Reveiws/Accounts/Perspective

36)"Silver Cluster Assembled Materials: A Model-Driven Perspective on Recent Progress, with a Spotlight on Ag12 Cluster Assembly"
　S. Biswas, Y. Negishi*
　Chem. Rec. in press.

The exploration of individual nanoclusters is rapidly advancing, despite stability concerns. To address this challenge, the assembly of cluster nodes through linker molecules has been successfully implemented. However, the linking of the cluster nodes itself introduces a multitude of possibilities, especially when additional factors come into play. While this method proves effective in enhancing material stability, the specific reasons behind its success remain elusive. In our laboratory, we have undertaken extensive studies on Ag cluster-assembled materials. So, here our goal is to establish a model system that allows for the discernment of various factors, eliminating unnecessary complexities during the linking approach. So, we hope that the systematic discourse presented in here will contribute significantly to future endeavors, helping to set clear priorities, and provide solutions to concerns that arise when working with a model system.






35)"Exploring the impact of various reducing agents on Cu nanocluster synthesis"
　S. Biswas, Y. Negishi*
　Dalton Trans. in press.
　Invited "Frontier article" (Invited by Sally Howells-Wyllie, Editor, Dalton Transactions)

The synthesis of copper (Cu) nanoclusters (NCs) has experienced significant advancements in recent years. Despite the exploration of metal NCs dating back almost two decades, challenges specific to Cu NC synthesis arise from the variable oxidation states and heightened reactivity of intermediate Cu complexes, distinguishing it from its analogous counterparts. In this study, we present a comprehensive overview of this newly evolving research domain, focusing on the synthetic aspects. We delve into various factors influencing the synthesis of Cu NCs, with specific emphasis on the role of reducing agents. The impact of the reducing agent is particularly pivotal in this synthetic process, ultimately influencing the formation of model M(0)-containing NCs, which are less readily accessible in the context of Cu NCs. We anticipate that this frontier article will pave the way for accelerated research in the field of Cu NCs. By aiding in the selection of specific reaction conditions and reducing agents, we believe that this work will contribute to a faster-paced exploration of Cu NCs, further advancing our understanding and applications in this exciting area of nanomaterial research.






34)"Structure and Application Portfolio of Intricately Architected Silver Cluster-Assembled Materials"
　R. Nakatani, S. Das,* Y. Negishi*
　Nanoscale in press.
　Invited "Review" (Invited by Edward Gardner, Development Editor)
　Highlighted in "Themed collection: Recent Review Articles"

Silver Cluster-Assembled Materials (SCAMs) represent a new frontier of crystalline extended solids hallmarked by their customizable structures, commendable stabilities, and unique physical/chemical properties. Since their discovery in 2017, the diversity of organic linkers has afforded the SCAMs with ingenious architectures and the application scenario has expanded beyond photoluminescence sensing to environmental sustainability and biomedical applications. It is critically important to chronicle these recent key advances and review the progress of SCAMs that can enable translating the material discoveries into real implementation. Herein, we provide a succinct overview of the trajectory of SCAM research, with crucial insights into atomic-level structural correlations with the phenomena at the nanoscale, and discuss the gaps and opportunities that are still open in addition to charting a roadmap for future research directions.






33)"Triplet-Triplet Annihilation-Based Photon Upconversion Using Nanoparticles and Nanoclusters"
　Y. Niihori,* T, Kosaka, Y. Negishi*
　Mater. Horiz. in press
　Selected as "Inside Front Cover"
　Highligted in "Themed Collection: Recent Review Articles"

The phenomenon of photon upconversion (UC), generating high-energy photons from low-energy photons, has attracted significant attention. In particular, triplet-triplet annihilation-based UC (TTA-UC) has been achieved by combining the excitation states of two types of molecules, called the sensitizer and emitter (or annihilator). With TTA-UC, it is possible to convert weak, incoherent near-infrared (NIR) light, which constitutes half of the solar radiation intensity, into ultraviolet and visible light that are suitable for the operation of light-responsive functional materials or devices such as solar cells and photocatalysts. Research on TTA-UC is being conducted worldwide, often employing materials with high intersystem crossing rates, such as metal porphyrins, as sensitizers. This review summarizes recent research and trends in triplet energy transfer and TTA-UC for semiconductor nanoparticles or nanocrystals with diameters in the nanometer range, also known as quantum dots, and for ligand-protected metal nanoclusters, which have even smaller well-defined sub-nanostructures. Concerning nanoparticles, transmitter ligands have been applied on the surface of the nanoparticles to efficiently transfer triplet excitons formed inside the nanoparticles to emitters. Applications are expanding to solid-state UC devices that convert NIR light to visible light. Additionally, there is active research in the development of sensitizers using more cost-effective and environmentally friendly elements. Regarding metal nanoclusters, methods have been established for the evaluation of excited states, deepening the understanding of luminescent properties and excited relaxation processes.






32)"Size-Dependent Carbon Dioxide Reduction Activity of Copper Nanoparticle and Nanocluster Electrocatalysts"
　T. Kawawaki,* T. Okada, K. Takemae, S. Tomihari, Y. Negishi*
　ChemNanoMat in press.
　Invited Review to Special Collection "Early Career Researchers 2023" (Invited by Dr. Claire M. Cobley, Editor-in-Chief)

The electrochemical carbon dioxide (CO2) reduction reaction (CRR, which can convert CO2 into useful compounds at room temperature and ambient pressure by using electricity derived from renewable energy source), has been attracting attention in recent years. This is because it can convert CO2 into useful compounds, which is pertinent to establishing a next-generation recycling-oriented energy society. However, further improvement of the electrocatalyst is required to improve its activity, selectivity, and durability. Among these, copper (Cu) can synthesize various hydrocarbons from CO2 and has been the most studied electrocatalyst for the CRR over many years. In particular, regarding ligand-protected Cu particles for the CRR, the size, shape, and ligands of Cu particles prepared by chemical reduction can be precisely controlled. In this review, we summarize previous research on the size-dependence of the CRR by using ligand-protected Cu particles (nanoparticles and nanoclusters) prepared by liquid-phase reduction, and discuss the current status of these studies for researchers on the electrochemical CRR.






31)"A Comprehensive Analysis of Luminescent Crystalized Cu Nanoclusters"
　S. Biswas, Y. Negishi*
　J. Phys. Chem. Lett. in press
　Invited Perspective (Invited by Prof. Gregory Scholes, Editor-in-Chief)
　Selected as "Supplementary Cover"

Photoluminescence (PL) emission is an intriguing characteristic displayed by atomically precise d10 metal nanoclusters (NCs), renowned for their meticulous atomic arrangements that have captivated the scientific community. Cu(I) NCs are a focal point in extensive research due to their abundance, cost-effectiveness, and unique luminescent attributes. Despite similar core sizes, their luminescent characteristics vary, influenced by multiple factors. Progress hinges on synthesizing new NCs and modifying existing ones, with post-synthetic alterations impacting emission properties. The rapid advancements in this field pose challenges in discerning essential points for excelling amidst competition with other d10 NCs. This perspective article explores the intricate origins of PL emission in Cu(I) NCs, providing a comprehensive review of their correlated structural architectures. Understanding the mechanistic origin of PL emission in each cluster is crucial for correlating diverse characteristics, contributing to a deeper comprehension from both fundamental and applied scientific perspectives.






30)"Covalent Organic Frameworks: Cutting-Edge Materials for Carbon Dioxide Capture and Water Harvesting from Air"
　H. Mabuchi, T. Irie, J. Sakai, S. Das,* Y. Negishi*
　Chem. Euro. J. 30, e202303474 (2024).
　Invited Review to special collection "Covalent Organic Frameworks: Green Synthesis and Cutting-Edge Applications"　
　Selected as "Inside Front Cover"
　Selected as "Hot Topic: Carbon Dioxide"

The implacable rise of carbon dioxide (CO2) concentration in the atmosphere and acute water stress are one of the central challenges of our time. Present-day chemistry is strongly inclined towards more sustainable solutions. Covalent organic frameworks (COFs), attributable to their structural designability with atomic precision, functionalizable chemical environment and robust extended architectures, have demonstrated promising performances in CO2 trapping and water harvesting from air. In this Review, we discuss the major developments in this field as well as sketch out the opportunities and shortcomings that remain over large-scale COF synthesis, device engineering, and long-term performance in real environments.






29)"Atomically Precise Metal Nanoclusters as Catalysts for Electrocatalytic CO2 Reduction"
　T. Kawawaki,* T. Okada, D. Hirayama, Y. Negishi*
　Green Chem. 26, 122-163 (2024).
　Invited Review to "Advances in Electrosynthesis for a Greener Chemical Industry"
　Selected as "Outside Back Cover"

Electrochemical carbon dioxide (CO2) reduction can be used to convert CO2 into various compounds at room temperature and ambient pressure using electricity generated from renewable energy sources. This technology is indispensable in establishing an environmentally responsible and sustainable society. However, further improvements in the activity and selectivity require the development of electrocatalysts that can directly serve as the actual reaction sites. In recent years, metal nanoclusters, which are metal particles with a size of approximately 1 nm, have been reported to be capable of electrochemical CO2 reduction with high activity and selectivity, owing to their unique geometric/electronic structure. This review summarizes the synthesis methods of atomically precise metal nanoclusters and their application in electrochemical CO2 reduction. We expect that this review will help clarify the current status of these studies and further accelerate the research on highly active and selective CO2 reduction catalysts using metal nanoclusters.






28)"Elucidation of the Electronic Structures of Thiolate-Protected Gold Nanoclusters by Electrochemical Measurements"
　T. Kawawaki,* Y. Negishi*
　Dalton Trans. 52, 15152-15167 (2023).
　Invited Review to "The New Talent: Asia-Pacific themed issue" (Invited by Associate editor, Prof. Takashi Uemura)
　Selected as "Inside Front Cover"
　Selected as "Dalton Transactions HOT Articles"
　Selected as "2023 Frontier and Perspective articles"

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






27)"Advances in Cu Nanocluster Catalyst Design: Recent Progress and Promising Applications"
　S. Biswas, S. Das,* Y. Negishi*
　Nanoscale Horiz. 8, 1509-1522 (2023).
　Selected as "Inside Front Cover"
　Selected as "Nanoscale Horizons Most Popular 2023 Articles"
　Selected as "Nanoscale Horizons HOT Papers"

The quest for cleaner pathways to the production of fuels and chemicals from non-fossil feedstock, efficient transformation of raw materials to value-added chemicals under mild conditions, and control over the activity and selectivity of chemical processes are driving the state-of-the-art approaches to the construction and precise chemical modification of sustainable nanocatalysts. As a burgeoning category of atomically precise noble metal nanoclusters, copper nanoclusters (Cu NCs) benefitting from their exclusive structural architecture, ingenious designability of active sites and high surface-to-volume ratio qualify as potential rationally-designed catalysts. In this Minireview, we present a detailed coverage of the optimal design strategies and controlled synthesis of Cu NC catalysts with a focus on tuning of active sites at the atomic level, the implications of cluster size, shape and structure, the ligands and heteroatom doping on catalytic activity, and reaction scope ranging from chemical catalysis to emerging photocatalysis and electrocatalysis.






26)"Progress and Prospects in the Design of Functional Atomically-precise Ag(I)-thiolate Nanoclusters and Their Assembly Approaches"
　S. Biswas, S. Das, Y. Negishi*
　Coord. Chem. Rev. 492, 215255 (2023).

Atomically-precise noble metal nanoclusters have raised nanomaterials research to new heights by merit of their distinct structural, optical and electronic properties, which can be leveraged for wide-ranging applications. The photoluminescence characteristics has given greater prominence to thiolate-protected silver nanoclusters (Ag NCs) for sensing, bioimaging and photovoltaics. Notwithstanding high potential, the susceptibility to oxidation has left Ag NCs with limited applicability. Reticular chemistry providing a platform for the transformation of discrete entities to extended structures, Ag NCs could be linked with organic struts via strong coordination bonds to form periodic networks called silver cluster-assembled materials (SCAMs) that revealed abundant catalytically active sites and unprecedented architectural stability. This Review highlights the recent developments in the synthesis of Ag(I)-thiolate NCs with special emphasis on size control, stability and optical properties, besides discussing the synergistic interactions between Ag NCs and organic linkers in coordination-assembled SCAMs facilitating the activity as well as morphology evolution from supramolecular self-assembly of Ag NCs. Finally, we provide an outlook of the challenges and opportunities covering from fundamentals to functional devices and applications.






25)"Anion-templated Silver Nanoclusters: Precise Synthesis and Geometric Structure"
　Y. Horita, M. Ishimi, Y. Negishi*
　Sci. Technol. Adv. Mater. 24, 2203832 (2023).
　Invited Review to "Special Issue"

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






24)"Vertex-Shared Linear Superatomic Molecules: Stepping Stones to Novel Materials Composed of Noble Metal Clusters"
　Y. Niihori, S. Miyajima, A. Ikeda, T. Kosaka, Y. Negishi*
　Small Sci. 3, 2370009 (2023).
　Invited Review (Invited by Editor, Dr. Sneha Rhode)
　Selected as "Outiside Front Cover"

Extremely small metal clusters composed of noble metal atoms (M) have the orbitals similar to those of atoms and therefore can be thought of as artificial atoms or superatoms. If these superatoms can be assembled into molecular analogues, it might be possible to create materials with new characteristics and properties that are different from those of existing substances. Therefore, the concept of superatomic molecules has attracted significant attention. The present review focuses on vertex-shared linear M12n+1 superatomic molecules formed via the sharing of a single metal atom between M13 superatoms having icosahedral cores and summarizes the knowledge obtained to date in this regard. This summary discusses the most suitable ligand combinations for the synthesis of M12n+1 superatomic molecules along with the valence electron numbers, stability, optical absorption characteristics and luminescence properties of the M12n+1 superatomic molecules fabricated to date. This information is expected to assist in the production of many M12n+1 superatomic molecules with novel structures and physicochemical properties in the future.






23)"Promoting Photocatalytic Carbon-Dioxide Reduction by Tuning the Properties of Cocatalysts"
　T. Kawawaki, Y. Akinaga, D. Yazaki, H. Kameko, D. Hirayama, Y. Negishi*
　Chem. Euro. J. 29, e202203387 (2023).
　Invited "Review" to special collection "Photocatalysis for a Brighter Future"
　Selected as "Inside Cover"
　Selected as "Chemistry Europe Reviews"
　Selected as "Editors' Choice: Spotlights"
　Selected as "Hot Topic: Carbon Dioxide"
　Selected as "Hot Topic: Photocatalysts"
　Selected as "Review Showcase"
　Highlighted in "Frontispiece"

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






22)"Atomically Precise Thiolate-Protected Gold Nanoclusters: Current Status of Designability of the Structure and Physicochemical Properties"
　S. Hossain, D. Hirayama, A. Ikeda, M. Ishimi, S. Funaki, A. Samanta, T. Kawawaki, Y. Negishi*
　Aggregate, 4, e255 (2023).
　Invited Review (Invited by Editor-in-Chief, Prof. Ben Zhong Tang)
　Selected as "Inside Front Cover"

Thiolate (SR)-protected gold nanoclusters (Aun(SR)m NCs) are a rare type of material capable of simultaneously exhibiting multiple physicochemical properties well-suited to specific applications, including photoluminescence, thermocatalysis, electrocatalysis, photocatalysis, magnetism and optical activity. Over the past several decades, there has been tremendous progress in our understanding of the structure and physicochemical properties of Aun(SR)m NCs, resulting in the ability to fine-tune the characteristics of these materials. It is therefore helpful to examine the extent to which the properties of Aun(SR)m and related metal NCs can now be adjusted based on design. This review presents representative examples of previous studies concerning the geometry, electronic structure, luminescence properties, catalysis, magnetism and optical activity of Aun(SR)m and related metal NCs, and discusses the current status of the designability of these NCs to impart specific structural and physicochemical characteristics. This information is expected to accelerate the fabrication of highly functional materials based on Aun(SR)m and related metal NCs.






21)"Metal-nanocluster Science and Technology: My Personal History and Outlook"
　Y. Negishi
　Phys. Chem. Chem. Phys, 24, 7569-7594 (2022).
　Invited "Perspective"
　Selected as "Outside Front Cover"
　Selected as "2022 HOT Article"

Metal nanoclusters (NCs) are among the leading targets in research of nanoscale materials, and elucidation of their properties (science) and development of control techniques (technology) have been continuously studied for the past 60 years or so. I have been continuously studying metal NCs since 1995 while changing my own interests, and thereby research topics, according to development of the field and the global situation. Specifically, I started my research on gas-phase metal NCs, and then investigated systematic isolation of thiolate-protected gold NCs. Thereafter, I worked on development of flexible and precise techniques for controlling ligand-protected metal NCs and creation of highly functional energy and environmental catalysts using ligand-protected metal NCs. This personal account summarizes my previous studies on metal NCs. Through this personal history, I would like to describe my motivation for each previous subject, what type of research I want to perform for each subject in the future, and how I consider the outlook of the field.






20)"Development and Functionalization of Visible-light-driven Water-splitting Photocatalysts"
　T. Kawawaki, M. Kawachi, D. Yazaki, Y. Akinaga, D. Hirayama, Y. Negishi*
　Nanomaterials, 12, 344 (2022).
　Invited Review to Special Issue "Advanced Nanomaterials for Water Splitting"
　Highlighted in "Inside Cover"
　Highlighted in "Nanomaterials MDPI Twitter"

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






19)"Supported, ~1-nm-Sized Platinum Clusters: Controlled Preparation and Enhanced Catalytic Activity"
　T. Kawawaki, N. Shimizu, Y. Mitomi, D. Yazaki, S. Hossain, Y. Negishi*
　Bull. Chem. Soc. Jpn, 94, 2853-2870 (2021).
　Invited Accunt to Special Issue "Masterpiece Materials with Functional Excellence"
　Highlighted in "Inside Cover"(Twittter 1/2)
　Highlighted in "Account/Review Collection 2020-2021"

We have been aiming to reduce the amount of platinum (Pt) needed in catalysts for automobile exhaust-gas purification and fuel cell electrodes. To achieve this, we have attempted to: 1) establish simple methods for synthesizing ligand-protected ~1-nm-sized Pt clusters with a narrow distribution in the number of constituent atoms; 2) load these clusters onto supports, while retaining their number of constituent atoms, to prepare supported ~1-nm-sized Pt clusters; and 3) elucidate the catalytic activity of each type of supported ~1-nm-sized Pt cluster. These studies have revealed that: 1) ligand-protected ~1-nm-sized Pt clusters stable in the atmosphere can be isolated with high purity by a combination of polyol reduction and ligand-exchange reaction; 2) ~1-nm-sized Pt clusters can be loaded onto the support without aggregation when the clusters are adsorbed on the support and then calcined at an appropriate temperature; and 3) Pt17 clusters loaded onto γ-alumina exhibit high activity and durability for exhaust-gas purification, whereas Ptn clusters (n = ~35, ~51, or ~66) loaded onto carbon black exhibit high activity and durability for the oxygen reduction reaction (which occurs at fuel cell electrodes). This account describes our previous studies and explores future prospects for supported ~1-nm-sized Pt clusters.






18)"Thiolate-Protected Metal Nanoclusters: Recent Development in Synthesis, Understanding of Reaction, and Application in Energy and Environmental Field"
　T. Kawawaki, A. Ebina, Y. Hosokawa, S. Ozaki, D. Suzuki, S. Hossain, Y. Negishi*
　Small 17, 202005328 (2021).
　Invited Review to "Special Issue"
　Selected as"Frontispiece"
　Selected as "Hot Topic: Water Splitting"

Metal nanoclusters (NCs), which are composed of about 250 or fewer metal atoms, possess great potential as novel functional materials. Fundamental research on metal NCs gradually started in the 1960s, and since 2000, thiolate (SR)-protected metal NCs have been the main metal NCs actively studied. The precise and systematic isolation of SR-protected metal NCs has been achieved in 2005. Since then, research on SR-protected metal NCs for both basic science and practical application has rapidly expanded. This review describes this recent progress in the field of SR-protected metal NCs in three areas: synthesis, understanding, and application. Specifically, the recent study of alloy NCs and connected structures composed of NCs is highlighted in the “synthesis” section, recent knowledge on the reactivity of NCs in solution is highlighted in the “understanding” section, and the applications of NCs in the energy and environmental field are highlighted in the “application” section. This review provides insight on the current state of research on SR-protected metal NCs and discusses the challenges to be overcome for further development in this field as well as the possibilities that these materials can contribute to solving the problems facing modern society.






17)"Toward the Creation of High-performance Heterogeneous Catalysts by Controlled Ligand Desorption from Atomically Precise Metal Nanoclusters"
　T. Kawawaki, Y. Kataoka, M. Hirata, Y. Iwamatsu, S. Hossain, Y. Negishi*
　Nanoscale Horiz. 6, 409-448 (2021).
　Invited "Review"
　Selected as "Outside Front Cover"
　Selected as "Nanoscale 2022 Lunar New Year Collection"
　Selected as "Nanoscale Most Popular 2021 Articles"
　Top 10% of highly cited articles from the Royal Society of Chemistry

Ligand-protected metal nanoclusters controlled by atomic accuracy (i. e. atomically precise metal NCs) have recently attracted considerable attention as active sites in heterogeneous catalysts. Using these atomically precise metal NCs, it becomes possible to create novel heterogeneous catalysts based on a size-specific electronic/geometrical structure of metal NCs and understand the mechanism of the catalytic reaction easily. However, to create high-performance heterogeneous catalysts using atomically precise metal NCs, it is often necessary to remove the ligands from the metal NCs. This review summarizes previous studies on the creation of heterogeneous catalysts using atomically precise metal NCs while focusing on the calcination as a ligand-elimination method. Through this summary, we intend to share state-of-art techniques and knowledge on (1) experimental conditions suitable for creating high-performance heterogeneous catalysts (e.g., support type, metal NC type, ligand type, and calcination temperature), (2) the mechanism of calcination, and (3) the mechanism of catalytic reaction over the created heterogeneous catalyst. We also discuss (4) issues that should be addressed in the future toward the creation of high-performance heterogeneous catalysts using atomically precise metal NCs. The knowledge and issues described in this review are expected to lead to clear design guidelines for the creation of novel heterogeneous catalysts.






16)"Creation of Active Water-splitting Photocatalysts by Controlling Cocatalysts Using Atomically Precise Metal Nanoclusters"
　T. Kawawaki,Y. Kataoka, S. Ozaki, M. Kawachi, M. Hirata, Y. Negishi*
　Chem. Commun. 57, 417-44 (2021).
　Invited "Feature Article"
　Selected as "Inside Front Cover"

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






15)"Atomically Precise Alloy Nanoclusters"
　T. Kawawaki, Y. Imai, D. Suzuki, S. Kato, I. Kobayashi, T. Suzuki, R. Kaneko, S. Hossain, Y. Negishi*
　Chem. Euro. J. 26, 16150-16193 (2020).
　Invited "Review"
　Selected as "Front Cover" and "Cover Profile"
　Selected as "Hot Topic: Gold"
　Highlighted in "Frontispiece"

Metal nanoclusters (NCs) have a particle size of about one nanometer, which makes them the smallest unit that can give a function to a substance. In addition, metal NCs possess physical and chemical properties that are different from those of the corresponding bulk metals. Metal NCs with such characteristics are expected to be important for use in nanotechnology. Research on the precise synthesis of metal NCs and elucidation of their physical/chemical properties and functions is being actively conducted. When metal NCs are alloyed, it is possible to obtain further various electronic and geometrical structures and functions. Thus, research on alloy NCs has become a hot topic in the study of metal NCs and the number of publications on alloy NCs has increased explosively in recent years. Such publications have provided much insight into the effects of alloying on the electronic structure and function of metal NCs. However, the rapid increase in knowledge has made it difficult for researchers (especially those new to the field) to grasp all of it. Therefore, in this review, we summarize the reported chemical composition, geometrical structure, electronic structure, and physical and chemical properties of Aun−xMx(SR)m, Agn−xMx(SR)m, Aun−xMx(PR3)l(SR)m, and Agn−xMx(PR3)l(SR)m (Au=gold, Ag=silver, M=heteroatom, PR3=phosphine, and SR=thiolate) NCs. This review is expected to help researchers understand the characteristics of alloy NCs and lead to clear design guidelines to develop new alloy NCs with intended functions.






14)"Controlled Colloidal Metal Nanoparticles and Nanoclusters: Recent Applications as Cocatalysts for Improving Photocatalytic Water-splitting Activity"
　T. Kawawaki, Y. Mori, K. Wakamatsu, S. Ozaki, M. Kawachi, S. Hossain, Y. Negishi*
　J. Mater. Chem. A, 8, 16081-16113 (2020)..
　Invited "Review"
　Top 5% of highly cited works from Japanese institutions

In recent years, research on the use of metal nanoparticles (NPs) and nanoclusters (NCs) synthesized by liquid-phase reduction in water-splitting photocatalysts has been actively conducted. Water-splitting photocatalysts have been attracting attention because they can produce hydrogen (H2), which is attractive as a next-generation energy source, from solar energy and water. However, further improvement of water-splitting photocatalysts is required for their practical use in society. Recent studies have demonstrated that the active sites (cocatalysts) of water-splitting photocatalysts can be controlled using the advanced NP/NC syntheses and structural modulation techniques established in the fields of colloid, NP, and NC chemistry and thereby highly active water-splitting photocatalysts can be developed. If such research progresses further, it is expected that a transition to a new society using H2 as the main energy source will become possible. However, such applied research has just started and examples of such research are currently limited. The purpose of this review is to introduce the importance of controlled colloidal NPs/NCs in research on water-splitting photocatalysis to readers by summarizing the existing research. We hope that this review will raise interest in the application of metal NPs/NCs in water-splitting photocatalysis and that a society actively addressing energy and environmental problems will become a reality as soon as possible.






13)"One-, Two-, and Three-Dimensional Self-Assembly of Atomically Precise Metal Nanoclusters"
　A. Ebina, S. Hossain, H. Horihata, S. Ozaki, S. Kato, T. Kawawaki, Y. Negishi*
　Nanomaterials, 10, 1105, (2020).
　Invited Review to "Special Issue"

Metal nanoclusters (NCs), which consist of several, to about one hundred, metal atoms, have attracted much attention as functional nanomaterials for use in nanotechnology. Because of their fine particle size, metal NCs exhibit physical/chemical properties and functions different from those of the corresponding bulk metal. In recent years, many techniques to precisely synthesize metal NCs have been developed. However, to apply these metal NCs in devices and as next-generation materials, it is necessary to assemble metal NCs to a size that is easy to handle. Recently, multiple techniques have been developed to form one-, two-, and three-dimensional connected structures (CSs) of metal NCs through self-assembly. Further progress of these techniques will promote the development of nanomaterials that take advantage of the characteristics of metal NCs. This review summarizes previous research on the CSs of metal NCs. We hope that this review will allow readers to obtain a general understanding of the formation and functions of CSs and that the obtained knowledge will help to establish clear design guidelines for fabricating new CSs with desired functions in the future.






12)"Atomic-level Separation of Thiolate-protected Metal Clusters"
　Y. Negishi,* S. Hashimoto, A. Ebina, K. Hamada, S. Hossain, T. Kawawaki
　Nanoscale, 12, 8017-8039 (2020).
　Invited "Review"
　Selected as "Inside Front Cover"

Fine metal clusters have attracted much attention from the viewpoints of both basic and applied science for many years because of their unique physical/chemical properties and functions, which differ from those of bulk metals. Among these materials, thiolate (SR)-protected gold clusters (Aun(SR)m clusters) have been the most studied metal clusters since 2000 because of their ease of synthesis and handling. However, in the early 2000s, it was not easy to isolate these metal clusters. Therefore, high-resolution separation methods were explored, and several atomic-level separation methods, including polyacrylamide gel electrophoresis (PAGE), high-performance liquid chromatography (HPLC), and thin-layer chromatography (TLC), were successively established. These techniques have made it possible to isolate a series of Aun(SR)m clusters, and much knowledge has been obtained on the correlation between the chemical composition and fundamental properties such as the stability, electronic structure, and physical properties of Aun(SR)m clusters. In addition, these high-resolution separation techniques are now also frequently used to evaluate the distribution of the product and to track the reaction process. In this way, high-resolution separation techniques have played an essential role in the study of Aun(SR)m clusters. However, only a few reviews have focused on this work. This review focuses on PAGE, HPLC, and TLC separation techniques, which offer high resolution and repeatability, and summarizes previous studies on the high-resolution separation of Aun(SR)m and related clusters with the purpose of promoting a better understanding of the features and the utility of these techniques.






11)"Gold Nanoclusters as Electrocatalysts for Energy Conversion"
　T. Kawawaki, Y. Negishi*
　Nanomaterials, 10, 238 (2020).
　Invited Review to "Special Issue"

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.






10)"Photo/electrocatalysis and Photosensitization Using Metal Nanoclusters for Green Energy and Medical Applications"
　T. Kawawaki, Y. Negishi*, H. Kawasaki*
　Nanoscale Adv. 2, 17-36 (2020).
　Invited "Review"
　Selected as "Inside FrontCover"
　Selected as a part of "Themed Collection: Photocatalysis and Photoelectrochemistry"
　Selected as "Editor's Choice: Single-atom and nanocluster catalysis"
　Top 5% of highly cited works from Japanese institutions

Owing to the rapidly increasing demand for sustainable technologies in fields such as energy, environmental science, and medicine, nanomaterial-based photo/electrocatalysis has received increasing attention. Recently, synthetic innovations have allowed the fabrication of atomically precise metal nanoclusters (NCs). These NCs show potential for green energy and medical applications. The present article primarily focuses on evaluation of the recent developments in the photo/electrocatalytic and photosensitizing characteristics of metal and alloy NCs. The review comprises two sections: (i) photo/electrocatalysis for green energy and (ii) photosensitization for biomedical therapy applications. Finally, the challenges associated with the use of metal NCs are presented on the basis of current developments.






9)"Deepening the Understanding on Thiolate-Protected Metal Clusters Using High-Performance Liquid Chromatography"
　Y. Niihori, K. Yoshida, S. Hossain, W. Kurashige, Y. Negishi*
　Bull. Chem. Soc. Jpn, 92, 664-695 (2019)..
　Invited Accunt to Special Issue"Material Innovation"
　HIghlighted in "Inside Cover"
　HIghlighted in "Account/Review Collection 2019-20"

Thiolate (SR)-protected metal clusters have been extensively studied by using various structural analysis methods since the start of research into these clusters. We have also studied these clusters based on common analysis methods used by many other research groups. However, we also have actively worked towards efficient application of high-performance liquid chromatography (HPLC) to study these clusters. Consequently, we have achieved high-resolution separations of several SR-protected gold and alloy clusters. By realizing such high-resolution separations, we have achieved a deeper understanding of a number of issues, including: 1) the transition size from bulk to non-bulk behavior in dodecanethiolate-protected gold clusters; 2) heteroatom substitution effects on the electronic structures and the dependence of isomer distributions on experimental conditions in hydrophobic SR-protected alloy clusters; 3) the mechanism of ligand-exchange reactions in hydrophobic metal clusters; and 4) the chemical composition of products in hydrophilic metal clusters. These results have clearly demonstrated that HPLC separation and analysis are extremely effective in helping to understand the fundamental properties of SR-protected metal clusters.






8)"Alloy Clusters: Precise Synthesis and Mixing Effects"
　S. Hossain, Y. Niihori, L. V. Nair, B. Kumar, W. Kurashige, Y. Negishi*
　Acc. Chem. Res., 51, 3114-3124 (2018).
　Invited Article to Special Issue"Toward Atomic Precision in Nanoscience"
　Selected as "Supplementary Cover"

Metal alloys exhibit functionalities unlike those of single metals. Such alloying has drawn considerable research interest, particularly for nanoscale particles (metal clusters/nanoparticles), from the viewpoint of creating new functional nanomaterials. In gas phase cluster research, generated alloy clusters can be spatially separated with atomic precision in vacuum. Thus, the influences of increases or decreases in each element on the overall electronic structure of the cluster can be elucidated. However, to further understand the related mixing and synergistic effects, alloy clusters need to be produced on a large scale and characterized by various techniques. Because alloy clusters protected by thiolate (SR) can be synthesized by chemical methods and are stable in both solution and the solid state, these clusters are ideal study materials to better understand the mixing and synergistic effects. Moreover, the alloy clusters thus created have potential applications as functional materials. Therefore, since 2008, we have been working on establishing a precise synthesis method for SR-protected alloy clusters and elucidating their mixing and synergistic effects.
Early research focused on the precise synthesis of alloy clusters wherein some of the Au in the stable SR-protected gold clusters ([Au25(SR)18]− and [Au38(SR)24]0) is replaced by Pd, Ag, or Cu. These studies have shown that Pd, Ag, or Cu substitute at different metal sites. We also have examined the as-synthesized alloy clusters to clarify the effect of substitution by each element on the physical and chemical properties of the clusters. However, in early studies, the number of substitutions could not be controlled with atomic accuracy for [Au25–xMx(SR)18]− (M = Ag or Cu). Then, in following research, methods have been established to obtain alloy clusters with control over the composition. We have succeeded in developing a method for controlling the number of Ag substitutions with atomic precision and thereby elucidating the effect of Ag substitution on the electronic structure of clusters with atomic precision. Concurrently, we also studied alloy clusters containing multiple heteroelements with different preferential substitution sites. These results revealed that the effects of substitution of each element can be superimposed on the cluster by combining multiple elemental substitutions at different sites. In addition, we successfully developed methods to synthesize alloy clusters with heterometal core. These findings are expected to lead to clear design guidelines for developing new functional nanomaterials.






7)"Understanding and Practical Use of Ligand and Metal Exchange Reactions in Thiolate-Protected Metal Clusters to Synthesize Controlled Metal Clusters"
　Y. Niihori, S. Hossain, S. Sharma, B. Kumar, W. Kurashige, Y. Negishi*
　Chem. Rec. (Personal Accounts), 17, 473-484 (2017).
　Invited "Personal Accounts"
　Selected as "Outside Front Cover"

It is now possible to accurately synthesize thiolate (SR)-protected gold clusters (Aun(SR)m) with various chemical compositions with atomic precision. The geometric structure, electronic structure, physical properties, and functions of these clusters are well known. In contrast, the ligand or metal atom exchange reactions between these clusters and other substances have not been studied extensively until recently, even though these phenomena were observed during early studies. Understanding the mechanisms of these reactions could allow desired functional metal clusters to be produced via exchange reactions. Therefore, we have studied the exchange reactions between Aun(SR)m and analogous clusters and other substances for the past four years. The results have enabled us to gain deep understanding of ligand exchange with respect to preferential exchange sites, acceleration means, effect on electronic structure, and intercluster exchange. We have also synthesized several new metal clusters using ligand and metal exchange reactions. In this account, we summarize our research on ligand and metal exchange reactions.






6)"Perspective: Exchange Reactions in Thiolate-Protected Metal Clusters"
　Y. Niihori, S. Hossain, B. Kumar, L. V. Nair, W. Kurashige, Y. Negishi*
　APL Mater. (Perspective), 5, 053201 (2017).
　Invited "Perspective" to "Special Issue"
　Highlighted in "Featured"

Thiolate-protected metal clusters can exchange ligands or metal atoms with other substances such as coexisting ligands, complexes, and metal clusters in solution. Using these reactions, it is possible to synthesize metal clusters with new physical and chemical properties. Although the occurrence of such reactions was recognized nearly 20 years ago, their details were not well understood. In recent years, techniques for the precise synthesis of metal clusters and their characterization have progressed considerably and, as a result, details of these reactions have been clarified. In this perspective, we focus on the most-studied thiolate-protected gold clusters and provide a summary of recent findings as well as future expectations concerning the exchange reactions of these clusters.






5)"Precise Synthesis, Functionalization and Application of Thiolate-Protected Gold Clusters"
　W. Kurashige, Y. Niihori, S. Sharma, Y. Negishi*
　Coord. Chem. Rev., 320-321, 238-250 (2016).
　Invited "Review" to "Special Issue"

Thiolate-protected gold clusters (Aun(SR)m) have received significant attention as new functional nanomaterials because they exhibit size-specific physical and chemical properties that are not seen in bulk gold. To date, our research group has studied the following three aspects of Aun(SR)m and related clusters: (1) the development of new methods allowing precise synthesis; (2) the establishment of new methods to impart high functionality; and (3) the utilization of these clusters as active sites in photocatalytic materials. This review summarizes our most recent work concerning these three subjects.






4)"High-Resolution Separation of Thiolate-Protected Gold Clusters by Reversed-Phase High-Performance Liquid Chromatography"
　Y. Niihori, C. Uchida, W. Kurashige, Y. Negishi*
　Phys. Chem. Chem. Phys. (Perspective), 18, 4251-4265 (2016).
　Invited "Perspective"
　Selected as "Outside Front Cover"
　Highly Cited Paper

Thiolate (RS)-protected gold clusters (Aun(SR)m) have attracted much attention as building blocks of functional nanomaterials. Our group has been studying the high-resolution separation of Aun(SR)m clusters using reversed-phase high-performance liquid chromatography. In this perspective, we summarize our recent results on the separation of Aun(SR)m clusters and their doped clusters according to the core size, charge state, ligand composition, and coordination isomers. Additionally, this perspective describes new findings obtained by using high-resolution separation and future prospects for the separation of such types of metal clusters. We believe that the techniques and knowledge gained in this study would contribute to the creation of Aun(SR)m clusters with the desired functions and associated functional nanomaterials.






3)"Recent Progress in the Functionalization Methods of Thiolate-Protected Gold Clusters"
　W. Kurashige, Y. Niihori, S. Sharma, Y. Negishi*
　J. Phys. Chem. Lett. (Perspective), 5, 4134-4142 (2014).
　Invited "Perspective"
　Selected as "Outside Front Cover"
　Highlighted in "Editorial"

Nanomaterials that exhibit both stability and functionality are currently considered to hold great promise as components of nanotechnology devices. Thiolate-protected gold clusters (Aun(SR)m) have long attracted attention as functional nanomaterials. Magic Aun(SR)m clusters are an especially stable group of thiolate-protected clusters that have particularly high potential as functional materials. Although numerous application experiments have been conducted for magic Aun(SR)m clusters, it is important that functionalization methods are also established to allow for effective utilization of these materials. The results of recent research on heteroatom doping and the use of other chalcogenide ligands strongly suggest that these strategies are promising as functionalization methods of magic Aun(SR)m clusters. In this Perspective, we focus on studies relating to three representative types of magic clusters—Au25(SR)18, Au38(SR)24, and Au144(SR)60—and discuss the recent progress and future issues.






2)"Towards the Creation of Functionalized Metal Nanoclusters and Highly Active Photocatalytic Materials Using Thiolate-Protected Magic Gold Clusters"
　Y. Negishi*
　Bull. Chem. Soc. Jpn (Award Accounts), 87, 375-389 (2014).
　Invited "Accounts" as "a Winner of CSJ Award for Young Chemists"
　Highlighted in "Back Cover"

Advances in developments in nanotechnology have encouraged the creation of highly functionalized nanomaterials. Thiolate-protected gold clusters (Aun(SR)m) less than 2 nm in size exhibit size-specific physical and chemical properties not observed in bulk metals. Therefore, they have attracted attention as functional units or building blocks in nanotechnology. The highly stable, magic Aun(SR)m clusters possess great potential as new nanomaterials. We are studying the following subjects related to magic Aun(SR)m clusters: (1) establishing methods to enhance their functionality, (2) developing high-resolution separation methods, and (3) utilizing the clusters as active sites in photocatalytic materials. Through these studies, we aim to create highly functional metal nanoclusters and apply them as highly active photocatalytic materials. The results of our efforts to date are summarized in this paper.






1)"Toward the Creation of Stable, Functionalized Metal Clusters"
　Y. Negishi,* W. Kurashige, Y. Niihori, K. Nobusada
　Phys. Chem. Chem. Phys. (Perspective), 15, 18736-18751 (2013).
　Invited "Perspective"
　Selected as "Outside Front Cover"
　Selected as "HOT article"

Nanomaterials which exhibit both stability and functionality are currently considered to hold the most promise as components of nanotechnology devices. Thiolate (RS)-protected gold nanoclusters (Aun(SR)m) have attracted significant attention in this regard and, among these, the magic clusters are believed to be the best candidates since they are the most stable. We have investigated the effects of heteroatom doping, protection by selenolate ligands and protection by photoresponsive thiolates on the stability and physical/chemical properties of these clusters. Through such studies, we have attempted to establish methods of modifying magic Aun(SR)m clusters as a means of creating metal clusters that are both robust and functional. This paper summarizes our studies towards this goal and the obtained results.