Zinc oxide / graphene quantum dot aerogel, preparation method thereof and NO2 gas sensor

Abstract

The invention relates to zinc oxide / graphene quantum dot aerogel, a preparation method thereof and an NO2 gas sensor. The preparation method of the zinc oxide / graphene quantum dot aerogel comprises the following steps: mixing sodium alginate, zinc oxide and a graphene quantum dot solution to obtain a suspension, and freeze-drying to obtain the zinc oxide / graphene quantum dot aerogel. According to the invention, unique physicochemical properties such as high specific surface area, good conductivity and chemical stability of the graphene quantum dots (GQDs) are fully exerted, and the problems of high working temperature, low sensitivity, slow response and the like of a gas sensitive sensor constructed by taking pure ZnO as a main body are improved.

Description

Technical Field

[0001] The invention belongs to the technical field of semiconductor gas sensors, and in particular relates to a zinc oxide / graphene quantum dot aerogel and a preparation method thereof, and a NO2 gas sensitive sensor. Background Art

[0002] NO2, as a highly reactive nitrogen oxide, is well known for its iconic brown-red appearance and pungent smell. In the atmospheric environment, it is not only the main component of acid rain, but also the core element that promotes the formation of ground ozone. NO2 comes from various sources, including the combustion of fossil fuels, industrial production processes, and even natural phenomena such as lightning. It poses a risk to human health, especially the respiratory system. Long-term exposure to NO2 may even cause heart disease. In the environmental monitoring system, the concentration of NO2 is one of the key indicators for measuring air quality. In view of this, many countries have introduced strict emission standards and implemented corresponding control measures to reduce the adverse effects of NO2 on the environment and human health. It can be seen that the accurate quantitative detection and effective monitoring of NO2 has become a key issue that needs to be solved in various environmental scenarios.

[0003] In recent years, semiconductor gas sensors have been widely used in many fields. Among them, the use of two-dimensional materials to prepare gas sensors has become the mainstream trend in this field. Zinc oxide (ZnO), as an n-type semiconductor material, has a series of unique physical and chemical properties, and is therefore widely used in many fields. It appears as a white or slightly yellow powder, has a wide band gap of about 3.2-3.37eV and a high exciton binding energy, which gives it important application value in the optoelectronic field. In addition, its piezoelectricity and photoconductivity also make it play a key role in devices such as sensors and actuators. Experimental verification shows that ZnO has good selectivity for NO2 gas. However, ZnO will only respond to NO2 gas under high temperature conditions, and its sensitivity is low. These factors limit its practical application to a certain extent. Summary of the invention

[0004] The first object of the present invention is to provide a zinc oxide / graphene quantum dot aerogel to solve the technical problems of high operating temperature and low sensitivity of ZnO.

[0005] The second object of the present invention is to provide a method for preparing zinc oxide / graphene quantum dot aerogel.

[0006] The third object of the present invention is to provide a NO2 gas sensor.

[0007] In order to achieve the above purpose, the technical solution adopted by the present invention is:

[0008] A preparation method of zinc oxide / graphene quantum dot aerogel, which is obtained by mixing sodium alginate, zinc oxide and graphene quantum dot solution to obtain a suspension and then freeze-drying.

[0009] Furthermore, 0.02 - 0.04 g of zinc oxide is added per mL of the sodium alginate solution; 0.4 - 0.12 mL of graphene quantum dot solution is added per mL of the sodium alginate solution.

[0010] Furthermore, after the suspension is freeze-dried, it is soaked in calcium chloride solution, then washed and freeze-dried to obtain the product.

[0011] Furthermore, the concentration of the calcium chloride solution is 0.9 - 1.1 M; the soaking time is 1 - 2 h.

[0012] Furthermore, the sodium alginate solution is an aqueous solution of sodium alginate with a mass fraction of 3 - 5%; the concentration of the graphene quantum dot solution is 0.015 - 0.025 mg / mL.

[0013] Furthermore, the preparation method of the zinc oxide is: dissolving zinc acetate dihydrate and urea in water, performing hydrothermal reaction and then freeze-drying, and then annealing in an inert gas to obtain the product.

[0014] Furthermore, the temperature of the hydrothermal reaction is 150 - 170 °C, the time of the hydrothermal reaction is 22 - 26 h; the temperature of the annealing is 380 - 420 °C, the time of the annealing is 1.5 - 2.5 h; the inert gas is argon.

[0015] A zinc oxide / graphene quantum dot aerogel is prepared by using the above-mentioned preparation method of zinc oxide / graphene quantum dot aerogel.

[0016] A NO2 gas sensor includes the above-mentioned zinc oxide / graphene quantum dot aerogel.

[0017] The beneficial effects of the present invention:

[0018] The present invention gives full play to the unique physical and chemical properties of graphene quantum dots (GQDs), such as high specific surface area, good electrical conductivity and chemical stability, and improves the problems of high working temperature, low sensitivity and slow response of the gas sensor constructed with pure ZnO as the main body.

[0019] The present invention composites zinc oxide and graphene quantum dots into zinc oxide / graphene quantum dot aerogel (ZnO / GQDs), and the zinc oxide / graphene quantum dot aerogel is exposed to NO2. From the perspective of physical adsorption, NO2 molecules will first be physically adsorbed on the surface of ZnO / GQDs due to van der Waals force. The crystal structure and surface properties of ZnO give it certain adsorption sites, and the large specific surface area of ​​GQDs also provides more adsorption space for NO2. From the perspective of electron transfer and chemical reaction, there is a certain energy level difference between the conduction band and the valence band of ZnO. When NO2 is adsorbed on the ZnO surface, it will capture electrons from the conduction band of ZnO, resulting in a decrease in the electron density on the ZnO surface, forming a space charge layer, and then changing its electrical properties. GQDs can serve as a bridge for electron transmission and accelerate the transfer of electrons from ZnO to NO2. The π-conjugated structure of GQDs can interact with the π orbit of the NO2 molecule to promote the transfer of electrons. At the same time, GQDs themselves may also generate some active intermediate states through charge transfer between NO2, further promoting the reaction. GQDs and ZnO interact weakly through weak van der Waals forces, so the intrinsic electronic properties of GQDs can be retained in ZnO / GQDs, which has the advantages of low operating temperature and high sensitivity.

[0020] The NO2 gas sensor of the present invention benefits from the synergistic effect of ZnO and GQDs, and has high sensitivity and fast response recovery rate. The NO2 gas sensor of the present invention can achieve high sensitivity and response value to NO2 at room temperature, which benefits from the room temperature conductivity and high carrier mobility of GQDs. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The X-ray diffraction analysis diagram (XRD) of the zinc oxide / graphene quantum dot aerogel in Examples 1, 2, and 3;

[0022] Figure 2 The Raman analysis graph (Raman) of the zinc oxide / graphene quantum dot aerogel in Examples 1, 2, and 3;

[0023] Figure 3 The ultraviolet-visible diffuse reflection absorption spectrum analysis diagram (UV-vis) of the zinc oxide / graphene quantum dot aerogel in Examples 1, 2, and 3;

[0024] Figure 4 are scanning electron microscope images (SEM) of the zinc oxide / graphene quantum dot aerogels in Examples 1, 2, and 3, wherein a and b are scanning electron microscope images of the zinc oxide / graphene quantum dot aerogel in Example 1, c and d are scanning electron microscope images of the zinc oxide / graphene quantum dot aerogel in Example 2, and e and f are scanning electron microscope images of the zinc oxide / graphene quantum dot aerogel in Example 3;

[0025] Figure 5 It is the transmission electron microscopy (TEM) image of the zinc oxide / graphene quantum dot aerogel in Example 2, where a is the TEM image and b is the partial enlarged view;

[0026] Figure 6 It is the X-ray energy spectrum (EDS) of the zinc oxide / graphene quantum dot aerogel in Example 2;

[0027] Figure 7 It is the X-ray photoelectron spectroscopy analysis (XPS) of the zinc oxide / graphene quantum dot aerogels of Examples 1, 2, and 3. Among them, a is the full spectrum of the zinc oxide / graphene quantum dot aerogels in Examples 1, 2, and 3, b is the Zn 2p spectrum of the zinc oxide / graphene quantum dot aerogels in Examples 1, 2, and 3, and c - e are the O1s spectra of the zinc oxide / graphene quantum dot aerogels in Examples 1, 2, and 3;

[0028] Figure 8 It is the NO2 gas sensing performance analysis of the zinc oxide / graphene quantum dot aerogels of Examples 1, 2, and 3. Among them, a is the dynamic response and recovery curve of the zinc oxide / graphene quantum dot aerogels of Examples 1, 2, and 3 to NO2 with a concentration of 100 ppb - 4 ppm at room temperature, b - d are the dynamic response and recovery curves of the zinc oxide / graphene quantum dot aerogels of Examples 1, 2, and 3 to NO2 with a concentration of 100 ppb at room temperature, e is the linear relationship diagram between the zinc oxide / graphene quantum dot aerogels of Examples 1, 2, and 3 and the response value for different concentrations of NO2; f is the repeatability diagram of the zinc oxide / graphene quantum dot aerogel of Example 2 to NO2 with a concentration of 100 ppb at room temperature, g is the stability diagram of the sensor prepared from the zinc oxide / graphene quantum dot aerogel of Example 2, and h is the selectivity diagram of the sensor prepared from the zinc oxide / graphene quantum dot aerogel of Example 2. Detailed implementation manners

[0029] The present invention will be further described below in conjunction with the embodiments of the present invention and the accompanying drawings.

[0030] Example 1

[0031] The preparation method of the zinc oxide / graphene quantum dot aerogel of Example 1 includes the following steps:

[0032] Step 1: Preparation of ZnO powder

[0033] S1: Add 3.696 g of zinc acetate dihydrate (Zn(CH3COO)2·2H2O) and 0.3603 g of urea (CO(NH2)2) to 30 mL of deionized water (DI), and magnetically stir for 30 min to obtain a mixed solution;

[0034] S2: Transfer the mixed solution to a 100 mL teflon autoclave, heat it to 160 °C in an oven, heat for 24 h, and naturally cool to room temperature;

[0035] S3: Centrifuge 6 times alternately with deionized water and ethanol to remove excess impurity ions;

[0036] S4: Freeze-dry for 24 hours;

[0037] S5: Put the freeze-dried sample into a porcelain boat, anneal at 400 °C for 2 h in argon to obtain the product.

[0038] Step 2: Synthesis of ZnO / GQDs aerogel

[0039] S1: Dissolve 1 g of sodium alginate in 25 mL of DI, stir magnetically at 70 °C until dissolved to obtain a 4 wt% sodium alginate solution;

[0040] S2: Add 0.75 g of ZnO to the sodium alginate solution, stir magnetically until homogeneous, add 10 mL of graphene quantum dot solution, and stir magnetically at 70 °C until homogeneous to obtain a suspension; The preparation method of the graphene quantum dot solution is: Add 5 g of citric acid to the reaction kettle, react at 210 °C for 3 h, and then dropwise add 1.5 M sodium hydroxide solution at room temperature to make the system pH 7 to obtain the graphene quantum dot solution; The concentration of the graphene quantum dot solution is 0.02 mg / mL.

[0041] S3: Pour the suspension into a mold, and freeze-dry for 48 h to remove the remaining liquid;

[0042] S4: To obtain a calcium ion-induced cross-linking process, immerse the freeze-dried sample in 1 M calcium chloride (CaCl2) solution for 1 h;

[0043] S5: Wash to remove unbound calcium ions, and freeze-dry again to obtain the zinc oxide / graphene quantum dot aerogel of Example 1. The zinc oxide / graphene quantum dot aerogel of Example 1 is named ZnO / GQDs-1.

[0044] Example 2

[0045] The preparation method of the zinc oxide / graphene quantum dot aerogel of Example 2 is substantially the same as that of Example 1. The difference between the preparation method of the zinc oxide / graphene quantum dot aerogel of Example 2 and that of Example 1 is that the addition amount of graphene quantum dots in Example 2 is 20 mL. The zinc oxide / graphene quantum dot aerogel of Example 2 is named ZnO / GQDs-2.

[0046] Example 3

[0047] The preparation method of the zinc oxide / graphene quantum dot aerogel of Example 3 is substantially the same as that of Example 1. The difference between the preparation method of the zinc oxide / graphene quantum dot aerogel of Example 3 and that of Example 1 is that the addition amount of graphene quantum dots in Example 3 is 30 mL. The zinc oxide / graphene quantum dot aerogel of Example 3 is named ZnO / GQDs-3.

[0048] Example 4

[0049] The preparation method of the zinc oxide / graphene quantum dot aerogel of Example 4 includes the following steps:

[0050] Step 1: Preparation of ZnO powder

[0051] S1: Add 3.396 g of zinc acetate dihydrate and 0.3553 g of urea to 35 mL of deionized water, and magnetically stir for 30 min to obtain a mixed solution;

[0052] S2: Transfer the mixed solution to a 100 mL teflon autoclave, heat it to 150 °C in an oven, heat for 26 h, and naturally cool to room temperature;

[0053] S3: Centrifuge 6 times alternately with deionized water and ethanol to remove excess impurity ions;

[0054] S4: Freeze-dry for 24 hours;

[0055] S5: Put the freeze-dried sample into a porcelain boat, and anneal it at 420 °C in argon for 2.5 h to obtain it.

[0056] Step 2: Synthesis of ZnO / GQDs aerogel

[0057] S1: Dissolve 1 g of sodium alginate in 25 mL of DI, and magnetically stir at 70 °C until dissolved to obtain a 4 wt% sodium alginate solution;

[0058] S2: Add 0.75 g of ZnO to the sodium alginate solution, magnetically stir until homogeneous, add 10 mL of graphene quantum dot solution, and magnetically stir at 70 °C until homogeneous to obtain a suspension; The preparation method of the graphene quantum dot solution is: Add 4 g of citric acid to the reaction kettle, react at 230 °C for 2.5 h, and then dropwise add 1.5 M sodium hydroxide solution at room temperature to make the pH of the system 7 to obtain the graphene quantum dot solution; The concentration of the graphene quantum dot solution is 0.02 mg / mL.

[0059] S3: Pour the suspension into a mold, and freeze-dry for 48 h to remove the remaining liquid;

[0060] S4: In order to obtain a calcium ion-induced cross-linking process, immerse the freeze-dried sample in 1.1 M calcium chloride solution for 1 h;

[0061] S5: Wash to remove unbound calcium ions, and then freeze-dry again to obtain the zinc oxide / graphene quantum dot aerogel of Example 4.

[0062] Example 5

[0063] The preparation method of the zinc oxide / graphene quantum dot aerogel of Example 5 comprises the following steps:

[0064] Step 1: Preparation of ZnO powder

[0065] S1: Add 3.996 g of zinc acetate dihydrate and 0.3653 g of urea into 30 mL of deionized water, and magnetically stir for 30 min to obtain a mixed solution;

[0066] S2: Transfer the mixed solution to a 100 mL teflon autoclave, heat it to 170 °C in an electric oven, heat for 22 h, and naturally cool to room temperature;

[0067] S3: Centrifuge 6 times alternately with deionized water and ethanol to remove excess impurity ions;

[0068] S4: Freeze-dry for 24 hours;

[0069] S5: Put the freeze-dried sample into a porcelain boat, anneal it at 380 °C in argon for 2 h to obtain the product.

[0070] Step 2: Synthesis of ZnO / GQDs aerogel

[0071] S1: Dissolve 1 g of sodium alginate in 25 mL of DI, and magnetically stir at 70 °C until dissolved to obtain a 4 wt% sodium alginate solution;

[0072] S2: Add 1 g of ZnO into the sodium alginate solution, magnetically stir until homogeneous, add 10 mL of graphene quantum dot solution, and magnetically stir at 70 °C until homogeneous to obtain a suspension; The preparation method of the graphene quantum dot solution is: Add 6 g of citric acid into a reaction kettle, react at 190 °C for 2.5 h, and then dropwise add 1.5 M sodium hydroxide solution at room temperature to make the pH of the system 7 to obtain the graphene quantum dot solution; The concentration of the graphene quantum dot solution is 0.02 mg / mL.

[0073] S3: Pour the suspension into a mold, and freeze-dry for 48 h to remove the remaining liquid;

[0074] S4: To obtain a calcium ion-induced crosslinking process, immerse the freeze-dried sample in 0.9 M calcium chloride solution for 1 h;

[0075] S5: Wash to remove unbound calcium ions, and freeze-dry again to obtain the zinc oxide / graphene quantum dot aerogel of Example 5.

[0076] Preparation of NO2 gas sensor: Weld the piezoelectric crystal onto the gas-sensing base and rinse it with deionized water, then dry and age it in an oven at 80 °C. Take 1 g of the zinc oxide / graphene quantum dot aerogel, put it into a blender to make powder, then put it into a mortar, grind it and add 3 mL of deionized water, stir evenly to obtain a paste, spray-print it onto the electrode plate with a spray gun, and dry it in an oven. Repeat this process 5 - 10 times to make the ZnO / GQDs on the electrode plate reach the standard thickness, thus obtaining the NO2 gas sensor.

[0077] From Figure 1 It can be seen that the diffraction peaks of the zinc oxide / graphene quantum dot aerogels of Examples 1 - 3 at 31.77, 34.42, 36.25, 56.6, 62.86, and 67.96 correspond to the (100), (002), (101), (110), (103), and (112) crystal planes of wurtzite ZnO respectively. In addition, no other characteristic impurity peaks were observed in the XRD patterns of the zinc oxide / graphene quantum dot aerogels of Examples 1 - 3, and there is no different crystal structure, indicating that the addition of GQDs does not change the ZnO composition.

[0078] From Figure 2 It can be seen that the Raman spectra of the zinc oxide / graphene quantum dot aerogels of Examples 1 - 3 show two strong peaks, corresponding to the D-band spectral line (1384 cm -1 ) and the G-band spectral line (1588 cm -1 ). The D-band is caused by edges or structural defects, which can break the selection rules and symmetry. The G-band is related to the in-plane vibration of sp 2 hybridized carbon atoms in the two-dimensional hexagonal lattice of graphene quantum dots. The ID / IG (intensity ratio) values of ZnO / GQDs are 0.91, 0.94, and 0.91 respectively, and the value of ZnO / GQDs-2 is the highest. The increase in the ID / IG value indicates an increase in the disorder of GQDs and more defects.

[0079] From Figure 3 It can be seen that the zinc oxide / graphene quantum dot aerogels of Examples 1 - 3 all have an obvious ultraviolet absorption peak at 306 nm, which is the transition of electrons from the valence band to the conduction band. According to the relationship between the direct semiconductor band gap (Eg) and the ultraviolet absorbance (a), combined with the theoretical formula (Formula Ⅰ), the measured absorbance is processed, and the band gap sizes of the zinc oxide / graphene quantum dot aerogels of Examples 1 - 3 are extrapolated. From Figure 3It can be seen that the band gaps of the zinc oxide / graphene quantum dot aerogels in Examples 1-3 are 3.09 eV, 3.07 eV, and 3.09 eV respectively, and the band gap of ZnO / GQDs-2 is smaller;

[0080] (αhν) 1 / r = A(hν - Eg), Equation I.

[0081] From Figure 4 it can be seen that there are many pores composed of small flakes on the surface of the zinc oxide / graphene quantum dot aerogels in Examples 1-3, indicating that the zinc oxide / graphene quantum dot aerogels in Examples 1-3 have a uniform 3D structure of interconnected pore graphene, and this structure increases the surface roughness and active sites of the zinc oxide / graphene quantum dot aerogels. Among them, the pore size of ZnO / GQDs-2 is the smallest, which is due to the hydrogen bond interaction between GQDs and sodium alginate, and this hydrogen bond interaction increases the resistance to the formation of larger ice particles by freeze-drying.

[0082] From Figure 5 and Figure 6 it can be seen that GQDs have an obvious crystal structure, and the lattice fringe spacing is measured to be 0.227 nm, corresponding to its (100) crystal plane; Figure 6 is the EDS spectrum of the zinc oxide / graphene quantum dot aerogel. Elements C, O, and Zn can be observed in the figure. The mass fractions of each element in the zinc oxide / graphene quantum dot aerogel are: C accounts for 69.54%, O accounts for 2.61%, and Zn accounts for 27.85%. The atomic percentages are C is 90.77%, O is 2.55%, and Zn is 6.68%. The standard deviation of the elemental quantitative analysis in the spectrum can reflect the quality and uniformity of the zinc oxide / graphene quantum dot aerogel to a certain extent, and at the same time confirm the successful preparation of the zinc oxide / graphene quantum dot aerogel.

[0083] Figure 7 a shows the full spectrum of the zinc oxide / graphene quantum dot aerogels in Examples 1-3. Obvious C1s, O 1s, and Zn 2p peaks can be seen, indicating that the zinc oxide / graphene quantum dot aerogel contains carbon, oxygen, and zinc elements. Among them, carbon comes from graphene quantum dots, and zinc and oxygen come from zinc oxide. The C 1s diffraction peak is introduced by accidentally occurring carbon-based contamination, and the C1s peak corresponding to the binding energy at 284.8 eV is usually used as a reference value during calibration. In Figure 7 b it can be seen that the Zn 2p 1 / 2 peak located at 1045.9 eV and the Zn 2p 3 / 2 peak located at 1022.8 eV correspond to the +2 valence state of Zn. The peak decomposition of the O 1s spectrum reveals the different chemical environments of oxygen in the zinc oxide / graphene quantum dot aerogel. From Figure 7From c-e, the peak at 531.7 eV is caused by oxygen coordination in Zn-O. The peak with a binding energy of 533.2 eV can be attributed to oxygen vacancies, while the peak at 537.4 eV is caused by the oxygen in -OH on the surface of zinc oxide / graphene quantum dot aerogel. As Figure 7 shown in c-e, the order of oxygen vacancy concentration is ZnO / GQDs-2 > ZnO / GQDs-1 > ZnO / GQDs-3. ZnO / GQDs-2 contains the highest concentration of oxygen vacancies, which can adsorb more oxygen molecules, thus improving the gas-sensing performance of NO2 gas.

[0084] At room temperature, the sensitivity curves of the zinc oxide / graphene quantum dot aerogels of Examples 1-3 for different concentrations of NO2 are as Figure 8 shown in a-e. The sensitivity of ZnO / GQDs-2 is much higher than that of Example 1 and Example 3. The sensitivities of the three at 100 ppb are 15.7, 108.6, and 33.5 respectively. Compared with the pure ZnO gas sensor (135 °C), the operating temperature of the GQDs-doped ZnO gas sensor is reduced to room temperature. Figure 8 b-d are the response and recovery time diagrams of ZnO / GQDs-1, ZnO / GQDs-2, and ZnO / GQDs-3 for 100 ppb NO2 at room temperature. The response and recovery times are 90 / 35 s, 80 / 22 s, and 76 / 30 s respectively. From Figure 8 e, it can be seen that the determination coefficients (R 2 ) of the fitting curves are all close to 1.0, indicating a high degree of linearity. From Figure 8 f, it can be seen that after 5 cycles in 100 ppb NO2 and air, ZnO / GQDs-2 can almost completely recover to the original value, indicating that ZnO / GQDs-2 has good repeatability. Figure 8 The periodicity of 100 ppb NO2 of ZnO / GQDs-2 is shown in g. The five broken lines in the figure correspond to the test data of 1 week, 2 weeks, 3 weeks, 4 weeks, and 5 weeks respectively. The fluctuation range and change trend of the broken lines between different weeks are relatively similar, reflecting the good stability characteristics of ZnO / GQDs-2. Figure 8 h is the selectivity Debra diagram of ZnO / GQDs-2. Selectivity tests were carried out on 100 ppb NO2, CO2, SO2, H2S, and CH4 at room temperature. The results show that ZnO / GQDs-2 has a very obvious selectivity for NO2.

Claims

1. A method for preparing zinc oxide / graphene quantum dot aerogel, characterized in that: Sodium alginate, zinc oxide and graphene quantum dot solution are mixed to obtain a suspension, which is then freeze-dried.

2. The method for preparing zinc oxide / graphene quantum dot aerogel according to claim 1, characterized in that: For each mL of the sodium alginate solution, 0.02 to 0.04 g of zinc oxide is added; for each mL of the sodium alginate solution, 0.4 to 0.12 mL of the graphene quantum dot solution is added.

3. The method for preparing zinc oxide / graphene quantum dot aerogel according to claim 1, characterized in that: The suspension is freeze-dried and then soaked in a calcium chloride solution, followed by washing and freeze-drying to obtain the product.

4. The method for preparing zinc oxide / graphene quantum dot aerogel according to claim 3, characterized in that: The concentration of the calcium chloride solution is 0.9-1.1 M; the soaking time is 1-2 hours.

5. The method for preparing zinc oxide / graphene quantum dot aerogel according to claim 1, characterized in that: The sodium alginate solution is a sodium alginate aqueous solution with a mass fraction of 3-5%; the concentration of the graphene quantum dot solution is 0.015-0.025 mg / mL.

6. The method for preparing zinc oxide / graphene quantum dot aerogel according to claim 1, characterized in that: The preparation method of the zinc oxide is as follows: zinc acetate dihydrate and urea are dissolved in water, freeze-dried after hydrothermal reaction, and then annealed in an inert gas to obtain the zinc oxide.

7. The method for preparing zinc oxide / graphene quantum dot aerogel according to claim 6, characterized in that: The temperature of the hydrothermal reaction is 150-170° C., and the time of the hydrothermal reaction is 22-26 hours; the temperature of the annealing is 380-420° C., and the time of the annealing is 1.5-2.5 hours; and the inert gas is argon.

8. The method for preparing zinc oxide / graphene quantum dot aerogel according to claim 1, characterized in that: The mass ratio of the zinc acetate dihydrate to urea is 3.396-3.996:0.3553-0.3653; 25-40 mL of water is added for every 3.696 g of the zinc acetate dihydrate.

9. A zinc oxide / graphene quantum dot aerogel, characterized in that: The zinc oxide / graphene quantum dot aerogel is prepared by the preparation method of any one of claims 1 to 8.

10. A NO2 gas sensor, characterized in that: Comprising the zinc oxide / graphene quantum dot aerogel as described in claim 9.

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