| Jun 07, 2022 |
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(Nanowerk News) Owing to their unique permeable, hollow shell structures with inner, movable cores, yolk-shell nanocrystals are suitable for a wide variety of applications. Yolk-shell nanocrystals consisting of a gold core with various semiconductor shells have been developed by Tokyo Tech researchers, using a novel sequential ion-exchange process. These metal-semiconductor yolk-shell nanocrystals can serve as highly effective photocatalysts for many applications.
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Yolk-shell nanocrystals are unique materials with fascinating structural properties, such as a permeable shell, interior void space, and movable yolk. These nanocrystals are suitable for a variety of applications, depending on the choice of materials used for their fabrication.
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For example, if the inner surface of their shells are reflective, yolk-shell nanocrystals can make for a reliable photovoltaic device. A mobile core can can act as a stirrer, capable of mixing solutions held within the shell. The inner and outer surfaces of the shell provide plenty of active sites for reactions, and the yolk-shell structure’s fascinating properties (a result of electronic interactions and charge-transfer between the surfaces of the structure) make these nanocrystals ideal for photocatalysis applications. Understandably, yolk-shell nanocrystals have earned the attention of researchers worldwide.
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Now, in a collaborative study published in ACS Applied Nano Materials (“Electronic Interactions and Charge-Transfer Dynamics for a Series of Yolk–Shell Nanocrystals: Implications for Photocatalysis”), which was also selected as the ACS Editors’ Choice, an international research team led by Associate Professor Tso-Fu Mark Chang and Assistant Professor Chun-Yi Chen at Tokyo Institute of Technology (Tokyo Tech) and Professor Yung-Jung Hsu at the National Yang Ming Chiao Tung University in Taiwan have developed several yolk-shell structures containing a metallic gold (Au) yolk with various semiconductor shells. Such structures have risen in popularity worldwide because of their fascinating properties, owing to their Au cores.
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“Yolk-shell nanocrystals comprising of a metal yolk and semiconductor shells are particularly interesting because they can be geared to mass transport-related utilizations, for example, photocatalysis,” says Professor Chen.
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To make the nanocrystals, the researchers employed a sequential ion-exchange process. The procedure involves delicate sulfidation on an [email protected]2O core-shell nanocrystal template (where Au contributes to the core, and Cu2O to the shell formation), followed by a kinetically controlled cation exchange reaction that enables conversion of the shell composition (i.e., Cu2O) into various metal sulfides, which are semiconductors. Four representative yolk-shell nanocrystal samples, including [email protected]7S4, [email protected], [email protected], and [email protected]3S4, were synthesized for investigation in this way, as shown in Figure 1.
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| Figure 1. (a) Schematic depiction of the synthetic procedure for [email protected]7S4, [email protected], [email protected], and [email protected]3S4. (b-e) shows the corresponding TEM images. The synthesis of yolk-shell nanostructures involves sulfidation on an [email protected]2O core-shell nanocrystal template to convert the shell composition to various metal sulphides. (Image: Tokyo Tech)
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The performance of these yolk-shell structures as photocatalysts was evaluated using x-ray photoelectron spectroscopy (XPS) and steady-state photoluminescence (PL) spectroscopy.
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Using XPS, the researchers found the metal cores and semiconducting shells of the nanocrystals to have electronic interactions favorable for photocatalysis applications. Time-resolved PL spectroscopy revealed the nanostructures to have high PL intensity, indicating high photocatalytic activity, implying that they were highly capable of absorbing light and generating electron-hole charge carriers (as shown in Figure 2).
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| Figure 2. Steady-state photoluminescence (PL) spectra of (a) [email protected]7S4, (b) [email protected], (c) [email protected], and (d) [email protected]3S4. The results of their pure counterparts are also included. (Image: Tokyo Tech)
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Under light illumination, the nanostructures were found to have high photoluminescence (PL) activity, revealing them to be highly capable of absorbing light and generating electrons and holes as charge carriers.
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“In a real-world scenario, the reactions facilitated by separated photoexcited electrons and holes play a role in environmental purification, by producing reactive oxygen species,” explains Prof. Chen, describing one scenario in which their novel yolk-shell photocatalysts could be used. These photoexcited electrons and holes can facilitate a multitude of reactions, making yolk-shell nanocrystals applicable in many fields such as environmental purification, hydrogen production, and carbon dioxide reduction.
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