Gas sensing technologies play a vital role in our modern world, from ensuring our safety in homes and workplaces to 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 revolutionize 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. As reported in their latest paper, which was published in ACS Applied Materials & Interfaces, 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 made available online on December 26, 2024, and was published on January 8, 2025, in Volume 17, Issue 1. This study was co-authored by Mr. Sogo Iwakami and Mr. 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 “functionalized” 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,” explains Assoc. Prof. Ohba, “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 functionalized 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, in particular, exhibited the greatest changes in sheet resistance (the inverse of conductivity) when exposed to NH₃—these changes were as high as 30%.

Worth noting, the team tested whether functionalized 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,” concludes Assoc. Prof. Ohba.

Overall, this study serves as an important stepping stone toward next-generation gas-sensing devices. Excited about their findings, Assoc. Prof. Ohba remarks: “As graphene is among the thinnest possible sheets with gas permeability, the functionalized graphene sheets developed in this work could be used in daily wearable devices. Thus, in the future, anyone would be able to detect harmful gases in their surroundings.” Hopefully, further work in this field will make this vision a reality and push graphene-based technology forward.

About Associate Professor Tomonori Ohba
Tomonori Ohba is an Associate Professor and Director of the Ohba Research Group at the Department of Chemistry of the Graduate School of Science at Chiba University, Japan. He primarily works in the field of physical chemistry, aiming to elucidate chemical phenomena at the nanomolecular level by employing advanced theoretical and experimental methods. He also explores nanospaces to control molecular motion, investigate molecular behavior, and discover new molecular reactivities. His extensive research work, published in numerous reputed journals, has been cited more than 5,000 times.

Funding:
This research was supported by JSPS KAKENHI Grant Number 23H01999.