Experimental and Theoretical Advances in MXene-Based Gas Sensors.

MXenes, two-dimensional (2D) transition metal carbides and nitrides, have been arousing interest lately in the field of gas sensing thanks to their remarkable features such as graphene-like morphology, metal-comparable conductivity, large surface-to-volume ratio, mechanical flexibility, and great hydrophilic surface functionalities. With tunable etching and synthesis methods, the morphology of the MXenes, the interlayer structures, and functional group ratios on their surfaces were effectively harnessed, enhancing the efficiency of MXene-based gas-sensing devices. MXenes also efficiently form nanohybrids with other nanomaterials, as a practical approach to revamp the sensing performance of the MXene sensors. This Mini-Review summarizes the recent experimental and theoretical reports on the gas-sensing applications of MXenes and their hybrids. It also discusses the challenges and provides probable solutions that can accentuate the future perspective of MXenes in gas sensors.

Introduction

Since the discovery of MXenes as novel two-dimensional (2D) compounds in 2011 by Gogotsi et al.,[1] they have gained increasing interest thanks to their exceptional properties such as graphene-like morphology, metallic conductivity, mechanical flexibility, tunable energy bandgap, and strong hydrophilicity.[2] MAX phases, MXene’s precursors, are layered ternary carbides or nitrides with a general formula of MAX (n = 1–3), where M is an early transition metal (Sc, Ti, V, Cr, Zr, Hf, Nb, Mo, Ta, and W); A is a IIIA and IVA group element (Al, Si, P, Ga, Ge, As, In, Sn, and Pb); and X represents N or C (Figure a). MXenes with a general formula of MXT are synthesized by selectively etching the A element from MAX phases using solutions containing fluoride ions, yielding multilayered MXene sheets which can then be delaminated into single or few-layered sheets (Figure b). Depending on the etching process, MXenes are separated by terminal groups such as hydroxyl (−OH), oxygen (−O), fluorine (−F), and/or chlorine (−Cl) ions located on their surfaces (called T), resulting in surface hydrophilicity.[2]

Figure 1

(a) Chemical elements for the formation of MAX phases. (b) Illustration of MXene synthesis from the MAX phase.

Gas sensing is crucial to diagnose disease, monitor air pollution, detect explosives, and control chemical processes. Among different gas-sensing methods, chemiresistive gas sensors have drawn substantial interest because of their superior performance and low cost. However, they suffer from high operating temperature, high power consumption, and limited selectivity.[3] Thanks to the large surface-to-volume ratio, excellent surface conductivity, and surface-terminated functionality and on top of all hydrophilicity, MXenes are promising candidates for chemiresistive gas sensor applications.[2] It should be mentioned that while hydrophilic gas molecules (known as polar molecules) may be absorbed satisfactorily by hydrophilic absorbents such as MXenes the applicability of MXenes for detection of hydrophobic gas molecules (known as nonpolar molecules) could be limited.

The objective of this Mini-Review is to provide a short and concise survey of scientific literature, the most recent progress, and prospects in sensing of gases by MXenes. To this end, we discuss both experimental and theoretical studies that have been reported so far.

Experimental Perspective

Following the successful synthesis of 2D titanium carbide MXene,[1] its gas-sensing applications have been investigated. Lee et al.[4] reported the first demonstration of MXene as a gas sensor by evaluating the gas-sensing properties of drop-casted Ti3C2T on Pt-interdigitated electrodes fabricated on a flexible polyimide substrate under ambient atmosphere (Figure a). They observed the random functionalization of the surface of the MXene with different terminal groups like −O, −OH, and −F, with the first two dominating (Figure a). The sensor indicated p-type semiconductor behavior, and its resistance enhanced after adsorption of ammonia, acetone, methanol, and ethanol gases (Figure b). The electron transfer from the gas molecules to the dominant surface terminal groups reduces the MXene film’s majority carrier (holes) by electron–hole recombination, increasing the resistance. The lowest response was achieved for acetone, while the sensor presented its highest response to ammonia with a theoretical limit of detection (LoD) of 9.27 ppm. Kim et al.[5] further investigated the volatile organic compound (VOCs) sensing performance of Ti3C2T by fabrication of a metallic sensor with an exceptionally high signal-to-noise ratio (SNR). The sensor was exposed to acetone, ethanol, ammonia, nitrogen dioxide, sulfur dioxide, and carbon dioxide, while its electrical resistance was monitored (Figure c). Due to metallic conductivity and full termination of the surface with −OH and −O functional groups, the sensitivity of the sensor exceeded the conventional sensors with semiconductor channel materials, offering a LoD of 50–100 ppb for VOCs at room temperature. Regardless of being oxidative or reductive, all the gases caused a resistance enhancement after exposure, ascribed to the metallic conductivity of the channel material in which the gas molecule adsorption limits its charge carrier transport. Additionally, the SNR of Ti3C2T sensors was 2 orders of magnitude larger than those reported for other 2D materials, which is highly desirable for gas-sensing utilizations.

Figure 2

(a) Photograph of 2D Ti3C2T-based gas sensor and schematic representation of the functionalized Ti3C2T structure. (Adapted with permission from ref (4). Copyright 2017 American Chemical Society.) (b) The resistance changes of the semiconducting Ti3C2T sensor. (Adapted with permission from ref (4). Copyright 2017 American Chemical Society.) (c) Gas sensor response of the metallic Ti3C2T sensor. (Adapted with permission from ref (5). Copyright 2018 American Chemical Society.) (d) SEM images of Ti3AlC2 synthesized from different carbon sources (top row). Ti3C2T MXene created from graphite, TiC, and carbon lampblack obtained MAX phases (bottom row) and (e) their corresponding gas responses. (Adapted with permission from ref (6). Copyright 2019 American Chemical Society.)

The influence of Ti3C2T MXene precursors (Ti3AlC2) on the MXene’s gas-sensing performance was studied by Shuck et al.[6] The Ti3AlC2 precursor (MAX phase) was synthesized by using different carbon sources (graphite, titanium carbide, and carbon lampblack) (Figure d). A minimal intensive layer delamination (MILD)-like process which involves etching in HCl and LiF was employed to convert MAX phase crystals into Ti3C2T MXene flakes (Figure d). The MXene flakes produced from graphite, titanium carbide, and carbon lampblack had an average size of 4.2, 2.6, and 0.5 μm, respectively (Figure d). It was discovered that the highest gas response to acetone, ethanol, and ammonia is attributed to MXene-derived titanium carbide, followed by graphite and carbon lampblack, revealing that the MXene precursor has a key role in the performance of its sensor (Figure e). The lower response of carbon lampblack based MXene can be related to its lower electrical conductivity (1020 S/cm) in comparison with graphite-based (4400 S/cm) and TiC-based (3480 S/cm) MXene.

The gas-sensing properties of sensors based on 2D Ti3C2T MXenes could be enhanced by engineering the structure and surface chemistry. Yuan et al.[7] reported the fabrication of a flexible and high-performance VOC sensor based on a three-dimensional (3D) Ti3C2T MXene framework. A 3D polymer (PVA/PEI mixture) framework containing cross-linked fibers was synthesized using an electrospinning process, transferred on Au-interdigitated electrodes, and then immersed into the MXene dispersion. Because of electrostatic interactions between the negatively charged MXene (caused by functional groups) and positively charged 3D polymer framework (originated from the PEI component), the surface of the fibers was functionalized by MXene through self-assembly (Figure a). Thanks to the highly interconnected porous structure, the sensor offered higher sensitivity toward VOCs, such as acetone, methanol, and ethanol in ppb level in comparison with pure MXene (Figure b–d). The sensor also showed a lower response toward polar inorganic gas molecules (NO2, NH3, and H2O) and almost no response to hydrocarbons (toluene and cyclohexane). The functional groups of MXene (mostly −OH and −F) interact with VOCs and polar inorganic gas molecules by forming hydrogen bonds, resulting in charge transfer. The amplitude of the gas response can be related to the strength of the formed hydrogen bond. Interestingly, the resistance of the sensor increased regardless of the type of the molecule as a result of the metallic conductivity of the MXene. Yang et al.[8] improved the sensing properties of the Ti3C2T MXene by increasing the surface oxygen–fluorine ratio and alkali metal ion (Na+) intercalation. After HF acid etching of Ti3AlC2, the produced organ-like Ti3C2T was added into 5 M NaOH under magnetic stirring at room temperature for 2 h for the purpose of Na+ intercalation. The fabricated alkalized Ti3C2T sensor was found to be very selective to ammonia with 28.87% response at 100 ppm, almost two times higher than bare Ti3C2T (Figure e). Unlike bare Ti3C2T, alkalized Ti3C2T showed a negative gas response to NH3 associated with change in the carrier type after oxygen functionalization brought by alkaline treatment (Figure f,g).

Figure 3

(a) SEM image of a 3D Ti3C2T MXene. (Adapted with permission from ref (7). Copyright 2018 Royal Society of Chemistry.) Resistance changes of the 3D MXene sensor and MXene sensor with exposure to 5 ppm of (b) acetone, (c) methanol, and (d) ethanol. (Adapted with permission from ref (7). Copyright 2018 Royal Society of Chemistry.) (e) Response of the sensors based on Ti3C2T and alkalized Ti3C2T to 100 ppm of different gases. (Adapted with permission from ref (8). Copyright 2019 American Chemical Society.) Response curve of the sensors based on (f) Ti3C2T and (g) alkalized Ti3C2T to the NH3 at different concentrations. (Adapted with permission from ref (8). Copyright 2019 American Chemical Society.) (h) In situ XRD characterization of Ti3C2T films and schematic view of the interlayer structure of Ti3C2T MXene nanosheets with intercalated water molecules and Na+ ions. (Adapted with permission from ref (9). Copyright 2019 American Chemical Society.) (i) Change in d-spacing of Ti3C2T films upon CO2 and ethanol exposure. (Adapted with permission from ref (9). Copyright 2019 American Chemical Society.) (j) Ethanol response of gas sensors over the CO2 response versus NaOH concentration. (Adapted with permission from ref (9). Copyright 2019 American Chemical Society.)

The gas-sensing mechanism in metallic MXenes is more complicated than the surface adsorption and typical charge transfer in conventional 2D materials. For stacked layered structures such as MXenes, interlayer swelling is also responsible for conductivity change and gas response.[9] Koh et al. investigated the impact of interlayer swelling on the gas detection performance of Ti3C2T MXene sensors.[9] Followed by the preparation of Ti3C2T MXene powders by etching Ti3AlC2 in LiF and HCl, few single Ti3C2T flakes were produced by sonication. A continuous film was formed on an anodized aluminum oxide (AAO) filter through vacuum filtration and was transferred onto SiO2/Si substrates. The filter was then etched away using NaOH solutions at different concentrations (0.03, 0.3, 5, and 100 mM), followed by washing using DI water and drying in a vacuum chamber, resulting in the occupation of the interlayer spacing with Na+ ions and water molecules (Figure h). The degree of Na+ ion intercalation was controlled by NaOH concentration. Using in situ X-ray diffraction (XRD), the dynamic change in interlayer distance of the MXene films (d-spacing) upon exposure to gas molecules was monitored (Figure h). The free interlayer spacing of Ti3C2T thin film was obtained to be 4.3 Å, associated with the trapped water molecules. A decrease in the interlayer distance of 0.84 Å after N2 purging of intercalated Ti3C2T MXene suggested that water and adsorbents were eliminated. While the interlayer distance of intercalated Ti3C2T MXene remained constant upon exposure to CO2, it significantly enlarged after interaction with ethanol (Figure i) and returned to almost its original value after N2 purge. It was reported that the degree of swelling because of ion intercalation corresponds well with the intensity of the gas response. Treatment of Ti3C2T MXene with 0.3 mM NaOH resulted in the maximum swelling as well as the largest gas selectivity to ethanol over CO2 (Figure j).

Gas-sensing improvement of MXene-based devices can also be achieved through their hybridization with other nanomaterials. Chen et al.[10] reported the fabrication of a flexible nanohybrid room-temperature sensor composed of MXene (Ti3C2T) and TMD (WSe2) via liquid-phase exfoliation and inject printing. The MXene scaffolds with a typical size of 300 nm were uniformly decorated by WSe2 flakes with a typical size of less than 30 nm, creating numerous heterojunction interfaces (Figure a). The sensitivity of Ti3C2T toward gas molecules was increased after its hybridization with WSe2 (Figure b). Selective and sensitive detection of O-containing VOCs (ethanol, methanol, and acetone) was achieved by a Ti3C2T/WSe2 sensor, while the detection of hydrocarbons was limited (Figure b). Specifically, the sensitivity of Ti3C2T/WSe2 toward ethanol was 12 times larger than that of Ti3C2T. Moreover, unlike a pure MXene sensor, the gas response of the MXene/TMD sensor did not reach saturation at 40 ppm of ethanol, suggesting the capability of the sensor for wide-range detection of ethanol (Figure c). The fabrication of a room-temperature ammonia sensor composed of Ti3C2T MXene and reduced graphene oxide (rGO) sheets was presented by Lee et al.[11] Through a scalable one-step wet-spinning process, the MXene/rGO hybrid fibers with superior mechanical and electronic properties were prepared (Figure d). The hybrid fibers showed an extremely high response of 6.77% to NH3 (7.9 and 4.7 times greater than those of MXene film and rGO fiber, respectively) (Figure e) but had a low response of almost 1% to H2S, SO2, acetone, ethanol, xylene, and benzene (Figure f). The enhanced ammonia response of the hybrid fibers was attributed to the three times increase in active sites of the MXene (Ti–O) after its hybridization with graphene.

Figure 4

(a) Low-magnification TEM image of a single Ti3C2T/WSe2 nanohybrid (scale bar, 100 nm). (Adapted with permission from ref (10). Copyright 2020 Springer Nature.) (b) Selectivity of the Ti3C2T and Ti3C2T/WSe2 sensors upon exposure to various VOCs at 40 ppm. (Adapted with permission from ref (10). Copyright 2020 Springer Nature.) (c) Ethanol response as a function of gas concentrations for Ti3C2T and Ti3C2T/WSe2 sensors. (Adapted with permission from ref (10). Copyright 2020 Springer Nature.) (d) Schematic view of the spinning process for a MXene/GO hybrid fiber. (Adapted with permission from ref (11). Copyright 2020 American Chemical Society.) (e) Comparison of the gas response of MXene film, rGO fiber, and MXene/rGO hybrid fiber. (Adapted with permission from ref (11). Copyright 2020 American Chemical Society.) (f) The gas selectivity comparison of rGO fiber and MXene/rGO hybrid fiber to various testing gases. (Adapted with permission from ref (11). Copyright 2020 American Chemical Society.)

Sun et al.[12] reported that the homogeneous distribution of the W18O49 nanorods on the surface of Ti3C2T significantly improves the acetone-sensing performance toward LoD of 170 ppb at 300 °C (Figure a). The high response (11.6) of the W18O49/Ti3C2T sensor to 20 ppm of acetone and low response (less than 2) to ammonia, formaldehyde, acetone, and ethanol confirmed its selectivity toward acetone (Figure b). The W18O49/Ti3C2T sensor’s gas response was also investigated in terms of the weight percent of MXene within the composite. For less than 2 wt % of MXene, the response was enhanced with increasing MXene content, attributing to the removal of −F groups after the solvothermal process and synergistic interfacial interactions between two components. The response of the W18O49/Ti3C2T sensor with more than 2 wt % of MXene decayed due to the presence of −F functional groups that were not fully eliminated after the solvothermal process, indicating the deteriorating effect of the −F functional group on the gas-sensing performance of MXene-based sensors. Due to the nonpolar structure of C-containing gas molecules such as toluene, MXenes do not show a good response to them.[7,10,13] Hermawan et al.[13] described the fabrication of hybrid heterostructures of CuO nanoparticles/Ti3C2T MXene via electrostatic self-assembly. It was reported that CuO nanoparticles have an average size of 7 nm and were uniformly distributed over the interlayers and surface of Ti3C2T MXene (Figure c). Although bare Ti3C2T exhibited a trivial response to 50 ppm of toluene at 250 °C, the CuO/Ti3C2T showed a 5-fold increase in toluene response at the same condition compared to pristine CuO (Figure d). The reason behind this is that the metallic MXene as the support layer for p-type semiconductor CuO nanoparticles plays a key role in improvement of the gas response and recovery time of the CuO nanoparticles by enhancing the charge carrier mobility. The CuO/Ti3C2T sensor also presented an adequate selectivity toward toluene in the presence of VOCs and hydrogen gas (Figure e); moreover, it showed higher response in comparison with CuO/MoS2 and CuO/rGO sensors toward toluene (Figure f).

Figure 5

Response of different W18O49/Ti3C2T sensors to (a) various concentrations of acetone and (b) 20 ppm of ammonia, formaldehyde, acetone, and ethanol. (Adapted with permission from ref (12). Copyright 2020 Elsevier.) (c) SEM image of CuO/Ti3C2T. (Adapted with permission from ref (13). Copyright 2020 American Chemical Society.) (d) Toluene response of CuO/T3C2T, T3C2T MXene, and CuO with respect to temperature. (Adapted with permission from ref (13). Copyright 2020 American Chemical Society.) (e) Response of CuO/Ti3C2T to 50 ppm of different gases. (Adapted with permission from ref (13). Copyright 2020 American Chemical Society.) (f) Toluene response of CuO/Ti3C2T, CuO/MoS2, and CuO/rGO. (Adapted with permission from ref (13). Copyright 2020 American Chemical Society.)

There are very few reports on the gas-sensing applications of other MXenes beyond Ti3C2T. The performance of V2CT for detection of nonpolar and polar gases was reported by Lee et al.[14] By selectively etching Al atoms from the V2AlC MAX phase using HF acid, multilayered V2CT MXene was produced, followed by intercalating with tetra n-butyl ammonium ions (TBA+) to facilitate the delamination process (Figure a). They discovered that the contents of −O and −OH functionals on the surface of V2CT MXene are much more than the undesirable −F terminal group. Due to the presence of −O atoms on the surface of V2CT MXene, the sensor showed excellent room-temperature sensitivity to both polar gases such as acetone and nonpolar gases such as hydrogen with a theoretical LoD of 11.16 and 1.735 ppm, respectively (Figure b-,). In comparison with their previous report on Ti3C2T MXene[4] in which under the same experimental conditions ammonia gained the highest response, V2CT MXene is very selective to hydrogen owing to the presence of V atoms, suggesting that the selectivity of the MXenes can be engineered by replacing the transition metal atoms. Molybdenum carbide is another MXene structure for which the gas-sensing properties were evaluated. Guo et al. studied the VOC sensing performance of Mo2CT MXene sensors.[15] By selective etching of Ga layers from the Mo2Ga2C precursor using HF, multilayer Mo2CT MXenes were produced. The concentration of the MXene (0.066 mg/mL) and the sonication time (8 h) were optimized in order to achieve a higher VOCs sensitivity. The optimized Mo2CT MXene sensor offered an excellent selectivity toward toluene against the other VOCs with sensitivity of 0.0366 Ω/ppm at 140 ppm and a LOD of 220 ppb. Cho et al. also reported the synthesis of two phases of pure molybdenum carbide materials (α-MoC1– and β-Mo2C) with structures similar to MXenes (without the expression T in the chemical formula) via a temperature-programmed reduction technique.[16] While both phases show metallic behavior, α-MoC1– MXene (200–400 ohm) has higher resistance compared to β-Mo2C (tens of ohm) (Figure d). Due to the excellent catalytic properties and high electrical conductivity, the molybdenum carbide-based sensors showed great sensing performance as well as ultrahigh SNR. The sensors were exposed to various gases like acetone, ethanol, propanal, hexane, toluene, NO2, SO2, and NH3. Irrespective of the gases’ reducing or oxidizing characteristics, the β-Mo2C sensor presented a positive response (Figure e). This phenomenon is correlated to the highly metallic properties and delocalized density of states (DOS) of β-Mo2C in which the flowing electrons are impeded by charge clouds formed by adsorbed gases (Figure f). However, α-MoC1– with a higher resistance than β-Mo2C revealed p-type sensing characteristics and various responses to different gases due to localized DOS (Figure e,f). Moreover, the gas response of the α-MoC1– sensor is dramatically larger than that of β-Mo2C. The fabricated α-MoC1– sensor was able to detect ppb–ppt levels of NO2 with outstanding stability of half-year (Figure g).

Figure 6

(a) SEM image of the multilayered V2CT. (Adapted with permission from ref (14). Copyright 2019 American Chemical Society.) (b) Gas response of a V2CT MXene sensor and (c) its theoretical LoD. (Adapted with permission from ref (14). Copyright 2019 American Chemical Society.) (d) Channel resistance, (e) response to various gases, (f) DOS, and (g) NO2 gas response of the α-MoC1– and β-Mo2C sensors. (Adapted with permission from ref (16). Copyright 2018 Royal Society of Chemistry.)

Table summarized the gas-sensing performance of MXene-based sensors.

Table 1

Summary of the Experimental Reports on the Gas-Sensing Performance of MXene-Based Devices

MXene	gas analytes	MXene sensors’ fabrication details (source, synthesis, and deposition)	concentration (ppm)	response (%)	recovery time	LoD (ppb)	ref
Ti3C2Tx	ammonia	Ti3AlC2 powder; LiF–HCl etching; drop coating on flexible polyimide films	100	21	NR	NR	(4)
	methanol		100	14.3		NR
	ethanol		100	11.5		NR
	acetone		100	7.5		25
	acetone	Ti3AlC2 powder; LiF–HCl etching; vacuum filtration on SiO2 substrates	100	0.97	NR	50	(5)
	ethanol		100	1.7		100
	ammonia		100	0.8		100
	propanal		100	0.88		NR
	nitrogen dioxide		100	0.25		NR
	sulfur dioxide		100	0.2		NR
	carbon dioxide		10000	0.1		NR
	ethanol	Ti3AlC2 powders (produced using graphite, TiC, and carbon lampblack); LiF–HCl etching; spin coating on SiO2/Si substrates	100	0.16	NR	NR	(6)
	acetone		100	0.23		NR
	ammonia		5	0.62		NR
	acetone	Ti3AlC2 powder; LiF–HCl etching; electrospinning on 3D polymer framework	10	1.4	less than 2 min to VOCs	50	(7)
	ethanol		10	1.7		50
	methanol		10	2.2		50
	ammonia		10	0.7		NR
	trichloromethane		10000	0.1		NR
	water		10000	0.5		NR
	nitrogen dioxide		10	0.9		NR
	ethanol	Ti3AlC2 powder; HF etching + alkaline treatment (Na+ intercalation); drip coating on Al2O3 ceramic substrate	100	2	NR	NR	(8)
	acetaldehyde		100	3		NR
	formaldehyde		100	5		NR
	methanol		100	2		NR
	methane		100	4		NR
	nitrogen dioxide		100	10		NR
	ammonia		100	28.87		NR
	ethanol	Ti3AlC2 powder; LiF–HCl etching + alkaline treatment (Na+ intercalation); vacuum filtration on SiO2/Si substrates	0.1%	9.995	NR	NR	(9)
	carbon dioxide		1%	0.53		NR
WSe2/Ti3C2Tx	ethanol	Ti3AlC2 powder; HF etching + mixing into CTA+–WSe2 solution; inkjet printing on polyimide substrates	40	9.4	6.6 s to 40 ppm of ethanol	NR	(10)
	methanol		40	7.4		NR
	acetone		40	4.3		NR
	hexane		40	2		NR
	benzene		40	1.4		NR
	toluene		40	2.2		NR
rGO/Ti3C2Tx	acetone	Ti3AlC2 powder; LiF–HCl etching + mixing into rGO solution; wet spinning to produce hybrid fibers, loaded on a glass substrate	50	0.5	NR	NR	(11)
	ethanol		50	0.45		NR
	ammonia		50	6.8		NR
	hydrogen sulfide		50	0.5		NR
	sulfur dioxide		50	0.59		NR
	xylene		50	0.35		NR
	benzene		50	0.46		NR
W18O49/Ti3C2Tx	ethanol	Ti3AlC2 powder; HF etching + mixing into acetylacetone and WCl6 dissolved in ethanol; drop coating on ceramic plates	20	1.8	6 s to 170 ppb acetone	NR	(12)
	acetone		20	11.6		170
	formaldehyde		20	1.2		NR
	ammonia		20	2		NR
CuO/Ti3C2Tx	ethanol	Ti3AlC2 powder; HF etching + mixing into CuO nanoparticles dispersed in ethanol; brush coating on silicate glass	50	7.2	10 s to 50 ppm toluene	NR	(13)
	toluene		50	11.4		320
	hydrogen		50	2.5		NR
	acetone		50	7		NR
	methanol		50	5.4		NR
V2CTx	hydrogen sulfide	V2AlC particles; HF etching; drop casting on flexible polyimide films	100	0.05	7 min to hydrogen and 5.5 min to CH4	3504	(14)
	hydrogen		100	24.35		1375
	methane		100	1.67		9399
	ammonia		100	1.66		NR
	acetone		100	2.26		1116
	ethanol		100	8.16		NR
Mo2CTx	toluene	MO2Ga2C powder; HF etching; drop coating on SiO2/Si substrates	140	2.81	NR	220	(15)
	benzene		140	0.97		NR
	ethanol		140	0.73		NR
	methanol		140	0.58		NR
	acetone		140	0.14		NR

Theoretical Perspective

The complexity of the sensing mechanism of the MXene sensors renders its experimental tuning challenging. Thus, there is a need for atomic-level modeling (density functional theory (DFT) and molecular dynamics (MD)) to complement the experimental data and deepen our insights into MXene and gas analyte interactions. The first theoretical study was done by Yu et al.[17] They investigated the interaction behavior of H2, CO2, O2, NH3, CO, N2, NO2, and CH4 gas molecules on the semiconductor Ti2CO2 (Figure a). Among all considered gases, only NH3 is chemisorbed on the surface of Ti2CO2 MXene by donating a relatively large charge of 0.174 e and forming a N–Ti bond. The adsorption energy of NH3 on Ti2CO2 (−0.37 eV) was found to be less than Ti2C(OH)2 (−0.48 eV) and V2CO2 (−0.81 eV) and comparable to Ti3C2O2 (−0.34). The superior sensitivity of Ti2CO2 toward NH3 in comparison with phosphorene and MoS2 was demonstrated by calculating current–voltage curves before and after adsorption of the gas (Figure b). It was also revealed that NH3–Ti2CO2 interactions could be further reinforced under strains. Under 3% biaxial strain, the energy of adsorption for NH3 on Ti2CO2 became −0.51 eV, while the adsorption energies of other gases were slightly changed. Interestingly, the NH3 capture was a reversible process due to the escape of the gas molecule from the MXene surface after the removal of the strain. In a different study, Xia et al.[18] probed the interactions of H2, NO, CO, CO2, N2, O2, and CH4 on Zr2CO2 MXene, finding that NH3 is strongly chemisorbed due to an apparent charge transfer (0.188 e) and large adsorption energy (−0.81 eV), while other gases are only physisorbed. The authors expected the same behavior for M2CO2 (M = Sc, Ti, Zr, and Hf) owing to their similar atomic and electronic structures. It was also discovered that chemisorption to physisorption transition in the adsorption of NH3 on Zr2CO2 happens when two extra electrons are introduced into the MXene sheet, presenting a practical way to release the gas molecule. Wang et al.[19] further analyzed the performance of the Hf2CO2 for sensing multiple gases like H2, O2, CO, NO2, CO2, SO2, NH3, HCN, and H2S. As expected from ref (18), Hf2CO2 is greatly sensitive and selective to NH3 with the adsorption energy of −0.834 eV and an outstanding charge transfer of 0.146 e. It was also revealed that preadsorption of the MXene surface with H2O, SO2, and CO2 molecules could significantly increase the adsorption of NH3 due to the formation of hydrogen bonds.

Figure 7

(a) Optimized configurations for gas molecule adsorption on Ti2CO2. (Adapted with permission from ref (17). Copyright 2015 American Chemical Society.) (b) I–V curve of Ti2CO2 MXene, MoS2, and phosphorene before and after exposure to NH3. Inset shows a schematic view of the Ti2CO2 MXene sensor for NH3 molecule detection. (Adapted with permission from ref (17). Copyright 2015 American Chemical Society.) (c). Optimized configurations for adsorbed acetone and ammonia on Ti3C2(OH)2. (Adapted with permission from ref (5). Copyright 2018 American Chemical Society.) (d) Binding energies of adsorbed acetone and ammonia on various 2D materials. (Adapted with permission from ref (5). Copyright 2018 American Chemical Society.)

To complement their experimental studies,[5] Kim et al. scrutinized the influence of the functional groups of Ti3C2T MXene on its sensing performance.[5] Using the DFT method, the interactions of acetone and NH3 on the −OH-, −O-, and −F-terminated Ti3C2T were investigated. They concluded that Ti3C2(OH)2 showed higher binding energy toward acetone and NH3 compared to Ti3C2O2 and Ti3C2F2 MXenes (Figure c). In addition to that, the binding energies between these gases and Ti3C2(OH)2 are more than twice the corresponding energies obtained for these gases on other 2D materials such as MoS2, graphene, and black phosphorus (Figure d). Hajian et al.[20] analyzed the effect of the ratio of functional groups on NH3 detection of Ti3C2T MXene. Considering two different ratios of −F functional groups in the form of Ti3C2(OH)0.44F0.88O0.66 and Ti3C2(OH)0.66F0.22O0.11, they found that low ratios of −F result in stronger ammonia adsorption. This can be attributed to the smaller charge transfer between ammonia and fluorine in comparison with ammonia and oxygen. Hence, the synthesis method and the type of MAX phase etchant which determine the ratios of functional group are important factors for the gas-sensing performance of the MXene sensors.

Apart from NH3 and VOCs, the MXene-based sensor showed great promise for detecting other inorganic gases. Zhang et al.[21] investigated the formaldehyde capture efficiency of Ti3C2O2 nanosheets around room temperature. The moderate adsorption energy of 0.45 eV, as well as an adsorption capacity of greater than 6 mmol/g, suggested the potential of Ti3C2O2 MXene for indoor formaldehyde removal. Ma et al.[22] considered M2CO2 (M = Sc, Ti, Zr, and Hf) MXenes to find the best toxic SO2 detection platforms. A strong chemical bond between SO2 and Sc2CO2 accompanied by adsorption energy of −0.646 and charge transfer of 0.453 e implied that monolayer Sc2CO2 is a desirable material for sensing SO2. Interestingly, the SO2 molecules were only physisorbed on M2CO2 MXenes (Figure a). The reason behind this is the orbital hybridization of SO2 and Sc2CO2 around the Fermi level in projected DOS (PDOS) (Figure b), which results in metallic behavior and consequently more charge transfer in the system. Due to a sharp increase in the conductance upon exposure to SO2 and low adsorption energies obtained for N2, CO, CO2, CH4, H2, H2S, and H2O, Sc2CO2 MXene is a highly sensitive and selective material to SO2. Besides that, the adsorption strength of SO2 on the monolayer could be strengthened or reduced by exerting the negative electric field or controlling the tensile strength. M2CO2 (M = Sc, Ti, Zr, and Hf) MXenes were also evaluated for other toxic gases such as NO and CO detection.[23] For NO, the strongest interactions took place between the molecule and Sc2CO2, which can be related to the orbital hybridization between MXene and the gas molecule around the Fermi level. The large charge transfer of 0.303 e and moderate adsorption energy of −0.47 eV proved that Sc2CO2 is highly suitable for NO sensing. On the other hand, CO was only physisorbed on Sc2CO2 MXene, and a small charge 0.017 e was transferred. However, the sensing performance of the Sc2CO2 MXene to CO was improved by introducing Mn dopants in the structure, leading to charge transfer of 0.199 e and strong adsorption energy of −0.85 eV.

Figure 8

(a) Optimized configurations and (b) their corresponding PDOS and charge difference density for M2CO2 (M = Hf, Zr, and Ti) MXenes with adsorbed SO2. (Adapted with permission from ref (22). Copyright 2017 American Chemical Society.) (c) Optimized configurations and (d) sensitivity and for different gases on Ti2NS2 and V2NS2 MXene sheets. (Adapted with permission from ref (24). Copyright 2020 Elsevier.)

Nitride MXenes were also inspected for gas-sensing applications. Naqvi et al.[24] analyzed M2NS2 (M = V, Ti) MXenes for detection of H2S, SO2, NH3, NO, NO2, CH4, CO, and CO2. It was found that gas molecules are mostly physisorbed (C-containing molecules) or weakly chemisorbed (N- and S-containing molecules) on MXene (Figure c), promising a reversible N- and S-containing molecule adsorption/desorption process. While both Ti2NS2 and V2NS2 MXenes could be utilized as platforms for NO2 and NO sensing, the latter exhibited higher sensitivity (Figure d).

The potential of double transition metal MXene in gas sensing was also studied. Khaledialidusti et al.[25] examined Mo2TiC2T MXene terminated with specific surface functional groups of fluorine, oxygen, or hydroxide for CO2 capture, while pristine and O-terminated MXenes are metallic, F-terminated, and OH-terminated MXenes that exhibit semiconducting behavior. The CO2 molecule is weakly physisorbed on the surface of perfect Mo2TiC2O2 MXene with a nonspontaneous and endothermic process with reaction energy of 0.21 eV. To address this limitation, the impact of atomic vacancy defects on the capture efficiency of MXene was explored. It was discovered that the O-terminated MXene requires more energy than the F- and OH-terminated MXenes, showing that the surface terminations have a key role in defect formation. Moreover, defect formation was likely to occur in the outer Mo layers than in the inner Ti layer. CO2 adsorption energy was significantly enhanced to −0.11 and −0.35 eV by trapping in the defects formed in Mo2TiC2O2 MXene.

Summary and Outlook

In this Mini-Review, we highlighted the state-of-the-art use of MXenes and their hybrids in advanced gas sensors from experimental and theoretical perspectives. MXenes have grabbed enormous attention for their gas-sensing applications due to their fascinating properties such as metal-comparable conductivity, graphene-like morphology, large surface-to-volume ratio, strong hydrophilicity, mechanical flexibility, and rich elemental compositions and surface terminations. Recent studies have shown that the morphology, the surface functional groups, the precursor, and the interlayer structures of MXenes could be tuned in order to enhance the gas-sensing performance of the MXene-based devices. Numerous MXenes have been introduced theoretically, among which around 20 distinct MXenes have been experimentally synthesized. To date, most of the experimental works on MXene-based gas sensors refer to titanium carbide, the first discovered MXene. However, few works appear in the literature showing the gas-sensing potential of vanadium carbide and molybdenum carbide. Ergo, it would be advantageous to explore the possibility of other synthesized MXenes for gas-sensing and VOC applications.

The hydrophilicity of the MXenes which originates from their hydroxyl or oxygen-terminated surfaces is an important property that ensures their high sensitivity and selectivity in unfavorable environments. Observations from theoretical studies revealed that fluorine functional groups have a deteriorating effect on gas-sensing performance; therefore, controlling the synthesis method and the type of MAX phase etchant, which determine the functional groups’ ratios, is of crucial significance to keeping the fluorine content low. Fabrication of high-quality MXenes with fewer defects, large lateral dimensions, and uniform terminations with only a single type of functional group is highly desirable to develop sensors with excellent performance. Incorporating MXenes with 2D nanomaterials, gold nanoparticles, nanotubes, metal oxides, polymers, etc., could also be considered a strategy to improve the gas sensors’ efficiency. The modulation of heterojunction interfaces and underlying gas-sensing mechanisms needs to be intensively explored.

The modulation of Debye length/the depth of the space charge layer on the flakes’ size and their gas-sensing performance need to be comprehensively studied. The frequency dependence of electrical conductivity and the gas-sensing aspects need further exploration. Current flake sizes using atomic force microscopy (AFM) revealed a thickness of ∼70 nm for gas-sensing applications.[5] This is larger than 10 atomic layers and cannot be called a 2D solid in strict definition, as anything above 10 layers is considered 3D. Thus, the gas-sensing properties reported are from a 3D nanostructure. Unlike graphene, which is straightforward to make single layers, the current methods of making MXenes are not straightforward, utilizing strong acids that are not conducive for the environment. New processing methods that enable a more benign environmental footprint are highly necessary to fully realize MXenes as 2D nanomaterials, which presents significant opportunities in the processing area.

The mechanism of gas sensing in metallic MXenes is more complex than the surface adsorption and typical charge transfer in conventional 2D materials. Regardless of being oxidative and reductive, it was observed that all the adsorbed gas molecules cause an enhancement or decrement in the resistance with a high signal-to-noise ratio. The high signal-to-noise ratio is attributed to the metallic conductivity. Interestingly, for highly metallic MXenes, the gas response was trivial, attributed to the difficulty of flowing electrons by charge clouds formed by adsorbed gases. Moreover, for stacked layered structures such as MXenes, interlayer swelling makes a significant contribution to conductivity change and gas response. Hence, computational assessments should be employed to deepen the understanding of MXene and gas–molecule interactions.

Finally, MXene research is still only in its infancy. As a 2D nanomaterial, MXenes present a fundamental building block on which exotic nanostructures can be constructed. Experimental data should be combined with computational material predictions to investigate the structure, process, and property relationships for sensor applications. With the choice of a 60 plus group of layered ternary carbides and nitrides available, MXenes as 2D material present tantalizing opportunities to enable new scientific discoveries that are yet to be realized.