Pulse-driven NO2 sensor based on In2O3 sensitive material modified with porous graphene oxide and preparation method thereof

Abstract

A pulse-driven NO2 sensor based on porous graphene oxide-modified In2O3 sensitive material and its preparation method belong to the technical field of semiconductor metal oxide gas sensors. It consists of an Al2O3 ceramic tube substrate with a pair of parallel annular gold electrodes on both sides of the outer surface, a sensitive material coated on the outer surface of the Al2O3 ceramic tube and the gold electrodes, and a nickel-chromium metal heating coil placed inside the Al2O3 ceramic tube; the nickel-chromium metal heating coil is connected to the current output end of the pulse drive circuit, and the sensitive material is porous graphene oxide-modified In2O3 material. In addition to high sensitivity, the sensor obtained by the present invention also has a low detection limit and good selectivity. In addition, a pulse drive circuit is designed for the pulse temperature modulation (PTM) strategy, which further improves the detection capability of the sensor. In PTM mode, the sensor response is 1.6 times that of the traditional constant temperature mode.

Description

Technical Field

[0001] The present invention belongs to the technical field of semiconductor metal oxide gas sensors, and particularly relates to a pulse-driven NO2 sensor based on an In2O3 sensitive material modified with porous graphene oxide (HGO) and a preparation method thereof. Background Art

[0002] With the development of society and the gradual improvement of people's health awareness, the effective detection and control of toxic and harmful gases in the environment has become increasingly important for protecting the environment and human health. Nitrogen dioxide (NO2) is a common toxic and harmful gas, mainly derived from motor vehicle exhaust and fossil fuel combustion. NO2 emissions not only cause acid rain and have serious negative impacts on water bodies, land, and artificial ecosystems, but also lead to the formation of ground-based photochemical haze, which poses a great threat to the environment and human health. Therefore, finding a simple and suitable method to prepare an efficient NO2 sensor is crucial.

[0003] Metal oxide semiconductor (MOS) nanomaterials, such as In2O3, WO3, SnO2, and ZnO, are commonly used gas-sensing materials due to their ease of fabrication, excellent stability, and high sensitivity. Among them, In2O3 has unique advantages, including a wide bandgap (3.55–3.75 eV), low resistivity (1 kΩ–160 kΩ), and higher conductivity (exceeding 1000 S / m), making it an ideal material for NO2 gas sensors. Furthermore, In2O3 has two crystalline phases: hexagonal (h-In2O3) and cubic (c-In2O3). Its morphology is easily tunable, allowing for interface manipulation through rational design. Porous graphene oxide (HGO) possesses numerous interconnected micro-, meso-, and macropores, providing open channels for faster carrier transport and, consequently, greater carrier accumulation. Furthermore, the porous structure of HGO provides numerous defect sites for the adsorption of target gases, effectively promoting the diffusion of gas molecules through its pores. Motivated by these advantages, utilizing the porous structure of graphene oxide may provide a feasible approach to improve the performance of In2O3-based NO2 gas sensors. Summary of the Invention

[0004] The present invention aims to provide a pulse-driven NO2 sensor based on a porous graphene oxide (HGO)-modified In2O3 sensitive material and its preparation method. The resulting sensor exhibits high sensitivity, a low detection limit, and good selectivity. Furthermore, a pulse drive circuit is designed for pulse temperature modulation (PTM) strategy, further enhancing the sensor's detection capabilities. In PTM mode, the sensor's response is 1.6 times that of conventional constant temperature mode.

[0005] The NO2 sensor involved in the present invention uses In2O3 modified with porous graphene oxide (HGO) as the sensitive material. On the one hand, the HGO and In2O3 composite form a heterogeneous interface, which is conducive to the effective improvement of gas sensing performance. On the other hand, the presence of pores in HGO reduces its adsorption energy for NO2 gas molecules, making NO2 molecules more easily adsorbed on HGO. The porous structure of HGO facilitates gas diffusion, providing an ideal gas transport channel and sufficient gas adsorption sites. In addition, pulse temperature modulation (PTM) provides a high-temperature zone that is conducive to electron activation. During the pulse off phase (low temperature), NO2 molecules adsorb on the surface of the sensing material, enhancing the surface charge transfer between NO2 molecules and the material, thereby improving NO2 sensing performance. The combined effect of these aspects promotes the reaction efficiency between the sensitive material and NO2 gas, improves the material's sensitivity and selectivity, and shortens the response and recovery time. The present invention prepares graphene oxide (GO) and porous graphene oxide (HGO) separately by a water bath method, and then uses HGO to modify In2O3 nanosheets through a hydrothermal method to obtain a composite material to improve the material's gas sensing performance. The In2O3 gas sensor based on the optimal HGO doping ratio (0.5wt%) can achieve sensitive detection of NO2 gas as low as 10ppb at an operating temperature of 50°C.

[0006] The present invention discloses a pulse-driven NO2 gas sensor based on In2O3 sensitive material modified with porous graphene oxide (HGO), such as Figure 1 As shown, it consists of an Al2O3 ceramic tube substrate with a pair of parallel annular gold electrodes on both sides of the outer surface and two platinum wires connected to each gold electrode, a sensitive material coated on the outer surface of the Al2O3 ceramic tube and the gold electrodes, and a nickel-chromium metal heating coil placed inside the Al2O3 ceramic tube; the nickel-chromium metal heating coil is connected to the current output end of the pulse drive circuit, and the sensitive material is an In2O3 material modified with porous graphene oxide (HGO), and is obtained by the following steps:

[0007] (1) Add 0.2-0.25g In(NO3)3 XH2O is dissolved in deionized water to obtain a uniform solution, which is then mixed with 30-35 mL of ethanol and ethylene glycol, followed by the addition of 0.1-0.3 g of polyvinyl pyrrolidone (PVP), followed by stirring for 25-35 min, and the obtained uniform solution is added to a polytetrafluoroethylene-lined stainless steel autoclave, which is well sealed and maintained at 160-200°C for 5-8 hours; after the autoclave is naturally cooled to room temperature, the precipitate is collected by centrifugation, and the precipitate is washed alternately with water and ethanol 5-8 times, and the obtained product is dried at 70-90°C for 10-20 hours to obtain a solid powder, which is calcined in air at 400-600°C for 1.5-3.0 hours at a heating rate of 3.0-6.0°C / min to obtain an In2O3 nanosheet sensitive material;

[0008] (2) 3-6 mL of a 30% mass fraction hydrogen peroxide (H2O2) solution and 40-60 mL of a 2 mg / mL graphene oxide dispersion were mixed uniformly, and then stirred in a water bath at 90-110°C for 3-5 h. The reaction solution was centrifuged and washed with ethanol and deionized water, respectively, and then dispersed in an aqueous solution and ultrasonicated to obtain a uniform porous graphene oxide (HGO) dispersion with a concentration of 2 mg / mL;

[0009] (3) 40-60 mg of In2O3 obtained in step (1) and 100-150 μL of 2 mg / mL HGO dispersion obtained in step (2) were dispersed in 5-20 mL of deionized water and stirred for 20-40 min. The resulting mixture was then transferred to a polytetrafluoroethylene-lined stainless steel autoclave, sealed well, heated at 140-180°C for 13-16 hours, cooled to room temperature, and dried to obtain a porous graphene oxide (HGO)-modified In2O3 nanosheet material with an HGO content of 0.25-1.0 wt%.

[0010] The present invention provides a method for preparing a pulse-driven NO2 gas sensor based on a porous graphene oxide (HGO)-modified In2O3 nanosheet sensitive material, comprising the following steps:

[0011] (1) 10-20 mg of porous graphite oxide (HGO) modified In2O3 nanosheet sensitive material was placed in an agate mortar, 3-5 drops of ethanol were added, the mixture was thoroughly mixed and ground into a paste, and then an appropriate amount of the paste was evenly coated on the outer surface of an Al2O3 ceramic tube to form a 15-30 μm thick layer of sensitive material, and the sensitive material completely covered a pair of parallel annular gold electrodes on both sides of the outer surface of the ceramic tube; the length of the Al2O3 ceramic tube was 4.0-4.5 mm, and the inner and outer diameters were 0.8-1.0 mm and 1.2-1.5 mm, respectively; the width of a single annular gold electrode was 0.4-0.5 mm, and the distance between the two gold electrodes was 0.5-0.6 mm; the length of the platinum wire extending from the gold electrode was 4-6 mm;

[0012] (2) calcining the Al2O3 ceramic tube with the sensitive material coated on the outer surface obtained in step (1) in air at 200-300°C for 1.5-3.0 hours to improve the stability of the sensitive material, with a heating rate of 3-6°C / min. After the temperature drops to room temperature, a nickel-chromium alloy heating coil with a resistance value of 27-33Ω is passed through the interior of the ceramic tube as a heating wire; finally, the above device is welded to a hexagonal tube seat, thereby preparing a indirectly heated NO2 gas sensor based on the In2O3 nanosheet sensitive material modified with porous graphite oxide (HGO);

[0013] (3) Connecting the nickel-chromium alloy heating coil of the indirectly heated NO2 gas sensor based on the porous graphite oxide (HGO)-modified In2O3 nanosheet sensitive material obtained in step (2) to the current output end of the pulse driving circuit to obtain an indirectly heated pulse-driven NO2 gas sensor based on the porous graphite oxide (HGO)-modified In2O3 sensitive material.

[0014] The pulse drive circuit described in step (3) (such as Figure 6As shown in (a), T. Yang, Q. Yang, Y. Xiao, P. Sun, Z. Wang, Y. Gao, J. Ma, Y. Sun, and G. Lu (A pulse-driven sensor based on ordered mesoporous Ag2O / SnO2 with improved H2S sensing performance, Sens Actutors Bchem. 228 (2016) 529-538) consists of a single-chip microcomputer (MCU), a keying circuit, and an output circuit consisting of a first relay and a second relay. The keying circuit is connected to the MCU input to set the cycle time and pulse width of the MCU output pulse signal; the MCU's two control signal outputs are connected to the first relay and the second relay, respectively, to control the on and off of the first and second relays. A nickel-chromium alloy heating coil, the first relay, and the first current source form a high-temperature heating circuit (high temperature range: 175-275°C, duration: 2-5 seconds). The nickel-chromium alloy heating coil, the second relay, and the second current source form a low-temperature heating circuit (low temperature range: 20-30°C, duration: 15-20 seconds). When the first relay is connected, the first current source heats the Nichrome heating coil; when the second relay is connected, the second current source heats the Nichrome heating coil. Driven periodically by these two current sources, the current in the sensor's Nichrome heating coil changes periodically, and the resistance between the sensor's two gold electrodes also changes steadily and periodically. During measurement, the resistance between the two gold electrodes at low temperatures, after temperature stabilization, is used to calculate the sensitivity.

[0015] The indirectly heated pulse-driven NO2 gas sensor using the porous graphite oxide (HGO)-modified In2O3 nanosheet sensitive material of the present invention has the following advantages:

[0016] (1) A simple water bath method and hydrothermal method were used to prepare porous graphite oxide (HGO) modified In2O3 nanosheet sensitive materials. The method is simple and low-cost.

[0017] (2) By utilizing the advantages of the porous structure of HGO to modify indium oxide nanosheets, the sensitivity of indium oxide to NO2 gas was improved (1859-1ppm), the response and recovery speed to NO2 were accelerated, and it had good selectivity and a low detection limit.

[0018] (3) Using a simple pulse drive circuit, a side-heated NO2 gas sensor based on porous graphite oxide (HGO)-modified In2O3 nanosheets was subjected to alternating high pulses (high temperature, ranging from 175 to 275°C) and low pulses (low temperature, ranging from 20 to 30°C). During the high-temperature pulse phase, gas molecules tended to be surface activated, while during the low-temperature pulse phase, they tended to diffuse and adsorb on the sensing layer. The alternating action of low-temperature and high-temperature pulses enhanced the surface charge transfer between NO2 molecules and the material, thereby improving the NO2 sensing performance.

[0019] (4) Use commercially available tubular sensors, which have simple device technology, low cost, small size, and are suitable for mass production. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 : Schematic diagram of the structure of the NO2 indirectly heated gas sensor based on the porous graphene oxide modified In2O3 nanorod sensitive material of the present invention;

[0021] Names of each part: Al2O3 ceramic tube 1, platinum wire 2, gold electrode 3, sensitive material 4, nickel-chromium alloy heating coil 5.

[0022] Figure 2 (a): Schematic diagram of the HGO formation mechanism and photos of GO and HGO dispersions; the color of the HGO dispersion is darker than that of the GO dispersion. Figure 2 (b) and Figure 2 (c) TEM morphology photos of GO and HGO sensitive materials respectively; it can be seen that holes appear in GO after hydrogen peroxide etching. Figure 2 (d1): SEM image of the sensitive material in Comparative Example 2; Figure 2 (d2) TEM image of the sensitive material in Comparative Example 2; Figure 3 (d3): High-magnification HRTEM of the sensitive material in Comparative Example 2; it can be seen that the indium oxide in Comparative Example 2 presents a regular flaky morphology, and smoothly wrinkled GO is observed attached to the surface of the indium oxide, proving the successful preparation of the sensitive material in Comparative Example 2. Figure 2 (e1): SEM image of the sensitive material in Example 1; Figure 2 (e2): TEM image of the sensitive material in Example 1; Figure 2 (e3): High-magnification HRTEM of the sensitive material in Example 1; it can be seen that the indium oxide in Example 1 also presents a regular flaky morphology, and HGO is observed to become rough and adhere to the surface of the indium oxide, proving the successful preparation of the sensitive material in Example 1.

[0023] Figure 3: XRD patterns of In2O3 nanosheets, graphene oxide (GO)-modified In2O3 nanosheets, and porous graphene oxide (HGO)-modified In2O3 nanosheets prepared in Comparative Example 1, Comparative Example 2, and Example 1 of the present invention; all diffraction peaks of all samples can be well pointed to the In2O3 standard card.

[0024] Figure 4 : Temperature variation curves of the gas sensors prepared in Comparative Example 1, Comparative Example 2 and Example 1; the optimal operating temperature of each device is 50°C; compared with the sensors in Comparative Example 1 and Comparative Example 2, the sensor in Example 1 has the highest sensitivity to NO2, with a sensitivity of 1859 to 1 ppm NO2.

[0025] Figure 5 (a): Response recovery curves of the gas sensors prepared in Comparative Example 1, Comparative Example 2 and Example 1 to 1 ppm NO2 at the optimal operating temperature; it shows that the gas sensor prepared in Example 1 has the fastest response recovery speed. Figure 5 (b): A comparison of the selectivity of the gas sensors prepared in Comparative Example 1, Comparative Example 2 and Example 1 to different gases at the optimal operating temperature; it shows that the sensors in the comparative example and the example both have good selectivity to NO2 gas.

[0026] Figure 6 (a) Schematic diagram of a pulse-driven gas sensor; the two control signal outputs of the microcontroller are connected to the first and second relays, respectively, to control their on and off states. When the first relay is on, a first external current source drives the sensor to heat; when the second relay is on, a second external current source drives the sensor to heat. Under the periodic drive of these two current sources, the current in the sensor's nickel-chromium alloy heating coil exhibits periodic changes, while the resistance between the sensor's gold electrodes also exhibits stable periodic changes.

[0027] Figure 6 (b) and Figure 6 (c) The response value curves and response recovery time curves of the sensor prepared in Example 1 to 1 ppm NO2 at different high temperature ratios under PTM mode; the results show that considering the response recovery time and response value of Example 1, a high temperature ratio of 15% (total pulse time of 20 s, high temperature pulse time of 3 s) is selected as the optimal high temperature ratio for PTM mode.

[0028] Figure 7 (a): Transient response curve of the sensor prepared in Example 1 to 1 ppm NO2 at a high temperature ratio of 15% in PTM mode; Figure 7 (b) and Figure 7(c) Response value curves and response recovery time curves of the sensor prepared in Example 1 to 1 ppm NO2 at different high temperatures under PTM mode; the results show that considering the response recovery time and response value of Example 1 at different high temperatures, 250°C is selected as the optimal operating temperature of PTM.

[0029] Figure 8 A comparison of the sensor's sensitivity to 1 ppm NO2 gas at a constant operating temperature (62.5°C) and optimal PTM mode (250°C) in Example 1 shows a 1.6-fold increase in response to 1 ppm NO2 under optimal PTM mode compared to the corresponding optimal isothermal mode. Furthermore, the response / recovery time was reduced from 479 seconds / 429 seconds to 322 seconds / 127 seconds.

[0030] Figure 9 (a) Schematic diagram of a pulse-driven NO2 sensor alarm system based on porous graphene oxide-modified In2O3 sensitive material; the system consists of an alarm circuit, a pulse drive circuit, and a gas sensor. The alarm circuit is composed of a power supply circuit, a single-chip microcomputer system, an EEPROM memory circuit, an LCD interface, and a sensor circuit (J. Han, D. Kong, W. Zhou, Y. Gao, Y. Gao, G. Liu and G. Lu, Interface-engineering in MOF-derived In2O3 for highly sensitive and dual-functional gas sensor towards NO2 and triethylamine. Sensors and Actuators B: Chemical, 2023.395). Figure 9 (b) Schematic diagram of the circuit structure of the pulse-driven alarm circuit of the present invention. The pulse-driven circuit, gas sensor and alarm circuit prepared in Example 1 are integrated together. Specifically, the platinum wire of the gas sensor and the nickel-chromium alloy heating coil are connected to the voltage comparator of the sensor circuit part of the alarm circuit (J. Han, D. Kong, W. Zhou, Y. Gao, Y. Gao, G. Liu and G. Lu, Interface-engineering in MOF-derived In2O3 for highly sensitive and dual-functional gas sensor towards NO2 and triethylamine. Sensors and Actuators B: Chemical, 2023.395)

[0031] Figure 10 : Schematic diagram of the application of NO2 sensors with safe and excessive concentrations under simulated conditions; first, the sensor was repeatedly tested at different NO2 concentrations (10, 20, 50, 100, 200, 500, and 1000 ppb). The resistance value R of the sensor at different NO2 concentrations was obtained by using the voltage divider of the sensor (equivalent to a variable resistor) and the resistor in the circuit (the total power supply voltage is 5V). g (When NO2 is not introduced, the resistance is R a ), and according to R g / R a The sensitivity is calculated. According to the maximum limit of ambient air pollutants, the NO2 threshold is set to 53ppb NO2; after testing, when 53ppb NO2 is introduced, the screen displays a voltage of 880mV (using the voltage division of the sensor (equivalent to a variable resistor) and the resistor in the circuit (the total power supply voltage is 5V), the resistance value R of the sensor at different NO2 concentrations is obtained. g (When NO2 is not introduced, the resistance is R a ), and according to R g / R a The voltage comparator in the alarm circuit compares the voltage between the two ends of the sensor and the set threshold value to realize the NO2 alarm. When the NO2 concentration is lower than the set threshold value (53ppb), the alarm light goes out; when the NO2 concentration exceeds the set threshold value (53ppb), the alarm light comes on.

[0032] Note: The device's sensitivity (response value) is defined as the ratio of the resistance between its two gold electrodes in the measured gas to the resistance in air. The response and recovery time are the time it takes for the sensor's resistance to change by 90% after changing the device's gas environment (from air to NO2 or vice versa). Testing is performed using a static test system: the device is placed in a 1-liter gas cylinder containing a certain concentration of the measured gas. The resistance change is observed and recorded, and the corresponding sensitivity value and response recovery time are calculated. DETAILED DESCRIPTION

[0033] Comparative Example 1:

[0034] 1. In2O3 nanosheets were synthesized by a hydrothermal method. 0.22 g of In(NO3)3·xH2O was added to 6 mL of deionized water and stirred vigorously for 0.5 h to make the solution homogeneous.

[0035] 2. Add 20 mL of ethanol, 13 mL of ethylene glycol, and 0.2 g of polyvinylpyrrolidone (PVP) to the solution obtained in step 1, and stir for 0.5 hours;

[0036] 3. Transfer the solution from step 2 to a 50 mL polytetrafluoroethylene-lined stainless steel autoclave, seal tightly, and maintain at 180°C for 6 hours. After the reaction is complete and the autoclave cools to room temperature, collect the precipitate by centrifugation and wash it six times with water and ethanol. Dry the resulting product in an oven at 80°C for 12 hours to obtain a yellow solid powder.

[0037] 4. The powder obtained in step 3 was transferred to a porcelain boat, placed in a muffle furnace, and calcined at 500°C in air for 2 hours to obtain In2O3 nanosheet sensitive material.

[0038] 5. Take 15 mg of the sensitive material obtained in step 4 and place it in an agate mortar. Add 3 drops of ethanol, mix thoroughly and grind into a paste. Then, take an appropriate amount of the slurry and evenly coat it on the outer surface of the Al2O3 ceramic tube to form a 20 μm thick layer of sensitive material. The sensitive material can completely cover the annular gold electrodes on both sides of the outer surface of the ceramic tube. The length of the Al2O3 ceramic tube is 4 mm, and the inner and outer diameters are 0.8 mm and 1.2 mm respectively. The width of a single annular gold electrode is 0.45 mm, and the distance between the two gold electrodes is 0.55 mm; a platinum wire is led out from the gold electrode, and its length is 5 mm.

[0039] 6. Transfer the Al2O3 ceramic tube coated with the sensitive material obtained in step 5 to a porcelain boat, place it in a muffle furnace, and calcine it in air at 200°C for 2 hours at a heating rate of 5°C / min to improve the stability of the sensitive material. After the temperature drops to room temperature, pass a nickel-chromium alloy heating coil with a resistance of 30Ω through the interior of the ceramic tube as a heating wire. Finally, weld the above device to a hexagonal tube socket to prepare a NO2 gas sensor. Comparative Example 2:

[0040] 1. In2O3 nanosheets were synthesized by a hydrothermal method. 0.22 g of In(NO3)3·xH2O was added to 6 mL of deionized water and stirred vigorously for 0.5 h to make the solution homogeneous.

[0041] 2. Add 20 mL of ethanol, 13 mL of ethylene glycol, and 0.2 g of PVP to the solution obtained in step 1, and stir for 0.5 hours;

[0042] 3. Transfer the solution from step 2 to a 50 mL polytetrafluoroethylene-lined stainless steel autoclave, seal tightly, and maintain at 180°C for 6 hours. After the reaction is complete and the autoclave cools to room temperature, collect the precipitate by centrifugation and wash it six times with water and ethanol. Dry the resulting product in an oven at 80°C for 12 hours to obtain a yellow solid powder.

[0043] 4. The powder obtained in step 3 was transferred to a porcelain boat, placed in a muffle furnace, and calcined at 500°C in air for 2 hours to obtain In2O3 nanosheet sensitive material.

[0044] 5. The In2O3 (50 mg) obtained in step 4 and 125 μL of GO suspension (2 mg / mL) were dispersed in 10 mL of deionized water and stirred for 30 min. The mixture was then transferred to a 30 mL polytetrafluoroethylene-lined stainless steel autoclave, sealed well, and heated at 100°C for 4 h. After cooling to room temperature, it was dried to obtain graphene oxide (GO)-modified In2O3 nanosheets with a GO content of 0.5 wt%.

[0045] 6. Take 15 mg of the sensitive material obtained in step 5 and place it in an agate mortar. Add 3 drops of ethanol, mix thoroughly and grind into a paste. Then, take an appropriate amount of the paste and evenly coat it on the outer surface of the Al2O3 ceramic tube to form a 20 μm thick layer of sensitive material. The sensitive material can completely cover the annular gold electrodes on both sides of the outer surface of the ceramic tube. The length of the Al2O3 ceramic tube is 4 mm, and the inner and outer diameters are 0.8 mm and 1.2 mm respectively. The width of a single annular gold electrode is 0.45 mm, and the distance between the two gold electrodes is 0.55 mm; a platinum wire is led out from the gold electrode, and its length is 5 mm.

[0046] 7. Transfer the Al2O3 ceramic tube coated with the sensitive material from step 6 to a porcelain boat and place it in a muffle furnace. Calcine it in air at 200°C for 2 hours at a heating rate of 5°C / min to improve the stability of the sensitive material. After the temperature drops to room temperature, insert a 30Ω nickel-chromium alloy heating coil through the ceramic tube as a heating filament. Finally, weld this device to a hexagonal tube socket to complete the NO2 gas sensor.

[0047] Embodiment 1:

[0048] 1. In2O3 nanosheets were synthesized by a hydrothermal method. 0.22g In(NO3)3·xH2O was added to 6mL deionized water and stirred vigorously for 0.5h to make the solution homogeneous.

[0049] 2. Add 20 mL of ethanol, 13 mL of ethylene glycol, and 0.2 g of PVP to the solution obtained in step 1, and stir for 0.5 hours;

[0050] 3. Transfer the solution from step 2 to a 50 mL polytetrafluoroethylene-lined stainless steel autoclave, seal tightly, and maintain at 180°C for 6 hours. After the reaction is complete and the autoclave cools to room temperature, collect the precipitate by centrifugation and wash it six times with water and ethanol. Dry the resulting product in an oven at 80°C for 12 hours to obtain a yellow solid powder.

[0051] 4. The powder obtained in step 3 was transferred to a porcelain boat, placed in a muffle furnace, and calcined at 500°C in air for 2 hours to obtain In2O3 nanosheet sensitive material.

[0052] 5. Mix 5 mL of 30% 2 mg / mL hydrogen peroxide (H2O2) solution and 50 mL of graphene oxide dispersion, then stir in a 100°C water bath for 4 h. Centrifuge and wash the prepared HGO, then ultrasonically disperse it to obtain a uniform HGO dispersion with a concentration of 2 mg / mL.

[0053] 6. In2O3 (50 mg) obtained in steps 4 and 5 and 125 μL of HGO suspension (2 mg / mL) were dispersed in 10 mL of deionized water and stirred for 30 min. The mixture was then transferred to a 30 mL polytetrafluoroethylene-lined stainless steel autoclave, sealed well, heated at 100°C for 4 hours, cooled to room temperature, and dried to obtain porous graphene oxide (HGO)-modified In2O3 nanosheets with an HGO content of 0.5 wt%.

[0054] 7. Take 15 mg of the sensitive material obtained in step 6 and place it in an agate mortar. Add 3 drops of ethanol, mix thoroughly and grind into a paste. Then, take an appropriate amount of the paste and evenly coat it on the outer surface of the Al2O3 ceramic tube to form a 20 μm thick layer of sensitive material. The sensitive material can completely cover the annular gold electrodes on both sides of the outer surface of the ceramic tube. The length of the Al2O3 ceramic tube is 4 mm, and the inner and outer diameters are 0.8 mm and 1.2 mm respectively. The width of a single annular gold electrode is 0.45 mm, and the distance between the two gold electrodes is 0.55 mm; a platinum wire is led out from the gold electrode, and its length is 5 mm.

[0055] 8. Transfer the Al2O3 ceramic tube coated with the sensitive material from step 7 to a porcelain boat and place it in a muffle furnace. Calcine it in air at 200°C for 2 hours at a heating rate of 5°C / min to improve the stability of the sensitive material. After the temperature drops to room temperature, insert a 30Ω nickel-chromium alloy heating coil through the ceramic tube as a heating filament. Finally, weld this device to a hexagonal tube socket to complete the NO2 gas sensor.

Claims

1. A pulse-driven NO2 gas sensor based on porous graphene oxide-modified In2O3 sensitive material, comprising an Al2O3 ceramic tube substrate with a pair of parallel annular gold electrodes on either side of its outer surface, each gold electrode connected to two platinum wires; a sensitive material coated on the outer surface of the Al2O3 ceramic tube and the gold electrodes; and a nickel-chromium metal heating coil placed inside the Al2O3 ceramic tube; characterized by: The nickel-chromium metal heating coil is connected to a pulse drive circuit, which is composed of a single-chip microcomputer, a key control circuit, and an output circuit composed of a first relay and a second relay; the key control circuit is connected to the single-chip microcomputer input to set the cycle time and pulse width of the single-chip microcomputer output pulse signal; the two control signal outputs of the single-chip microcomputer are respectively connected to the first relay and the second relay to control the on and off of the first relay and the second relay; the nickel-chromium alloy heating coil, the first relay, and the first current source form a high-temperature heating circuit; the nickel-chromium alloy heating coil, the second relay, and the second current source form a low-temperature heating circuit; the sensitive material is an In2O3 material modified with porous graphene oxide, and is obtained by the following steps, (1) Add 0.2~0.25 g In(NO3)3· X H2O was dissolved in deionized water to obtain a uniform solution, which was then mixed with 30-35 mL of ethanol and ethylene glycol. 0.1-0.3 g of polyvinyl pyrrolidone was then added and stirred for 25-35 min. The resulting uniform solution was then placed in a polytetrafluoroethylene-lined stainless steel autoclave, which was well sealed and maintained at 160-200°C for 5-8 hours. After the autoclave was naturally cooled to room temperature, the precipitate was collected by centrifugation and washed alternately with water and ethanol 5-8 times. The resulting product was then dried at 70-90°C for 10-20 hours to obtain a solid powder. The resulting solid powder was calcined in air at 400-600°C for 1.5-3.0 hours at a heating rate of 3.0-6.0°C / min to obtain an In2O3 nanosheet sensitive material. (2) 3-6 mL of 30% mass fraction hydrogen peroxide solution and 40-60 mL of 2 mg / mL graphene oxide dispersion were mixed evenly, and then stirred in a water bath at 90-110 °C for 3-5 h. The reaction solution was centrifuged and washed with ethanol and deionized water, respectively, and then dispersed in aqueous solution and ultrasonicated to obtain a uniform porous graphene oxide dispersion with a concentration of 2 mg / mL. (3) 40-60 mg of In2O3 obtained in step (1) and 100-150 μL of 2 mg / mL porous graphene oxide dispersion obtained in step (2) were dispersed in 5-20 mL of deionized water and stirred for 20-40 min. The resulting mixture was then transferred to a polytetrafluoroethylene-lined stainless steel autoclave, sealed well, and heated at 140-180°C for 13-16 hours. The mixture was cooled to room temperature and dried to obtain a porous graphene oxide-modified In2O3 nanosheet material with a graphene oxide content of 0.25-1.0 wt%.

2. A pulse-driven NO2 gas sensor based on porous graphene oxide-modified In2O3 sensitive material according to claim 1, characterized in that: The high temperature range of the high temperature heating circuit is 175~275℃, which lasts for 2~5 seconds; the low temperature range of the low temperature heating circuit is 20~30℃, which lasts for 15~20 seconds.

3. The pulse-driven NO2 gas sensor based on the porous graphene oxide-modified In2O3 sensitive material according to claim 1, characterized in that: The length of the Al2O3 ceramic tube is 4.0~4.5 mm, and the inner and outer diameters are 0.8~1.0 mm and 1.2~1.5 mm, respectively. The width of a single annular gold electrode is 0.4~0.5 mm, and the distance between the two gold electrodes is 0.5~0.6 mm. The length of the platinum wire leading from the gold electrode is 4~6 mm.

4. A method for preparing a pulse-driven NO2 gas sensor based on a porous graphene oxide-modified In2O3 nanosheet sensitive material according to any one of claims 1 to 3, comprising the following steps: (1) Take 10-20 mg of porous graphite oxide modified In2O3 nanosheet sensitive material in an agate mortar, add 3-5 drops of ethanol, mix thoroughly and grind into a paste, then dip an appropriate amount of the paste and evenly coat it on the outer surface of the Al2O3 ceramic tube to form a 15-30 μm thick sensitive material layer, and make the sensitive material completely cover a pair of parallel annular gold electrodes on both sides of the outer surface of the ceramic tube; (2) The Al2O3 ceramic tube obtained in step (1) and coated with a sensitive material on its outer surface is calcined in air at 200-300°C for 1.5-3.0 hours at a heating rate of 3-6°C / min; after the temperature drops to room temperature, a nickel-chromium alloy heating coil with a resistance value of 27-33 Ω is passed through the interior of the ceramic tube as a heating wire; finally, a pair of parallel annular gold electrodes and the heating wire are welded to a hexagonal tube seat, thereby preparing a indirectly heated NO2 gas sensor based on a porous graphite oxide modified In2O3 nanosheet sensitive material; (3) Connecting the nickel-chromium alloy heating coil of the indirectly heated NO2 gas sensor based on the porous graphite oxide modified In2O3 nanosheet sensitive material obtained in step (2) to a pulse driving circuit to obtain an indirectly heated pulse driven NO2 gas sensor based on the porous graphite oxide modified In2O3 sensitive material.

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