Design and Fabrication of Aerogel Composites for Oil Water Separation and Spilled Oil Cleaning

: Aerogel composites are multicomponent highly porous materials with air as the major dispersed phase. There are many kinds of aerogel composites including synthetic and natural aerogel composites. Aerogel composites have found wide applications in sorption, thermal insulation, vibration damping, and noise control. This mini review focuses on the aerogel composites with oil water separation function for spilled oil cleaning. The design and fabrication of various aerogel composites for oil cleaning are emphasized. The commonly used technologies including in-situ reaction, sol–gel spinning, templating, and self-assembling are introduced. The microstructure control through directional freeze casting, bio-inspired approach, coating, etc., are discussed.


Introduction
Aerogel composites are unique in view of their light weight, high porosity, and low thermal conductivity. As a result, they have found wide applications for chemical sorption, energy storage, catalysis, sensing, and thermal insulation. Numerous composite aerogels have been developed. There are two typical categories of aerogel composites for oil-water separation, including synthetic polymer-based and sustainable source-based aerogel composites. Among various synthetic aerogel composites, polyacrylonitrile (PAN) nanofiber with functional polymers, polyvinyl alcohol (PVA), and carbon nanotube (CNT) showed self-cleaning and oil/water separation capabilities [1]. Due to the hydrophobicity, polyurethane (PU) was used to make oil sorption foam for a long time, but the relatively low oil uptake, lack of mechanical strength and poor recyclability limit the application of the polymer-based aerogel for spilled oil cleaning [2]. Polyurethane (PU) and polyvinylidene fluoride (PVDF) composite aerogel showed high absorption capability and good recyclability [3]. Three-dimensional silica [4], graphene-added polystyrene [5], reduced graphene in porous silica [6], coated nickel mesh [7], graphene oxide added carbon nanotube [8], boron nitride and graphene [9], and carbon nitride and oxide graphene added PVDF [10] are some other types of synthesized composite aerogels. In the second category of aerogel composites made from sustainable sources, there are alginate composites [11][12][13], and composites containing agar [14], chitosan [15][16][17][18], cellulose [19][20][21][22][23][24][25] as high absorbents. Many other biomass-based aerogel composites were reviewed in [26]. Several aerogel composites can be produced directly from natural sources. For example, chitosan [27], lignin [28], cotton [29][30][31], pollen grains [32] and silk fibroin [33,34] are some of the naturally derived sources. Recycling fabric from car tire waste for aerogel composite manufacturing was also reported [35]. Aerogel composites are commonly produced from the freeze-drying process. The raw materials for the nanofiber aerogel composites may be prepared through spinning [36]. Post-heat treatment, for example, carbonization [37], is often used to change the compositions of aerogel composites. To improve the surface properties, coatings are generally applied [38].
Oil spilling releases petroleum hydrocarbons into ocean and on land, which causes serious pollution. It could result in disastrous consequences both environmentally and economically. Protecting the ocean, coastline, and land is the driving force for researching the oil/water separation technologies. Previous research has successfully generated relatively low-cost, new aerogel composite materials exhibiting excellent performances on absorbing oil and oil/water separation as will be discussed in greater detail in one of the sections (Section 4) in this review.
This mini review highlights recent progress in design, fabrication, and oil/water separation application of aerogel composites. First, several important manufacturing technologies and structure control of aerogel composites will be introduced. Second, the microstructures of various aerogel composites will be presented. In the third part, the oil/water separation efficiencies of aerogel products will be compared. Lastly, perspectives and concluding remarks will be given.

Hydrothermal Deposition on Porous Template
Hydrothermal synthesis has been considered as one of the most used methods for nanomaterial preparation [39]. Zhang et al. [7] applied the hydrothermal approach followed by hydrogel modification to produce a composite coating deposited on a nickel mesh. Sodium carboxymethyl cellulose (CMC-Na), K4Fe(CN)6‧3H2O, and nickel mesh were used as the starting materials. Other chemicals including ferric chloride hexahydrate and HCl were also used. During the in-situ growth of Prussian blue analogue (PBA) at the surface of Ni mesh (PBA@Ni mesh), hydrothermal method was used as shown in Figure  1 reproduced from [7]. The cleaned Ni mesh was placed into a Teflon-lined stainless-steel autoclave filled with a solution of K4Fe(CN)6‧3H2O and HCl. The hydrothermal reaction conditions were set at 95 °C for 5 h. Both nickel etching and in-situ deposition of PBA occurred to generate a rough nickel surface covered with the PBA. Next, the Fe(III)-CMC@PBA@Ni mesh composite was produced by immersing the PBA@Ni mesh into a solution containing 0.1 wt% FeCl3‧6H2O and 0.1 wt% CMC-Na solution, respectively. After the process being repeated for multiple times, the composite mesh was air-dried at 45 °C for 3 min to form a hydrophilic Fe(III)-CMC hydrogel layer. The final product, Fe(III)-CMC@PBA@Ni mesh, showed an aerogel composite structure as can be seen from the scanning electron microscopic (SEM) image at the lower right corner of Figure 1 [7].

Hydrogen Bond Induced Self-Assembling
Shi et al. [40] introduced hydrogen bonding self-assembling to produce materials for aerogel composites. Spherical beads consisting of the partially reduced graphene oxide (prGO) and titanium carbide, MXene (Ti 3 C 2 T x ) nanosheets, were constructed by hydrogen bond induced self-assembling. The titanium carbide (Ti 3 C 2 T x MXene) is a novel 2D nanosheet material which can be obtained by selective removal of atoms from the aluminum layers in Ti 3 AlC 2 . Practically, it can be performed by etching Ti 3 AlC 2 in HCl and LiF. The Ti 3 C 2 T x MXene compound has large specific surface areas and high electrical conductivity comparable with those of metals. Surface functional groups are ready to be added onto Ti 3 C 2 T x . Therefore, it has been recognized as an effective oil/water separation material for environment cleaning. Graphene oxide (GO) can be easily attached to the hydroxyl groups hanging on the surface of MXene through the abundant hydrogen bonds so that 3D structures can be generated.
The prGO/MXene spherical beads were prepared by a one-step hydrogen bondinduced self-assembly method [40]. Briefly, the GO and MXene suspensions were mixed at various GO/MXene mass ratios. The mixtures were ultrasonically agitated to form homogeneous GO/MXene dispersions. The GO/MXene dispersions were degassed and then poured into the coagulation bath containing 1 wt% cetyltrimethylammonium bromide (CTAB) surfactant to generate the GO/MXene beads. The obtained wet beads were freezedried to form aerogel composites. The procedures for making the aerogel composites are schematically illustrated in Figure 2 [40].

Hydrogen Bond Induced Self-Assembling
Shi et al. [40] introduced hydrogen bonding self-assembling to produce materials for aerogel composites. Spherical beads consisting of the partially reduced graphene oxide (prGO) and titanium carbide, MXene (Ti3C2Tx) nanosheets, were constructed by hydrogen bond induced self-assembling. The titanium carbide (Ti3C2Tx MXene) is a novel 2D nanosheet material which can be obtained by selective removal of atoms from the aluminum layers in Ti3AlC2. Practically, it can be performed by etching Ti3AlC2 in HCl and LiF. The Ti3C2Tx MXene compound has large specific surface areas and high electrical conductivity comparable with those of metals. Surface functional groups are ready to be added onto Ti3C2Tx. Therefore, it has been recognized as an effective oil/water separation material for environment cleaning. Graphene oxide (GO) can be easily attached to the hydroxyl groups hanging on the surface of MXene through the abundant hydrogen bonds so that 3D structures can be generated.
The prGO/MXene spherical beads were prepared by a one-step hydrogen bond-induced self-assembly method [40]. Briefly, the GO and MXene suspensions were mixed at various GO/MXene mass ratios. The mixtures were ultrasonically agitated to form homogeneous GO/MXene dispersions. The GO/MXene dispersions were degassed and then poured into the coagulation bath containing 1 wt% cetyltrimethylammonium bromide (CTAB) surfactant to generate the GO/MXene beads. The obtained wet beads were freezedried to form aerogel composites. The procedures for making the aerogel composites are schematically illustrated in Figure 2 [40]. Schematic showing the hydrogen bond-induced self-assembling of beads for oil/water separation aerogel composite preparation. CTAB stands for the cetyltrimethylammonium bromide surfactant. Reproduced with permission from [40], ©2021 Elsevier B.V.

Gas Foaming
This technique uses gas as the foaming agent. During the foaming process, a 2D nanofiber network or mat can be converted into a 3D porous aerogel. In the work performed by Jin et al. [41], polyacrylonitrile (PAN) and itaconic acid (IT) (at a ratio of 99:1) were dissolved into dimethylformamide (DMF) with a concentration of 10 wt%. The solution was electrospun as shown in Figure 3a. The spun nanofibers were in a 2D mat form as sketched in Figure 3b. The 3D foam as illustrated in Figure 3c was fabricated from the Schematic showing the hydrogen bond-induced self-assembling of beads for oil/water separation aerogel composite preparation. CTAB stands for the cetyltrimethylammonium bromide surfactant. Reproduced with permission from [40], ©2021 Elsevier B.V.

Gas Foaming
This technique uses gas as the foaming agent. During the foaming process, a 2D nanofiber network or mat can be converted into a 3D porous aerogel. In the work performed by Jin et al. [41], polyacrylonitrile (PAN) and itaconic acid (IT) (at a ratio of 99:1) were dissolved into dimethylformamide (DMF) with a concentration of 10 wt%. The solution was electrospun as shown in Figure 3a. The spun nanofibers were in a 2D mat form as sketched in Figure 3b. The 3D foam as illustrated in Figure 3c was fabricated from the electrospun 2D mat in a gas foaming process using 1.0 M sodium borohydride solution as the gas generating agent. The electrospun 2D mat was immersed into the sodium borohydride solution at room temperature for various periods of time (from 5 min to 4 h). Hydrogen gas generated by the hydrolysis of the sodium borohydride caused the 2D fiber mat being converted into 3D foam. The as-formed 3D foam was washed thoroughly with distilled water and frozen to −20 • C followed by freeze drying for 20 h to produce a cylindrical 3D PANIA foam. Following that, the foam was dip-coated with graphene oxide (GO). The GO coated sample was cooled down to −20 • C again followed by freeze drying for 20 h to obtain the foam as shown in Figure 3d. The GO-coated 3D PANIA foam was stabilized in air at 250 • C for 30 min and then carbonized in Ar at 800 • C for 30 min to generate the 3D graphene-carbon nanofiber (G-CNF) foam as schematically shown in Figure 3e [41]. electrospun 2D mat in a gas foaming process using 1.0 M sodium borohydride solution as the gas generating agent. The electrospun 2D mat was immersed into the sodium borohydride solution at room temperature for various periods of time (from 5 min to 4 h). Hydrogen gas generated by the hydrolysis of the sodium borohydride caused the 2D fiber mat being converted into 3D foam. The as-formed 3D foam was washed thoroughly with distilled water and frozen to −20 °C followed by freeze drying for 20 h to produce a cylindrical 3D PANIA foam. Following that, the foam was dip-coated with graphene oxide (GO). The GO coated sample was cooled down to −20 °C again followed by freeze drying for 20 h to obtain the foam as shown in Figure 3d. The GO-coated 3D PANIA foam was stabilized in air at 250 °C for 30 min and then carbonized in Ar at 800 °C for 30 min to generate the 3D graphene-carbon nanofiber (G-CNF) foam as schematically shown in Figure 3e [41].

Vacuum-Assisted Resin Transfer Molding and Particle Leaching
In [42], Zeng et al. prepared polylactic acid (PLA)-based aerogels using the technique involving procedures such as vacuum-assisted resin transfer molding (VARTM), phase separation, solvent extraction, and particle leaching. A chitosan (CS)-coated PLA scaffold was obtained by infiltrating the polymer chloroform solution into a porous preform made from compacted NaCl particles. Here, NaCl served as the porogen. To control the pore structure and porosity of the aerogel composite, the sizes of the NaCl particles and the concentrations of the PLA/chloroform solutions were changed. The physiochemical properties of PLA and CS-coated PLA aerogels were tested. Both PLA and CS-coated PLA showed the porosities of up to 94%. The highest water uptake of the aerogels reached 1200%. Hierarchical open pores with high interconnectivity were observed. The CS-coated PLA aerogel composite possesses mechanical properties and enhanced hydrophilicity compared to those of the pure PLA aerogel [42].
Using the same technique of vacuum infiltration followed by particle leaching, Cui et al. [3] prepared polyvinylidene difluoride/thermoplastic polyurethane (PVDF/TPU) aerogel composites with superoleophilic/hydrophobicity. Three-dimensional porous foam aerogel composites were found very efficient in separation of oils and organics from water. The space holder, NaCl, with various particle sizes was used. The PVDF powder and TPU particles were dissolved in warm 1, 4-dioxane by stirring to obtain an 8% PVDF/TPU/dioxane solution. Then, a small amount of deionized water was added

Vacuum-Assisted Resin Transfer Molding and Particle Leaching
In [42], Zeng et al. prepared polylactic acid (PLA)-based aerogels using the technique involving procedures such as vacuum-assisted resin transfer molding (VARTM), phase separation, solvent extraction, and particle leaching. A chitosan (CS)-coated PLA scaffold was obtained by infiltrating the polymer chloroform solution into a porous preform made from compacted NaCl particles. Here, NaCl served as the porogen. To control the pore structure and porosity of the aerogel composite, the sizes of the NaCl particles and the concentrations of the PLA/chloroform solutions were changed. The physiochemical properties of PLA and CS-coated PLA aerogels were tested. Both PLA and CS-coated PLA showed the porosities of up to 94%. The highest water uptake of the aerogels reached 1200%. Hierarchical open pores with high interconnectivity were observed. The CS-coated PLA aerogel composite possesses mechanical properties and enhanced hydrophilicity compared to those of the pure PLA aerogel [42].
Using the same technique of vacuum infiltration followed by particle leaching, Cui et al. [3] prepared polyvinylidene difluoride/thermoplastic polyurethane (PVDF/TPU) aerogel composites with superoleophilic/hydrophobicity. Three-dimensional porous foam aerogel composites were found very efficient in separation of oils and organics from water. The space holder, NaCl, with various particle sizes was used. The PVDF powder and TPU particles were dissolved in warm 1, 4-dioxane by stirring to obtain an 8% PVDF/TPU/dioxane solution. Then, a small amount of deionized water was added dropwise into the solution. Afterward, the NaCl particles with four size ranges of 0-150 µm, 150-212 µm, 212-300 µm, and 300-425 µm were poured into a plastic mold and compressed into the compact preforms. Then, the PVDF/TPU/dioxane/water mixture solution was infiltrated into the NaCl preform under the assistance of vacuum. After the infiltration, the prepared samples were quickly transferred to a refrigerator and kept overnight at −80 • C. After that, the frozen blocks were immersed into ethanol for 24 h to remove the solvent (1, 4-dioxane). The prepared PVDF/TPU/NaCl specimens were immersed into deionized water for 8 h to remove the NaCl porogen. Finally, the samples underwent freeze drying for 12 h to obtain the 3D porous PVDF/TPU aerogel composites [3].
The scanning electron microscopic (SEM) images of the aerogel composites are shown in Figure 4. All the images reveal the 3D hierarchical porous structures. As shown in Figure 4a, the composite produced by using the NaCl particles with a size range from 300 to 425 µm has the macroscale pore sizes close to that of the particle (at the order of 400 µm). In Figure 4b, the structure shows the macroscale pore sizes at the order of 300 µm. In Figure 4c, the size of macroscale pores decreased to the order of 200 µm. The image in Figure 4d shows the size of macroscale pores of around 150 µm. dropwise into the solution. Afterward, the NaCl particles with four size ranges of 0-150 µm, 150-212 µm, 212-300 µm, and 300-425 µm were poured into a plastic mold and compressed into the compact preforms. Then, the PVDF/TPU/dioxane/water mixture solution was infiltrated into the NaCl preform under the assistance of vacuum. After the infiltration, the prepared samples were quickly transferred to a refrigerator and kept overnight at −80 °C. After that, the frozen blocks were immersed into ethanol for 24 h to remove the solvent (1, 4-dioxane). The prepared PVDF/TPU/NaCl specimens were immersed into deionized water for 8 h to remove the NaCl porogen. Finally, the samples underwent freeze drying for 12 h to obtain the 3D porous PVDF/TPU aerogel composites [3]. The scanning electron microscopic (SEM) images of the aerogel composites are shown in Figure 4. All the images reveal the 3D hierarchical porous structures. As shown in Figure 4a, the composite produced by using the NaCl particles with a size range from 300 to 425 µm has the macroscale pore sizes close to that of the particle (at the order of 400 µm). In Figure 4b, the structure shows the macroscale pore sizes at the order of 300 µm. In Figure 4c, the size of macroscale pores decreased to the order of 200 µm. The image in Figure 4d shows the size of macroscale pores of around 150 µm.

In-situ Growth
Yuan et al. [43] reported the in-situ growth technique for making hierarchical hollow SiO2@MnO2 cubes. These cubes are the materials for making elastic polyurethane (PU) aerogel composites. The aerogel composites were found very effective in removing oil from water. The incorporation of hierarchical hollow SiO2@MnO2 cubes into the PU could change the structure by increasing the surface roughness and specific surface areas of the aerogel composites. Therefore, the hierarchical hollow SiO2@MnO2 cubes can also

In-situ Growth
Yuan et al. [43] reported the in-situ growth technique for making hierarchical hollow SiO 2 @MnO 2 cubes. These cubes are the materials for making elastic polyurethane (PU) aerogel composites. The aerogel composites were found very effective in removing oil from water. The incorporation of hierarchical hollow SiO 2 @MnO 2 cubes into the PU could change the structure by increasing the surface roughness and specific surface areas of the aerogel composites. Therefore, the hierarchical hollow SiO 2 @MnO 2 cubes can also improve the hydrophobic properties and enhance the adsorption capacity [44]. The mechanical properties of the aerogel composites can be improved as well.
To produce the hierarchical hollow SiO 2 @MnO 2 cubes, a sacrificial iron oxide hard template was synthesized first. Then, silica layer grew in-situ onto the iron oxide to form cubes. After that, the manganese oxide layer was deposited. Lastly, hydrophobic treatment of hierarchical hollow SiO 2 @MnO 2 cubes was conducted [43]. During the synthesis of the cube-shaped iron oxide (hematite) colloidal particles, a mixture of 6.0 M NaOH solution (90 mL) and 2.0 M well-stirred FeCl 3 ·6H 2 O solution (100 mL) were poured into a glass bottle. After stirring for 15 min, a suspension containing Fe(OH) 3 gel formed. The gel was transferred to a Teflon-lined stainless-steel autoclave to be treated at 100 • C for 8 days.
After the aging treatment, hematite colloidal (Fe 2 O 3 ) cubes were generated. The cubes were collected by filtration.
Following the preparation of hematite colloidal (Fe 2 O 3 ) cubes, the process for depositing SiO 2 layer on the iron oxide cubes was shown [43]. In a solution containing tetraethoxysilane (TEOS) and NH 3 ·H 2 O, the Fe 2 O 3 cubes were coated with silica. The silica-coated cubes were obtained upon the completion of the coating process. Then, the iron oxide cubes as sacrificial templates were removed through the etching in a 4.0 M HCl solution (20 mL) at 100 • C for 24 h. This generated the hollow SiO 2 cubes.
The hierarchical hollow SiO 2 @MnO 2 cubes were produced in a hydrothermal process using the previously fabricated hollow SiO 2 cubes as hard templates [43,44]. The hollow SiO 2 cubes were ultrasonically dispersed into water. Then, KMnO 4 was added to form a suspension. This suspension was sealed in a Teflon-lined stainless steel autoclave and heat treated at 150 • C for 48 h. The produced hierarchical hollow SiO 2 @MnO 2 cubes were collected by centrifugation, and washed multiple times using ethanol and deionized water. To improve the hydrophobicity of the hierarchical hollow SiO 2 @MnO 2 cubes, the hydrophobic modification was made on the surface of hierarchical hollow cubes, and the cubes were treated with KH570, a methacryl-functional silane. Briefly, the hierarchical hollow SiO 2 @MnO 2 cubes and KH570 were added into a flask. At pH = 3-4, the aerogel composites were added with ethanol. Ultrasonic dispersing for 15 min was carried out. Continued magnetic stirring was conducted for 2 h. The composites were kept in microwave to maintain 80 • C temperature at the same time. The obtained hydrophobic hierarchical hollow SiO 2 @MnO 2 cubes were filtrated, washed three times with ethanol and deionized water, and collected for further processing polyurethane aerogel composites [43]. Both scanning electron microscopic (SEM) and transmission electron microscopic (TEM) examinations on the hierarchical hollow SiO 2 and SiO 2 @MnO 2 cubes were performed, and the imaging results can be found in Figure 5. Figure 5A shows the hollow SiO 2 morphology under SEM. The solid crust of silica can barely be seen. Figure 5B reveals the hierarchical hollow SiO 2 @MnO 2 structure under SEM. Ligaments from the deposition of manganese oxide are observed. Figure 5C is a transmission electroscopic (TEM) image revealing the hierarchical hollow SiO 2 @MnO 2 at a relatively low magnification. Fluffy feature of the manganese oxide can be detected. To show the hierarchical feature more clearly, Figure 5D illustrates the hierarchical hollow SiO 2 @MnO 2 at a higher magnification than the previous TEM image of Figure 5C [43]. Transparent nanosheets of MnO 2 can be seen at the outer layer of the cubes.

Vapor Induced Phase Inversion
The vapor-induced phase inversion process involves the exchange of solvents under certain conditions. As shown in Figure 6a [45], the dimethyl sulfoxide (DMSO) solvent in PVDF/DMSO solution was exchanged with water. The photos in Figure 6b illustrate the PVDF aerogel formation process. In a typical experiment, PVDF was added into dimethyl sulfoxide (DMSO) solvent to produce a 10% transparent solution. The solution was placed into a sealed container partially filled with deionized (DI) water. The water vapor induced phase inversion allowed the formation of a PVDF gel. The water containing PVDF gel was frozen. Finally, freeze-drying generated PVDF aerogel. Although this technique was demonstrated using pure PVDF polymer, it is suitable for composite aerogel preparation.

Vapor Induced Phase Inversion
The vapor-induced phase inversion process involves the exchange of solvents unde certain conditions. As shown in Figure 6a [45], the dimethyl sulfoxide (DMSO) solvent in PVDF/DMSO solution was exchanged with water. The photos in Figure 6b illustrate the PVDF aerogel formation process. In a typical experiment, PVDF was added into dimethy sulfoxide (DMSO) solvent to produce a 10% transparent solution. The solution was placed into a sealed container partially filled with deionized (DI) water. The water vapor induced phase inversion allowed the formation of a PVDF gel. The water containing PVDF gel was frozen. Finally, freeze-drying generated PVDF aerogel. Although this technique was demonstrated using pure PVDF polymer, it is suitable for composite aerogel preparation

Vapor Induced Phase Inversion
The vapor-induced phase inversion process involves the exchange of solvents under certain conditions. As shown in Figure 6a [45], the dimethyl sulfoxide (DMSO) solvent in PVDF/DMSO solution was exchanged with water. The photos in Figure 6b illustrate the PVDF aerogel formation process. In a typical experiment, PVDF was added into dimethyl sulfoxide (DMSO) solvent to produce a 10% transparent solution. The solution was placed into a sealed container partially filled with deionized (DI) water. The water vapor induced phase inversion allowed the formation of a PVDF gel. The water containing PVDF gel was frozen. Finally, freeze-drying generated PVDF aerogel. Although this technique was demonstrated using pure PVDF polymer, it is suitable for composite aerogel preparation.

Directional Freeze Cast Oriented Structures
Directional freezing followed by drying induced structures are shown by a series of SEM images in Figure 7 [13]. The high biocompatibility, low cost, and hydrophobic composite aerogels produced through the directional freezing-drying technology contain chitosan (CS) as the continued phase, the reduced graphene oxide nanosheets (rGO) as the reinforcers and the hydrophobic silica particles/polydimethylsioxane (H-SiO 2 /PDMS) as the surface modifiers. The images in Figure 7a reveal the unidirectional structure of a pure chitosan (CS) aerogel at different magnifications. The elongated cell wall morphology indicates the solidification direction of the solvent (water). SEM images in Figure 7b show the structure of a chitosan (CS) aerogel composite reinforced by the reduced graphene oxide (rGO) at different magnifications. It seems that the addition of rGO in CS hindered the unidirectional cell wall structure formation. In Figure 7c, the three SEM images at different magnifications demonstrate the cell wall structures with varied orientations. Obviously, the enhanced hydrophobicity from adding H-SiO 2 /PDMS into the CS/rGO aerogel composite assisted the organization of the cell wall structural domains. In a typical experiment on making such aerogel composites, chitosan (CS) was dissolved in diluted acetic acid solution and stirred for 4 h to obtain a transparent solution with a CS concentration of 2 wt%. Graphene oxide (GO) was dispersed into deionized water at a concentration of 4.0 mg·mL −1 by ultrasonication treatment. The CS solution was mixed with the GO water dispersion at different mass ratios of 10:0, 10:0.5, 10:1, 10:2, 10:3 under continuous magnetic stirring for 12 h. Following that, ultrasonication for 1 h was performed to generate a homogeneous mixture solution. The mixture was transported into a Teflon mode with a diameter of 25 mm and a thickness of 5 mm. Freezing from bottom to top was achieved by placing the Teflon mold on a copper part dipped in liquid nitrogen. The frozen sample was freeze-dried for 72 h to obtain the elongated cell wall structured aerogel. The obtained GO/CS aerogel was fully immersed into the suspension of PDMS and H-SiO 2 . After extracting the excess solvent with ethanol, the specimen was reduced to generate the rGO/CS-Si aerogel composite with the honeycomb morphology as shown in Figure 7c [45].
Yi et al. [17] produced a chitosan (CS)-methyltrimethoxysilane (MTMS) aerogel composite by the unidirectional freezing method. The advantages of the aerogel composite for oil removal were shown as well. Due to the directional growth of ice crystals along the vertical direction as shown in the left-hand side of Figure 8, a special elongated honeycomb structure with the spring-like morphology was obtained, which can be seen from the SEM image presented in the upper middle section of Figure 8. The cross section of the aerogel composite along the longitudinal direction is featured by well-aligned, low-tortuosity through-channels as revealed by the SEM image located in the lower middle section of Figure 8. Such a special porous structure contributed to the superior mechanical properties of the aerogel composite. This structure is highly durable for large mechanical compression and fast shape recovery, which allows the oil to be squeezed out for recycling as shown by the photos in the upper right part of Figure 8. It was reported that the silylated-CS aerogel composite has a high oil absorption capacity of 63 g/g, along with the excellent recyclability via simple hand squeezing [17]. The unidirectional move in the through-channels provides fast oil transport in the aerogel composite as illustrated by the photos in the lower right part of Figure 8. This allows the CS-MTMS aerogel composite to be installed on portable facilities for fast absorption of oil from water. s. Sci. 2023, 7, x FOR PEER REVIEW 9 of 19
Yi et al. [17] produced a chitosan (CS)-methyltrimethoxysilane (MTMS) aerogel composite by the unidirectional freezing method. The advantages of the aerogel composite for oil removal were shown as well. Due to the directional growth of ice crystals along the vertical direction as shown in the left-hand side of Figure 8, a special elongated honeycomb structure with the spring-like morphology was obtained, which can be seen from the SEM image presented in the upper middle section of Figure 8. The cross section of the aerogel composite along the longitudinal direction is featured by well-aligned, low-tortuosity through-channels as revealed by the SEM image located in the lower middle section of Figure 8. Such a special porous structure contributed to the superior mechanical properties of the aerogel composite. This structure is highly durable for large mechanical compression and fast shape recovery, which allows the oil to be squeezed out for recycling as shown by the photos in the upper right part of Figure 8. It was reported that the silylated-CS aerogel composite has a high oil absorption capacity of 63 g/g, along with the excellent recyclability via simple hand squeezing [17]. The unidirectional move in the throughchannels provides fast oil transport in the aerogel composite as illustrated by the photos in the lower right part of Figure 8. This allows the CS-MTMS aerogel composite to be installed on portable facilities for fast absorption of oil from water.  Recent development of directional freezing has generated new approaches. Bidirectional freezing technique as shown in [16] represents one of such progresses. A bidirectional freeze-casting square mold with a right-angle copper part and the rest of polystyrene foam was designed for generating dual temperature gradients, for example, "bottom to top" (along z-direction) and "left to right" (along x-direction) as shown in Figure 9a. The copper surface of the cubic mold was placed on a steel plate in contact with the cold source (liquid nitrogen or dry ice ethanol bath). Due to the significant difference in the thermal conductivities between the polystyrene foam and copper, bidirectional freezing was achieved. The device was used to fabricate the aerogel composite consisting of the Recent development of directional freezing has generated new approaches. Bidirectional freezing technique as shown in [16] represents one of such progresses. A bidirectional freeze-casting square mold with a right-angle copper part and the rest of polystyrene foam was designed for generating dual temperature gradients, for example, "bottom to top" (along z-direction) and "left to right" (along x-direction) as shown in Figure 9a. The copper surface of the cubic mold was placed on a steel plate in contact with the cold source (liquid nitrogen or dry ice ethanol bath). Due to the significant difference in the thermal conductivities between the polystyrene foam and copper, bidirectional freezing was achieved. The device was used to fabricate the aerogel composite consisting of the elastic chitosan coated with a nonflammable and hydrophobic silane coating made of methyltrichlorosilane (MTCS) by chemical vapor deposition (CVD). The highly oriented structures along the dual temperature gradients, as shown by the two SEM images in Figure 9b, were obtained using the bidirectional freeze casting method. Well-aligned lamellae along both x-and z-directions are clearly shown in the SEM images. The pores are in the honeycomb shape consisting of numerous through-channels. These through-channels run all the way from the bottom to the top (along z-direction) or from the left to the right (along x-direction), which facilitates fast oil transport within the aerogel and results in the rapid absorption of oil by the composite. For comparison, the unidirectional freeze casting and ordinary freeze casting set-ups for aerogel composite preparation and resulted aerogel structures were also illustrated by Cao et al. [16]. Figure 9c schematically shows the unidirectional freeze casting to produce aerogel composites. The SEM images in Figure 9d

Stratified Structure
The work performed by Shi et al. [40] introduced the aerogel composites with stratified structures formed from partially reduced graphene oxide (prGO) and MXene (Ti3C2Tx) nanosheets. The aerogel composites are named after the major components. The pure prGO aerogel is known as prGSBDs, while the prGO and MXene aerogel composites are known as prGMSBDs. The prGO to MXene mass ratios of 20:1, 10:1, and 5:1 were adopted from the conditions for processing the aerogel composites, and the obtained aerogel composites are named as prGMSBDs-20, prGMSBDs-10, and prGMSBDs-5, respectively. Both prGSBDs and prGMBDS aerogels were observed by SEM. As shown in Figure   Figure 9.

Stratified Structure
The work performed by Shi et al. [40] introduced the aerogel composites with stratified structures formed from partially reduced graphene oxide (prGO) and MXene (Ti 3 C 2 T x ) nanosheets. The aerogel composites are named after the major components. The pure prGO aerogel is known as prGSBDs, while the prGO and MXene aerogel composites are known as prGMSBDs. The prGO to MXene mass ratios of 20:1, 10:1, and 5:1 were adopted from the conditions for processing the aerogel composites, and the obtained aerogel composites are named as prGMSBDs-20, prGMSBDs-10, and prGMSBDs-5, respectively. Both prGSBDs and prGMBDS aerogels were observed by SEM. As shown in Figure 10a-h, all the aerogel specimens have a unique porous structure with stratified feature formed by the single-layered or multi-layered GO and/or Ti 3 C 2 T x nanosheets. The strata or layers in the prGSBDs are relatively loose and the 3D randomly interconnected networks consisting of stacked rGO nanosheets can be seen from the images shown in Figure 10a,e. Numerous large pores are also observed in the two SEM images. Contrary to the prGSBDs, the prGMSBDs aerogel composites show much more ordered strata, indicating that the addition of Ti 3 C 2 T x nanosheets can effectively hinder the aggregation of rGO sheets and prevent them overlapping with each other. This feature is evidently revealed by the two SEM images in Figure 10b,f for the specimen with the relatively low content of Ti 3 C 2 T x nanosheets in the prGMSBDs-20 aerogel composite specimen. With the increase in titanium carbide nanosheet content, the strata in the prGMSBDs-10 aerogel composite become more densely aligned as shown in Figure 10c,g. For the prGMSBDs-5 aerogel composite with even higher content of MXene (Ti 3 C 2 T x ) nanosheets, the porous structure shows even densely aligned strata. Much smaller elongated pores can be seen in Figure 10d, while the number of pillars between the nanoplates increased a lot, as illustrated in Figure 10h.

Fiber Network Structure
Microfabrillated cellulose aerogels (MFCAs) made by Zhou et al. [25] have the fiber network structure as shown in Figure 11. The microfabrillated cellulose (MFC) suspension was isolated from softwood kraft pulp. The pulp was oxidized by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO). The TEMPO-oxidized cellulose was fibrillated by a homogenizer. The obtained MFC suspensions (0.2 wt%) were frozen in liquid nitrogen and subjected to freeze-drying using a freeze-dryer. The SEM images in Figure 11a,e at different magnifications clearly show the fiber network structure. To produce aerogel composites, the microfibrillated cellulose aerogel specimens with a diameter of 30 mm and height of 10 mm were immersed into 40 mL ethanol solutions with pre-added 0.5 mL water and different amounts of methyltriethoxysilane (MTES) (1 mL, 2 mL, and 3 mL). The pH of the solutions was adjusted to 2 by adding 99.5% acetic acid, and solutions were stirred at 60 °C for 90 min. After that, the pH of the mixtures was increased to 7.5 with the addition of 25% ammonia and stirred for another 60 min. The obtained aerogel composites were washed with ethanol and methanol to remove the residuals and byproducts from the silanization reactions. The aerogel composites were vacuum-dried at 105 °C for 2 h. The final products of hydrophobic microfabrillated cellulose aerogels (HMFCAs) were obtained. Specifically, the designated names HMFCA-1, HMFCA-2, and HMFCA-3 correspond to the different MTES loadings (1 mL, 2 mL, 3 mL in the solutions as mentioned before).

Fiber Network Structure
Microfabrillated cellulose aerogels (MFCAs) made by Zhou et al. [25] have the fiber network structure as shown in Figure 11. The microfabrillated cellulose (MFC) suspension was isolated from softwood kraft pulp. The pulp was oxidized by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO). The TEMPO-oxidized cellulose was fibrillated by a homogenizer. The obtained MFC suspensions (0.2 wt%) were frozen in liquid nitrogen and subjected to freeze-drying using a freeze-dryer. The SEM images in Figure 11a,e at different magnifications clearly show the fiber network structure. To produce aerogel composites, the microfibrillated cellulose aerogel specimens with a diameter of 30 mm and height of 10 mm were immersed into 40 mL ethanol solutions with pre-added 0.5 mL water and different amounts of methyltriethoxysilane (MTES) (1 mL, 2 mL, and 3 mL). The pH of the solutions was adjusted to 2 by adding 99.5% acetic acid, and solutions were stirred at 60 • C for 90 min. After that, the pH of the mixtures was increased to 7.5 with the addition of 25% ammonia and stirred for another 60 min. The obtained aerogel composites were washed with ethanol and methanol to remove the residuals and byproducts from the silanization reactions. The aerogel composites were vacuum-dried at 105 • C for 2 h. The final products of hydrophobic microfabrillated cellulose aerogels (HMFCAs) were obtained. Specifically, the designated names HMFCA-1, HMFCA-2, and HMFCA-3 correspond to the different MTES loadings (1 mL, 2 mL, 3 mL in the solutions as mentioned before). different amounts of methyltriethoxysilane (MTES) (1 mL, 2 mL, and 3 mL). The pH of the solutions was adjusted to 2 by adding 99.5% acetic acid, and solutions were stirred at 60 °C for 90 min. After that, the pH of the mixtures was increased to 7.5 with the addition of 25% ammonia and stirred for another 60 min. The obtained aerogel composites were washed with ethanol and methanol to remove the residuals and byproducts from the silanization reactions. The aerogel composites were vacuum-dried at 105 °C for 2 h. The final products of hydrophobic microfabrillated cellulose aerogels (HMFCAs) were obtained. Specifically, the designated names HMFCA-1, HMFCA-2, and HMFCA-3 correspond to the different MTES loadings (1 mL, 2 mL, 3 mL in the solutions as mentioned before).  The morphologies of the aerogel composites (silanization-treated) were illustrated by several sets of SEM images. The images in Figure 11b-d show the cellulose fiber networks and highly porous structures. Through comparison on the porous and network structures of MFCAs and HMFCAs (Figure 11a-d), it was determined that the silanization of MTES did not change the porous structures of the pristine MFCAs. However, with MTES silanization treatment, polysiloxane particles with diameters of approximately 200 nm at the surface of the modified aerogel composites (HMFCAs) were detected. The higher the concentration of MTES, the more polysiloxane nanoparticles as evident by comparing the SEM images in Figure 11f-h. The formation mechanism of the polysiloxane particles was discussed [25]. The little amount of water added in the ethanol solution triggered the hydrolysis of MTES. The hydrolysis-generated silanols reacted with the hydroxyl groups at the surface of MFCAs or other silanols. The reaction between the silanols and the hydroxyl groups led to a covalently bonded silane layer at the surface of the MFCAs, and the generation of the polysiloxane particles was attributed to the self-polymerization of silanols. Elemental composition analysis of the HMFCAs showed that the intensity of Si signal became stronger with the increase in the concentration of the MTES. This indicated the occurrence of the silanization reaction and the successful conversion of pristine MFCAs to the aerogel composites (HMFCAs).

Beaded Structure
Electrospraying can inject droplets on a substrate, and the dried droplets form beaded structures. He et al. [36] used chloroform (CF) and N,N-dimethylformamide (DMF) as the solvents to produce a polycaprolactone (PCL) solution for electrospraying. PCL electrospraying solutions were prepared by adding 5 wt% PCL in CF/DMF (9:1, v/v). Porous SiO 2 aerogel with a pore diameter of approximately 20 nm and particle sizes ranging from 2 to 40 µm was added into the electrospraying solution to increase the hydrophobicity of the PCL/SiO 2 aerogel composite. The SiO 2 aerogel with different contents (0.1%, 0.3%, 0.5%, 0.7%, and 1.0% w/v) was added into the PCL electrospraying solutions. The obtained PCL/SiO 2 aerogel composites were observed by SEM. Figure 12a-f shows the images of the aerogel composites with different SiO 2 aerogel contents. For all the specimens, a similar beaded structure can be seen. Hierarchical porous feature is also detected. Such a porous, beaded structure composed of numerous micro-nano beads, nanofibers, and microfibers increased the porosity of the aerogel composites, which can provide more adsorption space for oil clean-up [36].  [36]. Copyright ©2021 American Chemical Society.

Biological Carbon Frame Structure
For the aerogel composites directly derived from woods, leaves, flower pollen, and seeds, the biological structures may be well preserved. In [32], Rong et al. used rapeseed flower pollen grains as the starting materials for aerogel composite preparation. Natural rapeseed flower pollen contains hard exine and tender core with complex biotic components including hemicellulose, cellulose, amino acids, proteins, and nucleic acids. To generate porous carbon with high surface area from pollen grains, the rapeseed flower pollen was washed with ethanol, followed by ultrasonication. The pollen grains were filtered and further washed with deionized water multiple times. In order to preserve the carbon frame, pollen grains were immersed in a mixed solution consisting of formaldehyde and ethanol. Filtering and washing the grains with deionized water were conducted. Dehydration of the pollen grains in 12.0 M sulfuric acid under magnetic stirring at 80 °C for 4 h was performed. The pollen grains were washed with deionized water until the pH reached 7. Air-dried in 80 °C for 20 h, the pollen grains were calcined at 600 °C for 2 h in nitrogen. The carbonized pollen grains containing biological structure are shown in the SEM image located in the lower left part of Figure 13.  [36]. Copyright ©2021 American Chemical Society.

Biological Carbon Frame Structure
For the aerogel composites directly derived from woods, leaves, flower pollen, and seeds, the biological structures may be well preserved. In [32], Rong et al. used rapeseed flower pollen grains as the starting materials for aerogel composite preparation. Natural rapeseed flower pollen contains hard exine and tender core with complex biotic components including hemicellulose, cellulose, amino acids, proteins, and nucleic acids. To generate porous carbon with high surface area from pollen grains, the rapeseed flower pollen was washed with ethanol, followed by ultrasonication. The pollen grains were filtered and further washed with deionized water multiple times. In order to preserve the carbon frame, pollen grains were immersed in a mixed solution consisting of formaldehyde and ethanol. Filtering and washing the grains with deionized water were conducted. Dehydration of the pollen grains in 12.0 M sulfuric acid under magnetic stirring at 80 • C for 4 h was performed. The pollen grains were washed with deionized water until the pH reached 7. Air-dried in 80 • C for 20 h, the pollen grains were calcined at 600 • C for 2 h in nitrogen. The carbonized pollen grains containing biological structure are shown in the SEM image located in the lower left part of Figure 13.

Oil/Water Separation Efficiency
Traditionally, the surface contact behavior was evaluated by the wettability at the solid/fluid interface (the composite aerogels in this case and the fluid, either oil or water). The wettability of a liquid drop to a solid surface is assessed by the contact angle. A low contact angle corresponds to the wetting state, whereas a high contact angle of larger than 90 degrees represents the unwetted state of the fluid to the solid. In air, the values of contact angles of oil and/or water to a solid surface can be calculated by the Young's equation [46]. If a solid is immersed into an oil/water emulsion, there are multiple water-oil-solid interfaces. The contact angles still can be determined using the Young's equation. The surface energy terms should be selected according to the different solid/fluid systems. Since the surface tension of oil is much lower than that of water, most hydrophilic surfaces could be oleophobic at the solid-water-oil interface [47]. For oil/water separation and selfcleaning of oil in water environment, the oleophobic property is needed. For those solids with rough surfaces, Wenzel model [48] and Cassie-Baxter model [49] are suitable for the contact angle calculations. If there is no air trapped at the interfaces, Wentzel model can be used to determine the contact angle of the liquid drop to the rough solid surface using a surface roughness factor [48].
It must be indicated that the Wentzel model only considers the wetting to surface with uniform roughness details which are smaller than the size of the liquid drop. Cassie and Baxter [49] modified the Wentzel model to the solid surface with nonuniformity of roughness. In the Cassie-Baxter model, air bulbs are allowed to be trapped into the liquidsolid interfacial region. As a result, the contact angle can be calculated from the solidliquid contact area fraction in the region covered by the liquid drop and the air-liquid contact area fraction in the region covered by the liquid drop.
The wetting state of oil and water drops to solid surfaces is tunable by the environment of interfaces, the physical chemistry properties of materials, and the geometrical roughness at the surface of the solids. An oil drop may show low wetting angle at a nanofiber aerogel or membrane surface, but the nanofiber aerogel or membrane itself may be highly oleophobic in water. This is the foundation for achieving the antifouling and selfcleaning surface underwater for aerogel composites. Therefore, the so-called super oleophobic property is needed for efficient oil-water separation in many cases.
In addition to the contact angle, several other parameters were proposed to evaluate the performance of oil-water separation as described by Zhang et al. [50,51]. These parameters include oil permeation flux (J), the oil rejection rate (R), equilibrium adsorption

Oil/Water Separation Efficiency
Traditionally, the surface contact behavior was evaluated by the wettability at the solid/fluid interface (the composite aerogels in this case and the fluid, either oil or water). The wettability of a liquid drop to a solid surface is assessed by the contact angle. A low contact angle corresponds to the wetting state, whereas a high contact angle of larger than 90 degrees represents the unwetted state of the fluid to the solid. In air, the values of contact angles of oil and/or water to a solid surface can be calculated by the Young's equation [46]. If a solid is immersed into an oil/water emulsion, there are multiple wateroil-solid interfaces. The contact angles still can be determined using the Young's equation. The surface energy terms should be selected according to the different solid/fluid systems. Since the surface tension of oil is much lower than that of water, most hydrophilic surfaces could be oleophobic at the solid-water-oil interface [47]. For oil/water separation and self-cleaning of oil in water environment, the oleophobic property is needed. For those solids with rough surfaces, Wenzel model [48] and Cassie-Baxter model [49] are suitable for the contact angle calculations. If there is no air trapped at the interfaces, Wentzel model can be used to determine the contact angle of the liquid drop to the rough solid surface using a surface roughness factor [48].
It must be indicated that the Wentzel model only considers the wetting to surface with uniform roughness details which are smaller than the size of the liquid drop. Cassie and Baxter [49] modified the Wentzel model to the solid surface with nonuniformity of roughness. In the Cassie-Baxter model, air bulbs are allowed to be trapped into the liquidsolid interfacial region. As a result, the contact angle can be calculated from the solid-liquid contact area fraction in the region covered by the liquid drop and the air-liquid contact area fraction in the region covered by the liquid drop.
The wetting state of oil and water drops to solid surfaces is tunable by the environment of interfaces, the physical chemistry properties of materials, and the geometrical roughness at the surface of the solids. An oil drop may show low wetting angle at a nanofiber aerogel or membrane surface, but the nanofiber aerogel or membrane itself may be highly oleophobic in water. This is the foundation for achieving the antifouling and self-cleaning surface underwater for aerogel composites. Therefore, the so-called super oleophobic property is needed for efficient oil-water separation in many cases.
In addition to the contact angle, several other parameters were proposed to evaluate the performance of oil-water separation as described by Zhang et al. [50,51]. These parameters include oil permeation flux (J), the oil rejection rate (R), equilibrium adsorption capacities (Q c ), oil flux recovery ratio (FRR), the reversible fouling resistance (R r ), and irreversible fouling resistance (R ir ). The definitions of these parameters were presented (see [50]). To compare the efficiencies of nanofiber aerogels on separation of oil/water and organics/water, electrospun polyacrylonitrile (PAN), polyaniline coated polyacrylonitrile (PP), and polyaniline and nano molecular brush consisting of polyacrylic acid and polyethyleneimine on polyacrylonitrile (PPPP) were produced [52]. The water contact angle (WCA) is 0 o for the three nanofiber aerogel membranes. The underwater oil contact angle (UOCA) for them is over 150 • . The WCA and UOCA values indicate the superhydrophiliciy and oloephobicity of these nanofibers. There exist differences in the flow recovery rate (FRR) and flux (J) as shown in Table 1. Obviously, the PPPP nanofiber showed the highest efficiency in separating n-hexane and water because it showed the highest values in FRR, J 1 and J 2 among the three nanofibers.

Conclusions
Recent progress in studies of the aerogel composites for oil absorption and oil/water separation has been presented. For the preparation of aerogel composites, template approach, in situ reaction, sol-gel spinning are matured technologies. Applying coatings to aerogel composites is very effective in changing the surface properties of the composites. Among various coating processes, silanization is especially useful for increasing the hydrophobicity of aerogel composites. For controlling the microstructure of aerogel composites, oriented porous structures offer advantages including fast oil transport, good oil recovery capability, and high mechanical strength during the application in oil/water separation. Therefore, unidirectional and bidirectional freeze casting technologies have been emphasized. There are many research activities focusing on the preparation of sustainable source-derived or bio-based aerogel composites. Such aerogel composites are highly absorptive, non-toxic, and recyclable. Wood, cotton, plant stems and leaves are some of the most studied biomass sources for aerogel composite manufacturing. Any cellulose and lignin generating sources are considered to have value for oil absorption aerogel composites processing. Waste materials including used tire fabrics and recycled papers have been studied for producing oil sorption aerogel composites. In view of the future research directions in the aerogel composite field, carbon aerogel composites with functional additives are promising because of the high temperature stability and recyclability. Further studies on the durability of aerogel composites under various spilled oil cleaning service conditions should be addressed. In addition, energy recovery from the disposal of aerogel composites with absorbed oil should be investigated.