Atomic-level separation of thiolate-protected metal clusters

Assignment of individual structures from intermetalloid nickel gallium cluster ensembles

Poorly selective mixed-metal cluster synthesis and separation yield reaction solutions of inseparable intermetalloid cluster mixtures, which are often discarded. High-resolution mass spectrometry, however, can provide precise compositional data of such product mixtures. Structure assignments can be achieved by advanced computational screening and consideration of the complete structural space. Here, we experimentally verify structure and composition of a whole cluster ensemble by combining a set of spectroscopic techniques. Our study case are the very similar nickel/gallium clusters of M12, M13 and M14 core composition Ni6+xGa6+y (x + y ≤ 2). The rationalization of structure, bonding and reactivity is built upon the organometallic superatom cluster [Ni6Ga6](Cp*)6 = [Ga6](NiCp*)6 (1; Cp* = C5Me5). The structural conclusions are validated by reactivity tests using carbon monoxide, which selectively binds to Ni sites, whereas (triisopropylsilyl)acetylene selectively binds to Ga sites.

Tiara-like Hexanuclear Nickel-Platinum Alloy Nanocluster

Tiara-like metal nanoclusters (TNCs) have attracted a great deal of attention because of their high stability and easy synthesis under atmospheric conditions as well as their high activity in various catalytic reactions. Alloying is one of the methods that can be used to control the physicochemical properties of nanoclusters, but few studies have reported on alloy TNCs. In this study, we synthesized alloy TNCs [NixPt6-x(PET)12, where x = 1-5 and PET = 2-phenylethanethiolate] consisting of thiolate, nickel (Ni), and platinum (Pt). We further evaluated the stability, geometric structure, and electronic structure by high-performance liquid chromatography and density functional theory calculations. The results revealed that NixPt6-x(PET)12 has a distorted structure and is therefore less stable than single-metal TNCs.

Symmetry breaking of highly symmetrical nanoclusters for triggering highly optical activity

Developing new approaches to fulfill the enantioseparation of nanocluster racemates and construct cluster-based nanomaterials with optical activity remains highly desired in cluster science, because it is an essential prerequisite for fundamental research and extensive applications of these nanomaterials. We herein propose a strategy termed "active-site exposing and partly re-protecting" to trigger the symmetry breaking of highly symmetrical nanoclusters and to render cluster crystals optically active. The vertex PPh3 of the symmetrical Ag29(SSR)12(PPh3)4 (SSR = 1, 3-benzenedithiol) nanocluster was firstly dissociated in the presence of counterions with large steric hindrance, and then the exposed Ag active sites of the obtained Ag29(SSR)12 nanocluster were partly re-protected by Ag+, yielding an Ag29(SSR)12-Ag2 nanocluster with a symmetry-breaking construction. Ag29(SSR)12-Ag2 followed a chiral crystallization mode, and its crystal displayed strong optical activity, derived from CD and CPL characterizations. Overall, this work presents a new approach (i.e., active-site exposing and partly re-protecting) for the symmetry breaking of highly symmetrical nanoclusters, the enantioseparation of nanocluster racemates, and the achievement of highly optical activity.

Accelerated size-focusing light activated synthesis of atomically precise fluorescent Au22(Lys-Cys-Lys)16 clusters

Atomically precise metal nanoclusters are promising candidates for various biomedical applications, including their use as photosensitizers in photodynamic therapy (PDT). However, typical synthetic routes of clusters often result in complex mixtures, where isolating and characterizing pure samples becomes challenging. In this work, a new Au22(Lys-Cys-Lys)16 cluster is synthesized using photochemistry, followed by a new type of light activated, accelerated size-focusing. Fluorescence excitation-emission matrix spectroscopy (EEM) and parallel factor (PARAFAC) analysis have been applied to track the formation of fluorescent species, and to assess optical purity of the final product. Furthermore, excited state reactivity of Au22(Lys-Cys-Lys)16 clusters is studied, and formation of type-I reactive oxygen species (ROS) from the excited state of the clusters is observed. The proposed size-focusing procedure in this work can be easily adapted to conventional cluster synthetic methods, such as borohydride reduction, to provide atomically precise clusters.

Elucidation of the electronic structures of thiolate-protected gold nanoclusters by electrochemical measurements

Metal nanoclusters (NCs) with sizes of approximately 2 nm or less have different physical/chemical properties from those of the bulk metals owing to quantum size effects. Metal NCs, which can be size-controlled and heterometal doped at atomic accuracy, are expected to be the next generation of important materials, and new metal NCs are reported regularly. However, compared with conventional materials such as metal complexes and relatively large metal nanoparticles (>2 nm), these metal NCs are still underdeveloped in terms of evaluation and establishment of application methods. Electrochemical measurements are one of the most widely used methods for synthesis, application, and characterisation of metal NCs. This review summarizes the basic knowledge of the electrochemistry and experimental techniques, and provides examples of the reported electronic states of thiolate-protected gold NCs elucidated by electrochemical approaches. It is expected that this review will provide useful information for researchers starting to study metal NCs.

Insights into the synthesis of NHC-stabilized Au nanoclusters through real-time reaction monitoring

Atomically precise gold nanoclusters (AuNCs) are interesting nanomaterials with potential applications in catalysis, bioimaging and optoelectronics. Their compositions and properties are commonly evaluated by various analytical techniques, including UV-vis spectroscopy, NMR spectroscopy, ESI mass spectrometry, and single-crystal X-ray diffraction. While these techniques have provided detailed insights into the structure and properties of nanoclusters, synthetic methods still suffer from a lack of in situ and real-time reaction monitoring methodologies. This limits insight into the mechanism of formation of AuNCs and hinders attempts at optimization. We have demonstrated the utility of HPLC-MS as a monitoring methodology in the synthesis of two NHC-protected gold nanoclusters: [Au13(NHC)9Cl3]2+ and [Au24(NHC)14Cl2H3]3+. Herein we show that HPLC coupled with mass spectrometry and 13C NMR spectroscopy of labelled derivatives enables new insight into critical reaction dynamics of AuNCs synthesis and rapid reaction optimization.

Anion-templated silver nanoclusters: precise synthesis and geometric structure

Metal nanoclusters (NCs) are gaining much attention in nanoscale materials research because they exhibit size-specific physicochemical properties that are not observed in the corresponding bulk metals. Among them, silver (Ag) NCs can be precisely synthesized not only as pure Ag NCs but also as anion-templated Ag NCs. For anion-templated Ag NCs, we can expect the following capabilities: 1) size and shape control by regulating the central anion (anion template); 2) stabilization by adjusting the charge interaction between the central anion and surrounding Ag atoms; and 3) functionalization by selecting the type of central anion. In this review, we summarize the synthesis methods and influences of the central anion on the geometric structure of anion-templated Ag NCs, which include halide ions, chalcogenide ions, oxoanions, polyoxometalate, or hydride/deuteride as the central anion. This summary provides a reference for the current state of anion-templated Ag NCs, which may promote the development of anion-templated Ag NCs with novel geometric structures and physicochemical properties.

Promoting Photocatalytic Carbon Dioxide Reduction by Tuning the Properties of Cocatalysts

Suppressing the amount of carbon dioxide in the atmosphere is an essential measure toward addressing global warming. Specifically, the photocatalytic CO₂ reduction reaction (CRR) is an effective strategy because it affords the conversion of CO₂ into useful carbon feedstocks by using sunlight and water. However, the practical application of photocatalyst-promoting CRR (CRR photocatalysts) requires significant improvement of their conversion efficiency. Accordingly, extensive research is being conducted toward improving semiconductor photocatalysts, as well as cocatalysts that are loaded as active sites on the photocatalysts. In this review, we summarize recent research and development trends in the improvement of cocatalysts, which have a significant impact on the catalytic activity and selectivity of photocatalytic CRR. We expect that the advanced knowledge provided on the improvement of cocatalysts for CRR in this review will serve as a general guideline to accelerate the development of highly efficient CRR photocatalysts.

Analytical separation techniques: toward achieving atomic precision in nanomaterials science

The size- and shape-dependence of the properties are the most characteristic features of nanoscale matter. In many types of nanomaterials, there is a size regime wherein every atom counts. In order to fully realize the idea of 'maneuvering things atom by atom' envisioned by Richard Feynman, synthesis and separation of nanoscale matter with atomic precision are essential. It is therefore not surprising that analytical separation techniques have contributed tremendously toward understanding the size- as well as shape-dependent properties of nanomaterials. Fascinating properties of nanomaterials would not have been explored without the use of these techniques. Here we discuss the pivotal role of analytical separation techniques in the progress of nanomaterials science. We begin with a brief overview of some of the key analytical separation techniques that are of tremendous importance in nanomaterials research. Then we describe how each of these techniques has contributed to the advancements in nanomaterials science taking some of the nanosystems as examples. We discuss the limitations and challenges of these techniques and future perspectives.

A Review of State of the Art in Phosphine Ligated Gold Clusters and Application in Catalysis

Atomically precise gold clusters are highly desirable due to their well-defined structure which allows the study of structure-property relationships. In addition, they have potential in technological applications such as nanoscale catalysis. The structural, chemical, electronic, and optical properties of ligated gold clusters are strongly defined by the metal-ligand interaction and type of ligands. This critical feature renders gold-phosphine clusters unique and distinct from other ligand-protected gold clusters. The use of multidentate phosphines enables preparation of varying core sizes and exotic structures beyond regular polyhedrons. Weak gold-phosphorous (Au-P) bonding is advantageous for ligand exchange and removal for specific applications, such as catalysis, without agglomeration. The aim of this review is to provide a unified view of gold-phosphine clusters and to present an in-depth discussion on recent advances and key developments for these clusters. This review features the unique chemistry, structural, electronic, and optical properties of gold-phosphine clusters. Advanced characterization techniques, including synchrotron-based spectroscopy, have unraveled substantial effects of Au-P interaction on the composition-, structure-, and size-dependent properties. State-of-the-art theoretical calculations that reveal insights into experimental findings are also discussed. Finally, a discussion of the application of gold-phosphine clusters in catalysis is presented.

Homoleptic Silver-Rich Trimetallic M20 Nanocluster

Two silver-rich M₂₀ alloy nanoclusters (NCs), [Cu_3.5Ag_16.5{S₂P(OⁿPr)₂}₁₂] (1) and [Cu_2.5AuAg_16.5{S₂P(OⁿPr)₂}₁₂] (2), were synthesized and fully characterized by electrospray ionization mass spectrometry, NMR spectroscopy, and X-ray crystallography. Cluster 2, the first structurally characterized trimetallic M₂₀ NC, was produced by doping one Au atom into a bimetallic M₂₀ NC. Structural analyses showed the preferred positions of Group 11 metals in the yielded M₂₀ NCs. Their antioxidation ability has been investigated, and the time-dependent UV-vis spectrum shows that the presence of Cu^I atoms in structures 1 and 2 can improve the antioxidant ability.

Development and Functionalization of Visible-Light-Driven Water-Splitting Photocatalysts

With global warming and the depletion of fossil resources, our fossil fuel-dependent society is expected to shift to one that instead uses hydrogen (H₂) as a clean and renewable energy. To realize this, the photocatalytic water-splitting reaction, which produces H₂ from water and solar energy through photocatalysis, has attracted much attention. However, for practical use, the functionality of water-splitting photocatalysts must be further improved to efficiently absorb visible (Vis) light, which accounts for the majority of sunlight. Considering the mechanism of water-splitting photocatalysis, researchers in the various fields must be employed in this type of study to achieve this. However, for researchers in fields other than catalytic chemistry, ceramic (semiconductor) materials chemistry, and electrochemistry to participate in this field, new reviews that summarize previous reports on water-splitting photocatalysis seem to be needed. Therefore, in this review, we summarize recent studies on the development and functionalization of Vis-light-driven water-splitting photocatalysts. Through this summary, we aim to share current technology and future challenges with readers in the various fields and help expedite the practical application of Vis-light-driven water-splitting photocatalysts.

Metal-nanocluster Science and Technology: My Personal History and Outlook

Metal nanoclusters (NCs) are among the leading targets in research of nanoscale materials, and elucidation of their properties (science) and development of control techniques (technology) have been continuously studied for...

Controlling the Chemistry of Nanoclusters: From Atomic Precision to Controlled Assembly

Metal nanoclusters have gained prominence in nanomaterials sciences, owing to their atomic precision, structural regularity, and unique chemical composition. Additionally, the ligands stabilizing the clusters provide great opportunities for linking the clusters in higher order dimensions, eventually leading to the formation of a repertoire of nanoarchitectures. This makes the chemistry of atomic clusters worth exploring. In this mini review, we aim to focus on the chemistry of nanoclusters. Firstly, we summarize the important strategies developed so far for the synthesis of atomic clusters. For each synthetic strategy, we highlight the chemistry governing the formation of nanoclusters. Next, we discuss the key techniques in the purification and separation of nanoclusters, as the chemical purity of clusters is deemed important for their further chemical processing. Thereafter which we provide an account of the chemical reactions of nanoclusters. Then, we summarize the chemical routes to the spatial organization of atomic clusters, highlighting the importance of assembly formation from an application point of view. Finally, we raise some fundamentally important questions with regard to the chemistry of atomic clusters, which, if addressed, may broaden the scope of research pertaining to atomic clusters.

Critical Role of CF3 Groups in the Electronic Stabilization of [PdAu24(C≡CC6H3(CF3)2)18]2- as Revealed by Gas-Phase Anion Photoelectron Spectroscopy

The role of alkynyl ligands with electron-withdrawing nature in the stability of metal clusters was investigated by gas-phase anion photoelectron spectroscopy (PES) on heteroleptic cluster anions [PdAu₂₄(C≡CAr^F)_18-x(C≡CPh)_x]^2- (Ar^F = 3,5-(CF₃)₂C₆H₃). Gas-phase PES on the cluster anions with specific x (= 0-6) revealed that electron binding energies decreased linearly with x, indicating that the electron-withdrawing CF₃ substituents on the alkynyl ligand played a critical role in the electronic stabilization of [PdAu₂₄(C≡CAr^F)₁₈]^2-. Density functional theory calculations reproduced the decrease of electron binding energies and rationally explained the ligand effect by a mechanism similar to the modulation of the work function of gold films by organic monolayers.

Creation of High-Performance Heterogeneous Photocatalysts by Controlling Ligand Desorption and Particle Size of Gold Nanocluster

Recently, the creation of new heterogeneous catalysts using the unique electronic/geometric structures of small metal nanoclusters (NCs) has received considerable attention. However, to achieve this, it is extremely important to establish methods to remove the ligands from ligand-protected metal NCs while preventing the aggregation of metal NCs. In this study, the ligand-desorption process during calcination was followed for metal-oxide-supported 2-phenylethanethiolate-protected gold (Au) 25-atom metal NCs using five experimental techniques. The results clearly demonstrate that the ligand-desorption process consists of ligand dissociation on the surface of the metal NCs, adsorption of the generated compounds on the support and desorption of the compounds from the support, and the temperatures at which these processes occurred were elucidated. Based on the obtained knowledge, we established a method to form a metal-oxide layer on the surface of Au NCs while preventing their aggregation, thereby succeeding in creating a water-splitting photocatalyst with high activity and stability.

A comparative study of structural, electronic, and optical properties of thiolated gold clusters with icosahedral vs face-centered cubic cores

The structural, electronic, and optical properties of the protected Au clusters with icosahedral (Ih) and face-centered cubic (FCC)-like Au₁₃ cores were studied to understand the origin of the difference in the optical gaps of these clusters. It has been demonstrated that the choice of density functionals does not qualitatively affect the properties of Au₂₃ and Au₂₅ clusters with Ih and FCC cores. The density of states, molecular orbitals, and natural charges were analyzed in detail using the B3LYP functional. The substantial energy difference in the lowest-energy absorption peaks for the clusters with the Ih and FCC cores is attributed to the difference in the natural charges of the central Au atoms (Au_c) in the Ih and FCC cores, the former of which is more negative than the latter. Natural population analysis demonstrates that the excess negative charge of the Au_c atom in clusters with Ih cores occupies the 6p atomic orbitals. This difference in Au_c is attributed to the smaller size of the Ih core compared to the FCC core, as a less bulky ligand allows a smaller core with increased electron density, which, in turn, increases the highest occupied molecular orbital energy and decreases the optical gap.

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.

Design of a fluorescent and clickable Ag38(SRN3)24 nanocluster platform: synthesis, modeling and self-assembling

Fluorescent atomically precise Ag₃₈(11-azido-2-ol-undecane-thiolate)₂₄ nanoclusters are easily prepared using sodium ascorbate as a "green" reducer and are extensively characterized by way of elemental analyses, ATR-FTIR, XRD, SAXS, UV-vis, fluorescence spectroscopies, and theoretical modeling. The fluorescence and the atomically determined stoichiometry and structure, the facile and environmentally green synthesis, together with the novel presence of terminal azido groups in the ligands which opens the way to "click"-binding a wide set of molecular species, make Ag₃₈(11-azido-2-ol-undecane-thiolate)₂₄ nanoclusters uniquely appealing systems for biosensing, recognition and functionalization in biomedicine applications and in catalysis.

Size Exclusion Chromatography: An Indispensable Tool for the Isolation of Monodisperse Gold Nanomolecules

Highly monodisperse and pure samples of atomically precise gold nanomolecules (AuNMs) are essential to understand their properties and to develop applications using them. Unfortunately, the synthetic protocols that yield a single-sized nanomolecule in a single-step reaction are unavailable. Instead, we observe a polydisperse product with a mixture of core sizes. This product requires post-synthetic reactions and separation techniques to isolate pure nanomolecules. Solvent fractionation based on the varying solubility of different sizes serves well to a certain extent in isolating pure compounds. It becomes tedious and offers less control while separating AuNMs that are very similar in size. Here, we report the versatile and the indispensable nature of using size exclusion chromatography (SEC) as a tool for separating nanomolecules and nanoparticles. We have demonstrated the following: (1) the ease of separation offered by SEC over solvent fractionation; (2) the separation of a wider size range (∼5-200 kDa or ∼1-3 nm) and larger-scale separation (20-100 mg per load); (3) the separation of closely sized AuNMs, demonstrated by purifying Au₁₃₇(SR)₅₆ from a mixture of Au₃₂₉(SR)₈₄, Au₁₄₄(SR)₆₀, Au₁₃₇(SR)₅₆, and Au₁₃₀(SR)₅₀, which could not be achieved using solvent fractionation; (4) the separation of AuNMs protected by different thiolate ligands (aliphatic, aromatic, and bulky); and (5) the separation can be improved by increasing the column length. Mass spectrometry was used for analyzing the SEC fractions.

Creation of active water-splitting photocatalysts by controlling cocatalysts using atomically precise metal nanoclusters

With global warming and the depletion of fossil resources, our fossil-fuel-dependent society is expected to shift to one that instead uses hydrogen (H₂) as clean and renewable energy. Water-splitting photocatalysts can produce H₂ from water using sunlight, which are almost infinite on the earth. However, further improvements are indispensable to enable their practical application. To improve the efficiency of the photocatalytic water-splitting reaction, in addition to improving the semiconductor photocatalyst, it is extremely effective to improve the cocatalysts (loaded metal nanoclusters, NCs) that enable the reaction to proceed on the photocatalysts. We have thus attempted to strictly control metal NCs on photocatalysts by introducing the precise-control techniques of metal NCs established in the metal NC field into research on water-splitting photocatalysts. Specifically, the cocatalysts on the photocatalysts were controlled by adsorbing atomically precise metal NCs on the photocatalysts and then removing the protective ligands by calcination. This work has led to several findings on the electronic/geometrical structures of the loaded metal NCs, the correlation between the types of loaded metal NCs and the water-splitting activity, and the methods for producing high water-splitting activity. We expect that the obtained knowledge will lead to clear design guidelines for the creation of practical water-splitting photocatalysts and thereby contribute to the construction of a hydrogen-energy society.

Thin layer chromatography based extraction approaches for improved analysis of volatile compounds with gas chromatography-mass spectrometry and direct analysis with gas analyzer

In this study, thin-layer chromatography was applied for selective extraction of volatile compounds in perfume prior to analysis with solid phase microextraction and gas chromatography-mass spectrometry. The standard compounds were desorbed from the thin-layer chromatography plate and extracted at 80°C for 15 min showing good linearity of the calibration curves (R²  > 0.98) and acceptable recovery range (65-81%). The plate after the separation was cut into four smaller parts followed by solid phase microextraction/gas chromatography-mass spectrometry analysis, which revealed different compound profile in each part with the correlation between log P of the standard compounds and their positions along the thin-layer chromatography plate (R²  = 0.65). This approach was applied to analyze perfume compounds in the sample with strong matrix interference from the synthetic agarwood. Terpene hydrocarbons (woody-based odors), ketones/esters, aldehydes, ethers, and alcohols were mostly observed at 8 ± 1, 6 ± 1, 5 ± 2, 4 ± 2, and 3 ± 2 cm, respectively, from the bottom of the thin-layer chromatography plate. While, the conventional solid phase microextraction/gas chromatography-mass spectrometry analysis of this sample solution revealed only 62 compounds (including 35 perfume compounds), the four-piece approach resulted in 109 compounds (62 perfume compounds). Furthermore, the capability of thin-layer chromatography-Gas Analyzer approach to analyze the isomers in this complex sample was demonstrated.