Preparation and Gas-Sensing Properties of Two-Dimensional Molybdenum Disulfide/One-Dimensional Copper Phthalocyanine Heterojunction

Although 2D MoS2 alone shows excellent gas-sensing performance, it is prone to stacking when used as the sensitive layer, resulting in insufficient contact with the target gas and lower sensitivity. To solve this, a 2D-MoS2/1D-CuPc heterojunction was prepared with different weight ratios of MoS2 nanosheets to CuPc micro-nanowires, and its room-temperature gas-sensing properties were studied. The response of the 2D-MoS2/1D-CuPc heterojunction to a target gas was related to the weight ratio of MoS2 to CuPc. When the weight ratio of MoS2 to CuPc was 20:7 (7-CM), the gas sensitivity of MoS2/CuPc composites was the best. Compared with the pure MoS2 sensor, the responses of 7-CM to 1000 ppm formaldehyde (CH2O), acetone (C3H6O), ethanol (C2H6O), and 98% RH increased by 122.7, 734.6, 1639.8, and 440.5, respectively. The response of the heterojunction toward C2H6O was twice that of C3H6O and 13 times that of CH2O. In addition, the response time of all sensors was less than 60 s, and the recovery time was less than 10 s. These results provide an experimental reference for the development of high-performance MoS2-based gas sensors.


Introduction
Due to industrial development, large amounts of toxic and harmful gases pollute the environment and endanger people's health [1,2].Thus, there is a need to control and monitor air pollution in real time.Common harmful gases include volatile organic compounds (VOCs), nitrogen oxides, sulfur oxides, and carbon oxides, which can damage a person's sense of smell, vision, mucosa, lung function, liver function, and central nervous system [3,4].These gases are produced mainly during industrial production, transportation, and household activities [5].
Numerous strategies have been developed for air-quality monitoring in recent years, including semiconductor gas sensors, which are detection devices with high sensitivity, a fast response speed, and low cost [6,7].Commonly used sensing materials to construct semiconductor gas sensors include zero-dimensional [8], one-dimensional [9], twodimensional [10], and three-dimensional [11] materials.Layered molybdenum disulfide (MoS 2 ) is a two-dimensional (2D) material that has been widely studied because of its photoelectric properties, stable chemical properties, and simple preparation process [12][13][14].Two-dimensional MoS 2 has an ultra-high specific surface area, abundant surface defects, a tunable band gap (~1.8 eV), and excellent carrier mobility.These properties give it a high affinity for most harmful gas molecules [15,16], making it highly suitable for gas detection and gas sensor research.In the past decade, many studies have reported the gas-sensing properties of 2D MoS 2 and its composites.Hai et al. obtained monolayer and multilayered MoS 2 films via a mechanical peeling method using transparent tape.They prepared fieldeffect transistors (FETs) for NO gas detection for the first time by using photolithography.

Synthesis of MoS 2 Nanosheets
MoS 2 nanosheets were prepared via grinding-assisted liquid phase stripping [25].First, 100 mg of MoS 2 raw material was weighed and ground in an agate mortar for 2 h, and an appropriate amount of NMP was added during the grinding process.After that, the samples were placed in a vacuum oven and dried for 12 h after grinding.After drying, the samples were dispersed in 45 vol% absolute ethanol for 1 h, and then, the dispersion was centrifuged for 20 min (1500 r/min) to obtain MoS 2 nanosheets.Finally, the sample was dried in air for later use (Figure S1a).

Preparation of 1D-CuPc
CuPc (copper phthalocyanine) micro-nanowires (1D-CuPc) were prepared via physical vapor deposition (PVD) [26], as shown in Figure S1b.Purified CuPc (1 mg) was placed in a beaker, 20 mL ethanol was added, and the beaker was sealed and sonicated for 1 h to form a CuPc suspension in ethanol.The suspension was left to stand for 24 h at room temperature to obtain a supernatant for later use.A silicon wafer substrate was clamped with forceps and immersed in the supernatant so that the nanoparticles in the suspension were transferred to the substrate.The nanoparticles acted as crystal nuclei on the substrate surface to induce the growth of 1D-CuPc.Materials with a length of more than 100 µm were obtained at a growth time of 4 h and a growth temperature of 400 • C, and using N 2 as the carrier gas at a flow rate of 20 mL/min.

Material Characterization
A Raman spectrometer (inVia Renishaw) was used to analyze phases and the vibrational characteristics of the sample.The structures of all the samples were measured via X-ray diffraction (XRD) (Bruker D8 Advance, with Cu-Kα radiation), and Fourier-transform infrared (FTIR) spectrometry (Bruker VERTEX 70, Saarbrücken, Germany).Field emission scanning electron microscopy (JSM-7610F Plus) was used to study the topography of the sample.The surface chemistry of the sample was measured via X-ray photoelectron spectroscopy (XPS K-Alpha+, Thermo Fisher Scientific, Waltham, MA, USA).
In this experiment, ethanol (C 2 H 6 O), acetone (C 3 H 6 O), and formaldehyde (CH 2 O) solutions were used to obtain 1000 ppm C 2 H 6 O, C 3 H 6 O, and CH 2 O gas environments, respectively.In addition, a 98%-relative-humidity environment was obtained using KNO 3saturated saline solution.The gas-sensing performance of the sensors was evaluated using a constant voltage of 2 V at 24 • C using a Keithley 2636b workstation (Keithley Instruments, Cleveland, OH, USA).The gas sensors were placed in different types of gas environments for testing, as shown in Figure 1.When the current stabilized, we switched to the next gas environment to continue the test.The current response of the gas performance test was defined as Response = (I G -I R )/I R , where I R and I G are the current of the sensor in the reference gas and the target gas, respectively.The response time was defined as the time period during which the sensor current reached 90% of the response value after exposure to the target gas, and the recovery time was defined as the time period when the sensor current become 10% of the response value after the target gas was removed.
In this experiment, ethanol (C2H6O), acetone (C3H6O), and formaldehyde (CH2O) solutions were used to obtain 1000 ppm C2H6O, C3H6O, and CH2O gas environments, respectively.In addition, a 98%-relative-humidity environment was obtained using KNO3saturated saline solution.The gas-sensing performance of the sensors was evaluated using a constant voltage of 2 V at 24 °C using a Keithley 2636b workstation (Keithley Instruments, Cleveland, OH, USA).The gas sensors were placed in different types of gas environments for testing, as shown in Figure 1.When the current stabilized, we switched to the next gas environment to continue the test.The current response of the gas performance test was defined as Response = (IG-IR)/IR, where IR and IG are the current of the sensor in the reference gas and the target gas, respectively.The response time was defined as the time period during which the sensor current reached 90% of the response value after exposure to the target gas, and the recovery time was defined as the time period when the sensor current become 10% of the response value after the target gas was removed.

Material Characterization and Analysis
The surface morphology of the 2D-MoS2/1D-CuPc heterojunction was characterized via SEM.As shown in Figure 2a-f, the morphologies of all samples were composed of MoS2 nanosheets and CuPc.The different mass ratios of MoS2 and CuPc changed the morphology of the 2D-MoS2/1D-CuPc heterojunction samples.Figure 2a shows the morphology of 3-CM, in which MoS2 nanosheets wrapped CuPc micro-nanowires, and MoS2 nanosheets were the main material.Figure 2b shows the morphology of 5-CM.Upon increasing the higher content of 3-CM CuPc micro-nanowires, MoS2 nanosheets were stacked on CuPc micro-nanowires.Figure 2c shows the morphology of 7-CM, in which MoS2 nanosheets penetrated the network of CuPC micro-nanowires, which should have allowed gas molecules to fully contact the material, and thus, improved the gas-sensing performance.Figure 2d shows that the MoS2 nanosheets were tightly bonded to the CuPC micro-nanowires to form a 2D-MoS2/1D-CuPc heterojunction.Figure 2e,f show 10-CM and 20-CM, respectively, which both had roughly the same structure, in which CuPc micro-nanowires were stacked to form conductive micro-nanowires.

Material Characterization and Analysis
The surface morphology of the 2D-MoS 2 /1D-CuPc heterojunction was characterized via SEM.As shown in Figure 2a-f, the morphologies of all samples were composed of MoS 2 nanosheets and CuPc.The different mass ratios of MoS 2 and CuPc changed the morphology of the 2D-MoS 2 /1D-CuPc heterojunction samples.Figure 2a shows the morphology of 3-CM, in which MoS 2 nanosheets wrapped CuPc micro-nanowires, and MoS 2 nanosheets were the main material.Figure 2b shows the morphology of 5-CM.Upon increasing the higher content of 3-CM CuPc micro-nanowires, MoS 2 nanosheets were stacked on CuPc micro-nanowires.Figure 2c shows the morphology of 7-CM, in which MoS 2 nanosheets penetrated the network of CuPC micro-nanowires, which should have allowed gas molecules to fully contact the material, and thus, improved the gas-sensing performance.Figure 2d shows that the MoS 2 nanosheets were tightly bonded to the CuPC micro-nanowires to form a 2D-MoS 2 /1D-CuPc heterojunction.Figure 2e,f show 10-CM and 20-CM, respectively, which both had roughly the same structure, in which CuPc micro-nanowires were stacked to form conductive micro-nanowires.
The elemental composition and valence states of the composites were tested via XPS.It is obvious that MoS2 was composed of Mo, S, C, and O, and CuPc was composed of C, O, N, and Cu.The 2D-MoS2/1D-CuPc heterojunction was composed of Mo, S, Cu, C, and N elements (Figure S2). Figure 5a shows the Mo 3d spectra of the 2D-MoS2 nanosheet and the 2D-MoS2/1D-CuPc heterojunction.For the pure MoS2 nanosheet, two peaks can be seen at 232.21 eV and 229.27 eV, which correspond to Mo 3d3/2 and Mo 3d5/2, respectively [30].In addition, the two faint peaks at 232.82 eV and 236.32 eV are typical signals of Mo 6+ 3d5/2 and Mo 6+ 3d3/2 in Mo-O bonds, respectively, suggesting that the introduction of O was caused by defects or vacancies on the surface of MoS2 [31].The overall shape of the 2D-MoS2/1D-CuPc heterojunction's Mo 3d spectrum was almost identical to that of pure MoS2 nanosheets.However, all peaks moved to lower binding energies, suggesting an increase in the electron cloud density around MoS2.A shift to lower binding energies was also  Figure 4 shows the Raman spectra of 3-CM, 5-CM, 7-CM, 10-CM, and 20-CM, in which the strong signals at 375.3 cm −1 and 401.9 cm −1 in Figure 4a were attributed to the in-plane E 1 2g and out-of-plane A 1g vibration modes [28].The characteristic peaks of the 7-CM out-of-plane A 1g vibration mode underwent a blue shift and their intensity was reduced, indicating that the MoS2 nanosheets had fewer layers than the other samples.In Figure 4b, the strong signals at 1445.7 cm −1 and 1520.2 cm −1 were attributed to the asymmetric vibration of C=N and the telescopic vibration of C=C [29], either due to the presence of CuPc, residual NMP, or their combination.The elemental composition and valence states of the composites were tested via XPS.It is obvious that MoS2 was composed of Mo, S, C, and O, and CuPc was composed of C, O, N, and Cu.The 2D-MoS2/1D-CuPc heterojunction was composed of Mo, S, Cu, C, and N elements (Figure S2). Figure 5a shows the Mo 3d spectra of the 2D-MoS2 nanosheet and the 2D-MoS2/1D-CuPc heterojunction.For the pure MoS2 nanosheet, two peaks can be seen at 232.21 eV and 229.27 eV, which correspond to Mo 3d3/2 and Mo 3d5/2, respectively [30].In addition, the two faint peaks at 232.82 eV and 236.32 eV are typical signals of Mo 6+ 3d5/2 and Mo 6+ 3d3/2 in Mo-O bonds, respectively, suggesting that the introduction of O was caused by defects or vacancies on the surface of MoS2 [31].The overall shape of the 2D-MoS2/1D-CuPc heterojunction's Mo 3d spectrum was almost identical to that of pure MoS2 nanosheets.However, all peaks moved to lower binding energies, suggesting an increase in the electron cloud density around MoS2.A shift to lower binding energies was also found in the S 2p spectrum (Figure 5b).The results showed that the pure MoS2 nanosheets The elemental composition and valence states of the composites were tested via XPS.It is obvious that MoS 2 was composed of Mo, S, C, and O, and CuPc was composed of C, O, N, and Cu.The 2D-MoS 2 /1D-CuPc heterojunction was composed of Mo, S, Cu, C, and N elements (Figure S2). Figure 5a shows the Mo 3d spectra of the 2D-MoS 2 nanosheet and the 2D-MoS 2 /1D-CuPc heterojunction.For the pure MoS 2 nanosheet, two peaks can be seen at 232.21 eV and 229.27 eV, which correspond to Mo 3d 3/2 and Mo 3d 5/2 , respectively [30].In addition, the two faint peaks at 232.82 eV and 236.32 eV are typical signals of Mo 6+ 3d 5/2 and Mo 6+ 3d 3/2 in Mo-O bonds, respectively, suggesting that the introduction of O was caused by defects or vacancies on the surface of MoS 2 [31].The overall shape of the 2D-MoS 2 /1D-CuPc heterojunction's Mo 3d spectrum was almost identical to that of pure MoS 2 nanosheets.However, all peaks moved to lower binding energies, suggesting an increase in the electron cloud density around MoS 2 .A shift to lower binding energies was also found in the S 2p spectrum (Figure 5b).The results showed that the pure MoS 2 nanosheets had two typical peaks at 162.15 eV and 163.40 eV, attributed to S 2p 3/2 and S 2p 1/2 , respectively, while the 2D-MoS 2 /1D-CuPc heterojunction had peaks at lower binding energies of 162.73 eV and 163.96 eV [9].Based on the above test results, it can be inferred that the superior interfacial contact at the 2D-MoS 2 /1D-CuPc heterojunction effectively transferred electrons.

Gas-Sensing Studies
The gas-sensing performance of gas sensors based on the 2D-MoS 2 /1D-CuPc heterojunction was studied using a homemade gas-sensing detection platform.Figure 7 shows the effect of the MoS 2 -to-CuPc ratio on the gas-sensing response.The response curve shows that the 2D-MoS 2 /1D-CuPc heterojunction with different doping ratios had a relatively stable response to 1000 ppm of formaldehyde (CH 2 O), acetone (C 3 H 6 O), ethanol (C 2 H 6 O), and 98% RH at room temperature.The response of the 2D-MoS 2 /1D-CuPc heterojunction to the four target gases first increased, and then, decreased as the proportion of CuPc increased.Figure 8a shows that the best gas-sensing performance was obtained when the    A significant advantage of this sensor array over a single-sensor array is that an unknown analyte can be identified and detected based on mathematical analyses, kinetics, and thermodynamics [35] or via principal component analysis (PCA) [36,37].The sensor array was made of 2D-MoS2/1D-CuPc with different MoS2/CuPc ratios.The recognition In the above reaction [32][33][34], e − represents electron conduction, and O − 2 (s) represents surface-adsorbed oxygen ions.C 2 H 6 O(g), C 3 H 6 O(g), and CH 2 O(g), respectively represent adsorbed C 2 H 6 O, C 3 H 6 O, and CH 2 O molecules.The reduced gas molecules released electrons into 2D-MoS 2 /1D-CuPc during the reaction, and the current of 7-CM increased rapidly, indicating a good synergistic effect between the materials at this ratio.
To further evaluate the effect of the ratio of MoS 2 and CuPc on the sensing performance of 2D-MoS 2 /1D-CuPc, the average response size, response time, and recovery time of the six sensors to four target analytes was analyzed.Figure 8b,c show that the response of the 2D-MoS 2 /1D-CuPc heterojunction to the analyte increased first, and then, decreased as the mass ratio of MoS 2 to CuPc decreased.The response time of all sensors was less than 60 s, and the recovery time was less than 10 s.This fast recovery was due to the good crystallinity of the sensitive structural elements, which provided the gas sensors with greater electron mobility.The one-dimensional structure facilitated the transport and transfer of electrons, as well as rapid response and recovery.
A significant advantage of this sensor array over a single-sensor array is that an unknown analyte can be identified and detected based on mathematical analyses, kinetics, and thermodynamics [35] or via principal component analysis (PCA) [36,37].The sensor array was made of 2D-MoS 2 /1D-CuPc with different MoS 2 /CuPc ratios.The recognition performance of the sensor array was further evaluated using 3D PCA (Figure 9).The coordinates of the four test samples could be distinguished, and the sensor identified all four samples.The results show that the 2D-MoS 2 /1D-CuPc heterojunction had high sensitivity and selectivity.
To further investigate the gas-sensing properties of the sensor array, its thermodynamic and kinetic parameters were combined, converted into a graphical signal, and combined with image recognition technology.The reaction amplitude and reaction time were taken as the thermodynamic and kinetic parameters, respectively.Six sensors showed six reaction amplitudes and six reaction times for each analyte.The ratio of the six reaction values to the six reaction times yielded six new thermodynamic and kinetic parameters, which were used to construct visually distinct hexagons, as shown in Figure 10.This shows that the sensor array had high recognition performance.performance of the sensor array was further evaluated using 3D PCA (Figure 9).The coordinates of the four test samples could be distinguished, and the sensor identified all four samples.The results show that the 2D-MoS2/1D-CuPc heterojunction had high sensitivity and selectivity.To further investigate the gas-sensing properties of the sensor array, its thermodynamic and kinetic parameters were combined, converted into a graphical signal, and combined with image recognition technology.The reaction amplitude and reaction time were taken as the thermodynamic and kinetic parameters, respectively.Six sensors showed six reaction amplitudes and six reaction times for each analyte.The ratio of the six reaction values to the six reaction times yielded six new thermodynamic and kinetic parameters, which were used to construct visually distinct hexagons, as shown in Figure 10.This shows that the sensor array had high recognition performance.
To study the real-time monitoring performance of 7-CM, dynamic tests with different concentrations of C3H6O and C2H6O were carried out.The relationship between response and vapor concentration at room temperature is shown in Figure 11. Figure 11a,c show that the surface C3H6O and C2H6O vapors of 7-CM had good gas-sensing characteristics.Figure 11b,d demonstrate that 7-CM had good linearity toward C3H6O and C2H6O vapors.The device had outstanding advantages in terms of quantitative detection, direct readout, simplified correction, and auxiliary circuitry.
To evaluate the performance of 2D-MoS2/1D-CuPc heterojunction-based sensors, Table 1 compares the sensing performance of C2H6O sensors with different sensing materials.Three-dimensional (MoS2)/ZnO and MoS2/TiO2 detected C2H6O, but their response time and recovery time were longer than those of 7-CM and MoS2/CeO2.MoS2/NiCo2O4 and CdS/MoS2 exhibited a faster response and recovery, but they only operated at temperatures above 100 °C, which means higher power consumption and a more complex operating environment.Compared with these composites, the 2D-MoS2/1D-CuPc heterojunction showed higher responses and shorter response times at room temperature.Especially, the responses of the 2D-MoS2/1D-CuPc composite were better than all the sensing materials in Table 1, showing good application potential.To evaluate the performance of 2D-MoS 2 /1D-CuPc heterojunction-based sensors, Table 1 compares the sensing performance of C 2 H 6 O sensors with different materials.Three-dimensional (MoS 2 )/ZnO and MoS 2 /TiO 2 detected C 2 H 6 O, but their response time and recovery time were longer than those of 7-CM and MoS 2 /CeO 2 .MoS 2 /NiCo 2 O 4 and CdS/MoS 2 exhibited a faster response and recovery, but they only operated at temperatures above 100 • C, which means higher power consumption and a more complex operating environment.Compared with these composites, the 2D-MoS 2 /1D-CuPc heterojunction showed higher responses and shorter response times at room temperature.Especially, the responses of the 2D-MoS 2 /1D-CuPc composite were better than all the sensing materials in Table 1, showing good application potential.

Analysis of Gas-Sensing Mechanism
Figure 12 shows the current-voltage (I-V) characteristic curves of MoS 2 , 3-CM, 5-CM, 7-CM, 10-CM, and 20-CM sensors, in which the conductivity of 3-CM to 7-CM decreased sequentially.This was attributed to an increase in the heterojunction barrier between MoS 2 nanosheets and CuPc micro-nanowires.In addition, as the mass ratio of CuPc increased, the electrical conductivity of the material decreased due to the lower electrical conductivity of CuPc compared with MoS 2 .The 7-CM surface contained dispersed CuPc micro-nanowires that ensured the penetration and diffusion of gas molecules, which improved the sensitivity of the sensor [46].
Figure 12 shows the current-voltage (I-V) characteristic curves of MoS2, 3-CM, 5-CM, 7-CM, 10-CM, and 20-CM sensors, in which the conductivity of 3-CM to 7-CM decreased sequentially.This was attributed to an increase in the heterojunction barrier between MoS2 nanosheets and CuPc micro-nanowires.In addition, as the mass ratio of CuPc increased, the electrical conductivity of the material decreased due to the lower electrical conductivity of CuPc compared with MoS2.The 7-CM surface contained dispersed CuPc micro-nanowires that ensured the penetration and diffusion of gas molecules, which improved the sensitivity of the sensor [46].Enhancements in the gas-sensing performance of the 2D-MoS2/1D-CuPc heterojunction were attributed to the heterojunction between MoS2 and CuPc.CuPc is a p-type semiconductor, while MoS2 is an n-type semiconductor.Figure 13a shows a schematic diagram of the band structure of MoS2 and CuPc, where Ef and Φ are the Fermi level and work function, respectively.The work functions of MoS2 and CuPc were 4.39 eV and 2.96 eV, respectively.Figure 13a shows that CuPc had a higher Fermi level than MoS2, allowing electrons to quickly transfer from CuPc to MoS2.Thus, the bands of CuPc and MoS2 began to shift until their Fermi levels reached a new equilibrium (Figure 13d) and generated a built-in Schottky barrier (qV0).Thus, a depletion layer existed at the interface between MoS2 and CuPc [47].When the 2D-MoS2/1D-CuPc composite contacted the target gas (e.g., ethanol), electron exchange occurred (Figure 13c).As a result, the conductivity of the MoS2/CuPc heterojunction was greatly increased, which improved its response.In addition, the CuPC micro-nanowires on the surface of the MoS2 nanosheets acted as a p-type dopant and increased the specific surface area of the 2D-MoS2/1D-CuPc heterojunction, Enhancements in the gas-sensing performance of the 2D-MoS 2 /1D-CuPc heterojunction were attributed to the heterojunction between MoS 2 and CuPc.CuPc is a p-type semiconductor, while MoS 2 is an n-type semiconductor.Figure 13a shows a schematic diagram of the band structure of MoS 2 and CuPc, where E f and Φ are the Fermi level and work function, respectively.The work functions of MoS 2 and CuPc were 4.39 eV and 2.96 eV, respectively.Figure 13a shows that CuPc had a higher Fermi level than MoS 2 , allowing electrons to quickly transfer from CuPc to MoS 2 .Thus, the bands of CuPc and MoS 2 began to shift until their Fermi levels reached a new equilibrium (Figure 13d) and generated a built-in Schottky barrier (qV 0 ).Thus, a depletion layer existed at the interface between MoS 2 and CuPc [47].When the 2D-MoS 2 /1D-CuPc composite contacted the target gas (e.g., ethanol), electron exchange occurred (Figure 13c).As a result, the conductivity of the MoS 2 /CuPc heterojunction was greatly increased, which improved its response.In addition, the CuP C micro-nanowires on the surface of the MoS 2 nanosheets acted as a p-type dopant and increased the specific surface area of the 2D-MoS 2 /1D-CuPc heterojunction, which provided more adsorption sites for target gas molecules, thus improving its sensitivity.The enhanced charge transfer and increased number of active sites improved the sensing performance of the 2D-MoS 2 /1D-CuPc heterojunction.

Conclusions
We prepared 2D-MoS2/1D-CuPc heterojunction composites with different mass ratios of MoS2 to CuPc via a heating/magnetic reflux stirring method, and then, used them to construct 2D-MoS2/1D-CuPc-based gas sensors.The results showed that the response of

Figure 1 .
Figure 1.Schematic of sensing test system.

Figure 1 .
Figure 1.Schematic of sensing test system.

Figure 5 .
Figure 5. XPS spectra of (a) Mo 3d and (b) S 2p of the different samples.

Figure 5 .
Figure 5. XPS spectra of (a) Mo 3d and (b) S 2p of the different samples.

Figure 5 .
Figure 5. XPS spectra of (a) Mo 3d and (b) S 2p of the different samples.

Figure 10 .
Figure 10.Characteristic fingerprints derived from kinetic and thermodynamic parameters from (a) CH 2 O, (b) C 3 H 6 O, (c) C 2 H 6 O, and (d) 98% RH at 1000 ppm at room temperature.To study the real-time monitoring performance of 7-CM, dynamic tests with different concentrations of C 3 H 6 O and C H 6 O were carried out.The relationship between response and vapor concentration at room temperature is shown in Figure 11. Figure 11a,c show that the surface C 3 H 6 O and C 2 H 6 O vapors of 7-CM had good gas-sensing characteristics.Figure 11b,d demonstrate that 7-CM had good linearity toward C 3 H 6 O and C 2 H 6 O vapors.The device had outstanding advantages in terms of quantitative detection, direct readout, simplified correction, and auxiliary circuitry.

Figure 11 .
Figure 11.(a) Sensing curves of the 7-CM sensor to different concentrations of C3H6O; (b) fitted plots of the response vs. C3H6O concentration; (c) sensing curves of the 7-CM sensor to different concentrations of C2H6O; (d) fitted plots of the response vs. C2H6O concentration.

Figure 11 .
Figure 11.(a) Sensing curves of the 7-CM sensor to different concentrations of C 3 H 6 O; (b) fitted plots of the response vs. C 3 H 6 O concentration; (c) sensing curves of the 7-CM sensor to different concentrations of C 2 H 6 O; (d) fitted plots of the response vs. C 2 H 6 O concentration.

Sensors 2023 ,
23, x FOR PEER REVIEW 13 of 15which provided more adsorption sites for target gas molecules, thus improving its sensitivity.The enhanced charge transfer and increased number of active sites improved the sensing performance of the 2D-MoS2/1D-CuPc heterojunction.

Figure 13 .
Figure 13.Schematic diagrams of (a) the band structure of MoS2 and CuPc; (b) the gas-sensing mechanism of MoS2/CuPc composites in the air; (c) the gas-sensing mechanism of MoS2/CuPc composites in ethanol; (d) the band structure of MoS2/CuPc composites in the air; (e) the band structure of MoS2/CuPc composites in ethanol.

Figure 13 .
Figure 13.Schematic diagrams of (a) the band structure of MoS 2 and CuPc; (b) the gas-sensing mechanism of MoS 2 /CuPc composites in the air; (c) the gas-sensing mechanism of MoS 2 /CuPc composites in ethanol; (d) the band structure of MoS 2 /CuPc composites in the air; (e) the band structure of MoS 2 /CuPc composites in ethanol.

Table 1 .
Comparison of C 2 H 6 O-sensing performance using different sensors.