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A Structural Model of Ultrathin Gold Nanorods Based on High-Resolution Transmission Electron Microscopy: Twinned 1D Oligomers of Cuboctahedrons Ryo Takahata, Seiji Yamazoe, Kiichirou Koyasu, and Tatsuya Tsukuda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12198 • Publication Date (Web): 24 Jan 2017 Downloaded from http://pubs.acs.org on January 25, 2017
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The Journal of Physical Chemistry
A Structural Model of Ultrathin Gold Nanorods Based on High-Resolution Transmission Electron Microscopy: Twinned 1D Oligomers of Cuboctahedrons Ryo Takahata,1 Seiji Yamazoe,1,2,3 Kiichirou Koyasu,1,2 Tatsuya Tsukuda1,2* 1
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Katsura, Kyoto 615-8520 3 CREST, JST, Tokyo 102-0076, Japan 2
Supporting Information Placeholder ABSTRACT: 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 twined 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.
INTRODUCTION Atomic structures of thiolate-protected Au clusters Aun(SR)m and their size-dependent evolution have been determined successfully thanks to the close cooperation among atomically-precise synthesis, state-of-the-art characterization techniques and theoretical calculation methods.1,2 Powder X-ray diffraction (XRD) analysis and density functional theory calculations on a series of Aun(SR)m (n = 38–520) showed the appearance of nonbulk-like Au core upon changing from Au187(SR)68 to Au144(SR)60.3 Formation of icosahedral or decahedral-based Au cores protected by Au-SR oligomers has been proved by single-crystal XRD analysis on Au25(SR)18, Au28(SR)20, Au36(SR)24, Au38(SR)24, Au102(SR)44, Au130(SR)50, and Au133(SR)52.1,2 High-resolution transmission electron microscopy (HRTEM) is a powerful tool for the study of atomic structures because it does not require the growth of single crystals.4–8 Three-dimensional atomic structure of Au68 was determined using low-dose TEM method.4 High angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) imaging confirmed the structures of Au38(SR)24, Au130(SR)50 and Au144(SR)60.5–7 Statistical analysis using HAADF-STEM suggested polydispersity in cluster sizes and structures for Au144(SR)60.8 Recently, Au nanostructures with a variety of shapes such as spheres, cubes, cages, stars, ellipsoids, rods, and wires have been developed as the building blocks for novel functional materials.9,10 Among these, Au nanowires (AuNWs) have gained notable attention because of their potential applications in plasmonic,11 sensing,12 and mechanical devices.13 Nearly a decade ago, chemical synthesis of a new class of AuNWs with diameters smaller than ~2 nm, namely ultrathin Au nanowires (AuUNWs), was reported.14–17 It was proposed from HRTEM observations that the majority of the AuUNWs have
single-crystalline structures that grew along the [111] direction,14–17 whereas AuUNWs with twined crystalline14 and polycrystalline15 structures were also observed. An aberration-corrected (AC) HRTEM revealed that AuUNWs exhibit atomically wavy surface and contain large amounts of defects and that the (111) atomic planes are wrinkled.18,19 These structures affect not only electronic structure, but also the breakdown and coalescence processes. Recently, our group has developed a method for controlling the length of ultrathin Au nanorods (AuUNRs) and demonstrated the length-dependent evolution of the longitudinal mode of localized surface plasmon resonance (LSPR) in the IR region.20 These AuUNRs exhibited a Rayleigh-like instability at reduced concentration of surfactant as well as at elevated temperatures.21 Although literature reports have demonstrated that AuUNRs show (111) planes perpendicular to the longitudinal axis,22 a model structure of AuUNRs at the atomic level has not been proposed to date. In this work, we examined the atomic structures of AuUNRs using AC-HRTEM. Statistical analysis of HRTEM images revealed that the majority of AuUNRs (diameter of ~1.8 nm) has multiply twined crystal structure, with a period of ~1.4 nm. We propose that the AuUNRs consist of one-dimensional oligomers of cuboctahedrons connected via twin boundaries.
EXPERIMENTAL DETAILS Synthesis. AuUNRs were synthesized using a method reported previously.21 Briefly, 700 mg of oleylamine purified by distillation and 10 mg of HAuCl4·4H2O were dissolved in 10 mL of cyclohexane. The mixture was stirred for 2 h until the color of the solution changed from orange to yellow. Then, 500 mL of cyclohexane was added to this solution and the mixed solution was stirred for a few minutes. Next, 700 mg of
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triisopropylsilane was added to this solution and the mixture was left to stand. After 36 h, the resulting brown solution was rotary evaporated to
