The hydrated ions become steerable and what can it bring to us?

Recently, Chinese scientists have led the world, and for the first time obtained atomic-scale resolution images of hydrated sodium ions, and discovered a magic number effect of hydrated ion transport. This research opens a new door for the research of hot topics such as ion battery research and development, seawater desalination and biological ion channels.

The research results were published on May 14th in the world's top academic journal Nature. The results were completed by the Jiang Ying Group of the Center for Quantum Materials Science of Peking University, Limei Xu's research group, Yiqin Gao's research group of the School of Chemistry and Molecular Engineering of Peking University, and the Enge Wang Group of the Chinese Academy of Sciences/Peking University.

Uncover the most mysterious layer of water molecules

Water is one of the most abundant, most familiar, and least understood in nature. Why is water so mysterious? "This is related to its composition." Wang Enge, an academician of the Chinese Academy of Sciences, told reporters that because the hydrogen atoms in water molecules are the lightest atoms in the periodic table, they cannot be directly applied. The classical particle model is to study it, but it needs to simulate "full quantization", that is, its nucleus and electron must be regarded as quantum, which greatly increases the difficulty of research.

"The interaction of water with other substances is also a very complicated process." Ying Jiang, a professor of the Center for Quantum Materials Science at Peking University's School of Physics, one of the authors of the article, said that the most common is the hydration process of ions. When the salt is dissolved in water, the ions formed after dissolution is not free in water, but are combined with water molecules to form a "cluster" called an ion hydrate. "Ion hydration can be said to be ubiquitous and play an important role in many physical, chemical, and biological processes, such as salt dissolution, electrochemical reactions, ion transfer in living organisms, air pollution, seawater desalination, corrosion, etc."

What kind of microstructure does ion hydrate have and how does it move? These issues have been the focus of academic debate. It is understood that as early as the end of the 19th century, people realized the existence of ion hydration and began systematic research, but after more than one hundred years of efforts, the number of hydrated shells of ions, the number and structure of water molecules in each hydration layer The types, the effects of hydrated ions on the water-hydrogen bond structure, and the microscopic factors that determine the transport properties of hydrated ions have been inconclusive.

Drifting out the fog, humans see ion hydrate clear images for the first time

In recent years, Enge Wang and Ying Jiang have collaborated with colleagues and students to develop high-resolution scanning probe technology at the atomic level and full-quantization calculation method for light element systems, which has accumulated rich experimental and theoretical foundations for research.

To perform high-resolution imaging of hydrated ions at the atomic scale, it is first necessary to "separate" individual hydrated ions.

This is a very difficult thing. In order to solve this problem, the researchers have continually tried and explored, based on scanning tunneling microscopy developed a unique ion manipulation technology to prepare a single ion hydrate - using a very sharp metal tip in the sodium chloride film The surface moves, draws a single sodium ion, and then "drags" the water molecules to bind to it. This results in a single "hydrated sodium ion" containing a different number of water molecules.

After the experiment to prepare a single ion hydrate cluster, the next challenge is to clarify its geometric adsorption configuration through high-resolution imaging.

In response to this problem, the researchers developed a non-invasive AFM imaging technique based on carbon monoxide tip modification that can rely on extremely weak high-order electrostatic forces to scan the image. They applied this technique to the ion hydrate system, obtained atomic resolution imaging for the first time, and successfully determined its atomic adsorption configuration.

This is the first time in the world to obtain atomic-level images of ion hydrates in real space. Moreover, this image is quite clear: not only the adsorption position of water molecules and ions can be accurately determined, but even small changes in the orientation of water molecules can be directly identified. It can be said that the spatial resolution is almost the limit of the atom.

Discover the wonderful dynamics of the "magic number effect"

After obtaining microscopic images of ion hydrates, the researchers further studied their kinetic transport properties and found an interesting effect: sodium ions containing a specific number of water molecules when moving on the surface of sodium chloride crystals. Hydrates seem to suffer from "hyperactivity disorder" - an abnormally high diffusion rate that is 10-100 times faster than other hydrates. Researchers refer to this feature as the "magic number effect" of dynamics.

Why is this strange phenomenon? Through simulation calculations, the researchers found that this magic number effect is derived from the degree of symmetry matching between ion hydrate and surface lattice. Simply put, sodium ion hydrates containing 1, 2, 4, and 5 water molecules are easily "stuck" by the surface of sodium chloride crystals, while ion hydrates containing 3 water molecules, due to symmetry and substrate It doesn't match, but it's hard to be "stuck", so it will "slide" very quickly on its surface.

For the first time, this work established a direct correlation between the microstructure and transport properties of ion hydrates, refreshing the traditional understanding of ion transport in confined systems.

The hydrated ions become steerable and can bring us something?

It is understood that this research work has been well received and appreciated by reviewers in three different fields of Nature. They believe that the work "will immediately lead to widespread interest in the field of theory and applied surface science", "providing new ways to transport hydrated ions on the surface of nanoscale control and can be extended to other hydration systems."

Academician Enge Wang said, "The results of this study show that we can achieve the purpose of selectively enhancing or weakening the transport capacity of certain ions by changing the symmetry and periodicity of the surface of the material. This has important implications for many related applications. "

For example, a new type of ion battery can be developed. Jiang Ying told reporters that the lithium-ion battery we use today is generally composed of macromolecular polymers, and based on this latest research, it is possible to develop a new type of battery based on hydrated lithium ions. "This battery will greatly increase the ion transfer rate, thus shortening the charging time and increasing the battery power, making it more environmentally friendly and costly."

In addition, this achievement has opened up a new way for research in the frontier areas such as anti-corrosion, electrochemical reaction, seawater desalination and biological ion channel. At the same time, the high-precision experimental technology developed by this work is expected to be applied to more extensive hydrate systems in the future.

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