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Superhydrophobic Graphene/Cellulose/Silica Aerogel with Hierarchical Structure as Superabsorbers for High Efficiency Selective Oil Absorption and Recovery Hao-Yang Mi, Xin Jing, Han-Xiong Huang, Xiangfang Peng, and Lih-Sheng Turng Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04388 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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Industrial & Engineering Chemistry Research

Superhydrophobic Graphene/Cellulose/Silica Aerogel with Hierarchical Structure as Superabsorbers for High Efficiency Selective Oil Absorption and Recovery

Hao-Yang Mi1,2,3, Xin Jing1,2,3*, Han-Xiong Huang1, Xiang-Fang Peng1, Lih-Sheng Turng2,3*

1

Department of Industrial Equipment and Control Engineering, South China University of

Technology, Guangzhou, 510640, China 2

Wisconsin Institutes for Discovery, University of Wisconsin–Madison, Madison, WI 53715,

USA 3

Department of Mechanical Engineering, University of Wisconsin–Madison, Madison, WI

53706, USA

Corresponding Authors: L.S. Turng E-mail: [email protected] X. Jing E-mail: [email protected]

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Carbon-based aerogels have been recognized as a promising 3D superabsorbent material for oil absorption due to their high absorption capacity. However, their selective absorption efficiency is limited because of their relatively low hydrophobicity. Here, we report the fabrication of fluorinated hybrid aerogel (FHA) consisting of graphene oxide (GO), cellulose nanofibrils (CNF), and silica nanoparticles by one-pot hydrothermal synthesis and subsequent freeze drying and chemical vapor deposition modification. The CNF in FHA prevented volume shrinkage, greatly reduced the bulk density, and increased the surface area, while the GO retained its mechanical strength and the free-standing characteristic of FHA. Silica particles and CNF fibrils created hierarchical structures on pore walls, and the grafted fluorochains reduced the surface energy. The synergistic effect of hierarchical structure and low surface energy contributed to the excellent superhydrophobicity (water contact angle of 157°) and water repellency (contact angle hysteresis less than 1°) of FHA. Meanwhile, FHA maintained superoleophilicity and showed extraordinary absorption efficiency (~ 100%) and relatively high absorption capacity (39 ~ 68 times weight gain) to various oils and chemical solvents. This high performance can be maintained in repetitive use. Furthermore, a selfdriven oil collection device assembled with FHA demonstrated its diverse applications in water remediation.

Keywords: Superhydrophobicity; selective oil absorption; aerogel; graphene; cellulose nanofibrils


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1. Introduction Water pollution from oil leaks and organic chemicals has been a serious environmental problem and is calling for high performance absorbent materials that are capable of selectively recovering oil from water with high capacity and efficiency.1, 2 Nature-inspired superhydrophobic materials have received considerable attention in recent years due to their excellent water repellency and have been used in many applications like oil/water separation, self-cleaning, corrosion resistance, anti-icing, and drag reduction.3-6 However, there are more requirements for an ideal superabsorbent material, such as the following: (1) light weight and porous structure that allows for the absorption of a great amount of oil, (2) superhydrophobic/superoleophilic properties that ensure high efficiency in selective oil absorption, (3) high property stability and reusability, and (4) low fabrication cost and easy to handle. Although numerous absorbent materials such as mesoporous silica, resin, expanded graphite, activated carbon, modified sponge, and carbonaceous aerogels have been developed so far, challenges still remain in producing high performance superabsorbers that fulfill all requirements.7-11 Low oil absorption capacity, poor water/oil selectivity, and high fabrication cost limit the wide use of these materials.12 Recently, carbon-based three-dimensional (3D) aerogels have become a rising star as a new generation of superabsorbent materials because of their ultralow density, high surface area, and remarkable oil absorption capacity.13, 14 Bi et al. reported a carbon fiber aerogel prepared by pyrolysis of raw cotton and showed a high absorption capacity of 50 ~110 g/g.15 Similarly, Wu et al. prepared a carbon nanofiber aerogel through pyrolysis of bacterial cellulose and achieved an absorption capacity of 100 ~ 300 g/g and a water contact angle (WCA) of 110 ~ 130°.16 Carbon nanotube (CNT) sponges with an absorption capacity of 90 ~175 g/g were prepared by Gui et al. via chemical vapor deposition (CVD) using dichlorobenzene as the carbon source.17 Gao et al. prepared ultra-flyweight carbon aerogel 3 ACS Paragon Plus Environment


using CNT and giant graphene oxide (GO) through freeze drying. This low density aerogel showed an extremely high absorption capacity of 200 ~ 750 g/g.11 In these studies, either high quality giant GO and CNT were required as constructing materials, or inert gas protected pyrolysis and high temperature (~ 1000 °C) particle growth were employed as fabrication method, which have high fabrication cost and energy consumption. Moreover, although these carbonaceous aerogels showed high absorption capacities, their WCAs were usually less than 140°, which is not in the superhydrophobic region. Low WCAs typically correspond to low selectivity in separation of oil from water. The selectivity of aerogels was not reported in most studies, but carbonized sponges derived from melamine foam, which had a WCA ranging from 120° to 140°, did show a 20 ~ 30 g/g water absorption rate.18 Aerogels made of hydrophobic polymer or cellulose have also been used as absorbent materials. For example, a poly (vinylidene fluoride) (PVDF) aerogel prepared through vapor-induced phase inversion showed a high WCA of 151°, but only achieved an absorption capacity of 3~7 g/g.19 A cellulose aerogel modified with alkyl ketene dimmers achieved hydrophobicity with a WCA ranging from 100° to 140°, and a decreased water absorption rate was found as WCA increased.20 Mulyadi et al. fabricated cellulose aerogel absorbent materials by coating a hydrophobic polymer. Although the aerogel showed a relatively high WCA of 149°, its absorption capacity reached 25 ~45 g/g, and this capacity decreased by more than half in its second use.21 Duan et al. modified chitin sponges with methyltrichlorosilane (MTCS) via CVD and achieved a WCA of 145° and an absorption capacity of 30 ~ 55 g/g.22 It can be seen from these studies that fabricating aerogel that simultaneously have high absorption capacity and superhydrophobicity is fairly difficult. The fundamental basis for structured non-wetting surfaces is the well known Cassie–Baxter theory, which explains the relationship of apparent contact angle with the intrinsic contact angle and the structural parameters of a rough surface.23 Based on this theory, recent experimental studies have confirmed several approaches—including (1) introducing re-entrant surface textures, (2) 4 ACS Paragon Plus Environment

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creating two-tire or hierarchical structures, and (3) reducing surface energy—to enhance superhydrophobicity and even obtain superamphiphobic surfaces.24-27 The first two approaches enhance the fraction of liquid–gas interfacial area, which is referred to as “air pocket” in some studies.28, 29 The third criterion increases the intrinsic contact angle of the material. Although controlling the microstructure and surface energy has been very successful in designing superhydrophobic 2D surfaces, it remains a challenge and has been rarely researched for porous 3D aerogels. This is mainly because carbonaceous aerogels usually have smooth pore walls and their surface energy cannot be altered due to a lack of functional groups. In order to manipulate the hierarchical structures and surface energy, and to fabricate aerogels with superhydrophobicity and high selective oil absorption efficiency, in this study, a novel material system containing GO, cellulose nanofibrils (CNF), and silica (abbreviated to GCS particles for simplicity) was used to produce hybrid aerogels that have both hierarchical structures and functional groups that can be further modified. The GCS aerogel (GCSA) was prepared by one-pot hydrothermal reaction (HTR) followed by solvent exchange and freeze drying. Silica nanoparticles and CNF fibrils were preserved on the pore walls creating hierarchical structures together with the microsized pores. The surface energy was reduced by the grafting of perfluorodecyltriethoxysilane (PDTS). Using an optimized formula, the obtained fluorinated hybrid aerogel (FHA) showed unprecedented superhydrophobicity (WCA = 157.3°) and water repellency (CAH < 1°); meanwhile, it was still superoleophilic with an oil contact angle of 0°. Due to its high surface area and superhydrophobicity, FHA showed a relatively high absorption capacity (38 ~ 68 g/g) to various oils and chemical solvents and, most importantly, it demonstrated an extraordinary separation efficiency (~ 100%) for selective oil absorption from water. FHA also showed improved mechanical properties, as compared to CNF aerogel, as well as high reusability and property stability. Moreover, FHA was assembled into a simple device that can be used for self-driven oil 5 ACS Paragon Plus Environment


collection from water surfaces, thus demonstrating its versatile application in water remediation.

2. Experimental Section 2.1 Materials Graphite was purchased from Fisher Scientific. Silicon dioxide (silica) (particle size of 10–20 nm) was purchased from Sigma–Aldrich. CNF was prepared by the Forest Products Laboratory (FPL) according to our previously reported method.30 All other chemical reagents were purchased from Sigma–Aldrich and used as received. Deionized water was used throughout our research. 2.2 Synthesis of GO GO was synthesized according to the improved Hummer’s method reported by Marcano.31 Briefly, a mixture of graphite flakes (2 g) and potassium permanganate (KMnO4) (12 g) were added into a mixture of concentrated sulfuric acid (H2SO4)/phosphoric acid (H3PO4) (360:40 ml). The resulting mixture was stirred at 50 °C for 12 h. Afterwards, the mixture was cooled to room temperature and then poured onto a mixture of ice (~300 g) with hydrogen peroxide (H2O2) (5 ml, 30 wt.%). The mixture was centrifuged (8000 rpm for 30 min), and the supernatant was decanted away. The remaining solid was then washed in succession with DI water, 30% HCl, ethanol, and DI water again. For each wash, the filtrate was centrifuged (8000 rpm for 30 min) and the supernatant was decanted away. The resulting solid was further purified by dialysis against DI water for 3 days. After that, the solution was freeze-dried at –50 °C and 8 mbar for 5 days to obtain GO. 2.3 Synthesis of GCSA A quantity of 120 mg of GO, CNF, and silica particles with different ratios were dispersed in 20 mL of DI water with ultrasonication at 50 W for 30 min using a probe sonicator (Hielscher) in an ice bath. Then, 2% butanetetracarboxylic acid (BTCA) and 1.5% 6 ACS Paragon Plus Environment

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sodium hypophosphite (SHP) were dissolved in the solution by vortex. The GCS hydrogel was obtained by one-pot HTR at 180 °C for 6 hours. The hydrogel was quenched in terbutanol/water (50/50) mixture over night and subsequently frozen at –80 °C followed by freeze drying for 3 days to obtain GSCA. For comparison, seven aerogels with different formulas were prepared in this study, and detailed material nomenclature and formulas are listed in Table 1.

Table 1. Formula to prepare different aerogels for comparison. Unit: mg Formula GO CNF Silica

GOA 120 0 0

GSA 110 0 10

CNFA 0 120 0

CSA 0 110 10

G8C3SA 80 30 10

G1C1SA 55 55 10

G3C8SA 30 80 10

2.4 Modification of GCSA Fabricated aerogels were co-heated with 50 µL of perfluorodecyltriethoxysilane (PDTS) in a sealed desiccator at 175 °C under vacuum for 10 h to allow grafting of PDTS on the hydroxyl groups of CNF and silica. An “M” was added in the sample abbreviation for modified aerogels. 2.5 General Characterizations Transmission electron microscopy (TEM) analysis of GO and CNF was carried out on a FEI Tecnai 12 G2 at an acceleration voltage of 120 kV. Bruker bioscope catalyst atomic-force microscopy (AFM) was used to analyze the synthesized GO in a tapping mold. The morphology of the fabricated aerogels was observed using a fully digital LEO GEMINI 1530 scanning electron microscope (SEM) (Zeiss, Germany) at a voltage of 3 kV. The pore size was measured from SEM images using the Image Pro-Plus software. Fourier transform infrared spectroscopy (FTIR) spectra were recorded in transmittance mode using a Bruker Tensor 27 spectrometer (Thermo Scientific Instrument) in the range of 4000–600 cm-1 with a resolution of 4 cm-1. Raman spectra analysis of synthesized GO and GO aerogel obtained by 7 ACS Paragon Plus Environment


HTR was performed with a DXR Raman microspectrometer (Thermo Scientific). Raman spectra were recorded in the range of 100 to 3500 cm-1. The X-ray photoelectron spectroscopy (XPS) measurements were performed on an X-ray photoelectron spectrometer with a focused, monochromatic K-alpha X-ray source and a monoatomic/cluster ion gun (Thermo Scientific). The bulk densities of modified aerogels were measured by dividing the specimen mass by the volume via the volume displacement method in water. The volume retention was calculated by dividing the specimen volume with the volume of solution used in HTR. The Brunauer– Emmett–Teller (BET) surface area was measured using a Gemini VII 2390 surface area analyzer (Micromeritics Instrument Corp.). Compression tests were carried out using an Instron universal testing machine equipped with a 50 N load cell. Cylindrical specimens were compressed at a crosshead speed of 5 mm/min. The contact angle measurements were performed at room temperature with a Dataphysics OCA 15 optical contact angle measuring system using the sessile drop method. 2.6 Absorption Capacity The absorption capacities of modified aerogels to various oils and chemical solvents were assessed by weighing the sample before and after saturation. The weight gain (g/g) was calculated by,

q = (m − m0 ) / m0 where m0 and m represent the mass of the specimen before and after saturation. The reusability of FHA was evaluated over 10 absorption–desorption cycles to hexane and chloroform. Specimens were saturated in the solvents for 10 min until a constant weight was attained, then they were distilled at 150°C under vacuum in the absorption–desorption cycle. The weight changes in each cycle were recorded. 2.7 Selective Absorption Efficiency


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A quantity of 1 mL oil (hexane or chloroform) and 5 mL water were mixed in a 10 mL beaker. FHA was weighed and used to selectively absorb oil atop water (for hexane) or underwater (for chloroform). The weight change of the sample and the residual water were measured to calculate the efficiency in absorbing the oil and preventing water loss. The test was repeated 10 times on the same sample to investigate the stability of selectivity. The oil absorption rate was calculated by,

eo = 100 × (mo+ s − ms ) / mo where mo is the mass of the oil added to the water, ms is the mass of the sample, and mo+s is the mass of the sample after oil absorption. The water loss rate was calculated by,

ew = 100 × (mw − mrw ) / mw where mw is the mass of water initially added and mrw is the mass of residual water after oil absorption.

3. Results and Discussion 3.1. Microstructure and Chemical Characterization Figure 1 illustrates the synthesis procedure of a fluorinated hybrid aerogel (FHA) with a cylindrical geometry via a three step approach. (1) GO, CNF, and silica particles were dispersed in water by ultrasonication and self-assembled into a hydrogel during a hydrothermal reaction (HTR). (2) The GCS hydrogel was quenched in a terbutanol/water (50/50) solution for solvent exchange and subsequently lyophilized using a freeze drier to obtain GSCA. (3) MGSCA (or FHA) was obtained by grafting perfluorodecyltriethoxysilane (PDTS) onto GSCA through the CVD method. The FHA fabricated via this approach overcame several challenges faced by carbonaceous aerogels. First, the hybrid material system involved 2D graphene sheets and CNF amorphous regions, 1D CNF crystal fibrils, 9 ACS Paragon Plus Environment


and 0D silica nanoparticles that constructed a multidimensional topography and hierarchical structure composed of 3D micro-size pores and nano-sized fabrils and particles on FHA. Second, HTR induced an isotropic self-assembled porous structure that aided the freestanding ability and dimensional stability of the 3D material. Third, hydroxyl groups present on CNF and silica offered grafting sites to PDTS, and the introduction of hydrophobic chains reduced the surface energy of FHA. Fourth, the fabrication approach uses low cost constructing materials and mild processing conditions. Therefore, this method gave more flexibility to alter the structure and surface energy of the aerogels, thus opening a new path toward low-density, non-wetting 3D material fabrication. The following sections elaborate on the morphological, chemical, mechanical, wettability, and selective oil absorption properties of FHA, and provide a detailed understanding of superwettable surface manipulation and the versatile water remediation applications of FHA.

Figure 1. Schematic illustration of the FHA fabrication process. Figure 2 shows the morphology characteristics of GO and CNF. The AFM image (Figure 2a) and height profile (Figure 2b) indicate that the synthesized GO has a size of ~1 µm and a thickness of ~1 nm, which is comparable with commonly synthesized GO.32, 33 Although large area and giant GOs have been reported to have superior properties, they are generally difficult to synthesize and purify.11, 34 Figure 2c indicates that the synthesized GOs were successfully exfoliated and showed a 2D sheet morphology. It has been found that CNF consisted of crystalline nanofibrils and amorphous domains, which supports the process-induced kink 10 ACS Paragon Plus Environment

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formation.35 As shown in Figure 2d, the CNF fibrils used in this study have a diameter of ~10 nm and an aspect ratio of over 1000. Raman spectra showed that the G band shifted to a lower frequency and the ID/IG ratio decreased after GO was processed into GOA, thus indicating the reduction of GO into rGO (Figure S1a).36 However, XRD results proved that GO was only partially reduced since two broad peaks were present at 12.2° and 24.7° on GOA representing the content of GO and rGO, respectively (Figure S1b).37 Hence, some hydrophilic groups on GO were preserved during HTR, and the π–π interactions among the partially reduced GO (rGO) drove the self-assembly of the graphene nanosheets into a monolithic 3D hydrogel.38 CNF was able to be crosslinked by BTCA and formed into a hydrogel during HTR. FTIR results of CNF and CNFA verified that ester bonds were formed in CNFA and no obvious degradation occurred (Figure S2).

Figure 2. (a) AFM image of synthesized GO and (b) height profile of an individual GO sheet. TEM images of synthesized (c) GO and (d) CNF. The morphology and digital photo of fabricated aerogels are shown in Figure 3. The porous structure was very different for the aerogels fabricated using different formulas. During HTR, GO was partially reduced, and the π–π interactions caused the assembly of 11 ACS Paragon Plus Environment


graphene sheets and shrinkage of the volume. Meanwhile, water was able to be encapsulated into the assembled 3D construct due to the residual hydrophilic oxygenated groups.39 CNF was crosslinked by BTCA during HTR forming a hydrogel containing vast amounts of water. Obtained hydrogels were subjected to a solvent exchange by soaking them in a 50/50 terbutanol/water solution prior to lyophilization. Figure S3 demonstrated the effect of solvent exchange. GOA cracked and CNFA deformed without solvent exchanged, while the samples with solvent exchange well maintained cylindrical shape. Similar solvent exchange was used elsewhere to regulate aerogel shape as well. 40

Figure 3. Morphology of aerogels fabricated: (a) GOA, (b) CNFA, (c) GSA, (d) CSA, (e) G8C3SA, (f) G1C1SA, and (g) G3C8SA. Insets show morphology at high magnification. The volume retention from the solution used to prepare the aerogels (Figure 4a) and the average pore size (Figure 4b) were measured to quantitatively compare the differences among the various aerogels. The pores of GOA were in the nanometer and submicron meter range, while the pore size of CNFA was over 40 µm. CNFA was able to retain more than 80% of the CNF solution before freeze drying, whereas the GOA shrunk to only 6% of the GO solution volume. Both volume retention and pore size did not show significant changes when silica nanoparticles were introduced. Although silica particles aggregated in GSA (Figure S4), they were dispersed and embedded in cellulose sheets of CSA and created rough surfaces (Figure 3d). For the case of GCSAs, the volume retention as well as the average pore size increased 12 ACS Paragon Plus Environment

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with the increase of cellulose content. Regions of GO and CNF were undistinguishable on the pore walls suggesting compatibility between GO and CNF. As more CNF was introduced, the aerogel relies less on the stacking of graphene sheets, and more on the crosslinked CNF networks to construct the monolith, thus preventing volume shrinkage. At the same time, the pore size was increase and more silica particles were exposed on the pore walls as the increase of CNF content as shown in Figure 3e–g.

Figure 4. (a) Volume retention and (b) average pore size statistical results for the aerogels fabricated. Insets in (a) show digital photo of the corresponding aerogels. (c) XPS survey scans and peak fitted C1s scans of (d) GCSA and (e) MGCSA. The aerogels were characterized using XPS before and after modification. A strong F1s peak was detected on MGSCA (Figure 4c), and CF3 and CF2 peaks appeared on the high resolution C1s core-level scan of MGSCA (Figure 4e), which indicated the successful grafting of fluorochains on the GSCA surface. In addition, the statistical data (Table S1) suggested that the percentage of C–C bonds was significantly higher, while the hydrophilic bonds, C–O and C=O, were lower for GSCA compared with MGSCA. The thermal properties of the modified aerogels were investigated by TGA in air. The results (Figure S5 and Table S2) showed that the Tonset of MGOA (427.2 °C) was much higher than that of MCNFA (208.4 °C). 13 ACS Paragon Plus Environment


Furthermore, the decomposition temperatures (Tonset, T50%, and Toffset) all increased, and the weight loss curves shifted to higher temperatures when GO was added to CNF, which was attributed to the high thermal conductivity of graphene.41, 42

3.2 Bulk Properties and Wettability The bulk density and surface area are important properties affecting the absorption capacity of materials. The bulk density of MGOA and MGSA were over 40 mg/cm3, while they were only about 10 mg/cm3 for MCNFA and MCSA. As expected, the density of MGCSAs decreased as the CNF proportion increased; nevertheless, they were still ultralight and could be supported by a dandelion (Figure 5a). The BET surface area (Figure 5b) showed an inverse trend which was in agreement with the volume retention and average pore size results, thus indicating that low densities contributed to high surface areas. The BET surface area of MGCSAs ranged from 29.5 to 93.5 m2/g, which was much higher than MGOA and MGSA.

Figure 5. (a) Bulk density and (b) BET surface area measurement results of different modified aerogels. Although the hybrid aerogels had low densities, they were free-standing and had relatively high strengths because of their isotropic porous structures and the reinforcement of graphene. Figure 6a shows that a small piece of MGCSA can easily support 154 g of weight without causing obvious deformation. The statistical results of all of the aerogels (Figure 6b 14 ACS Paragon Plus Environment

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and c) showed that MGOA and MGSA had much higher moduli and strengths than other aerogels since graphene sheets are one of the strongest materials with a high Young’s modulus (about 1.1 TP) and fracture strength (about 125 GPa). Furthermore, the partial overlapping or coalescing of flexible graphene sheets via π-π stacking interactions provided strong physical crosslinking to the aerogel.39, 43 However, the significant volume shrinkage in the self-assembled HTR caused GOA to have a high density and low surface area, which restricted its direct application as an absorbent material. Moreover, the high cost of high quality GO limits the mass production of GOA. When combined with CNF, the fabrication cost can be greatly reduced and the hybrid aerogels show high surface areas and low densities. Moreover, the compressive modulus (30.0 ~ 44.6 kPa) and strength at 50% strain (16.5 ~ 45.2 kPa) of MGCSAs were significantly higher than CNFAs and other graphene aerogels fabricated via different methods.34, 44, 45 Hence, MGCSAs should be suitable for applications that require weight bearing and could be used to replace metallic foam-based absorbers.46

Figure 6. (a) Photographs of an MGCSA supporting a weight of 154 g with little deformation, and the compressive stress vs. strain curves of different modified aerogels. (b) Compressive modulus and (c) compressive strength statistical results of different modified aerogels.



Besides high absorption capacity, ideal absorbent materials need to be superhydrophobic and superoleophilic in order to achieve high efficiency in the selective absorption of oil from water. The WCAs of modified aerogels were measured to evaluate their wettability. Figure 7a shows that the contact angle of MGOA and MGSA were 141.5° and 144.1°, respectively, which was higher than those reported for GO/CNT aerogels whose WCAs were around 140°.12 The WCAs of MCNFA and MCSA were in the range of 130° to 135°. The contact angle hysteresis (CAH) of these aerogels exceeded 7°. However, it was surprisingly to find that the MGCSAs were all superhydrophobic and super repellent with WCAs over 150° and CAHs less than 5°. As shown in Figure 7b and c, MG8C3SA showed an extremely high WCA of 157.3° and a water droplet immediately rolled off the sample with a sliding angle less than 1°. Moreover, the sample was still superoleophilic with a hexane contact angle of 0° (Figure 7b) and its WCA was still over 150° when the sample absorbed hexane (Figure S6), suggesting extraordinary selectivity in oil absorption. To the best of our knowledge, this high WCA and excellent water repellency have never been achieved by other carbon-based aerogels.


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Figure 7. (a) WCA and CAH of different modified aerogels. (b) WCA and oil CA of MG8C3SA showing superhydrophobicity and superoleophilicity. (c) Water droplet rolling off MG8C3SA with a roll-off angle less than 1° To understand the unprecedented superhydrophobicity of MGCSA, a universal model was created to describe the contact situations of liquid to structured substrates. The fundamental theory in textured non-wetting surfaces is described by the well know Cassie– Baxter equation, Equation (1), from which it is obvious that increases in θ and fLG are the suggested approaches to enhance θ*.

cos θ * = f SL cos θ − f LG

(1)

where, θ* is the apparent contact angle, θ is the contact angle on the flat surface (intrinsic contact angle), fSL is the fraction of the projected area of the solid in contact with the liquid, and fLG is the fraction of the projected area of the liquid in contact with the gas. Normally, f SL = 1 − f LG . However, this equation only provides general concepts of factors affecting θ*. Recent studies have revealed that the geometry of microstructures also have a tremendous effect on θ*.25,

47

A sphere model is used here as the microstructure on the structured surface for

explanation. As shown in Figure 8a, when liquid comes into contact with the sphere, it will sag along the sphere profile and the resultant force of the interfacial tensions (γSL, γSG, and γLG) will continuously increase until equilibrium is established; otherwise, the whole sphere would be wet by the liquid. If equilibrium is established at the lower semisphere, it will resemble the re-entrant texture, and the required θ will be less than 90°. If equilibrium is established at the equator plane, it will resemble the widely used column texture and the required θ will be 90°. If equilibrium is established at the upper semisphere, it will resemble a trapezoid texture and the required θ will be over 90°. Since the WCAs of most materials are less than 90°, many surfaces with pillar and trapezoid stage microstructures failed to show 17 ACS Paragon Plus Environment


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superhydrophobicity.48 However, the re-entrant structure is fairly difficult to create, and surfaces with re-entrant textures always exhibit repellency to oils which is adverse for oil absorption applications.49-51 Therefore, reducing the surface energy (increasing θ) is a feasible approach to enhancing hydrophobicity according to equation (1). In recent years, hierarchically structured surfaces have shown excellent water repellency.29, 52 For example, creating a second tier structure on micro pillars imparted superhydrophobicity to the textured surface.27 This improvement is always associated with the increase of “air pocket” volume.53 We believe the fundamental reason is the increased “solid–liquid– gas” composite interfaces due to the hierarchical structures, which reduce the required θ for equilibrium as depicted in Figure 8b. A modified Cassie–Baxter equation, Equation (2), was proposed by Tuteja to account for this phenomenon.26,

54

This equation evidently

showed that the apparent contact angles on the hierarchically structured substrate (θ*n) increased with an increase in the number of layers of hierarchical structures (n).

cosθ *n = (1 − f LG ,n )cosθ *n −1 − f LG ,n

(2)

Most carbon-based aerogel absorbers, which are normally fabricated by freeze drying, HTR, and sintering, lack hierarchical structures on the pore walls and do not have functional groups that can be further modified. Their hydrophobicity is supplied by the porous structures and the carbon–hydrogen chains so that their WCAs are usually below 140° and their separation efficiency is also limited. On the contrary, the MGCSAs fabricated in this study possess hierarchical structures formed by silica nanoparticles and CNF fibrils on micro-sized pores as shown in Figure 8c, as well as hydroxyl groups from unreduced GO, CNF, and silica that can be modified with low surface energy fluorochains. Together, these characteristics contributed to the excellent superhydrophobicity of MGCSAs.


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Figure 8. (a) Theoretical diagram showing the possible equilibrium situations of water in contact with different substrates. (b) Illustration of equilibrium established on a hierarchically structured substrate. (c) Silica nanoparticles and CNF fibrils on MGCSA pore surfaces.

3.3 Applications for Water Remediation When choosing an absorbent material, there is always a trade-off between absorption capacity and selective absorption efficiency. The absorption capacity to hexane and chloroform of different modified aerogels was measured and it was found that the absorption capacity showed the same trend as the surface area (Figure S7). Although MG8C3SA had the highest WCA of 157.3°, its absorption capacity was 20.5 ~ 45.5 g/g, which was lower than many reported absorbent materials. The MG1C1SA had a WCA of 155.5° and a relatively high absorption capacity of 38.7 ~ 68.2 g/g. Although its absorption capacity was still lower than reported carbon nanofiber aerogel,15 CNT/giant GO aerogel,11 and reduced GO foam,55 it was superior to the Kymene-containing cellulose aerogel,

21

FeOOH-containing graphene

aerogel56, and poly (vinylidene fluoride) aerogel.19 Most importantly, it showed the highest superhydrophobicity among all reported aerogels as compared in Table S3. Therefore, 19 ACS Paragon Plus Environment


MG1C1SA was used as the absorbent material in the following experiments representing an example of FHA. As a demonstration, FHA was used to absorb light oil (hexane) atop water (Figure 9a) and heavy oil (chloroform) under water (Figure 9b). FHA quickly absorbed the hexane around it within 1 s after being placed in the beaker, thus indicating excellent superoleophilicity and an ultrafast absorption rate (Movie 1). When FHA was immersed in water, an air layer formed, protecting it from wetting, and chloroform was rapidly absorbed once FHA touched it (Movie 2). Moreover, FHA was able to immediately float back to the surface once the forceps were released, thus facilitating the collection of the material after use. The separation efficiency was investigated by measuring the weight percentage of the collected oil and purified water. As shown in Figure 9c, the oil absorption rate of FHA was close to 100% for both hexane and chloroform. Furthermore, almost no water was absorbed (loss of water ~ 0%) by FHA in both water surface absorption and underwater absorption tests. This excellent separation efficiency was maintained over 10 cycles of repetitive measurements. In another absorption and desorption test, FHA maintained 92% and 95% of its initial absorption capacity to hexane and chloroform, respectively, when repetitively used 10 times, thus suggesting high sustainability and reusability (Figure S8). In addition, FHA showed a remarkable absorption ability towards various oils and chemical solvents as shown in Figure 9d. In consideration of its excellent separation efficiency, FHA has the potential to be used as a new high performance superabsorbent material.


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Figure 9. Demonstration of the selective absorption of FHA to (a) light oil atop water and (b) heavy oil under water. Oils were dyed red with oil O red. (c) Separation efficiency of FHA to hexane and chloroform over 10 cycles. (d) Absorption capacity of FHA to various oils and chemical solvents. Attributed to the free-standing ability of FHA, a simple self-driven device was developed to collect oil from the surface of water. As shown in Figure 10a, the device consists of a straight vial, some weights placed in the vial, FHA assembled at the vial neck, and a needle used for venting. When the device was placed in the beaker, FHA was quickly saturated with oil, then the oil started to flow into the vial slowly and the vial leaned gradually as more oil was collected. Interestingly, the vial was able to chase the oil trace and keep collecting until it turned upright, and the whole setup was still able to float on the water, which facilitated recycling (Figure 10b). Figure 10c showed that the setup had a significant weight gain within 5 min mainly due to the fast oil absorption of FHA driven by capillary action.57, 58 Then the weight gain decreased significantly since the inflow of oil was driven by gravitational force and needed to overcome the interfacial tension from FHA. The collection stopped in ~25 min and the collected oil was about three times that of the absorption capacity of FHA. This



simple device demonstrated the capability of FHA to be assembled into an oil collection device, thus expanding its versatile applications in water remediation.

Figure 10. (a) Illustration of the device developed for oil collection. (b) Photos demonstrating the oil collection process. (c) The weight of recovered oil over time with the device.

4. Conclusion In this study, a simple low cost approach was proposed to fabricate GO/CNF/silica hybrid aerogel with a hierarchical porous structure and low surface energy. Combining CNF with GO overcame the defect of volume shrinkage of the GO aerogel. It also greatly reduced the bulk density and increased the surface area of the hybrid aerogel while maintaining its freestanding characteristics and relatively high mechanical strengths due to the isotropic porous structure and reinforcement effect of the graphene sheets. Silica nanoparticles and CNF fibrils appeared on the surface of the micro-pore walls, thereby creating complex hierarchical structures. The synergistic effect of CNF, GO, and silica contributed to the stiffness and property stability of the aerogel. Together with the low surface energy contributed by PDTS grafting, FHA showed unprecedented superhydrophobicity with a WCA = 157.3° and super 22 ACS Paragon Plus Environment

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water repellency (CAH < 1°) among all reported carbon-based aerogels. Meanwhile, FHA was still superoleophilic with an oil contact angle of 0°. Attributed to its high surface area and superhydrophobicity, FHA showed a relatively high absorption capacity to various oils and chemical solvents, and, most importantly, it possessed extraordinary separation efficiency in the selective absorption of oil from water, as well as high property stability and reusability. Moreover, a simple device was assembled using FHA and was used to easily collect oil from the surface of water, thus demonstrating some of the diverse applications of high performance FHA for water remediation.

Acknowledgements The authors would like to acknowledge the financial support of the National Natural Science Foundation of China (51603075; 21604026), the Office of the Vice Chancellor for Research and Graduate Education, and the Wisconsin Institute for Discovery (WID) at the University of Wisconsin–Madison.

Appendix A. Supplementary data Supplementary data related to this article can be found online.

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