Gas-Sensing Mechanisms of MXenes and MXene-Based Heterostructures

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1		Angga Hermawan	--	4420	2023-12-06 16:43:48	\|
2	Reference format revised.	Lindsay Dong	+ 1 word(s)	4421	2023-12-08 01:51:02	\|

This entry is adapted from the peer-reviewed paper 10.3390/s23218674

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.

2D MXenes heterostructure gas sensor gas-sensing mechanism

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, Ti₃C₂T_x MXenes were obtained by selective etching of the A element (e.g., aluminium) from their corresponding MAX phases (e.g., Ti₃AlC₂) 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 (NO₂), ammonia (NH₃), and hydrogen (H₂) ^[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 Ti₃C₂T_x 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 Ti₃C₂T_x (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, Ti₃C₂T_x stands out as the most extensively explored member for its applications in gas sensors. The behaviour of a Ti₃C₂T_x 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 Ti₃C₂ 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 Ti₃C₂T_x sensors. The reaction mechanism of p-type Ti₃C₂T_x MXene and reducing gas is depicted in Equations (1) and (2).

2NH₃ + 3O⁻→N₂ + 3H₂O + 3e⁻(1)

NH₃ + OH⁻→NH₂ + H₂O + e⁻(2)

This scenario mirrors the behaviour observed in V₂CT_x MXenes. However, in the case of the V₂CT_x sensor exposed to oxidising gases like NO₂, the dynamic changes are distinct. NO₂ molecules attach to the active sites of V₂CT_x through surface terminations like single-bonded O and single-bonded OH. Consequently, a transfer of electrons from V₂CT_x to NO₂ molecules transpires, augmenting hole concentration within V₂CT_x (Equations (3)–(5)). This, in turn, amplifies conductivity and diminishes resistance in the V₂CT_x sensor. Furthermore, the reaction between NO₂ and O²⁻ yields NO²⁻, consuming electrons and thereby reducing the resistance of the V₂CT_x sensor ^[22]. This intricate process can be encapsulated in the following equations.

O_{2 (ads)} + e⁻ → O₂⁻, (at RT) (3)

NO_{2 (gas)} + e⁻ → NO₂⁻ _(ads)(4)

2NO_2(g) + O₂⁻ _(ads) + e⁻ → 2NO₂⁻ _(ads) + O₂(5)

Based on the results of these experimental studies, the gas-sensing mechanism in pristine MXenes can be comprehensively depicted in a general model, as illustrated in Figure 1. This model effectively illustrates the intricate interplay between molecular interactions and the surface of MXenes, and how these interactions relate to changes in electrical resistance and the band diagram.

Figure 1. Gas sensing mechanism of the pristine 2D MXenes and the corresponding energy band diagram and electrical conductivity changes in reducing and oxidising gas.

Typically, MXene-based gas sensors function at room temperature to prevent surface oxidation. As a result, the most prevalent adsorbed oxygen ions are of the O₂⁻ type. When external gas is introduced, the reaction between the adsorbed O₂⁻ ions on the surface and the gas leads to changes in the conductivity of the MXenes. In the presence of a reducing gas, the above-mentioned reaction results in the return of electrons to the MXenes, generating both holes and electron recombination. This phenomenon leads to an increase in resistivity due to the depletion of holes.

3.2. Sensing Mechanism in 2D MXene Heterostructures

The gas-sensing mechanism in MXene heterostructures is inherently more complex due to their ability to be combined with various types of materials, such as metal oxides, polymers, metal nanoparticles, and others. This combination leads to diverse configurations in the energy-band diagram when these components come into contact, interacting with target molecules of either reducing or oxidising gases. This complexity is further heightened when heterostructures consist of more than two materials, introducing additional challenges and uncertainties into the gas-sensing mechanism.

Hermawan et al. ^[31] fabricated a sensor device consisting of p-type CuO semiconductor with metallic Ti₃C₂OH₂ and found that the work function played a significant role in regulating the gas-sensing mechanism, charge transfer, and energy-band alignment. The Ti₃C₂T_x MXene with –OH termination exhibits a work function of around 3.9 eV, lower than CuO’s work function of 4.7 eV, creating a Schottky barrier at their interface and aligning the Fermi energy levels. Charge transfer occurs bidirectionally between metal and semiconductor across the interface, limited by the barrier height ΔΦB, resulting in high room-temperature resistivity. During the gas-sensing mechanism, O⁻ ion adsorption reduces interface band bending, aiding charge transfer with the temperature rise. The Schottky barrier leads to poorer mobility in p-type/metallic CuO/Ti₃C₂T_x than in p-type CuO, explaining the slightly higher resistance despite Ti₃C₂T_x’s better conductivity. O⁻ removal by reducing gas thins the depletion region (HALs), raises band bending, and reinstates the Schottky barrier, causing hole trapping in Ti₃C₂T_x.

Now, turning to the discussion of the gas-sensing mechanism in MXene/non-oxide heterostructures, it is worth noting that conducting polymers also exhibit n-type or p-type conductivity. The energy levels of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) in polymers, whether they act as donors or acceptors, are crucial factors. Given this, the gas-sensing mechanism in MXene-polymer heterostructures is likely to bear similarity to that observed in MXene/metal-oxide heterostructures. PEDOT:PSS is categorised as a donor (n-type) conjugated polymer. The formation of a band depletion stems from the absorption and ionisation of oxygen molecules, achieved by capturing electrons from the conduction band of PEDOT:PSS/MXene heterostructures. This depletion results in elevated sensor resistances. Upon exposure to reducing gas, the depletion thins, permitting electron flow and leading to a decrease in electrical resistance (Figure 2).

Figure 2. Gas sensing mechanism in Ti₃C₂T_x MXene/PEDOT:PSS heterostructures.

Interestingly, in additional MXene composites, such as those involving carbonaceous or chalcogenide materials, the gas-sensing mechanism is similarly governed by the band-gap and work-function interactions between MXenes and the accompanying materials. In particular to the number of captured electrons, when the molecules of the adsorbed gas species are less numerous than the available electrons on the sensor surface, the sensing behaviours are influenced by the quantity of adsorbed gas species.

The stability of MXenes is highly important for device fabrication as the prepared sensor undergoes different environmental conditions. However, the long-term oxidation stability is very low for Ti₃C₂T_x aqueous solution at room or high temperature and titanium carbide converts into its oxidised form (TiO₂), which affects its electronic properties. A separate study shows that MXenes can be stored for a long time at low temperature in addition to some solvents like isopropylalcohol (IPA), ethanol, etc. Lipatov and coworkers fabricated field-effect transistors (FETs) with single-layered Ti₃C₂T_x flakes as the conductive channels to study the electronic properties and environmental stability of Ti₃C₂T_x ^[32]. The results showed a field-effect electron mobility of 2.6 ± 0.7 cm²V¹s¹ and resistivity of 2.31 μΩm (4600 Scm⁻¹) for single-layered Ti₃C₂T_x flakes, which is one order of magnitude higher than that of the bulk Ti₃C₂T_x. The environmental stability data envisaged that Ti₃C₂T_x FET remains stable and highly conductive even after exposure to humid air for 70 h. Reports claimed that the interlayer spacing can be modulated in multi-layered MXenes using K⁺ and Mg²⁺ ions, thereby increasing the adsorption of the target gas/VOC.

Upon careful consideration of the multitude of empirical data available in many reports, scholars elucidate the intricate gas-sensing mechanism inherent within the MXene heterostructures. This mechanism manifests through electrical responses and the consequential alignment of band energies, as visually depicted in Figure 3. Notably, the nature of the electrical response, signifying alterations in resistivity or current flow, is intrinsically tied to the inherent properties of the paired materials constituting the tandem structure. Moreover, the band-energy alignment during contact is a pivotal aspect of this sensing mechanism. This alignment, crucially influenced by the work function, serves as a critical determinant of the ensuing gas–surface interactions. Specifically, it is noteworthy that band-depletion events correspondingly coincide with the presence of adsorbed oxygen species, a phenomenon of profound significance in the context of gas-sensing mechanisms within MXenes heterostructures.

Figure 3. Gas sensing mechanism in MXene heterostructures; electrical response to different gas and band-energy alignment.

4. Engineering Approach to Enhance 2D MXene Sensing Performances

4.1. Surface Functionalisation

In recent years, researchers have explored various approaches to functionalise pure MXenes and enhance their surface properties and reactivity. Among these approaches are ion intercalation using K⁺ and Mg²⁺ ions, decoration with noble metals, fluoroalkyl-silane treatment, and transition-metal oxyfluoride modification, to name a few ^[33]^[34]^[35]^[36]. By improving the VOC-sensing attributes of MXenes, functionalisation has opened up exciting possibilities for these materials in diverse applications. Notably, functionalisation using nanostructures and ion intercalation has led to significant improvements in sensor performance.

Noble metal decoration is another effective methodology for improving the sensor characteristics of MXenes. For instance, Li et al. ^[37] suggested Ag-modified MXenes as humidity sensors with a polymer adhesive layer. The novel approach increased the ionic conductivity and thereby introduced a flexible sensor with a high sensitivity of 106%, which records the fluctuations in humidity with the change in human voices and motion of hands, which shows its potential for application in robotics.

Surface functionalisation with various strategies has been found to increase the adsorption rates and sensing capability of MXenes for various analytes. The detailed analysis reflected that the sensor characteristics of pure MXenes are enhanced by surface functionalisation with strategies such as ion intercalation, doping with sulfur or noble metals, and modification of terminal groups. Overall, the diverse functionalisation techniques have the potential to expand the range of applications for MXene-based sensors.

4.2. Layering Structures

With regards to their functional attributes, MXenes are prospective materials for gas-sensing applications. However, the stacked sheets of MXene can lead to reduced specific surface areas, thereby diminishing the sensing performance, as evidenced by previous studies ^[25]^[38]. The influence of 2D layered materials is particularly noteworthy because of their diverse benefits, including a significant surface-to-volume ratio, exceptional flexibility, modifiable electronic structure, and remarkable mechanical stability ^[39]. The etching technique used for structuring MXene from the MAX phase typically leads to the development of layered structures. To achieve mono-layered MXene, an additional exfoliation approach is necessary, usually followed by mechanical shaking or sonication ^[40]. Delamination or intercalation allows for the exploration of exclusive characteristics of 2D materials.

The appropriate selection of the intercalant, along with the parameters used for the process, significantly impact the sensing and functional attributes of the material. The most commonly reported intercalants include dimethyl sulfoxide, isopropyl amine, urea, and hydrazine ^[41]^[42]^[43]^[44]. Mashtalir et al. investigated the effects of hydrazine intercalation on the characteristics of MXene and found a decrease in surface groups, resulting in surface modification. Additionally, the intercalation increased the number of active sites on the surface, leading to improved adsorption and sensor parameters of the material ^[44].

5. Two-Dimensional MXene-Based Heterostructures as High-Performance Gas-Sensing Materials

5.1. MXene/Metal-Oxide Heterostructures

Metal-oxide semiconductors (MOS) have been widely investigated for gas sensing due to their exceptional specific surface area, facile synthesis, and high response to toxic gases ^[45]^[46]^[47]^[48]. However, MOS gas detectors have drawbacks such as cross-selectivity, low selectivity, and high working temperature ^[31]^[49]^[50]^[51]^[52]. Nevertheless, recent research has shown that hybridising metal oxides with MXenes can overcome these limitations and improve the performance of volatile organic compound (VOC) sensors ^[53].

The use of MXene/metal-oxide composites has also been explored for NH₃ detection, with promising results. Tai et al. ^[52] designed a TiO₂/Ti₃C₂T_x film for NH₃ detection, which showed a significantly improved sensor response and response/recovery times compared to pure MXene due to modulation of a self-built electric field. Ranjbar et al. ^[54] proposed a non-invasive breath analyser for early detection of chronic kidney diseases, using a marigold-flower-shaped V₂O₅/CuWO₄ integrated with Ti₃C₂T_x sheets for NH₃ sensing. This sensor is portable, cost-effective, and has high sensitivity due to transduction in electrical resistance (Shottky junction) when exposed to air and NH₃.

In addition, the MXene/metal-oxide composites have successfully detected various VOCs that were previously difficult to detect, including triethylamine, toluene, acetone, nitrogen dioxide, methanol, hexanal, and formaldehyde. Liang et al. ^[55] proposed using SnO₂/Ti₃C₂T_x composites for detecting triethylamine gas at 140 °C. The MXene’s interconnected porous composite structure resulted in enhanced sensing attributes toward triethylamine. Hermawan et al. ^[31] discovered a sensor using CuO/Ti₃C₂T_x to detect toluene gas, which is the least detected gas using metal oxide to date. The work proposes a work-function matching strategy as a sensing mechanism and highlights electronic self-assembly’s impact on enhancing sensing parameters. The proposed work shows a speedy response time of 5 s and a good response of 11.4 to toluene.

Nanocomposites consisting of MXene and In₂O₃ were investigated by Liu et al. ^[56] to determine their ability to sense methanol. The formation of the Schottky junction contributed to the enhanced surface area, mesoporous nature, and improved band gap (ranging from 1.18 eV to 2.08 eV). Meanwhile, Kuang et al. ^[57] demonstrated that Ti₃C₂T_x/TiO₂, which they synthesised and analysed structurally and morphologically, was capable of sensing various volatile organic compounds (VOCs) with a response improvement ranging from 1.5 to 12.6 times that of purely MXene-based sensors. The altered carrier density and interfacial heterojunction attributed to the heightened sensitivity. The MXene skeleton’s presence in the sensor facilitated a high signal-to-noise ratio. This, combined with the facile fabrication technique and low power consumption, rendered the sensor ideal for health monitoring purposes.

MXenes with p-type semiconducting behaviour showed a positive gas response where the resistance of the device increases with the addition of a reducing gas. The thickness of the MXene film dramatically affects the sensing performance. With an increase of the thickness of the MXene sheets, the sensor response declines for both reducing and oxidising gases. Variations in carbon precursors (graphite, TiC, and lampblack) and Ti₃C₂T_x flake size directly influence the NH₃ sensing response at room temperature, and it was found that small-size flakes show a better response than the larger flakes because they produced a shorter gas-diffusion path. The Ti₃C₂T_x film as a metallic channel was used to provide good electrical conductivity, to yield low noise and a high signal induced by tremendous adsorption sites for ethanol sensing reported by Kim et al. ^[30]. High selectivity of single-layered Ti₃C₂T_x towards NH₃ with reasonable sensitivity at room temperature was correlated with first-principles calculations and it was found that the high negative adsorption energy, charge transfer, and smaller size of NH₃ among all other test gases caused the strongest interaction and highest deformation in the Ti₃C₂O₂ surface. The N-atom from NH₃ lies just above the Ti of Ti₃C₂O₂, which results in a strong bond between Ti and N, and this interaction contributes to the high adsorption capacity.

5.2. MXene/Polymer Heterostructures

Polymers belong to a class of materials that showcase various properties like conductivity, flexibility, sensitivity, and an array of functional groups for surface reactions, target gases, and low-temperature operating conditions. These characteristics make them ideal for chemical-sensor applications when combined with MXene ^[58]^[59]^[60].

While pure MXene-based NH₃ sensors are known to have excellent sensing attributes, they are limited by their long recovery times due to the adsorption energy of NH₃, which causes drift in the baseline resistance of the sensor. To address these limitations, researchers have developed MXene composites with polyaniline (PANI) ^[59], poly(styrene sulfonic acid) (PEDOT:PSS) ^[61], and other materials. For instance, Li et al. ^[62] created a flexible polymer (PANI)/MXene composite using an in situ method to develop an NH₃ sensor with enhanced sensing characteristics compared to pure Ti₃C₂T_x. The sensor showed 20–80% humidity dependence at a lower operating temperature, which makes it a viable option for agricultural applications. The sensor’s superior sensing attributes are due to Schottky-junction formation, and simulation studies have confirmed its practicality for use ^[62] (Figure 4).

MXene/polymer composites have been shown to have excellent sensing properties for various gases, VOCs, CO₂, and humidity, exhibiting fast response and recovery times, high sensitivity, and good repeatability. The use of MXene/polymer composites in sensor technology has opened up new possibilities for the development of high-performance sensors for various applications, including real-time electronic sensing and physiological monitoring. The flexibility and enhanced surface area provided by the composites make them suitable for the development of flexible sensors that can be used for wearable technology.

Figure 4. (a) A room-temperature ammonia (NH₃) sensor based on PEDOT:PSS/MXene composites ^[61]. (b) Dynamic transient for response of Ti₃C₂T_x and PANI/Ti₃C₂T_x-based flexible sensors exposed to 50–200 ppm ethanol gases. Response and recovery times of PANI/Ti₃C₂T_x-based flexible sensors to 150 ppm ethanol gases ^[58]. (c) A methanol gas-sensor performance based on a blend of PEDOT:PSS and Ti₃C₂T_x ^[60].

5.3. MXene/Carbonaceous Heterostructures

Carbonaceous materials refer to any materials containing carbon, including graphite, carbon nanotubes, graphene, activated carbon, and carbon fibres, among others. These materials are known for their high thermal and electrical conductivities, making them useful in a variety of applications, such as electrodes for batteries and fuel cells, conductive additives for polymers and composites, and sensors. Graphene, for example, is considered one of the most conductive materials known, with a conductivity that is orders of magnitude higher than copper. Carbon nanotubes and carbon fibres are also highly conductive and are used in applications where strength and conductivity are required, such as in the aerospace and automotive industries. Activated carbon, on the other hand, is highly porous and is commonly used for adsorption and purification processes due to its large surface area and ability to selectively adsorb certain molecules.

Graphene and its derivatives have been widely studied for their potential in detecting volatile organic compounds (VOCs) and humidity due to their high surface area, porosity, and thermal stability. Liu et al. ^[63] developed a VOC sensor at room temperature using a CuO/Ti₃C₂T_x/rGO composite. The porous networks, high surface areas, uniform CuO dispersion, and high electron conductivity of the composite led to acetone sensing at room temperature with excellent sensing properties. The sensor showed a response of 52% to 100 ppm acetone gas with a quick response/recovery time (6.5 s/7.5 s) and exceptional selectivity and reproducibility. This approach using 3D rGO/MXene structures with MOS offers a new way for the development of room-temperature VOC sensors.

5.4. MXene/Noble-Metal Heterostructures

Theoretical research has suggested that substitution of surface groups of MXenes with noble metals can potentially enhance the gas-sensing abilities of MXene/noble-metal compounds. The incorporation of noble metals brings about environmental resistance to corrosion and oxidation and contributes to the catalytic nature, thereby improving the sensor reactions. Additionally, noble metals act as electron traps, halting the rapid electron-hole recombination, which is beneficial to the sensor performance. Therefore, noble-metal functionalised MXenes represent a promising solution for improved sensing mechanisms, charge carriers, and VOC selectivity ^[64].

5.5. MXene/Metal Chalcogenide Heterostructures

Out of various gas-sensing materials, metal chalcogenides are commonly used for the detection of different VOCs, but there is a lack of research on MXene/metal-chalcogenide composites for gas-sensing applications. Only a handful of articles in the literature highlight on MXene/metal-chalcogenide composites’ potential for detecting gases such as ethanol and nitrogen dioxide. Chen et al. ^[65] proposed a flexible ethanol sensor employing Ti₃C₂T_x/WSe₂, which demonstrated good reproducibility and repeatability. The band diagram showed an n-type response and modulation of the Schottky barrier, with the sensitivity being 11 times greater than that of pure MXene.

6. Conclusions

In conclusion, MXene-based heterostructures hold great promise for gas-sensing applications due to their unique properties. However, there are still several technical challenges and limitations that must be addressed, including improving the stability of MXenes in different gas environments and developing highly sensitive and selective gas sensors using these materials. In addition to technical challenges, the economic feasibility and environmental impact of MXene-based gas sensors must also be considered. The large-scale synthesis of MXene-based heterostructures requires cost-effective and environmentally friendly synthesis routes. Additionally, the disposal of MXenes after use must be handled in an environmentally sustainable manner. Therefore, researchers must work towards developing scalable and sustainable synthesis routes and explore environmentally friendly methods for the disposal of MXenes. Despite these challenges, the potential applications of MXene-based heterostructures in gas sensing are vast, ranging from environmental monitoring to medical diagnostics. The continued exploration and optimisation of these materials for gas-sensing applications can lead to the development of highly sensitive, selective, and reliable gas sensors, ultimately contributing to the advancement of various industries and the betterment of society as a whole.

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