In-situ preparation method of modified graphene quantum dot / ZnO gas sensitive material

Abstract

The application belongs to the technical field of semiconductor gas sensitive material preparation, and particularly relates to an in-situ preparation method of modified graphene quantum dot / ZnO gas sensitive material. Al2O3 gas sensitive ceramic tubes pretreated by lye are used as substrates, ZnO seeds are loaded on the gas sensitive ceramic tubes, then the Al2O3 gas sensitive ceramic tubes loaded with the ZnO seeds are used as substrates, and hydrothermal reaction is carried out by taking zinc chloride, sodium carbonate and graphene quantum dots as main raw materials, so that the graphene quantum dot / ZnO gas sensitive material is obtained, and then the graphene quantum dot / ZnO gas sensitive material is modified by using ammonia water through a solvothermal method. Compared with pure ZnO gas sensitive elements, the modified graphene quantum dot / ZnO gas sensitive element has a best working temperature reduced by about 170 DEG C, a sensitivity obviously increased, an excellent selectivity to ethanol in ethanol, acetone, benzene, ammonia and methanol gas atmospheres, and a greatly shortened response and recovery time.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor gas-sensitive material preparation technology, and specifically relates to an in-situ preparation method of modified graphene quantum dot / ZnO gas-sensitive material. Background Technology

[0002] With the rapid development of modern industrial technology, an increasing number of toxic and harmful gases are being emitted, posing a threat to the human living environment. Therefore, the detection of these gas molecules is essential in many fields, particularly environmental monitoring. Metal oxide sensors are among the preferred candidates for detecting low concentrations of volatile organic compounds (VOCs), offering advantages such as ease of measurement, durability, ease of fabrication into small sizes, and low cost. In the research of gas-sensitive materials for metal oxide sensors, the study of nano-ZnO has attracted considerable attention. ZnO is a semiconductor material with a wide direct bandgap (3.37 eV). Due to its good response to various reducing and oxidizing gases, low cost, good chemical and physical stability under sensor operating conditions, and environmental friendliness, it has received widespread attention in gas sensing applications. However, ZnO as a gas-sensitive material still faces some challenges, including low sensitivity, poor selectivity, and high operating temperature for the detection of harmful chemicals and VOCs, thus hindering its practical application.

[0003] Graphene quantum dots (GQDs) have small lateral dimensions and relatively high electron mobility. Due to defects in their structure and functional groups, they can provide many active sites, promoting the adsorption of gas molecules. Therefore, modifying ZnO with GQDs can improve its gas-sensing performance. Furthermore, to further alter the charge transfer between GQDs and metal oxides and provide more active sites, different methods have been used to modify GQDs to improve the performance of gas sensors. Current research shows that modifying SnO2 with nitrogen-doped GQDs can effectively increase its adsorption of NO2, thereby improving the response speed of the composite material and reducing its operating temperature. Existing technology has successfully prepared N-GQDs-modified ZnO gas-sensing materials using a hydrothermal method and fabricated gas-sensing elements using traditional thick-film processes. It was found that the operating temperature of these elements was significantly reduced, and the sensitivity to NO2 gas was increased. Currently, there is very little research on modified GQDs / ZnO gas-sensitive materials, especially no reports on the direct preparation of corresponding gas-sensitive materials on Al2O3 gas-sensitive ceramic tubes using the seed-induced method.

[0004] Currently, thick-film gas-sensitive elements are still often produced using traditional manual coating methods, which are time-consuming and cumbersome, resulting in poor uniformity of the gas-sensitive elements. Therefore, this invention explores a simple, environmentally friendly, and low-cost seed-induced method to directly prepare gas-sensitive materials on Al2O3 gas-sensitive ceramic tubes. Summary of the Invention

[0005] The purpose of this invention is to provide a method for in-situ preparation of modified graphene quantum dot / ZnO gas-sensitive materials on an Al2O3 gas-sensitive ceramic tube. This invention utilizes a seed-induced method, using an Al2O3 gas-sensitive ceramic tube as a substrate, and employs a simple hydrothermal method with graphene quantum dots, zinc chloride, and sodium carbonate as the main raw materials to directly prepare graphene quantum dot / ZnO gas-sensitive materials in situ on the Al2O3 gas-sensitive ceramic tube. Furthermore, ammonia water is used to modify the graphene quantum dot / ZnO gas-sensitive material, which has the advantages of simple operation, environmental friendliness, and low cost. It can be used to detect ethanol gas, thereby improving the performance of ZnO gas-sensitive elements in ethanol detection. This invention discloses an in-situ preparation method for modified graphene quantum dot / ZnO gas-sensitive materials, including the following steps:

[0006] (1) Pretreatment of Al2O3 gas-sensitive ceramic tube: The Al2O3 gas-sensitive ceramic tube is immersed in NaOH solution to remove organic contaminants on its surface, and hydroxyl groups are introduced to its surface to activate the ceramic tube. Then, the gas-sensitive ceramic tube after being immersed in alkaline solution is ultrasonically cleaned with deionized water. After 45-55 minutes, it is taken out and dried in an oven at 70-80℃ for later use.

[0007] (2) Preparation of ZnO seeds and their loading on gas-sensitive ceramic tubes: Weigh zinc acetate dihydrate and place it in a beaker, add anhydrous ethanol, and after the zinc acetate dihydrate dissolves, add polyvinylpyrrolidone and ultrasonically disperse for 20-30 min. Then, use a disposable pipette to quickly add 0.5 mol / L NaOH ethanol solution to the above solution and continue ultrasonic dispersion for 30 min to obtain ZnO seed solution. The mass ratio of hydrated zinc acetate, anhydrous ethanol, polyvinylpyrrolidone and NaOH ethanol solution is 1:90-100:1-2:200-220. Take the Al2O3 gas-sensitive ceramic tube described in step (1) and make the ZnO seed solution uniformly form a film on its surface. After drying, ZnO seeds can be loaded on the gas-sensitive ceramic tube.

[0008] (3) Preparation of graphene quantum dot solution: Dissolve a certain amount of citric acid in deionized water, stir for a certain time, place in an oven and heat to melt at 180-200℃, adjust the pH of the resulting liquid to 7 with NaOH solution, centrifuge and take the supernatant to obtain graphene quantum dot solution.

[0009] (4) In-situ preparation of graphene quantum dot / ZnO gas-sensitive material: Sodium dodecyl sulfate and the graphene quantum dot solution prepared in step (3) were added to the zinc chloride aqueous solution and stirred for 30-40 min. Then, sodium carbonate solution was rapidly added dropwise to the above mixed solution under magnetic stirring and stirring was continued for 1-1.5 h. The resulting mixed solution and the gas-sensitive ceramic tube loaded with ZnO seeds were placed in the lining of the reactor for hydrothermal reaction. After the reaction was completed and the reactor was cooled, the gas-sensitive ceramic tube was taken out, washed several times with deionized water and anhydrous ethanol, and dried in an oven at 70-80℃. Finally, the gas-sensitive ceramic tube was calcined at 300-320℃ for 1-2 h to prepare the graphene quantum dot / ZnO gas-sensitive material in situ on the gas-sensitive ceramic tube.

[0010] (5) Preparation of modified graphene quantum dot / ZnO gas-sensitive material: Measure ammonia water and add it to ethylene glycol solution at a molar ratio of 1:300-350. After stirring evenly, transfer it into the liner of the reaction vessel and put the ceramic tube from step (4) into it at the same time. Place the reaction vessel in an oven for solvothermal reaction. After the reaction is completed, take out the ceramic tube, wash it with ethanol several times, and dry it to obtain the modified graphene quantum dot / ZnO gas-sensitive material.

[0011] Preferably, in step (1), the concentration of the NaOH solution is 0.1 mol / L, and the soaking time is 5 to 10 min.

[0012] Preferably, in step (2), ZnO seed crystals are loaded onto the Al2O3 gas-sensitive ceramic tube by drop coating. Generally, the drop coating-drying cycle is repeated 5 to 8 times, and the tube is placed in an 80°C oven to dry for later use.

[0013] Preferably, in step (3), the amount of citric acid used is 0.5 to 1.5 g, and the heating and melting time at 180°C is 2 h.

[0014] Preferably, in step (4), the graphene quantum dot solution is prepared using 0.76g of citric acid, and the amount used is 1-4mL.

[0015] Preferably, in step (4), the concentration of the zinc chloride aqueous solution is 1.50 mol / L, the concentration of the sodium carbonate solution is 0.80 mol / L, and the molar ratio of sodium dodecyl sulfate to zinc chloride is 1:65 to 1:70.

[0016] Preferably, in step (4), the temperature of the hydrothermal reaction is 110-170°C, and the time of the hydrothermal reaction is 12-24 hours.

[0017] Preferably, in step (5), the amount of ammonia water used is 200 μL, the amount of ethylene glycol solution used is 50 mL, the solvothermal reaction temperature is 150–180 °C, and the reaction time is 2–4 h.

[0018] The present invention also provides a gas-sensitive element comprising a modified graphene quantum dot / ZnO gas-sensitive material prepared by the method of the present invention.

[0019] Preferably, compared to a pure ZnO gas sensor, the modified graphene quantum dot / ZnO gas sensor exhibits an optimal operating temperature that is reduced by approximately 170°C, and can be used for the detection of ethanol gas. This gas sensor demonstrates excellent selectivity for ethanol gas in atmospheres containing ethanol, acetone, benzene, ammonia, and methanol.

[0020] The advantages and effects of this invention are:

[0021] This invention discloses for the first time a method for in-situ preparation of modified graphene quantum dot / ZnO gas-sensitive materials on Al2O3 gas-sensitive ceramic tubes using a seed-induced method. The method is simple to operate, environmentally friendly, and low-cost. The modified graphene quantum dot / ZnO gas-sensitive element prepared by this invention has a low optimal operating temperature and exhibits excellent selectivity for ethanol gas. Attached Figure Description

[0022] Figure 1 Scanning electron microscope (SEM) images of the modified graphene quantum dot / ZnO gas-sensitive materials prepared under different conditions in Examples 1 to 4.

[0023] Figure 2 The image shows the energy dispersive spectroscopy (EDS) spectrum of the modified graphene quantum dot / ZnO gas-sensitive material prepared in Example 2.

[0024] Figure 3 The sensitivity-temperature curves of the gas-sensitive elements prepared in Examples 1 to 4 and Comparative Example 1 are shown.

[0025] Figure 4 The sensitivity test results of the gas-sensitive elements prepared in Example 2 and Comparative Example 1 to five different gases at 100 ppm are shown.

[0026] Figure 5 The response recovery time of the N-2 gas-sensitive element prepared in Example 2.

[0027] Figure 6 The response recovery time of the PZ-150 gas-sensitive element prepared in Comparative Example 1 is given. Detailed Implementation

[0028] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings.

[0029] Example 1

[0030] (1) Pretreatment of Al2O3 gas-sensitive ceramic tube: Immerse the Al2O3 gas-sensitive ceramic tube in 0.1mol / L sodium hydroxide solution for 5min, clean it with deionized water by ultrasonication, take it out after 45min, and dry it in an 80℃ oven for later use.

[0031] (2) Preparation of ZnO seeds and loading them onto the gas-sensitive ceramic tube: Weigh 0.176 g of zinc acetate dihydrate and place it in a beaker. Add 20 mL of anhydrous ethanol. After the zinc acetate dihydrate dissolves, add 0.176 g of polyvinylpyrrolidone and ultrasonically disperse for 20 min. Then, use a disposable pipette to quickly add 50 mL of 0.5 mol / L sodium hydroxide ethanol solution to the above solution and continue ultrasonic dispersion for 30 min to obtain the ZnO seed solution. Take the Al2O3 gas-sensitive ceramic tube pretreated in step (1) and use the drop coating method to make the ZnO seed solution uniformly form a film on its surface. Repeat the drop coating-drying cycle 8 times. Finally, put it in an 80℃ oven to dry, and ZnO seeds can be loaded onto the gas-sensitive ceramic tube.

[0032] (3) Preparation of graphene quantum dot solution: Weigh 0.76g of citric acid using an electronic balance, dissolve it in 20mL of deionized water, stir for 10min, place it in an oven and heat it at 180℃ to melt it, adjust the pH of the resulting liquid to 7 with NaOH solution, centrifuge and take the supernatant liquid, which is the graphene quantum dot solution.

[0033] (4) In-situ preparation of graphene quantum dot / ZnO gas-sensitive materials:

[0034] Weigh 3.28 g of zinc chloride using an electronic balance and dissolve it in 60 mL of deionized water to obtain a zinc chloride solution. Add 0.40 g of sodium dodecyl sulfate and 1.0 mL of the graphene quantum dot solution from step (3) to the zinc chloride solution and stir with a magnetic stirrer for 30 min. Simultaneously weigh 5.08 g of anhydrous sodium carbonate and dissolve it in 60 mL of deionized water under magnetic stirring to obtain a sodium carbonate solution. Continuously add the sodium carbonate solution dropwise to the above mixed solution using a disposable pipette and continue stirring for 1 h.

[0035] The mixed solution and the gas-sensitive ceramic tube loaded with ZnO seeds were placed together in the lining of the reactor, and the reactor was placed in an oven at 150°C for hydrothermal reaction for 12 hours. After the reactor cooled, the gas-sensitive ceramic tube was removed, washed three times with deionized water and three times with anhydrous ethanol, and then dried in an oven at 80°C.

[0036] Finally, the gas-sensitive ceramic tube is placed in a muffle furnace and calcined at 300°C for 2 hours to prepare graphene quantum dot / ZnO gas-sensitive materials in situ on the gas-sensitive ceramic tube.

[0037] (5) Preparation of modified graphene quantum dot / ZnO gas-sensitive materials:

[0038] Measure 200 μL of ammonia water and add it to 50 mL of ethylene glycol solution. After stirring evenly, transfer the solution into the liner of the reaction vessel, and simultaneously place the ceramic tube from step (4) into the liner. Place the reaction vessel in a 150°C oven for a solvothermal reaction for 4 hours. After the reaction is complete, remove the ceramic tube, wash it several times with ethanol, and then dry it to obtain the modified graphene quantum dot / ZnO gas-sensitive material, which is labeled as N-1. The scanning electron microscope image of N-1 is shown below. Figure 1 (a), by Figure 1 (a) It can be seen that the morphology of the modified graphene quantum dot / ZnO gas-sensitive material is an irregularly shaped and differently sized nanosheet structure, which allows for more gaps between particles, thus improving its gas-sensing performance.

[0039] Examples 2-4

[0040] The amount of graphene quantum dot solution used in step (4) of Example 1 was changed to 2.0, 3.0 and 4.0 mL respectively, and the remaining steps were the same as in Example 1.

[0041] The modified graphene quantum dot / ZnO gas-sensitive materials prepared in Examples 2-4 are labeled as N-2, N-3, and N-4, respectively.

[0042] SEM images of the modified graphene quantum dot / ZnO gas-sensitive material prepared in Example 2 are shown below. Figure 1 As shown in (b), its morphology remains an irregularly shaped nanosheet structure, but its size uniformity is significantly improved. Furthermore, the nanosheets exhibit a three-dimensional spatial distribution, with more voids formed between nanosheets of different orientations. This is undoubtedly beneficial for improving its gas-sensing performance. SEM images of the modified graphene quantum dot / ZnO gas-sensing materials prepared in Examples 3 and 4 are shown below. Figure 1 (c) and Figure 1 As shown in (d), it can be clearly seen that as the amount of graphene quantum dots added increases, the size of the nanosheets tends to increase, and the contact between the sheets becomes closer, and the distribution orientation of the nanosheets tends to be consistent.

[0043] Comparative Example 1

[0044] In step (4) of Example 1, no graphene quantum dot solution is added, and the modification treatment in step (5) is not performed. The remaining steps are the same as in Example 1. Pure ZnO gas-sensitive material can be prepared in situ on the gas-sensitive ceramic tube and labeled as PZ-150.

[0045] Application testing of gas-sensitive elements:

[0046] Roughen the electrode tips of the sensor base with a file, and apply flux to the tips of the four signal electrodes of the base. Take the gas-sensitive ceramic tube described in Examples 1-4 and Comparative Example 1, insert a needle through the ceramic tube, place it on the sensor base, and solder one signal electrode of the base to the signal wire of the ceramic tube. Remove the needle, hold the remaining signal wires with tweezers, and solder them in sequence. After the four signal wires are soldered, insert the nichrome heating wire into the ceramic tube and solder it; finally, use a multimeter to measure the resistance between the heating electrodes of the base. First, age the gas-sensitive element on an aging bench at 80mA for 48 hours, and then age it at 160mA for 8 hours.

[0047] The response values ​​of five gas-sensitive elements to 100 ppm ethanol gas at different temperatures were tested, and the results are as follows: Figure 3 As shown in the figure, the operating temperature of the modified graphene quantum dot / ZnO gas sensor is significantly lower than that of pure ZnO. Specifically, the optimal operating temperature of the N-2 gas sensor is 170℃ lower than that of the pure ZnO (PZ-150) sensor. At their respective optimal operating temperatures, the modified graphene quantum dot / ZnO gas sensor exhibits higher sensitivity to 100ppm ethanol than the pure ZnO gas sensor, with the N-2 sensor showing nearly twice the sensitivity of the pure ZnO sensor.

[0048] The response values ​​of N-2 and PZ-150 gas sensors to different gases such as ethanol, acetone, benzene, ammonia, and methanol were tested at a gas concentration of 100 ppm. Each gas sensor was tested at its optimal operating temperature. The results are as follows: Figure 4 As shown, it is clear that the N-2 gas sensor exhibits excellent selectivity for ethanol gas.

[0049] The response recovery times of N-2 and PZ-150 gas sensors in a 100ppm ethanol atmosphere were tested, and the results are as follows: Figure 5 and Figure 6 As shown in the figure, the comparison shows that the addition of modified graphene quantum dots shortens the device's response time from 7s to 5.7s, and greatly reduces the device's recovery time from 154s to 21.5s.

Claims

1. A method for in-situ preparation of a modified graphene quantum dot / ZnO gas-sensitive material, characterized in that: Includes the following steps: S1. Immerse the Al2O3 gas-sensitive ceramic tube in NaOH solution, then perform ultrasonic cleaning. After 45-55 minutes, remove it and dry it in an oven at 70-80℃ for later use. S2. Weigh zinc acetate hydrate and place it in a beaker. Add anhydrous ethanol and wait for the zinc acetate dihydrate to dissolve. Then add polyvinylpyrrolidone and ultrasonically disperse for 20-30 minutes. Add 0.5 mol / L NaOH ethanol solution dropwise to the above solution and continue ultrasonic dispersion for 30-40 minutes to obtain the ZnO seed solution. The mass ratio of zinc acetate hydrate, anhydrous ethanol, polyvinylpyrrolidone, and NaOH ethanol solution is 1:90-100:1-2:200-220. Take the Al2O3 gas-sensitive ceramic tube obtained in step S1 and make the ZnO seed solution form a uniform film on its surface. Dry it for later use. S3. Citric acid is dissolved in deionized water and heated in an oven at 180-200℃ to melt it. The resulting liquid is adjusted to pH 7 with NaOH solution. After centrifugation, the supernatant is taken to obtain a graphene quantum dot solution. S4. Add sodium dodecyl sulfate and the graphene quantum dot solution prepared in step S3 to the zinc chloride aqueous solution, and stir for 30-40 min. Then, under magnetic stirring, rapidly add sodium carbonate solution dropwise to the above mixed solution and continue stirring for 1-1.5 h. Place the resulting mixed solution and the gas-sensitive ceramic tube obtained in step S2 into the liner of the reactor for hydrothermal reaction. After the reaction is completed and the reactor has cooled, remove the ceramic tube, wash it several times with deionized water and anhydrous ethanol, and dry it in an oven at 70-80℃. Finally, calcine the ceramic tube at 300-320℃ for 1-2 h. S5. Add ammonia water to the ethylene glycol solution at a molar ratio of 1:300-350, stir evenly, and then transfer it into the lining of the reactor. At the same time, put the ceramic tube obtained in step S4 into the reactor. Place the reactor in an oven for solvothermal reaction. After the reaction is completed, take out the ceramic tube, wash it with ethanol several times, and then dry it to obtain the modified graphene quantum dot / ZnO gas-sensitive material.

2. The in-situ preparation method of the modified graphene quantum dot / ZnO gas-sensitive material according to claim 1, characterized in that: The concentration of the NaOH solution in step S1 is 0.1 mol / L, and the soaking time is 5 to 10 minutes.

3. The in-situ preparation method of the modified graphene quantum dot / ZnO gas-sensitive material according to claim 1, characterized in that: In step S2, ZnO seed crystals are loaded onto the Al2O3 gas-sensitive ceramic tube using a drop-coating method. The drop-coating-drying cycle is repeated 5 to 8 times, and the tube is then placed in a 100°C oven to dry for later use.

4. The in-situ preparation method of the modified graphene quantum dot / ZnO gas-sensitive material according to claim 1, characterized in that: The amount of citric acid used in step S3 is 0.5 to 1.5 g, and the heating and melting time at 180°C is 2 h.

5. The in-situ preparation method of the modified graphene quantum dot / ZnO gas-sensitive material according to claim 1, characterized in that: The amount of graphene quantum dot solution used in step S4 is 1 to 4 mL.

6. The in-situ preparation method of the modified graphene quantum dot / ZnO gas-sensitive material according to claim 1, characterized in that: In step S4, the concentration of the zinc chloride aqueous solution is 1.50 mol / L, the concentration of the sodium carbonate aqueous solution is 0.80 mol / L, and the molar ratio of sodium dodecyl sulfate to zinc chloride is 1:65-70.

7. The in-situ preparation method of the modified graphene quantum dot / ZnO gas-sensitive material according to claim 1, characterized in that: In step S4, the temperature of the hydrothermal reaction is 110–170°C, and the time of the hydrothermal reaction is 12–24 h.

8. The in-situ preparation method of the modified graphene quantum dot / ZnO gas-sensitive material according to claim 1, characterized in that: In step S5, the solvothermal reaction temperature is 150–180°C, and the reaction time is 2–4 h.

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