Abstract
The development of shape-controlled noble metal nanocrystals such as nanowires (NWs) is progressing steadily owing to their potentially novel catalytic properties and the ease with which they can be prepared by reducing the metal ions in a particular solution as capping agents. Recently, many reports have been presented on the preparation of shape-controlled Au nanocrystals, such as nanostars and nanoflowers, by a one-pot method using 2-[4-(2-hydroxyethyl)-1-piperazinyl] ethanesulfonic acid (HEPES) as capping and reducing agents. The catalytic activity is depressed due to the adsorption of the capping agent onto a Au surface. Since HEPES has low binding affinities on the Au surface, shape-controlled nanocrystals obtained using HEPES are effective for application as nanocatalysts because HEPES was easily removed from the Au surface. In this study, we report the preparation of AuNWs, with an average diameter of 7.7 nm and lengths of a few hundred nanometers, in an aqueous solution containing HEPES and sodium borohydride. A γ-Al2O3-supported AuNW (AuNW/γ-Al2O3) catalyst was obtained using catalytic supporters and a water extraction method that removed HEPES from the Au surface without morphological changes. AuNW/γ-Al2O3 was then utilized to catalyze the oxidation of 1-phenylethyl alcohol to acetophenone. The formation rate of acetophenone over AuNW/γ-Al2O3 was 3.2 times that over γ-Al2O3-supported spherical Au nanoparticles (AuNP/γ-Al2O3) with almost the same diameter.
Highlights
The study of noble metal nanocrystals is very important in several areas of nanosciences, such as electrochemistry, electronics, magnetic storage sensing, and catalysis.[1−10] The properties of metal nanocrystals are strongly dependent on their sizes and shapes.[1,2,11] an effective synthesis technique is essential to obtain nanocrystals with desired properties
Transmission electron microscopy (TEM) images showed that AuNWs with an average diameter of 7.7 nm were obtained by mixing an aqueous solution of NaBH4 (110 mM) with an aqueous solution containing HEPES (15 mM) and HAuCl4 (Figure 2a)
TEM-energy-dispersive X-ray (EDX) revealed that the AuNWs were composed of pure Au (Figure S2)
Summary
The study of noble metal nanocrystals is very important in several areas of nanosciences, such as electrochemistry, electronics, magnetic storage sensing, and catalysis.[1−10] The properties of metal nanocrystals are strongly dependent on their sizes and shapes.[1,2,11] an effective synthesis technique is essential to obtain nanocrystals with desired properties. Shape-controlled nanocrystals, such as nanowires (NWs) and nanoflowers, are synthesized by reducing the noble metal ions in a solution containing a surfactant, a polymer, or low molecular organic components as capping agents, which inhibit the precipitation of nanocrystals.[12−22] for gold nanoparticles (NPs), spherical NPs, such as decahedral, icosahedral, and truncated octahedral nanocrystals, are identified as stable structures because the surface area per volume of spherical NPs is lower than that of shape-controlled nanocrystals.[1,23] Recently, many studies have reported that NWs are prepared using the diffusion-limited aggregation method.[1,24−27] Such NWs tend to exhibit high catalytic activities compared to the original spherical NPs owing to the disordered state of the metal atoms within the aggregated domains, namely, grain boundaries, which often exhibit high catalytic activity.[25−30] To prepare NWs by this method, it needs high NP concentration because of the difficulty to aggregate NPs under low concentration (Figure 1). We report the preparation of AuNWs obtained by reducing Au ions in aqueous HEPES and NaBH4 and its catalytic performance for the oxidation reaction of 1-phenylethyl alcohol to acetophenone
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