Seiji Yamazoe

Nonscalable Oxidation Catalysis of Gold Clusters

Small, negatively charged gold clusters isolated in vacuum can oxidize CO via electron-transfer-mediated activation of O2. This suggests that Au clusters can act as aerobic oxidation catalysts in the real world when their structure parameters satisfy given required conditions. However, there is a technical challenge for the development of Au cluster oxidation catalysts; the structural parameters of the Au clusters, such as size and composition, must be precisely controlled because the intrinsic chemical properties of the clusters are strongly dependent on these parameters. This Account describes our efforts to achieve precision synthesis of small (diameter <2 nm) Au clusters, stabilized by polymers and immobilized on supports, for a variety of catalytic applications. Since we aim to develop Au cluster catalysts by taking full advantage of their intrinsic, size-specific chemical nature, we chose chemically inert materials for the stabilizers and supports. We began by preparing small Au clusters weakly stabilized by polyvinylpyrrolidone (PVP) to test the hypothesis that small Au clusters in the real world will also show size-specific oxidation catalysis. The size of Au:PVP was controlled using a microfluidic device and monitored by mass spectrometry. We found that only Au clusters smaller than a certain critical size show a variety of aerobic oxidation reactions and proposed that the reactions proceed via catalytic activation of O2 by negatively charged Au clusters. We also developed a method to precisely control the size and composition of supported Au clusters using ligand-protected Au and Au-based bimetallic clusters as precursors. These small Au clusters immobilized on mesoporous silica, hydroxyapatite, and carbon nanotubes acted as oxidation catalysts. We have demonstrated for the first time an optimal Au cluster size for the oxidation of cyclohexane and a remarkable improvement in the oxidation catalysis of Au25 clusters by single-atom Pd doping. The non-scalable catalysis of Au clusters that we reported here points to the possibility that novel catalysis beyond that expected from bulk counterparts can be developed simply by reducing the catalyst size to the sub-2 nm regime.

A Critical Size for Emergence of Nonbulk Electronic and Geometric Structures in Dodecanethiolate-Protected Au Clusters

We report on how the transition from the bulk structure to the cluster-specific structure occurs in n-dodecanethiolate-protected gold clusters, Au(n)(SC12)m. To elucidate this transition, we isolated a series of Au(n)(SC12)m in the n range from 38 to ∼520, containing five newly identified or newly isolated clusters, Au104(SC12)45, Au(∼226)(SC12)(∼76), Au(∼253)(SC12)(∼90), Au(∼356)(SC12)(∼112), and Au(∼520)(SC12)(∼130), using reverse-phase high-performance liquid chromatography. Low-temperature optical absorption spectroscopy, powder X-ray diffractometry, and density functional theory (DFT) calculations revealed that the Au cores of Au144(SC12)60 and smaller clusters have molecular-like electronic structures and non-fcc geometric structures, whereas the structures of the Au cores of larger clusters resemble those of the bulk gold. A new structure model is proposed for Au104(SC12)45 based on combined approach between experiments and DFT calculations.

A New Binding Motif of Sterically Demanding Thiolates on a Gold Cluster

A gold cluster, Au(41)(S-Eind)(12), was synthesized by ligating the bulky arenethiol 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacene-4-thiol (Eind-SH) to preformed Au clusters. Extended X-ray absorption fine structure, X-ray photoelectron spectroscopy, and the fragmentation pattern in the mass spectrometry analysis indicated that formation of gold-thiolate oligomers at the interface was suppressed, in sharp contrast to conventional thiolate-protected Au clusters.

Binding motif of terminal alkynes on gold clusters.

Gold clusters protected by terminal alkynes (1-octyne (OC-H), phenylacetylene (PA-H) and 9-ethynyl-phenanthrene (EPT-H)) were prepared by the ligand exchange of small (diameter <2 nm) Au clusters stabilized by polyvinylpyrrolidone. The bonding motif of these alkynes on Au clusters was investigated using various spectroscopic methods. FTIR and Raman spectroscopy revealed that terminal hydrogen is lost during the ligand exchange and that the C≡C bond of the alkynyl group is weakened upon attachment to the Au clusters. Acidification of the water phase after the ligand exchange indicated that the ligation of alkynyl groups to the Au clusters proceeds via deprotonation of the alkynes. A series of precisely defined Au clusters, Au34(PA)16, Au54(PA)26, Au30(EPT)13, Au35(EPT)18, and Au(41-43)(EPT)(21-23), were synthesized and characterized in detail to obtain further insight into the interfacial structures. Careful mass analysis confirmed the ligation of the alkynes in the dehydrogenated form. An upright configuration of the alkynes on Au clusters was suggested from the Au to alkyne ratios and photoluminescence from the excimer of the EPT ligands. EXAFS analysis implied that the alkynyl carbon is bound to bridged or hollow sites on the cluster surface.

Hierarchy of bond stiffnesses within icosahedral-based gold clusters protected by thiolates

Unique thermal properties of metal clusters are believed to originate from the hierarchy of the bonding. However, an atomic-level understanding of how the bond stiffnesses are affected by the atomic packing of a metal cluster and the interfacial structure with the surrounding environment has not been attained to date. Here we elucidate the hierarchy in the bond stiffness in thiolate-protected, icosahedral-based gold clusters Au25(SC2H4Ph)18, Au38(SC2H4Ph)24 and Au144(SC2H4Ph)60 by analysing Au L3-edge extended X-ray absorption fine structure data. The Au–Au bonds have different stiffnesses depending on their lengths. The long Au–Au bonds, which are more flexible than those in the bulk metal, are located at the icosahedral-based gold core surface. The short Au–Au bonds, which are stiffer than those in the bulk metal, are mainly distributed along the radial direction and form a cyclic structural backbone with the rigid Au–SR oligomers.

Reversible Photocontrol of Surface Wettability between Hydrophilic and Superhydrophobic Surfaces on an Asymmetric Diarylethene Solid Surface

By alternate UV and visible light irradiation, reversible topographical changes were observed on a newly synthesized diarylethene microcrystalline surface between the rough crystalline surface of an open-ring isomer and flat eutectic surfaces. The contact angle changes of a water droplet between 80° and 150° and peak intensities changes of the open-ring isomer in XRD patterns within 2 h of repeating cycle were observed. The results indicated that reversibly photogenerated rod-shaped crystals on the surface were produced based on the lattice of the open-ring isomer crystals in the subphase.

The effect of SrTiO3 substrate orientation on the surface morphology and ferroelectric properties of pulsed laser deposited NaNbO3 films

Orientated NaNbO3 (NN) films were grown on SrRuO3/(001)SrTiO3 [SRO/(001)STO], SRO/(110)STO, and SRO/(111)STO substrates by pulsed laser deposition. Scanning electron microscopy images showed that the surface morphologies of the NN/SRO/(001)STO, NN/SRO/(110)STO, and NN/SRO/(111)STO took the form of a stepped structure, a striped pattern, and trigonal pyramidal-like structures, respectively. The dielectric and ferroelectric properties of the films were characterized. The NN/SRO/(110)STO film showed the lowest relative dielectric constant and the largest remanent polarization of 30.8 μC/cm2 among all films. These were interpreted as being due to the orientation direction of the NN film grown on (110)STO being parallel to the polarization direction of NN.

Au25 Clusters Containing Unoxidized Tellurolates in the Ligand Shell

We report herein the synthesis and characterization of Au25 clusters containing tellurolates (TePh) in the ligand shell ([Au25(TePh)n(SC8H17)18-n](-); n = 1-18). [Au25(TePh)n(SC8H17)18-n](-) clusters were synthesized by reacting [Au25(SC8H17)18](-) with diphenyl ditelluride ((PhTe)2) in solution. Characterization of the products by mass spectrometry and X-ray absorption fine structure analysis revealed that the tellurolates in [Au25(TePh)n(SC8H17)18-n](-), unlike those in tellurolate-protected gold nanoparticles, were not oxidized. Various experiments on the products and theoretical calculations on related clusters revealed that protection by the tellurolates distorts (expands) the central Au13 core and decreases the HOMO-LUMO gap of the Au25 clusters.

Slow-Reduction Synthesis of a Thiolate-Protected One-Dimensional Gold Cluster Showing an Intense Near-Infrared Absorption

Slow reduction of Au ions in the presence of 4-(2-mercaptoethyl)benzoic acid (4-MEBA) gave Au76(4-MEBA)44 clusters that exhibited a strong (3 × 10(5) M(-1) cm(-1)) near-infrared absorption band at 1340 nm. Powder X-ray diffraction studies indicated that the Au core has a one-dimensional fcc structure that is elongated along the {100} direction.

Surface Plasmon Resonance in Gold Ultrathin Nanorods and Nanowires

We synthesized and measured optical extinction spectra of Au ultrathin (diameter: ∼1.6 nm) nanowires (UNWs) and nanorods (UNRs) with controlled lengths in the range 20-400 nm. The Au UNWs and UNRs exhibited a broad band in the IR region whose peak position was red-shifted with the length. Polarized extinction spectroscopy for the aligned Au UNWs indicated that the IR band is assigned to the longitudinal mode of the surface plasmon resonance.