intermetallic L1₂ and disordered fcc Pd₃Sn nanorods with similar composition and shape, but different crystal phases were synthesized, respectively (Figure 5B).¹⁰⁵ Besides, by tuning the metal precursor ratios and reaction temperatures, Pd-Sn nanoparticles with various structures were obtained, including the intermetallic hexagonal Pd₃Sn₂, intermetallic orthorhombic Pd₂Sn and disordered fcc Pd₃Sn nanoparticles.¹¹¹ As another example, simultaneous control over crystal phase and size of Pt-Bi intermetallic nanoparticles was achieved in microfluidic reactors.¹¹² Specifically, the employment of reaction temperatures of 260°C and 350°C resulted in the formation of hexagonal Pt₁Bi₁ and trigonal Pt₁Bi₂ intermetallic nanoparticles, respectively. By tuning the solvents (i.e., ethylene glycol, PEG400, PEG600) and the length of the reaction channel (i.e., 10–120 cm), the size of the Pt₁Bi₂ nanoparticles was controlled in the range of 4–33.5 nm. Besides, for the Pt-Sn intermetallic nanoparticles synthesized via the seeded growth method, composition control was realized by tuning the size of the Pt seeds and the amount of Sn precursor, leading to the formation of PtSn, PtSn₂, and PtSn₄ intermetallic nanoparticles.¹¹³

Shape control has been achieved as well, with the assistant of capping agents that can selectively adsorb onto specific facets. So far, noble metal-based intermetallic nanocrystals with diverse shapes, including one-dimensional (1D) nanorods/nanowires,^{41,105,114,115} 2D nanosheets/nanoplates,^{25,39,44,49,116,117} and 3D polyhedra,^43,118–120 have been prepared via wet-chemical synthesis. For example, L1₂ intermetallic Pt₃Co hierarchical nanowires were prepared by reducing Pt(acac)₂ and Co(acac)₃ in oleylamine, where the formation of nanowires was ascribed to the usage of the cetyltrimethylammonium chloride (CTAC) shape-directing agent (Figure 6A).⁴¹ Orthorhombic intermetallic Pd₂Sn nanorods were prepared by reducing Pd(acac)₂ and Sn(acac)₂ in oleylamine, where the usage of trioctylphosphine (TOP) and Cl⁻ ions together contributed to the formation of nanorods.¹¹⁴ The aspect ratio of nanorods was tuned by changing the concentration of TOP and Cl⁻ ions, where higher concentration of TOP resulted in thicker rod, while higher concentration of Cl⁻ ions resulted in longer rod (Figure 6B). L1₀ intermetallic PdZn nanosheets with thickness less than 5 nm were synthesized by reducing Pd(acac)₂ and Zn(acac)₂ in oleylamine, where the formation of sheets was due to the usage of Mo(CO)₆ that could release the shape-directing molecule CO under high temperature (Figure 6C).³⁹ Rhombohedral intermetallic Pd₈Sb₃ hexagonal nanoplates with thickness of approximately 15.3 nm were prepared by reducing Pd(acac)₂ and SbCl₃ in benzyl alcohol via the solvothermal reaction, where polyvinyl pyrrolidone and NH₄Br functioned as the shape-directing agents (Figure 6D).¹¹⁶ Interestingly, with the addition of W(CO)₆ while maintaining other reaction parameters unchanged, rhombohedral intermetallic Pd₂₀Sb₇ rhombohedra were obtained, which were bounded by six rhombic faces that were obliquely intersected (Figure 6E).¹¹⁸ Three types of L1₂ intermetallic Pt₃Sn nanocubes with cubic, concave cubic (Figure 6F), and defect-rich cubic shapes were synthesized via the solvothermal method.⁴³ The large difference in electronegativity between Pt and Sn, as well as the chemical etching process induced by Cl⁻ ions and oxygen enabled the shape control.

Figure 6. Shape-controlled preparation of intermetallic nanocrystals via wet-chemical synthesis. (A) STEM image of L12 hierarchical Pt3Co nanowires (left panel), and HAADF-STEM image of a Pt3Co nanowire (right panel). Reproduced with permission,41 copyright 2016 Nature Publishing Group. (B) TEM image of orthorhombic Pd2Sn nanorods (left panel), HRTEM image of a Pd2Sn nanorod (middle panel), and structural model showing the adsorption of TOP and Cl− ions on a Pd2Sn nanorod (right panel). Reproduced with permission,114 copyright 2015 American Chemical Society. (C) TEM image of L10 PdZn nanosheets (left panel), and HAADF-STEM image of a PdZn nanosheet (right panel). Reproduced with permission,39 copyright 2019 American Chemical Society. (D) STEM image of rhombohedral Pd8Sb3 hexagonal nanoplates (left panel), and HAADF-STEM image of a Pd8Sb3 nanoplate (right panel). Reproduced with permission,116 copyright 2022 Wiley-VCH. (E) STEM image of rhombohedral Pd20Sb7 rhombohedra (left panel), and HAADF-STEM image of a Pd20Sb7 rhombohedron (right panel). Reproduced with permission,118 copyright 2022 Wiley-VCH. (F) STEM image of a concave cubic L12 Pt3Sn nanocube (left panel), and HAADF-STEM image of a Pt3Sn nanocube (right panel). Reproduced with permission,43 copyright 2016 Wiley-VCH.

Nevertheless, there are still several limitations associated with the wet-chemical synthetic method. First, the boiling point of solvents greatly limits the range of reaction temperatures, which could cause incomplete disorder-to-order transition. Besides, when applying the as-obtained intermetallic nanocrystals as electrocatalysts, additional steps, such as loading of the nanocrystals on substrate and removal of capping ligands to expose active sites via thermal annealing or UV irradiation, are normally required, which may add complexity and increase cost.^121,122

Other methods

In addition to the above-mentioned methods, other approaches, such as physical vapor deposition,⁴⁸ electrochemical method,^98,123,124 microwave-assisted method,¹²⁵ and sputtering,^126,127 have also been reported to synthesize binary intermetallic nanocrystals. For example, electrochemical deposition was used to directly synthesize the metastable intermetallic Pd₃₁Bi₁₂ nanocrystals at room temperature from a solution containing Bi(C₂H₃O₂)₃, Na₂PdCl₄, acetic acid and ethylenediaminetetraacetic acid.⁹⁸ Since electrodeposition can be performed at large overpotentials, corresponding to conditions far from equilibrium, the metastable Pd₃₁Bi₁₂ structure that is difficult to be obtained via the conventional methods can be accessed. As another example, through an electrochemical dealloying process, monoclinic PdBi₂ was converted into orthorhombic Pd₃Bi.^124,128 Its formation could be attributed to the low melting point of PdBi₂ (~480°C), indicating a low vacancy formation energy, such that the dealloying of Bi and atom inter-diffusion could be promoted via the vacancy-promoted diffusion mechanism.¹²⁸

SYNTHESIS OF BINARY INTERMETALLIC NANOCRYSTAL-BASED HETEROSTRUCTURES

It has been reported that, for many noble metal-based binary intermetallic nanocrystals, a thin noble metal shell is observed coating over the binary intermetallic core, forming a core-shell structure.¹²⁹ The formation of the thin noble metal shell arises from the dealloying of non-noble metals from the binary intermetallic structures. In some cases, the noble metal shell forms concurrently with the growth of intermetallic nanocrystals or in-situ during the electrocatalytic test. In other cases, an additional step is performed to induce the formation of noble metal-rich surfaces, such as electrochemical dealloying,^59,130,131 acid leaching,^95,132,133 and high-temperature annealing.^31,134 For example, L1₀ PtFe nanoparticles were immersed in 0.1 mol/L HClO₄ solution at 60°C for 12 h, such that Fe atoms in surface layers were removed and a defective Pt shell was formed over the L1₀ PtFe core.⁹⁵ Subsequently, the defective Pt layer was smoothed by annealing under Ar/H₂ at 400°C, leading to the formation of L1₀ PtFe@Pt core-shell nanoparticles with the presence of an atomically thin (5 Å) Pt shell (Figure 7A).

Figure 7. Synthesis of binary intermetallic nanocrystal-based heterostructures. (A) Scheme showing the formation of Pt skin over L10 intermetallic PtFe via acid etching and thermal annealing. Reproduced with permission,95 copyright 2018 American Chemical Society. (B) HRTEM images of intermetallic Pd3Pb nanosheets coated with the tensile-strained Pd shell. Reproduced with permission,135 copyright 2019 American Chemical Society. (C) HAADF-STEM images of PdCu@PtCu viewed along the [111], [100] and [110] zone axes (left panel), and the 2D atomic models showing the core-shell interfaces with different lattice spacings (right panel). Reproduced with permission,136 copyright 2017 American Chemical Society.

Since electrocatalytic reactions take place on the catalyst surface, such noble metal shells have two significant effects. First, the noble metal-rich surface endows the catalyst with enhanced durability, such that the dissolution of non-noble metals during the electrochemical process can be largely alleviated. Second, because of the lattice mismatch between the binary intermetallic core and monometallic shell, compressive or tensile strain will be developed in the overlayer. As a result of the strain effect, the electronic structure of the catalytic active metal will be changed, such that the binding energy of reaction species could be manipulated to improve the catalytic activity. For example, coating of an ultrathin Pt skin over the intermetallic Pt₃Co nanoparticles could introduce a 4.4% compressive strain in the Pt shell.²⁴ This led to a downshift of the antibonding state between O–Pt and a weaker binding energy between the oxide intermediates and Pt, which improved the intrinsic ORR activity of Pt. On the other hand, coating of an ultrathin Pd skin over the intermetallic Pd₃Pb nanosheets could introduce a 5.4% tensile strain in the Pd shell, because the lattice parameter of Pd was smaller than that of the intermetallic Pd₃Pb (Figure 7B).¹³⁵ To highlight, such strain effect will decrease with the increase of shell thickness, because strain can be relaxed when atoms have large distance.¹³⁷ Therefore, synthesis of thin metal shells with controllable thickness enables the precise control over strain. For example, by means of temperature-controlled Fe etching in acetic acid, three types of PdFe@Pd core-shell nanoparticles were prepared, in which PdFe core had size of approximately 8 nm and Pd shells exhibited controllable thickness of 0.27, 0.65, or 0.81 nm. It was found that the ORR activity was optimal when the Pd shell thickness was 0.65 nm.¹³⁸

Monometallic shells having compositions different from the intermetallic cores can also be coated via the seed-mediated growth method.^48,139,140 For example, to maximize the utilization efficiency of the expensive noble metal Pt, L1₀ intermetallic AuCu nanoparticles were used as seed, onto which a thin Pt skin, consisting of 1.5 monolayers of Pt, was grown through the galvanic replacement reaction between Pt precursors and Cu atoms in surface layers.¹³⁹ As another example, to suppress the CO poisoning effect in methanol oxidation reaction (MOR), sub-monolayer Pb was grown on hexagonal PtBi intermetallic nanoplates via the seed-mediated growth.¹⁴¹

In addition to monometallic overlayers, bimetallic shells have been grown over the intermetallic cores as well, providing more room for strain modulation.^142–146 For example, PtFe@PtBi core-shell nanoparticles were synthesized by using L1₀ PtFe nanoparticles as seeds, followed by the leaching of Fe and the incorporation of Bi in acidic solutions.¹⁴⁵ Subsequently, atom rearrangement was triggered by thermal annealing under NH₃ at 500°C, leading to the formation of PtFe@PtBi core-shell nanoparticles consisting of the intermetallic PtFe core and 2–3 atomic layers of PtBi shell. Impressively, in such a PtFe@PtBi core-shell structure, both the core and shell endowed the surface Pt with a compressive strain. For the core, an interlayer compressive strain was induced due to the smaller atomic radius of Fe (0.127 nm) than that of Pt (0.139 nm). For the shell, an additional surface compression was induced by in-plane shearing due to the presence of large Bi atoms (0.155 nm). As another example, PdCu@PtCu core-shell nanoparticles, comprising of the B2 intermetallic PdCu core and the fcc disordered PtCu shell, were prepared via the seed-mediated growth.¹³⁶ TEM analyses showed that an orientation-dependent surface strain was developed in the PtCu overlayer, i.e., the compressive strain on PtCu (200) was greater than that on PtCu (111) facets, due to their different degrees of lattice mismatch (Figure 7C). PtBi@PtRh₁ core-shell nanoplates, comprising of the intermetallic PtBi core and Pt shell alloyed with discrete Rh atoms, were prepared.¹⁴⁷ Briefly, Rh single atoms were incorporated first via galvanic replacement between RhCl₃ and the surface Bi in PtBi nanoparticles, which were subsequently alloyed with Pt upon electrochemical dealloying of Bi atoms.

SYNTHESIS OF TERNARY INTERMETALLIC NANOCRYSTALS

With the tremendous progress that has been made in preparing a library of binary intermetallic nanocrystals via diverse synthetic strategies, attention is being paid to incorporate more than two elements to form ternary or even multinary intermetallic nanocrystals. The synthetic methods of ternary noble metal-based intermetallic nanocrystals are similar to those for preparing their binary counterparts, except for the addition of the third metal precursor into the reaction system.^{51,92,148,149} The conventional thermal annealing method and their modifications,^150–154 wet-chemical synthesis method,^39,155 and other strategies¹⁵⁶ have been reported to prepare ternary intermetallic nanocrystals. For example, L1₀ PtCoNi nanoparticles were prepared by impregnating the metal precursors (H₂PtCl₆·6H₂O, CoCl₂·6H₂O, and NiCl₂·6H₂O) with controlled molar ratios onto the carbon support, followed by thermal annealing under H₂ at 700°C.¹⁵⁷ Alternatively, the L1₀ PtNiCo nanoparticles with sub-6 nm were also prepared by annealing the presynthesized Pt@NiCoO_x core-shell nanoparticles under Ar/H₂ at 600°C, in which the shell structures with rich oxygen vacancies not only promoted the atomic diffusion and ordering process but also protected the particles against sintering during annealing.¹⁵⁰ Intermetallic Pd_2-xNi_xGe nanoparticles with controllable Pd/Ni ratios were prepared via a wet-chemical solvothermal metho, in which the metal precursors (K₂PdCl₄, GeCl₄, and NiCl₂) were co-reduced in the tetra-ethylene glycol solvent by the superhydride (Li(Et₃BH)) reducing agent.¹⁵⁵ Besides, the disordered ternary alloys can also be prepared first via the wet-chemical method, followed by transformation to the ordered ternary intermetallics via high-temperature thermal annealing, such as the cases of intermetallic PdCuM (M = Ni and Cu),¹⁵⁸ PtFeCu,¹⁵⁹ and PtCoM (M = Mn, Fe, Ni, Cu, and Zn).¹⁶⁰ In addition to simply extending the synthetic strategies of binary intermetallic nanocrystals, ternary intermetallic nanocrystals can also be prepared by using the binary counterparts as seeds, followed by the introduction of the third metal through galvanic replacement. For example, intermetallic PdZnAu nanoparticles were prepared by incorporating Au into intermetallic PdZn nanoparticles via a galvanic replacement method, which could be realized due to the more positive redox potential of AuCl₄²⁻/Au (0.93 V vs SHE) than those of PdCl₄²⁻/Pd (0.59 V vs. SHE) and Zn²⁺/Zn (−0.76 V v.s SHE).⁵⁵

In most cases, the third metal can be viewed as a substitution of either element in the binary system and shares the crystallographic position with that element. As a result, the XRD pattern of the ternary intermetallic nanocrystals is consistent with their corresponding binary one, while shift in peak positions can be observed and the peak shifting direction depends on the atomic radius of the third metal. For example, hcp intermetallic Pt₄₅Sn₂₅Bi₃₀ nanoplates were prepared by co-reducing Pt(acac)₂, SnCl₂ and Bi(act)₃ in oleylamine/octadecene via a one-pot wet-chemical method.¹⁶¹ The XRD pattern of Pt₄₅Sn₂₅Bi₃₀ was consistent with that of the standard intermetallic PtSn, while all peaks shifted to lower angles, due to the integration of Bi with a large atomic radius into the PtSn lattice (Figure 8A). As another example, L1₀ intermetallic PtZnCu nanoparticles were prepared by using ZIF-8 as the sacrificial template, onto which H₂PtCl₆ and Cu(NO₃)₂ were impregnated, followed by thermal annealing under H₂ at 750°C.¹⁶² The XRD pattern of PtZnCu was consistent with that of the L1₀ PtZn, while all peaks shifted to higher angles because of the incorporation of Cu with a small atomic radius.

Figure 8. Synthesis of ternary intermetallic nanocrystals. (A) XRD pattern of hcp intermetallic Pt45Sn25Bi30 nanoplates (upper left panel), HAADF-STEM image (upper right panel) and EDS elemental mappings of a Pt45Sn25Bi30 nanoplate (lower panel). Reproduced with permission,161 copyright 2019 Wiley-VCH. (B) Scheme showing the synthesis of fct Pt0.4Ir0.1Fe0.5 nanowires (upper left panel), HAADF-STEM image (upper right panel) and EDS elemental mappings of a Pt0.4Ir0.1Fe0.5 nanowire (lower panel). Reproduced with permission,163 copyright 2022 Wiley-VCH.

Interestingly, unconventional phases that do not match any phases in the ternary phase diagrams or the constituent unary and binary systems can also be accessed in ternary intermetallic structures. A typical example is the synthesis of AuCuSn₂ intermetallic nanoparticles via the co-reduction of HAuCl₄, Cu(C₂H₃O₂)₂ and SnCl₂ in tetraethylene glycol using NaBH₄ as the reducing agent.^164,165 Based on the XRD pattern, it exhibited an unconventional NiAs-type ordered structure, which was different from either its bulk counterpart or the constituent binary systems. Such observation has also been extended to synthesize the unconventional AuNiSn₂ intermetallic nanoparticles.

It is generally believed that with the incorporation of more elements, the electronic structure of the catalytic active metals can be further optimized to enhance the catalytic performance.^{150,157,158,161,162,166,167} Apart from it, the introduction of a third metal also has several merits in terms of the synthesis of intermetallic structures. First, in some cases, the third metal could facilitate the disorder-to-order transition.^66,167–169 Such third metals can be those with lower surface energy and/or lower solubility relative to the binary alloys, such that they tend to migrate to the surface during thermal annealing, causing the creation of rich lattice vacancies and thereby promoting the ordering process. For example, by partially replacing Fe in PtFe nanoparticles with Cu, a lower annealing temperature of 500°C was sufficient to transform the disordered fcc PtFe_xCu_1-x to the ordered bct PtFe_xCu_1-x, while 700°C was needed for transforming the binary fcc PtFe nanoparticles.¹⁶⁷ A much lower annealing temperature of approximately 300°C was adopted for the partial ordering transformation of Sb-doped PtFe (Fe₃₂Pt₄₅Sb₂₃) nanoparticles.⁶⁶ Such Sb-promoted disorder-to-order transition could be attributed to the incorporation of Sb atoms with low surface energy and low solubility, which were prone to escape from the PtFe lattice, leaving behind rich lattice vacancies, thereby promoting the ordering transformation via enhancing the atom mobility of Pt and Fe. Similar effect has also been observed for Ag¹⁶⁸ or Au-doped PtFe nanoparticles.¹⁶⁹ Besides, the third metal can also help to preserve the morphology of the intermetallic nanocrystals during high-temperature annealing. For example, fct Pt_0.4Ir_0.1Fe_0.5 nanowires with an average diameter of 2.6 nm were prepared via high-temperature annealing of the disordered counterpart at 690°C (Figure 8B).¹⁶³ The introduction of the high-melting-point metal Ir into the PtFe nanowires and the coating of SiO₂ protective shell before annealing were crucial for maintaining the 1D morphology. Otherwise, the thin nanowires would break into fragments and nanoparticles upon annealing.

SYNTHESIS OF HEI NANOCRYSTALS

High-entropy materials, owing to their unique structures and exceptional physicochemical properties, have triggered broad interests in diverse research fields.^170,171 As a typical group of high-entropy materials, high-entropy alloys (HEAs) refer to those having at least five principal elements with nearly equal atomic ratio. In contrast to HEAs, which exhibit compositional complexity but disordered distribution of the constituent elements, in HEIs, five or more elements show ordered arrangement while maintaining the compositional complexity. The crystal structure of an HEI normally follows the structure of its “parent” binary intermetallic counterpart. The “parent” lattice provides two sets of crystallographic positions, such that metal atoms with analogous characteristics (e.g., radius and electronegativity) share one sublattice. For example, the crystal structure of (PtCoNi)(SnInGa) HEI follows the structure of its “parent” binary intermetallic PtSn, i.e., B8₁, while the Pt and Sn atoms are partially substituted by Co/Ni and In/Ga atoms, respectively (Figure 9A).¹⁷² To date, HEIs have been used as catalysts or electrocatalysts toward various reactions, such as propane dehydrogenation reaction,^172,173 acetylene semihydrogenation reaction,¹⁷⁴ ORR,¹⁷⁵ HER,¹⁷⁶ and ethanol oxidation reaction (EOR).^177,178

Figure 9. Synthesis of HEI nanocrystals. (A) Structural models of binary intermetallic PtSn and (PtCoNi)(SnInGa) HEI. Reproduced with permission,172 copyright 2022 Nature Publishing Group. (B) HAADF-STEM image and EDS elemental mappings of the CoFeNiCuPd HEI nanoparticle. Reproduced with permission,175 copyright 2022 Wiley-VCH. HEI, high-entropy intermetallic.

So far, intermetallic nanocrystals have been greatly limited to binary or ternary compositions, as discussed above. When five or more elements are included, phase segregation is prone to occur because of the thermodynamic immiscibility of the constituent elements.¹⁷⁹ Therefore, the controlled synthesis of HEIs remains challenging. Methods such as the direct co-reduction of metal precursors,^{172,174,175,177} disorder-to-order transition upon thermal annealing,^178,179 and melt-spinning,¹⁷⁶ have been reported. Co-reduction of metal precursors can occur in either liquid or gas phase. For example, a one-pot wet-chemical synthetic method was used to prepare hcp PtRhBiSnSb HEI nanoplates, in which Pt(acac)₂, Rh(acac)₃, Bi(act)₃, SnCl₂, and SbCl₃ were thermally decomposed in oleylamine/octadecene at 220°C.¹⁷⁷ The successful formation of such HEI derived from the same hcp crystalline structures of PtBi, RhBi, PtSn, and PtSb intermetallics. The crystal structure followed the structure of PtBi, i.e., hcp, while Pt and Bi atoms were partially substituted by Rh and Sn/Sb atoms, respectively. A co-impregnation of metal precursors followed by high-temperature annealing method was developed to prepare (NiFeCu)(GaGe) HEI nanoparticles, whose crystal structure followed the structure of NiGa, i.e., CsCl-type.¹⁷⁴ The co-impregnation method has also been applied to synthesize PtCoNiInGaSn HEIs supported on CeO₂, whose crystal structure was consistent with that of the “parent” PtSn intermetallic.¹⁷² As another example, PtCoCuGeGaSn HEIs supported on SiO₂ were prepared by impregnating all the metal precursors into the pores of SiO₂, which were then dried and reduced under H₂ at 700°C.¹⁷³ Besides, a 2D organic-inorganic superstructure was prepared first, in which metal precursors were homogeneously dispersed.¹⁷⁵ Upon annealing under NH₃ atmosphere, CoFeNiCuPd HEI nanoparticles supported on 2D N-rich mesoporous carbon nanosheets were obtained (Figure 9B). It exhibited an L1₂ structure, in which Fe and Co atoms were located at the face-centered sites, Pd and Cu atoms were located at the vertex sites, while Ni atoms were randomly distributed over the entire lattice. In contrast, annealing under N₂ resulted in the formation of its disordered HEA counterpart.

Besides, the disorder-to-order transition under high-temperature annealing for the synthesis of binary/ternary intermetallic nanoparticles can also be extended to HEIs. For example, the disordered HEA nanoparticles containing at least five elements were first prepared by heating metal precursors on the carbon substrate at approximately 1100 K.¹⁷⁹ Then, the corresponding HEI nanoparticles were obtained via annealing HEAs at approximately 1100 K followed by rapid cooling to lock the ordered structures. Such method enabled the preparation of several ultrasmall HEI nanoparticles with size of 4–5 nm, including the L1₀ Pt(Fe_0.7Co_0.1Ni_0.1Cu_0.1), L1₂ (Pt_0.8Au_0.1Pd_0.1)₃(Fe_0.9Co_0.1), and L1₀ (Pt_0.8Pd_0.1Au_0.1)(Fe_0.6Co_0.1Ni_0.1Cu_0.1Sn_0.1). As another example, PtRhFeNiCu HEAs supported on carbon were first synthesized by the impregnation-reduction method, which then experienced the disorder-to-order transition under H₂/Ar at 700°C, leading to the formation of ordered PtRhFeNiCu HEIs.¹⁷⁸

ELECTROCATALYTIC APPLICATIONS OF INTERMETALLIC NANOCRYSTALS ORR

ORR is the key reaction occurring at the cathode of the proton exchange membrane fuel cells (PEMFCs). It takes place via either the two-electron (2e⁻) pathway, in which O₂ is partially reduced to H₂O₂ in acidic or HO₂⁻ in alkaline media, or the four-electron (4e⁻) pathway, in which O₂ is completely reduced to H₂O in acidic or OH⁻ in alkaline media. The latter is desirable for PEMFCs due to its higher current efficiency. One of the bottlenecks for the commercialization of PEMFCs is the sluggish ORR process, which requires high-efficiency electrocatalysts. An optimal adsorption of O* on the catalyst surface is desirable, since too strong O* adsorption could limit the removal of the adsorbed intermediates, while too weak O* adsorption could limit the dissociation of O₂.³³ So far, platinum-group-metal-based catalysts are considered as the most efficient ORR catalysts among other elements. Nevertheless, considering the scarcity and high cost of Pt, the instability issue arising from metal dissolution during long-term operation, and the ambition to further boost the ORR activity, continuous research efforts are urgently required.

An effective strategy is to develop binary or even multinary Pt-based nanocatalysts with intermetallic structures. The introduction of 3d transition metals or p-block metals can not only reduce the usage of expensive Pt, but also enable the modulation of Pt electronic structure through the alloying effect. Besides, through dealloying of the less noble metals, nanocatalysts containing the Pt-based intermetallic core coated with few layers of Pt skin can be formed, exhibiting a compressive strain in the Pt overlayer when the lattice of the metal core is smaller than that of the Pt skin. As a result of the compressive strain, the Pt d-band center shifts downward, leading to the decreased binding energy of the oxygenated intermediates (OH*, O*, OOH*, etc.) on Pt, thereby promoting the ORR activity.³⁴

The ORR performance of intermetallic binary Pt-M (M = Fe, Co, Ni, Cu, etc) with L1₀,^{56,130,138,150,180} L1₁,¹⁸¹ and L1₂^{32,41,87,96,151} structures as well as the intermetallic nanocrystal-based heterostructures^51,145 has been studied extensively. For example, L1₀ PtCo coated with 2–3 atomic layers of strained Pt skins exhibited enhanced specific activity (8.26 mA/cm²) and mass activity (2.26 A/mg_Pt), which were approximately 12 and approximately 15 times, respectively, higher than those measured on the disordered counterparts.³⁷ L1₁ PtCu nanoframes with thin Pt skins exhibited superior activity and stability than the disordered counterpart, with a high mass activity of 2.47 A/mg_Pt and enhanced durability after 10,000 accelerated durability test (ADT) cycles.⁴⁰ L1₂ Pt₃Co nanoparticles coated with 2–3 atomic layers of strained Pt skins showed over 200% increase in mass activity and over 300% increase in specific activity than the disordered Pt₃Co and Pt/C.³¹ In contrast to the conventional Pt-rich intermetallics, such as Pt₃M and PtM, PtCo₃@Pt nanoparticles, due to the increased amount of the non-noble metal Co in the core, exhibited a larger compressive strain in the Pt shell.⁵⁹ The L1₂ PtCo₃@Pt catalyst showed a superior mass activity (0.72 A/mg_Pt) than the L1₀ PtCo catalyst (0.45 A/mg_Pt), and both of them behaved better than their disordered counterparts. Besides, the Pt skins also endowed PtCo₃ with good durability, despite its high content of Co which was prone for leaching during long-term operation. In addition, the compressive strain can be further increased by alloying the Pt overlayer with elements having larger size. For example, by coating PtBi shell over the L1₀ intermetallic PtFe nanoparticles, the resultant PtFe@PtBi core-shell nanoparticles exhibited stronger compressive strain in the Pt overlayer (−4.6%), as compared to the uncoated PtFe (−2.6%).¹⁴⁵ Accordingly, the PtFe@PtBi catalyst showed the best ORR performance among the uncoated ordered PtFe and ordered ternary PtFeBi nanoparticles, with a high mass activity of 0.96 A/mg_Pt and specific activity of 2.06 mA/cm² (Figure 10A).

Figure 10. Electrocatalytic performance of intermetallic nanocrystals toward ORR. (A) HAADF-STEM image and EDS line scan of a PtFe@PtBi core-shell nanoparticle (upper panel), comparison in terms of mass and specific activities of various catalysts (middle panel), and the color-coded strain distributions of Pt, PtFe and PtFe@PtBi (lower panel). Reproduced with permission,145 copyright 2021 Wiley-VCH. (B) Structural models with top and side views of a PtPb nanoplate (upper left panel), HAADF-STEM image showing the interface between the PtPb core and Pt shell (upper right panel), and comparison in terms of mass and specific activities of various catalysts (lower panel). Reproduced with permission,25 copyright 2019 AAAS. (C) Unit cell of the L10 PtFe (left panel), and change of Fe composition with time for ordered and disordered PtFe nanoparticles (right panel). Reproduced with permission,22 copyright 2010 American Chemical Society. ORR, oxygen reduction reaction.

Although the compressive strain in Pt skins has been well recognized to enhance the ORR activity of Pt-based nanocatalysts, a few studies also emphasize the importance of tensile strain, which is induced when the lattice of the intermetallic core is larger than that of the Pt shell. A typical example is the B8₁ PtPb@Pt nanoplates, in which Pt skins with (110) planes were coated over the (010) and (001) planes of the intermetallic PtPb core.²⁵ As a result of the lattice mismatch, a high tensile strain (~7.5%) and a little compressive strain (~1%) were induced in the Pt skins. It was found that the strain effect on the binding energy of the oxygenated intermediates was dependent on the exposed facet of the nanocatalysts. Although the tensile strain on the Pt (111) facet was not favorable, it became favorable when the tensile strain was built up on low-coordination facets, in this case, the (110) facet. As a result of such tensile strain on the Pt (110) planes, the σ* antibonding states shifted downward, leading to a decreased binding energy of the oxygenated species, thereby enhancing the overall ORR performance, with a high specific activity of 7.8 mA/cm² and mass activity of 4.3 A/mg_Pt (Figure 10B).

In addition, featuring the more negative formation enthalpy of intermetallic structures, higher durability is normally achieved for intermetallic nanocatalysts compared to their disordered counterparts, especially in acidic media. For example, the Fe atoms in L1₀ PtFe nanoparticles exhibited stronger tolerance to acid leaching compared to those in the disordered fcc PtFe in 0.5 mol/L H₂SO₄ solution, i.e., only 3.3% loss of Fe in the L1₀ PtFe after immersion in acid for 1 h, in contrast to a heavy loss of Fe (36.5%) in the fcc PtFe (Figure 10C).²² After 1000 potential cycles of stability test, the L1₀ PtFe showed a slight compositional change (Fe/Pt = 42:58), while a severe loss of Fe was observed in fcc PtFe (Fe/Pt = 11:89). As another example, for the nanocatalyst containing the L1₀ intermetallic PtCo core coated with Pt skins, only 5% loss of Co was observed after operating at 60°C in acidic media for 24 h, in contrast to 34% loss of Co after 7 h for the disordered counterpart.³⁷

By incorporating additional elements, ternary and HEI intermetallic nanocrystals offer more room to tune the ORR performance.^90,169,182 For example, Pt-skin-coated L1₀ PtNiCo nanoparticles with controllable Ni/Co ratios, i.e., 9/1, 8/2, 6/4, 4/6, and 2/8, were synthesized by annealing the pre-synthesized Pt@NiCoO_x core-shell nanoparticles.¹⁵⁰ Their ORR activities were studied and compared with the binary L1₀ PtNi and L1₀ PtCo nanoparticles. Among them, L1₀ PtNi_0.8Co_0.2, with a Ni/Co ratio of 8/2, exhibited the most positive half-wave potential (E_1/2), while L1₀ PtNi showed the most negative E_1/2. DFT calculations suggested that the intermetallic PtNi_0.8Co_0.2 core could optimize the compressive strain in the Pt shell, thereby optimizing the Pt−O binding energy. L1₂ intermetallic Pt₃Co_0.6Ti_0.4 nanoparticles supported on ZIF-8-derived mesoporous carbon exhibited a superior ORR mass activity (1.49 A/mg_Pt) and enhanced durability (20.1% loss of mass activity after 20,000 potential cycles) than the Pt₃Co counterpart (0.83 A/mg_Pt, 42.2% loss of mass activity after 20,000 potential cycles).¹⁵¹ The improved ORR performance of the ternary Pt₃Co_0.6Ti_0.4 nanoparticles could be attributed to the partial substitution of Ti with a lower electronegativity (1.54) for Co (1.88), leading to enhanced electron transfer from Co/Ti to Pt, thereby resulting in an optimized binding energy between Pt and the oxygenated intermediates. The L1₂ CoFeNiCuPd HEI nanoparticles supported on nitrogen-rich mesoporous carbon exhibited a large half-wave potential (0.90 V), a high mass activity (2.037 mA/μg_Pd), and remarkable durability (only 10 mV decay after 10,000 cycles), much superior to the disordered HEA counterpart and the commercial Pt/C.¹⁷⁵ The outstanding activity and stability were attributed to several factors, including the 2D mesoporous nitrogen-doped carbon support, which promoted the mass transfer and electron conductivity, as well as the ordered HEI nanoparticles, which possessed modulated electronic structures and a highly robust structural configuration.

Considering the scarcity and high cost of Pt, low- or non-Pt ORR catalysts have been searched as substitutions. Pd is a typical alternative for ORR in alkaline media.^44,108 Pd has intrinsically lower ORR activity than Pt, because of the insufficient overlap between the t_2g component in Pd-4d¹⁰ orbital and the O-2p orbital of oxygenated species, leading to a slower electron transfer kinetics and a weakened oxygen binding energy.¹⁸³ Therefore, various strategies have been developed to enhance the ORR activity of Pd, one of which is to construct Pd-based intermetallic nanocrystals. For example, L1₂ Pd₃Pb nanosheets and nanocubes coated with Pd skins showed a homogeneous tensile strain in the Pd (100) facets, resulting in an upshift of the Pd d-band center and the optimized oxygen binding energy.¹³⁵ As a result, the Pd₃Pb nanosheets and nanocubes exhibited over 160% and 140% increases in mass activity and over 114% and 98% increases in specific activity compared with their uncoated counterparts, respectively. L1₂ Pd₃Pb tripods with the preferential exposure of (110) facets exhibited a high mass activity of 0.56 A/mg_Pd and specific activity of 1.76 mA/cm² in alkaline media, outperforming the commercial Pt/C and Pd/C.⁴⁵ DFT calculations suggested that the enhanced ORR activity originated from the strongly coupled Pd-4d and Pb-sp orbitals on the (110) facets, such that the orbital configuration of Pd-Pb became similar to that of Pt. Ternary intermetallic Au₁₀Pd₄₀Co₅₀ nanoparticles exhibited comparable activity to the conventional Pt/C in both acidic and alkaline electrolytes, while the durability in alkaline media was superior to that of Pt/C.¹⁴⁸ After 10,000 potential cycles, the E_1/2 decayed approximately 30 mV for Pt/C, while no obvious change in E_1/2 was observed for the intermetallic Au₁₀Pd₄₀Co₅₀. Both the ordered arrangement of Pd and Co atoms as well as the protective effect of the surface-rich gold atoms contributed to the enhanced durability. As another example, trace amount of Pt substituted Pd, forming intermetallic Pt_0.2Pd_1.8Ge nanocrystals.¹⁸⁴ Such a ternary nanocatalyst exhibited an improved E_1/2 of 0.95 V, a high selectivity of H₂O over H₂O₂, and high stability (i.e., mass activity of 320 mA/mg_Pt remained unchanged after 50,000 ADT cycles), outperforming the intermetallic Pd₂Ge counterpart and Pt/C. DFT calculations suggested that upon Pt substitution, the adsorption of OH* became weaker and that of OOH* became stronger, resulting in diminished OH poisoning and thus less H₂O₂ formation and enhanced stability.

Nevertheless, intermetallic nanocrystals do not always catalyse ORR better than their disordered counterparts. The structure, facet, shape and size are all important factors to be taken into considerations. For example, the ORR activities of Pt₃Sn nanocubes with different degrees of ordering were compared.¹⁸¹ Pt₃Sn nanocubes with 60%- and 30%-ordering exhibited a 2.3-fold enhancement in specific activity than their ordered counterpart with 95%-ordering. In this reaction system, the ordered Pt₃Sn was directly synthesized via the wet-chemical method, while the disordered counterparts were obtained by the subsequent thermal annealing. In such a unique order-to-disorder transition process, rich vacancies were developed on the surface of the less ordered particles during thermal annealing, thereby promoting the ORR activity. However, the 95%-ordering Pt₃Sn catalyst still showed better stability than the disordered counterparts.

Small molecule oxidation reactions

Small molecule oxidation reactions are important anodic reactions in direct liquid-feed fuel cells, such as direct methanol fuel cells, direct ethanol fuel cells (DEFCs) and direct formic acid fuel cells. Such reactions are complicated and kinetically sluggish, which involve the transfer of multiple electrons and many reaction intermediates and products. Taking EOR as an example, the C1-pathway, which involves the complete oxidation of ethanol and transfer of 12 electrons to form CO₂, is desired for high-efficiency DEFCs.¹⁸⁵ However, this process is usually suppressed by the C2-pathway, which involves the incomplete oxidation of ethanol and transfer of either 4 electrons to form acetic acid/acetate or 2 electrons to form acetaldehyde. Besides, various intermediates, such as CH₃CHO* and CO*, are produced during the reactions. These intermediates, especially CO*, would block the catalytic active metals, usually Pt and Pd, leading to undesired poisoning and deteriorated catalytic performance. Therefore, the goal is to develop catalysts that can achieve high atom utilization of noble metals, high mass activity at low overpotentials, high C1-pathway selectivity, long-term durability, and superior anti-poisoning ability.

One of the effective strategies to mitigate the CO poisoning is to construct intermetallic nanocatalysts, such that the active Pt or Pd sites can be isolated using non-noble metals.¹⁸⁶ Such non-noble metals are usually oxophilic, including Sn, Pb, Cu, Fe Co, Zn, etc. This is because the oxygenated species, such as the hydroxyl ions from the electrolyte, have a higher affinity to adsorb on the more oxophilic metal sites. The OH* will then spill-over to the adjacent Pt/Pd sites where CO* is strongly adsorbed, and react with CO*, leading to the oxidative removal of CO on the Pt/Pd sites.¹⁷ For example, by constructing the orthorhombic PdBi intermetallic nanocrystals, the catalytically active Pd sites were isolated by Bi atoms, greatly suppressing the adsorption of CO.¹⁸⁷ The remarkable CO tolerance was clearly demonstrated by the in-situ attenuated total reflection-infrared (ATR-IR) spectroscopy, in which peaks corresponding to the adsorbed CO* were observed for Pd/C with continuous Pd sites, while negligible CO* adsorption peaks were present for the intermetallic PdBi nanocrystals. As a result, the orthorhombic PdBi intermetallic nanocrystals exhibited superior activity toward formic acid oxidation reaction than the disordered counterpart and Pd/C (Figure 11A). In addition to active site isolation, another mechanism for improving the CO tolerance of intermetallic nanocatalysts is through electronic structure modification. Due to the strong bonding between Pt/Pd and non-noble metals, the position of the Pt/Pd d-band center could be optimized to lower the CO binding energy. For example, by synthesizing intermetallic Pd₃Pb nanoparticles, the d-band center of Pd was shifted downward.¹⁸⁸ Accordingly, the CO binding energy on the Pd₃Pb (111) facet was calculated to be 1.05 eV, which was lower than that on the Pd (111) facet, i.e., 1.29 eV, leading to stronger CO tolerance of Pd₃Pb than Pd.

Figure 11. Electrocatalytic performance of intermetallic nanocrystals toward small molecule oxidation reactions. (A) Scheme showing the usage of Pd and intermetallic PdBi nanocatalysts toward FAOR (upper left panel), in-situ ATR-IR spectra for Pd/C and intermetallic PdBi/C during the FAOR test (lower left panel), Pd mass-normalized CV curves in 0.5 mol/L H2SO4 + 0.5 mol/L HCOOH (upper right panel), and comparison in terms of mass and specific activities of various catalysts (lower right panel). Reproduced with permission,187 copyright 2020 American Chemical Society. (B) Structural model of PtBi@PtRh1 (upper left panel), comparison in terms of mass activity and selectivity of different catalysts (lower left panel), energy profiles for breaking the C − C bond of the CH2CO* on Pt (110) and Rh-doped strained Pt (110), respectively (right panel). Reproduced with permission,147 copyright 2021 Wiley-VCH. FAOR, formic acid oxidation reaction.

For nanocatalysts containing an intermetallic core coated with few atomic layers of Pt/Pd skins, the strain effect also contributes to the enhanced catalytic activity toward small molecule oxidation reactions. For example, intermetallic PtBi nanoplates covered with a tensile-strained Pt shell alloyed with Rh single atoms (PtBi@PtRh₁) showed excellent EOR performance in alkaline electrolyte, exhibiting high mass activities of 5417 mA/mg_Pt+Rh at 0.6 V and 13,020 mA/mg_Pt+Rh at the peak potential.¹⁴⁷ Significantly, such catalyst delivered a high C1-pathway selectivity of 24.6%, superior anti-poisoning abilities toward the CO* and CH₃CHO* intermediates, and excellent durability during 100,000 s operation. DFT calculations suggested that the tensile strain in the PtRh₁ overlayer and the presence of Rh single atoms together resulted in an enhanced adsorption of ethanol and key surface intermediates, as well as a lower activation barrier for the C − C bond cleavage (Figure 11B). Intermetallic Pt₃Sn nanocubes covered with defect-rich Pt atomic layers displayed tensile strain of approximately 4.4% along the [001] direction, leading to an upshift of the Pt d-band center, thereby facilitating the C − C bond cleavage for complete ethanol oxidation and suppressing the generation of poisoned CO intermediates.¹⁸⁹ Accordingly, it showed a superior specific activity (5.83 mA/cm²) and mass activity (1166.6 mA/mg_Pt) for EOR, compared to the unstrained Pt₃Sn intermetallic nanocubes, Pt nanocubes and the commercial Pt/C. Intermetallic Pt₃Ga nanoparticles covered with 2–3 atomic layers of Pt skins exhibited tensile strain in the Pt skins, which were calculated to be more energetically favorable for MOR than the unstrained counterpart.¹⁹⁰ The stronger OH* binding on the strained Pt skins also enabled easier removal of CO*. As a result, it showed superior specific activity (7.195 mA/cm²) and mass activity (1.094 mA/μg_Pt) than the unstrained counterpart and the commercial Pt/C. In addition to the tensile strain, some studies also highlighted the contribution from the compressive strain. For example, when Pt₃Mn nanoparticles coated with Pt skins were used as the catalyst toward MOR, the MOR activity was found to be dependent on the degree of ordering of the Pt₃Mn core, i.e., a higher degree of ordering corresponded to a better MOR activity.¹⁹¹ DFT calculations suggested that the compressive strain induced in the Pt skins resulted in a weakened CO binding and a smaller free energy change from CO* to COOH*, thereby promoting the MOR activity.

Very recently, studies have also demonstrated the promising catalytic performance of HEI nanocrystals toward small molecule oxidation reactions. For example, the hcp PtRhBiSnSb HEI nanoplates exhibited outstanding mass activities of 19.529, 15.558, and 7.535 A/mg_Pt+Rh toward the methanol, ethanol, and glycerol oxidation reactions, respectively, in alkaline media, superior to the quaternary PtBiSnSb, binary PtBi, and the commercial Pt/C.¹⁷⁷ Remarkably, the PtRhBiSnSb HEI nanoplates possessed outstanding stability, i.e., retaining 70.2% of the initial MOR activity even after 5000 cycles, maintaining stable current densities over 20000 s, and suppressing CO poisoning. DFT calculations showed that the excellent alcohol oxidation performance could be mainly ascribed to two factors, i.e., the presence of Rh atoms, which optimized the electronic structures of the active sites (Pt and Rh), thereby facilitating electron transfer and electroactivity, as well as the synergistic contributions of Bi, Sn and Sb atoms, which enabled the stabilization of valence states on the active sites via p − d orbital coupling. As another example, PtRhFeNiCu HEI nanoparticles also possessed a higher mass activity (914 mA/mg_Pt), better CO tolerance, and enhanced stability than the HEA counterpart and the commercial Pt/C.¹⁷⁸ DFT calculations indicated that compared to the disordered HEA, in the ordered HEI structure, the adsorption configuration of key reaction intermediates was changed, leading to reduced energy barrier and stronger C − C bond-breaking ability.

Nevertheless, intermetallic nanostructures are not always superior to their disordered counterparts toward small molecule oxidation reactions. Especially, when both metals are functional in a reaction, more care should be taken. For example, Pt₃Sn nanocubes with 60%-ordering exhibited 5.6 times higher activity toward MOR than those with 95%-ordering.¹⁹² This is because both Pt and Sn were active toward MOR. For the 60%-ordered Pt₃Sn, due to its lower compositional stability, oxidation of Sn⁰ to Sn⁴⁺ occurred during the electrochemical oxidation reaction. Compared to Sn⁰, the Sn⁴⁺ sites could bind OH* and oxidize the CO* intermediate adsorbed on Pt more efficiently, thereby leading to the enhanced activity of the 60%-ordered Pt₃Sn.

HER

HER is the key cathodic reaction for electrocatalytic water splitting. It requires high-performance electrocatalysts with lower overpotentials at the industrial-level current density, fast HER kinetics, high turnover frequency (TOF), and outstanding durability in both acidic and alkaline media. Pt is recognized as the most active metal toward HER, and great research efforts have been devoted to exploring high-efficiency Pt-based nanocatalysts.¹⁹³ Through constructing Pt-based binary or multinary intermetallic nanocrystals, the electronic structure of Pt could be modulated to optimize the adsorption and desorption of hydrogen, thereby promoting the HER activity. For example, intermetallic Pt₃Ge nanocrystals with the exposure of (202) facet exhibited superior catalytic performance toward HER, with low overpotentials of 21.7 and 96 mV at the current density of 10 mA/cm² in acidic and alkaline media, respectively.¹⁹⁴ Using an electrochemical flow cell, it exhibited a long-term durability of >75 h in alkaline media under the industrial-level current density of >500 mA/cm². Several factors, such as the electron transfer from Ge to Pt and the optimal hydrogen binding on the Pt₃Ge (202) facet, together contributed to its superior activity. Intermetallic Pt₃Ti nanoparticles, in situ formed by reducing Pt loaded Ti₃C₂T_x MXenes, exhibited superior HER performance in acidic media, with a low overpotential of 32.7 mV at 10 mA/cm² and a low Tafel slope of 32.3 mV/dec.¹⁹⁵ DFT calculations suggested that the (111) and (100) facets of Pt₃Ti nanoparticles had a comparable hydrogen binding as the Pt (111) facet, accounting for the promising activity. Furthermore, by incorporating a third metal, Co, into the ordered mesoporous PtZn nanoparticles, ternary intermetallic PtZnCo nanoparticles were obtained, which exhibited superior HER activity in alkaline media than the bimetallic PtZn nanoparticles, showing a high mass activity of 1.77 A/mg_Pt, a high specific activity of 3.07 mA/cm_Pt² at −0.1 V, high TOF values of 3.34 H₂/s at 20 mV and 11.88 H₂/s at 50 mV, and remarkable stability (only 7 mV loss after 50,000 cycles).¹⁹⁶ XPS analyses and the electrochemical CO stripping experiment together revealed that with the incorporation of Co, the ternary PtZnCo catalyst possessed a more electron-rich Pt surface than its binary counterpart, due to the enhanced electron transfer from Zn/Co to Pt atoms, contributing to the desorption of H₂ and thereby accelerating the HER kinetics (Figure 12A).

Figure 12. Electrocatalytic performance of intermetallic nanocrystals toward HER. (A) HAADF-STEM image and its corresponding EDS elemental mappings of a PtZnCo ternary nanoparticle (upper left panel), polarization curves (upper right panel), XPS Pt 4f spectra (lower left panel), and electrochemical CO stripping voltametric curves (lower right panel) of various catalysts. Reproduced with permission,196 copyright 2022 Wiley-VCH. (B) EDS elemental mappings of Pd3Pb@Pt nanoplates (upper left panel), polarization curves (upper right panel), and charge density difference between Pt and Pd3Pb (lower panel). Reproduced with permission,197 copyright 2019 American Chemical Society.

Considering the scarcity and high cost of Pt, low- and non-Pt HER electrocatalysts have been extensively explored. One of the strategies to maximize the Pt atom utilization efficiency is to grow thin layers of Pt skins over the non-Pt intermetallic nanocrystals. For example, intermetallic Pd₃Pb nanoplates coated with Pt sub-monolayer exhibited promising HER performance with a low overpotential of 13.8 mV at 10 mA/cm², a high mass activity of 7834 A/g_Pd+Pd at −0.05 V and excellent stability in acidic media.¹⁹⁷ DFT calculations suggested that electron transfer from the Pd₃Pb core to the Pt shell resulted in an upshift of the Pt d-band center, leading to stronger hydrogen binding on the Pt overlayer and thus enhanced HER activity (Figure 12B).

Besides, non-Pt intermetallic HER nanocatalysts have been explored as substitutions. For example, B2 intermetallic PdCu nanowires exhibited better HER performance in both acidic and alkaline media than the disordered counterpart, arising from the optimal hydrogen binding energy on the B2 PdCu surface.¹²³ Intermetallic Ir₃V exhibited a low overpotential of 9 mV at 10 mA/cm² and a high mass activity of 1200 A/g_Ir at the overpotential of 20 mV in alkaline media, which was 6.7, 9.4, and 3.3 times greater than those of the commercial Pt/C, Ir/C, and disordered Ir₃V catalysts, respectively.¹⁹² The enhanced HER kinetics was ascribed to the charge redistribution on the ordered Ir₃V surface with a maximized number of neighboring Ir/V sites compared to the disordered counterpart, thereby promoting water dissociation on the electron-deficient V sites. bcc intermetallic IrGa nanoparticles on N-doped rGO exhibited excellent performance toward both HER and OER, thus having a low cell voltage of 1.51 V to achieve 10 mA/cm² for the overall water splitting in alkaline condition.⁶¹ DFT calculations suggested that the high performance could be ascribed to the creation of electron-rich Ir via incorporating Ga with lower electronegativity, such that the electron transfer between Ir and the adsorbed species could be promoted, thereby decreasing the energy barriers of the potential determining steps. As another example, intermetallic IrMo nanoparticles supported on carbon nanotubes were demonstrated to be an efficient HER catalyst in both acidic and alkaline media, with a specific activity of 0.95 mA/cm_Ir² and a Tafel slope of 38 mV/dec in 1.0 mol/L KOH, which were close to those in 0.5 mol/L H₂SO₄ (i.e., specific activity of 2 mA/cm_Ir² and Tafel slope of 14 mV/dec).¹⁹⁸ In contrast, the commercial Ir/C and Pt/C catalysts showed a drastic drop of activities when the electrolyte was changed from acid to base. The small activity difference in acidic and alkaline electrolytes could be ascribed to the existence of Mo sites in the intermetallic IrMo nanocrystals that could stably adsorb OH*, thereby stabilizing the water dissociation product and decreasing the thermodynamics barrier. Besides, the L1₂ FeCoNiAlTi HEI prepared via the physical metallurgical technique exhibited promising HER activity in alkaline media, with an overpotential of 88.2 mV at 10 mA/cm², a Tafel slope of 40.1 mV/dec and outstanding durability, which were comparable to those of noble metal catalysts.¹⁷⁶ Theoretical calculations suggested that the excellent performance could be attributed to the electronic structure modification through incorporating five metal elements within a highly ordered L1₂ structure, as well as the optimization of hydrogen adsorption/desorption through the site-isolation effect induced by the intermetallic L1₂ structure.

CO₂/CO reduction reactions

Electrochemical CO₂/CO reduction reactions refer to the processes that convert CO₂/CO into value-added chemicals, during which C − H and C − C bonds are built to form hydrocarbons, acids and alcohols. In such processes, electrocatalysts are essential to activate the CO₂/CO molecules and achieve high production yield with high product selectivity. Capitalizing on the remarkable catalytic performance of Cu toward CO₂ reduction, Cu-based intermetallic nanocrystals have been widely explored. For example, three types of bimetallic Pd-Cu catalysts with ordered, disordered, and phase-separated atomic arrangements (atomic ratio of Pd/Cu = 1:1) were studied toward CO₂ reduction.¹⁹⁹ The ordered PdCu catalyst exhibited the highest selectivity for the C1 products such as CO and CH₄ (>80%), while the phase-separated PdCu catalyst exhibited the highest selectivity for the C2 products such as ethylene and ethanol (>60%), suggesting that C − C coupling was more favorable on surface with continuous Cu atoms. Four types of AuCu nanoparticles with similar size but different degrees of ordering were synthesized by annealing the disordered AuCu nanoparticles, in which the degree of ordering was controlled by tuning the annealing temperature and period.²⁰⁰ When they were used as catalysts for CO₂ reduction, CO and H₂ were the main products. With increasing degree of ordering, the selectivity for CO increased while that for H₂ decreased, i.e., a high CO Faradaic efficiency (FE) of 80% for the ordered AuCu nanoparticles and a low CO FE of 34% for the disordered counterpart. DFT calculations suggested that the superior catalytic performance of the ordered AuCu nanoparticles could be attributed to the formation of the compressively strained three-atom-thick Au layers over the intermetallic AuCu core during the thermal annealing-induced disorder-to-order transition. Interestingly, when the intermetallic core was changed to the ordered AuCu₃ nanoparticles, the Au overlayer exhibited an unconventional face-centered tetragonal (fct) phase, which was stabilized by the large interface strain.²⁰¹ Compared to the conventional fcc Au, the fct Au exhibited an upshift of the Au d-band center, thereby decreasing the energy barrier of COOH* formation and facilitating the CO₂-to-CO reaction kinetics. Accordingly, the AuCu₃@fct Au nanoparticles exhibited a high CO FE of 94.5% at −0.8 V, superior to the fcc Au nanoparticles (72.2%). In addition, Au-Cu intermetallic systems with three types of structures, i.e., Au₃Cu, AuCu and AuCu₃ nanoparticles, were compared toward CO₂ reduction.²⁰² In general, more types of products were generated with increasing amount of Cu and/or more negative potentials, while Au₃Cu nanoparticles gave the highest CO production rate compared to the others.

Besides Cu, other intermetallic nanocrystals have also been investigated as catalysts to electrochemically reduce CO₂ into value-added chemicals, such as formate. For example, intermetallic Pd₃Bi nanocrystals were reported as the efficient catalyst to produce formate, showing a high formate FE of approximately 100% and remarkable stability even at potentials more negative than −0.35 V, superior to the disordered Pd₃Bi which gave a lower formate FE of <60%.²⁰³ DFT calculations suggested that compared to pure Pd and the disordered Pd₃Bi, the atomic ordering of Pd and Bi within the intermetallic Pd₃Bi structure could enhance the *CO₂ adsorption, suppress the *CO binding and promote the *OCHO adsorption, leading to a high formate selectivity (Figure 13A). Intermetallic Ag₃Sn coated with an ultrathin SnO_x layer (Ag₃Sn@SnO_x) enabled the production of formate with a high FE of approximately 80% and partial current density of −16 mA/cm² at −0.8 V at an optimal SnO_x shell thickness of approximately 1.7 nm.²⁰⁴ DFT calculations suggested that the high electronic conductivity of the intermetallic Ag₃Sn core and the favorable stabilization of the *OCHO intermediate on the SnO (101) facet with oxygen vacancies together contributed to the good catalytic performance.

Figure 13. Electrocatalytic performance of intermetallic nanocrystals toward CO2/CO reduction reactions. (A) Scheme showing the transformation from ordered Pd3Bi to disordered Pd3Bi and their CO2 reduction products (upper panel), formate FEs and partial current densities at different potentials for various catalysts (middle panel), comparison in terms of binding energy of *CO2, *CO and *H on various catalysts (lower left panel), and the energetic trend of CO2 reduction to formic acid on various catalysts (lower right panel). Reproduced with permission,203 copyright 2021 Wiley-VCH. (B) HAADF-STEM image of a B2 PdCu nanoparticle (upper panel), current densities and FEs at different potentials (lower panel). Reproduced with permission,58 copyright 2022 Nature Publishing Group. FE, Faradaic efficiency.

As an example for CO reduction, intermetallic B2 PdCu nanoparticles were reported as a promising catalyst, showing a high acetate FE of 70 ± 5% and partial current density of 425 mA/cm² at −1.03 V (Figure 13B).⁵⁸ The intermetallic PdCu structure featured alternately aligned rows of Pd and Cu atoms, providing abundant Pd-Cu pairs. These Pd-Cu pairs were the catalytic active sites, promoting the absorption of *CO and stabilization of ethenone as a key intermediate, inhibiting the competing HER, thereby facilitating the formation of acetate.

Nitrogen reduction reaction

Electrochemical nitrogen reduction at ambient conditions has been developed as a sustainable approach to produce ammonia (NH₃), an important raw material in chemical industry and a promising carbon-free energy carrier, alternative to the energy-intensive Haber-Bosch process. Unfortunately, such approach is greatly limited by the weak N₂ adsorption on catalyst and the sluggish cleavage of the intrinsically inactive N ≡ N bond, leading to the production of NH₃ with low yield and selectivity. Therefore, it is essential to develop high-efficiency catalysts with enhanced adsorption and activation of N₂. Only a few papers have touched on the utilization of intermetallic nanocrystals as the NRR catalyst. A typical example is the nanoporous intermetallic Pd₃Bi nanocrystals, which were prepared by chemically etching the nanoporous PdBi₂.¹²⁴ When it was used as the NRR catalyst in 0.05 mol/L H₂SO₄ electrolyte, it exhibited a high NH₃ yield of 59.05 ± 2.27 µg/(h·mg_cat) and FE of 21.52 ± 0.71% at −0.2 V. Operando X-ray absorption spectroscopy studies and DFT calculations together suggested that the strong coupling between the Pd and Bi sites in the intermetallic structure was critical, such that N₂ was preferentially adsorbed and activated on the Bi sites, followed by the hydrogenation with protons adsorbed on the surrounding Pd sites, leading to the formation of NH₃ (Figure 14A).

Figure 14. Electrocatalytic performance of intermetallic nanocrystals toward NH3 synthesis. (A) Crystal structure of Pd3Bi (upper left panel), NH3 yield rates at different potentials for various catalysts (upper right panel), NH3 FEs at different potentials for various catalysts (lower left panel), and the adsorption energy of N2 on different catalyst surfaces (lower right panel). Reproduced with permission,124 copyright 2021 Wiley-VCH. (B) HAADF-STEM image of an intermetallic RuGa nanocrystal (upper left panel), ammonia yield rates and FEs at different potentials (upper right panel), and the reaction Gibbs free energy diagrams for bcc RuGa and hcp Ru (lower panel). Reproduced with permission,205 copyright 2022 Wiley-VCH. FE, Faradaic efficiency.

Considering the low solubility of N₂ in water and the intrinsically inert N ≡ N bond, other nitrogenous molecules can be used as substitutions, such as nitrate (NO₃⁻) that is abundant in wastewater and NO that can be found in air pollutants from the combustion of fossil fuels. The reduction of nitrate or NO into NH₃ enables the simultaneous production of valuable NH₃ and elimination of wastes, demonstrating a waste-to-valuable-resources concept. B2 intermetallic PdCu nanocubes have been used as catalyst for the reduction of nitrate into NH₃, showing a high FE of 92.5% at −0.5 V and a high yield rate of 6.25 mol/(h·g) at −0.6 V.²⁰ Machine learning and DFT calculations suggested that on the (100) facet of the B2 PdCu nanocubes, Cu atoms stabilized *NO₃ due to the upshift of the d-band center, while Pd atoms destabilized *N due to the larger interatomic coupling from the subsurface Pd, facilitating NO₃⁻ addsorption onto the catalyst surface and protonation of N-bonded species toward NH₃. The bcc intermetallic RuGa nanocrystals have been used as catalyst for the reduction of NO into NH₃, exhibiting a high NH₃ yield rate of 320.6 μmol/(h·mg_Ru) ad FE of 72.3% at −0.2 V in neutral media.²⁰⁵ DFT calculations suggested that in the intermetallic bcc RuGa structure, Ru atoms were electron-rich, as a result of the electron donation from Ga to Ru. The electron-rich Ru atoms facilitated the adsorption and activation of the *HNO intermediate, thereby reducing the energy barrier of the potential-determining step in NO reduction(Figure 14B).

CHALLENGES AND OPPORTUNITIES

In this minireview, the very recent progress of noble metal-based intermetallic nanocrystals, including binary, ternary, and HEI nanocrystals, has been summarized. Various synthetic strategies, including the conventional thermal annealing approach and its modifications, as well as the wet-chemical synthetic method, have been discussed, highlighting their strengths and limitations. Their electrocatalytic applications toward ORR, small molecule oxidation reactions, HER, CO₂/CO reduction reactions, and nitrogen reduction reaction have been discussed, with emphasis on how the intermetallic structures contribute to the enhanced catalytic performance. Despite these remarkable achievements, challenges and opportunities remain in many aspects.

In terms of synthesis, first, both the thermal annealing and wet-chemical synthesis methods for the preparation of intermetallic nanocrystals have their own merits and limitations. The conventional thermal annealing approach enables control over the degree of ordering by tuning the annealing temperature and period for many systems, but most of the products are limited to spherical nanoparticles because the high-index facets tend to reconstruct at high temperatures to minizine surface energy, disabling anisotropy in nanocrystals. The wet-chemical method, on the contrary, enables control over shape and architecture by using shape-directing agents. However, achieving high degree of ordering for some systems is difficult, because high temperature is usually required to overcome the thermodynamic energy barrier for ordering, which is, however, constrained by the solvent boiling point. To tackle these problems, continuous efforts are expected toward improving the existing methods and creating new synthesis protocols. It is essential to develop effective methods to achieve precise control over the nucleation and growth processes, and to facilitate atom diffusion for ordering under milder conditions.

Second, most reported works hitherto have been limited to bimetallic nanocrystals with ordered intermetallic structures, while the syntheses of intermetallic ternary, multinary and HEI nanocrystals remain challenging. Especially, due to the high tendency for phase segregation when more than five elements are integrated within one particle, it remains extremely difficult to construct HEI nanoparticles with high compositional homogeneity. Therefore, it is imperative to develop effective and general protocols for their syntheses, expanding the family of intermetallic nanomaterials.

Third, since the catalytic performance of metal-based nanocrystals can be affected by several structural parameters, such as size, shape, composition, architecture, phase and defect, synthesizing metal-based nanocrystals with different degrees of ordering, while maintaining other structural parameters the same is a prerequisite for investigating the ordering-dependent properties. Nonetheless, the construction of noble metal-based nanocrystals with simultaneous control over the degree of ordering and other structural parameters remains difficult, which calls for further studies. Besides, to further boost the intrinsic catalytic activity of noble metal-based intermetallic nanocrystals, more delicate control over the structural features of intermetallic nanocrystals is expected, such as the introduction of defects and the exposure of reactive facets.

Forth, the formation mechanisms of intermetallic nanocrystals need to be clearly understood, which can in turn guide the design and synthesis of new and unconventional intermetallic nanostructures. In-situ techniques, such as in situ TEM, XRD, and X-ray absorption spectroscopy, have been utilized to study the disorder-to-order transition mechanisms of bimetallic nanocrystals.^67,206 On the other hand, thermodynamic and kinetic studies on the nucleation and growth processes of intermetallic nanocrystals could also offer insights in their formation mechanisms. More studies are expected to unveil the general rules of intermetallic formation mechanisms.

In terms of electrocatalytic applications, first, since the nanocatalyst surface will come into contact with the gas and electrolyte molecules during the electrochemical reactions, surface reconstruction from their pristine state to a more active state would happen.²⁰¹ Besides, nanocatalysts may experience structural degradation and performance deterioration during long-term operation. Therefore, understanding the dynamic behavior of the catalyst surface and the degradation mechanism through advanced ex situ and in situ techniques is essential to reveal the real catalytic active sites, structural and electronic changes of catalysts during electrochemical reactions, etc. Coupled this information with theoretical calculations, the catalytic reaction pathways, the adsorption modes of intermediate species, and the rate determining steps can also be unveiled.

Second, so far, most of the noble metal-based intermetallic nanocatalysts are designed for ORR and small molecule oxidation reactions, while their electrocatalytic applications toward CO₂/CO reduction reactions, nitrogen reduction reaction, and C − N coupling reactions via nitrogen-integrated CO₂ reduction reactions are rarely explored. Considering the abilities to tune the electronic structures of active sites and improve catalytic durability via the construction of intermetallic nanocrystals, it is anticipated that intermetallic nanocrystals with well-controlled compositions and structures could serve as effective catalysts toward these reactions.

Third, to realize industrial commercialization, it is important to examine the as-prepared nanocatalysts in the industrially relevant conditions, e.g., high current densities, high temperatures, harsh acidic or alkaline media, and long-term operation. This is highly important, because the real industrial operation conditions can be quite different compared to the lab-scale conditions. Besides, to be more cost effective in industrial applications, it is necessary to partially or completely replace noble metals with the non-noble ones in intermetallic nanocrystals, while maintaining an acceptable catalytic performance. To this end, designing intermetallic cores covered with thin noble metal shells, intermetallic nanocrystals doped with noble metal single atoms, or even non-noble metal intermetallic nanocrystals could be possible solutions.

Overall, intermetallic nanocrystals, featuring their fascinating ordered structures and catalytic performance, have created considerable opportunities in the fields of material sciences and chemistry. Yet challenges exist in many aspects, which call for more research efforts to tackle the problems. It is envisioned that more breakthroughs in the controlled syntheses of intermetallic nanocrystals and utilization of them toward industrial-level energy-related applications will be achieved in the future.

ACKNOWLEDGMENTS

Qingyu Yan acknowledges funding support from Singapore MOE AcRF Tier 1 (Grant No. 2020-T1-001-031).

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.