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 × 10⁵ 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

Structural Model of Ultrathin Gold Nanorods Based on High-Resolution Transmission Electron Microscopy: Twinned 1D Oligomers of Cuboctahedrons

Recently, we have developed a synthetic method of ultrathin gold nanorods (AuUNRs) with a fixed diameter of ∼1.8 nm and variable lengths in the range of 6–400 nm. It was reported that these AuUNRs exhibited intense IR absorption assigned to the longitudinal mode of localized surface plasmon resonance and broke up into spheres owing to Rayleigh-like instability at reduced surfactant concentration and at elevated temperatures. In order to understand the structure–property correlation of AuUNRs, their atomic structures were examined in this work using aberration-corrected high-resolution transmission electron microscopy. Statistical analysis revealed that the most abundant structure observed in the AuUNRs (diameter ≈ 1.8; length ≈ 18 nm) was a multiply twinned crystal, with a periodicity of ∼1.4 nm in length. We propose that the AuUNRs are composed of cuboctahedral Au₁₄₇ units, which are connected one-dimensionally through twin defects

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

Rayleigh Instability and Surfactant-Mediated Stabilization of Ultrathin Gold Nanorods

Ultrathin gold nanorods (AuUNRs; diameter ∼2 nm) stabilized by oleylamine (OA) were spheroidized when dispersed in chloroform containing a small amount of OA. Time-resolved optical spectroscopy and TEM analysis indicated that the AuUNRs were gradually shortened with the release of small Au nanospheres (AuNSs) because of Rayleigh instability, followed by transformation into plasmonic AuNSs (diameter >2 nm). The OA surfactants play an essential role in stabilizing the morphology of AuUNRs by suppressing the diffusion of Au surface atoms

Selenolate-Protected Au₃₈ Nanoclusters: Isolation and Structural Characterization

We report the isolation and structural characterization of dodecaneselenolate-protected Au₃₈ clusters (Au₃₈(SeC₁₂H₂₅)₂₄). These clusters were synthesized via the reaction of phenylethanethiolate-protected Au₃₈ clusters (Au₃₈(SC₂H₄Ph)₂₄) with didodecyldiselenide ((C₁₂H₂₅Se)₂). Characterization of the product by mass spectrometry and thermogravimetric analysis confirmed that highly pure Au₃₈(SeC₁₂H₂₅)₂₄ had been obtained. The electronic and geometrical structures, bonding characteristics, and stability of the Au₃₈(SeC₁₂H₂₅)₂₄ clusters were assessed using extended X-ray fine structure and X-ray absorption near edge structure measurements, optical absorption spectroscopy, electrochemical measurements, and stability testing

Gold Ultrathin Nanorods with Controlled Aspect Ratios and Surface Modifications: Formation Mechanism and Localized Surface Plasmon Resonance

We synthesized gold ultrathin nanorods (AuUNRs) by slow reductions of gold­(I) in the presence of oleylamine (OA) as a surfactant. Transmission electron microscopy revealed that the lengths of AuUNRs were tuned in the range of 5–20 nm while keeping the diameter constant (∼2 nm) by changing the relative concentration of OA and Au­(I). It is proposed on the basis of time-resolved optical spectroscopy that AuUNRs are formed via the formation of small (<2 nm) Au spherical clusters followed by their one-dimensional attachment in OA micelles. The surfactant OA on AuUNRs was successfully replaced with glutathionate or dodecanethiolate by the ligand exchange approach. Optical extinction spectroscopy on a series of AuUNRs with different aspect ratios (ARs) revealed a single intense extinction band in the near-IR (NIR) region due to the longitudinal localized surface plasmon resonance (LSPR), the peak position of which is red-shifted with the AR. The NIR bands of AuUNRs with AR < 5 were blue-shifted upon the ligand exchange from OA to thiolates, in sharp contrast to the red shift observed in the conventional Au nanorods and nanospheres (diameter >10 nm). This behavior suggests that the NIR bands of thiolate-protected AuUNRs with AR < 5 are not plasmonic in nature, but are associated with a single-electron excitation between quantized states. The LSPR band was attenuated by thiolate passivation that can be explained by the direct decay of plasmons into an interfacial charge transfer state (chemical interface damping). The LSPR wavelengths of AuUNRs are remarkably longer than those of the conventional AuNRs with the same AR, demonstrating that the miniaturization of the diameter to below ∼2 nm significantly affects the optical response. The red shift of the LSPR band can be ascribed to the increase in the effective mass of electrons in AuUNRs

Preferential Location of Coinage Metal Dopants (M = Ag or Cu) in [Au_{25–<i>x</i>}M_<i>x</i>(SC₂H₄Ph)₁₈]⁻ (<i>x</i> ∼ 1) As Determined by Extended X‑ray Absorption Fine Structure and Density Functional Theory Calculations

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 [Au_23.8Ag_1.2(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 [Au_23.6Cu_1.4(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

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

Small Copper Nanoclusters Synthesized through Solid-State Reduction inside a Ring-Shaped Polyoxometalate Nanoreactor

Cu nanoclusters exhibit distinctive physicochemical properties and hold significant potential for multifaceted applications. Although Cu nanoclusters are synthesized by reacting Cu ions and reducing agents by covering their surfaces using organic protecting ligands or supporting them inside porous materials, the synthesis of surface-exposed Cu nanoclusters with a controlled number of Cu atoms remains challenging. This study presents a solid-state reduction method for the synthesis of Cu nanoclusters employing a ring-shaped polyoxometalate (POM) as a structurally defined and rigid molecular nanoreactor. Through the reduction of Cu2+ incorporated within the cavity of a ring-shaped POM using H2 at 140 °C, spectroscopic studies and single-crystal X-ray diffraction analysis revealed the formation of surface-exposed Cu nanoclusters with a defined number of Cu atoms within the cavities of POMs. Furthermore, the Cu nanoclusters underwent a reversible redox transformation within the cavity upon alternating the gas atmosphere (i.e., H2 or O2). These Cu nanoclusters produced active hydrogen species that can efficiently hydrogenate various functional groups such as alkenes, alkynes, carbonyls, and nitro groups using H2 as a reductant. We expect that this synthesis approach will facilitate the development of a wide variety of metal nanoclusters with high reactivity and unexplored properties