[Image above] Bottles containing exfoliated graphene obtained from a new method based on plasma spraying. Reprinted with permission from Islam et al., ACS Nano, Vol. 15 Iss. 1 (1775–1784). Copyright 2021 American Chemical Society.
Almost two decades have passed since the discovery of graphene in 2004. Yet use of this material in industry remains limited.
Part of the reason for graphene’s limited use is the lack of standards regulating quality of the material. But another major factor is that we still haven’t developed an ideal method for graphene production.
There are four essential factors of an ideal graphene production method: 1) it produces high-quality graphene, 2) it distributes graphene in narrow layers, 3) it uses a fast and reproducible technique, and 4) it allows for high throughput. Unfortunately, current methods of graphene production struggle to fulfill all four factors.
Bottom-up approaches such as chemical vapor deposition and epitaxial growth are the leading methods for graphene production. They oﬀer the ability to grow high-quality graphene in a large area, but they currently lack bulk production protocols and remain expensive due to multistep processes.
Top-down approaches that involve exfoliating graphite to obtain graphene are scalable thanks to cheap and abundant graphite sources. But quality and narrow layer distribution of the graphene typically suffer, for example, in electrochemical exfoliation and mechanical exfoliation, respectively.
Researchers have conducted some studies on synthesizing graphene by exposing graphene precursors to high temperatures, but this approach tends to suffer from some of the same problems as other exfoliation methods, namely producing graphene with topological defects or multilayers.
However, a recent study by four researchers from the Indian Institute of Technology Patna that proposes a new high-temperature exfoliation method for graphene may fulfill all four essential factors.
Their method is based on plasma spraying, a well-established technique for depositing metal or ceramic coatings. In this process, powders of the coating materials are fed via an inert gas stream into a plasma jet, which melts the coating materials and then sprays them over the substrate to be coated.
In their study, the researchers used argon gas to introduce graphite directly into a plasma plume at a powder feed rate of 120 g/h. Then, they collected the plasma exposed graphite and introduced it to mild centrifugation in deionized water to remove unexfoliated large agglomerates.
The researchers used various microscopy and spectroscopy methods to analyze the exfoliated graphene, and the results were promising. The graphene flakes were up to 3 µm in diameter, with 85% being a single atomic layer and the rest having a few layers. The material was free of defects and had a high carbon-to-oxygen ratio of 21, which is comparable to graphene made using the bottom-down approach of chemical vapor deposition.
In a C&EN article, senior author Anup Kumar Keshri, assistant professor and head of the Department of Metallurgical and Materials Engineering at the Indian Institute of Technology Patna, says the technique yielded 48 grams of graphene in 1 hour, which suggests it should be easy to scale up. In addition, because the method does not require any solvents, intercalants, or purification steps, cost of the lab-made graphene is only $1.12 per gram, which “is competitive or even lower than commercially available graphene” and should go down further when mass-produced.
“We believe that this work could be a game changer in the production of pristine graphene in large scale for numerous applications,” the researchers conclude their paper.
The paper, published in ACS Nano, is “Ultra-fast, chemical-free, mass production of high quality exfoliated graphene” (DOI: 10.1021/acsnano.0c09451).