Gradient graphene aerogel as well as preparation method and application thereof

Abstract

The invention relates to gradient graphene aerogel as well as a preparation method and application thereof. The preparation method comprises the following steps: dispersing graphene oxide and a reducing agent in a solvent to obtain a mixed dispersion liquid; taking a part of the mixed dispersion liquid, and carrying out a first heating reaction to obtain a first layer of graphene hydrogel; adding a part of the mixed dispersion liquid to the first layer of graphene hydrogel, carrying out a second heating reaction to form a second layer of graphene hydrogel on the first layer of graphene hydrogel, and repeating the step to obtain gradient graphene hydrogel; and carrying out freeze drying on the gradient graphene hydrogel to obtain the gradient graphene aerogel. The improved gradient graphene aerogel obtained by the preparation method disclosed by the invention has good integrity, high interlayer binding force and more excellent mechanical properties, and has the characteristics of continuous deformation and discontinuous displacement field on the whole.

Description

Technical Field

[0001] The present application relates to the field of aerogel technology, and in particular to a gradient graphene aerogel and a preparation method and application thereof. Background Art

[0002] Flexible wearable pressure sensors have been widely used in real-time monitoring of human physiological parameters and various motion states. These sensors provide comfort, good fit and wearability, providing reliable data for promoting a healthy lifestyle. Graphene aerogels are widely used in flexible piezoresistive sensors due to their remarkable properties, including robustness, elasticity, conductivity and light weight.

[0003] Functionally gradient materials are a new type of composite material that combines two or more materials into a continuous gradient of components and structures. There is no obvious interface, which reduces interface damage. The characteristics of gradient materials are the heterogeneity of organization, multi-scale and continuity of mechanical properties, as well as the multi-level nature of the structure.

[0004] Although related work has proposed a method of preparing gradient graphene aerogels by combining layer-by-layer assembly and unidirectional freezing, and combining the advantages of graphene aerogels with customized structural design in FATPS (freeze-assisted transfer printing strategy), the bonding force between each layer of gradient structures in the gradient graphene aerogels prepared by the above methods is weak, and a relatively complete integrated structure cannot be formed. At the same time, since the freezing process regulates each layer of structure separately, the overall structure cannot be regulated by the ice template method, which is not conducive to performance improvement. Therefore, it is of great significance to propose a preparation method for gradient graphene aerogels that are integrated and have adjustable overall structure. Summary of the invention

[0005] The present application aims to solve one of the technical problems in the related art at least to some extent. To this end, one purpose of the present application is to propose a method for preparing a gradient graphene aerogel, which can prepare a gradient graphene aerogel with integrity and controllable overall structure.

[0006] In a first aspect, an embodiment of the present application provides a method for preparing a gradient graphene aerogel, comprising the following steps:

[0007] dispersing graphene oxide and a reducing agent in a solvent to obtain a mixed dispersion;

[0008] Taking a portion of the mixed dispersion for a first heating reaction to obtain a first layer of graphene hydrogel;

[0009] adding a portion of the mixed dispersion onto the first layer of graphene hydrogel, and performing a second heating reaction, thereby forming a second layer of graphene hydrogel on the first layer of graphene hydrogel, and repeating this step to obtain a gradient graphene hydrogel;

[0010] The gradient graphene hydrogel is freeze-dried to obtain a gradient graphene aerogel.

[0011] In a second aspect, an embodiment of the present application provides a gradient graphene aerogel, which is obtained by the preparation method of the first aspect of the present application.

[0012] In a third aspect, an embodiment of the present application provides the application of the gradient graphene aerogel of the second aspect of the present application in the field of wave absorbing structure, sensor monitoring, construction or energy.

[0013] Compared with the prior art, the present invention has the following beneficial effects:

[0014] 1. The present application provides an improved method for preparing gradient graphene aerogels, which has a simple process, does not require the development of additional new equipment, and does not require the addition of additional new substances. It is convenient, fast, low-cost, and can be mass-produced. The preparation method of the present application forms a gradient graphene hydrogel through a step-by-step hydrothermal reaction. The gradient graphene hydrogel includes multiple layers of hydrogel, each layer of hydrogel undergoes a hydrothermal reaction for a different period of time, and then the gradient graphene hydrogel is freeze-dried as a whole, and the overall structure is regulated by the ice template method, thereby forming an integrated graphene aerogel with a gradient structure. Due to the hydrothermal reaction, π-π bonds are formed between each layer of aerogel in the aerogel, and the interlayer bonding force is very strong, which is conducive to improving the mechanical properties.

[0015] 2. The improved gradient graphene aerogel obtained by the preparation method of the present application has good integrity. π-π bonds are formed between each layer of different aerogels due to the hydrothermal reaction, which improves the interlayer bonding force and makes the gradient graphene aerogel have better mechanical properties. The overall deformation is continuous and the displacement field is discontinuous.

[0016] In addition, the improved gradient graphene aerogel has a gradient structure, which makes the graphene aerogel show the heterogeneity of organization, multi-scale and continuity of mechanical properties, as well as the multi-level nature of structure.

[0017] 3. The gradient graphene aerogel of the present application can be widely used in the fields of wave absorbing structures, sensor monitoring, construction, energy, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a flow chart of the steps of the method for preparing the gradient graphene aerogel of the present application.

[0019] Figure 2 This is a schematic diagram of the gradient graphene aerogel structure of Example 1 of the present application.

[0020] Figure 3 This is a physical picture of the gradient graphene aerogel of Example 1 of the present application.

[0021] Figure 4 This is a microscopic morphology image (optical microscope image) of different gradient layers of the gradient graphene aerogel of Example 1 of the present application; Figure 4 (a) is a 1-hour hydrothermal layer, Figure 4 (b) is a 2-hour hydrothermal layer, Figure 4 (c) is a 3-hour hydrothermal layer. DETAILED DESCRIPTION

[0022] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application clearer, the present application is further described in detail below in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not intended to limit the present application. Unless otherwise defined, the technical terms used in the following examples have the same meanings as those generally understood by those skilled in the art to which the present application belongs. In the embodiments, unless otherwise specified, the raw materials used are commercially available, and the methods used in the following embodiments are conventional methods in the art unless otherwise specified.

[0023] In the first aspect, the present invention provides a method for preparing a gradient graphene aerogel, the steps of which are shown in the flowchart as follows: Figure 1 As shown, the following steps are included:

[0024] dispersing graphene oxide and a reducing agent in a solvent to obtain a mixed dispersion;

[0025] Taking a portion of the mixed dispersion for a first heating reaction to obtain a first layer of graphene hydrogel;

[0026] adding a portion of the mixed dispersion onto the first layer of graphene hydrogel, and performing a second heating reaction, thereby forming a second layer of graphene hydrogel on the first layer of graphene hydrogel, and repeating this step to obtain a gradient graphene hydrogel;

[0027] The gradient graphene hydrogel is freeze-dried to obtain a gradient graphene aerogel.

[0028] The preparation method of the present application forms a gradient graphene hydrogel through a step-by-step hydrothermal reaction. The gradient graphene hydrogel includes multiple layers of hydrogel, and each layer of hydrogel undergoes a hydrothermal reaction for a different period of time. The gradient graphene hydrogel is then freeze-dried as a whole, and the overall structure is regulated by an ice template method, thereby forming an integrated graphene aerogel with a gradient structure. Due to the hydrothermal reaction, π-π bonds are formed between each layer of aerogel in the aerogel, and the interlayer bonding force is very strong, which is beneficial to improving the mechanical properties.

[0029] In some embodiments, the preparation method includes multiple heating reactions, the temperature of the first heating reaction is 70-90°C (e.g., 70°C, 75°C, 80°C, 85°C, or 90°C), and the time is 0.5-2h (e.g., 0.5h, 1h, 1.5h, or 2h); the temperature of the second heating reaction is 70-90°C (e.g., 70°C, 75°C, 80°C, 85°C, or 90°C), and the time is 0.5-2h (e.g., 0.5h, 1h, 1.5h, or 2h), and so on. Preferably, the temperature of the last heating reaction is 110-130°C (e.g., 110°C, 115°C, 120°C, 125°C, or 130°C), and the time is 0.5-2h (e.g., 0.5h, 1h, 1.5h, or 2h). In other words, except for the last heating reaction temperature of 110-130°C and the time of 0.5-2h, the temperature of the heating reaction (i.e., hydrothermal reaction) in other steps is 70-90°C and the time is 0.5-2h. The reaction temperature of the present application is conducive to the full progress of the hydrothermal reaction, which is conducive to the conversion of the mixed dispersion from a solution state to a hydrogel state. The temperature of the last heating reaction of the present application is higher than the previous heating reaction temperature, which is conducive to the full hydrothermal reaction and the effective formation of π-π bonds between the layers, thereby improving the bonding strength between the layers.

[0030] In some specific embodiments, after the second layer of graphene hydrogel is formed, this step may be repeated 0-3 times (eg, 1 time, 2 times, or 3 times).

[0031] In some specific embodiments, after forming the second layer of graphene hydrogel, repeat this step once, thereby forming a third layer of graphene hydrogel on the second layer of graphene hydrogel to obtain a gradient graphene hydrogel. The preparation method includes three heating reactions. The temperature of the first heating reaction and the second heating reaction are both 70-90°C (e.g., 70°C, 75°C, 80°C, 85°C or 90°C), and the time is 0.5-2h (e.g., 0.5h, 1h, 1.5h or 2h). The temperature of the third heating reaction is 110-130°C (e.g., 110°C, 115°C, 120°C, 125°C or 130°C), and the time is 0.5-2h (e.g., 0.5h, 1h, 1.5h or 2h).

[0032] In some specific embodiments, the last heating reaction is carried out under closed conditions. Gas is generated during the reaction. Under closed conditions, as the amount of gas increases, the system pressure increases. At a higher temperature, it is more conducive to the full hydrothermal reaction and the effective formation of π-π bonds between the layers, thereby improving the bonding strength between the layers.

[0033] In some embodiments, after each heating reaction is completed, the preparation method further includes cooling. After each thermal reaction is completed, cooling is performed before proceeding to the next step, which is beneficial to making the hydrogel state formed by the thermal reaction more stable and preventing the mixed dispersion added later from destroying the already formed gel structure.

[0034] In some embodiments, in each step, the volume of the mixed dispersion is the same, preferably 5-10 ml (e.g., 5 ml, 6 ml, 7 ml, 8 ml, 9 ml or 10 ml). Ensuring the same volume can make the structure of the prepared aerogel more controllable, and at the same time, during the use of the aerogel, the change of the aerogel structure is more uniform when it is under pressure, and there will be no sudden change in mechanical properties due to excessive volume differences of each layer of the aerogel structure.

[0035] In some embodiments, in each step, the portion of the mixed dispersion is added dropwise.

[0036] In some embodiments, the reducing agent includes one or more of ethylenediamine, ascorbic acid, polydopamine, sodium dodecyl sulfate, and sodium sulfide. These reducing agents are beneficial to reducing graphene oxide to graphene, and during the reduction process, the state of the reaction system is transformed from a solution state to a hydrogel state.

[0037] In some embodiments, the mass ratio of the reducing agent to the graphene oxide is 0.3:1 to 5:1, for example, 0.3:1, 0.5:1, 0.7:1, 1:1, 2:1, 3:1, 4:1 or 5:1. Preferably, when the reducing agent is ethylenediamine, the mass ratio may be 0.3:1 to 0.8:1. Preferably, when the reducing agent is ascorbic acid, the mass ratio may be 3:1 to 5:1. The amount of the reducing agent affects the progress of the heating reaction, and thus affects the change in the state of the reaction system. Too little reducing agent will cause the reaction system to be unable to completely transform from a solution state to a hydrogel state, causing the mixed dispersion added later to destroy the gel structure that has been generated.

[0038] In some embodiments, the solvent comprises water.

[0039] In some embodiments, the preparation method further includes: subjecting the gradient graphene aerogel to high-temperature reduction to obtain a highly reduced gradient graphene aerogel. Preferably, the temperature of the high-temperature reduction is 900-1100°C (e.g., 900°C, 950°C, 1000°C, 1050°C or 1100°C), and the time is 10-20 minutes (e.g., 10 minutes, 15 minutes or 20 minutes). Preferably, the high-temperature reduction is carried out in an inert protective atmosphere (e.g., an argon atmosphere). The highly reduced gradient graphene aerogel has a more stable structure and higher conductivity, which is beneficial to improving the overall performance of the aerogel. The temperature range of the present application is conducive to the full implementation of the reduction reaction. If the reduction temperature is too low, it is not conducive to the thorough implementation of the reduction reaction, and if the reduction temperature is too high, it may cause graphene carbonization.

[0040] In some embodiments, freeze drying includes cryogenic freezing and vacuum drying. Preferably, the temperature of cryogenic freezing is -80°C to -100°C (e.g., -80°C, -85°C, -90°C, -95°C or -100°C), and the time is 1-4h (e.g., 1h, 2h, 3h or 4h). Preferably, the vacuum degree of vacuum drying is 1-6Pa (e.g., 1-3Pa, 3-6Pa or 2-4Pa), and the time is 60-84h (e.g., 60h, 66h, 72h, 78h or 84h). In some specific embodiments, cryogenic freezing is cryogenic centripetal freezing. The temperature, pressure and time of freeze drying in this application are conducive to completely freezing and fully drying the gradient graphene hydrogel, so that the hydrogel can be completely transformed into an aerogel and improve the stability of the aerogel structure.

[0041] In some embodiments, the preparation of the mixed dispersion comprises:

[0042] dispersing graphene oxide in a solvent to obtain a graphene oxide dispersion;

[0043] The graphene oxide dispersion liquid and the reducing agent are mixed and dispersed to obtain the mixed dispersion liquid.

[0044] In some specific embodiments, the concentration of the graphene oxide dispersion is 5-10 mg / ml, for example, 5 mg / ml, 6 mg / ml, 7 mg / ml, 8 mg / ml, 9 mg / ml or 10 mg / ml. If the concentration of the graphene oxide dispersion is too low, the hydrogel cannot be formed during the reaction.

[0045] In some specific embodiments, ultrasound is used for dispersion. Ultrasonic dispersion is beneficial to improve dispersion uniformity. The ultrasonic dispersion time is 5-15 min (e.g., 5 min, 10 min, or 15 min). Ultrasonic dispersion is performed in a mode of working time 15-50 s and interval time 2-4 s.

[0046] In a second aspect, an embodiment of the present application provides a gradient graphene aerogel, which is obtained by the preparation method of the first aspect of the present application.

[0047] In a third aspect, an embodiment of the present application provides the application of the gradient graphene aerogel of the second aspect of the present application in the field of wave absorbing structure, sensor monitoring, construction or energy.

[0048] The present application is described below with reference to specific embodiments. It should be noted that these embodiments are merely illustrative and do not limit the present application in any way.

[0049] Example 1

[0050] (1) A certain concentration of graphene oxide aqueous solution is dispersed by an ultrasonic cell crusher to obtain a uniform dispersion; specifically, 30 ml of a 5 mg / ml graphene oxide aqueous solution is transferred into a beaker using a pipette, the beaker is placed in the ultrasonic cell crusher, the ultrasonic cell crusher power is set to 300 W, the working time is 20 s, the interval time is 3 s, and ultrasonic dispersion is performed for 10 min to obtain 30 ml of a uniform graphene oxide dispersion.

[0051] (2) Add a certain volume of reducing agent to the graphene oxide uniform dispersion obtained in step 1, and disperse it by ultrasonic cell crusher for ten minutes to obtain a mixed dispersion; specifically, add 90 μl of ethylenediamine (analytical grade) to the graphene oxide uniform dispersion obtained in step 1 using a pipette, and the volume ratio of ethylenediamine to graphene oxide aqueous solution is 30 μl:10 ml (ethylenediamine density 0.899 g / cm 3 , the mass ratio of ethylenediamine to graphene oxide is 0.54:1), the mixed solution is placed in an ultrasonic cell crusher, the ultrasonic cell crusher power is set to 400 W, the working time is 40 s, the interval time is 3 s, and the ultrasonic dispersion is performed for 10 min to obtain a mixed dispersion.

[0052] (3) The mixed dispersion obtained in step 2 is transferred to a pharmaceutical bottle, sealed and placed in a drying oven for heating, reacted for 1 hour, and a 1-hour graphene hydrogel is obtained; specifically, 10 ml of the mixed dispersion obtained in step 2 is transferred to the pharmaceutical bottle using a dropper, the pharmaceutical bottle is sealed with a bottle cap, and then transferred to an oven for reaction, the oven temperature is set to 80° C., the reaction time is 1 hour, and a gel-like product is obtained, which is recorded as a 1-hour graphene hydrogel (i.e., the first layer of graphene hydrogel).

[0053] (4) Add the same volume of the mixed dispersion obtained in step 2 onto the 1-hour graphene hydrogel layer, seal it and heat it in a drying oven; specifically, take out the medicine bottle and cool it to room temperature, add 10 ml of the mixed dispersion obtained in step 2 onto the 1-hour graphene hydrogel. Since the 1-hour graphene hydrogel is a colloidal substance, the mixed dispersion added later will not destroy the gel structure that has been generated. After sealing the medicine bottle with a bottle cap, transfer it to an oven to continue the reaction. The oven temperature is set to 80°C and the reaction time is 1 hour to obtain a gradient gel product, including 2 layers of hydrogel, which are respectively recorded as 1-hour graphene hydrogel (i.e., the second layer of graphene hydrogel) and 2-hour graphene hydrogel (i.e., the first layer of graphene hydrogel) transformed from the 1-hour graphene hydrogel in step 3.

[0054] (5) adding the same volume of the mixed dispersion obtained in step 2 onto the 1-hour graphene hydrogel layer, sealing the bottle and heating it in an oven; specifically, taking out the medicine bottle and cooling it to room temperature, adding 10 ml of the mixed dispersion obtained in step 2 onto the above two layers of gradient hydrogel, sealing the medicine bottle with a bottle cap, placing the medicine bottle in a reactor, keeping the air pressure valve closed, rotating and sealing the reactor, placing it in an oven and setting the temperature to 120°C, reacting for 1 hour, cooling the reactor to room temperature, and obtaining a gradient gel-like product, including 3 layers of hydrogel, respectively recorded as 1-hour graphene hydrogel (i.e., the third layer of graphene hydrogel), 2-hour graphene hydrogel (i.e., the second layer of graphene hydrogel) transformed from the 1-hour graphene hydrogel obtained in step 4, and 3-hour graphene hydrogel (i.e., the first layer of graphene hydrogel) transformed from the 2-hour graphene hydrogel obtained in step 3.

[0055] (6) The gradient gel-like product obtained in step 5 is cryogenically frozen by a freeze dryer; specifically, the gradient gel-like product obtained in step 5 is placed in a cold trap of the freeze dryer, the cold trap is set at a temperature of -90°C, pre-cooled for 3 hours in advance to ensure that the internal temperature is uniform, and frozen for 2 hours under this temperature environment to ensure that the gradient graphene hydrogel is completely frozen.

[0056] (7) Further vacuum drying is performed to obtain a gradient graphene aerogel; specifically, the frozen gradient graphene hydrogel is placed in a freeze dryer for vacuum drying, the vacuum pump is turned on, the vacuum degree inside the freeze dryer is maintained at 1-6 Pa, the drying time is 72 hours, and the gradient graphene aerogel is obtained.

[0057] (8) The gradient graphene aerogel obtained in step seven is further subjected to high-temperature reduction in a high-temperature tubular furnace to obtain a highly reduced gradient graphene aerogel; specifically, the gradient graphene aerogel obtained in step seven is placed in a high-temperature tubular Maboro furnace for high-temperature reduction treatment to improve its conductivity, and an argon atmosphere of one atmosphere of pressure is maintained in the Maboro furnace, the temperature is set to 1000° C., and the reaction time is 15 min to obtain a highly reduced gradient graphene aerogel.

[0058] The schematic diagram of the structure of the highly reduced gradient graphene aerogel prepared in Example 1 is as follows Figure 2 As shown in the actual picture Figure 3 The microscopic morphology of different gradient layers in the highly reduced gradient graphene aerogel is shown in Figure 4 shown.

[0059] Example 2

[0060] (1) A certain concentration of graphene oxide aqueous solution is dispersed by an ultrasonic cell crusher to obtain a uniform dispersion; specifically, 30 ml of a 5 mg / ml graphene oxide aqueous solution is transferred into a beaker using a pipette, the beaker is placed in the ultrasonic cell crusher, the ultrasonic cell crusher power is set to 300 W, the working time is 20 s, the interval time is 3 s, and ultrasonic dispersion is performed for 10 min to obtain 30 ml of a uniform graphene oxide dispersion.

[0061] (2) Add a certain volume of reducing agent to the graphene oxide uniform dispersion obtained in step 1, and disperse it by ultrasonic cell crusher for ten minutes to obtain a mixed dispersion; specifically, add 105 μl of ethylenediamine (analytical grade) to the graphene oxide uniform dispersion obtained in step 1 using a pipette, and the volume ratio of ethylenediamine to graphene oxide aqueous solution is 35 μl:10 ml (ethylenediamine density 0.899 g / cm 3 , the mass ratio of ethylenediamine to graphene oxide is 0.63:1), the mixed solution is placed in an ultrasonic cell crusher, the ultrasonic cell crusher power is set to 400 W, the working time is 40 s, the interval time is 3 s, and the ultrasonic dispersion is performed for 10 min to obtain a mixed dispersion.

[0062] (3) The mixed dispersion obtained in step 2 is transferred to a pharmaceutical bottle, sealed and placed in a drying oven for heating and reacting for 2 hours to obtain a 2-hour graphene hydrogel; specifically, 10 ml of the mixed dispersion obtained in step 2 is transferred to the pharmaceutical bottle using a dropper, the pharmaceutical bottle is sealed with a bottle cap, and then transferred to an oven for reaction, the oven temperature is set to 70°C, the reaction time is 2 hours, and a gel-like product is obtained, which is recorded as a 2-hour graphene hydrogel (i.e., the first layer of graphene hydrogel).

[0063] (4) Add the same volume of the mixed dispersion obtained in step 2 onto the 2-hour graphene hydrogel layer, seal it and heat it in a drying oven; specifically, take out the medicine bottle and cool it to room temperature, add 10 ml of the mixed dispersion obtained in step 2 onto the 2-hour graphene hydrogel. Since the 2-hour graphene hydrogel is a colloidal substance, the mixed dispersion added later will not destroy the gel structure that has been generated. After sealing the medicine bottle with a bottle cap, transfer it to an oven to continue the reaction. The oven temperature is set to 70°C and the reaction time is 2 hours to obtain a gradient gel product, including 2 layers of hydrogel, which are respectively recorded as 2-hour graphene hydrogel (i.e., the second layer of graphene hydrogel) and 4-hour graphene hydrogel (i.e., the first layer of graphene hydrogel) transformed from the 2-hour graphene hydrogel in step 3.

[0064] (5) adding the same volume of the mixed dispersion obtained in step 2 onto the 2-hour graphene hydrogel layer, sealing the bottle and heating it in an oven; specifically, taking out the medicine bottle and cooling it to room temperature, adding 10 ml of the mixed dispersion obtained in step 2 onto the above two layers of gradient hydrogel, sealing the medicine bottle with a bottle cap, placing the medicine bottle in a reactor, keeping the air pressure valve closed, rotating the reactor to seal it, placing it in an oven and setting the temperature to 110° C., reacting for 2 hours, cooling the reactor to room temperature, and obtaining a gradient gel-like product, including 3 layers of hydrogel, respectively recorded as 2-hour graphene hydrogel (i.e., the third layer of graphene hydrogel), 4-hour graphene hydrogel (i.e., the second layer of graphene hydrogel) transformed from the 2-hour graphene hydrogel obtained in step 4, and 6-hour graphene hydrogel (i.e., the first layer of graphene hydrogel) transformed from the 4-hour graphene hydrogel obtained in step 3.

[0065] (6) The gradient gel-like product obtained in step 5 is cryogenically frozen by a freeze dryer; specifically, the gradient gel-like product obtained in step 5 is placed in a cold trap of the freeze dryer, the cold trap is set at a temperature of -80°C, pre-cooled for 3 hours in advance to ensure that the internal temperature is uniform, and frozen for 4 hours under this temperature environment to ensure that the gradient graphene hydrogel is completely frozen.

[0066] (7) Further vacuum drying is performed to obtain a gradient graphene aerogel; specifically, the frozen gradient graphene hydrogel is placed in a freeze dryer for vacuum drying, the vacuum pump is turned on, the vacuum degree inside the freeze dryer is maintained at 1-6 Pa, the drying time is 72 hours, and the gradient graphene aerogel is obtained.

[0067] (8) The gradient graphene aerogel obtained in step seven is further subjected to high-temperature reduction in a high-temperature tubular furnace to obtain a highly reduced gradient graphene aerogel; specifically, the gradient graphene aerogel obtained in step seven is placed in a high-temperature tubular Maboro furnace for high-temperature reduction treatment to improve its conductivity, and an argon atmosphere of one atmosphere of pressure is maintained in the Maboro furnace, the temperature is set to 900° C., and the reaction time is 20 min to obtain a highly reduced gradient graphene aerogel.

[0068] The structural schematic diagram of the highly reduced gradient graphene aerogel prepared in Example 2 is similar to Figure 2 , the physical picture is similar to Figure 3 , the microscopic morphology of different gradient layers is similar Figure 4 .

[0069] Example 3

[0070] (1) A certain concentration of graphene oxide aqueous solution is dispersed by an ultrasonic cell crusher to obtain a uniform dispersion; specifically, 30 ml of a 5 mg / ml graphene oxide aqueous solution is transferred into a beaker using a pipette, the beaker is placed in the ultrasonic cell crusher, the ultrasonic cell crusher power is set to 300 W, the working time is 20 s, the interval time is 3 s, and ultrasonic dispersion is performed for 10 min to obtain 30 ml of a uniform graphene oxide dispersion.

[0071] (2) Add a certain volume of reducing agent to the graphene oxide uniform dispersion obtained in step 1, and disperse it by ultrasonic cell crusher for ten minutes to obtain a mixed dispersion; specifically, add 90 μl of ethylenediamine (analytical grade) to the graphene oxide uniform dispersion obtained in step 1 using a pipette, and the volume ratio of ethylenediamine to graphene oxide aqueous solution is 30 μl:10 ml (ethylenediamine density 0.899 g / cm 3 , the mass ratio of ethylenediamine to graphene oxide is 0.54:1), the mixed solution is placed in an ultrasonic cell crusher, the ultrasonic cell crusher power is set to 400 W, the working time is 40 s, the interval time is 3 s, and the ultrasonic dispersion is performed for 10 min to obtain a mixed dispersion.

[0072] (3) The mixed dispersion obtained in step 2 is transferred to a pharmaceutical bottle, sealed and placed in a drying oven for heating, reacted for 0.5 hours, and a 0.5-hour graphene hydrogel is obtained; specifically, 10 ml of the mixed dispersion obtained in step 2 is transferred to the pharmaceutical bottle using a dropper, the pharmaceutical bottle is sealed with a bottle cap, and then transferred to an oven for reaction, the oven temperature is set to 90° C., the reaction time is 0.5 hours, and a gel-like product is obtained, which is recorded as a 0.5-hour graphene hydrogel (i.e., the first layer of graphene hydrogel).

[0073] (4) Add the same volume of the mixed dispersion obtained in step 2 onto the 0.5 hour graphene hydrogel layer, seal it and heat it in a drying oven; specifically, take out the medicine bottle and cool it to room temperature, add 10 ml of the mixed dispersion obtained in step 2 onto the 0.5 hour graphene hydrogel. Since the 0.5 hour graphene hydrogel is a colloidal substance, the mixed dispersion added later will not destroy the gel structure that has been generated. After sealing the medicine bottle with a bottle cap, transfer it to an oven to continue the reaction. The oven temperature is set to 90°C and the reaction time is 0.5h to obtain a gradient gel product, including 2 layers of hydrogel, which are respectively recorded as 0.5 hour graphene hydrogel (i.e., the second layer of graphene hydrogel) and 1 hour graphene hydrogel (i.e., the first layer of graphene hydrogel) transformed from the 0.5 hour graphene hydrogel in step 3.

[0074] (5) adding the same volume of the mixed dispersion obtained in step 2 onto the 0.5-hour graphene hydrogel layer, sealing the mixture and heating it in an oven; specifically, taking out the medicine bottle and cooling it to room temperature, adding 10 ml of the mixed dispersion obtained in step 2 onto the above two layers of gradient hydrogel, sealing the medicine bottle with a bottle cap, placing the medicine bottle in a reactor, keeping the air pressure valve closed, rotating and sealing the reactor, placing it in an oven and setting the temperature to 130° C. for a reaction time of 0.5 h, and cooling the reactor to room temperature to obtain a gradient gel-like product, including 3 layers of hydrogel, respectively recorded as 0.5-hour graphene hydrogel (i.e., the third layer of graphene hydrogel), 1-hour graphene hydrogel (i.e., the second layer of graphene hydrogel) transformed from the 0.5-hour graphene hydrogel obtained in step 4, and 1.5-hour graphene hydrogel (i.e., the first layer of graphene hydrogel) transformed from the 1-hour graphene hydrogel obtained in step 3.

[0075] (6) The gradient gel-like product obtained in step 5 is cryogenically frozen by a freeze dryer; specifically, the gradient gel-like product obtained in step 5 is placed in a cold trap of the freeze dryer, the cold trap is set to a temperature of -100°C, pre-cooled for 3 hours in advance to ensure that the internal temperature is uniform, and frozen for 1 hour under this temperature environment to ensure that the gradient graphene hydrogel is completely frozen.

[0076] (7) Further vacuum drying is performed to obtain a gradient graphene aerogel; specifically, the frozen gradient graphene hydrogel is placed in a freeze dryer for vacuum drying, the vacuum pump is turned on, the vacuum degree inside the freeze dryer is maintained at 1-6 Pa, the drying time is 72 hours, and the gradient graphene aerogel is obtained.

[0077] (8) The gradient graphene aerogel obtained in step seven is further subjected to high-temperature reduction in a high-temperature tubular furnace to obtain a highly reduced gradient graphene aerogel; specifically, the gradient graphene aerogel obtained in step seven is placed in a high-temperature tubular Maboro furnace for high-temperature reduction treatment to improve its conductivity, and an argon atmosphere of one atmosphere of pressure is maintained in the Maboro furnace, the temperature is set to 1100° C., and the reaction time is 10 min to obtain a highly reduced gradient graphene aerogel.

[0078] The structural schematic diagram of the highly reduced gradient graphene aerogel prepared in Example 3 is similar to Figure 2 , the physical picture is similar to Figure 3 , the microscopic morphology of different gradient layers is similar Figure 4 .

[0079] Example 4

[0080] Gradient graphene aerogel was prepared according to the method of Example 1, except that step 2 was different. Specifically, (2) a certain volume of a reducing agent was added to the graphene oxide uniform dispersion obtained in step 1, and the mixture was dispersed by an ultrasonic cell crusher for ten minutes to obtain a mixed dispersion; specifically, ascorbic acid powder (the mass ratio of ascorbic acid to graphene oxide was 4:1) was added to the graphene oxide uniform dispersion obtained in step 1 using a pipette, and the mixed solution was placed in an ultrasonic cell crusher, and the ultrasonic cell crusher power was set to 400 W, the working time was 40 s, the interval time was 3 s, and the ultrasonic dispersion was performed for 10 min to obtain a mixed dispersion.

[0081] The structural schematic diagram of the highly reduced gradient graphene aerogel prepared in Example 4 is similar to Figure 2 , the physical picture is similar to Figure 3 , the microscopic morphology of different gradient layers is similar Figure 4 .

[0082] Comparative Example 5

[0083] Gradient graphene aerogel was prepared according to the method of Example 1, except that the amount of ethylenediamine used in step 2 was 60 μl, and the volume ratio of ethylenediamine to graphene oxide aqueous solution was 20 μl:10 ml (ethylenediamine density 0.899 g / cm 3 , the mass ratio of ethylenediamine to graphene oxide is 0.36:1).

[0084] Since the amount of ethylenediamine used was too small, the mixed dispersion in step three could not be transformed from a solution state to a hydrogel state, and ultimately no gradient graphene aerogel was obtained.

[0085] Comparative Example 6

[0086] Gradient graphene aerogel was prepared according to the method of Example 1, except that the concentration of the graphene oxide aqueous solution in step 2 was 2 mg / ml (ethylenediamine density 0.899 g / cm 3 , the mass ratio of ethylenediamine to graphene oxide is 1.3:1).

[0087] Since the concentration of the graphene oxide aqueous solution was too low, the mixed dispersion in step three could not be transformed from a solution state to a hydrogel state, and ultimately no gradient graphene aerogel was obtained.

[0088] Through the above examples and comparative examples, it can be seen that the preparation method of the present application forms a gradient graphene hydrogel through a step-by-step hydrothermal reaction. The gradient graphene hydrogel includes multiple layers of hydrogel, each layer of hydrogel undergoes a hydrothermal reaction for a different time, and then the gradient graphene hydrogel is freeze-dried as a whole, and the overall structure is regulated by the ice template method, thereby forming an integrated graphene aerogel with a gradient structure. The preparation method of the present application has a simple process, does not require additional research and development of new equipment, and does not require additional addition of new substances. It is convenient, fast, low-cost, and can be mass-produced.

[0089] The terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of this application, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0090] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0091] Although the embodiments of the present application have been shown and described above, it can be understood that the above embodiments are exemplary and cannot be understood as limitations on the present application. Ordinary technicians in this field can change, modify, replace and modify the above embodiments within the scope of the present application.

Claims

1. A method for preparing a gradient graphene aerogel, characterized in that: The following steps are involved: dispersing graphene oxide and a reducing agent in a solvent to obtain a mixed dispersion; Taking a portion of the mixed dispersion for a first heating reaction to obtain a first layer of graphene hydrogel; adding a portion of the mixed dispersion onto the first layer of graphene hydrogel, and performing a second heating reaction, thereby forming a second layer of graphene hydrogel on the first layer of graphene hydrogel, and repeating this step to obtain a gradient graphene hydrogel; The gradient graphene hydrogel is freeze-dried to obtain a gradient graphene aerogel.

2. The preparation method according to claim 1, characterized in that: The preparation method comprises multiple heating reactions, wherein the temperature of the first heating reaction is 70-90°C and the time is 0.5-2h; the temperature of the second heating reaction is 70-90°C and the time is 0.5-2h, and so on; preferably, the temperature of the last heating reaction is 110-130°C and the time is 0.5-2h; Preferably, after each heating reaction is completed, the preparation method further comprises cooling; Preferably, after forming the second layer of graphene hydrogel, this step is repeated 0-3 times.

3. The preparation method according to claim 1 or 2, characterized in that: In each step, the volume of the mixed dispersion is the same, and the volume is preferably 5-10 ml; Preferably, in each step, the portion of the mixed dispersion is added dropwise.

4. The preparation method according to claim 1 or 2, characterized in that: The reducing agent includes one or more of ethylenediamine, ascorbic acid, polydopamine, sodium lauryl sulfate and sodium sulfide; Preferably, the solvent comprises water.

5. The preparation method according to claim 1 or 2, characterized in that: The mass ratio of the reducing agent to graphene oxide is 0.3:1 to 5:

1.

6. The preparation method according to claim 1 or 2, characterized in that: Also includes: The gradient graphene aerogel is subjected to high-temperature reduction to obtain a highly reduced gradient graphene aerogel; preferably, the temperature of the high-temperature reduction is 900-1100° C., and the time is 10-20 minutes; preferably, the high-temperature reduction is performed in an inert protective atmosphere.

7. The preparation method according to claim 1 or 2, characterized in that: Freeze drying includes low-temperature freezing and vacuum drying; preferably, the temperature of low-temperature freezing is -80°C to -100°C, and the time is 1-4 hours; preferably, the vacuum degree of vacuum drying is 1-6Pa, and the time is 60-84 hours.

8. The preparation method according to claim 1 or 2, characterized in that: The preparation of the mixed dispersion comprises: dispersing graphene oxide in a solvent to obtain a graphene oxide dispersion; Mixing and dispersing the graphene oxide dispersion liquid and the reducing agent to obtain the mixed dispersion liquid; Preferably, the concentration of the graphene oxide dispersion is 5-10 mg / ml; preferably, ultrasound is used for dispersion.

9. A gradient graphene aerogel, characterized in that: The method is obtained by the preparation method according to any one of claims 1 to 8.

10. Application of the gradient graphene aerogel according to claim 9 in wave absorbing structures, sensor monitoring, construction or energy fields.

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