Conductometric Gas Sensor Based on MoO3 Nanoribbon Modified with rGO Nanosheets for Ethylenediamine Detection at Room Temperature

An ethylenediamine (EDA) gas sensor based on a composite of MoO3 nanoribbon and reduced graphene oxide (rGO) was fabricated in this work. MoO3 nanoribbon/rGO composites were synthesized using a hydrothermal process. The crystal structure, morphology, and elemental composition of MoO3/rGO were analyzed via XRD, FT-IR, Raman, TEM, SEM, XPS, and EPR characterization. The response value of MoO3/rGO to 100 ppm ethylenediamine was 843.7 at room temperature, 1.9 times higher than that of MoO3 nanoribbons. The MoO3/rGO sensor has a low detection limit (LOD) of 0.235 ppm, short response time (8 s), good selectivity, and long-term stability. The improved gas-sensitive performance of MoO3/rGO composites is mainly due to the excellent electron transport properties of graphene, the generation of heterojunctions, the higher content of oxygen vacancies, and the large specific surface area in the composites. This study presents a new approach to efficiently and selectively detect ethylenediamine vapor with low power.


Introduction
Ethylenediamine (C 2 H 8 N 2 , EDA) is an important chemical feedstock widely used in petrochemical and pharmaceutical applications, printing, dyeing, electroplating, and fine chemical intermediates [1,2]. However, EDA is a hazardous chemical that is volatile, corrosive, and flammable, causing environmental pollution and representing a serious threat to human health [3]. EDA vapors can invade the body through the respiratory system and skin, thus causing serious health problems such as conjunctivitis, pneumonia, contact dermatitis, asthma, liver and kidney dysfunction, and even tumors [4,5]. The permissible concentration-short term exposure limit (PC-STEL) for EDA is 10 ppm [6]. Therefore, achieving real-time, effective detection and monitoring of EDA is essential to production life safety. Currently, most methods for detecting EDA rely on expensive and complex instruments such as mass spectrometry, gas chromatography, liquid chromatography, and fluorescence probes [7,8]. Therefore, there is a critical challenge in developing economical, portable, realtime, and selective EDA inspection devices that offer room-temperature operation.

Preparation of MoO 3 /rGO
In the preparation of MoO 3 /rGO nanocomposites, GO (7 mg) was ultrasonically dispersed in 10 mL of deionized water (DIW) for 2 h to obtain the GO solution. A quantity of 0.618 g of AHM was dissolved in 15 mL of DIW and then added to the above GO solution with continuous stirring for 0.5 h. Subsequently, 2.5 mL of HNO 3 was added into the mixed solution under stirring conditions and maintained for 0.5 h. The solution was transferred to a 50 mL stainless steel reactor and heated at 180 • C for 20 h. The precipitate was washed several times with DIW and ethanol and then dried overnight at 70 • C in a vacuum oven. Pure MoO 3 nanoribbons were synthesized without the addition of GO using the procedure described above.

Sensor Fabrication and Measurement
The as-synthesized MoO 3 and MoO 3 /rGO powders were ultrasonically dispersed in ethanol, which was applied directly onto Au-interdigitated electrodes (spacing: about 50 µm) through the drop-cast method. After drop casting, the prepared devices were heated at 70 • C for 12 h in a vacuum atmosphere to evaporate the water molecules completely, thus generating a dry thin film to bridge the interdigitated electrodes.
The static gas-sensing performance of the obtained products was tested using a WS-30B measurement system (China), and the bias voltage of 5 V, relative humidity (RH) of 20 ± 2%, and room temperature (23 ± 2 • C) were controlled during measurements. The operational process and test principles for the sensors are similar to those in previous studies [26]. S = R a /R g or S = R g /R a represents the response (S) of the sensor, where R a and R g are the resistance value of the sensor in the air and the target gas, respectively. Response/recovery time is defined here as the time required to achieve 90% of the total change in resistance value.

Electrochemical Measurements
All electrochemical tests were performed in a three-electrode system using a CHI660E electrochemical workstation (Shanghai, China) with 0.1 M sodium sulfate (Na 2 SO 4 ) as the electrolyte. The saturated Ag/AgCl electrode, Pt electrode, and Glassy carbon electrode (GCE) were used as the reference electrode, counter electrode, and working electrode, respectively. An amount of 5 mg of the sample was dispersed in 2 mL of ethanol and 10 µL of Nafion solution, and then the solution was drop coated on the GCE and dried at room temperature. The prepared electrodes were immersed in the electrolyte for 1 h to ensure that the open-circuit voltage (OCP) was stabilized before starting electrochemical measurements. The electrochemical impedance spectroscopy (EIS) measurement assay settings were as follows: OCP bias 0.34 V, frequency range 0.1-100 kHz, and ac amplitude 5 mV. For the Mott-Schottky (MS) measurements, the increments were 20 mV, the frequency was 1000 Hz, and amplitude was 5 mV.

Structural and Morphological Characterization
The XRD spectra (Figure 1a)   , respectively. The intensity of the diffraction peaks in the (020), (040), and (060) crystal planes of MoO 3 /rGO were found to be clearly higher than those of MoO 3 , indicating the presence of a layered crystal structure for the anisotropic growth of MoO 3 in composites [27,28]. The characteristic diffraction peak of rGO was not found in the XRD pattern of MoO 3 /rGO composites. This is mainly attributed to the relatively low content and peak intensity of rGO [29]. Figure 1b indicates the FT-IR spectra of pure MoO 3 and MoO 3 /rGO composites. Here, the three absorption peaks in the range of 500-1000 cm −1 for MoO 3 at approximately 559, 875, and 998 cm −1 correspond to the stretching vibra-tions of (Mo 3 -O), (Mo 2 -O), and (Mo=O), respectively [11,30]. For MoO 3 /rGO, two new absorption peaks are visible at about 1231 and 1613 cm −1 and assigned to the C=C and C-O-C stretching vibrations of rGO [31]. The FT-IR test results confirmed the presence of rGO in the composites. Raman spectroscopy is an effective means of characterizing carbon materials with the usual features of G-band and D-band, and that of MoO 3 and MoO 3 /rGO is shown In Figure 1c. The characteristic peaks at 994 and 818 cm −1 are ascribed to the stretching vibrations of the Mo=O bond, and at 665 cm −1 , they correspond to the asymmetrical stretching vibrations of the Mo 2 -O bond [32]. The peaks in the range of 100-400 cm −1 are related to the various bending modes of α-MoO 3 crystals [33]. Besides the peaks of MoO 3 , the D (1350 cm −1 ) and G (1598 cm −1 ) characteristic peaks of rGO also exist in MoO 3 /rGO. The D band is assigned to the breathing mode of sp 3 -hybridized carbon, structural defects, and amorphous carbon, whereas the G band corresponds to the scattering mode of sp 2 carbon [34]. Raman spectroscopy further confirmed the successful preparation of MoO 3 /rGO composites.
MoO3/rGO composites. Here, the three absorption peaks in the range of 500-1000 cm −1 for MoO3 at approximately 559, 875, and 998 cm −1 correspond to the stretching vibrations of (Mo3-O), (Mo2-O), and (Mo=O), respectively [11,30]. For MoO3/rGO, two new absorption peaks are visible at about 1231 and 1613 cm −1 and assigned to the C=C and C-O-C stretching vibrations of rGO [31]. The FT-IR test results confirmed the presence of rGO in the composites. Raman spectroscopy is an effective means of characterizing carbon materials with the usual features of G-band and D-band, and that of MoO3 and MoO3/rGO is shown In Figure 1c. The characteristic peaks at 994 and 818 cm −1 are ascribed to the stretching vibrations of the Mo=O bond, and at 665 cm −1 , they correspond to the asymmetrical stretching vibrations of the Mo2-O bond [32]. The peaks in the range of 100-400 cm −1 are related to the various bending modes of α-MoO3 crystals [33]. Besides the peaks of MoO3, the D (1350 cm −1 ) and G (1598 cm −1 ) characteristic peaks of rGO also exist in MoO3/rGO. The D band is assigned to the breathing mode of sp 3 -hybridized carbon, structural defects, and amorphous carbon, whereas the G band corresponds to the scattering mode of sp 2 carbon [34]. Raman spectroscopy further confirmed the successful preparation of MoO3/rGO composites.  [35,36]. Together, the TEM and SAED suggest that orthorhombic MoO3 nanoribbons grow mainly along the [001] direction [37].  [35,36]. Together, the TEM and SAED suggest that orthorhombic MoO 3 nanoribbons grow mainly along the [001] direction [37].
The full XPS spectra (Figure 3a) of MoO 3 and MoO 3 /rGO indicate the presence of Mo, O, and C elements in both materials. The C 1s peak at 284.8 eV in the XPS full spectrum of pure MoO 3 was caused by the C contamination of the analyzer. Figure 3b presents the C 1s high-resolution spectra of MoO 3 /rGO. The fitted peaks at 284.6, 286.1, and 288.7 eV were ascribed to the C-C, C-O, and C=O groups [18,31]. Figure 3c shows high-resolution Mo 3d XPS spectra of pure MoO 3 and MoO 3 /rGO. The peaks at 233.1 and 236.2 eV are assigned to the binding energies of Mo 3d 3/2 and Mo 3d 5/2 orbital electrons of Mo 6+ [35]. Moreover, the binding energies at about 232.1 eV (Mo 3d 5/2 ) and 235.2 eV (Mo 3d 3/2 ) correspond to Mo 5+ [35]. Table 1 presents the relative content of Mo 5+ and Mo 6+ in the two materials. The relative content of Mo 5+ in the composites increased from 4.6% to 9.7% compared to pure MoO 3 . This result demonstrates the higher content of oxygen vacancies in the composites [38]. Figure [39]. The content of oxygen species in MoO 3 and MoO 3 /rGO is listed in Table 1. Evidently, the O C and O V content in the composites is higher than that of MoO 3 . The full XPS spectra (Figure 3a) of MoO3 and MoO3/rGO indicate the presence of Mo, O, and C elements in both materials. The C 1s peak at 284.8 eV in the XPS full spectrum of pure MoO3 was caused by the C contamination of the analyzer. Figure 3b presents the C 1s high-resolution spectra of MoO3/rGO. The fitted peaks at 284.6, 286.1, and 288.7 eV were ascribed to the C-C, C-O, and C=O groups [18,31]. Figure 3c shows high-resolution Mo 3d XPS spectra of pure MoO3 and MoO3/rGO. The peaks at 233.1 and 236.2 eV are assigned to the binding energies of Mo 3d3/2 and Mo 3d5/2 orbital electrons of Mo 6+ [35]. Moreover, the binding energies at about 232.1 eV (Mo 3d5/2) and 235.2 eV (Mo 3d3/2) correspond to Mo 5+ [35]. Table 1 presents the relative content of Mo 5+ and Mo 6+ in the two materials. The relative content of Mo 5+ in the composites increased from 4.6% to 9.7% compared to pure MoO3. This result demonstrates the higher content of oxygen vacancies in the composites [38]. Figure 3d illustrates the high-resolution spectra of O 1s. The peaks of binding energy around 530.4, 531.4, and 532.5 eV are ascribed to the lattice oxygen (OL), oxygen vacancies (OV), and chemisorbed oxygen (OC) of the samples, respectively [39]. The content of oxygen species in MoO3 and MoO3/rGO is listed in Table 1. Evidently, the OC and OV content in the composites is higher than that of MoO3.    XPS analysis indicated that the complexes contained more oxygen vacancies and chemisorbed oxygen. Both materials were examined via EPR and O 2 -TPD to further examine the content of oxygen vacancies and chemisorbed oxygen. Figure 4a provides the EPR spectra of MoO 3 and MoO 3 /rGO. Both samples display a Lorentz line (g = 2.001).
Here, the signal intensity of the MoO 3 /rGO composite is significantly higher than that of pure MoO 3 . This result means that MoO 3 /rGO has more oxygen vacancies [40]. Figure 4b presents the O 2 -TPD curves of MoO 3 and MoO 3 /rGO composites. The resolved peaks at lower temperatures (<100 • C) are attributed to physisorbed oxygen, and the peaks at higher temperatures (250-550 • C) are associated with chemisorbed oxygen [41]. Compared to pure MoO 3 , the chemisorbed oxygen peak of the MoO 3 /rGO composite presented a lower resolution temperature and larger peak area. This result illustrates that the chemisorbed oxygen is more active in the composite [42]. The analysis of the EPR and O 2 -TPD spectra is in agreement with the XPS results. It is well known that the presence of more adsorbed oxygen species corresponds to more gas-sensing properties in sensors [43].
x FOR PEER REVIEW 7 of 16 The N2 adsorption-desorption isotherms of MoO3 and MoO3/rGO are depicted in Figure 5. The specific surface areas of MoO3 and MoO3/rGO were 7.7 and 39.3 m 2 /g, respectively. It is evident that MoO3/rGO has a larger specific surface area. According to the IUPAC classification, the isotherms of both samples can be classified as type IV isotherms with H3-type hysteresis loops, indicating the presence of mesoporous structures [44]. The average pore sizes of MoO3 and MoO3/rGO were 3.46 and 3.39 nm, respectively, when calculated by the BJH method. This result demonstrates that MoO3 and MoO3/rGO are mesoporous materials. In addition, MoO3/rGO (0.047 m 3 /g) has a larger pore volume than MoO3 (0.037 m 3 /g). The larger specific surface area and rich pore channels of MoO3/rGO allow for the exposure of many active sites to interact with the target gas [9,45]. The N 2 adsorption-desorption isotherms of MoO 3 and MoO 3 /rGO are depicted in Figure 5. The specific surface areas of MoO 3 and MoO 3 /rGO were 7.7 and 39.3 m 2 /g, respectively. It is evident that MoO 3 /rGO has a larger specific surface area. According to the IUPAC classification, the isotherms of both samples can be classified as type IV isotherms with H3-type hysteresis loops, indicating the presence of mesoporous structures [44]. The average pore sizes of MoO 3 and MoO 3 /rGO were 3.46 and 3.39 nm, respectively, when calculated by the BJH method. This result demonstrates that MoO 3 and MoO 3 /rGO are mesoporous materials. In addition, MoO 3 /rGO (0.047 m 3 /g) has a larger pore volume than MoO 3 (0.037 m 3 /g). The larger specific surface area and rich pore channels of MoO 3 /rGO allow for the exposure of many active sites to interact with the target gas [9,45].
IUPAC classification, the isotherms of both samples can be classified as type IV isotherms with H3-type hysteresis loops, indicating the presence of mesoporous structures [44]. The average pore sizes of MoO3 and MoO3/rGO were 3.46 and 3.39 nm, respectively, when calculated by the BJH method. This result demonstrates that MoO3 and MoO3/rGO are mesoporous materials. In addition, MoO3/rGO (0.047 m 3 /g) has a larger pore volume than MoO3 (0.037 m 3 /g). The larger specific surface area and rich pore channels of MoO3/rGO allow for the exposure of many active sites to interact with the target gas [9,45].  The comparison shows that the Rct of the MoO3/rGO composites is obviously smaller than that of pure MoO3, which suggests that rGO helps to increase the charge migration rate in the composite [46]. The MS test results show that the slope of the curve is positive for both materials. This result illustrates that both MoO3 and MoO3/rGO have the conductive characteristics of n-type semiconductors.  The comparison shows that the Rct of the MoO 3 /rGO composites is obviously smaller than that of pure MoO 3 , which suggests that rGO helps to increase the charge migration rate in the composite [46]. The MS test results show that the slope of the curve is positive for both materials. This result illustrates that both MoO 3 and MoO 3 /rGO have the conductive characteristics of n-type semiconductors.      (5) and (6), respectively [26]. The test results indicate that MoO 3 /rGO has a wider detection range and higher sensitivity than the pure MoO 3 sensor.  Response/recovery time and repeatability of gas-sensitive materials to target gas are crucial indexes to assess the sensitivity of gas-sensitive sensors. Therefore, the response/recovery curves of the MoO3 and MoO3/rGO sensors were compared for 100 ppm EDA gas at room temperature (see Figure 8a,b). Response/recovery time for the MoO3 and MoO3/rGO sensors to 100 ppm EDA gas were 18/901 s and 8/357 s, respectively. The MoO3/rGO sensor yielded a shorter response/recovery time than the pure MoO3 sensor. Figure 8c,d show the cyclic response curves for MoO3 and MoO3/rGO to 100 ppm EDA gas at room temperature. The sensor response values do not vary significantly over the five cycles, indicating that both sensors have good cycling stability. K represents the slope of the fitted curve at low concentrations, and Z represents the standard deviation of the response values.

Gas-Sensing Properties
Response/recovery time and repeatability of gas-sensitive materials to target gas are crucial indexes to assess the sensitivity of gas-sensitive sensors. Therefore, the response/recovery curves of the MoO 3 and MoO 3 /rGO sensors were compared for 100 ppm EDA gas at room temperature (see Figure 8a  Selectivity and long-term stability are key parameters for measuring sensor performance in practical applications. The results of the selectivity tests for the MoO3 and MoO3/rGO gas sensors are plotted in Figure 9a. Obviously, the gas response values of MoO3 and MoO3/rGO sensors are much higher for 100 ppm EDA gas than for the other interfering gases (100 ppm triethylamine, NH3, ethanol, formaldehyde, and acetone gas). This result demonstrates the excellent selectivity of MoO3 and MoO3/rGO sensors. The long-term stability of MoO3 and MoO3/rGO tests was evaluated once a week for eight weeks, and the results are summarized in Figure 9b. The response values of MoO3 and MoO3/rGO varied less than 5% over time, indicating that the sensors have excellent longterm stability. The response and recovery times of these two sensors during long-term stability tests are shown in the inset of Figure 9b. The response and recovery time of the MoO3 sensor in general increased with the increase in the test period, which is caused by the agglomeration of the MoO3 nanoribbons. The response and recovery times of the MoO3/rGO sensor fluctuated in magnitude, though not significantly, compared to the first week. MoO3/rGO is more stable compared to MoO3 nanoribbons, which is mainly due to the incorporation of rGO, which can effectively prevent the agglomeration of nanoribbons in the composites. Overall, the MoO3/rGO sensor showed high response values, good function matching, fast responses, excellent selectivity, and long-term stability for EDA gas detection with potential for practical applications. Selectivity and long-term stability are key parameters for measuring sensor performance in practical applications. The results of the selectivity tests for the MoO 3 and MoO 3 /rGO gas sensors are plotted in Figure 9a. Obviously, the gas response values of MoO 3 and MoO 3 /rGO sensors are much higher for 100 ppm EDA gas than for the other interfering gases (100 ppm triethylamine, NH 3 , ethanol, formaldehyde, and acetone gas). This result demonstrates the excellent selectivity of MoO 3 and MoO 3 /rGO sensors. The long-term stability of MoO 3 and MoO 3 /rGO tests was evaluated once a week for eight weeks, and the results are summarized in Figure 9b. The response values of MoO 3 and MoO 3 /rGO varied less than 5% over time, indicating that the sensors have excellent long-term stability. The response and recovery times of these two sensors during long-term stability tests are shown in the inset of Figure 9b. The response and recovery time of the MoO 3 sensor in general increased with the increase in the test period, which is caused by the agglomeration of the MoO 3 nanoribbons. The response and recovery times of the MoO 3 /rGO sensor fluctuated in magnitude, though not significantly, compared to the first week. MoO 3 /rGO is more stable compared to MoO 3 nanoribbons, which is mainly due to the incorporation of rGO, which can effectively prevent the agglomeration of nanoribbons in the composites. Overall, the MoO 3 /rGO sensor showed high response values, good function matching, fast responses, excellent selectivity, and long-term stability for EDA gas detection with potential for practical applications. We compared the MoO3/rGO sensor with other sensors used for EDA detection (Table 2). The MoO3/rGO sensor has good gas-sensitive performance for EDA with high response values and low detection limits. The MoO3/rGO sensor fabricated in this work is potentially valuable for industrial applications.

Gas-Sensing Mechanism
The resistance changes in the gas-sensitive characteristics of metal oxide semiconductors arise from the chemisorption and desorption of gases on the surfaces of materials [22]. The MoO3/rGO composite exhibited an n-type nature in performance tests measuring sensitivity to EDA vapor. Therefore, the resistance changes in MoO3/rGO are caused by variation in the electron concentration in the material [54]. Figure 10 illustrates the sensing mode of MoO3/rGO composites in the air and the EDA vapor. When the MoO3/rGO sensor was exposed to the air atmosphere, O2 molecules in the air were adsorbed onto the composite surface and captured electrons from the material to form adsorbed oxygen species (O − 2 ) (Equations (7) and (8)) [15]. Simultaneously, an electron depletion layer formed on the MoO3 surface, causing a decrease in the charge carrier density of the composite and increasing sensor resistance [29]. When the sensor was exposed to reduced EDA vapor, the EDA molecules adsorbed onto the material's surface and interacted with the adsorbed oxygen species (Equation (9)) [55]. Meanwhile, the electrons were released from the reaction and back into the material, thereby inducing a decrease in the thickness of the depletion layer and reducing the resistance of the composites [43]. When the sensor returned to We compared the MoO 3 /rGO sensor with other sensors used for EDA detection ( Table 2). The MoO 3 /rGO sensor has good gas-sensitive performance for EDA with high response values and low detection limits. The MoO 3 /rGO sensor fabricated in this work is potentially valuable for industrial applications.

Gas-Sensing Mechanism
The resistance changes in the gas-sensitive characteristics of metal oxide semiconductors arise from the chemisorption and desorption of gases on the surfaces of materials [22]. The MoO 3 /rGO composite exhibited an n-type nature in performance tests measuring sensitivity to EDA vapor. Therefore, the resistance changes in MoO 3 /rGO are caused by variation in the electron concentration in the material [54]. Figure 10 illustrates the sensing mode of MoO 3 /rGO composites in the air and the EDA vapor. When the MoO 3 /rGO sensor was exposed to the air atmosphere, O 2 molecules in the air were adsorbed onto the composite surface and captured electrons from the material to form adsorbed oxygen species (O − 2 ) (Equations (7) and (8)) [15]. Simultaneously, an electron depletion layer formed on the MoO 3 surface, causing a decrease in the charge carrier density of the composite and increasing sensor resistance [29]. When the sensor was exposed to reduced EDA vapor, the EDA molecules adsorbed onto the material's surface and interacted with the adsorbed oxygen species (Equation (9)) [55]. Meanwhile, the electrons were released from the reaction and back into the material, thereby inducing a decrease in the thickness of the depletion layer and reducing the resistance of the composites [43]. When the sensor returned to the air, O 2 molecules were adsorbed back onto the surface of the nanobelts. This caused the electron depletion layer to rebuild and the resistance to return to its initial value.
the air, O2 molecules were adsorbed back onto the surface of the nanobelts. This caused the electron depletion layer to rebuild and the resistance to return to its initial value.
O2(gas) → O2(ads) The excellent gas-sensitive performance of MoO3/rGO with EDA vapor at room temperature can be attributed to three main factors. First, 1D MoO3 features a layered structure formed by the alternating stacking of octahedral MoO6 bilayer planes in the [010] direction, and the [010] crystal planes of MoO3 nanoribbons have higher catalytic activity [27]. This facilitates the adsorption and diffusion of gas molecules, exposes more active sites, and provides a fast transport path for electrons along the axial direction, thus improving gas-sensitive performance [56]. Secondly, rGO nanosheets can prevent the stacking and agglomeration of MoO3 nanobelts, and rGO itself has high electron mobility, which gives the composites a larger specific surface area, more abundant pore channels, and higher electron transport capacity [24]. This not only provides more adsorption sites and effective diffusion pathways for EDA gases but also shortens the response/recovery time, further improving the sensing performance [20]. Third, heterogeneous structures are formed between the two materials when MoO3 is combined with rGO. The work function of the n-type material MoO3 (5.3 eV) [57] is different from that of the p-type material rGO (4.8 eV) [19]. The electrons in rGO are transferred to MoO3 to balance the Fermi energy level (Ef), which causes the energy band to bend and increases the electron concentration of MoO3 in the composite [58]. This process allows for more electrons to be trapped by O2 molecules adsorbed on the MoO3 surface, thereby forming more adsorbed oxygen species and providing more active sites for the material [21,59]. Hence, the gas-sensitive performance is considerably enhanced.

Conclusions
In summary, MoO3/rGO composites were fabricated using a hydrothermal method to develop the first MOS-based resistive gas sensor for the detection of EDA gases. At The excellent gas-sensitive performance of MoO 3 /rGO with EDA vapor at room temperature can be attributed to three main factors. First, 1D MoO 3 features a layered structure formed by the alternating stacking of octahedral MoO 6 bilayer planes in the [010] direction, and the [010] crystal planes of MoO 3 nanoribbons have higher catalytic activity [27]. This facilitates the adsorption and diffusion of gas molecules, exposes more active sites, and provides a fast transport path for electrons along the axial direction, thus improving gas-sensitive performance [56]. Secondly, rGO nanosheets can prevent the stacking and agglomeration of MoO 3 nanobelts, and rGO itself has high electron mobility, which gives the composites a larger specific surface area, more abundant pore channels, and higher electron transport capacity [24]. This not only provides more adsorption sites and effective diffusion pathways for EDA gases but also shortens the response/recovery time, further improving the sensing performance [20]. Third, heterogeneous structures are formed between the two materials when MoO 3 is combined with rGO. The work function of the n-type material MoO 3 (5.3 eV) [57] is different from that of the p-type material rGO (4.8 eV) [19]. The electrons in rGO are transferred to MoO 3 to balance the Fermi energy level (Ef), which causes the energy band to bend and increases the electron concentration of MoO 3 in the composite [58]. This process allows for more electrons to be trapped by O 2 molecules adsorbed on the MoO 3 surface, thereby forming more adsorbed oxygen species and providing more active sites for the material [21,59]. Hence, the gas-sensitive performance is considerably enhanced.

Conclusions
In summary, MoO 3 /rGO composites were fabricated using a hydrothermal method to develop the first MOS-based resistive gas sensor for the detection of EDA gases. At room temperature, the MoO 3 /rGO composites exhibited higher response values (834.7), shorter response/recovery times (8/357 s), and lower detection limits (0.235 ppm) for EDA compared to pure MoO 3 nanobelts. In addition, the MoO 3 /rGO composites exhibited good selectivity and long-term stability. The outstanding gas-sensitive performance of this sensor mainly contributed to the formation of heterojunctions between MoO 3 nanoribbons and rGO alongside the large specific surface area, abundant oxygen vacancies, and good electron transport properties of rGO. This study provides a new direction for the design and application of highly selective and responsive ethylenediamine sensors at room temperature. Data Availability Statement: Data will be made available on request.

Conflicts of Interest:
The authors declare no conflict of interest.