A molecule-like PtAu24(SC6H13)18 nanocluster as an electrocatalyst for hydrogen production

Alternating Array Stacking of Ag26 Au and Ag24 Au Nanoclusters

An assembly strategy for metal nanoclusters using electrostatic interactions with weak interactions, such as C-H⋅⋅⋅π and π⋅⋅⋅π interactions in which cationic [Ag₂₆ Au(2-EBT)₁₈ (PPh₃ )₆ ]⁺ and anionic [Ag₂₄ Au(2-EBT)₁₈ ]^- nanoclusters gather and assemble in an unusual alternating array stacking structure is presented. [Ag₂₆ Au(2-EBT)₁₈ (PPh₃ )₆ ]⁺ [Ag₂₄ Au(2-EBT)₁₈ ]^- is a new compound type, a double nanocluster ion compound (DNIC). A single nanocluster ion compound (SNIC) [PPh₄ ]⁺ [Ag₂₄ Au(2-EBT)₁₈ ]^- was also synthesized, having a k-vector-differential crystallographic arrangement. [PPh₄ ]⁺ [Ag₂₄ Au(2,4-DMBT)₁₈ ]^- adopts a different assembly mode from both [Ag₂₆ Au(2-EBT)₁₈ (PPh₃ )₆ ]⁺ [Ag₂₄ Au(2-EBT)₁₈ ]^- and [PPh₄ ]⁺ [Ag₂₄ Au(2-EBT)₁₈ ]^- . Thus, the striking packing differences of [Ag₂₆ Au(2-EBT)₁₈ (PPh₃ )₆ ]⁺ [Ag₂₄ Au(2-EBT)₁₈ ]^- , [PPh₄ ]⁺ [Ag₂₄ Au(2-EBT)₁₈ ]^- and the existing [PPh₄ ]⁺ [Ag₂₄ Au(2,4-DMBT)₁₈ ]^- from each other indicate the notable influence of ligands and counterions on the self-assembly of nanoclusters.

Reversible Switching of Catalytic Activity by Shuttling an Atom into and out of Gold Nanoclusters

It is challenging to control the catalyst activation and deactivation by removal and addition of only one central atom, as it is almost impossible to precisely abstract an atom from a conventional catalyst and analyze its catalysis. Here we report that the loss of one central atom in Au₂₅ (resulting in Au₂₄ ) enhances the catalytic activity in the oxidation of methane compared to the original Au₂₅ . More importantly, the activity can be readily switched through shuttling the central atom into Au₂₄ and out of Au₂₅ . This work will serve as a starting point for design rules on how to control catalytic performance of a catalyst by an atom alteration.

Aggregation induced non-emissive-to-emissive switching of molecular platinum clusters

We show here for the first time the Aggregation Induced Emission (AIE) mechanism and solvatochromic impact on Pt-SG (SG-deprotonated glutathione) nanoclusters. In this work, the AIE properties of Pt-SG clusters were investigated through computational and spectroscopic investigations. Computational data established that aggregation triggers a distinct change in the frontier molecular orbitals (FMOs) from metal d-orbital centered FMOs in the monomer to metal-thiolate and thiolate centered FMOs in the dimer improving the radiative decay process. Solvent dependent photoluminescence studies proved that a Lewis-acidic environment can significantly perturb the metal-thiolate and thiolate centered FMOs that are involved in the electronic transitions as predicted by our computational work. These semiconducting clusters exhibit a large Stokes shift and zero spectral overlap between absorption and emission which makes this Pt-SG cluster an excellent material for solar concentrators and solid-state light emitters. This AIE-OFF-ON emission was utilized to delineate a proof-of-concept sensor device that is sensitive to temperature and an acid/base.

Atomically Tailored Gold Nanoclusters for Catalytic Application

Recent advances in the synthetic chemistry of atomically precise metal nanoclusters (NCs) have significantly broadened the accessible sizes and structures. Such particles are well defined and have intriguing properties, thus, they are attractive for catalysis. Especially, those NCs with identical size but different core (or surface) structure provide unique opportunities that allow the specific role of the core and the surface to be mapped out without complication by the size effect. Herein, we summarize recent work with isomeric Au_n NCs protected by ligands and isostructural NCs but with different surface ligands. The highlighted work includes catalysis by spherical and rod-shaped Au₂₅ (with different ligands), quasi-isomeric Au₂₈ (SR)₂₀ with different R groups, structural isomers of Au₃₈ (SR)₂₄ (with identical R) and Au₃₈ S₂ (SR)₂₀ with body-centred cubic (bcc) structure, and isostructural [Au₃₈ L₂₀ (PPh₃ )₄ ]²⁺ (different L). These isomeric and/or isostructural NCs have provided valuable insights into the respective roles of the kernel, surface staples, and the type of ligands on catalysis. Future studies will lead to fundamental advances and development of tailor-made catalysts.

Electrochemistry of Atomically Precise Metal Nanoclusters

Thiolate-protected metal nanoparticles containing a few to few hundred metal atoms are interesting materials exhibiting unique physicochemical properties. They encompass the bulk-to-molecule transition region, where discrete electronic states emerge and electronic band energetics yield to quantum confinement effects. Recent progresses in the synthesis and characterization of ultrasmall gold nanoparticles have opened up new avenues for the isolation of extremely monodispersed nanoparticles with atomically precision. These nanoparticles are also called nanoclusters to distinguish them from other regular metal nanoparticles with core diameter >2 nm. These nanoclusters are typically identified by their actual molecular formulas; prominent among these are Au₂₅(SR)₁₈, Au₃₈(SR)₂₄, and Au₁₀₂(SR)₄₄, where SR is organothiolate. A number of single crystal structures of these nanoclusters have been disclosed. Researchers have effectively utilized density functional theory (DFT) calculations to predict their atomic and electronic structures, as well as their physicochemical properties. The atomically precise metal nanoclusters have been the focus of recent studies owing to their novel size-specific electrochemical, optical, and catalytic properties. In this Account, we highlight recent advances in electrochemistry of atomically precise metal nanoclusters and their applications in electrocatalysis and electrochemical sensing. Compared with gold nanoclusters, much less progress has been made in the electrochemical studies of other metal nanoclusters, and thus, we mainly focus on the electrochemistry and electrochemical applications of gold-based nanoclusters. Voltammetry has been extremely powerful in investigating the electronic structure of metal nanoclusters, especially near HOMO and LUMO levels. A sizable opening of HOMO-LUMO gap observed for Au₂₅(SR)₁₈ gradually decreases with increasing nanocluster size, which is in line with the change in the optical gap. Heteroatom-doping has been a powerful strategy to modify the optical and electrochemical properties of metal nanoclusters at the atomic level. While the superatom theory predicts 8-electron configuration for [Au₂₅(SR)₁₈]^- and many doped nanoclusters thereof, Pt- and Pd-doped [PtAu₂₄(SR)₁₈]⁰ and [PdAu₂₄(SR)₁₈]⁰ nanoclusters show dramatically different electronic structures, as manifested in their optical spectra and voltammograms, suggesting the occurrence of the Jahn-Teller distortion in these doped nanoclusters. Furthermore, metal-doping may alter their surface binding properties, as well as redox potentials. Metal nanoclusters offer great potential for attaining high activity and selectivity in their electrocatalytic applications. The well-defined core-shell structure of a metal nanocluster is of special advantage because the core and shell can be independently engineered to exhibit suitable binding properties and redox potentials. We discuss recent progress made in electrocatalysis based upon metal nanoclusters tailored for water splitting, CO₂ conversion, and electrochemical sensing. A well-defined model nanocatalyst is absolutely necessary to reveal the detailed mechanism of electrocatalysis and thereby to lead to the development of a new efficient electrocatalyst. We envision that atomically controlled metal nanoclusters will enable us to systematically optimize the electrochemical and surface properties suitable for electrocatalysis, thus providing a powerful platform for the discovery of finely tuned nanocatalysts.

Reaction mechanisms at the homogeneous–heterogeneous frontier: insights from first-principles studies on ligand-decorated metal nanoparticles

The frontiers between homogeneous and heterogeneous catalysis are progressively disappearing.

Water-soluble metal nanoclusters: recent advances in molecular-level exploration and biomedical applications

Recent advances of water-soluble metal nanoclusters (MNCs) in designing highly luminescent MNCs, ligand shell engineering, tracking MNC's growth processes, and biomedical applications are highlighted.

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.

Intrinsically fluorescent gold nanoclusters stabilized within a copper storage protein that follow the Irving–Williams trend in metal ion sensing

A copper storage protein is used to synthesize gold clusters with tunable emission that follow the Irving–Williams series for metal detection.

Tailoring the photoluminescence of atomically precise nanoclusters

Due to their atomically precise structures and intriguing chemical/physical properties, metal nanoclusters are an emerging class of modular nanomaterials. Photo-luminescence (PL) is one of their most fascinating properties, due to the plethora of promising PL-based applications, such as chemical sensing, bio-imaging, cell labeling, phototherapy, drug delivery, and so on. However, the PL of most current nanoclusters is still unsatisfactory-the PL quantum yield (QY) is relatively low (generally lower than 20%), the emission lifetimes are generally in the nanosecond range, and the emitted color is always red (emission wavelengths of above 630 nm). To address these shortcomings, several strategies have been adopted, and are reviewed herein: capped-ligand engineering, metallic kernel alloying, aggregation-induced emission, self-assembly of nanocluster building blocks into cluster-based networks, and adjustments on external environment factors. We further review promising applications of these fluorescent nanoclusters, with particular focus on their potential to impact the fields of chemical sensing, bio-imaging, and bio-labeling. Finally, scope for improvements and future perspectives of these novel nanomaterials are highlighted as well. Our intended audience is the broader scientific community interested in the fluorescence of metal nanoclusters, and our review hopefully opens up new horizons for these scientists to manipulate PL properties of nanoclusters. This review is based on publications available up to December 2018.

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.

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.

Silver Doping-Induced Luminescence Enhancement and Red-Shift of Gold Nanoclusters with Aggregation-Induced Emission

A deep understanding on the luminescence property of aggregation-induced emission (AIE) featured metal nanoclusters (NCs) is highly desired. This paper reports a systematic study on enhancing the luminescence of AIE-featured Au NCs, which is achieved by Ag doping to engineer the size/structure and aggregation states of the Au^I -thiolate motifs in the NC shell. Moreover, by prolonging the reaction time, the luminescence of the as-synthesized AuAg NCs could be further tailored from orange to red, which is also due to the variation of the Au^I -thiolate motifs of NCs. This study can facilitate a better understanding of this AIE-featured luminescent probe and the design of other synthetic routes for this rising family of functional materials.

Insights into Interfaces, Stability, Electronic Properties, and Catalytic Activities of Atomically Precise Metal Nanoclusters from First Principles

Atomically precise, ligand-protected metal nanoclusters are of great interest for their well-defined structures, intriguing physicochemical properties, and potential applications in catalysis, biology, and nanotechnology. Their structure precision provides many opportunities to correlate their geometries, stability, electronic properties, and catalytic activities by closely integrating theory and experiment. In this Account, we highlight recent theoretical advances from our efforts to understand the metal-ligand interfaces, the energy landscape, the electronic structure and optical absorption, and the catalytic applications of atomically precise metal nanoclusters. We mainly focus on gold nanoclusters. The bonding motifs and energetics at the gold-ligand interfaces are two main interests from a computational perspective. For the gold-thiolate interface, the -RS-Au-SR- staple motif is not always preferred; in fact, the bridging motif (-SR-) is preferred at the more open facets such as Au(100) and Au(110). This finding helps understand the diversity of the gold-thiolate motifs for different core geometries and sizes. A great similarity is demonstrated between gold-thiolate and gold-alkynyl interfaces, regarding formation of the staple-type motifs with PhC≡C- as an example. In addition, N-heterocyclic carbenes (NHCs) without bulky groups also form the staple-type motif. Alkynyls and bulky NHCs have the strongest binding with the gold surface from comparing 27 ligands of six types, suggesting a potential to synthesize NHC-protected gold clusters. The energy landscape of nanosystems is usually complex, but experimental progress in synthesizing clusters of the same Au-S composition with different R groups and isomers of the same Au _n(SR) _m formula have made detailed theoretical analyses of energetic contributions possible. Ligand-ligand interactions turn out to play an important role in the cluster stability, while metastable isomers can be obtained via kinetic control. Although the superatom-complex theory is the starting point to understand the electronic structure of atomically precise gold clusters, other factors also greatly affect the orbital levels that manifest themselves in the experimental optical absorption spectra. For example, spin-orbit coupling needs to be included to reproduce the splitting of the HOMO-LUMO transition observed experimentally for Au₂₅(SR)₁₈^-, the poster child of the family. In addition, doping can lead to structural changes and charge states that do not follow the superatomic electron count. Atomically precise metal nanoclusters are an ideal system for understanding nanocatalysis due to their well-defined structures. Active sites and catalytic mechanisms are explored for selective hydrogenation and hydrogen evolution on thiolate-protected gold nanoclusters with and without dopants. The behavior of H in nanogold is analyzed in detail, and the most promising site to attract H is found to be coordinately unsaturated Au atoms. Many insights have been gained from first-principles studies of atomically precise, ligand-protected gold nanoclusters. Interesting and important questions remaining to be addressed are pointed out in the end.

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.

Hydrogen evolution reaction (HER) on Au@Ag ultrananoclusters as electro-catalysts

We propose and investigate computationally Ag-Au subnanometer clusters as catalysts for the hydrogen evolution reaction (HER). Focusing on Ag12Au, we conduct a complete first-principles study of the HER process on this Au@Ag ultrananocatalyst. After determining the hydrogen-saturated resting state under standard conditions as Ag12AuH11, HER reaction energies and barriers are predicted also including solvent effects using both implicit and explicit models. We find that Ag12Au is a good candidate as a HER catalyst, with good stability and an overall reaction energy barrier of 0.89 eV as an upper bound. We also draw indications for the design of HER subnanometer catalysts.

Ligand Structure Determines Nanoparticles' Atomic Structure, Metal-Ligand Interface and Properties

The nature of the ligands dictates the composition, molecular formulae, atomic structure and the physical properties of thiolate protected gold nanomolecules, Aun(SR)m. In this review, we describe the ligand effect for three classes of thiols namely, aliphatic, AL or aliphatic-like, aromatic, AR, or bulky, BU thiol ligands. The ligand effect is demonstrated using three experimental setups namely: (1) The nanomolecule series obtained by direct synthesis using AL, AR, and BU ligands; (2) Molecular conversion and interconversion between Au38(S-AL)24, Au36(S-AR)24, and Au30(S-BU)18 nanomolecules; and (3) Synthesis of Au38, Au36, and Au30 nanomolecules from one precursor Aun(S-glutathione)m upon reacting with AL, AR, and BU ligands. These nanomolecules possess unique geometric core structure, metal-ligand staple interface, optical and electrochemical properties. The results unequivocally demonstrate that the ligand structure determines the nanomolecules' atomic structure, metal-ligand interface and properties. The direct synthesis approach reveals that AL, AR, and BU ligands form nanomolecules with unique atomic structure and composition. Similarly, the nature of the ligand plays a pivotal role and has a significant impact on the passivated systems such as metal nanoparticles, quantum dots, magnetic nanoparticles and self-assembled monolayers (SAMs). Computational analysis demonstrates and predicts the thermodynamic stability of gold nanomolecules and the importance of ligand-ligand interactions that clearly stands out as a determining factor, especially for species with AL ligands such as Au38(S-AL)24.

Toward Total Synthesis of Thiolate-Protected Metal Nanoclusters

Total synthesis, where desired organic- and/or biomolecules could be produced from simple precursors at atomic precision and with known step-by-step reactions, has prompted centuries-lasting bloom of organic chemistry since its conceptualization in 1828 (Wöhler synthesis of urea). Such expressive science is also highly desirable in nanoscience, since it represents a decisive step toward atom-by-atom customization of nanomaterials for basic and applied research. Although total synthesis chemistry is less established in nanoscience, recent years have witnessed seminal advances and increasing research efforts devoted into this field. In this Account, we discuss recent progress on introducing and developing total synthesis routes and mechanisms for atomically precise metal nanoclusters (NCs). Due to their molecular-like formula and properties (e.g., HOMO-LUMO transition, strong luminescence and stereochemical activity), atomically precise metal NCs could be regarded as "molecular metals", holding potential applications in various practical sectors such as biomedicine, energy, catalysis, and many others. More importantly, the molecular-like properties of metal NCs are sensitively dictated by their size and composition, suggesting total synthesis of them as an indispensable basis for reliably realizing their practical applications. Atomically precise thiolate-protected Au, Ag and their alloy NCs are employed as model NCs to exemplify design strategies and governing principles in total synthesis of inorganic nanoparticles. This Account starts with a brief summary of total synthesis methodologies of atomically precise metal NCs. Following the methodological summary is a detailed discussion on the mechanisms governing these synthetic strategies, which is the main focus of this Account. Based on unprecedented precision (at atomic resolution) and ease (ensured by size-dependent properties) of tracking clusters' size/structure changes, mechanisms driving growth (e.g., reduction growth and seeded growth) and functionalization (e.g., alloying reaction and ligand exchange) of metal NCs have been explored at molecular level. With definitive step-by-step reaction routes, two-electron (2 e^-) reduction (driving the growth reactions) and surface motif exchange (SME, prompting alloying and ligand exchange reactions) are discussed in depth and details. In addition to those sub- and/or individual-cluster level understandings, the self-assembly chemistry delivering high orderliness and enhanced materials performance in NC assemblies/supercrystals is also deciphered. This Account is then concluded with our perspectives toward potential development of cluster chemistry. Advances in total synthesis chemistry of metal NCs could not only serve as guidelines for future synthetic practice of NCs, but also provide molecular-level clues for many pending fundamental puzzles in nanochemistry, including nucleation growth, alloying chemistry, surface engineering and evolution of metamaterials.

Carbon nitride supported Fe2 cluster catalysts with superior performance for alkene epoxidation

Sub-nano metal clusters often exhibit unique and unexpected properties, which make them particularly attractive as catalysts. Herein, we report a "precursor-preselected" wet-chemistry strategy to synthesize highly dispersed Fe2 clusters that are supported on mesoporous carbon nitride (mpg-C3N4). The obtained Fe2/mpg-C3N4 sample exhibits superior catalytic performance for the epoxidation of trans-stilbene to trans-stilbene oxide, showing outstanding selectivity of 93% at high conversion of 91%. Molecular oxygen is the only oxidant and no aldehyde is used as co-reagent. Under the same condition, by contrast, iron porphyrin, single-atom Fe, and small Fe nanoparticles (ca. 3 nm) are nearly reactively inert. First-principles calculations reveal that the unique reactivity of the Fe2 clusters originates from the formation of active oxygen species. The general applicability of the synthesis approach is further demonstrated by producing other diatomic clusters like Pd2 and Ir2, which lays the foundation for discovering diatomic cluster catalysts.

Au25(SR)18: the captain of the great nanocluster ship

Noble metal nanoclusters are in the intermediate state between discrete atoms and plasmonic nanoparticles and are of significance due to their atomically accurate structures, intriguing properties, and great potential for applications in various fields. In addition, the size-dependent properties of nanoclusters construct a platform for thoroughly researching the structure (composition)-property correlations, which is favorable for obtaining novel nanomaterials with enhanced physicochemical properties. Thus far, more than 100 species of nanoclusters (mono-metallic Au or Ag nanoclusters, and bi- or tri-metallic alloy nanoclusters) with crystal structures have been reported. Among these nanoclusters, Au25(SR)18-the brightest molecular star in the nanocluster field-is capable of revealing the past developments and prospecting the future of the nanoclusters. Since being successfully synthesized (in 1998, with a 20-year history) and structurally determined (in 2008, with a 10-year history), Au25(SR)18 has stimulated the interest of chemists as well as material scientists, due to the early discovery, easy preparation, high stability, and easy functionalization and application of this molecular star. In this review, the preparation methods, crystal structures, physicochemical properties, and practical applications of Au25(SR)18 are summarized. The properties of Au25(SR)18 range from optics and chirality to magnetism and electrochemistry, and the property-oriented applications include catalysis, chemical imaging, sensing, biological labeling, biomedicine and beyond. Furthermore, the research progress on the Ag-based M25(SR)18 counterpart (i.e., Ag25(SR)18) is included in this review due to its homologous composition, construction and optical absorption to its gold-counterpart Au25(SR)18. Moreover, the alloying methods, metal-exchange sites and property alternations based on the templated Au25(SR)18 are highlighted. Finally, some perspectives and challenges for the future research of the Au25(SR)18 nanocluster are proposed (also holding true for all members in the nanocluster field). This review is directed toward the broader scientific community interested in the metal nanocluster field, and hopefully opens up new horizons for scientists studying nanomaterials. This review is based on the publications available up to March 2018.

Revealing isoelectronic size conversion dynamics of metal nanoclusters by a noncrystallization approach

Atom-by-atom engineering of nanomaterials requires atomic-level knowledge of the size evolution mechanism of nanoparticles, which remains one of the greatest mysteries in nanochemistry. Here we reveal atomic-level dynamics of size evolution reaction of molecular-like nanoparticles, i.e., nanoclusters (NCs) by delicate mass spectrometry (MS) analyses. The model size-conversion reaction is [Au₂₃(SR)₁₆]⁻ → [Au₂₅(SR)₁₈]⁻ (SR = thiolate ligand). We demonstrate that such isoelectronic (valence electron count is 8 in both NCs) size-conversion occurs by a surface-motif-exchange-induced symmetry-breaking core structure transformation mechanism, surfacing as a definitive reaction of [Au₂₃(SR)₁₆]⁻ + 2 [Au₂(SR)₃]⁻ → [Au₂₅(SR)₁₈]⁻ + 2 [Au(SR)₂]⁻. The detailed tandem MS analyses further suggest the bond susceptibility hierarchies in feed and final Au NCs, shedding mechanistic light on cluster reaction dynamics at atomic level. The MS-based mechanistic approach developed in this study also opens a complementary avenue to X-ray crystallography to reveal size evolution kinetics and dynamics.

How metal nanoclusters evolve in size is poorly understood, particularly at the atomic level. Here, the authors use mass spectrometry to study the size conversion dynamics between two isoelectronic gold nanoclusters with atomic resolution, revealing that the growth reaction proceeds through a distinct balanced equation.

Electrophoretic Mechanism of Au25(SR)18 Heating in Radiofrequency Fields

Gold nanoparticles in radiofrequency (RF) fields have been observed to heat. There is some debate over the mechanism of heating. Au₂₅(SR)₁₈ in RF is studied for the mechanistic insights obtainable from precise synthetic control over exact charge, size, and spin for this nanoparticle. An electrophoretic mechanism can adequately account for the observed heat. This study adds a new level of understanding to gold particle heating experiments, allowing for the first time a conclusive connection between theoretical and experimentally observed heating rates.

Kernel Tuning and Nonuniform Influence on Optical and Electrochemical Gaps of Bimetal Nanoclusters

Fine tuning nanoparticles with atomic precision is exciting and challenging and is critical for tuning the properties, understanding the structure-property correlation and determining the practical applications of nanoparticles. Some ultrasmall thiolated metal nanoparticles (metal nanoclusters) have been shown to be precisely doped, and even the protecting staple metal atom could be precisely reduced. However, the precise addition or reduction of the kernel atom while the other metal atoms in the nanocluster remain the same has not been successful until now, to the best of our knowledge. Here, by carefully selecting the protecting ligand with adequate steric hindrance, we synthesized a novel nanocluster in which the kernel can be regarded as that formed by the addition of two silver atoms to both ends of the Pt@Ag₁₂ icosohedral kernel of the Ag₂₄Pt(SR)₁₈ (SR: thiolate) nanocluster, as revealed by single crystal X-ray crystallography. Interestingly, compared with the previously reported Ag₂₄Pt(SR)₁₈ nanocluster, the as-obtained novel bimetal nanocluster exhibits a similar absorption but a different electrochemical gap. One possible explanation for this result is that the kernel tuning does not essentially change the electronic structure, but obviously influences the charge on the Pt@Ag₁₂ kernel, as demonstrated by natural population analysis, thus possibly resulting in the large electrochemical gap difference between the two nanoclusters. This work not only provides a novel strategy to tune metal nanoclusters but also reveals that the kernel change does not necessarily alter the optical and electrochemical gaps in a uniform manner, which has important implications for the structure-property correlation of nanoparticles.

Dopant-Dependent Electronic Structures Observed for M2Au36(SC6H13)24 Clusters (M = Pt, Pd)

Heteroatom doping is a powerful means to tune the optical and electronic properties of gold clusters at the atomic level. We herein report that doping a Au₃₈ cluster with Pt and Pd atoms leads to core-doped [Pt₂Au₃₆(SC₆H₁₃)₂₄]^2- and [Pd₂Au₃₆(SC₆H₁₃)₂₄]⁰, respectively. Voltammetric investigations show that these clusters exhibit drastically different electronic structures; whereas the HOMO-LUMO gap of [Pt₂Au₃₆(SC₆H₁₃)₂₄]^2- is found to be 0.95 V, that of [Pd₂Au₃₆(SC₆H₁₃)₂₄]⁰ is drastically decreased to 0.26 V, suggesting Jahn-Teller distortion of the 12-electron cluster. Density functional investigations confirm that the HOMO-LUMO gap of the Pd-doped cluster is indeed reduced. Analysis of the optimized geometry for the 12-electron [Pd₂Au₃₆(SC₆H₁₃)₂₄]⁰ reveals that the rod-like M₂Au₂₁ core becomes more flattened upon Pd-doping. Reversible geometrical interconversion between [Pt₂Au₃₆(SC₆H₁₃)₂₄]⁰ and [Pt₂Au₃₆(SC₆H₁₃)₂₄]^2- is clearly demonstrated by manipulating the oxidation state of the cluster.

Design of atomically precise Au2Pd6nanoclusters for boosting electrocatalytic hydrogen evolution on MoS2

A new Au–Pd alloy nanocluster (NC) – Au₂Pd₆S₄(PPh₃)₄(C₆H₄F₂S)₆is synthesized. The NC is applied to enhance the electrocatalytic HER activity of MoS₂compared with a single Pd or Au component.

Enhanced hydrogen evolution reaction activity of hydrogen-annealed vertical MoS2 nanosheets

Vertical configuration and H₂ annealing enhanced the hydrogen evolution reaction activity of MoS₂ nanosheets.

Controlling Ag-doping in [AgxAu25-x(SC6H11)18]- nanoclusters: cryogenic optical, electronic and electrocatalytic properties

Doping metal nanoclusters with a second type of metal is a powerful method for tuning the physicochemical properties of nanoclusters at the atomic level and it also provides opportunities for a fundamental understanding of alloying rules as well as new applications. Herein, we have devised a new, one-phase strategy for achieving heavy Ag-doping in Au₂₅(SR)₁₈ nanoclusters. This strategy overcomes the light doping of silver by previous methods. X-ray crystallography together with ESI-MS determined the composition of the product to be [Ag_xAu_25-x(SC₆H₁₁)₁₈]^- with x ∼ 21. Cryogenic optical spectroscopy (80-300 K) revealed fine features in optical absorption peaks. Interestingly, the heavy doping of silver does not significantly change the electron-phonon coupling strength and the surface phonon frequency. DFT simulations reproduced the experimentally observed trend of electronic structure evolution with Ag doping. We further investigated the electrocatalytic performance of such heavily Ag-doped nanoclusters for oxygen reduction in alkaline solutions. The mass activity of ligand-off [Ag_xAu_25-x(SC₆H₁₁)₁₈]^- nanoclusters (217.4 A g^-1_metal) was determined to be higher than that of ligand-on nanoclusters (29.6 A g^-1_metal) at a potential of -0.3 V (vs. Ag/AgCl). The rotating disk electrode (RDE) studies revealed the tunable kinetic features of the nanoclusters by ligand removal.

Two-Way Transformation between fcc- and Nonfcc-Structured Gold Nanoclusters

Precisely tuning the structure of nanomaterials, especially in a two-way style, is challenging but of great importance for regulating properties and for practical applications. The structural transformation from nonfcc to fcc (face center cubic) in gold nanoclusters has been recently reported; however, the reverse process, that is, the structural transformation from fcc to nonfcc, not to mention the two-way structural transformation between fcc and nonfcc, remains unknown. We developed a novel synthesis method, successfully fulfilled the two-way structure transformation, and studied the stability of gold nanoclusters with different structures. Additionally, a novel gold nanocluster was synthesized and structurally resolved by single-crystal X-ray crystallography. This work has important implications for structure and property tuning of gold nanoclusters and might open up some new potential applications for gold nanoclusters.

Atomically Precise Gold Nanoclusters Accelerate Hydrogen Evolution over MoS2 Nanosheets: The Dual Interfacial Effect

Hydrogen generation via electrocatalytic water splitting holds great promise for future energy revolution. It is desirable to design abundant and efficient catalysts and achieve mechanistic understanding of hydrogen evolution reaction (HER). Here, this paper reports a strategy for improving HER performance of molybdenum disulfide (MoS₂ ) via introducing gold nanoclusters as a cocatalyst. Compared to plain MoS₂ nanosheets, the Au₂₅ (SR)₁₈ /MoS₂ nanocomposite exhibits enhanced HER activity with a small onset potential of -0.20 V (vs reversible hydrogen electrode) and a higher current density of 59.3 mA cm^-2 at the potential of -0.4 V. In addition to the interfacial interaction between nanoclusters and MoS₂ , the interface between the Au₂₅ core and the surface ligands (thiolate vs selenolate) is also discovered to distinctly affect the catalytic performance. This work highlights the promise of metal nanoclusters in boosting the HER performance via tailoring the interfacial electronic interactions between gold nanoclusters and MoS₂ nanosheets, as well as the interface between metal core and surface ligands.

Synthesis of Au38(SCH2CH2Ph)24, Au36(SPh-tBu)24, and Au30(S-tBu)18 Nanomolecules from a Common Precursor Mixture

Phenylethanethiol protected nanomolecules such as Au₂₅, Au₃₈, and Au₁₄₄ are widely studied by a broad range of scientists in the community, owing primarily to the availability of simple synthetic protocols. However, synthetic methods are not available for other ligands, such as aromatic thiol and bulky ligands, impeding progress. Here we report the facile synthesis of three distinct nanomolecules, Au₃₈(SCH₂CH₂Ph)₂₄, Au₃₆(SPh-tBu)₂₄, and Au₃₀(S-tBu)₁₈, exclusively, starting from a common Au_n(glutathione)_m (where n and m are number of gold atoms and glutathiolate ligands) starting material upon reaction with HSCH₂CH₂Ph, HSPh-tBu, and HStBu, respectively. The systematic synthetic approach involves two steps: (i) synthesis of kinetically controlled Au_n(glutathione)_m crude nanocluster mixture with 1:4 gold to thiol molar ratio and (ii) thermochemical treatment of the purified nanocluster mixture with excess thiols to obtain thermodynamically stable nanomolecules. Thermochemical reactions with physicochemically different ligands formed highly monodispersed, exclusively three different core-size nanomolecules, suggesting a ligand induced core-size conversion and structural transformation. The purpose of this work is to make available a facile and simple synthetic method for the preparation of Au₃₈(SCH₂CH₂Ph)₂₄, Au₃₆(SPh-tBu)₂₄, and Au₃₀(S-tBu)₁₈, to nonspecialists and the broader scientific community. The central idea of simple synthetic method was demonstrated with other ligand systems such as cyclopentanethiol (HSC₅H₉), cyclohexanethiol(HSC₆H₁₁), para-methylbenzenethiol(pMBT), 1-pentanethiol(HSC₅H₁₁), 1-hexanethiol(HSC₆H₁₃), where Au₃₆(SC₅H₉)₂₄, Au₃₆(SC₆H₁₁)₂₄, Au₃₆(pMBT)₂₄, Au₃₈(SC₅H₁₁)₂₄, and Au₃₈(SC₆H₁₃)₂₄ were obtained, respectively.

Energy Gap Law for Exciton Dynamics in Gold Cluster Molecules

The energy gap law relates the nonradiative decay rate to the energy gap separating the ground and excited states. Here we report that the energy gap law can be applied to exciton dynamics in gold cluster molecules. Size-dependent electrochemical and optical properties were investigated for a series of n-hexanethiolate-protected gold clusters ranging from Au₂₅ to Au₃₃₃. Voltammetric studies reveal that the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps of these clusters decrease with increasing cluster size. Combined femtosecond and nanosecond time-resolved transient absorption measurements show that the exciton lifetimes decrease with increasing cluster size. Comparison of the size-dependent exciton lifetimes with the HOMO-LUMO gaps shows that they are linearly correlated, demonstrating the energy gap law for excitons in these gold cluster molecules.

Anion photoelectron spectroscopy of free [Au25(SC12H25)18]. NANOSCALE 2017;9:13409-13412. [PMID: 28861577 DOI: 10.1039/c7nr04641c]

Previous theoretical studies have shown that the thiolated gold cluster compound [Au₂₅(SR)₁₈]^- can be viewed as a prototypical superatom with a closed electronic structure. The quantized electronic structure of [Au₂₅(SR)₁₈]^- has been experimentally demonstrated by optical and electrochemical methods in the dispersed state. Nevertheless, no direct information is available on the energy levels and densities of occupied states. Here, we report the photoelectron spectrum of [Au₂₅(SC₁₂H₂₅)₁₈]^- isolated under vacuum for the first time. The spectrum exhibits two distinct peaks, corresponding to electron detachment from the superatomic 1P orbitals and Au 5d orbitals of the Au₁₃ core. The adiabatic electron affinity of [Au₂₅(SC₁₂H₂₅)₁₈]⁰ was experimentally determined to be 2.2 eV, which is significantly smaller than that of [Au₂₅(SCH₃)₁₈]⁰ predicted theoretically.