Scientists successfully synthesize graphyne

Though scientists have tried with limited success for more than a decade to synthesize a new form of carbon called "graphyne," a team of researchers at the University of Colorado Boulder has finally succeeded in creating the material in bulk.

Graphyne has long interested the scientific community because of its similarities to graphene, another form of carbon highly valued by industry. Yet despite decades of work and theorizing, only a few fragments of graphyne have ever been created before now.

Reported May 9 in "Nature Synthesis," the breakthrough potentially opens brand-new possibilities for electronics, optics, and semiconducting material research.

"The whole audience, the whole field, is really excited that this longstanding problem, or this imaginary material, is finally getting realized," said Yiming Hu, lead author of a research paper detailing the success.

Scientists have long been interested in the construction of new or novel forms, or allotropes, of carbon because of the element's usefulness to industry, as well as its versatility. Allotropes of carbon include graphite, diamond, and graphene.

These various forms of carbon – denoted as sp2, sp3 and sp hybridized carbon (or the different ways carbon atoms can bind to other elements) – and their corresponding bonds, are widely used. The most well-known carbon allotropes are graphite (used in making tools like pencils and batteries) and diamonds. They are made of sp2 carbon and sp3 carbon, respectively.

Carbon allotropes have distinct physical properties that arise from the unique combination and arrangement of multiple types of bonds that have various lengths, strengths, geometry, and electronic properties. For instance, graphite is opaque and soft, whereas diamond is transparent and the hardest known natural substance.

Carbon research pays dividends

Graphynes are two-dimensional carbon allotropes, like graphene, which are optically transparent and mechanically flexible, yet strong and electronically conductive. The discovery of graphene ushered in a new era of 2D materials and quantum technology. Unlike graphenes, which consist solely of sp2-hybridized carbons, graphynes contain sp-hybridized carbons periodically integrated into an sp2-hybridized carbon framework.

Scientists predicted that graphyne would exhibit intriguing and unique electron-conducting, mechanical, and optical properties. Specifically, the electron conduction in graphynes would be exceptionally fast, as it is in graphene. Yet, the electron conduction in some graphyne could be controlled in a defined direction, unlike the multidirectional conduction in graphene.

Despite many earlier attempts, large-scale synthesis of graphynes with long-range crystallinity over a large area remains elusive.

"Moreover, to the best of our knowledge, the most stable graphyne structure, 'γ-graphyne,' which consists of alternating phenylene (sp2-hybridized carbons only) and ethynylene (sp-hybridized carbons only) building blocks, has not been fully experimentally realized. Additionally, the lack of knowledge on the stacking order and orientation of adjacent graphyne layers has impeded the study of graphyne properties," Hu wrote.

"We also observed the sheet-like morphology of γ-graphyne and its folding behavior. Understanding such folding behavior will open many possibilities to explore the unique mechanical and electronic properties of this periodically sp–sp2-hybridized γ-graphyne, an intriguing new member added to the 2D material family, he concluded.

Tremendous research efforts have been devoted to constructing novel carbon allotropes, which include sp2-hybridized carbon allotropes, fullerene (awarded the Nobel Prize in Chemistry in 1996), carbon nanotubes graphene (awarded Nobel Prize in Physics in 2010), a biphenylene network and an sp-hybridized carbon allotrope, cyclocarbon. All known carbon forms are composed of a single type of carbon atom. However, there are many more composed of various combinations of sp3-, sp2-, and sp-hybridized carbon atoms yet to be discovered, according to the research paper.

However, these methods do not allow for the diverse types of carbon to be synthesized together in any sort of large capacity, like what's required for graphyne, which has left the theorized material to remain that – a theory, Hu said.

Thinking outside the box

The need for a nontraditional approach led researchers to reach out to Wei Zhang's lab group.

Zhang, a professor of chemistry at CU Boulder and a co-author of the paper reporting the breakthrough, studies reversible chemistry, or chemistry that allows bonds to self-correct, allowing for the creation of novel ordered structures, or lattices, such as synthetic DNA-like polymers.

"We brought out the problem again and used a new tool to solve an old problem that is really important," he said.

In the research, the periodically sp–sp2-hybridized carbon allotrope, γ-graphyne, was synthesized in bulk.

Using a process called alkyne metathesis, which is an organic reaction that entails the redistribution, or cutting and reforming, of alkyne chemical bonds (a type of hydrocarbon with at least one carbon-carbon triple covalent bond) – as well as thermodynamics and kinetic control, the group was able to successfully create a new material, a substance that could rival the conductivity of graphene but with control.

"There's a pretty big difference (between graphene and graphyne) but in a good way," Zhang said in an interview. "This could be the next generation wonder material. That's why people are very excited."

Plans for more study

While the team has successfully created graphyne, its members still want to study the "particular details" of the material, including how to create it on a large scale and how it can be manipulated.

"We are really trying to explore this novel material from multiple dimensions, both experimentally and theoretically, from atomic-level to real devices," Zhang said of the next steps.

These efforts, in turn, should aid in figuring out how the material's electron-conducting and optical properties can be used in industry applications like lithium-ion batteries, according to the researchers.

"We hope in the future we can lower the costs and simplify the reaction procedure, and then, hopefully, people can really benefit from our research," said Hu.

Zhang said the synthesis of graphyne could never have been accomplished without the support of an interdisciplinary team.

"Without the support from the physics department, without some support from colleagues, this work probably couldn't be done," he said.

Other co-authors of the paper include Chenyu Wu, Qingyan Pan and Yingjie Zhao from Qingdao University of Science and Technology; and Yinghua Jin, Rui Lyu, Vikina Martinez, Shaofeng Huang, Jingyi Wu, Lacey J. Wayment, Noel A. Clark, Markus B. Raschke from CU Boulder.