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

Abstract

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.

1. 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], two-dimensional [10], and three-dimensional [11] materials. Layered molybdenum disulfide (MoS₂) 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 MoS₂ 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 MoS₂ and its composites. Hai et al. obtained monolayer and multilayered MoS₂ films via a mechanical peeling method using transparent tape. They prepared field-effect transistors (FETs) for NO gas detection for the first time by using photolithography. When 2–4 layers of MoS₂ were used, the FET was stable and showed a LOD of 0.8 ppm for NO, but the stability when using a MoS₂ monolayer was poor [17]. Li et al. prepared MoS₂ via a hydrothermal method and obtained resistive devices. At an operating temperature of 100 °C, the device showed good selectivity for carbon monoxide (CO) and hydrogen sulfide (H₂S), showing good repeatability and linearity [18]. Li Wenbo et al. prepared three different morphologies of molybdenum disulfide sensors for ammonia detection via a hydrothermal method. The results showed that the nanoflower-like MoS₂ sensor had a better response to ammonia and showed excellent repeatability, stability, and selectivity, with a response of 7.41% and a recovery time of 874 s toward NH₃ at 10 ppm. The response to NH₃ was 4.5 times as high as methanol and isopropanol and 8 times higher than acetone. Under the same conditions, the response values of the other two morphologies were 2.01% and 5.11%, respectively [19].

The above studies showed that although 2D-MoS₂ showed excellent gas-sensing performance when used alone, it was prone to stacking when used as the sensitive layer. This resulted in insufficient contact with the target gas, which reduced the sensitivity of the sensor and prolonged the response-recovery time of the sensor. Building on the research on 2D-MoS₂ sensors, researchers have found that constructing heterojunctions is a feasible research method for solving the above shortcomings. The heterostructure formed of composite materials and MoS₂ nanosheets can inhibit the interlayer agglomeration of MoS₂ sheets, thereby providing sufficient active sites for the adsorption of gas molecules. This can help improve the gas detection performance of MoS₂-based sensors [20]. Bu Xin et al. prepared MoS₂/Cd₂SnO₄ composites via water heating–calcination and studied the effect of MoS₂ doping on the gas-sensing properties. The experimental results showed that when the mass ratio between MoS₂ and Cd₂SnO₄ was 2.5%, the response of the gas-sensing element prepared using MoS₂/Cd₂SnO₄ composites to 100 ppm formaldehyde gas was 40 at 170 °C. This was 29 times that of pure MoS₂ for the same concentration of formaldehyde gas [21]. Zhang et al. prepared CeO₂/MoS₂ composites via a hydrothermal method, which exhibited a better room-temperature NO₂-sensing response and a 69.1% higher sensitivity to 5 ppm NO₂ than pure MoS₂. In total, 85% of the resistance was recovered within 900 s, and the sensor showed a fast response, long-term stability, and selectivity [22]. Hemalatha et al. designed an NH₃ sensor consisting of two-dimensional transition-metal dichalcogenide (mainly WS₂ and MoS₂), and polyaniline (PANI). The test results showed that the WS₂/PANI and MoS₂/PANI composites had higher sensitivity to NH₃ than individual MoS₂ and PANI. The response and recovery times of the composite films were 10/16 s and 14/16 s, respectively [23].

The above results show that 2D MoS₂ composites have better gas-sensing performance than monolayer MoS₂ but still have the following problems when used in traditional composite thin-film sensors: (1) the thin-film material is highly ordered, which is not conducive to gas diffusion in the membrane and affects the response speed of the sensor; (2) they have a low flexibility, which affects the practical applications of the sensor. Most research has used inorganic nanomaterials and 2D-MoS₂ composites, but there has been no research on organic one-dimensional (1D) materials and 2D-MoS₂ composites. Copper phthalocyanine (CuPc) is a typical organic semiconductor material [24] with good thermal and chemical stability. Its structure can be easily modified, and it has been widely used to design and research thin-film gas sensors with good selectivity and low operating temperatures. Moreover, one-dimensional CuPc exhibits better gas-sensing properties than bulk CuPc. Therefore, a heterojunction strategy was used to improve the performance of gas sensors by combining the advantages of 2-D MoS₂ and 1-D CuPc.

Here, we report a gas sensor based on a heterojunction between synthetic CuPc micro-nanowires and exfoliated MoS₂ nanosheets. One-dimensional CuPc was prepared via physical vapor deposition (PVD), and MoS₂/CuPc composites with different mass ratios were prepared via a magnetic reflux stirring method. The performance of 2D-MoS₂/1D-CuPc heterojunction gas sensing was studied using XRD, SEM, XPS, FTIR, and Raman spectroscopy.

2. Materials and Methods

2.1. Materials

Molybdenum disulfide (MoS₂) and copper phthalocyanine (CuPc) were purchased from Sigma Aldrich. Potassium sulfate (K₂SO₄) was purchased from Xuzhuangzi, Dongli District, Tianjin. Absolute ethanol (C₂H₆O), acetonitrile (CH₃CN), N-methyl pyrrolidone (C₅H₉NO), formaldehyde (CH₂O), and acetone (C₃H₆O) were purchased from Tianjin Zhiyuan Chemical Reagent Co., Ltd. (Tianjin, China). The above chemicals were of analytical grade (AR) and did not require further purification.

2.2. Synthesis of MoS₂ Nanosheets

MoS₂ nanosheets were prepared via grinding-assisted liquid phase stripping [25]. First, 100 mg of MoS₂ 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 MoS₂ nanosheets. Finally, the sample was dried in air for later use (Figure S1a).

2.3. 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 N₂ as the carrier gas at a flow rate of 20 mL/min.

2.4. Preparation of 2D-MoS₂/1D-CuPc Heterojunction

Composites were prepared via heating with magnetic reflux stirring. The prepared MoS₂ nanosheets (20 mg) were dispersed in 20 mL of absolute ethanol. Then, CuPc micro-nanowires were added, subjected to magnetic reflux stirring, and heated for 2 h. Finally, 2D-MoS₂/1D-CuPc heterojunction samples were obtained via centrifugation at 8000 r/min. A series of 2D-MoS₂/1D-CuPc heterojunction samples at different mass ratios were synthesized using the same method. The preparation of the 2D-MoS₂/1D-CuPc heterojunction is shown in Figure S1. The weight ratios of MoS₂ to CuPc were controlled at 20:3, 20:5, 20:7, 20:10, and 20:20, and the samples were named 3-CM, 5-CM, 7-CM, 10-CM, and 20-CM, respectively.

2.5. 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).

2.6. Gas-Sensing Performance Evaluation

First, 2D-MoS₂/1D-CuPc heterojunctions with different mass ratios (20:3, 20:5, 20:7, 20:10, and 20:20) were dispersed in absolute ethanol at a concentration of 10 mg/mL. Then, the dispersions were used to carry out uniform coating (2 μL MoS₂/CuPc composite dispersion) to fabricate a 2D-MoS₂/1D-CuPc-based sensing chip (13 mm × 7 mm, 0.5 mm thick) with Ag-Pd interdigital electrodes (IDEs). The minimum width and spacing of the electrodes was 0.2 mm). Finally, the interdigital electrode was dried at 60 °C and aged for 48 h at a voltage of 2 V to obtain a sensing chip with good stability (Figure S1c).

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 KNO₃-saturated 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.

3. Results and Discussion

3.1. 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.

Figure 3 shows the X-ray diffraction (XRD) patterns of CuPc micro-nanowires, MoS₂ nanosheets, and 2D-MoS₂/1D-CuPc (7-CM), which were used to determine the crystallinity and phase structure of the samples. Typical diffraction peaks at 7° and 9.2° were observed in the 2D-MoS₂/1D-CuPc pattern, which corresponded to the (−1,0,1) and (1,0,1) crystal planes of β-phase CuPc, respectively. The diffraction peaks at 14.4°, 32.6°, 33.5°, 35.8°, 39.5°, 44.2°, and 49.8° corresponded to the (0,0,2), (1,0,0), (1,0,1), (1,0,2), (1,0,3), (1,0,4), and (1,0,5) crystal planes of MoS₂. The results in Figure 4 show that CuPc was compounded onto the MoS₂ surface [27].

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 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 Mo⁶⁺ 3d_5/2 and Mo⁶⁺ 3d_3/2 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 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-MoS₂/1D-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-MoS₂/1D-CuPc heterojunction effectively transferred electrons.

Figure 6 shows the FTIR spectra of CuPc, MoS₂ NSs, 3-CM, 5-CM, 7-CM, 10-CM, and 20-CM. In Figure 6a, the spectra of MoS₂ nanosheets, 3-CM, 5-CM, 7-CM, 10-CM, and 20-CM all showed an absorption peak at 468 cm⁻¹. The magnification in Figure 6b shows that the intensity of the MoS₂ absorption peak increased with the MoS₂-to-CuPc ratio. Figure 6a also shows that CuPc, 3-CM, 5-CM, 7-CM, 10-CM, and 20-CM all contained similar absorption peaks, whose intensity decreased upon increasing the MoS₂-to-CuPc ratio. Among them, the peaks at 726 cm⁻¹, 752 cm⁻¹, and 780 cm⁻¹ belonged to the -C-H- out-of-plane bending vibration, and the peaks at 874 cm⁻¹, 900 cm⁻¹, 1065 cm⁻¹, 1086 cm⁻¹, and 1119 cm⁻¹ belonged to -C-C-telescopic vibrations. The peaks at 1166 cm⁻¹ and 1333 cm⁻¹ belonged to -C-O- telescopic vibration. The peak at 1285 cm⁻¹ belonged to -O-H-in-plane telescopic vibrations. The peaks at 1418 cm⁻¹, 1465 cm⁻¹, 1507 cm⁻¹, and 1611 cm⁻¹ belonged to telescopic vibrations of the -C=C- bonds in the phthalocyanine skeleton [30].

3.2. Gas-Sensing Studies

The gas-sensing performance of gas sensors based on the 2D-MoS₂/1D-CuPc heterojunction was studied using a homemade gas-sensing detection platform. Figure 7 shows the effect of the MoS₂-to-CuPc ratio on the gas-sensing response. The response curve shows that the 2D-MoS₂/1D-CuPc heterojunction with different doping ratios had a relatively stable response to 1000 ppm of formaldehyde (CH₂O), acetone (C₃H₆O), ethanol (C₂H₆O), and 98% RH at room temperature. The response of the 2D-MoS₂/1D-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 ratio of MoS₂ to CuPc was 20:7. Two-dimensional MoS₂/one-dimensional CuPc had the highest sensitivity toward C₂H₆O. The average responses of pure MoS₂ to 1000 ppm CH₂O, C₃H₆O, C₂H₆O, and 98% RH were 2.9, 18.4, 5.3, and 31.1, respectively. For 3-CM, the mean response of 1000 ppm CH₂O, C₃H₆O, C₂H₆O, and 98% RH increased to 5.2, 69.5, 630.6, and 43.3, respectively. The mean responses of 5-CM toward 1000 ppm CH₂O, C₃H₆O, C₂H₆O, and 98% RH were 12.4, 285.1, 745.5, and 400.7, respectively. The average responses of 7-CM to 1000 ppm CH₂O, C₃H₆O, C₂H₆O, and 98% RH were 125.6, 753.1, 1645.1, and 471.7, respectively. Compared with pure MoS₂, the response of 7-CM to 1000 ppm CH₂O, C₃H₆O, C₂H₆O, and 98% RH increased by 122.7, 734.6, 1639.8, and 440.5, respectively. When the ratio of MoS₂ to CuPc further decreased, the sensitivity of 2D-MoS₂/1D-CuPc began to decrease. The results showed that the responses of 10-CM to 1000 ppm CH₂O, C₃H₆O, C₂H₆O, and 98% RH were 30.9, 251.4, 72.9, and 396.8, respectively, which were all better than those of pure MoS₂. The responses of 20-CM to 1000 ppm CH₂O, C₃H₆O, C₂H₆O, and 98%RH were 4.2, 117.9, 16.8, and 32.1, respectively, which were also better than those of pure MoS₂. These results show that the ratio of MoS₂ and CuPc had a significant effect on the gas sensitivity of the 2D-MoS₂/1D-CuPc heterojunction.

When the 2D-MoS₂/1D-CuPc heterojunction sensor was exposed to C₂H₆O, C₃H₆O, and CH₂O gas, surface-adsorbed oxygen (${\mathrm{O}}_2^{-}$ < 100 °C, ${\mathrm{O}}_2^{-}$ 100–300 °C, ${\mathrm{O}}_2^{-}$ > 300 °C) reacted with C₂H₆O, C₃H₆O, and CH₂O as follows:

$${\mathrm{C}}_2{\mathrm{H}}_6\mathrm{O}\left(\mathrm{g}\right)+3{\mathrm{O}}_2^{-}\left(\mathrm{s}\right)\to 2\mathrm{C}{\mathrm{O}}_2+3{\mathrm{H}}_2\mathrm{O}+3{\mathrm{e}}^{-}$$

(1)

$${\mathrm{C}}_3{\mathrm{H}}_6\mathrm{O}\left(\mathrm{g}\right)+4{\mathrm{O}}_2^{-}\left(\mathrm{s}\right)\to 3\mathrm{C}{\mathrm{O}}_2+3{\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}$$

(2)

$$\mathrm{C}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)+{\mathrm{O}}_2^{-}\left(\mathrm{s}\right)\to \mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}$$

(3)

In the above reaction [32,33,34], e⁻ represents electron conduction, and ${\mathrm{O}}_2^{-}\left(\mathrm{s}\right)$ represents surface-adsorbed oxygen ions. ${\mathrm{C}}_2{\mathrm{H}}_6\mathrm{O}\left(\mathrm{g}\right)$, ${ \mathrm{C}}_3{\mathrm{H}}_6\mathrm{O}\left(\mathrm{g}\right)$, and$\mathrm{C}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)$, respectively represent adsorbed C₂H₆O, C₃H₆O, and CH₂O molecules. The reduced gas molecules released electrons into 2D-MoS₂/1D-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 MoS₂ and CuPc on the sensing performance of 2D-MoS₂/1D-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-MoS₂/1D-CuPc heterojunction to the analyte increased first, and then, decreased as the mass ratio of MoS₂ 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-MoS₂/1D-CuPc with different MoS₂/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-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. Comparison of C₂H₆O-sensing performance using different sensors.

Materials	Working Temperature	Concentration (ppm)	Response	Res/Rec Time (s)	Ref.
SnO2QD/MoS2	180 °C	50	0.49	25/85	[38]
MoS2/CeO2	RT	50	7.78	7/5	[39]
3D(MoS2)/ZnO	220 °C	500	12.08	30/10	[40]
MoS2/TiO2	350 °C	100	0.62	52/155	[41]
MoS2/PSi	RT	40	0.17	---	[42]
MoS2/NiCo2O4	170 °C	100	9.00	15/6	[43]
MoS2Nanosheets	RT	114	1.14	2.4/29	[44]
CdS/MoS2	210 °C	100	0.95	20/9	[45]
2D-MoS2/1D-CuPc	RT	100	710.42	<10/<4	This work
		50	497.69

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.

4. Analysis of Gas-Sensing Mechanism

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 E_f 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 CuP_C 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, 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-MoS₂/1D-CuPc heterojunction.

5. 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 the 2D-MoS₂/1D-CuPc heterojunction to gas analytes was related to the mass ratio of MoS₂ to CuPc. When the ratio of MoS₂ to CuPc was 20:7, the best gas-sensing performance of the 2D-MoS₂/1D-CuPc heterojunction was obtained. Compared with the MoS₂ sensor, the responses of 7-CM to 1000 ppm CH₂O, C₃H₆O, C₂H₆O, and 98% RH were increased by 122.7, 734.6, 1639.8, and 440.5, respectively, at room temperature. According to PCA and radar analysis, a 2D-MoS₂/1D-CuPc heterojunction sensor array could identify and detect four target gases. After further data processing, the sensor array showed higher recognition ability. These results provide an experimental reference for the development of high-performance MoS₂-based gas sensors.