Graphene and its derivatives represent an exciting area of chemical discovery. With it’s range of exciting properties graphene based technology has the potential to transform our lives.
Graphene oxide membranes have been receiving attention for their extremely powerful separation abilities and the ease at which it can be modified, allowing for membrane permittivity to be fine-tuned. These membranes show the potential to be used for water purification, ‘green’ gas purification and greenhouse gas capture.
AZoNano spoke to Dr. Yang Su, of the University of Manchester’s prestigious National Graphene Institute, about the work he, and the rest of the team he is part of, are conducting on these revolutionary membranes.
Why is graphene oxide a powerful candidate for use in next-generation membranes?
Next-generation membranes need to be highly selective, with a high permeability to a select few molecules, whilst also being inexpensive and stable enough for wide use. Graphene oxide shows the potential to be strong in all of these areas.
This is because the physical structure of graphene oxide sheets lends itself easily to being used as a membrane.
The structure of graphene oxide (GO). GO has oxidative 'defects' which break the perfect strucutre of graphene. Whilst this means GO is not as conductive as graphene the holes resulting from these defects make it perfect for use in membrane technology.
There are numerous pores inside membranes. It is the shape and size of these pores that defines the selectivity and permittivity of a membrane. Whereas the identity of the materials used to construct the membrane and the way in which it is processed determines its stability and cost.
Graphene oxide (GO) membranes consist of many layers of two dimensional graphene oxide layers stacked on top of each other to give a laminate of two dimensional GO sheets. These stacked sheets have interconnected channels running through them which act as the membranes pores.
The pores are uniform in size and only 0.9 nm in width. This small width means the membrane is highly selective as only ions and molecules smaller than 0.9 nm can permeate through. Any ions or molecules larger than 0.9 nm are prevented from passing through the membrane by a process called physical extrusion.
The channels inside the GO membrane can be modified with different methods based on what type of membrane is required. Modifying the membrane channels allows membrane permeability to be fine-tuned meaning certain compounds and ions can be selected in such a way that the membrane is highly permeable to them.
Additionally, unlike polymer membranes, GO membranes are more chemically inert, which means they have a longer service lifetime.
Graphene oxide is not as expensive as pure graphene and it can, in fact, be made easily using only graphite powder and some inexpensive chemistry which could even be carried out at home. (though I wouldn’t recommend it!)
This high selectivity coupled with a low cost and long operational lifetime is why there is so much interest in GO advanced membrane technology.
An image of multilayer graphene. Sheets in GO membranes are organised in a similar fashion. Layered one over another to give a laminate. Image Credits: shutterstock.com/Bessarab
Can unadulterated, pure graphene be used for this purpose?
In short, no.
Pure graphene is highly impermeable. Graphene will occasionally be permeable to very small particles such as protons (one of the components of an atomic nucleus) but this is only under very specific conditions that you wouldn’t find outside of a lab.
Any matter larger than a proton, such as liquids and gases, will be completely impermeable to graphene. However, this impermeability has its own uses - for example graphene would work very well as a barrier or protective membrane.
As pure graphene is completely defect free there are no gaps in its structure making it completely impervious to all but the smallest of particles. Image Credits: shutterstock.com/ktsdesign
What molecules and liquids are GO membranes capable of separating?
The unique pore structure of GO membrane provides a good platform for us to play with. By changing the structure of the membrane we can adjust the membranes permeability with respect to different molecules.
Depending on the membrane structure they can be used for gas separation, such as hydrogen purification, CO2 capture, or gas drying by removing water.
The GO membranes can also be used for the separation of aqueous mixtures, such as in the dehydration and purification of organic solvents. This technology has the potential to be used to separate certain solute/ions from solvents, which could potentially be used for water purification.
How can the membrane separation properties be modified? Can the membrane be functionalised or the pore size controlled in other ways?
The membrane separation properties can be modified using many different methods with the main methods being chemical functionalization and physical pore size control.
Attaching external molecules to the sheets by chemical functionalization can be used to expand the pore sizes. This results in faster transport through the membranes and different selectivities.
In contrast, our recent results show that by removing oxygen atoms from GO sheets we could induce the channels to collapse. This type of GO membrane could be used as a barrier or protective coating for the packaging industry.
A schematic showing a GO laminate (in orange) acting as a membrane. Small molcules, such as water (in green), can pass through whereas larger molecules (in red) are blocked due to their larger size.
How do you see this technology impacting the real world?
Due to its amazing separation properties we expect this technology to have a huge impact in the fields of energy reduction and environmental conservation.
For example, this technology could be used for the dehydration and purification of biofuels. Currently water is produced as a side product when biofuels are produced. This water damages the biofuel yield and the final product quality, and its removal is difficult. Our GO membranes could help solve this problem.
GO membranes also have the potential to be used for portable water purifiers. The small pore size of the membranes means all of the bacteria present in dirty water, and most of the other impurities, will be sieved out with only pure water passing through. This technology would be hugely useful in the developing world. The membranes also show promise for use in water desalination, but we’re not quite there yet.
The membranes also have the ability to separate gases, meaning in the future they could be used to control greenhouse gas emissions and to purify hydrogen related clean energy gases.
GO membrane technology could assist in the production and purification of clean biofuels. Image Creidts: shutterstock.com/Toa55
Do you think graphene oxide membranes have the potential to be used for large scale water filtration or do you believe this would be impractical?
The membrane technology that we are developing could definitely be used for large scale water filtration.
Firstly, graphene oxide can be manufactured on a large scale meaning the materials are readily available. Secondly, the membranes are produced with little difficulty, even simple membranes formed by evaporating water from a GO suspension results in a GO membrane that performs well.
Finally, and most importantly, the membranes already show amazing properties for sieving impurities from water. Of course, at our current stage, we still need to work hard to address several issues on its practical applications, but our research team are working on this and I’m confident we will get there.
How does your research on graphene membranes fit into the wider picture, with the other research being undertaken at the National Graphene Institute?
We collaborate widely within and outside the Institute.
We work with a lot of different research groups as our membranes could be used in a lot of different technologies with a lot of different uses. There is particular interest from research groups working on biomedical, fuel cell and environmental research.
The National Graphene Institute at the University of Manchester
When do you expect this technology to become widely available? What obstacles must be overcome before this happens?
We don't have a detailed timeline yet. At our current stage, among a lot of possible applications, we are assessing them for one or two applications which could be realized on a large scale.
Our main obstacle is that we need more industrial partners to fill the gap between the lab experiments and pilot scale productions, we then could conduct application-oriented research to finalize the end products.
What do your research team have planned for the future?
As there are a lot of interesting things to explore, we have no detailed plan. We have too much to choose from!
However, as our membrane based research could have such a huge impact on human welfare we are determined that our 2D materials based membranes will be realized in real life.
Where can our readers find out more about the NGI and the research that you are undertaking there?
As well as our research there is a lot of interesting research happening at the NGI.
About Dr. Yang Su
Dr. Yang Su has worked as a post-doctoral researcher at the University of Mancheser for the past three years, following obtaining his PhD at the Chinese Academy of Sciences. Following working as part of the Nobel Prize winning group (awarded for their work with graphene) Yang is now based in Manchester University’s National Graphene Institute.
Yang has worked in the field of carbon nanomaterials, with a particular focus on 2D materials. His work has been featured in 12 peer reviewed papers with over 850 citations at the time of writing. His previous research has been varied focusing on subjects such as the inkjet printing of 2D materials to his current membrane based research.
Yang is currently focusing on using 2D materials to help solve energy and environment related challenges. He hopes his research on 2D membranes will enable better welfare that will benefit people on a global scale.
About the National Graphene Institute
The National Graphene Institute is the leading centre for graphene research in the UK. It enables academics and industry to work side-by-side on the graphene applications of the future. More than 50 companies from across the world have already chosen to partner with The University of Manchester working on graphene-related projects.
The 7,825 square metre, five-storey building features cutting-edge facilities and equipment throughout to create a world-class research hub. The NGI is a significant first step in the vision to create a Graphene City in Manchester. The £60m Graphene Engineering Innovation Centre (GEIC) is currently under construction and will complement the NGI with further industry-led development in graphene applications.
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