Researchers from Japan have developed an improved gas-sensing technology by treating graphene sheets with plasma under different conditions, creating structural and chemical defects that enhance ammonia detection.
These functionalised graphene sheets exhibited superior sensing performance compared to pristine graphene, potentially paving the way for wearable gas detection devices for everyday use.
Gas sensing technologies play a vital role in monitoring environmental pollution and industrial processes.
Traditional gas sensors, while effective, often face limitations in their sensitivity, response time, and power consumption.
To account for these drawbacks, recent developments in gas sensors have focused on carbon nanomaterials, including the ever-popular graphene.
This versatile and relatively inexpensive material can provide exceptional sensitivity at room temperature while consuming minimal power. Thus, graphene holds the potential to revolutionise gas detection systems.
Against this backdrop, a research team led by Associate Professor Tomonori Ohba from the Graduate School of Science, Chiba University, Japan, explored a promising avenue to improve graphene’s sensing properties even further.
The team investigated how and why graphene sheets treated by plasma with different gases can lead to enhanced sensitivity for ammonia (NH3), a toxic compound.
The study was co-authored by Sogo Iwakami and Shunya Yakushiji, also from Chiba University.
The researchers produced graphene sheets and applied a plasma treatment to them under argon (Ar), hydrogen (H2), or oxygen (O2) environments.
This treatment “functionalised” graphene, meaning that it modified the surface of the graphene sheets by attaching specific chemical groups and creating controlled defects, serving as additional binding sites for gas molecules like NH3.
After treatment, the researchers employed a variety of advanced spectroscopic techniques and theoretical calculations to shed light on the precise chemical and structural changes the graphene sheets underwent.
The team found that the gas used during plasma treatment led to the creation of different types of defects on the graphene sheets. “The O2 plasma treatment induced oxidation of the graphene, producing graphoxide, whereas the H2 plasma treatment induced hydrogenation, producing graphane,” Ohba explained.
“Spectroscopic analysis suggested that graphoxide had carbon vacancy-type defects, graphane had sp3-type defects, and Ar-treated graphene had both types of defects.”
To clarify, an sp3-type defect is a structural change where a carbon atom in graphene shifts from having three bonds in a flat plane to forming four bonds in a tetrahedral arrangement, often due to hydrogen atoms attaching to the surface.
Interestingly, introducing these defects into the graphene sheets greatly enhanced their performance for sensing NH3. Since NH3 binds more easily to defects rather than to pristine graphene, the electrical conductivity of functionalised sheets changed more noticeably when exposed to NH3.
This property can be leveraged in gas-sensing devices to detect and quantify the presence of NH3.
Graphoxide exhibited the greatest changes in sheet resistance (the inverse of conductivity) when exposed to NH3—these changes were as high as 30 per cent.
Worth noting, the team tested whether functionalised graphene sheets could withstand repeated exposure to NH3 without degrading their gas-sensing performance. Although some irreversible changes in sheet resistance were observed, some significant changes were fully reversible and cyclable.
“The results showed that functionalizing graphene structures with plasma generated noble materials with a superior NH3 gas-sensing performance compared with pristine graphene,” Ohba said adding that the study serves as an important stepping stone toward next-generation gas-sensing devices.