Biao Huang
*b,
Yiming Wanga,
Fukai Fengd,
Nailiang Yang
e,
Yiyao Ge
*a and
Ming Zhao
*bc
aState Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, No. 30 Xueyuan Road, Haidian District, Beijing 100083, China. E-mail: yiyaoge@ustb.edu.cn
bDepartment of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore. E-mail: biaoh01@nus.edu.sg; mingzhao@nus.edu.sg
cCentre for Hydrogen Innovations, National University of Singapore, Singapore 117580, Singapore
dSchool of Materials Science and Engineering, University of Science and Technology Beijing, No. 30 Xueyuan Road, Haidian District, Beijing 100083, China
eState Key Laboratory of Biopharmaceutical Preparation and Delivery, Institute of Process Engineering, Chinese Academy of Sciences, University of Chinese Academy of Sciences, P. R. China
First published on 7th August 2025
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.
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.
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., Au0.5Rh0.5) 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., Au0.85Rh0.15 and Au0.15Rh0.85). 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–CrOx 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 CrOx 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 CrOx 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-CrOx 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.
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.
![]() | ||
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–MxSny heterodimers were first formed with an increasing ratio of MxSny domain in the heterodimers, finally obtaining a pure MxSny 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.
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.
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 |
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 |
![]() | ||
Fig. 6 (a) HOR polarization curves of different catalysts. (b) Comparison of half-wave potential E1/2, mass- and ECSA-normalized activities of Ru–CrOx@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.
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).
![]() | ||
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. |
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.
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–CuI, Au–CuII, and Au–CuIII 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–CuI, Au–CuII, and Au–CuIII) and one core–shell structure (Au–CuIV) by increasing the contact area between Au and Cu.58 With the optimal Au–Cu contact area, Au–CuIII 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–CuI (72.1% FE in liquid products), C2H5OH on Au–CuII (78.1% FE), and C2H4 on Au–CuIII (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.
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.
This journal is © The Royal Society of Chemistry 2025 |