Unveiling the potential of Ti₃C₂T_x MXene for gas sensing: recent developments and future perspectives

Abstract

Ti₃C₂T_x MXene, a two-dimensional transition metal carbide, has garnered significant attention as a promising material for gas sensing applications due to its exceptional properties. In this comprehensive review article, we provide an in-depth overview of the recent advancements in the field of Ti₃C₂T_x-based gas sensors. We summarize the various synthesis methods employed for Ti₃C₂T_x MXene production, including selective etching of MAX phase precursor materials and controlled annealing processes to customize its surface chemistry and properties. We thoroughly review the gas sensing properties of Ti₃C₂T_x MXene towards a wide range of gases, including volatile organic compounds (VOCs), toxic gases, and other target gases. We provide detailed insights into the gas sensing performance of Ti₃C₂T_x-based gas sensors, including sensitivity, selectivity, response and recovery times, and stability, under various temperature and gas concentration conditions. The gas sensing mechanisms involving charge transfer and surface chemisorption processes between gas molecules and the Ti₃C₂T_x MXene surface are also discussed based on experimental and theoretical studies. Furthermore, we highlight the strategies employed to enhance the gas sensing performance of Ti₃C₂T_x-based gas sensors, such as surface functionalization, doping, and hybridization with other materials. We discuss the influence of various factors, such as annealing temperature, surface modification, and device configurations, on the gas sensing properties of Ti₃C₂T_x MXene-based gas sensors. Finally, we address the challenges and future perspectives of Ti₃C₂T_x-based gas sensors, including the need for standardized testing protocols, long-term stability, and scalability of fabrication processes. Our comprehensive review aims to inspire further research efforts and promote the development of Ti₃C₂T_x MXene-based gas sensors with enhanced performance and broader applications, highlighting the promising potential of Ti₃C₂T_x MXene as a cutting-edge material for gas sensing applications.

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Nitesh K. Chourasia

Dr Nitesh K. Chourasia is working as a National Postdoctoral Fellow at the School of Physical Sciences, Jawaharlal Nehru University (JNU), New Delhi, India. He has obtained his MSc degree from Banaras Hindu University (BHU), and his PhD degree from the School of Materials Science and Technology, Indian Institute of Technology, Banaras Hindu University (IIT-BHU), Varanasi, India. His research interest mainly focuses on new types of ion-conducting dielectrics for electronic and optoelectronic devices (e.g. transistors, photodetectors, sensors, etc.). His current research activities are to synthesize Ti3C2Tx MXene for toxic gas sensors.

Ankita Rawat

Ankita Rawat is a PhD student at the School of Physical Sciences, Jawaharlal Nehru University (JNU), New Delhi, India. She has completed her MSc from the Department of Physics, IIT Delhi. Her research interest is mainly focused on 2D materials-based electronic and optoelectronics devices (e.g. transistors and photodetectors). She is also working on Ti3C2Tx MXene-based gas sensors.

Ritesh Kumar Chourasia

Dr Ritesh Kumar Chourasia earned his Master's in Physics (Specialization-Electronics) and PhD from the Institute of Science, Banaras Hindu University (Varanasi, India). He has qualified for the highly competitive exam CSIR-JRF (junior research fellow) in Physical Sciences for getting a PhD fellowship and eligibility required for a permanent faculty position in India. He has also qualified for UGC-NET in Electronic Sciences, an international examination GATE-Physics, and JEST-Physics. He joined as an Assistant Professor at Lalit Narayan Mithila University, Darbhanga, in 2017. His current research interest is theoretical modeling including DFT studies of some novel materials, simulation, and device fabrication in the photonics and photovoltaics field. He has published 24 research papers in international journals of repute, 2 patents, and 8 book chapters.

Pawan Kumar Kulriya

Pawan Kumar Kulriya is working as an Associate Professor of Physics at the School of Physical Sciences at the Jawaharlal Nehru University (JNU) New Delhi. Before joining JNU, he was also associated as a Scientist-F in the Materials Science Group, Inter-University Accelerator Centre (IUAC) New Delhi, India for 17 years. He holds both a PhD in Physics and an MTech in Solid State Materials from the Indian Institute of Technology (IIT) Delhi and an MSc in Physics from the University of Rajasthan, Jaipur. He was awarded a prestigious Fulbright-Nehru Postdoctoral Fellowship by the U.S. Department of State at the Rensselaer Polytechnic Institute in New York, USA where he has carried out a research project on the materials for the immobilization of high level radioactive waste. Dr Kulriya has more than 140 peer-reviewed papers in leading international journals. His research on Experimental Condensed Matter Physics includes nano-materials for nuclear energy systems, materials for radiation shielding, development of advanced ceramic and high entropy alloys as well as their performance evaluation under the extreme environment of high temperature and radiation. Apart from this, he is currently working on 2D materials for gas sensing applications.

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1. Introduction

In our contemporary world, marked by swift industrial growth and urban development, the significance of sensors capable of precisely identifying a wide range of hazardous gases has grown considerably. Consequently, sensitive materials, serving as pivotal elements in gas sensor technology, have garnered significant attention in ongoing research endeavors. This heightened focus is driven by their crucial roles in enhancing industrial safety, safeguarding the environment, and preserving personal well-being.^1,2 At present, several materials have been investigated and established for gas sensors, such as metal oxides (MO), carbon-based materials, metal–organic frameworks (MOF), conductive polymers, rare-earth oxides, etc.^3–12 Metal oxide semiconductor (MOS) can be classified into two types, n-type (e.g., ZnO, SnO₂, TiO₂, etc.) and p-type (e.g., SnO, CuO, WO₃, etc.). MO is the preferred choice for researchers as a transistor, photodetector, and gas sensor owing to its high sensitivity, high electron mobility, good transparency, low off-state current, chemical stability, compact dimensions, facile fabrication, and affordability.^13–24 For instance, Saini et al. used nanowires of Zn(II) 2,3,9,10,16,17,23,24-octakis(octyloxy)-29H,31H-phthalocyanine (ZnPcOC₈) and nanoflowers of Cu(II) 2,3,9,10,16,17,23,24-octakis(octyloxy)-29H,31H-phthalocyanine (CuPcOC₈) for a Cl₂ sensor at room temperature.²⁵ The fabricated sensor has a detection limit as low as 5 ppb and with responses as high as 715% and 550% within 10 s and 15 s by using simple and cost-effective self-assembly techniques. Chaloeipote et al. employed 3D printing technology for the synthesis of CuO, and the resulting CuO sensor demonstrated a favorable response to the detection of NO₂, registering a response of 14.17% when detecting 200 ppm NO₂ at room temperature. Nonetheless, conventional gas sensors constructed with MO as gas-sensitive materials often attain higher response levels within the temperature range of 200 °C to 500 °C.²⁶ This requirement for elevated temperatures not only escalates the cost of gas detection but also introduces potential safety hazards due to the elevated operating temperatures. Wang et al. employed KIT-6 as the rigid template for the synthesis of WO₃ with incorporated Au particles. The resulting Au–WO₃ sensor exhibited remarkable sensitivity in detecting trimethylamine, with a rapid response time of just 1 second and a response rate of 42.56% when exposed to 100 ppm trimethylamine at 268 °C.²⁷ Sun et al. reported H₂ gas sensor using ZnO nanofiber film (average diameter of ∼60 nm). Within the temperature range of 210–330 °C, upon exposure to 20 ppm H₂ gas, the resistance of the ZnO nanofiber film increased, and the sensitivity ∼28.84% at 270 °C.²⁸ Kumar et al. reported room temperature chemiresistive sensors using hexadecafluorinated copper phthalocyanine (F16CuPc) and graphene oxide (rGO) hybrid materials. The fabricated sensor is highly sensitive to Cl₂ gas with a detection limit up to ppb level.²⁹ Generally, gas sensors utilizing MOSs typically demand elevated operating temperatures to attain their peak gas-sensing capabilities. This not only results in heightened energy consumption but also raises the potential danger of explosions when detecting flammable and explosive gases. Moreover, the requirement for high operating temperatures restricts their suitability for use in wearable gas sensors. Hence, there is a need to innovate and create new materials for gas sensing that can operate at ambient room temperature.

Recently, two-dimensional (2D) materials have attracted much attention to research society in different scientific applications. Since the discovery of Ti₃C₂T_x MXene in 2011, 2D transition metal carbides/nitrides (together referred to as MXenes) have involved significant research consideration.^30,31 Owing to their outstanding mechanical, biological and electrical properties such as efficient electromagnetic interference (EMI) shielding, large surface-to-volume ratio, extraordinary electrical conductivity, water dispersibility, high energy capacity, and excellent antibacterial activity, this newly discovered category of materials are important in a wide range of different areas of research.^32–37 In the case of sensors, the replacement of traditional sensor materials (e.g., metal alloys and carbon) has been crucial in light of the fact that unadventurous technology is progressively reaching its limits in terms of potential improvements to sensor performance. As a result, significant exertions have been made to investigate the use of MXenes in a variety of sensor technologies for biological, chemical, mechanical, optical, and gas sensors.^38–42 MXene's 2D structure, having diverse terminal functionalization groups, provides a large number of active surface sites that can help as quick-to-respond sensor platforms for numerous exterior boosts. Apart from this, the high electrical conductivity of MXene is advantageous to attaining low noise.⁴³ Because of these unique characteristics, MXenes appear to be a promising alternative sensor material that can provide high sensitivity, extraordinarily low-level detectivity (ppb level), high selectivity, and low power consumption as well as can operate at ambient conditions. Last but not least, the fact that MXenes are easily dispersed in water enables environmentally responsible manufacture as well as amendment treatments, which further increases their utility in the processing industry. M_n+1X_nT_X is the general formula of MXenes, where n demonstrates structures. The structure of MXene is known to be influenced by its MAX (M_n+1X_nT_X) phase precursor, which has previously constrained the MXene family to materials containing 2, 3, or 4 atomic layers of transition metals.^44,45 Recently introducing a new MXene variant, M₅X₄, which contains five atomic layers of transition metals.⁴⁶ M represents an early transition metal (e.g., Ti, Mo, and V), and X refers to carbon/nitrogen or their mixture. At the same time, T_X stands for numerous surface terminations, e.g., oxygen (=O), hydroxyl (–OH), and fluorine (–F).⁴⁷ MXene research has sparked interest due to its superior features and ease of processing; hence till now over 30 compositions of MXene have already been reported.^48,49 Furthermore, the remarkable electrical properties and elevated electron density at the Fermi level exhibited by Ti₃C₂T_x make it highly amenable to surface modification through the introduction of specific terminating species. As a consequence, these distinctive characteristics of Ti₃C₂T_x MXene make it particularly promising for sensitive interactions with toxic gas molecules, rendering it a viable candidate for gas-sensing applications. One effective approach involves the integration of Ti₃C₂T_x nanomaterials with other substances, such as transition metal dichalcogenides (TMDs), metal oxide semiconductors (MOS), and graphene, resulting in potential hybrid materials well-suited for gas sensing applications.⁵⁰ In brief, Ti₃C₂T_x MXene materials have emerged as a promising advancement in the realm of gas sensors, offering numerous advantages over conventional metal oxides. Notably, Ti₃C₂T_x MXenes display highest sensitivity to various gases, enabling the detection of even trace concentrations with greater accuracy. Moreover, Ti₃C₂T_x MXene-based sensors exhibit faster response times, crucial for rapid gas detection in safety-critical applications. Unlike many metal oxide sensors that require elevated operating temperatures, Ti₃C₂T_x MXenes can function effectively at room temperature, reducing energy consumption and eliminating the safety risks associated with high temperatures. Their superior selectivity, durability, and flexibility, along with the ability to engineer their properties, make Ti₃C₂T_x MXene materials a compelling choice for gas sensors, especially in applications demanding precision, reliability, and versatility.

Herein, we systematically review the synthesis process, physical properties, and sensing applications in different areas of Ti₃C₂T_x MXenes. Fig. 1 shows the structure of this review article. At first, we will start with the synthesis of MXenes using different approaches (e.g. top down and bottom up approach), further our focus will be the properties of MXenes that will be useful of sensing applications. At last, this review article emphasizes the different types of Ti₃C₂T_x MXenes (e.g. pristine, functionalized and composite) for gas sensing applications. Although we will discuss different types of possible MXenes but our goal is to cover the state-of-the-art research accomplishments of using Ti₃C₂T_x MXene and their application in gas sensors, including novel device architecture fundamental sensing mechanisms, and offer wise prospects for upcoming endeavours. This review article offers a fresh perspective on the utilization of Ti₃C₂T_x MXene in gas sensing, surpassing earlier review articles in terms of its comprehensive coverage of recent developments and its forward-looking insights. Unlike previous reviews, this article provides a more up-to-date and detailed examination of the latest advancements in MXene-based gas sensors. It highlights the cutting-edge techniques and applications that have emerged, shedding light on how Ti₃C₂T_x MXene materials can be tailored for enhanced gas-sensing performance. This review articles also summarize theoretical studies of MXene-based gas sensors using Density Functional Theory (DFT) and Molecular Dynamics (MD) simulations, which provide insights into the adsorption mechanisms and dynamic behavior of gas molecules on MXene surfaces, allowing for a comprehensive understanding of sensor performance, selectivity, and stability at the atomic level. By offering future perspectives, it goes beyond summarizing existing knowledge and sets a forward-looking agenda for researchers and engineers, helping to shape the future of Ti₃C₂T_x MXene-based gas sensing technology. This focus on recent developments and future directions positions the article as a valuable resource in the field of gas sensing, providing a more current and forward-thinking perspective compared to earlier review articles.

Fig. 1 Structure of the review article.

2. Technique for synthesis of MXenes

The techniques used to synthesize the MXene significantly affect its structure and surface functional groups, which in turn impacts the sensor's performance. The top-down etching from MAX phases is now the most widely utilized method for making MXene nanosheets. The layered structure of the MAX phase is made up of densely packed M layers and X atoms that occupy the octahedral sites and are bonded together with A atoms. In general, the M–A bond is typically thought of as a metallic bond, while the M–X bond can exhibit ionic, metallic, or covalent properties.⁵¹ There are mainly two steps involved in the synthesis of MXene, i.e., (1) selective etching of A layer from MAX phase and (2) delamination of MXenes from MAX phase;⁵² both steps will discuss one by one.

2.1 Top-down approach

2.1.1 Etching. In the very first synthesis of MXene, it was suggested that an aqueous solution of hydrofluoric acid (HF) could etch the MAX phase of Ti₃AlC₂, and Ti₃C₂T_x-based MXene can be formed after selective removal of the Al layer. This synthesis method is known as top-down synthesis and is also referred to as the main synthesis route nowadays. The etching of Ti₃AlC₂ involves three reaction steps, as mentioned below

2Ti₃AlC₂ + 6HF →2AlF₃ + 3H₂+ 2Ti₃C₂ (1)

Ti₃C₂ + 2H₂O → Ti₃C₂ (OH)₂ + H₂ (2)

Ti₃C₂ + 2HF → Ti₃C₂F₂ + H₂ (3)

As one can see from eqn (1), during etching, AlF₃ will be a byproduct that is water-insoluble; it's vital to pick the right temperature and time to avoid precipitation of AlF₃. For chemical etching, one needs to consider different reaction parameters like HF concentration, reaction time, optimum temperature, etc. According to the literature, M_n+1AX_n, with a large n number, necessitates a long etching time, a low pH value, and high etching temperature.⁵³ Therefore, a high concentration of HF is always beneficial for preferential etching of the Al layer, which can be confirmed from the structural investigation. XRD pattern exhibits a clear downshift of the (002) peak and displays the successful Al etching in Ti₃AlC₂.⁵⁴ Furthermore, the experimental results showed that the HF concentration can be used to rectify vacancy defects caused by the removal of surface Ti atoms.

As we know that handling HF is quite difficult, and apart from that, HF is highly corrosive in nature and not environment friendly. Furthermore, HF has a proclivity for over-corroding, resulting in defects in MXene. In 2014, NH₄HF₂ was used as an etchant to avoid using toxic HF directly.⁵⁵ This is a significantly safer and milder alternative. In the same year, researchers also utilized LiF and HCl mixture for the etching of the Al layer in Ti₃AlC₂ to form Ti₃C₂T_x.⁵⁶ The reaction time was 45 hours at a constant temperature of 40 °C, and the results showed that the presence of protons and fluorine ions is essential for effective etching. Later on, researchers also put more effort into refining this method by varying the feeding ratio of LiF and MAX (LiF : MAX = 5 : 1 or 7.5 : 1).⁵⁷ Excess LiF was found to promote Al etching and Li⁺ intercalation, resulting in Ti₃C₂T_x MXene nanosheets with identical thickness, larger size, and fewer defects. In most cases, the etching process transforms solid dense MAX into structures resembling accordions with loosely collected layers, and these formations are also referred to as multilayer MXenes. As a result, an exfoliation step is compulsory for multilayer MXenes into monolayers.

2.1.2 Exfoliation. Etching and delamination are the two primary processes that make up the majority of the MXene exfoliation process from the MAX phase. In the first step, a chemical etching process is used to remove the “A” layer from the middle of the MAX layers. Each MXene layer must be delaminated from the etched MAX phase in the second step. This procedure for the synthesis of MXene was carried out in a number of different ways, as shown in Fig. 2.⁵⁸ Each technique either incorporates HF into the MAX solution directly or introduces it indirectly through the use of LiF. The chemical connection that holds the M and X layers together can be broken apart if they are subjected to sonication, intercalants like dimethyl sulfoxide (DMSO), or ionic solutions. In any of these three scenarios, the layers are more likely to delaminate. Apart from this, LiF and HF are also favorable for non-Ti-based MXene (e.g., niobium (Nb), vanadium (V), etc.)^59,60

Fig. 2 Schematic diagram of the synthesis route of Ti₃C₂T_x MXene. Reproduced with permission from ref. 58 Copyright 2020. MDPI (open Access).

The nanosheets of MXene were able to be exfoliated into a single layer by first inserting large organic molecules into the interlayers of the accordion-like structure and then subjecting the structure to ultrasonication or mechanical vibration.⁵² This allowed the nanosheets to be rearranged into a single layer. The frequently used intercalants comprise NH₄⁺, dimethyl sulfoxide, hydrazine, urea, and tetrabutylammonium hydroxide (TBAOH).^61–63 Recently, Chia et al. reported that by using TBAOH as the intercalant, one could exfoliate Ti₃C₂T_x MXene from HF etching products.⁶⁴ After removing the A layer, expanding MAX powder that contained micron-sized flakes led to the formation of Ti₃C₂–HF, which was then delaminated into MXenes with one or more than one layers by reducing the strength of the interlayer contact with TBAOH.

2.2 Bottom-up approach

In addition to the top-down tactics that were discussed above, a multifunctional bottom-up approach known as chemical vapor deposition (CVD) was developed for the purpose of synthesizing high-quality and large-area MXenes. In contrast to the top-down method, which has a complex combination of groups and low controllability, the bottom-up approach has the advantage of accurately manipulating the morphology and termination groups of MXenes. The synthesis process is crucial for understanding the inherent characteristics of MXenes. Researchers reported ultrathin Mo₂C, WC, and TaC MXene crystals on Cu/Mo foil using CVD, and all of them have fewer defects with larger lateral dimensions.⁶⁵ The bottom-up synthesis technique of MXenes is still a big problem due to the costly equipment and tedious procedures.

In addition, if the MXene was subjected to mechanical vibration or sonication, it would contract and develop further defects. The inherent properties of 2D TMCs and TMNs have been hindered due to the chemical modification and mechanical damage that has been done to MXenes. The constituent parts of the mother 3D MAX phase have a direct influence on the structure and species of MXenes and vice versa. All of the MXenes that have been synthesized up to this point contain structures of the M₂X, M₃X₂, and M₄X₃ types. New synthesis methods are urgently required to improve the quality of 2D TMCs and TMNs as well as to expand the family of these materials because these MX-structured materials make up a large class of TMCs and TMNs and have a variety of fascinating properties.

The CVD production of high-quality ultrathin TMCs crystals utilizing a bilayer metal foil as a growth substrate was initially reported in 2015 by Xu et al., who was also the first to reveal the technique.⁶⁵ The stack of Cu/Mo foils was annealed in hydrogen (H₂) to a temperature that was higher than 1085 °C, which is the melting point of copper. Following this step, methane was injected, which led to the formation of Mo₂C crystals on the surface of the liquid copper. During the course of development, the top layer of liquid Cu plays several crucial tasks that are essential to the process. Meanwhile, it participates in the process of converting methane into carbon atoms by acting as a catalyst, and it serves as a pathway for controlling the diffusion of molecular molybdenum from the molybdenum foil to the surface of the liquid copper. On the surface of the liquid Cu, ultrathin Mo₂C crystals form due to a process involving carbon and molybdenum atoms. In addition, due to the significant differences in the chemical reactivity of copper and molybdenum carbide (Mo₂C), copper acts as a sacrificial layer after Mo₂C is developed. This allows the transfer of Mo₂C to desired target substrate through etching. A bottom-up method, such as CVD, can produce high-quality films on various substrates; however, this method is not typically utilized in the production of MXenes because the films produced using this method are not mono-layers but relatively thinner films. The CVD was the method Xu and colleagues used to fabricate thin films of Mo₂C, WC, and TaC, for example.⁶⁶ In spite of this, even the most imperceptibly thin Mo₂C films consisted of at least six layers of Mo₂C, as opposed to a single layer of MXenes. To synthesize ultrathin TaN, Wang et al. used NH₃ as a nitrogen source and Cu/Ta foil as a growth substrate at 1077 °C. Copper foil maintains a solid state throughout CVD development. According to the images obtained from the TEM, both TaN crystals are single crystals. In addition, ultrathin TaB crystals were produced by employing boron powder as the source of element Wang et al. synthesised zirconium based MXenes (e.g. Zr₂CCl₂ and Zr₂CBr₂) by utilizing direct CVD techniques.⁶⁷ These two zirconium MXenes seemed in the same common morphology as that of the Ti–MXenes. In summary, the choice between bottom-up and top-down synthesis methods for MXene can influence its gas sensing properties. Bottom-up synthesis offers precise control over composition and structure, while top-down synthesis provides scalability but may yield less controlled surfaces. Both approaches have their advantages and considerations depending on the specific gas sensing requirements and desired performance characteristics.

3. Properties of MXene

High electrical conductivity, unique layered structure, wide specific surface area, and excellent dispersity distinguish MXenes from other functional materials, paving the way for further research and development in the sensor industry.⁶⁸ In addition to its superior conductivity, the most well-liked Ti₃C₂T_x MXene also has a large surface area and an abundance of surface functional groups.⁶⁹

3.1 Stability

The key parameter which decides the stability of MXene is; reaction conditions, solutions, and composition. Since, Ti₃C₂T_x MXene is easily oxidized in the water and air, its environmental stability is an important study topic, as is its stability and service life during the sensor fabrication and utilization. Because the oxidation of MXene often begins at the edge of the molecule and then spreads throughout the molecule, it is impossible for the entire molecule to be oxidized at the same time. Oxidation proceeds most slowly in the solid state and most quickly in the liquid state. The reaction of oxidation can be understood from the below reaction

Ti₃C₂O₂ + 4H₂O → 3TiO₂ + 2C + 4H₂

MXene's fragile oxidation resistance is one of the challenges it faces in the process of usage, and it's worth looking into ways to alleviate the performance loss affected by oxidation. Ti₃C₂T_x MXene can be effortlessly transformed into TiO₂ through annealing at either 1150 °C for thirty seconds or 400 °C for a longer time.^70,71 Some of MXene's benefits, like conductivity and capacitance, are often sacrificed during oxidation. As reported earlier, the ionic group of polyphenylene has the ability to effectively occupy the edge of the MXene sheet, which protects it from being oxidized by the water molecules.⁷²

3.2 Electrical properties

The electrical properties of a single layer of Ti₃C₂T_x MXene are worth researching in order to better understand its electrical performance and potential application in sensors. The band gap in MXene arises due to surface functionalization; otherwise, all MXenes exhibit metallic behaviour. The metallic Ti₃C₂T_x material really does have the capacity to serve as an electrode for super capacitors. The performance of electrochemical capacitor devices is significantly influenced by the electronic conductivity of MXene. After being functionalized, some MXenes transform into semiconductors with energy gaps ranging from 0.25 to 2.0 eV.⁷³

3.3 Mechanical properties

Because of their exceptional in-plane tensile characteristics, bending tolerance, and flexibility, MXenes are perfect for use in the fabrication of bendable electrodes and other electrical devices.⁵⁶ The surface terminations of MXenes have a significant impact on the mechanical characteristics. MXenes with an O termination are expected to have extremely high stiffness; however those with F and OH terminations exhibit lower elastic stiffness than O-terminated MXenes. That is why bonding strength between Ti–O being stronger than in the situations of Ti–OH and Ti–F. The flexibility of the surface-functionalized MXenes is greater than that of bare MXenes. The Ti₃C₂T_x materials also have a remarkable degree of flexibility and tensile resistance. The 5 μm thick Ti₃C₂T_x sheet can tolerate 1.3 MPa or roughly 4000 times of own weight, with no any observable distortion or fracture.⁷⁴

4. Important characteristics of gas sensors

The main characteristic of a gas sensor is determined by a few critical parameters such as sensitivity, selectivity, stability, resolution, and response/retrace time shown in Fig. 3.

Fig. 3 Important characteristics of a typical metal oxide-based gas sensor.

• Sensitivity or response: it is defined as a relative change in sensor signal per unit concentration of the given analyte. Sensitivity (S) is often expressed as R_a/R_g for reducing gases or R_g/R_a for oxidizing gases, where R_a denotes the resistance of the gas sensors in the reference gas (typically air) and R_g denotes the resistance in the reference gas containing the target gases.

• Selectivity: the ability to selectively identify the particular analyte in the presence of other analytes.

• Stability: the time period over which the sensor retains its original characteristics.

• Detection limit: the lowermost concentration difference can be eminent by the sensors.

• Response time and recovery time: the response and recovery time was defined as the time required for 90% of the total resistance change to occur while introducing and removing the analyte.

4.1 Working principle and mechanism of chemiresistive gas sensor

A chemiresistive gas sensor is a remarkable device designed to detect the presence of specific gases by measuring changes in the electrical resistance or conductivity of a sensing material when it interacts with the target gas. The best example of chemiresistive gas sensor are metal oxide (MO) gas sensor. The heart of any chemiresistive gas sensor is the sensing material. The choice of sensing material is pivotal to the sensor's performance because different materials exhibit varying levels of sensitivity to different gases. Common sensing materials include metal oxides, conducting polymers, and organic materials, each with its unique gas-sensing mechanisms. The gas sensing mechanism of a chemiresistive gas sensor primarily involves two fundamental processes: adsorption and chemisorption.

4.1.1 Absorption mechanism. Adsorption is the process by which gas molecules adhere to the surface of the sensing material without forming strong chemical bonds. It occurs due to weak van der Waals forces, electrostatic interactions, or physisorption. When a gas molecule interacts with the surface of the sensing material, it may physically adsorb onto the surface or get trapped in surface defects or pores. This adsorption process can lead to a change in the electronic structure of the sensing material. In the case of semiconducting materials like MOs or carbon nanotubes, gas adsorption can alter the concentration of charge carriers (electrons or holes) in the material's crystal lattice.

4.1.2 Chemisorption mechanism. Chemisorption is a more specific and stronger form of gas–surface interaction than adsorption. It involves the formation of chemical bonds between gas molecules and surface atoms of the sensor material. In chemisorption, the gas molecules undergo a chemical reaction with the surface atoms, resulting in the creation of new chemical entities on the sensor's surface. The formation of these chemical bonds can lead to significant changes in the electronic structure of the sensing material, including changes in the number of charge carriers and their energy levels. The strength of chemisorption depends on the nature of the gas and the surface chemistry of the sensing material. Some gases chemisorb readily, while others may only adsorb weakly or not at all.

4.2 Factors affecting gas sensing performance

Several factors can influence the gas sensing mechanism and the performance of sensor:

1. Sensing material: the choice of sensing material is crucial and depends on the target gas or gases. Different materials have varying degrees of sensitivity to specific gases.

2. Target gas: the type of gas to be detected plays a significant role. Different gases have distinct chemical properties, and their interactions with the sensing material vary accordingly.

3. Temperature: temperature can influence the gas-sensing mechanism by altering the kinetics of adsorption and desorption processes. Many chemiresistive sensors operate at elevated temperatures to enhance sensitivity.

4. Humidity: humidity levels can affect the adsorption and chemisorption processes. Some sensors are sensitive to changes in humidity, which can lead to false readings.

5. Interfering gases: the presence of other gases in the environment can interfere with the sensor's response to the target gas. Selectivity is a critical consideration in sensor design.

In conclusion, chemiresistive gas sensors operate on the fundamental principles of adsorption and chemisorption. These mechanisms lead to changes in the electrical conductivity or resistance of a sensing material when it interacts with a specific target gas. The choice of sensing material, target gas, environmental conditions, and the presence of interfering gases all influence the sensor's performance. Understanding the gas sensing mechanism is essential for designing and optimizing chemiresistive gas sensors for various applications.

4.3 Sensing mechanism of Ti₃C₂T_x MXene based gas sensor

The gas-sensing mechanisms in typical MO gas sensors are closely tied to the exchange of electrons, which occurs as a result of electron donation or acceptance from the target gas. Nevertheless, the gas-sensing mechanisms of MXenes exhibit greater complexity compared to the conventional charge-transfer model. In the previous report on Ti₃C₂T_x MXene based gas sensor, p-type sensing behavior was observed for ethanol, methanol, acetone, and ammonia gas at room temperature.⁷⁵ However, in subsequent investigations, it was observed that Ti₃C₂T_x MXene consistently exhibited an increase in resistance when exposed to either oxidizing or reducing gases.^74,76 Based on the above observations, researchers proposed two possible mechanism. (1) MXene exhibits metallic characteristics rather than semiconductor behavior, and the adsorption of gas can diminish the number of carriers while raising the channel resistance of MXene,⁷⁷ and (2) the expansion of MXene interlayers upon exposure to gases obstructs its electron transport in the out-of-plane direction.⁷⁸ Both of these mechanisms can clarify why MXenes consistently display a positive response to all gases. Though, according to Yang et al. report, the response signal of Ti₃C₂T_x MXene had a shift from positive to negative followed by NaOH treatment.⁷⁹ The change in response direction is ascribed to the rise in the O/F ratio, suggesting that surface terminations also play a noteworthy role in gas-sensing behavior. However, the fundamental mechanism behind this transformation remains unclear.

In sensors, employing hybrid structures combining MXenes and other materials, the sensor's response direction frequently hinges on the MXene's concentration within the composite. When the composite features a notably high MXenes content in conjunction with MOs, the gas response of the composite sensor aligns with that of a pure MXenes sensor, and the composite sensor exhibits significantly lower resistance compared to a pure MO sensor.⁸⁰ In this scenario, the presence of MOs enhances the specific surface area and facilitates the adsorption of gases.⁸¹ Conversely, in situations where the MXenes content is relatively low, the gas response of the composite sensor aligns with that of the MO sensor, and the composite sensor exhibits considerably higher resistance compared to a pure MXene sensor.⁸² In this instance, the MO take on a central role in facilitating carrier conduction during the gas-sensing process.

5. Why MXene for sensing applications?

The integration of sophisticated nanomaterials has ushered in a new era in the realm of sensors. When it comes to the process of signal transduction, selecting the appropriate transducer (material) is of the utmost importance. Traditional transducers suffer from the necessity of a high over-potential in the case of electrochemical sensors because of their extremely small surface area. Additionally, the target analyte's slow charge kinetics and the mass transfer constraint make it difficult for the transducer to transfer enough mass. The materials for strain sensors, instead, must preserve conductivity and mechanical strength. As a result, any viable sensor device's transducer material should have significant properties such as high surface area and high conductivity, superior mechanical strength, and a simple synthesis process. To accomplish this, a wide range of materials, including metals, polymers, metal oxides, and carbonaceous materials, as well as their combinations, have been investigated. The race to identify the ideal combination with a high signal throughput is still ongoing despite this. In 2011, Drexel University was credited with discovering MXenes, a novel type of 2D material.³⁰ Although their primary application is in the field of energy materials, such as batteries and supercapacitors, the community of sensor researchers is showing an unusual interest in these 2D materials.⁸³ The diversity of their surface and chemical properties is directly tied to the growing application of sensor technology, which is directly related to their increasing reach. MXenes have relatively high hydrophilicity and are simple to functionalize on the surface. This is in contrast to other 2D materials like graphene, which, despite having a high conductivity, is challenging to integrate with electrode systems due to its low hydrophilicity and the fact that its sheets can be folded or re-stacked.⁸⁴ In addition, because of their exceptionally high conductivity, MXenes are an excellent substrate for the research and development of electrochemical sensors.³³ Because of their superior mechanical qualities and conductivity, MXenes provide a great conductive filler that may be used in strain and pressure sensors. It is an attractive transducing material for various sensors due to MXenes' excellent surface characteristics, metal-like conductivity, and flexibility with just the right amount of stiffness. These characteristics make MXenes suitable for electrochemical, strain bio-electrochemical, photochemical, and pressure sensors.⁸⁵

6. Application of Ti₃C₂T_x MXene as a gas sensor

In order to make an accurate comparison of Ti₃C₂T_x MXene's application in gas sensors, the nanomaterial can be divided into three distinct classes: pure Ti₃C₂T_x, functionalized Ti₃C₂T_x, and composite Ti₃C₂T_x. This review article investigates the functionalization of Ti₃C₂T_x in terms of the alteration of functional groups and the intercalation of metal ions. Additionally, combining Ti₃C₂T_x with other materials is a successful tactic to enhance its gas-sensing performance. There are many various subgroups of materials that can be composited with Ti₃C₂T_x, including transition-metal dichalcogenides (TMDs), graphene derivatives, metal oxides, polymers, etc. Ti₃C₂T_x can be used to make a variety of different composites. The application in gas sensors of all three different categories of Ti₃C₂T_x will be discussed one by one.

6.1 Pure Ti₃C₂T_x for gas sensor

After the discovery of MXene in 2011, Ti₃C₂T_x was initially used as a gas sensor, as reported by Lee et al.⁷⁵ They reported the Ti₃C₂T_x-based volatile organic compounds (VOC) gas sensor at room temperature by integrating the Ti₃C₂T_x nanosheets with solution-processed on flexible polyimide film. At ambient temperature, the Ti₃C₂T_x sensors demonstrated p-type sensing behaviour while successfully measuring methanol, ethanol, acetone, and ammonia gas. The fabricated sensor exhibited the highest and lowest response when exposed to ammonia gas due to large adsorption energy, as shown in Fig. 4(b). Theoretically, the estimated value of the limit of detection (LOD) for acetone gas is 9.27 ppm. To detect VOC gases at parts per billion (10⁻⁹) level, Kim et al. proposed a Ti₃C₂T_x gas sensor with an extremely high signal-to-noise ratio (SNR).⁷⁴ They deposited a thin film of fully functionalized and high metallic conductivity of multi-layered Ti₃C₂T_x on a pre-printed Au electrode SiO₂ substrate. The Ti₃C₂T_x gas sensor's response and SNR were compared to those of typical 2D materials (including MoS₂, black phosphorus (BP), and reduced graphene oxide (r-GO)). It was revealed that the Ti₃C₂T_x sensor's SNR was two orders of magnitude higher than that of other 2D materials because of their incredibly low noise level. Ti₃C₂T_x MXene gas sensors showed a very low limit of detection (LOD) for VOC gases of 50–100 ppb at room temperature. A single-layer Ti₃C₂-based NH₃ sensor was proposed by Wu et al. at ambient temperature with high selectivity [Fig. 4(c) and (d)].⁸⁶ Following intercalation, ultrasonic dispersion, and delamination, Ti₃C₂ was coated on the ceramic tube's surface to make the gas sensor ready to detect a variety of gases, including NO, CH₄, NH₃, H₂S, H₂O, acetone ethanol, and methanol with 500 ppm concentration at room temperature. According to their research, the single-layer Ti₃C₂ sensor had the highest response, which was adequately explained by first-principles calculations. Several investigations on NH₃ showed that the manufactured Ti₃C₂-based sensors have good repeatability to ammonia at room temperature, with a detection limit of 10 ppm. Additionally, utilising practical first principles, Khakbaz et al. thoroughly investigated the possibility of pure Ti₃C₂T_x for gas sensing.⁸⁷ On Ti₃C₂T_x, gas molecules such as NH₃, CO, N₂O, NO, NO₂, CH₄, CO₂, and H₂S were adsorbed, and the effects of various ratios of numerous functional groups were examined. The results showed that Ti₃C₂T_x is especially sensitive to NH₃ relative to the other gas molecules studied, with a charge transfer of 0.098 e and adsorption energy of 0.36 eV, respectively. Using first-principles analysis, Zeng et al. also reported that Ti₃C₂T_x MXene is foreseen to be an encouraging sensor for SF₆ decomposition characteristic components.⁸⁸ The adsorption strength of analytes is unevenly exhibited by different terminated Ti₃C₂T_x MXene, and hydroxyl groups greatly enhance the adsorption strength on the Ti₃C₂T_x MXene surface. Based on their finding they also suggested that following the adsorption of the analytes, a clear charge transfer is what primarily causes variations in Ti₃C₂T_x conductivity.

Fig. 4 (a) Schematic illustration of pure Ti₃C₂T_x VOCs, (b) results of ethanol, methanol, acetone, and ammonia gas bubbling at 100 ppm using Ti₃C₂T_x based sensor at 300 K. Reproduced with permission from ref. 75 Copyright 2017. American Chemical Society, (c) pictorial representation of absorption of NH₃ on the surface of Ti₃C₂T_x MXene and, (d) room temperature response of Ti₃C₂ MXene gas sensors toward 500 ppm CH₄, H₂S, H₂O, ethanol, methanol, acetone, NH₃, and NO. Reproduced with permission from ref. 86 Copyright 2019. American Chemical Society.

Numerous variables, including film thickness, MAX phase precursors, flake size, and degree of oxidation, have an impact on the gas sensing capabilities of pure Ti₃C₂T_x.^89,90 For gas (acetone, ethanol, and NH₃) detection, Kim et al. also described an interfacial assembly of ultrathin Ti₃C₂T_x films.⁹⁰ In particular, the addition of ethyl acetate to the Ti₃C₂T_x solution promotes the production of a self-assembled layer, which had an excellent plane adhesion and stacking order. It was discovered that the self-assembly technique could improve the connectivity of small flakes, leading to minimal electrical noise in the gas sensor [Fig. 5(a) and (b)]. The gas sensor made of ultrathin Ti₃C₂T_x responded to NH₃ the best, as shown in Fig. 5(c). According to research by Shuck et al., the MAX phase carbon sources had an impact on the Ti₃C₂T_x's structure and characteristics.⁵⁴ The three different types of Ti₃C₂T_x that were produced through etching varied in terms of chemical composition, lateral flake size, electrical conductivity, and stability. They reported that Ti₃C₂T_x made from graphite had the highest conductivity (4400 S cm⁻¹) and was the most stable (time constant, 10.1 days), whereas MXene made from TiC had a similar conductivity (3480 S cm⁻¹) but had the lowermost colloidal stability (4.8 days) as well as the lowest chemical stability (5.1 days). The most responsive to gases and with good selectivity to NH₃ was the TiC-produced Ti₃C₂T_x, as shown in Fig. 5(d). In spite of this, Ti₃C₂T_x has not made significant progress in the field of gas sensors due to the fact that it is easily oxidized and has a tendency to be unstable. Chae et al. looked into the key variables (including water molecules, temperature, and solvents) controlling the oxidation of Ti₃C₂T_x in order to get around this constraint. They proposed that low temperature and ethanol solvent can help in maintaining the intrinsic properties of prepared Ti₃C₂T_x.⁹¹ For the purpose of detecting VOCs at room temperature, Liu et al. recently proposed termination-modified Ti₃C₂T_x MXene (Ti₃C₂T_x–M) through alkali pre-treatment and trimethylacetic anhydride functionalization [photograph is shown in Fig. 5(e)].⁹² In comparison to its unprepared counterpart, the Ti₃C₂T_x sensor showed a fivefold increase in response to ethanol with a 29% reduction in response to water vapor followed by the spray coating methods, as shown in Fig. 5(f).

Fig. 5 (a) Schematic of the interfacial self-assembly technique for preparation of ultrathin MXene films. Gas sensing response of, (b) large Ti₃C₂ flakes, (c) small Ti₃C₂ flakes. Reproduced with permission from ref. 90 Copyright 2019. American Chemical Society, (d) Ti₃C₂T_x's ability to detect gases after being made from various MAX precursors which showed different sensitivities with ethanol, acetone, and ammonia. Reproduced with permission from ref. 54 Copyright 2019. American Chemical Society, (e) photographs of pure Ti₃C₂T_x, Ti₃C₂T_x-M1, and Ti₃C₂T_x-M2 colloid solution and, (f) gas sensing performance of VOCs at 40 ppm concentration. Reproduced with permission from ref. 92 Copyright 2021. Wiley.

In summary, one notable advantage of pristine Ti₃C₂T_x MXene-based gas sensors is their exceptional sensitivity and rapid response to a wide range of gases. These sensors exhibit high surface area, enabling effective gas adsorption, and their 2D layered structure provides numerous active sites for gas molecules to interact with, resulting in enhanced detection capabilities. Additionally, MXene-based sensors often possess excellent electrical conductivity, facilitating quick and precise signal readouts. These properties make them highly versatile for gas sensing applications, ranging from environmental monitoring to industrial safety, where the ability to detect and respond to diverse gas species swiftly and accurately is of paramount importance. However, one significant drawback of pristine Ti₃C₂T_x MXene-based gas sensors is their limited selectivity when detecting multiple gases simultaneously. The inherent reactivity of MXene materials can lead to cross-sensitivity issues, making it challenging to distinguish between different gas species accurately. The MXene's hydrophilic nature can lead to water absorption, which interferes with gas sensing performance and reduces sensitivity. To overcome this limitation, researchers have been exploring strategies such as surface functionalization and composite material design, which we will discuss one by one in next section. Surface modification with specific functional groups or the incorporation of other nanomaterials can enhance the selectivity of MXene-based sensors by tailoring their chemical affinity to target gases. These advancements in MXene-based gas sensors hold promise for enhancing their practical utility in real-world applications.

6.2 Functionalized Ti₃C₂T_x for gas sensor

Even though Ti₃C₂T_x has metallic properties when it is pure, its electronic properties can be changed by functionalization and become semiconducting. The functionalization can be done through metal ion intercalation,⁹³ sulphur doping,⁹⁴ oxidation,⁹⁵ noble metal modification,⁹⁶ and functional group modification.^97,98 Researchers have started to pay more attention to functionalization because it can improve the adsorption effect of Ti₃C₂T_x, which is favorable for gas sensing applications. Metal ion intercalation was studied by Koh et al. as part of their investigation into how gas-induced interlayer swelling of Ti₃C₂T_x influences the performance of gas sensing.⁹³ When NaOH was utilized as the ionic intercalating agent, the in situ X-ray diffraction (XRD) experiments demonstrated a significant relationship between the gas response and the interlayer transport caused by the interlayer ions as shown in Fig. 6(a). The highest level of sensitivity was reported in Ti₃C₂T_x after treatment with 0.3 mmol L⁻¹ NaOH, and the highest response was achieved for ethanol, as shown in Fig. 6(b). Similar to this, Yang et al. enhanced the gas sensing performances of the organ-like Ti₃C₂T_x by –OH functional alteration and alkaline metal ion (Na) intercalation.⁷⁹ The sensing components were uniformly coated on the Al₂O₃ ceramic substrate by drop-coating process to create the pristine Ti₃C₂T_x and alkalized Ti₃C₂T_x gas sensors to study the alkalized Ti₃C₂T_x's gas sensing performance and the schematic is shown in Fig. 6(c). It was discovered that the alkalized Ti₃C₂T_x exhibited the opposite tendency to the pristine Ti₃C₂T_x, whose resistance rose with the concentration of NH₃. The highest response with alkalized Ti₃C₂T_x was achieved in NH₃ gas (around 28.7% more than pristine Ti₃C₂T_x) and the selectivity also enhanced with alkalized Ti₃C₂T_x (ethanol, acetaldehyde, formaldehyde, methanol, acetone, methane, and NO₂) as shown in Fig. 6(d).

Fig. 6 (a) Gas sensor schematic based on Ti₃C₂T_x, where mM stands for mmol L⁻¹, (b) gas response of Ti₃C₂T_x sensors with different concentrations of NaOH, inset: magnified gas response of CO₂. Reproduced with permission from ref. 93 Copyright 2019. American Chemical Society, (c) schematic of an organ-like Ti₃C₂T_x-based gas sensor with alkaline treatment, (d) selectivity test of alkalized Ti₃C₂T_x and Ti₃C₂T_x with a 100 ppm concentration to various test gases. Reproduced with permission from ref. 79 Copyright 2019. American Chemical Society.

The inherently metallic Ti₃C₂T_x was functionalized with oxygen by Pazniak et al. to increase gas sensitivity. Fig. 7(a)–(g)⁹⁵ shows that Ti₃C₂T had been partially oxidized kept its conductivity at the optimum annealing temperature of 350 °C, and the TiO₂ formed during oxidation improved its detection capacities to gases at concentrations as low as 10⁻⁶. The partially oxidized multisensor array-based Ti₃C₂T_x demonstrated strong response values and selectivity for acetone. Recently, Yao et al. also demonstrated a partially oxidized Ti₃C₂T_x MXene-based sensor which showed a high response and excellent selectivity for NH₃.⁹⁹ The heat treatment of the detecting ceramic tube, which eliminates the adsorbed water and partially oxidizes the material at a temperature of 280 °C, is an essential aspect of this work. The Ti₃C₂T_x MXene-280 has a 147% response to 500 ppm NH₃ in ambient conditions, whereas the equivalent response/recovery time are 67/157 s, respectively.⁹⁹

Fig. 7 A partially oxidised Ti₃C₂T_x MXene film's gas sensing capabilities on a multisensor chip: (a) image of a water-based, stable delaminated MXene solution, (b) monolayer Ti₃C₂T_x MXene flakes' schematic structure, (c) Ti₃C₂T_x MXene flakes on Si/SiO₂ as depicted in an AFM image with scale bar 1 μm, (d) schematic of a multielectrode chip with a coating of drop-cast MXene flakes, (e) SEM image of a MXene film with contact area between Si/SiO₂ substrate and Pt electrode, (f) variation of MXene G/(G_air) with time, (g) the multisensor array vector signals are anticipated by LDA into a two-component taste diagram for three organic vapours (10 ppm) and dry air; the circles associated to the gases are constructed with a 0.7 confidence around the relevant gravity centres (Main figure), dependence of the chemiresistive response with different concentration of gases (inset). Reproduced with permission from ref. 95 Copyright 2020. American Chemical Society.

The electrochemical performance of the 2D Ti₃C₂T_x sheet decorated with Pt nanoparticles (PtNPs) was studied by Lorencova et al.¹⁰⁰ The findings demonstrated that Ti₃C₂T_x/PtNP nanocomposite exhibited significantly superior and more stable redox activity in an anodic potential window compared to GCE treated with pure Ti₃C₂T_x MXene. The H₂O₂ sensor fabricated with Ti₃C₂T_x/PtNP on GCE provided LOD of 448 nM and a potential at which reduction begins of +250 mV (vs. Ag/AgCl), in contrast to 883 M and 160 mV observed for Ti₃C₂T_x modified GCE. A flexible hydrogen sensor was also fabricated by Zhu et al., employing Ti₃C₂T_x MXene that had been modified with the noble metal palladium (MXene@Pd CNC).⁹⁶ This sensor had a response time of (32 ± 7) s and a sensitivity range of S = (23 ± 4)% to 4% H₂.

In summary, functionalized Ti₃C₂T_x MXenes hold great promise for gas sensors due to their versatility and tunable surface chemistry, offering distinct advantages and potential drawbacks. One significant advantage is their enhanced selectivity and sensitivity towards specific gases. By introducing functional groups like –OH, –F, or –NH₂, the MXene surface can be tailored to interact selectively with target gas molecules, leading to improved detection capabilities for specific gases of interest. Moreover, functionalization can increase the active sites available for gas adsorption, further enhancing sensitivity. However, a notable drawback of excessive functionalization is the potential reduction in electrical conductivity. This can hinder electron flow within the sensor, impacting its overall performance. To overcome this limitation, researchers are actively working on optimizing the degree of functionalization to strike a balance between improved selectivity and maintaining good conductivity. Additionally, innovative techniques such as in situ functionalization and the development of hybrid structures that combine functionalized and pristine MXenes are being explored. These approaches aim to harness the benefits of functionalization while mitigating its negative impact on conductivity, thereby ensuring the gas sensor's overall effectiveness. Furthermore, the integration of functionalized MXenes into composite materials or heterostructure with other conductive materials like graphene or carbon nanotubes has shown promise in overcoming conductivity challenges while preserving the enhanced gas-sensing properties. In brief, functionalized Ti₃C₂T_x MXenes offer tailored selectivity in gas sensors, but careful consideration of conductivity issues and the exploration of hybrid structures and composite materials are essential for optimizing their performance in practical applications.

6.3 Ti₃C₂T_x composites-based gas sensor

A key tactic to enhance the gas sensing capabilities of Ti₃C₂T_x is the formation of the composite. The diversity of materials, including metal oxides,¹⁰¹ TMDs,¹⁰² graphene,¹⁰³ and polymers,¹⁰⁴ have been effectively combined with Ti₃C₂T_x. The Ti₃C₂T_x composite-based gas sensor attracts much attention because of the formation of a Schottky barrier due to the combination of metal oxide and Ti₃C₂T_x, which improves the gas sensing performance of the device. To advance the NH₃ sensing properties of pure Ti₃C₂T_x, Tai et al. reported a gas sensor based on a bilayer film of TiO₂ and Ti₃C₂T_x.⁸¹ In accordance with the findings, the TiO₂/Ti₃C₂T_x sensor responds to 10 ppm NH₃ at room temperature with a response value that is 1.63 times greater and response/recovery times that are 0.65 and 0.52 times faster than those of a pure Ti₃C₂T_x sensor as shown in Fig. 8(a) and (b). The TiO₂/Ti₃C₂T_x sensor, meanwhile, has an excellent linear response (R² = 0.95596) at NH₃ concentrations of 2 to 10 ppm and can detect NH₃ concentrations as low as 0.5 ppm. Sun et al. demonstrated how to synthesise W₁₈O₄₉/Ti₃C₂T_x composites using an easy solvothermal technique to grow 1D W₁₈O₄₉ nanorods (NRs) on Ti₃C₂T_x MXene sheets as shown in Fig. 8(c).¹⁰⁵ The reported W₁₈O₄₉/Ti₃C₂T_x composites have excellent selectivity, enduring stability, a low LOD (170 ppb acetone), and quick response and recovery time (5.6 s and 6 s to 170 ppb acetone). They also reported a good response to low concentrations of acetone (11.6 to 20 ppm) and the operated temperature of the device was 300 °C as shown in Fig. 8(d).

Fig. 8 (a) Response/recovery curve at a concentration of 10 ppm NH₃ for TiO₂/Ti₃C₂T_x, Ti₃C₂T_x, and TiO₂ gas sensors, (b) response and recovery times of the Ti₃C₂T_x and TiO₂/Ti₃C₂T_x gas sensors. Reproduced with permission from ref. 81 Copyright 2019. Elsevier, (c) schematic of reaction between acetone and Ti₃C₂T_x/W₁₈O₄₉ composite and (d) response–recovery curves of different sensors at a concentration of 0.17–500 ppm acetone at 300 °C temperature, Inset: Response–recovery curves towards 0.17–2 ppm acetone (enlarged view). Reproduced with permission from ref. 105 Copyright 2019. Elsevier.

TMDs have received attention as prospective composite materials because of their numerous redox reaction active sites and high surface reactivity. To enhance the sensing capabilities of TMDs at ambient temperature, MXene can be combined with other TMDs, including In₂O₃, WS₂, SnO₂, graphene, CoS, WSe₂, and MOS₂, to generate heterojunctions.^{39,102,106–110} Ti₃C₂T_x and WSe₂ were the model materials that Chen et al. decided to use in their implementation of the detection of various volatile organic compound (VOC) gases.¹⁰² The nanohybrids gas sensor based on Ti₃C₂T_x/WSe₂ was synthesized by combining liquid-phase exfoliation Ti₃C₂T_x with WSe₂ that had been functionalized with cetyltrimethyl ammonium bromide (CTAB), as shown in Fig. 9(a) and (b). The gas sensor that was fabricated employing inkjet printing materials and a wireless monitoring system displayed a 12-fold improvement in ethanol sensitivity, very fast response and recovery time (9.7 s and 6.6 s), and a very low level of electrical noise. In addition, graphene and its derivatives have been the subject of a significant amount of research owing to the high surface area of these materials and the fact that their conductivity can be controlled. As a result, composites made of graphene and Ti₃C₂T_x are deserving of future investigation in the realm of gas sensing. Yuan et al. have demonstrated a VOC sensor that is both high-performing and versatile, and it is built on a 3D MXene framework (3D-M).¹¹¹ The 3D-M was fabricated by combining a robust electrospinning method. The 3D-M sensor is able to function at ambient temperature thanks to its extremely interconnected porous structure. It is especially sensitive to extremely low concentrations of volatile organic compounds (ppb level) due to its capacity to quickly access and disperse gas molecules. In addition, the sensor has a very wide sensing range, a quick response and recovery time (less than two minutes), and excellent reversibility and versatility for a variety of volatile organic compounds. The researchers led by Lee et al. announced the fabrication of Ti₃C₂T_x MXene/graphene hybrid fibres by a wet-spinning approach.¹¹² The remarkable mechanical and electrical qualities of these hybrid fibers make them ideally suited for use in flexible wearable gas sensors. The ability of the produced fibers to demonstrate significant sensitivity to NH₃ gas at ambient temperature is made possible by the synergistic impacts of the electrical characteristics and gas-adsorption capacities of MXene and graphene. In comparison to MXene and graphene on their own, the hybrid fibers demonstrated an NH₃ sensing response that was 6.77% more sensitive (ΔR/R₀ = 6.77%). Even after being bent over a period of 2000 times, the hybrid fibres showed an exceptionally high degree of mechanical flexibility, with a resistance that changed by ±0.2% and a low level of noise resistance. This enabled the hybrid fibres to detect gas even as they were deformed, as shown in Fig. 9(c) and (d). Recently, Yang et al. demonstrated the inherent process of the improvement of gas-sensing capabilities of oxidized Ti₃C₂T_x by analyzing the performance of crumpled Ti₃C₂T_x spheres.¹¹³ This was done in order to show that the crumpled Ti₃C₂T_x spheres have better gas-sensing properties. After explaining the structural and compositional modifications caused by the oxidation of Ti₃C₂T_x, it was found that the increase in gas-sensing capabilities was due to the presence of a large number of Ti atom defects that had been introduced by the oxidation process. This was discovered after it was established that oxidation had brought about changes in structure and composition. Calculations using the first principles of density functional theory (DFT) were carried out, and the findings revealed that the adsorption capacity of Ti atom defect sites for gas molecules was significantly improved, particularly with respect to NO₂. Meanwhile, a reducing agent known as Ti₃C₂T_x was utilized to assist in the process of GO reduction. The gas sensing performance was further improved by forming a Ti₃C₂T_x/TiO₂/rGO heterostructure, which resulted in high sensitivity, low level of detection limit, and a remarkable selectivity to NO₂, reaching up to 19.85% for 5 ppm NO₂ at ambient temperature. All of these improvements were made possible by the formation of the heterostructure.

Fig. 9 (a) A schematic representation of the steps involved in the preparation of Ti₃C₂T_x/WSe₂ nanohybrids, (b) illustration of inkjet-printed gas sensors for the detection of VOC using a wireless monitoring system. Reproduced with permission from ref. 102 Copyright 2020. Springer Nature (Open Access), (c) schematic of the spinning process for MXene/GO hybrid fiber, and (d) comparison of the gas response of MXene film, rGO fiber, and MXene/rGO hybrid fiber (40 wt% MXene). Reproduced with permission from ref. 112 Copyright 2020. American Chemical Society.

Since polymers are sensitive, cheap, versatile, and may be used at room temperature, they have frequently been used to fabricate Ti₃C₂T_x composites. Flexible gas sensors based on the Ti₃C₂T_x/polyaniline (PANI) nanocomposite were studied by Zhao et al. using density functional theory in conjunction with electrosensitive experiments on large samples.¹⁰⁴ This composite sensing material showed a high sensitivity for ethanol (41.1% at 200 ppm) and fast response/recovery time (0.4/0.5 second) at room temperature, as shown in Fig. 10(a) and (b), which makes it the next-generation sensing material that can quickly detect real-time VOC gases. This is because the properties of composites and highly active Ti₃C₂T_x MXene work synergistically with one another. To track NH₃ volatilization in farming operations, Li et al. report the development of a polyaniline (PANI)/Ti₃C₂T_x hybrid sensitive film-based flexible chemiresistive gas sensor.¹¹⁴ An in situ method of self-assembly was utilized during the process of depositing the hybrid film onto the flexible polyimide substrate, as shown in Fig. 10(c). High sensitivity, excellent repeatability, a low detection limit, and strong air stability are all characteristics of this sensor, which is enabled by the gas-sensing enhancing effects of the PANI/Ti₃C₂T_x Schottky junction and the enhanced protonation degree of PANI in the hybrids. Particularly, the sensor demonstrates outstanding NH₃-sensing properties in situations with relative humidity (RH) of 20–80% at a 10–40 °C temperature range. This makes it attractive for real-world agricultural applications because it can operate in a wide temperature range. Wang et al. investigated the possibility of using a conductive poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonic acid) (PEDOT:PSS) composite Ti₃C₂T_x in a gas sensing application, as shown in Fig. 10(d).¹¹⁵ In comparison to the pristine materials, the Ti₃C₂T_x/PEDOT:PSS sensor, which was fabricated using the process of drop-coating, exhibited superior sensing behaviors when exposed to VOC vapors at ambient temperature. They also investigated the effect of composite ratio on gas sensing performance, and the findings revealed that the ideal mass ratio for PEDOT : PSS and Ti₃C₂T_x was 4 : 1. This ratio produced the maximum response from both of these gas sensing components. Recently, Chen et al. reported a nanocomposite of Ti₃C₂T_xMXene and polypyrrole (PPy) by using in situ polymerization reaction for gas sensing application.¹¹⁶ The PPy/MXene-12, a nanocomposite-based gas sensor, displayed the best possible gas-detecting performance. It had a high response of 31.9% to 100 ppm NH₃, a low detection limitation, and unfailing selectivity.

Fig. 10 (a) Interdigitated electrode schematic before and after coating with PANI/Ti₃C₂T_x nanocomposites, (b) selectivity of Ti₃C₂T_x and PANI/Ti₃C₂T_x-based flexible sensors exposed to 200 ppm. Reproduced with permission from ref. 104 Copyright 2019. John Wiley & Sons (Open Access), (c) agricultural NH₃ volatilization monitoring based on Ti₃C₂T_x/PANI NH₃ sensor. Reproduced with permission from ref. 114 Copyright 2020. Elsevier, (d) schematic of fabrication of Ti₃C₂T_x/PEDOT:PSS materials and gas sensor devices. Reproduced with permission from ref. 115 Copyright 2020. Elsevier.

In summary, the Ti₃C₂T_x composites-based gas sensors offer a range of advantages and drawbacks, necessitating innovative solutions for their optimal performance. One significant advantage is their improved conductivity and stability compared to pristine Ti₃C₂T_x, making them highly suitable for practical gas sensing applications. These composites often integrate Ti₃C₂T_x with other materials like polymers, MOs, or carbon-based nanomaterials, harnessing the synergistic effects of each component. This enhances the sensor's electrical properties and provides a larger surface area for gas adsorption, leading to enhanced sensitivity and response times. However, a notable drawback is the potential loss of selectivity when using composites, as the additional materials may introduce cross-sensitivity to various gases. To address this, researchers are actively developing advanced composites with tailored properties that maintain or even improve selectivity while preserving high conductivity. Surface modification techniques and precise control over the composition and morphology of the composite materials allow for a fine-tuning of gas-sensing characteristics. Another challenge lies in the potential degradation of the composites over time due to environmental factors or harsh operating conditions. To overcome this, protective coatings or encapsulation techniques are being explored to shield the sensitive composites from external influences, thus improving their long-term stability and reliability. Moreover, ongoing research into the optimization of composite fabrication processes and the development of novel composite combinations are continuously pushing the boundaries of gas sensor technology, aiming to strike a balance between enhanced performance and selectivity, as well as long-term durability for a wide range of gas detection applications.

Recently, lots of research going on Ti₃C₂T_x MXene based self-powered gas sensor. One of the most remarkable aspects of MXene-based gas sensors is their ability to operate without the need for external power sources, such as batteries. This is achieved through the concept of self-powering, where the interaction between the gas molecules and the MXene-based sensing layer generates electricity. Two primary mechanisms have been explored for self-powering MXene-based gas sensors.

(1) Piezoelectric effect: when gas molecules adsorb or react with the MXene surface, they induce mechanical deformation or strain in the sensing layer. This strain can be harvested as piezoelectric energy, which is then converted into an electrical signal. This approach allows for the continuous and sustainable operation of gas sensors without the need for batteries.

(2) Triboelectric effect: in the triboelectric effect, friction or contact between the gas molecules and the MXene surface generates static electricity. This effect can also be harnessed to power gas sensors. As gas molecules interact with the MXene layer, they create charge imbalances that can be tapped into for sensing purposes.

Sardana et al. reported a self-powered humidity sensor using an electrospun anisotropic triboelectric nanogenerator (A-TENG) composed of aligned Ti₃C₂T_x MXene nanofibers (NFs) and cellulose acetate (CA) NFs.¹¹⁷ Significantly, anisotropic structure of TENG displays its potential in sensitive moisture detection upon fast adsorption/desorption of water molecules due to increased pore length of prepared NFs. They also demonstrated the applied application of detecting the skin moisture. The same group also reported, a new kind of MXene/TiO₂/C-NFs heterojunction-based sensory component for detection of NH₃ by using cellulose nanofibers (C-NFs) as a substrate. At room temperature, the proposed sensor shows excellent reproducibility, high selectivity, and sensitivity toward NH₃ (1–100 ppm) along with a fast response/recovery time (76 s/62 s).¹¹⁸ Wang et al. demonstrated self-powered and low-temperature resistant Ti₃C₂T_x MXene-modified electronic-skin (e-skin) for multifunctional sensing.¹¹⁹ The reported e-skin displays decent sensitivity to pressure (1.29 mV Pa⁻¹) and frequency (5.60 V Hz⁻¹) at a low temperature (−15 °C). In the meantime, utilizing this flexible e-skin, they successfully demonstrated temperature sensing over the range from −15 °C to 25 °C with a sensitivity of 76.6 nA cm⁻² °C⁻¹. Gasso et al. reported SnO₂/Ti₃C₂T_x–MXene nanocomposite-based self-powered gas sensor for NO₂ sensing. The fabricated sensor has detection limit of 0.03 ppb NO₂ and power consumption as low as 1.2 μW. Moreover, they also reported SnO₂/MXene nanocomposite-based piezoelectric pressure sensor (PPs) with a high sensitivity of 2.088 kPa⁻¹ under the pressure range of 1.63–6.23 kPa with a fast response time (265 ms) and recovery time (75.5 ms).¹²⁰ Wang et al. reported high-performance Ti₃C₂T_x MXene/tungsten oxide nanofibers-based NO₂ sensor. The as fabricated self-powered Ti₃C₂T_x MXene/WO₃ sensor driven by TENG has an outstanding response at room temperature (ΔU_s/U_sa = 510% at 50 ppm) for NO₂ gas, which is 15 times larger than that of the resistive MXene/WO₃ sensor.¹²¹

While MXene-based self-powered gas sensors have made significant strides, ongoing research is focused on further enhancing their sensitivity, selectivity, and stability. Additionally, efforts are underway to reduce production costs and facilitate large-scale manufacturing for wider adoption. In conclusion, MXene-based self-powered gas sensors are poised to play a pivotal role in shaping the future of gas detection technology. Their unique combination of material properties, self-powering mechanisms, and diverse applications make them a promising solution for addressing environmental, industrial, and healthcare challenges. As research in this field continues to evolve, we can anticipate even more sophisticated and accessible MXene-based gas sensors that contribute to a safer and healthier world.

7. MXene gas sensor: theoretical approach

The intricacy of the sensing mechanism employed by MXene sensors introduces considerable difficulties when attempting to fine-tune them through experimental methods. MXene sensors are equipped with a sophisticated and multifaceted system for detecting various environmental changes or target analytes, and this complexity can make it a formidable task to optimize their performance through practical experimentation. This complexity arises from the intricate interplay of materials and physical processes within the sensor, requiring a deep understanding of its underlying principles to effectively modify and enhance its sensing capabilities. Consequently, researchers face significant challenges in achieving the desired sensor performance due to the complicated nature of the MXene sensing mechanism, which necessitates a meticulous and thorough approach to experimentation and tuning. Hence, there exists a requirement for atomic-level modeling, specifically utilizing techniques such as density functional theory (DFT) and molecular dynamics (MD), to complement the empirical data and advance our understanding of the interactions between MXene materials and gas analytes.

The first computational study was conducted by Yu et al., who demonstrated the potential of MXenes as a gas sensor using monolayer Ti₂CO₂.¹²² They investigated the interaction behavior of H₂, N₂, CO₂, CO, NH₃, O₂, NO₂, and CH₄ gas molecules on the semiconductor Ti₂CO₂. Because Ti₂CO₂ is the only one among its counterparts [Ti₂C, Ti₂CF₂, and Ti₂C(OH)₂] that exhibits semiconductor properties, the researchers focused their investigation on Ti₂CO₂ through first-principles simulations. Out of all the gases studied, NH₃ is the sole gas that chemisorbs onto the surface of the Ti₂CO₂ MXene. It accomplishes this by donating a relatively substantial charge of 0.174 e and establishing a N–Ti bond. In DOS analysis, there is a significant overlap of electronic charges, leading to orbital mixing and substantial charge transfer between NH₃ and Ti₂CO₂. This observation strongly indicates chemisorption, where chemical bonds are formed between NH₃ and Ti₂CO₂. Conversely, when examining other gas molecules, it was evident that they exhibited significantly lower gas adsorption energies and charge transfer compared to NH3. This points to the absence of overlapping electronic charges and signifies weak interactions between these molecules and the Ti₂CO₂ surface. The adsorption energy of NH₃ on Ti₂CO₂ was determined to be −0.37 electron volts (eV), which is quite less as compared to Ti₂C(OH)₂ [−0.48 eV], V₂CO₂ [−0.81 eV] and analogous to Ti₃C₂O₂ [−0.34]. The higher sensitivity of Ti₂CO₂ toward NH₃ was confirmed by calculating I–V curves before and after absorption of gas. Xiao et al. investigated the interactions of various gases, including NO, CO, H₂, CO₂, O₂, N₂, and CH₄, with Zr₂CO₂ MXene.¹²³ Their findings revealed that NH₃ exhibited strong chemisorption characterized by a noticeable charge transfer 0.188 e and a substantial adsorption energy −0.81 eV, whereas other gases were primarily physisorbed. The authors anticipated a similar response for M₂CO₂ (where M = Sc, Ti, Zr, and Hf) due to their analogous atomic and electronic structures. Additionally, it was observed that the transition from chemisorption to physisorption in NH₃ adsorption on Zr₂CO₂ occurred when two additional electrons were introduced into the MXene sheet, offering a practical means to release the gas molecule. In a subsequent study, Wang et al. conducted a comprehensive analysis of Hf₂CO₂'s performance in detecting multiple gases, including H₂, O₂, NO₂, SO₂, CO, CO₂, HCN, NH₃, and H₂S.¹²⁴ Consistent with the earlier report, Hf₂CO₂ exhibited remarkable sensitivity and selectivity toward NH₃, characterized by an exceptional charge transfer of 0.146 e and substantial adsorption energy of −0.834 eV. Additionally, it was observed that preadsorption of the MXene surface with H₂O, SO₂, and CO₂ molecules significantly enhanced NH₃ adsorption by fostering the formation of hydrogen bonds.

In addition to their experimental investigations, Kim et al. conducted a detailed examination of how the functional groups of Ti₃C₂T_x MXene influence its sensing capabilities.⁷⁴ Employing the DFT approach, they analyzed the interactions between acetone and NH₃ with Ti₃C₂T_x surfaces terminated by –OH, –O, and –F groups. Their findings indicated that Ti₃C₂(OH)₂ exhibited stronger binding energy with both acetone and NH₃ compared to Ti₃C₂O₂ and Ti₃C₂F₂ MXenes. Hajian et al. investigated the impact of the ratio of functional groups on NH₃ detection using Ti₃C₂T_x MXene.¹²⁵ They examined two different ratios of –F functional groups in and observed that lower ratios of –F groups led to stronger NH₃ adsorption. This effect was attributed to the smaller charge transfer between NH₃ and fluorine in comparison to NH₃ and oxygen. Consequently, the synthesis method and the choice of MAX phase etchant, which dictate the functional group ratios, emerged as critical factors influencing the gas-sensing performance of MXene sensors. Zhang et al. examined the ability of Ti₃C₂O₂ nanosheets to capture formaldehyde effectively at room temperature.¹²⁶ The findings revealed a moderate adsorption energy of 0.45 eV and an adsorption capacity exceeding 6 mmol g⁻¹, indicating the promising potential of Ti₃C₂O₂ MXene for indoor formaldehyde removal. Ma et al. explored M₂CO₂ (where M = Sc, Ti, Zr, and Hf) MXenes to identify the most effective platforms for detecting toxic SO₂.¹²⁷ Their investigation revealed that monolayer Sc₂CO₂ exhibited a robust chemical bond with SO₂, characterized by an adsorption energy of −0.646 eV and a charge transfer of 0.453 e, establishing it as a favorable material for SO₂ sensing. The M₂CO₂ MXene was also explored for other gas detection such as NO and CO. Regarding NO, the most significant interactions were observed between the molecule and Sc₂CO₂, likely due to orbital hybridization occurring between the MXene and the gas molecule near the Fermi level. This was substantiated by a substantial charge transfer of 0.303 e and a reasonably moderate adsorption energy of −0.47 eV, affirming Sc₂CO₂ as a highly suitable material for NO sensing. Conversely, in the case of CO, it was primarily physisorbed onto Sc₂CO₂ MXene, with a minimal charge transfer of 0.017 e. However, the sensing capabilities of Sc₂CO₂ MXene for CO were enhanced through the introduction of Mn dopants into the structure, resulting in a charge transfer of 0.199 e and a strong adsorption energy of −0.85 eV.¹²⁸ Banu et al. utilized M₂CO₂ MXene for the detection of NH₃via oxygen functionalization.¹²⁹ Based on the detailed DFT study, they reported the adsorption behaviour of NH₃ on Cr₂C/Fe₂C went through the alteration from chemisorption (E_ad = −1.40/−1.33 eV) to physisorption (E_ad = −0.29/−0.22 eV). Recently Quan et al. reported fully flexible Ti₃C₂T_x/WS₂ based gas sensor for NO₂ detection at room temperature.¹³⁰ To delve further into the sensing mechanism, adsorption models for NO₂ on WS₂(001), Ti₃C₂T_x(001), and Ti₃C₂T_x(001)/WS₂(001) were computed using the DFT method. The calculated adsorption energies for NO₂ molecules on the surfaces of bare WS₂(001), Ti₃C₂T_x(001), and Ti₃C₂T_x(001)/WS₂(001) were found to be −0.20 eV, −3.37 eV, and −3.73 eV, respectively. Weng et al. reported highly sensitive and selective Ti₃C₂O₂-based gas sensors for NH₃ sensing.¹³¹ Through DFT study, they calculated adsorption energy of NH₃ on Ti₃C₂O₂ yields an appropriate value (−0.42 eV), and the charge transfer rate of 0.18 e. Based on their theoretical results, only NH₃ adsorbs onto the nanosheet through chemisorption. Reji et al. demonstrated the Sc₂CO₂ MXene nanosheets for detection of VOCs using first-principles DFT calculations.¹³² They reported that 2-propanol (−0.63 eV), ethanol (−0.59 eV), and acetonitrile (−0.51 eV) show higher interaction with the Sc₂CO₂ MXene nanosheets. At the end, they also predict that Sc₂CO₂ MXene nanosheets can be utilized as a dual-mode sensor to detect potential VOC biomarkers in the exhaled breath.

8. Advantage and drawback of Ti₃C₂T_x MXene gas sensors and other common materials

Ti₃C₂T_x MXene is a promising material for gas sensors, but like any material, it has its advantages and drawbacks when compared to common materials used in gas sensing applications. The comparison of Ti₃C₂T_x MXene with some common materials is given in Table 1.

Table 1 Advantage and drawback of Ti₃C₂T_x MXene with some common material

S. no.	Materials	Advantages	Drawbacks
1.	Ti3C2Tx MXene based gas senor	✓ High sensitivity	• Complex synthesis
		✓ Tunable properties	• Limited gas selectivity
		✓ Good conductivity	• Poor humidity sensitivity
		✓ Chemical stability
2.	Metal oxide-based gas sensors (e.g., SnO2 and ZnO)	✓ Low cost	• High operating temperature
		✓ Commercial availability	• Slow response/recovery
		✓ Good selectivity	• Limited sensitivity
3.	Organic polymer-based gas sensors	✓ Low cost	• Limited sensitivity
		✓ Low power consumptions	• Limited durability
		✓ Tailorability	• Limited operating range
Other 2D materials (e.g., graphene, TMDs like MoS2)
4.	Graphene	✓ Excellent electrical conductivity	• Limited gas selectivity.
		✓ Low noise levels	• Difficulty in functionalizing for specific gas detection
		✓ High surface area	• Susceptibility to environmental factors, such as humidity
5.	TMDs	✓ Good gas selectivity due to varying band structures	• Moderate surface area compared to MXenes.
		✓ Layer-dependent properties for tenability	• Synthesis challenges
		✓ Compatibility with existing semiconductor technologies.	• Sensitivity to environmental conditions, including humidity.

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In summary, Ti₃C₂T_x MXene-based gas sensors offer unique advantages such as high surface area, tunability, and fast response times. However, they also face challenges related to synthesis complexity, moisture sensitivity, and limited gas range. The choice of gas sensor material depends on the specific application and requirements for sensitivity, selectivity, and operational conditions.

Our detailed study suggested that Ti₃C₂T_x MXene-based gas sensors have a high response value for NH₃ gas. The response of NH₃ gas based on Ti₃C₂T_x MXene with different literature has been shown in Fig. 11.

Fig. 11 Gas response of NH₃ gas with Ti₃C₂T_x MXene-based gas sensors.

We have summarised the different types of Ti₃C₂T_x MXene (pure, functionalized and compoite) based gas sensors in terms of sensitivity, limit of detection and working range in Table 2.

Table 2 Summary of Ti₃C₂T_x MXene-based gas sensor

MXene	MXene type	Target gas	Sensitivity in %	Limit of detection (LOD)	Working range	Ref.
Ti3C2Tx	Pure	Ammonia	0.21	NR	25 to 200 ppm	75
	Pure	Acetone	0.97	0.011 ppb	50 ppb to 100 ppm	74
	Pure	Nitrogen dioxide	4.5	NR	5 ppm	91
	Pure	Ammonia	0.62	NR	5 ppm	54
	Pure	Ammonia	6.13	NR	10 to 700 ppm	86
	Pure	Ammonia	1.22	NR	1 to 100 ppm	133
	Pure	Methane	68	NR	200 to 10 000 ppm	134
	Pure	Toluene	19.32	NR	1 to 50 ppm	94
	Functionalized	Ethanol	9.99	NR	0.3 mM L^−1 NaOH	93
	Functionalized	Hydrogen	23 ± 4	NR	0.5 to 40% H2	96
	Functionalized	Toluene	79.5	NR	1 to 50 ppm	94
	Functionalized	Ammonia	28.87	NR	10 to 500 ppm	79
	Composite	Ethanol	14	NR	5 to 120 ppm	135
	Composite	Toluene	79.5	NR	1 to 50 ppm	94
	Composite	Hydrogen	70	NR	100 to 500 ppm	136
	Composite	Ammonia	6.2	NR	1 to 80 ppm	137
	Composite	Ammonia	3.1	NR	10 ppm	81
	Composite	Ammonia	0.4	NR	1 to 100 ppm	138
	Composite	Ammonia	24.8	NR	1 to 100 ppm	139
	Composite	Ammonia	22.3	NR	1 to 5 ppm	80
	Composite	Ammonia	40	4.29 ppb	0.5 to 100 ppm	39
	Composite	Ammonia	100.7	NR	20 ppm	140
	Composite	Toluene	11.4	NR	10 to 50 ppm	141
	Composite	Nitrogen dioxide	41.93	NR	5 to 100 ppm	142
	Composite	Nitrogen dioxide	37	125 ppb	0.125 to 5 ppm	101
	Composite	Nitrogen dioxide	165	50 ppb	0.05 to 20 ppm	143
	Composite	Acetone	0.25	0.05 ppm	0.05 to 30 ppm	144
	Composite	Acetone	52	NR	10 to 500 ppm	145
	Composite	Ethanol	27.4	NR	50 to 200 ppm	104
	Composite	Carbon dioxide	1	NR	8 to 3000 ppm	146
	Composite	Hexanal	3.4	217 ppb	10 to 40 ppm	147
	Composite	Methanol	29.6	NR	5 to 100 ppm	106

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9. Conclusions and future prospective

MXenes are a newly discovered family of 2D materials that have been the subject of intensive research due to the exceptional properties that make them exceedingly valuable for use in various applications. MXene-based sensors have been among the most extensively researched applications because they are able to exploit the full potential of the one-of-a-kind 2D structure of MXenes. This is because MXenes have an inherently large specific surface area and surface-mediated properties that can be controlled. Thus, investigations on MXene-based gas sensors have been enhancing the selectivity and sensitivity using approaches such as composite formation, surface functionalization, and macro-structuring. The fact that the alternation of the MXene surface does not forfeit its fundamental outstanding electrical conductivity and improves its other physical and chemical properties makes it one of the most promising materials for sensors. For instance, it is conceivable to boost the adsorption energy of analytes on the MXene surface while maintaining the material's conductivity. The aforementioned methods have proven to be successful in the majority of the research aimed at enhancing the performance of sensors.

The development of MXene-based sensors is still in its early stages; therefore, there are still issues that need to be resolved before they can be used commercially. There needs to be more information about these requirements because the performance standards and other requirements for putting these sensors on the market have not yet been developed in a systematic way. In this review, we have discussed the recent developments in the field of gas sensors based on 2D Ti₃C₂T_x from three different perspectives: etching and delamination of Ti₃C₂T_x; pure, functional, and composite Ti₃C₂T_x applied in gas sensors; and their sensing mechanism. The Ti₃C₂T_x MXene has garnered a lot of attention and has been demonstrated to offer a lot of potential in the field of gas sensor. This is because of their one-of-a-kind structure and features. However, the application of Ti₃C₂T_x in the real world is constrained by a number of different reasons. The first difficulty is associated with the process of preparing the MXene precursor, also known as the MAX phase. In most cases, the formation of a MAX phase occurs as a result of the high-temperature processing of combustible materials in powder forms, such as titanium, aluminum, and others. To obtain a MAX powder of the desired fineness, multiple milling processes are necessary. If HF is to be used as an etchant, the handling of HF and treatment of waste must be done correctly. The second obstacle is the mass production of MXene-based sensors, which is difficult because MXene does not have sufficient mechanical stability to be processed by industrial machines. In order to overcome this obstacle, researchers have been concentrating on the fabrication of nanocomposites by combining MXenes with a variety of other materials, particularly polymers. The third obstacle is determining the ideal proportion of MXene to incorporate into a polymer matrix in order to produce a high-performance sensor. The fourth obstacle pertains to applications that take place in the real world and require sensors to be durable and functional even after being damaged. This can be accomplished by modifying the surface of the components that make up a sensor, which in most cases results in improved mechanical properties of those components. The fifth obstacle is the requirement for clean and flawless MXene surfaces, which increases the sensitivity of MXene-based sensors. It is necessary to develop etchants that are superior to both HF and HCl/LiF. In general, the focus of research efforts on this subject at the moment is on exploring the maximal capability and potential of Ti₃C₂T_x MXenes in terms of transferring their advantageous properties to sensors in order to obtain high performance of sensitivity and LOD. These aspects are absolutely necessary for the successful commercialization of MXene-based sensors. However, in order for MXenes to be successfully employed in the industry, several practical characteristics, including productivity, stability, and multifunctionality, should be examined and improved.

As a result, the area of future research should be broadened, and the primary objective should be to enhance sensor performance in terms of the variables described above so that it is compatible with industry requirements. In order to speed up the development of next-generation sensor technologies based on MXenes, it will be necessary to address the requirements listed above for commercialization. This will eventually pave the way for the creation of low-cost, high-performance, and multimodal sensors that can be used in soft-electronics applications. We hope this work will provide a roadmap for the commercialization of Ti₃C₂T_x MXene-based gas sensors and encourage a wide range of in-depth and applicable studies in this area.

Conflicts of interest

All authors declare that they have no conflicts of interest. We certify that the submission is original work and is not under review at any other publication.

Acknowledgements

This work was supported by “Science and Engineering Research Board,”. Nitesh K. Chourasia acknowledges “Science and Engineering Research Board” for National Post-Doctoral Fellowship (PDF/2021/001490). Ankita Rawat thanks CSIR-UGC NET with File No. 09/263(1233)/2020-EMR-I for providing Junior Research Fellowship (JRF).

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