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Controlled Preparation of Graphene-Based Aerogel Beads

Xiaoxiao Xiang , Xiaowen Tian , Huie Liu , Shuang Chen , Yanan Zhu , Yuqin Bo

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (7) : 1092 -1099.

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Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (7) : 1092-1099 DOI: 10.7536/PC200825
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Controlled Preparation of Graphene-Based Aerogel Beads
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Abstract

Graphene aerogel is a three-dimensional macroscopic material assembled from two-dimensional graphene sheets. Due to its high porosity, large specific surface area and low density, it has a broad application prospect in the adsorption and removal of water pollutants and has become a research hotspot. However, most of the related researches focused on bulk graphene aerogels, and few researches on graphene-based aerogel beads. This review summarizes the preparation methods of graphene-based aerogel beads, including electrostatic spray, electrostatic spinning, microfluidic technology and wet spinning. Taking the wet spinning method as the representative, the influencing factors for aerogel beads formation are analyzed, such as the concentration and viscosity of GO dispersion, extrusion parameters, the type and concentration of coagulation bath, etc. Furthermore, the factors of adjusting the pore size of the material are further analyzed. For example, the pore size of the material can be adjusted within a certain range by controlling the GO concentration, the GO layer sizes, and the temperature of freezing treatment. In view of the different treatment objects in the wastewater, the design of graphene-based aerogel beads with excellent adsorption performance, recycling performance and controllable micromorphology and the optimization and innovation of preparation methods are still the key points of exploration in future.

Contents

1 Introduction

2 Preparation of graphene-based aerogel beads

2.1 Preparation methods

2.2 Drying methods

3 Influencing factors of graphene-based aerogel beads formation

3.1 Influencing factors for wet spinning method

3.2 Influencing factors for other methods

4 Methods for adjusting the pore size of graphene-based aerogel beads

4.1 GO concentration

4.2 Temperature of freezing treatment

4.3 GO layer sizes

4.4 The speed of homogenizer

5 Conclusion and outlook

Graphical abstract

Keywords

graphene / aerogel beads / controllable preparation / shaping control / pore size control / adsorption

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Xiaoxiao Xiang, Xiaowen Tian, Huie Liu, Shuang Chen, Yanan Zhu, Yuqin Bo. Controlled Preparation of Graphene-Based Aerogel Beads. Progress in Chemistry, 2021, 33(7): 1092-1099 DOI:10.7536/PC200825

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1 Introduction

With the progress of society and the development of science and technology, water pollution has become a global problem to be solved urgently, mainly including oily wastewater, dye wastewater and heavy metal ion wastewater[1⇓⇓~4]. Among the wastewater treatment methods, adsorption method is widely used because of its simple operation and no secondary pollution. Three-dimensional graphene aerogels with large specific surface area and high porosity have been widely studied as efficient adsorbents in recent years[5⇓~7]. However, most of the related studies are focused on the traditional bulk graphene aerogels, and the preparation and adsorption performance analysis of aerogels with different shapes and sizes are lacking. Graphene-based aerogel beads are spherical aerogel materials made of graphene as the main skeleton unit, and their particle size is generally divided into micron (170-300 μm) and millimeter (0.5-5 mm), specific surface area is 60~425 m2/g, porosity is 86.2% -97.4%, density is 2.8~92 mg·cm-3[8,9,11], pore volume is 0.195~1.028 cm3·g-1, and pore size range is 4 nm-100 μm[8,9][10,11][12,13][12,14][9,14,15]. Making graphene aerogel into small spheres has many advantages. First, it is easy to meet the needs of practical applications. Chemical spills may flow into sewers, pipes or cracks in the ground, and the large volume of bulk aerogel may cause inconvenience, so it is better to use smaller aerogel spheres[11]. In addition, the graphene aerogel beads can also be used in the fields of drug delivery, solar steam power generation, filler membranes and the like[9]. Second, it provides the possibility for large-scale production. The bulk aerogel is generally prepared in a high-pressure reaction kettle, while the aerogel beads can be continuously prepared in batches in an open container, so that the equipment requirement is lower; and meanwhile, the graphene aerogel beads have smaller particle sizes and shorter diffusion channels, so that the washing and drying time required in the preparation process is shortened, and the preparation method is expected to be applied to industrial production[16][17⇓⇓⇓~21]. Third, the adsorption rate is fast. When the graphene aerogel beads are used for water adsorption and purification, the adsorption rate is greatly accelerated and the adsorption time is saved due to the larger external surface area of the graphene aerogel beads.
According to the requirements of practical application, the graphene-based aerogel beads with different sizes can be obtained by selecting an appropriate preparation method, so that the application field of the aerogel is further expanded, and the use efficiency of the aerogel is improved[22⇓⇓⇓~26]. In this paper, the preparation methods, molding factors and pore size adjustment methods of graphene-based aerogel beads are summarized and analyzed based on the latest research progress, in order to provide some guidance for further research in this field.

2 Preparation of Graphene-based Aerogel Beads

The shape of the bulk aerogel depends on the container in which the graphene oxide dispersion is contained during its preparation, while the aerogel beads need to be realized by means of appropriate spheroidization techniques[27⇓⇓⇓⇓⇓⇓~34]. Electrostatic spraying, electrospinning, microfluidic technology and wet spinning are commonly used preparation methods, and then the solvent in the pores of the prepared graphene-based hydrogel beads is replaced by gas through supercritical drying, freeze drying, atmospheric drying and other drying methods, and finally the graphene-based aerogel beads with a porous network structure are obtained[9,16,35,36][37⇓~39][10,40,41][12,15,42⇓⇓⇓⇓⇓~48].

2.1 Method for preparing graphene-based aerogel bead

2.1.1 Electrostatic spray

Electrostatic spraying refers to the method of atomizing GO dispersion into droplets under high voltage and then drying to prepare aerogel beads, which generally uses liquid nitrogen as the receiving liquid of droplets.The lower temperature of liquid nitrogen can make ice crystals grow radially to the center of the droplet, and the prepared graphene-based aerogel beads have a radially divergent microchannel structure, thus realizing the controllable formation of material channels. In addition, the particle size of the graphene-based aerogel beads can be adjusted by optimizing the process parameters of the spraying device, and the particle size is generally micron-sized.
Liao et al. prepared graphene aerogel beads (rGOAMs) by combining high voltage electroatomization with freeze casting, freeze drying and two-step thermal reduction. The device is shown in Figure 1A. When a high voltage is applied, the precursor solution droplets flowing out of the injector are charged to form Taloy cones, which are ejected from the Taloy cones and dispersed into smaller charged droplets, which are then collected in liquid nitrogen[9]. The prepared microsphere has a dandelion-like central divergent microchannel structure. The size of microspheres can be adjusted by changing the flow rate of precursor solution.When the flow rate increases from 3.75 mL·h-1 to 11.25 mL·h-1, the average diameter of microspheres increases from 183 μm to 413 μm. The prepared rGOAMs showed good adsorption performance for both oil and organic solvents, and the adsorption capacity increased with the decrease of the size of the microspheres. Yu et al, Liu et al, and He et al all used the same method to prepare graphene-based aerogel beads, and obtained a radial microchannel structure similar to the above rGOAMs[16][35][36]. For example, the graphene/chitosan composite aerogel beads prepared by Yu et al. Can rapidly adsorb heavy metal ions and water-soluble organic pollutants in water, and compared with the graphene/chitosan composite aerogel bulk material, the regular pore channels of the composite aerogel significantly shorten the internal diffusion path and significantly improve the adsorption rate[16]. However, none of the above researchers has systematically investigated the influence of the size of aerogel beads on its performance, so it is necessary to pay attention to it in future research.

2.1.2 Electrospinning

The principle and device of electrostatic spinning and electrostatic spraying are basically the same, but the viscosity of the precursor solution is different[37]. Electrospray is suitable for precursor solution with low viscosity, while electrospinning is suitable for precursor solution with high viscosity. Due to the similarity of the two preparation devices, the graphene-based aerogel beads prepared by the electrospinning method also show a similar radial microchannel structure.
Meng et al. Obtained GO/Fe3O4 aerogel microspheres by using GO/FeCl3·6H2O solution as precursor, electrostatic spinning-freeze-drying technology and high temperature calcination. The microspheres have relatively uniform morphology and layered structure with radial micropores. The particle size can be adjusted by changing the content of Fe3+[38]. When switched to a coaxial electrostatic spinning device, the GO/Fe3O4@SiO2 composite aerogel beads with a core-shell structure can be obtained by using the SiO2 precursor as the chitin solution and the GO/FeCl3·6H2O solution as the core solution. On the basis of coaxial electrospinning, Li et al. Developed a multi-axis electrospinning-freeze-drying-calcination process, which can prepare graphene aerogel beads in large quantities, and control the particle size by changing the spinning speed[39].

2.1.3 Microfluidic method

Microfluidic chip is the core part of microfluidic devices, which can be divided into four types according to the types of microchannels: T-shaped, Y-shaped, fluid focusing and stepped microchannels[40]. The formation of droplets in the microchannel is caused by the shear action between two immiscible liquids. The dispersed phase is dispersed into droplets at the intersection of the two fluids and injected into the continuous phase to form water-in-oil or oil-in-water droplets. The particle size can be controlled by adjusting the velocity of the two fluids. When the multi-channel composite microfluidic chip is used, functional microspheres such as core-shell structure microspheres and porous capsules can be prepared[40]. Zhao et al. Synthesized graphene aerogel beads using a T-type microfluidic device, which had high adsorption capacity for perfluorooctane sulfonate in water. The dispersed phase was mainly GO dispersion, and the continuous phase was fluorinated oil. The microspheres with good monodispersity could be obtained by adjusting the flow rate of the two-phase fluid[41]. Li et al. replaced the fluid in the continuous phase with gas. Thanks to the low viscosity of gas, the preparation of graphene/sodium alginate aerogel beads can be operated at a higher flow rate, thus realizing its rapid batch preparation. Compared with the traditional liquid-liquid phase microfluidic process (flow rate of 2 μL/min), the yield is greatly improved, and the flow rate can reach 800 μL/min[10]. In addition, the particle size of the aerogel beads can be adjusted by changing the gas pressure.

2.1.4 Wet Spinning

The basic principle of the wet spinning method is that the precursor solution is dropped into the coagulation bath one by one, the gel is cross-linked to obtain hydrogel beads, and then the aerogel beads are prepared by drying. When the droplet contacts the surface of the coagulation bath, the surface of the droplet will first assemble to form a GO film, while the interior of the hydrogel sphere will continue to gel in the coagulation bath until it is completely gelated[11]. The difference of the gelation rate inside and outside the aerogel sphere makes the aerogel sphere generally have a core-shell structure, and the interior is mostly a honeycomb structure, so the particle size can be adjusted by changing the droplet size. Because the preparation of aerogel beads by wet spinning mainly involves two stages of free falling and gelation of droplets, it is also called "titration-gelation method" or "ball-drop method" in some literatures[49][50].
Zhao et al. Dropped the graphene dispersion into the CaCl2 solution, and then dried and annealed under normal pressure to prepare the super-elastic graphene aerogel spheres (RGOs). The internal morphology of the RGOs is shown in Figure 2, from which it can be seen that the graphene aerogel spheres show an obvious core-shell structure, with a thin shell and wrinkles on the surface, and a honeycomb structure with a pore size of 50 ~ 100 μm in the core[15].
Ouyang et al. prepared graphene/chitosan aerogel beads (CTS-GO) with special core-shell structure by wet spinning method combined with two-step freeze casting, which has uniform particle size and high sphericity, and is composed of CTS shell with micro-channel structure and GO core with random honeycomb structure. The macroscopic morphology is shown in Figure 3[14]. The presence of the GO-core enhances its adsorption capacity, while the CTS shell coated on the GO-core can improve the surface wettability, which is beneficial to the adsorption of pollutants in water.
Compared with the above methods, the diameter of the aerogel spheres prepared by the wet spinning method is larger, generally in the order of millimeters, and the process is simple and controllable without complex devices such as high voltage electrostatic and microfluidic devices, which has attracted wide attention. When the precursor solution properties and related parameters are determined, the GO droplets immersed in the coagulation bath have the same size, so that aerogel beads with high uniformity and similar sphericity can be obtained, and in addition, the batch continuous preparation of the aerogel beads can be realized through different degrees of amplification.

2.2 Drying methods of graphene-based aerogel bead

In the preparation process of graphene-based aerogel beads, it is very important to select the appropriate drying method for its performance, the fundamental purpose is to avoid the volume shrinkage and structural cracking caused by the surface tension of the solvent when the wet aerogel beads are dried. At present, that dry method is mainly divided into three type of supercritical drying, vacuum freeze drying and normal pressure drying, wherein the supercritical dry mainly controls the temperature and the pressure to enable the solvent to reach the critical point of the solvent in the drying process,To complete the transition from liquid phase to supercritical fluid, Wang et al. Used inkjet printing-liquid marble-supercritical fluid technology to prepare monodisperse graphene aerogel beads with uniform and controllable size, which reduced the volume shrinkage of the gel during drying[51⇓~53][54,55][15,56][8]. Vacuum freeze-drying is to realize the transformation of solvent from solid to gas in wet gel beads under low temperature and vacuum. Both methods can ensure the integrity of the internal skeleton of the gel beads, and are widely used. However, in order to achieve large-scale preparation of graphene-based aerogel beads, supercritical drying and vacuum freeze drying have obvious disadvantages, such as long cycle, high cost and complex equipment, while atmospheric pressure drying has more advantages[57⇓⇓~60]. Yao et al. Used a low surface tension solvent to displace the water in the wet gel beads, and then dried them at normal pressure to prepare highly elastic graphene/carbon nanotube composite aerogel beads.Groups of composite aerogel beads can be completely restored to their original state after compression at 95% strain, and there is no obvious structural damage of aerogel beads after 1000 times of compression and release at 70% strain[44]. At present, atmospheric drying is still immature, and surface tension is still inevitable in the drying process. How to achieve as little volume shrinkage as possible under atmospheric drying is still the focus of future exploration.

3 Factors influencing the formation of graphene-based aerogel spheres

Due to the diversity of preparation methods, the factors affecting the formation of graphene-based aerogel beads are also different. In this paper, the factors affecting the formation of graphene-based aerogel beads were analyzed based on the preparation method represented by wet spinning method, which can provide basic guidance for the preparation of graphene-based aerogel beads.

3.1 Influencing factors on the formation of aerogel beads prepared by wet spinning

3.1.1 Concentration and viscosity of GO dispersion

The viscosity of the dispersion will significantly affect the formation of the aerogel spheres.When the viscosity is high, more solution will adhere to the nozzle, which will cause "tailing" phenomenon when dripping, and may even fail to form spheres[61,62]. When the viscosity is small, it is difficult for the droplets to maintain their spherical morphology after contacting the coagulation bath, so it is necessary to select the GO solution with appropriate viscosity. The viscosity of GO dispersion is mainly dependent on the concentration of GO. Zhang Liangzhong et al. Found that when the concentration of GO dispersion was in the range of 6 ~ 14 mg/mL and the viscosity was in the range of 210 ~ 350 mPa · S, uniform and regular graphene aerogel beads with good mechanical strength could be prepared[63]. When the viscosity is too high, it will be unable to form and irregular. Bao et al. Also pointed out that the concentration of GO should be maintained at 0.5 wt% ~ 2.0 wt% to prepare well-formed aerogel beads[11].
In addition, it should be pointed out that when GO is compounded with viscous liquids such as cellulose and sodium alginate, the concentration of GO should be reduced appropriately to maintain appropriate viscosity and promote molding[63].

3.1.2 Extrusion parameter

The extrusion parameters of wet spinning method include the size of nozzle channel, the extrusion speed of GO solution and the distance between the nozzle and the surface of coagulation bath, which are the key to the formation of graphene-based aerogel beads.
The size of the nozzle channel directly affects the size of the aerogel beads. Feng et al prepared graphene/sodium alginate aerogel beads using nozzles with different diameters of 0.3, 0.7 and 1.0 mm. It was found that within a certain range, the smaller the nozzle diameter, the more uniform the formation of aerogel beads, but when the nozzle diameter was too small (0.3 mm), it could not form a regular sphere[61]. When the nozzle diameter is larger, the spherical morphology is also affected, because the contact surface between the droplet and the air is larger, and the cohesion force that the droplet needs to overcome when falling increases, so that the mass of the droplet is larger and teardrop-like, and the sphericity is reduced. In the present study, the aerogel spheres with good formation can be obtained when the nozzle channel is between 0. 5 mm and 7 mm[11,61].
The extrusion speed is the key to achieve the spherical morphology, when the extrusion speed is within a certain range, it will form a more regular ball, when the extrusion speed exceeds this range, it may hit the ball dropped before, or even get a short rod or fiber structure, which is not conducive to the formation of the ball. Zhao et al. Prepared graphene aerogels with different shapes, including spherical, cylindrical and worm-like shapes, by adjusting the extrusion speed[15]. The suitable extrusion speed described in the literature is 1 ~ 16 mL/min, which should be adjusted according to the actual equipment and the properties of the solution[45,61,64].
The distance of the nozzle from the surface of the coagulating bath is also critical for shaping. Bao et al. Pointed out that the distance of 5 ~ 30 mm is better for forming, when the distance is larger, the potential energy of the droplet is larger, the falling speed is faster, and the moment of contact with the coagulation bath is impacted greatly, which makes the gel ball deformed and flat[11]. When the distance is small, the potential energy of the droplet is not enough to overcome the surface tension of the coagulation bath, which makes it difficult for the droplet to completely immerse in the coagulation bath, but to float on the surface of the coagulation bath and has a greater impact on the sphericity.

3.1.3 Coagulation bath type and concentration

In the preparation process of wet spinning, the type and concentration of coagulation bath is also a factor that can not be ignored for the formation of pellets. CaCl2, H2SO4 and polyethyleneimine polymer are commonly used in the coagulation bath for the preparation of graphene-based aerogel beads. Because GO contains abundant oxygen-containing functional groups and is negatively charged when dispersed in water, the positively charged components in the coagulation bath can be used to crosslink with the oxygen-containing functional groups in GO, thus promoting the gelation[15][65][43][66⇓~68][69,70][11]. Thus, acids, salts, and positively charged surfactants and polymers can be used as a coagulation bath to promote formation. Li et al. Used a mixture of deionized water, ethanol and calcium chloride as a coagulation bath, and covered the surface of the coagulation bath with dimethicone as a forming agent, resulting in more regular aerogel beads than in previous studies[12]. This is because the Ca2+ can cross-link with the oxygen-containing functional groups to promote gelation, and the strong hydrophobicity of dimethicone plays a greater role in assisting the formation of beads. In addition, the use of ethanol can reduce the surface tension of the coagulation bath, making it easier for the droplets to completely immerse in the coagulation bath.
In fact, the gelation process is accompanied by the diffusion of coagulant. Increasing the concentration of coagulation bath within a certain range can accelerate the internal gelation and shorten the time of complete gelation, but too high concentration may cause the volume contraction of GO spheres[11]. The suitable coagulation bath concentration is 0.5 wt% to 5 wt%[11,45,55,71].

3.2 Influencing factors on the formation of aerogel beads prepared by other method

In addition, for different preparation methods of graphene aerogel beads, there are also specific molding factors, for example, in addition to the above-mentioned GO dispersion concentration, extrusion speed and other factors,Parameters such as voltage in electrostatic spraying and electrospinning, microflow velocity, microchannel material and microchannel geometry in microfluidics are also important factors affecting the formation[9][41]. In addition, the drying method also has a certain influence on the formation of graphene-based aerogel beads. Supercritical drying and vacuum freeze drying can reduce the surface tension in the drying step, thus reducing the adverse effects of volume shrinkage on the formation of aerogel beads, while the aerogel beads prepared by atmospheric pressure drying are prone to volume shrinkage and structural cracking[51][54][11].

4 Pore size adjustment of graphene-based aerogel bead

The application fields of graphene-based aerogel beads are different, and the required pore size is also different, so how to effectively control the pore size is of great significance for the performance optimization of the material. For example, when the graphene-based aerogel beads are used for adsorbing leaked oil products on the sea, the larger pore diameter has small diffusion resistance and can quickly react to the leaked oil products, and at this time, the oil products can be quickly filled into the pores of the aerogel beads to complete the adsorption process; When graphene-based aerogel beads are used to adsorb organic dyes and heavy metal ions in wastewater, smaller pore size can obtain larger adsorption capacity, but the diffusion resistance of adsorption will increase accordingly, resulting in lower adsorption rate, so it is necessary to select the appropriate pore size to achieve the balance between adsorption capacity and adsorption rate[16]. At present, there are few studies on controlling the pore size of graphene-based aerogel beads, and the following factors have great potential in pore size control.

4.1 GO concentration

In the study of preparation of graphene-based aerogel beads, it is found that the concentration of GO has a great influence on the pore size. Liao et al. prepared graphene oxide aerogel beads (GOAMs) by electrospraying combined with freeze-drying technology. As shown in Figure 4, the corresponding pore size decreased with the increase of GO concentration[9]. Compared with 6 mg/mL, the pore structure of GOAMs prepared with 4 mg/mL GO was looser and the pore size was larger. Li et al. Obtained the same conclusion when preparing graphene aerogel beads. With the increase of GO concentration from 8 mg/mL to 16 mg/mL, the average pore size gradually decreased from 14.3 nm to 12.9 nm[12]. Due to the higher concentration of GO, more lamellae are stacked and the pore structure is arranged more closely. Therefore, it is feasible to control the pore size of the graphene-based aerogel beads by changing the concentration of GO, but it is difficult to achieve a wide range of pore size control due to the limited concentration of GO suitable for spheroidization.

4.2 Freezing temperature

Graphene-based hydrogel beads usually undergo a freeze-casting process before drying, and the freezing temperature affects the rate of ice crystal growth and nucleation, thereby affecting the pore size of the aerogel beads, and the lower the freezing temperature, the smaller the pore size. Zhu et al. placed the prepared cylindrical graphene hydrogel (GH) on a metal tray, and controlled the freezing temperature by adjusting the distance between the metal tray and liquid nitrogen. The temperature was set at -10, -40, -70, -100 and -196 ℃, and the graphene aerogel (GA) was obtained after vacuum freeze-drying[72]. As shown in fig. 5, as the freezing temperature decreases from -10 ° C to -196 ° C, the average pore size of GA decreases from 240 μm to 6 μm, and the graphene lamellae are arranged more closely. Jung et al. Also froze graphene hydrogels at different freezing temperatures and dried them into graphene aerogels[73]. When the freezing temperature increases from -200 ℃, -80 ℃ to -20 ℃, the pore size increases gradually. The pore size ranges from 2 to 100 nm at-200 ℃, and the maximum pore size can reach 60 μm at-20 ℃.
Because the size of ice crystal can directly reflect the size of aerogel pore, the lower the freezing temperature is, the faster the growth rate of ice crystal is, which leads to the smaller size of ice crystal, and the opposite is true when the freezing temperature is increased. By changing the freezing temperature, the pore size of aerogels can be controlled in a wide range. At present, the research on using this method to control the pore size focuses on block aerogels, but it also has great potential in the aspect of pore size control of aerogel beads.

4.3 GO slice size

In the process of self-assembly, the GO layers are connected and stacked to form the pore walls of the graphene aerogel beads, and the pore size is closely related to the size of the GO layers. By changing the experimental conditions of the GO exfoliation process, i.e., mechanical oscillation and ultrasonic treatment with different durations (5 min, 1 H), the size of GO lamellae was reduced in turn. The GO suspensions with different lamellae sizes (L-GOs, M-GOs, S-GOs) were subjected to hydrothermal reduction and freeze-drying to prepare graphene aerogels (L-GF, M-GF, S-GF). It was found that the pore diameters of L-GF, M-GF and S-GF were 10 μm, 2 ~ 5 μm and 500 nm, respectively[74]. The transverse size of GO lamellae is close to the pore size of aerogels, and the more severe the experimental conditions during the delamination process, the smaller the corresponding pore size of aerogels. Therefore, the GO sheet size plays a decisive role in the pore size regulation of graphene aerogels, and has great application potential in accurately controlling the pore size of porous graphene-based aerogel beads.

4.4 Homogenizer speed

In the study of bulk aerogels, our group used Pickering emulsion soft template method to prepare graphene-based aerogels, and studied the feasibility of adjusting the pore size of graphene aerogels by homogenizer speed[5,75,76].
For example, Diao Shuai et al. Prepared an oil-in-water Pickering emulsion with GO solution as the water phase and cyclohexane as the oil phase, and then prepared graphene aerogel by hydrothermal reduction, replacement of the oil phase in the pores, and freeze-drying[75]. When the stirring speed of high speed homogenizer was 10 000,12 000 and 15 000 R/min, the average pore size of graphene aerogel was 45,35 and 30 μm, respectively, which was similar to the particle size of emulsion (43. 46,34.84 and 28. 99 μm). This is because the emulsion droplets can be filled in the pores of graphene aerogel to play a supporting role, and the size of the emulsion droplets can reflect the size of the pores. With the increase of the rotation speed of the homogenizer, the size of the droplets obtained by shearing the emulsion becomes smaller, and the pore size of the graphene aerogel also decreases.
Therefore, in the Pickering emulsion soft template method, the pore size of the graphene aerogel can be precisely controlled by adjusting the rotating speed of the homogenizer. At present, the preparation of graphene aerogel by this method is mainly completed by freeze-drying, which has high energy consumption and long production cycle.Using the support of Pickering emulsion droplets, combined with atmospheric pressure drying to prepare graphene-based aerogel beads with controllable pore size and excellent performance will be the direction of future exploration.

5 Conclusion and prospect

At present, there are various methods to prepare graphene-based aerogel beads, mainly including electrostatic spraying, electrostatic spinning, microfluidic method and wet spinning method, and the particle size of graphene-based aerogel beads can be controlled by adjusting the corresponding parameters. The properties of the dispersion, the extrusion parameters, the type and concentration of the coagulation bath and the drying method are all the influencing factors. The pore size can be effectively controlled by controlling the concentration of GO, the freezing temperature, the rotating speed of the homogenizer and the size of the GO sheet.
According to the different treatment objects in wastewater, the design of graphene-based aerogel beads with excellent adsorption performance, recycling performance and controllable morphology, and the optimization and innovation of preparation methods are still the focus of future exploration. While ensuring the performance, further simplifying the preparation process can provide the possibility for industrial production and application, such as atmospheric pressure drying, which has a good application prospect.However, the volume shrinkage and cracking of graphene-based aerogel beads prepared by ambient pressure drying are still unavoidable, which reduce the adsorption capacity and even limit its application, and solving these problems is still the key.

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Funding

Natural Science Foundation of Shandong Province(ZR2017MB015)

National Natural Science Foundation of China(22078366)

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