Ti₃C₂ MXene-Based Composites for Hydrogen and Ammonia Gas Sensing: A Review

Abstract

The unique contributions of 2D Ti₃C₂ MXenes surface, electrical, and chemical features play a crucial role in determining toxic and flammable gas-sensing behavior. Specifically, its high electrical conductivity (metallic nature), layered nanosheet structure (nanosheets), and surface termination groups (–O, –F, and –OH) collectively contribute to excellent hydrogen (H₂) and ammonia (NH₃) gas-sensing behavior. This review systematically explores the impact of pristine and modified Ti₃C₂ MXene, including its interfaces with various metals and metal oxides for enhancing H₂ and NH₃ detection. Furthermore, the significance of room temperature operation and flexible gas sensing mechanisms is explored. Notably, integration of Ti₃C₂ MXene and sulfur nanosheets demonstrates rapid response and recovery times with detection limits at ppt level. Ti₃C₂ MXene-based interfaces also exhibit excellent long-term stability under various relative humidity conditions. The selective surface termination groups (–OH and –O) facilitate the formation of hydrogen bonds with NH₃ molecules for enhancing gas adsorption and sensing selectivity. In addition, the expansion of the interlayer spacing plays a vital role in improving the gas-sensing performance. Partial oxidation of Ti₃C₂ MXene into TiO₂ increases the interlayer distance, promoting faster diffusion of gas molecules and quicker sensor response. Overall, the intrinsic properties of Ti₃C₂ MXene and its composites significantly achieve high-performance room-temperature H₂ and NH₃ gas-sensing performance.

1. Introduction

In recent decades, rapid industrialization has led to a significant increase in release of toxic gases into the atmosphere, which results in air pollution. Interaction and exposure to these toxic gases poses serious risks to human health, particularly by causing respiratory disorders. It is to be noted that H₂ and NH₃ gases play a crucial role in advancing a sustainable and clean energy future [1]. To achieve global net-zero emissions targets, H₂ fuel is increasingly being adopted as a clean energy carrier for fuel cell vehicles and other energy applications [2]. However, the safe production, storage, and transportation of H₂ fuel remain major challenges due to high flammability. It is necessary to avoid the interaction of H₂ concentrations above the lower explosive limit of 4% with explosive materials, which makes the rapid detection of H₂ leaks essential for ensuring operational safety [3]. Similarly, NH₃ is a toxic gas and its permissible exposure limit is restricted to 25 ppm in 8 h, according to the standards of the Occupational Safety and Health Administration (OSHA) [4,5]. Interaction between metal and metal oxides strengthens gas-sensing behavior [6]. Therefore, the development of highly sensitive, selective, and reliable gas sensors for H₂ and NH₃ detection is of considerable importance. Semiconductor metal oxides and their heterostructures have been extensively investigated for gas sensing because of their excellent chemical stability and tunable electronic properties [7]. In particular, transition metal oxides such as ZnO, WO₃, MoO₃, TiO₂, and SnO₂ (both n- and p-type) have demonstrated promising H₂ and NH₃ gas-sensing performance [8,9,10,11,12]. Nevertheless, conventional metal oxide sensors generally require elevated operating temperatures; they exhibit high sensitivity to ambient humidity, which decreases their H₂ gas-sensing ability. Meanwhile, decrements in the recovery process (desorption) of transition metal oxides during the NH₃ gas-sensing process have been noted. To address these limitations, forms of 2D Ti₃C₂ MXene have emerged as promising gas-sensing materials owing to their exceptional metallic conductivity, ultrathin layered structure, large specific surface area with abundant exposed active sites, and surface termination groups (–O, –OH, and –F), which provide favorable adsorption sites for gas molecules [13]. These unique characteristics make 2D Ti₃C₂ MXene an attractive material for room temperature H₂ and NH₃ gas sensing. Interestingly, Ti₃C₂ MXene exhibits excellent H₂ storage capability when integrated with various metal hydrides for highlighting its potential for hydrogen-related applications [14]. Its layered architecture facilitates rapid gas diffusion and high electrical conductivity (fast charge transport), which results in enhanced sensitivity, rapid response/recovery times, and improved room-temperature gas-sensing performance. Rapid charge transport kinetics also play a crucial role in determining the gas-sensing behavior of Ti₃C₂ MXene [15]. The adsorption energy of NH₃ on Ti₃C₂O₂ has been reported to be 0.42 eV accompanied by a charge transfer rate of 0.18 e [16], which indicates a favorable interaction between NH₃ molecules and the MXene surface. In detail, as NH₃ contains a lone electron pair, it acts as an electron donor facilitating electron transfer to Ti₃C₂O₂ and inducing a significant change in its electrical resistance for strengthened gas-sensing response/recovery signals. Interestingly, the surface –OH termination groups on Ti₃C₂ MXene promote the formation of strong hydrogen bonds with NH₃ molecules to modulate the electrical signals [17]. In other words, the metallic or p-type semiconductor behavior in the Ti₃C₂ MXene effectively gains electrons from NH₃ and recombines with holes, which results in increase of resultant resistance [18]. In addition to H₂ and NH₃ detection, the intrinsic properties of 2D Ti₃C₂ MXene also facilitate remarkable room-temperature NO₂ gas sensing [19]. Therefore, Ti₃C₂ MXene-based composites have attracted increasing attention as promising materials for room-temperature gas-sensing applications.

This review comprehensively summarizes and evaluates the room-temperature NH₃ and H₂ gas-sensing performance of various 2D Ti₃C₂ MXene-integrated semiconductor nanostructures. Particular emphasis is placed on the remarkable improvement of the limit of detection (LOD), ranging from ppm to ppt levels. In addition, this review discusses the excellent cyclic durability and long-term stability of these composites under harsh humid environmental conditions. The influence of enlarged interlayer spacing on facilitating gas diffusion and increasing the accessibility of active adsorption sites is also highlighted. Moreover, the role of surface termination groups (–O, and –OH) in promoting hydrogen-bond formation and enhancing gas adsorption is critically examined to provide a comprehensive understanding of the improved gas-sensing performance of Ti₃C₂ MXene-based materials.

2. Importance of Ti₃C₂ MXene for Gas Sensing

As discussed above, Ti₃C₂ MXene exhibits several attractive properties, including a 2D layered structure, high specific surface area, excellent electrical conductivity, and tunable surface terminal groups. These characteristics facilitate active adsorption sites for targeting gas molecules, which result in electrical signal variation and efficient hazardous-gas-sensing performance [20]. In particular, the layered structure promotes gas diffusion by maximizing the number of accessible active sites. The availability of a high surface-area-to-volume ratio provides abundant active edges for gas adsorption, while the surface termination groups serve as chemically active sites for strong interaction with target gas molecules. Furthermore, the high electron density of Ti₃C₂ MXene facilitates rapid charge carrier transport during gas adsorption, which leads to enhanced sensing performance. The fabrication of few-layer or monolayer Ti₃C₂ MXene nanosheets further increases the exposure of basal-plane active sites, thereby improving gas diffusion and adsorption across the nanosheet surface. Additionally, interlayer expansion achieved through the incorporation of nanoparticles or other 2D nanosheets creates additional diffusion pathways, resulting in faster response and recovery times. Collectively, these unique structural and electronic characteristics of 2D Ti₃C₂ MXene establish a highly promising material for room-temperature gas-sensing applications. Figure 1 illustrates the NH₃ and H₂ gas-sensing performance ability of Ti₃C₂ MXene integrated with various nanostructures.

Figure 1.

Room temperature NH₃ and H₂ gas-sensing performance of various Ti₃C₂ MXene nanosheet-integrated nanostructures.

3. Ti₃C₂ MXene-Based Composites for H₂ Gas Sensing

3.1. Ti₃C₂/Pd Nanostructures

The effective interaction between Ti₃C₂ MXene and ultra-small Pd colloidal nanostructures (Pd CNC) (5 ± 1.9 nm) significantly enhances H₂-sensing performance [21]. in this example, upon H₂ exposure, the formation of PdH_x altered the Pd work function (3.2 eV), facilitating electron transfer from Pd to the Ti₃C₂ MXene. Furthermore, the intercalation of Pd CNCs between Ti₃C₂ MXene nanosheets increased the interlayer spacing, resulting in a larger effective surface area for gas adsorption. The surface morphology of the Ti₃C₂ MXene@Pd CNC film is presented in Figure 2a. Accordingly, the Ti₃C₂ MXene@Pd CNC film sensor exhibited stable response and recovery characteristics during H₂ detection. In this system, Ti₃C₂ MXene exhibited n-type semiconducting behavior and effectively accepted electrons from Pd the to Ti₃C₂ MXene, which led to a decrease in electrical resistance. The sensor demonstrated response and recovery times of 32 ± 7 s and 161 ± 23 s at a low H₂ concentration of 4%. In detail, accelerated electron transfer from Pd CNC to Ti₃C₂ MXene was observed after insertion of H₂ under high H₂ adsorption. Under these conditions, the flexible paper-based Ti₃C₂@Pd CNC sensor film achieved a sensitivity of 23.0 ± 4.0% at 4% H₂, as shown in Figure 2b. In addition, the sensor maintained stable performance under bending angles of 30, 60, 90, and 120°, demonstrating excellent mechanical flexibility. For practical application, an H₂ gas leak detection system incorporating an alarm circuit and LED indicator was developed. As illustrated in Figure 2c, after exposure to 4% H₂, the LED was activated, which confirmed the sensor’s capability for real-time leak detection. Overall, the flexible Ti₃C₂ MXene@Pd CNC composite demonstrated considerable potential for practical H₂ gas-sensing applications due to its high sensitivity, mechanical flexibility, and reliable detection performance.

In another study, Doan et al. [22] fabricated a room-temperature H₂ gas sensor by decorating multilayer Ti₃C₂T_x MXene with Pd nanoparticles at different loadings (30, 60, and 90 mg) on silicon substrates. The authors demonstrated that both the Ti₃C₂T_x MXene loading (60 mg) and Pd nanoparticle content significantly influenced the H₂ gas-sensing performance at 10, 50, and 100 ppm concentrations. The incorporation of Pd nanoparticles enhanced the H₂ adsorption and promoted intimate interfacial contact with Ti₃C₂T_x for facilitating the formation of efficient charge transport pathways. Among the investigated samples, the sensor containing 60 mg of Ti₃C₂T_x MXene with a lower Pd nanoparticle density exhibited highest response of 56% at 100 ppm H₂. Furthermore, the sensor showed excellent cyclic stability over five consecutive cycles and maintained a strong sensing response after approximately 90 days of storage under ambient conditions at 50 ppm H₂. The enhanced sensing performance was attributed to the formation of PdH and TiH₂ during H₂ exposure, which increased the electrical resistance of the Pd–Ti₃C₂T_x composite. Overall, the optimized combination of Ti₃C₂T_x MXene loading and Pd nanoparticle content resulted in a stable, sensitive, and reliable room-temperature H₂ gas sensor.

Figure 2.

(a) Surface morphology of the Ti₃C₂ MXene@Pd CNC composite film, (b) H₂ gas-sensing ability of Ti₃C₂ MXene@Pd CNC at various H₂ (v/v) concentrations, (c) demonstration of the practical applicability of Ti₃C₂@Pd CNC for H₂ gas sensing using an LED indicator (Reprinted from Ref. [21], copyright with permission from Elsevier), (d) surface morphology of Pd nanoparticles integrated Ti₃C₂ MXene-derived sodium titanate nanoribbons, (e) rapid response and recovery times of 1.1 s and 13.5 s by the Pd-NTO NRs composite, and (f) long-term stable response of the Pd-NTO NRs composite over 8 days (Reprinted from Ref. [23], copyright with permission from Elsevier).

Similarly, Wu et al. [23] combined Ti₃C₂ MXene-derived sodium titanate nanoribbons (NTO NRs) and nanotubes (NTO NTs) with Pd nanoparticles (3.5 nm) for room temperature H₂ gas sensing. Notably, the abundant oxygen vacancies in the NTO NRs facilitated rapid H₂ adsorption and dissociation, resulting in ultrafast H₂ sensing performance. The surface morphology of the Pd-NTO NRs is shown in Figure 2d. During H₂ exposure, formation of PdH_x resulted in a lower work function than n-type NTO, which created a favorable Schottky junction that promoted electron transfer from PdH_x to the NTO, which enhanced the sensing response. Furthermore, the 1D nanoribbon architecture provided a shorter charge diffusion path than the nanotube structure, which enabled more efficient charge transport under reduced scattering at multiple interconnections. XPS studies (O 1s spectrum) of the Pd-NTO NRs showed the presence of –OH and oxygen vacancy peaks, which influenced the gas adsorption process. Accordingly, the Pd-NTO NRs achieved an ultrafast response time of 1.1 s (recovery-13.5 s) with a response of 12.0 toward 1% H₂, as shown in Figure 2e. The sensor also demonstrated a wide detection range from 0.8 ppm to 10% H₂. The enhanced sensing mechanism was mainly attributed to the catalytic activity of Pd nanoparticles, which dissociated H₂ molecules into atomic H. These H atoms spilled over onto the NTO surface, which reacted with adsorbed O₂⁻ ions and released the electrons, thereby increasing the sensor current. Meanwhile, the formation of O–H and Ti–H surface bonds further contributed to the enhanced electrical response. Moreover, the Pd-NTO NR sensor exhibited excellent long-term stability, maintaining its H₂ sensing performance even after 8 days, as illustrated Figure 2f. Overall, the synergistic interaction between Pd nanoparticles and Ti₃C₂ MXene-derived NTO nanoribbons significantly enhanced the ultrafast and highly sensitive detection of H₂ gas.

In addition to the above incorporation of Pd nanostructures, Wang et al. [24] modified the Ti₃C₂T_x MXene surface by integrating bimetallic=Pt nanoparticles and Pd-Pt alloy for the elevation of room-temperature H₂ gas sensing. The Pd=Pt/Ti₃C₂T_x exhibited a response time of 6 s and recovery time of 8 s with a remarkable response of 24.6% towards 200 ppm H₂. Notably, the sensor demonstrated excellent micro-leak detection capability with LOD as low as 1 ppm at an ultrafast response time of 1 s. This response time was slightly faster than that of reported Pd-NTO NRs sensor (1.1 s) at 1% H₂ [23]. The enhanced sensing performance was attributed to the modulation in the Schottky junction formed between the Pd=Pt nanoparticles and Ti₃C₂T_x MXene. In air, adsorbed oxygen molecules extracted electrons from the MXene surface, leading to the formation of an electron depletion layer. Upon exposure to H₂, the catalytic dissociation of H₂ on the Pd–Pt nanoparticles promoted electron transfer back to the Ti₃C₂T_x surface, altering the depletion layer and decreasing the sensor resistance. Among the two metals, Pd exhibited a higher H₂ adsorption capability than Pt, playing a dominant role in improving the sensing performance of the bimetallic composite. The Pd=Pt/Ti₃C₂T_x sensor maintained stable sensing characteristics over 10 consecutive operating cycles, demonstrating excellent repeatability. Overall, the incorporation of bimetallic Pd=Pt nanoparticles significantly improved the room-temperature H₂-sensing performance, enabling highly sensitive detection of H₂ micro-leaks down to 1 ppm.

3.2. Ti₃C₂ MXene Completely Derived C/TiO₂

Ti₃C₂ MXene can be completely converted into C/TiO₂ at certain elevated temperatures, offering a promising route for developing environmental safety devices [25]. In this context, Ti₃C₂ MXene-derived C/TiO₂ composites prepared at 400 and 500 °C were systematically investigated for H₂ gas sensing at room temperature as well as at elevated temperatures (100, 150, 200, 250 and 300 °C) [26]. Decrease in corban and fluorine content along with an increase in oxygen content (TiO₂ formation) was observed following increased calcination temperatures (400 and 500 °C). Here, oxygen was diffused into the Ti₃C₂ MXene, confirming the partial oxidation of Ti₃C₂ MXene into TiO₂. The O 1s spectra revealed oxygen vacancies at 533.7 eV. In detail, H₂ interaction with lattice oxygen of C/TiO₂ transformed into oxygen vacancy, which released electrons into the conduction band and lowered the resistance. Among the prepared samples, the C/TiO₂ composite calcined at 500 °C exhibited the best sensing performance, delivering a response of 92.6% toward 40,000 ppm H₂ at 300 °C, with response and recovery times of 45 and 20 s, respectively. Remarkably, the same sensor also demonstrated excellent room-temperature performance by achieving a response of 62.2%, with response and recovery times of 13 and 42 s, respectively. The improved sensing performance at elevated temperatures was mainly attributed to enhanced surface reaction kinetics, reduced interference from humidity and volatile impurities, and the effective removal of reaction by-products. Overall, the controlled thermal transformation of Ti₃C₂ MXene into a C/TiO₂ composite significantly improved the stability of H₂ sensing performance over a wide range of operating temperature.

3.3. Ti₃C₂ MXene/Organic Frameworks

In addition to noble metal-functionalized Ti₃C₂ MXene sensors, Nallamalla et al. [27] developed a flexible room-temperature H₂ gas sensor by integrating a covalent organic framework (DAAQ-COF) with Ti₃C₂ MXene on OHP sheet substrate. The resulting DAAQ-COF/Ti₃C₂ MXene composite exhibited enhanced structural, chemical, and morphological stability owing to strong interfacial electronic modulation at the COF/MXene interface. Notably, optimizing the Ti₃C₂ MXene content to 50 wt% effectively regulated the interfacial charge transfer and H₂ adsorption behavior, leading to superior sensing performance. The adsorption strength (chemisorption) and heterointerface phenomena significantly contributed to improved room-temperature H₂ gas sensing. In this process, the flexible sensor achieved a LOD of 1 ppm with response and recovery times of 29 and 18 s, respectively, at room temperature. The response and recovery times were increased at an elevated gas-sensing temperature of 150° (74/88 s) and bending angle of 97.45° (85/87 s). Overall, the integration of DAAQ-COF with Ti₃C₂ MXene provided an effective strategy for developing flexible, highly sensitive H₂ sensors, capable of detecting H₂ at ppm-level concentrations while maintaining good operational stability under thermal and mechanical stress.

3.4. Ion-Beam Irradiated Ti₃C₂ MXene

Previously, ion-beam irradiation has been demonstrated as an effective approach for modifying the physical properties of semiconductor thin films, such as Al and Cu-doped ZnO systems [28,29]. Following this strategy, low-energy N⁺ ion-beam irradiation with fluences of 1 × 10¹⁵, 5 × 10¹⁵, and 1 × 10¹⁶ ions cm⁻² was employed to modify Ti₃C₂ MXene nanosheets to achieve room-temperature H₂ gas sensing [30]. The ion-beam treatment induced subtle structural modifications in the Ti₃C₂ MXene nanosheets, including reduced crystallite size, improved nanosheet ordering, and increased interlayer d-spacing. The surface morphology of Ti₃C₂ MXene irradiated with 1 × 10¹⁶ ions cm⁻² is shown in Figure 3a. Among the different irradiation conditions, the Ti₃C₂ MXene treated at ion fluence of 1 × 10¹⁶ ions cm⁻² exhibited higher H₂ sensing response of 2.1 toward 3000 ppm H₂, with response and recovery times of 98 and 109 s, respectively (Figure 3b). In comparison, pure Ti₃C₂ MXene displayed a lower response of 1.37 and significantly longer response and recovery times of 170 and 244 s, respectively. Furthermore, the ion-beam-modified Ti₃C₂ MXene demonstrated excellent cyclic stability over 10 sensing cycles and retained 94% of its initial response after 10 days (Figure 3c,d). After the interaction of Ti₃C₂ MXene with H₂, the electrons effectively transferred from H₂ to the Ti₃C₂ MXene, which increased the electron density in the sensor. Overall, ion-beam-induced structural engineering effectively optimized the surface and electronic properties of Ti₃C₂ MXene, resulting in improved room-temperature H₂-sensing performance with enhanced stability.

Figure 3.

(a) Surface morphology of Ti₃C₂ MXene nanosheets irradiated with an N⁺ ion-beam at a fluence of 1 × 10¹⁶ ions cm⁻², (b) response and recovery characteristics of Ti₃C₂ MXene irradiated under different N⁺ ion-beam energies. (c) room temperature cyclic stability over 10 sensing cycles for Ti₃C₂ MXene irradiated at a fluence of 1 × 10¹⁶ ions cm⁻², (d) remarkable long-term stability over 1 to 10 days for Ti₃C₂ MXene irradiated at a fluence of 1 × 10¹⁶ ions cm⁻² (Reprinted from Ref. [30], copyright with permission from Elsevier).

Overall, Ti₃C₂ MXene-based sensors have exhibited enhanced H₂ gas-sensing performance through structural modification and hybridization. Integration with Pd nanoparticles, Pd-Pt alloys, MXene-derived sodium titanate nanoribbons, COF frameworks, and ion-beam treatment features significantly improved their sensitivity, response/recovery times, stability, and LOD. Specifically, Pd facilitated H₂ dissociation and electron transfer, while engineered interfaces enhanced charge transportation and adsorption. Flexible Ti₃C₂/Pd composites detected 4% H₂ with stable operation under bending conditions. Interestingly, bi-metallic Pd-Pt/Ti₃C₂ composite achieved 1 ppm LOD, and DAAQ-COF/Ti₃C₂ also sensed 1 ppm H₂ at room temperature. On the other hand, Ti₃C₂ MXene-derived C/TiO₂ and ion-beam-modified MXene further demonstrated improvement in H₂-sensing performance and durability. The progress in Ti₃C₂T_x MXene-based composites for room temperature H₂ gas sensing is presented in Table 1.

Table 1.

Room-temperature H₂ gas-sensing ability of Ti₃C₂ MXene-based nanostructures.

Composite	Working Temperature	Response/ Recovery Time (s)	Response (%)	Gas Concentration /LOD (ppm)	Gas Sensing Equation	Stability (Cycles)/Long-Term Stability (Days)	Reference
Ti3C2-MXene based composites for H2 sensing
Ti3C2/Pd nanostructures
Ti3C2@Pd CNC	Room temperature	32 ± 7/161 ± 23 s	23.0 ± 4.0 (Sensitivity)	4% H2	(Ig − I0)/I0 × 100	8/—	[21]
Pd-Ti3C2Tx	Room temperature	—	56	100	|Rg − Ra|/Ra × 100	5/90 at 50 ppm	[22]
Pd-NTO NRs	Room temperature	1.1/13.5	12	1% H2/0.8	(Igas − Iair)/Iair	11 at 5000 ppm/8	[23]
Pd=Pt/Ti3C2Tx	Room temperature	1/—	24.6	200/1	|Ra − Rg|/Ra × 100	10/60 at 200 ppm	[24]
Ti3C2 MXene derived C/TiO2
Ti3C2 MXene derived C/TiO2	Room temperature	13/42	62.2	40,000/50	|Ra − Rg|/Ra × 100	—/30 at 300 °C	[26]
Ti3C2 MXene/organic frameworks
DAAQ-COF/with Ti3C2	Room temperature	29/18	—	1 ppm/—	—	5/25	[27]
Ion beam irradiated Ti3C2 MXene
N+ ion-beam induced Ti3C2 MXene	Room temperature	98/109	2.1	3000	Ro/Rg	10/10	[30]

4. Ti₃C₂ MXene-Based Composites for NH₃ Gas Sensing

In addition to H₂ gas sensing, Ti₃C₂-based composites have been used for NH₃ gas sensing at room temperature. Specifically, ultrafast response and recovery processes were achieved in the developed composites. The dominant performance of pure Ti₃C₂ and Ti₃C₂-based composites is described below.

4.1. Pure Ti₃C₂ MXene

The room-temperature NH₃ gas-sensing performance of Ti₃C₂T_x MXene films with thickness ranging from 2.5 (0.01 mg·mL⁻¹) to 15 nm (2.56 mg·mL⁻¹) was investigated [31]. The authors reported that thinner Ti₃C₂T_x MXene films (2.5 nm) exhibited higher sensitivity toward NH₃ molecules than thicker films. The 2.5 nm Ti₃C₂T_x MXene films displayed semiconducting behavior due to the presence of surface termination groups, which induced dipole polarization. Thus, a remarkable difference in NH₃ sensing performance between the thinner and thicker films was observed. Notably, the 2.5 nm film exhibited a response of 1.22% toward 1 ppm NH₃, whereas the thicker films showed no detectable response at this concentration. Furthermore, the thinnest film achieved a maximum response of 9.4% at 100 ppm, with excellent cyclic stability over five sensing cycles. As the thickness increased from 2.5 to 15 nm, the NH₃ gas sensing rapidly decreased from 9.4 to 0.35%, respectively. These findings demonstrate that precise control of Ti₃C₂T_x MXene film thickness plays a crucial role in enhancing room-temperature NH₃ gas-sensing performance.

Interestingly, the strategic combination of MXene and its parent MAX phase (Ti₃C₂T_x/Ti₃AlC₂) enabled efficient room-temperature NH₃ gas sensing [32]. The lower work function of Ti₃AlC₂ compared to Ti₃C₂T_x/Ti₃AlC₂ composite confirmed the formation of a planar heterojunction formation at the Ti₃C₂T_x/Ti₃AlC₂ interface. Furthermore, the Ti₃C₂T_x MXene surface contained higher concentrations of –OH and –O termination groups than –F groups, which provided abundant active sites for NH₃ adsorption. According to the O1 s XPS spectrum, the binding energies of adsorbed oxygen species, Ti–O–Ti, Ti–OH, and C–OH were located at 529.51, 530.22, 531.95, and 532.32 eV, respectively. Upon exposure to NH₃, these oxygen-containing functional groups facilitated electron transfer to the Ti₃C₂T_x/Ti₃AlC₂ composite, resulting in an increase in sensor resistance. Moreover, the formation of a Schottky barrier at the heterointerface promoted efficient charge carrier separation and enhanced interaction between the sensing surface and NH₃ molecules, which improved sensing performance. In this context, the Ti₃C₂T_x/Ti₃AlC₂ composite exhibited a response of 1.2% at a LOD of 50 ppb. At 200 ppm, the sensor achieved a high response of 11.0% along with rapid response and recovery times of 12 and 60 s, respectively. Furthermore, the responses towards CO₂ and ethanol were lower at about 1.5 and 2.2% compared to NH₃ (8.3%) at 100 ppm, which confirmed the high selectivity of the Ti₃C₂T_x/Ti₃AlC₂ heterostructure towards NH₃. In addition, the synergistic interaction between Ti₃C₂T_x and Ti₃AlC₂ contributed to excellent long-term stability with negligible degradation after 120 days. Furthermore, DFT calculations demonstrated that the –OH-terminated Ti₃C₂(OH)₂/Ti₃AlC₂ heterostructure had higher NH₃ adsorption capability compared with pristine Ti₃C₂O₂ and Ti₃C₂F₂, owing to stronger binding interactions. Among these, Ti₃C₂(OH)₂/Ti₃AlC₂ exhibited higher NH₃ adsorption energy (−0.950 eV) than Ti₃C₂O₂ (−0.705 eV) and Ti₃C₂F₂ (−0.467 eV), which indicated superior adsorption performance and potential for improving NH₃ gas-sensing performance. Overall, the combined effects of (i) Ti₃C₂T_x/Ti₃AlC₂ heterostructure, (ii) abundant –OH and –O surface termination groups, and (iii) Schottky barrier formation enhanced the room-temperature NH₃-sensing performance.

Recently, Mirzaei et al. [33] explored the NH₃-sensing performance of pure Ti₃C₂T_x MXene (24 h etched) under self-heating conditions between 1 to 6 V applied voltages. With regard to NH₃, acetone, methanol, ethanol, benzene, toluene, and H₂S sensing, Ti₃C₂T_x MXene was highly selective towards 100 ppm NH₃, with a response of 0.67% at 25 °C and 4 V. Partial oxidation of Ti₃C₂T_x MXene into TiO₂ revealed –O surface termination groups. According to the XPS O 1 s spectra, lattice oxygen from Ti–O and adsorbed –OH species at 530.1 and 532.2 eV enhanced the NH₃ adsorption. The presence of –OH, –O, and –F accelerated the NH₃ adsorption and electron transfer and improved NH₃ selectivity. The response to higher humidity conditions (30 to 90%) was not much altered compared with 0% (dry air) conditions. In detail, the response at 90% relative humidity was 0.502% (22% reduction) compared with 0% humidity (0.67%) at 100 ppm NH₃, demonstrating tolerance under harsh humid conditions. The response was unaltered at 0.613%, with no significant drift even after six cycles. Also, the sensor was sustained through 500 bending and 1000 tilting cycles, with no structural changes after mechanical testing.

4.2. Metal-Ions Intercalated Ti₃C₂ MXene

Kim et al. [34] strategically intercalated the K⁺, Na⁺, Ca²⁺, and Mg²⁺ cations between Ti₃C₂T_x MXene nanosheets for effective room-temperature NH₃ gas sensing from 10 to 100 ppm. The Na⁺ ions-intercalated Ti₃C₂T_x MXene showed a better response of about 0.36% at 100 ppm compared with the other metal cation counterparts. A signal-to-noise ratio of 531 was achieved for Na⁺-Ti₃C₂T_x MXene, which was higher than K⁺-Ti₃C₂T_x (192), Mg²⁺-Ti₃C₂T_x (333), and Ca²⁺-Ti₃C₂T_x (371) MXenes. The intercalation process facilitated preparation of transparent and highly conductive films for attaining stable electrical pathways during the gas-sensing process. This strategy did not lead to oxidation and TiO₂ formation, thereby preserving the pure chemical structure of Ti₃C₂T_x MXene. Overall, metal ion intercalation increased the gas-sensing performance of Ti₃C₂T_x MXene.

4.3. Alkalized Ti₃C₂ MXene

The electrical properties of Ti₃C₂T_x MXene can be effectively altered by alkalization [35]. In this context, alkalization of Ti₃C₂T_x MXene has been explored to enhance room-temperature NH₃ gas-sensing performance over a concentration range of 5 to 500 ppm [36]. In one study, Ti₃C₂T_x MXene was successfully alkalized using 2/5/8M NaOH solutions for 12 h, which significantly improved its rapid response/recovery characteristics. The alkalization process promoted the intercalation of Na⁺ ions between the Ti₃C₂T_x MXene nanosheets, facilitating the replacement of –F termination groups with more hydrophilic –OH terminations, which produced additional active sites for NH₃ adsorption. When exposed to 200 ppm NH₃, the alkalized Ti₃C₂T_x MXene exhibited faster response/recovery times (73/38 s) compared to pure Ti₃C₂T_x MXene (83/84 s) at 40% relative humidity and room temperature. The alkalized samples demonstrated excellent cyclic stability over five response/recovery cycles and long-term stability for 17 days, but pristine Ti₃C₂T_x MXene showed a gradual decline in sensing performance. As a result, the alkalized Ti₃C₂T_x MXene achieved a high response of 100.3% at 500 ppm with LOD of 5 ppm. The enhanced sensing mechanism was attributed to the interaction of NH₃ molecules with –OH and –O terminations along with H₂O-assisted electron donation towards the Ti₃C₂T_x MXene, contributing to an increase in sensor resistance. Overall, the strategic alkalization approach effectively improved the room-temperature NH₃-sensing capability of Ti₃C₂T_x MXene.

Following a similar strategy, Zhang et al. [37] modified the Ti₃C₂T_x MXene surface through KOH alkalization followed by annealing under nitrogen atmosphere to enhance NH₃ gas-sensing performance. These combined effects increased the surface –OH and –O termination groups and expanded the interlayer spacing from 9.1 to 12.1 A°, which effectively facilitated the NH₃ adsorption and diffusion within the Ti₃C₂T_x MXene nanosheets. After the annealing process, the characteristic layered morphology of Ti₃C₂T_x MXene was preserved, accompanied by improved crystallinity and grain growth. As a result, the annealed Ti₃C₂T_x MXene exhibited a superior room-temperature NH₃ response of 35.7 at 200 ppm, which was higher than that of alkalized (30.1) or pure Ti₃C₂T_x MXene (18.7) with response times of 170, 300, and 200 s, respectively. Initially, alkalization improved the interlayer spacing and generated abundant reactive sites, which contributed to improved NH₃ selectivity and adsorption. The heat-treated samples achieved superior NH₃ gas sensing compared with the alkalized Ti₃C₂T_x MXene. For annealed samples, linear increment in the O/F ratio and interlayer spacing advanced the NH₃ gas-sensing performance even after 50 days, with a response of 36.11 at 5 ppm. These findings further demonstrate the significant role of alkalization and post-treatment strategies in advancing the room-temperature NH₃ gas-sensing performance of Ti₃C₂T_x MXene.

4.4. Partially Oxidized Ti₃C₂ MXene

Controlled oxidation of Ti₃C₂T_x MXene (at 280 °C) significantly enhanced room-temperature NH₃ gas-sensing performance [38]. Specifically, the improved sensing behavior was attributed to (i) enhancement of hydrogen bonding between NH₃ molecules and surface termination groups (–OH and –O) and (ii) synergistic interaction between Ti₃C₂T_x MXene and in situ formed TiO₂ (p-n junction), which promoted efficient charge transport behavior during NH₃ sensing. The abundant surface termination groups provided active adsorption sites for NH₃ molecules, while the preserved layered structure in Ti₃C₂T_x MXene facilitated rapid electron transport. The XPS O 1s spectra revealed adsorbed oxygen, –O, and –OH termination groups located at 530.3, 532.1, and 534 eV, respectively. Notably, the oxidation process did not disrupt the intrinsic layered morphology of the Ti₃C₂T_x MXene, thereby retaining its excellent electrical conductivity. As a result, the oxidized Ti₃C₂T_x MXene exhibited a high response of 21.3% at 10 ppm NH₃. At a higher concentration of 500 ppm, the sensor achieved further improvement, with a response of 147% and response/recovery times of 67/157 s. The sensing mechanism involved the reaction of NH₃ molecules with adsorbed O₂⁻ (in air) on the sensor surface, which released electrons towards the material, reduced the width of the electron depletion layer, and decreased the sensor resistance. The formation of TiO₂ (i) increased the charge transfer through the p-n heterojunction, (ii) provided abundant oxygen-related defects to increase NH₃ adsorption, and (iii) created active sites for the stabilization of the Ti₃C₂T_x MXene for higher NH₃-sensing capability. The oxidized Ti₃C₂T_x MXene showed a negligible response (2%) towards ethanol at 500 ppm. Responses of 138, 71, 56, 47, and 41% (at 500 ppm) were observed at various relative humidity conditions of 28, 42, 50, 62, and 70%, respectively. It should be noted that the response level was relatively stable under higher humid conditions (62 to 70%). Overall, the controlled oxidation process effectively optimized the surface chemistry and interfacial charge transportation of the Ti₃C₂T_x MXene, which enabled highly sensitive room-temperature NH₃ detection with LOD of 10 ppm.

4.5. Ti₃C₂ MXene/Sulfur

Shen et al. [39] demonstrated ultra-sensitive NH₃ gas sensing at the ppt level (3, 5, 7, 10, and 15) by integrating Ti₃C₂ MXene with sulfur nanosheets. The interlayer spacing of Ti₃C₂ MXene nanosheets (2–3 μm) was effectively covered by smaller sulfur nanosheets with sizes ranging from 200 to 300 nm. With an increase in NH₃ concentration from 3 to 15 ppt, the sensing response exhibited linear enhancement accompanied by rapid adsorption and desorption kinetics. The integration of sulfur nanosheets with Ti₃C₂ MXene nanosheets is shown in Figure 4a. Notably, the response/recovery times of Ti₃C₂ MXene@sulfur nanosheets (5/4 s) were significantly lower than those of pure sulfur nanosheets (60/18 s). The superior electrical conductivity of Ti₃C₂ MXene compared with sulfur nanosheets improved the charge transfer and gas-sensing performance at the ppt level. Thus, the Ti₃C₂ MXene@sulfur composite exhibited remarkable NH₃ sensing by achieving a response current of about 15 pA (sulfur nanosheets-3 pA) with ultra-rapid response and recovery times of 5 and 4 s, respectively (Figure 4b). Under these conditions, a sensing response of 375% was achieved at NH₃ concentration of 3 ppt. This structural integration again proved cyclic stability (across five cycles) and long-term stability over 20 days at 3 ppt level. The composite architecture also maintained a stable NH₃ response under various relative humidity conditions (20–70%), as shown in Figure 4c. In that study, water molecules did not interfere with NH₃ adsorption sites under relative humidity conditions between 20 to 70%. At higher conditions (>80%), excessive H₂O molecules compete with NH₃, thereby reducing the availability of adsorption sites. In this system, NH₃ molecules acted as electron donors, which facilitated charge transfer at the Ti₃C₂ MXene@sulfur interface and resulted in improved sensing performance. The enhanced NH₃ sensing mechanism was mainly attributed to the synergistic effects of sulfur vacancies, high electrical conductivity of Ti₃C₂ MXene, and surface termination groups, which promoted efficient adsorption and desorption of NH₃ molecules even under highly humid conditions.

Figure 4.

(a) Surface morphology of sulfur nanosheets integrated Ti₃C₂ MXene, (b) ultrafast response and recovery times of about 5 and 4 s, (c) stable NH₃ gas sensing under relative humidity ranging from 20 to 70% (Reprinted from Ref. [39], copyright with permission from Elsevier), (d) formation of Ti₃C₂T_x/graphene flexible gel fiber, (e) unaltered NH₃ gas-sensing performance of Ti₃C₂T_x/graphene composite under straight and bent conditions, (f) flexible Ti₃C₂T_x MXene/graphene hybrid fibers woven into a laboratory coat (Reprinted from Ref. [40], copyright with permission from ACS Publication).

4.6. Ti₃C₂ MXene/Graphene

The integration of two carbon-based materials (Ti₃C₂T_x MXene and graphene) into a wearable and flexible fiber architecture demonstrated efficient room-temperature NH₃ gas sensing [40]. Figure 4d shows a photograph of the Ti₃C₂T_x/rGO flexible gel fiber. The synergistic interaction between Ti₃C₂T_x MXene and rGO sheets effectively suppressed the severe aggregation of Ti₃C₂T_x MXene nanosheets at the molecular level under pH 6 conditions, which resulted in a well-defined hybrid structure. At selective content of about 40% Ti₃C₂T_x MXene, significant enhancement in the number of active adsorption sites for NH₃ molecules was achieved. In particular, the density of Ti–O active sites in Ti₃C₂T_x MXene increased three-fold after strategic hybridization with graphene. Accordingly, the resultant composite facilitated stronger NH₃ adsorption and improving the sensing performance. Thus, the dominance of Ti₃C₂T_x MXene and Ti–O active sites achieved a high response of 7.21% toward 100 ppm NH₃, as shown in Figure 4e. The selectivity towards ethanol was very low at about 1%, which suggests preferential adsorption of NH₃ molecules by the Ti₃C₂T_x MXene//rGO hybrid fibers. In addition, the sensor achieved LOD of 10 ppm with response of 4.26%. The mechanically robust Ti₃C₂T_x MXene//rGO hybrid fibers maintained consistent gas-sensing performance under repeated bending and releasing cycles. The practical demonstration of wearable Ti₃C₂T_x/graphene hybrid fiber integrated into a laboratory coat is presented in Figure 4f. Overall, the synergistic combination of Ti₃C₂T_x MXene and graphene in a flexible fiber architecture provided an effective strategy for the advancement in next-generation wearable NH₃ gas sensors operating at room temperature.

Li et al. [41] significantly improved the room-temperature NH₃ gas-sensing performance of Ti₃C₂T_x/graphene composite compared to pure Ti₃C₂T_x and graphene. The authors attributed the enhanced sensing characteristics to the abundant surface termination groups on the Ti₃C₂T_x/graphene surface, which promoted the adsorption of NH₃ molecules. In particular, the –O and –OH termination groups effectively interacted with adsorbed NH₃ molecules, which released three electrons and one electron, respectively. The surface morphology of the Ti₃C₂T_x/graphene composite film is shown in Figure 5a. As a p-type sensing material, the Ti₃C₂T_x/graphene composite accepted electrons from NH₃ molecules, which led to increase in sensor resistance. The sensing response was investigated over an NH₃ concentration range of 0.5 to 100 ppm, exhibiting a linear increment from 2.0 to 25.5%, respectively, as shown in Figure 5b. The response towards NO₂ (5.4%) and H₂ (3.7%) was lower compared to NH₃, which confirmed the developed composite’s high selectivity towards NH₃. At 100 ppm NH₃, the composite exhibited response and recovery times of 26 and 148 s, respectively. Furthermore, the Ti₃C₂T_x/graphene sensor demonstrated excellent repeatability over three consecutive sensing cycles, with a stable response of 25% and outstanding long-term stability, maintaining an average response of 24.9% after 16 days under 80% relative humidity (Figure 5c). The hydrophilic behavior favored stable NH₃ sensing at higher humidity conditions. These results successfully demonstrated that the synergistic integration between p-type Ti₃C₂T_x MXene and graphene substantially enhanced the room-temperature NH₃-sensing performance under highly humid environmental conditions.

4.7. Ti₃C₂ MXene/Semiconductors

Furthermore, He et al. [42] enhanced the room-temperature NH₃ gas-sensing performance by decorating Ti₃C₂T_x MXene nanosheets with SnO₂ nanoparticles. The incorporation of SnO₂ nanoparticles increased the amount of surface-adsorbed oxygen species, which created additional active sites for NH₃ adsorption and facilitated improvement in gas sensing performance. Interestingly, the Ti₃C₂T_x/SnO₂ composite exhibited the highest sensing performance at room temperature, whereas the sensor showed almost no response at 100 °C. A linear decrease in sensor response was observed as the operating temperature increased from 25 to 100 °C. This deterioration in sensing performance at elevated temperatures was attributed to the gradual reduction in hydroxyl (–OH) surface terminations. The O 1s peak showed integrated lattice oxygen (530.1 eV) and adsorbed oxygen (532 eV). The improvement in surface adsorbed oxygen created more active sites, increasing sensing performance. Accordingly, the Ti₃C₂T_x/SnO₂ composite achieved an excellent response of 40% toward 50 ppm at room temperature under relative humidity of 46%, with response and recovery times of 36 and 44 s, respectively. The introduction of SnO₂ increased NH₃ exposure towards the interfacial regions, density of reactive oxygen species on the surface, and baseline stability. Furthermore, the response towards ethanol was very low (3.12%), which confirmed high selectivity towards NH₃. The sensor also exhibited an ultralow LOD of 4.29 ppb, demonstrating its capability for highly sensitive NH₃ detection. In addition, the composite maintained stable sensing performance for 60 days at NH₃ concentration of 50 ppm, indicating excellent long-term stability. The electrons transfer from NH₃ to Ti₃C₂T_x increased the sensor resistance and NH₃ sensing capacity. As the relative humidity increased from 25 to 85%, the response was higher at moderate conditions (45% humidity) under increased NH₃ adsorption and reaction pathways. Overall, the synergistic interaction between Ti₃C₂T_x and SnO₂ nanoparticles significantly enhanced NH₃ adsorption, which enabled superior room-temperature sensing performance, high tolerance of humidity, and excellent long-term operational stability.

Yao et al. [43] synthesized Ti₃C₂T_x MXene/SnO nanocomposite by integrating ultrathin SnO nanoflakes with Ti₃C₂T_x MXene through a one-step hydrothermal method, for room-temperature NH₃ vapor response over a wide range of 1 to 1000 ppm concentrations. The formation of p-n heterojunctions improved the charge carrier concentration, which led to superior NH₃-sensing performance compared to individual p-type or n-type components. Moreover, the matrix effectively suppressed the oxidation of SnO into SnO₂, preserving the structural and chemical stability of the composite. Furthermore, the conductive Ti₃C₂T_x skeleton increased the surface area of the composite (14.27 m²/g), which generated abundant oxygen adsorption sites and active reaction centers for NH₃ molecules. In addition to lattice oxygen (530.3 eV), formation of surface adsorbed oxygen species at 531.8 eV was noticed. Here, the adsorbed oxygen species mainly contributed to the improved gas-sensing performance. In addition, the ultrathin SnO nanoflakes with a thickness of 9 nm facilitated rapid electron transportation and accelerated the sensing kinetics. The synergistic interaction between Ti₃C₂T_x MXene and SnO effectively modulated the electrical conductivity of the p-n heterojunctions, which resulted enhanced response and recovery characteristics. Furthermore, SnO greatly contributed to increased surface reactions through adsorbed oxygen, charge transfer through heterostructure, and abundant surface-active sites to achieve long-term sensing stability. The nanocomposite exhibited a response of 7.8 for 200 ppm (35.49 for 1000 ppm), with response and recovery times of 61 and 119 s, respectively. The response of about 1.59 towards ethanol was very low compared to NH₃, which confirmed high selectivity towards NH₃. Furthermore, the sensor achieved LOD of 1 ppm and demonstrated excellent repeatability over four sensing cycles and stable long-term performance for 22 days. The interaction of NH₃ with adsorbed oxygen transferred the electrons towards the Ti₃C₂T_x MXene/SnO, improving the NH₃ sensing performance. According to the gas-sensing signal direction, SnO behaved like an n-type semiconductor, which created p-n junction at the interface of p-type Ti₃C₂T_x MXene. Overall, the Ti₃C₂T_x MXene/SnO heterostructure exhibited a broad NH₃-detection range (1–1000 ppm), high sensitivity, rapid sensing kinetics, and excellent long-term stability for advancing the room-temperature NH₃ sensing applications.

Figure 5.

(a) Surface morphology of the Ti₃C₂T_x/graphene composite film, (b) NH₃ gas response over a concentration range of 10–100 ppm, (c) remarkable long-term stability of the In₂O₃/Ti₃C₂T_x MXene composite with a stable response over 16 days at 80% relative humidity, (Reprinted from Ref. [41], copyright with permission from Elsevier), (d) TEM image of In₂O₃ microtubes interfaced with Ti₃C₂T_x MXene, (e) ultrafast response and recovery times of 3 s and 2 s for the In₂O₃/Ti₃C₂T_x MXene composite, (f) excellent cyclic and long-term stability of the In₂O₃/Ti₃C₂T_x MXene composite, (Reprinted from Ref. [44], copyright with permission from Elsevier).

Achieving ultrafast response and recovery is crucial for realizing high-performance gas sensors with rapid adsorption and desorption kinetics. In this context, Liu et al. [44] developed a metal–organic frameworks (MOFs)-derived In₂O₃ microtube (2–5 μm)/Ti₃C₂T_x MXene heterostructure for room-temperature NH₃ gas sensing. The TEM image of the MOFs-derived In₂O₃ microtubes/Ti₃C₂T_x MXene heterostructure is shown in Figure 5d. The composite exhibited a highly mesoporous architecture with a large specific surface area of 446.63 m²/g and pore size distribution of 5–10 nm, which provided abundant active sites for NH₃ adsorption. The O 1s spectrum revealed lattice oxygen, oxygen defects, and chemisorbed oxygen at 528.9, 530.5, and 532.0 eV, respectively. These features created more gas-sensing capability. As shown in Figure 5e, the heterostructure achieved ultrafast response and recovery times of 3 and 2 s toward 5 ppm NH₃ at room temperature, whereas the corresponding values increased to 28 and 65 s at 200 °C. The significantly shortened response time at room temperature indicated the enhancement in NH₃ adsorption kinetics and efficient electron transfer within the heterostructure. However, at 100 °C, the response/recovery time slightly increased to 5/3 s due to the partial oxidation of the Ti₃C₂T_x MXene, which reduced the electrical conductivity of the composite. The NH₃ molecules interacted with adsorbed oxygen and released electrons to the In₂O₃/Ti₃C₂T_x heterostructure, reducing the sensor resistance. Introduced In₂O₃ behaved like an active sensing component for the detection of NH₃ through surface adsorption. Due to its mesoporous architecture, In₂O₃ provided numerous active sites at the interface of Ti₃C₂T_x MXene, leading to enhancement in charge transfer and sensor response at room temperature. Thus, the MOFs-derived In₂O₃/Ti₃C₂T_x MXene composite exhibited outstanding room-temperature NH₃ gas sensing of 60.6% with ultra-fast response and recovery characteristics (Figure 5f). The response towards NH₃ was significantly higher than ethanol, CO, and NO₂, which suggests high selectivity towards NH₃. Furthermore, the sensor retained a stable NH₃ response up to relative humidity of 60%, demonstrating excellent humidity tolerance. At higher relative humidity conditions (>60%), decreases in response were noticed due to the interference of water molecules. The work function of p-type Ti₃C₂T_x MXene (4.28 eV) was higher than n-type In₂O₃ (4.28 eV). Thus, electrons transferred from In₂O₃ to the surface functional groups modulated the p-type Ti₃C₂T_x MXene. Overall, the formation of p-n heterojunctions between In₂O₃ and Ti₃C₂T_x MXene significantly enhanced charge transfer and gas-adsorption features for exceptional room-temperature NH₃-sensing performance.

The interaction of NiO with a Ni₂P nanosheet heterostructure enhanced H₂S gas sensing [45]. In this context, Yang et al. [46] highlighted the incorporation of NiO nanoparticles into Ti₃C₂T_x MXene (NiO/Ti₃C₂T_x) for room-temperature NH₃ gas sensing. The NiO/Ti₃C₂T_x heterostructure exhibited faster recovery than response over an NH₃ concentration range of 10–50 ppm. More importantly, the sensing response of NiO/Ti₃C₂T_x composite was 1.18 and 1.38 times higher than those of pristine Ti₃C₂T_x MXene and NiO, respectively. This enhanced sensing performance was attributed to the increased density of active adsorption sites and improved electron transport across the NiO/Ti₃C₂T_x heterojunction. Furthermore, pristine Ti₃C₂T_x MXene exhibited a higher response than pure NiO owing to its abundant surface termination groups (–O, –OH, and –F), which provided favorable adsorption sites for NH₃ molecules. According to the O 1s spectra, Ni-O and –OH groups on the NiO corresponded to lattice oxygen and adsorbed oxygen, respectively. The effectively adsorbed NH₃ molecules interacted with adsorbed oxygen and transferred electrons towards the sensor, increasing the sensor resistance. Specifically, NiO increased the NH₃ adsorbed oxygen species, surface area, and Schottky heterojunction, increasing the adsorption sites for room-temperature NH₃ gas sensing. Upon forming the heterostructure, the NiO/Ti₃C₂T_x composite achieved a response of 6.13% toward 50 ppm NH₃ with rapid response and recovery times of 60 and 19 s, respectively. Responses towards ethanol and IPA were observed to be 2.26 and 0.53%, respectively, which suggests high selectivity towards NH₃ (6.13%). The sensor was also capable of detecting NH₃ concentrations as low as 10 ppm and demonstrated excellent repeatability over 15 sensing cycles at 50 ppm (long-term stability for 15 days). In addition, the NiO nanoparticles increased the specific surface area and number of active sites available for oxygen and NH₃ adsorption. The NH₃ response was highest at 10% humidity, gradually decreasing at 30 to 50%, with a significant reduction between 70 to 90% relative humidity conditions. Combined with the abundant surface termination groups (–O, –OH, and –F) and high charge-carrier mobility in Ti₃C₂T_x MXene, enhanced room-temperature NH₃-sensing performance was achieved in the NiO/Ti₃C₂T_x heterostructure.

In addition to previously reported SnO₂ [42], SnO [43], and NiO [46] semiconductor nanostructures, Pasupuleti et al. [47] demonstrated sub-ppb-level NH₃ detection at room temperature under UV illumination using a ZnO@Ti₃C₂T_x MXene composite. In this composite, ZnO nanoparticles were uniformly anchored onto Ti₃C₂T_x MXene nanosheets, which resulted in a larger specific surface area (47.55 m²/g) than that of pristine ZnO (23.61 m²/g) and Ti₃C₂T_x (13.86 m²/g). The increased surface area provided abundant active sites for NH₃ adsorption, which facilitated both sensitivity and selectivity. Key features such as lattice oxygen, oxygen vacancy, and chemisorbed oxygen were observed in the O 1s spectrum, and these promoted NH₃ adsorption. Furthermore, the Schottky junction formed at the ZnO/Ti₃C₂T_x interface reduced the electron depletion layer upon NH₃ exposure, which played a crucial role in enhancing the sensing performance. Introduction of ZnO nanoparticles created oxygen vacancies (active adsorption sites), Schottky junctions (charge transfer), and suppressed Ti₃C₂T_x nanosheet restacking (high surface area), increasing the long-term stability of NH₃ gas sensing. Under UV illumination, photogenerated holes promoted the neutralization of adsorbed O₂⁻ ions, promoting the desorption of NH₃ molecules and accelerating the recovery process. As a result, the ZnO@Ti₃C₂T_x exhibited response and recovery times of 92 and 104 s, respectively, with an impressive LOD of 89.41 ppb. At an NH₃ concentration of 20 ppm, the sensor achieved a remarkable response of 39.16%, higher than for CO (6.415) and NO₂ (16.25%) gases. These findings demonstrated that ZnO nanoparticles effectively modulated the interfacial electronic structure of Ti₃C₂T_x MXene for efficient photo-assisted room-temperature NH₃ gas sensing.

In another study, Lu et al. [48] successively fabricated a Ti₃C₂T_x MXene/MoS₂/PPy ternary composite for efficient room-temperature NH₃ gas sensing. In this hybrid structure, PPy provided abundant hydrogen-bonding active sites for enhanced NH₃ adsorption, while MoS₂ nanosheets increased the interlayer spacing of Ti₃C₂T_x MXene nanosheets for effective exposure of additional active sites and gas adsorption. The surface morphology of the Ti₃C₂T_x/MoS₂/PPy is shown in Figure 6a. Owing to the p-type semiconducting behavior of Ti₃C₂T_x, electron donation from NH₃ molecules resulted in increased sensor resistance. The composite exhibited a detectable response of 2.31% at 10 ppm NH₃ and a high response of 21% at 100 ppm with rapid response/recovery times of 43.6/40.8 s at room temperature under 45% relative humidity (Figure 6b). The response decreased from 23 to 4.7% (at 100 ppm) by increasing the relative humidity from 30 to 90%, respectively. Adsorption of water molecules on active sensing sites prevented the NH₃ molecules from reaching the sensor surface. The response towards ethanol and NO₂ was 5.4 and 2.5%, respectively. This suggests that the Ti₃C₂T_x MXene/MoS₂/PPy composite was highly selective towards NH₃ compared with ethanol and NO₂. In addition, the sensor displayed excellent repeatability over five sensing cycles and maintained stable long-term performance for 7 days under the same humidity conditions (Figure 6c). Overall, the synergistic effects of Ti₃C₂T_x MXene, MoS₂ nanosheets, and PPy significantly improved NH₃ sensing performance in harsh humid environments.

In another study, Bi₂O₃ was integrated with Ti₃C₂T_x (Bi₂O₃/Ti₃C₂T_x) for room-temperature NH₃ gas sensing [49]. This composite was highly selective to NH₃ with a response of 61% at 100 ppm, compared with ethanol (10%), methanol (3%), and acetic acid (4%). High surface area, electrical conductivity, interfacial charge transfer, and surface functional groups (–O, –OH, and –F) accelerated the NH₃ adsorption and interaction. Response and recovery times of 61 and 164 s were observed for the Bi₂O₃/Ti₃C₂T_x composite at 100 ppm. Notably, the response linearly increased from 61 to 124% with an increase in relative humidity from 50 to 90%, which suggested high humidity tolerance. Identical response signals were achieved after six cycles and 20 days (five cycles each day), showing long-term stability. Thus, the developed Bi₂O₃/Ti₃C₂T_x composite provided remarkable adsorption/desorption kinetics at the interface with minimal degradation of active sensing sites.

Seekaew et al. [50] developed a Ti₃C₂T_x MXene/GO/CuO/ZnO (9:1:5:5 wt%) heterojunction composite by hydrothermal process, for room-temperature NH₃ gas sensing in the range of 25 to 200 ppm. This combination included p-n junctions. The intimate contact between various semiconductors achieved abundant adsorption sites, charge transfer, and oxygen vacancies, thereby enhancing NH₃ gas sensing. Notably, stable response and complete recovery were achieved with response time and recovery of 26 and 26 s, respectively, at 100 ppm. The LOD was about 4.1 ppm. The presence of –OH and –O groups accelerated the NH₃ adsorption. Thus, an NH₃ response of 96.3% was achieved at 200 ppm (25 ppm-30.7%). The response towards NH₃ was significantly higher than ethanol, methanol, and acetone. No response towards NO₂ was achieved by the selective composite. These results suggest that the surface chemistry favored NH₃ selectivity compared with other volatile organic compounds. At 80 to 90% relative humidity, the sensor showed a highly stable response with slight decrease in resistance. At 200 ppm, this composite achieved stable response and recovery curves after six repeated cycles and long-term stability after 45 days. Overall, the formation of multiple-semiconductor-heterojunction-integrated Ti₃C₂T_x MXene contributed to excellent response and recovery in the detection of NH₃ at room temperature.

Figure 6.

(a) Surface morphology of the Ti₃C₂T_x MXene/MoS₂/PPy composite, (b) response and recovery times of 43.6 and 40.8 s with a response of 21% under 100 ppm NH₃, (c) long-term stability over 7 days at 45% relative humidity (Reprinted from Ref. [48], copyright with permission from Elsevier), (d) surface morphology of Ag nanoparticle-integrated oxygen-rich p-type Ti₃C₂T_x MXene, (e) enhanced response toward 15 ppm NH₃ under 40% relative humidity, and (f) remarkable long-term cyclic stability and NH₃ sensing performance over 30 days (Reprinted from Ref. [51], copyright with permission from Elsevier).

4.8. Ti₃C₂ MXene/Ag and Au Metals

The incorporation of Ag nanoparticles into oxygen-rich p-type Ti₃C₂T_x MXene (AgT) significantly enhanced room-temperature NH₃ gas-sensing performance [51]. The surface morphology of an AgT composite is presented in Figure 6d. During the sensing process, NH₃ gas molecules were initially adsorbed onto the Ag nanoparticles, followed by their diffusion into the Ti₃C₂T_x MXene layers, resulting in an increment in the sensor resistance. Interestingly, the rapid recovery behavior was attributed to the abundant surface termination groups, where the –O terminations exhibited weaker adsorption energy and –OH groups facilitated faster NH₃ desorption. XPS studies showed that lattice oxygen and adsorbed oxygen were observed through Ti–O–Ti formation at 530.0 eV. Furthermore, –O and –OH termination groups were observed through C–Ti–OH formation at 531.9 eV. Accordingly, the AgT composite exhibited enhanced NH₃ gas-sensing performance up to 10 ppm (stable at 15 and 20 ppm). In this process, the AgT sensor achieved responses of 4.35% and 4.96% toward 10 and 15 ppm NH₃, respectively, which outperformed pristine single-layer Ti₃C₂T_x MXene (2.93%) and multilayer Ti₃C₂T_x MXene (4.30%), as shown in Figure 6e. In addition, the AgT composite achieved LOD of 0.5 ppm. At a relative humidity of 40%, the AgT sensor exhibited response and recovery times of 511 and 605 s, respectively, compared to 319/578 s for single-layer and 446/659 s for multilayer Ti₃C₂T_x MXene. Moreover, the synergistic effects of the multilayer architecture, Ag nanoparticles, and abundant surface termination groups enabled excellent long-term stability over 30 days, even under 40% relative humidity (Figure 6f). DFT studies highlighted the superior NH₃ adsorption of Ag-Ti₃C₂O₂ (–O termination groups), with adsorption energy of −0.445 eV (dry) and −0.490 eV (pre-adsorbed H₂O) at adsorption distances of 2.494 and 2.269 Å, respectively. Pre-adsorbed H₂O increased the charge transfer to 0.08 e, which confirmed enhanced NH₃ adsorption and sensing performance compared with pristine Ti₃C₂O₂. Overall, defect engineering combined with surface functionalization through Ag nanoparticles effectively improved the room-temperature NH₃-sensing performance of Ti₃C₂T_x MXene.

Interestingly, the selective interaction of noble Au (1.92 at%) and Pt (0.83%) metallic nanoparticles with Ti₃C₂T_x MXene nanosheets for NH₃ gas-sensing applications was investigated [52]. The sensor thickness was maintained within the range of 10 to 20 μm. Among the two-metal nanoparticle-decorated composites, Au-modified Ti₃C₂T_x MXene exhibited superior NH₃ response and selectivity compared to Pt-modified Ti₃C₂T_x MXene. The surface morphology of the Au nanoparticle-interfaced Ti₃C₂T_x MXene nanosheets is presented in Figure 7a. Ohmic contact between sensors and electrodes with negligible polarization did not influence the sensing signal. Accordingly, Au (1.92 at%) and Pt (0.83%)-decorated Ti₃C₂T_x MXene sensors demonstrated room-temperature NH₃ responses (100 ppm) of 16 and 9%, respectively. The response/recovery times for Au- and Pt-decorated sensors were recorded as 87/217 s and 77/248 s, respectively, over six consecutive sensing cycles. Furthermore, the flexibility of the sensors was evaluated under 0 to 120° bending conditions. Remarkably, after bending at 120°, the flexible NH₃ gas sensors retained responses of 9.9% and 6% for Au (1.92 at%) and Pt (0.83 at%) nanoparticle-decorated Ti₃C₂T_x MXene, respectively (Figure 7b). These composite sensors also demonstrated stable NH₃-sensing performance under highly humid conditions. As an electron-donating gas, NH₃ molecules effectively transferred electrons towards the sensor, which altered the sensor resistance. The proposed charge transfer mechanism and interaction between NH₃ molecules and the sensor surface is illustrated in Figure 7c. Overall, the incorporation of noble metal nanoparticles into Ti₃C₂T_x MXene nanosheets enhanced the sensing characteristics and enabled flexible, room-temperature NH₃ gas detection.

4.9. Ti₃C₂ MXene/Polymers

Qiu et al. [53] enhanced room-temperature NH₃ gas sensing by developing an organic–inorganic p-n heterojunction based on PEDOT:PSS/nitrogen-doped Ti₃C₂T_x MXene composite film. A selective mass ratio of 1:0.5 (N-MXene:PEDOT:PSS) maintained stable NH₃ gas-sensing performance owing to the dominant electron extraction by the PEDOT matrix compared to N-MXene. The incorporated N-atoms acted as electron donors, which increased the density of active adsorption sites within the PEDOT:PSS matrix. Furthermore, the formation of TiO₂ nanoparticles between the N-MXene layers enlarged the interlayer space and suppressed the retacking of MXene nanosheets, which facilitated efficient NH₃ diffusion and improved response and recovery characteristics. Consequently, the well-defined layered architecture promoted rapid gas transportation, resulting in response and recovery times of 280 and 393.6 s, respectively. Under these optimized conditions, the PEDOT:PSS/N-MXene composite exhibited a response of about 13% toward 10 ppm at 20 °C and 36%RH. The NO₂ and CO response was negligible compared to NH₃, which suggests PEDOT:PSS/N-MXene’s high selectivity towards NH₃ gas. In addition, the sensor demonstrated excellent repeatability over four sensing cycles and maintained stable performance up to 17 days. These results confirmed that the synergistic integration between PEDOT:PSS and N-doped Ti₃C₂T_x MXene significantly improved the room-temperature NH₃-sensing performance.

Arkoti et al. [54] reported improvement in NH₃ gas-sensing performance by developing Ti₃C₂T_x MXene-based sensors with and without a mixed-matrix membrane (MMM) filter composed of PVDF-ZIF-67. The authors demonstrated that the incorporation of the MMM filter improved the long-term stability of the sensor. In comparison, pure Ti₃C₂T_x MXene dominated the sensor behavior than with the MMM filter. Interestingly, strong hydrogen-bonding interactions between NH₃ gas molecules and –OH termination groups influenced the gas-sensing behavior. The minimum detectable NH₃ concentrations were 9 ppt and 1 ppb for sensors without and with the MMM filter. At 5 ppm NH₃ concentration, the Ti₃C₂T_x MXene sensor exhibited response/recovery times of 37.8/34.5 s and 49.5/51.2 s without and with MMM filter, respectively. Furthermore, at 35% relative humidity, the NH₃-sensing responses of Ti₃C₂T_x MXene with and without the MMM filter were about 10.6% and 11.86%, respectively. Responses towards ethanol and NO₂ were observed to be 2.8 and 1.2% without the MMM filter, confirming high selectivity towards NH₃. The MMM filter facilitated controlled diffusion of NH₃ molecules toward the Ti₃C₂T_x MXene layer, resulting in improved long-term stability over 35 days. Overall, the introduction of the MMM filter effectively reduced interference from water molecules and suppressed the oxidation of Ti₃C₂T_x MXene for enhancing the durability of NH₃ gas-sensing devices.

Room-temperature NH₃ gas sensing using PANI/Ti₃C₂T_x was studied [55]. The formation of –OH and –O functional groups led to effective interaction with NH₃. The constructive interface between conductive Ti₃C₂T_x MXene and semiconducting PANI created a potential Schottky barrier, which enhanced the NH₃ adsorption properties under altered sensor resistance. According to DFT studies, the adsorption energy of NH₃ (−5.351 eV) was lower than CO (0.057), NO₂ (−4.498 eV), and ethanol (0.127 eV), which confirmed the remarkable adsorption capacity of NH₃ on the PANI/Ti₃C₂T_x surface. Under these conditions, the flexible PANI/Ti₃C₂T_x composite achieved a room-temperature NH₃ response of 55.90% at 20 ppm (pure PANI-24.20%). Interestingly, the response increased from 52.32 to 58.54% under relative humidity conditions of 25 and 68%, respectively. This organized flexible PANI/Ti₃C₂T_x composite achieved excellent cyclic repeatability without decaying even after five cycles. Moreover, long-term stability over 21 days was maintained without altering the response. Overall, remarkable NH₃ adsorption capacity in the flexible PANI/Ti₃C₂T_x composite was achieved.

Figure 7.

(a) Surface morphology of Au nanoparticle-decorated Ti₃C₂T_x MXene nanosheets, (b) NH₃ gas response over the concentration range of 10–50 ppm at an Au concentration of 1.92%, (c) charge transfer mechanism and NH₃ gas sensing mechanism of the Au/Ti₃C₂T_x MXene composite, (Reprinted from Ref. [52], copyright with permission from Elsevier), (d) TEM image of Ni(OH)₂ nanoparticles interfaced with Ti₃C₂T_x MXene nanosheets, (e) enhanced gas-sensing response of the Ni(OH)₂ nanoparticles interfaced with Ti₃C₂T_x compared with pure Ti₃C₂T_x MXene, and (f) stable long-term response over 30 days at 50 ppm NH₃ for the Ni(OH)₂ nanoparticle-interfaced Ti₃C₂T_x MXene composite (Reprinted from Ref. [56], copyright with permission from Elsevier).

4.10. Ti₃C₂ MXene/Metal Hydroxides

The incorporation of Ni(OH)₂ nanoparticles onto Ti₃C₂T_x MXene nanosheets significantly enhanced room-temperature NH₃ gas-sensing performance [56]. The introduction of optimized Ni(OH)₂ loading (7.8 wt.%) increased the interlayer spacing of Ti₃C₂T_x MXene nanosheets, which provided additional diffusion pathways and abundant adsorption sites for NH₃ molecules. Consequently, the composite exhibited a high specific surface area of 53.0591 m² g⁻¹, which further promoted gas adsorption. The TEM image of the Ni(OH)₂/Ti₃C₂T_x MXene composite is presented in Figure 7d. During the hybridization process, some of the –OH surface termination groups were replaced through the incorporation of Ni ions (XPS studies), which modified the surface chemistry of the MXene. Owing to the dominant Ti₃C₂T_x MXene content relative to low Ni(OH)₂ loading (7.8 wt.%), electrical resistance of the composite remained primarily governed by the conductive MXene network and enhanced the surface reactivity introduced by Ni(OH)₂. Thus, the Ni(OH)₂/Ti₃C₂T_x composite exhibited a room-temperature NH₃ response of 13.69% at 80 ppm, as shown in Figure 7e. Furthermore, the sensor demonstrated stable response and recovery times of 78 and 108 s, with a response of 10.3% after five consecutive sensing cycles and excellent long-term stability over 30 days at 50 ppm, as shown in Figure 7f. The response value towards 70 ppm NO (2.4%) was lower than towards NH₃ (11.6%), which suggests that the Ni(OH)₂/Ti₃C₂T_x composite was highly selective towards NH₃. Notably, the sensor maintained excellent sensing performance under humid conditions, exhibiting responses of 5.1 and 6.2% toward 10 ppm NH₃ at relative humidities of 40 and 60%, respectively. At higher humidity of 80%, the response decreased to 3.9% due to the excessive water molecules blocking the NH₃ adsorption sites. The adsorbed NH₃ transferred electrons to the Ni(OH)₂ and Ti₃C₂T_x, leading to a decrease in the concentration of holes, which increased the sensor resistance and increased the NH₃ sensing capacity. Overall, the synergistic integration of Ni(OH)₂ nanoparticles with Ti₃C₂T_x MXene effectively enhanced the NH₃ adsorption, gas diffusion, and humidity tolerance.

Similarly, Huang et al. [57] developed a Co(OH)₂/Ti₃C₂T_x nanocomposite for room-temperature NH₃ gas sensing. The incorporation of optimized Co(OH)₂ content (2.13%) along with NaOH interaction (alkalization) altered the characteristic (002) diffraction peak position of Ti₃C₂T_x MXene towards lower diffraction angles, which indicated an increase in the interlayer spacing [35]. This structural modification promoted the diffusion of NH₃ molecules into the enlarged interlayer channels. In addition, the specific surface area of the Co(OH)₂/Ti₃C₂T_x nanocomposite (19.0984 m²/g) was substantially higher than that of pure MXene (3.9900 m²/g), providing a greater number of active adsorption sites for NH₃ molecules. Lattice oxygen in the Co(OH)₂/Ti₃C₂T_x nanocomposite was observed, with O–H and Ti–O bonds at 529.8 and 531.1 eV, respectively. As a result, the nanocomposite containing 2.13% Co(OH)₂ exhibited NH₃ gas response of 14.7% at 100 ppm, with response and recovery times of 39 and 49 s, respectively. The response towards ethanol was negligible a 0.7%, which confirmed the potential response and selectivity of Co(OH)₂/Ti₃C₂T_x composite towards NH₃. Furthermore, the sensor demonstrated excellent repeatability over five sensing cycles and maintained stable sensing performance up to 30 days. The synergistic interaction between Co(OH)₂ and Ti₃C₂T_x also enhanced the NH₃ sensing performance under humid conditions of 30% (high response), 40–60% (decreasing), and 70% (increase again). At higher humidity conditions of 70%, NH₃ interacted with water molecules and formed hydrated NH₃ molecules, which increased the Ti₃C₂T_x nanosheets’ interlayer spacing and sensing ability. The adsorbed NH₃ transferred the electrons to the Co(OH)₂ and Ti₃C₂T_x, which reduced the concentration of holes. Overall, the surface modification of Ti₃C₂T_x MXene with Co(OH)₂ effectively enhanced the selectivity and room-temperature NH₃ sensing performance.

4.11. Ti₃C₂ MXene/MOFs

Very recently, Qiu et al. [58] successfully demonstrated ppb level NH₃ gas sensing at room temperature by integrating Ni₃(HITP)₂ MOF and Ti₃C₂ MXene. In this hybrid structure, Ni₃(HITP)₂ MOF crystals were directly grown on the Ti₃C₂ MXene surface, enabling intimate interfacial contact between two components. The resulting Ni₃(HITP)₂/Ti₃C₂ composite exhibited a high specific surface area of 431.2 m²/g, which provided abundant active sites for NH₃ adsorption, diffusion, and transportation. This unique architecture significantly advanced the molecular capture capability of the sensing platform. Furthermore, the formation of Ti–O–Ni and/or Ti–F–Ni bonds at the Ni₃(HITP)₂ MOF/Ti₃C₂ MXene interface facilitated interfacial charge transfer, improving the gas-sensing performance. This structural integration enhanced the NH₃ gas-sensing response to 33% at 30 ppm, with rapid response/recovery times of 9/13 s, which demonstrated efficient and reversible NH₃ adsorption and desorption processes. Also, composite sensors demonstrated excellent cyclic stability over five cycles under NH₃ concentrations of 1, 5, and 15 ppm, along with long-term stability for more than 50 days at 15 ppm at 30% relative humidity. A remarkably LOD of 100 ppb at room temperature was achieved. After NH₃ exposure, NH₃ gas molecules behaved like electron donors and experienced potential redox reaction with adsorbed oxygen ions. Overall, the integration of MOF and Ti₃C₂ MXene provided a promising strategy for developing highly sensitive, stable, and room-temperature NH₃ gas sensors with ppb-level detection capability.

4.12. Ti₃C₂ MXene/Single Atoms

In addition to the above studies, Ni single-atom active sites coordinated with N and C (Ni–N–C) on Ti₃C₂T_x MXene (Ni–N–C/Ti₃C₂T_x) have been studied with regard to room-temperature NH₃ gas sensing [59]. A composite integrated with a gold electrode (AuE-Ni–N–C/Ti₃C₂T_x) exhibited a response of 20.1% toward 5 ppm, with LOD of 29.3 ppb. Furthermore, Ti₃C₂T_x-MXene (ME) and Ti₃C₂T_x-MXene/NMP/PEDOT:PSS (MNPE) electrode inks were employed to fabricate ME-Ni–N–C/Ti₃C₂T_x and MNPE-Ni–N–C/Ti₃C₂T_x sensors, which achieved responses of 33.2 and 27.3% at 5 ppm, respectively. The ME-Ni–N–C/Ti₃C₂T_x and MNPE-Ni–N–C/Ti₃C₂T_x composites exhibited response/recovery times of 30/80 s and 30/150 s with LOD of 27.0 and 12.1 ppb, respectively. This improved NH₃ sensing performance was attributed to the oxygen-enriched groups in the MNPE and ME electrodes, which facilitated higher density of surface-active sites than the AuE-Ni–N–C/Ti₃C₂T_x. Moreover, the ME-Ni–N–C/Ti₃C₂T_x sensor demonstrated superior long-term stability by retaining 35.8% of its sensing response after 4 weeks. These findings highlight the effectiveness of ME and MNPE electrode inks for developing flexible and high-performance NH₃ gas sensors.

4.13. Ti₃C₂ MXene/Cationic Polyacrylamide

Rapid NH₃ gas response/recovery process was achieved by developing flexible cationic polyacrylamide (CPAM)=interfaced Ti₃C₂T_x MXene nanosheets (CPAM/Ti₃C₂T_x) [60]. The structural defects and surface termination groups (–OH, –O, and –NH₂) in the CPAM/Ti₃C₂T_x composite played a crucial role in enhancing the adsorption of NH₃ gas molecules. Among these functional groups, the bond length (1.89 A°) between the NH₃ and –OH terminations at adsorption energy of −0.515 eV contributed to Ti₃C₂T_x MXene’s role in the NH₃ sensing mechanism. The strong hydrogen bonding between the –OH termination groups and NH₃ molecules facilitated electron transfer from NH₃ to CPAM/Ti₃C₂T_x. In this context, the sensor exhibited rapid response/recovery time of 12.7/14.6 s and response of 3.1% at 150 ppm, which further increased to 4.7% at 200 ppm. The presence of abundant functional groups also enabled stable NH₃-sensing performance at 200 ppm at a relative humidity of 35%. The sensor achieved high tolerance at relative humidity of under 45%, with negligible changes, and 50% decreased response at relative humidity of 95%. Furthermore, CPAM acted as an effective interfacial binder, which promoted strong adhesion between the Ti₃C₂T_x MXene nanosheets and mechanical stability, ensuing a stable response at 60 °C over 140 operating cycles. Overall, the synergistic effect of surface termination groups and the CPAM interface significantly enhanced the flexibility, stability, and rapid room-temperature NH₃-sensing performance of the CPAM/Ti₃C₂T_x composite.

Overall, Ti₃C₂T_x MXene-based composites exhibited remarkable room-temperature NH₃ gas sensing, with abundant surface terminations (–OH, –O, and –F), enlarged interlayer spacing, gas diffusion, and adsorption. Functionalization with polymers, graphene, metal oxides, MOFs, noble metals, sulfur nanosheets, and hydroxides greatly enhanced the key features such as adsorption, charge transfer, selectivity, and stability by suppressing oxidation. Results revealed NH₃ responses up to 147%, ultrafast response/recovery as low as 3/2 s, LOD from ppm to ppt levels, stable operation under humid conditions, bending, and long-term stability. Interestingly, strategies such as oxidation, alkalization, thickness control, and defect engineering further optimized the active sites, enabling highly sensitive, flexible, and wearable NH₃ gas-sensing platforms at room temperature. Developments in Ti₃C₂T_x MXene-based composites for room-temperature NH₃ gas sensing are presented in Table 2.

Table 2.

Room-temperature NH₃ gas-sensing ability of Ti₃C₂ MXene-based nanostructures.

Composite	Working Temperature	Response/ Recovery Time (s)	Response (%)	Gas Concentration /LOD (ppm)	Gas Sensing Equation	Stability (Cycles)/Long-Term Stability (Days)	Reference
Ti3C2 MXene based composites for NH3 sensing
Pure Ti3C2-MXene
Ti3C2Tx	Room temperature	—	1.22	1/—	|Rg − Ra|/Ra × 100	5 at 100 ppm	[31]
Ti3C2Tx/Ti3AlC2	Room temperature	12/60	11	200/50 ppb	|Rg − Ra|/Ra × 100	—/120	[32]
Ti3C2Tx	Room temperature	—	0.67	100/—	ΔR/Ra × 100	6/—	[33]
Metal ions intercalated Ti3C2 MXene
K+, Na+, Ca2+ and Mg2+ cations intercalated Ti3C2Tx MXene	Room temperature	—	0.36	100/10	ΔR/R0 × 100	—	[34]
Alkalized Ti3C2 MXene
NaOH alkalized Ti3C2Tx	Room temperature	73/38 (200 ppm)	100.3	500/5	|Rg − Ra|/Ra × 100	5/17	[36]
KOH alkalized Ti3C2Tx	Room temperature	170/— at 100 ppm	35.7	200/	—	8/50	[37]
Partially alkalized Ti3C2 MXene
Partially oxidized Ti3C2Tx	Room temperature	67/157	21.3	10/—	ΔR/Rg	4/21 at 500 ppm	[38]
Ti3C2 MXene/sulfur
Ti3C2@Sulfur	Room temperature	5/4	375	3 ppt/—	—	5/20	[39]
Ti3C2 MXene/Graphene
Ti3C2Tx MXene/graphene	Room temperature	—	7.21	100/10	ΔR/R0	2000 (bending)	[40]
Ti3C2Tx/graphene	Room temperature	26/148	25.5	100/0.5	rm − rg/rg × 100	3/16	[41]
Ti3C2 MXene/semiconductors
Ti3C2Tx/SnO2	Room temperature	36/44	40 at 46% RH	50/4.29 ppb	|Ra − Rg|/Ra × 100	5 at 30 ppm/60 at 50 ppm	[42]
Ti3C2Tx/SnO	Room temperature	61/119	7.8	200/1	Ra/Rg	4/22	[43]
In2O3/Ti3C2Tx	Room temperature	3/2	60.6	5/—	|Ra − Rg|/Ra × 100	3/30	[44]
NiO/Ti3C2Tx	Room temperature	60/19	6.13	50/10	|Rg − Ra|/Ra × 100	15/15	[46]
ZnO@Ti3C2Tx	Room temperature	92/104	39.16	20/89.1 ppb	|Ra − Rg|/Ra × 100	10/—	[47]
Ti3C2Tx/MoS2/PPy	Room temperature	43.6/40.8	23.1	10/—	|Rg − Ra|/Ra × 100	5/7	[48]
Bi2O3/Ti3C2Tx	Room temperature	61/164	61	100/5	|Rg − Ra|/Ra × 100	6/20	[49]
Ti3C2Tx/GO/CuO/ZnO	Room temperature	26/25	96.3	200/4.1	|Ra − Rg|/Ra × 100	6/45	[50]
Ti3C2 MXene/Ag and Au metals
Ag-loaded Ti3C2Tx	Room temperature	511/605	4.96	10/0.5	|Rg − Ra|/Ra × 100	—/30	[51]
Au/Ti3C2Tx	Room temperature	87/217	16	100/—	|Rg − Ra|/Ra × 100	6/—	[52]
Ti3C2 MXene/polymers
PEDOT:PSS/nitrogen doped Ti3C2Tx	Room temperature	280/393.6	13 at 36% RH	10/—	|Rg − R0|/R0 × 100	4/17	[53]
PVDF-ZIF-67/Ti3C2Tx	Room temperature	37.8/34.5	10.56	5/(9 ppt and 1 ppb)	|Rg − Ra|/Ra × 100	5/35	[54]
PANI/Ti3C2Tx	Room temperature	—	55.90	20/5	|Rg − Ra|/Ra × 100	6/21	[55]
Ti3C2 MXene/metal hydroxides
Ni(OH)2/Ti3C2Tx	Room temperature	78/108 s	13.69	80/1	|Rg − Ra|/Ra × 100	5/30 at 50 ppm	[56]
Co(OH)2/Ti3C2Tx	Room temperature	39/49	14.7	100/5	|Ra − Rg|/Ra × 100	5/30	[57]
Ti3C2 MXene/MOFs
Ni3(HITP)2/Ti3C2	Room temperature	9/13	33	30/0.1	|R − R0|/R0	5/50	[58]
Ti3C2 MXene/single atoms
ME-Ni–N–C/Ti3C2Tx	Room temperature	30/80 s	33.2	5/27.0 ppb	|Ra − Rg|/Ra × 100	5/24	[59]
Ti3C2 MXene/cationic polyacrylamide
CPAM/Ti3C2Tx	Room temperature	12.7/14.6	3.1	150	(Ig − I0)/I0	2 at 200 ppm/10	[60]

5. Stability of Ti₃C₂ MXene-Based Composites Under Humid Conditions

Achieving a stable composite with rapid adsorption/desorption kinetics for target molecules is highly desirable for practical detection of toxic gas at room temperature. The unique advantages of Ti₃C₂ MXene, including its layer structure, tunable surface termination groups, and excellent electrical conductivity have significantly progressed the use of Ti₃C₂ MXene-based composites for NH₃ gas sensing across a range of ppm to ppt levels. More importantly, these composites have demonstrated reliable room-temperature NH₃ sensing performance under harsh environmental conditions, particularly in humid atmospheres. The coordination of Ni(OH)₂/Ti₃C₂T_x composite exhibited enhanced NH₃ responses of 5.1 and 6.2% at 10 ppm under humidity levels of 40 and 60%, respectively [56]. Similarly, a Ti₃C₂T_x/SnO₂ composite achieved room-temperature NH₃ response of 40% towards 50 ppm NH₃ at 46% relative humidity with response and recovery times of 36 and 44 s, respectively [42]. Furthermore, a p-n heterojunction-based MOFs-derived In₂O₃/T_i3C₂T_x MXene composite demonstrated an unaltered NH₃ response of 60.6% with ultra-fast response/recovery times of 5/3 s [44]. Additionally, Ag nanoparticle-decorated oxygen-rich Ti₃C₂T_x MXene maintained long-term NH₃ sensing stability over 30 days even under 40% relative humidity conditions [51]. A p-type Ti₃C₂T_x/graphene composite achieved NH₃ response of 24.9% under an elevated humidity level of 80% [41]. Interestingly, alkalization treatment of Ti₃C₂T_x MXene improved the response/recovery characteristics, achieving 73/38 s at 200 ppm NH₃ under 40% relative humidity [36]. Compared with pristine Ti₃C₂T_x MXene, the alkalized samples exhibited excellent cyclic stability over five sensing cycles and long-term stability for 17 days without significant response degradation. Furthermore, a Ti₃C₂ MXene@sulfur composite demonstrated stable NH₃ sensing performance over a broad humidity range of 20–70% with ultrafast response and recovery times of 5 and 4 s, respectively [39]. Meanwhile, Ti₃C₂ MXene-based sensors demonstrated lower responses towards ethanol (0.7–5.4%), NO₂ (1.2–5.4%), CO₂ (1.5%), H₂ (3.7%), IPA (0.53%), and NO (2.4%) compared to NH₃ response behavior. These results confirm the excellent selectivity and durability of Ti₃C₂ MXene-based composites for reliable NH₃ detection at room temperature. These advancements in the NH₃ gas-sensing abilities of Ti₃C₂ MXene-based composites are summarized in Figure 8.

Figure 8.

Key features contributing to the enhanced room-temperature NH₃ and H₂ gas-sensing performance of Ti₃C₂MXene-based nanostructures.

6. Conclusions

The incorporation of Ti₃C₂T_x MXene into various nanostructured interfaces has successfully contributed to room-temperature NH₃ and H₂ gas-sensing applications. The intrinsic properties of Ti₃C₂T_x MXene, including high electrical conductivity (facilitating charge transfer), surface termination groups (promoting hydrogen bonding and molecular interaction), and layered architecture (enhancing gas diffusion), play crucial roles in improving sensing performance at room temperature. The structural and chemical features have enabled development of robust gas sensors with high sensitivity ranging from ppm to ppt concentrations and ultrafast response characteristics. As a proof of concept, Ti₃C₂ MXene@sulfur nanosheets achieved exceptional response/recovery times of 5/4 s under ppt-level of NH₃ [39]. Moreover, the Ti₃C₂ MXene-based composites maintained stable sensing performance under humid environments and exhibited long-term stability exceeding 50 days. For practical H₂ leak detection, the integration of Ti₃C₂ MXene-based sensors with an alarm circuit and LED indicator was successfully demonstrated, where the LED was activated upon exposure to 4% H₂ gas [21]. Additionally, flexible Ti₃C₂T_x MXene/graphene hybrid fibers woven into laboratory coats have demonstrated the potential for wearable room-temperature NH₃ sensing applications [40]. Overall, the synergistic coupling of 2D Ti₃C₂ MXene with diverse semiconductor nanostructures has significantly enhanced the selectivity and sensing capability of H₂ and NH₃ gas sensors at room temperature.

7. Future Perspectives

Although Ti₃C₂T_x MXene-based composites have demonstrated outstanding room-temperature H₂ and NH₃-sensing performance, the following key strategies can further strengthen their H₂ and NH₃-sensing capabilities.

1. The anchoring of isolated single metal atoms onto Ti₃C₂ MXene surfaces can accelerate gas molecule dissociation and activation through tunable adsorption energies. Highly dispersed metallic centers act as adsorption intermediates, which modify electrical properties through enhanced electron transfer and improved sensitivity.

2. Controlled engineering of selective surface termination groups can promote selective gas adsorption and facilitate charge transfer between gas molecules and sensing materials to improve sensing performance.

3. Advanced in situ characterization techniques, including in-situ X-ray absorption spectroscopy, Raman spectroscopy, and FTIR are essential for understanding gas adsorption mechanisms and identifying intermediate species formed during room-temperature gas sensing.

4. Theoretical investigations into Ti₃C₂ MXene-based H₂ and NH₃ gas sensors remain relatively limited. Therefore, comprehensive DFT studies are required in order to understand the adsorption behavior of gas molecules on different surface terminations, including –OH, –O, and –F-functionalized Ti₃C₂ MXenes.

5. Further modification of the Ti₃C₂ MXene structure (into a monolayer or few layers) can enable ultrafast response and recovery kinetics for ppb-level detection through improved gas adsorption, charge trapping, and charge transport mechanisms. Also, intercalation of various nanostructures between Ti₃C₂ MXene nanosheets expands the interlayer spacing and increases the accessible surface area, thereby enhancing the gas-sensing performance.

Overall, Ti₃C₂ MXene-based composites provide a promising platform for advancing room-temperature H₂ and NH₃ gas sensing from ppm to ppb detection regimes.

Author Contributions

Conceptualization, A.S.; Writing-original draft preparation, A.S.; Writing-review and editing, A.S.; validation, A.S.; Writing-review and editing, J.-S.N.; supervision, J.-S.N.; funding acquisition, J.-S.N. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data will be made available on request.

Conflicts of Interest

The authors declare no conflicts of interest.

Funding Statement

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. RS-2026-25473753).