Synthesis, Stability, and Photoluminescence Properties of PdAu10(PPh3)8Cl2 Clusters

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.

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...

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.

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.

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.

The synthesis and structure of the [PdAu13(PPh3)3(SR)7]+ nanocluster

Metal alloy nanoclusters have attracted increasing attention due to the synergistic effect of the foreign atoms. For the first time the synthesis and crystal structure of the [PdAu13(PPh3)3(SR)7]+ nanocluster is reported. The crystal structure of the nanocluster was determined by single crystal X-ray diffraction. The [PdAu13(PPh3)3(SR)7]+ nanocluster has a concave polyhedron Au9Pd kernel, which looks like a girl dancing ballet. The structure shows that [PdAu13(PPh3)3(SR)7]+ has an open shell. Meanwhile, we also carried out ultraviolet-visible (Uv-vis) absorption spectroscopy and fluorescence spectroscopy to study the optical properties of the [PdAu13(PPh3)3(SR)7]+ nanocluster.

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.

Tailoring the stability, photocatalysis and photoluminescence properties of Au11 nanoclusters via doping engineering

Dopants in gold nanoclusters have been proved to mediate the intrinsic electronic properties of homo-clusters. In this work, we report the precise synthesis of atomically precise Au₈Ag₃(PPh₃)₇Cl₃ alloy nanoclusters with multiple Ag dopants for the first time. Their structure was resolved by single-crystal X-ray crystallography. Au₈Ag₃(PPh₃)₇Cl₃ nanoclusters possessed a similar structure topology to the well-known Au₁₁(PPh₃)₇Cl₃ nanoclusters. It is observed that the three Ag atoms were fixed at the cluster surface and bound selectively with the chlorine ligands in a C₃-axis manner. The alloy nanoclusters exhibited a closed-shell electronic structure (i.e., 8(Au 6s¹) + 3(Ag 5s¹) - 3(Cl) = 8e), as evidenced by electrospray ionization-mass spectrometry (ESI-MS). The photothermodynamic stability of alloy clusters was remarkably improved (e.g., full decomposition after 7 days under sunlight irradiation vs. 3 days for Au₁₁(PPh₃)₇Cl₃ clusters). DFT calculations indicated that the Ag dopants in a C₃-axis manner could obviously delocalize the electrons of Au to the orbitals of P atoms and then mediate the electronic property of the clusters. Shrinkage of the HOMO-LUMO gap to 1.67 eV of Au₈Ag₃(PPh₃)₇Cl₃ was observed as compared with that of homo-nanoclusters of Au₁₁(PPh₃)₇Cl₃ (2.06 eV). The electrochemical gap of Au₈Ag₃(PPh₃)₇Cl₃ alloy nanoclusters was 1.272 V, which was higher than that of Au₁₁(PPh₃)₇Cl₃ nanoclusters, which indicated higher electrochemical stability, as evidenced by the differential pulse voltammetry (DPV) method. Au₈Ag₃(PPh₃)₇Cl₃ clusters exhibited three specific photoluminescence peaks at 405, 434 and 454 nm. AuAg alloy clusters exhibited twofold greater activity than homo gold clusters in the photooxidation of benzylamine, which was mainly due to the unique electronic properties of the alloy clusters. Controllable heteroatom doping engineering is a powerful method to tune the electronic properties of clusters, and then improve their photothermodynamic and electrochemical stability simultaneously for potential photocatalytic applications.

Synthesis of Trimetallic (HPd@M2Au8)3+ Superatoms (M = Ag, Cu) via Hydride-Mediated Regioselective Doping to (Pd@Au8)2

We have recently reported that hydride (H^-) doped superatom (HPd@Au₈)⁺ protected by eight PPh₃ ligands selectively grew into (HPd@Au₁₀)³⁺ by the nucleophilic addition of two Au(I)Cl units. In the present study, (HPd@Au₈)⁺ was successfully converted to unprecedented trimetallic (HPd@M₂Au₈)³⁺ superatoms (M = Ag, Cu) by controlled doping of two Ag(I)Cl or Cu(I)Cl units, respectively. Single-crystal X-ray diffraction analysis demonstrated that two Ag(I) or Cu(I) ions were regioselectively incorporated. Theoretical calculations suggested that hydrogens in (HPd@M₂Au₈)³⁺ (M = Au, Ag, Cu) occupy the same bridging site between the central Pd atom and the surface Au atom. (HPd@Ag₂Au₈)³⁺ exhibited photoluminescence at 775 nm, with the enhanced quantum yield of 0.09%, although it is structurally and electronically equivalent with (HPd@Au₁₀)³⁺. This study demonstrates that hydride-mediated growth process is a promising atomically-precise bottom-up synthetic method of new multimetallic superatoms.

Hydride Doping of Chemically Modified Gold-Based Superatoms

Atomically size-selected gold (Au) clusters protected by organic ligands or stabilized by polymers provide an ideal platform to test fundamental concepts and size-specific phenomena, such as the superatomic concept and metal-to-nonmetal transition. Recent studies revealed that these stabilized Au clusters take atomlike quantized electronic structures and can be viewed as chemically modified Au superatoms. An analogy between Au and hydrogen (H) atoms is an interesting proposal made for bare Au clusters: a Au atom at a low-coordination site of a Au cluster can be replaced with a H atom while retaining the structural motif and electronic structure. However, this proposal has not been experimentally proved in chemically modified Au superatoms while a recent theoretical study predicted the formation of [HAu₂₅(SR)₁₈]⁰ (RS = thiolate). This Account summarizes our recent studies on the interaction of hydride(s) with two types of chemically modified Au-based superatoms: (1) the Au cores of [Au₉(PPh₃)₈]³⁺ and [PdAu₈(PPh₃)₈]²⁺ formally described as (Au₉)³⁺ and (PdAu₈)²⁺, respectively, and (2) Au₃₄ cluster stabilized by poly( N-vinyl-2-pyrrolidone) (PVP). The (Au₉)³⁺ and (PdAu₈)²⁺ cores correspond to oblate-shaped superatoms with six electrons and a coordinatively unsaturated site at the center, whereas the Au₃₄ cluster in PVP is viewed as a nearly spherical superatom having a closed electronic structure with 34 electrons and multiple uncoordinated sites on the surface. Through this study, we aimed to deepen our understanding on the role of a hydride in the formation processes of Au superatoms, the effect of adsorbed hydride(s) on the electronic structure of Au superatoms, and the activity of adsorbed hydrogen species for hydrogenation catalysis. Mass spectrometry and nuclear magnetic resonance spectroscopy demonstrated that a single hydride (H^-) was selectively doped to (Au₉)³⁺ and (PdAu₈)²⁺ upon reactions with BH₄^- to form (HAu₉)²⁺ and (HPdAu₈)⁺, respectively. Density functional theory (DFT) calculations showed that (HAu₉)²⁺ and (HPdAu₈)⁺ were more spherical than the original superatoms and had a closed electronic structure with eight electrons. The hydride-doped (HAu₉)²⁺ was selectively converted to the well-known (Au₁₁)³⁺ by electrophilic addition of two Au(I) units whereas (HPdAu₈)⁺ was converted to a new hydride-doped (HPdAu₁₀)³⁺. A two-step mechanism was proposed for hydride-mediated growth of Au-based superatoms: closure of the electronic structures by adsorption of a hydride, followed by the addition of two Au(I) units. The selective formation of Au₃₄ superatoms in PVP is also explained by assuming that hydride-doped Au clusters with 34 electrons were involved as key intermediates. The Au₃₄ superatom exhibited the localized surface plasmon resonance (LSPR) band by reacting with BH₄^- due to the electron donation by multiply adsorbed hydrides. The LSPR band disappeared by exposing hydride-doped Au₃₄ to dissolved O₂, but reappeared by reaction with BH₄^-. Catalysis for hydrogenation of C═C bonds was generated by doping a single Pd or Rh atom to Au₃₄. The results reported here demonstrate that the hydride doped to chemically modified Au superatoms mimics Au^- in terms of electron count. The hydride-mediated growth processes observed will contribute to the development of an atomically precise, bottom-up method of synthesizing new artificial elements in a periodic table for nanoscale materials. The interaction of hydride(s) with Au superatoms will find application in hydrogenation catalysis and hydrogen sensing.

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.

Hydride-Mediated Controlled Growth of a Bimetallic (Pd@Au8)2+ Superatom to a Hydride-Doped (HPd@Au10)3+ Superatom

A hydride (H^-)-doped bimetallic superatom (HPdAu₈)⁺ was produced by reacting BH₄^- with an oblate (PdAu₈)²⁺ superatom protected by PPh₃. The H atom in (HPdAu₈)⁺ survived during the sequential addition of Au(I)Cl to form an (HPdAu₁₀)³⁺ superatom, in sharp contrast to the proton release from a H^--doped pure gold superatom (HAu₉)²⁺ in the growth process to (Au₁₁)³⁺. Single-crystal X-ray diffraction analysis and density functional theory calculations on (HPdAu₁₀)³⁺ showed that the interstitially doped H atom induced a notable deformation of the core.

Optical properties of nanoalloys

Optical absorption spectra of bare (left) and monolayer-protected (right) metal nanoalloys.

Isolation and Tandem Mass Spectrometric Identification of a Stable Monolayer Protected Silver-Palladium Alloy Cluster

A selenolate-protected Ag-Pd alloy cluster was synthesized using a one-pot solution-phase route. The crude product upon chromatographic analyses under optimized conditions gave three distinct clusters with unique optical features. One of these exhibits a molecular peak centered at m/z 2839, in its negative ion mass spectrum assigned to Ag5Pd4(SePh)12(-), having an exact match with the corresponding calculated spectrum. Tandem mass spectrometry of the molecular ion peak up to MS(9) was performed. Complex isotope distributions in each of the mass peaks confirmed the alloy composition. We find the Ag3Pd3(-) core to be highly stable. The composition was further supported by scanning electron microscopy, energy-dispersive spectroscopy, and X-ray photoelectron spectroscopy.