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Application for Exhaled Gas Sensor Based on Novel Mxenes Materials*

Jixiu Zhu , Qiaofen Chen , Titong Ni , Aimin Chen , Jianmin Wu

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (2) : 232 -242.

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Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (2) : 232-242 DOI: 1.7536/PC200506
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Application for Exhaled Gas Sensor Based on Novel Mxenes Materials*
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Abstract

In order to detect, analyze, and identify exhaled gas, electronic-nose combined with artificial intelligence has become a hot spot in the field of non-invasive medical detection. However, gas sensing materials cannot meet the requirements of high sensitivity as well as high selectivity at room temperature, which seriously hinders the application of gas sensors in the field of health care. It is still challenging to find suitable materials for the electronic-nose. With many unique properties: a wide variety, large specific surface area, strong electrical conductivity, rich functional groups on the surface, and adjustable bandwidth, novel two-dimensional MXenes material has become a star candidate for the gas sensor of highly sensitive and low energy consumption. In this review, we summarize the latest research achievements of MXenes based materials with the special structure in gas sensing, focus on the gas sensing mechanism and modification methods, and probe into the problems and challenges still existing in the application of MXenes materials in gas sensing.

Contents

1 Introduction

2 Synthesis of MXenes

3 Structure and properties of MXenes

3.1 Structure of MXenes

3.2 MXenes electronic characteristics for gas sensing

4 Application of MXenes in gas sensing

4.1 Surface adsorption calculation of MXenes

4.2 Gas sensing performance of MXenes

4.3 Gas sensing mechanism of MXenes

5 Conclusion and outlook

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Keywords

MXenes / gas sensor / gas adsorption / gas sensing mechanism

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Jixiu Zhu, Qiaofen Chen, Titong Ni, Aimin Chen, Jianmin Wu. Application for Exhaled Gas Sensor Based on Novel Mxenes Materials*. Progress in Chemistry, 2021, 33(2): 232-242 DOI:1.7536/PC200506

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1 Introduction

With the rapid development of the world economy and the rapid increase of population, the contradiction between the shortage of medical resources and the huge demand for diagnosis and treatment is prominent. Therefore, the development of a portable and inexpensive diagnostic platform is particularly urgent[1]. Blood analysis, as a routine and invasive diagnostic method, inevitably involves pain, high cost and high risk. As an alternative, there is an increasing demand for non-invasive diagnosis by detecting biomarkers derived from metabolites such as breath, sweat and urine[2]. Human exhalation is composed of volatile inorganic gases (oxygen, nitrogen, nitric oxide, carbon dioxide, water vapor, etc.) And volatile organic compounds VOCs (acetone, isoprene, ethanol, etc.). Metabolites in the body are decomposed into gas molecules, which then enter the lung tissue through blood and lymphatic vessels to exchange with air, and then are discharged by respiratory metabolism[3]. Changes in the metabolic process of the human body can affect the composition of exhaled air, and some diseases can be diagnosed by detecting and identifying exhaled air. Clinical studies have shown that breath gas analysis can be used to diagnose diseases such as cancer, multiple sclerosis, Parkinson's disease, tuberculosis and diabetes[4]. At present, the detection of exhaled VOCs has been realized by GC-MS, PTR-MS and electronic nose[5,6]. Generally, GC-MS and PTR-MS require various pre-treatments including dehumidification, filtration and gas concentration to achieve accurate analysis of exhaled gas, which is expensive, bulky and time-consuming, and seriously hinders real-time and cost-effective disease diagnosis. However, electronic noses based on nanosensor arrays have become powerful in vitro diagnostic tools. It uses pattern recognition methods and clinical data to establish characteristic response maps of exhaled VOCs related to specific diseases for in vitro diagnosis, which can be applied to any stage of life and has great attraction in biomedicine[7,8]. For example, our group used a variety of ionic liquids to modify graphene oxide to construct a sensor array with good recognition ability for a variety of standard gases and mixed gases[9]. In addition, a flexible 2D g-C3N4/rGO gas sensing paper chip was prepared by layer-by-layer self-assembly technology, and a virtual gas sensing array was constructed by controlling the light switch, which realized the distinction between inorganic gas NO2 and SO2 at room temperature[10]. Recently, our group has constructed an electronic nose using metal ions to induce graphene oxide to assemble gas sensing arrays, which can diagnose lung cancer by detecting human exhaled gases[11]. It is found that the core component of the gas sensing array is the gas sensing material, and its performance characteristics for human exhaled gas detection mainly include: (1) low power consumption (room temperature detection); (2) high response sensitivity; (3) Ultra-fast response/recovery time; (4) ultra-high sensitivity and ultra-low detection limit; (5) ultra-high selectivity; (6) Good long-term stability. Therefore, it is more challenging and important to find sensing materials that can meet the requirements of high sensitivity, high selectivity and long-term stability at room temperature.
At present, due to the significant surface effect of nanomaterials, their photoelectric properties are highly sensitive to surface adsorption substances, and they are ideal materials for detecting low concentration gases. A variety of nanomaterials, such as metal oxides, conducting polymers, noble metals, and two-dimensional nanomaterials, have been used to construct gas sensors with small size, low power consumption, high sensitivity, and long-term stability[12,13][14,15][16,17][18⇓~20]. Semiconductor gas sensors and catalytic combustion gas sensors based on traditional gas sensitive materials, metal oxides, have good stability, but they have the defects of high operating temperature, low sensitivity and easy poisoning, which limit the detection of ultra-low concentration VOCs exhaled by human body[21⇓~23]. Gas sensors based on conductive polymers and noble metals have the advantages of high sensitivity and low working temperature, but they have poor thermal stability and low selectivity for VOCs, which can not meet the requirements of stable and efficient devices[24,25]. In order to obtain high-performance gas sensors with low power consumption, some two-dimensional materials, such as graphene, transition metal sulfides, black phosphorus and so on, have become candidates for carrying gas sensing elements due to their excellent characteristics such as two-dimensional structure with single atom thickness, high conductivity, high carrier transport capacity and large effective specific surface area[26,27][28,29][30]. Two-dimensional materials have the ability to detect gases rapidly at room temperature, which provides a better choice for the preparation of gas sensors with low energy consumption. Recently, MXenes, a novel two-dimensional layered nanomaterial with graphene-like structure, has been widely used as an attractive material in energy storage, supercapacitors, electrocatalysis, photocatalysis, adsorption, biosensing and other fields[31⇓~33][34,35][36,37][38,39][40][41]. As a new two-dimensional material, MXenes has many advantages as gas sensing materials, such as wide variety, large specific surface area, strong conductivity, high electron mobility, adjustable energy band width, narrow band semiconductor properties, good flexibility and plasticity[42,43]. Therefore, MXenes material has become a star candidate for high-performance gas sensing materials[44].
At present, the research on the application of MXenes in gas sensing is still in its infancy, and most of them focus on theoretical analysis and preliminary experimental exploration. The key problems in the application of MXenes in gas sensing are how to promote the selective adsorption of gases, improve the gas selectivity of MXenes, and enhance the room temperature stability of MXene. In this review, the latest progress in the preparation of MXenes and its application in gas sensing is systematically described, and the gas sensing mechanism of MXenes is analyzed from the perspective of contact resistance.Finally, the existing problems in the application of MXenes in gas sensing are pointed out, and some possible research directions in the field of gas sensing are prospected.

2 Synthesis of MXenes

MXenes is a general term for several transition metal carbides, nitrides, or carbonitrides, which are prepared from precursors by phase selective etching. The general formula of MAX is Mn+1AXn(n=1~3), wherein M is a transition metal (such as Sc, Ti, Zr, Nb, etc.), A is an element with an atomic number of 13, 14, etc. (Such as Si, Al, Ge or Sn), and X is carbon or nitrogen, or both[45]. Unlike other two-dimensional materials, the MAX phase has strong chemical bonds and, therefore, cannot be delaminated by shear or other mechanical means. There are not only ionic bonds, but also metallic bonds and covalent bonds between M-X, and pure metallic bonds are the binding mode between M-A, so A-layer atoms are more active[46,47]. The metal bond M-a in MAX can be destroyed by the reaction of HF solution with MAX to generate a AFx, and the a layer atoms are selectively etched and removed to obtain the multilayer MXenes with a Mn+1XnTx structure (Figure 1A), wherein T represents the functional groups such as hydroxyl, oxygen or fluorine on the surface of the laminate[48]. The interlayer force can be further adjusted by intercalating cations into the MXenes, thereby realizing the efficient exfoliation of the multilayer MXenes. Figs. 1b-d are scanning electron microscope (SEM) images of the Ti2CTx, the Ti3C2Tx, and the Ta4C3Tx material, respectively, which have an obvious two-dimensional layered structure.
So far, in the reports about the synthesis of MXenes, the main method is to etch the MAX phase directly with hydrofluoric acid (HF). In a pioneering experiment on the synthesis of Ti3C2Tx, Gogotsi et al. immersed Ti3AlC2 powder in 50 wt% hydrofluoric acid (HF) and reacted for 2 H at room temperature to synthesize Ti3C2TxMXenes[48]. The principle of this etching method can also be extended to other MAX phases. During the chemical etching process, the a layer of the MAX phase reacts with HF to generate AlF3 and H2 (Formula 1). In addition, surface transition metals spontaneously react with water or fluoride ions to form OH, O, or F surface functional groups (Formulas 2, 3)[48⇓~50].
M n + 1 Al X n ( s ) + 3 H F ( aq ) → M n + 1 X n ( s ) + Al F 3 ( s ) + 3 / 2 H 2 ( g )
M n + 1 X n ( s ) + 2 H 2 O ( l ) → M n + 1 X n ( OH ) 2 ( s ) + H 2 ( g )
M n + 1 X n ( s ) + 2 H F ( g ) → M n + 1 X n ( F ) 2 ( s ) + H 2 ( g )
Although the HF etching method can be applied to the preparation of partial MXenes, for example,Ti2C T x [ 51 ] 、V2C T x [ 52,53 ] 、V4C3 T x [ 54 ] 、Nb2C T x [ 53 ] 、MXenes of Nb4C3 T x [ 55 ], UTa4C3 N T x [ 49 ],However, due to the unique stability and reactivity of each MAX to different etchants, the HF etching method cannot be generalized to the synthesis of other MXenes. In general, the M4AX3(413 phase) requires stronger etching conditions than the M3AX2(312 phase) and the M2AX(211 phase). Table 1 lists the reaction conditions for HF solution-based etching of some MXenes. During wet etching with HF, it should be noted that too high a concentration of HF and too high an experimental temperature or too long an etching time can result in the common deletion of the A and M sites in MXenes, resulting in the formation of carbon derivatives[56,57]. Therefore, a balance must be established between etching and maximum stability in order to obtain 2D MXenes with the desired performance. In addition, HF can also be used to etch non-MAX to produce MXenes. For example, two Ga layers are etched from a Mo2Ga2C to produce a Mo2CTxMXene[58,59]. In addition, Huang et al. Reported the fabrication of Zr3C2TxMXene by etching a Al3C2 layer from a Zr3Al3C5[60]; They also reported the successful fabrication of Hf3C2TxMXene by substituting Si on the Al site of the Hf3Al4C6 and then etching the [Al(Si)]4C4 layer[61]. This indicates that it is possible to produce MXenes even if the precursor is not the MAX phase, which provides more options for the synthesis of MXenes.
Due to the harm of hydrogen fluoride to human body and environment, it is of great significance to find a greener synthesis process to prepare MXene materials. Gogotsi et al. Used a safer mixed solution of hydrochloric acid and fluoride salt (HCl) and lithium fluoride (LiF), in which the Ti3AlC2 powder was soaked at 35 ℃ for 24 H to obtain the Ti3C2Tx[63]. Later, a mixture of LiF and HCl solutions was shown to successfully synthesize other types of MXenes, such as Ti2CTx and Ti3CNTx[64][65]. In fact, other compounds containing fluoride salts (NaF, FeF3, KF, CsF, CaF2) and acids can also be used to synthesize MXenes[63,66,67]. It has been proved that the etching method of mixing fluoride salt and acid solution will release a part of HF in the reaction process, so it has the same etching mechanism as HF solution etching, but the reaction process of this method is milder than that of HF etching method. In addition, more environmentally friendly etching solutions than the mixed solution of fluoride salt and acid have also been found, for example, Hliam et al. Found that weakly acidic ammonium hydride fluoride (NH4HF2) can synthesize Ti3C2TxMXenes.Feng et al. Further found that the use of NH4HF2 etching solution can increase the lattice parameters of Ti3C2Tx, because the existence of functional groups on the surface of Ti3C2Tx makes its surface charge negative, thus attracting cations (NH4+) and anchoring on the surface of Ti3C2Tx, playing a supporting role[68][69].

3 Structure and Properties of MXenes

3.1 Structure of MXenes

Because the M and X layers in the MAX phase of the precursor of MXenes are interlaced with the A layer to form a hexagonal crystal structure, the MXenes phase also has the same symmetrical hexagonal lattice. The M atoms in MXenes are arranged in a tight structure, with X atoms filling the octahedral interstitial positions. There are three arrangements in the MXenes structure: :BγA-AγB(M2X-M2X), BγAβC-CβAγB(M3X2-M3X2), and BαCβAγB-BγAβCαB(M4X3-M4X3)[70]. As shown in fig. 2, the 2D MXene consists of a sheet with hexagonal cells in which the X layer is sandwiched by two M transition metal layers.

3.2 Electronic Properties of MXenes for Gas Sensing

In the water environment of the chemical etching solution, the outer surface of the exfoliated MX layer is usually functionalized by O, F or OH functional groups, and these abundant groups (O, F or OH) on the surface can become attachment sites for the direct growth of other nanostructured materials or functional molecules, thereby modifying them and providing feasibility for improving the selectivity of gas sensors[72,73]. In addition, this surface functionalization has a significant impact on the electronic and ionic transport properties of MXenes, that is, the conductivity of MXenes is directly related to the electron transfer process occurring on its surface[74]. MXenes has certain metal characteristics and narrow band gap semiconductor characteristics, which make it have the inherent advantage of good conductivity, such as Ti3C2Tx, whose room temperature conductivity can reach 10 000 S·cm-1[75,76]. Due to the Fermi level of the d orbital of the transition metal M in MXenes, the original MXenes(Mn+1Xn) are essentially metallic, but when their surfaces are functionalized, some MXenes show semiconducting properties. Table 2 lists the bandgap of some MXenes. MXenes with different band gaps can be used to prepare gas sensing arrays to realize the specific recognition of human exhaled gases. To sum up, MXenes has certain inherent advantages as a gas sensing material.

4 Gas Sensing Applications of MXenes

Among the many electrical gas sensors, the resistive gas sensor has a simple structure and is most likely to be developed into a standardized microdevice for portable platforms. However, the reported resistive gas sensors are basically high-temperature sensors based on semiconductor metal oxides, and the additional high power consumption requirements hinder their application in portable consumer electronics products. As a star material similar to two-dimensional graphene, MXenes has abundant active sites for gas adsorption on its surface, which makes it have high application value in the field of gas sensing. As a gas sensing material, MXenes can be used to detect VOCs at ppb level at room temperature, which provides an option to solve the problem of high energy consumption of traditional resistive gas sensors. At present, both theoretical calculation and experimental research show that MXenes has excellent gas sensing performance for NH3 and VOCs.

4. 1 M Xenes surface adsorption calculation

It has been theoretically demonstrated that MXenes(M2CO2,M=Sc, Ti, Zr, Hf) with semiconductor properties are highly sensitive to NH3, as shown in fig. 3A. Xiao et al. Found that after the NH3 was adsorbed on the M2CO2 as an electron donor, the charge transfer mainly occurred between the M atom of the M2CO2 and the N atom of the NH3.When the MXene adsorbs the NH3, the charge of the NH3 molecule is transferred to the transition metal atom on the surface of the MXene, and the conductivity of the Ti2CO2 is obviously improved, as shown in fig. 3B and fig. 3C[81]. They also found that the desorption of NH3 can be simply achieved by adjusting the electrons injected into the M2CO2, making the NH3 sensor reversible[82]. For example, the lowest unoccupied electronic state (LUES) of the Zr2CO2 is mainly from the Zr atom, which means that when an additional electron is introduced into the Zr2CO2, the electron will fill up the unoccupied electronic orbital of the Zr atom. Therefore, the injected electrons are mainly distributed on the transition metals, which leads to the increase of the metal bond length and adsorption energy of the NH3-M and the decrease of the adsorption energy of the NH3 on the MXene surface. Other research groups have found that the Sc2CO2 of the monolayer has good adsorption strength and obvious charge transfer for SO2.Charge transfer from the SO2 to the Sc2CO2 increases the energy above the Fermi level of the Sc2CO2 and the conductivity of the Sc2CO2[83]; By applying external tensile strain or electric field, high selectivity, high sensitivity, controllable capture or reversible desorption can be obtained, and the Sc2CO2 is predicted to have good sensing performance for toxic SO2 gases, as shown in fig. 3 (d). The surface functional groups of MXenes have significant contribution or influence on the gas sensing performance. Junkaew et al studied the reactivity and selectivity of four O-functionalized MXenes (M2CO2(M=Ti, V, Nb, Mo) to gas molecules using density functional theory (DFT) calculations[84]. According to the calculated adsorption energy results, among the 11 gas molecules, the adsorption force of Ti2CO2 and Nb2CO2 to NH3 is stronger, while Mo2CO2 and V2CO2 are more sensitive to NO. The surface functional groups of the Ti3C2TxMXene material are a combination of -F, = O and -OH, and the existence and the content change of the functional groups can realize selective sensing of gas molecules. For example, Pourfath et al. studied that the contribution of surface functional groups to charge transfer is different through charge difference calculation, and the contribution of fluorine atoms to charge transfer is smaller than that of oxygen atoms.There is a strong electrostatic attraction between the lone pair electrons of the O atom in the = O functional group on the MXenes and the positively charged part of the exposed hydrogen atom in the NH3 molecule, so the selectivity for the NH3 molecule can be improved by controlling the content of the = O functional group on the MXenes surface[85]. Recently, Naqvi et al.Several gases (such as CH4, CO, CO2, NH3, NO, NO2,Sensing behavior of H2S and SO2) on S-functionalized MXenes M2NS2(M=Ti, V)[86]. Charge transfer analysis shows that a considerable amount of charge is transferred from the NO and NO2 gas molecules to the Ti2NS2 and V2NS2, indicating that the presence of the S-group functional group in the 2D M2NS2MXenes has superior sensitivity to NO and NO2 gases.

4. Gas sensing performance of 2 MXenes

Exhaled ammonia is a biomarker for the diagnosis of urea imbalance due to kidney disease or gastric ulcer caused by Helicobacter pylori[4]. Respiratory VOCs such as alcohols, aldehydes, acids, and ketones are produced by different biochemical and physiological processes, and abnormal levels indicate an abnormal metabolic state[87]. By monitoring the levels of these biomarkers in exhaled breath, patients can be distinguished from healthy individuals. Exhaled breath contains both organic volatile gases and inorganic gases, and the concentration is at the level of ppm(10-6), ppb(10-9) or even lower. The detection of VOCs and inorganic gases at low concentrations is essential for early diagnosis of diseases. Beidaghi et al. And Maleski et al. Respectively reported that the Ti3C2TxMXenes sensing material showed relatively sensitive sensing performance for gases such as acetone, ethanol, methanol and ammonia at low concentrations at room temperature, and its signal-to-noise ratio was two orders of magnitude higher than that of other 2D materials, as shown in Figure 4 (a, B)[88][89]. Among them, Ti3C2Tx gas sensing materials reported by Gogotsi et al. Can detect acetone as low as 50 ppb at room temperature[89]. Moreover, compared with acid gases such as NO2 and SO2, Ti3C2Tx is more sensitive to VOCs, as shown in Figure 4 (C, d). Because the research on the sensing performance of MXene is still in the early stage, the current research mainly focuses on the Ti3C2Tx, while other types of MXenes with good performance are relatively rare. Beidaghi et al. Used two-dimensional V2CTxMXene as gas sensing materials, which have ultra-high sensitivity to non-polar gases (hydrogen and methane) at room temperature, with a theoretical LOD of 1 ppm for hydrogen and 9 ppm for methane[90]. Zhao et al. prepared a high-performance acetone sensor using a V4C3TxMXene sensing material with an operating temperature of 25 ° C and a LOD of 1 ppm[91].
Table 3 summarizes the sensing performance of various MXenes based gas sensors. It can be seen intuitively that the lowest detection limit of MXenes based gas sensors is as low as ppb level, and their working temperature is basically room temperature, especially for NH3 and VOCs gases with high sensitivity. Based on the highly sensitive response of Ti3C2Tx to NH3 and VOCs, many researchers use Ti3C2Tx as a sensitizer to synthesize a composite material with other semiconductor materials to enhance its single selectivity or sensitivity to a certain gas. Chang et al. Proposed to synthesize a W18O49/Ti3C2Tx composite by in-situ growth of one-dimensional W18O49 nanorods (NRs) on the surface of two-dimensional Ti3C2TxMxenes sheets through a solvothermal process[92]. The W18O49/Ti3C2Tx composite material is sensitive to low concentration acetone (11.6 to 20 ppm) at the working temperature of 300 deg C, has good selectivity and good long-term stability, and has the response/recovery time of 5.6/6 s at the concentration with the detection limit of 170 ppb. Although that composite material improve the sensitivity and the selectivity of acetone, the operation temperature is high, the energy consumption is high, and the advantage that the two-dimensional layere nanomaterial Ti3C2Tx can work at room temperature is not brought into play. Moreover, the thermal stability of Ti3C2Tx is poor, and it is easy to oxidize to TiO2 at high temperature, which can not guarantee the effective application of MXenes materials. In order to improve the NH3 sensing performance of Ti3C2Tx, Jiang et al. Prepared gas sensing materials based on TiO2/Ti3C2Tx bilayer nanostructures[93]. Under the condition of room temperature 25 ℃ (relative humidity 60.8%), the response value of the TiO2/Ti3C2Tx to the 10 ppm NH3 is 1.63 times larger than that of the pure Ti3C2Tx, and the response/recovery time is 0.65/0.52 s shorter than that of the pure Ti3C2Tx. The response to NH3 is as low as 0.5 ppm. However, the Ti3C2Tx gas sensing material still has the problems of low response value and poor moisture resistance.
Achieving highly selective recognition in the detection of exhaled gas analytes is a great challenge for MXenes sensing materials with metal conductivity because the conductance of MXenes produces a similar response regardless of the nature of the gas. Chen et al. And Pazniak et al. Respectively prepared Ti3C2Tx/WSe2 and partially oxidized Ti3C2TxMXene, and used them as gas sensing materials to realize the inversion of gas response in the detection of ethanol, that is, the pure Ti3C2Tx showed a positive resistance change (p-type sensing behavior) to ethanol, while the Ti3C2Tx/WSe2 sensor and partially oxidized Ti3C2TxMXene showed a negative resistance change (n-type sensing behavior) to ethanol, as shown in Figure 5A[94][95]. In addition, compared with the original Ti3C2Tx, the sensitivity of the Ti3C2Tx/WSe2 to ethanol is increased by more than 12 times. The improvement of its sensing performance is mainly attributed to the formation of numerous heterojunction interfaces by the Ti3C2Tx/WSe2 composite nanomaterial, which promotes charge transfer, and the surface of the WSe2 nanosheet provides a large number of active sites for gas adsorption, as shown in Fig. 5B. In addition, the combination of MXenes and WSe2 overcomes the instability and oxidation problems of a single MXenes sensor chip, and provides a scientific idea for expanding the sensing applications of other two-dimensional MXenes materials. Pazniak et al. integrated the partially oxidized Ti3C2TxMXene material into the multi-sensor array, and processed the vector signals from all sensors on the chip with linear discriminant analysis (LDA) to achieve highly selective identification of alcohol gases, as shown in Fig. 5C, d[95].
Portable and wearable real-time online diagnosis platform can better apply sensing technology to early diagnosis of diseases and monitoring of physical conditions. In recent years, flexible miniaturized gas sensors have attracted much attention due to their low cost and the ability to be integrated into smart wearable and portable electronic devices[96]. Yuan et al. Prepared 3D-MXenes(Ti3C2Tx) materials by electrospinning technology combined with self-assembly method[97]. A flexible VOCs gas sensor based on 3D-MXenes material was fabricated, which has high sensitivity to ppb VOCs (acetone, methanol and ethanol) at room temperature, and provides a broad prospect for the development of practical and wearable VOC sensors. Kim et al. prepared MXene/GO hybrid fibers by layer-by-layer self-assembly of GO and exfoliated Ti3C2TxMXene through a binderless wet spinning fiber process, as shown in Figure 6a[98]. MXene/GO hybrid fibers were further thermally annealed at 200 ℃ in Ar atmosphere to convert GO to rGO. The gas sensor prepared by the MXene/rGO hybrid fiber has excellent mechanical durability, flexibility and chemical activity, and the gas sensing capability of the MXene/rGO gas sensor for ammonia gas does not change during structural deformation (bending/release state), as shown in fig. 6B. The highly flexible MXene/rGO gas sensor is woven into a lab coat and connected to a multimeter to detect 100 ppm of NH3 with a gas response of 7.21%, as shown in Figure 6 C, d. MXene/rGO fibers have high sensitivity and mechanical stability to gas molecules, making them an ideal platform for wearable gas sensors.

4.3 Gas Sensing Mechanism of MXenes

The basic principle of gas sensing of two-dimensional nanomaterials is different from that of traditional metal oxides. Two-dimensional nanomaterials are mainly based on the charge transfer process between gas molecules and the surface of sensing materials. Taking p-type two-dimensional nanomaterials as an example, when reducing gas is adsorbed, the resistance of the sensing material is usually increased. When the reduced gas molecules are desorbed, the resistance of the sensing material returns to its initial value. Therefore, the amount of change in resistance of the two-dimensional nanomaterial depends on the intensity of electron donating or electron withdrawing of the target detection gas.
However, the gas sensing response of MXenes in the current experimental study is different from the calculation prediction. No matter whether the oxidizing gas or the reducing gas is introduced, MXenes shows a response mode of increasing resistance. This may be due to the use of MXenes with metallic properties in the experimental study, rather than MXenes with semiconductor properties in the computational study. Therefore, the gas sensing mechanism of MXenes can not be explained by p-type or n-type two-dimensional materials. Due to the high metal conductivity of MXenes, its conductivity is dominated by the resistance between the lamellae and the resistance within the lamellae[89,90,102,103]. Therefore, the gas sensing mechanism of MXenes can be explained by the electron tunneling effect, as shown in Equations 4 and 5[104]. Equation 4 shows the conductance (.sigma.) As a function of distance (.delta.) And activation energy (Ec), and Equation 5 shows the electron crossing activation energy as a function of dielectric constant (.epsilon.).
Ec ≈ e 2 4 π ε r ε 0 r
σ ∝ e - 2 δβ e - Ec / ( kT )
Where σ is the conductance of the material and represents the crossing ability of electrons, β is the tunnel decay constant, δ is the distance between Ti3C2Tx sheets, R is the gas constant, Ec is the activation energy of electron crossing, e is electron charge, εr is dielectric constant of the medium, and ε0 is vacuum dielectric constant. According to Formula 4, the gas is adsorbed on the surface of the MXenes material and combines with the charged group — OH in the MXenes material, resulting in a decrease in the dielectric constant εr of the material, an increase in the electron crossing activation energy Ec of the MXenes material, and a decrease in the electrical conductivity σ. According to Formula 5, when VOCs gas is introduced, the distance δ between the MXenes nanosheets increases due to the insertion of VOCs gas molecules between the MXenes nanosheets, which results in an increase in the contact resistance between MXenes material layers, that is, a decrease in the electrical conductivity σ of the MXenes material.
In order to study the sensing mechanism of Ti3C2Tx for acetone and NH3, Maleski et al. calculated the binding energy of acetone and ammonia on Ti3C2Tx, MoS2, RGO and BP by using density functional theory (DFT) simulation, as shown in Fig. 7A[89]. For acetone and ammonia, the strongest binding energy of Ti3C2(OH)2 is more than twice that of other two-dimensional materials. It is speculated that the superior gas adsorption performance of hydroxyl in Ti3C2Tx is the main reason for its high sensitivity to acetone and ammonia. This work demonstrates that there is a charge transfer induced by gas adsorption in the gas sensing mechanism of MXenes. In addition, Zhou et al. Used Ti3C2Tx as a gas sensing material.The gases of CH4, H2S, H2O, NH3, nitric oxide, ethanol, methanol and acetone were tested at room temperature.It was found that the selectivity of Ti3C2Tx to NH3 was very high[99]. In order to understand the reasons for this high selectivity, they also studied the adsorption behavior, adsorption energy, adsorption geometry, charge transfer and other aspects by using first-principles calculation methods, and confirmed that the charge transfer caused by the adsorption of NH3 on the Ti3C2Tx was the main reason for the change of Ti3C2Tx resistance. However, MXenes has metallic conductivity and contains interlayer water molecules, which means that gas molecules may interact in a more complex way than typical charge transfer. Koh et al. Intercalated the Ti3C2Tx by Na+ ions, and the swelling effect of gas on the Ti3C2TxMXene material was proved by in-situ XRD technology: after 70 min of ethanol purging, the (002) peak of the Ti3C2Tx shifted to a smaller angle, and the interlayer spacing increased by 0.8282 Å.After purging with N2 for 120 min, the adsorbed ethanol was desorbed and taken away, the (002) peak of the Ti3C2Tx was shifted to a larger angle and recovered, and the interlayer spacing of the Ti3C2Tx membrane was reduced by 0.5151 Å compared with that after ethanol swelling, as shown in Fig. 7B[105]. Therefore, it is also very important to control the interlayer distance of the Ti3C2TxMXene to improve the selectivity of gas sensing.

5 Conclusion and prospect

The gas sensor based on MXenes has high sensitivity and ultra-low detection limit for VOCs, NH3 and other respiratory markers related to various human diseases, which provides a reference technical support for respiratory diagnosis and an advanced detection scheme for many fields such as environment and health. In addition, the low noise, room temperature operability, small size and easy integration into wearable nanoelectronic devices of MXenes-based gas sensors make up for the shortcomings of traditional gas sensors. In recent years, researchers have made some progress in improving the selectivity and stability of MXenes by semiconductor recombination and acid and alkali treatment.
Compared with graphene, the gas sensing research of MXenes is at an early stage and still faces many problems and challenges. Therefore, we believe that the following areas are important directions for the future development of MXenes based gas sensing:
(1) At present, only Ti-MXenes and V-MXenes have been used as gas sensing materials, and there are few reports on other gas sensing materials based on transition metal MXenes. Considering the excellent gas sensing properties of MXenes materials, more scientific work is needed to develop other transition metal-based MXenes gas sensing applications in the future.
(2) The ability of MXenes to identify VOCs still needs to be further improved. So far, there are few reports on the use of MXenes to construct gas sensor arrays for the identification and detection of VOCs in exhalation. As a sensing material, MXenes has unique advantages in gas sensing arrays: the M and X elements in MXense are adjustable, and its abundant end groups (-OH, -F and = O) can be further functionalized, so the band gap can be adjusted in a certain energy band range; The MXenes has a layered structure, and the interlayer distance or the pore size or the hydrophobicity of the material can be adjusted through interlayer intercalation modification, so that the selective adsorption of the material to VOCs gas can be adjusted, which has great potential for further improving the sensitivity and the recognition degree of the material to the gas.
(3) Because the exhaled gas contains a large amount of water vapor, the collected exhaled gas is generally dehumidified in the early stage, such as condensation and other measures. In addition, in order to reduce the impact of humidity on the sensing material, the humidity resistance of the material itself should be further improved. However, due to its excellent two-dimensional morphology and layered structure, MXenes is easy to form a variety of composite materials with other materials, such as MXenes/metal oxide, MXenes/TMD, MXenes/conductive polymer, and MXenes/carbon nanomaterials, which may be an effective solution to the problems of poor long-term stability and poor moisture resistance of MXene in gas sensing applications.
(4) The mechanism of high sensitivity to gas based on MXenes is not clear, and the gas sensing of MXenes is based on the change of electrical properties of MXenes caused by the adsorption of external gas molecules. In the future, theoretical and experimental studies should be combined to study the potential mechanism of the influence of gas molecules on the electrical properties of various MXenes, which is also an important step in developing their applications in gas sensing.

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Funding

National Natural Science Foundation of China(21575127)

Natural Science Foundation of Zhejiang Province(LY19B050004)

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