Currently graphene has neither global standard nor reference material. So, how can the quality of graphene be assessed?
Graphene Quality : A key factor
“Hundreds of companies in the world claim to produce graphene.”
Firstly, the type of analysis methods differ a lot and secondly it can be questionable if the analysis is systematic.
Research conducted by the National University of Singapore (NUS) proposes a protocol to analyse and to compare the quality of graphene.
The quality of graphene from 60 manufacturers was analysed with the conclusion that the graphene is generally low quality and non-optimal for many applications.
It tends to show that most graphene manufacturers produce graphite micro-platelets.
Nowadays several local initiatives are taking shape, in order to develop programs for the verification of graphene quality.
In Europe, this work is setted up in cooperation with the LNE and INERIS.
1-Ultraviolet–visible spectroscopy (UV-Vis)
With only absorbing 2% of the visible light, one layer of graphene is transparent to the human eye. Graphene absorbs however at a very characteristic wavelength in the ultra violet range.
It is well described in the literature that pristine graphene exhibits a characteristic absorbance peak at 269 nm. This peak can be attributed to π→π* transitions of aromatic C bonds.
Graphene oxide on the other hand has many defects and functional groups interrupting electronic conjunction within honeycomb structure.
Therefore, more energy is required for π→π* transition of the aromatic C bonds, explaining why the absorbance peak is shifted to shorter wavelengths.
The shoulder found at 300 nm is described in the literature as n→π* transitions of the carbonyl groups typical of graphene oxide.
UV-Vis spectroscopy is a very simple tool to get a first idea of the quality of a graphene dispersion and to investigate the stability over time.
-Blue : Chemically exfoliated graphene
-Red : Graphene Oxide
When discussing the quality of graphene, Raman spectroscopy is the technique that needs to be taken into consideration.
Since the first isolation of SLG (Single Layer Graphene) in 2004, it is an ongoing effort by many research groups to decipher the Raman spectrum of graphene in order to extract the integrality of information available.
This is not a simple undertaking and expertise is required to interprate correctly the information.
From a more simple point of view, the three main modes (see below) provide already many information about the quality of the material.
Meaning the higher the intensity the thinner the material. This is especially evident in comparison to the Raman spectrum of graphite.
The Raman spectrum of graphene oxide is evidently different compared to graphene, due to the presence of numerous functional groups.
The G mode is the reference of all comparison because it represents the carbon honycomb structure of graphene.
The D mode induced through sp³ via functional groups or edges.
The 2D mode is very sensitive to the thickness of the graphene.
It is important to remember that the efficiency of graphene in different applications relies directly on its structure and properties as shown on the figure presented above.
Graphene a worderful material
As raw materials are limited around the globe, there is a surge to minimize their use and to increase the shelf life of products. Although we are still limited in our understanding of their impact. If used wisely and in their optimal form, nanomaterials offer very significant savings in materials, energy or water.
As a result, they can lead to a dramatic reduction in waste and carbon dioxide footprint compared to more traditional materials or processes.
Recent progresses realized on graphene production process and environmental-friendly graphene play an important role on this perspective.
By associating these technological breakthroughs with the possibility to increase the shelf life or performance of coatings, polymers or composites, industrials are now convinced to consider graphene as an environmental alternative.
The main goal for the years to come will be to scale-up an industrial and energy-efficient process as well as producing few or even no waste.
Graphene would then answer to a very modern and urgent need: ally a high performance for the industry with a neutral or positive impact on the environment.