Graphene oxide reduction under the microscope with Raul Arenal
Raul Arenal, ARAID Researcher and Group Leader at Graphene Flagship partner the University of Zaragoza, Spain, used electron microscopy to study the changes taking place during GO’s reduction. His two recent papers could help scientists to design new materials with carefully tailored properties.
What happens to graphene oxide when we reduce it?
Graphene oxide (GO) is now commonly used in the Graphene Flagship and beyond. We reduce GO to make its properties more uniform, with fewer defects, making it more similar in terms of structure and properties to pristine graphene. This transformation improves its thermal and electrical conductivity, among other properties – useful for applications in fields like energy storage, electronics, water treatment and sensing.
Please tell me a bit about your background. How did you get to where you are today?
I did my PhD at the University of Paris, while I was working at Graphene Flagship partner CNRS, France. Afterwards, I moved to Chicago to pursue a postdoc, then returned to CNRS and worked there as a researcher for four years. It was then that I went on sabbatical to the Graphene Flagship partner the University of Zaragoza, in Spain, which is where I am today.
My research focuses on carbon-based materials like GO. I am really interested in understanding the chemical and physical properties of these materials – their electron configurations, and so-on. Electron microscopy is a great tool for this.
What have you been working on lately?
I am pleased to talk about two recent papers published by my group, both of which use in situ electron microscopy to investigate how GO’s properties change as it transforms into reduced graphene oxide (RGO).
There are several methods to reduce GO, and we investigated two of them: thermal and electrochemical reduction.
Why is electron microscopy so useful, and what information can it reveal?
It is one of the best ways to understand the transformation from GO into RGO. The changes are local, occurring at the sub-nanometre level. The information we obtained from electron microscopy showed us how to have much better control over the processes occurring during this transformation.
Using microscopy, we monitored the sub-nanometre changes in real time – in situ – and we tracked what happens to the material on the molecular level, under controlled conditions, in terms of temperature or time.
What can you tell me about the paper you published in Carbon, focusing on thermal reduction?
We studied the thermal reduction of GO to RGO. By heating the sample at different temperatures, from 70 to 1200 °C, we were able to stop at various temperature increments and analyse the changes that had occurred in the material.
We investigated what happens to the water in the sample at each of these temperatures. We also tracked the different functional groups, like carbonyls and epoxides, and at which temperatures they desorb from the parent structure. Once water and functional groups were removed, as we kept increasing the temperature, the sp2 carbon atoms became more ordered and more similar to the sp2 carbon atoms in graphene.
These studies allowed us to paint a picture of what happens to GO as it transforms under the effects of thermal reduction, explaining why RGO has properties closer to pristine graphene than GO.
How about your other paper, published in 2D Materials, investigating the effects of electrochemical reduction?
We studied the same parameters using transient electron microscopy, and introduced some new protocols to record what happens to the sp2 and sp3 carbon atoms.
Instead of heating the sample, we deposited GO on two electrodes and applied a current between them. The electrical current reduces GO to RGO. Electrochemical reduction gives a more homogeneous RGO, but the other processes, like the removal of water and desorption of functional groups, match those of thermal reduction.
Our approach was different, but the two studies – and their results – are complementary.
What does this mean for Graphene Flagship scientists, and other researchers working with GO and RGO?
Researchers will have more information, and that can only be a good thing. For instance, about which temperature to use for a desired outcome. Or, if they wanted to remove only epoxides, for example, 200 °C is more than enough, whilst preserving the other types of functional groups.
This lays a solid groundwork for future investigations and could lead to new applications and developments.
What is next for your group?
These two papers both came from a long investigation that kicked off years ago. There is a long history behind our research, and there is much more to come in the future. Now, we have all the parameters we need. We learnt a lot from these experiments and will continue in the same direction to learn more. We will also look at other layered materials as well.