In June 2023, Wu Zhongshuai received a review from Nature. Opening the email, one of the reviewers' comments was particularly sharp, with a plethora of questions that left him somewhat bewildered. Yet, in the realm of scientific research, akin to climbing a mountain, he believed, "Once the research direction is set, there's no reason for us to give up." He and his team embarked on in-depth research and supplemented it with over 80 pages of responses.
On April 3, 2024, after over three years of persistent exploration, Researcher Wu Zhongshuai's team at the Dalian Institute of Chemical Physics, Chinese Academy of Sciences (hereafter referred to as DICP), in collaboration with the Shenzhen Advanced Technology Research Institute of the Chinese Academy of Sciences, Academician Cheng Huiming from the Institute of Metal Research of the Chinese Academy of Sciences, and Associate Professor Kang Ning from the School of Electronics, Peking University, made the latest breakthrough in the macroscopic production of two-dimensional transition metal telluride materials, paving the way for the scalable production of metal telluride nano-2D materials.
The related findings were published in Nature, with reviewers praising the method for its simplicity, speed, efficiency, and universal significance.
Following the publication, the first author, Zhang Liangzhu, was extremely excited. He said, "When I first entered this field, I read a paper on graphene exfoliation published in 1958, which has now been cited over thirty thousand times. This proves that the research content is at the forefront of the field. I hope my paper will also be cited thirty thousand times in the future!"
Scientists Achieve Large-Scale Production of Two-Dimensional Metal Telluride Materials
The "Sandwich" Approach to "Top-Down" Fabrication
Since the discovery of graphene in 2004, two-dimensional materials have demonstrated immense potential in various fields such as physics, energy storage, catalysis, and optoelectronics.
Two-dimensional transition metal tellurides are a new class of emerging materials composed of tellurium atoms (Te) and transition metal atoms (such as molybdenum, tungsten, and niobium). Their microscopic structure resembles a "sandwich," with transition metal atoms sandwiched between top and bottom layers of tellurium atoms, forming layered two-dimensional materials. Due to their unique physical and chemical properties such as superconductivity, magnetism, and catalytic activity, two-dimensional transition metal telluride materials have shown significant potential applications in quantum communication, catalysis, energy storage, optics, and have garnered widespread attention in the international academic community.
However, achieving large-scale production of high-quality two-dimensional transition metal telluride materials remains a significant challenge.
"Two-dimensional transition metal telluride materials are typically prepared using a 'top-down' approach, akin to dismantling building blocks, by mechanically or chemically peeling off layers one by one to produce single-layer two-dimensional nanosheets," explained Zhang Liangzhu.
Currently, common "top-down" methods include chemical intercalation exfoliation, ball milling, tape exfoliation, liquid-phase ultrasonication, and so on. Among them, chemical intercalation exfoliation is the most efficient method, garnering significant attention. However, its exfoliation process still requires several hours.
Apart from the waiting time, safety concerns often linger in the minds of scientists.
Previously, scientists mostly used organic lithium reagents as intercalating agents, inserting lithium ion-containing intercalating agents into the layers of multi-layered bulk materials, and utilizing the reaction between lithium and water to cause the intercalating agent to "expand," forming a "gas pressure column" between each layer. This would separate the stacked nanosheets layer by layer, akin to using a "chemical scraper" to peel off the nanosheets. This interlayer gas expansion force is much greater than mechanical peeling force, significantly improving the exfoliation efficiency. However, organic lithium is a flammable and explosive solution reagent, posing significant safety hazards. Therefore, achieving safe and efficient chemical exfoliation became the goal of scientists' efforts.
Based on these challenges, Wu Zhongshuai and his team came up with an innovative material—lithium borohydride.
"2 vs. 1" Reviewer Comments
"Because previous exfoliation methods required several hours or even days to produce nanosheets, we have been trying various reagents, and finally found that lithium borohydride can greatly shorten the time," Zhang Liangzhu told the China Science Daily.
The first time lithium borohydride reagents were used, Zhang Liangzhu inserted them into multi-layered bulk materials at ten o'clock in the evening. According to his usual calculations, it would take several days to obtain nanosheets, so he locked the laboratory and left after turning off the lights. However, the next day when he returned to the laboratory and examined the samples under a scanning electron microscope, he found many nanosheets.
"I was very surprised at the time. Nanosheets were peeled off in such a short time. I thought this was a direction worth exploring further. However, this method also produced some nanoribbons, which were not the pure nanosheet morphology we wanted. At the time, we speculated that the presence of nanoribbons might be due to either too high temperature or prolonged reaction time," Zhang said.
The team subsequently adjusted the reaction process continuously, from 500 degrees, 400 degrees, 300 degrees... By continuously varying the temperature, they eventually found that nanosheets could be perfectly exfoliated at 350 degrees for 10 minutes.
Furthermore, lithium borohydride reagents possess strong reducing properties and remain stable in dry air, making them suitable for achieving high-temperature solid-phase lithium intercalation reactions, significantly enhancing safety compared to previous methods.
When the team excitedly submitted the paper to Nature, they received a major revision review. The opinions of several reviewers were sharply divided, with two recommending minor revisions for acceptance, while another suggested rejection, resulting in a "2 vs. 1" situation.
"Two reviewers gave relatively positive responses, but one raised many questions."
The negative reviewer was an expert in the field of synthesizing two-dimensional materials using the "top-down" method, mainly employing chemical vapor deposition methods to synthesize high-quality two-dimensional material nanosheets. Hence, he was concerned about the defects in the quality and size of nanosheets produced by chemical intercalation methods. While he agreed that high-quality nanosheets would greatly assist research in many fields, he believed that the advantages and characteristics of the preparation method relative to other methods should be demonstrated. He also suggested showcasing the applications of transition metal telluride nanosheets in catalysis and energy storage to demonstrate the application scenarios of nanosheet powders.
The advantage of two-dimensional materials lies in their large market application, so the efficiency and quantity of preparation are crucial. Receiving the reviewers' opinions provided the team with many new ideas. They immediately sat down to analyze the paper line by line and supplemented experiments.
Breakthrough from 1g to 108g
The team further conducted experiments and conducted in-depth comparisons with methods such as glass liquid-phase ultrasonic exfoliation, ball milling exfoliation, electrochemical intercalation exfoliation, and liquid-phase ultrasonic exfoliation, confirming that lithium borohydride reagents could exfoliate nanosheets at 350 degrees for 10 minutes, showing a significant improvement in efficiency compared to other methods.
At the same time, the team also attempted large-scale exfoliation preparation using this method and successfully produced 108g of niobium telluride nanosheets, which was two orders of magnitude higher than the production volume of less than 1 gram using the previous liquid-phase chemical intercalation exfoliation method. Additionally, the team used this method to prepare nanosheets of five different transition metals and twelve different alloy compounds, demonstrating its universality.
Macroscopically controllable 2D transition metal telluride nanosheets have been successfully synthesized by Dalian Institute of Chemical Physics, as illustrated in their provided images.
The team further processed the synthesized nanosheet powder into solutions, thin films, screen printing inks, 3D printing devices, micro-supercapacitors for photolithography, demonstrating excellent processability. The monolayer 2D nanosheets prepared by this method are expected to play a significant role in high-performance quantum devices, battery materials, supercapacitors, composite materials, and other fields.
After completing the modifications, they confidently submitted the supplemented 80-plus-page dataset to the journal Nature. In February 2024, they received an acceptance letter.
"I feel particularly happy because my research has been recognized, and it also proves the academic level of our team," said Zhang Liangzhu from the Wu Zhongshuai team at the Dalian Institute of Chemical Physics. His greatest feeling is the positive academic atmosphere, where everyone is clear about the focus of the field and wants to achieve innovative and breakthrough results, rather than just following existing directions as "followers."
In the future, the team will continue to scale up the production of nanosheets and actively apply them in the field of battery energy storage, such as using them as catalysts for lithium-oxygen batteries. These catalysts can catalyze the reversible conversion between lithium peroxide and lithium, greatly increasing the energy density of batteries and bringing new opportunities for the development of next-generation high-performance batteries.
For more information, refer to the related paper: Nature Article Link.
