Recent advances in metal-based Janus nanomaterials: synthesis and electrocatalytic applications

Abstract

Metal-based heterostructures have attracted increasing research interest due to their intriguing properties and promising applications, especially in catalysis. Regulating the spatial configuration of distinct components in metal-based heterostructures has been considered a promising route to modulate their properties, functions, and performances. In particular, metal-based Janus heterostructures with their components exhibiting unique asymmetric configurations have shown some novel merits and properties that are unattainable in other traditional symmetric architectures. This review highlights the most recent progress in metal-based nanomaterials with Janus architectures, focusing on their synthesis strategies and electrocatalytic applications. First, their typical synthetic approaches, including co-reduction synthesis, seed-mediated growth, post-synthetic treatment, and other methods, are systematically summarized. Then, the applications of metal-based Janus nanomaterials in a range of electrocatalytic reactions, including hydrogen electrocatalysis, oxygen electrocatalysis, small-molecule oxidation, nitrate reduction, and carbon dioxide reduction, are presented by highlighting their structure–performance relationship. Finally, current challenges and future directions in this exciting field are discussed.

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Biao Huang

Dr. Biao Huang is currently a Research Fellow at the Department of Materials Science and Engineering in the National University of Singapore. He obtained his B.E. and M.S. degrees from Chongqing University (China) in 2017 and 2020, respectively. Then, he received his PhD under the supervision of Prof. Hua Zhang from City University of Hong Kong (China) in 2024. His research focuses on the synthesis of metal-based nanomaterials and their catalytic applications.

Yiyao Ge

Yiyao Ge is currently a Full Professor at the State Key Laboratory for Advanced Metals and Materials in the University of Science and Technology Beijing. He obtained his B.E. degree in Engineering in 2012 at the University of Science and Technology Beijing (China) and received his PhD in Engineering in 2017 from Tsinghua University (China). As a Research Fellow, he joined Prof. Hua Zhang's group at Nanyang Technological University (Singapore) in 2017 and then at City University of Hong Kong (China) in 2020. After working at School of Materials Science and Engineering in Peking University (China) from 2022, he moved to the University of Science and Technology Beijing in 2024. His research interests focus on the rational synthesis and structural engineering of novel inorganic micro-/nanomaterials.

Ming Zhao

Dr. Ming Zhao is currently an assistant professor in the Department of Materials Science and Engineering at the National University of Singapore. He received his B.S. and M.E. degrees in Materials Physics and Chemistry from Nanjing University, China. In 2019, he obtained his PhD from the Georgia Institute of Technology. Before joining NUS, he worked as a postdoctoral associate at Cornell University. His current research interests include the rational design and chemical imaging of advanced nanocatalysts for sustainability applications.

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1. Introduction

With the rapid development of nanotechnology, the last two decades have witnessed a dramatic growth in research on metal-based nanomaterials because of their fascinating physicochemical properties as well as exceptional performances for different applications such as catalysis,^1–7 sensing,^8,9 energy storage,¹⁰ Raman scattering,^11–13 and drug delivery.^14,15 For instance, the high surface-to-volume ratio and tunable electronic structures of metal nanomaterials enable enhanced catalytic activity, and the localized surface plasmon resonance of plasmonic metals, such as Au and Ag, contributes to sensitive optical sensing and surface-enhanced Raman scattering.¹⁶ Besides, the diverse nanostructures, structure-dependent light response, and good biocompatibility of Au-based nanomaterials have opened new avenues for drug delivery with high drug loading and controllable targeting in bioscience.¹⁵ Especially, metal-based nanomaterials show great potential as high-performance catalysts for versatile electrocatalytic reactions owing to their good chemical stability and excellent intrinsic activity.^17–22 To date, numerous efforts have been dedicated to developing novel metal-based catalysts with controlled sizes, shapes, compositions, and nanostructures for achieving highly efficient catalysis.^2,23–26 Recently, upgrading the composition of metal-based nanomaterials from monometallic to multi-metallic heterostructures with different components has been considered a promising strategy to obtain diverse functionalities and enhanced catalytic performances in metal-based electrocatalysts,^27–31 owing to the synergistic effects between these components. Typically, the spatial configuration of the distinct constituents in these complex heterostructures plays a crucial role in determining their electrocatalytic performances, and various configurations in metal-based nanomaterials have been explored.^32,33

The Janus architecture, named after the Roman god with two faces heading in opposite directions, has attracted a lot of attention in numerous applications since it was first proposed at the Nobel Lecture of P. G. de Gennes in 1991 owing to its unique asymmetric configuration,^33–35 thus integrating different physicochemical properties in a single architecture. The broken symmetry of the Janus architecture also endows it with some merits and properties that are unattainable in other traditional symmetric architectures, such as the synergistic combination of various electrochemical properties with minimal mutual interference from different components and promoted functionalization with an anisotropic structure that could facilitate the transfer of intermediates during the reaction. These unique merits could be beneficial in achieving highly efficient catalysis using metal-based Janus nanocatalysts.^36–38 In particular, Janus nanocatalysts can be ideal candidates for tandem catalysis, which involves multiple catalytic steps occurring sequentially or simultaneously in one system to yield target products, such as the electrochemical carbon dioxide reduction reaction and nitrate reduction reaction.^39–42 By integrating components suitable for different catalytic steps during tandem catalysis, Janus nanocatalysts exhibit enhanced activity and superior selectivity for specific products. More importantly, their separated configuration makes it possible to distinguish the role of different components during catalysis and build structure–performance correlations, which are difficult to accomplish on homogeneous catalysts.

In this review, we provide a comprehensive and focused summary of the recent advances in metal-based Janus nanomaterials with particular attention to their synthesis strategies and electrocatalytic applications across a variety of reactions (Scheme 1). Firstly, the typical synthetic strategies to obtain metal-based Janus nanomaterials will be discussed, including co-reduction synthesis, seed-mediated growth, post-synthetic treatment, and other methods. The principles of the formation of Janus nanostructures in these synthetic strategies are also summarized. Then, we introduce the application of these metal-based Janus nanomaterials for various electrocatalytic reactions, including hydrogen electrocatalysis, oxygen electrocatalysis, small-molecule oxidation reaction, nitrate reduction reaction, and carbon dioxide reduction reaction, highlighting the unique advantages of asymmetric Janus structures for highly efficient electrocatalysis. Lastly, we give our understanding of the challenges and promising research directions in the future in this intriguing field. Compared to previous reviews, our work not only systematically summarizes the synthetic strategies of Janus metal nanomaterials, but also correlates the structural features of metal-based Janus architectures with their catalytic performances across various electrocatalytic reactions. Moreover, by highlighting the underlying mechanism for the formation of Janus architectures and the structure–performance relationships of Janus metal nanocatalysts, this review aims to provide new insights into the design of novel Janus metal nanomaterials and how their asymmetric structure contributes to enhanced activity and selectivity, which can not only help clarify the unique advantages of Janus nanostructures but also provide guidance for the rational design of next-generation electrocatalysts.

Scheme 1 Schematic of the overview of this review.

2. Synthesis of metal-based Janus nanomaterials

In this section, an overview of the primary synthesis approaches used to prepare metal-based Janus nanomaterials will be presented, including co-reduction synthesis, seed-mediated growth, post-synthetic treatment, and other emerging strategies. Each approach offers unique pathways to tailor the size, shape, interface, and composition of Janus nanomaterials. The mechanism of their formation process will be summarized, and the key factors that influence the formation of Janus nanostructures during their synthesis will also be highlighted.

2.1. Co-reduction synthesis

Co-reduction synthesis refers to the one-step method where the asymmetric feature of Janus metal-based nanomaterials is formed spontaneously during the nucleation and growth process without using pre-synthesized seeds, which usually relies on the phase separation of different components during the reduction or precise modulation of the reaction kinetics. This method is simple and scalable, making it attractive for large-scale applications. However, achieving uniformity and reproducibility remains a challenge due to the complexity of the reaction kinetics and thermodynamics. To successfully prepare Janus nanostructures, the experimental conditions, such as the choice of surfactants/solvents/metal precursors, the concentration of the precursors, and reaction temperature, should be precisely controlled during co-reduction synthesis to avoid the alloying of different components.

The direct synthesis of Janus nanostructures via the co-reduction of thermodynamically immiscible metals represents a robust and widely adopted strategy. The intrinsic immiscibility between the metals can prevent the alloy formation, favoring phase separation and leading to the formation of Janus nanostructures after the co-reduction of the metal precursors. In a typical work recently reported by Chen et al.,⁴³ the structural evolution of the Au–Rh bimetallic system was systematically investigated, and the formation mechanism of Janus Au–Rh nanostructures was also studied. A block co-polymer micelle-templated synthesis strategy was employed to prepare various Au–Rh nanostructures, wherein polymer micelles served as nanoreactors to confine the co-reduction of the Au and Rh precursors during high-temperature heating. By carefully tuning the ratio between the metal precursors and the annealing conditions, a library of Au–Rh nanoparticles with diameters ranging from 4 to 1 nm and varying compositions was obtained. It was found that the particle size played an important role in influencing the mixing of Au and Rh and the formation of a Janus structure. As shown in Fig. 1a, at larger particle sizes of about 3 to 4 nm, the Au and Rh elements were observed to separate into distinct domains within individual particles due to the immiscible nature of the Au–Rh system, leading to the formation of well-defined Au–Rh Janus nanostructures with sharp phase boundaries, as confirmed by the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image (Fig. 1b) and corresponding elemental energy-dispersive spectroscopy (EDS) mapping (Fig. 1c). As the particle size decreases to below 2 nm, the proportion of surface atoms increases significantly, enhancing surface effects that lower the mixing enthalpy between Au and Rh to form Au–Rh alloys. Besides, the composition of the Au–Rh system is another critical factor in obtaining Janus heterostructures. Specifically, nearly equal atomic compositions (e.g., Au_0.5Rh_0.5) exhibited the strongest repulsive interactions between the Au and Rh elements, thus favoring the formation of phase-separated Janus structures at larger sizes. In contrast, the Au–Rh alloy structure could be readily formed, even at slightly larger particle sizes in the case of a high proportion of Au or Rh (e.g., Au_0.85Rh_0.15 and Au_0.15Rh_0.85). In another work, Lou et al. demonstrated the synthesis of a Janus Cu–Ni dimer by carefully controlling the temperature during the annealing process of Co and Ni precursors.⁴⁴ Unlike the Co–Ni solid solution alloy, which was obtained by annealing at 700 °C, the Janus Co–Ni structure was prepared by annealing at a lower temperature of 500 °C. It was claimed that this lower annealing temperature reduced the inter-diffusion of atoms, and thus prevented the complete mixing of the Cu and Ni atoms, leading to phase separation and the formation of a Janus structure with Cu-rich and Ni-rich domains. These two works highlight the importance of regulating the phase separation by finely tuning the heating temperature for the successful preparation of Janus metal nanomaterials.

Fig. 1 (a) HAADF-STEM images of Au–Rh nanoparticles with sizes ranging from 4 to 1 nm. A schematic of the transition from Janus Au–Rh to Au–Rh alloy by decreasing the size of the Au–Rh nanoparticles is shown below. Scale bars, 1 nm. (b and c) HAADF-STEM images (b) and EDS elemental mapping (c) of a representative Janus Au–Rh nanoparticle. Scale bars, 2 nm. Reproduced with permission from ref. 43. Copyright 2024, Springer Nature. (d) EELS mapping images and (e) line scanning EELS spectra across the Ru–CrO_x heterostructure. Reproduced with permission from ref. 45. Copyright 2024, Springer Nature. (f) Schematic illustration of the synthesis of Co/Co₉S₈ Janus heterostructure. Reproduced with permission from ref. 46. Copyright 2022, Wiley-VCH.

The combination of metals with their corresponding compounds (e.g., oxides, sulfides, and phosphides) offers a direct and effective route for constructing Janus structures, primarily driven by the intrinsic differences in their crystal structures and surface energies, which collectively drive phase separation and directional growth during co-reduction synthesis. As a typical example, Zhang et al. successfully prepared a crystalline Ru-amorphous CrO_x Janus cluster–cluster heterostructure on nitrogen-doped carbon by engineering interfacial bonding between the clusters.⁴⁵ The synthesis was conducted via the controlled pyrolysis of mixed metal precursors at 400 °C under a reductive atmosphere. The formation of this crystalline-amorphous Janus structure with the separation between Ru and CrO_x was indicated by electron energy loss spectroscopy (EELS) mapping (Fig. 1d) and line scans (Fig. 1e). The reaction temperature as well as the ratio between the Ru and Cr precursors were found to be crucial for obtaining Janus Ru-CrO_x clusters with a well-defined interface. Specifically, temperatures lower than and higher than 400 °C would lead to insufficient nucleation and growth of the Ru and Cr–O species and large crystalline Ru clusters due to significant sintering, respectively. Besides, the optimal Ru/Cr precursor ratio was determined to be 1/1, while other ratios induced an excess of one component, resulting in poor interface coupling and fewer heterostructures. In another work, Suryanto et al. demonstrated the preparation of Ni–Fe₂O₃ Janus nanoparticles by annealing Ni-oleate and Fe-oleate reverse micelles under Ar gas at 350 °C, where rapid Fe₂O₃ nucleation was followed by Ni reduction and selective overgrowth to yield Ni–Fe₂O₃ Janus nanoparticles.⁴⁷ By contrast, changing the annealing conditions to H₂ gas and a higher temperature of 700 °C drove full mixing and alloy formation of Ni and Fe. In another work, Lu et al. presented the general preparation of a series of Janus heterostructures composed of transition metal alloys and their corresponding sulfides (TM/TMS) through an ultrafast high-temperature shock (HTS) strategy.⁴⁶ In this method, mixed precursors of metals and sulfur species were deposited onto carbon nanofibers and subjected to a rapid Joule-heating process under an inert atmosphere. During the synthesis, sulfide phases were formed at the beginning through sulfurization at high temperatures. Subsequently, the strong reductive environment and ultrafast heating selectively reduced parts of the sulfides back to metallic states, particularly at high-energy crystal facets, while the other part maintained the sulfide phase during rapid cooling, ultimately forming a TM/TMS Janus heterostructure (Fig. 1f). It was found that the ultrafast heating (∼3000 K s⁻¹) and cooling rates (2000 K s⁻¹) enabled by the HTS process played a decisive role, in which ultrafast heating ensured the partial reduction of sulfides to metals, while the rapid cooling prevented extensive diffusion and recrystallization, kinetically freezing the Janus configuration. Interestingly, the formation of different Janus configurations was controllable by tuning the experimental parameters. Specifically, the heating temperature and the reaction duration critically determined the degree of sulfide reduction, and thus the metal-to-sulfide ratio in the Janus heterostructure. Smaller applied voltages induced lower heating temperatures and led to a higher ratio of sulfide, whereas higher voltages resulted in the greater reduction of sulfide and a larger proportion of metal domains. By changing the HTS conditions, the elemental composition of the TM/TMS Janus structures could be modulated from unary (e.g., Co/Co₉S₈) to binary, ternary, and even quaternary alloy/sulfide systems (e.g., FeCoNiCu/(FeCoNiCu)₉S₈), demonstrating the versatility of this approach.

2.2. Seed-mediated growth

Seed-mediated growth is a powerful and widely used strategy for the controlled fabrication of metal-based Janus nanomaterials, owing to its excellent versatility and tunability.^48–50 Typically, monodisperse metal nanocrystals with different morphologies are first prepared and used as seeds, onto which other materials are selectively deposited under precisely controlled reaction conditions. By modulating factors such as the choice of surfactant, precursor concentration, reaction temperature, and the injection rate, asymmetric nucleation and directional growth can be achieved, resulting in the formation of Janus architectures rather than core–shell or alloyed structures. Compared to co-reduction synthesis, seed-mediated growth can decouple the nucleation and growth steps, which allows precise control over the morphology, composition, and phase distribution of the final Janus metal-based nanomaterials, thus offering much higher versatility for preparing desired Janus nanostructures.

Rational control over the reaction kinetics by tuning parameters such as using a low precursor injection rate with a syringe pump, reductant with weak strength, and appropriate surfactants can effectively slow down the supply of metal precursors and atoms during the growth of the secondary materials, thereby promoting the asymmetric growth and formation of Janus heterostructures. For example, the injection rate of the Au precursor was reported to be a decisive factor that determined the growth pattern of Au on pre-synthesized penta-twinned Pd decahedral seeds during the seed-mediated synthesis of a Pd–Au Janus heterostructure.⁵¹ The reaction temperature during the growth of Au was kept at a low value of 37 °C, which minimizes the surface diffusion of Au atoms on Pd seeds. A schematic illustration of the seed-mediated growth of Au on Pd nanocrystals using different injection rates of Au precursors is shown in Fig. 2a, where with slow injection of the Au precursor, the deposition kinetics of Au atoms was low enough to achieve asymmetric growth but outperformed surface diffusion, thereby yielding Pd–Au Janus icosahedra. In contrast, fast injection induced rapid reaction kinetics, driving the symmetric lateral growth of Au on Pd and yielding concave core–shell starfishes. In another work, Ma et al. reported the seeded synthesis of Ag–Cu Janus nanostructures with exposed (100) facets (JNS-100) via the confined growth of Cu on one face of pre-synthesized Ag nanocubes.⁵² The formation of JNS-100 was achieved by finely tuning the reduction kinetics using a syringe pump to slowly add the Cu precursor, reductant, and surfactant during the growth of Cu on Ag. By modulating the amount and injection speed of Cu precursors, the seed-mediated growth of Cu on Ag nanocubes could be confined to one face, while the size of Cu domains changed accordingly, yielding three different Ag–Cu Janus nanostructures with varying Ag/Cu atomic ratios (Ag₆₅–Cu₃₅, Ag₅₀–Cu₅₀, and Ag₂₅–Cu₇₅ JNS-100), as confirmed by the HAADF-STEM image and EDS mapping results (Fig. 2b–d), respectively. More recently, the same group extended their strategy to fabricate fcc-2H-fcc (fcc: face-centered cubic, 2H: hexagonal close-packed) Au–Cu Janus nanostructures (JNSs) via the epitaxial growth of Cu on one side of Au nanorods featuring an unconventional fcc-2H-fcc heterophase by carefully tuning the reaction kinetics enabled by modulating the experimental parameters during the seed-mediated growth of Cu on Au nanorods (Fig. 2e–i).⁵³ Importantly, it was found that the choice of reductant and injection rate of Cu precursors significantly influenced the configuration of the obtained Au–Cu heterostructures. Specifically, using 1,2-hexanediol (HDO) as the reductant and a syringe pump to deliver the Cu precursor at 0.094 mL min⁻¹ yielded Janus structures, in which a Cu island decorated only one side of an Au nanorod. Varying the reductant to 1,2-hexadecanediol (HDD) with a slower injection rate (0.062 mL min⁻¹) produced Au–Cu co-axial heterostructures in which Cu covers the Au nanorod except for its two ends, while the one-shot addition of the Cu precursor with 1,2-butanediol (BDO) as the reductant gave an Au–Cu core–shell heterostructure, where the Au nanorod is completely covered by a Cu shell.

Fig. 2 (a) Schematic of two growth pathways of Au on Pd decahedra during seed-mediated growth by modulating the injection rate of the Au precursor. Slow injection promotes uniform shell growth, while rapid injection induces asymmetric deposition, leading to Janus Pd–Au structures. Reproduced with permission from ref. 51. Copyright 2023, the American Chemical Society. (b–d) HAADF-STEM images and corresponding EDS elemental mapping of Ag–Cu Janus heterostructures (Ag–Cu JNS-100), synthesized by seed-mediated growth of Cu on Ag nanocubes under kinetically controlled conditions using syringe-pump-assisted injection of Cu precursors and surfactants: Ag₆₅–Cu₃₅ JNS-100 (b), Ag₅₀–Cu₅₀ JNS-100 (c), and Ag₂₅–Cu₇₅ JNS-100 (d). Reproduced with permission from ref. 52. Copyright 2022, Wiley-VCH. (e) HAADF-STEM image and (f) EDS mapping of fcc-2H-fcc Au–Cu Janus nanostructures (Au–Cu JNSs), prepared via seed-mediated growth of Cu on one side of fcc-2H-fcc Au nanorods. (g and h) Schematic of the unit cell (top panel) and characteristic crystal plane (bottom panel) for the fcc phase (g) and 2H phase (h). (i) Crystal model of the obtained fcc-2H-fcc Au–Cu JNSs, showing the spatially segregated domains formed under carefully optimized reductant and Cu precursor injection conditions. Reproduced with permission from ref. 53. Copyright 2024, Wiley-VCH.

Besides controlling the supply rate of metal precursor, changing other experimental conditions, such as the reduction potential of metal ions and pH value, is also effective for suppressing the reaction kinetics, thus promoting the formation of Janus nanostructures. For instance, Ag–Cu Janus nanostructures were constructed via the asymmetric epitaxial deposition of Cu on pre-synthesized two-dimensional (2D) Ag nanoplates,⁵⁴ during which iodide ions (I⁻) were introduced to slow down the deposition kinetics of Cu atoms by altering the reduction potential of the Cu²⁺ precursor, favoring the nucleation of Cu at the high-energy edge sites of the Ag nanoplate. As shown in Fig. 3a, this well-controlled reaction kinetics during the seed-mediated growth, along with the large lattice mismatch (11.4%) between Ag and Cu, promoted the asymmetrical growth of Cu on Ag nanocrystals, leading to the formation of spatially segregated 2D–2D Janus configurations and yielding a 2D Ag–Cu Janus heterostructure (Fig. 3b). In contrast, an Ag@Cu core–shell structure with conformal Cu shell growth was formed without adding I⁻ during the reaction, which resulted in fast reaction kinetics and a conventional symmetrical growth mode of Cu (Fig. 3c), highlighting the critical role of kinetic control in steering the anisotropic growth. Besides, the size of the Ag nanoplates and the Cu domains in the Ag–Cu Janus heterostructures could be modulated by varying the size of the Ag seeds and the atomic ratio of Ag/Cu, respectively. By using a similar strategy, Sang et al. demonstrated the seed-mediated synthesis of Pd–Au Janus heterodimers by precisely controlling the nucleation dynamics of Au onto pre-synthesized Pd nanocubes to achieve asymmetrical growth.⁵⁵ The key to the formation of the Janus nanostructure is to slow down the reaction kinetics by lowering the concentration of the Au precursor and modulating the pH of the growth solution. Specifically, a lower concentration of Au precursor would lead to the slower deposition of Au atoms, and thus shift Au deposition from symmetrical growth to site-selective growth, forming Pd–Au Janus heterostructures. In contrast, increasing the Au concentration would result in the more dispersed deposition of Au on different sites of Pd seeds. Besides, high acidity (adding HCl) led to reduced reaction kinetics, thus favoring the formation of a Pd–Au Janus dimer, whereas a higher pH value (adding NaOH) accelerated the kinetics, promoting a uniform Au coating all over the Pd seed.

Fig. 3 (a) Schematic of seed-mediated growth of Cu on 2D Ag nanoplates to form Ag–Cu 2D–2D Janus nanostructures, enabled by the addition of iodide ions (I⁻) to lower the reduction potential of Cu²⁺ and promote nucleation at high-energy edge sites. (b and c) TEM images with the corresponding model of Ag–Cu core@shell (b) and Ag–Cu Janus nanoplate (c). Reproduced with permission from ref. 54. Copyright 2022, Elsevier Inc. (d) Schematic showing modulation of the Au–Ag interfacial contact area through selective surface functionalization of Au seeds using PDDA and FSDNA ligands, enabling controlled growth of Au–Ag Janus heterostructures. (e–i) Corresponding TEM images of dimers (e), Janus (f), acorn (g), eccentric core–shell (h), and core–shell (i) Au–Ag heterostructures. Reproduced with permission from ref. 56. Copyright 2022, Wiley-VCH. (j and l) Schematic of Au–Cu Janus nanostructures grown on different Au seed morphologies using a thiol-ligand-assisted seed-mediated method: (j) Au nanosphere (AuNS)–Cu Janus structures. (k) Au nanorod (AuNR)-Cu heterostructures. (l) Au nanoplate (AuNPL)-Cu heterostructures. Reproduced with permission from ref. 57. Copyright 2022, the American Chemical Society.

Another effective strategy for the seed-mediated synthesis of Janus metal heterostructures is to adjust the surface energy of the seeds and create specific deposition sites on the metal seeds, thereby enabling the asymmetrical growth of the secondary metal. For example, Zeng et al. introduced two ligands, including poly(diallyldimethylammonium) chloride (PDDA) and fish sperm DNA (FSDNA), to selectively functionalize the surface of Au seeds for the overgrowth of Ag.⁵⁶ Since the FSDNA-modified Au surface possesses a higher atomic adsorption energy than the PDDA-modified surface, Ag atoms would deposit and grow preferentially on the PDDA-rich areas of Au, thus forming Au–Ag Janus nanostructures. This strategy also enabled continuous control over the Au–Ag interfacial area by the selective functionalization of the surface of Au by tuning the amount of PDDA (Fig. 3d). With an increase in PDDA concentration, the deposition of Ag during the seed-mediated growth would occur on a larger area of the Au surface, and the structure of the obtained Au–Ag gradually changed from dimer (Fig. 3e), Janus (Fig. 3f), acorn (Fig. 3g), and eccentric core–shell (Fig. 3h) to a concentric core–shell architecture (Fig. 3i) with enhanced contact areas. Moreover, this strategy showed good versatility, allowing the seed-mediated growth of Ag islands on Au nanocrystals including octahedra, decahedra, and bipyramid. Fan et al. introduced a ligand-assisted, seed-mediated strategy to construct Au–Cu Janus heterostructures by exploiting strong thiol ligands to elevate the interfacial energy between Au seeds and Cu, thereby driving island-type Cu overgrowth on the Au surface.⁵⁷ By varying the surface coverage of thiol ligands on the Au seeds, continuous modulation of the Janus asymmetry with varying Cu domain sizes was achieved. Notably, this ligand-assisted approach for the seed-mediated growth of Cu on Au proved broadly applicable across diverse Au seeds with different morphologies, including Au nanospheres (Fig. 3j), Au nanorods (Fig. 3k), and Au nanoplates (Fig. 3l), demonstrating its versatility in preparing shape-dependent Au–Cu Janus architectures. Recently, Zhang et al. introduced a semi-affinity, seed-mediated protocol using Au nanospheres as the seed, which was modified with two ligands, including 1-hexadecylamine (HDA) and PDDA, to prepare Au–Cu Janus heterostructures with continuously adjustable interfaces.⁵⁸ It was found that the amount of HDA and the reaction temperature were the two decisive factors that influenced the growth mode of Cu on the Au seed. Specifically, adding more HDA and increasing the temperature would promote the reduction kinetics of Cu²⁺ and the coverage of HDA on the Cu surface, which led to an enhanced contact area between the Au and Cu domains and achieved the formation of a series of Au–Cu nanostructures from isolated Au–Cu dimer and Janus structures to acorn-like Janus structure and core–shell structures. Zheng et al. introduced N-oleyl-1,3-propanediamine (OPDA) as the ligand during the seeded growth of an Au–Cu Janus heterostructure, wherein Cu atoms were selectively deposited onto one side of concave cubic Au seeds.⁵⁹ It was claimed that the existence of OPDA led to high interfacial energy, which promoted the island growth mode of Cu and facilitated the formation of a Janus nanostructure. The reductivity of the Cu precursor in the growth solution was also found to be an important parameter that influenced the architecture of the Au–Cu products. Unlike water-soluble Cu precursors, offering a fast supply of Cu atoms, the use of Cu(acac)₂ as the Cu precursor, which possesses limited solubility, and thus low reductivity, could keep the slow reduction, and thus single-site nucleation pathway of Cu, ensuring the high purity of Au–Cu Janus nanostructures. In another work, Xia et al. proposed a Bi underpotential deposition (UPD)-assisted seed-mediated method for the synthesis of Pd@Bi–PdBi Janus nanodimers.⁶⁰ Specifically, Bi was deposited as a sub-monolayer across all facets of pre-synthesized Pd octahedral seeds by Bi UPD, creating high-energy Pd–Bi surface sites that dramatically lower the barrier for heterogeneous PdBi nucleation, which switched the following growth of PdBi from layer-by-layer mode (core–shell structure) to island growth mode (Janus or core-satellite structure). In addition, the degree of supersaturation was also claimed as a vital factor that determines the multi-site or single-site growth pathways during the island growth process. Increasing the pH (adding more HCl) or decreasing the ascorbic acid concentration would result in slower reduction and lower degree of supersaturation, which transformed the growth from multi-site to single-site, leading to a transformation from core-satellite to Janus dimers.

The intrinsic lattice mismatch between different metals can harness interfacial strain to direct the selective nucleation and island growth of a secondary metal on the seed, therefore yielding an asymmetric Janus nanostructure. As a typical example, Lyu et al. reported the seed-mediated synthesis of Pd–Cu Janus nanocrystals using Pd icosahedral seeds.⁶¹ The large lattice mismatch (7.1%) between Pd and Cu was the key to the preferential deposition of Cu from one particular site on the Pd seed. Moreover, the shape of the obtained Pd–Cu Janus nanocrystals could be controlled by tuning the reduction rate of the Cu precursor. Under slow reduction, Cu atoms preferentially nucleate at a single vertex of the Pd icosahedron and proliferate into Janus Pd–Cu pentagonal bipyramids (Fig. 4a1–a3) or decahedra (Fig. 4b1–b3), whereas faster kinetics shift the nucleation to the edges to yield singly twinned, truncated bitetrahedra (Fig. 4c1–c3). The TEM, scanning TEM (STEM), and EDS mapping results confirm the Janus architecture of these Pd–Cu nanostructures with clear elemental separation. In another work, Zhu et al. demonstrated the growth of Cu on Au@Ag core–shell nanorods with different Au/Ag ratios.⁶² Due to the lattice mismatch of Cu and Ag, the overgrowth of a large amount of Cu induced the reconstruction of the Ag shell of different Au@Ag nanorods, leading to the formation of various AuAg–Cu Janus nanostructures with Cu domains on the tip of the AuAg nanorods. In another work, a novel strategy was employed to fabricate Janus Au–Cu nanojellyfish (JNF) structures using Au nanoflowers with twinned tips as the seed (Fig. 4d).⁶³ This asymmetrical growth was achieved through the reduction of Cu²⁺ ions in the presence of hexadecylamine (HDA), which stabilized the Cu domain and suppressed oxidation. The selective deposition of Cu onto one end of the Au nanoflower was directed by the presence of lattice mismatch and local strain fields at the twin defects, which created energetically favorable sites for selective Cu nucleation. During the reaction, several reaction parameters, including the type and concentration of Cu precursors, the concentration of the surfactant, and the solution pH, were found critical for the generation of Janus nanostructures due to their influence on the reaction kinetics. For instance, under a low concentration of surfactant, less dense coverage of the surfactant could be achieved at the tips of Au seeds, which became the preferred nucleation site for Cu and promoted the subsequent overgrowth of a Cu domain on one tip. Jia et al. developed a general seed-mediated strategy to synthesize a series of Janus Au–Cu nanostructures (Au–Cu JNCs) with different morphologies by exploiting the substantial lattice mismatch between Au and Cu to induce asymmetric growth, as illustrated in Fig. 4e.⁶⁴ Using penta-twinned Au nanobipyramids (Au NBPs) as the seeds, Cu nanodomains were selectively deposited on one side of the Au surface under carefully tuned kinetic conditions. The growth followed the island growth mode, where Cu nucleated preferentially on the high-index facets of the Au NBPs, leading to spatially separated Au and Cu domains with a well-defined interface, as shown in the TEM image (Fig. 4f). The asymmetric architecture was achieved through a combination of large lattice mismatch between Au and Cu with optimized reaction parameters such as controlled surfactant concentrations and selection of appropriate Cu precursors. For instance, the use of appropriate concentrations of cetyltrimethylammonium bromide (CTAB) and hexadecylamine (HDA) was found to regulate the site-selective deposition and prevent random nucleation or aggregation. This synthetic approach was shown to be highly versatile, enabling the formation of Janus heterostructures on various Au nanocrystal geometries, including nanospheres (Fig. 4g), nanorods (Fig. 4h), and nanoplates (Fig. 4i), thereby demonstrating the robustness and broad applicability of the symmetry-breaking growth method.

Fig. 4 (a1) TEM, (a2) STEM, and (a3) EDS mapping images of Janus Pd–Cu pentagonal bipyramids; (b1) TEM, (b2) STEM, and (b3) EDS mapping images of Janus Pd–Cu decahedra; (c1) TEM, (c2) STEM, and (c3) EDS mapping images of Janus Pd–Cu truncated bitetrahedra. The red dashed lines indicate twin boundaries. Reproduced with permission from ref. 61. Copyright 2020, the American Chemical Society. (d) Schematic of the selective growth of Cu on the tip of Au nanoflowers. Reproduced with permission from ref. 63. Copyright 2022, Elsevier Inc. (e) Schematic of the growth of Cu on Au nanocrystals with different morphologies. (f–i) TEM images of various Janus Au–Cu heterostructures obtained using Au nanobipyramids (f), nanospheres (g), nanorods (h), and nanoplates (i) as the seed. Reproduced with permission from ref. 64. Copyright 2021, Wiley-VCH.

Besides metals with an intrinsic large mismatch, some effective strategies have been developed to modulate the lattice mismatch between metals to favor the growth of Janus nanostructures. For example, Kong et al. introduced a surface-doping-mediated seeded growth method to construct Au–Ag Janus nanomaterials from trisoctahedral Au seeds,⁶⁵ despite the near-zero lattice mismatch between Au and Ag. It was found that the doping of trace amounts of Pt and Ag atoms on the Au surface could induce lattice strain, which disrupted the epitaxial symmetry and altered the growth mode of Ag from conformal growth to island growth, leading to the directional deposition of Ag on only one side of the Au seeds. In contrast, Au@Ag core–shell nanoparticles were obtained without Pt/Ag doping of the Au seeds. Moreover, the size and shape of the Janus Au–Ag nanomaterials could be further modulated by varying the concentration of the Ag precursors or the size/quantity of the Au seeds. This work provides an effective strategy to increase the lattice mismatch by atom doping and alter the growth mode of metals. Due to the unique long-range disordered structure of amorphous nanomaterials, the lattice mismatch between amorphous and crystalline nanomaterials could also be used to guide the synthesis of Janus nanomaterials with an amorphous-crystalline heterophase. In a typical work, an amorphous NiFeP-crystalline Ag Janus nanostructure was synthesized via the asymmetric growth of crystalline Ag on amorphous NiFeP seeds.⁶⁶ Initially, amorphous NiFeP seeds were formed by reducing Ni(acac)₂ and Fe(acac)₂ precursors in the presence of tri-n-octylphosphine (TOP), which acted as both a phosphorus source and a stabilizing ligand. Subsequently, the Ag precursor was introduced to selectively nucleate Ag domains on the pre-synthesized NiFeP seeds, yielding dumbbell-like nanoparticles with distinct Ag and NiFeP regions. It was claimed that the large lattice mismatch between the amorphous and the crystalline regions resulted in the island growth mode of crystalline Ag, thus yielding a Janus heterostructure. More recently, Cheng et al. reported a site-selective growth approach based on a galvanic replacement reaction to prepare amorphous Se-crystalline Au Janus nanostructures.⁶⁷ Amorphous Se nanospheres were first synthesized and dispersed in aqueous solution. Upon the addition of HAuCl₄ under mildly acidic conditions, Au³⁺ ions were reduced by the Se atoms on the surface of amorphous Se nanospheres through a galvanic replacement reaction. Due to the large mismatch between crystalline Au and amorphous Se, as well as the strong binding between Au and Se, the nucleation of Au atoms would follow the island growth mode. The size of the crystalline Au domain was tunable by adjusting the reaction time and amount of HAuCl₄, with a larger Au domain obtained at longer reaction times and a higher amount of HAuCl₄ added. The number of Au domains formed on the surface could be precisely controlled by adjusting the pH of the solution, where a lower pH resulted in slow reduction, and thus fewer nucleation sites, favoring the formation of Se–Au Janus particles with only one Au domain, while higher pH allowed fast reduction and multiple nucleation sites, yielding core-satellite Se–Au nanostructures with more Au domains.

During seed-mediated growth, Janus nanostructures can be prepared by partially masking the surface of the seeds with an inert material, followed by the deposition of metals on the unmasked region and removal of the mask. For example, Tuff et al. demonstrated an inorganic mask substrate-based strategy for fabricating aligned arrays of Au–Ag Janus nanostructures.⁶⁸ Firstly, Au nanocubes were immobilized on the substrate and conformally coated with Al₂O₃ as the mask via atomic layer deposition. Then, a directional ion beam was used to selectively thin the Al₂O₃ layer on one side of each particle. Upon immersion in the growth solution, the asymmetrically protected Au cores were exposed to the Ag precursor, leading to selective Ag deposition and the formation of side-by-side Janus nanostructures. During the growth of Ag, the Al₂O₃ dissolved, and the Ag–Au Janus nanostructure was finally obtained.

2.3. Post-synthetic treatment

In addition to symmetry breaking during the growth stage, Janus architectures can also be obtained through post-synthetic treatment. A typical example is the phase separation of multicomponent alloy or core–shell metal nanomaterials induced by thermal annealing or solvent-assisted compositional reorganization, where thermodynamically unstable domains can undergo selective redistribution under external stimulation, yielding a well-defined Janus configuration. This method offers a powerful post-growth route to Janus structures when direct synthesis is challenging due to the lattice compatibility or uniform deposition tendencies during the initial growth stage. For example, Xie et al. reported the formation of Janus structures as a result of high-temperature-induced phase separation in the initial core–shell systems.⁶⁹ Using Au nanoplates coated with a mesoporous SiO₂ shell, they sequentially deposited Pt or Pd to form core–shell Au@Pt and Au@Pd nanoplates, respectively, which were subjected to thermal annealing treatment. It was found by in situ STEM imaging (Fig. 5a) that upon alloying at intermediate temperatures of up to 650 °C, the Au@Pt nanoplates transformed into the Au–Pt alloy. Then, at elevated temperatures of up to 1100 °C, distinct phase segregation occurred in the Au–Pt alloy, as indicated by the HADDF-STEM image and corresponding EDS mapping in Fig. 5b, leading to the formation of Au–Pt Janus heterostructures, while their morphology was well maintained due to the protection from the SiO₂ shell. A similar transformation from core–shell to alloy, and finally to Janus structure was also observed in the Au–Pd system.

Fig. 5 (a) STEM images of the products obtained under different temperatures during in situ heating of Au@Pt nanoplates and (b) STEM image with EDS mapping images of the Janus Au–Pt. Reproduced with permission from ref. 69. Copyright 2024, Elsevier Inc. (c) Schematic of different nucleation pathways during the transformation of Sn nanoparticles to PtSn₄. Reproduced with permission from ref. 70. Copyright 2022, the American Chemical Society. (d) HAADF-STEM image and EDS mapping images of PtSn₄–PdSn₄ Janus nanostructure. The bright and dark areas in the HAADF-STEM image represent the PtSn₄ and PdSn₄ domains, respectively. Reproduced with permission from ref. 71. Copyright 2023, Springer Nature. (e) Schematic of the synthesis of a Janus Pt–Ir nanocage. (f) TEM image of the Janus Pt–Ir nanocages. Scale bar: 50 nm. Reproduced with permission from ref. 72. Copyright 2021, Wiley-VCH.

In addition to thermal annealing, the controllable oxidation of pre-synthesized metal nanomaterials is another typical post-synthetic treatment for the preparation of metal–metal oxide Janus nanomaterials. Li et al. developed a hydroxycarbonate-assisted pyrolysis method for the preparation of Janus Ni/Ni₂P nanoparticles,⁷³ which involved pyrolyzing a mixture of Ni precursor, glucose, urea, and zinc carbonate hydroxide at 1000 °C in an Ar atmosphere. The process yielded ultrafine Ni nanoparticles embedded in N-doped carbon nanofibers. Subsequent mild oxidation treatment partially oxidized the Ni nanoparticles into NiO, forming the Ni/NiO Janus heterostructure. Finally, phosphorization treatment converted the NiO component into Ni₂P, resulting in the formation of Janus-structured Ni/Ni₂P nanoparticles.

Galvanic replacement offers another effective post-synthetic strategy for transforming nanostructures into Janus architectures through localized redox reactions. In this method, pre-synthesized materials composed of a metal with a more negative reduction potential serve as both a template and reductant, which selectively reduce other metal precursors in solution. With delicate control over the reaction parameters, this process induces the asymmetrical deposition of the secondary metal, leading to the formation of a Janus heterostructure. For example, Yu et al. developed a one-pot, two-step method using galvanic replacement between Sn and other metals to achieve the controllable preparation of Sn-based Janus heterostructures.⁷⁴ Usually, Sn nanoparticles were obtained by the reduction of the Sn²⁺ precursor in oleylamine, and then other metal ions (Mⁿ⁺) were injected using a syringe pump into the colloidal solution containing the formed Sn nanoparticles, which initiated the galvanic replacement between Sn and Mⁿ⁺. With the continuous addition of Mⁿ⁺, Sn–M_xSn_y heterodimers were first formed with an increasing ratio of M_xSn_y domain in the heterodimers, finally obtaining a pure M_xSn_y phase after the complete replacement of the Sn nanoparticles. Following this strategy, Sn–Cu₆Sn₅ Janus intermetallic alloy-metal nanoparticles with various ratios between Sn and Cu₆Sn₅ were prepared by adding different amounts of Cu precursor. It is worth noting that the injection rate of the Mⁿ⁺ and the replacement temperature play an important role in the formation of the Janus heterostructure. Using the Sn–PtSn₄ system as a typical example,⁷⁰ Yu et al. reported that the injection rate should be slow enough to enable the single nucleation growth of PtSn₄ from one side of the Sn nanoparticles, whereas the fast addition of Pt²⁺ would likely induce multiple nucleation sites on the Sn nanoparticles for PtSn₄, forming a sandwiched PtSn₄–Sn–PtSn₄ structure (Fig. 5c). Besides, the low replacement temperature would reduce the replacement kinetics and favor the formation of Sn–PtSn₄ Janus heterodimers. Later, Yu et al. further extended this powerful strategy to synthesize Janus intermetallic nanomaterials with a more complex composition and distribution of intermetallic alloys by precisely tuning the experimental conditions during the galvanic replacement and the sequential addition of various metal precursors.⁷¹ For instance, adding Pt²⁺ first to Sn nanoparticles would lead to the formation of Sn–PtSn₄, and the subsequent addition of Pd²⁺ further converted the remaining Sn into a PdSn₄ phase, while the PtSn₄ intermetallic alloy was well preserved due to the high stability of intermetallic alloys, ultimately forming PtSn₄–PdSn₄ Janus heterodimers, as shown by the STEM and EDS mapping images in Fig. 5d. With the addition of different metal precursors (Pt²⁺, Pd²⁺, Au³⁺, and Cu²⁺) and adjustable addition sequences, heterostructured intermetallic nanomaterials with a combination of up to four types of intermetallic domains and distinct distribution of the segments could be combined in one nanoparticle, significantly expanding the library of complex metal heterostructures with precisely controlled interfaces and compositions.

2.4. Other methods

In addition to the widely explored symmetry-breaking strategies such as interfacial energy modulation, kinetic control, lattice mismatch utilization, and post-synthetic phase separation, some alternative approaches have been developed to synthesize metal-based Janus nanomaterials. For example, Zhu et al. developed a sequential deposition and selective etching strategy for synthesizing Janus metal nanocages composed of two types of platinum-group metals (PGMs), including Rt, Ir, Rh, and Ru, using a sequential deposition and selective etching strategy.⁷² This process began with the preparation of Pd nanocubes as the seeds for the growth of other metals, onto which ultrathin layers of several atomic layers of different PGMs were conformally deposited by injecting the metal precursor using a syringe pump. During the deposition, the thickness of the metal layers could be controlled by altering the concentration of the corresponding metal precursors, whereas the slow injection rate and high temperature ensured the uniform growth of metal layers. Subsequently, the selective etching of the Pd core resulted in the formation of Janus nanocages with asymmetric, porous walls and well-defined (100) facets. Using the synthesis of Pt–Ir Janus nanocages as a typical example, the procedure is schematically illustrated in Fig. 5e, and the TEM image of the resultant Pt–Ir nanocages is shown in Fig. 5f. Interestingly, this method was extended to fabricate 11 different types of Janus nanocages with different selections of PGMs and varying atomic thicknesses. In another work, Huang et al. reported the preparation of a Janus Au–Pd catalyst loaded on a carbon support using a modified sol immobilization method, where Au and Pd colloids were independently prepared in the first stage, and then combined immediately before support immobilization.⁷⁵ In contrast, the direct co-reduction of Au and Pd colloids would lead to the formation of the Au–Pd alloy. Therefore, it is believed that the separate reduction of Au and Pd ensures the spatial separation of the Pd and Au phases within individual nanoparticles rather than forming an alloy. Zhang et al. reported a multi-step method involving sequential galvanic replacement and Ostwald ripening to prepare an Au–AgPd Janus nanostructure with AgPd nanoparticle deposited at one end of Au nanobipyramids. Specifically, an Ag shell was grown on Au nanobipyramids and formed Au@Ag core–shell nanorods.⁷⁶ Then, Pd²⁺ and ascorbic acid were added to the aforementioned system to initiate the stepwise galvanic replacement between Pd²⁺ and Ag, co-reduction, and Ostwald ripening processes, ultimately yielding Au–AgPd Janus nanodarts. The anisotropic deposition of AgPd at one end of Au nanobipyramids was attributed to the cooperative interplay between Ostwald ripening and directional ion migration, which was modulated by the reaction parameters such as the amount of Ag precursor, choice of surfactant, solution pH, and growth dynamics. A transition from dumbbell structures to high-yield nanodarts was observed with an increase in the amount of Ag precursor, which enhanced the ion migration. Notably, the use of CTAB, a mild alkaline environment, and the absence of stirring were both essential in promoting the Ostwald ripening and directional ion migration to the nanoparticle tip, thereby favoring tip-selective Pd deposition. These findings support a mechanism wherein Ag oxidation during galvanic replacement generates localized electrons that accumulate at the sharp Au tips, guiding the site-selective reduction of Pd²⁺. This tip-favored electron density, combined with Ag consumption and metal redistribution via Ostwald ripening, led to the structural evolution into Janus Au–AgPd nanodarts.

3. Electrocatalytic applications

Generally, the electrocatalytic reaction process involves three consecutive steps. Firstly, the reactant molecules are adsorbed onto the surface of an electrocatalyst.^77,78 Then, electron and proton transfer occur at the interface, facilitating the formation of the reaction intermediates. Finally, the desired product is formed and desorbed from the catalyst surface to the electrolyte. Each of these steps plays a critical role in determining the activity, selectivity, and efficiency of the overall electrocatalytic process. Due to their unique asymmetrical architecture and multifunctionality, Janus nanomaterials allow the integration of components that are specifically effective for different steps to maximize the catalytic performance, especially for electrocatalytic reactions with multiple steps. Their intrinsic asymmetry also enables the spatial separation and optimization of different catalytic functions, which can facilitate the fundamental steps of electrocatalytic processes, including reactant adsorption, intermediate transformation, and product desorption. The advantages of Janus nanomaterials in different electrocatalytic reactions can be briefly summarized. Firstly, the cooperative effect of different domains allows the integration of complementary catalytic properties, enabling different regions to independently optimize the reaction steps. Secondly, the spatial separation can also facilitate directional charge and mass transport, improving the reaction kinetics and efficiency. Thirdly, the interface coupling between the two domains can facilitate efficient charge transfer, stabilize key intermediates, and promote favorable reaction pathways. Moreover, Janus structures are ideal platforms for tandem catalysis, such as NO₃RR and CO₂RR, allowing sequential reaction steps to occur on different faces, thereby improving the overall reaction kinetics and selectivity. More importantly, Janus nanomaterials allow precise control over the composition, morphology, surface properties, and interface of adjacent domains. Therefore, Janus nanomaterials hold great promise to match the requirements of different electrocatalytic reactions.

In this section, the recent progress on the electrocatalytic applications of metal-based Janus nanomaterials is summarized, including hydrogen electrocatalysis, oxygen electrocatalysis, small-molecule oxidation reaction, nitrate reduction reaction, and carbon dioxide reduction reaction. Also, the unique advantages of asymmetric Janus structures for highly efficient electrocatalysis are discussed. Additionally, we list the catalytic performances of some recently reported metal-based Janus nanostructures for hydrogen and oxygen electrocatalysis in Table 1, as well as for CO₂RR and NO₃RR in Table 2.

Table 1 The catalytic performances of recently reported metal-based Janus nanostructures for hydrogen and oxygen electrocatalysis

Catalyst	Reaction	Catalytic performances						Ref.
		Electrolyte	Mass activity (A mg^−1)	Current density (mA cm^−2)	Overpotential (mV)	Tafel slope (mV dec^−1)	Stability
Ru–CrOx	HER	1 M KOH	—	10	7	30.1	20 h (@100 mA cm^−2)	45
Janus Ni/W	HER	1 M KOH	—	10	62.6	66.09	60 h (@10 mA cm^−2)	81
Co/Fe3O4	HER	1 M KOH	—	10	53.9	43.7	10 h (@10 mA cm^−2)	82
Ni–Fe2O3	HER	1 M KOH	—	10	46	58	10 h (@10 mA cm^−2)	47
Ru–CrOx	HOR	0.1 M KOH	13.76	5.1	—	—	40 h (@0.02 V)	45
Janus Ni/W	HOR	0.1 M KOH	—	2.19	—	—	48 h (@0.1 V)	81
Pt/Ir Janus nanocage	ORR	0.1M HClO4	0.15	—	—	—	—	72
Pt/Ir Janus nanocage	OER	0.1M HClO4	—	10	271	—	—	72
Co/Co9S8	OER	1 M KOH	—	10	274	68.7	10 h (@10 mA cm^−2)	46
FeCo/(FeCo)S	OER	1 M KOH	—	10	270	53.4	10 h (@10 mA cm^−2)	46
FeCoNi/(FeCoNi)9S8	OER	1 M KOH	—	10	238	70.4	10 h (@10 mA cm^−2)	46
Ni–Fe2O3	OER	1 M KOH	—	10	210	53	10 h (@100 mA cm^−2)	47
Ni/Ni2P	OER	1 M KOH	—	10	285	45.2	36 h (@10 mA cm^−2)	73
Co/Fe3O4	OER	1 M KOH	—	10	272	50.2	10 h (@1.56 V)	82

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Table 2 The catalytic performances of recently reported metal-based Janus nanostructures for the NO₃RR and CO₂RR

Catalyst	Application	Catalytic performances					Ref.
		Target product	FE of target product (%)	FE of C2^+ (%)	Potential (V vs. RHE)	Stability
Cu–Ni	NO3RR	NH3	92.5	—	−0.2	—	44
Janus CoCu–Ti3C2Tx	NO3RR	NH3	93.6	—	−0.7	10 cycles (@−0.7 V)	90
Cu6Sn5–Sn	NO3RR	NO2^−	90	—	0	—	74
Ag–Cu	CO2RR	C2H4	54	72	−1.2	10 h (@−1.2 V)	52
Au nanobipyramid-Cu	CO2RR	C2H4	41.5	46.4	−1.0	10 h (@-0.981 V)	64
fcc-2H-fcc Au–Cu	CO2RR	C2H4	55.5	84.3	−1.1	10 h (@1.1 V)	53
Typical Janus Au–CuII	CO2RR	C2H5OH	∼40	67.3	−0.78	—	58
Acorn-like Janus Au–CuIII	CO2RR	C2H4	∼35	80	−0.87	—	58

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3.1. Hydrogen electrocatalysis

Hydrogen electrocatalysis enables both the clean generation of H₂ by the hydrogen evolution reaction (HER) and its efficient utilization in fuel cells through the hydrogen oxidation reaction (HOR), which makes it important for sustainable energy conversions.^79,80 In hydrogen electrocatalysis, Janus nanomaterials are usually designed with hydrogen-affinitive domains, such as noble metals, to optimize the dissociation of water as well as H* adsorption and highly conductive domains to facilitate rapid electron/proton transfer, thereby achieving boosted reaction kinetics. Metal-based Janus nanomaterials have shown excellent performances for hydrogen electrocatalysis due to their combination of distinct metal domains, where one component typically facilitates hydrogen adsorption and activation, while the other could enhance the intermediate stabilization, as well as the presence of well-defined interfaces, leading to reduced energy barriers and improved catalytic efficiency. Also, the modulation of electronic structure enabled by the electron transfer in Janus metal nanomaterials could optimize the adsorption energy of the intermediates during hydrogen electrocatalysis. For example, an Ru–CrO_x cluster–cluster Janus heterostructure loaded on nitrogen-doped carbon nanosheets (Ru–CrO_x@CN) with a strongly coupled interface was prepared and used for HER and HOR in alkaline electrolytes.⁴⁵ Specifically, Ru–CrO_x@CN exhibited a significantly enhanced current density than that of Ru@CN, CrO_x@CN, and commercial Pt/C (Fig. 6a), showing a mass activity of 13.76 A mg_Ru⁻¹ and exchange current density of 2.8 A mg_Ru⁻¹ for HOR (Fig. 6b). Ru–CrO_x@CN also displayed significantly enhanced anti-poisoning ability from carbon monoxide (CO) compared to Pt/C and excellent long-term stability, showing negligible decay over 40 h of HOR and good maintenance of its Janus structure. The hydroxide exchange membrane fuel cell (HEMFC) assembled by using Ru–CrO_x@CN as the anode and Pt/C as the cathode also showed high mass activities and unprecedented stability over 105 h under a high current density of 500 mA cm⁻² (Fig. 6b). In HER, Ru–CrO_x@CN required a small overpotential of only 7 mV to reach a current density of 10 mA cm⁻² (Fig. 6c), with higher mass activity, lower Tafel slope, and larger turnover frequency (TOF) than Ru@CN and Pt/C. These results demonstrate the advantage of Janus metal-based heterostructures over single-component catalysts for hydrogen electrocatalysis, and the Ru nanocluster in Ru–CrO_x@CN should play a dominant role in delivering excellent catalytic performances for hydrogen electrocatalysis, whereas the CrO_x cluster mainly contributes to the regulation of intermediate binding. It was revealed by control experiments and theoretical calculations that the excellent catalytic performances should be attributed to the cluster–cluster interface with Ru penetration, which enhances the Ru–O–Cr interfacial bonding, facilitates H–OH bond cleavage and formation, and thus enables faster water splitting and reformation, respectively. Besides, the strong cluster–cluster coupling simultaneously optimizes hydrogen adsorption on Ru and hydroxyl binding on CrO_x, as well as reduces the energy barrier for the reaction barrier of the Volmer step in HOR. In another work, a Janus non-noble Ni/W catalyst was constructed on nickel foam (NF), denoted as Ni/W@NF, and used for alkaline HER and HOR.⁸¹ The overpotential of the Janus Ni/W@NF catalyst for HER was comparable to that of commercial Pt/C and significantly superior to that of monometallic Ni and W catalysts. In HOR, the Janus Ni/W@NF catalyst also delivered a high exchange current density of 1.83 mA cm⁻², which was over three times that of Ni@NF (0.55 mA cm⁻²) and nine times that of W@NF (0.20 mA cm⁻²). Density functional theory (DFT) results demonstrated that the enhanced HER/HOR performances originated from the combination of Ni and W in the Janus Ni/W catalyst, which optimally bind hydrogen and favor OH⁻ adsorption, respectively, as well as the existence of an interface between Ni and W, where adsorbed hydrogen and OH⁻ readily recombine to form water. It was also claimed that an internal electric field at the interface further promoted charge transfer, as confirmed by the smaller charge transfer resistance, thus accelerating both the HER and HOR kinetics and leading to enhanced HER/HOR performances.

Fig. 6 (a) HOR polarization curves of different catalysts. (b) Comparison of half-wave potential E_1/2, mass- and ECSA-normalized activities of Ru–CrO_x@CN (red), Ru@CN (green), and Pt/C (black). (c) HER polarization curves of different catalysts. Reproduced with permission from ref. 45. Copyright 2024, Springer Nature. (d) ORR polarization curves of different catalysts. (e) OER polarization curves of different catalysts. Reproduced with permission from ref. 46. Copyright 2022, Wiley-VCH. (f) OER polarization curves of distinct catalysts. Reproduced with permission from ref. 47. Copyright 2019, Springer Nature.

The unique asymmetrical feature and the elemental separation within Janus metal-based nanomaterials are also beneficial for boosting their HER performances. For instance, a Co/Fe₃O₄ Janus nanocatalyst supported on carbon fiber paper (denoted as J–CoFe–CFP) exhibited a low overpotential of 53.9 mV at 10 mA cm⁻² and a favorable Tafel slope of 43.7 mV dec⁻¹ for alkaline HER, which are superior to that of other Co/Fe-based catalysts including CoFe alloy (CoFe alloy–CFP), mixture of Co and Fe (Co/Fe–CFP), monometallic Co (Co–CFP) and monometallic Fe (Fe–CFP).⁸² In another work, a Janus Ni–Fe₂O₃ catalyst (denoted as Ni–Fe NP) showed great promise for alkaline HER, delivering an overpotential of just 46 mV to reach 10 mA cm⁻² with a Tafel slope of 58 mV dec⁻¹, which are close to that of commercial Pt/C.⁴⁷ More importantly, the HER performance of the Janus Ni–Fe₂O₃ catalyst was enhanced over the other control samples with distinct compositions and spatial configurations, including Ni nanoparticles (260 mV), Fe nanoparticles (410 mV), physical mixtures of Ni and Fe nanoparticles (112 mV), and Ni–Fe alloy nanoparticles (307 mV). The superior HER activity of the Janus Ni–Fe₂O₃ arises from its Ni/Fe₂O₃ interface, where an Ni–O–Fe bridge is formed. DFT calculations reveal that this interface modulates the hydrogen adsorption free energies to more optimal values, in contrast to less favorable values on pure Ni or Fe₂O₃ surfaces, thus reducing the energy barrier for the initial Volmer step and facilitating efficient hydrogen adsorption and desorption during HER. The aforementioned results indicate the decisive role of the spatial configuration engineering of metal heterostructures in determining their electrocatalytic performances.

3.2. Oxygen electrocatalysis

Oxygen electrocatalysis refers to both the oxygen reduction reaction (ORR), which is the key reaction for fuel cells, and the oxygen evolution reaction (OER) for water splitting and rechargeable metal–air batteries, rendering it indispensable for sustainable energy conversion and storage.^83–85 Currently, the development and practical application of oxygen electrocatalysis are still substantially limited by the sluggish reaction kinetics.^22,86 To design Janus nanomaterials for oxygen electrocatalysis, it is important to provide surfaces optimized for O–O bond formation and cleavage, while maintaining high electronic conductivity. Generally, one side of Janus nanomaterials may offer strong adsorption of oxygen intermediates (e.g., OH*), while the other regulates desorption or charge redistribution. Janus metal-based nanomaterials have been reported as promising candidates as highly efficient catalysts with enhanced performances for oxygen electrocatalysis. For instance, Zhu et al. reported the preparation of Janus nanocages composed of platinum-group metals,⁷² enabling the fabrication of electrocatalysts with asymmetric, ultrathin, and porous walls for dual functionality. In particular, the Pt–Ir Janus nanocages exhibited an exceptional bifunctional electrocatalytic performance toward both the ORR and OER in acidic media. Three types of Pt–Ir Janus nanocages with distinct Pt/Ir layer thickness and position were tested, including Pt_1.4L/Ir_3.1L, Pt_3.1L/Ir_1.3L, and Ir_3.2L/Pt_1.4L (L means atomic layers). Among these Pt–Ir Janus nanocages, Pt_1.4L/Ir_3.1L exhibited high activity towards both ORR and OER, delivering a mass activity of 0.15 A mg⁻¹_Pt+Ir for ORR and an overpotential of 271 mV at 10 mA cm⁻² for OER, outperforming the mixture of commercial Pt/C and Ir/C. This enhancement is primarily attributed to the increased atomic utilization efficiency of Pt and Ir afforded by the open, ultrathin structure of the nanocages. Besides, it was found that the ORR/OER performances also depended on the thickness and spatial arrangement of the Pt and Ir layers in the Pt–Ir Janus nanocages. For example, Pt_1.4L/Ir_3.1L promoted higher ORR activity compared to Ir_3.2L/Pt_1.4L, which may be because the inner-surface Pt in Pt_1.4L/Ir_3.1L was less susceptible to PVP-induced site blocking at the outer surface of the Janus nanocage. Besides, Pt_1.4L/Ir_3.1L showed higher OER activity at a high potential (>1.53 V), whereas Ir_3.2L/Pt_1.4L delivered a better OER performance at a low potential (>1.53 V). DFT studies further revealed that the superior activity of the Janus nanocages is driven by the unique surface structure arising from Pt–Ir segregation. Pt-rich surfaces containing Ir islands promote selective OH adsorption at the Ir sites, which in turn destabilizes OH binding at the adjacent Pt sites due to OH–OH repulsion, mitigating the poisoning effect of OH on Pt, and thereby accelerating ORR kinetics.

Janus non-noble metal-based nanomaterials have also shown great promise for oxygen electrocatalysis, providing a low-cost, efficient alternative for noble metal nanomaterials. For example, Lu et al. present a new class of Janus heterostructures composed of transition metal alloys and their corresponding sulfides with precise control over the composition from unary (e.g., Co/Co₉S₈) to binary, ternary, and even quaternary alloy/sulfide systems (e.g., FeCoNiCu/(FeCoNiCu)₉S₈).⁴⁶ Among the various Janus heterostructures, FeCo/(FeCo)S showed an excellent bifunctional electrocatalytic performance for both the ORR and OER in alkaline media. Specifically, FeCo/(FeCo)S exhibited a half-wave potential of 808 mV (Fig. 6d), which is comparable to that of the commercial Pt/C catalyst and significantly better than single-component FeCo alloy and (FeCo)S. FeCo/(FeCo)S also delivered a favorable OER performance with an overpotential of only 270 mV at 10 mA cm⁻², outperforming the FeCo alloy (334 mV) and (FeCo)S (298 mV) (Fig. 6e). The Tafel slopes for ORR and OER of FeCo/(FeCo)S were also significantly lower than that of their single-component counterparts, demonstrating the enhanced reaction kinetics of Janus heterostructures. Other Janus structures, such as Co/Co₉S₈ and FeCoNi/(FeCoNi)₉S₈, also displayed similar bifunctional catalytic performances for ORR and OER. It was claimed that the superior performance of the Janus metal/metal sulfide heterostructure was attributed to the optimized binding energy of the oxygen-containing intermediates and bimetallic sites for oxygen electrocatalysis. DFT studies further revealed that charge redistribution occurred across the Janus interface, with electrons transferred from the FeCo domain to the sulfide domain, creating electron-rich S sites and electron-deficient metal centers. A significantly reduced energy barrier was found at the interfacial Fe–Co site compared to the Fe site and Co site, leading to a boosted oxygen electrocatalytic performance. Moreover, the construction of the Janus heterostructure also led to a modulated electronic structure in FeCo/(FeCo)S, which could be confirmed by its optimized d-band center compared to the FeCo alloy and (FeCo)S, which is beneficial for regulating the adsorption of the reaction intermediates during ORR and OER. Owing to its good bifunctionality, FeCo/(FeCo)S was further implemented as a cathode in both aqueous and flexible Zn-air batteries, which demonstrated a power density of 261.8 mW cm⁻² and stable performance over 470 cycles, both exceeding the Pt/C + IrO₂ benchmarks. In another work, Li et al. report the preparation of Janus Ni/Ni₂P nanoparticles loaded on a nitrogen-doped carbon nanofiber (N–CNF) matrix.⁷³ This engineered interface between metallic Ni and Ni₂P within a conductive and robust carbon support led to exceptional alkaline OER performances, including an overpotential of only 285 mV to reach 10 mA cm⁻², a small Tafel slope of 45.2 mV dec⁻¹, which is superior to that of the commercial RuO₂ catalyst (64.1 mV dec⁻¹), and long-term durability over 36 h at 10 mA cm⁻². The successful construction of the Janus Ni/Ni₂P structure was claimed as the main reason for this enhanced OER performance, which enabled electronic modulation, creating abundant active sites and enhancing intrinsic activity. Suryanto et al. reported the fabrication of a Janus Ni–Fe₂O₃ catalyst that could be used as a highly efficient OER catalyst along with favorable HER performances, as discussed in the previous section.⁴⁷ Specifically, the Janus Ni–Fe₂O₃ catalyst supported on nickel foam exhibited overpotentials of 210 and 270 mV to reach 10 and 100 mA cm⁻², respectively (Fig. 6f), as well as a high TOF of 0.052 s⁻¹ at 350 mV, outperforming Ni nanoparticles, physical mixtures of Ni and Fe nanoparticles, and Ni–Fe alloy nanoparticles and demonstrating the key role of the Janus heterostructure in achieving efficient OER. The high OER activity was attributed to the interfacial synergy between metallic Ni and Fe₂O₃ as discovered by DFT calculation, where the Ni–Fe interface optimized the binding energies of the key OER intermediates (HO*, O*, and HOO*), and the unique interfacial Ni–O–Fe configuration enabled a multi-site functional mechanism that could break the limit of energy scaling relations of OER for a lower overpotential. Considering the excellent performances of the Janus Ni–Fe₂O₃ for both HER and OER, a two-electrode electrolyzer was assembled based on Ni–Fe₂O₃ for overall water splitting, which delivered a low cell voltage of 1.47 V at 10 mA cm⁻² and outstanding long-term stability, outperforming conventional noble-metal-based systems (Pt/C + Ir/C).

3.3. Small-molecule oxidation reaction

Small-molecule oxidation reactions (SMOR), such as alcohol oxidation reaction and 5-hydroxymethylfurfural oxidation reaction (HMFOR), involve the electrochemical conversion of small organic compounds into value-added chemicals and fuels, offering a sustainable pathway for both green chemical synthesis and energy production.^87–89 However, despite their potential, the practical deployment of SMOR is hindered by their complex reaction pathways and sluggish kinetics. In the case of SMOR, Janus nanomaterials should combine active domains that favor the adsorption and activation of organic species with domains that facilitate the removal of poisoning intermediates (e.g., *CO). One side may provide oxophilic or hydroxyl-rich surfaces to generate –OH groups, while the other enhances electron transport or stabilizes the transient oxidation states. The modulation of the electronic structure of Janus nanomaterials is also important to regulate the adsorption of the key reaction intermediates during SMOR. Janus metal-based nanomaterials have recently emerged as promising catalysts for SMOR, owing to their unique asymmetric architectures and interfacial effects, which facilitate improved catalytic activity and selectivity for target products. For example, a Janus Pd@Bi–PdBi nanocrystal (PdBi HNCs), comprising a Bi-modified Pd octahedral domain (Pd@Bi) and a PdBi alloy domain, could be used for the ethylene glycol oxidation reaction (EGOR) with high selectivity towards the production of glycolic acid (GA).⁶⁰ Interestingly, the Janus PdBi HNC nanocatalyst showed higher mass activity than Pd octahedra and PdBi alloy, as well as better anti-poisoning ability for CO than Pd octahedra due to the introduction of oxophilic Bi. More importantly, the Janus PdBi HNC nanocatalyst achieved high selectivity for GA, reaching a maximum value of 93% at −0.1 V versus the saturated calomel electrode (SCE), which is superior to that of the Pd octahedra and PdBi alloy (Fig. 7a). Generally, there are two different pathways during EGOR, including the C–C cleavage pathway and the partial oxidation pathway to produce GA without C–C cleavage. It was claimed that PdBi HNCs could inhibit C–C cleavage to retain high selectivity for GA. In situ Fourier transform infrared spectroscopy further confirmed the presence of 2-hydroxyacetyl intermediates and the absence of adsorbed CO species, suggesting that the preferred pathway is via GA formation rather than C–C cleavage. The electron transfer from Bi to Pd and the shift in the d-band center enabled by interfacial strain and dislocations between the two domains are also beneficial for preventing the overoxidation of GA, leading to enhanced selectivity. In another work, Huang et al. reported the preparation of a Janus Au–Pd nanocatalyst with separated Au and Pd domains for SMORs.⁷⁵ Impressively, the carbon-supported Janus Au–Pd nanocatalyst (Au@Pd/C) showed superior HMFOR performances with a much-enhanced oxidation rate of HMF compared to the corresponding Au–Pd alloy catalyst (Au–Pd/C), a mixture of Au/C and Pd/C (Au/C + Pd/C), single-component Au/C and Pd/C. This enhancement in the reaction rate on Au@Pd/C with Au–Pd separation was also observed in other SMORs (Fig. 7b), including the oxidation reaction of 5-formyl-2-furan carboxylic acid (FFCA), 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) and alcohols such as 1,3-propanediol, glycerol, ethanol, 2-propanol, and 1-propanol, demonstrating the versatility of this powerful strategy for promoting the reaction kinetics in SMORs by virtue of the maximum synergistic effect of Au (dehydrogenation) and Pd (oxygen reduction reaction) with minimal mutual interference and rapid electron transfer in Janus metal-based nanomaterials. A series of control experiments and electrochemical investigations were conducted, and a cooperative redox enhancement (CORE) effect was proposed for the superior catalytic performances of the carbon-supported Janus Au–Pd nanocatalyst, in which the coupling of independent redox processes occurs at the isolated Au and Pd sites due to the elemental separation in Janus heterostructures. This work demonstrates the great potential of Janus metal-based nanomaterials as multi-component heterogeneous catalysts.

Fig. 7 (a) Selectivity toward the production of GA of different catalysts. Reproduced with permission from ref. 60. Copyright 2024 Wiley-VCH. (b) Reaction rates of different catalysts for the oxidation of various small organic molecules. Reproduced with permission from ref. 75. Copyright 2022, Springer Nature. (c) Schematic of the reaction mechanism of the NO₃RR process of the Janus Cu–Ni catalyst. Reproduced with permission from ref. 44 Copyright 2024, the American Chemical Society. (d) NH₃ FE of different catalysts at various potentials. Reproduced with permission from ref. 90 Copyright 2024, Wiley-VCH.

3.4. Nitrate reduction reaction

The electrocatalytic nitrate reduction reaction (NO₃RR) has garnered significant attention as a sustainable strategy for the sustainable production of ammonia, which addresses the challenges of nitrate pollution and provides an effective complement to the energy-consuming Haber–Bosch process.^91–93 However, the multi-electron and multi-step nature of NO₃RR results in sluggish kinetics, poor selectivity due to the competing HER, and inefficient intermediate conversion. In this case, Janus metal-based nanomaterials with spatially separated domains showing distinct catalytic functionalities have emerged as promising candidates by offering cooperative tandem effects during NO₃RR, which enable optimized intermediate adsorption, electron transfer, and enhanced selectivity. Usually, one side of a Janus nanocatalyst is composed of highly active metallic sites (e.g., Cu and Cu-based alloys) for NO₃⁻ activation, while its other side incorporates materials that can moderately supply active H* species for the reduction to suppress competitive side reactions and enhance the overall reaction efficiency. For instance, Lou et al. constructed a Janus Cu–Ni tandem catalyst with phase separation of Cu and Ni.⁴⁴ The Janus Cu–Ni catalyst loaded on carbon (denoted as Co₅₀Ni₅₀-Janus/C) delivered an impressive faradaic efficiency (FE) of 92.5% and a production rate of 1127 mmol h⁻¹ g⁻¹ at −0.2 V vs. RHE in NH₃ production, which showed a significant enhancement compared to CuNi solid-solution alloys (Cu₅₀Ni₅₀-SSA/C) and a physical mixture of Cu/C and Ni/C catalysts (Cu₅₀Ni₅₀-Mix/C). It was claimed that the excellent NO₃RR performance could be attributed to the unique phase-separated Janus heterostructure. The modulation of electronic structure within the Cu–Ni heterostructure could facilitate electron transportation from the catalyst to the adsorbed species and promote the rate-limiting step of NO₃RR (i.e. *NO₃⁻ + 2e⁻ + H₂O → *NO₂⁻ + 2OH⁻). In situ electrochemical Fourier transform infrared spectroscopy showed a bridge-bonded *NO configuration on Janus Cu–Ni, in contrast to the linearly adsorbed *NO on the CuNi alloy, as well as the detection of *H, which confirmed hydrogen spillover from the Ni to Cu domains. DFT calculations showed reduced reaction barriers for NO₃RR over the Janus Cu–Ni compared to the CuNi alloy. Then, a tandem process was proposed accordingly, as shown in Fig. 7c, which involves the efficient generation of *H species on the Ni site and their subsequent migration to the Cu domain by hydrogen spillover to facilitate the hydrodeoxygenation of *NO₃⁻ on Co₅₀Ni₅₀-Janus/C due to the synergistic interaction between Cu and Ni. Therefore, Cu sites are the predominant active center for the adsorption and reduction of *NO₃⁻, where the Ni sites mainly supply *H to modulate the reaction pathway and reduce the reaction barrier. In another study, Cui et al. prepared Janus CoCu nanoparticles supported on MXene (Ti₃C₂T_x) as an NO₃RR electrocatalyst.⁹⁰ The Janus CoCu–Ti₃C₂T_x achieved a high NH₃ yield of 8.08 mg h⁻¹ mg_cat⁻¹ and a maximum FE of 93.6% at −0.7 V vs. RHE, significantly outperforming its single-component Co–Ti₃C₂T_x and Cu–Ti₃C₂T_x counterparts (Fig. 7d). It was found that the Janus CoCu–Ti₃C₂T_x exhibited a tandem catalytic mechanism, wherein the Cu sites were responsible for the adsorption and activation of NO₃⁻, while the Co sites facilitated the generation of abundant adsorbed hydrogen (*H) by accelerating H₂O dissociation. This spatial separation of the Cu and Co sites enabled a cooperative interaction, effectively lowering the energy barriers for both NO₃⁻ adsorption and the subsequent hydrogenation of nitrogenous intermediates, thus leading to boosted NO₃RR performances.

Besides NH₃, the selective generation of other products during NO₃RR has also been reported for Janus metal-based catalysts. For example, Yu et al. synthesized Cu₆Sn₅–Sn Janus nanoparticles with a controllable ratio between the Cu₆Sn₅ and Sn domains.⁷⁴ Among them, the Cu₆Sn₅–Sn 1/1 Janus nanoparticles with a hemispherical morphology and equal ratio between the Cu₆Sn₅ and Sn domains demonstrated an outstanding NO₃RR performance, exhibiting a high mass activity of 1125 mA mg⁻¹, which is superior to that of pure Sn and Cu₆Sn₅ catalysts. Interestingly, unlike the typical Cu- or Sn-based catalysts that reduce NO₃⁻ predominantly to NH₃ or N₂, the Janus Cu₆Sn₅–Sn catalyst selectively produced NO₂⁻ as the main product, with high selectivity across a broad potential window. Specifically, the superior catalytic performance and high nitrite selectivity of the Cu₆Sn₅–Sn Janus nanoparticles stem from their unique anisotropic structure, where the intermetallic Cu₆Sn₅ domain serves as the primary active phase for nitrate reduction, and the metallic Sn domain acts as a promoter. The intimate interface between Cu₆Sn₅ and Sn facilitates efficient electron transfer and intermediate stabilization. This Janus architecture not only accelerates the rate-determining step of nitrate-to-nitrite conversion but also weakens nitrite adsorption, promoting its rapid desorption and preventing its further reduction.

3.5. Carbon dioxide reduction reaction

The electrochemical carbon dioxide reduction reaction (CO₂RR) has emerged as a promising strategy for converting CO₂ into value-added products especially multicarbon (C₂⁺) products, which is one of the important ways to achieve carbon neutrality and retard the greenhouse effect.^94–96 However, the highly selective production of C₂⁺ products is still challenging due to the complex reaction pathways involving multiple proton-electron transfer steps during CO₂RR, which makes it difficult to simultaneously optimize the adsorption energies of different intermediates at a single catalytic site. Janus nanostructures, characterized by their spatially segregated compositions and asymmetric architecture, offer an ideal platform for the production of C₂⁺ products through tandem CO₂RR.^62,97 Unlike alloys or core–shell architectures, Janus structures feature physically and electronically distinct domains, enabling different catalytic steps to occur at isolated but adjacent active sites. To design Janus nanomaterials for CO₂RR and maximize the tandem effect, CO-producing (e.g., Au and Ag) and C–C coupling domains (Cu-based materials) are usually incorporated. For instance, Ag or Au can efficiently reduce CO₂ to CO, while Cu domains facilitate CO-to-C₂⁺ transformations. Moreover, the CO spillover from the CO-producing domain to Cu should also be optimized by tuning the structural parameters of Janus nanomaterials, such as the size ratio of the two domains and interface engineering.

Especially, Janus catalysts based on the combination of Cu with other noble metals allow efficient CO spillover from CO-generating noble metals (e.g., Ag, Au, and Pd) to the active Cu domains for C–C coupling, leading to enhanced selectivity towards C₂⁺ products by following the tandem approach, which outperform their monometallic and alloy counterparts. For instance, Ma et al. systematically investigated the tandem CO₂RR performance of Janus Ag–Cu nanostructures with well-defined (100) facets (Ag–Cu JNS-100) and different Ag/Cu ratios, which demonstrated significantly enhanced catalytic selectivity for the CO₂RR toward C₂⁺ products, particularly ethylene and ethanol (Fig. 8a–c).⁵² Among the Janus Ag–Cu nanostructures with different Ag/Cu ratios, Ag₆₅–Cu₃₅ JNS-100 exhibited the best performance, achieving an FE of 54% for ethylene and 72% for total C₂⁺ products (Fig. 8b), substantially outperforming both monometallic Cu nanocubes and physical mixtures of Ag and Cu. Mechanistically, the high selectivity was attributed to the tandem catalysis effect (Fig. 8d), where the Ag domain efficiently reduces CO₂ to *CO intermediates at low overpotentials, which then spills over to the Cu (100) facet, where C–C coupling occurs to produce C₂⁺ products. DFT calculations further indicated that the enhanced activity originates from the electronic modulation and efficient CO spillover effect in the Janus Ag–Cu nanostructures, which reduced the energy barrier for C–C coupling. More recently, the same group extended their work to Au–Cu Janus nanostructures with an unconventional fcc-2H-fcc phase.⁵³ Due to the efficient tandem process enabled by the asymmetric architecture and the introduction of the unconventional phase, the fcc-2H-fcc Au–Cu Janus nanostructure delivered the maximum FEs of 55.5% and 84.3% for producing ethylene and C₂⁺ products, respectively, which are superior to that of Au–Cu heterostructures with other symmetrical configurations such as Au–Cu core–shell heterostructure (Fig. 8e), demonstrating the importance of constructing metal heterostructures for enhanced CO₂RR performance. It was claimed that the construction of the unconventional fcc-2H-fcc phase in the Au–Cu Janus nanostructure could diversify the CO* adsorption configuration, facilitate *CO spillover, and promote the C–C coupling process by modulating its electronic structures.

Fig. 8 (a–c) Electrocatalytic performance of Ag–Cu JNS-100: Ag₆₅–Cu₃₅ JNS-100 (a), Ag₅₀–Cu₅₀ JNS-100 (b), and Ag₂₅–Cu₇₅ JNS-100 (c) and (d) CO₂RR mechanism for Ag–Cu JNS-100. Reproduced with permission from ref. 52. Copyright 2022, Wiley-VCH. (e) Comparison of FEs of C₂⁺ products and C₂H₄ on different catalysts. Au–Cu JNSs: Au–Cu Janus nanostructures; Au–Cu CAHs: Au–Cu co-axial heterostructures; Au–Cu CSNs: Au–Cu core–shell nanostructures. Reproduced with permission from ref. 53. Copyright 2024, Wiley-VCH. (f) FEs for C₂⁺ products of Au–Cu catalysts in different potentials and (g) maximum FEs and corresponding partial current densities of CH₃OH, C₂H₅OH, and C₂H₄ in Au–Cu_I, Au–Cu_II, and Au–Cu_III catalysts. Reproduced with permission from ref. 58. Copyright 2024, Elsevier Inc.

During the tandem CO₂RR, the rational design of the morphology of Janus metal-based nanomaterials also provides a promising approach for further promoting their performance. For example, Jia et al. reported a novel class of Au–Cu Janus nanocrystals (JNCs) by growing Cu domains on Au nanocrystals with different morphologies for tandem CO₂RR.⁶⁴ Among these Janus Au–Cu catalysts, the Au nanobipyramid-Cu Janus nanocrystals (Au NBP-Cu JNCs) demonstrated a remarkable FE of 46.4% for C₂ products (C₂H₄ + C₂H₆), outperforming a series of Au/Cu-based catalysts with other configurations including monometallic Au/Cu, core–shell Au@Cu, and a mixture of Au and Cu, indicating the importance of the asymmetric Janus architecture and the interfacial contact between Au and Cu for promoting tandem CO₂RR. Interestingly, Au NBP-Cu JNCs also exhibited a superior FE for C₂ products than Au nanosphere-Cu Janus nanocrystals (Au NS-Cu JNCs), which could be ascribed to the high-index (116) facets of the Au NBPs with abundant atomic steps that enhance CO production on the Au surface, while also facilitating stronger interfacial coupling and optimal intermediate binding on the Cu domain, thus leading to enhanced C₂ product formation. In another work, the engineering of the metal interface between metal domains in Janus metal-based nanomaterials was reported to regulate the reaction pathway towards different products during CO₂RR. By varying the interfacial contact between Au and Cu, Zhang et al. synthesized a series of Au–Cu nanostructures with three Janus configurations (Au–Cu_I, Au–Cu_II, and Au–Cu_III) and one core–shell structure (Au–Cu_IV) by increasing the contact area between Au and Cu.⁵⁸ With the optimal Au–Cu contact area, Au–Cu_III achieved a maximum FE of 80% for C₂⁺ products (Fig. 8f) and a high partial current density of 466.1 mA cm⁻². In contrast, monometallic Au only generated CO and H₂, confirming the critical role of the Au–Cu interface in enabling *CO intermediate coupling and suppressing hydrogen evolution. Notably, the primary product during CO₂RR was tunable via interfacial engineering of the Janus Au–Cu nanocatalysts (Fig. 8g), where CH₃OH was dominant on Au–Cu_I (72.1% FE in liquid products), C₂H₅OH on Au–Cu_II (78.1% FE), and C₂H₄ on Au–Cu_III (63% FE in gas products). Operando surface-enhanced Raman spectroscopy, supported by DFT calculations, revealed that distinct reaction pathways emerge across different Au–Cu nanostructures, enabling the selective formation of the desired products through interface modulation.

4. Summary and outlook

In summary, this review highlights the recent progress in the development of metal-based Janus nanomaterials, with a particular focus on their synthesis strategies and electrocatalytic applications. Various approaches to achieve symmetry-breaking synthesis are systematically discussed, including co-reduction, seed-mediated growth, post-synthetic modifications, and other emerging techniques. The unique structural asymmetry and well-defined interfaces of Janus nanomaterials are shown to offer significant advantages in a wide range of electrocatalytic reactions, such as hydrogen electrocatalysis, oxygen electrocatalysis, small-molecule oxidation, nitrate reduction, and carbon dioxide reduction. However, although remarkable advances have been achieved in the last five years, substantial challenges persist in the synthesis and application of Janus metal-based heterostructures, which in turn open up a range of promising avenues for future research.

Firstly, current synthetic strategies for Janus metal-based nanomaterials are often complex and involve multistep procedures. Achieving anisotropic growth typically demands precise control over the reaction parameters. Even minor fluctuations in the experimental conditions can significantly affect the size, morphology, and interfacial structure of the resulting Janus nanoparticles. This high sensitivity not only limits the scalability of these methods but also results in poor reproducibility across different batches, thus hindering their systematic study and practical application. To this end, the development of more robust and generalized synthetic protocols is urgently needed. Future efforts should focus on exploring reaction systems with broader tolerance to experimental variation, leveraging automated synthesis platforms, and incorporating in situ monitoring techniques to enable real-time control over the nucleation and growth processes. These advancements would improve the reliability, yield, and uniformity of Janus nanostructures, facilitating their large-scale application in wider areas.

Secondly, most of the reported Janus metal-based nanomaterials are limited to binary or ternary compositions with conventional crystalline phases. To expand the library of Janus metal-based nanomaterials, it is highly desirable to broaden the compositional and structural complexity of Janus architectures. For example, high-entropy alloys (HEAs), with unique properties such as lattice distortion and cocktail effect, can be integrated into Janus heterostructures, which may offer a rich platform for tuning their catalytic properties across multiple electrochemical reactions. With the rapid development of phase engineering of nanomaterials (PEN), engineering Janus nanostructures with phase asymmetry, or so-called Janus-phase structures, represents a novel and exciting avenue for obtaining new properties and functionalities by creating asymmetric regions with different phases (e.g., amorphous and crystalline). These unconventional Janus heterophases can combine the advantages of unconventional phases and asymmetric architectures, and thus may show unprecedented performances for catalysis or other applications. Future work should explore novel synthetic routes that can create and stabilize unconventional phases within Janus architectures.

Thirdly, although Janus nanomaterials often demonstrate enhanced electrocatalytic performances, the fundamental mechanisms governing these improvements are not yet fully understood. The intrinsic asymmetry of Janus structures featuring spatially separated compositions provides an ideal platform for differentiating the roles of distinct domains during catalytic reactions. However, in most cases, the enhanced performance of Janus nanomaterials is attributed to “synergistic effects”, without detailed identification of their actual active sites. An in-depth and quantitative understanding of the catalytic mechanisms by the integration of advanced operando characterization techniques with theoretical simulations, including theoretical calculations and machine learning-assisted modeling, is urgently needed. By unraveling the distinct functions of different domains in Janus structures, researchers can gain deeper insights into their underlying mechanisms and the development of next-generation Janus catalysts with tailored functionalities.

Finally, the concept of Janus structures can be further extended by scaling down their building blocks from conventional nanoparticles to clusters, sub-nanometer species, or even single atoms. Constructing atomic-scale Janus heterostructures such as metal clusters and dual-site single-atom configurations would enable precise control over their active site geometry, electronic structure, and reaction pathways, offering an exciting strategy for the design of advanced catalysts.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

No primary research results, software or code has been included and no new data were generated or analysed as part of this review.

Acknowledgements

This work was supported by the National University of Singapore start-up grant, Centre for Hydrogen Innovations (Grant No. CHI-P2023-04), National Research Foundation of Singapore (Grant No. U2411D4005), Ministry of Education of Singapore (Grant No. 23-0646-A0001 and 24-1770-A0002). Y. G. acknowledges the financial support from the National Natural Science Foundation of China (No. 52471219) and the Fundamental Research Funds for the Central Universities (No. 00007838). M.Z. acknowledges the National Research Foundation, Prime Minister's Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) programme (Development of advanced catalysts for electrochemical carbon abatement, Project Code: 370184872). N. Y. acknowledges the financial support from the National Natural Science Foundation of China (No. 92163209) and Beijing Natural Science Foundation (No. JQ22004).

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