Synthesis of Cu2O-Modified Reduced Graphene Oxide for NO2 Sensors

Nowadays, metal oxide semiconductors (MOS)-reduced graphene oxide (rGO) nanocomposites have attracted significant research attention for gas sensing applications. Herein, a novel composite material is synthesized by combining two p-type semiconductors, i.e., Cu2O and rGO, and a p-p-type gas sensor is assembled for NO2 detection. Briefly, polypyrrole-coated cuprous oxide nanowires (PPy/Cu2O) are prepared via hydrothermal method and combined with graphene oxide (GO). Then, the nanocomposite (rGO/PPy/Cu2O) is obtained by using high-temperature thermal reduction under Ar atmosphere. The results reveal that the as-prepared rGO/PPy/Cu2O nanocomposite exhibits a maximum NO2 response of 42.5% and is capable of detecting NO2 at a low concentration of 200 ppb. Overall, the as-prepared rGO/PPy/Cu2O nanocomposite demonstrates excellent sensitivity, reversibility, repeatability, and selectivity for NO2 sensing applications.


Introduction
NO 2 , as a major air pollutant, is responsible for acid rain and hazardous to human respiratory tracts. According to the World Health Organization (WHO), the safety limit for NO 2 gas is 410 ppb per hour [1]. Hence, monitoring the trace amounts of NO 2 is necessary from health perspective and plays an important role in environmental pollution [2], air quality, and industrial safety [3][4][5][6][7].
The two-dimensional graphene, discovered by Novoselov et al. in 2004 [8], is widely employed as a promising sensing material due to its high specific surface area (2.6 × 10 3 m 2 /g) [9][10][11][12][13], ultra-high room-temperature electron mobility (2.0 × 10 5 cm 2 /Vs), and chemical stability [14,15]. Additionally, graphene can be easily and cost-effectively prepared by a wide range of techniques, such as mechanical peeling [8], chemical vapor deposition [16,17], silicon carbide (SiC) epitaxial growth [18], redox method [19], and other methods. It has been reported that the changes in the external chemical environment result in significant differences in the sensing performance of graphene [20]. Schedin et al. have first reported the performance of graphene-based gas sensors in 2007 [21], however, the asprepared sensors exhibited distinct disadvantages, such as slow as well as low response and poor selectivity [22,23].
Similar to graphene, metal oxides (MO) can also be used as sensing materials; however, MO-based sensors also possess some defects. The first MO-based commercial sensor appeared in the 1960s [24]. Moreover, the operating temperature of MO-based sensors ranges

Materials
All chemical reagents were of analytical grade and used as received without further purification. Pyrrole, acetic acid, ethanol, acetone, concentrated sulphuric acid, and hydrogen peroxide were purchased from the Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Copper acetate monohydrate was obtained from the Gretel Pharmaceutical Technology Co., Ltd., Suzhou, China.

Synthesis of Polypyrrole-Coated Cu 2 O Nanowires
PPy-coated Cu 2 O nanowires were prepared by a one-step hydrothermal method [47,48], where copper acetate monohydrate was used as a precursor and pyrrole acted as a reducing and structure-directing agent under weak acidic conditions. Initially, 0.2 g of copper acetate monohydrate were added in a certain amount of deionized water and ultrasonicated for 5-10 min for complete dissolution. Then, the pyrrole monomer (0.075 mL) was added into a small amount of deionized water. After transient ultrasonication, it was slowly added to the above solution and stirred with a glass rod. Then, 0.15 mL of acetic acid (1 mol/L) was added to the above mixture to make sure the dissolution of pyrrole monomers. The resulting mixture was transferred to an autoclave and placed in an oven at 120 • C for 12 h. Finally, the reaction solution was cooled down to room temperature and polypyrrole-coated copper oxide nanowires were obtained by sequentially washing with deionized water, ethanol, and acetone, followed by filtration and drying. The hydrothermal temperature of 120, 140, and 160 • C was also used to investigate the influence of growth temperature on structure and morphology.

Preparation of rGO/PPy/Cu 2 O Nanocomposites
GO was prepared by the modified Hummers method [49]. First, a certain amount of PPy/Cu 2 O nanowires was added into ethanol and ultrasonicated for 30 min to obtain a concentration of 2 mg/mL. A certain amount of 2 mg/mL GO solution was added into the above solution, ultrasonicated for 30 min and magnetically stirred for 3-5 h. Finally, the completely dispersed and uniformly mixed solution was filtered, washed, and dried to obtain GO/PPy/Cu 2 O nanocomposite. The as-prepared GO/PPy/Cu 2 O nanocomposites were placed in a tube furnace and heated at 350 • C for 1 h under the protection of Ar gas, resulting in the reduction in GO and formation of rGO/PPy/Cu 2 O nanocomposites.

Fabrication of rGO/PPy/Cu 2 O-Based Gas Sensor
Herein, the interdigitated electrode for gas sensing was prepared by the lift-off process [13]. First, a silicon wafer was heated at 90 • C in a mixture of concentrated H 2 SO 4 and H 2 O 2 for a certain time to obtain a hydrophilic silicon substrate. Then, the photoresist was applied to the silicon substrate. After a series of operations, we have obtained the planar interdigital electrodes by using the self-designed mask plate after exposure, sputtering and peeling. The as-prepared rGO/PPy/Cu 2 O nanocomposite was ultrasonically dispersed in ethanol to obtain a suspension with a concentration of 1 mg/mL. The same amount of dispersion was measured by a micropipette and applied to the interdigital electrode. The two poles of the interdigital electrode were connected through the gas sensing material. To observe it visually, SEM characterization of the device is shown in Figure 1. Finally, the as-prepared sensor was vacuum-dried before further characterization.
peeling. The as-prepared rGO/PPy/Cu2O nanocomposite was ultrasonically dispersed in ethanol to obtain a suspension with a concentration of 1 mg/mL. The same amount of dispersion was measured by a micropipette and applied to the interdigital electrode. The two poles of the interdigital electrode were connected through the gas sensing material. To observe it visually, SEM characterization of the device is shown in Figure 1. Finally, the as-prepared sensor was vacuum-dried before further characterization.

Gas Sensor Sensitivity
In order to simulate the real detection environment, we have utilized compressed air as a background and dilution gas during the gas sensitivity test. The test temperature was set at 25 °C and mild test conditions were used, which are comparable to the practical applications. Figure 2 shows a sketch of the gas sensing set-up. Prior to the test, the background gas was introduced for a certain time to remove the residual exhaust gas from the gas path. Then, the desired concentration of NO2 gas was introduced at the beginning of the test. The NO2 concentration (CN) can be given as: The concentration of the NO2 cylinder was 5000 ppm. FN (sccm) refers to the flow rate of the NO2 gas and FC (slm) represents the flow rate of diluted gas (air).

Gas Sensor Sensitivity
In order to simulate the real detection environment, we have utilized compressed air as a background and dilution gas during the gas sensitivity test. The test temperature was set at 25 • C and mild test conditions were used, which are comparable to the practical applications. Figure 2 shows a sketch of the gas sensing set-up. Prior to the test, the background gas was introduced for a certain time to remove the residual exhaust gas from the gas path. Then, the desired concentration of NO 2 gas was introduced at the beginning of the test. The NO 2 concentration (C N ) can be given as:  When NO2 gas passed through the sensor, the hole concentration of as-prepared rGO/PPy/Cu2O nanocomposite increased and the resistance decreased due to the adsorption between NO2 gas and as-prepared rGO/PPy/Cu2O nanocomposite. The gas sensitivity (S, %) can be calculated from the change in resistance using the I-T curve, as given below: where Ra represents the resistance of sensor in air and Rg corresponds to the resistance after the introduction of the NO2 gas. The concentration of the NO 2 cylinder was 5000 ppm. F N (sccm) refers to the flow rate of the NO 2 gas and F C (slm) represents the flow rate of diluted gas (air).

Characterization of as-Prepared Nanocomposites
When NO 2 gas passed through the sensor, the hole concentration of as-prepared rGO/PPy/Cu 2 O nanocomposite increased and the resistance decreased due to the adsorption between NO 2 gas and as-prepared rGO/PPy/Cu 2 O nanocomposite. The gas sensitivity (S, %) can be calculated from the change in resistance using the I-T curve, as given below: where R a represents the resistance of sensor in air and R g corresponds to the resistance after the introduction of the NO 2 gas.

Characterization of as-Prepared Nanocomposites
Scanning electron microscopy (SEM) is employed to explore the influence of different growth temperatures on the morphology and microstructure of PPy/Cu 2 O nanowires. As shown in Figure 3a, the length of copper oxide nanowires, grown at 120 • C, ranges from tens to hundreds of microns. Additionally, a smooth surface with uniform thickness is achieved (Figure 3b). However, when the growth temperature is increased to 140 • C, the copper oxide nanowires started to bend and exhibited different lengths (Figure 3c). One should note that the shorter copper oxide nanowires are not desirable for subsequent preparation of conductive films. As shown in Figure 3d, the further increase in growth temperature to 160 • C resulted in shorter copper oxide nanowires. Hence, the lower growth temperature is more favorable for copper oxide nanowires. One should also note that the growth temperature of <120 • C is not sufficient to produce copper oxide nanowires. In addition, Cu 2 O with a completely linear structure can be obtained at these reaction temperatures.  Furthermore, the graphene content also influences the morphology of resulting nanocomposites. It can be readily observed that graphene facilitates the recombination and coating of PPy/Cu2O nanowires. On the other hand, the excessive amount of graphene leads to the stacking of graphene sheets, which is highly undesirable for sensing applications. SEM is utilized to observe the morphology and microstructure of as-prepared rGO/PPy/Cu2O nanocomposites ( Figure 4). Overall, the utilization ratio of graphene increased with increasing graphene content in as-prepared rGO/PPy/Cu2O nanocomposites. However, the excess of graphene will decrease the exposure of PPy-coated Cu2O nanowires and result in uneven dispersion and stacking of graphene sheets. Furthermore, the graphene content also influences the morphology of resulting nanocomposites. It can be readily observed that graphene facilitates the recombination and coating of PPy/Cu 2 O nanowires. On the other hand, the excessive amount of graphene leads to the stacking of graphene sheets, which is highly undesirable for sensing applications. SEM is utilized to observe the morphology and microstructure of as-prepared rGO/PPy/Cu 2 O nanocomposites ( Figure 4). Overall, the utilization ratio of graphene increased with increasing graphene content in as-prepared rGO/PPy/Cu 2 O nanocomposites. However, the excess of graphene will decrease the exposure of PPy-coated Cu 2 O nanowires and result in uneven dispersion and stacking of graphene sheets. applications. SEM is utilized to observe the morphology and microstructure of as-prepared rGO/PPy/Cu2O nanocomposites ( Figure 4). Overall, the utilization ratio of graphene increased with increasing graphene content in as-prepared rGO/PPy/Cu2O nanocomposites. However, the excess of graphene will decrease the exposure of PPy-coated Cu2O nanowires and result in uneven dispersion and stacking of graphene sheets.  The structure of as-prepared nanocomposites was confirmed by X-ray diffraction (XRD). Figure [50,51]. In the XRD patterns of three hybrid structures, we can clearly see the diffraction peaks of Cu 2 O and the (111) and (200) peaks exhibit a relatively high intensity. Similarly, the characteristic diffraction peaks of graphene oxide and reduced graphene oxide are observed at 2θ = 10.2 • and 23.1 • , respectively. Since the relative quantity of Cu 2 O is much higher than rGO, the diffraction peaks of Cu 2 O are significantly stronger than the rGO. In addition, we have not observed phases other than rGO and Cu 2 O. These results confirm that the rGO/PPy/Cu 2 O nanocomposites have been successfully prepared after high-temperature reduction. The structure of as-prepared nanocomposites was confirmed by X-ray diffraction (XRD). Figure [50,51]. In the XRD patterns of three hybrid structures, we can clearly see the diffraction peaks of Cu2O and the (111) and (200) peaks exhibit a relatively high intensity. Similarly, the characteristic diffraction peaks of graphene oxide and reduced graphene oxide are observed at 2θ = 10.2° and 23.1°, respectively. Since the relative quantity of Cu2O is much higher than rGO, the diffraction peaks of Cu2O are significantly stronger than the rGO. In addition, we have not observed phases other than rGO and Cu2O. These results confirm that the rGO/PPy/Cu2O nanocomposites have been successfully prepared after high-temperature reduction. Moreover, Raman spectroscopy is carried out to confirm that the graphene oxide is successfully transformed into the reduced graphene oxide (rGO) [52][53][54][55]. Figure 6 shows two characteristic Raman peaks at 1333 and 1582 cm −1 , corresponding to D-and G-bands, respectively. The D-band is related to defect scattering and electron/hole recombination Moreover, Raman spectroscopy is carried out to confirm that the graphene oxide is successfully transformed into the reduced graphene oxide (rGO) [52][53][54][55]. Figure 6 shows two characteristic Raman peaks at 1333 and 1582 cm −1 , corresponding to D-and G-bands, respectively. The D-band is related to defect scattering and electron/hole recombination during oxidation and reduction processes. Overall, the intensity of D-band represents the degree of disorder in graphene. On the other hand, the G-band is related to the bond stretching of all pairs of sp 2 atoms, indicating the integrity of sp 2 hybridized structure. In general, the reduction in graphene is analyzed by measuring the intensity ratio of D-to G-bands (I D /I G ).  Figure 6 exhibits that the ID/IG ratio of GO/PPy/Cu2O and rGO/PPy/Cu2O nanocomposite is 1.133 and 1.153, respectively. Theoretically, when GO is reduced, the oxygen-containing functional groups on the graphene sheets are removed [56], the ordering of sp 2 carbon network structure is increased, sp 2 region is widened and the ID/IG ratio is decreased. In fact, a large number of sp 3 hybridized carbon atoms deoxidize to form a new sp 2 hybridized region, and the re-formed sp 2 region is smaller than GO, minimizing the average sp 2 region of rGO, which is reflected by the enhancement of ID/IG. To further illustrate the successful fabrication of rGO/PPy/Cu2O nanocomposites, we have employed Fourier transform infrared spectroscopy (FTIR) to characterize the changes in functional groups before and after high-temperature thermal reduction (Figure 7). The absorption peak near 3250 cm −1 can be attributed to N-H stretching vibrations of PPy and O-H stretching vibrations of GO. The absorption peak at 1552 cm −1 can be assigned to the vibrations of C=C skeleton, whereas the absorption peaks at 1323 and 1074 cm −1 can be attributed to the stretching vibrations of C-N, confirming the existence of PPy in as-prepared rGO/PPy/Cu2O nanocomposites. In the case of GO/PPy/Cu2O nanocomposite, the absorption peaks at 1625 and 1716 cm −1 correspond to the vibrational absorption of -COOH and C=O in carboxylic acids, respectively. One should note that the absorption intensity of -COOH and C=O groups in the FTIR spectrum of rGO/PPy/Cu2O nanocomposites is weakened, whereas the absorption peak of C-O at 1040 cm −1 is disappeared, indicating the reduction in oxygen-containing functional groups from the graphene surface and confirming the successful transformation of GO into rGO.   133 and 1.153, respectively. Theoretically, when GO is reduced, the oxygencontaining functional groups on the graphene sheets are removed [56], the ordering of sp 2 carbon network structure is increased, sp 2 region is widened and the ID/IG ratio is decreased. In fact, a large number of sp 3 hybridized carbon atoms deoxidize to form a new sp 2 hybridized region, and the re-formed sp 2 region is smaller than GO, minimizing the average sp 2 region of rGO, which is reflected by the enhancement of ID/IG. To further illustrate the successful fabrication of rGO/PPy/Cu 2 O nanocomposites, we have employed Fourier transform infrared spectroscopy (FTIR) to characterize the changes in functional groups before and after high-temperature thermal reduction (Figure 7). Furthermore, we have utilized X-ray photoelectron spectroscopy (XPS) to qualitatively analyze the elemental composition of as-prepared rGO/PPy/Cu2O nanocomposites. Figure 8 shows the wide-range and high-resolution C 1s XPS spectra of GO and rGO/PPy/Cu2O. The characteristic peaks of C-C/C=C (284.6 eV), C-O (286.9 eV), C=O (287.8 eV), and COOH (289.0 eV) can be clearly observed in the high-resolution C 1s spectrum (Figure 8b) [57]. The characteristic peak of C-N (285.5 eV) is observed in the high-resolution C 1s spectrum of the as-prepared rGO/PPy/Cu2O nanocomposite ( Figure  8d). Compared with the graphene oxide, the intensity of C-O, C=O, and COOH peaks is weakened in rGO/PPy/Cu2O nanocomposites due to the high-temperature thermal reduction in GO.  Furthermore, we have utilized X-ray photoelectron spectroscopy (XPS) to qualitatively analyze the elemental composition of as-prepared rGO/PPy/Cu 2 O nanocomposites. Figure 8 shows the wide-range and high-resolution C 1s XPS spectra of GO and rGO/PPy/Cu 2 O. The characteristic peaks of C-C/C=C (284.6 eV), C-O (286.9 eV), C=O (287.8 eV), and COOH (289.0 eV) can be clearly observed in the high-resolution C 1s spectrum (Figure 8b) [57]. The characteristic peak of C-N (285.5 eV) is observed in the highresolution C 1s spectrum of the as-prepared rGO/PPy/Cu 2 O nanocomposite (Figure 8d). Compared with the graphene oxide, the intensity of C-O, C=O, and COOH peaks is weakened in rGO/PPy/Cu 2 O nanocomposites due to the high-temperature thermal reduction in GO. Furthermore, we have utilized X-ray photoelectron spectroscopy (XPS) to qualitatively analyze the elemental composition of as-prepared rGO/PPy/Cu2O nanocomposites. Figure 8 shows the wide-range and high-resolution C 1s XPS spectra of GO and rGO/PPy/Cu2O. The characteristic peaks of C-C/C=C (284.6 eV), C-O (286.9 eV), C=O (287.8 eV), and COOH (289.0 eV) can be clearly observed in the high-resolution C 1s spectrum (Figure 8b) [57]. The characteristic peak of C-N (285.5 eV) is observed in the high-resolution C 1s spectrum of the as-prepared rGO/PPy/Cu2O nanocomposite ( Figure  8d). Compared with the graphene oxide, the intensity of C-O, C=O, and COOH peaks is weakened in rGO/PPy/Cu2O nanocomposites due to the high-temperature thermal reduction in GO.   Overall, SEM, XRD, Raman spectroscopy. FTIR and XPS confirm the successful synthesis of rGO/PPy/Cu 2 O nanocomposites, confirming the elemental composition and structure. Figure 9 presents the response curves of PPy-coated Cu 2 O nanowires sensor and rGO/PPy/Cu 2 O nanocomposite sensors, with different graphene contents, to NO 2 flow of 50 ppm. The mass ratio of GO to PPy/Cu 2 O nanowires was set at 0.08, 0.1, 0.12, 0.15, and 0.2, and the resulting rGO/PPy/Cu 2 O nanocomposites are named as D 0 , E 0 , F 0 , G 0 , and J 0 , respectively. Herein, the resistance response reached the maximum value within 300 sec after the introduction of NO 2 gas. The gas-sensitive response values of D 0 , E 0 , F 0 , G 0 , and J 0 were found to be 25.0, 42.5, 35.9, 30.0, and 25.1%, respectively. The ratio of 0.1 composite presents a maximum response, which is about 2.7 times of the sensor based on pure PPy-coated Cu 2 O nanowires (15.7%). Additionally, the sensor recovered the initial resistance level after ≈200 sec under the auxiliary irradiations of an ultraviolet lamp. The experimental results reveal that the rGO/PPy/Cu 2 O-based sensor renders superior gas sensing response, reaching the maximum response value of 42.5% at GO content of E 0 . The further increase in graphene content leads to restacking of graphene sheets and loss of excellent graphene properties, resulting in an inferior gas sensing response. Overall, SEM, XRD, Raman spectroscopy. FTIR and XPS confirm the successful synthesis of rGO/PPy/Cu2O nanocomposites, confirming the elemental composition and structure. Figure 9 presents the response curves of PPy-coated Cu2O nanowires sensor and rGO/PPy/Cu2O nanocomposite sensors, with different graphene contents, to NO2 flow of 50 ppm. The mass ratio of GO to PPy/Cu2O nanowires was set at 0.08, 0.1, 0.12, 0.15, and 0.2, and the resulting rGO/PPy/Cu2O nanocomposites are named as D0, E0, F0, G0, and J0, respectively. Herein, the resistance response reached the maximum value within 300 sec after the introduction of NO2 gas. The gas-sensitive response values of D0, E0, F0, G0, and J0 were found to be 25.0, 42.5, 35.9, 30.0, and 25.1%, respectively. The ratio of 0.1 composite presents a maximum response, which is about 2.7 times of the sensor based on pure PPy-coated Cu2O nanowires (15.7%). Additionally, the sensor recovered the initial resistance level after ≈200 sec under the auxiliary irradiations of an ultraviolet lamp. The experimental results reveal that the rGO/PPy/Cu2O-based sensor renders superior gas sensing response, reaching the maximum response value of 42.5% at GO content of E0. The further increase in graphene content leads to restacking of graphene sheets and loss of excellent graphene properties, resulting in an inferior gas sensing response.  [6]. One should note that the E0-based gas sensor rendered excellent gas sensing response at low NO2 concentrations, which indicates the superior NO2 adsorption effect of the as-prepared rGO/PPy/Cu2O nanocomposite, resulting in adsorption saturation in a relatively small time and high gas sensitivity. The sensor response with respect to NO2 concentration is mainly nonlinear [58,59] because of the Langmuir adsorption of NO2 on the surface of active substance. As the concentration of the target gas increases, the adsorption reaches saturation level and results in a decrease in response.  [6]. One should note that the E 0 -based gas sensor rendered excellent gas sensing response at low NO 2 concentrations, which indicates the superior NO 2 adsorption effect of the as-prepared rGO/PPy/Cu 2 O nanocomposite, resulting in adsorption saturation in a relatively small time and high gas sensitivity. The sensor response with respect to NO 2 concentration is mainly nonlinear [58,59] because of the Langmuir adsorption of NO 2 on the surface of active substance. As the concentration of the target gas increases, the adsorption reaches saturation level and results in a decrease in response. From a practical viewpoint, sustainable reuse is of great significance for gas sensors. Figure 11 shows the repeated gas sensing response evaluation of the E0-based gas sensor at NO2 flow of 50 ppm, showing excellent repeatability with one cycle consisting of almost 600 s. First of all, the response reaches the saturation level after 300 s of NO2 gas injection. Then, the NO2 gas is turned-off and background gas is turned-on at the same time. Under the illumination of ultraviolet lamp, NO2 gas is gradually desorbed and blown away by air. The sensor begins to gradually recover the initial resistance level. In this way, three cycles of cyclic testing are carried out to detect the repeatability of the E0-based gas sensor. Figure 11 confirms that the gas sensing response of the E0-based sensor at 50 ppm of NO2 gas is stable at ~43.0%. After three cycles, the response sensitivity does not decrease significantly, which further confirms that the as-prepared E0-based gas sensor possesses excellent response stability and repeatability. Under normal usage conditions, the GO/PPy/Cu2O-based gas sensor demonstrates excellent stability with only a slight decline in the response of 1.5% after 30 days, indicating good long-term stability. Figure 11. The repeatability and cyclic stability of the E0-based gas sensor. From a practical viewpoint, sustainable reuse is of great significance for gas sensors. Figure 11 shows the repeated gas sensing response evaluation of the E 0 -based gas sensor at NO 2 flow of 50 ppm, showing excellent repeatability with one cycle consisting of almost 600 s. First of all, the response reaches the saturation level after 300 s of NO 2 gas injection. Then, the NO 2 gas is turned-off and background gas is turned-on at the same time. Under the illumination of ultraviolet lamp, NO 2 gas is gradually desorbed and blown away by air. The sensor begins to gradually recover the initial resistance level. In this way, three cycles of cyclic testing are carried out to detect the repeatability of the E 0 -based gas sensor. Figure 11 confirms that the gas sensing response of the E 0 -based sensor at 50 ppm of NO 2 gas is stable at~43.0%. After three cycles, the response sensitivity does not decrease significantly, which further confirms that the as-prepared E 0 -based gas sensor possesses excellent response stability and repeatability. Under normal usage conditions, the GO/PPy/Cu 2 O-based gas sensor demonstrates excellent stability with only a slight decline in the response of 1.5% after 30 days, indicating good long-term stability. From a practical viewpoint, sustainable reuse is of great significance for gas sensors. Figure 11 shows the repeated gas sensing response evaluation of the E0-based gas sensor at NO2 flow of 50 ppm, showing excellent repeatability with one cycle consisting of almost 600 s. First of all, the response reaches the saturation level after 300 s of NO2 gas injection. Then, the NO2 gas is turned-off and background gas is turned-on at the same time. Under the illumination of ultraviolet lamp, NO2 gas is gradually desorbed and blown away by air. The sensor begins to gradually recover the initial resistance level. In this way, three cycles of cyclic testing are carried out to detect the repeatability of the E0-based gas sensor. Figure 11 confirms that the gas sensing response of the E0-based sensor at 50 ppm of NO2 gas is stable at ~43.0%. After three cycles, the response sensitivity does not decrease significantly, which further confirms that the as-prepared E0-based gas sensor possesses excellent response stability and repeatability. Under normal usage conditions, the GO/PPy/Cu2O-based gas sensor demonstrates excellent stability with only a slight decline in the response of 1.5% after 30 days, indicating good long-term stability. Figure 11. The repeatability and cyclic stability of the E0-based gas sensor. Figure 11. The repeatability and cyclic stability of the E 0 -based gas sensor.
Furthermore, it is of utmost importance to assess the selectivity of adsorbed gas in practical applications. Therefore, we have investigated the adsorption of different industrial and laboratory gases, such as chloroform, formaldehyde, ethanol, acetone, and ethyl acetate, by the E 0 -based gas sensor. The saturation vapor pressure of 1% is obtained by the bubbling method and the response of NO 2 gas (50 ppm) is used as a comparison point to assess sensor selectivity ( Figure 12). Figure 12 shows that the response of E 0 -based sensor to other gases is extremely low. For instance, formaldehyde exhibited the highest response of 2.5% among the tested gases, which is much lower than the response of 50 ppm NO 2 gas (42.5%). One should note that the concentration of these gases is much higher than 50 ppm. Still, the E 0 -based sensor demonstrated superior selectivity to the NO 2 gas.
Furthermore, it is of utmost importance to assess the selectivity of adsorbed gas in practical applications. Therefore, we have investigated the adsorption of different industrial and laboratory gases, such as chloroform, formaldehyde, ethanol, acetone, and ethyl acetate, by the E0-based gas sensor. The saturation vapor pressure of 1% is obtained by the bubbling method and the response of NO2 gas (50 ppm) is used as a comparison point to assess sensor selectivity ( Figure 12). Figure 12 shows that the response of E0-based sensor to other gases is extremely low. For instance, formaldehyde exhibited the highest response of 2.5% among the tested gases, which is much lower than the response of 50 ppm NO2 gas (42.5%). One should note that the concentration of these gases is much higher than 50 ppm. Still, the E0-based sensor demonstrated superior selectivity to the NO2 gas.

Sensing Mechanism
Cu2O, rGO, and PPy have similar p-type nature [37,60]. When the composite material is exposed to air, O2 molecules could be adsorbed on the material surface in the form of adsorbed After the introduction of NO2, NO2 molecules could be directly adsorbed on the surface by capturing electrons from the material (Equation (4)). In addition, NO2 also gains electrons from adsorbed oxygen ions (Equation (5) Figure 13 illustrates the gas sensing mechanism. After the above process [61], the hole concentration of the device increases. Herein, p-type polypyrrole completes the process of doping and de-doping by gas adsorption and desorption, respectively [62,63]. Meanwhile, graphene and polypyrrole provide a large number of binding sites for gas adsorption. The high charge mobility of conducting polymer, i.e., polypyrrole, and graphene facilitates carrier transport and migration to the electrode for collection. These

Sensing Mechanism
Cu 2 O, rGO, and PPy have similar p-type nature [37,60]. When the composite material is exposed to air, O 2 molecules could be adsorbed on the material surface in the form of adsorbed After the introduction of NO 2 , NO 2 molecules could be directly adsorbed on the surface by capturing electrons from the material (Equation (4)). In addition, NO 2 also gains electrons from adsorbed oxygen ions (Equation (5)). Figure 13 illustrates the gas sensing mechanism. After the above process [61], the hole concentration of the device increases. Herein, p-type polypyrrole completes the process of doping and de-doping by gas adsorption and desorption, respectively [62,63]. Meanwhile, graphene and polypyrrole provide a large number of binding sites for gas adsorption. The high charge mobility of conducting polymer, i.e., polypyrrole, and graphene facilitates carrier transport and migration to the electrode for collection. These processes lead to the rapid decrease in electron concentration within the rGO/PPy/Cu 2 O composites. In general, hole-assisted carrier transport is responsible for the conduction of p-type semiconductors. The hole concentration significantly increases with the decrease in electron concentration in rGO/PPy/Cu 2 O composite due to NO 2 adsorption, which increases sensor conductivity. processes lead to the rapid decrease in electron concentration within the rGO/PPy/Cu2O composites. In general, hole-assisted carrier transport is responsible for the conduction of p-type semiconductors. The hole concentration significantly increases with the decrease in electron concentration in rGO/PPy/Cu2O composite due to NO2 adsorption, which increases sensor conductivity. According to the principle of complementary feedback of gas sensor [64], the combination of p-type semiconductors in gas sensors renders a synergistic influence on gas sensing characteristics and temperature coefficients of both materials, reducing zero drift, shortening initial relaxation time, and rendering superior selectivity and stability. Herein, the interdigital electrode is equivalent to the parallel connection of a sensor and several resistors, which reduces the initial resistance of the sensor. The decrease in initial resistance of sensor increased the change in resistance, which corresponds to the response value. During the recovery stage of gas sensor, the newly adsorbed air molecules eliminate residual NO2 molecules from the surface of rGO/PPy/Cu2O nanocomposite by introducing air and auxiliary irradiations under an ultraviolet lamp [65], increasing the resistivity of p-type semiconductor and recovering to the initial resistance.

Conclusions
In summary, PPy-coated Cu2O nanowires have been prepared by the hydrothermal reaction and combined with graphene oxide to obtain rGO/PPy/Cu2O nanocomposites after high-temperature thermal reduction. Moreover, a p-p-type gas sensor has been fabricated using rGO/PPy/Cu2O nanocomposite as an electrode and room temperature sensing is realized. The results revealed that the rGO/PPy/Cu2O-based gas sensor renders better NO2 sensing performance than the PPy/Cu2O-based sensor, confirming the positive influence of graphene addition. When the mass ratio of graphene to PPy-coated Cu2O nanowires was 0.1, the rGO/PPy/Cu2O-based sensor demonstrated the highest response value of 42.5% for NO2 gas (50 ppm). When the concentration of NO2 was as low as 200 ppb, the rGO/PPy/Cu2O-based sensor still exhibited a response value of 20.3%. Moreover, the rGO/PPy/Cu2O-based sensor has also rendered stable repeatability and excellent selectivity at the NO2 concentration of 50 ppm.  According to the principle of complementary feedback of gas sensor [64], the combination of p-type semiconductors in gas sensors renders a synergistic influence on gas sensing characteristics and temperature coefficients of both materials, reducing zero drift, shortening initial relaxation time, and rendering superior selectivity and stability. Herein, the interdigital electrode is equivalent to the parallel connection of a sensor and several resistors, which reduces the initial resistance of the sensor. The decrease in initial resistance of sensor increased the change in resistance, which corresponds to the response value. During the recovery stage of gas sensor, the newly adsorbed air molecules eliminate residual NO 2 molecules from the surface of rGO/PPy/Cu 2 O nanocomposite by introducing air and auxiliary irradiations under an ultraviolet lamp [65], increasing the resistivity of p-type semiconductor and recovering to the initial resistance.

Conclusions
In summary, PPy-coated Cu 2 O nanowires have been prepared by the hydrothermal reaction and combined with graphene oxide to obtain rGO/PPy/Cu 2 O nanocomposites after high-temperature thermal reduction. Moreover, a p-p-type gas sensor has been fabricated using rGO/PPy/Cu 2 O nanocomposite as an electrode and room temperature sensing is realized. The results revealed that the rGO/PPy/Cu 2 O-based gas sensor renders better NO 2 sensing performance than the PPy/Cu 2 O-based sensor, confirming the positive influence of graphene addition. When the mass ratio of graphene to PPy-coated Cu 2 O nanowires was 0.1, the rGO/PPy/Cu 2 O-based sensor demonstrated the highest response value of 42.5% for NO 2 gas (50 ppm). When the concentration of NO 2 was as low as 200 ppb, the rGO/PPy/Cu 2 O-based sensor still exhibited a response value of 20.3%. Moreover, the rGO/PPy/Cu 2 O-based sensor has also rendered stable repeatability and excellent selectivity at the NO 2 concentration of 50 ppm.