ENGINEERING TECHNOLOGY

Ten years to sharpen a sword! Chinese scientists have obtained new artificial carbon crystals


Constantly discovering new structures, new properties, and realizing new applications has always been the research theme of materials scientists. In the field of carbon material research, the search for new crystal structures other than graphite and diamond is of great significance for obtaining carbon materials with newer properties and better performance, so it has attracted a wide range of interest from researchers.

On January 12, the international academic journal Nature published the research results of the team of Professor Zhu Yanwu of the Department of Materials Science and Engineering of the University of Science and Technology of China (hereinafter referred to as USTC). After ten years of efforts and exploration, they discovered a new technology for charge injection of fullerene C60 molecular crystals, constructed C60 polymer crystals and long-range ordered porous carbon crystals under atmospheric pressure conditions, and realized their grammagnitude preparation.

“Long-range ordered porous carbon crystals are a new class of artificial carbon crystals that have not been previously reported. The process of getting it is like treating fullerene molecules as pieces of ‘Lego bricks’, and the key is to design a way to ‘jam’ fullerenes together, which is the charge injection technology developed in the study. In fact, charge injection technology is quite universal, and is expected to become a new means to control crystal structure at the atomic level. Zhu Yanwu, the corresponding author of the paper, introduced to China Science News.

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Group photo of some members of the research team of Professor Zhu Yanwu (middle row) of China University of Science and Technology Photo by Dai Rui

A new “LEGO” approach to building carbon materials

Carbon is one of the most common elements in nature, and carbon atoms can form a variety of structures through different arrangements, such as graphite, diamond and amorphous carbon, which have been widely used in various fields.

In recent years, the discovery and development of new carbon materials such as fullerenes, carbon nanotubes, graphene and graphdiyne have attracted widespread attention and research boom. “Further, if we can use the above nanomaterials as basic units arranged into an orderly structure to ‘build’ new carbon crystal materials, we may explore more novel properties and develop greater application potential.” Zhu Yanwu said.

In fact, theories have previously predicted that such carbon materials can exist stably, possibly semiconductors or even superconducting properties, with rich and tunable electronic properties. However, there are still great challenges in the macro preparation of such carbon materials, their in-depth characterization, and systematic exploration of their application range.

The challenge mainly stems from the fact that in previous reports on the preparation of such carbon materials, researchers either use extreme conditions such as high temperature and high pressure, or use ultraviolet light, electron beam irradiation and other difficult to scale microscopic processing techniques, resulting in very low yield and impure products, which hinders people’s deeper and systematic exploration of such materials.

In this study, Zhu Yanwu’s team creatively used lithium nitride to charge the fullerene C60 molecular crystal, and heat treated it at an appropriate temperature, and finally obtained C60 polymer crystal and long-range ordered porous carbon crystal, and realized its grammagnitude preparation.

Nature reviewers believe that “the results presented in the paper are convincing and important for the fields of crystallography and materials science.” ”

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At the experimental line station of the National Synchrotron Radiation Laboratory, Pan Fei (first from left), the first author of the paper and a specially appointed associate researcher at the University of Science and Technology of China, and the testers are collecting the X-ray absorption spectrum of the sample

Charge allows molecules to “hold hands gently”

There are 60 carbon atoms in a fullerene C60 molecule, and if you want to use it as a structural unit to obtain a new carbon structure, it is necessary to form a stable connection between adjacent molecules without seriously destroying the cage-like structure of the fullerene, and it is necessary to achieve: a certain number of covalent bonds are formed between some carbon atoms in the C60 molecule and the carbon atoms in adjacent molecules.

In order to obtain a stable amount of this carbon structure, the team tried every conceivable approach. By chance, they noticed a material called lithium nitride. This material is chemically active, will produce an open flame when exposed to water, and has a strong ability to lose electrons. Further research found that the material was able to charge the graphite at about 400°C, inducing its rapid stacking morphological phase transition.

In the study, Pan Fei, the first author of the paper and a special associate researcher at the University of Science and Technology of China, is mainly responsible for the exploration of the preparation conditions and structural characterization of new carbon crystals. He said, “Charge injection is a commonly used method in physics research, generally used to change the electronic properties of materials (such as electronic energy levels), but rarely used to change and regulate the microstructure of materials.” ”

Based on the above research, they extended the method to the structural regulation of fullerene C60 molecular crystals. “We found that at the right temperature, electrons are transferred from lithium nitride to fullerene C60 molecules to achieve charge injection, and these additional charges will change the electronic structure of C60 molecules, making it easy to form covalent bonds between adjacent molecules, and it is easier for the two molecules to successfully ‘hold hands’.” Pan Fei said.

It is worth mentioning that the work has achieved a deep integration of theoretical simulation and experimental research. “The possibilities for combining 60 carbon atoms as a unit to make up a crystal are very numerous. We used neural network potential functions combined with machine learning methods to extensively search the evolution path from fullerenes to other carbon structures, obtained more than 300,000 possible intermediate state structures, and then compared these intermediate structure characteristics with the experimental results, which helped us further clarify the direction of experimental regulation of preparation conditions and improve the accuracy and efficiency of experimental work. Ni Kun, co-first author of the paper and a specially appointed associate researcher of China University of Science and Technology, introduced.

Why can such a crystal structure be obtained under such experimental conditions? In the process of research, they also used theoretical calculations to explain the mechanism of experimental results synchronously, and believed that charge injection may be a very useful microstructure control technique in the future.

It is worth mentioning that a more detailed structure search shows that long-range ordered porous carbon-based crystals represent a large class of metastable crystal structures in the transition from fullerene molecular crystals to graphite carbon crystals, which is expected to receive continuous attention and follow-up research in the field of carbon materials.

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Ni Kun (middle), co-first author of the paper and a specially appointed associate researcher of the University of Science and Technology of China, discusses the theoretical calculation scheme with his supervisor and colleagues Photo by Dai Rui

Develop Chinese carbon material systems

In fact, as early as 2011, when Zhu Yanwu was conducting postdoctoral research at the University of Texas at Austin, he found a chemical “activation” way to “activate” graphene, and successfully reconstructed graphene sheets into “living graphene” with high specific surface area, high conductivity and negative curvature structure, which showed excellent performance as an electrode material for supercapacitors. The research results were published in the journal Science.

“At that time, I began to think that using carbon-based nanocells with well-defined structures as building blocks, we could Chinese whether we could develop a new class of carbon material systems.” With this goal in mind, Zhu Yanwu returned to China to join the University of Science and Technology of China, and established the “Advanced Carbon Materials Research Group” in 2012 to focus on the preparation and application of new carbon materials. In the past ten years, although the team has devoted a considerable part of its energy to the study of the application of graphene materials and related industrialization technologies, and has made important progress, it has never forgotten the basic research goal of “preparing new carbon crystal materials”.

Ten years of research has finally paid off, and long-range ordered porous carbon-based crystals are one of the new artificial carbon crystals that their team wants to get.

“It is often said that ten years sharpen a sword. In fact, at the beginning, we didn’t have a ‘sword’, and we didn’t even know what a ‘sword’ looked like, it was just a hazy dream. Zhu Yanwu said frankly. But on the road of exploration, they never forgot their goal, and when they thought of new methods and technologies, they immediately returned to their “original intention”. They believe that the structural regulation of new materials from simple to complex, from single function to multi-level gradient function, is an inevitable trend and requires a long-term research process.

“In the study, Professor Zhu gave a very tolerant space for exploration, which is the basic condition for nurturing some original discoveries. If it is all prescribed, we must do this today and that tomorrow, it may be difficult to get a result that we are slightly satisfied with now. Pan Fei also mentioned that when encountering problems in scientific research, do not blindly “stumble”, it is best to adjust the research direction in the short term or go out to relax, look at the literature in other research fields, there may be unexpected gains.

Similarly, Ni Kun believes that basic research also needs a relatively “tolerant” environment, and cannot rush to produce results because of some short-term factors. He believes that “doing basic research, doing one thing of greater value is more meaningful than doing ten repetitive tasks, and often the breakthrough point of the problem may come from an immediate inspiration or multidisciplinary intersection.” As long as this ‘1’ is achieved, there will be more ‘0’ later. And if you can’t catch ‘1’, no matter how much ‘0’ you do, it’s still ‘0’. ”

Zhu Yanwu believes that the team can persist in this direction for so long thanks to the relatively relaxed scientific research environment of the University of Science and Technology of China. At the same time, he said, “to do basic research, you must have your own time for deep thinking and multidisciplinary communication, and you must not be busy with short-term goals.” ”

Speaking about the application of long-range ordered porous carbon crystals, Zhu Yanwu said, “This type of crystal may have potential applications in energy storage, ion screening, catalytic loading and other fields in the future, but this requires long-term in-depth research and engineering efforts.” Next, the team will study the properties of this new carbon-based crystal in depth and systematically, hoping to obtain a crystal structure with atomic-level accuracy periodicity through further experimental parameter tuning, and explore more properties and applications. (Source: Wang Min, China Science News)

Related paper information:https://doi.org/10.1038/s41586-022-05532-0



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