Gas sensors and their applications, methods for detecting H2S concentration in gases

Abstract

This invention relates to the field of materials and discloses a gas sensor and its application. The gas sensor includes a ceramic package and a three-dimensional graphene / metal oxide composite microsphere suspended in a groove of the ceramic package. The three-dimensional graphene / metal oxide composite microsphere is connected to the pins of the ceramic package via wires. The three-dimensional graphene / metal oxide composite microsphere includes a graphene microsphere and a metal oxide loaded on the graphene microsphere. The specific surface area of ​​the three-dimensional graphene / metal oxide composite microsphere is 50-500 m² / g. 2 / g. This gas sensor is based on the Joule principle and utilizes the three-dimensional structure and synergistic effect of three-dimensional graphene / metal oxide composite microspheres to reduce operating temperature and sensor power consumption, achieving low-power gas detection. This facilitates the miniaturization and portability of the sensor, making it highly suitable for applications in the field of gas monitoring.

Description

Technical Field

[0001] This invention relates to the field of materials, and more specifically to a gas sensor and its applications. Background Technology

[0002] In recent years, with increasing public concern for the environment, safety, and health, research on gas sensors capable of accurately monitoring toxic and harmful gases has intensified. Resistive gas sensors, a widely used type, convert gas concentration signals into electrical signals by detecting changes in resistance generated between oxygen anions on the device surface and the target gas, thus enabling the monitoring and analysis of toxic and harmful gas concentrations. Common gas-sensitive materials used in the fabrication of resistive gas sensors are nano-metal oxides, including nano-ferric oxide (Fe2O3), tin dioxide (SnO2), tungsten trioxide (WO3), molybdenum trioxide (MoO3), copper oxide (CuO), and zinc oxide (ZnO). However, due to the low room-temperature conductivity of metal oxides, metal oxide-based gas sensors require heating to 200-400℃ to operate normally. Statistics show that nearly 60% of the sensor's energy is used for heating the sensing material, which not only increases energy consumption but also hinders sensor miniaturization and long-term use, while also posing safety hazards. Therefore, how to fabricate sensors with low energy consumption and excellent gas-sensing performance has become one of the research hotspots in the field of gas sensors in recent years. This requires combining them with nanomaterials such as noble metals, nanomaterials, and conductive polymers to lower their operating temperature and reduce power consumption. Among these, graphene stands out due to its excellent electrical conductivity (reaching 10⁸ S·m). -1 Thermal conductivity (thermal conductivity can reach 5300 W·m) -1 K -1 It also boasts superior mechanical properties (Young's modulus reaches 1 TPa) and a large specific surface area (2630 m²). 2 g -1 This has become one of the hot topics in the research of nano-gas-sensitive materials.

[0003] Common graphene materials can be classified according to their dimensionality into one-dimensional nanofibers, two-dimensional nanofilms, and three-dimensional graphene microspheres. The porous structure of three-dimensional graphene microspheres is highly advantageous for improving their adsorption, energy storage, and gas-sensing properties: First, the three-dimensional structure effectively reduces the stacking of graphene sheets, preserving graphene's high specific surface area and increasing the number of active sites for reactions; second, the interconnected channels facilitate molecular diffusion into the material's interior, enhancing its sensitivity; finally, the three-dimensional graphene framework not only provides support sites for other nanomaterials but also accelerates electron flow within the material, enhancing its conductivity. Combining three-dimensional graphene with nano-metal oxides allows for complementary advantages, while further enhancing their gas-sensing properties through heterojunctions and synergistic effects, thus expanding their application range.

[0004] However, the current three-dimensional graphene / metal oxide gas sensors have high activation energies when reacting with target gases at room temperature, resulting in low sensitivity and slow response speed, which cannot meet people's needs in real life. Summary of the Invention

[0005] The purpose of this invention is to overcome the problems of low sensitivity, slow response speed, high operating temperature and high power consumption of existing gas sensors under room temperature conditions, and to provide a gas sensor and its application. This invention reduces the power consumption of the sensor through the Joule effect, which is beneficial to the miniaturization and portability of the sensor.

[0006] To achieve the above objectives, the present invention provides a gas sensor comprising a ceramic package and a three-dimensional graphene / metal oxide composite sphere suspended in a groove of the ceramic package, wherein the three-dimensional graphene / metal oxide composite sphere is connected to the pins of the ceramic package via a wire.

[0007] The three-dimensional graphene / metal oxide composite microspheres include graphene microspheres and metal oxides loaded on the graphene microspheres;

[0008] The specific surface area of ​​the three-dimensional graphene / metal oxide composite microspheres is 50-500 m². 2 / g.

[0009] Preferably, the three-dimensional graphene / metal oxide composite microspheres have a multi-level porous structure with an average pore diameter of 0.1-50 nm.

[0010] Preferably, the diameter of the three-dimensional graphene / metal oxide composite microspheres is 0.5-5 mm.

[0011] Preferably, the metal oxide is selected from one or more of WO3, CuO, ZnO, Co3O4, MoO3 and SnO2.

[0012] Preferably, the preparation method of the three-dimensional graphene / metal oxide composite microspheres includes the following steps:

[0013] (1) Mix graphene oxide, sodium alginate and water, then sonicate and heat to obtain a mixed dispersion;

[0014] (2) The mixed dispersion is dripped into a solution containing Ca using an injection pump. 2+ The graphene oxide / sodium alginate hydrogel was obtained by soaking in an aqueous solution. The graphene oxide / sodium alginate hydrogel was then washed to obtain graphene oxide microspheres.

[0015] (3) The graphene oxide microspheres, metal salt and water are mixed to obtain a mixture;

[0016] (4) The mixture is subjected to a hydrothermal reaction, followed by solid-liquid separation, washing, freeze drying, and sintering.

[0017] Preferably, in step (1), the weight ratio of the graphene oxide, sodium alginate and water is 1-15:0.1-50:1000, more preferably 5-10:0.1-40:1000.

[0018] Preferably, in step (1), the ultrasound duration is 10-120 min.

[0019] Preferably, in step (1), the heating conditions include: a temperature of 70-95°C and a time of 3-6 hours.

[0020] Preferably, in step (2), the flow rate of the syringe pump is 10-200 μL / min.

[0021] Preferably, in step (2), the substance containing Ca 2+ Ca in aqueous solution 2+ The concentration is 1-10% by weight.

[0022] Preferably, in step (2), the soaking time is 1-24 hours.

[0023] Preferably, in step (2), the washing process includes rinsing the graphene oxide / sodium alginate hydrogel with water and then soaking it in water for 12-48 hours.

[0024] Preferably, in step (3), the metal salt is selected from one or more of tungstate, copper salt, cobalt salt, zinc salt, molybdate and tin salt.

[0025] Preferably, the metal salt is selected from one or more of sodium tungstate, copper sulfate, zinc nitrate, cobalt nitrate, ammonium molybdate, and stannous chloride.

[0026] Preferably, in step (3), the weight ratio of the graphene oxide microspheres, metal salt and water is 1:1-20:2000-2600.

[0027] Preferably, in step (4), the hydrothermal reaction is a microwave hydrothermal reaction.

[0028] Preferably, the conditions for the microwave hydrothermal reaction include: a temperature of 150-220℃, a time of 0.5-6h, and a microwave power of 400-800W.

[0029] Preferably, in step (4), the freeze-drying conditions include a temperature of 25-60°C and a time of 12-48h.

[0030] Preferably, in step (4), the sintering conditions are: temperature of 200-700℃, time of 1-5h, and atmosphere of inert atmosphere.

[0031] A second aspect of the present invention provides an application of the above-mentioned gas sensor in detecting the concentration of H2S in a gas.

[0032] A third aspect of the present invention provides a method for detecting the concentration of H2S in a gas, the method comprising the following steps: applying a voltage to a gas sensor, then introducing a gas to be tested into contact with the three-dimensional graphene / metal oxide composite microspheres, and determining the concentration of H2S by measuring the response value after contact;

[0033] The gas sensor is the gas sensor described above.

[0034] Preferably, the voltage is 0.1-15V.

[0035] This invention employs a two-step method to prepare three-dimensional graphene / metal oxide composite microspheres, which are then used to fabricate a self-heating gas sensor. Based on the Joule principle, this gas sensor utilizes the three-dimensional structure and synergistic effect of the three-dimensional graphene / metal oxide composite microspheres to reduce operating temperature and sensor power consumption, achieving low-power gas detection. This facilitates the miniaturization and portability of the sensor, making it highly suitable for application in the field of gas monitoring. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of a gas sensor;

[0037] Figure 2 This is an optical scanning image of the three-dimensional graphene / metal oxide composite microspheres in Example 1;

[0038] Figure 3 These are images showing the nitrogen adsorption and desorption performance of the three-dimensional graphene / metal oxide composite microspheres in Example 1;

[0039] Figure 4 These are scanning electron microscope images of the three-dimensional graphene / metal oxide composite microspheres in Example 1;

[0040] Figure 5 This is a graph showing the power of the gas sensor in Example 1 as a function of voltage.

[0041] Figure 6 This is a graph showing the power of the gas sensor as a function of voltage in Example 2;

[0042] Figure 7This is a graph showing the change in the response value of the gas sensor to 20 ppm hydrogen sulfide as a function of voltage in Example 1;

[0043] Figure 8 This is a graph showing the response of the gas sensor to 20 ppm hydrogen sulfide as a function of voltage in Example 2.

[0044] Figure 9 This is a graph showing the temperature of the gas sensor as a function of voltage in Example 1;

[0045] Figure 10 This is a graph showing the response time of the gas sensor to 20 ppm hydrogen sulfide as a function of voltage in Example 1.

[0046] Figure 11 This is a graph showing the response values ​​of the gas sensor in Example 1 to different gases at a voltage of 2V.

[0047] Figure 12 This is a graph showing the change in the response value of the gas sensor in Example 1 to 20 ppm hydrogen sulfide over 30 days at a voltage of 2V;

[0048] Figure 13 This is a graph showing the temperature of the gas sensor as a function of voltage in Example 2;

[0049] Figure 14 This is a graph showing the response values ​​of the gas sensor in Example 2 to different concentrations of hydrogen sulfide at a voltage of 9V.

[0050] Figure 15 This is a graph showing the response values ​​of the gas sensor to different gases at 9V voltage in Example 2;

[0051] Figure 16 This is a graph showing the response values ​​of the gas sensor in Example 1 to different concentrations of hydrogen sulfide at a voltage of 2V.

[0052] Figure 17 This is a graph showing the response values ​​of the gas sensor in Example 1 to different concentrations of hydrogen sulfide at a voltage of 5V.

[0053] Figure 18 This is a graph showing the response values ​​of the gas sensor in Example 1 to different concentrations of hydrogen sulfide at a voltage of 0.3V.

[0054] Explanation of reference numerals in the attached figures

[0055] 1. Ceramic encapsulation; 2. Three-dimensional graphene / metal oxide composite microspheres; 11. Groove; 12. Leads. Detailed Implementation

[0056] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0057] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0058] The present invention provides a gas sensor, which includes a ceramic package 1 and a three-dimensional graphene / metal oxide composite microsphere 2 suspended in a groove 11 of the ceramic package 1, and the three-dimensional graphene / metal oxide composite microsphere 2 is connected to the pin 12 of the ceramic package 1 through a wire.

[0059] The three-dimensional graphene / metal oxide composite microsphere 2 includes graphene microspheres and metal oxides loaded on the graphene microspheres;

[0060] The specific surface area of ​​the three-dimensional graphene / metal oxide composite microspheres 2 is 50-500 m². 2 / g.

[0061] The gas sensor of the present invention is shown in the schematic diagram below. Figure 1 As shown, the ceramic package 1 can be a common ceramic package in the art, which can be purchased commercially. There are no special requirements for the size and shape of the ceramic package 1, as long as the three-dimensional graphene / metal oxide composite microsphere 2 can be suspended in the groove 11 of the ceramic package 1.

[0062] In one specific embodiment of the present invention, the method for fabricating the gas sensor includes: using a wire drawing machine to draw wires from the three-dimensional graphene / metal oxide composite microsphere 2 and then connecting them to the pins 12 of the ceramic package 1.

[0063] In one specific embodiment of the present invention, the conductor is a gold wire to improve conductivity.

[0064] Preferably, the three-dimensional graphene / metal oxide composite microspheres 2 have a multi-level pore structure with an average pore diameter of 0.1-50 nm, for example, 0.1 nm, 1 nm, 5 nm, 20 nm, 30 nm, 40 nm or 50 nm.

[0065] Preferably, the diameter of the three-dimensional graphene / metal oxide composite microsphere 2 is 0.5-5 mm.

[0066] In this invention, in order to ensure that the three-dimensional graphene / metal oxide composite microspheres 2 possess good gas-sensing properties, in a preferred embodiment, the specific surface area of ​​the three-dimensional graphene / metal oxide composite microspheres 2 needs to be controlled within the aforementioned range. In a specific embodiment, the specific surface area of ​​the three-dimensional graphene / metal oxide composite microspheres 2 can be 50 m². 2 / g, 100m 2 / g, 150m 2 / g、200m 2 / g、250m 2 / g, 350m 2 / g or 500m 2 / g.

[0067] Preferably, the metal oxide is selected from one or more of WO3, CuO, ZnO, Co3O4, MoO3 and SnO2.

[0068] A second aspect of the present invention provides a method for preparing the above-mentioned three-dimensional graphene oxide / metal oxide composite microspheres 2, the method comprising the following steps:

[0069] (1) Mix graphene oxide, sodium alginate and water, then sonicate and heat to obtain a mixed dispersion;

[0070] (2) The mixed dispersion is dripped into a solution containing Ca using an injection pump. 2+ The graphene oxide / sodium alginate hydrogel was obtained by soaking in an aqueous solution. The graphene oxide / sodium alginate hydrogel was then washed to obtain graphene oxide microspheres.

[0071] (3) The graphene oxide microspheres, metal salt and water are mixed to obtain a mixture;

[0072] (4) The mixture is subjected to a hydrothermal reaction, followed by solid-liquid separation, washing, freeze drying, and sintering.

[0073] In this invention, graphene oxide, as a monolayer material peeled from graphene oxide, can exist stably in aqueous solutions and polar solvents due to the introduction of numerous oxygen-containing groups on its surface and edges. After oxidation treatment, graphene oxide retains the layered structure of graphite, but many oxygen-containing functional groups are introduced into each graphene monolayer. The introduction of these oxygen-containing functional groups makes the structure of a single graphene very complex. Given the importance of graphene oxide in the field of graphene materials, many scientists have attempted to provide a detailed and accurate description of its structure to facilitate further research on graphene materials. Although computer simulations, Raman spectroscopy, and nuclear magnetic resonance have been used to analyze its structure, the precise structure of graphene oxide remains undetermined due to various reasons (different preparation methods, experimental conditions, and different graphite sources all have a certain influence on the structure of graphene oxide). The generally accepted structural model is that hydroxyl and epoxy groups are randomly distributed on the graphene oxide monolayer, while carboxyl and carbonyl groups are introduced at the edge of the monolayer. However, related theoretical analysis shows that the surface functional groups of graphene oxide are not randomly distributed, but have a high degree of correlation.

[0074] Sodium alginate is a byproduct of the extraction of iodine and mannitol from brown algae such as kelp or Sargassum. Its molecule is composed of β-D-mannuronic acid (M) and α-L-guluronic acid (G) linked by (1→4) bonds. Sodium alginate aqueous solutions have high viscosity and readily form gels under mild conditions. When Ca... 2+ 、Sr 2+ When cations are present, Na on the G unit + It undergoes an ion exchange reaction with divalent cations, and the G units stack up to form a cross-linked network structure, thereby forming a hydrogel.

[0075] In this invention, to facilitate the formation of graphene / sodium alginate hydrogels and obtain graphene microspheres with better performance, it is necessary to rationally control the dosage ranges of graphene oxide, sodium alginate, and water. The addition of sodium alginate can increase the viscosity and strength of the dispersion, preventing the dispersion from dripping from the injection pump into the Ca... 2+ In solution, it deforms due to low strength; however, the amount of sodium alginate should not be too high, as excessive sodium alginate will lead to excessive viscosity of the dispersion and blockage of the injection pump.

[0076] Therefore, in a preferred embodiment, in step (1), the weight ratio of the graphene oxide, sodium alginate and water is 1-15:0.1-50:1000, and more preferably 5-10:0.1-40:1000.

[0077] In a specific implementation, in step (1), the weight ratio of graphene oxide to water can be 5:1000, 6:1000, 7:1000, 8:1000, 9:100 or 10:1000, and the weight ratio of sodium alginate to water can be 0.1:1000, 1:1000, 5:1000, 10:1000, 15:1000, 20:1000, 25:1000, 30:1000, 35:1000 or 40:1000.

[0078] In this invention, in step (1), due to the increase in viscosity of the entire system after the addition of sodium alginate, it is necessary to completely disperse the graphene oxide by ultrasound. The ultrasound time can be adjusted according to the amount of sodium alginate added. Preferably, in step (1), the ultrasound time is 10-120 min, specifically 10 min, 20 min, 30 min, 40 min, 50 min, 60 min, 70 min, 80 min, 90 min, 100 min, 110 min, or 120 min.

[0079] In this invention, in step (1), the heating operation can completely dissolve sodium alginate and prevent agglomeration. The heating time is related to the amount of sodium alginate added. In a preferred case, the heating conditions include a temperature of 70-95°C and a time of 3-6 hours.

[0080] In a specific implementation, in step (1), the heating temperature can be 70°C, 75°C, 80°C, 85°C, 90°C or 95°C, and the heating time can be 3h, 3.5h, 4h, 4.5h, 5h, 5.5h or 6h.

[0081] Preferably, in step (2), the flow rate of the syringe pump is 10-200 μL / min, specifically 10 μL / min, 30 μL / min, 50 μL / min, 70 μL / min, 90 μL / min, 110 μL / min, 130 μL / min, 150 μL / min, 180 μL / min or 200 μL / min.

[0082] In this invention, step (2) contains Ca. 2+ In an aqueous solution, if Ca 2+ If the concentration of Ca is too low, the resulting gel strength will be too low; if the concentration of Ca is too low, the gel strength will be too low. 2+ If the concentration is too high, too much will remain in the gel, affecting its structure and performance. Therefore, in a preferred embodiment, preferably, in step (2), the Ca-containing... 2+ Ca in aqueous solution 2+The concentration is 1-10% by weight, specifically 1% by weight, 2% by weight, 3% by weight, 4% by weight, 5% by weight, 6% by weight, 7% by weight, 8% by weight, 9% by weight or 10% by weight.

[0083] In this invention, the substance containing Ca 2+ An aqueous solution can be obtained by mixing water with various water-soluble calcium salts conventionally used in the art. In one specific embodiment, the Ca-containing... 2+ The solution is an aqueous solution of calcium chloride.

[0084] In step (2) of the present invention, for the substance containing Ca 2+ There are no special requirements for the amount of aqueous solution used; it is sufficient to completely soak the mixed dispersion dripped in by the syringe pump.

[0085] In this invention, in step (2), the soaking time is calculated from the start of the dripping process until the end of the soaking. Preferably, in step (2), the soaking time is 1-24 hours, more preferably 6-24 hours. Specifically, the soaking time can be 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 12 hours, 14 hours, 16 hours, 18 hours, 20 hours, 2 hours, or 24 hours.

[0086] In this invention, in order to fully remove calcium ions from the hydrogel, the product can be washed by rinsing and soaking. Therefore, in a preferred embodiment of step (2), the washing process includes: rinsing the graphene oxide / sodium alginate hydrogel with water, and then soaking it in water for 12-48 hours. The specific soaking time can be 12 hours, 16 hours, 18 hours, 20 hours, 24 hours, 30 hours, 32 hours, 36 hours, 40 hours, or 48 hours.

[0087] Preferably, in step (2), the number of rinsing operations during the washing process is 1-5 times.

[0088] In step (3) of the present invention, the metal salt is selected from one or more of tungstate, copper salt, cobalt salt, zinc salt, molybdate and tin salt, and is more preferably a soluble salt.

[0089] In a preferred embodiment of the present invention, the tungstate is used to provide tungsten and can be a soluble tungstate commonly used in the art, such as one or more of potassium tungstate, sodium tungstate, and ammonium tungstate.

[0090] In a preferred embodiment of the present invention, the copper salt is used to provide copper element and can be a soluble copper salt commonly used in the art, such as one or more of copper chloride, copper sulfate and copper nitrate.

[0091] In a preferred embodiment of the present invention, the cobalt salt is used to provide cobalt element and can be a soluble cobalt salt commonly used in the art, such as one or more of cobalt chloride, cobalt carbonate, cobalt sulfate and cobalt nitrate.

[0092] In a preferred embodiment of the present invention, the zinc salt is used to provide zinc element and can be a soluble zinc salt commonly used in the art, such as one or more of zinc chloride, zinc sulfate and zinc nitrate.

[0093] In a preferred embodiment of the present invention, the molybdate is used to provide molybdenum and can be a soluble molybdate commonly used in the art, such as one or more of potassium molybdate, ammonium molybdate, and sodium molybdate.

[0094] In a preferred embodiment of the present invention, the tin salt is used to provide tin element and can be a soluble tin salt commonly used in the art.

[0095] Preferably, in step (3), the metal salt is selected from one or more of sodium tungstate, copper sulfate, zinc nitrate, cobalt nitrate, ammonium molybdate, and stannous chloride.

[0096] Preferably, in step (3), the weight ratio of the graphene oxide microspheres, metal salt and water is 1:1-20:2000-2600.

[0097] In the specific implementation of step (3), the weight ratio of the graphene oxide microspheres to the metal salt can be 1:1, 1:5, 1:10, 1:15 or 1:20, and the weight ratio of the graphene oxide microspheres to water can be 1:2000, 1:2100, 1:2200, 1:2300, 1:2400, 1:2500 or 1:2600.

[0098] In step (3) of the present invention, due to the different metal salts used, a certain amount of acid or alkali needs to be added to adjust the pH value to a suitable range in order to form a stable and dispersed hydrosol, thereby ensuring uniform growth into nano metal oxides during the heating process.

[0099] Therefore, in a specific embodiment of the present invention, when the metal salt is sodium tungstate, the specific process of step (3) includes: mixing the graphene oxide microspheres, the metal salt and water, and then adding a 2 mol / L hydrochloric acid solution dropwise under stirring to adjust the pH value to 1.5-2.5, and then obtaining the mixture, wherein the dropwise addition time of the hydrochloric acid solution is controlled to be 10-30 min; wherein there are no special requirements for the dropwise addition rate of the hydrochloric acid solution, it is sufficient to add it within the limited time.

[0100] In a specific embodiment of the present invention, when the metal salt is copper sulfate, the specific process of step (3) includes: mixing the graphene oxide microspheres, the metal salt and water, and then adding a sodium hydroxide solution with a concentration of 2 mol / L dropwise under stirring, adjusting the pH value to 9.5-10.5, and then obtaining a mixture, wherein the dropwise addition time of the sodium hydroxide solution is controlled to be 10-30 min; wherein there are no special requirements for the dropwise addition rate of the sodium hydroxide solution, it is sufficient to complete the dropwise addition within the limited time.

[0101] In a specific embodiment of the present invention, when the metal salt is zinc nitrate, the specific process of step (3) includes: mixing the graphene oxide microspheres, the metal salt and water, and then adding a sodium hydroxide solution with a concentration of 2 mol / L dropwise under stirring, adjusting the pH value to 9.5-10.5, and then obtaining a mixture, wherein the dropping time of the sodium hydroxide solution is controlled to be 10-30 min; wherein there are no special requirements for the dropping rate of the sodium hydroxide solution, and it is sufficient to drop it completely within the limited time.

[0102] In a specific embodiment of the present invention, when the metal salt is cobalt nitrate, the specific process of step (3) includes: mixing the graphene oxide microspheres, the metal salt and water, and then adding a 2 mol / L sodium hydroxide solution dropwise under stirring to adjust the pH value to 9.5-10.5, and then obtaining the mixture. The dropwise addition time of the sodium hydroxide solution is controlled to be 10-30 min. There are no special requirements for the dropwise addition rate of the sodium hydroxide solution, as long as it is added within the specified time.

[0103] In a specific embodiment of the present invention, when the metal salt is ammonium molybdate, the specific process of step (3) includes: mixing the graphene oxide microspheres, the metal salt and water, and then adding a nitric acid solution with a concentration of 2 mol / L dropwise under stirring, adjusting the pH value to 1.5-2.5, and then obtaining a mixture, wherein the dropping time of the nitric acid solution is controlled to be 10-30 min; wherein there are no special requirements for the dropping rate of the nitric acid solution, and it is sufficient to drop it completely within the limited time.

[0104] In a specific embodiment of the present invention, when the metal salt is stannous chloride, stannous chloride can hydrolyze on its own and form a stable dispersed hydrosol, so there is no need to add acid or alkali. Therefore, when the metal salt is stannous chloride, the specific process of step (3) includes: mixing the graphene oxide microspheres, the metal salt and water, and then stirring for 10-30 minutes to obtain a mixture.

[0105] In a specific embodiment of the present invention, in step (3), when sodium hydroxide solution, hydrochloric acid solution or nitric acid solution is added, the amount of water used is calculated as the total amount of water added and the water contained in the added solution. When the water content in the added sodium hydroxide solution, hydrochloric acid solution or nitric acid solution meets the ratio requirements of the amount of graphene oxide microspheres, metal salt and water, no additional water needs to be added.

[0106] In a preferred embodiment, the hydrothermal reaction in step (4) is a microwave hydrothermal reaction. Using a microwave hydrothermal method is beneficial for obtaining nanomaterials with more uniform size distribution, improving material stability, and significantly shortening the reaction time.

[0107] Preferably, the conditions for the microwave hydrothermal reaction include: a temperature of 150-220℃, a time of 0.5-6h, and a microwave power of 400-800W.

[0108] In the specific implementation of step (4), the temperature of the microwave hydrothermal reaction can be 150℃, 160℃, 170℃, 180℃, 190℃, 200℃, 210℃ or 220℃, the time of the microwave hydrothermal reaction can be 0.5h, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h or 6h, and the microwave power can be 400W, 500W, 600W, 700W or 800W.

[0109] In a preferred embodiment, in step (4), when washing the solid phase obtained from solid-liquid separation, anhydrous ethanol and deionized water are used in sequence.

[0110] More preferably, the washing is performed 2-5 times with anhydrous ethanol and 2-5 times with deionized water.

[0111] In this invention, the freeze-drying method used in step (4) can prevent the collapse of the pore structure of the gel and preserve the three-dimensional structure of its microspheres to the greatest extent.

[0112] Therefore, preferably, in step (4), the freeze-drying conditions include: a temperature of 25-60°C and a time of 12-48h.

[0113] In the specific implementation of step (4), the freeze-drying temperature can be 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C or 60°C, and the freeze-drying time can be 12h, 18h, 24h, 30h, 36h, 42h or 48h.

[0114] In step (4) of the present invention, the sintering operation can remove sodium alginate on the one hand, and reduce graphene oxide on the other hand, thereby improving its electrical conductivity and mechanical properties, while improving the crystal structure of the metal oxide.

[0115] Therefore, preferably, in step (4), the sintering conditions are: temperature of 200-700℃, time of 1-5h, and atmosphere of inert atmosphere. Specifically, the sintering temperature can be 200℃, 250℃, 300℃, 350℃, 400℃, 450℃, 500℃, 550℃, 600℃, 650℃ or 700℃, and the sintering time can be 1h, 2h, 3h, 4h or 5h.

[0116] More preferably, in step (4), the inert atmosphere may be provided by one or more gases selected from nitrogen, helium, neon, argon, krypton and xenon.

[0117] A second aspect of the present invention provides an application of the above-mentioned gas sensor in detecting the concentration of H2S in a gas.

[0118] A third aspect of the present invention provides a method for detecting the concentration of H2S in a gas, the method comprising the following steps: applying a voltage to a gas sensor, then introducing a gas to be tested into contact with the three-dimensional graphene / metal oxide composite microsphere 2, and determining the concentration of H2S by measuring the response value after contact;

[0119] The gas sensor is the gas sensor described above.

[0120] In the method for detecting H2S concentration in a gas according to the present invention, a voltage is applied to the gas sensor, causing the three-dimensional graphene / metal oxide composite microsphere 2 to become charged. Based on the Joule heating principle (Joule heating: the heat generated when an electric current passes through a conductor), the charged three-dimensional graphene / metal oxide composite microsphere 2 undergoes self-heating. The resistance change of the gas sensor is measured before and after the charged three-dimensional graphene / metal oxide composite microsphere 2 comes into contact with the gas to be measured, and then the response value S (Sensitivity) of the gas sensor to the gas to be measured is calculated as (R0 - R...). x ) / R0, where R0 is the resistance value of the gas sensor in air, R x This is the resistance value of the gas sensor after the gas to be measured is introduced.

[0121] In this invention, the voltage is 0.1-15V.

[0122] It should be noted that, due to differences in the fabrication process of the three-dimensional graphene / metal oxide composite microspheres 2 in the gas sensor, the resistance of the resulting three-dimensional graphene / metal oxide composite microspheres 2 will also be different. Therefore, the range of voltage that the gas sensor can apply (i.e., withstand voltage) will also be different depending on the three-dimensional graphene / metal oxide composite microspheres 2.

[0123] In one specific embodiment, the external standard method can be used to detect the H2S concentration in a gas. The specific process includes: mixing H2S with air in different proportions to prepare H2S standard gases with gradient concentrations; applying voltage to the gas sensor through a power supply; then introducing the H2S standard gases into contact with charged three-dimensional graphene / metal oxide composite microspheres 2; measuring the resistance value of the gas sensor; and then calculating the response value of the gas sensor; preparing an H2S concentration-response value standard curve under this voltage with H2S concentration as the abscissa and the gas sensor response value as the ordinate; then, under the same voltage, testing the gas to be tested using the same method to obtain the response value of the gas sensor under the gas to be tested; substituting the obtained gas sensor response value into the H2S concentration-response value standard curve under this voltage to obtain the H2S concentration in the gas to be tested; wherein, the gas sensors used for the H2S standard gases and the gas to be tested are from the same batch of gas sensors.

[0124] The method for detecting H2S concentration in gas described in this invention can be used to detect low concentrations of H2S, and still exhibits excellent detection performance for test gases with H2S concentrations of 5-50 ppm.

[0125] The gas sensor described in this invention utilizes the Joule heating principle for self-heating. Heating is achieved by adjusting the resistance and the applied measurement voltage, using the current provided by the measurement circuit. Moreover, since the heating is directly generated by the three-dimensional graphene / metal oxide composite microspheres themselves, no heat conduction is required, which greatly reduces heat dissipation. Furthermore, by eliminating the external heating circuit, energy consumption can be further reduced.

[0126] Since traditional gas sensors use ceramic tubes and MEMS chips with external heating elements, this invention utilizes the Joule heat of the material for heating, thus eliminating these external heating elements, simplifying their design, and facilitating fabrication and circuit miniaturization.

[0127] Compared with the prior art, the main advantages of the present invention are as follows:

[0128] 1. Gas sensors are fabricated using three-dimensional graphene / metal oxide composite microspheres, eliminating the need for coating onto ceramic tubes or MEMS chips, thus simplifying the process and reducing device size;

[0129] 2. By using self-heating, the performance of the sensor is improved, while the external heating element is eliminated, heat dissipation is reduced, power consumption is lowered, and low-power rapid detection is achieved.

[0130] The present invention will be described in detail below through embodiments, but the scope of protection of the present invention is not limited thereto.

[0131] In the following examples, the ceramic package used was purchased from Kyocera, with part number CLCC28.

[0132] All room temperatures mentioned below refer to 25°C.

[0133] Example 1

[0134] (1) Disperse 50mg of graphene oxide into 10ml (10g) of deionized water, add 0.1g of sodium alginate, and stir evenly. The weight ratio of graphene oxide, sodium alginate and deionized water is 5:10:1000. Then sonicate for 30min to completely peel off the graphene oxide sheets and disperse them evenly in water. Then heat at 70℃ for 4h to obtain a mixed dispersion.

[0135] (2) Add the mixed dispersion to a micro-injection pump, controlling the pump flow rate at 30 μL / min, so that the mixed dispersion drops evenly into the calcium chloride aqueous solution (Ca). 2+ The graphene oxide / sodium alginate hydrogel was formed by soaking in deionized water at a concentration of 5% by weight for 12 hours. The graphene oxide / sodium alginate hydrogel was rinsed three times with deionized water and then soaked in deionized water for 24 hours to obtain graphene oxide microspheres.

[0136] (3) Add 0.1g of graphene oxide microspheres to a 500mL beaker, then add 190mL (190g) of deionized water, and then add 1.06g of sodium tungstate. Under the conditions of room temperature and stirring, add hydrochloric acid solution with a concentration of 2mol / L dropwise to adjust the pH value to 2. The dropwise addition time of hydrochloric acid solution is controlled to be 30min to obtain a mixture. The weight ratio of graphene oxide microspheres, sodium tungstate and water is 1:10.6:2400 (at this time, the amount of water used is the amount of deionized water plus the water content in the hydrochloric acid solution). At this time, the mixture is a uniform suspension.

[0137] (4) The mixture was transferred to a hydrothermal reactor and then placed in a microwave synthesizer. It was then subjected to microwave-assisted hydrothermal treatment at 180°C (800W power) for 0.5 hours. After natural cooling to room temperature, the solid obtained from solid-liquid separation was washed three times with anhydrous ethanol and deionized water, respectively. It was then freeze-dried at 25°C for 24 hours and sintered at 500°C for 2 hours under nitrogen protection to obtain three-dimensional graphene / metal oxide composite microspheres (three-dimensional graphene / WO3 composite microspheres). The morphology of the obtained three-dimensional graphene / metal oxide composite microspheres was determined by scanning electron microscopy and nitrogen adsorption-desorption method (BET method). The morphology of the obtained three-dimensional graphene / metal oxide composite microspheres is shown in the figure below. Figure 2 as well as Figure 4 As shown, according to Figure 2 Yes, the three-dimensional graphene / metal oxide composite microspheres are uniformly spherical with a diameter of 1.78 mm. Figure 4 As can be seen, WO3 nanosheets were successfully loaded onto the surface of graphene microspheres using the in-situ loading method, and the WO3 nanosheets were uniformly distributed on the surface of the graphene microspheres. The BET test results are as follows: Figure 3 As shown, the specific surface area is 206 m². 2 / g, with a very high specific surface area, and multi-level channels ranging from 2 nanometers to 100 nanometers, with an average channel diameter of 10.38 nm;

[0138] (5) Using a wire drawing machine, lead wires (gold wires) from the three-dimensional graphene / metal oxide composite microspheres obtained in step (4) and connect them to the pins of the ceramic package to obtain gas sensor A1, as shown in the schematic diagram. Figure 1 As shown, the gas sensor includes a ceramic package 1 and a three-dimensional graphene / metal oxide composite sphere suspended in a groove 11 of the ceramic package 1, and the three-dimensional graphene / metal oxide composite sphere is connected to the pin 12 of the ceramic package 1 through a wire.

[0139] Example 2

[0140] The method of Example 1 was followed, except that step (3) was performed as follows: 0.1 g of graphene oxide microspheres were added to a 500 mL beaker, followed by 220 mL (220 g) of deionized water, and then 0.55 g of copper sulfate. A 2 mol / L sodium hydroxide solution was added dropwise at room temperature with stirring. The pH was adjusted to 10, and the sodium hydroxide solution was added for 30 min. A mixture was then obtained, in which the weight ratio of graphene oxide microspheres, copper sulfate, and water was 1:5.5:2400 (the amount of water used was the amount of deionized water plus the water content in the sodium hydroxide solution). Finally, three-dimensional graphene / metal oxide composite microspheres (three-dimensional graphene / CuO composite microspheres) were obtained. The obtained three-dimensional graphene / metal oxide composite microspheres were detected by scanning electron microscopy and nitrogen adsorption-desorption method (BET method). The obtained three-dimensional graphene / metal oxide composite microspheres were uniform spheres with a diameter of 1.2 mm and a specific surface area of ​​190 m². 2 / g, with a high specific surface area, and multi-level channels ranging from 2 nanometers to 100 nanometers, with an average channel diameter of 11.71 nm) and gas sensor A2.

[0141] Example 3

[0142] The method of Example 1 was followed, except that step (3) was performed as follows: 0.1 g of graphene oxide microspheres were added to a 500 mL beaker, followed by 220 mL (220 g) of deionized water, and then 1.428 g of zinc nitrate was added. A 2 mol / L sodium hydroxide solution was added dropwise under room temperature and stirring conditions. The pH value was adjusted to 10, and the dropwise addition time of the sodium hydroxide solution was controlled to be 30 min. A mixture was then obtained, in which the weight ratio of graphene oxide microspheres, zinc nitrate and water was 1:14.28:2400 (at this time, the amount of water used was the amount of deionized water plus the water content in the sodium hydroxide solution). Finally, three-dimensional graphene / metal oxide composite microspheres (three-dimensional graphene / ZnO composite microspheres) were obtained. The obtained three-dimensional graphene / metal oxide composite microspheres were detected by scanning electron microscopy and nitrogen adsorption-desorption method (BET method). The obtained three-dimensional graphene / metal oxide composite microspheres were uniform spheres with a diameter of 1.3 mm and a specific surface area of ​​193 m². 2 / g, with a high specific surface area, and multi-level channels ranging from 2 nanometers to 100 nanometers, with an average channel diameter of 10.9 nm) and gas sensor A3.

[0143] Example 4

[0144] The method of Example 1 was followed, except that step (3) was performed as follows: 0.1 g of graphene oxide microspheres were added to a 500 mL beaker, followed by 220 mL (220 g) of deionized water, and then 1.164 g of cobalt nitrate was added. A 2 mol / L sodium hydroxide solution was added dropwise under room temperature and stirring conditions. The pH value was adjusted to 10, and the dropwise addition time of the sodium hydroxide solution was controlled to be 30 min. A mixture was then obtained, in which the weight ratio of graphene oxide microspheres, cobalt nitrate and water was 1:11.64:2400 (at this time, the amount of water used was the amount of deionized water plus the water content in the sodium hydroxide solution). Finally, three-dimensional graphene / metal oxide composite microspheres (three-dimensional graphene / Co3O4 composite microspheres) were obtained. The obtained three-dimensional graphene / metal oxide composite microspheres were detected by scanning electron microscopy and nitrogen adsorption-desorption method (BET method). The obtained three-dimensional graphene / metal oxide composite microspheres were uniform spheres with a diameter of 1.7 mm and a specific surface area of ​​188 m². 2 / g, with a high specific surface area, and multi-level channels ranging from 2 nanometers to 100 nanometers, with an average channel diameter of 14.4 nm) and a gas sensor A4.

[0145] Example 5

[0146] The method was implemented according to Example 1, except that step (3) was performed as follows: 0.1g of graphene oxide microspheres were added to a 500mL beaker, followed by 220mL (220g) of deionized water, and then 1.76g of ammonium molybdate was added. Under the conditions of room temperature and stirring, a 2mol / L nitric acid solution was added dropwise to adjust the pH value to 2. The dropwise addition time of the nitric acid solution was controlled to be 30min, and then a mixture was obtained. The weight ratio of graphene oxide microspheres, ammonium molybdate and water was 1:17.6:2400 (at this time, the amount of water used was the amount of deionized water plus the water content in the nitric acid solution); finally, three-dimensional graphene / metal oxide composite microspheres (three-dimensional graphene / MoO3 composite microspheres) were obtained. The obtained three-dimensional graphene / metal oxide composite microspheres were detected by scanning electron microscopy and nitrogen adsorption-desorption method (BET method). The obtained three-dimensional graphene / metal oxide composite microspheres were uniform spheres with a diameter of 1.5mm and a specific surface area of ​​202m². 2 / g, with a high specific surface area, and multi-level channels ranging from 2 nanometers to 100 nanometers, with an average channel diameter of 10.57 nm) and a gas sensor A5.

[0147] Example 6

[0148] The method of Example 1 was followed, except that step (3) was performed as follows: 0.1g of graphene oxide microspheres were added to a 500mL beaker, followed by 240mL (240g) of deionized water, and then 1.2g of stannous chloride was added. The weight ratio of graphene oxide microspheres, stannous chloride and deionized water was 1:12:2400. The mixture was stirred at room temperature for 30min to obtain a mixture. Finally, three-dimensional graphene / metal oxide composite microspheres (three-dimensional graphene / SnO2 composite microspheres) were obtained. The three-dimensional graphene / metal oxide composite microspheres were detected by scanning electron microscopy and nitrogen adsorption-desorption method (BET method). The obtained three-dimensional graphene / metal oxide composite microspheres were uniform spheres with a diameter of 0.9mm and a specific surface area of ​​189m². 2 / g, with a high specific surface area, and multi-level channels ranging from 2 nanometers to 100 nanometers, with an average channel diameter of 11.87 nm) and a gas sensor A6.

[0149] Example 7

[0150] The method is implemented according to Example 1, except that step (1) is performed as follows:

[0151] 50 mg of graphene oxide was dispersed in 5 mL of deionized water, and 0.1 g of sodium alginate was added. The mixture was stirred until homogeneous, resulting in a weight ratio of graphene oxide, sodium alginate, and water of 10:20:1000. The mixture was then sonicated for 100 min to completely exfoliate the graphene oxide sheets and uniformly disperse them in water. The solution was then heated at 70 °C for 4 h to obtain a mixed dispersion. Finally, three-dimensional graphene / metal oxide composite microspheres (three-dimensional graphene / WO3 composite microspheres) were obtained. Scanning electron microscopy and nitrogen adsorption-desorption (BET) analysis showed that the obtained three-dimensional graphene / metal oxide composite microspheres were uniformly spherical with a diameter of 1.7 mm and a specific surface area of ​​197 m². 2 / g, with a high specific surface area, and multi-level channels ranging from 2 nanometers to 100 nanometers, with an average channel diameter of 10.52 nm) and a gas sensor A7.

[0152] Example 8

[0153] The method is implemented according to Example 1, except that step (2) is performed as follows:

[0154] The mixed dispersion was added to a micro-injection pump, and the flow rate of the pump was controlled at 100 μL / min, so that the mixed dispersion was evenly dropped into the calcium chloride aqueous solution (Ca). 2+The graphene oxide / sodium alginate hydrogel was formed by soaking the sample in a solution of 10% by weight for 12 hours. The hydrogel was then rinsed three times with deionized water and soaked in deionized water for 24 hours to obtain graphene oxide microspheres. Finally, three-dimensional graphene / metal oxide composite microspheres (WO3 composite microspheres) were obtained. These microspheres were analyzed by scanning electron microscopy and nitrogen adsorption-desorption (BET) method. The obtained three-dimensional graphene / metal oxide composite microspheres were found to be uniformly spherical with a diameter of 1.6 mm and a specific surface area of ​​191 m². 2 / g, with a high specific surface area, and multi-level channels ranging from 2 nanometers to 100 nanometers, with an average channel diameter of 11.55 nm) and a gas sensor A8.

[0155] Example 9

[0156] The method is implemented according to Example 1, except that step (4) is performed as follows:

[0157] The mixture was transferred to a hydrothermal reactor and then placed in a microwave synthesizer. It was subjected to microwave-assisted hydrothermal treatment at 200°C (800W power) for 3 hours. After natural cooling to room temperature, the solid obtained from solid-liquid separation was washed three times each with anhydrous ethanol and deionized water. It was then freeze-dried at 50°C for 24 hours and sintered at 500°C for 4 hours under nitrogen protection to obtain three-dimensional graphene / metal oxide composite microspheres (three-dimensional graphene / WO3 composite microspheres). Scanning electron microscopy and nitrogen adsorption-desorption (BET) analysis showed that the obtained three-dimensional graphene / metal oxide composite microspheres were uniformly spherical with a diameter of 1.9 mm and a specific surface area of ​​192 m². 2 / g, with a high specific surface area, and multi-level channels ranging from 2 nanometers to 100 nanometers, with an average channel diameter of 11.16 nm) and a gas sensor A9.

[0158] Example 10

[0159] The method was implemented according to Example 1, except that in step (4), the sintering temperature was 200℃, resulting in three-dimensional graphene / metal oxide composite microspheres (three-dimensional graphene / WO3 composite microspheres). These microspheres were examined by scanning electron microscopy and nitrogen adsorption-desorption method (BET method). The obtained three-dimensional graphene / metal oxide composite microspheres were found to be uniformly spherical with a diameter of 1.9 mm and a specific surface area of ​​196 m². 2 / g, with a high specific surface area, and multi-level channels ranging from 2 nanometers to 100 nanometers, with an average channel diameter of 10.73 nm) and a gas sensor A10.

[0160] Comparative Example 1

[0161] The three-dimensional graphene / metal oxide composite microspheres obtained in Example 1 were ground into powder and coated onto a ceramic tube to obtain gas sensor D1.

[0162] Comparative Example 2

[0163] The method of Example 1 was implemented, except that the three-dimensional graphene / metal oxide composite microspheres were replaced with graphene oxide microspheres obtained in step (2) to obtain gas sensor D2.

[0164] Comparative Example 3

[0165] The method was implemented according to Example 1, except that the specific surface area of ​​the three-dimensional graphene / WO3 composite microspheres used was 32 m². 2 / g, thus obtaining gas sensor D3;

[0166] The three-dimensional graphene / WO3 composite microspheres with this specific surface area were prepared according to the following method:

[0167] (1) Disperse 50mg of graphene oxide into 10ml (10g) of deionized water, add 0.1g of sodium alginate, and stir evenly. The weight ratio of graphene oxide, sodium alginate and deionized water is 5:10:1000. Then sonicate for 30min to completely peel off the graphene oxide sheets and disperse them evenly in water. Then heat at 70℃ for 4h to obtain a mixed dispersion.

[0168] (2) Add the mixed dispersion to a micro-injection pump, controlling the pump flow rate at 30 μL / min, so that the mixed dispersion drops evenly into the calcium chloride aqueous solution (Ca). 2+ The graphene oxide / sodium alginate hydrogel was formed by soaking in deionized water at a concentration of 5% by weight for 12 hours. The graphene oxide / sodium alginate hydrogel was rinsed three times with deionized water and then soaked in deionized water for 24 hours to obtain graphene oxide microspheres.

[0169] (3) Add 0.1g of graphene oxide spheres to a 500mL beaker, then add 240mL of deionized water, then add 0.5g of WO3, stir at room temperature for 30min, and then obtain the mixture;

[0170] (4) The mixture was transferred to a hydrothermal reactor and then placed in a microwave synthesizer. It was then subjected to microwave-assisted hydrothermal treatment at 180°C (800W power) for 0.5 hours. After natural cooling to room temperature, the solid obtained from solid-liquid separation was washed three times with anhydrous ethanol and deionized water, respectively. It was then freeze-dried at 25°C for 24 hours and sintered at 500°C for 2 hours under nitrogen protection to obtain three-dimensional graphene / metal oxide composite microspheres (three-dimensional graphene / WO3 composite microspheres). Scanning electron microscopy and nitrogen adsorption-desorption (BET) analysis showed that the obtained three-dimensional graphene / metal oxide composite microspheres were uniformly spherical with a diameter of 6.2 mm and a specific surface area of ​​32 m². 2 / g, with an average pore diameter of 63.12nm.

[0171] Test Example 1

[0172] The power changes of A1-A10 and D1-D3 under different applied voltages and their maximum response values ​​to hydrogen sulfide at a concentration of 20 ppm were measured respectively. The detection methods and results are as follows:

[0173] 1.1 Electrical performance testing:

[0174] A power supply unit (Keithley 2601B) was used to apply voltages of 0-25V to A1-A10 and D1-D3 respectively (with voltage intervals of 0.1V between 0-1V, 0.5V between 1-10V, and 1V between 10-16V). An infrared thermometer was used to measure the temperature of the three-dimensional graphene / metal oxide composite spheres in each gas sensor at different voltages. The minimum applied voltage V for A1-A10 and D1-D3 was then determined. min and the maximum applied voltage V max Then V min ~V max The applicable voltage of this gas sensor, i.e., the voltage it can withstand, is shown in Table 1. The applicable voltages for A1-A10 and D1-D3 are as follows, where V min This refers to the voltage applied when the temperature of the three-dimensional graphene / metal oxide composite microspheres first exceeds 25°C, V. max This refers to the maximum voltage that the gas sensor can withstand. If this voltage is exceeded, the temperature of the three-dimensional graphene / metal oxide composite microspheres will be too high, causing the gas sensor to burn out and become unusable.

[0175] Taking A1 as an example, when a voltage less than 0.3V is applied to A1, the temperature of the three-dimensional graphene / metal oxide composite spheres in A1 is ≤25℃. When a voltage of 0.3V is applied to A1, the temperature of the three-dimensional graphene / metal oxide composite spheres is >25℃, that is, the voltage of A1 is... minThe voltage of A1 is 0.3V. When a voltage of 5V is applied to A1, A1 does not burn out and can be used normally. However, when a voltage of 5.5V is applied to A1, A1 burns out and becomes unusable. Therefore, the voltage of A1 is... max It is 5V.

[0176] 1.2 Power Detection:

[0177] Apply V to A1 using the power supply meter. min ~V max A voltage of (0.3~5V) was applied, and the resistance of the gas sensor was measured at different voltages. The power of the gas sensor was calculated using P=U2 / R. The power change is shown in the figure. Figure 5 As shown;

[0178] Depend on Figure 5 Yes, the power of the gas sensor A1 varies between 0.23 and 29.4 mW when a voltage between 0.3 and 5 V is applied.

[0179] Apply V to A2 using the power supply meter. min ~V max A voltage of (0.1~9V) was applied to measure the resistance of the gas sensor at different voltages, and the power of the gas sensor was calculated using P=U2 / R. The power change is shown in the figure. Figure 6 As shown;

[0180] Depend on Figure 6 Yes, the power of the gas sensor A2 varies from 0.46 to 125.78 mW when a voltage between 0.1 and 9 V is applied.

[0181] The same method was used to detect the power variation range of A3-A10 and D1-D3, and the results are shown in Table 1.

[0182] 1.3 Maximum response value to hydrogen sulfide at a concentration of 20 ppm:

[0183] Apply V to A1 using the power supply meter. min ~V max A voltage of 0.3–5V was applied, and a mixture of hydrogen sulfide and air (hydrogen sulfide concentration of 20 ppm) was introduced. The response value S of the gas sensor under different voltages was measured, and the maximum response value was obtained. The results are as follows: Figure 7 As shown;

[0184] according to Figure 7 Yes, the sensing performance of A1 gradually increases with increasing voltage. When a voltage of 0.3V is applied, its response value to H2S is relatively small. When the voltage reaches 5V, the response value can reach 16%; the maximum response value is 16%.

[0185] Apply V to A2 using the power supply meter. min ~Vmax A voltage of (0.1–9 V) was applied, and a mixture of hydrogen sulfide and air (hydrogen sulfide concentration of 20 ppm) was introduced. The response value S of the gas sensor at different voltages was measured, and the maximum response value was obtained. The results are as follows: Figure 8 As shown,

[0186] according to Figure 8 Yes, the sensing performance of A2 gradually increases with increasing voltage. When the voltage reaches 9V, the response value can reach more than 25%; the maximum response value is 26.1%.

[0187] The same method was used to detect the response values ​​S of gas sensors A2-A10 and D1-D3 under different voltages, and the maximum response value was obtained. The results are shown in Table 1.

[0188] Table 1

[0189]

[0190] Based on the above results, the gas sensor obtained by this invention has a low power, far lower than the 500mW of existing sensors, indicating that the gas sensor obtained by this invention has low energy consumption and good response performance. In Comparative Example 1, the three-dimensional graphene / WO3 microspheres were ground into powder, which destroyed their three-dimensional structure and conductive pathways, thus reducing the response performance. In Comparative Example 2, due to the lack of metal oxides, the graphene oxide microspheres lacked active sites for hydrogen sulfide, thus showing no response to hydrogen sulfide. In Comparative Example 3, WO3 was composited onto graphene through a physical mixing method. Compared with WO3 grown in situ using precursors in the examples, physically mixed WO3 is more prone to agglomeration, causing a sharp decrease in the specific surface area of ​​the material, thus reducing the exposed active sites. At the same time, the lack of chemical connection between WO3 and graphene obstructs the conductive pathways, resulting in reduced response performance.

[0191] Test Example 2

[0192] The self-heating performance, response time to hydrogen sulfide gas, selectivity, and stability of A1 were tested. The specific test methods and results are as follows:

[0193] 2.1 Self-heating performance:

[0194] A power supply was used to apply voltages of 1.5-5V to the gas sensor A1 obtained in Example 1. An infrared thermometer was then used to measure the temperature of the three-dimensional graphene / metal oxide composite microspheres obtained in Example 1 under different voltages. Figure 9 As shown, the temperature of the three-dimensional graphene / metal oxide composite microspheres gradually increases with increasing voltage, and exhibits a good linear relationship (y = 12.628x + 76.593, R). 2=0.9809, where x represents voltage and y represents temperature), indicating that the three-dimensional graphene / metal oxide composite microspheres in A1 have good self-heating properties.

[0195] 2.2 Response time to hydrogen sulfide gas:

[0196] A voltage of 0.3-4V was applied to A1 using a power supply meter, and a mixture of hydrogen sulfide and air (hydrogen sulfide concentration of 20ppm) was introduced. The response time of the gas sensor was measured at different voltages. The response time of the gas sensor was defined as the time it took for the resistance change of the gas sensor to reach 90% of the total change after the mixture was introduced. The results are as follows: Figure 10 As shown.

[0197] according to Figure 10 Yes, as the voltage increases, the response time first decreases and then increases, with the shortest response time being 42s at a voltage of 2V.

[0198] 2.2 Selectivity:

[0199] A 2V voltage was applied to A1 using a power supply meter. Mixtures of hydrogen sulfide and air (hydrogen sulfide concentration 20ppm), hydrogen and air (hydrogen concentration 1000ppm), carbon monoxide and air (carbon monoxide concentration 1000ppm), methane and air (methane concentration 1000ppm), and ethylene and air (ethylene concentration 1000ppm) were then introduced, respectively, bringing the three-dimensional graphene / metal oxide composite microspheres into contact with each mixture. The corresponding response value S was measured, and the results are as follows: Figure 11 As shown, the response values ​​are S 硫化氢 =1.6%, S 氢气 =0.04%, S 一氧化碳 =0.06%, S 甲烷 =0.01%, S 乙烯 =0.03%, which shows that the gas sensor of the present invention has a low response value to H2, CO, CH4 and C2H4, but a high response value to H2S, indicating that the gas sensor of the present invention has good selectivity to H2S.

[0200] 2.3 Stability:

[0201] A 2V voltage was applied to A1 using a power supply meter, and a mixture of hydrogen sulfide and air (hydrogen sulfide concentration of 20ppm) was introduced. Its response value S to 20ppm hydrogen sulfide was measured. The first measurement was recorded as day 1, the second measurement was recorded on day 5, and measurements were taken every 5 days thereafter until day 30. The results are as follows... Figure 12 As shown, according to Figure 12After 30 days of testing, the gas sensor of this invention showed that its response value to 20 ppm hydrogen sulfide changed by less than 5%, indicating that the gas sensor obtained by this invention has good stability.

[0202] Test Example 3

[0203] The self-heating performance, response performance to different concentrations of hydrogen sulfide, and selectivity of A2 were tested. The specific test methods and results are as follows:

[0204] 3.1 Self-heating performance:

[0205] A voltage of 0.1-9V was applied to the gas sensor A2 obtained in Example 2 using a power supply meter. The temperature of the three-dimensional graphene / metal oxide composite microspheres obtained in Example 2 under different voltages was measured using an infrared thermometer. Figure 13 As shown, the temperature of the three-dimensional graphene / metal oxide composite microspheres gradually increases with the increase of voltage. At 9V, the temperature can reach 91℃, indicating that the three-dimensional graphene / metal oxide composite microspheres in A2 have good self-heating characteristics.

[0206] 3.2 Response performance to different concentrations of hydrogen sulfide:

[0207] A 9V voltage was applied to A2 using a power supply meter, and a mixture of hydrogen sulfide and air (hydrogen sulfide concentrations of 1ppm, 5ppm, 10ppm, 20ppm, 30ppm, 40ppm, and 50ppm) was introduced to measure its response value S to H2S concentrations of 1-50ppm. The results are as follows: Figure 14 As shown;

[0208] according to Figure 14 Yes, A2 has a good response to H2S concentrations of 5-50 ppm. Below 20 ppm, the response value and concentration change are relatively linear; above 20 ppm, the response value changes less with concentration and enters a plateau period. Therefore, the gas sensor obtained by this invention is suitable for measuring low concentrations of H2S.

[0209] 3.3 Selectivity:

[0210] A 9V voltage was applied to A2 using a power supply meter, and its response value S was measured for a mixture of hydrogen sulfide and air (H2S concentration 20ppm), a mixture of hydrogen and air (H2 concentration 1000ppm), a mixture of carbon monoxide and air (CO concentration 1000ppm), a mixture of methane and air (CH4 concentration 1000ppm), and a mixture of ethylene and air (C2H4 concentration 1000ppm). The results are as follows: Figure 15 As shown, S 硫化氢 =26.1%, S 氢气=4.1%, S 一氧化碳 =6.2%, S 甲烷 =1.3%, S 乙烯 =1.6%, indicating that the gas sensor of the present invention has a low response value to H2, CO, CH4 and C2H4, but a high response value to H2S, indicating that the gas sensor obtained by the present invention has good selectivity to H2S.

[0211] Application Example 1

[0212] Different ratios of H2S were mixed with air to prepare H2S standard gases with concentrations of 5 ppm, 10 ppm, 20 ppm, 30 ppm, 40 ppm, and 50 ppm. A 2V voltage was applied to the gas sensor A1 obtained in Example 1 using a power supply. Then, the aforementioned concentrations of H2S standard gas were introduced into contact with charged three-dimensional graphene / metal oxide composite microspheres. The resistance value of the gas sensor A1 was measured, and the response value of the gas sensor A1 under different concentrations of H2S standard gas was calculated. A standard curve of H2S concentration-response value under this voltage was constructed with the H2S concentration x in the H2S standard gas as the abscissa and the response value y of the gas sensor A1 as the ordinate. Figure 16 As shown, the fitted relationship is y = 0.00004x 3 -0.0048x 2 +0.1835x - 0.5346, R 2 =0.9959.

[0213] Application Example 2

[0214] Different ratios of H2S were mixed with air to prepare H2S standard gases with concentrations of 5 ppm, 10 ppm, 20 ppm, 30 ppm, 40 ppm, and 50 ppm. A 5V voltage was applied to the gas sensor A1 obtained in Example 1 using a power supply. Then, the aforementioned concentrations of H2S standard gas were introduced into contact with charged three-dimensional graphene / metal oxide composite microspheres. The resistance value of the gas sensor A1 was measured, and the response value of the gas sensor A1 under different concentrations of H2S standard gas was calculated. A standard curve of H2S concentration-response value under this voltage was constructed with the H2S concentration x in the H2S standard gas as the abscissa and the response value y of the gas sensor A1 as the ordinate. Figure 17 As shown, the fitted relationship is y = 0.0004x 3 -0.0481x 2 +1.8348x-5.3459, R 2 =0.9921.

[0215] Application Example 3

[0216] Different ratios of H2S were mixed with air to prepare H2S standard gases with concentrations of 5 ppm, 10 ppm, 20 ppm, 30 ppm, 40 ppm, and 50 ppm. A voltage of 0.3 V was applied to the gas sensor A1 obtained in Example 1 using a power supply. Then, the aforementioned concentrations of H2S standard gas were introduced, and after contacting the charged three-dimensional graphene / metal oxide composite microspheres, the resistance value of the gas sensor A1 was measured. The response value of the gas sensor A1 under different concentrations of H2S standard gas was then calculated. A standard curve of H2S concentration-response value under this voltage was constructed, with the H2S concentration x in the H2S standard gas as the abscissa and the response value y of the gas sensor A1 as the ordinate. Figure 18 As shown, the fitted relationship is y = 0.00001x 3 -0.0012x 2 +0.0459x - 0.1336, R 2 =0.9987.

[0217] The results above show that the gas sensor obtained by the present invention has the advantages of fast response speed and low power consumption to hydrogen sulfide, and can be used to detect the H2S concentration in the gas with significantly better performance.

[0218] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A gas sensor, characterized in that, The gas sensor includes a ceramic package (1) and a three-dimensional graphene / metal oxide composite microsphere (2) suspended in the groove (11) of the ceramic package (1), and the three-dimensional graphene / metal oxide composite microsphere (2) is connected to the pin (12) of the ceramic package (1) through a wire. The three-dimensional graphene / metal oxide composite microspheres (2) include graphene microspheres and metal oxides loaded on the graphene microspheres; The specific surface area of ​​the three-dimensional graphene / metal oxide composite microspheres is 50-500 m². 2 / g; The preparation method of the three-dimensional graphene / metal oxide composite microspheres (2) includes the following steps: (1) Mix graphene oxide, sodium alginate and water, then sonicate and heat to obtain a mixed dispersion; (2) The mixed dispersion is dripped into a solution containing Ca using an injection pump. 2+ The graphene oxide / sodium alginate hydrogel was obtained by soaking in an aqueous solution. The graphene oxide / sodium alginate hydrogel was then washed to obtain graphene oxide microspheres. (3) The graphene oxide microspheres, metal salt and water are mixed to obtain a mixture; (4) The mixture is subjected to hydrothermal reaction, and then solid-liquid separation, washing, freeze drying and sintering are carried out in sequence.

2. The gas sensor according to claim 1, characterized in that, The three-dimensional graphene / metal oxide composite microspheres have a multi-level porous structure with an average pore diameter of 0.1-50 nm.

3. The gas sensor according to claim 1, characterized in that, The diameter of the three-dimensional graphene / metal oxide composite microspheres is 0.5-5 mm.

4. The gas sensor according to any one of claims 1-3, characterized in that, The metal oxide is selected from one or more of WO3, CuO, ZnO, Co3O4, MoO3 and SnO2.

5. The gas sensor according to claim 1, characterized in that, In step (1), the weight ratio of the graphene oxide, sodium alginate and water is 1-15:0.1-50:1000.

6. The gas sensor according to claim 1 or 5, characterized in that, In step (1), the weight ratio of the graphene oxide, sodium alginate and water is 5-10:0.1-40:1000.

7. The gas sensor according to claim 1 or 5, characterized in that, In step (1), the ultrasound duration is 10-120 min.

8. The gas sensor according to claim 1, characterized in that, In step (1), the heating conditions include a temperature of 70-95°C and a time of 3-6 hours.

9. The gas sensor according to claim 1, characterized in that, In step (2), the flow rate of the syringe pump is 10-200 μL / min.

10. The gas sensor according to claim 1 or 9, characterized in that, In step (2), the substance containing Ca 2+ Ca in aqueous solution 2+ The concentration is 1-10 by weight.

11. The gas sensor according to claim 1, characterized in that, In step (2), the soaking time is 1-24 hours.

12. The gas sensor according to claim 1 or 11, characterized in that, In step (2), the washing process includes rinsing the graphene oxide / sodium alginate hydrogel with water and then soaking it in water for 12-48 hours.

13. The gas sensor according to claim 1, characterized in that, In step (3), the metal salt is selected from one or more of tungstate, copper salt, cobalt salt, zinc salt, molybdate and tin salt.

14. The gas sensor according to claim 1, characterized in that, The metal salt is selected from one or more of sodium tungstate, copper sulfate, zinc nitrate, cobalt nitrate, ammonium molybdate, and stannous chloride.

15. The gas sensor according to any one of claims 1, 13, or 14, characterized in that, In step (3), the weight ratio of the graphene oxide microspheres, metal salt and water is 1:1-20:2000-2600.

16. The gas sensor according to claim 1, characterized in that, In step (4), the hydrothermal reaction is a microwave hydrothermal reaction.

17. The gas sensor according to claim 16, characterized in that, The conditions for the microwave hydrothermal reaction include: a temperature of 150-220℃, a time of 0.5-6h, and a microwave power of 400-800W.

18. The gas sensor according to claim 1, characterized in that, In step (4), the freeze-drying conditions include a temperature of 25-60°C and a time of 12-48h.

19. The gas sensor according to claim 1, characterized in that, In step (4), the sintering conditions are: temperature of 200-700℃, time of 1-5h, and atmosphere of inert atmosphere.

20. The application of the gas sensor according to any one of claims 1-19 in detecting the concentration of H2S in a gas.

21. A method for detecting the concentration of H2S in a gas, characterized in that, The method includes the following steps: applying voltage to the gas sensor, then introducing the gas to be tested, and contacting it with the three-dimensional graphene / metal oxide composite microspheres (2), and determining the concentration of H2S by measuring the response value after contact; The gas sensor is the gas sensor described in any one of claims 1-19.

22. The method according to claim 21, characterized in that, The voltage is 0.1-15V.

Images