MXenes are a class of 2D transition-metal carbides, nitrides, and carbonitrides with exceptional properties, including substantial electrical and thermal conductivities, outstanding mechanical strength, and a considerable surface area, rendering them an appealing choice for gas sensors.
1. Introduction
In recent years, the discovery of 2D materials has revolutionised the field of materials science and engineering
[1]. Among these, MXenes have emerged as a promising class of 2D materials with diverse properties and applications. MXenes, a class of two-dimensional (2D) transition-metal carbides, nitrides, and carbonitrides, have emerged as a fascinating material family with exceptional properties, which are promising for a wide range of applications
[2][3][4][5]. The story of MXenes began in 2011, when researchers at Drexel University discovered a novel method to etch and exfoliate MAX phases, layered ternary carbides, and nitrides, resulting in the creation of a new family of 2D materials
[6]. For example, Ti
3C
2T
x MXenes were obtained by selective etching of the A element (e.g., aluminium) from their corresponding MAX phases (e.g., Ti
3AlC
2) using hydrofluoric acid (HF) or other etchants
[6].
MXenes are typically composed of transition metals, such as titanium (Ti), vanadium (V), niobium (Nb), chromium (Cr), zirconium (Zr), etc., and have the chemical formula M
n+1X
nT
x, where M represents the transition metal, X represents carbon and/or nitrogen, and T symbolises surface termination groups, such as hydroxyl (–OH), fluorine (–F), or oxygen (–O)
[5]. The number n is determined by the number of X layers, and can range from 1 to 4, while n+1 represents the number of transition metal layers
[7]. The T groups on the surfaces of MXenes can be chemically modified, leading to tuneable surface chemistry and wettability. Due to this, MXenes exhibit a range of unique physical, chemical, electronic, electrical, and mechanical properties, including high electrical conductivity, high thermal conductivity, high mechanical strength, and large surface area. Additionally, they have excellent chemical stability, good biocompatibility, and good optical properties
[8][9]. These properties make MXenes attractive candidates for various applications, such as supercapacitors, batteries, electrocatalysts, and sensors.
One of the most promising applications of MXenes is in gas sensors
[10][11][12]. Gas sensors play a crucial role in many aspects of our lives, including environmental monitoring, industrial process control, and medical diagnostics
[13][14]. A gas sensor is an electronic device that detects the presence of various gases in the environment and converts this information into a measurable signal
[15]. The critical parameters for gas sensors include sensitivity, selectivity, response time, and stability
[16][17][18]. Currently, the most common materials used for gas sensors are metal oxides such as tin oxide, zinc oxide, and titanium dioxide
[19][20]. However, these materials have some limitations, including low selectivity, poor stability, and the need for high operating temperatures
[19][20]. MXenes, on the other hand, have demonstrated exceptional gas-sensing properties, making them a promising candidate for replacing the current materials. For example, MXenes have exhibited high sensitivity and selectivity towards various gases, including nitrogen dioxide (NO
2), ammonia (NH
3), and hydrogen (H
2)
[11][21][22]. Moreover, MXenes have shown excellent stability and low power consumption, making them ideal for long-term use and portable applications. Furthermore, MXene-based heterostructures have shown significant advancements in gas-sensor performance compared to state-of-the-art technologies. A heterostructure is a material consisting of two or more different materials with different electronic properties
[23][24]. The combination of MXenes with other materials in heterostructures can enhance gas-sensing properties by increasing sensitivity and selectivity. For example, MXenes can be combined with metal oxides or polymers to create heterostructures that exhibit enhanced gas-sensing performance
[23][24][25]. Despite these advantages, MXenes and MXene-based-heterostructure gas sensors face several notable challenges. One key challenge lies in achieving optimal selectivity towards specific gas analytes while maintaining high sensitivity, as gas mixtures in real-world environments can be complex. Furthermore, the long-term stability and robustness of these sensors need improvement to ensure reliable and continuous operation over extended periods. Integration into practical devices and scaling-up of production processes are also areas of concern for the commercialisation of MXene-based gas sensors. Lastly, addressing issues related to the cost-effectiveness and availability of MXene materials at a larger scale is essential for widespread adoption. Overcoming these challenges is pivotal in realising the full potential of MXenes and MXene-based heterostructures in gas-sensing applications
[23][25][26].
2. Synthesis and Properties of 2D MXenes
Generally, MXenes emerge through the selective elimination of A layers from MAX phases, giving rise to two-dimensional materials that are usually composed of three or more atomic layers. These 2D materials possess distinct properties when compared to their three-dimensional (3D) precursor counterparts [27]. In their early synthesis, the primary etching agents predominantly comprised fluorine-containing compounds, such as HF, LiF+HCl, bifluoride salts, and molten salts containing fluorine. These etchants dictated the surface terminations of MXenes, yielding three primary variations: -F, -OH, and -O. However, in 2017, alternative non-fluorine etching methods, including electrochemical etching and concentrated alkaline hydrothermal etching, were introduced, resulting in the production of fluorine-free MXenes. More recently, a non-aqueous molten salt etching approach, employing Lewis acidic
In contrast to the stacked configuration of MXenes, single-layer MXene nanosheets exhibit superior chemical properties, such as a notable increase in specific surface area, favourable hydrophilicity, and a wealth of surface chemistry. In fact, the initial report on MXenes employed ultrasonic treatment to disassemble accordion-like MXenes into layers, albeit with limited success due to the robust bonding between these layers, resulting in low yields and impractical outcomes [28]. The process of obtaining single-layer nanosheets from accordion-like MXenes can be accomplished through appropriate delamination techniques, with the ease of this process directly influenced by the composition of surface functional groups. Furthermore, increasing the interlayer spacing of MXene flakes through ion intercalation is a common strategy for delaminating multilayered MXenes [28].
Following this, depending on the synthetic route, the produced MXenes can have significantly different properties because these properties rely on the functional groups, defects, interlayer structures, etc. MXenes are versatile materials that fulfil the essential requirements for fully functional gas-sensing devices [29].
MXenes offer high transparency and light absorbance, with Ti3C2Tx MXenes in the range of 1–2 nm thickness achieving up to 91.2% optical transparency and excellent light-to-heat conversion efficiency. Different terminations impact their optical properties, with -F and -OH groups reducing visible light absorption and reflectivity but enhancing reflectivity in the UV region. They demonstrate excellent thermal stability, with Ti3C2Tx (T = F, OH) remaining stable up to 500 °C, or 800 °C in argon atmosphere. Surface functionalisation with functional groups helps mitigate surface oxidation.
3. Fundamental Sensing Mechanisms in 2D MXenes and MXene Heterostructures
3.1. Sensing Mechanism in Pristine 2D MXenes
Among the MXene family, Ti3C2Tx stands out as the most extensively explored member for its applications in gas sensors. The behaviour of a Ti3C2Tx sensor typically resembles that of p-type semiconductors, a trait attributed to the presence of functional groups such as −F, −OH, and −O. In the realm of gas sensing, the transfer of charge carriers is governed by two primary mechanisms: physisorption and chemisorption. At ambient temperatures, physisorption takes precedence, involving the gas molecules’ physisorption onto the surface, circumventing the participation of adsorbed oxygen species. Consequently, alterations in the electrical signal stem from the adsorption and subsequent desorption processes.
The nature of gas analytes significantly influences these changes in the electrical signal. For instance, highly reducing gases elevate electrical resistance, whereas highly oxidising gases lead to its decline. To illustrate, consider the research conducted by Kim et al.
[30]. When Ti
3C
2 MXene encountered ammonia, the electrons generated will get combined with the holes in MXene, which decreases the majority charge-carrier concentration and thereby increases the resistance of Ti
3C
2T
x sensors. The reaction mechanism of p-type Ti
3C
2T
x MXene and reducing gas is depicted in Equations (1) and (2).