Computational and Experimental Elucidation of the Charge-Dependent Acid-Etching Dynamics and Electrocatalytic Performance of Au25(SR)18 q(q = -1, 0, +1) Nanoclusters

Using thiolate-protected Au25(SR)18 nanocluster (NC) with different charge states as the test candidate, how the charge effect affects the etching dynamics of thiolate ligands in acid and the electrocatalytic performance is explored. The ab initio molecular dynamics (AIMD) simulations revealed the charge-dependent reaction kinetics in acid, where the anionic and neutral Au25(SCH3)18 q (q = -1, 0) favorably react with the acid and partially remove the thiolate ligands via two-step proton attack, while the cationic Au25(SCH3)18 + NC is acid-resistant with no tendency for -SR removal. Density functional theory (DFT) calculations further predict that the dethiolated Au sites exhibit enhanced catalytic activity for CO2 electroreduction to CO, with the anionic Au25 - showing significantly superior activity. Acid etching and electrocatalytic experiments further confirmed partial removal of thiolate ligands in Au25(SCH3)18 q (q = -1, 0), with dethiolated Au25 NCs showing enhanced catalytic performance in CO2 electroreduction, particularly Au25 - exhibiting better activity than Au25 0. This work revealed an interesting charge state-mediated interface dynamics and electrocatalytic behaviors in Au25 NCs, which can be applied to modulate the interface and catalytic properties of other atomically precise metal nanoclusters.

Tuning Atomically Precise Gold Nanoclusters for Selective Electroreduction of CO2

The electroreduction of greenhouse gas CO2 into high-value-added chemicals using renewable electricity is a promising way to mitigate climate change and realize carbon cycling. Atomically precise thiolate-protected gold nanoclusters have shown great potential for selective electrochemical conversion of CO2 toward CO due to their quantum confinement effect and unique electronic structures. Additionally, the atomic precision of gold nanocluster is advantageous for investigating the CO2 reduction mechanism, which is typically challenging to understand due to the complexity of the catalytic interface, and unknown structure of the active site in more conventional catalysts. By summarizing CO2 reduction catalyzed by gold nanoclusters, we aim to identify key factors that contribute to the activity, selectivity, and stability of nanocluster catalysts, as well as elucidate the CO₂ reduction pathway, thereby contributing to the design of more active and selective nanocluster catalysts for CO2 reduction.

Remove the innermost atom of a magnetic multi-shell gold nanoparticle for near-unity conversion of CO2 to CO

Few reports on paramagnetic metal nanoparticles with atomic precision and their difficult tailoring retard the insightful investigation of metal nanoparticle paramagnetism. Herein, we introduced a thiol-iodine mixture ligand-protecting strategy to successfully synthesize multi-shell paramagnetic [Au127I4(TBBT)48 (I: iodine, TBBT: 4-tert-butylphenylthiolate)]. The innermost Au atom was successfully removed via thiol induction without altering the structure framework to produce diamagnetic Au126I4(TBBT)48 with local ligand arrangement changed (butterfly effect), which could be further transformed into paramagnetic [Au126I4(TBBT)48]+ via hydrogen peroxide oxidation. The spin populations of both paramagnetic nanoparticles are more densely distributed on surface iodine than sulfur. Diamagnetic Au126I4(TBBT)48 exhibited a Faradaic efficiency of ~100% at -0.57 volt during the electrocatalytic reduction of carbon dioxide to carbon monoxide, while paramagnetic Au127I4(TBBT)48 and [Au126I4(TBBT)48]+ exhibited the maximum Faradaic efficiency of 87% at -0.67 volt and 90% at -0.57 volt, respectively, indicating the spin-catalytic activity correlation.

Structural Evolution of Stapes Controls the Electrochemical CO2 Reduction on Bimetallic Cu-doped Gold Nanoclusters

Ligand protected gold nanoclusters have been proposed for electrochemical CO2 reduction (eCO2R) as an alternative to polycrystalline catalysts, showing higher selectivity control due to the tailored composition and precise microenvironment. Here, two gold cluster families are studied with different staple motifs (Au25(SR)18 and Au144(SR)60, where SR = thiolate) doped with Ag or Cu to understand the interplay between the composition and the performance of these catalysts. Detailed cluster characterization and Density Functional Theory simulations demonstrate that the dynamic aspects involving ligand removal are crucial to unraveling the role of the dopant, the cluster curvature, and the staple structure. The best activity performance toward CO is obtained for Cu-doped Au144(SR)60 at U = -0.8 VRHE as ligands are only partially depleted and the staple can bend to stabilize *CO intermediate, while Cu-containing Au25(SR)18 can produce formate but shows worse CO selectivity. This study points toward the importance of ligand stability during eCO2R on bimetallic gold nanoclusters, paving the way for improving the design and operation of this family of catalysts.

Recent Progress in Atomically Precise Cu-M Alloy Nanoclusters

Metal nanoclusters (NCs) with dimensions of approximately 3 nm serve as a crucial link between metal-organic complexes and metal nanoparticles, garnering significant interest due to their distinctive molecule-like characteristics. These include well-defined molecular structures, clear HOMO-LUMO transitions, quantized charge, and robust luminescence emission. Atomically precise alloy NCs, in contrast to homometallic NCs, exhibit a wealth of structures and intriguing properties, with their novel attributes often intricately tied to the positions of alloyed elements within the structure, facilitating the exploration of structure-property relationships. A notable subgroup within this category comprises Cu-M (where M represents metals such as Au, Ag, Rh, Ir, Pd, Pt, Zn, Al etc.) alloy NCs. In this review, we initially outline recent advancements in the development of efficient synthetic techniques for Cu-M alloy NCs, emphasizing the underlying physical and chemical properties that enable precise control over their sizes and surface characteristics. Subsequently, we delve into recent progress in structural elucidation techniques for Cu-M alloy NCs. This structural insight is instrumental in comprehensively understanding the structure-property correlations at the molecular level. Finally, we showcase various examples of Cu-M alloy NCs to illustrate their photoluminescent and catalytic properties, shedding light on their diverse functionalities and potential applications.

Electronic state modulation of Ag30 nanoclusters within a ring-shaped polyoxometalate

Atomically precise Ag nanoclusters display distinctive properties that are dictated by their structures and electronic states. However, manipulating the electronic states of Ag nanoclusters is challenging owing to their inherent instability and susceptibility to undesired structural changes, decomposition, and aggregation. Recently, we reported the synthesis of a body-centered cubic {Ag30}22+ nanocluster encapsulated within a ring-shaped polyoxometalate (POM) [P8W48O184]40- by reacting 16 Ag+-containing [P8W48O184]40- with Ag+ using N,N-dimethylformamide (DMF) as a mild reducing agent. This led to a redox-induced structural transformation into a face-centered cubic {Ag30}16+ nanocluster. In this study, we demonstrated the modulation of the electronic states of Ag30 nanoclusters within the ring-shaped POM through two different approaches. A face-centered cubic {Ag30}18+ nanocluster, featuring distinct oxidation states compared to previously reported {Ag30}22+ and {Ag30}16+ nanoclusters, was synthesized using tetra-n-butylammonium borohydride, a stronger reducing agent than DMF, in the reaction of 16 Ag+-containing [P8W48O184]40- and Ag+. Additionally, by leveraging the acid-base properties of POMs, we demonstrated the reversible, stepwise modulation of the charge distribution in the Ag30 nanocluster through controlling protonation states of the ring-shaped POM ligand. These results highlight the potential of engineering POM-stabilized Ag nanoclusters with diverse structures and electronic states, thereby facilitating the exploration of novel properties and applications utilizing the unique characteristics of the POM ligands.

Revisiting the activity origin of the PtAu24(SR)18 nanocluster for enhanced electrocatalytic hydrogen evolution by combining first-principles simulations with the experimental in situ FTIR technique

Thiolate-protected metal nanoclusters (NCs) have been widely used in various electrocatalytic reactions, yet the dynamic evolution of metal NCs during electrocatalysis has been rarely explored and the activity origin remains largely ambiguous. Herein, using a PtAu24(SCH3)18 NC as a prototype model, we combined advanced first-principles calculations and attenuated total reflection surface-enhanced infrared spectroscopy (ATR-SEIRAS) to re-examine its active site and reaction dynamics in the hydrogen evolution reaction (HER). It has been previously assumed that the central Pt is the only catalytic center. However, differently, we observed the spontaneous desorption of thiolate ligands under moderate potential, and the dethiolated PtAu24 exhibits excellent HER activity, which is contributed not only by the central Pt atom but also by the exposed bridged Au sites. Particularly, the exposed Au exhibits high activity even comparable to Pt, and the synergistic effect between them makes dethiolated PtAu24 an extraordinary HER electrocatalyst, even surpassing the commercial Pt/C catalyst. Our predictions are further verified by electrochemical activation experiments and in situ FTIR (ATR-SEIRAS) characterization, where evident adsorption of Au-H* and Pt-H* bonds is monitored. This work detected, for the first time, the Au-S interfacial dynamics of the PtAu24 nanocluster in electrocatalytic processes, and quantitatively evaluated the essential catalytic role of the exposed Au sites that has been largely overlooked in previous studies.

Optimizing the Activation Energy of Reactive Intermediates on Single-Atom Electrocatalysts: Challenges and Opportunities

Single-atom catalysts (SACs) have made great progress in recent years as potential catalysts for energy conversion and storage due to their unique properties, including maximum metal atoms utilization, high-quality activity, unique defined active sites, and sustained stability. Such advantages of single-atom catalysts significantly broaden their applications in various energy-conversion reactions. Given the extensive utilization of single-atom catalysts, methods and specific examples for improving the performance of single-atom catalysts in different reaction systems based on the Sabatier principle are highlighted and reactant binding energy volcano relationship curves are derived in non-homogeneous catalytic systems. The challenges and opportunities for single-atom catalysts in different reaction systems to improve their performance are also focused upon, including metal selection, coordination environments, and interaction with carriers. Finally, it is expected that this work may provide guidance for the design of high-performance single-atom catalysts in different reaction systems and thereby accelerate the rapid development of the targeted reaction.

Chemical Flexibility of Atomically Precise Metal Clusters

Ligand-protected metal clusters possess hybrid properties that seamlessly combine an inorganic core with an organic ligand shell, imparting them exceptional chemical flexibility and unlocking remarkable application potential in diverse fields. Leveraging chemical flexibility to expand the library of available materials and stimulate the development of new functionalities is becoming an increasingly pressing requirement. This Review focuses on the origin of chemical flexibility from the structural analysis, including intra-cluster bonding, inter-cluster interactions, cluster-environments interactions, metal-to-ligand ratios, and thermodynamic effects. In the introduction, we briefly outline the development of metal clusters and explain the differences and commonalities of M(I)/M(I/0) coinage metal clusters. Additionally, we distinguish the bonding characteristics of metal atoms in the inorganic core, which give rise to their distinct chemical flexibility. Section 2 delves into the structural analysis, bonding categories, and thermodynamic theories related to metal clusters. In the following sections 3 to 7, we primarily elucidate the mechanisms that trigger chemical flexibility, the dynamic processes in transformation, the resultant alterations in structure, and the ensuing modifications in physical-chemical properties. Section 8 presents the notable applications that have emerged from utilizing metal clusters and their assemblies. Finally, in section 9, we discuss future challenges and opportunities within this area.

Advanced Functionalized Nanoclusters (Cu, Ag, and Au) as Effective Catalyst for Organic Transformation Reactions

A considerable amount of research has been carried out in recent years on synthesizing metal nanoclusters (NCs), which have wide applications in the field of optical materials with non-linear properties, bio-sensing, and catalysis. Aside from being structurally accurate, the atomically precise NCs possess well-defined compositions due to significant tailoring, both at the surface and the core, for certain functionalities. To illustrate the importance of atomically precise metal NCs for catalytic processes, this review emphasizes 1) the recent work on Cu, Ag, and Au NCs with their synthesis, 2) the parameters affecting the activity and selectivity of NCs catalysis, and 3) the discussion on the catalytic potential of these metal NCs. Additionally, metal NCs will facilitate the design of extremely active and selective catalysts for significant reactions by elucidating catalytic mechanisms at the atomic and molecular levels. Future advancements in the science of catalysis are expected to come from the potential to design NCs catalysts at the atomic level.

Structure control and evolution of atomically precise gold clusters as heterogeneous precatalysts

Metal clusters have distinct features from single atom and nanoparticle (>1 nm) catalysts, making them effective catalysts for various heterogeneous reactions. Nevertheless, the ambiguity and complexity of the catalyst structure preclude in-depth mechanistic studies. The evolution of metal species during synthesis and reaction processes represents another challenge. One effective solution is to precisely control the structure of the metal cluster, thus offering a well-defined pre-catalyst. The well-defined chemical formula and configurations make atomically precise metal nanoclusters optimal choices. To fabricate an atomically precise metal nanocluster-based heterogeneous catalyst with enhanced performance, careful structural design of both the nanocluster and support material, an effective assembling technique, and a pre-treatment method for these hybrids need to be developed. In this review, we summarize recent advances in in the development of heterogeneous catalysts using atomically precise gold and alloy gold nanoclusters as precursors. We will begin with a brief introduction to the structural properties of atomically precise nanoclusters and structure determination of cluster/support hybrids. We will then introduce heterogeneous catalysts prepared from medium size (tens to hundreds of metal atoms) and low nuclearity nanoclusters. We will illustrate how ligand modification, support-cluster interaction, hybrid fabrication, and heteroatom (Pt, Pd Ag, Cu, Cd, Fe) introduction affect the structural properties and pretreatment/reaction-induced structural evolution of gold nanocluster pre-catalysts. Lastly, we will highlight the synthetic method of NCs@MOF hybrids and their effectiveness in circumventing the adverse cluster structural evolution. These findings are expected to shed light on the structure-activity relationship studies and future catalyst design strategies using atomically precise metal nanocluster pre-catalysts.

Engineering ligand chemistry on Au25 nanoclusters: from unique ligand addition to precisely controllable ligand exchange

Au25 nanoclusters (NCs) protected by 18 thiol-ligands (Au25SR18, SR is a thiolate ligand) are the prototype of atomically precise thiolate-protected gold NCs. Studies concerning the alteration of the number of surface ligands for a given Au25SR18 NC are scarce. Herein we report the conversion of hydrophobic Au25PET18 (PET = 2-phenylethylthiolate) NCs to Au25SR19 [Au25PET18(metal complex)1] induced by ligand exchange reactions (LERs) with thiolated terpyridine-metal complexes (metal complex, metal = Ru, Fe, Co, Ni) under mild conditions (room temperature and low amounts of incoming ligands). Interestingly, we found that the ligand addition reaction on Au25PET18 NCs is metal dependent. Ru and Co complexes preferentially lead to the formation of Au25SR19 whereas Fe and Ni complexes favor ligand exchange reactions. High-resolution electrospray ionization mass spectrometry (HRESI-MS) was used to determine the molecular formula of Au25SR19 NCs. The photophysical properties of Au25PET18(Ru complex)1 are distinctly different from Au25PET18. The absorption spectrum is drastically changed upon addition of the extra ligand and the photoluminescence quantum yield of Au25PET18(Ru complex)1 is 14 times and 3 times higher than that of pristine Au25PET18 and Au25PET17(Ru complex)1, respectively. Interestingly, only one surface ligand (PET) could be substituted by the metal complex when neutral Au25PET18 was used for ligand exchange whereas two ligands could be exchanged when starting with negatively charged Au25PET18. This charge dependence provides a strategy to precisely control the number of exchanged ligands at the surface of NCs.

Programmable kernel structures of atomically precise metal nanoclusters for tailoring catalytic properties

The unclear structures and polydispersity of metal nanoparticles (NPs) seriously hamper the identification of the active sites and the construction of structure-reactivity relationships. Fortunately, ligand-protected metal nanoclusters (NCs) with atomically precise structures and monodispersity have become an ideal candidate for understanding the well-defined correlations between structure and catalytic property at an atomic level. The programmable kernel structures of atomically precise metal NCs provide a fantastic chance to modulate their size, shape, atomic arrangement, and electron state by the precise modulating of the number, type, and location of metal atoms. Thus, the special focus of this review highlights the most recent process in tailoring the catalytic activity and selectivity over metal NCs by precisely controlling their kernel structures. This review is expected to shed light on the in-depth understanding of metal NCs' kernel structures and reactivity relationships.

Modulating Catalytic Activity and Stability of Atomically Precise Gold Nanoclusters as Peroxidase Mimics via Ligand Engineering

Metal nanoclusters (NCs), composed of a metal core and protecting ligands, show promising potentials as enzyme mimics for producing fuels, pharmaceuticals, and valuable chemicals, etc. Herein, we explore the critical role of ligands in modulating the peroxidase mimic activity and stability of Au NCs. A series of Au₁₅(SR)₁₃ NCs with various thiolate ligands [SR = N-acetyl-l-cysteine (NAC), 3-mercaptopropionic acid (MPA), or 3-mercapto-2-methylpropanoic acid (MMPA)] are utilized as model catalysts. It is found that Au₁₅(NAC)₁₃ shows higher structural stability than Au₁₅(MMPA)₁₃ and Au₁₅(MPA)₁₃ against external stimuli (e.g., pH, oxidants, and temperature) because of the intramolecular hydrogen bonds. More importantly, detailed enzymatic kinetics data show that the catalytic activity of Au₁₅(NAC)₁₃ is about 4.3 and 2.7 times higher than the catalytic activity of Au₁₅(MMPA)₁₃ and Au₁₅(MPA)₁₃, respectively. Density functional theory (DFT) calculations reveal that the Au atoms on the motif of Au NCs should be the active centers, whereas the superior peroxidase mimic activity of Au₁₅(NAC)₁₃ should originate from the emptier orbitals of Au atoms because of the electron-withdrawing effect of acetyl amino group in NAC. This work demonstrates the ligand-engineered electronic structure and functionality of atomically precise metal NCs, which afford molecular and atomic level insights for artificial enzyme design.

Electrocatalytic CO2 Reduction over Atomically Precise Metal Nanoclusters Protected by Organic Ligands

The electrochemical carbon dioxide reduction reaction (CO₂RR) is a promising method to realize carbon recycling and sustainable development because of its mild reaction conditions and capability to utilize the electric power generated by renewable energy such as solar, wind, or tidal energy to produce high-value-added liquid fuels and chemicals. However, it is still a great challenge to deeply understand the reaction mechanism of CO₂RRs involving multiple chemical processes and multiple products due to the complexity of the traditional catalyst's surface. Organic ligand-protected metal nanoclusters (NCs) with accurate compositions and definite atom packing structures show advantages for revealing the reaction mechanism of CO₂RRs. This Review focuses on the recent progress in CO₂RRs catalyzed by atomically precise metal NCs, including gold, copper, and silver NCs. Particularly, the influences of charge, ligand, surface structure, doping of Au NCs, and binders on the CO₂RR are discussed in detail. Meanwhile the reaction mechanisms of CO₂RRs including the active sites and the key reaction intermediates are also discussed. It is expected that progress in this research area could promote the development of metal NCs and CO₂RRs.

Ligand-Shell Engineering of a Au28 Nanocluster Boosts Electrocatalytic CO2 Reduction

Subtle tailoring of gold nanoclusters (NCs) could significantly change their physicochemical properties. However, direct comparison of the catalytic performance of gold NCs with identical metal cores but distinct ligand shells is rarely elucidated. In this work, a novel gold NC, Au₂₈ (C₂ B₁₀ H₁₁ S)₁₂ (tht)₄ Cl₄ (Au₂₈ -S), was isolated by a facile self-reducing synthesis. Au₂₈ -S adopts an identical Au₂₈ metal framework to that of the reported alkynyl-protected Au₂₈ -C. The different protective layers lead to distinctions in their electronic structure and optical properties. Furthermore, Au₂₈ -S shows better catalytic activity for the electrochemical reduction of CO₂ to CO. Theoretical calculations identified the active sites and shed light on the catalytic mechanism to elucidate the different catalytic performances. This work provides an ideal platform to study the protective layer-activity relationship of gold NCs, and may also provide guidance in the design of metal NC-based catalysts.

The Factors Dictating Properties of Atomically Precise Metal Nanocluster Electrocatalysts

Metal nanoparticles occupy an important position in electrocatalysis. Unfortunately, by using conventional synthetic methodology, it is a great challenge to realize the monodisperse composition/structure of metal nanoparticles at the atomic level, and to establish correlations between the catalytic properties and the structure of individual catalyst particles. For the study of well-defined nanocatalysts, great advances have been made for the successful synthesis of nanoparticles with atomic precision, notably ligand-passivated metal nanoclusters. Such well-defined metal nanoclusters have become a type of model catalyst and have shown great potential in catalysis research. In this review, the authors summarize the advances in the utilization of atomically precise metal nanoclusters for electrocatalysis. In particular, the factors (e.g., size, metal doping/alloying, ligand engineering, support materials as well as charge state of clusters) affecting selectivity and activity of catalysts are highlighted. The authors aim to provide insightful guidelines for the rational design of electrocatalysts with high performance and perspectives on potential challenges and opportunities in this emerging field.

High-Performance Ligand-Protected Metal Nanocluster Catalysts for CO2 Conversion through the Exposure of Undercoordinated Sites

Previous experimental breakthroughs reveal the potential to create novel heterogeneous catalysts for the electroreduction of CO2 to a high-value product CO using ligand-protected Au-based nanoclusters. Since the chemical composition and geometric structures have been precisely defined, it is possible to adopt robust design guidelines for the development of practical catalysts and to fundamentally elucidate the underlying reaction mechanism. In this short review, the computational progress made to understand the experimentally observed reduction process on the following subset of materials—Au25(SR)18−, Au24Pd(SR)18, Au23(SR)16− and Au21Cd2(SR)16−—is described. A significant finding from our first-principles mechanistic studies is that CO2 conversion on the fully ligand protected nanoclusters is thermodynamically unfavorable due to the very weak binding of intermediates on the surface region. However, the reaction becomes feasible when either Au or S sites are exposed through the removal of a ligand. The results particularly point to the role of undercoordinated S sites in the creation of highly functional heterogeneous catalysts that are both active and selective for the CO2 conversion process. The incorporation of dopants could significantly influence the catalytic reactivity of the nanoclusters. As demonstrated in the case of the monopalladium substitution in Au25(SR)18−, the presence of the foreign atom leads to an enhancement of CO production selectivity due to the greater stabilization of the intermediates. With the Cd substitution doping of Au23(SR)16−, the improvement in performance is also attributed to the enhanced binding strength of the intermediates on the geometrically modified surface of the nanocluster.

Heterointerface Created on Au-Cluster-Loaded Unilamellar Hydroxide Electrocatalysts as a Highly Active Site for the Oxygen Evolution Reaction

The oxygen evolution reaction (OER) is a critical element for all sorts of reactions that use water as a hydrogen source, such as hydrogen evolution and electrochemical CO₂ reduction, and novel design principles that provide highly active sites on OER electrocatalysts push the limits of their practical applications. Herein, Au-cluster loading on unilamellar exfoliated layered double hydroxide (ULDH) electrocatalysts for the OER is demonstrated to fabricate a heterointerface between Au clusters and ULDHs as an active site, which is accompanied by the oxidation state modulation of the active site and interfacial direct OO coupling ("interfacial DOOC"). The Au-cluster-loaded ULDHs exhibit excellent activities for the OER with an overpotential of 189 mV at 10 mA cm^-2 . X-ray absorption fine structure measurements reveal that charge transfer from the Au clusters to ULDHs modifies the oxidation states of trivalent metal ions, which can be active sites on the ULDHs. The present study, supported by highly sensitive spectroscopy combining reflection absorption infrared spectroscopy and modulation-excitation spectroscopy and density functional theory calculations, indicates that active sites at the interface between the Au clusters and ULDHs promote a novel OER mechanism through interfacial DOOC, thereby achieving outstanding catalytic performance.

Direct Formation of Colloidal All-Inorganic Metal Nanocrystals from Magic-Size Clusters

All-inorganic metal nanocrystals (NCs) offer an ideal platform for atomically precise surface design and show improved electrocatalytic activity compared to conventional NCs capped with organic ligands. Here, we show the possibility of obtaining colloidal all-inorganic Au NCs directly from magic-size clusters (MSCs) with the assistance of inorganic molecules, NOBF4 or Na2S2O8. The unique advantages of NOBF4 or Na2S2O8 as both oxidizing agents and stripping ligands are taken to tune the surface state of Au25(PET)18-TOA+ MSCs and assemble them to form either positively or negatively charged all-inorganic Au NCs. We show that positively charged all-inorganic Au NCs can be further modified with different functional groups, which provide the possibility to meet the target requirements. We found that the negatively charged NCs exhibit improved faradaic efficiency (FE = 92%) for the reduction of CO2 to CO at -0.369 V (vs RHE) and a 5-fold increase in current density compared to organic-capped Au NCs (FE = 67%). In addition, we extended this approach to other MSCs and formed all-inorganic metal NCs with different compositions and morphologies. The use of simple inorganic ligands to induce the conversion from MSCs to metal NCs enriches the current solution process of synthesizing all-inorganic NCs and can open up more opportunities for designing colloidal nanocrystal catalysts.

Homoleptic Alkynyl-Protected Ag15 Nanocluster with Atomic Precision: Structural Analysis and Electrocatalytic Performance toward CO2 Reduction

We report the fabrication of homoleptic alkynyl-protected Ag₁₅ (C≡C-^t Bu)₁₂ ⁺ (abbreviated as Ag₁₅ ) nanocluster and its electrocatalytic properties toward CO₂ reduction reaction. Crystal structure analysis reveals that Ag₁₅ possesses a body-centered-cubic (BCC) structure with an Ag@Ag₈ @Ag₆ metal core configuration. Interestingly, we found that Ag₁₅ can adsorb CO₂ in the air and spontaneously self-assembled into one-dimensional linear material during the crystal growth process. Furthermore, Ag₁₅ can convert CO₂ into CO with a faradaic efficiency of ca. 95.0 % at -0.6 V and a maximal turnover frequency of 6.37 s^-1 at -1.1 V along with excellent long-term stability. Finally, density functional theory (DFT) calculations disclosed that Ag₁₅ (C≡C-^t Bu)₁₁ ⁺ with one alkynyl ligand stripping off from the intact cluster can expose the uncoordinated Ag atom as the catalytically active site for CO formation.

Ligand-Protected Au55 with a Novel Structure and Remarkable CO2 Electroreduction Performance

A Au₅₅ nanocluster with the composition of [Au₅₅ (p-MBT)₂₄ (Ph₃ P)₆ ](SbF₆ )₃ (p-MBT=4-methylbenzenethiolate) is synthesized via direct reduction of gold-phosphine and gold-thiolate precursors. Single-crystal X-ray diffraction reveals that this Au₅₅ nanocluster features a face-centered cubic (fcc) Au₅₅ kernel, different from the well-known two-shell cuboctahedral arrangement in Au₅₅ (Ph₃ P)₁₂ Cl₆ . The Au₅₅ cluster shows a wide optical absorption band with optical energy gap (E_g =1.28 eV). It is found that the exclusion of chloride is crucial for the formation of the title cluster, otherwise rod-like [Au₂₅ (SR)₅ (PPh₃ )₁₀ Cl₂ ]²⁺ is obtained. The strategy to run synthetic reaction in the absence of halide leads to new members of phosphine/thiolate co-protected metal nanoclusters. The Au₅₅ nanocluster exhibits high catalytic activity and selectivity for electrochemical reduction of CO₂ to CO; the Faradaic efficiency (FE) reaches 94.1 % at -0.6 V vs. reversible hydrogen electrode (RHE).

Distinct chemical fixation of CO2 enabled by exotic gold nanoclusters

Atomically precise metal nanoclusters, especially the metal nanoclusters with an exotic core structure, have given rise to a great deal of interest in catalysis, attributing to their well-defined structures at the atomic level and consequently unique electronic properties. Herein, the catalytic performances of three gold nanoclusters, such as Au₃₈S₂(S-Adm)₂₀ with a body-centered cubic (bcc) kernel structure, Au₃₀(S-Adm)₁₈ with a hexagonal close-packed (hcp) core structure, and Au₂₁(S-Adm)₁₅ with a face-centered cubic (fcc) kernel structure, were attempted for the CO₂ cycloaddition with epoxides toward cyclic carbonates. Due to the excess positive charge with a strong Lewis acidity and large chemical adsorption capacity, the bcc-Au₃₈S₂(S-Adm)₂₀ nanocluster outperformed the hcp-Au₃₀(S-Adm)₁₈ and fcc-Au₂₁(S-Adm)₁₅ nanoclusters. Additionally, the synergistic effect between the gold nanocluster and co-catalyst played a crucial role in CO₂ cycloaddition.

Light-Induced Charge Transfer from Transition-Metal-Doped Aluminum Clusters to Carbon Dioxide

Charge transfer between molecules and catalysts plays a critical role in determining the efficiency and yield of photochemical catalytic processes. In this paper, we study light-induced electron transfer between transition-metal-doped aluminum clusters and CO₂ molecules using first-principles time-dependent density-functional theory. Specifically, we carry out calculations for a range of dopants (Zr, Mn, Fe, Ru, Co, Ni, and Cu) and find that the resulting systems fall into two categories: Cu- and Fe-doped clusters exhibit no ground-state charge transfer, weak CO₂ adsorption, and light-induced electron transfer into the CO₂. In all other systems, we observe ground-state electron transfer into the CO₂ resulting in strong adsorption and predominantly light-induced electron back-transfer from the CO₂ into the cluster. These findings pave the way toward a rational design of atomically precise aluminum photocatalysts.

Controlled syngas production by electrocatalytic CO2 reduction on formulated Au25(SR)18 and PtAu24(SR)18 nanoclusters

Syngas, a gaseous mixture of CO and H₂, is a critical industrial feedstock for producing bulk chemicals and synthetic fuels, and its production via direct CO₂ electroreduction in aqueous media constitutes an important step toward carbon-negative technologies. Herein, we report controlled syngas production with various H₂/CO ratios via the electrochemical CO₂ reduction reaction (CO₂RR) on specifically formulated Au₂₅ and PtAu₂₄ nanoclusters (NCs) with core-atom-controlled selectivities. While CO was predominantly produced from the CO₂RR on the Au NCs, H₂ production was favored on the PtAu₂₄ NCs. Density functional theory calculations of the free energy profiles for the CO₂RR and hydrogen evolution reaction (HER) indicated that the reaction energy for the conversion of CO₂ to CO was much lower than that for the HER on the Au₂₅ NC. In contrast, the energy profiles calculated for the HER indicated that the PtAu₂₄ NCs have nearly thermoneutral binding properties; thus, H₂ production is favored over CO formation. Based on the distinctly different catalytic selectivities of Au₂₅ and PtAu₂₄ NCs, controlled syngas production with H₂/CO ratios of 1 to 4 was demonstrated at a constant applied potential by simply mixing the Au₂₅ and PtAu₂₄ NCs based on their intrinsic catalytic activities for the production of CO and H₂.

Multiple Ways Realizing Charge-State Transform in AuCu Bimetallic Nanoclusters with Atomic Precision

Thiolate-protected nanoclusters with different charge states usually show similar structure frameworks but different electronic configurations, which are proved to dramatically affect their properties such as magnetism, photoluminescence, and catalytic activity. Until now, few nanoclusters with alterable charge states have been reported and only some of them are structurally solved, limiting the in-depth studies on their interesting properties. Here, a new AuCu alloy nanocluster [Au₁₈ Cu₃₂ (SPhCl)₃₆ ]^2- (HSPhCl = 4-chlorophenylthiophenol) is synthesized and structurally solved by X-ray crystallography. Interestingly, it is found that this nanocluster can be reduced to another nanocluster with a different charge state, that is, [Au₁₈ Cu₃₂ (SPhCl)₃₆ ]^3- . This change in charge states is clearly proved by X-ray crystallography, electrospray ionization mass spectrometry, thermogravimetric analysis, and electron paramagnetic resonance. Furthermore, several redox methods are carried out to realize the reversible interconversion between these two nanoclusters, including electrochemical redox, introduction of H₂ O₂ /NaBH₄ , and oxidation with silica under air atmosphere. This work offers new insight into the transform progress of charge states with AuCu alloy nanoclusters which contributes to the understanding of the relationship between electronic structure and properties of nanoclusters and further development of AuCu nanoclusters with excellent performance.

Site-Specific Electronic Properties of [Ag25 (SR)18 ]- Nanoclusters by X-Ray Spectroscopy

Silver nanoclusters (NCs) are of significant interest owing to their interesting structural, electronic, and catalytic properties. Among these NCs, Ag₂₅ (SR)₁₈ is particularly attractive due to its identical geometry as its Au counterpart, Au₂₅ (SR)₁₈ . Herein, the site-specific electronic properties of Ag₂₅ (SR)₁₈ and Au₂₅ (SR)₁₈ using X-ray spectroscopy experiments and quantum simulations are presented. To overcome the final state effect observed in X-ray photoelectron spectroscopy (XPS), a unique method is developed to reliably analyze the charge transfer behavior of the NCs. Density functional theory calculations are combined with XPS to provide more insight into the electronic properties of the NCs. The differences in the XPS valence bands of these two NCs are further compared and interpreted using the relativistic effect. The first derivative of the X-ray absorption near-edge structure (XANES) spectrum is further used as a tool to sensitively probe the bonding properties of Ag₂₅ (SR)₁₈ . By combining the experimental XANES data and their site-specific quantum simulations, the large impact of the staple motif on the bonding properties of the NC is demonstrated. These findings highlight the unique electronic properties of each atomic site in Ag₂₅ (SR)₁₈ ; the effective X-ray analysis techniques developed here can offer new opportunities for the site-specific study of other NCs.

Atomically Precise Gold Nanoclusters as Model Catalysts for Identifying Active Sites for Electroreduction of CO2

Accurate identification of active sites is critical for elucidating catalytic reaction mechanisms and developing highly efficient and selective electrocatalysts. Herein, we report the atomic-level identification of active sites using atomically well-defined gold nanoclusters (Au NCs) Au₂₅ , Au₃₈ , and Au₁₄₄ as model catalysts in the electrochemical CO₂ reduction reaction (CO₂ RR). The studied Au NCs exhibited remarkably high CO₂ RR activity, which increased with increasing NC size. Electrochemical and X-ray photoelectron spectroscopy analyses revealed that the Au NCs were activated by removing one thiolate group from each staple motif at the beginning of CO₂ RR. In addition, density functional theory calculations revealed higher charge densities and upshifts of d-states for dethiolated Au sites. The structure-activity properties of the studied Au NCs confirmed that dethiolated Au sites were the active sites and that CO₂ RR activity was determined by the number of active sites on the cluster surface.

The ligand effect on the interface structures and electrocatalytic applications of atomically precise metal nanoclusters

Metal nanoclusters, also known as ultra-small metal nanoparticles, occupy the gap between discrete atoms and plasmonic nanomaterials, and are an emerging class of atomically precise nanomaterials. Metal nanoclusters protected by different types of ligands, such as thiolates, alkynyls, hydrides, and N-heterocyclic carbenes, have been synthesized in recent years. Moreover, recent experiment and theoretical studies also indicated that the metal nanoclusters show great promise in many electrocatalytic reactions, such as hydrogen evolution, oxygen reduction, and CO₂reduction. The atomically precise nature of their structures enables the elucidation of structure-property relationships and the reaction mechanisms, which is essential if nanoclusters with enhanced performances are to be rationally designed. Particularly, the ligands play an important role in affecting the interface bonding, stability and electrocatalytic activity/selectivity. In this review, we mainly focus on the ligand effect on the interface structure of metal nanoclusters and then discuss the recent advances in electrocatalytic applications. Furthermore, we point out our perspectives on future efforts in this field.

Electrocatalysis of gold-based nanoparticles and nanoclusters

Gold (Au)-based nanomaterials, including nanoparticles (NPs) and nanoclusters (NCs), have shown great potential in many electrocatalytic reactions due to their excellent catalytic ability and selectivity. In recent years, Au-based nanostructured materials have been considered as one of the most promising non-platinum (Pt) electrocatalysts. The controlled synthesis of Au-based NPs and NCs and the delicate microstructure adjustment play a vital role in regulating their catalytic activity toward various reactions. This review focuses on the latest progress in the synthesis of efficient Au-based NP and NC electrocatalysts, highlighting the relationship between Au nanostructures and their catalytic activity. This review first discusses the parameters of Au-based nanomaterials that determine their electrocatalytic performance, including composition, particle size and architecture. Subsequently, the latest electrocatalytic applications of Au-based NPs and NCs in various reactions are provided. Finally, some challenges and opportunities are highlighted, which will guide the rational design of Au-based NPs and NCs as promising electrocatalysts.

Quantification of the Electrostatic Effect on Redox Potential by Positive Charges in a Catalyst Microenvironment

Charged functional groups in the secondary coordination sphere (SCS) of a heterogeneous nanoparticle or homogeneous electrocatalyst are of growing interest due to enhancements in reactivity that derive from specific interactions that stabilize substrate binding or charged intermediates. At the same time, accurate benchmarking of electrocatalyst systems most often depends on the development of linear free-energy scaling relationships. However, the thermodynamic axis in those kinetic-thermodynamic correlations is most often obtained by a direct electrochemical measurement of the catalyst redox potential and might be influenced by electrostatic effects of a charged SCS. In this report, we systematically probe positive charges in a SCS and their electrostatic contributions to the electrocatalyst redox potential. A series of 11 iron carbonyl clusters modified with charged and uncharged ligands was probed, and a linear correlation between the ν_CO absorption band energy and electrochemical redox potentials is observed except where the SCS is positively charged.

Dynamics of weak interactions in the ligand layer of meta-mercaptobenzoic acid protected gold nanoclusters Au68(m-MBA)32 and Au144(m-MBA)40

Atomically precise metal nanoclusters, stabilized and functionalized by organic ligands, are emerging nanomaterials with potential applications in plasmonics, nano-electronics, bio-imaging, nanocatalysis, and as therapeutic agents or drug carriers in nanomedicine. The ligand layer has an important role in modifying the physico-chemical properties of the clusters and in defining the interactions between the clusters and the environment. While this role is well recognized from a great deal of experimental studies, there is very little theoretical information on dynamical processes within the layer itself. Here, we have performed extensive molecular dynamics simulations, with forces calculated from the density functional theory, to investigate thermal stability and dynamics of the ligand layer of the meta-mercaptobenzoic acid (m-MBA) protected Au68 and Au144 nanoclusters, which are the first two gold nanoclusters structurally solved to atomic precision by electron microscopy [Azubel et al., Science, 2014, 345, 909 and ACS Nano, 2017, 11, 11866]. We visualize and analyze dynamics of three distinct non-covalent interactions, viz., ligand-ligand hydrogen bonding, metal-ligand O[double bond, length as m-dash]C-OHAu interaction, and metal-ligand Ph(π)Au interaction. We discuss their relevance for defining, at the same time, the dynamic stability and reactivity of the cluster. These interactions promote the possibility of ligand addition reactions for bio-functionalization or allow the protected cluster to act as a catalyst where active sites are dynamically accessible inside the ligand layer.

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

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

Reactivity and Lability Modulated by a Valence Electron Moving in and out of 25-Atom Gold Nanoclusters

The emergence of atomically precise metal nanoclusters with unique electronic structures provides access to currently inaccessible catalytic challenges at the single-electron level. We investigate the catalytic behavior of gold Au₂₅ (SR)₁₈ nanoclusters by monitoring an incoming and outgoing free valence electron of Au 6s¹ . Distinct performances are revealed: Au₂₅ (SR)₁₈ ^- is generated upon donation of an electron to neutral Au₂₅ (SR)₁₈ ⁰ and this is associated with a loss in reactivity, whereas Au₂₅ (SR)₁₈ ⁺ is generated from dislodgment of an electron from neutral Au₂₅ (SR)₁₈ ⁰ with a loss in stability. The reactivity diversity of the three Au₂₅ (SR)₁₈ clusters stems from different affinities with reactants and the extent of intramolecular charge migration during the reactions, which are closely associated with the valence occupancies of the clusters varied by one electron. The stability difference in the three clusters is attributed to their different equilibria, which are established between the AuSR dissociation and polymerization influenced by one electron.

Gold nanoclusters as electrocatalysts: size, ligands, heteroatom doping, and charge dependences

To establish an ultimate energy conversion system consisting of a water-splitting photocatalyst and a fuel cell, it is necessary to further increase the efficiencies of the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), and the oxygen reduction reaction (ORR). Recently, it was demonstrated that thiolate (SR)-protected gold clusters, Aun(SR)m, and their related alloy clusters can serve as model catalysts for these three reactions. However, as the previous data have been obtained under different experimental conditions, it is difficult to use them to gain a deep understanding of the means to attain higher activity in these reactions. Herein, we measured the HER, OER, and ORR activities of Aun(SR)m and alloy clusters containing different numbers of constituent atoms, ligand functional groups, and heteroatom species under identical experimental conditions. We obtained a comprehensive set of results that illustrates the effect of each parameter on the activities of the three reactions. Comparison of the series of results revealed that decreasing the number of constituent atoms in the cluster, decreasing the thickness of the ligand layer, and substituting Au with Pd improve the activities in all reactions. Taking the stability of the cluster into consideration, [Au24Pd(PET)18]0 (PET = 2-phenylethanethiolate) can be considered as a metal cluster with high potential as an HER, OER, and ORR catalyst. These findings are expected to provide clear design guidelines for the development of highly active HER, OER, and ORR catalysts using Aun(SR)m and related alloy clusters, which would allow realization of an ultimate energy conversion system.

Gold Nanoclusters as Electrocatalysts for Energy Conversion

Gold nanoclusters (Aun NCs) exhibit a size-specific electronic structure unlike bulk gold and can therefore be used as catalysts in various reactions. Ligand-protected Aun NCs can be synthesized with atomic precision, and the geometric structures of many Aun NCs have been determined by single-crystal X-ray diffraction analysis. In addition, Aun NCs can be doped with various types of elements. Clarification of the effects of changes to the chemical composition, geometric structure, and associated electronic state on catalytic activity would enable a deep understanding of the active sites and mechanisms in catalytic reactions as well as key factors for high activation. Furthermore, it may be possible to synthesize Aun NCs with properties that surpass those of conventional catalysts using the obtained design guidelines. With these expectations, catalyst research using Aun NCs as a model catalyst has been actively conducted in recent years. This review focuses on the application of Aun NCs as an electrocatalyst and outlines recent research progress.

A 42-metal Yb(iii) nanowheel with NIR luminescent response to anions

One Yb42 nanowheel [Yb42L14(OH)28(OAc)84] was constructed using a tridentate vanillin ligand. The external diameter of the wheel-like structure is about 3.6 nm, which allows direct visualization by TEM. It shows interesting NIR lanthanide luminescence sensing towards anions, especially to fluoride at the ppm level.