Observing electron transfer in solids

Electron transfer (ET) is a process in which an electron is transferred from one atom or molecule to another. It is fundamental to electrochemical reactions with applications in many fields. Nanoscale ET, which involves the transfer of electrons in the range of 1–100 nanometres in solids, is fundamental to the design of multifunctional materials. However, this process is not yet fully understood.

Nanotubes — nanomaterials with unique cylindrical nanostructures — offer a range of ET properties that can be realised through electron and hole injections into the nanotubes, making them a suitable candidate for studying nanoscale ET. However, carbon-based nanotubes can be difficult to control in terms of their shape and size due to conditions such as high temperatures, required for their synthesis. A viable approach for fabricating well-defined tuneable nanotubes is bottom-up fabrication of non-covalent nanotubes, which sometimes results in crystalline-form nanotubes.

In a recent study, researchers from the Tokyo University of Science used a novel approach to directly observe solid-state ET. Professor Junpei Yuasa said the researchers developed crystalline nanotubes with a double-walled structure. “By incorporating electron donor molecules into the pores of these crystalline nanotubes through a solid-state oxidation reaction, we succeeded in directly observing the electron transfer reaction in the solid using X-ray crystal structure analysis,” Yuasa said.

The researchers used a supramolecular crystallisation method, which involves oxidation-based crystallisation, to fabricate zinc-based double-walled crystalline nanotubes. This double-walled structure with large windows in the nano-tube walls made the crystal robust and flexible enough to maintain its crystalline state when subjected to ET oxidation processes. This structure also allows the crystal to absorb electron donor molecules.

The researchers used ferrocene and tetrathiafulvalene as electron donor molecules, which were absorbed through the windows of the nanotube crystals. This allows electrons to be removed from the absorbed electron donors through solid-state ET oxidation reactions, resulting in the accumulation of holes in the donors inside the nanotube. The robustness of the crystals also enabled the researchers to observe this ET oxidation process using X-ray crystal structure analysis, uncovering key insights.

Yuasa said understanding ET can lead to the development of novel functional materials, which in turn can lead to the design of more efficient semiconductors, transistors and other electronic devices. “Optoelectronic devices, such as solar cells, rely heavily on ET. Hence, direct observation of ET can help improve these devices' performance. Additionally, this approach can lead to advancements in energy storage, nanotechnology and materials science research,” Yuasa said.

This study into the direct observation of solid-state ET could be expanded to enable researchers to observe ET and its related phenomena in other nanomaterials. The research findings were published in the journal Nature Communications.

Image credit: iStock.com/Oselote