The Preferential Location of Coinage Metal Dopants (M = Ag or Cu) in [Au25-xMx(SC2H4Ph)18] (x~1) as Determined by EXAFS and DFT Calculations

Seiji Yamazoe, Wataru Kurashige, Katsuyuki Nobusada, Yuichi Negishi, and Tatsuya Tsukuda*
J. Phys. Chem. C 118, 25284-25290 (2014).

The preferential locations of Ag and Cu atoms in the initial stage of doping into [Au25(SC2H4Ph)18] were studied by X-ray absorption spectroscopy and density functional theory computations. The extended X-ray absorption fine structure (EXAFS) spectra of [Au23.8Ag1.2(SC2H4Ph)18] at the Ag K-edge were reproduced using a model structure in which the Ag dopant occupied a surface site in the icosahedral Au13 core that was computationally the most stable site. In contrast, the Cu K-edge EXAFS spectra of [Au23.6Cu1.4(SC2H4Ph)18] indicated that the Cu dopant was preferentially located at the oligomer site that was computationally less stable than the surface site. This discrepancy between the Cu location experimentally determined and that theoretically predicted was explained in terms of variations in the stability of the Cu dopant at the two sites against aerobic oxidation. These results demonstrate that the mixing patterns of bimetallic clusters are determined not only by the thermodynamic stability but also by the durability of the mixed structure under synthetic and storage conditions.

Thiolate-Mediated Selectivity Control in Aerobic Alcohol Oxidation by Porous Carbon-Supported Au25 Clusters

Tatchamapan Yoskamtorn, Seiji Yamazoe, Ryo Takahata, Jun-ichi Nishigaki, Anawat Thivasasith, Jumras Limtrakul, and Tatsuya Tsukuda*
ACS Catalysis 4, 3696-3700 (2014).

Supported Au25 clusters were prepared through the calcination of Au25(SC12H25)18 on hierarchically porous carbon nanosheets under vacuum at temperatures in the range of 400–500 C for 2–4 h. TEM and EXAFS analyses revealed that the thiolate coverage on Au25 gradually decreased with increasing calcination temperature and period and became negligibly small when the calcination temperature exceeded 500 C. The catalysis of these Au25 clusters was studied for the aerobic oxidation of benzyl alcohol. Interestingly, the selectivity for benzaldehyde formation was remarkably improved with the increase in the amount of residual thiolates on Au25, while the activity was reduced. This observation is attributed to the dual roles of the thiolates: the reduction of the oxidation ability of Au25 by electron withdrawal and the inhibition of the esterification reaction on the cluster surface by site isolation.

A Face-Sharing Bi-icosahedral Model for Al23

Kiichirou Koyasu and Tatsuya Tsukuda*
Phys. Chem. Chem. Phys. 16, 21717-21720 (2014).

A face-sharing bi-icosahedral motif is proposed as a candidate structure of the magic cluster, Al23, on the basis of DFT calculations. The structure can be viewed as a quasi-molecule made of two Al13 (D3d) superatoms with an open electronic configuration via constructive overlap of 1F and 2P superatomic orbitals. A face-sharing tri-icosahedral motif is also predicted forAl33.

Electron Microscopy of Gold Nanoparticles at Atomic Resolution

Maia Azubel, Jaakko Koivisto, Sami Malola, David Bushnell, Greg L. Hura, Ai Leen Koh, Hironori Tsunoyama, Tatsuya Tsukuda, Mika Pettersson, Hannu Häkkinen, and Roger D. Kornberg*
Science 345, 909-912 (2014).

Structure determination of gold nanoparticles (AuNPs) is necessary for understanding their physical and chemical properties, but only one AuNP larger than 1 nanometer in diameter [a 102–gold atom NP (Au102NP)] has been solved to atomic resolution. Whereas the Au102NP structure was determined by x-ray crystallography, other large AuNPs have proved refractory to this approach. Here, we report the structure determination of a Au68NP at atomic resolution by aberration-corrected transmission electron microscopy, performed with the use of a minimal electron dose, an approach that should prove applicable to metal NPs in general. The structure of the Au68NP was supported by small-angle x-ray scattering and by comparison of observed infrared absorption spectra with calculations by density functional theory.

Surface Plasmon Resonance in Gold Ultrathin Nanorods and Nanowires

Ryo Takahata, Seiji Yamazoe, Kiichirou Koyasu, and Tatsuya Tsukuda*
J. Am. Chem. Soc. 136, 8489-8491 (2014).

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.

Au25 Clusters Containing Unoxidized Tellurolates in the Ligand Shell

Wataru Kurashige, Seiji Yamazoe, Masaki Yamaguchi, Keisuke Nishido, Katsuyuki Nobusada, Tatsuya Tsukuda, and Yuichi Negishi*
J. Phys. Chem. Lett. 5, 2072-2076 (2014).

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.

Chemically-Modified Gold Superatoms and Superatomic Molecules

Jun-ichi Nishigaki, Kiichirou Koyasu, and Tatsuya Tsukuda*
Chem. Rec., 14, 897-909 (2014).

Clusters of gold atoms can be viewed as superatoms, in which valence electrons confined in the particles occupy atomic-like, discrete electronic levels. Chemical modification of the gold surperatoms and their aggregated molecules (superatomic molecules) with organic ligands is a promising approach for their application as building units of new functional materials. This account surveys the present status of the rapidly growing field of gold superatoms and superatomic molecules protected by thiolates and phosphines. The major aim of this article is to provide a simple picture for the structure, stability and bonding scheme of chemically-modified superatoms and superatomic molecules for the development of a new class of hierarchical materials.

Hydrogen-Induced Structural Transformation of AuCu Nanoalloys Probed by Synchrotron X-ray Diffraction Techniques

Miho Yamauchi*, Kazuya Okubo, Tatsuya Tsukuda, Kenichi Kato, Masaki Takata and Sadamu Takeda
Nanoscale, 6, 4067-4071 (2014).

In situ X-ray diffraction measurements reveal that the transformation of a AuCu nanoalloy from a face-centered-cubic to an L10 structure is accelerated under hydrogen atmosphere. The structural transformation rate for the AuCu nanoalloy under hydrogen above 433 K was found to be 100 times faster than that in vacuum, which is the first quantitative observation of hydrogen-induced ordering of nanoalloys.

Nonscalable Oxidation Catalysis of Gold Clusters

Seiji Yamazoe, Kiichirou Koyasu, and Tatsuya Tsukuda*
Accounts of Chemical Research, 47, 816-824 (2014).

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.

Selective Hydrogenation of 4-Nitrobenzaldehyde to 4-Aminobenzaledehyde by Colloidal RhCu Bimetallic Nanoparticles

Md. J. Sharif, Seiji Yamazoe, and Tatsuya Tsukuda*
Topics Catal., 57, 1049-1053 (2014).

Monodisperse RhCu bimetallic nanoparticles (NPs) with various compositions were prepared by the co-reduction of Rh and Cu ions in the presence of poly(N-vinyl-2-pyrrolidone) (PVP). Powder X-ray diffraction analysis of the PVP-stabilized RhCu NPs revealed the formation of a novel solid-solution structure in which the Rh and Cu atoms are randomly distributed. The catalytic properties of the colloidal RhCu NPs were studied using hydrogenation of 4-nitrobenzaldehyde as a test reaction. We found that the RhCu NPs efficiently reduced 4-nitrobenzaldehyde using atmospheric hydrogen at room temperature and that the selectivity to 4-aminobenzaldehyde was enhanced from ~70% with monometallic Rh NPs to ~97% with RhCu NPs with a Cu content of 50%.

A Twisted Bi-icosahedral Au25 Cluster Enclosed by Bulky Arenethiolates

Jun-ichi Nishigaki, Seiji Yamazoe, Shinji Kohara, Akihiko Fujiwara, Wataru Kurashige, Yuichi Negishi and Tatsuya Tsukuda*
Chem. Commun., 50, 839-841 (2014).

Ligation of 2,6-diphenylbenzenethiol (DppSH) onto Au clusters stabilized by poly(N-vinyl-2-pyrrolidone) and subsequent core etching yielded a single cluster Au25(SDpp)11. High-energy X-ray diffraction measurement showed that Au25 constitutes a core in Au25(SDpp)11. We propose a bi-icosahedral Au25 core whose 22 Au surface atoms are capped directly by 11 bulky arenethiolates.

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