One-Pot Synthesis of Pd Nanorings Using a Soft Template of Spindle

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C: Physical Processes in Nanomaterials and Nanostructures

One-Pot Synthesis of Pd Nanorings Using a Soft Template of Spindle-Shaped Amphiphilic Molecular Assembly Makoto Nakagawa, Sayaka Watanabe, Yoshiro Imura, KeHsuan Wang, and Takeshi Kawai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07315 • Publication Date (Web): 21 Sep 2018 Downloaded from http://pubs.acs.org on September 21, 2018

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The Journal of Physical Chemistry

One-Pot Synthesis of Pd Nanorings Using a Soft Template of Spindle-Shaped Amphiphilic Molecular Assembly Makoto Nakagawa, Sayaka Watanabe, Yoshiro Imura, Ke-Hsuan Wang and Takeshi Kawai * Department of Industrial Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan.

ABSTRACT

Metallic nanorings have attracted much attention from researchers owing to their unique optical, electrical, and magnetic properties, and now, simple and large-scale synthesis methods must be developed for their practical application in various fields. Here, we report one-pot synthesis of ultrathin Pd nanorings through a simple and novel soft-template approach based on wet-chemical synthesis. A spindle-shaped assembly of a long-chain amidoamine derivative (C18AA) served as the soft template of Pd nanorings that were prepared in an aqueous solution containing the Pd precursor and a suitable amount of C18AA and toluene, and the chemical reduction of the Pd precursor in the aqueous solution produced an array of almost circular Pd nanorings covering the assembly. Pd nanorings that were parallel to the amine group, which is the head group of

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C18AA, appeared on the assembly surface. The amount of toluene strongly affected the formation of the spindle-shaped assembly and Pd nanorings. Furthermore, transmission electron microscope images were obtained at several points in time during the reaction; they revealed that the Pd nanorings were produced by the fusion of Pd nanoparticles formed on the spindle-shaped molecular assembly during an early stage of the reaction. The Pd nanorings were also found to have high structural stability against organic solvents, high temperatures, and high and low pH.

INTRODUCTION Metallic nanocrystals have been extensively used in applications ranging from catalysis to electronics, photonics, and medicine.1−4 Controlling the shape and size of the metal nanocrystals is one of key issues in the fabrication of nanoscale materials because their shapes directly affect their physicochemical characteristics such as catalytic activity and optical properties.5,6 To learn more about the relationship between their shape and function, researchers in the field of nanotechnology and nanomaterials have energetically attempted to fabricate metal nanocrystals in various shapes, including thin plates, dendrimers, nanorods, and nanowires through advanced nanotechnology7 or wet-chemical methods.8−10 Recently, metallic nanorings have attracted much attention because they exhibit unique optical, electronic, and magnetic properties that are suitable for a number of novel applications, including metamaterials and nanoscale manipulation of light.11 Ag nanorings, for instance, can be used as highly efficient antennas to focus light onto the interior of the ring cavity.12 Furthermore, light waves can form standing waves with an interference pattern inside the nanorings,13 and Au nanoring arrays can convert incident propagating light into a strongly localized magnetic field.14


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The methods used to fabricate metal nanorings are mainly divided into two approaches: (i) advanced nanotechnology13–20 such as ion-beam lithography or electron-beam lithography; (ii) nanotemplating techniques14,20–45 using rigid and hard nanomaterials such as inorganic nanoparticles (NPs) or nanoporous materials. The first approach is extremely favorable for preparing metal nanorings with the desired diameters and thicknesses; however, the productivity is low because of the inherently two-dimensional (2D) fabrication method. The second approach has the potential for large-scale synthesis because it can be applied to dispersions of nanotemplates. However, many studies on nanotemplating use 2D substrates decorated with nanotemplates as the platform of nanoring production because fabrication methods such as electroplating,20–25 chemical reduction,26–30 and selective etching,31–34 except for Galvanic replacements, all require 2D substrates. The Galvanic replacement technique35–45 can be applied to inorganic NPs dispersed in a solution, but the applicable NPs are restricted to those with anisotropic structures such as plate-like NPs. Furthermore, the thickness of nanorings fabricated by both advanced nanotechnology and nanotemplating remains above ~10 nm.12–49 Wet-chemical synthesis using a soft template is advantageous for one-pot large-scale preparation of metal nanomaterials, and it has been successfully used to prepare metal nanocrystals of various shapes, including ultrathin metallic nanowires with diameters below ~3 nm, such as straight, meandering, or dendritic nanowires.50–55 Furthermore, coil-shaped ultrathin Au or Au–Cu nanowires have been fabricated with the assistance of polymer shells, stirring, and ultrasonic treatment.56–58 There has been few reports59 on wet-chemical synthesis of metallic nanorings measuring a few nanometers in thickness. Even though nanorings can apparently be prepared by connecting opposite ends of nanowires, the present technology has not progressed beyond the connection between the two ends.


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In previous papers,51–53 we reported that a lamellar molecular assembly of an amphiphilic compound bearing two amidoamine groups served as a useful template for preparing noble-metal nanowires with diameters of less than 3 nm and lengths of a few micrometers. Rational templates, such as spherical, cylindrical, or conical molecular assemblies of N-(2-aminoethyl)-3((2-(2-aminoethylcarbamoyl)ethyl)octadecylamino)propionamide (C18AA), which are produced by rolling up the lamellar assemblies, enable the preparation of noble-metal nanowires with closed-loop structures that are deposited on the assemblies (Scheme 1). In this paper, we demonstrate that C18AA can form spindle-shaped molecular assemblies in water, and these assemblies can be used as templates of Pd nanorings. To the best of our knowledge, this is the first report of this type of closed ultrathin nanorings that are prepared by a wet-chemical approach. Scheme 1. Soft template formed by rolling up a lamellar C18AA assembly for synthesis of metal nanorings.

EXPERIMENTAL SECTION Chemicals and materials


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Potassium tetrachloropalladate(II) (>95.0%), sodium borohydride (>92.0%), hydrochloric acid (1.0 M aqueous solution), and sodium hydroxide (1.0 M aqueous solution) were purchased from Kanto Chemical, Japan, and were used as received. 12Tungsto(IV)phosphoric acid n-hydrate (also known as phosphotungstic acid n-hydrate) was purchased from Wako Pure Chemical, Japan, and was used as received. Distilled water was used in all experiments. C18AA was synthesized according to a previously reported procedure.52 Preparation of Pd nanorings Pd nanorings were prepared from potassium tetrachloropalladate(II) (K2PdCl4), C18AA, toluene, sodium borohydride (NaBH4) and water. Potassium tetrachloropalladate(II) (2.5 mg, 7.7 µmol) and C18AA (7.4 mg, 15 µmol) were first dissolved in water (5.0 mL). After homogenization (45 kHz, 10 min) using an ultrasonic bath (W-113, Honda Electronics Co., Ltd., Japan), toluene (7.0 µL) was injected using a microsyringe. A fresh aqueous solution of 0.74 mM sodium borohydride (0.10 mL) was added to the homogenized solution; after vortex mixing, the resultant solution was left to stand for 24 h at room temperature (~25 °C). Morphological stability test of Pd nanorings The dispersions of Pd nanorings were centrifuged at 5000 rpm for 10 min, and the supernatant was removed by decantation. The precipitation was redispersed in methanol by ultrasonication for 10 min. To examine the morphological stability of the Pd nanorings, the as-prepared Pd nanoring dispersions were kept at 10, 25, 50, and 80 °C, and at pH values of 1, 5, 9 (original pH), and 13 for 24 h. The pH values of the dispersions were adjusted with solutions of hydrochloric acid or sodium hydroxide.


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Characterization X-ray diffraction (XRD) measurements of dried Pd nanorings were performed with a diffractometer (Ultima IV, Rigaku, Japan). Images of the Pd nanorings were acquired on a transmission electron microscope (TEM; JEM-2100, JEOL, Japan) operating at 200 kV or a second TEM (JEM-1011, JEOL, Japan) operating at 100 kV. High-resolution TEM (HR-TEM) images, high-angle annular dark-field scanning transmission electron microscope (HAADF– STEM) images, and TEM–energy-dispersive X-ray spectrometer (TEM–EDS) mapping images were acquired on a TEM (JEM-2100, JEOL, Japan) operating at 200 kV. Electron tomography images were recorded using the SYSTEM IN FRONTIER TEMography option on the JEM-2100 TEM. A series of tilted TEM images were acquired at angles between −56° and +62° in tilt increments of 1° to obtain three-dimensional (3D) tomography image of Pd nanorings and between −70° and +59° in tilt increments of 4° to obtain 3D tomography image of Pd NPs array (Supporting Information; SI). RESULTS AND DISCUSSION Soft-template synthesis of Pd nanorings Spindle-shaped molecular assemblies of C18AA were formed by adding a small quantity of toluene to an aqueous solution of C18AA containing K2PdCl4. Figure 1a, b and S1 show TEM images of the molecular assemblies stained with phosphotungstic acid. Interestingly, periodical dark stripes of the spindle-shaped template are clearly visible in Figure 1a and b, and these stripes probably represent a high-density domain of PdCl42– and WO42– ions. Because such ions favor binding to cationic amine groups of C18AA, the dark and light stripes probably correspond to the hydrophilic head group and hydrophobic hydrocarbon chains, respectively, of C18AA.


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The average distance between the stripes was 3.5 ± 0.3 nm (Figure S1), which is roughly equal to the d-spacing of 3.48 nm in the lamellar structure of C18AA, in which the hydrocarbon chains were arranged in an interdigitated fashion (Figure S2).53,60 The spindle-shaped template was formed by adding a small amount of toluene and rolling up the lamellar assembly of C18AA. The Pd nanorings were prepared by chemical reduction of Pd ions in the spindle-shaped template dispersion. Figure 1c, f and S3 show the representative TEM images of Pd nanorings obtained under optimal conditions; the product consisted of an assembly of nanorings with an average diameter of 3.5 nm (Figure S3). The concentrations of C18AA and NaBH4 were crucial for the formation of the nanorings, and lower or higher concentration of either could result in short nanowires or NPs (Figure S4). Furthermore, a preparation solution without toluene produced only short nanowires (Figure 1e), while a preparation solution containing a large quantity of toluene provided only longer nanowires (Figure 1g), even though both solutions contained the proper concentrations of C18AA and NaBH4. This result is consistent with the fact that we could not find spindle-shaped molecular assemblies in the aqueous solutions with either a large quantity of toluene or no toluene. Therefore, a proper quantity of toluene was essential to the formation of nanorings (Figure 1e–g).


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Figure 1. (a) Representative TEM image of dried and stained molecular self-assembly of C18AA; (b) high-magnification image of (a). TEM images of resultant Pd nanorings: (c) Pd nanorings at low magnification; (d) broken assembly of Pd nanorings. TEM images of nanoring samples prepared (e) without toluene, (f) with a proper amount (7 µL) of toluene, and (g) with 100 µL of toluene. The optimum conditions for the preparation of Pd nanorings were [K2PdCl4] = 7.5 µmol, [C18AA]/[K2PdCl4] = 2, [NaBH4]/[K2PdCl4] = 10, and a small quantity (7 µL) of toluene. The nanoring assemblies produced under these conditions had a wide range of sizes; the diameter at the center and the length of the assemblies were 50–300 nm and 100–2000 nm, respectively. STEM–EDS measurements confirmed that the nanorings were composed of pure palladium (Figure S5). XRD patterns of the nanorings show broad peaks at 39.4°, 45.2°, 66.6°, and 80.1°,


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which are assigned to diffraction from the (111), (200), (220), and (311) planes, respectively, of face-centered-cubic Pd (Figure 2a).61 Furthermore, the periodical fringes of 0.227 nm in the HRTEM images correspond to the (111) lattice spacing of Pd, indicating that the Pd nanorings grew predominantly in the (111) direction (Figure 2b, c).

Figure 2. (a) XRD pattern of Pd nanorings. (b, c) HR-TEM images of Pd nanorings; the yellow dashed lines indicate crystal grain boundaries in the Pd nanorings; the white arrows in (b) indicate the directions of the (111) crystal facets in each domain. 3D structure of Pd nanorings As it is difficult to clearly determine the 3D structure from conventional two-dimensional TEM images, tilted TEM images and TEM 3D tomography of a nanoring were used to reveal the shape of the nanoring assemblies. Figure S6 shows TEM images of a nanoring assembly rotating


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about the major axis at a 45° angle in the left or right direction. The minor axis of the assembly measured 133 nm in the original position, 132 nm for left rotation, and 137 nm for right rotation, indicating that it had an ellipsoidal shape and each nanoring had an almost perfect circular structure. In addition, 3D tomography of an assembly (Figure 3) revealed that there was no Pd product inside the spindle-shaped molecular assemblies, but Pd nanorings were formed on their surface. Thus, it is reasonable to conclude that the surface of the C18AA assemblies, as shown in Figure 1a, could serve as the template of Pd nanorings, and the nanorings grew along the short axis of the spindle-shaped template.

Figure 3. (a) TEM image of Pd nanorings and (b) TEM 3D tomography image of (a). (c) x–y computational slices of (b). Formation mechanism of Pd nanorings


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To clarify the growth mechanism of Pd nanorings, we examined TEM images of the products obtained after different periods of reaction (Figure 4a–e). After a reaction duration of

One-Pot Synthesis of Pd Nanorings Using a Soft Template of Spindle

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