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

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.

Tailoring silver nanoclusters via doping: advances and opportunities

Atomically precise noble metal nanoclusters (especially Au and Ag) have been pursued due to their fascinating molecule-like properties. In spite of the significant progress on Au nanoclusters (NCs), the structure and property evolution of Ag NCs is still in high demand. Doping is a useful strategy for improving the physicochemical performances of Ag NCs. Herein we summarize the recent advances in tailoring silver NC structures and properties via doping. First, we reviewed the recent studies on the synthesis of hetero metal atom doped silver bimetallic nanoclusters, which are classified by the dopants, including Au, Pt, Pd, Cu, Ni and Cd. Second, the doping effects on their properties were reviewed, including the locations of hetero metal atoms, the influence on their stability, and the charge state evolution. Moreover, we highlighted the doping-dependent improvement of the photo-luminescence (PL) performance and catalytic activity of Ag NCs.

Fast and high-yield synthesis of thiolate Ag44 and Au12Ag32 nanoclusters via the CTAB reverse micelle method

To advance the development of atomically precise Ag and Ag-alloyed nanoclusters, it is critical to develop effective synthetic methods. Herein, we successfully extend the CTAB (cetyl trimethyl ammonium bromide) reverse micelle method to synthesize a high-purity Ag₄₄(p-MBA)₃₀ (p-MBA = para-mercaptobenzoic acid) nanocluster and its corresponding alloy cluster Au₁₂Ag₃₂(p-MBA)₃₀ in a short time (15 min and 5 min), with a high yield of ∼83% and ∼85%, respectively. Furthermore, the mechanism regarding the reverse micelle method has been clearly elucidated. Through characterizing the reaction system by Raman spectroscopy and NMR spectroscopy techniques, it can be revealed that employing CTAB to form reverse micelles to construct a sealed chemical environment is critical for realizing the fast and high-yield synthesis.

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.

Crystal Structure of Au30-xAgx(S-tBu)18 and Effect of the Ligand on Ag Alloying in Gold Nanomolecules

We report the X-ray crystal structure of the Au_30-xAg_x(S-tBu)₁₈ alloy and the effect of the ligand on alloying site preferences. Gold-silver nanoalloys prepared by co-reduction of metal salts are known to have only partial Ag occupancies. Interestingly, Au_30-xAg_x(S-tBu)₁₈ has 100% Ag occupancy at two sites on the core surface as well as partial Ag occupancies on the surface, capping, and staples sites. The Au_30-xAg_x(S-tBu)₁₈ (x = 1-5) composition was confirmed by X-ray diffraction and electrospray ionization mass spectrometry studies. Thiolate ligands can be categorized into three classes on the basis of the groups at the α-position as aliphatic, aromatic, and bulky thiols. The effect of the ligand on Ag doping can be clearly seen in the crystal structures of Au_36-xAg_x(SPh-tBu)₂₄ and Au_38-xAg_x(SCH₂CH₂Ph)₂₄ when compared with that of Au_30-xAg_x(S-tBu)₁₈. Ag is preferentially doped onto the core surface when the ligand is aliphatic, and Ag is doped in both core surface and staple metal sites when the ligand is aromatic or bulky.

Interactions between Ultrastable Na4Ag44(SR)30 Nanoclusters and Coordinating Solvents: Uncovering the Atomic-Scale Mechanism

Recently, silver nanoclusters have garnered considerable attention after the high-yield synthesis and crystallization of a thiolate-protected silver nanocluster, Na₄Ag₄₄(SR)₃₀ (SR, protecting thiolate ligand). One intriguing feature of Na₄Ag₄₄(SR)₃₀ is its outstanding stability and resistance to chemical reactions, in striking difference from other silver nanostructures whose susceptibility to oxidation (tarnishing) has been commonly observed and thus limits their applications in nanotechnology. Herein, we report the mechanism on the ultrahigh stability of Na₄Ag₄₄(SR)₃₀ by uncovering how coordinating solvents interact with the Na₄Ag₄₄(SR)₃₀ nanocluster at the atomic scale. Through synchrotron X-ray experiments and theoretical calculations, it was found that strongly coordinating aprotic solvents interact with surface Ag atoms, particularly between ligand bundles, which compresses the Ag core and relaxes surface metal-ligand interactions. Furthermore, water was used as a cosolvent to demonstrate that semiaqueous conditions play an important role in protecting exposed surface regions and can further influence the local structure of the silver nanocluster itself. Notably, under semiaqueous conditions, aprotic coordinating solvent molecules preferentially remain on the metal surface while water molecules interact with ligands, and ligand bundling persisted across the varied solvation conditions. This work offers an atomic level mechanism on the ultrahigh stability of the Na₄Ag₄₄(SR)₃₀ nanoclusters from the nanocluster-coordinating solvent interaction perspective, and implies that nanocluster-solvent interactions should be carefully considered moving forward for silver nanoclusters, as they can influence the electronic/chemical properties of the nanocluster as well as the surface accessibility of small molecules for potential catalytic and biomedical applications.

Insights into Charge Transfer at an Atomically Precise Nanocluster/Semiconductor Interface

The deposition of an atomically precise nanocluster, for example, Ag₄₄(SR)₃₀, onto a large‐band‐gap semiconductor such as TiO₂ allows a clear interface to be obtained to study charge transfer at the interface. Changing the light source from visible light to simulated sunlight led to a three orders of magnitude enhancement in the photocatalytic H₂ generation, with the H₂ production rate reaching 7.4 mmol h⁻¹ g_catalyst⁻¹. This is five times higher than that of TiO₂ modified with Ag nanoparticles and even comparable to that of TiO₂ modified with Pt nanoparticles under similar conditions. Energy band alignment and transient absorption spectroscopy reveal that the role of the metal clusters is different from that of both organometallic complexes and plasmonic nanoparticles: A type II heterojunction charge‐transfer route is achieved under UV/Vis irradiation, with the cluster serving as a small‐band‐gap semiconductor. This results in the clusters acting as co‐catalysts rather than merely photosensitizers.

The stability enhancement factor beyond eight-electron shell closure in thiacalix[4]arene-protected silver clusters

Destroying coordination open sites may significantly enhance the stability of metal nanoclusters.

We report the synthesis and structures of two 34-atom metal nanoclusters, namely [Ag₃₄(BTCA)₃(C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CBu^t)₉(tfa)₄(CH₃OH)₃]SbF₆ and [AuAg₃₃(BTCA)₃(C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CBu^t)₉(tfa)₄(CH₃OH)₃]SbF₆, where H₄BTCA is p-tert-butylthiacalix[4]arene and tfa is trifluoroacetate. Their compositions and structures have been determined by single-crystal X-ray structural analysis and ESI-MS. The cationic cluster consists of a centered icosahedron M@Ag₁₂ (M = Ag or Au) core that is surrounded by 21 peripheral silver atoms. Surrounding protection is provided by four kinds of ligands, including three BTCA, nine ^tBuC Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C, four tfa, and three methanol solvent ligands. It was found that the Ag₅@BTCA μ₅-coordination motif of thiacalixarene is critical for high stability of the title clusters, and extra stability enhancement can be achieved by doping a gold atom at the center of the silver cluster. This work suggests that coordination saturation should be taken into account in addition to electronic and geometric factors for analyzing metal nanocluster stabilities.

Chemistry and Structure of Silver Molecular Nanoparticles

Silver and gold molecular nanoparticles (mNPs) are a relatively new class of molecular materials of fundamental interest. They are high-nuclearity metal-organic compounds, with ligated metal cores, where the different character of bonding in the ligand shell and metal core gives rise to many of the unique properties of these materials. Research has primarily focused on gold mNPs, due to their good stability and the ease with which they may be synthesized and processed. To understand these materials as a general class, however, it will be necessary to broaden research efforts to other metals. Gold and silver are isoelectronic and have the same atomic radius, making the comparison of gold and silver mNPs attractive. The optical and chemical differences of the two metals provide useful contrasts, however, as well as a means to access a wider range of properties. In this Account, we focus on the synthesis, structure, and reactivity of silver mNPs. First, we review the origins and history of the field, from the ill-defined gas-phase metal clusters of the 1980s to the precisely defined mNPs of 1996 and onward. Next, we discuss the role of silver as a complement to gold mNPs in the effort to generalize lessons learned from either material and extend them into new metals. The synthesis of silver mNPs is covered in some detail, noting the choices made as the chemistry and the materials were developed. The importance of coordinating solvents and thermodynamic stability are also noted. The need to reduce solvent use is discussed and a new approach to achieving this goal is presented. Next, the structures of silver mNPs are discussed, including the Ag₄₄ and Ag₁₇ archetypes, and focusing on the successful de novo structure prediction of the latter. Structure and prediction of ligand shell motifs are also discussed. Finally, the postsynthetic chemistry and reactivity of silver mNPs are presented, including some of the first efforts to elucidate reaction mechanisms, beginning in 2012. Silver nanoparticles are gaining in popularity, particularly compared with gold, as the potential for silver to make a technological and economic impact is recognized. The superior optical properties of silver already make it a valuable material for plasmonics, but this may also translate to molecular species for nonlinear optics, sensors, and optoelectronics. The higher reactivity may also lead to a greater diversity of chemistry for silver compared to gold, including as an important broad-spectrum antimicrobial. Conversely, the "ultrastability" of the Ag₄₄ archetype has already enabled unprecedented scale up with molecular precision, and may lead to the first industrial-scale production of metal mNPs. Clearly, silver mNPs are one of the most promising and significant new materials being studied today.

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.

3D to 2D reorganization of silver-thiol nanostructures, triggered by solvent vapor annealing

Metal-organic composites are of great interest for a wide range of applications. The control of their structure remains a challenge, one of the problems being a complex interplay of covalent and supramolecular interactions. This paper describes the self-assembly, thermal stability and phase transitions of ordered structures of silver atoms and thiol molecules spanning from the molecular to the mesoscopic scale. Building blocks of molecularly defined clusters formed from 44 silver atoms, each particle coated by a monolayer of 30 thiol ligands, are used as ideal building blocks. By changing solvent and temperature it is possible to tune the self-assembled 3D crystals of pristine nanoparticles or, conversely, 2D layered structures, with alternated stacks of Ag atoms and thiol monolayers. The study investigates morphological, chemical and structural stability of these materials between 25 and 300 °C in situ and ex situ at the nanoscale by combining optical and electronic spectroscopic and scattering techniques, scanning probe microscopies and density-functional theory (DFT) calculations. The proposed wet-chemistry approach is relatively cheap, easy to implement, and scalable, allowing the fabricated materials with tuned properties using the same building blocks.

Customizing the Structure, Composition, and Properties of Alloy Nanoclusters by Metal Exchange

The properties of metal materials can be greatly enriched by including various elements to generate alloys. The galvanic replacement represents a classical method for the preparation of both bulk- and nanoalloy materials. The difference of the electrochemical potential between the two metals acts as the driving force for the galvanic replacement reaction. However, this classical rule partially fails at the ultrasmall size scale, for that novel chemistry emerges by the decrease of the size of materials down to less than 3 nm due to the strong quantum effect. In this Account, we discuss an emerging topic of nanochemistry, the metal exchange in atomically precise ultrasmall (<3 nm) metal nanoparticles (or nanoclusters). The metal exchange method uses different types of metal sources (e.g., AuBrPPh₃ or AgSR complexes) to react with templating metal nanoclusters (e.g., Au₂₅(SR)₁₈^-), and finally alloy nanoclusters are produced. We demonstrate that the metal exchange reaction between metal nanoclusters and metal complexes does not follow the classical metal activity sequence (i.e., Fe > Cd > Co > Ni > Pb > Cu > Hg > Ag > Pd > Pt > Au) and such metal exchange reactions in the nanocluster range is, to a large extent, related with the electron shell closing and the structural stability of nanoclusters. In the subsequent sections, we present effective control over the number, position, and distribution of the dopants. The shape and structure of the final alloy products can be tailored by recently developed metal exchange methods. More importantly, modulation and enhancement of the properties of NCs through metal exchange are realized. For example, the largely increased quantum yield and the significantly improved catalytic activity. In addition, we shall also discuss the real-time characterization of the metal exchange reaction by the combination of UV-vis absorption spectroscopy, high resolution electrospray ionization mass spectrometry (ESI-MS), matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS), and single crystal X-ray diffraction (SC-XRD). By controlling the charge of the templating metal nanoclusters and the different types of metal complexes, the driving force of metal exchange has been studied, which is considered to be the thermodynamics rather than the electrochemical potential. In summary, the metal exchange reactions in the ultrasmall nanocluster range are totally different compared with the case of larger-sized metal nanoparticles. Depending on this novel method, atomically precise alloy nanoclusters can be prepared by reacting the nanocluster composed of inert metal (such as Au) with complexes of high-activity metals (e.g., Cd/Hg/Cu/Ag). We anticipate that future research on the metal exchange will contribute to the fundamental understanding of reaction behavior of metal atoms in ultrasmall nanoclusters and to the design of alloy nanoclusters with enhanced properties.

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.