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Abstract： Two-dimensional （2D） graphene oxide （GO） nanosheets and 1D 2，2，6，6-tetramethylpiperidin-1-oxyl （TEMPO）-oxidized cellulose nanofibers （TOCN） were assembled into GO/TOCN aerogels via a low temperature hydrothermal and freeze-drying process. The as-prepared GO/TOCN aerogels exhibited interconnected 3D network microstructures， a low density of 6.8 mg/cm3， a high porosity up to 99.2% and excellent mechanical flexibility. The high porosity in conjunction with their hydrophobicity （contact angle of 121.5°）， allowed the aerogels to absorb different organic liquids with absorption capacities up to 240 times of their own weight， depending on the density of the liquids. These results indicated that the aerogels were excellent candidates as sorbent materials for the clean-up of organic liquids. After five absorption-desorption cycles， the absorption capacity of the TOCN carbon aerogels could be regenerated up to 97% of the initial absorption capability， which demonstrated their excellent recyclability.
Keywords： graphene oxide； cellulose nanofiber； carbon aerogel
1 Introduction
Marine pollution resulting from organic pollutants or oil spills is a global concern because of its environmental and economic impact[1]. Efficient sorbent materials are more attractive for oil-spill cleanup because of the selective adsorption property and complete removal of the oil while causing no adverse effect to the environment[2]. Graphene oxide （GO）-based aerogels is an ideal sorbent material for marine oil-spill recovery owing to their low density， hydrophobicity， and excellent physical and chemical resistance[3-5]. However， there are several disadvantages that limit the application of GO-based aerogels. Owing to the strong π-π interactions between the graphene sheets， the GO layers tend to aggregate together， which inevitably reduces the specific surface area of the GO aerogels[6]. In addition， the GO-based aerogels are fragile because their three-dimensional （3D） structure is easily destroyed and collapses during their assembly and follow-up using process[7].
There are several synthetic strategies for fabricating 3D graphene aerogels， including hydrothermal treatment[8-9]， chemical vapor deposition（CVD）[10-11] ， and in situ reducing-assembly method[6，12-14]. In a recent report， graphene sheets were deposited directly on nickel foam by CVD methods under high temperature （1000℃） and harsh atmosphere conditions （Ar and H2）[15]. Ruoff and co-workers utilized hydrothermal reduction to produce 3D graphene aerogels that showed excellent mechanical and electrical properties[8]. However， relatively high temperatures （～200℃） and pressure conditions were required during the hydrothermal process， which were both time-consuming and energy-consuming. In general， most of the reported methods require harsh conditions or toxic chemicals， which motivated us to find a facile and green method of preparing GO-based aerogels. Cellulose nanofibers have recently attracted interest because of their superior properties such as high strength， high Young’s modulus， high aspect ratio， high specific surface area， low thermal expansion， and low density[16-18]. 2，2，6，6-tetramethyl-1-piperidinyloxy （TEMPO）-oxidized cellulose nanofibers （TOCNs）， which are made from native wood celluloses by TEMPO-mediated oxidation， exhibit a long aspect ratio （>100）. This implies that the TOCN are very flexible and can be easily interwoven into a strong network for the preparation of aerogels[19]. Therefore， TOCN are excellent candidate materials for obtaining mechanically flexible aerogels.
Despite the recent progress in aerogel preparative methods， it is still a challenge to develop a green synthetic strategy for environmentally-friendly 3D graphene aerogels with the excellent mechanical flexible property under relatively mild conditions. Herein， we report a new strategy of the in situ incorporation of functional TOCN into graphene aerogels by a low-temperature hydrothermal process. In this strategy， the TOCNs were uniformly dispersed in GO-based aerogels and efficiently prevented the stacking of GO nanosheets. The mechanical properties of the GO/TOCN aerogels were significantly improved as a result of the incorporation of the TOCN. The one-step， green， and facile synthetic method did not require any reducing agents to fabricate the GO/TOCN aerogels. These aerogels possessed a very low apparent density of 6.8 mg/cm3 and a high porosity of 99.2%. In addition， they showed excellent mechanical stability， high hydrophobicity， and super-oleophilicity， which allowed them to be used as sorbents. The GO/TOCN aerogels could absorb a variety of oils and organic liquids， demonstrating high sorption capacities approaching 240 times their own weight， and excellent recyclability.
2 Experimental
2.1 Materials
Natural graphite powder （325 mesh） was purchased from Qingdao Huatai Lubricant Sealing S&T Co.， Ltd. （Qingdao， China）. A commercial softwood kraft pulp sample was provided by Northwood Pulp and Timber Limited （Canada）. The pulp was composed of approximately 95 wt% of α-cellulose. It was thoroughly washed with deionized water and used in its never-dried form. TEMPO was purchased from Sigma-Aldrich. Sodium bromide （NaBr）， sodium hydroxide （NaOH）， sodium hypochlorite （NaClO） solution， potassium permanganate （KMnO4）， sulfuric acid （H2SO4， 98%）， hydrogen peroxide （H2O2）， and sodium nitrate （NaNO3） were procured from Longood Medicine （Beijing）. All chemicals were of laboratory grade and used without further purification. Ultrapure water （18.2 MΩ·cm） was produced by a Heal Force purification system and used to prepare all aqueous solutions. 2.2 Preparation of GO
GO is prepared according to Hummer’s method[20] . Specifically， 3.0 g of graphite flakes were mixed with 1.5 g of NaNO3 and cooled to 0℃. Concentrated H2SO4 （69 mL） was then added to the mixture and stirred uniformly， followed by the slow addition of 9.0 g of KMnO4 to the mixture. The reaction temperature was kept below 20℃ during the addition of the KMnO4. Subsequently， the reaction was warmed to 35℃ and stirred for 30 min. Next， 138 mL of distilled water was added slowly to the mixture while maintaining the reaction temperature at 98℃ for 15 min. The mixture was then cooled down and an additional 420 mL of water and 3 mL of 30% H2O2 were added. After cooling in air， the mixture was washed with distilled water over a filter. The aqueous GO solution was obtained after 30 min of sonication.
2.3 Preparation of TOCN
The preparation method of TOCN has been previously reported[21]. Briefly， the cellulose fibers （1 g） were suspended in water （100 mL） containing TEMPO （0.016 g， 0.1 mmol） and sodium bromide （0.1 g， 1 mmol）. The 12% NaClO solution was adjusted to pH value 10 by the addition of 0.1 mol/L HCl. The TEMPO-mediated oxidation was initiated by the addition of the desired amount of the NaClO solution （5.0 mmol NaClO per gram of cellulose） and continued at room temperature with stirring at 500 r/min. The reaction mixture was maintained at pH value 10 by adding 0.5 mol/L NaOH using a pH stat until no further NaOH consumption could be observed. The TEMPO-oxidized cellulose was thoroughly washed with water by centrifugation and then treated several times with high-pressure homogenization.
2.4 Preparation of GO/TOCN aerogels
The aqueous dispersions of GO （6 mg/mL） and TOCN （3 mg/mL） were mixed in equivalent volumes to form a suspensions. The mixture was then homogeneously dispersed by ultrasonic treatment for 20 min. Subsequently， the sample was heated to 95℃ in an oil bath for 6 h to yield the gel which was submerged in liquid nitrogen to rapidly freeze the solution. The frozen sample was placed under vacuum （freeze-dried） to produce the GO/TOCN aerogel. The shape of the mold controlled the shape of the aerogel.
2.5 Characterization
Transmission electron microscopy （TEM JEM-1010） images were observed at an accelerated voltage of 80 kV. The surface morphology of the GO/TOCN aerogels was observed by scanning electron microscopy （SEM， S-3400 N II）. The fractured specimens were attached to the holders with conductive double-sided carbon tape and sputter coated with gold to avoid charging during the observation. Fourier transform infrared （FT-IR） spectra were obtained using a spectrophotometer （Nicolet iN10-MX， ThermoScientific）. X-ray diffraction （XRD） patterns were recorded using a D8-Advance X-ray diffraction analyzer. The wetting properties were evaluated by contact angle tests， which were performed using by the CAST2.0 contact angle analysis system at room temperature （Solon Information Technology Co. Ltd.， Shanghai， China）. 2.6 Absorption capacity
The absorption capacity of the GO/TOCN aerogels was measured by immersing a piece of the aerogel in an organic liquid for 5 min until the saturation state was reached. The saturated aerogels were then weighed and the organic solvent absorption capacity （g/g） of the GO/TOCN aerogels was calculated as：
Qa=（msa-m0）/m0 （1）
Where Qa is absorption capacity， and msa and m0 are the weights of the aerogel saturated with organic liquids and the pristine GO/TOCN aerogel， respectively.
3 Results and discussion
3.1 The morphology of the GO， TOCN， and GO/TOCN aerogels
The GO/TOCN aerogels were readily prepared by mixing equal volumes of the aqueous dispersions of GO （6 mg/mL） and the TOCN aqueous （3 mg/mL） followed by hydrothermal treatment （Fig.1）. The details of this process are given in the exprimental section. The stable mixing aqueous dispersions were obtained because of the negative charges on both the GO nanosheets and the TOCN （Fig.1b）. After heating for 6 h， the GO/TOCN mixture changed in color from dark brown to black and the graphene monolith floated to the top of the water phase. Interestingly， except for the integrated cylinder gel， there were no separated graphene sheets and cellulose elsewhere， and a transparent solution is left at the bottom of the vessel （Fig.1c）. After pouring out the transparent solution， the GO/TOCN gels were attached to the bottom of the slant vessel （Fig.1d）. Owing to the reduction of GO， the π-π interactions between the graphene sheets increase， which led to the formation of a compact GO/TOCN gel. After freeze-drying， ultra-light GO/TOCN aerogels were obtained （Fig.1e， Fig.1f）. The shape of the molds was changed to get different shapes of GO/TOCN aerogels. The GO/TOCN aerogel was ultra-light； its density was （6.8±0.2） mg/cm3， which was comparable to that of CNT sponges （5～10 mg/cm3）[22] and graphene foam grown by CVD （5 mg/cm3）[15]. The GO/TOCN aerogel showed a high porosity of （99.2±0.2）%.
The morphologies of TOCN， GO， and GO/TOCN aerogel were characterized by transmission electron microscopy （TEM）. As shown in Fig.2a， cellulose nanofibrils with an average width of 2～5 nm and lengths of up to several micrometers were prepared via coupled TEMPO-mediated oxidation and homogenization. Owing to strong mechanical disintegration， the cellulose fibers were entangled with each other to form a dense web-like structure. Fig.2b shows that the GO nanosheets were 300～800 nm in width. After low temperature hydrothermal treatment， the GO nanosheets stacked into networks to form a hybrid gel， with the TOCN being entrapped within the networks and fixed between the GO nanosheets （Fig.2c）. SEM images revealed that the freeze-dried GO/TOCN aerogels had an inter-connected 3D macro-porous network with pore sizes ranging from sub-micrometers to several micrometers and pore walls consisting of ultrathin layers of stacked graphene sheets （Fig.2d）， which was similar to the typical structure of graphene aerogels reported in literatures[23-24]. 3.2 Chemical structure and interactions of the GO/TOCN aerogels
To investigate the chemical structure and possible interactions between the GO nanosheets and TOCN in the aerogels， FT-IR spectroscopic analyses were performed and XRD patterns were studied. Fig.3 shows the FT-IR spectra of GO， TOCN， and GO/TOCN aerogels. GO exhibited several absorption bands at 3340， 1730， 1606， 1218， and 1047 cm-1， which were attributed to the O—H， C=O， aromatic C=C， C—O—C， and C—O stretching vibrations， respectively[25]. The peaks at 3340 cm-1 and 1047 cm-1 were attributed to the O—H stretching vibrations and the C—O bond of cellulose[26]， and the peaks at 2890 cm–1 and 1417 cm–1 corresponded to the C—H stretching and bending of the —CH2 groups， respectively[27]. In addition， the peak at 1606 cm–1 was assigned to the H—O—H stretching vibrations of absorbed water in the carbohydrate and the peak at 1730 cm–1 confirmed the presence of carboxylic acid groups in the TOCN. For the GO/TOCN aerogels， the peak at 1730 cm–1 became significantly weaker after the hydrothermal process and was similar to that of graphite， indicating that GO may have been reduced to graphene. Nevertheless， there was still a part of oxygen–containing functional groups， deriving from the TOCN in the GO/TOCN hybrid aerogels.
The XRD patterns （Fig.3b） showed three characteristic peaks for
the TOCN centered at 2q=14.7°， 16.8°， and 22.7°， which corresponded to the （1-10）， （110）， and （200） planes of cellulose I， respectively[28]. Pure GO exhibited a sharp characteristic diffraction peak at 2q=10.6°[29] . For the GO/TOCN aerogel， the original crystal structure of the TOCN did not change and the diffraction peak of GO completely disappeared， indicating the removal of most of the oxygen-containing groups of GO. Instead， a new broad diffraction peak appeared at 2q=25°， which was similar to that of graphite， indicating that GO was successfully reduced to graphene.
3.3 Mechanical properties of GO/TOCN aerogels
The GO/TOCN aerogels showed good mechanical properties and were able to endure an outside compression up to 80% volume reduction. They recovered almost their entire original volume without any obvious changes after the release of the compression （Fig.4b）. As shown in Fig.4a， the stress-strain curves of the GO/TOCN aerogel were set at a maximum strain of 80%. The loading process showed two distinct regions， including a linear elastic region at compressive strain （ε）<55% during which the stress increased linearly with the compressive strain. In the linear elastic region， the compressive stress gradually increased with the strain， owing to the elastic bending of the GO nanosheets and nanofibers. The densification region at ε>55% was marked by a rapid increase of compressive stress because of the continuously decreasing pore volume. Hysteresis loops can represent the typical stress-strain diagrams of elastomeric open cell foams because of energy dissipation[30]. We also tested the cyclic compression recovery of the GO/TOCN aerogels under the loading stress at 80% （Fig. 4a inset）. The GO/TOCN aerogels retained their initial shape after twenty cyclic compression processes. The excellent mechanical properties of the GO/TOCN aerogels illustrate their potential application for the absorption of organic liquids by simple squeezing methods （Fig.4c）. The inset in Fig.4a shows the recovery capacity of the GO/TOCN aerogels at maximum strains of 80% for twenty cycles.
3.4 Organic liquids absorption by GO/TOCN aerogels
The GO/TOCN aerogels are hydrophobic and oleophilic with a water contact angle of 121.5° and an oil contact angle of 0° （Fig.5b inset）， which endows them with high selectivity for absorbing organic pollutants and oil from water. As shown in Fig.5a， when the aerogel was immersed into the n-hexane （colored with Sudan III） water mixture， it floated on the water surface and selectively absorbed n-hexane completely in 15 s， exhibiting excellent selectivity adsorption. The average absorption rate of the aerogel was 23 g/（g·s） for the aerogel， which is faster than the previous reported absorption rate for the pure graphene （0.57 g/（g·s））[31] and comparable to the ultra-light graphene aerogels （27 g/（g·s））[5]. This may attributed to the hydrophobic and highly porous nature of the aerogel， which allowed the organic liquids to rapidly enter the 3D pores.
As shown in Fig.5b， a variety of organic liquids and oils were used to investigate the sorption capacity of the GO/TOCN aerogels. These solvents and oils are common pollutants in our daily life as well as from industry. The weight of the absorbed substance per unit weight of the dried GO/TOCN aerogels was calculated to evaluate the sorption capacity quantitatively. It was found that the sorption capacity of the GO/TOCN aerogels reached up to 100～240 times its own weight depending on the density of the organic solvents. We also compared the sorption capacities of previously reported sorbents with that of the GO/TOCN aerogels and it is obvious that the GO/TOCN aerogels show much higher sorption capacity than many absorbents reported previously （Table 1）； the sorption capacity of the prepared aerogels was increased over that of reduced graphene oxide foam by 5～40 times[32]， and exfoliated graphite by 60～90 times[33] ， although it was still lower than these of carbon aerogels from bacterial cellulose[3]， nitrogen-doped graphene foam by 200～600 times[34] and ultra-flyweight aerogels by 215～913 times[35] . However， compared with carbon， graphene， and CNT aerogels[34-35] ， the GO/TOCN aerogels were prepared in a facile manner at low temperatures， which implies better energy conservation and environmental protection.
Since most organic pollutants such as crude oil and toluene are either useful or expensive raw materials， their recycling has become more meaningful. There are two common and simple methods for recycling organic liquids. The first method is distillation， which is suitable for the removal of valuable pollutants or those with low boiling points such as alcohols and esters. For pollutants with high boiling points， squeezing is an attractive alternative method. Considering the low boiling points of ethanol， absorption and distillation at 80℃ of the aerogel are performed repeatedly for five times （Fig.5c）. All the ethanol can be driven out of the aerogel by distillation and with less than 3% loss in the Qwt after five cycles of testing including the experimental error during the absorption–distillation process. This indicated that the absorption-distillation cycle is well applicable for the low boiling organic liquids. However， the high energy consumption and the inefficiency of absorption-distillation for organic solvents with high boiling points make this method less attractive compared to the simple squeezing method. The adsorption-squeezing method was also tested using gasoline with the strain set at 50%. As shown in Fig.5d， it was evident that the absorption capacity of the aerogels suffered less than 5% loss in Qwt after five cycles of testing. This was within the limits of experimental error because of the super-elastic property of the aerogel， which was comparable with the stability under the distillation process. 4 Conclusions
In conclusion， we successfully prepared graphene oxide （GO） and TOCN aerogels via a one-step facile hydrothermal process. The TOCN interacted with GO nanosheets via π-π interactions to form a strong network， which functioned as a template for the assembly of GO nanosheets into a 3D GO/TOCN aerogel. In addition， the TOCN retained in the skeleton of GO also acted as an intensive component， which endowed the GO/TOCN aerogels with mechanical strength. The as-prepared GO/TOCN aerogels， with a low density of 6.8 mg/cm3 and hydrophobicity （water contact angle of 121.5°） were able to absorb different organic liquids with a capacity varying from 100 to 240 times of their weight， depending on the density of the organics. The cellulose component efficiently prevented the GO aerogel from collapsing and the 3D structure of the GO/TOCN aerogels was able to fully recover without fracture even after 90% compression. It also showed outstanding recyclability via distillation and direction-squeezed methods， illustrating its promising application in the absorption of oil and organic liquids.
Notes
The authors declare no competing financial interest.
Acknowledgments
This work was supported by the State Key Laboratory of Pulp and Paper Engineering （201750）， Chinese Ministry of Education （113014A）， National Natural Science Foundation of China （21404011， 21674013）， and Chinese Ministry of Education （113014A）.
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Keywords： graphene oxide； cellulose nanofiber； carbon aerogel
1 Introduction
Marine pollution resulting from organic pollutants or oil spills is a global concern because of its environmental and economic impact[1]. Efficient sorbent materials are more attractive for oil-spill cleanup because of the selective adsorption property and complete removal of the oil while causing no adverse effect to the environment[2]. Graphene oxide （GO）-based aerogels is an ideal sorbent material for marine oil-spill recovery owing to their low density， hydrophobicity， and excellent physical and chemical resistance[3-5]. However， there are several disadvantages that limit the application of GO-based aerogels. Owing to the strong π-π interactions between the graphene sheets， the GO layers tend to aggregate together， which inevitably reduces the specific surface area of the GO aerogels[6]. In addition， the GO-based aerogels are fragile because their three-dimensional （3D） structure is easily destroyed and collapses during their assembly and follow-up using process[7].
There are several synthetic strategies for fabricating 3D graphene aerogels， including hydrothermal treatment[8-9]， chemical vapor deposition（CVD）[10-11] ， and in situ reducing-assembly method[6，12-14]. In a recent report， graphene sheets were deposited directly on nickel foam by CVD methods under high temperature （1000℃） and harsh atmosphere conditions （Ar and H2）[15]. Ruoff and co-workers utilized hydrothermal reduction to produce 3D graphene aerogels that showed excellent mechanical and electrical properties[8]. However， relatively high temperatures （～200℃） and pressure conditions were required during the hydrothermal process， which were both time-consuming and energy-consuming. In general， most of the reported methods require harsh conditions or toxic chemicals， which motivated us to find a facile and green method of preparing GO-based aerogels. Cellulose nanofibers have recently attracted interest because of their superior properties such as high strength， high Young’s modulus， high aspect ratio， high specific surface area， low thermal expansion， and low density[16-18]. 2，2，6，6-tetramethyl-1-piperidinyloxy （TEMPO）-oxidized cellulose nanofibers （TOCNs）， which are made from native wood celluloses by TEMPO-mediated oxidation， exhibit a long aspect ratio （>100）. This implies that the TOCN are very flexible and can be easily interwoven into a strong network for the preparation of aerogels[19]. Therefore， TOCN are excellent candidate materials for obtaining mechanically flexible aerogels.
Despite the recent progress in aerogel preparative methods， it is still a challenge to develop a green synthetic strategy for environmentally-friendly 3D graphene aerogels with the excellent mechanical flexible property under relatively mild conditions. Herein， we report a new strategy of the in situ incorporation of functional TOCN into graphene aerogels by a low-temperature hydrothermal process. In this strategy， the TOCNs were uniformly dispersed in GO-based aerogels and efficiently prevented the stacking of GO nanosheets. The mechanical properties of the GO/TOCN aerogels were significantly improved as a result of the incorporation of the TOCN. The one-step， green， and facile synthetic method did not require any reducing agents to fabricate the GO/TOCN aerogels. These aerogels possessed a very low apparent density of 6.8 mg/cm3 and a high porosity of 99.2%. In addition， they showed excellent mechanical stability， high hydrophobicity， and super-oleophilicity， which allowed them to be used as sorbents. The GO/TOCN aerogels could absorb a variety of oils and organic liquids， demonstrating high sorption capacities approaching 240 times their own weight， and excellent recyclability.
2 Experimental
2.1 Materials
Natural graphite powder （325 mesh） was purchased from Qingdao Huatai Lubricant Sealing S&T Co.， Ltd. （Qingdao， China）. A commercial softwood kraft pulp sample was provided by Northwood Pulp and Timber Limited （Canada）. The pulp was composed of approximately 95 wt% of α-cellulose. It was thoroughly washed with deionized water and used in its never-dried form. TEMPO was purchased from Sigma-Aldrich. Sodium bromide （NaBr）， sodium hydroxide （NaOH）， sodium hypochlorite （NaClO） solution， potassium permanganate （KMnO4）， sulfuric acid （H2SO4， 98%）， hydrogen peroxide （H2O2）， and sodium nitrate （NaNO3） were procured from Longood Medicine （Beijing）. All chemicals were of laboratory grade and used without further purification. Ultrapure water （18.2 MΩ·cm） was produced by a Heal Force purification system and used to prepare all aqueous solutions. 2.2 Preparation of GO
GO is prepared according to Hummer’s method[20] . Specifically， 3.0 g of graphite flakes were mixed with 1.5 g of NaNO3 and cooled to 0℃. Concentrated H2SO4 （69 mL） was then added to the mixture and stirred uniformly， followed by the slow addition of 9.0 g of KMnO4 to the mixture. The reaction temperature was kept below 20℃ during the addition of the KMnO4. Subsequently， the reaction was warmed to 35℃ and stirred for 30 min. Next， 138 mL of distilled water was added slowly to the mixture while maintaining the reaction temperature at 98℃ for 15 min. The mixture was then cooled down and an additional 420 mL of water and 3 mL of 30% H2O2 were added. After cooling in air， the mixture was washed with distilled water over a filter. The aqueous GO solution was obtained after 30 min of sonication.
2.3 Preparation of TOCN
The preparation method of TOCN has been previously reported[21]. Briefly， the cellulose fibers （1 g） were suspended in water （100 mL） containing TEMPO （0.016 g， 0.1 mmol） and sodium bromide （0.1 g， 1 mmol）. The 12% NaClO solution was adjusted to pH value 10 by the addition of 0.1 mol/L HCl. The TEMPO-mediated oxidation was initiated by the addition of the desired amount of the NaClO solution （5.0 mmol NaClO per gram of cellulose） and continued at room temperature with stirring at 500 r/min. The reaction mixture was maintained at pH value 10 by adding 0.5 mol/L NaOH using a pH stat until no further NaOH consumption could be observed. The TEMPO-oxidized cellulose was thoroughly washed with water by centrifugation and then treated several times with high-pressure homogenization.
2.4 Preparation of GO/TOCN aerogels
The aqueous dispersions of GO （6 mg/mL） and TOCN （3 mg/mL） were mixed in equivalent volumes to form a suspensions. The mixture was then homogeneously dispersed by ultrasonic treatment for 20 min. Subsequently， the sample was heated to 95℃ in an oil bath for 6 h to yield the gel which was submerged in liquid nitrogen to rapidly freeze the solution. The frozen sample was placed under vacuum （freeze-dried） to produce the GO/TOCN aerogel. The shape of the mold controlled the shape of the aerogel.
2.5 Characterization
Transmission electron microscopy （TEM JEM-1010） images were observed at an accelerated voltage of 80 kV. The surface morphology of the GO/TOCN aerogels was observed by scanning electron microscopy （SEM， S-3400 N II）. The fractured specimens were attached to the holders with conductive double-sided carbon tape and sputter coated with gold to avoid charging during the observation. Fourier transform infrared （FT-IR） spectra were obtained using a spectrophotometer （Nicolet iN10-MX， ThermoScientific）. X-ray diffraction （XRD） patterns were recorded using a D8-Advance X-ray diffraction analyzer. The wetting properties were evaluated by contact angle tests， which were performed using by the CAST2.0 contact angle analysis system at room temperature （Solon Information Technology Co. Ltd.， Shanghai， China）. 2.6 Absorption capacity
The absorption capacity of the GO/TOCN aerogels was measured by immersing a piece of the aerogel in an organic liquid for 5 min until the saturation state was reached. The saturated aerogels were then weighed and the organic solvent absorption capacity （g/g） of the GO/TOCN aerogels was calculated as：
Qa=（msa-m0）/m0 （1）
Where Qa is absorption capacity， and msa and m0 are the weights of the aerogel saturated with organic liquids and the pristine GO/TOCN aerogel， respectively.
3 Results and discussion
3.1 The morphology of the GO， TOCN， and GO/TOCN aerogels
The GO/TOCN aerogels were readily prepared by mixing equal volumes of the aqueous dispersions of GO （6 mg/mL） and the TOCN aqueous （3 mg/mL） followed by hydrothermal treatment （Fig.1）. The details of this process are given in the exprimental section. The stable mixing aqueous dispersions were obtained because of the negative charges on both the GO nanosheets and the TOCN （Fig.1b）. After heating for 6 h， the GO/TOCN mixture changed in color from dark brown to black and the graphene monolith floated to the top of the water phase. Interestingly， except for the integrated cylinder gel， there were no separated graphene sheets and cellulose elsewhere， and a transparent solution is left at the bottom of the vessel （Fig.1c）. After pouring out the transparent solution， the GO/TOCN gels were attached to the bottom of the slant vessel （Fig.1d）. Owing to the reduction of GO， the π-π interactions between the graphene sheets increase， which led to the formation of a compact GO/TOCN gel. After freeze-drying， ultra-light GO/TOCN aerogels were obtained （Fig.1e， Fig.1f）. The shape of the molds was changed to get different shapes of GO/TOCN aerogels. The GO/TOCN aerogel was ultra-light； its density was （6.8±0.2） mg/cm3， which was comparable to that of CNT sponges （5～10 mg/cm3）[22] and graphene foam grown by CVD （5 mg/cm3）[15]. The GO/TOCN aerogel showed a high porosity of （99.2±0.2）%.
The morphologies of TOCN， GO， and GO/TOCN aerogel were characterized by transmission electron microscopy （TEM）. As shown in Fig.2a， cellulose nanofibrils with an average width of 2～5 nm and lengths of up to several micrometers were prepared via coupled TEMPO-mediated oxidation and homogenization. Owing to strong mechanical disintegration， the cellulose fibers were entangled with each other to form a dense web-like structure. Fig.2b shows that the GO nanosheets were 300～800 nm in width. After low temperature hydrothermal treatment， the GO nanosheets stacked into networks to form a hybrid gel， with the TOCN being entrapped within the networks and fixed between the GO nanosheets （Fig.2c）. SEM images revealed that the freeze-dried GO/TOCN aerogels had an inter-connected 3D macro-porous network with pore sizes ranging from sub-micrometers to several micrometers and pore walls consisting of ultrathin layers of stacked graphene sheets （Fig.2d）， which was similar to the typical structure of graphene aerogels reported in literatures[23-24]. 3.2 Chemical structure and interactions of the GO/TOCN aerogels
To investigate the chemical structure and possible interactions between the GO nanosheets and TOCN in the aerogels， FT-IR spectroscopic analyses were performed and XRD patterns were studied. Fig.3 shows the FT-IR spectra of GO， TOCN， and GO/TOCN aerogels. GO exhibited several absorption bands at 3340， 1730， 1606， 1218， and 1047 cm-1， which were attributed to the O—H， C=O， aromatic C=C， C—O—C， and C—O stretching vibrations， respectively[25]. The peaks at 3340 cm-1 and 1047 cm-1 were attributed to the O—H stretching vibrations and the C—O bond of cellulose[26]， and the peaks at 2890 cm–1 and 1417 cm–1 corresponded to the C—H stretching and bending of the —CH2 groups， respectively[27]. In addition， the peak at 1606 cm–1 was assigned to the H—O—H stretching vibrations of absorbed water in the carbohydrate and the peak at 1730 cm–1 confirmed the presence of carboxylic acid groups in the TOCN. For the GO/TOCN aerogels， the peak at 1730 cm–1 became significantly weaker after the hydrothermal process and was similar to that of graphite， indicating that GO may have been reduced to graphene. Nevertheless， there was still a part of oxygen–containing functional groups， deriving from the TOCN in the GO/TOCN hybrid aerogels.
The XRD patterns （Fig.3b） showed three characteristic peaks for
the TOCN centered at 2q=14.7°， 16.8°， and 22.7°， which corresponded to the （1-10）， （110）， and （200） planes of cellulose I， respectively[28]. Pure GO exhibited a sharp characteristic diffraction peak at 2q=10.6°[29] . For the GO/TOCN aerogel， the original crystal structure of the TOCN did not change and the diffraction peak of GO completely disappeared， indicating the removal of most of the oxygen-containing groups of GO. Instead， a new broad diffraction peak appeared at 2q=25°， which was similar to that of graphite， indicating that GO was successfully reduced to graphene.
3.3 Mechanical properties of GO/TOCN aerogels
The GO/TOCN aerogels showed good mechanical properties and were able to endure an outside compression up to 80% volume reduction. They recovered almost their entire original volume without any obvious changes after the release of the compression （Fig.4b）. As shown in Fig.4a， the stress-strain curves of the GO/TOCN aerogel were set at a maximum strain of 80%. The loading process showed two distinct regions， including a linear elastic region at compressive strain （ε）<55% during which the stress increased linearly with the compressive strain. In the linear elastic region， the compressive stress gradually increased with the strain， owing to the elastic bending of the GO nanosheets and nanofibers. The densification region at ε>55% was marked by a rapid increase of compressive stress because of the continuously decreasing pore volume. Hysteresis loops can represent the typical stress-strain diagrams of elastomeric open cell foams because of energy dissipation[30]. We also tested the cyclic compression recovery of the GO/TOCN aerogels under the loading stress at 80% （Fig. 4a inset）. The GO/TOCN aerogels retained their initial shape after twenty cyclic compression processes. The excellent mechanical properties of the GO/TOCN aerogels illustrate their potential application for the absorption of organic liquids by simple squeezing methods （Fig.4c）. The inset in Fig.4a shows the recovery capacity of the GO/TOCN aerogels at maximum strains of 80% for twenty cycles.
3.4 Organic liquids absorption by GO/TOCN aerogels
The GO/TOCN aerogels are hydrophobic and oleophilic with a water contact angle of 121.5° and an oil contact angle of 0° （Fig.5b inset）， which endows them with high selectivity for absorbing organic pollutants and oil from water. As shown in Fig.5a， when the aerogel was immersed into the n-hexane （colored with Sudan III） water mixture， it floated on the water surface and selectively absorbed n-hexane completely in 15 s， exhibiting excellent selectivity adsorption. The average absorption rate of the aerogel was 23 g/（g·s） for the aerogel， which is faster than the previous reported absorption rate for the pure graphene （0.57 g/（g·s））[31] and comparable to the ultra-light graphene aerogels （27 g/（g·s））[5]. This may attributed to the hydrophobic and highly porous nature of the aerogel， which allowed the organic liquids to rapidly enter the 3D pores.
As shown in Fig.5b， a variety of organic liquids and oils were used to investigate the sorption capacity of the GO/TOCN aerogels. These solvents and oils are common pollutants in our daily life as well as from industry. The weight of the absorbed substance per unit weight of the dried GO/TOCN aerogels was calculated to evaluate the sorption capacity quantitatively. It was found that the sorption capacity of the GO/TOCN aerogels reached up to 100～240 times its own weight depending on the density of the organic solvents. We also compared the sorption capacities of previously reported sorbents with that of the GO/TOCN aerogels and it is obvious that the GO/TOCN aerogels show much higher sorption capacity than many absorbents reported previously （Table 1）； the sorption capacity of the prepared aerogels was increased over that of reduced graphene oxide foam by 5～40 times[32]， and exfoliated graphite by 60～90 times[33] ， although it was still lower than these of carbon aerogels from bacterial cellulose[3]， nitrogen-doped graphene foam by 200～600 times[34] and ultra-flyweight aerogels by 215～913 times[35] . However， compared with carbon， graphene， and CNT aerogels[34-35] ， the GO/TOCN aerogels were prepared in a facile manner at low temperatures， which implies better energy conservation and environmental protection.
Since most organic pollutants such as crude oil and toluene are either useful or expensive raw materials， their recycling has become more meaningful. There are two common and simple methods for recycling organic liquids. The first method is distillation， which is suitable for the removal of valuable pollutants or those with low boiling points such as alcohols and esters. For pollutants with high boiling points， squeezing is an attractive alternative method. Considering the low boiling points of ethanol， absorption and distillation at 80℃ of the aerogel are performed repeatedly for five times （Fig.5c）. All the ethanol can be driven out of the aerogel by distillation and with less than 3% loss in the Qwt after five cycles of testing including the experimental error during the absorption–distillation process. This indicated that the absorption-distillation cycle is well applicable for the low boiling organic liquids. However， the high energy consumption and the inefficiency of absorption-distillation for organic solvents with high boiling points make this method less attractive compared to the simple squeezing method. The adsorption-squeezing method was also tested using gasoline with the strain set at 50%. As shown in Fig.5d， it was evident that the absorption capacity of the aerogels suffered less than 5% loss in Qwt after five cycles of testing. This was within the limits of experimental error because of the super-elastic property of the aerogel， which was comparable with the stability under the distillation process. 4 Conclusions
In conclusion， we successfully prepared graphene oxide （GO） and TOCN aerogels via a one-step facile hydrothermal process. The TOCN interacted with GO nanosheets via π-π interactions to form a strong network， which functioned as a template for the assembly of GO nanosheets into a 3D GO/TOCN aerogel. In addition， the TOCN retained in the skeleton of GO also acted as an intensive component， which endowed the GO/TOCN aerogels with mechanical strength. The as-prepared GO/TOCN aerogels， with a low density of 6.8 mg/cm3 and hydrophobicity （water contact angle of 121.5°） were able to absorb different organic liquids with a capacity varying from 100 to 240 times of their weight， depending on the density of the organics. The cellulose component efficiently prevented the GO aerogel from collapsing and the 3D structure of the GO/TOCN aerogels was able to fully recover without fracture even after 90% compression. It also showed outstanding recyclability via distillation and direction-squeezed methods， illustrating its promising application in the absorption of oil and organic liquids.
Notes
The authors declare no competing financial interest.
Acknowledgments
This work was supported by the State Key Laboratory of Pulp and Paper Engineering （201750）， Chinese Ministry of Education （113014A）， National Natural Science Foundation of China （21404011， 21674013）， and Chinese Ministry of Education （113014A）.
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