Here, we discuss the unique properties of MXenes, which make them suitable for applications in gas sensors, and highlight the need to explore advanced materials for gas sensor applications.
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MXenes are two-dimensional (2D) materials with active sites, tunable surface chemistry, metallic conductivity, and outstanding stability, which makes them highly desirable candidates for the fabrication of gas sensors.
Gas Sensors and Their Significance
Gas sensors are electronic devices that detect different types of gasses, particularly toxic and explosive ones. They help identify gas leaks, smoke, and carbon monoxide in residential premises. Air contaminants of volatile organic compounds (VOC) can be detected through air quality and odor monitors that are integrated with an air cleaning or ventilation control system.
Oxygen sensors are used in the biotechnology industry to control anaerobic workstations, food packaging, medical ventilation equipment, fire detectors, and for flue gas emission monitoring. These gas sensors are also useful for monitoring oxygen deficiencies in confined and enclosed spaces.
Similarly, ammonia detectors help prevent the accumulation of this toxic gas in poultry farms, livestock, and sewage treatment plants. Other gas sensors monitor gasses emitted by various industrial processes, such as oxygen, sulfur dioxide, carbon monoxide, and nitrogen oxide.
What are MXenes?
MXene is a graphene-like 2D material whose chemical composition constitutes metal transition carbide or nitride. Among various types of MXenes, titanium carbide (Ti3C2Tx) is the most widely used. Stripping of the metal in titanium aluminum carbide (Ti3AlC2) resulted in the formation of Ti3C2Tx with empty layers.
Modifying the etching conditions tunes the physical and chemical properties of Ti3C2Tx. It has been widely explored in electrocatalysis research owing to its conductive metal core and highly functionalized surface. Moreover, integrating Ti3C2Tx with molybdenum sulfide via hydrothermal reaction results in a three-dimensional (3D) structure with outstanding hydrogen evolution activity.
Suitability of 2D MXenes in Gas Sensors
Currently, many materials, such as metal oxides, conductive polymers, carbon-based materials, metal-organic frameworks (MOFs), and rare-earth oxides, are used to develop gas sensors. Among the various materials used in fabricating gas sensors, 2D MXene has large surface areas with many adsorption sites, making it desirable for fabricating gas sensors with high sensitivity and specificity.
2D MXene (transition metal carbide, nitride, or carbonitrides) has rich functional groups on its surface, high conductivity, large specific surface area, large porosity, high hydrophilicity, and rich organic bonds, which are suitable properties for the adsorption of gasses. Additionally, the lower resistance of MXenes at room temperature with less noise makes them favorable for room-temperature sensing applications.
Types of MXene Gas Sensors
Delaminated MXene Gas Sensors
Ti3C2Tx MXene nanosheets are prepared from Ti3AlC2 and deposited in flexible polyimide (PI) for sensing ammonia gas at room temperature. The functional groups present on the surface of Ti3C2Tx MXene allowed the adsorption of ammonia gas on its surface. The ammonia gas bonded strongly, transferring electrons to Ti3C2Tx, causing an increase in the resistance and creating a sensing signal.
Gas sensors based on metal oxides exhibit a high signal and low noise at high temperatures because of their activation energy and lack of gas adsorption sites. However, MXenes have both high conductivity and abundant adsorption sites, which make them promising gas sensors.
Previous studies have shown that modifying the surface of MXene, inducing interlayer swelling (by treatment with an alkaline solution), and leveraging specific gas interactions improves the effectiveness of gas sensors.For instance, modifying the surface of MXene with plasma increases the number of oxygen functional groups, which enhances the response of the sensor to nitrogen dioxide gas. Similarly, reducing the hydrophilic surface groups on MXene by introducing hydrocarbon groups improves its sensitivity to alcohol detection.
Multilayered MXene Gas Sensors
The van der Waals forces that exist between the layers of MXene nanosheets lead to material agglomeration owing to self-stacking, making it difficult to adsorb gas molecules on their surfaces. Accordion-like Ti3C2Tx MXenes, synthesized using hydrogen fluoride etching, have improved acetone sensing owing to their high surface area and the presence of a large number of hydrogen atoms.
Compared to Ti-based MXenes, Mo-based MXenes have more conductance and higher reactivity, yet this material has not received much attention. Among the fabricated gas sensors based on multilayered Mo2CTx MXenes on glass, a gas sensor based on a porous silicon (pSi) substrate showed better carbon dioxide sensing.
MXene–Metal Oxide Composites
Combining MXenes with metal oxides to fabricate gas sensors enhances their performance at room temperature. In a MXene–metal oxide composite, 2D MXene contributed to the high conductivity at room temperature.
The Ti3C2Tx - tin oxide (SnO2) composite, prepared using a hydrothermal reaction, was used for nitrogen dioxide sensing at room temperature. Compared to pristine MXene, gas sensors based on Ti3C2Tx -SnO2 composites exhibited superior performance to nitrogen dioxide gas.
MXene-transition Metal Dichalcogenides (TMDs) Composites
The unique properties of 2D TMDs, such as abundant adsorption sites, high surface areas, and high surface reactivities, make MXene-TMD composites highly desirable for gas-sensing studies. For instance, the Ti3C2Tx -WSe2 composite exhibited enhanced sensitivity to ethanol gas owing to the presence of numerous heterojunctions between Ti3C2Tx and WSe2.
Recent Studies
An article published in ACS Applied Materials and Interfaces fabricated a hydrogen sulfide (H2S) gas sensor based on Ti3C2Tx MXene organic composites. Cost-effective and high-performance sensors are required for environmental monitoring and human health.
Here, the (poly[3,6-diamino-10-methylacridinium chloride-co-3,6-diaminoacridine -squaraine] (PDS-Cl) polymer was used to preserve the selectivity and enhance the gas-sensing response by thirty-fold.
Another article published in ACS Sensors reported the fabrication of fully flexible paper-based gas sensors integrated with a Ti3C2Tx-MXene nonmetallic electrode and the Ti3C2Tx/ tungsten sulfide (WS2) gas sensing film was designed to form an Ohmic contact and Schottky heterojunction in a single gas-sensing channel.
The Ti3C2Tx/WS2 gas-sensing film exhibited outstanding chemical and physical properties because both Ti3C2Tx and WS2 nanoflakes exhibited effective charge transfer, high conductivity, and abundant active sites for gas sensing. The fabricated sensor showed sensitivity towards nitrogen dioxide at room temperature, 76 times higher than that of the Ti3C2Tx/WS2 sensor-integrated gold interdigital electrode.
Conclusion
Overall, the use of 2D MXenes in the development of gas sensors has marked a significant stride in the field of sensor technology. Their exceptional properties, such as large surface area, tunable electronic properties, high conductivity, and chemical stability, make MXene highly desirable for gas-sensing applications.
References and Further Reading
Mirzaei, A., et al. (2023). Room Temperature Chemiresistive Gas Sensors Based on 2D MXenes. Sensors, 23(21), p. 8829. doi.org/10.3390/s23218829
Hosseini-Shokouh, S. H., et al. (2023). Highly Selective H2S Gas Sensor Based on Ti3C2Tx MXene–Organic Composites. ACS Applied Materials & Interfaces, 15(5), pp. 7063-7073. doi.org/10.1021/acsami.2c19883
Quan, W., et al. (2023). Fully Flexible MXene-based Gas Sensor on Paper for Highly Sensitive Room-Temperature Nitrogen Dioxide Detection. ACS Sensors, 8(1), pp. 103-113. doi.org/10.1021/acssensors.2c01748
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