Tiara-like Hexanuclear Nickel-Platinum Alloy Nanocluster.

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

Intrinsic Visible Plasmonic Properties of Colloidal PtIn2 Intermetallic Nanoparticles.

Materials that intrinsically exhibit localized surface plasmon resonance (LSPR) in the visible region have been predominantly researched on nanoparticles (NPs) composed of coinage metals, namely Au, Ag, and Cu. Here, as a coinage metal-free intermetallic NPs, colloidal PtIn2 NPs with a C1 (CaF2 -type) crystal structure are synthesized by the liquid phase method, which evidently exhibit LSPR at wavelengths similar to face-centered cubic (fcc)-Au NPs. Computational simulations pointed out differences in the electronic structure and photo-excited electron dynamics between C1-PtIn2 and fcc-Au NPs; reduces interband transition and stronger screening with smaller number of bound d-electrons compare with fcc-Au are unique origins of the visible plasmonic nature of C1-PtIn2 NPs. These results strongly indicate that the intermetallic NPs are expected to address the development of alternative plasmonic materials by tuning their crystal structure and composition.

Clarifying the Electronic Structure of Anion-Templated Silver Nanoclusters by Optical Absorption Spectroscopy and Theoretical Calculation.

Electronic structures of anion-templated silver nanoclusters (Ag NCs) are not well understood compared to conventional, template-free Ag NCs. In this study, we synthesized three new anion-templated Ag NCs, namely [S@Ag17(S-4CBM)15(PPh3)5]0, [S@Ag18(S-4CBM)16(PPh3)8]0, and [Cl@Ag18(S-4CBM)16(PPh3)8][PPh4], where S-4CBM = 4-chlorobenzene methanethiolate, and single-crystal X-ray crystallography revealed that they have S@Ag6, S@Ag10, and Cl@Ag10 cores, respectively. Investigation of their electronic structures by optical spectroscopy and theoretical calculations elucidated the following unique features: (1) their electronic structures are different from those of template-free Ag NCs described by the superatomic concept; (2) optical absorption in the range of 550-400 nm for S2--templated Ag NCs is attributed to the charge transitions from S2--templated Ag-cage orbitals to the s-shaped orbital in the S2- moiety; (3) the Cl--templated Ag NCs can be viewed as [Cl@Ag18(S-4CBM)16(PPh3)8]0[PPh4]0 rather than the ion pair [Cl@Ag18(S-4CBM)16(PPh3)8]-[PPh4]+; and (4) singlet-coupled singly occupied orbitals are involved in the optical absorption of the Cl--templated Ag NC.

A new two-dimensional luminescent Ag12 cluster-assembled material and its catalytic activity for reduction of hexacyanoferrate(III).

Silver cluster-assembled materials (SCAMs) have garnered significant interest as promising platforms for different functional explorations. Their atomically precise structures, intriguing chemical/physical properties, and remarkable luminescent capabilities make them highly appealing. However, the properties of these materials are primarily determined by their structural architecture, which is heavily influenced by the linker molecules used in their assembly. The choice of linker molecules plays a pivotal role in shaping the structural characteristics and ultimately determining the unique properties of SCAMs. To this end, the first SCAM with an intriguing (3,6)-connected kgd topology, [Ag12(StBu)6(CF3COO)6(TPBTC)6]n (termed TUS 3), TPBTC = benzene-1,3,5-tricarboxylic acid tris-pyridin-4-ylamide, has been synthesized by reticulating C6-symmetric Ag12 cluster cores with C3-symmetric tripodal pyridine linkers. Due to the structutural architecture of the linker molecule, TUS 3 posseses a luminescent porous framework structure where each two-dimensional (2D) layers are non-covalently linked with each other to form a three dimensional (3D) framework and ultimately offers uniaxial open channels. The compact mesoporous structural architecture not only gives the excellent surface area but also offers impressive stability of this material even in water medium. Taking advantage of these properties, TUS 3 shows brilliant catalytic activity in the reduction of hexacyanoferrate(III) using sodium borohydride in aqueous solutions.

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

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

Five Novel Silver-Based Coordination Polymers as Photoluminescent Sensing Platforms for the Detection of Nitrobenzene.

Nitrobenzene (NB) is a highly toxic chemical and a cause for concern to human health and the environment. Hence, it is worth designing new efficient and robust sensing platforms for NB. In this study, we present three newly synthesized luminescent silver cluster-based coordination polymers, {[Ag10 (StBu)6 (CF3 COO)4 (hpbt)] (DMAc)2 (CH3 CN)2 }n (hpbt=N,N,N',N'N",N"-hexa(pyridine-4-yl)benzene-1,3,5-triamine), [Ag12 (StBu)6 (CF3 COO)6 (bpva)3 ]n (bpva=9,10-Bis(2-(pyridin-4-yl)vinyl)anthracene), and {[Ag12 (StBu)6 (CF3 COO)6 (bpb)(DMAc)2 (H2 O)2 ] (DMAc)2 }n (bpb=1,4-Bis(4-pyridyl)benzene) composed of Ag10 , Ag12 and Ag12 cluster cores, respectively, connected by multidentate pyridine linkers. In addition, two new luminescent polymorphic silver(I)-based coordination polymers, [Ag(CF3 COO)(dpa)]n (dpa=9,10-di(4-pyridyl)anthracene) referred to as Agdpa (H) and Agdpa (R), where H and R denote hexagon- and rod-like crystal shapes, respectively, have been prepared. The coordination polymers exhibit highly sensitive luminescence quenching effects to NB, attributed to the π-π stacking interactions between the polymers and NB as well as the electron-withdrawing character of NB.

Carbon Nitride Loaded with an Ultrafine, Monodisperse, Metallic Platinum-Cluster Cocatalyst for the Photocatalytic Hydrogen-Evolution Reaction.

For the realization of a next-generation energy society, further improvement in the activity of water-splitting photocatalysts is essential. Platinum (Pt) is predicted to be the most effective cocatalyst for hydrogen evolution from water. However, when the number of active sites is increased by decreasing the particle size, the Pt cocatalyst is easily oxidized and thereby loses its activity. In this study, a method to load ultrafine, monodisperse, metallic Pt nanoclusters (NCs) on graphitic carbon nitride is developed, which is a promising visible-light-driven photocatalyst. In this photocatalyst, a part of the surface of the Pt NCs is protected by sulfur atoms, preventing oxidation. Consequently, the hydrogen-evolution activity per loading weight of Pt cocatalyst is significantly improved, 53 times, compared with that of a Pt-cocatalyst loaded photocatalyst by the conventional method. The developed method is also effective to enhance the overall water-splitting activity of other advanced photocatalysts such as SrTiO3 and BaLa4 Ti4 O15 .

Pt17 nanocluster electrocatalysts: preparation and origin of high oxygen reduction reaction activity.

We recently found that [Pt17(CO)12(PPh3)8]z (Pt = platinum; CO = carbon monoxide; PPh3 = triphenylphosphine; z = 1+ or 2+) is a Pt nanocluster (Pt NC) that can be synthesized with atomic precision in air. The present study demonstrates that it is possible to prepare a Pt17-supported carbon black (CB) catalyst (Pt17/CB) with 2.1 times higher oxygen reduction reaction (ORR) activity than commercial Pt nanoparticles/CB by the adsorption of [Pt17(CO)12(PPh3)8]z onto CB and subsequent calcination of the catalyst. Density functional theory calculation strongly suggests that the high ORR activity of Pt17/CB originates from the surface Pt atoms that have an electronic structure appropriate for the progress of ORR. These results are expected to provide design guidelines for the fabrication of highly active ORR catalysts using Pt NCs with a diameter of about 1 nm and thereby enabling the use of reduced amounts of Pt in polymer electrolyte fuel cells.

Key factors for connecting silver-based icosahedral superatoms by vertex sharing.

Metal nanoclusters composed of noble elements such as gold (Au) or silver (Ag) are regarded as superatoms. In recent years, the understanding of the materials composed of superatoms, which are often called superatomic molecules, has gradually progressed for Au-based materials. However, there is still little information on Ag-based superatomic molecules. In the present study, we synthesise two di-superatomic molecules with Ag as the main constituent element and reveal the three essential conditions for the formation and isolation of a superatomic molecule comprising two Ag13-xMx structures (M = Ag or other metal; x = number of M) connected by vertex sharing. The effects of the central atom and the type of bridging halogen on the electronic structure of the resulting superatomic molecule are also clarified in detail. These findings are expected to provide clear design guidelines for the creation of superatomic molecules with various properties and functions.

Improved activity for the oxygen evolution reaction using a tiara-like thiolate-protected nickel nanocluster.

Practical electrochemical water splitting and carbon-dioxide reduction are desirable for a sustainable energy society. In particular, facilitating the oxygen evolution reaction (OER, the reaction at the anode) will increase the efficiency of these reactions. Nickel (Ni) compounds are excellent OER catalysts under basic conditions, and atomically precise Ni clusters have been actively studied to understand their complex reaction mechanisms. In this study, we evaluated the geometric/electronic structure of tiara-like metal nanoclusters [Nin(PET)2n; n = 4, 5, 6, where PET refers to phenylethanethiolate] with the same SR ligand. The geometric structure of Ni5(SR)10 was determined for the first time using single-crystal X-ray diffraction. Additionally, combined electrochemical measurements and X-ray absorption fine structure measurements revealed that Ni5(SR)10 easily forms an OER intermediate and therefore exhibits a high specific activity.

Promoting Photocatalytic Carbon Dioxide Reduction by Tuning the Properties of Cocatalysts.

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

Composition control of alloy nanoparticles consisting of bulk-immiscible Au and Rh metals via an ionic liquid/metal sputtering technique for improving their electrocatalytic activity.

AuRh bimetallic alloy nanoparticles (NPs) were successfully prepared by simultaneous sputtering of Au and Rh in a room-temperature ionic liquid (RTIL) of N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate (DEME-BF4). Bimetallic AuRh alloy NPs of 1-2 nm in size were formed in the RTIL. The alloy composition was controllable by changing the surface areas of Au and Rh plates used as sputtering targets. Loading thus-obtained AuRh NPs on carbon black (CB) powders increased the size of AuRh NPs to ca. 2-8 nm, depending on the Au/Rh ratio. The electrocatalytic activity for oxygen reduction reaction (ORR) of AuRh NP-loaded CB catalysts showed a volcano-type dependence on their composition, in which AuRh NPs with Au surface coverage of 62% exhibited the optimal ORR activity, the specific activity being ca. 5 times higher than that of pure Rh NPs.

CeO2 Supported Gold Nanocluster Catalysts for CO Oxidation: Surface Evolution Influenced by the Ligand Shell.

Monolayer protected Au nanocluster catalysts are known to undergo structural changes during catalytic reactions, including dissociation and migration of ligands onto the support, which strongly affects their activity and stability. To better understand how the nature of ligands influences the catalytic activity of such catalysts, three types of ceria supported Au nanoclusters with different kinds of ligands (thiolates, phosphines and a mixture thereof) have been studied, employing CO oxidation as model reaction. The thiolate-protected Au25/CeO2 showed significantly higher CO conversion after activation at 250 °C than the cluster catalysts possessing phosphine ligands. Temperature programmed oxidation and in situ infrared spectroscopy revealed that while the phosphine ligands seemed to decompose and free Au surface was exposed, temperatures higher than 250 °C are required to efficiently remove them from the whole catalyst system. Moreover, the presence of residues on the support seemed to have much greater influence on the reactivity than the gold particle size.

Selective formation of [Au23(SPh t Bu)17]0, [Au26Pd(SPh t Bu)20]0 and [Au24Pt(SC2H4Ph)7(SPh t Bu)11]0 by controlling ligand-exchange reaction.

To use atomically precise metal nanoclusters (NCs) in various application fields, it is essential to establish size-selective synthesis methods for the metal NCs. Studies on thiolate (SR)-protected gold NCs (Au n (SR) m NCs) revealed that the atomically precise Au n (SR) m NC, which has a different chemical composition from the precursor, can be synthesized size-selectively by inducing transformation in the framework structure of the metal NCs by a ligand-exchange reaction. In this study, we selected the reaction of [Au25(SC2H4Ph)18]- (SC2H4Ph = 2-phenylethanethiolate) with 4-tert-butylbenzenethiol ( t BuPhSH) as a model ligand-exchange reaction and attempted to obtain new metal NCs by changing the amount of thiol, the central atom of the precursor NCs, or the reaction time from previous studies. The results demonstrated that [Au23(SPh t Bu)17]0, [Au26Pd(SPh t Bu)20]0 (Pd = palladium) and [Au24Pt(SC2H4Ph)7(SPh t Bu)11]0 (Pt = platinum) were successfully synthesized in a high proportion. To best of our knowledge, no report exists on the selective synthesis of these three metal NCs. The results of this study show that a larger variety of metal NCs could be synthesized size-selectively than at present if the ligand-exchange reaction is conducted while changing the reaction conditions and/or the central atoms of the precursor metal NCs from previous studies.

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

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

Simple and high-yield preparation of carbon-black-supported ∼1 nm platinum nanoclusters and their oxygen reduction reactivity.

The improvement of oxygen reduction reaction (ORR) catalysts is essential before polymer electrolyte fuel cells can be used widely. To this end, we established a simple method for the size-selective synthesis of a series of ligand-protected platinum nanoclusters with ∼1 nm particle size (Ptn NCs; n = ∼35, ∼51, and ∼66) and narrow size distribution (±âˆ¼4 Pt atoms) under atmospheric conditions. Using this method, each ligand-protected ∼1 nm Pt NC was obtained in a relatively high yield (nearly 80% for Pt∼66). We succeeded in adsorbing each ligand-protected ∼1 nm Pt NC on carbon black (CB) and then removing most of the ligands from the surface of the Pt NCs via calcination while maintaining the original size. The obtained Pt∼35/CB, Pt∼51/CB, and Pt∼66/CB exhibited ORR mass activities that were 1.6, 2.1, and 1.6 times higher, respectively, than that of commercial CB supported-Pt nanoparticles, and also display high durability.

[Ag23Pd2(PPh3)10Cl7]0: A new family of synthesizable bi-icosahedral superatomic molecules.

Icosahedral noble-metal 13-atom nanoclusters (NCs) can form connected structures, which can be regarded as superatomic molecules, by vertex sharing. However, there have been very few reports on the superatomic molecules formed using silver (Ag) as the base element. In this study, we synthesized [Ag23Pd2(PPh3)10Cl7]0 (Pd = palladium, PPh3 = triphenylphosphine, Cl = chloride), in which two icosahedral 13-atom NCs are connected, and elucidated its geometric and electronic structures to clarify what type of superatomic molecules can be synthesized. The results revealed that [Ag23Pd2(PPh3)10Cl7]0 is a synthesizable superatomic molecule. Single crystal x-ray diffraction analysis showed that the metal-metal distances in and between the icosahedral structures of [Ag23Pd2(PPh3)10Cl7]0 are slightly shorter than those of previously reported [Ag23Pt2(PPh3)10Cl7]0, whereas the metal-PPh3 distances are slightly longer. On the basis of several experiments and density functional theory calculations, we concluded that [Ag23Pd2(PPh3)10Cl7]0 and previously reported [Ag23Pt2(PPh3)10Cl7]0 are more stable than [Ag25(PPh3)10Cl7]2+ because of their stronger superatomic frameworks (metal cores). These findings are expected to lead to clear design guidelines for creation of new superatomic molecules.

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

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

Toward the creation of high-performance heterogeneous catalysts by controlled ligand desorption from atomically precise metal nanoclusters.

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

Thiolate-Protected Metal Nanoclusters: Recent Development in Synthesis, Understanding of Reaction, and Application in Energy and Environmental Field.

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.