Graphene micron sieve, preparation method thereof and application of graphene micron sieve in gas sensor

Abstract

The invention discloses a graphene micron sieve, a preparation method thereof and application in a gas sensor, introduced water vapor is liquefied on the surface of a substrate to form water drops by utilizing a condensation phenomenon, and an oxygen-enriched structure for inhibiting graphene nucleation / growth can be formed in an area in contact with the water drops through high-temperature rapid heating treatment; and other regions are kept unchanged, and the oxygen-enriched structure can inhibit nucleation / growth of graphene in the CVD process, so that the finally grown graphene film is of a sieve-hole-shaped structure. By adopting the method disclosed by the invention, the graphene micron sieve film with the controllable sieve pore diameter can be directly grown on the non-metal substrate, the adverse effect of the conventional template etching process on the device performance is avoided, and a foundation is laid for realizing the in-situ construction of the patterned graphene in the field of electronic devices. The invention also relates to an application of the graphene micron sieve in a gas sensor, and the existence and concentration of gas are detected by monitoring the change of resistance or conductivity by utilizing the adsorption capacity of the graphene micron sieve on gas molecules.

Description

Technical Field

[0001] The present invention relates to graphene derivative materials and chemical vapor deposition (CVD) preparation technology thereof, and in particular to a graphene micron sieve, a preparation method thereof and application in a gas sensor. Background Art

[0002] Graphene is a sp 2 The single-atom-layer ultra-thin two-dimensional material that is closely hybridized into a hexagonal honeycomb lattice has excellent optical, electrical, thermal and mechanical properties due to its unique crystal structure, which has very broad application prospects in many fields such as transparent conductive films, flexible electronic devices, electrochemical energy storage, graphene composite materials, biomedicine, and sensor detection. Graphene sieve is a special structure of single-layer or few-layer graphene with high-density perforations. It has a richer specific surface area and more boundaries than ordinary graphene films, so it can provide a large number of active sites for adsorbing molecules, making it have great application potential in gas sensors.

[0003] There are two main methods for preparing graphene sieves. One is a top-down method, which usually involves transferring graphene grown by mechanical exfoliation or metal CVD to a non-metallic substrate such as SiO2 / Si, and etching it into a graphene sieve with the help of a porous mask. The other is a bottom-up method, which uses a porous metal template or self-assembled metal spheres as a catalyst to grow graphene sieves on a SiO2 / Si substrate. However, both methods have the prominent problem of contamination and doping of the graphene sieve by residues when removing conventional "hard" templates, and the pretreatment steps are numerous and the production cycle is long. Therefore, it is of great significance to develop a method for directly growing graphene sieves on non-metallic substrates such as SiO2 / Si. Summary of the invention

[0004] The purpose of the present invention is to provide a method for preparing a graphene micron sieve. The method has the advantages of simple operation, good controllability, green and pollution-free, and effectively solves the problems of easy pollution and damage of the border of the current preparation of patterned graphene, complex process, high production cost, etc., and lays a foundation for studying the in-situ construction of electronic devices without transfer of graphene micron sieve. Another purpose of the present invention is to provide the application of graphene micron sieve as a sensitive material of a gas sensor in detecting gas molecules, and detect the presence and concentration of gas by monitoring the change of resistance or conductivity.

[0005] Based on the above purpose, the technical solution adopted by the present invention is: A method for preparing a graphene micron sieve comprises the following steps: 1) Place the cleaned non-metallic substrate in a quartz tube at room temperature, fill it with heated carrier gas and water vapor in a non-oxidizing atmosphere, and form water droplets on the non-metallic substrate; 2) The substrate pretreated in step 1) is heated from room temperature to a growth temperature of 900-1200° C. for a holding time of not less than 1 minute to form a reaction system; 3) Keeping the temperature in step 2) unchanged, turning off the carrier gas and water vapor, charging the carbon source and hydrogen into the reaction system in step 2), and growing the graphene micron sieve on the substrate treated in step 2) for a growth time of 20 to 180 minutes; 4) After the reaction is completed, turn off the carbon source and hydrogen, cool to room temperature under the protection of a non-oxidizing atmosphere, and obtain a graphene micron sieve.

[0006] Furthermore, in step 1), the non-metallic substrate used includes but is not limited to a silicon wafer, a quartz wafer, boron nitride, alumina, a mica wafer or a silicon wafer with a silicon dioxide coating.

[0007] Furthermore, in step 1) and step 4), the non-oxidizing atmosphere is nitrogen or an inert gas or a mixture of the two.

[0008] Furthermore, in step 1), the method of heating the carrier gas and water vapor includes but is not limited to a water bath, an alcohol bath, an oil bath, a sand bath, electric heating and microwave heating.

[0009] Furthermore, in step 1), water vapor is brought into the reaction system through a carrier gas, and the carrier gas used is at least one of nitrogen and an inert gas, and the volume purity is greater than 99.9%. The amount of water vapor added is adjusted by changing the carrier gas flow rate, and the carrier gas flow rate is greater than 0 to 100 ml / min.

[0010] Furthermore, in step 1), the time required for condensation to form water droplets on the substrate is 1 to 20 minutes.

[0011] Furthermore, in step 2), the time required for the substrate to be heated from room temperature to the growth temperature is 1 to 5 minutes. After reaching the growth temperature, the substrate is kept at the growth temperature for 1 to 10 minutes.

[0012] Furthermore, in step 3), the carbon source used is a hydrocarbon: at least one of methane, ethane, acetylene, ethylene, methanol, ethanol, benzene, toluene, and cyclohexane, the volume ratio of the carbon source to hydrogen is (0.1~10):1, and the flow rate of hydrogen is 1~500 ml / min.

[0013] Furthermore, when the carbon source is liquid, it is placed in a Monterey bottle and brought into the reaction system by bubbling with a carrier gas. The molar ratio of the carbon source to the carrier gas is (0.1~10):1, and the carrier gas is nitrogen or an inert gas or a mixture of the two.

[0014] Furthermore, in step 4), the cooling rate is not less than 1°C / s. The cooling rate is generally 1°C / s to 10°C / s.

[0015] The graphene micron sieve prepared by the above preparation method has graphene nucleation points mainly formed in a circle. As the growth time increases, the nucleation points grow and the inside of the spliced ​​film becomes sieve-shaped, and the average pore size ranges from submicron to tens of microns.

[0016] The above graphene micron sieve is used as a sensitive material of a gas sensor in detecting gas molecules.

[0017] Furthermore, the gas molecules are nitrogen dioxide, hydrogen, carbon monoxide, ammonia, hydrogen sulfide, volatile organic compounds or other harmful gases.

[0018] The process of detecting gas molecules is as follows: a. Connecting electrodes at both ends of the graphene micro-sieve to obtain a sensor, ensuring good contact between the electrodes and the graphene micro-sieve, so as to apply voltage and measure the conductivity change of the graphene micro-sieve; b. Calibrate the sensor using a series of standard gases of known concentration to establish the relationship between conductivity change and gas concentration; c. Place the graphene micrometer sieve in a gas environment to be tested, apply a known voltage, measure the conductivity change of the graphene micrometer sieve, and record the conductivity change data of the graphene micrometer sieve under different gas concentrations; d. Analyze the collected data, determine the relationship between gas concentration and resistivity change, and calculate the concentration of the gas to be measured; e. After each test, clean the graphene micron sieve using clean gas or vacuum system to remove adsorbed gas molecules to ensure the reusability and accuracy of the sensor.

[0019] The design concept of the present invention is: The condensation phenomenon is used to liquefy the introduced water vapor on the substrate surface to form water droplets. After high-temperature rapid heating treatment, the area in contact with the water droplets on the growth substrate can form an oxygen-rich structure, while other areas remain unchanged. During the CVD process, the oxygen-rich structure will inhibit the nucleation / growth of graphene, so that the final grown graphene film has a sieve-like structure.

[0020] The characteristics and beneficial effects of the present invention are: 1. The present invention proposes a green method for preparing a graphene micron sieve. The water droplet mask formed by liquefaction of water vapor does not need to be removed. The method has the advantages of simple operation, good controllability and no pollution.

[0021] 2. The graphene micron sieve prepared by the present invention can realize the in-situ construction of gas sensor devices on non-metallic substrates, avoiding problems such as graphene material contamination and damage caused by the transfer process. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The schematic diagram of the experimental device for growing graphene micrometer sieve on non-metallic materials by CVD method is shown in FIG. 11 is a gas inlet; 12 is a quartz tube; 13 is a substrate; 14 is a gas outlet; 15 is a horizontal reactor; 16 is a conical flask; and 17 is a mass flow meter.

[0023] Figure 2 This is a scanning electron microscope photograph of graphene nucleation points growing on SiO2 / Si substrate in a circular distribution. The scale in the figure is 1 micron. The experimental conditions are shown in Example 1.

[0024] Figure 3 This is an optical microscope photo of a graphene micrometer mesh grown on a SiO2 / Si substrate. The scale in the figure is 10 micrometers. The experimental conditions are shown in Example 2.

[0025] Figure 4 This is a scanning electron microscope photo of a graphene micrometer sieve grown on a SiO2 / Si substrate. The scale in the figure is 1 micrometer. The experimental conditions are shown in Example 3.

[0026] Figure 5 This is a transmission electron microscope photo of a graphene micrometer screen grown on a SiO2 / Si substrate and transferred to a microgrid. The scale in the figure is 2 micrometers. The experimental conditions are shown in Example 3.

[0027] Figure 6 This is an optical microscope photo of a graphene micrometer sieve grown on a SiO2 / Si substrate. The scale in the figure is 10 micrometers. The experimental conditions are shown in Example 3.

[0028] Figure 7 This is a Raman spectrum of a graphene micro-sieve grown on a SiO2 / Si substrate, with a laser wavelength of 532 nanometers. The experimental conditions are shown in Example 3.

[0029] Figure 8 This is an atomic force microscope photo of a graphene micrometer sieve grown on a SiO2 / Si substrate. The scale in the figure is 10 micrometers. The experimental conditions are shown in Example 3.

[0030] Fig. 9 The C1s X-ray photoelectron spectrum of the graphene micro-sieve grown on SiO2 / Si substrate. The experimental conditions are shown in Example 3.

[0031] Fig.10This is an optical microscope photo of a graphene micrometer mesh grown on a SiO2 / Si substrate. The scale in the figure is 10 micrometers. The experimental conditions are shown in Example 4.

[0032] Fig.11 The C1s X-ray photoelectron spectrum of the graphene micro-sieve grown on SiO2 / Si substrate. The experimental conditions are shown in Example 4.

[0033] Fig.12 The resistance dynamic change of the graphene micrometer sieve under different nitrogen dioxide gas concentrations. The experimental conditions are shown in Example 5.

[0034] Fig.13 is the fitting curve of the graphene micro-sieve resistance response rate changing with the nitrogen dioxide concentration. The experimental conditions are shown in Example 5. DETAILED DESCRIPTION

[0035] Below, the present invention is further described in detail by examples.

[0036] like Figure 1 As shown, a quartz tube 12 is provided in a horizontal reactor 15, one end of the quartz tube 12 is a gas inlet 11, the outer end of the gas inlet 11 is connected to a mass flowmeter 17, the other end of the quartz tube 12 is a gas outlet 14, and a substrate 13 is placed at the front end of the quartz tube 12. The water vapor in the conical flask 16 is brought into the reaction system by a carrier gas (such as an inert gas such as argon, nitrogen, or a mixed gas of the two). The gas inlet 11 is provided with a plurality of mass flowmeters 17, which can selectively control the introduction of gases such as hydrogen, methane, ethylene, acetylene or argon. The liquid carbon source (such as ethanol, methanol, benzene, toluene or cyclohexane) is placed in a Monte Carlo washing bottle and brought in by bubbling argon or a mixed gas of argon and nitrogen.

[0037] Example 1 A method for preparing a graphene micron sieve, such as Figure 1As shown, the present invention adopts a horizontal reactor to grow graphene micron sieve, a quartz tube 12 is arranged in the horizontal reactor 15, a gas inlet 11 and a gas outlet 14 are arranged at both ends of the quartz tube 12, an air inlet pipe is arranged on the gas inlet 11, a plurality of branch pipes are connected to the air inlet pipe, each branch pipe is provided with a mass flowmeter 17, one of the branch pipes is connected to the conical flask 16. First, a substrate (in this embodiment, a SiO2 / Si substrate, i.e., a substrate formed by depositing 290nm thick silicon dioxide on the surface of a silicon wafer) 13 is sequentially placed in deionized water, acetone, and isopropanol for ultrasonic cleaning for 30 minutes. After cleaning, the SiO2 / Si substrate 13 is placed at the front end of the quartz tube 12 (room temperature environment, tube diameter 20 mm, length 1200 mm); first, argon gas is passed through a branch pipe (not through the conical flask 16) to exhaust the air in the quartz tube 12, and then 20 ml / min of argon gas (as a carrier gas) is passed into the conical flask 16, and the deionized water vapor heated in the water bath in the conical flask 16 is introduced into the reaction system, and the liquefied water droplets on the SiO2 / Si substrate are kept for 5 minutes; it is quickly placed in a horizontal reaction furnace (furnace tube diameter 22 mm, constant temperature zone length 20 mm) 15 central area (temperature 1100 ℃) was heat treated for 5 minutes; after the heat treatment, the argon carrier gas containing water vapor was turned off, methane and hydrogen were introduced (gas flow rates of methane 15 ml / min and hydrogen 60 ml / min, respectively, hydrogen alone was used as a reducing gas), and the growth of graphene micron sieve began, and the growth time was 30 minutes. After the growth was completed, the methane and hydrogen were turned off, argon was introduced, and it was quickly cooled to room temperature at a rate of 10 ℃ / s, and the obtained graphene nucleation points were distributed in a circular shape.

[0038] Scanning electron microscopy Figure 2 ,Depend on Figure 2 It can be seen that the obtained graphene nucleation points are mainly formed around a circle, there are very few graphene islands inside the circle, and there are a large number of graphene islands outside the circle that are not spliced ​​into a film.

[0039] Example 2 A method for preparing a graphene micron sieve, such as Figure 1As shown, the present invention adopts a horizontal reactor to grow graphene micron sieve, a quartz tube 12 is arranged in the horizontal reactor 15, a gas inlet 11 and a gas outlet 14 are arranged at both ends of the quartz tube 12, an air inlet pipe is arranged on the gas inlet 11, a plurality of branch pipes are connected to the air inlet pipe, each branch pipe is provided with a mass flowmeter 17, one of the branch pipes is connected to the conical flask 16. First, a substrate (in this embodiment, a SiO2 / Si substrate, i.e., a substrate formed by depositing 290nm thick silicon dioxide on the surface of a silicon wafer) 13 is sequentially placed in deionized water, acetone, and isopropanol for ultrasonic cleaning for 30 minutes. After cleaning, the SiO2 / Si substrate 13 is placed at the front end of the quartz tube 12 (room temperature environment, tube diameter 20 mm, length 1200 mm); first, argon gas is passed through a branch pipe (not through the conical flask 16) to exhaust the air in the quartz tube 12, and then 20 ml / min of argon gas (as a carrier gas) is passed into the conical flask 16, and the deionized water vapor heated in the water bath in the conical flask 16 is introduced into the reaction system, and the liquefied water droplets on the SiO2 / Si substrate are kept for 2 minutes; it is quickly placed in a horizontal reaction furnace (furnace tube diameter 22 mm, constant temperature zone length 20 mm) 15 central area (temperature 1100 ° C) heat treatment for 5 minutes; after the heat treatment, turn off the argon carrier gas containing water vapor, introduce methane and hydrogen (gas flow rates are 15 ml / min for methane and 60 ml / min for hydrogen, hydrogen alone is used as a reducing gas), start to grow graphene micron sieve, the growth time is 60 minutes, after the growth is completed, turn off the methane and hydrogen, introduce argon, and quickly cool to room temperature at a rate of 10 ° C / s to obtain a graphene film with a sieve-like structure.

[0040] Optical microscope photos are shown in Figure 3 ,Depend on Figure 3 It can be seen that the interior of the obtained graphene film is in the shape of a sieve hole, and the diameter of the sieve hole is about 0.6 microns.

[0041] Example 3 A method for preparing a graphene micron sieve, such as Figure 1As shown, the present invention adopts a horizontal reactor to grow graphene micron sieve, a quartz tube 12 is arranged in the horizontal reactor 15, a gas inlet 11 and a gas outlet 14 are arranged at both ends of the quartz tube 12, an air inlet pipe is arranged on the gas inlet 11, a plurality of branch pipes are connected to the air inlet pipe, each branch pipe is provided with a mass flowmeter 17, one of the branch pipes is connected to the conical flask 16. First, a substrate (in this embodiment, a SiO2 / Si substrate, i.e., a substrate formed by depositing 290nm thick silicon dioxide on the surface of a silicon wafer) 13 is sequentially placed in deionized water, acetone, and isopropanol for ultrasonic cleaning for 30 minutes. After cleaning, the SiO2 / Si substrate was placed at the front end of the quartz tube 12 (room temperature environment, tube diameter 20 mm, length 1200 mm); first, argon was passed through a branch pipe (not through the conical flask 16) to exhaust the air in the quartz tube 12, and then 20 ml / min of argon (as a carrier gas) was passed into the conical flask 16, and the deionized water vapor heated in the water bath in the conical flask 16 was introduced into the reaction system, and the liquefied water droplets on the SiO2 / Si substrate were kept for 5 minutes; it was quickly placed in a horizontal reaction furnace (furnace tube diameter 2 2 mm, constant temperature zone length 20 mm) 15 central area (temperature 1100℃) was heat treated for 5 minutes; after the heat treatment, the argon carrier gas containing water vapor was turned off, and methane and hydrogen were introduced (gas flow rates of methane 15 ml / min and hydrogen 60 ml / min, respectively, and hydrogen was used alone as a reducing gas), and the growth of graphene micron sieve began, and the growth time was 60 minutes. After the growth was completed, the methane and hydrogen were turned off, and argon was introduced, and it was quickly cooled to room temperature at a rate of 10℃ / s to obtain a graphene film with a sieve-like structure.

[0042] The graphene film was analyzed and characterized using optical microscopy, Raman spectroscopy, atomic force microscopy, and transmission electron microscopy. The results are detailed in Figures 4 to 9 ,Depend on Figure 4 It can be seen that the interior of the graphene film obtained by this method is sieve-shaped, and there is a clear boundary between the graphene inside the pores and the graphene outside the pores. Figure 5 It can be seen from the transmission electron microscope photos that the graphene film obtained by this method has a sieve-like shape after being transferred to the microgrid. Figure 6 As shown in FIG. 1 , it can be seen from the optical microscope photo that the mesh structure array inside the graphene film obtained by this method has a pore size of about 3 microns. Figure 7 As shown in Figure 2, from the Raman spectrum of the graphene micro-sieve, it can be seen that the quality of graphene crystals inside and outside the pores is different, and there is a significant difference in thickness. Figure 8 As shown in the figure, it can be seen from the atomic force microscope photo that the thickness of the graphene micro-sieve film obtained by this method is about 0.86 nm, and the average layer thickness is about three layers. Fig. 9 As shown in the C1s X-ray photoelectron spectrum, the graphene film obtained by this method has sp2 and sp 3 Hybridized carbon-carbon bonds and carbon-oxygen bonds.

[0043] Example 4 A method for preparing a graphene micron sieve, such as Figure 1 As shown, the present invention adopts a horizontal reactor to grow graphene micron sieve, a quartz tube 12 is arranged in the horizontal reactor 15, a gas inlet 11 and a gas outlet 14 are arranged at both ends of the quartz tube 12, an air inlet pipe is arranged on the gas inlet 11, a plurality of branch pipes are connected to the air inlet pipe, each branch pipe is provided with a mass flowmeter 17, one of the branch pipes is connected to the conical flask 16. First, a substrate (in this embodiment, a SiO2 / Si substrate, i.e., a substrate formed by depositing 290nm thick silicon dioxide on the surface of a silicon wafer) 13 is sequentially placed in deionized water, acetone, and isopropanol for ultrasonic cleaning for 30 minutes. After cleaning, the SiO2 / Si substrate was placed at the front end of the quartz tube 12 (room temperature environment, tube diameter 20 mm, length 1200 mm); first, argon gas was passed through a branch pipe (not through the conical flask 16) to exhaust the air in the quartz tube 12, and then 20 ml / min of argon gas (as a carrier gas) was passed into the conical flask 16, and the deionized water vapor heated in the water bath in the conical flask 16 was introduced into the reaction system, and the liquefied water droplets on the SiO2 / Si substrate were kept for 10 minutes; it was quickly placed in a horizontal reaction furnace (furnace tube straight The central area of ​​the film (diameter 22 mm, constant temperature zone length 20 mm) was heat treated (temperature 1100°C) for 5 minutes; after the heat treatment, the argon carrier gas containing water vapor was turned off, and methane and hydrogen were introduced (gas flow rates of methane 15 ml / min and hydrogen 60 ml / min, respectively, and hydrogen alone was used as a reducing gas), and the growth of graphene micron sieve began for 60 minutes. After the growth was completed, the methane and hydrogen were turned off, argon was introduced, and the film was quickly cooled to room temperature at a rate of 10°C / s to obtain a graphene film with a sieve-like structure.

[0044] Optical microscope observation Fig.10 ,Depend on Fig.10 It can be seen that the interior of the obtained graphene film is sieve-shaped, and the sieve hole diameter is about 17 microns. Fig.11 As shown in the C1s X-ray photoelectron spectrum, the graphene film obtained by this method has sp 2 and sp 3 Hybridized carbon-carbon bonds and a heavier proportion of carbon-oxygen bonds.

[0045] Example 5 Gold electrodes with a width of 200 microns and a thickness of 50 nanometers were deposited on both ends of the graphene micron sieve obtained in Example 3 to ensure good electrical contact between the electrode and the graphene micron sieve. At room temperature, the sensor was placed in a sealed test chamber to ensure stable environmental conditions. 1000 ml / min of argon was introduced to remove the air in the chamber for 5 minutes; gas molecules with concentrations of 1, 2, 4, 8, 12, 16, and 20 ppm (nitrogen dioxide gas in this embodiment) were introduced in sequence through a precise mass flow controller, and the time of each introduction of different gas concentrations was controlled so that the response time was 3 minutes and the recovery time was 20 minutes. Finally, a relationship curve between the resistance change of the graphene micron sieve and the gas concentration was established. Fig.12 and 13 The dynamic change of the gas sensor's resistance in the range of 1 to 20 ppm of nitrogen dioxide gas concentration, as well as the fitting curve of the response rate changing with the nitrogen dioxide concentration, are shown. The sensor sensitivity is as high as 7.25% ppm. -1 .

[0046] The results of the embodiments show that the present invention utilizes the condensation phenomenon to liquefy the introduced water vapor on the substrate surface to form water droplets. After high-temperature rapid heating treatment, the area in contact with the water droplets can form an oxygen-rich structure that inhibits the nucleation / growth of graphene. Using the method described in the present invention, a graphene micron sieve film with controllable sieve diameter can be directly grown on a non-metallic substrate, avoiding the adverse effects of conventional template etching processes on device performance, and laying the foundation for the in-situ construction of patterned graphene in the field of electronic devices. The present invention also utilizes the adsorption capacity of graphene micron sieves for gas molecules to detect the presence and concentration of gas by monitoring changes in resistance or conductivity.

Claims

1. A method for preparing a graphene micron sieve, characterized in that: The following steps are involved: 1) Place the cleaned non-metallic substrate in a quartz tube at room temperature, fill it with heated carrier gas and water vapor in a non-oxidizing atmosphere, and form water droplets on the non-metallic substrate; 2) heating the substrate pretreated in step 1) from room temperature to a growth temperature of 900-1200° C. for at least 1 minute to form a reaction system; 3) Keeping the temperature in step 2) unchanged, turning off the carrier gas and water vapor, charging the carbon source and hydrogen into the reaction system in step 2), and growing the graphene micron sieve on the substrate treated in step 2) for a growth time of 20 to 180 minutes; 4) After the reaction is completed, turn off the carbon source and hydrogen, cool to room temperature under the protection of a non-oxidizing atmosphere, and obtain a graphene micron sieve.

2. The method for preparing the graphene micron sieve according to claim 1, characterized in that: In step 1), the non-metallic substrate used is a silicon wafer, a quartz wafer, boron nitride, alumina, a mica wafer or a silicon wafer with a silicon dioxide coating.

3. The method for preparing the graphene micron sieve according to claim 1, characterized in that: In step 1), the carrier gas and water vapor are heated in a water bath, an alcohol bath, an oil bath, a sand bath, electric heating or microwave heating; the carrier gas is at least one of nitrogen and an inert gas, and the amount of water vapor added is adjusted by changing the carrier gas flow rate. The carrier gas flow rate is greater than 0 to 100 ml / min, and the time required for condensation to form water droplets on the substrate is 1 to 20 minutes.

4. The method for preparing the graphene micron sieve according to claim 1, characterized in that: In step 2), the time required for the substrate to rise from room temperature to the growth temperature is 1 to 5 minutes.

5. The method for preparing the graphene micron sieve according to claim 1, characterized in that: In step 3), the carbon source used is at least one of methane, ethane, acetylene, ethylene, methanol, ethanol, benzene, toluene and cyclohexane, the volume ratio of the carbon source to hydrogen is (0.1~10):1, and the flow rate of hydrogen is 1~500 ml / min; when the carbon source is a liquid, it is placed in a container bottle and brought into the reaction system by bubbling with a carrier gas, and the molar ratio of the carbon source to the carrier gas is (0.1~10):

1.

6. The method for preparing the graphene micron sieve according to claim 1, characterized in that: In step 1) and step 4), the non-oxidizing atmosphere is nitrogen or an inert gas or a mixture of the two; in step 4), the cooling rate is not less than 1°C / s.

7. The graphene micron sieve obtained by the preparation method according to any one of claims 1 to 6.

8. Use of the graphene microsieve described in claim 7 as a sensitive material of a gas sensor in detecting gas molecules.

9. The use according to claim 8, characterized in that: The gas molecules are nitrogen dioxide, hydrogen, carbon monoxide, ammonia, hydrogen sulfide or volatile organic compounds.

10. The use according to claim 8, characterized in that: The detection process is as follows: a. Connect electrodes at both ends of the graphene micron sieve to obtain a sensor; b. Calibrate the sensor using a series of standard gases of known concentration to establish the relationship between conductivity change and gas concentration; c. Place the graphene micrometer sieve in a gas environment to be tested, apply a known voltage, measure the conductivity change of the graphene micrometer sieve, and record the conductivity change data of the graphene micrometer sieve under different gas concentrations; d. Analyze the collected data, determine the relationship between gas concentration and conductivity change, and calculate the concentration of the gas to be tested; e. After each test, clean the graphene micron screen using clean gas or vacuum system to remove adsorbed gas molecules.

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