Nanotechnology

Engineers reveal the secret behind green graphene

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July 13, 2023

(Nanowerk News) When Ange Nzihou, an expert in turning society’s waste into valuable products, visited Princeton in 2022, she brought with her a technique for turning waste biomass into graphene, a material with many uses from batteries to solar cells. He knew his approach using non-toxic iron catalysts offered advantages over existing methods that rely on hazardous chemicals, precious metals or fossil fuels.

There was just one problem: Nzihou didn’t know exactly how the process worked.

“In my work as a chemical engineer, I am often interested in the final properties of materials and how they can be applied in the real world,” said Nzihou, distinguished professor of chemical engineering at IMT Mines Albi – CNRS in France who visited Princeton through the Fulbright Visiting Scholar Program. “But if you want to optimize the properties of the material you’re producing, you have to understand what’s happening at the nano and atomic scales to bring about the transformation.”

That’s where Claire White, professor of civil and environmental engineering and Andlinger Center for Energy and the Environment, comes to help.

As the host of the Nzihou faculty, White contributed his expertise in atomic and nanoscale characterization of materials to uncover the mechanisms that allow iron to help transform waste biomass into graphene.

The result was not only two papers, the first of which was published in ChemSusChem (“Synthesis and Growth of Green Graphene from Biochar Revealed by Magnetic Properties of Iron Catalysts”) and another in Applied Nano Materials (“Iron Nanoparticles to Catalyze Graphitization of Cellulose for Energy Storage Applications”), which details the mechanism and promise of using iron as a catalyst to convert waste biomass, such as wood chips and other cellulose-rich biomass, into value-added carbon materials. It was also the launching pad for the ongoing collaboration between the two groups, which combined each group’s expertise to add new dimensions to their research programs.

Discovery of nanoscale proportions

Graphene, a sheet of pure carbon one atom thick, is usually prepared by chemical vapor deposition, a process often used in the semiconductor industry to produce a uniform layer. However, Nzihou said chemical vapor deposition often relies on hazardous chemicals and expensive technology. Similarly, he said alternatives to graphene production are usually using toxic or expensive materials, as well as the use of petroleum-based sources.

In search of an environmentally friendly way to produce graphene, Nzihou and White turned to an underutilized source of biomass as a starting material for the process. Unfortunately, most of this biomass is rich in cellulose, an abundant polymer found in plant cell walls. Cellulose has proven difficult to convert into highly ordered carbon materials such as graphene without the use of toxic metal or rare earth catalysts due to its structure and arrangement of chemical bonds.

But Nzihou found that an iron oxide catalyst could help. By incorporating iron into biomass and heating it in an oxygen-limited environment through a process known as carbonization, Nzihou demonstrated the possibility to convert cellulose-rich biomass into final materials with large regions of ordered graphene sheets.

“Ange has shown that it is possible to use iron as a catalyst,” said White. “But the real question is trying to understand how iron imparts this catalytic behavior.”

White turned to his expertise in atomic and nanoscale characterization for the answer. Using techniques such as total X-ray scattering, Raman spectroscopy, transmission electron microscopy, and magnetic measurements, the researchers found that during the heating process, the iron oxide catalyst first breaks down to form nanoparticles within the biomass. When the cellulose-rich biomass starts to dissolve at higher temperatures, it precipitates as a layer of graphene sheets onto the surface of the iron particles.

“We were actually able to observe the shells of carbon atoms that formed around the iron nanoparticles during the process,” said White.

Interestingly, Nzihou and White found that some of the larger iron nanoparticles support a wider graphene-forming region than many of the smaller ones, a useful clue that may inform future efforts to improve the process of converting waste biomass to graphene. Researchers are also continuously refining the process to increase the size of pristine graphene regions while reducing the number of defects in the final material.

“Now that we have an understanding of the mechanism, we can figure out how to improve the process and optimize the properties of graphene sheets compared to conventional chemical vapor deposition methods, and even consider ways to scale this up in the near future,” said Nzihou. “Because ultimately, our work is about developing advanced carbon materials that are environmentally friendly while closing the carbon cycle and reducing carbon dioxide emissions.”

Launchpad for fruitful collaboration

The researchers say the project allowed them to leverage each other’s expertise to advance the field of sustainable carbon use, and the initial partnership has since merged with several ongoing research projects.

“This is an exciting collaboration,” said White. “I would never see myself working on sustainable carbon materials this way, but this project with Ange has provided a major opportunity to expand my work and add a new dimension to my research.”

For Nzihou, his time as a visiting Fulbright Scholar turned out to be just a preview of what was to come. He will return to the Andlinger Center in March 2024 as a Gerhard R. Andlinger Visiting Fellow to continue exploring ways of converting underutilized biomass sources into advanced carbon materials with specific properties for applications ranging from agriculture to energy storage and CO2 sequestration.

With White, he plans to broaden the scope of his work by bringing together the expertise of other Princeton faculty members such as Craig Arnold, Michele Sarazen and Rodney Priestley to develop strategies for sustainable carbon use. He also aims to collaborate with the Princeton Plasma Physics Laboratory (PPPL) to explore the use of plasma to drive various production processes.

“I believe that the carbon research we are conducting will be very impactful, because there are still many interesting challenges that need to be overcome on the ground,” said Nzihou. “And I believe Princeton is the right place to do it. When I looked at the structure of the Andlinger Center, I saw that it had everything I needed to develop my research.”



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