Shell-yellow nanocrystals with mobile golden yellow: new generation of photocatalysts

June 07, 2022

(News from Nanowerk) Due to their unique permeable hollow shell structures with movable inner cores, yellow shell nanocrystals are suitable for a wide variety of applications. Yellow-shelled nanocrystals consisting of a gold core with various semiconductor shells have been developed by researchers at Tokyo Tech, using a novel sequential ion-exchange process. These yellow-shelled metal-semiconductor nanocrystals can serve as highly efficient photocatalysts for many applications.

Yellow-shelled nanocrystals are unique materials with fascinating structural properties, such as permeable shell, interior void space, and mobile yolk. These nanocrystals are suitable for a variety of applications, depending on the choice of materials used to manufacture them.

For example, if the inner surface of their shells is reflective, yellow shell nanocrystals can be a reliable photovoltaic device. A moveable core box can act as an agitator, capable of mixing solutions contained within the shell. The inner and outer surfaces of the shell provide many active sites for reactions, and the fascinating properties of the yellow-shell structure (the result of electronic interactions and charge transfer between the surfaces of the structure) make these nanocrystals ideal for photocatalysis applications. Naturally, the yellow-shelled nanocrystals have attracted the attention of researchers around the world.

Now, in a collaborative study published in Nanomaterials applied by ACS (“Electronic Interactions and Charge-Transfer Dynamics for a Series of Yolk-Shell Nanocrystals: Implications for Photocatalysis”), which was also selected as an Editors’ Choice for ACS, an international research team led by Associate Professor Tso- Fu Mark Chang and assistant Professor Chun-Yi Chen of Tokyo Institute of Technology (Tokyo Tech) and Professor Yung-Jung Hsu of National Yang Ming Chiao Tung University in Taiwan developed several yellow-shell structures containing metallic golden yellow (Au) with various semiconductor shells. Such structures have gained worldwide popularity due to their fascinating properties, due to their Au cores.

“Yellow-shelled nanocrystals composed of a metallic yellow and semiconducting shells are of particular interest because they can be adapted for uses related to mass transport, for example photocatalysis,” says Professor Chen.

To make the nanocrystals, the researchers used a sequential ion exchange process. The procedure involves gentle sulfurization on a [email protected]2Model of O core-shell nanocrystal (where Au contributes to the core and Cu2O to shell formation), followed by a kinetically controlled cation exchange reaction that allows conversion of shell composition (i.e. Cu2O) into various metal sulfides, which are semiconductors. Four representative samples of yellow shell nanocrystals, including [email protected]sevenS4, [email protected], [email protected]and [email protected]3S4were synthesized for investigation in this way, as shown in Figure 1.

Figure 1. (a) Schematic representation of the synthetic procedure for [email protected]sevenS4, [email protected], [email protected]and [email protected]3S4. (be) shows the corresponding TEM images. Synthesis of yellow shell nanostructures involves sulfurization on a [email protected]2O matrix of core-shell nanocrystals to convert the composition of the shell into various metal sulfides. (Picture: Tokyo Tech)

The performance of these yellow-shell structures as photocatalysts was evaluated using X-ray photoelectron spectroscopy (XPS) and steady-state photoluminescence (PL) spectroscopy.

Using XPS, the researchers discovered that the metallic cores and semiconducting shells of the nanocrystals had electronic interactions favorable to photocatalysis applications. Time-resolved PL spectroscopy revealed that the nanostructures had high PL intensity, indicating high photocatalytic activity, implying that they were highly capable of absorbing light and generating electron-hole charge carriers (like the shown in Figure 2).

Steady state photoluminescence (PL) spectra of (a) Au@Cu7S4, (b) Au@CdS, (c) Au@ZnS and (d) Au@Ni3S4 Figure 2. Steady state photoluminescence (PL) spectra of (a) [email protected]sevenS4(b) [email protected](vs) [email protected]and D) [email protected]3S4. The results of their pure counterparts are also included. (Picture: Tokyo Tech)

Under bright illumination, the nanostructures were found to have high photoluminescence (PL) activity, which showed them to be highly capable of absorbing light and generating electrons and holes as charge carriers.

“In a real-life scenario, the reactions facilitated by separated photoexcited electrons and holes play a role in purifying the environment, producing reactive oxygen species,” says Professor Chen, describing a scenario in which their new yellow shell photocatalysts could be used. . These photoexcited electrons and holes can facilitate a multitude of reactions, making yellow shell nanocrystals applicable in many fields such as environmental purification, hydrogen production and carbon dioxide reduction.

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