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

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

Synthesis and Catalytic Application of Ag₄₄ Clusters Supported on Mesoporous Carbon

4-(Fluoro­phenyl)­thiolate-protected Ag₄₄ clusters (Ph₄P)₄[Ag₄₄(SC₆H₄F)₃₀] were calcined on mesoporous carbon (MPC) under vacuum at 300–500 °C for 2 h. X-ray absorption spectroscopy, transmission electron microscopy, and thermal-desorption mass spectrometry revealed that sulfur-free Ag₄₄ clusters were successfully produced by the calcination of [Ag₄₄(SC₆H₄F)₃₀]^4– at 300 °C, in sharp contrast to the formation of silver sulfide nanoparticles by the calcination of dodecanethiolate-protected Ag nanoparticles (3.0 ± 0.6 nm). Ag₄₄/MPC was applied in the catalytic dehydrogenation of ammonia–borane (NH₃BH₃) as a test reaction. It turned out that the Ag₄₄/MPC catalysts produced 1 equiv of H₂ from NH₃BH₃ but only in the presence of O₂ (turnover frequency 1.9 × 10³ h^–1 Ag atom^–1). Given that nanoparticles of other metals (Pt, Pd, Rh, Ni, or Ru) produced 3 equiv of H₂ under an inert atmosphere, this result indicates that the Ag₄₄/MPC-catalyzed dehydrogenation of NH₃BH₃ proceeds by a different mechanism from that on other nanoparticles

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

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

Tuning the Direction of Photoinduced Electron Transfer in Porphyrin-Protected Gold Clusters

The interfacial electron-transfer reaction in ligand-protected gold clusters (AuCs) has been extensively investigated, but there are limited reports on organic chromophore ligands for photoinduced electron-transfer reactions of chromophore-attached AuCs. Here, we focused on porphyrins as chromophore ligands because of their tunable redox properties through the insertion of metal ions. We synthesized 1.3 nm diameter AuCs face-coordinated by free-base porphyrin (H2P) or AuIII porphyrin (AuP+) as photofunctional ligands. The synthesized H2P- and AuP+-protected AuCs (H2P-AuCs and AuP+-AuCs) were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, and ultraviolet–visible–near-infrared absorption spectroscopy. Femtosecond transient absorption measurements revealed the photodynamics of H2P-AuCs and AuP+-AuCs. The AuCs in H2P-AuCs and AuP+-AuCs act as electron acceptors and electron donors, respectively, achieving control of the photoinduced electron-transfer direction by inserting the metal ion into the porphyrin ligand. This drastic change is caused by the high electrophilicity of AuP+, indicating that the precise design of the protecting ligand can expand the potential of AuCs as photofunctional materials