Advanced nanoarchitectures of carbon aerogels for multifunctional environmental applications

2.1 RF carbon aerogel

The sol-gel polycondensation of RF is a conventional way to prepare precursors of CAs, which was developed by Pekala’s group in 1989 [15]. In a typical sol-gel procedure, organics are polymerized to form a crosslinked network in the pH range of 5.4–7.6 in two stages: (a) the formation of hydroxymethyl derivatives of resorcinol (mono-, di- or tri-substituted) and (b) the condensation of these intermediates to form methylene and methylene ether bridged oligomers, which make up the clusters to compose the gel structure (Figure 1) [15, 16]. RF-CAs are obtained by pyrolysis and carbonization of as-prepared organogels in an inert atmosphere.

Figure 1:

A two-step polymerization process through the addition reaction of resorcinol and formaldehyde, followed by subsequent polycondensation of the hydroxymethyl derivatives. Reprinted with permission from Ref. [16].

In the gelation process, sodium carbonate is generally used as a catalyst, as well as alternative cations like lithium, potassium and cesium [1, 16]. It was found that cations play a vital role in determining the porous characteristics of RF-CAs. Larger cesium cation makes the oligomers less soluble and leads to large clusters and increased pore sizes, while lithium, sodium and potassium have appropriately equal ability to stabilize RF colloidal suspension and result in small clusters. In addition, other metal cations, such as Ni²⁺, Co²⁺ and Fe³⁺, are reported to be substitutes for the traditional catalysts (Na₂CO₃) [17–19]. Although acid, like nitric acid, acetic acid and oxalic acid, can be used as catalysts, it is common to obtain RF-CAs with relatively higher density and lower meso- and macroporosity than base-catalyzed RF-CAs, probably due to the formation of lager aggregated clusters during the polymerization reaction [1, 20, 21]. The pore texture can also be tailored by resorcinol-to-catalyst (R/C) molar ratio, and higher R/C ratios result in less clusters, longer gelation time and larger pore sizes but decreased Brunauer-Emmett-Teller (BET) surface areas [16].

The flexibility of CA syntheses provides possibility to modify network structures through incorporation of additives during the synthesis processes. Templates are preferable to construct hierarchical pore texture. Hard templating based on silica particles can be utilized to facilely tailor pore size by altering particle sizes [17, 22]. However, strongly corrosive hydrofluoric acid should be involved in etching the inorganic silica templates. Soft templating based on polymer backbones such as polystyrene (PS) and polymethylmethacrylate, and triblock copolymers such as Pluronic F127 and P123, is superior because these organic templates can be simply removed through pyrolysis [23–25]. Triblock copolymers are applicable to generate ordered mesoporous structures in RF resin due to their self-assembly in water [26]. RF-CAs with bimodal porous structure can be obtained by using two or more different templates in which macropores are continuous and interconnected, while meso- and microporous channels run through the pore wall of RF-CAs [17, 27]. Such hierarchical porous structures lead to high specific surface area and diffusion efficiency, which is beneficial for adsorption and catalysis applications.

The addition of graphene oxide (GO) is another way to modify the framework of RF-CAs, where GO becomes a part of the textural structure. GO acts as an anti-shrinkage additive in the composite RF-CAs, which could not only effectively strengthen the RF skeleton but also reduce the aerogel density [28]. The BET surface area, pore volume and mechanical property depend much on GO loadings [29]. Moreover, due to the reduction of GO, the electrochemical performance of composite RF-CAs is much enhanced, which is promising for electrode materials [30, 31].

Proper post-treatments also significantly improve the final physicochemical properties of CAs. In physical treatments, as-prepared RF organogels are usually thermally treated in a flow containing activating gas such as CO₂. Oxidation by such an activating gas could increase the microporosity, total pore volume and specific surface area of RF-CAs but avoid any obvious destruction of the scaffolds [32]. On the other hand, in chemical treatments, precursor RF-CAs are treated by oxidizing agents, like KOH and HNO₃ [33]. This process could not only enhance porosity but also regulate the hydrophobicity of final CAs. For example, CAs subjected to surface modification with HNO₃ would introduce oxygen-containing functional groups and improve their wettability, while aging in HCl would consume carboxyl groups and increase the hydrophobicity [27, 34].

2.2 Graphene aerogel

Graphene, a two-dimensional sheet of sp²-hybrized carbon atoms, is a basic building unit for a range of carbon allotropes, which has attracted tremendous attention in the past few years due to its ultra-large specific surface area and excellent mechanical, chemical and electrical properties. Monolithic graphene aerogels (GAs) have been created through various approaches, such as hydrothermal reduction [35], chemical reduction [36] and template-directed chemical vapor deposition [37]. Among them, chemical reduction of GO is the most facile way because of its low cost and scalable production. Typically, a homogeneous colloidal GO suspension is thermally treated in the presence of reducing agents, like ascorbic acid, ethylenediamine and hydrazine [3, 36]. With the elimination of oxygen-containing functional groups on the GO, the hydrophobicity and the π-conjugated structure of reduced GO (rGO) increase. Then the phase separation of GO sheets occurs, and the crosslinks form through the surface interaction of Van der Waals and residual π-π stacking between the faces of each sheets, as well as hydrogen bonds among remaining oxygen-containing groups [35, 38].

Additives in the syntheses can be applied to tune the mechanical and chemical features of GAs. Divalent metal ions (Ca²⁺, Mg²⁺, Co²⁺, Ni²⁺ etc.) are reported to serve as linkers that form coordinative bonds with oxygen-containing groups and facilitate the mechanical stiffness of GO aggregates, which are indicated by both experimental and theoretical investigations [19, 39–41]. On the other hand, reinforcers like polyvinyl alcohol [38], glucose [42] and dopamine [43] would form polymers during the hydrothermal treatment, which could either fill in the pores or react with the functional groups on GO and therefore enhance the mechanical strength of GAs. Ye et al. reported that GA crosslinked by epoxy resin not only exhibits excellent mechanical strength (0.231 MPa) but also achieves high elasticity [44]. It is well known that the zero band gap of pristine graphene weakens its catalytic activity in the absence of metal components. However, recent achievements demonstrated that chemical doping of graphene or GO with substituent heteroatoms could effectively modulate the electronic and surface chemical features [45, 46]. By solvothermal treatment of GO sheets and precursors containing specific element atoms and further calcination, the heteroatoms would be integrated in the structure of GO. GO sheets doped by nitrogen, sulfur and boron as metal-free catalysts are efficient in catalytic activation of peroxymonosulfate and oxygen reduction reaction due to the different electronegativities of heteroatoms and carbon atom [45, 47].

3.1 Adsorption

Adsorption is considered as a facile, effective and low-cost approach to separate and remove contaminants from aqueous phase. Till now, many kinds of adsorbents have been developed, including AC, molecular sieves, macroporous adsorptive resins and so forth. However, these materials face some disadvantages, like limited adsorption capacities and difficulties in separation, recovery and reusability. Adsorption processes are strongly affected by the porous structures of the adsorbents and the sizes of the adsorbates. CAs have interconnected and open frameworks with tunable pore size distributions and large surface areas, which allow fast adsorption kinetics in the removal of various organic molecules such as dyes, organic solvents and pesticides, and heavy metal ions. Their monolithic morphology and chemical stability also guarantee the simplicity in separation and recovery. Therefore, CAs are becoming potential candidates as adsorbents for water purification.

Extensive investigations have been carried out on the removal of organic dyes or oils through adsorption using CAs, and a series of novel methods have been developed to regulate the porosity of CAs. Qian et al. synthesized GAs from self-assembly of GO sheets and resol-type phenolic prepolymers [48]. The resol prepolymers coating on GO sheets not only lessen the aggregation of graphene sheets by reducing their Van der Waals interactions but also result in the formation of an interconnected network because of the crosslinking reactions at the interface between overlapping graphene sheets. The as-prepared GA obtained from the least amount of precursors reaches a BET surface area of 640 m²/g and a pore volume of 2.92 cm³/g and could adsorb organic solvents with 200–400 times its original mass (Table 1). Chi et al. prepared 3D hierarchical porous GAs (HGAs) using PS particles with different sizes as the sacrificial templates to tailor their microstructure (porosity and pore size distribution) [49]. The HGAs are highly hydrophobic, and the maximum specific surface area and pore size are 237.47 m²/g and 120 nm, respectively. The adsorption capacities for oils and organic solvents depend much on the PS sizes, which might be closely related to the pore sizes, specific surface areas and pore volumes of the aerogels, as well as the molecular dimension of the target adsorbates (Figure 2). Wei et al. used Ni²⁺ to promote the gelation process of GO and RF and obtained a Ni-doped GO/carbon cryogel (NGCC) [19]. The metal ions perform as crosslinking agents and provide active sites for the assembly of layers, which result in the formation of corrugations in the NGCC and a well-defined interconnected porous structure with pore size ranging from submicrometers to tens of micrometers. The adsorption capacities of NGCC for oil are higher than those of samples without Ni²⁺ or/and GO. It is also worth pointing out that due to the high degree of electron delocalization conjugated system on the surface of graphene, NGCCs are more inclined to couple with organic dye molecules.

Figure 2:

(A) Adsorption efficiency of the HGAs for different oils and organic solvents. (B) Surface area and pore size and (C) pore volume of the HGAs with different PS sizes. Reprinted with permission from Ref. [49].

Hydrophobicity is another vital feature for CAs to have a good affinity toward organics. Generally, according to the preparation procedures, either the pyrolysis of RF precursors in an inert atmosphere or the reduction of GO sheets leads to trace amounts of residual oxygen functional groups remaining on the substrates. This creates a hydrophobic surface and facilitates the adsorption activity due to the superwetting behavior of organic matters [19, 51]. Zhou et al. developed a novel approach to enhance the hydrophobicity of GAs [50]. They incorporated porous Fe₃O₄ nanoparticles (with sizes of 350–500 nm) and PS microspheres (with sizes of about 1 μm) to produce PS/Fe₃O₄/graphene hybrid aerogel composites (CPFAs) with a porous micro-nano substructure within the reticulated graphene network. The particulate incorporation increases the interconnected micropores exiting in the composites and the surface roughness in the nano- and microscale on graphene surfaces, leading to a relatively low surface energy and promoted hydrophobicity. The average BET surface area of CPFAs is 430 m²/g, slightly lower than that of pure GA (512 m²/g), but the water contact angle of CPFAs could even increase with the content of Fe₃O₄ nanoparticles in the range of 0–40 wt%. The diesel adsorption experiments showed that about 27, 37 and 40 times the original mass of CPFA for diesel oil, lubricating oil and crude oil, respectively, could be completely adsorbed in 60 s even after 10 water-oil separation cycles.

In addition, the adsorption capacity of CAs may be related with the zeta potential of target compounds and aerogel composites. Zhang et al. found that methyl blue (MB) has a much higher total adsorption capacity than methyl orange (MO) and orange G (OG) [51]. It could be explained as that MB reduces the electrostatic repulsion with negatively charged GA (-1.57 V) due to its relatively more positive zeta potential (-1.45 V) than those of MO (-4.15 V) and OG (-9.79 V). However, electrostatic interaction is not always a determining factor. Although the point of zero charge pH (pH_PZC) of RF cryogel is 10.5, the adsorption capacity of imidacloprid (IMI) is not significantly influenced by solution pHs, which indicated that the main factor of IMI bonding is Van der Waals forces instead of electrostatic interactions with the cryogel surface [52].

The recovery and regeneration of CAs after the adsorption process are other vital issues for practical applications. A simple approach to remove the adsorbed organics is direct combustion in air, which is closely related to the chemical stability of CAs [53]. Owing to the incorporation of Fe₃O₄ nanoparticles, CPFA, which is mentioned above, is magnetic and compressible [50]. After the oil soaking operation, CPFA could be readily moved with a magnet and collected. More importantly, it could be regenerated by a simple squeezing operation, and useful organics could be recycled.

To capture heavy metal ions from wastewater, diverse functionalized CAs are developed [54]. Different from the surface feature required for organics removal, hydrophilicity is preferable for inorganic ions removal, which promises a full contact between the CAs and target contaminants in aqueous solutions. The oxygen-containing functional groups exposed to the outer surface of CAs, such as hydroxyl, carboxyl and sulfo groups, not only can improve the hydrophilicity but also are essential for the high adsorption capacity toward heavy metal ions [55]. Thus, the main efforts have been focused on the functionalization of CAs with optimized functional groups, as well as the increase of specific surface area. Although several functionalized GO sheets have been found to exhibit good performances, their assembly into 3D aerogels is more practical because of their high mechanical strength, avoidance of agglomeration, multi-dimensional adsorption sites and easy reusability.

Polymers are multi-functional additives. Their macromolecules can strengthen the skeleton and maintain the porous structure of CAs. GO-chitosan (GO-CS) composite aerogel fabricated by lyophilization shows a higher adsorption capacity of Cu²⁺ (21.6 mg/g) than that of pure GO aerogel (14.9 mg/g) [56]. This might be explained by the fact that CS serves as the support to avoid excessive stacking of GO sheets. Besides, more importantly, polymers are usually utilized to decorate GO sheets with desired functional groups to enhance their adsorption capacities of heavy metal ions depending on the chelate effect [57]. However, high uniformity and grafting rate of polymers are usually difficult to be achieved through direct functionalization of GO. One novel approach to solve this problem is to modify GO with a compatible mediator first, on which target polymers are implanted subsequently. According to this strategy, Dong et al. synthesized nano-thick polyethylenimine (PEI) layer-coated polydopamine (PD) modified GO (PEI-PD/GO) composite nanosheets, which were then assembled into PEI-PD/rGO aerogel by a hydrothermal method (Figure 3A) [58]. The uniform coating of ultrathin PD layer on GO not only improves the hydrophilicity of GO sheets but also provides abundant active sites for further high loading of PEI via the Michael-Addition reaction. PEI-PD/GO aerogel possesses a higher BET surface area (373 m²/g) and a more uniform porous structure than rGO, PD/rGO and PEI/rGO aerogels (Figure 3B). Moreover, with the help of PD interlayer and adequate amine groups of PEI, PEI-PD/GO aerogel exhibits a stronger affinity toward various heavy metal ions, resulting in a superior adsorption performance (Figure 3C and D).

Figure 3:

(A) Schematic illustration for the fabrication of PEI-PD/GO composite. (B) Scanning electron microscope (SEM) image of PEI-PD/GO aerogel (scale bar: 5 μm). (C) Comparison of adsorption capacity of Pb²⁺ on rGO, PD/rGO, PEI-PD/GO and PEI/rGO aerogels, and (D) Cu²⁺, Cd²⁺, Pb²⁺ and Hg²⁺ on active carbon (black), PEI/rGO (red) and PEI-PD/GO (blue) aerogels. Reprinted with permission from Ref. [58].

Iron-derived nanoparticles are accounted for the removal of several inorganic ions, like arsenic (As) and phosphate, but tend to aggregate [59, 60]. GO sheets have been proven to be a good candidate to act as the support for the immobilization of nanoparticles. GAs decorated with iron oxides are ideal adsorbents due to their synergistic effectiveness of adsorption [61, 62]. Along this thought, Andjelkovic et al. developed a GA immobilized with α-FeOOH to eliminate As in wastewater [63]. In the synthesis procedures, Fe(II) was directly utilized as the reductant, and the rGO sheets anchored with Fe nanoparticles were self-assembled into a 3D hydrogel. The well-developed porous structure and large specific surface area (220±5 m²/g) of the prepared aerogels is attributed to the spacer of α-FeOOH. Arsenic ions are removed by the adsorbent at a fast rate through two mechanisms, electrostatic attraction with surface groups and ligand interaction between As and α-FeOOH nanoparticles, but the adsorption capacities are varied significantly depending on the valence of As and solution pHs.

3.2 Capacitive deionization

Capacitive deionization, otherwise known as electrosorption, is an emerging batch electrodeionization technology for brackish water that contains 500–5000 ppm of salt [64]. Compared with conventional deionization methods, such as reverse osmosis, distillation, electrodialysis and ion exchange, CDI can be conducted at ambient conditions and low external direct voltage (<2 V) without secondary pollutions and does not need extra equipment (high-pressure pumps, membranes, distillation columns or thermal heaters) [65]. CDI is developed based on the same principle as electrical double-layer (EDL) capacitors, in which ions move towards the oppositely charged electrodes and are temporarily electroadsorbed in EDL between the solution and electrode interface by applying an external electrostatic field. After electrode saturation, electrosorption is reversible by removing the applied potential, and the consumed energy can be partially recovered during desorption (Figure 4) [65, 67, 68]. Typically, CDI electrodes are made of porous carbon materials, including AC, CNTs, carbon nanofibers, mesoporous carbon, graphene, CAs and so forth. Among them, CAs electrodes exhibit a superior electrosorption performance due to their high specific surface area, high electrical conductivity, chemical inertness and hierarchically porous structures [69].

Figure 4:

Schematic illustration of CDI. Reprinted with permission from Ref. [66].

Although specific surface area concerns the adsorption capacities, the pore size distribution of CAs has a greater influence on ion removal and selectivity during CDI [70]. The simplified representation of EDL formation and development for ideal planar surfaces is not applicable for such porous surfaces. Electrosorption can occur in both micro- and mesoporosity but via different mechanisms. When the pore size is greater than the diffuse layer thickness, the distribution of ions follows the Gouy-Chapman-Stern theory. In mesopores, the pore size decreases to a dimension similar to the diffuse layer thickness, and the EDL on the walls begins to affect each other, which is called EDL overlapping effect, and the electrosorption density (sorption capacity normalized by pore volume, μmol/cm³) is reduced. However, when pore size further decreases to micro-dimension and approaches the ion hydrated radius, ions tend to form a single layer in the center of the pore due to the ion hydration effect, and the capacitance is enhanced, which involves more sophisticated models [70–72]. On the other hand, ion selectivity also depends on the relationships between pore size and ion hydrated radius. When the ion hydrated radius is similar to pore size, ionic mobility would decrease due to an increase in water viscosity and/or coulombic interactions in the charged pore walls, which causes an ion selectivity [70]. Chao et al. verified that the electrosorption performance and capacities of three monovalent ions, Li⁺, K⁺ and Rb⁺, increase with the decreases of hydrated radius [73].

As mentioned above, physical activation by CO₂ and chemical activation by KOH are usually applied to tailor the porous features of CAs. Both activation processes give rise to an increase in the micropore volumes, accompanied by the enlargement of mesopores. Mesoporosity plays an important role in that it overcomes diffusional limitation and ensures the migration of the ions to the inner porosity, while the improved surface areas and micropore volumes provide more accessible active sites to accommodate ions, both of which contribute to the stable and superior specific capacitance and efficient electrosorption performance [74, 75]. Kohli et al. achieved NaCl adsorption capacity of 8.4 mg/g (initial concentration: 500 mg l^-1, applied voltage: 1.2 V) for CO₂-activated RF-CA, compared to the capacity of untreated one (4.1 mg/g), because of the synergistic effect of higher specific surface area and appropriate pore size distribution after activation. In addition, chemical KOH treatment of GAs not only forms in-plane micropores but also generates a large amount of edge carbon, which has much higher specific capacitance than that of the basal plane, leading to an increased electrosorption capacity [76].

Incorporation of heteroatoms, such as oxygen, nitrogen, sulfur and boron, in the framework of CAs is a popular way to improve the surface characteristics and electrochemical properties. CAs functionalized by the polar groups show an improved wettability and basicity, which favors the electrode/electrolyte interactions and accelerates the diffusion of the ions from the bulk solution towards the electrode surface. Moreover, the heteroatoms would alter the electron configuration of pristine carbon matrix, so as to facilitate the electron transport and enhance the electrical conductivity. In the field of supercapacitors, heteroatom-doped carbon materials are beneficial for attractive pseudo-capacitance and better energy storage performance arising from the Faradaic reactions between the functional groups and ions in the electrolytes [77]. Inspired by this, several investigations have been carried out on CA electrodes doped by heteroatoms, especially nitrogen, which were applied to increase the electrosorption capacity [78, 79]. Figure 5 shows the scheme of the electrochemical process of N-doped graphene materials in NaCl solution [80]. The electrically active pyridinic and pyrrolic nitrogen atoms show affinity to Na⁺ ions, while a possible oxidation/reduction reaction between H₂O and pyridinic and pyrrolic nitrogen could happen to improve wettability. Consequently, more Na⁺ can be easily accessible to the surface of graphene and form more effective EDL. Besides, graphite nitrogen atoms are favorable to the electron transport and decrease the electrical resistance, which also enhance the electrosorption capacity. Experiments revealed that an ultrahigh NaCl adsorption capacity of 21.0 mg/g (initial concentration=~500 mg l^-1, applied voltage=1.2 V) for N-doped graphene sponge could be achieved, which was higher than those of graphene sponge (14.6 mg/g) and pristine graphene (4.5 mg/g), accompanied by a relatively high charge efficiency (the ratio of adsorbed ions and consumed charges) and an excellent reversibility [81]. However, it should be pointed out that high heteroatom contents and specific functional groups seem to make charges participate in Faradaic reactions rather than the formation of EDL, causing less charge efficiency [82]. More mechanistic aspects of the role the heteroatoms of CAs play in the CDI should be further studied.

Figure 5:

Scheme of the electrochemical process of N-doped graphene materials in NaCl solution. Reprinted with permission from Ref. [80].

Metal oxides and conducting polymers are two extensively investigated electrode materials to generate supercapacitance based on the principle of Faraday pseudo-capacitors. Nevertheless, they usually display low stability due to the Faradaic reactions. Combination of carbon materials and metal oxides or conducting polymers not only overcomes the above-cited deficiency but also provides much higher specific capacitance [83, 84]. Due to the similar principle of supercapacitor and CDI, those hybrid electrodes are also applicable in CDI applications. Yin et al. synthesized a 3D GA/TiO₂ nanoparticle composite by a new strategy, where low valence ions (Ti³⁺) were oxidized to high valence metal oxide (TiO₂) by GO, resulting in powerful interactions arising from the chemical adsorption of Ti³⁺ and GO sheets [85]. TiO₂ possesses a high dielectric constant to hold more electric charges. The GA/TiO₂ with open porous structures demonstrated an ideal double-layer capacitor behavior with a high specific capacitance and low ion diffusion resistance. NaCl adsorption capacity of GA/TiO₂ reaches 24.2 mg/g at the feeding concentration of 6000 mg l^-1. Furthermore, the desalination cycling experiment revealed both fast response and high reversibility of electroadsorption-desorption behavior of GA/TiO₂.

On the other hand, polyaniline and polypyrrole are two conducting polymers commonly incorporated into carbonaceous materials by in situ polymerization of corresponding aromatic monomers [86]. Besides their high pseudo-capacitance, the improvement in electrochemical property is attributed to the facilitation of π-π interaction between the conducting polymers and aromatic skeleton of graphite materials [86, 87]. Recently, as a simulation of membrane CDI, carbon electrodes coated with ion-exchange polymers have been developed to overcome the inherent disadvantage of CDI that, during the adsorption of counter-ions, a simultaneous desorption of co-ions occurs [88, 89]. Although a high electrical resistance is detected, the composite carbon electrodes integrate the advantage of the high capacitance and an ion-exchange membrane, which enhances the charge efficiency due to the selective transport of ions between the electrode surface and bulk solutions. However, up to date, rare studies have focused on the conducting polymer-decorated CAs. It can be expected that their preparation and application would have no complications due to the flexibility of CA syntheses.

3.3 Catalysis

Recalcitrant organic pollution remains to be a severe problem which concerns water safety a lot. Adsorption seems to be a simple way to eliminate contaminants in water, but after all, it is only a physical separation and subsequent treatment is still necessary. Nowadays, many studies have been carried out on advanced oxidation processes (AOPs), and remarkable achievements are gained. Generally, powder-like heterogeneous catalysts are utilized. However, nano-scaled catalysts are easy to aggregate, which significantly affects their performance. More importantly, the efficient separation of the nanocatalysts from aqueous solutions is regarded as a major obstacle for their scale-up applications in water remediation. As catalyst supports, CAs are attracting more and more attention. Owing to their large specific surface area and macroscopic morphology, nanocatalysts can be uniformly immobilized on the substrates, and the monoliths can be removed from reaction systems without difficulties. Furthermore, the outstanding electrical features of CAs contribute a lot to the enhancement of catalytic performance, which makes CAs more than rigid substrates.

As an important branch of AOPs, the mechanism of semiconductor photocatalysis with TiO₂, Cu₂O, AgBr, ZnS and so on have been well studied [90–93]. The electrons in valence band can be excited to conduction band by photons depending on the band gap. How to expand the available wavelength range of light and how to efficiently separate the photogenerated electron-hole pair are always the urgent tasks to further enhance the photocatalytic performance. Several studies have discovered that the combination of semiconductors and GAs could increase the intensity of light absorbance and shift the adsorption edge to the higher wavelength region [92, 94–96]. This could be ascribed to the contribution from the absorption of graphene sheets and the modified fundamental processes of exciton formation upon irradiation, which suggests a strong interaction between the semiconductor nanoparticles and graphene sheets. Furthermore, due to the excellent electrical conductivity, the GAs also serve as an electron reservoir, which promotes the charge transfer and facilitates the separation of the electrons and holes [94, 97]. These merits of GAs are beneficial for photocatalysis in the aspect of photocatalytic mechanism.

Qiu et al. reported novel GAs with mesoporous TiO₂ nanocrystals grown in situ (TiO₂/GAs) [98]. During the hydrothermal process, glucose was utilized as both linkers and face-growth inhibitors. The hydroxyl groups at both ends of glucose created strong connection of graphene and TiO₂, which not only provided an ultradispersed distribution of mesoporous TiO₂ nanocrystals with (0 0 1) facets (Figure 6A and B) but also improved the electron transport efficiency. Under the solar light irradiation, the photogenerated electrons transfer from the TiO₂ nanocrystals to the graphene sheets through glucose linkers, and the holes accumulate on the (0 0 1) facets, enhancing the separation of electrons and holes (Figure 6C). The TiO₂/GAs prepared in the presence of glucose maintain the macroporous structure and exhibit higher and more stable photocatalytic activity in the degradation of MO than those without glucose. The MO (10 mg l^-1) could be degraded up to 90% by TiO₂/GAs (67 wt%) after irradiation for 5 h, and the catalysts maintain high photoactivity (83%) after five cycles (Figure 6D). As shown in Figure 6E, the photocatalytic degradation process is simple and the monolithic catalysts could be easily recycled by a tweezer.

Figure 6:

(A) High- and (B) low-magnification transmission electron microscope (TEM) images of TiO₂/GAs (67 wt%) synthesized in the presence of glucose. (C) Schematic illustration of photo-electrons transfer between TiO₂ and graphene. (D) Cycling photodegradation of MO under simulated solar light irradiation. (E) Pictures of the photodegradation process and the recycle of TiO₂/GAs by tweezer. Reprinted with permission from Ref. [98].

On the other hand, the inherent physicochemical properties of CAs also play an important role in photocatalytic degradation of organic pollutants. As compared to the powder-like photocatalysts, the low mass density of the composite aerogels makes them possible to float on the surface of the aqueous reaction systems to absorb more light without vigorous stirring [99]. The large surface area, interconnected open pores and outstanding adsorption ability could enrich the pollutants on the surface or in the porosity of the aerogels, which ensures a full contact to the catalysts [14, 100]. Similar to CDI, when an external potential was applied, ionized dye molecules (e.g. alizarin red) could be electrosorbed on the surface of TiO₂/CA, followed by photocatalytic degradation of dyes and regeneration of the electrode [101]. The low anode voltage (0.6 V) also served as bias potential that inhibited the recombination of electron-hole pairs. The results indicated that the photocatalysis-enhanced electrosorption process is superior to either photocatalysis or electrosorption alone in removal capability and stability and is suitable for high-concentration wastewater treatment.

Wettability of CAs should be taken into account for selective degradation of various organics in aqueous systems. Without complex surface modification, Liu et al. fabricated TiO₂/rGO aerogels with controllable and continuously tunable surface wettability from super-hydrophobic to super-hydrophilic according to the amount of incorporated TiO₂ (Figure 7A) [102]. When the TiO₂ content is relatively low (less than 75.8 wt%), the nanoparticles with uniform size of 20–30 nm are well dispersed on the rGO sheets. However, when the TiO₂ content further increases (more than 90.4 wt%), the subsequent self-assembly of TiO₂ nanoparticles would occur and nano-hemispheres with a mean diameter of ca. 250 nm on ultrathin rGO sheets are observed (Figure 7B–E). The specific surface areas also decrease from 232 to 108 m²/g accordingly with the incorporated TiO₂ content but are still larger than either GA (36 m²/g) or pure TiO₂ (106 m²/g) prepared similarly. The degradation experiments revealed that these aerogels show significantly different photocatalytic performances in the system containing both lipophilic oleic acid (layered with Sudan III dye) and water-soluble MO. When the hydrophobic TiO₂/rGO aerogel is applied, hydrophilic MO molecules are kept away from its surface, while the oleic acid on the surface of the solution is adsorbed fast and a complete decomposition under the irradiation could be achieved in 8 h monitored by the Fourier transform infrared spectra (Figure 7F). On the contrary, the hydrophilic TiO₂/rGO aerogel could adsorb 49% of MO without irradiation, but a completed degradation occurred in 4 h under the irradiation, leaving oleic acid unchanged (Figure 7G). The selective photocatalysis of different contaminants derived from surface wettability makes TiO₂/rGO aerogels especially useful in the systems which contain extremely poisonous pollutants in a low concentration together with a large quantity of unpoisonous chemicals.

Figure 7:

(A) Photographs of the TiO₂/rGO aerogels and their water contact angles with various TiO₂ loading. (B) SEM and (C) TEM images of TiO₂/rGO aerogel (TiO₂ wt%=75.8%). (D) SEM and (E) TEM images of TiO₂/rGO aerogel (TiO₂ wt%=96.7%). The adsorption and photocatalytic degradation of mixed contaminants over (F) hydrophobic and (G) hydrophilic TiO₂/rGO aerogels. Reprinted with permission from Ref. [102].

Fenton reaction and its derivatives are very well-known techniques in AOPs, and the explorations on metal-doped CAs as catalysts have already been conducted [103–105]. Ramirez et al. researched the degradation of azo dye Orange II in the presence of H₂O₂ using iron-impregnated RF-CA [103]. It was found that the adsorption capacities of the catalysts are closely related with the pore size distribution, and mesoporosity favors the adsorption of large macromolecules. Moreover, the fine iron species dispersion could also be attributed to high mesopore volume and large external surface area, which leads to an improved catalytic performance. It is noteworthy that the iron leaching from iron-impregnated RF-CA is relatively considerable, and the incorporation of iron in the framework is suggested.

Fenton reactions integrated with other techniques have been developed to further enhance the organics degradation performance. Recently, Peng et al. fabricated a novel 3D-ordered macroporous Fe₂O₃/CA (3DOM-Fe₂O₃/CA) electrode applied in a neutral solar photo-electro-Fenton (SPEF) system [106]. Multilayer deposition of PS spheres (500 nm) was constructed on the RF-CA substrate, followed by immersion of Fe(NO₃)₃ ethanol solution. Then the as-prepared sample was thermally treated to remove PS templates, and 3DOM-Fe₂O₃/CA with well-defined 3D hexagonal Fe₂O₃ array (390–425 nm) on the CA substrate was obtained (Figure 8A). In addition, a hollow “cylindrical tube” formed by the holes with a diameter of 120 nm in the vertical direction could be clearly observed (Figure 8B). Such ordered hierarchical porosity exhibits multiple merits in SPEF system. Basically, it facilitates the diffusion and mass transport of pollutants and provides a large BET surface area of 540 m²/g. Under irradiation, stronger light absorption intensity could be realized due to the multiple scattering effect in the 3D ordered macroporous structure, and the separation efficiency of photogenerated electrons and holes would be enhanced because of the small-size Fe₂O₃. Furthermore, the electrochemical impedance spectroscopy showed that 3DOM-Fe₂O₃/CA displays a lower resistivity than traditional Fe₂O₃/CA. During the SPEF process, more electrons and holes could be generated by 3DOM-Fe₂O₃/CA under visible light irradiation and then could transfer to the surface of the electrode and participate in the degradation of IMI (Figure 8C). The results manifested the improved degradation efficiency (95% of IMI could be removed in 2.5 h at pH 7) and reduced energy consumption of novel 3DOM-Fe₂O₃/CA electrode.

Figure 8:

(A) and (B) SEM images of as-prepared 3DOM-Fe₂O₃ on the CA substrate. (C) Schematic illustration of efficient degradation of IMI with 3DOM-Fe₂O₃/CA electrode in SPEF process. Reprinted with permission from Ref. [106].