, offer more conductance and higher reactivity than Ti-based MXenes; however, less attention has been paid to them. Three gas sensors based on multilayered Mo
MXenes on glass, crystalline Si (cSi), and porous Si (pSi) substrates were used for CO
sensing. The sensor deposited on glass Si substrate displayed the best response to CO
. However, at higher temperatures, the sensor deposited on pSi exhibited an enhanced response to CO
gas. The enhanced gas response was justified by the lack of charge transfer from either the cSi or pSi substrates to MXene at RT. However, at higher temperatures, the charge transfer from these substrates to MXene leads to a decrease in resistance, which ultimately contributes to the sensing response.
The combination of MXenes with metal oxides is a promising strategy for enhancing the RT-sensing properties of the resultant composite, which generally leads to high-performance gas sensors at RT. Therefore, a SnO
2/Ti
3C
2T
x composite was synthesized hydrothermally. A mixture of MXene powder and stannic chloride pentahydrate (SnCl
4·5H
2O) was prepared. Then, it was put into a 50 mL Teflon-lined autoclave and heated at 180 °C for 12 h. The fabricated sensor offered a response of 40% ([(|R
g − R
a|)/R
a] × 100) to 40 ppm NH
3 at RT, which was higher than a pristine sensor. In the composite, the 2D MXene provided a matrix with high conductivity, which enabled RT sensing. NH
3 absorption at the defect sites on the MXene surface, as well as the interaction with functional groups, resulted in an enhanced gas response. Furthermore, the formation of heterojunctions between MXene and SnO
2 NPs, which acted as resistance modulation sources, contributed to the sensing enhancement
[32].
A MXene/NiO composite was synthesized via an in situ precipitation method. A NiSO
4·6H
2O and MXene solution was prepared, and then a NaOH aqueous solution was dropped into the solution. After being stirred for 2 h, the precipitates were collected and washed three times with deionized water and three times with ethanol. Then, the samples were dried at 60 °C for 24 h. Finally, the MXene/NiO composite materials were obtained after being calcined at 350 °C for 2 h under N
2 atmosphere. The sensor response to 50 ppm HCHO gas was 8.8 at RT. Based on FTIR analysis, numerous hydroxyl and other oxygen-containing functional groups were present on the sensor surface, which are important for NH
3 gas sensing.
2.2.2. MXene-TMD Composites
Two-dimensional transition metal dichalcogenides (TMDs) have high surface areas, abundant adsorption sites, and high surface reactivities; therefore, their composites with MXenes are promising for sensing studies
[33][34]. In a relevant study, a Ti
3C
2T
x-WSe
2 composite was chemically prepared and the fabricated sensor displayed a response of 9% ([(|R
g − R
a|)/R
a] × 100) to 40 ppm ethanol at RT. In addition, fast t
res (9.7 s) and t
rec (6.6 s) were recorded. The enhanced response to ethanol gas is related to the numerous heterojunctions generated between Ti
3C
2T
x and WSe
2. The enhanced response to ethanol gas is related to the numerous heterojunctions generated between Ti
3C
2T
x and WSe
2. In heterojunctions, band bending occurs, and as a result, potential barriers will be formed between two materials, leading to difficulty of flow of the charge carriers. Upon injection of the target gas, the height of potential barriers changes, contributing to significant resistance changes in heterojunctions. More heterojunctions result in higher modulation of the sensor resistance. In addition, after 1000 bending cycles, the performance not only did not decrease, but also slightly increased owing to the creation of microcracks and wrinkles by the strain forces, which acted as adsorption sites
[35].
The influence of the electrode type on sensing performance was explored. A flexible paper-based sensor using a Ti
3C
2T
x/WS
2 composite was fabricated using either a Ti
3C
2T
x-MXene electrode (ME) or a Au electrode (AE). The ME + Ti
3C
2T
x/WS
2 gas sensor exhibited the highest response of 15.2% ([(|R
g − R
a|)/R
a] × 100) to 1 ppm NO
2 gas at RT, owing to the formation of Ohmic contact between the sensing layer and ME, in contrast to the Schottky contact formed between the sensor and AE. When Au and Ti
3C
2T
x/WS
2 were in contact, the formation of Schottky potential barriers prevented the transport of charges between the two materials, and only a small number of carriers were able to cross the junction. In contrast, when Ti
3C
2T
x/WS
2 contacted the ME, the height of the barrier between the nonmetal ME and the Ti
3C
2T
x/WS
2 sensor was much lower, which allowed the easy transport of charge carriers across the junction. Furthermore, the flexible 2D ME has a large specific surface area and offers adequate adsorption and reaction sites for oxygen and the target gas. In addition, numerous surface groups are present on the ME surface, which affect the sensing performance. Finally, the excellent conductivity of Ti
3C
2T
x accelerated the electron flow during the sensing process and shortened t
rec. The optimized sensor showed good flexibility by maintaining its performance even after bending 500 times by 60°
[36].
2.2.3. MXene-Conducting Polymers Composites
Conducting polymers (CPs) are promising materials for gas sensors because of their high conductivity, possibility of working at RT, tunable chemical composition, easy doping, and low price
[37][38][39]; therefore, they can be used with MXene to boost the RT gas-sensing properties of the resultant composite. A sensor was fabricated for RT ammonia sensing by the in situ polymerization of PEDOT and PSS on Ti
3C
2T
x MXene. The sensor showed a high response of 36.6% ([(|R
g − R
a|)/R
a] × 100) to 100 ppm of NH
3 with t
res and t
rec of 2 min and 40 s, respectively. In addition, the sensor on the flexible PI substrate exhibited good mechanical flexibility by maintaining its performance at different bending angles. Charge flow occurred between the NH
3 molecules and the sensor surface, leading to a change in the electrical conductivity. Furthermore, the high specific surface area of the composite, along with π = π interactions, increased the concentration of charge carriers
[40].
A Ti
3C
2T
x MXene/urchin-like polyaniline (PANI) composite was produced using a template method by employing sulfonated PS nanosphere templates and in situ polymerization on flexible polyethylene terephthalate (PET). The sensor disclosed a high response of 3.70 to 10 ppm NH
3 at RT, which was higher than that of the pristine sensor. The enhanced sensing was related to the hollow urchin-like morphology of PANI and the NS morphology of Ti
3C
2T
x, both of which were beneficial for providing more adsorption sites for NH
3 gas. Second, Schottky heterojunctions were generated by the intimate contact between PANI and the Ti
3C
2T
x NS, which shortened the diffusion length for charges and led to fast charge flow. Furthermore, the degree of protonation of PANI increased through its connection with the Ti
3C
2T
x NS. The increased -NH
2+ and = NH
+ groups in the composite led to an enhanced response to NH
3 gas. As NH
3 is an indicator of meat freshness, the fabricated sensor was successfully used to evaluate pork meat freshness. After 36 h, the sensor was able to indicate an increase in NH
3 concentration in the meat, confirming spoilage
[41]. PANI NPs were decorated with Ti
3C
2T
x NSs via in situ polymerization. The sensor displayed a response (ΔI/I
0) of 40 to 200 ppm ethanol gas at RT. In addition, it exhibited good mechanical flexibility; under bending from 0° to 120°, it exhibited almost the same performance, demonstrating good flexibility. In particular, under bending to ~120° it showed a high response of 27.4% ([(|R
g − R
a|)/R
a] × 100) to 150 ppm ethanol. In addition, the t
res and t
rec were 0.6 and 0.8 s, respectively, after bending. Based on DFT calculations, the adsorption energies of −0.985, −0.689, and −0.544 were calculated for OH-terminated Ti
3C
2, O-terminated Ti
3C
2, and F-terminated Ti
3C
2, respectively. This demonstrates that the OH-terminated Ti
3C
2 had the strongest binding energy for ethanol
[42].
2.2.4. Ternary Composites
MXene-based ternary composites have been studied less for gas-sensing applications than binary composites because of the complexity of the synthesis procedure and the need for optimization of the three components. However, they exhibit superior sensing properties because there are more resistance-modulation sources inside the sensing materials.
A hamburger-like SnO-SnO2/Ti3C2Tx MXene nanocomposite was hydrothermally prepared at 120 °C for 8 h. It revealed a high response of 12.1 to 100 ppm acetone at RT, which was higher than that of the pristine sensors. Moreover, it revealed a trec of 9 s. The improved response was related to the higher surface area of the SnO-SnO2-Ti3C2Tx MXene composite (46.7 m2/g), relative to pristine Ti3C2Tx, (13.6 m2/g), and SnO-SnO2 (38.6 m2/g) gas sensors. Furthermore, there are more resistance modulation sources in the nanocomposite than in the other sensors.
Ternary 2D Ti
3C
2T
x MXene@TiO
2/MoS
2 composites were prepared using the hydrothermal method for NH
3 sensing at RT. It showed a response of 164% ([(|R
g − R
a|)/R
a] × 100) to 100 ppm NH
3 gas at RT, which was higher than that of the pristine sensor counterparts. The improved sensing of NH
3 was attributed to the layered nanostructure with a unique morphology and p-n heterojunctions. Furthermore, DFT studies indicated that NH
3 was able to transfer more charge to the composite surface than to pristine Ti
3C
2T
x MXene and MoS
2, resulting in a higher modulation of the resistance
[43].
A 2D Ti3C2Tx MXene-MoO2/MoO3 NSs composite was fabricated using the hydrothermal method at 180 °C/10 h for ethanol detection at RT. It revealed a high response of 19.77 to 200 ppm to ethanol, and fast tres and trec (46 s/276 s). The high surface area (13.54 m2/g) and abundant surface groups on MXene provided more active sites for the adsorption of oxygen and ethanol molecules.
Various nanocomposites such as MXenes with GO, ZnO, CuO, GO/ZnO, GO/CuO, ZnO/CuO, and GO/ZnO/CuO have been hydrothermally synthesized for NH
3 sensing at RT. Among them, Ti
3C
2T
x MXene/GO/CuO/ZnO with an optimal ratio of 9:1:5:5 exhibited the best NH
3 gas sensing without resistance drift. The response to 200 ppm was 96% ([(|R
g − R
a|)/R
a] × 100) along with good humidity independence. The improved sensing response was related to the generation of multiple p-n and p-p heterojunctions, as well as the presence of many functional groups on the surfaces of MXene and GO
[44].
A 3D Ti
3C
2T
x MXene/rGO/SnO
2 aerogel was fabricated using a facile solvothermal approach at 140 °C for 24 h. It exhibited a response of 54.97% ([(|R
g − R
a|)/R
a] × 100) to 10 ppm formaldehyde at RT. In addition, it indicated short t
res and t
rec (2.9 and 2.2 s) along with high stability. The high surface area of 103 m
2/g and the generation of p-n junctions between rGO and SnO
2 and p-p junctions between MXene and rGO contributed to the sensing mechanism. Based on DFT calculations, the adsorption energy of HCHO on Ti
3C
2T
x MXene/rGO/SnO
2 was −5.7 eV, which was larger than that for other sensors
[45].
2.2.5. Other MXene-Based Composites
A hollow nanofiber GaN/Ti
3C
2T
x composite was synthesized by hydrothermal nitridation at 120 °C/12 h. Ti
3C
2T
x, which has metallic properties, acts as a conductive channel and decreases the overall resistance at RT. In addition, Ti
3C
2T
x accelerated charge flow during the sensing reactions, resulting in fast sensor dynamics at RT. Accordingly, the response of the composite sensor to 50 ppm NH
3 was 3.5 times higher than that of the bare Ti
3C
2T
x. The large specific surface area and unique hollow porous morphology of the GaN NFs provide sufficient adsorption sites for NH
3 gas. The formation of p-n Ti
3C
2T
x-GaN heterojunctions is beneficial for resistance modulation. The responses to NH
3 were not affected by 20–80%RH. At high humidity, the sensor was covered with multilayered physisorbed water, leading to the inhibition of the direct reaction between the adsorbed oxygen and NH
3 [46].
Ni(OH)
2 has features such as non-toxicity, low cost, ease of synthesis, and semiconducting properties. Ni(OH)
2/Ti
3C
2T
x composites were synthesized via in situ electrostatic self-assembly. The sensor with ~7.8 wt% Ni(OH)
2 revealed the highest response, of 13% ([(|R
g − R
a|)/R
a] × 100) to 50 ppm NH
3 gas at RT. A further increase in Ni(OH)
2 resulted in the partial aggregation of Ni(OH)
2, causing a decrease in the number of adsorption sites and the sensing response. The formation of interfacial Schottky junctions between the two components and the increase in adsorption sites owing to the high surface area (54 m
2/g) are attributed to the sensing mechanism
[47].
A BiOCl-Ti
3C
2T
x MXene composite with an NS morphology, excellent homogeneity, and good electronic characteristics was synthesized for sensing studies. It revealed a high response to 34.58 to 100 ppm NO
2 gas at 80%RH. The high response of the gas sensor was attributed to the formation of p-p heterojunctions between BiOCl and MXene
[48].
The SnS2/Ti3C2 MXene composites were produced via electrostatic interactions. The response to 50 ppm acetone was 29.8% ([(|Rg − Ra|)/Ra] × 100) at RT, and the tres and trec were ~90 and 355 s, respectively. The oxygen-containing functional groups on Ti3C2 formed hydrogen bonds with acetone. Electrons flowed from Ti3C2Tx to SnS2 to form heterojunctions with potential barriers, the heights of which were changed upon exposure to target gas. The sensor could detect acetone in both the optical and electrical modes. To demonstrate the optical mode of the sensor, the sensor signal was connected to an LED, and the blue light evolution images of the LED at various acetone concentrations were processed.
2.2.6. Doped/Decorated MXenes
Doping is a popular method for enhancing the gas-sensing properties of metal oxides
[49]. Few studies on the doping of MXenes for gas-sensing applications have been reported. Generally, noble metals with catalytic activity are used for decoration on the sensing materials, and since they are much more expensive than other materials, fewer studies have been conducted using noble metal decoration on MXenes. Also, doped MXenes are less studied relative to composite-based MXenes for gas-sensing application due to the lower impact of doping on the gas response relative to heterojunctions. However, in future studies much more attention should be paid to doped and decorated MXenes for gas-sensing studies. Heteroatom additions to MXenes can go to lattice sites, functional group sites or become adsorbed on surfaces. In general, element doping effects are as follows: (i) generation of active species and increase in conductance; (ii) adjustment of the electronic structure by introduction of defects; (iii) changing of the surface nature and chemical bonds in MXene; (iv) adjustment of the surface chemical properties to increase catalytic performance
[50]. In this regard, S atoms with high electronegativity can decrease the electron density of the Ti atom, leading to a higher binding energy than that of Ti-C bonds. In a relevant study, it was demonstrated that the S doping of Ti
3C
2T
x MXene led to a higher gas-sensing response to toluene than that of the pristine sensor. An enhanced response of 214% ([(|R
g − R
a|)/R
a] × 100) to 1 ppm toluene was obtained after sulfur doping. Expansion of the interlayer spacing after sulfur doping has been reported; therefore, a larger surface area resulted in effective gas diffusion and provided more sites for toluene gas. Furthermore, the S at the surface of the MXenes acted like oxygen ions, leading to the expansion of the electron depletion layer (EDL) on the MXene. Upon interaction of toluene gas with these adsorbed sulfur species, they react with sulfur ions, and the liberated electrons increase the concentration of electrons, leading to the appearance of a sensor signal. Moreover, owing to the donating effect of the ethyl group, a remarkable enhancement in the activity of the H
2 atoms on the benzene ring was observed, leading to enhanced selectivity for toluene gas. In addition, DFT calculations revealed an increase in the binding energy of toluene to the S-doped MXenes
[51].
3. Conclusions
Pristine MXene gas sensors without any modification often exhibit poor performance; hence, their surfaces can be modified to increase the number of surface functional groups or add new functional groups. Therefore, the response and selectivity can be increased. Composite fabrication with other materials, such as metal oxides, TMDS, and CPs, is a very popular and promising strategy for enhancing the RT performance of gas sensors. In particular, because of the highly intrinsic sensing properties of metal oxides, their composites with MXenes have led to the realization of high-performance gas sensors that can work at RT. Composites based on MXenes-TMDs have high surface areas and abundant surface groups, both of which are beneficial for gas sensing. Furthermore, composites with CPs are highly sensitive to NH3 because of the high intrinsic sensitivities of both MXenes and CPs to this gas. Compared to MXene composites, less attention has been paid to doped MXenes, and more studies are needed in the future. Ternary composites are also promising for sensing applications; however, the optimization of all components is often difficult, and in this regard, more detailed studies are needed. In addition, the use of UV light to promote surface reactions and increase the number of active surface sites is a promising technique for enhancing the RT-sensing properties of MXene-based gas sensors.