Crystal Structure of the PdAu24(SR)18(0) Superatom

Cyclic ion mobility of doped [MAu24L18]2- superatoms and their fragments (M = Ni, Pd and Pt; L = alkynyl)

Collision-induced dissociation and high-resolution cyclic ion mobility mass spectrometry, along with quantum chemical calculations and trajectory simulations, were used to compare the structures of isolated [MAu24(CCR)18]2-, M = Ni, Pd, or Pt, and their associated fragment ions. The three different alkynyl ligand-stabilized (CCR, R = 3,5-(CF3)2C6H3), transition metal-doped, gold cluster dianions showed mutually resolvable collision cross sections (CCS), which were ordered consistently with their molecular structures from X-ray crystallography. All three [MAu24(CCR)18]2- species fragment by sequential diyne loss to form [MAu24(CCR)18-n]2-, with n up to 12. The resultant fragment isomer distributions are significantly n- and M-dependent, and hint at a process involving concerted elimination of adjacent ligands. In particular [NiAu24(CCR)18]2- also fragments to generate alkyne-oligomers, an inference supported by the parallel observation of precursor dianion isomerization as collision energy is increased.

Recent developments in the investigation of driving forces for transforming coinage metal nanoclusters

Metal nanoclusters serve as an emerging class of modular nanomaterials. The transformation of metal nanoclusters has been fully reflected in their studies from every aspect, including the structural evolution analysis, physicochemical property regulation, and practical application promotion. In this review, we highlight the driving forces for transforming atomically precise metal nanoclusters and summarize the related transforming principles and fundamentals. Several driving forces for transforming nanoclusters are meticulously reviewed herein: ligand-exchange-induced transformations, metal-exchange-induced transformations, intercluster reactions, photochemical transformations, oxidation/reduction-induced transformations, and other factors (intrinsic instability, pH, temperature, and metal salts) triggering transformations. The exploitation of transforming principles to customize the preparations, structures, physicochemical properties, and practical applications of metal nanoclusters is also disclosed. At the end of this review, we provide our perspectives and highlight the challenges remaining for future research on the transformation of metal nanoclusters. Our intended audience is the broader scientific community interested in metal nanoclusters, and we believe that this review will provide researchers with a comprehensive synthetic toolbox and insights on the research fundamentals needed to realize more cluster-based nanomaterials with customized compositions, structures, and properties.

Probing the surface-active sites of metal nanoclusters with atomic precision: a case study of Au5Ag11

The determination of surface-active sites in metal nanoclusters is of great significance for the in-depth understanding of structural evolutions and physicochemical property mechanisms. In this work, the surface-active sites of the Au5Ag11(DMBT)8(DPPOE)2 cluster template towards metal-/ligand-exchange reactions were unambiguously identified at the atomic level. The active-site tailoring of this nanocluster gave rise to three derivative nanoclusters, Au5Ag9Cu2(DMBT)8(DPPOE)2, Au5Ag11(DMBT)6(DCBT)2(DPPOE)2, and Au5Ag11(DCBT)8(DPPOE)2. The single-crystal structural analysis revealed that all these M16 (M = Au/Ag/Cu) clusters exhibited almost the same framework. Besides, the surface-active site tailoring contributed to significant changes in optical absorptions and emissions of these metal nanoclusters. The findings in this work not only provide an in-depth understanding of the active-site tailoring of cluster surface structures but also develop an intriguing template that enables us to grasp the structure-property correlations at the atomic level.

Assembling Atomically Precise Noble Metal Nanoclusters Using Supramolecular Interactions

Supramolecular chemistry (SC) of noble metal nanoclusters (NMNCs) is one of the fascinating areas of contemporary materials science. It is principally concerned with the noncovalent interactions between NMNCs, as well as between NMNCs and molecules or nanoparticles. This review focuses on recent advances in the supramolecular assembly of NMNCs and applications of the resulting structures. We have divided the topics into four distinct subgroups: (i) SC of NMNCs in gaseous and solution phases, (ii) supramolecular interactions of NMNCs in crystal lattices, (iii) supramolecular assemblies of NMNCs with nanoparticles and NMNCs, and (iv) SC of NMNCs with other molecules. The last explores their interactions with fullerenes, cyclodextrins, cucurbiturils, crown ethers, and more. After discussing these topics concisely, various emerging properties of the assembled systems in terms of their mechanical, optical, magnetic, charge-transfer, etc. properties and applications are presented. SC is seen to provide a crucial role to induce new physical and chemical properties in such hybrid nanomaterials. Finally, we highlight the scope for expansion and future research in the area. This review would be useful to those working on functional nanostructures in general and NMNCs in particular.

Controlled Synthesis of Au25 Superatom Using a Dendrimer Template

Superatoms are promising materials for their potential in elemental substitution and as new building blocks. Thus far, various synthesis methods of thiol-protected Au clusters including an Au₂₅ superatom have been investigated. However, previously reported methods were mainly depending on the thermodynamic stability of the aimed clusters. In this report, a synthesis method for thiol-protected Au clusters using a dendrimers template is proposed. In this method, the number of Au atoms was controlled by the stepwise complexation feature of a phenylazomethine dendrimer. Therefore, synthesis speed was increased compared with the case without the dendrimer template. Hybridization for the Au₂₅ superatoms was also achieved using the complexation control of metals.

Selective formation of [Au23(SPh t Bu)17]0, [Au26Pd(SPh t Bu)20]0 and [Au24Pt(SC2H4Ph)7(SPh t Bu)11]0 by controlling ligand-exchange reaction

To use atomically precise metal nanoclusters (NCs) in various application fields, it is essential to establish size-selective synthesis methods for the metal NCs. Studies on thiolate (SR)-protected gold NCs (Au n (SR) m NCs) revealed that the atomically precise Au n (SR) m NC, which has a different chemical composition from the precursor, can be synthesized size-selectively by inducing transformation in the framework structure of the metal NCs by a ligand-exchange reaction. In this study, we selected the reaction of [Au25(SC2H4Ph)18]- (SC2H4Ph = 2-phenylethanethiolate) with 4-tert-butylbenzenethiol ( t BuPhSH) as a model ligand-exchange reaction and attempted to obtain new metal NCs by changing the amount of thiol, the central atom of the precursor NCs, or the reaction time from previous studies. The results demonstrated that [Au23(SPh t Bu)17]0, [Au26Pd(SPh t Bu)20]0 (Pd = palladium) and [Au24Pt(SC2H4Ph)7(SPh t Bu)11]0 (Pt = platinum) were successfully synthesized in a high proportion. To best of our knowledge, no report exists on the selective synthesis of these three metal NCs. The results of this study show that a larger variety of metal NCs could be synthesized size-selectively than at present if the ligand-exchange reaction is conducted while changing the reaction conditions and/or the central atoms of the precursor metal NCs from previous studies.

The size-dependent influence of palladium doping on the structures of cationic gold clusters

The physicochemical properties of small metal clusters strongly depend on their precise geometry. Determining such geometries, however, is challenging, particularly for clusters formed by multiple elements. In this work, we combine infrared multiple photon dissociation spectroscopy and density functional theory calculations to investigate the lowest-energy structures of Pd doped gold clusters, PdAu n-1 + (n ≤ 10). The high-quality experimental spectra allow for an unambiguous determination of the structures adopted by the clusters. Our results show that the Pd-Au interaction is so large that the structures of PdAu n-1 + and Au n + are very different. Pd doping induces a 2D to 3D transition at much smaller cluster sizes than for pure Au n + clusters. PdAu n-1 + clusters are three-dimensional from n = 4, whereas for Au n + this transition only takes place at n = 7. Despite the strong Au-Pd interaction, the Au n-1 + cluster geometries remain recognizable in PdAu n-1 + up to n = 7. This is particularly clear for PdAu6 +. In PdAu8 + and PdAu9 +, Pd triggers major rearrangements of the Au clusters, which adopt pyramidal shapes. For PdAu4 + we find a geometry that was not considered in previous studies, and the geometry found for PdAu8 + does not correspond to the lowest-energy structure predicted by DFT, suggesting kinetic trapping during formation. This work demonstrates that even with the continuous improvement of computational methods, unambiguous assignment of cluster geometries still requires a synergistic approach, combining experiment and computational modelling.

Exploring the structure of atom-precise silver-palladium bimetallic clusters prepared via improved single-pot co-reduction synthesis protocol

Designing atom-precise bimetallic clusters with a relatively cost-effective and more abundant metal than Au (i.e., Ag) is desirable for the development of heterogeneous bimetallic cluster catalysts for industrial applications. Atom-precise Ag-based bimetallic clusters, which are analogs of the well-studied Au based clusters, are yet to be fully explored as catalysts. Establishing the Pd loading limit and the position of the Pd dopant in AgPd bimetallic clusters will further give an insight into the structure-activity relationships for these atom-precise AgPd heterogeneous catalysts. In this study, an improved single-pot co-reduction strategy was employed to prepare the bimetallic clusters, which were then characterized by mass spectrometry, x-ray photoelectron spectroscopy (XPS), and x-ray absorption spectroscopy (XAS) to identify the loading and position of the dopant metal. Our results show that only a single dopant Pd atom can be incorporated, and in comparison with monometallic Ag₂₅ clusters, the absorption peaks of Ag₂₄Pd₁(SPhMe₂)₁₈ ^2- bimetallic clusters are blue shifted due to the incorporation of Pd. The XPS and XAS results show that the Ag₂₄Pd₁(SPhMe₂)₁₈ ^2- bimetallic clusters have multivalent Ag(0) and Ag(I) atoms and surprisingly show Pd(II) species with significant Pd-S bonding, despite the prevailing wisdom that the Pd dopant should be in the center of the cluster. The XAS results show that the singly doped Pd atom predominantly occupies the staple position, albeit we cannot unambiguously rule out the Pd atom in an icosahedral surface position in some clusters. We discuss the ramifications of these results in terms of possible kinetically vs thermodynamically controlled cluster formation.

An insight, at the atomic level, into the polarization effect in controlling the morphology of metal nanoclusters

The polarization effect has been a powerful tool in controlling the morphology of metal nanoparticles. However, a precise investigation of the polarization effect has been a challenging pursuit for a long time, and little has been achieved for analysis at the atomic level. Here the atomic-level analysis of the polarization effect in controlling the morphologies of metal nanoclusters is reported. By simply regulating the counterions, the controllable transformation from Pt₁Ag₂₈(S-PhMe₂)_x(S-Adm)_18−x(PPh₃)₄ (x = 0–6, Pt1Ag28-2) to Pt₁Ag₂₄(S-PhMe₂)₁₈ (Pt1Ag24) with a spherical configuration or to Pt₁Ag₂₈(S-Adm)₁₈(PPh₃)₄ (Pt1Ag28-1) with a tetrahedral configuration has been accomplished. In addition, the spherical or tetrahedral configuration of the clusters could be reversibly transformed by re-regulating the proportion of counterions with opposite charges. More significantly, the configuration transformation rate has been meticulously manipulated by regulating the polarization effect of the ions on the parent nanoclusters. The observations in this paper provide an intriguing nanomodel that enables the polarization effect to be understood at the atomic level.

Based on the inter-conversion between Pt₁Ag₂₄(SR)₁₈ and Pt₁Ag₂₈(SR)₁₈(PPh₃)₄, an insight into the polarization effect in controlling the morphology of metal nanoparticles is presented.

Size-Scalable Near-Infrared Photoluminescence in Gold Monolayer Protected Clusters

Near-infrared photoluminescence of a series of three gold monolayer protected clusters (MPCs) with volumes spanning 50-200 ų was studied by using variable-temperature photoluminescence (VT-PL) spectroscopy. The three MPCs, which included Au₂₀(SC₈H₉)₁₅-diglyme, Au₂₅(SC₈H₉)₁₈, and Au₃₈(SC₁₂H₂₅)₂₄, all exhibited temperature-dependent intensities that reflected a few-millielectronvolt energy gap that separated bright emissive and dark nonradiative electronic states. All clusters showed increased PL intensities upon raising the sample temperature from 4.5 K to a cluster-specific value, upon which increased sample temperature resulted in emission quenching. The increased PL in the low-temperature range is attributed to thermally activated carrier transfer from dark to bright states. The quenching at elevated temperatures is attributed to nonradiative vibrational relaxation through Au-Au stretching of the MPCs metal core. Importantly, the results show evidence of a common and size scalable metal-centered intraband PL mechanism that is general for ultrasmall metal nanoclusters, which are expected to show nonscalable optical properties.

[Au16Ag43H12(SPhCl2)34]5-: An Au-Ag Alloy Nanocluster with 12 Hydrides and Its Enlightenment on Nanocluster Structural Evolution

The structural determination of alloy hydride nanoclusters with high nuclearity remains challenging. We herein report the synthetic procedure and the structural elucidation of an Au-Ag alloy nanocluster with 12 hydride ligands-[Au₁₆Ag₄₃H₁₂(SPhCl₂)₃₄]^5-. The structure of [Au₁₆Ag₄₃H₁₂(SPhCl₂)₃₄]^5- comprises an Au₁₆Ag₃ kernel that is stabilized by 12 hydride ligands, 8 thiol bridges, and 6 Ag_m(SR)_n motif units. The 12 hydride ligands in Au₁₆Ag₄₃ have been confirmed by both ²H NMR and ESI-MS measurements, and their positions have been theoretically evaluated, located at the interlayer between the Au₁₆Ag₃ kernel and the Ag-SR shell. The metastable [Au₁₆Ag₄₃H₁₂(SPhCl₂)₃₄]^5- can convert to [Au₁₂Ag₃₂(SPhCl₂)₃₀]^4- spontaneously. Structurally, the Au₁₆Ag₃ kernel of [Au₁₆Ag₄₃H₁₂(SPhCl₂)₃₄]^5- could be regarded as the overlapping of two hollow Au₈Ag₃ cages via sharing an Ag₃ line, which is in contrast to the solely icosahedral Au₁₂ kernel of [Au₁₂Ag₃₂(SPhCl₂)₃₀]^4-. Besides, the overall construction of Au₁₆Ag₄₃ or Au₁₂Ag₃₂ follows a complementing or overlapping assembly mode, respectively. Overall, the structural anatomy of Au₁₆Ag₄₃H₁₂(SPhCl₂)₃₄ sheds some new insight into the structural evolution of metal nanoclusters.

Synthesis and Optical Properties of Unique Pt1Ag24 Nanoclusters with Mixed Exterior Motif Structures

The atomic arrangement of metal nanoclusters plays a significant role in the structure-property correlation. Herein, we present a novel Pt₁Ag₂₄(SR)₁₆(PPh₃)₃ nanocluster with a unique structure, different from two reported Pt₁Ag₂₄ nanoclusters. The nanocluster was prepared via one-pot synthesis and solvent extraction. It has a centered icosahedral Pt₁Ag₁₂ kernel and an open shell composed of three Ag₂(SR)₃(PPh₃) staple motifs and a unique trefoil-like Ag₆(SR)₇ motif. The three kinds of Pt₁Ag₂₄ nanoclusters have the same kernel but different shell configurations. The fine-tuning of structures is necessary and significant for the investigation of the relationship of structures and properties. The different UV-vis absorption spectra indicate that the optical properties of three Pt₁Ag₂₄ nanoclusters mainly depend on the exterior shell configuration and the metal-ligand interface. This work provides insights toward growth mechanism and the structure-property correlation of metal nanoclusters.

Operando Surface Spectroscopy and Microscopy during Catalytic Reactions: From Clusters via Nanoparticles to Meso-Scale Aggregates

Operando characterization of working catalysts, requiring per definitionem the simultaneous measurement of catalytic performance, is crucial to identify the relevant catalyst structure, composition and adsorbed species. Frequently applied operando techniques are discussed, including X-ray absorption spectroscopy, near ambient pressure X-ray photoelectron spectroscopy and infrared spectroscopy. In contrast to these area-averaging spectroscopies, operando surface microscopy by photoemission electron microscopy delivers spatially-resolved data, directly visualizing catalyst heterogeneity. For thorough interpretation, the experimental results should be complemented by density functional theory. The operando approach enables to identify changes of cluster/nanoparticle structure and composition during ongoing catalytic reactions and reveal how molecules interact with surfaces and interfaces. The case studies cover the length-scales from clusters via nanoparticles to meso-scale aggregates, and demonstrate the benefits of specific operando methods. Restructuring, ligand/atom mobility, and surface composition alterations during the reaction may have pronounced effects on activity and selectivity. The nanoscale metal/oxide interface steers catalytic performance via a long ranging effect. Combining operando spectroscopy with switching gas feeds or concentration-modulation provides further mechanistic insights. The obtained fundamental understanding is a prerequisite for improving catalytic performance and for rational design.

Toward Controlling the Electronic Structures of Chemically Modified Superatoms of Gold and Silver

Atomically precise gold/silver clusters protected by organic ligands L, [(Au/Ag)_x L_y ]^z , have gained increasing interest as building units of functional materials because of their novel photophysical and physicochemical properties. The properties of [(Au/Ag)_x L_y ]^z are intimately associated with the quantized electronic structures of the metallic cores, which can be viewed as superatoms from the analogy of naked Au/Ag clusters. Thus, establishment of the correlation between the geometric and electronic structures of the superatomic cores is crucial for rational design and improvement of the properties of [(Au/Ag)_x L_y ]^z . This review article aims to provide a qualitative understanding on how the electronic structures of [(Au/Ag)_x L_y ]^z are affected by geometric structures of the superatomic cores with a focus on three factors: size, shape, and composition, on the basis of single-crystal X-ray diffraction data. The knowledge accumulated here will constitute a basis for the development of ligand-protected Au/Ag clusters as new artificial elements on a nanometer scale.

Observable but Not Isolable: The RhAu24 (PET)181+ Nanocluster

The synthesis and characterization of RhAu₂₄ (PET)₁₈ (PET = 2-phenylethanethiol) is described. The cluster is cosynthesized with Au₂₅ (PET)₁₈ and rhodium thiolates in a coreduction of RhCl₃ , HAuCl₄ , and PET. Rapid decomposition of RhAu₂₄ (PET)₁₈ occurs when purified from the other reaction products, precluding the study of isolated cluster. Mixtures containing RhAu₂₄ (PET)₁₈ , Au₂₅ (PET)₁₈ , and rhodium thiolates are therefore characterized. Mass spectrometry, X-ray photoelectron spectroscopy, and chromatography methods suggest a combination of charge-charge and metallophilic interactions among Au₂₅ (PET)₁₈^1- , rhodium thiolates and RhAu₂₄ (PET)₁₈ resulting in stabilization of RhAu₂₄ (PET)₁₈ . The charge of RhAu₂₄ (PET)₁₈ is assigned as 1+ on the basis of its stoichiometric 1:1 presence with anionic Au₂₅ (PET)₁₈ , and its stability is contextualized within the superatom electron counting rules. This analysis concludes that the Rh atom absorbs one superatomic electron to close its d-shell, giving RhAu₂₄ (PET)₁₈¹⁺ a superatomic electron configuration of 1S² 1P⁴ . Overall, an updated framework for rationalizing open d-shell heterometal dopant electronics in thiolated gold nanoclusters emerges.

Toward the creation of high-performance heterogeneous catalysts by controlled ligand desorption from atomically precise metal nanoclusters

Ligand-protected metal nanoclusters controlled by atomic accuracy (i. e. atomically precise metal NCs) have recently attracted considerable attention as active sites in heterogeneous catalysts. Using these atomically precise metal NCs, it becomes possible to create novel heterogeneous catalysts based on a size-specific electronic/geometrical structure of metal NCs and understand the mechanism of the catalytic reaction easily. However, to create high-performance heterogeneous catalysts using atomically precise metal NCs, it is often necessary to remove the ligands from the metal NCs. This review summarizes previous studies on the creation of heterogeneous catalysts using atomically precise metal NCs while focusing on the calcination as a ligand-elimination method. Through this summary, we intend to share state-of-art techniques and knowledge on (1) experimental conditions suitable for creating high-performance heterogeneous catalysts (e.g., support type, metal NC type, ligand type, and calcination temperature), (2) the mechanism of calcination, and (3) the mechanism of catalytic reaction over the created heterogeneous catalyst. We also discuss (4) issues that should be addressed in the future toward the creation of high-performance heterogeneous catalysts using atomically precise metal NCs. The knowledge and issues described in this review are expected to lead to clear design guidelines for the creation of novel heterogeneous catalysts.

Controlling the Crystallographic Packing Modes of Pt1Ag28 Nanoclusters: Effects on the Optical Properties and Nitrogen Adsorption-Desorption Performances

We herein report the manipulation of the crystallographic packing modes of Pt₁Ag₂₈(S-Adm)₁₈(PPh₃)₄ nanoclusters by altering counterions as different polyoxometalates (POMs). Specifically, the Cl^- anion of the presynthesized Pt₁Ag₂₈ nanocluster was substituted by POM anions including [Mo₆O₁₉]^2-, [W₆O₁₉]^2-, or [PW₁₂O₄₀]^3-. The crystal lattices of these Pt₁Ag₂₈ nanoclusters with diverse anions showed distinct packing modes and thus manifested remarkably distinguishable crystalline-state optical properties and nitrogen adsorption-desorption performances. Overall, the combination of intercluster control in this work and intracluster control reported previously (the control over metal-ligand within the nanocluster framework) accomplished a more comprehensive manipulation over the M₂₉(SR)₁₈(PR'₃)₄ nanocluster system, which enables us to further grasp the structure-property correlations at the atomic level.

Molecular reactivity of thiolate-protected noble metal nanoclusters: synthesis, self-assembly, and applications

Thiolate-protected noble metal (e.g., Au and Ag) nanoclusters (NCs) are ultra-small particles with a core size of less than 3 nm. Due to the strong quantum confinement effects and diverse atomic packing modes in this ultra-small size regime, noble metal NCs exhibit numerous molecule-like optical, magnetic, and electronic properties, making them an emerging family of "metallic molecules". Based on such molecule-like structures and properties, an individual noble metal NC behaves as a molecular entity in many chemical reactions, and exhibits structurally sensitive molecular reactivity to various ions, molecules, and other metal NCs. Although this molecular reactivity determines the application of NCs in various fields such as sensors, biomedicine, and catalysis, there is still a lack of systematic summary of the molecular interaction/reaction fundamentals of noble metal NCs at the molecular and atomic levels in the current literature. Here, we discuss the latest progress in understanding and exploiting the molecular interactions/reactions of noble metal NCs in their synthesis, self-assembly and application scenarios, based on the typical M(0)@M(i)-SR core-shell structure scheme, where M and SR are the metal atom and thiolate ligand, respectively. In particular, the continuous development of synthesis and characterization techniques has enabled noble metal NCs to be produced with molecular purity and atomically precise structural resolution. Such molecular purity and atomically precise structure, coupled with the great help of theoretical calculations, have revealed the active sites in various structural hierarchies of noble metal NCs (e.g., M(0) core, M-S interface, and SR ligand) for their molecular interactions/reactions. The anatomy of such molecular interactions/reactions of noble metal NCs in synthesis, self-assembly, and applications (e.g., sensors, biomedicine, and catalysis) constitutes another center of our discussion. The basis and practicality of the molecular interactions/reactions of noble metal NCs exemplified in this Review may increase the acceptance of metal NCs in various fields.

Dynamics of Pd Dopant Atoms inside Au Nanoclusters during Catalytic CO Oxidation

Doping gold nanoclusters with palladium has been reported to increase their catalytic activity and stability. PdAu24 nanoclusters, with the Pd dopant atom located at the center of the Au cluster core, were supported on titania and applied in catalytic CO oxidation, showing significantly higher activity than supported monometallic Au25 nanoclusters. After pretreatment, operando DRIFTS spectroscopy detected CO adsorbed on Pd during CO oxidation, indicating migration of the Pd dopant atom from the Au cluster core to the cluster surface. Increasing the number of Pd dopant atoms in the Au structure led to incorporation of Pd mostly in the S-(M-S) n protecting staples, as evidenced by in situ XAFS. A combination of oxidative and reductive thermal pretreatment resulted in the formation of isolated Pd surface sites within the Au surface. The combined analysis of in situ XAFS, operando DRIFTS, and ex situ XPS thus revealed the structural evolution of bimetallic PdAu nanoclusters, yielding a Pd single-site catalyst of 2.7 nm average particle size with improved CO oxidation activity.

Atomically precise alloy nanoclusters: syntheses, structures, and properties

Metal nanoclusters fill the gap between discrete atoms and plasmonic nanoparticles, providing unique opportunities for investigating the quantum effects and precise structure-property correlations at the atomic level. As a versatile strategy, alloying can largely improve the physicochemical performances compared to the corresponding homo-metal nanoclusters, and thus benefit the applications of such nanomaterials. In this review, we highlight the achievements of atomically precise alloy nanoclusters, and summarize the alloying principles and fundamentals, including the synthetic methods, site-preferences for different heteroatoms in the templates, and alloying-induced structure and property changes. First, based on various Au or Ag nanocluster templates, heteroatom doping modes are presented. The templates with electronic shell-closing configurations tend to maintain their structures during doping, while the others may undergo transformation and give rise to alloy nanoclusters with new structures. Second, alloy nanoclusters of specific magic sizes are reviewed. The arrangement of different atoms is related to the symmetry of the structures; that is, different atoms are symmetrically located in the nanoclusters of smaller sizes, and evolve into shell-by-shell structures at larger sizes. Then, we elaborate on the alloying effects in terms of optical, electrochemical, electroluminescent, magnetic and chiral properties, as well as the stability and reactivity via comparisons between the doped nanoclusters and their homo-metal counterparts. For example, central heteroatom-induced photoluminescence enhancement is emphasized. The applications of alloy nanoclusters in catalysis, chemical sensing, bio-labeling, and other fields are further discussed. Finally, we provide perspectives on existing issues and future efforts. Overall, this review provides a comprehensive synthetic toolbox and controllable doping modes so as to achieve more alloy nanoclusters with customized compositions, structures, and properties for applications. This review is based on publications available up to February 2020.

Atomic-level separation of thiolate-protected metal clusters

Fine metal clusters have attracted much attention from the viewpoints of both basic and applied science for many years because of their unique physical/chemical properties and functions, which differ from those of bulk metals. Among these materials, thiolate (SR)-protected gold clusters (Au_n(SR)_m clusters) have been the most studied metal clusters since 2000 because of their ease of synthesis and handling. However, in the early 2000s, it was not easy to isolate these metal clusters. Therefore, high-resolution separation methods were explored, and several atomic-level separation methods, including polyacrylamide gel electrophoresis (PAGE), high-performance liquid chromatography (HPLC), and thin-layer chromatography (TLC), were successively established. These techniques have made it possible to isolate a series of Au_n(SR)_m clusters, and much knowledge has been obtained on the correlation between the chemical composition and fundamental properties such as the stability, electronic structure, and physical properties of Au_n(SR)_m clusters. In addition, these high-resolution separation techniques are now also frequently used to evaluate the distribution of the product and to track the reaction process. In this way, high-resolution separation techniques have played an essential role in the study of Au_n(SR)_m clusters. However, only a few reviews have focused on this work. This review focuses on PAGE, HPLC, and TLC separation techniques, which offer high resolution and repeatability, and summarizes previous studies on the high-resolution separation of Au_n(SR)_m and related clusters with the purpose of promoting a better understanding of the features and the utility of these techniques.

Activation of atom-precise clusters for catalysis

The use of atom-precise, ligand-protected metal clusters has exceptional promise towards the fabrication of model supported-nanoparticle heterogeneous catalysts which have controlled sizes and compositions. One major challenge in the field involves the ease at which metallic clusters sinter upon removal of protected ligands, thus destroying the structural integrity of the model system. This review focuses on methods used to activate atom-precise thiolate-stabilized clusters for heterogeneous catalysis, and strategies that can be used to mitigate sintering. Thermal activation is the most commonly employed approach to activate atom-precise metal clusters, though a variety of chemical and photochemical activation strategies have also been reported. Material chemistry methods that can mitigate sintering are also explored, which include overcoating of clusters with metal oxide supports fabricated by sol-gel chemistry or atomic layer deposition of thin oxide films or encapsulating clusters within porous supports. In addition to focusing on the preservation of the size and morphology of deprotected metal clusters, the fate of the removed ligands is also explored, because detached and/or oxidized ligands can also greatly influence the overall properties of the catalyst systems. We also show that modern characterization techniques such as X-ray absorption spectroscopy and high-resolution electron microscopy have the capacity to enable careful monitoring of particle sintering upon activation of metal clusters.

Nanocluster growth via "graft-onto": effects on geometric structures and optical properties

The concept of “graft-onto” has been exploited to facilitate nanocluster growth from Pt₁Ag₂₈ to Pt₁Ag₃₁.

Atomically precise engineering on the nanocluster surface remains highly desirable for the fundamental understanding of how surface structures of a nanocluster contribute to its overall properties. In this paper, the concept of “graft-onto” has been exploited to facilitate nanocluster growth on surface structures. Specifically, the Ag₂(DPPM)Cl₂ complex is used for re-constructing the surface structure of Pt₁Ag₂₈(SR)₁₈(PPh₃)₄ (Pt₁Ag₂₈, SR = 1-adamantanethiolate) and producing a size-growth nanocluster – Pt₁Ag₃₁(SR)₁₆(DPPM)₃Cl₃ (Pt₁Ag₃₁). The grafting effect of Ag₂(DPPM)Cl₂ induces both direct changes on the surface structure (e.g., size growth, structural transformation, and surface rotation) and indirect changes on the kernel structure (from a fcc configuration to an icosahedral configuration). Remarkable differences have been observed by comparing optical properties between Pt₁Ag₂₈ and Pt₁Ag₃₁. Significantly, Pt₁Ag₃₁ exhibits high photo-luminescent intensity with a quantum yield of 29.3%, which is six times that of the Pt₁Ag₂₈. Overall, this work presents a new approach (i.e., graft-onto) for the precise dictation of nanocluster surface structures at the atomic level.

Metal Nanoclusters Stabilized by Selenol Ligands

The past decades have witnessed great advances in controllable synthesis, structure determination, and property investigation of metal nanoclusters. Selenolated nanoclusters, a special branch in the nanocluster family, have attracted great interest in these years. The electronegativity and atomic radius of selenium is different from sulfur, and thus the selenolated nanoclusters are anticipated to display different electronic/geometric structures and distinct chemical/physical properties relative to their thiolated analogues. This review covers the syntheses, structures, and properties of selenolated nanoclusters (including Au, Ag, Cu, and alloy nanoclusters). Ligand effects (between SeR and SR) on nanocluster properties, including optical absorption, stability, and electrochemical properties, are disclosed as well. At the end of the review, a scope for improvements and future perspectives of selenolated nanoclusters is highlighted. The review hopefully opens up new horizons for cluster scientists to synthesize more selenolated nanoclusters with novel structures and properties. This review is based on publications available up to May 2019.

Rational construction of a library of M29 nanoclusters from monometallic to tetrametallic

Exploring intermetallic synergy has allowed a series of alloy nanoparticles with prominent chemical-physical properties to be produced. However, precise alloying based on a maintained template has long been a challenging pursuit, and little has been achieved for manipulation at the atomic level. Here, a nanosystem based on M₂₉(S-Adm)₁₈(PPh₃)₄ (where S-Adm is the adamantane mercaptan and M is Ag/Cu/Au/Pt/Pd) has been established, which leads to the atomically precise operation on each site in this M₂₉ template. Specifically, a library of 21 species of nanoclusters ranging from monometallic to tetrametallic constitutions has been successfully prepared step by step with in situ synthesis, target metal-exchange, and forced metal-exchange methods. More importantly, owing to the monodispersity of each nanocluster in this M₂₉ library, the synergetic effects on the optical properties and stability have been mapped out. This nanocluster methodology not only provides fundamental principles to produce alloy nanoclusters with multimetallic compositions and monodispersed dopants but also provides an intriguing nanomodel that enables us to grasp the intermetallic synergy at the atomic level.

Free Valence Electron Centralization Strategy for Preparing Ultrastable Nanoclusters and Their Catalytic Application

Metal nanoclusters have attracted extensive interests owing to their atomically precise structures as well as intriguing properties. However, silver nanoclusters are not as stable as their gold counterparts, impeding the practical applications of Ag nanoclusters. In this work, a strategy of free valence electron centralization was exploited to render parent Ag nanoclusters highly stable. The stability of Ag₂₉(SSR)₁₂(PPh₃)₄ (SSR: benzene-1,3-dithiol) was controllably enhanced by stepwisely alloying the Ag₂₉ nanocluster to Ag₁₇Cu₁₂(SSR)₁₂(PPh₃)₄ and Au₁Ag₁₆Cu₁₂(SSR)₁₂(PPh₃)₄. Specifically, the trimetallic Au₁Ag₁₆Cu₁₂ is ultrastable even at 175 °C, which is close to the nanocluster decomposition temperature. The structures of Ag₁₇Cu₁₂ and Au₁Ag₁₆Cu₁₂ nanoclusters are determined by single-crystal X-ray diffraction. Furthermore, a combination of X-ray photoelectron spectroscopy measurements and density functional theory calculations demonstrates that the enhanced stability is induced by the centralization of the free valence electrons to the interior of the nanocluster. More importantly, the Au₁Ag₁₆Cu₁₂ enables the multicomponent A³ coupling reaction at high temperatures, which remarkably shortens the catalytic reaction time from ∼5 h to 3 min. Overall, this work presents a strategy for enhancing the thermal stability of nanoclusters via centralizing the free valence electrons to the nanocluster kernels.

Superatom spin-state dynamics of structurally precise metal monolayer-protected clusters (MPCs)

Electronic spin-state dynamics were studied for a series of Au₂₅(SC₈H₉)₁₈ ^q and Au₂₄Pd(SC₈H₉)₁₈ monolayer-protected clusters (MPCs) prepared in a series of oxidation states, q, including q = -1, 0, +1. These clusters were chosen for study because Au₂₅(SC₈H₉)₁₈ ^-1 is a closed-shell superatomic cluster, but Au₂₅(SC₈H₉)₁₈ ⁰ is an open-shell (7-electron) system; Au₂₅(SC₈H₉)₁₈ ⁺¹ and PdAu₂₄(SC₈H₉)₁₈ ⁰ are isoelectronic (6-electron) closed-shell systems. Carrier dynamics for electronic fine structure spin states were isolated using femtosecond time-resolved circularly polarized transient-absorption spectroscopy (fs-CPTA). Excitation energies of 1.82 eV and 1.97 eV were chosen for these measurements on Au₂₅(SC₈H₉)₁₈ ⁰ in order to achieve resonance matching with electronic fine structure transitions within the superatomic P- and D-orbital manifolds; 1.82-eV excited an unpaired P_z electron to D states, whereas 1.97-eV was resonant with transitions between filled P_x and P_y subshells and higher-energy D orbitals. fs-CPTA measurements revealed multiple spin-polarized transient signals for neutral (open shell) Au₂₅(SC₈H₉)₁₈, following 1.82-eV excitation, which persisted for several picoseconds; time constants of 5.03 ± 0.38 ps and 2.36 ± 0.59 ps were measured using 2.43 and 2.14 eV probes, respectively. Polarization-dependent fs-CPTA measurements of PdAu₂₄(SC₈H₉)₁₈ clusters exhibit no spin-conversion dynamics, similar to the isoelectronic Au₂₅(SC₈H₉)₁₈ ⁺¹ counterpart. These observations of cluster-specific dynamics resulted from spin-polarized superatom P to D excitation, via an unpaired P_z electron of the open-shell seven-electron Au₂₅(SC₈H₉)₁₈ MPC. These results suggest that MPCs may serve as structurally well-defined prototypes for understanding spin and quantum state dynamics in nanoscale metal systems.

Total structural determination of [Au1Ag24(Dppm)3(SR)17]2+ comprising an open icosahedral Au1Ag12 core with six free valence electrons

Herein, we report the first silver-rich nanocluster containing an open icosahedral Au₁Ag₁₂ core.

Single Au Atom Doping of Silver Nanoclusters

Ag₂₉ nanoclusters capped with lipoic acid (LA) can be doped with Au. The doped clusters show enhanced stability and increased luminescence efficiency. We attribute the higher quantum yield to an increase in the rate of radiative decay. With mass spectrometry, the Au-doped clusters were found to consist predominantly of Au₁Ag₂₈(LA)₁₂^3-. The clusters were characterized using X-ray absorption spectroscopy at the Au L₃-edge. Both the extended absorption fine structure (EXAFS) and the near edge structure (XANES) in combination with electronic structure calculations confirm that the Au dopant is preferentially located in the center of the cluster. A useful XANES spectrum can be recorded for lower concentrations, or in shorter time, than the more commonly used EXAFS. This makes XANES a valuable tool for structural characterization.

Effects of Metal-Doping on Hydrogen Evolution Reaction Catalyzed by MAu24 and M2Au36 Nanoclusters (M = Pt, Pd)

This paper describes the effects of doped metals on hydrogen evolution reaction (HER) electrocatalyzed by atomically controlled MAu₂₄ and M₂Au₃₆ nanoclusters, where M = Pt and Pd. HER performances, such as onset potential ( E_onset), catalytic current density, and turnover frequency (TOF), are comparatively examined with respect to the doped metals. Doping Pt or Pd into gold nanoclusters not only changes the electrochemical redox potentials of nanoclusters but also considerably improves the HER activities. E_onset is found to be controlled by the nanocluster's reduction potential matching the reduction potential of H⁺. The higher catalytic current and TOF are observed with the doped nanoclusters in the order of PtAu₂₄ > PdAu₂₄ > Au₂₅. The same trend is observed with the Au₃₈ group (Pt₂Au₃₆ > Pd₂Au₃₆> Au₃₈). Density functional theory calculations have revealed that the hydrogen adsorption free energy (Δ G_H) is significantly lowered by metal-doping in the order of Au₂₅ > PdAu₂₄ > PtAu₂₄ and Au₃₈ > Pd₂Au₃₆ > Pt₂Au₃₆, indicating that hydrogen adsorption on the active site of nanocluster is thermodynamically favored by Pd-doping and further by Pt-doping. The doped metals, albeit buried in the core of the nanoclusters, have profound impact on their HER activities by altering their reduction potentials and hydrogen adsorption free energies.

Atomic-Level Doping of Metal Clusters

Atomically precise noble metal (mainly silver and gold) nanoclusters are an emerging category of promising functional materials for future applications in energy, sensing, catalysis, and nanoelectronics. These nanoclusters are protected by ligands such as thiols, phosphines, and hydride and have sizes between those of atoms and plasmonic nanoparticles. In metallurgy, the properties of a pure metal are modified by the addition of other metals, which often offers augmented characteristics, making them more utilizable for real-life applications. In this Account, we discuss how the incorporation of various metal atoms into existing protected nanoclusters tunes their structure and properties. The process of incorporating metals into an existing cluster is known as doping; the product is known as a doped cluster, and the incorporated metal atom is called a dopant/foreign atom. We first present a brief historical overview of protected clusters and the need for doping and explain (with examples) the difference between an "alloy" and a "doped" cluster, which are two frequently confused terms. We then discuss several commonly observed challenges in the synthesis of doped clusters: (i) doping produces a mixture of compositions that prevents the growth of single crystals; (ii) doping with foreign atoms sometimes changes the overall composition and structure of the parent cluster; and (iii) doping beyond a certain number of foreign atoms decomposes the doped cluster. After delineating the challenges, we review a few potential synthetic methods for doped clusters: (i) the co-reduction method, (ii) the galvanic exchange method, (iii) ligand-induced conversion of bimetallic clusters to doped clusters, and (iv) intercluster reactions. As a foreign atom is able to occupy different positions within the structure of the parent cluster, we examine the structural relationship between the parent clusters and their different foreign-atom-doped clusters. We then show how doping enhances the stability, luminescence, and catalytic properties of clusters. The enhancement factor highly depends on the number and nature of the foreign atoms, which can also alter the charge state of the parent cluster. Atomic-level doping of foreign atoms in the parent cluster is confirmed by high-resolution electrospray ionization and matrix-assisted laser desorption ionization mass spectrometry techniques and single-crystal X-ray diffraction methods. The photophysical properties of the doped clusters are investigated using both time-dependent and steady-state luminescence and optical absorption spectroscopies. After presenting an overview of atomic-level doping in metal clusters and demonstrating its importance for enriching the chemistry and photophysics of clusters and extending their applications, we conclude this Account with a brief perspective on the field's future.

Alloy Clusters: Precise Synthesis and Mixing Effects

Metal alloys exhibit functionalities unlike those of single metals. Such alloying has drawn considerable research interest, particularly for nanoscale particles (metal clusters/nanoparticles), from the viewpoint of creating new functional nanomaterials. In gas phase cluster research, generated alloy clusters can be spatially separated with atomic precision in vacuum. Thus, the influences of increases or decreases in each element on the overall electronic structure of the cluster can be elucidated. However, to further understand the related mixing and synergistic effects, alloy clusters need to be produced on a large scale and characterized by various techniques. Because alloy clusters protected by thiolate (SR) can be synthesized by chemical methods and are stable in both solution and the solid state, these clusters are ideal study materials to better understand the mixing and synergistic effects. Moreover, the alloy clusters thus created have potential applications as functional materials. Therefore, since 2008, we have been working on establishing a precise synthesis method for SR-protected alloy clusters and elucidating their mixing and synergistic effects. Early research focused on the precise synthesis of alloy clusters wherein some of the Au in the stable SR-protected gold clusters ([Au₂₅(SR)₁₈]^- and [Au₃₈(SR)₂₄]⁰) is replaced by Pd, Ag, or Cu. These studies have shown that Pd, Ag, or Cu substitute at different metal sites. We also have examined the as-synthesized alloy clusters to clarify the effect of substitution by each element on the physical and chemical properties of the clusters. However, in early studies, the number of substitutions could not be controlled with atomic accuracy for [Au_{25- x}M _x(SR)₁₈]^- (M = Ag or Cu). Then, in following research, methods have been established to obtain alloy clusters with control over the composition. We have succeeded in developing a method for controlling the number of Ag substitutions with atomic precision and thereby elucidating the effect of Ag substitution on the electronic structure of clusters with atomic precision. Concurrently, we also studied alloy clusters containing multiple heteroelements with different preferential substitution sites. These results revealed that the effects of substitution of each element can be superimposed on the cluster by combining multiple elemental substitutions at different sites. In addition, we successfully developed methods to synthesize alloy clusters with heterometal core. These findings are expected to lead to clear design guidelines for developing new functional nanomaterials.

Rethinking Heterometal Doping in Ligand-Protected Metal Nanoclusters

Heterometal doping is a promising avenue toward tailoring properties of ligand-protected metal nanoclusters for specific applications. Though successful doping has been demonstrated in several structures, the underlying reasons for the dopant preference on occupying specific locations on the nanocluster with different concentrations remain unclear. In this study we apply our thermodynamic stability model, originally developed for ligand-protected monometallic nanoclusters, to rationalize the synthetic accessibility, dopant location, and concentrations of various heterometals on ligand-protected Au nanoclusters. Importantly, we demonstrate that the thermodynamic stability theory is a significant step forward in accurately describing doping effects on nanoclusters using first-principles calculations. With our computational predictions being in excellent agreement with a series of experiments, we introduce the thermodynamic stability theory as a new method for bimetallic nanocluster prediction.

Engineering Functional Metal Materials at the Atomic Level

With continuous research efforts devoted into synthesis and characterization chemistry of functional nanomaterials in the past decades, the development of metal materials is stepping into a new era, where atom-by-atom customization of property-dictating structural attributes is expected. Herein, the state-of-the-art modulation of functional metal nanomaterials at the atomic level, by size- and structure-controlled synthesis of thiolate-protected metal (e.g., Au and Ag) nanoclusters (NCs), is exemplified. Metal NCs are ultrasmall (<3 nm) particles with hierarchical primary, secondary, and tertiary structures, reminiscent of natural proteins or enzymes. Given the proven dependence of their physicochemical properties on their size and structure, documented synthetic methodologies delivering NCs with atomic-level monodispersity and tailorable size and structural attributes at individual hierarchical levels are categorized and discussed. Such assured atomic-level modulation could confer metal NCs with novel application opportunities in diverse fields, which are also exemplified by their size- and structure-dictated catalytic and biomedical performance. The precise synthesis and application chemistry developed based on the hierarchical structure scheme of metal NCs could increase the acceptance of metal NCs as a new family of functional materials.

Hetero-biicosahedral [Au24Pd(PPh3)10(SC2H4Ph)5Cl2]+ nanocluster: selective synthesis and optical and electrochemical properties

A recent study implied that a hetero-biicosahedral 25-atom cluster composed of two kinds of icosahedral 13-atom clusters could serve as a molecular rectifier and dipole material. However, no hetero-biicosahedral 25-atom clusters containing three types of ligands, in this case, phosphines, halogens, and thiolates, have been reported. In this study, we selectively synthesized [Au24Pd(PPh3)10(SC2H4Ph)5Cl2]Cl (Au = gold, Pd = palladium, PPh3 = triphenylphosphine, SC2H4Ph = phenylethanethiolate, Cl = chloride), in which one Au was replaced with a Pd. The single-crystal X-ray structural analysis demonstrated that [Au24Pd(PPh3)10(SC2H4Ph)5Cl2]Cl was a hetero-biicosahedral 25-atom cluster in which the central atom of one icosahedral Au13 core was replaced by a Pd atom. Optical absorption spectroscopy suggested that the electronic structure of each individual icosahedral 13-atom core in [Au24Pd(PPh3)10(SC2H4Ph)5Cl2]+ was reasonably well maintained, similar to the case of [Au25(PPh3)10(SC2H4Ph)5Cl2]2+. Density functional theory calculation revealed that the peak splitting in the region below 2.2 eV of the optical absorption spectrum of [Au24Pd(PPh3)10(SC2H4Ph)5Cl2]+ is due to the splitting of HOMOs and also suggested that this cluster has dipole moment. Electrochemical measurements showed that [Au24Pd(PPh3)10(SC2H4Ph)5Cl2]+ was relatively stable to reduction. These results are expected to contribute to the development of molecular rectifiers and dipole materials based on hetero-biicosahedral 25-atom clusters.

Atomic and Isomeric Separation of Thiolate-Protected Alloy Clusters

Techniques to control the chemical compositions and geometric structures of alloy clusters are indispensable to understand the correlation between the structures and physical/chemical properties of alloy clusters. In this study, we established a method to separate thiolate-protected 25-atom gold-silver alloy clusters (Au_{25- x}Ag _x(SR)₁₈) according to their chemical composition and structural isomer. Furthermore, using this method, we revealed that an isomeric distribution of the products exists in Au_{25- x}Ag _x(SR)₁₈ ( x ≥ 2) and that the distribution of these isomers depends on the synthesis method and standing time in solution. In this study, it was also demonstrated that the continuous discretization of the electronic structure is induced by the Ag substitution. This method can also be used to separate mixtures of [Au₂₄M(SR)₁₈]⁰ (M = Au, Pt, or Pd) and other Au-Ag alloy clusters ([Au_{36- x}Ag _x(SR)₂₄]⁰ and [Au_{38- x}Ag _x(SR)₂₄]⁰). This method is expected to be used to obtain comprehensive knowledge of the structural-property correlation of alloy clusters.

M4Au12Ag32(p-MBA)30 (M = Na, Cs) bimetallic monolayer-protected clusters: synthesis and structure

Crystals of M₄Au₁₂Ag₃₂(p-MBA)₃₀ bimetallic monolayer-protected clusters (MPCs), where p-MBA is p-mercapto-benzoic acid and M⁺ is a counter-cation (M = Na, Cs) have been grown and their structure determined. The mol-ecular structure of triacontakis[(4-carboxylatophenyl)sulfanido]dodecagolddotriacontasilver, Au₁₂Ag₃₂(C₇H₅O₂S)₃₀ or C₂₁₀H₁₅₀Ag₃₂Au₁₂O₆₀S₃₀, exhib-its point group symmetry at 100 K. The overall diameter of the MPC is approximately 28 Å, while the diameter of the Au₁₂Ag₂₀ metallic core is 9 Å. The structure displays ligand bundling and inter-molecular hydrogen bonding, which gives rise to a framework structure with 52% solvent-filled void space. The positions of the M⁺ cations and the DMF solvent mol-ecules within the void space of the crystal could not be determined. Three out of the five crystallographically independent ligands in the asymmetric unit cell are disordered over two sets of sites. Comparisons are made to the all-silver M₄Ag₄₄(p-MBA)₃₀ MPCs and to expectations based on density functional theory.