Terminology Explanation:To ensure terminological consistency and conceptual clarity throughout the text, the terms used in the document are defined.
Atomic Condensed States: Refers to the spatial arrangement and aggregation characteristics of metal atoms within multi-component metallic nanoparticles. In these nanoparticles, metal atoms form "stable adhesion" relationships through metallic bonds. Different atomic condensed states can induce differentiated electronic interactions, thereby significantly influencing the catalytic performance of the nanoparticles.
Uniform alloy: Refers to a state in which different metal atoms are highly uniformly mixed at the atomic scale within nanoparticles, forming a single solid solution phase.
Heterogeneous structure: Refers to a multiphase structure in nanoparticles where different components are spatially separated and form distinct phase interfaces.
Core–shell structure: A special type of heterogeneous structure characterized by a metal core within a nanoparticle, with another metal epitaxially deposited on its surface to form a multiphase structure featuring a core–shell spatial distribution.
Electronic coupling effect: In multi-component metal nanoparticles, differences in electronegativity or work function among atoms of different components induce electron redistribution, thereby modulating the electronic structure of key reaction sites and significantly influencing their catalytic activity and selectivity.
Stress Effects: In multicomponent metallic nanoparticles, compressive or tensile strain is introduced into the structure due to atomic size differences or interfacial lattice mismatch. This strain can alter the surface atomic spacing and the position of the d-band center, thereby modulating the adsorption energy of guest molecules and significantly influencing catalytic reaction performance.
Surface Geometric Effects: On the surfaces of multicomponent metal nanoparticles, the arrangement of different metal atoms can form diverse coordination environments and geometric configurations. These local structural features significantly influence the adsorption patterns of reaction intermediates and the reaction pathways, thereby playing a crucial regulatory role in catalytic processes.
1 Introduction
Noble metal nanomaterials, particularly the platinum-group metals such as platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh), and iridium (Ir), occupy a pivotal position in the field of heterogeneous catalysis. In the petrochemical industry, these platinum-group precious metals play an irreplaceable role in key reactions including hydro- and dehydrogenation of hydrocarbons, reforming, and isomerization
［1-5］; in the automotive industry, three-way catalytic converters extensively employ platinum-group metals (Pt, Pd, and Rh) to convert harmful NO
x, CO, and hydrocarbons in automobile exhaust gases into harmless gases
［6-7］; in the new energy sector, Pt-based noble metal nanocatalysts have demonstrated outstanding performance in proton-exchange-membrane electrolysis for hydrogen production and in fuel cells, and are regarded as core components of future energy technologies
［8-11］. The widespread application of noble metal nanocatalysts is primarily attributed to their unique electronic structures and nanoscale effects. At the nanoscale, these materials exhibit a significantly enhanced specific surface area, resulting in a greater number of atoms exposed on the surface and thus more catalytically active sites. Furthermore, the increased abundance of low-coordinated atoms at the surfaces of nanocrystals—such as those located at edges, corners, or defects—endows these sites with distinctive electronic structures and adsorption properties, which play crucial roles in catalytic reactions
［12-13］. More importantly, when the particle size of these metals decreases below 3 nm, the materials begin to exhibit molecular-like characteristics, and even minor changes in size can lead to substantial alterations in catalytic properties. Consequently, they demonstrate markedly superior catalytic activity compared with bulk materials or larger-sized nanoparticles
［14-19］. These characteristics render noble metal nanomaterials exceptionally high-performing catalysts across a wide range of catalytic applications. However, the scarcity of noble metals and their highly uneven global distribution pose significant constraints on their widespread adoption. China possesses relatively limited resources of platinum-group metals and is heavily reliant on imports. Therefore, the development of highly efficient, low-loading novel noble metal nanocatalysts that enhance catalytic activity and atom utilization efficiency has become a major focus of current research efforts.
In this context, introducing additional metal elements into noble-metal nanoparticles to construct multi-component metallic nanomaterials has become an effective strategy for enhancing catalytic performance. Compared with single-metal systems, such multi-component metallic nanomaterials can exhibit significantly improved catalytic activity. This enhancement originates from pronounced synergistic effects among the different metal components, which are primarily manifested in the following aspects. (1) Ligand effect
［20-21］. Differences in electronegativity among various metal atoms or in work functions between different metallic phases lead to charge transfer and redistribution among metals, thereby altering the electronic structure of active sites. Such changes in electronic structure can significantly influence the adsorption strength of reactants, thus modulating the catalytic reaction activity. (2) Strain effect
［22-23］. Discrepancies in atomic radii or lattice parameters among different metal components result in substantial deviations in interatomic distances or metal–metal bond lengths from their intrinsic values, giving rise to tensile or compressive strain. This strain can markedly alter the shape and energy positions of the d-orbital electronic states of metals, thereby regulating both the adsorption behavior of reactants and the catalytic activity. (3) Geometric effect
［24-26］. The specific atomic arrangements or active-site configurations formed by different types of metal atoms on the surface of nanocrystals can affect the adsorption patterns of reactants on the surface or influence the formation of particular reaction intermediates, thereby impacting the reaction pathway and selectivity. These synergistic effects enable multi-component metallic nanomaterials to achieve optimization of catalytic performance through fine-tuned compositional control, thereby enhancing their application potential.
It is worth emphasizing that, in multicomponent metallic nanomaterials, relying solely on “composition” to define their properties is far from sufficient. This is because the spatial arrangement of different atoms within the material—namely, the “atomic condensed state”—is a key factor influencing its catalytic behavior. Therefore, precise control over the atomic condensed states of each metal atom in nanoparticles constitutes an important approach for modulating catalytic properties. Specifically, multicomponent metallic nanoparticles can exhibit various forms of atomic condensed states: ranging from atomically homogeneous mixing to form solid-solution alloy structures, to spatially separated core–shell or multiphase heterogeneous structures. These distinct atomic condensed states significantly affect the manner in which electronic coupling occurs within the material. In homogeneous alloys, different metal atoms interact directly through bonding, thereby influencing the geometric characteristics and electronic structure of active sites
［27-29］. In contrast, in heterogeneous structures, the metals interact via epitaxial relationships across different phases and through electron transfer, thus regulating the geometric characteristics and electronic structure of active sites
［21, 30-32］. These distinct mechanisms of electronic structure regulation have greatly expanded the range of strategies available for controlling the catalytic properties of noble-metal active sites. By precisely tuning the atomic condensed states within multicomponent metallic nanoparticles, it may be possible to create novel noble-metal nanocatalysts and discover new functionalities.
However, traditional wet-chemical synthesis still faces challenges in precisely controlling the atomic aggregation states of multicomponent metal nanoparticles. The root of this challenge lies in the substantial property differences among various metal elements, primarily manifested as significant disparities in their intrinsic reduction potentials
［33］. This factor leads to the following consequences: during alloy synthesis, the differences in intrinsic reduction potentials between metal salts result in marked disparities in reduction kinetics. Some metals are readily reduced, whereas others are difficult to reduce, leading to non-uniform compositional distributions within alloy nanoparticles and making it challenging to precisely regulate surface compositions based on overall compositions
［34-37］. In the synthesis of heterogeneous structures, specific requirements regarding intrinsic reduction potentials or metal reactivity sequences must be met. Typically, only noble metals with high intrinsic reduction potentials and inert chemical properties can serve as substrates for the epitaxial growth of reactive metal nanostructures with lower intrinsic reduction potentials. Conversely, if reactive metals are used as substrates to attempt the epitaxial growth of noble metal nanostructures, displacement reactions are highly likely to occur, resulting in partial corrosion or even complete dissolution of the reactive metals
［38-39］. Consequently, core–shell structures with relatively reactive metals as cores and noble metals as shells are often difficult to achieve. This limitation significantly restricts the controllability of oriented growth of heterogeneous nanocrystals.
This paper, based on the research efforts of our team and related research groups both domestically and internationally, outlines the fundamental principles and recent advances in the atomic-scale condensation-state regulation of multi-component metallic nanomaterials. The core concept involves constructing synthesis systems that deviate from thermodynamic equilibrium, thereby mitigating or eliminating the thermodynamic constraints imposed by intrinsic reduction potential differences among metals. This approach enhances the homogeneity of mixing and the precise tunability of composition for different metal atoms within alloyed nanoparticles. On the other hand, by employing coordination chemistry techniques and introducing appropriate ligands to modulate the reduction potentials of metal salts, it is possible to effectively suppress displacement reactions between noble-metal salts and reactive metal nanostructures, thus enabling the controlled synthesis of novel core–shell structures with reactive metals as the core and noble metals as the shell. These strategies have significantly improved the precision of atomic-scale condensation-state control in multi-component nanoparticles, endowing the materials with tunable electronic structures and superior catalytic performance. Finally, the paper offers a perspective on future development directions in this field, focusing on the opportunities and challenges associated with achieving structural design and regulation at the atomic level.
2 Alloy Nanomaterials
Uniform alloy nanomaterials refer to alloy phases in which different metallic components are highly mixed at the atomic scale, forming a single solid solution. Precisely controlling the compositional tunability, uniformity, size, and structure of alloy nanomaterials is crucial for realizing their high-performance applications.
2.1 Traditional Co-Reduction Synthesis Method: Challenges in Atomic Condensed-State Regulation
Alloy nanomaterials are typically synthesized using co-reduction strategies such as the impregnation method and wet chemical reduction.
The impregnation method is a conventional approach for preparing supported metal nanocatalysts, particularly suitable for the large-scale synthesis of single-metal catalysts (
Figure 1a)
［40］. However, this method faces significant challenges when it comes to the preparation of multicomponent alloy nanocatalysts. As a dry-process technique, impregnation struggles to achieve uniform distribution of multiple metal precursors on the support surface; moreover, the substantial differences in reduction potentials among various metals often lead to sequential reduction behavior. These factors collectively result in non-uniform compositions of the resulting multicomponent metal nanoparticles: some particles are alloys with varying compositions, while others consist of single-metal species
［41］. In addition, the obtained nanoparticles typically exhibit a broad particle-size distribution. Although several improved strategies have been developed in recent years—such as adsorption impregnation
［41］, mixed-salt impregnation
［42］, and instantaneous high-temperature carbothermal reduction
［43］—these approaches have, to some extent, enhanced the size and compositional uniformity of alloy nanoparticles, overall, the impregnation method still lacks an effective mechanism for the precise synthesis of alloy nanomaterials.
Compared with the impregnation method, the wet-chemical co-reduction strategy offers greater controllability in the size, morphology, and structural regulation of noble metal nanomaterials. The fundamental principle of this method involves the simultaneous reduction of two or more metal precursors in solution to zero-valent metal atoms, which subsequently undergo nucleation and crystal growth to form alloy nanoparticles. However, significant intrinsic differences in reduction potentials often exist among different metal precursors, leading to the preferential reduction of metal ions with higher reduction potentials and thereby compromising the uniformity of the alloy composition
［33］. For metal systems with similar reduction potentials, this approach can achieve a favorable match in reduction rates, thus facilitating the formation of homogeneous alloys. For example, Professor Yu Huang’s team at the University of California, Los Angeles, used K
2PtCl
4 and Na
2PdCl
4 as precursors, polyvinylpyrrolidone (PVP) as a surfactant, and
N, N-dimethylformamide (DMF) as the reducing agent, successfully synthesizing Pt-Pd alloy nanomaterials with uniform elemental distribution
［44］. Given that Pd
2+(
E 0 = +0.90 V) and Pt
2+(
E 0 = +1.18 V) possess relatively close reduction potentials, synchronous co-reduction and the synthesis of a homogeneous alloy phase were achieved.
However, when there is a large difference in reduction potentials among different metal ions, the reduction rates are difficult to match, often leading to a restricted alloying process. This results in a narrow range of controllable alloy compositions and non-uniform distribution within particles. To achieve coordinated reduction kinetics among precursors, researchers have attempted to regulate their synchronous reduction behavior by adjusting the molar feed ratios of each metal precursor, thereby promoting alloy formation. For example, the team led by Shouheng Sun at Brown University in the United States
［47］found that significantly increasing the feed ratio of the Ag precursor in the synthesis system can effectively reduce the difference in reduction rates between Ag and Au precursors, thus enabling co-reduction. Specifically, when the molar feed ratio of AgNO
3to HAuCl
4reaches 10:1, an Au
60Ag
40alloy nanomaterial can be obtained. In addition, to overcome the challenges posed by differences in reduction potentials, some studies have employed strong reducing agents such as sodium borohydride (NaBH
4) in aqueous media to promote the synchronous reduction of different metal ions, thereby achieving alloying even in systems with large reduction potential differences (
Figure 1b)
［45］. However, it should be noted that the vigorous reactions induced by strong reducing agents are often difficult to precisely control in terms of nucleation and growth processes, which limits the ability to regulate the final nanoparticle size, morphology, and composition. Currently, oil-phase synthesis is the primary method for wet-chemical preparation of alloy nanocrystals (
Figure 1c)
［46］. This strategy typically uses mild reducing agents such as oleylamine, carbon monoxide, and glucose to reduce oil-soluble metal precursors (e.g., metal acetylacetonate complexes) at approximately 200 ℃. Furthermore, zero-valent metal carbonyl compounds are also frequently employed as precursors. This approach demonstrates significant advantages in morphology control. Nevertheless, oil-phase synthesis still faces certain limitations, primarily resulting in relatively large nanocrystal sizes and difficulties in precisely controlling the order of metal reduction. For instance, the team led by Xiaoqing Huang at Xiamen University
［34］discovered during the synthesis of Pt–Pb alloy nanooctahedra that Pb salts are preferentially reduced at the initial stage of the reaction, leading to non-uniform compositional distribution within the alloy nanocrystals (
Figure 1d). Similarly, the team led by Minhua Shao at the Hong Kong University of Science and Technology
［35］observed during their investigation of the oil-phase growth mechanism of Pt–Ni nanooctahedra that spherical Pt cores initially form as the temperature increases, followed by the gradual reduction and deposition of Ni on the Pt surface, ultimately yielding non-uniform alloy nanocrystals with a Pt core/Pt–Ni shell structure (
Figure 1e). Moreover, similar phenomena of non-uniform compositional distribution within particles have also been observed in the synthesis of high-entropy alloy nanomaterials
［36–37］.
2.2 Wet-Chemical Synthesis Method Based on Active Hydrogen–Interface Co-Reduction
To overcome the influence of reduction potential differences among various metal precursors during the synthesis of alloy nanomaterials, our team has proposed a wet-chemical synthesis strategy based on an active hydrogen–mediated interfacial co-reduction mechanism
［29, 48–50］. This strategy draws inspiration from conventional electroless nickel plating processes. In typical electroless plating, hypophosphite ions (H
2PO
2 -) are introduced into the system to facilitate the efficient reduction of nickel ions (Ni
2+) to metallic Ni on heterogeneous metal surfaces. Studies have shown that H
2PO
2 -can generate highly reductive active hydrogen species—namely, atomic hydrogen or hydrogen radicals (H·)—on metal surfaces, thereby driving the efficient conversion of Ni
2+to Ni
0. Building upon this mechanism, we propose employing metal-catalyzed active hydrogen as a reducing agent in the synthesis of alloy nanomaterials. As an exceptionally strong reducing agent (
E 0 = −2.31 V), H· possesses a standard reduction potential significantly lower than those of noble-metal salts and common non-noble-metal salts (
E 0 typically falls within −1 V), enabling it to effectively surmount intrinsic reduction potential differences among various metal salts and achieve their synchronous co-reduction. Furthermore, since H· is confined to the metal surface and does not undergo homogeneous mixing with metal salts in solution, the reduction reaction shifts from the traditional solution phase to the solid–liquid interface, thereby further enhancing both the kinetics of metal-salt reduction and the controllability of alloy nanocrystal growth. This approach holds promise as a viable pathway for the precise synthesis of alloy nanomaterials. In practical applications, we have introduced inorganic small molecules structurally analogous to H
2PO
2 -, such as bisulfite ions (HSO
3 -) and nitrous acid (HNO
2), which are non-high-valence oxyacids, to construct noble–non-noble metal co-reduction systems (
Figure 2a, b)
［29, 48］. Research indicates that this strategy offers significant advantages in controlling alloy composition: by generating active hydrogen *in situ* on metal surfaces, it can effectively mitigate the impact of reduction potential differences between noble-metal salts (e.g., Pt) and non-noble-metal salts (e.g., Fe, Co, Ni, Cu, Ga, In, Sn, Pb, Bi, etc.) on reduction kinetics, allowing these salts to be co-reduced at nearly synchronous rates and ultimately yielding alloy nanostructures with highly uniform compositions. Based on this method, we have successfully synthesized a variety of alloy nanomaterials, including Pt–M alloy nanowires and ultrathin alloy shells on Pd octahedra (
Figure 2c)
［29, 48］, while also achieving continuous compositional tuning over a broad range.
Inspired by the phenomenon of metal-catalyzed organic dehydrogenation reactions that generate hydrogen gas, we further expanded the implementation pathways of the active hydrogen–interface–co-reduction mechanism
［50］. We hypothesized that in organic catalytic dehydrogenation reactions, active hydrogen might serve as a precursor to hydrogen gas; consequently, these active hydrogens could be utilized for the synthesis of alloyed nanomaterials. Based on this conjecture, we introduced the dehydrogenation-active organic small molecule DMF to replace inorganic precursors. Studies have shown that DMF can catalyze dehydrogenation on metal surfaces, generating active hydrogen species. Leveraging the strong reducing power of these species and their interface-confined characteristics, it is possible to achieve efficient co-reduction of multiple metal ions, successfully constructing high-entropy alloy nanoparticles with highly uniform elemental distribution (
Figure 2d)
［50］. More importantly, this strategy endows high-entropy alloy nanomaterials with unprecedented compositional tunability: while keeping other components constant, each metallic component within the alloy nanocrystals can be independently and precisely regulated according to the precursor concentrations in the synthesis system, greatly enhancing both the freedom of material composition design and the catalytic tuning capabilities (
Figure 2e, f)
［50］.
The active hydrogen species in the interfacial co-reduction synthesis system have been experimentally verified. In this synthesis system, we introduced the radical scavenger 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and detected the characteristic signal of the DMPO-H· adduct using electron paramagnetic resonance (EPR) spectroscopy, thereby confirming that active hydrogen exists in the form of a hydrogen radical (H·) within the system (
Figure 2b)
［29, 50］. Further studies demonstrated that when HNO
2is used as the precursor of active hydrogen, substantial amounts of H
2and N
2gases can be detected during the synthesis process, which are attributed to the coupling between active hydrogen atoms and the reduction of nitrous acid to nitrogen gas on the metal surface, respectively. Isotope labeling experiments revealed that DMF and H
2O each contribute one H·, and their coupling ultimately results in the formation of H
2. These findings confirm the presence of highly reductive active hydrogen species in this synthesis system and elucidate the formation mechanism of active hydrogen.
The introduction of active hydrogen significantly enhances the uniformity of alloy component distribution within nanoparticles, ensuring consistency between their surface composition and bulk phase composition. This characteristic makes it possible to precisely regulate surface elements by controlling the bulk phase composition, thereby providing a solid foundation for optimizing catalytic performance. In contrast, traditional wet-chemical co-reduction methods often result in pronounced radial element gradients within alloy particles due to differences in reduction rates, leading to significant disparities between surface and bulk compositions and limiting effective regulation of catalytic active sites. On the other hand, this interfacial co-reduction strategy markedly strengthens the controllability of alloy components. It not only enables the synthesis of alloy nanomaterials across a broad compositional range but also allows for flexible adjustment of specific element ratios, thus overcoming the limitations of conventional wet-chemical co-reduction methods in terms of compositional control. It is worth noting that, as an extension of traditional approaches, the team led by Xia Younan at the Georgia Institute of Technology proposed a strategy involving the slow and continuous injection of mixed metal salts into the reduction system to construct alloy nanoshell layers with specific compositions on the surfaces of metal nanocrystal seeds, which to some extent improved the precision of alloy composition control
［51］. Compared with this method, the active hydrogen–mediated interfacial reduction mechanism offers greater operational simplicity and higher reaction controllability, particularly demonstrating outstanding advantages in the precise fabrication of multicomponent systems or high-entropy alloys.
Therefore, the wet-chemical synthesis strategy based on the interfacial co-reduction mechanism of active hydrogen provides a universal and efficient synthetic platform for alloy nanomaterials with uniform component distribution and precise controllability. This strategy not only enriches the theoretical framework of nanomaterial synthesis but also offers strong support for the construction and application expansion of high-performance nanoalloys in complex multicomponent systems.
2.3 Vacancy Diffusion Induced Alloying Method
Void diffusion-induced alloying is a synthetic strategy that leverages point defects within materials to facilitate effective mixing between dissimilar metals, particularly suitable for achieving uniform alloying in immiscible metal systems under low-temperature conditions.
In crystalline materials, vacancies are the most common point defects. Their presence provides channels for atomic diffusion, significantly reducing the diffusion energy barrier. Atoms move along vacancy migration pathways, thereby achieving elemental mixing in the solid state. The Gezelte team at the University of Notre Dame in the United States
［52］found that in Au–Ag nanoparticle systems, vacancies at the particle interfaces markedly accelerate radial diffusion of metal atoms, enabling effective mixing of the two elements even at room temperature. This vacancy-dominated diffusion mechanism enhances interatomic diffusion efficiency by effectively lowering the atomic diffusion energy barrier. The Skrabalak team at Indiana University in the United States
［53］observed during their study of the displacement reaction between Ag–Pd heterostructures and HAuCl
4that the large number of Ag vacancies generated during the reaction can significantly promote alloying between Ag and Pd (
Figure 3a). These findings validate the promoting effect of atomic vacancies on solid-state diffusion between the two metals and underscore the critical role of point defect engineering in constructing alloy structures.
Based on the insights from the aforementioned research, our research team
［54］developed a synthetic strategy for alloy nanomaterials based on vacancy-mediated diffusion (
Figure 3b). Taking the preparation of PdCu alloy nanocages as an example, a Pd nanoshell was first grown on the surface of Cu nanocrystals to construct a Cu@Pd core–shell structure. Subsequently, the internal Cu core was removed via oxidative etching. During this process, a large number of Cu atoms migrated out of the crystal lattice, leaving behind abundant vacancies. These vacancies served as diffusion pathways, effectively reducing the atomic diffusion energy barrier and significantly accelerating the atomic exchange rate between Cu and Pd. Ultimately, this led to the homogeneous mixing of the two elements at the nanoscale, resulting in a PdCu homogeneous alloy nanomaterial (
Figure 3c). In addition, we used Ag as a template to fabricate an Ag@RuIr core–shell structure and introduced vacancies within the Ag core through etching, thereby inducing effective diffusion and alloying among Ru, Ir, and Ag. Despite the fact that Ag is immiscible with Ru and Ir under conventional thermodynamic conditions, this strategy still enabled the homogeneous mixing of the three metals, yielding a structurally stable AgRuIr alloy nanoframework with uniform composition.
The alloying strategy driven by vacancy diffusion has enriched the synthesis system of alloy nanomaterials, helping to overcome the limitations imposed by traditional synthesis approaches in terms of elemental miscibility and diffusion kinetics. This approach significantly expands the range of element selection and the diversity of structural design for alloy materials.
2.4 Synthesis Method of Alloy Nanoclusters Based on Nanoreactors
When the size of metal particles decreases below 3 nm, the number of atoms is reduced to several hundred or even dozens. Unlike single-atom or large-sized nanoparticles, these nanoclusters exhibit molecular-like electronic structures, and even slight variations in their size or atomic number can significantly affect their catalytic properties, thereby providing important opportunities for the development of highly efficient catalysts
［41］. However, the synthesis of alloy nanoclusters at this scale still faces significant challenges. In the impregnation method for constructing supported alloy nanoclusters, due to the non-uniform distribution of metal precursors on the support surface and differences in reduction potentials among metals, substantial inconsistencies in size and composition often arise between different clusters
［41］. Moreover, in traditional colloidal chemical synthesis systems, the mismatch in reduction kinetics caused by intrinsic differences in reduction potentials among metals makes it extremely difficult to achieve synchronous reduction and uniform alloying of multiple metals within clusters containing only a few atoms. Even in the relatively mature field of "nanocluster chemistry," the alloy nanoclusters obtained are often mixtures composed of various components and structures, lacking compositional consistency among clusters
［55-59］. This compositional inconsistency not only limits the clear identification of catalytic active sites but also hinders the precise control and optimization of catalytic performance.
To overcome the challenges in the precise synthesis of alloy nanoclusters, our research team has proposed a synthesis strategy based on nanoreactors (
Figure 4a)
［19］. The core of this strategy lies in shifting the synthesis process of alloy nanoclusters from conventional liquid-phase systems to water-in-oil (W/O) microemulsion systems
［18-19, 60-62］. The unique thermodynamic equilibrium characteristics of microemulsions ensure that metal salts are uniformly dispersed into nanoscale droplets—referred to as “nanoreactors”—thereby achieving highly consistent species and concentrations of metal salts within each droplet. Subsequently, the microemulsion droplets are microencapsulated and solidified via sol–gel chemistry, followed by high-temperature reduction to yield alloy nanoclusters. The sub-nanometer size of these clusters further promotes uniform distribution of metallic components throughout their interiors. By employing this synthetic method, we have successfully prepared various alloy nanoclusters, including PtCo and PtFe, enabling independent and precise control over both composition and size, while demonstrating excellent versatility and tunability.
Compared with traditional synthesis methods, the alloy nanocluster synthesis method based on nano-reactors possesses distinct advantages: (1) Composition uniformity and tunability. This strategy leverages thermodynamic equilibrium to drive the uniform distribution of metal precursors within each nano-reactor, ensuring highly consistent compositional ratios across all alloy nanoclusters (
Figure 4b, c). Meanwhile, by adjusting precursor concentrations, flexible control over different elemental ratios can be achieved, endowing the approach with excellent compositional designability. (2) Size uniformity and tunability. The synthesized nanoclusters exhibit an extremely narrow size distribution, with a size uncertainty of less than 0.2 nm, and their sizes can be continuously tuned within the sub-3 nm range (
Figure 4d). (3) Nanocluster stability. Confined within porous cavities, the nanoclusters effectively suppress aggregation and sintering at elevated temperatures, thereby imparting superior thermal stability.
2.5 Summary and Outlook
This chapter systematically explores various synthesis strategies for homogeneous alloy nanomaterials (
Table 1). First, it elucidates that the inherent differences in reduction potentials among metals pose significant challenges in conventional methods, thereby rendering the precise synthesis of homogeneous alloy nanomaterials rather difficult. Subsequently, several novel synthesis strategies are highlighted, including active-hydrogen–mediated interfacial co-reduction, vacancy-diffusion–induced alloying, and nano-reactor synthesis approaches, which enable the precise fabrication of alloy nanomaterials with uniform composition and controllable size. Nevertheless, precisely controlling the surface and bulk compositions of complex multicomponent alloys at the atomic scale, as well as real-time monitoring their dynamic evolution during synthesis and catalytic processes, remains a core challenge that urgently needs to be addressed in this field.
3 Heterogeneous Structural Metal Nanomaterials
In multi-component metallic nanomaterials, different metal components can spatially form heterogeneous structures with distinct interfaces and specific atomic arrangements, representing another important form of their atomic condensed states. Among these, core–shell structures are a typical type of heterogeneous structure, consisting of an inner core and an outer shell layer. The metal components in the core and shell exist as separate phases, with well-defined interfaces and typically exhibiting good epitaxial relationships. These structural features give rise to significant electronic coupling effects between the core and the shell, thereby effectively tuning the overall catalytic performance.
3.1 Traditional Core–Shell Structures: Limitations of the Metal Reactivity Series on Core–Shell Structures
In the synthesis of conventional core–shell structures, the order of metal reactivity is typically followed, which is fundamentally determined by the standard reduction potentials of their corresponding precursor salts. Generally, the higher the reduction potential of a metal salt, the more inert the corresponding metal; conversely, the lower the reduction potential, the more reactive the metal. Therefore, chemically inert noble metals are often used as the core, while more reactive metals are employed to form the shell. For example, Au and Pd are frequently utilized as seed materials to effectively resist oxidative etching by the shell-layer metal precursors. For instance, the team led by Peidong Yang at the University of California, Berkeley
［63］successfully fabricated shape-tunable Pt@Pd core–shell nanocrystals by epitaxially growing Pd on the surface of Pt nanocubes. Similarly, Au@Ag core–shell structures can also be synthesized through Ag deposition onto Au seed crystals
［64–65］.
However, constructing a "reverse activity-order" core–shell structure—where a relatively reactive metal (such as Ag, Cu, or Ni) serves as the core and a noble metal (such as Au, Pt, or Pd) forms the shell—presents significant challenges. This is because the reactive metal core can readily be oxidized and dissolved by noble metal salts with higher reduction potentials, triggering spontaneous displacement reactions that lead to the destruction of the core structure. Such reactions involve the oxidative dissolution of the reactive metal and the reductive deposition of noble metal ions. Taking the reaction between Ag nanocrystals and HAuCl
4 as an example, Ag atoms lose electrons and are oxidized to Ag
+, entering the solution, while AuCl
4- gains electrons and is reduced to Au atoms, which then deposit on the surface of the Ag nanocrystals. Due to the specific stoichiometric relationships inherent in displacement reactions—for instance, the reduction of one AuCl
4- ion requires the oxidation of three Ag atoms—the dissolution rate of the core often exceeds the deposition rate of the shell. Consequently, the final products typically exhibit hollow structures, and in some cases may even fragment or completely disintegrate
[38–39]. This uncontrollable displacement reaction severely hinders the formation of core–shell structures with well-defined phase interfaces. Therefore, synthesizing core–shell structures in the reverse activity order is relatively difficult, which largely restricts the diversity of atomic aggregation states in multi-metal heterogeneous structures and limits the exploration of the unique properties of such non-traditional core–shell materials.
3.2 Synthesis of Core–Shell Structured Nanomaterials Based on the Activity Series of Metals
Given the unique properties that core–shell structures based on the reactivity series of metals may exhibit, it is necessary to develop reliable strategies for suppressing displacement reactions in order to achieve the controlled synthesis of such structures
［66］. Currently, the following two types of strategies have been proven effective.
(1) Kinetic control strategy.
In 2014, the team led by Qin Dong at the Georgia Institute of Technology in the United States
［67］reported a kinetic control strategy that successfully achieved the controllable growth of gold nanoshell layers on silver nanocube surfaces, thereby constructing Ag@Au core–shell nanocubes with an inverse activity order (
Figure 5). The key to this synthesis method lies in increasing the pH of the reaction system to promote the deprotonation of the reducing agent ascorbic acid (AA), thereby significantly enhancing its reducing power. In this system, the Au precursor can undergo both reduction reactions with AA to generate zero-valent Au atoms and displacement reactions with silver nanocrystals; these two processes are kinetically competitive. By strengthening the reducing ability of AA, the reduction reaction of the Au precursor becomes kinetically dominant, effectively suppressing its displacement reaction with Ag nanocrystals. Using this strategy, Ag@Au core–shell nanocubes with an Au shell thickness of only 0.6 nm were successfully prepared. These nanostructures exhibit complete morphology, with no obvious etching or hollowing observed. Furthermore, this method has been extended to the synthesis of other core–shell structures such as Ag@Pt, demonstrating good structural controllability and broad applicability
［68–70］.
In addition, by reducing the concentration of the metal precursor in the synthesis system, effective kinetic control of the displacement reaction can also be achieved. The Gun’ko team at the National University of Ireland
［71］successfully epitaxially grew an Au nanoshell layer on the surface of citrate (TSC)-modified Ag nanosheets by slowly adding a low-concentration HAuCl
4solution with AA as the reducing agent. In this synthesis, the strong adsorption of TSC onto the Ag surface played an important role in inhibiting the displacement reaction between the Au precursor and the Ag nanosheets. The Qin Dong team
［72］controlled the synthesis of a Pd–Ag alloy nanoshell on the surface of Ag nanocubes by gradually introducing low-concentration Na
2PdCl
4and AgNO
3into an AA reduction system. This method enables the sequential deposition of Pd and Ag atoms on the edges, corners, and facets of the nanocubes, thereby achieving fine-tuned control over the crystal growth process of the alloy shell.
(2) Thermodynamic control strategy.
The core of this strategy lies in introducing specific ligands, which effectively reduce the reduction potential of noble metal precursors through coordination interactions, making it lower than the oxidation potential of the reactive metal core. This thermodynamically inhibits the occurrence of displacement reactions. Compared with kinetic control methods that rely on differences in reaction rates, this strategy offers higher controllability and reliability, and holds promise for more effectively achieving the controlled synthesis of core–shell structures with an inverse reactivity sequence.
Our team proposed the prototype of this strategy as early as 2012, successfully achieving the controlled construction of Ag@Au core–shell nanosheets and significantly enhancing the chemical stability of Ag nanosheets
［73］. Specifically, by introducing iodide ions (I
-) into the reaction system, ligand exchange with the Au precursor HAuCl
4occurred, yielding a stable HAuI
4complex that effectively lowered the reduction potential of the Au precursor. Since the standard reduction potential of HAuI
4(
E 0 = +0.54 V) is lower than that of Ag⁺ (
E 0 = +0.80 V), the thermodynamic driving force for the displacement reaction between the Au precursor and Ag nanocrystals was suppressed. Experimental results demonstrated that Au could be uniformly deposited on the surface of Ag nanosheets, forming a complete, non-hollow core–shell structure while preserving the original triangular or hexagonal flake morphology, thereby validating the effectiveness of this thermodynamic control strategy. It should be noted, however, that I
-still exhibits a certain oxidizing-promoting effect on Ag nanocrystals; consequently, this method is only suitable for constructing extremely thin Au shells, which limits the flexibility in controlling shell thickness. To overcome this limitation, we further developed an improved strategy employing sulfite ions (SO
3 2-) as ligands
［74］. SO
3 2-can strongly coordinate with AuCl
4 -to form a highly stable Au(SO
3)
2 3-complex, whose standard reduction potential is reduced to +0.111 V. This exceptionally low reduction potential reliably prevents the displacement reaction between the gold precursor and silver nanocrystals. Meanwhile, SO
3 2-exhibits no significant coordination with Ag, thus avoiding ligand-assisted oxidative etching and ensuring the integrity of the Ag core structure. This strategy enabled the layer-by-layer epitaxial growth of Au atoms on the surface of Ag nanocrystals, resulting in Ag@Au core–shell nanostructures with uniform shells and well-defined morphologies, thereby expanding the versatility and controllability of the thermodynamic control approach (Fig. 6a).
This method is suitable for constructing inverse activity series core–shell structures with silver as the core and platinum-group metals as the shell. We found that when acetonitrile (CH
3CN) is used as a ligand for H
2PtCl
6, it can significantly reduce the reduction potential of the Pt precursor, thereby inhibiting its displacement reaction with Ag nanocrystals and enabling the controlled synthesis of Ag@Pt core–shell nanocrystals (nanosheets, nanowires, etc.) (
Figure 6b)
[75–76]. Similarly, glucose (C
6H
12O
6) as a ligand can lower the reduction potential of Na
2PdCl
4, thus achieving the controlled synthesis of Ag@Pd core–shell structures (
Figure 6c)
[30]. Based on this approach, the Li team at Purdue University in the United States
[77] employed organic ligands such as oleyl amine (OAm) and triphenylphosphine (PPh
3) to regulate the reduction potentials of platinum-group metal salts, successfully realizing the controlled crystal growth of Pd, Pt, and other metals on Ag nanoparticles.
Despite significant progress in the construction of Ag@M (M: noble metal) core–shell structures, extending the core materials to more inexpensive and reactive 3d transition metals (such as Cu, Ni, Fe, and Co) still poses challenges. The team led by Peidong Yang
［78］found that using trioctylphosphine (TOP) as a ligand can effectively reduce the reduction potential of Au salts, thereby inhibiting the displacement reaction between Au salts and Cu nanowires and enabling the controlled synthesis of Cu@Au core–shell nanowires. Based on a thermodynamic control strategy, our team successfully constructed hexagonal-close-packed (hcp) Ni@Pt core–shell dendritic nanocrystals (
Figure 6d)
［31］. By introducing strong coordination between OAm and H
2PtCl
6, the reduction potential of the Pt precursor was significantly lowered, thus suppressing its displacement reaction with nickel nanocrystals. Furthermore, due to the substantial lattice mismatch (~5%) between Ni and Pt, Pt atoms encounter a high migration energy barrier on the Ni surface, which tends to result in island-like growth. To achieve uniform epitaxial growth, this synthesis was carried out at a relatively high temperature (144 °C), with the Pt precursor being introduced at a low rate to promote the diffusion of Pt atoms across the Ni surface, thereby forming a continuous Pt shell layer. Atom-resolved electron microscopy analysis revealed that the Pt shell layer exhibits anisotropic phase replication behavior on different crystal facets of the hcp-Ni nanodendrites: on the (0001) facet, the Pt shell maintains its intrinsic face-centered cubic (fcc) structure; whereas on non-(0001) facets, the Pt shell successfully replicates the hcp lattice of Ni and displays pronounced compressive strain along the [0001] direction. A clear phase boundary forms between Ni and Pt, with almost no atomic-level mixing occurring. The thickness of the Pt shell can be precisely tuned within the range of 2–4 atomic layers. The controllable construction of this structure provides a pathway for efficient electronic structure modulation of platinum-group metals in core–shell architectures, while also enhancing the utilization efficiency of Pt atoms and significantly reducing both the amount and cost of catalysts.
3.3 Synthesis of Other Heterogeneous Nanostructures
Besides the core–shell structure, multi-component metal nanomaterials also include various other heterogeneous structures, such as core–framework structures and Janus structures. These heterogeneous structures are typically prepared through synthesis methods based on symmetry breaking
［79］.
Xia Younan’s team
［80］proposed the “deposition–diffusion model” during the seed-mediated growth process (
Figure 7a). In this model, atoms of the second metal first deposit at high-surface-energy sites on the metal seeds, such as edges, corners, and defects, and then gradually diffuse to other sites like crystal facets. When the surface diffusion rate of metal atoms is much greater than the deposition rate, a core–shell structure with a uniform shell thickness tends to form. However, when the atomic deposition rate significantly exceeds the surface diffusion rate, newly deposited atoms tend to “stick upon impact” to the high-surface-energy sites rather than distributing uniformly, thereby forming a core–frame structure (
Figure 7b). Using this approach, core–frame nanomaterials such as Pd@Rh and Au@Pt have been synthesized
［81–82］.
Controlling symmetry breaking during growth can greatly increase the diversity of heterogeneous-structure nanocrystals. This asymmetric growth mode typically occurs under low deposition rates and high surface diffusion energy barriers. The team led by Xia Younan
［79, 83］achieved asymmetric selective deposition of Ag by regulating the growth rate of Ag on Pd nanocubes (
Figure 7c, d). When the injection rate of AgNO
3was sufficiently slow, Ag atoms deposited only on one of the six {100} facets of the Pd nanocubes, thereby forming Pd–Ag Janus dimers. As the injection rate of AgNO
3was increased, Ag atoms could deposit on three facets of the Pd nanocubes and even on all six facets, resulting in eccentric Pd–Ag nanocubes and Pd@Ag core–shell nanocubes.
3.4 Summary and Outlook
This chapter systematically reviews the synthetic advances of several typical heterogeneous nanostructured materials, with a particular focus on analyzing the challenges faced by conventional methods in constructing core–shell structures that defy the activity order. It also elucidates how advanced strategies such as kinetic and thermodynamic control can effectively suppress displacement reactions, thereby enabling the precise fabrication of these non-traditional core–shell architectures (
Table 2). Looking ahead, research in this field will increasingly emphasize extending these strategies to lower-cost transition-metal systems and exploring precise atomic-scale manipulation of interfacial structures, with the aim of endowing heterogeneous nanostructures with enhanced functionality and higher catalytic efficiency.
4 Catalytic Regulation Based on the Condensed State of Metal Atoms
Precise control of the atomic condensed state in multicomponent metallic nanomaterials has laid the foundation for tuning their catalytic properties. The exceptional catalytic performance of these materials cannot be simply attributed to the additive effects of their individual components. At the core of this phenomenon is the unique spatial arrangement of different metal atoms at the nanoscale—the atomic condensed state—which determines the material's electronic structure, lattice strain, and surface atomic coordination environment. These factors, in turn, influence the adsorption strength of reactants, the reaction pathways of intermediates, and the desorption kinetics of products during catalytic reactions. In this section, we will elaborate on the impact of the atomic condensed state in multicomponent metallic nanomaterials on their catalytic properties from three perspectives: electronic coupling (or ligand) effects, strain effects, and surface geometric effects.
4.1 Regulation of Electronic Coupling Effects
In multi-component metallic nanomaterials, the condensed-phase forms of different metal atoms—such as the formation of homogeneous alloys or heterogeneous structures—determine their distinct electronic coupling modes, which in turn endow the materials with varied catalytic properties.
In homogeneous alloy nanomaterials, different metal atoms are highly mixed at the atomic scale. The constituent atoms interact through direct bonding, leading to electron redistribution and hybridization of the electronic band structure, thereby altering the electronic structure of key reaction sites. Consequently, by controlling the types and proportions of alloy components, the electronic coupling effects can be effectively tuned, enabling fine optimization of catalytic performance
［20-21］. For example, research conducted by Markovic’s team at the Argonne National Laboratory in the United States
［27］has demonstrated that Pt
3M alloys formed between platinum and 3d transition metals (such as Fe, Co, Ni, Ti, and V) exhibit significantly different catalytic activities in the electrocatalytic oxygen reduction reaction (ORR). Specifically, when Pt is alloyed with metals that have markedly different electronegativities or d-band center positions (e.g., Ni, Co, and Fe), electron transfer occurs between these metals and Pt, resulting in a downward shift of Pt’s d-band center. This shift effectively modulates the adsorption energies of key ORR intermediates (*O and *OH). Since pure Pt typically exhibits excessively strong adsorption of these oxygen species, which limits the ORR reaction kinetics, the d-band center shift induced by alloying adjusts the adsorption strength between Pt and oxygen species closer to the optimal region on the ORR volcano plot, thereby significantly enhancing catalytic activity. Notably, Pt
3Ni(111) demonstrates approximately tenfold higher catalytic activity in ORR than Pt(111) and roughly ninetyfold higher activity than commercial Pt/C catalysts, clearly illustrating the enhancing effect of alloying on Pt’s catalytic performance (
Figure 8a)
［28］. Our research group
［29］systematically constructed controllable Pt–Ni alloy shells on Pd nanocrystal surfaces and investigated the influence of the Pt/Ni ratio within the shell on the electrocatalytic hydrogen evolution reaction (HER) activity (
Figure 8b). The results indicate that as the Ni/Pt ratio increases, the electronic structure of Pt sites undergoes continuous changes, characterized by a decrease in charge density. This change is attributed to the ligand effect of Ni on Pt, which reduces the binding energy (Δ
G H∗) between Pt sites and hydrogen intermediates (H*). When Δ
G H∗approaches zero, HER activity reaches its optimum. Among the samples examined, Pd@PtNi (with a Ni/Pt ratio of 1.1:1) exhibited the lowest Δ
G H∗value and the best HER activity. This further confirms that precise control over alloy composition can significantly optimize electronic structure and catalytic activity, providing an effective pathway for the screening of high-performance catalysts.
Unique electronic coupling effects distinct from those in homogeneous alloy systems have been demonstrated in heterogeneous structured metal nanomaterials. At the heterojunction, differences in work function or electronegativity induce redistribution and migration of electrons among the constituent components, thereby significantly modulating the electronic structure of active sites near the interface. For example, the Tsang group at the University of Oxford
［21］developed an Ag@Pd core–shell structured catalyst for efficient hydrogen production via formic acid decomposition. In this structure, the Ag core exerts an electron-injection effect on the Pd shell through electronic coupling. Since the work function of Ag (approximately 4.26 eV) is lower than that of Pd (approximately 5.6 eV), electrons spontaneously migrate from the Ag core to the Pd shell, enhancing the adsorption of reaction intermediates on the Pd surface during the formic acid decomposition process and thus improving hydrogen production efficiency. Notably, when the thickness of the Pd shell is controlled within 1–2 atomic layers, the catalytic performance reaches its optimum, indicating that this electronic coupling effect operates over a very short range, confined to only a few atomic layers near the interface. Our team has found that in hcp-Ni@Pt core–shell nanostructures, the Ni core exerts a significant electronic modulation on the Pt shell via core–shell electron transfer (
Figure 8c–e)
［31］. Given that the work function of Pt (5.93 eV) exceeds that of Ni (5.35 eV), electrons transfer from the Ni core to the Pt shell. X-ray photoelectron spectroscopy (XPS) results show that as the number of Pt atomic layers decreases, the Pt 4f peak gradually shifts toward lower binding energies (
Figure 8c). This phenomenon indicates that the electron transfer process is layer-dependent: as the number of Pt layers diminishes, the outermost Pt atoms exhibit increasingly pronounced electron-rich characteristics. This electronic reconstruction leads to an upward shift of the Fermi level of Pt, thereby reducing Δ
G H∗and resulting in markedly enhanced electrocatalytic HER performance (
Figure 8d, e). When the thickness of the hcp-Pt shell is 2.6 atomic layers, the Ni@Pt catalyst exhibits optimal HER activity. These findings demonstrate that by precisely constructing heterogeneous interfaces and controlling their electronic coupling effects, substantial improvements in catalytic performance can be achieved.
4.2 Strain Effect Regulation
Multicomponent metallic nanomaterials often exhibit structural distortions that deviate from their intrinsic lattice constants, manifesting as tensile or compressive strains, which significantly alter their electronic structures and catalytic properties
［22-23］. These strains typically arise from mismatches in lattice constants or atomic radii among the constituent components. According to the d-band center theory, tensile strain can lead to d-band contraction and upward shift, thereby enhancing the binding affinity of catalytic sites toward adsorbed intermediates; conversely, compressive strain causes d-band broadening and downward shift, weakening adsorption strength. Therefore, by precisely controlling the atomic condensed states of multicomponent metallic nanomaterials, it is possible to effectively tune surface electronic states and adsorption behaviors, thus optimizing catalytic performance.
In uniform alloy nanomaterials, the mixing of different types of metal atoms can induce strain in the overall crystal lattice. This effect is particularly pronounced when there are significant differences in atomic sizes, often resulting in substantial tensile or compressive strains. For example, in the Mo-doped PdPtNiCuZn high-entropy alloy nanosheets synthesized by Guo Shaojun’s team at Peking University
［84］, the combined effects of atomic size mismatch, the ultrathin structure of the nanosheets, and surface concave sites give rise to an intrinsic tensile strain of approximately 4.4%. Density functional theory (DFT) calculations indicate that this strain effect, in synergy with the presence of isolated Mo atoms, significantly optimizes the adsorption behavior of methanol oxidation reaction (MOR) intermediates, thereby lowering the reaction energy barrier and enhancing resistance to CO poisoning. It should be emphasized that strain effects in alloys frequently coexist with electronic coupling, working together to enhance catalytic performance.
In heterogeneous nanostructured materials (core–shell structures), strain primarily originates from lattice mismatch between the core and shell. When the intrinsic lattice constant of the shell metal does not match that of the inner core, epitaxial crystal growth will induce passive tensile or compressive strain. By rationally designing the core material and its crystal structure, the strain state in the shell layer can be effectively controlled. For example, our research team
［30］fabricated Ag@AgPd core–shell nanostructures, introducing 4.5% tensile strain into the Pd shell layer (
Figure 9a). Experimental and theoretical calculations demonstrate that this tensile strain significantly reduces the energy barrier for electrocatalytic ethanol dehydrogenation and enhances the reaction selectivity toward CO
2as the final product via C—C bond cleavage of ethanol molecules. Compared with Ag–Pd alloys, the core–shell architecture provides greater lattice tensile strain, thereby delivering superior catalytic performance. Additionally, by constructing an hcp-Ni@Pt core–shell structure, we introduced approximately 5% uniaxial compressive strain along the [0001] direction in the hcp-Pt shell layer (
Figure 9b)
［31］. This compressive strain effectively lowered Δ
G H∗, thus reducing the HER energy barrier and markedly improving catalytic activity. The Jin Mingshang group at Xi’an Jiaotong University
［32］achieved continuous strain modulation of the Pt(100) shell layer from −5.1% to +5.9% by adjusting the degree of phosphidation/dephosphidation of Pd nanocubes, resulting in an “M-shaped” activity profile in electrocatalytic MOR. Furthermore, the Zhang Hua group at City University of Hong Kong
［85］induced approximately 2.5% tensile strain in the Pt shell layer of a Pd@Pt core–shell structure by introducing hydrogen atoms into the Pd core, thereby enhancing Pt’s adsorption capacity for OH* species and improving both the performance and stability of alcohol oxidation reactions (
Figure 9c). These findings strongly validate the feasibility and high efficiency of strain engineering based on core–shell architectures in modulating catalyst electronic structures and substantially enhancing their catalytic performance.
4.3 Surface Geometric Effect Regulation
Surface geometric effects provide an important means to regulate the catalytic behavior of multi-component metallic nanomaterials. By controlling the coordination configuration of active centers at the atomic level, reaction pathways can be precisely directed, thereby enhancing catalytic efficiency and product selectivity.
In multi-component metallic nanomaterials, spatial isolation of active metal atoms (such as Pt or Pd) from inert or low-active metal atoms (such as Hg or Au) can effectively alter the adsorption configuration or binding energy toward guest molecules. For example, in the oxygen reduction reaction (ORR), Pt and Pd typically favor a four-electron pathway to fully reduce O
2 into H
2O; however, when their atoms are isolated by Hg or Au to form discrete active sites, they tend to proceed via a two-electron pathway, yielding H
2O
2.
[86–87]. This is because isolated Pt or Pd atoms lack the continuous atomic arrangement and specific electronic structure required to induce O–O bond cleavage, thereby inhibiting the formation of key intermediates O* and OH* in the four-electron pathway. Research by the Rossmeisl group at the Technical University of Denmark
[87] has demonstrated that Pt–Hg alloy nanoparticles exhibit up to 96% selectivity toward H
2O
2 in ORR, accompanied by significantly enhanced catalytic activity. Similarly, in the electrocatalytic formic acid oxidation reaction (FAOR), the formation of CO intermediates often leads to catalyst poisoning. Our team employed a crystal self-terminating growth mechanism to controllably grow high-density isolated Pt single atoms on the surface of Au nanoparticles, thereby effectively suppressing the CO-forming reaction pathway under a high Pt surface coverage (up to 26%) and achieving markedly improved catalytic performance (
Figure 10a)
[24]. The aforementioned strategies achieve precise control over reaction pathways through atomic-level spatial isolation, fully highlighting the critical role of surface geometric effects in electrocatalysis.
Surface geometric effects are not only manifested in the construction of isolated active sites, but also in the realization of synergistic catalysis through the formation of diverse surface sites. This enables reaction intermediates to migrate and undergo cooperative transformations among different active sites, effectively overcoming the catalytic limitations imposed by single sites and thereby enhancing overall reaction performance. For example, the PtCo alloy nanoclusters synthesized by our team exhibited outstanding activity and selectivity during the hydrogenolysis of 5-hydroxymethylfurfural (HMF) to produce 2,5-dimethylfuran (
Figure 10b)
［19］. On the alloy surface, electron transfer from Co to Pt occurs, forming a surface dipole structure composed of electron-rich Pt and electron-deficient Co. This Pt–Co dipole adsorbs and activates the C
O bond of the HMF molecule, while the Pt sites efficiently activate H
2. The synergy between these two processes significantly enhances both the efficiency of the hydrogenolysis reaction and the selectivity toward 2,5-dimethylfuran. Furthermore, our team investigated the catalytic behavior of Au–Pd nanoparticles in the ethanol oxidation reaction (EOR) and found that their surfaces contain Au sites, Pd sites, and Pd sites adjacent to Au (Pd(−Au)), which provide a series of continuously varying adsorption energies (
Figure 10c, d)
［88］. Density functional theory (DFT) calculations indicate that different intermediates in the EOR can migrate among these sites with varying affinities based on their optimal binding energies, thereby independently optimizing the energy barriers of each elementary step. This type of synergistic catalysis achieved through intermediate migration effectively breaks the linear scaling relationship and markedly improves the overall kinetics of the EOR.
High-entropy alloy nanomaterials have provided a new opportunity for constructing diversified surface active sites (
Figure 10 e)
[89]. On the surface of high-entropy alloys, there exist millions of multi-element active sites with diverse coordination environments, where the binding energy of guest molecules is synergistically influenced by the central atom and its surrounding atomic environment. This complex interaction leads to a broadening of the d-band, causing the adsorption energy to no longer be a single value but rather to exhibit an almost continuous distribution. Consequently, high-entropy alloys can provide a series of adsorption sites with varying strengths for target reactions, better accommodating multiple reaction intermediates and identifying reaction pathways with binding energies close to the ideal values. The continuous distribution of adsorption energies endows high-entropy alloy catalysts with the following advantages: (1) By selecting and tuning the types and ratios of constituent elements, the binding energy can be precisely adjusted to approach a pre-set ideal value, thereby enabling the rational design of electrochemical catalysts; (2) Different sites on the surface of high-entropy alloys can function as independent catalytic units, each corresponding to a specific binding-energy peak, allowing them to simultaneously catalyze multiple reactions on the same surface, particularly suitable for cascade reactions; (3) Traditional catalysts are often constrained by linear scaling relationships, which limit further enhancement of activity. In contrast, on the surface of high-entropy alloys, different reaction intermediates can adsorb onto various types of sites, weakening or even breaking such constraints and providing a new avenue for overcoming the inherent limitations of catalytic activity. For example, our team has found that the PtCuNiCoFe high-entropy alloy exhibits a significant promotional effect on MOR
[50]. In this high-entropy alloy nanomaterial, the complexity and disorder of the surface atomic arrangement result in diverse surface active sites. Among these, Fe–Co–Ni sites possess weak binding energies for MOR intermediates, whereas Pt–Cu sites exhibit strong binding energies. The complexity of the active sites facilitates the migration and transformation of reaction intermediates under different atomic environments, thereby enhancing reaction efficiency. Collectively, these studies reveal that fine-tuning surface geometric effects through the regulation of atomic condensed states represents an effective strategy for improving the catalytic performance of multi-component metallic nanomaterials.
4.4 Summary and Outlook
This chapter elucidates how the atomic condensed states of multicomponent metallic nanomaterials influence catalytic performance by modulating electronic coupling effects, stress effects, and surface geometric effects. A wealth of examples demonstrates that both direct bonding interactions within homogeneous alloys and electronic reconstructions at heterogeneous interface structures can precisely tune the electronic structure of active sites and the adsorption energies of guest molecules, thereby significantly enhancing catalytic activity and selectivity. Looking ahead, research in this field will place greater emphasis on integrating advanced in-situ characterization techniques with theoretical calculations to design and construct multifunctional synergistic active sites at the atomic scale, in order to meet the demands of more complex and efficient catalytic reactions.
5 Conclusion and Future Work
This paper systematically reviews the latest research progress on atomic aggregation state regulation in multicomponent metallic nanomaterials and its impact on catalytic performance. At the nanoscale, metal atoms can form various aggregated states, including homogeneous alloys and diverse heterogeneous structures. These distinct atomic arrangements significantly influence the electronic structure and catalytic properties of the materials.
The intrinsic reduction potential difference among metal salts poses a significant challenge for precise control of atomic condensed states. In the synthesis of homogeneous alloy nanomaterials, conventional wet-chemical methods are constrained by this intrinsic reduction potential difference, making it difficult to achieve uniform alloying and fine-tuned compositional regulation. To overcome this challenge, this paper highlights a wet-chemical synthesis strategy based on an active-hydrogen-mediated interfacial co-reduction mechanism. By in situ generating active hydrogen species on the metal surface, this approach effectively mitigates the influence of the reduction potential difference on alloy formation, enabling the simultaneous co-reduction of multiple metal salts and substantially enhancing both the homogeneity and controllability of the alloy composition. Furthermore, vacancy-diffusion-induced alloying offers a new pathway for achieving homogeneous alloying in immiscible metal systems, while nano-reactor-based synthesis strategies have addressed the challenges associated with size and compositional uniformity in alloy nanoclusters. In the synthesis of heterogeneous-structure metal nanomaterials, traditional methods are typically limited by the intrinsic reduction potential differences (or the reactivity sequence) between metals, which hinder the formation of core–shell structures with a reactive metal core and a noble-metal shell arranged in reverse order of reactivity. This paper provides a detailed discussion of strategies that combine kinetic control—such as enhancing the reducing power of the reductant or lowering the precursor concentration—and thermodynamic control—such as introducing ligands to modulate the reduction potentials of metal ions—to effectively suppress displacement reactions and successfully construct these novel core–shell architectures. These strategies have significantly expanded the design space for heterogeneous nanocrystals and opened up new opportunities for developing high-performance catalysts.
The condensed-phase form of atoms in multicomponent metallic nanomaterials directly influences the pathways of electronic interactions. In brief, in homogeneous alloys, different metal atoms achieve electronic coupling through direct bonding; differences in electronegativity drive electron transfer and redistribution, thereby modulating the material’s electronic structure. In heterogeneous structures, such as core–shell architectures, the work-function difference between the core and shell induces electron migration from the low-work-function side to the high-work-function side, resulting in interfacial electronic reconstruction and significantly altering the electronic states of catalytic active sites. Based on this, multicomponent metals exhibit pronounced electronic coupling effects, strain effects, and surface-geometry effects, which markedly influence their catalytic performance.
Despite significant progress in the atomic condensed-state regulation of multi-component metallic nanomaterials, this field still faces numerous opportunities and challenges:
(1) Atomic-level precision control and characterization: Although wet-chemical synthesis methods have already enabled compositional and structural control, achieving finer structural design and precise control at the atomic level—particularly for complex multicomponent high-entropy alloys and more intricate heterogeneous structures—remains a core challenge. Meanwhile, it is necessary to develop more advanced in-situ characterization techniques to monitor in real time and elucidate the atomic evolution and condensed-phase formation mechanisms during the synthesis of nanomaterials.
(2) The close integration of theoretical computation and experimentation: By combining computational methods such as density functional theory calculations and machine learning, we can gain a deeper understanding of the influence rules of different atomic condensed phases on the electronic structure and catalytic performance of materials. This, in turn, can guide experimental design and accelerate the development of novel, highly efficient catalysts.
(3) Expanding novel multi-component systems: Extending the research focus from traditional precious metals to more inexpensive and abundant non-precious metals, exploring atomic condensed-phase regulation within multi-component systems, with the aim of developing low-cost catalysts.
(4) Challenges in Practical Catalytic Applications: Although laboratory-synthesized nanocatalysts exhibit excellent performance, how to stably and efficiently apply them to real industrial catalytic processes while overcoming issues such as agglomeration, sintering, and segregation remains a topic for further investigation. For example, alloy nanomaterials often undergo segregation during catalysis, forming a single-metal shell layer that results in a surface composition different from the bulk material, which is detrimental to the synergistic catalytic reactions involving multiple metal sites. Therefore, effectively inhibiting or regulating this segregation phenomenon and precisely controlling the surface alloy composition during catalysis constitute an important research challenge.
In summary, the atomic condensed-state regulation of multi-component metallic nanomaterials has brought revolutionary opportunities to the fields of materials science and catalysis. Through continuous innovation and multidisciplinary collaboration, it is expected that in the future, the performance of nanocatalysts can be more precisely controlled, providing key technological support for addressing major challenges in energy, environmental protection, and chemical engineering.
the National Natural Science Foundation of China(22371222)
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