H2S/Butane Dual Gas Sensing Based on a Hydrothermally Synthesized MXene Ti3C2Tx/NiCo2O4 Nanocomposite

Real-time sensing of hydrogen sulfide (H2S) at room temperature is important to ensure the safety of humans and the environment. Four kinds of different nanocomposites, such as MXene Ti3C2Tx, Ti3AlC2, WS2, and MoSe2/NiCo2O4, were synthesized using the hydrothermal method in this paper. Initially, the intrinsic properties of the synthesized nanocomposites were studied using different techniques. P-type butane and H2S-sensing behaviors of nanocomposites were performed and analyzed deeply. Four sensor sheets were fabricated using a spin-coating method. The gas sensor was distinctly part of the chemiresistor class. The MXene Ti3C2Tx/NiCo2O4-based gas sensor detected the highest response (16) toward 10 ppm H2S at room temperature. In comparison, the sensor detected the highest response (9.8) toward 4000 ppm butane at 90 °C compared with the other three fabricated sensors (Ti3AlC2, WS2, and MoSe2/NiCo2O4). The MXene Ti3C2Tx/NiCo2O4 sensor showed excellent responses, minimum limits of detection (0.1 ppm H2S and 5 ppm butane), long-term stability, and good reproducibility compared with the other fabricated sensors. The highest sensing properties toward H2S and butane were accredited to p–p heterojunctions, higher BET surface areas, increased oxygen species, etc. These simply synthesized nanocomposites and fabricated sensors present a novel method for tracing H2S and butane at the lowest concentration to prevent different gas-exposure-related diseases.


Introduction
Developing novel gas sensors for detecting hazardous gases (H 2 S, C 2 H 4 , butane, NO 2 , SO 2 , and NH 3 ) is essential due to their variety of applications for human safety and the ecological environment.Hydrogen sulfide (H 2 S), with an unpleasant pungent smell, is one of the most nocuous air pollutants.It is also called industrial exhaust gas, which can be released from eggs, meat, wastewater treatment centers, sewers, and oil and gas fields [1,2].The Occupational Safety and Health Administration (OSHA) regulation limits the allowed concentration of 100 ppb H 2 S in air [3].Different concentrations of H 2 S exposure such as 2-5 ppm, 100-150 ppm, 200-300 ppm, 500-700 ppm, and 700-2000 ppm cause headaches, nausea, loss of smell, pulmonary edema, loss of consciousness, and sudden death within a minute, respectively [4].That is why it is very important to establish real-time H 2 S detection.On the other hand, butane (C 4 H 10 ) is a colorless and flammable gas that normally exists in wet/cracking gas, which could be considered one of the primary components of liquefied petroleum gas (LPG).Similar to H 2 S, when the butane concentration exceeds 800 ppm in the air, it causes dizziness, syncope, and nausea in humans.Fan et al. stated that the mixture of gas (1.6-8.5%) of butane and air can cause an explosion [5].So, the detection of butane is essential for human safety.
Semiconductor metal oxides (SMOs) have been strongly suggested and extensively studied as gas sensors because of their advantages, such as simple fabrication, real-time sensing, low cost, portability, and ability to synthesize different nanocomposites [6].Conventional SMOs have abundant oxygen vacancies, which provide extra active sites for enhancing gas-sensing properties [7].Cobalt-containing spinel oxides (ACo 2 O 4 , A = Ni, Zn, Cu) are an emerging class of ternary SMOs, owing to their widespread attention in different research areas such as gas sensors [8,9], super-capacitors [10], lithium-ion batteries [11], photocatalysis [12], etc.Among different spinel oxides, NiCo 2 O 4 , a typical p-type material, has found interesting applications in the field of gas sensors [13].NiCo 2 O 4 has the oxidation states of Ni (Ni 3+ /Ni 2+ ) and Co (Co 3+ /Co 2+ ) in most studies, and Ni immerses 16d octahedral sites, while Co is assorted in 16d octahedral as well as 8a tetrahedral sites [14].Du et al. stated the gas-sensing properties of NiCo 2 O 4 hollow microtubules and found that a sensor of NiCo 2 O 4 detected 100 ppm xylene at the high operating temperature of 220 • C, and the response was 9.2 [15].NiCo 2 O 4 was used to study the gas-sensing properties of n-butanol, xylene, and H 2 S [16][17][18].Dang et al. also detected 100 ppm n-butanol gas at a temperature of 165 • C in layered nanoflower-like NiCo 2 O 4 [16].However, the abovediscussed metal oxide-based gas sensor was found to have poor selectivity, high operating temperature, etc. [19,20].Thus, a gas sensor based on a nanocomposite of 2D material and pure metal oxide is necessary to detect hazardous gases at lower temperatures.
A member of 2D layered materials, MXenes (M n+1 X n T x , where M represents the transition metals including Ti, Nb, Zr, Cr, etc.; X stands for C or N; and T x is designated for terminated functional groups such as fluorine (-F), hydroxyl (-OH), or oxygen (-O)) have been used for outstanding achievements in different fields, such as gas sensors [21][22][23][24][25], energy storage devices [26], super-capacitors [27], and humidity sensing [28].Among the MXene family, Ti 3 C 2 T x , which was discovered in 2011, comprises transition metal carbides and nitrides that have received great attraction from researchers because of their relatively low density (4.91 g/cm −3 ) [29], sizeable BET-specific surface area, unique tunable electronic structure, excellent hydrophilicity features, and abundant surface terminals [30].Critically, MXene maintains excellent metallic conductivity during surface modifications [31], which makes MXene different from other 2D materials such as graphene, MoS 2 , WS 2 , etc.More importantly, the distinct character of MXenes is that, with their surface modifications, their excellent metallic conductivity is not sacrificed.For example, Kim et al. stated that the signal-to-noise ratio of Ti 3 C 2 T x is two times higher than that of traditional 2D materials (MoS 2 , graphene, black phosphorus, etc.) [31,32].Various examples from the literature based on Ti 3 C 2 T x and metal oxides were studied to highlight the impact of Ti 3 C 2 T x as gas sensors.Pasupuleti et al. and Wang et al. synthesized different nanocomposites with MXene, such as CuO/Ti 3 C 2 T x MXene and SnO-SnO 2 /Ti 3 C 2 T x ; these sensors improved the gas sensing properties five times more than those of pure CuO sensors and SnO-SnO 2 sensors [25,33].Due to the enhanced gas sensing properties of MXene-based nanocomposites, it can be expected that a nanocomposite of MXene Ti 3 C 2 T x and NiCo 2 O 4 could improve gas sensing properties.Until now, there have been rare instances in the literature reporting H 2 S sensing using MXene Ti 3 C 2 T x and NiCo 2 O 4 nanocomposites.
Herein, four different nanocomposites based on MXene Ti 3 C 2 T x /NiCo 2 O 4 , Ti 3 AlC 2 / NiCo 2 O 4 , WS 2 /NiCo 2 O 4 , and MoSe 2 /NiCo 2 O 4 were synthesized using the hydrothermal method.Supportive characterizations such as XRD, SEM, TEM, HRTEM, BET, FTIR, and XPS were performed to check the microstructure and morphology of the nanocomposites.The sensor chips were synthesized with nanocomposites and gas-sensing properties were analyzed.The response (S) was defined as the ratio of the resistance in target gas Rg and resistance in air Ra, S = R g /R a .The sensor of MXene/NiCo 2 O 4 detected the highest responses toward H 2 S and butane at room temperature and 90 • C, respectively.Significantly fewer responses to all gases were detected using other sensor sheets, suggesting the better selectivity of MXene/NiCo 2 O 4 toward H 2 S and butane; additionally, the sensor of MXene/NiCo 2 O 4 also showed minimum responses to 0.1 ppm H 2 S and 5 ppm butane, respectively.The current work provides a broad perspective on the application of H 2 S sensors.

Morphology and Structure of Products
In Figure 1 butane, respectively.The current work provides a broad perspective on the application of H2S sensors.

Morphology and Structure of Products
In Figure 1, XRD diffraction peaks were performed to find the structural information of MXene Ti3C2Tx/NiCo2O4, Ti3AlC2/NiCo2O4, WS2/NiCo2O4, and MoSe2/NiCo2O4.The XRD patterns of all products stated that the diffraction peaks located at the 2θ values of 18.90°, 31.14°,36.59°,38.40°, 44.62°, 55.43°, 59.09°, and 64.98° consisted of (111), ( 220), (311), ( 222), (400), (422), (511), and (440) planes of NiCo2O4, respectively.Small peaks of Ti3C2Tx, Ti3AlC2, WS2, and MoSe2 were also found, and no other peaks were revealed, suggesting the successful synthesis of designed nanocomposites.The estimated crystallite sizes of NiCo2O4 were 13.36, 10.86, 11.4, and 11.5 nm based on the (311) peak in the nanocomposites of MXene Ti3C2Tx/NiCo2O4, Ti3AlC2/NiCo2O4, WS2/NiCo2O4, and MoSe2/NiCo2O4, respectively.To find the morphologies of the synthesized samples, SEM images were introduced.Figure 2a-d shows the SEM images and EDS spectra (mappings) of MXene/NiCo2O4, Ti3AlC2/NiCo2O4, WS2/NiCo2O4, and MoSe2/NiCo2O4.Spherical NiCo2O4 nanoparticles with an average particle size around 30-40 nm were found in all the products and clear layered structure of MXene.SEM confirmed the attachment of spherical NiCo2O4 nanoparticles with the layered structure of MXene.The EDS spectra and scattering of each element also verified the successful synthesis of the synthesized nanocomposites.To find the morphologies of the synthesized samples, SEM images were introduced.Figure 2a-d  The TEM and HRTEM images also described the morphology and particle size of the samples.Figure 3a-d   The TEM and HRTEM images also described the morphology and particle size of the samples.Figure 3a-d shows different TEM and HRTEM images for MXene/NiCo2O4, Ti3AlC2/NiCo2O4, WS2/NiCo2O4, and MoSe2/NiCo2O4.The clear particle size of NiCo2O4 of about 30-40 nm was checked in all nanocomposites.Figure 3a shows the lattice stripe spacing of 0.829 nm, 0.335 nm, and 0.369 nm, corresponding to the (311), (220), and (002) planes of NiCo2O4 and MXene.Figure 3b shows clear spacing of 0.30 nm and 0.21 nm, related to the (311) and (002) planes of NiCo2O4 and Ti3AlC2.Figure 3c,d shows the clear spacing of 0.312 nm and 0.45 nm for the (311) and (002) planes of NiCo2O4 and WS2, while the spacing of 0.252 nm and 0.362 nm correspond to the (311) and (002) planes of NiCo2O4 and MoSe2.N2 adsorption-desorption experiments were performed, as shown in Figure 4a,b, to study the BET-specific area and pore size distribution of the synthesized products including MXene/NiCo2O4, Ti3AlC2/NiCo2O4, WS2/NiCo2O4, and MoSe2/NiCo2O4.The data showed that the BET-specific surface areas and pore sizes of MXene/NiCo2O4, Ti3AlC2/NiCo2O4, WS2/NiCo2O4, and MoSe2/NiCo2O4 were 41.92, 40.92, 34.74, and 26.98 m 2 /g, and 25.95 nm, 25.62 nm, 20.89 nm, and 16.13 nm, respectively.The pore volumes of MXene/NiCo2O4, Ti3AlC2/NiCo2O4, WS2/NiCo2O4, and MoSe2/NiCo2O4 were 0.2254 cm 3 /g, 0.2237 cm 3 /g, 0.1691 cm 3 /g, and 0.1409 cm 3 /g, respectively.The type IV hysteresis loop was determined from the N2 adsorption-desorption isotherm, indicating the porous characteristic of spherical NiCo2O4.The BET surface area and pore size of MXene/NiCo2O4 are greater than other nanocomposites, suggesting that porous structures offer extra tunnels In Figure 5, the surface chemical composition and electronic state of the nanocomposite MXene/NiCo 2 O 4 were studied using X-ray photoelectron spectra characteristics.The full XPS spectrum is shown in Figure 5a, which suggests the presence of all the elements.The spectrum of Ni 2p is shown in Figure 5b, which specifies two peaks corresponding to Ni 2p 3/2 and Ni 2p 1/2 , respectively, while the main spin-orbit doublets Ni 2p 3/2 were further divided into Ni 3+ and Ni 2+ [38].Two satellite peaks were found as well.The spectrum of Co 2p is shown in Figure 5c, which shows two peaks at values of 779.9 eV and 794.9 eV, which are related to Co 2p 3/2 and Co 2p 3/2 , respectively.The main peak of Co 2p 3/2 was further split into two Co 3+ and Co 2+ , and a satellite peak was also detected [39]. Figure 5d shows the O 1s spectra of Ti 3 C 2 /NiCo 2 O 4 , which states that the O 1s spectrum was divided into three oxygen species (O L , O V , and O C ).The Ti 2p spectrum of MXene/NiCo 2 O 4 showed four peaks at 452.3, 455.1, 458.2, and 459.7 eV, which are related to C-Ti, Ti 2+ , Ti 3+ , and Ti-O, respectively [40].The C 1s spectrum showed three peaks (C-C, C-O or C-H, C=O) at the values of (284.7 eV, 285.5 eV, and 288.7 eV), respectively [41].
that stimulate the penetration of gas molecules into the sensitive particle and surface reaction, which in turn increase test gas adsorption as well as enhance the gas sensing properties of MXene/NiCo2O4 [34].The FTIR spectrum of MXene/NiCo2O4 is shown in Figure 4c.The vibration peak cited at 3404.02 cm −1 corresponded to the hydrogen-bonded OH or coordinated H2O.The peaks at the values of 555.98 cm −1 and 1623 cm −1 were related to the bending vibrations (OH-Ti) and (Ti-O), suggesting oxygen-containing functional groups on the surface of MXene [35,36].The adsorption peaks at the values of 555.98 cm −1 and 654.21 cm −1 corresponded to the bonding formation of Ni-O and Co-O in NiCo2O4 material [37].In Figure 5, the surface chemical composition and electronic state of the nanocomposite MXene/NiCo2O4 were studied using X-ray photoelectron spectra characteristics.The full XPS spectrum is shown in Figure 5a, which suggests the presence of all the elements.The spectrum of Ni 2p is shown in Figure 5b, which specifies two peaks corresponding to Ni 2p3/2 and Ni 2p1/2, respectively, while the main spin-orbit doublets Ni 2p3/2 were further divided into Ni 3+ and Ni 2+ [38].Two satellite peaks were found as well.The spectrum of Co 2p is shown in Figure 5c, which shows two peaks at values of 779.9 eV and 794.9 eV, which are related to Co 2p3/2 and Co 2p3/2, respectively.The main peak of Co 2p3/2 was further split into two Co 3+ and Co 2+ , and a satellite peak was also detected [39]. Figure 5d shows the O 1s spectra of Ti3C2/NiCo2O4, which states that the O 1s spectrum was divided into three oxygen species (OL, OV, and OC).The Ti 2p spectrum of MXene/NiCo2O4 showed four peaks at 452.3, 455.1, 458.2, and 459.7 eV, which are related to C-Ti, Ti 2+ , Ti 3+ , and Ti-O, respectively [40].The C 1s spectrum showed three peaks (C-C, C-O or C-H, C=O) at the values of (284.7 eV, 285.5 eV, and 288.7 eV), respectively [41].

Gas-Sensing Properties
The gas-sensing properties of four different sensor sheets were studied, and the sensors were fabricated with the synthesized materials such as MXene/NiCo The results showed that the response was improved by increasing the concentrations of butane.Figure 6d shows that the responses of the MXene/NiCo 2 O 4 -based gas sensor were decreased with the enhancement in relative humidity (RH); the attachment of water molecules on the surface of the sensing material at higher RH might be the reason for the decreasing response.
Figure 7a-c shows that the gas sensor of MXene/NiCo 2 O 4 yielded responses such as 16, 13, 10, 8, 4, 2.4, and 1.8 to 10 ppm, 8 ppm, 6 ppm, 4 ppm, 2 ppm, 1 ppm, and 0.1 ppm, H 2 S, respectively, at room temperature.The minimum detection limit was 0.1 ppm.The response/recovery times for 10 ppm H 2 S were 10 s/40 s, respectively, while the response/recovery times for 8 ppm H 2 S, 6 ppm H 2 S, 4 ppm H 2 S, 2 ppm H 2 S, 1 ppm H 2 S, and 0.1 ppm H 2 S were 9 s/38 s, 10 s/39 s, 8 s/41 s, 9 s/42 s, 8 s/40 s, and 7 s/36 s, respectively.With the enhancement in H 2 S concentration, the response was increased.Figure 7d describes the relationship between the response of the MXene/NiCo 2 O 4 composite-based gas sensor and relative humidity (RH).The results demonstrated that when the RH was increased in the test chamber, by values such as 45%, 65%, and 85%, the response decreased.
Ti3AlC2/NiCo2O4, WS2/NiCo2O4, and MoSe2/NiCo2O4.The gas sensor of MXene/NiCo2O4 detected the highest responses toward H2S and butane at different operating temperatures compared with the other sensors.In Figure 6a-c, the gas sensor based on MXene/NiCo2O4 shows that the responses to 4000 ppm, 3000 ppm, 2000 ppm, 1000 ppm, 500 ppm, 100 ppm, 50 ppm, 20 ppm, 15 ppm, 10 ppm, and 5 ppm butane were 9.8, 8.8, 7.4, 6.6, 5.2, 4.6, 3.8, 2.9, 2.1, 1.9, and 1.2, respectively, at the operating temperature of 90 °C.The minimum detection limit was 5 ppm.The response/recovery times for 4000 ppm butane were ~200/~180 s, and the response/recovery times for 3000 ppm, 2000 ppm, 1000 ppm, 500 ppm, 100 ppm, 50 ppm, 20 ppm, 15 ppm, 10 ppm, and 5 ppm butane were 199 s/35s, 190 s/36s, 195 s/40s, 200 s/42s, 201 s/34s, 192 s/32s, 189 s/32s, 198 s/38s, 195 s/38s, and 197 s/32s, respectively.The results showed that the response was improved by increasing the concentrations of butane.Figure 6d shows that the responses of the MXene/NiCo2O4-based gas sensor were decreased with the enhancement in relative humidity (RH); the attachment of water molecules on the surface of the sensing material at higher RH might be the reason for the decreasing response.The sensor sheet depicted a high response toward 10 ppm H 2 S at room temperature and 4000 ppm butane at 90 • C, and the increased operating temperature decreased the response.The results showed that the operating temperature of 90 • C was the optimal temperature for butane sensing and that room temperature was optimal for H 2 S sensing.The highest response may relate to the highest BET surface area, the p-p heterojunction between Ti 3 C 2 T x and NiCo 2 O 4 , and abundant active sites.H 2 S-sensing and butane-sensing properties have been studied by many researchers, who suggested that in the gas sensing of H 2 S/butane based on the reaction of H 2 S/butane and adsorbed oxygen on the surface of NiCo 2 O 4 , at low temperatures, oxygen adsorbs on the surface of sensitive material and captures extra electrons from the conduction band.Figure 8 shows the responses of the gas sensors based on spherical MXene/NiCo2O4 for 10 ppm H2S and 4000 ppm butane at different operating temperatures, which shows the relationship between the responses of the sensor based on MXene/NiCo2O4 and the operating temperatures.Sun et al. stated that the operating temperature affects the species of adsorption oxygen, the carrier concentration/resistance, and response-recovery time [42].The sensor sheet depicted a high response toward 10 ppm H2S at room temperature and 4000 ppm butane at 90 °C, and the increased operating temperature decreased the  response.The results showed that the operating temperature of 90 °C was the optimal temperature for butane sensing and that room temperature was optimal for H2S sensing.
The highest response may relate to the highest BET surface area, the p-p heterojunction between Ti3C2Tx and NiCo2O4, and abundant active sites.H2S-sensing and butane-sensing properties have been studied by many researchers, who suggested that in the gas sensing of H2S/butane based on the reaction of H2S/butane and adsorbed oxygen on the surface of NiCo2O4, at low temperatures, oxygen adsorbs on the surface of sensitive material and captures extra electrons from the conduction band.Figure 9a,b shows the reproducibility of the MXene/NiCo2O4, Ti3AlC2/NiCo2O4, WS2/NiCo2O4, and MoSe2/NiCo2O4 gas sensors to 10 ppm H2S and 4000 ppm butane at room temperature and at the operating temperature of 90 °C.The responses of the MXene/NiCo2O4 primary sensor to 10 ppm H2S dropped slightly, but for butane, it was quite stable.Overall, the sensor showed good reproducibility for both gases compared with the other sensors.Long-term stability tests are also an essential factor in gas sensors.The long-term stabilities of the gas sensors based on MXene/NiCo2O4, Ti3AlC2/NiCo2O4, WS2/NiCo2O4, and MoSe2/NiCo2O4 to 4000 ppm butane at 90 °C and 10 ppm H2S at room temperature are shown in Figure 10a,b.The responses of the MXene/NiCo2O4-based gas sensor for 4000 ppm butane were quite stable, but the response for 10 ppm H2S dropped slightly by 16 to 14 in the first 10 days, but after that, it was stable, implying the better stability of the sensor.Long-term stability tests are also an essential factor in gas sensors.The long-term stabilities of the gas sensors based on MXene/NiCo2O4, Ti3AlC2/NiCo2O4, WS2/NiCo2O4, and MoSe2/NiCo2O4 to 4000 ppm butane at 90 °C and 10 ppm H2S at room temperature are shown in Figure 10a,b.The responses of the MXene/NiCo2O4-based gas sensor for 4000 ppm butane were quite stable, but the response for 10 ppm H2S dropped slightly by 16 to 14 in the first 10 days, but after that, it was stable, implying the better stability of the sensor.11a, and at 90 • C, as shown in Figure 11b.These graphs demonstrate that the sensor of MXene/NiCo 2 O 4 showed good selectivity while testing different gases of 10 ppm at room temperature and testing different gases of 5 ppm at the operating temperature of 90 • C, respectively.Various gas sensors for H 2 S and butane sensing were studied.As listed in Table 1, the sensor fabricated with MXene/NiCo 2 O 4 showed a high response, short response/recovery time, and minimum limit of detection at room temperature and the operating temperature of 90 The gas sensors based on MXene/NiCo2O4, (b) Ti3AlC2/NiCo2O4, (c) WS2/NiCo2O4, and (d) MoSe2/NiCo2O4 showed responses toward the different gases at room temperature, as shown in Figure 11a, and at 90 °C, as shown in Figure 11b.These graphs demonstrate that the sensor of MXene/NiCo2O4 showed good selectivity while testing different gases of 10 ppm at room temperature and testing different gases of 5 ppm at the operating temperature of 90 °C, respectively.Various gas sensors for H2S and butane sensing were studied.As listed in Table 1, the sensor fabricated with MXene/NiCo2O4 showed a high response, short response/recovery time, and minimum limit of detection at room temperature and the operating temperature of 90 °C. Figure 12a,b shows that the responses increased with the increase in H2S concentration and butane concentration, indicating that the sensors showed a linear relationship between their responses and the H2S/butane concentrations.The gas sensor of MXene/NiCo2O4 showed linearity between its responses and the concentrations of H2S/butane, which showed the promising applicability of the current sensor.

Gas-Sensing Mechanism
The typical p-type NiCo2O4 is widely used as gas sensors, although Ti3C2Tx MXene also displayed p-type sensing behavior in our case [43].So, the p-p hetero-junction based on the nanocomposite of MXene/NiCo2O4 showed the p-type gas sensing mechanism.The mechanism is based on the chemical reaction between the target gas and chemisorbed

Gas-Sensing Mechanism
The typical p-type NiCo 2 O 4 is widely used as gas sensors, although Ti 3 C 2 T x MXene also displayed p-type sensing behavior in our case [43].So, the p-p hetero-junction based on the nanocomposite of MXene/NiCo 2 O 4 showed the p-type gas sensing mechanism.The mechanism is based on the chemical reaction between the target gas and chemisorbed oxygen on the sensitive material [44].The mechanism was similar for H 2 S and butane in the current manuscript, so it is explained for H 2 S only.The hypothesized gas-sensing mechanism of MXene/NiCo 2 O 4 is shown in Figure 13.When the MXene/NiCo 2 O 4 -based gas sensor was introduced into the air, the oxygen molecules captured electrons from sensing material and generated O 2 − (less than 100 • C) (Equations ( 1)-( 4)), this whole process created hole accumulation layers (HALs), which reduced the resistance of the sensing material.After HAL creation, the sensor was placed into H 2 S; the electrons were released back into the conduction band of the sensing mechanism with the existence of SO 2 and 2H 2 O, thus making the HAL thinner, increasing the height of the Schottky barrier, and resulting in an enhancement in resistance.
O 2 (ads.)+ e − → O 2 After HAL creation, the sensor was placed into H 2 S. The electrons were then released back into the conduction band of sensing mechanism with the existence of SO 2 and 2H 2 O (Equation ( 5)), thus making the HAL thinner, increasing the height of the Schottky barrier, and resulting in an enhancement in resistance.The reaction was as follows.
The mechanism was basically based on the adsorption/desorption mechanism and oxidation/reduction mechanism.The charge carrier transfer also played a critical role in the gas-sensing properties.The gas-sensing mechanism was followed by one of the previous studies [23].As it is well-known, MXene plays a p-type sensing behavior and NiCo 2 O 4 also shows p-type behavior, so the sensing behavior would be typical p-type in this study, and the response was calculated as S = R g /R a .A Schottky barrier formed at the p-p heterojunction interface to equalize the Fermi level due to the transfer of carriers.The work function of NiCo 2 O 4 (5.5 eV) was higher than Ti 3 C 2 T x MXene (3.9 eV) [45,46].The electrons flow from MXene to NiCo 2 O 4 and holes from NiCo 2 O 4 to MXene; this reaction would reduce the depletion layer as well as enhance the resistance of the sensitive material.The band gap of p-type NiCo 2 O 4 was approximately 3.1 eV, which was higher than MXene (1.1 eV) [47].Another factor that enhances the gas sensing properties of MXene/NiCo 2 O 4 is the abundant functional groups present in MXene, as proved with XPS such as (-O, -Cl, and -Na), which also enhanced active sites as well as oxygen adsorption sites [48].High carrier mobility and electrical conductivity of MXene also improved the gas sensing properties of MXene/NiCo 2 O 4 .The highest BET surface area of MXene/NiCo 2 O 4 corresponded to the adsorption of oxygen species and H 2 S, resulting in enhanced absorption and diffusion capacity of H 2 S molecules.
eV) [47].Another factor that enhances the gas sensing properties of MXene/NiCo2O4 is the abundant functional groups present in MXene, as proved with XPS such as (-O, -Cl, and -Na), which also enhanced active sites as well as oxygen adsorption sites [48].High carrier mobility and electrical conductivity of MXene also improved the gas sensing properties of MXene/NiCo2O4.The highest BET surface area of MXene/NiCo2O4 corresponded to the adsorption of oxygen species and H2S, resulting in enhanced absorption and diffusion capacity of H2S molecules.

Synthesis of Layered Ti 3 C 2 T x
In total, 20 mL of hydrochloric acid (HCL, 38%) was poured into a 100 mL PTFE beaker.Then, 2.96 g NH 4 F was added to it while magnetically stirring for 30 min until the solution was uniformly transparent.After that, 0.5 g of Ti 3 AlC 2 was slowly added above the suspension, and the stirring was continued.After three days, the mixture was washed many times with ethanol and water in centrifuge tubes at 8500 rpm until the supernatant reached a pH of approximately 7, and then it was dried for 5 h at 80 • C in an oven.

Synthesis of Materials
The synthesis of nanocomposites is described in Figure 14.The details were as follows: NiCl 2 .6H 2 O (2 mmol) and CoCl 2 .6 H 2 O (4 mmol) were added into four beakers.In the black suspension, 5% of different 2D materials such as Ti 3 C 2 T x , Ti 3 AlC 2 , WS 2 , and MoSe 2 were added into the suspensions during stirring; after 5-10 min, 2M NaOH was added into the suspensions to adjust the pH to 12. Subsequently, after 24 h of stirring, the solutions were transferred into 50 mL stainless steel autoclaves, and the oven was adjusted to 22 h and 190 • C.After the autoclave process, the samples were washed 3-4 times with DI water and ethanol using centrifugation (8500 rpm).After this stage, the samples were divided into two parts: one with a solid and liquid mixture was used to fabricate sensor sheets, and the second was dried in an oven for 11 h and 80 • C. The final step was the calcination process at 400 • C for 2 h and 2 • C/min.

Fabrication of Sensor Sheets
As discussed above, one part of the samples (the mixture of solid and liquid) after the autoclave process was used to fabricate sensor sheets.The sensor sheet was bought from Wei Sheng Electronics, Zhenjiang, Jiangsu, China.The alumina substrate of the sensor sheet had a thickness of 0.20 mm with two Ag inter-digital electrodes (1.9 mm width).The length and width of the sensor sheet were 13.4 mm and 7 mm, respectively.Moreover, the sensor sheet had a total of 16 inter-digits; the thickness of a single digit was 0.18 mm, and the distance between the two digits was 0.25 mm.The details were as follows: the ceramic sensor chip was fixed on a spin coating machine, the suspension liquid in the test tube was shaken evenly, a small amount of the suspension liquid was drawn with a disposable dropper, and 4~5 drops were dropped on the side of the ceramic negative with a cross-fingered electrode to spin the coating into a film.After that, the coated sensor sheet was placed into a glass dish and put in an oven at 130 • C for 10 min.The purpose was to volatilize the alcohol and dry the sensor sheet.After drying twice, the dried sensor sheet was put into a crucible and placed in a muffle furnace for sintering.The temperature was 400 • C, the duration was 2 h, and the heating rate was 2 • C/min.It naturally cooled to room temperature to obtain the hetero-structure thin film sensor sheet for detecting different gases in this paper.

Conclusions
In summary, various nanocomposites based on different 2D materials (Ti 3 C 2 , Ti 3 AlC 2 , WS 2 , and MoSe 2 ) and spherical NiCo 2 O 4 were synthesized using the hydrothermal method.Different characterization strategies such as XRD, SEM, EDS, TEM, HRTEM, BET, FTIR, and XPS were used to examine the synthesized materials' crystal structures, morphologies, and chemical states.The gas sensing properties were investigated thoroughly, four sensor sheets were fabricated from the synthesized samples, and the results showed that the MXene Ti 3 C 2 T x /NiCo 2 O 4 -based gas sensor detected the highest response (16) toward 10 ppm H 2 S at room temperature.In comparison, the sensor detected the highest response (9.8) toward 4000 ppm butane at 90 • C compared with the other three fabricated sensors (Ti 3 AlC 2 , WS 2 , and MoSe 2 /NiCo 2 O 4 ).The Ti 3 C 2 T x /NiCo 2 O 4 -based gas sensor also showed minimum detection limits of 0.1 ppm H 2 S at room temperature and 5 ppm butane at 90 • C; furthermore, the sensor generated sensational selectivity and great stability/reproducibility.Moreover, the sensor detected the highest response toward 10 ppm H 2 S when the relative humidity was 25%, and the response was decreased by increasing the RH.The gas-sensing detection at low temperatures with the current sensor could be a promising candidate for H 2 S and butane detection because the sensor showed an almost linear relationship between H 2 S/butane concentrations and its response.
Author Contributions: S.S.: conceptualization, methods, and writing-review and editing.H.Z.: conceptualization, analysis, methods, resources, writing-review and editing, and funding acquisition.A.A.: conceptualization, analysis, methods, resources, and writing-review and editing.All authors have read and agreed to the published version of the manuscript.
shows the SEM images and EDS spectra (mappings) of MXene/NiCo 2 O 4 , Ti 3 AlC 2 /NiCo 2 O 4 , WS 2 /NiCo 2 O 4 , and MoSe 2 /NiCo 2 O 4 .Spherical NiCo 2 O 4 nanoparticles with an average particle size around 30-40 nm were found in all the products and clear layered structure of MXene.SEM confirmed the attachment of spherical NiCo 2 O 4 nanoparticles with the layered structure of MXene.The EDS spectra and scattering of each element also verified the successful synthesis of the synthesized nanocomposites.

Figure 6 .
Figure 6.Resistance changes in the MXene/NiCo2O4 composite-based gas sensor for different concentrations of butane at 90 °C (a-c) and the relationship between the response of MXene/NiCo2O4 and different RHs at 90 °C (d).

Figure 6 .
Figure 6.Resistance changes in the MXene/NiCo 2 O 4 composite-based gas sensor for different concentrations of butane at 90 • C (a-c) and the relationship between the response of MXene/NiCo 2 O 4 and different RHs at 90 • C (d).

Figure 8
Figure8shows the responses of the gas sensors based on spherical MXene/NiCo 2 O 4 for 10 ppm H 2 S and 4000 ppm butane at different operating temperatures, which shows the relationship between the responses of the sensor based on MXene/NiCo 2 O 4 and the operating temperatures.Sun et al. stated that the operating temperature affects the species of adsorption oxygen, the carrier concentration/resistance, and response-recovery time[42].The sensor sheet depicted a high response toward 10 ppm H 2 S at room temperature and 4000 ppm butane at 90 • C, and the increased operating temperature decreased the response.The results showed that the operating temperature of 90 • C was the optimal temperature for butane sensing and that room temperature was optimal for H 2 S sensing.The highest response may relate to the highest BET surface area, the p-p heterojunction between Ti 3 C 2 T x and NiCo 2 O 4 , and abundant active sites.H 2 S-sensing and butane-sensing properties have been studied by many researchers, who suggested that in the gas sensing of H 2 S/butane based on the reaction of H 2 S/butane and adsorbed oxygen on the surface of NiCo 2 O 4 , at low temperatures, oxygen adsorbs on the surface of sensitive material and captures extra electrons from the conduction band.

Figure 7 .
Figure 7. Resistance changes in the MXene/NiCo2O4 composite-based gas sensor at different concentrations of H2S at room temperature (a-c) and the relationship between the response of MXene/NiCo2O4 and different RH at room temperature (d).

Figure 7 .
Figure 7. Resistance changes in the MXene/NiCo 2 O 4 composite-based gas sensor at different concentrations of H 2 S at room temperature (a-c) and the relationship between the response of MXene/NiCo 2 O 4 and different RH at room temperature (d).

Figure 8 .
Figure 8. Responses of MXene/NiCo 2 O 4 to 10 ppm H 2 S and 4000 ppm butane at different operating temperatures.

Figure 21 Figure 9 .
Figure 9a,b shows the reproducibility of the MXene/NiCo 2 O 4 , Ti 3 AlC 2 /NiCo 2 O 4 , WS 2 /NiCo 2 O 4 , and MoSe 2 /NiCo 2 O 4 gas sensors to 10 ppm H 2 S and 4000 ppm butane at room temperature and at the operating temperature of 90 • C. The responses of the MXene/NiCo 2 O 4 primary sensor to 10 ppm H 2 S dropped slightly, but for butane, it was quite stable.Overall, the sensor showed good reproducibility for both gases compared with the other sensors.Molecules 2024, 29, x FOR PEER REVIEW 11 of 21

Figure 9 . 21 Figure 9 .
Figure 9.The reproducibility of the gas sensors based on MXene/NiCo 2 O 4 , Ti 3 AlC 2 /NiCo 2 O 4 , WS 2 /NiCo 2 O 4 , and MoSe 2 /NiCo 2 O 4 to 4000 ppm butane at 90 • C (a) and 10 ppm H 2 S at room temperature (b).Long-term stability tests are also an essential factor in gas sensors.The long-term stabilities of the gas sensors based on MXene/NiCo 2 O 4 , Ti 3 AlC 2 /NiCo 2 O 4 , WS 2 /NiCo 2 O 4 , and MoSe 2 /NiCo 2 O 4 to 4000 ppm butane at 90 • C and 10 ppm H 2 S at room temperature are shown in Figure10a,b.The responses of the MXene/NiCo 2 O 4 -based gas sensor for 4000 ppm butane were quite stable, but the response for 10 ppm H 2 S dropped slightly by 16 to 14 in the first 10 days, but after that, it was stable, implying the better stability of the sensor.

Figure 21 Figure 12 .
Figure 12a,b shows that the responses increased with the increase in H 2 S concentration and butane concentration, indicating that the sensors showed a linear relationship between their responses and the H 2 S/butane concentrations.The gas sensor of MXene/NiCo 2 O 4 showed linearity between its responses and the concentrations of H 2 S/butane, which showed the promising applicability of the current sensor.Molecules 2024, 29, x FOR PEER REVIEW 13 of 21

Figure 12 .
Figure 12.Relationship between the response of MXene/NiCo 2 O 4 and H 2 S concentrations at room temperature (a) and the relationship between the response of MXene/NiCo 2 O 4 and butane concentrations at 90 • C (b).

Molecules 2024 , 21 Figure 14 .
Figure 14.The synthesis method and the sensor fabrication method.

Figure 14 .
Figure 14.The synthesis method and the sensor fabrication method.

Funding:
This work was supported by the National Natural Science Foundation of China (51679022), the Natural Science Foundation of China (52271303), the Dalian Science Technology Innovation Fund (2019J12GX023), Liaoning Revitalization Talents Program (XLYC2002074), Fundamental Research Funds for the Central Universities (3132022219), Fundamental Research Funds for the Central Universities (3132021501), the Technology Innovation Foundation of Dalian (2022JJ11CG010), the "Pioneer" and "Leading Goose" R&D Program of Zhejiang (Grant No. 2022C03084), the National Natural Science Foundation of China (Grant Nos.62205296 and 62205297), and the Zhejiang Provincial Natural Science Foundation of China (Grant Nos.LQ22F050007 and LQ23F050004).Institutional Review Board Statement: Not applicable.Informed Consent Statement: Not applicable.

Table 1 .
comparison of different gas sensors.

Table 1 .
Comparison of different gas sensors.