Research Article www.acsami.org
Direct Oil Recovery from Saturated Carbon Nanotube Sponges Xiying Li,† Yahui Xue,† Mingchu Zou,‡ Dongxiao Zhang,§ Anyuan Cao,*,‡ and Huiling Duan*,†,∥ †
State Key Laboratory for Turbulence and Complex Systems, Department of Mechanics and Engineering Science, College of Engineering, ‡Department of Materials Science and Engineering, College of Engineering, §ERE & SKLTCS, College of Engineering, and ∥CAPT, HEDPS and IFSA Collaborative Innovation Center of MoE, BIC-ESAT, Peking University, Beijing 100871, P. R. China S Supporting Information *
ABSTRACT: Oil adsorption by porous materials is a major strategy for water purification and industrial spill cleanup; it is of great interest if the adsorbed oil can be safely recovered from those porous media. Here, direct oil recovery from fully saturated bulk carbon nanotube (CNT) sponges by displacing oil with water in controlled manner is shown. Surfactant-assisted electrocapillary imbibition is adopted to drive aqueous electrolyte into the sponge and extrude organic oil out continuously at low potentials (up to −1.2 V). More than 95 wt % of oil adsorbed within the sponge can be recovered, via a single electrocapillary process. Recovery of different oils with a wide range of viscosities is demonstrated, and the remaining CNT sponge can be reused with similar recovery capacity. A direct and efficient method is provided to recover oil from CNT sponges by water imbibition, which has many potential environmental and energy applications. KEYWORDS: oil recovery, carbon nanotube sponge, electrocapillary, imbibition, surfactant
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INTRODUCTION With rapid industry development, widespread use of organic oils and solvents has become a major source of water contamination and brought serious impact to our environment.1 Many methods have been attempted for water purification including physical methods such as oil collection via skimmer vessels and adsorbent materials,2,3 chemical approaches such as the use of dispersants4 and controlled burning, as well as biological methods such as bioremediation.1,5 Among those, oil adsorption by porous materials is commonly used because of its simplicity, low cost, and high efficiency. Superhydrophobic or superoleophilic materials in the form of meshes, films and membranes,6,7 and aerogels or sponges8−14 made from carbon nanostructures have been exploited to remove oil from water. In particular, nanostructure-based porous materials show excellent adsorption capacity for a variety of organic solvents and oils, as well as high selectivity in an oil−water mixture environment.8−14 It would bring significant economic and environmental benefit if one can recover oil from saturated porous media, in a clean and efficient way. Recycling methods such as mechanical extrusion, heat treatment and in situ burning have been adopted to regenerate adsorbents.13−17 Such mechanical, thermal, or chemical treatments usually deteriorate the material structure, resulting in degradation of performance in subsequent use. Moreover, the adsorbed oil in the porous media has been wasted/consumed and cannot be recovered safely by those methods. Although nanostructured aerogels possess certain mechanical strength and flexibility, it is difficult to squeeze out all adsorbed oil and their porous network may collapse under severe compression. Smart materials which can © XXXX American Chemical Society
change wetting property under external stimuli (e.g., pH, irradiation, temperature, electric, or magnetic field) have been proposed, but usually involving a complicated manufacturing process.18−23 Here, we show direct oil recovery from carbon nanotube (CNT) sponges after full adsorption. We find that electrocapillary water imbibition can displace adsorbed oil in the porous sponge and extrude oil out smoothly and completely. We introduced a small amount of surfactant to assist electrocapillary imbibition at controlled speed depending on the surfactant concentration and applied voltage, and have recovered different types of oils with a range of viscosities. Thus, our CNT sponges, as a promising adsorbent candidate, also can achieve high recovery capacity and recyclability via a simple and efficient method.
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RESULTS AND DISCUSSION Previously, our team has reported highly porous, lightweight CNT sponges consisting of stacked multiwalled nanotubes, synthesized by chemical vapor deposition.24 These naturally superhydrophobic CNT sponges adsorb various organic solvents and oils with high capacity and selectivity, but can only be recycled by mechanical extrusion or combustion.24,25 Recently, electrocapillary imbibition phenomenon was observed in nanoporous gold (NPG), in which water could be pumped into an empty NPG as well as an oil-saturated NPG.26 Received: February 16, 2016 Accepted: April 27, 2016
A
DOI: 10.1021/acsami.6b01623 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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generated in the oil layer. In order to observe the oil extrusion process, we completely immersed the oil-saturated sponge into the electrolyte solution and then started the imbibition process (Figure 1D). At the beginning, some gas bubbles (coming from residual air trapped near the sponge surface) emerged immediately after applying the bias (−1.2 V). Then, oil droplets were raised up from the sponge very quickly and continuously (Movie S1). We observed that oil droplets mainly emerged through the upper side of cylindrical face and the top surface of the sponge, indicating that water has been flowing into the sponge through the bottom part and driving the oil upward. We have carried out linear potential sweeping (0.1 V to −1.2 V, within capacitive charging region determined by cyclic voltammogram) on oil-saturated sponge samples to examine the electrocapillary conditions for oil recovery. To enable water imbibition, an appropriate amount of surfactant (1 wt % DTAB) must be introduced into the aqueous electrolyte (1 M KOH). Successful oil recovery was evident from the recorded mass change (Δm) above a threshold voltage of −0.24 V (versus Ag/AgCl in 3 M KCl) as water started to flow in and the lower density oil came out, resulting in a net increase of the total sponge mass (Figure 2A). This indicates that the oil encapsulated within the sponge pores has been gradually exchanged by water, from the bottom part (in contact with electrolyte) to upper region of the sponge. Without DTAB, there is no mass change through the entire voltage window, indicating that water has been resisted by the oil contained in the sponge thus oil recovery cannot be triggered. In addition, cyclic potential (E) sweeping (between −1.2 to 0.1 V) showed that electrocapillary imbibition occurred at E = −1.2 V to −0.24 V with monotonic mass increase, but has stopped in the range of E = −0.24 to 0.1 V producing horizontal plateaus along the curve (Figure 2B). On the basis of the measured mass increase (Δm), the mass of extruded oil (m−o ) with the same volume as invaded water during the process can be calculated by m−o = Δmρo/(ρw − ρo), where ρ is density, and subscripts “w’ and ‘o” denote water and oil, respectively. To eliminate the effect of sample size (see eq 2 below), we plotted the variable of mo−L/A versus the electrocapillary time (t) at different potentials (from −0.2 V to −1.2 V) (Figure 3A). Here, A is cross-sectional area of the sample, and L is sample length. At the beginning, each curve maintains a linear relationship for a long period, indicating that the oil outflow was stable with a nearly constant recovery rate. Larger (absolute) potentials result in higher oil recovery rate, as can be seen from the increased slope. During the process of water imbibition and simultaneous oil extrusion, the height of the rising water front (h) follows26−29
The same phenomenon was also observed in CNT sponge, in which water could be pumped into an empty sponge at high speed under a threshold voltage of −0.52 V.27 On such basis, our idea is to use an oil-filled (versus empty) sponge for water imbibition, to explore whether oil adsorbed inside the sponge can be extruded out and thus directly recovered. Electrocapillary imbibition were carried out in a threeelectrode potentiostat. A cylindrical CNT sponge (diameter, D = 8 mm, average length, L = 2 cm) grown on a copper wire was connected as the working electrode (see Experimental Section for details). The reference electrode was a commercial Ag/AgCl in 3 M KCl. The aqueous electrolyte solution contains 1 M KOH and 1 wt % surfactant dodecyl trimethylammonium bromide (DTAB, a kind of cationic surfactant). An oil layer (cyclohexane, density 0.78 g/cm3) with thickness larger than the sponge height was laid on top of the electrolyte without mixing. The test was performed in two steps: 1) oil adsorption into the CNT sponge, and 2) oil recovery by electrocapillary imbibition (Figure 1B, C). First, the oil layer was lifted upward
Figure 1. Oil displacement by electrocapillary imbibition. (A) Illustration of the experimental setup in which an aqueous electrolyte solution was sucked into a saturated CNT sponge and oil inside the sponge was displaced. The vertical arrow indicates the flow direction of electrolyte front through the sponge pores. (B, C) Photos of a CNT sponge in contact to blue-dyed cyclohexane (step 1: oil imbibition) and then moved down to contact the electrolyte (step 2: oil recovery). (D) Snapshots of an oil-saturated CNT sponge completely immersed in the electrolyte, from which gas bubbles (at 1 s) and oil droplets were extruded out when a potenital of −1.2 V was applied.
r2 Pc dh = c dt 8 ηw h + ηo(L − h)
(1)
where rc is effective capillary radius and equals the mean pore radius (r) divided by the tortuosity (τ), Pc is effective capillary pressure and equals 2σwo cos θ/rc, and η is viscosity. Here, σwo is water−oil interface tension, and θ is contact angle of water on CNT surfaces. For as-grown sponges, rc was measured to be ∼81 ± 5 nm.27 At any instant, the mass of extruded oil (m‑o) is related to the level of the water front (h) by h = m−o /(ρoAφ), where φ is porosity of CNT sponges (close to 99%).24 Considering the very close values between ηo (0.993 mPa s) and ηw (1.054 mPa s), eq 1 may be simply rewritten as
to soak and immerse the sponge, which was fully infiltrated by cyclohexane during the process (Figure 1B). In the second step, the underlying electrolyte solution was raised further to make contact with the bottom surface of the sponge (Figure 1C). At this stage, a bias was applied between the sponge and counter electrode to trigger electrocapillary imbibition, and the adsorbed oil within the sponge was extruded out into the oil layer. This is a rather smooth process as no disturbance was B
DOI: 10.1021/acsami.6b01623 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 2. Mass change of the CNT sponge during electrocapillary imbibition. (A) Measured mass change in a surfactant-added electrolyte (1 M KOH + 1 wt % DTAB) showing a threshold voltage of −0.24 V, above which (absolute value) water flows into the sponge and displaces oil. There is no mass change in KOH electrolyte (without DTAB) through the potential sweep. (B) Mass change recorded during cyclic potential sweeping (between −1.2 to 0.2 V), in which electrolyte imbibition is off in the potential range of −0.2 to 0.2 V, resulting in periodic horizontal plateaus along the curve.
Figure 3. Influence of voltage and surfactant’s concentration on oil displacement. (A) Measured mass change of displaced oil (m‑oL/A) during imbibition under different potentials (E = −0.2 V to −1.1 V) with 1 wt % DTAB in the electrolyte. (B). Capillary pressure Pc derived from data in Figure 3A (experimental) and theoretical calculation depending on the potential. (C) Measured mass change of displaced oil (m−o L/A) under −1.2 V using different DTAB concentrations (0 to 2 wt %). (D) Capillary pressure Pc derived from data in Figure 3C (experimental) and theoretical fitting. − r2 P d ⎛ mo L ⎞ ⎜ ⎟ = ρφ c c o dt ⎝ A ⎠ 8 η
pressure (Pc) as a function of potential (E), which is close to the theoretical calculation based on the Lippmann equation (see details in Supporting Information) at lower potentials (Figure 3B). The deviation at higher potentials is due to the contact-angle saturation in electrowetting between electrolyte and CNTs in the sponge.30 In addition, we have explored the oil recovery process based on two another electrolyte systems (1 M KCl solution and saturated Na2SO4 solution at 20 °C) and measured the mass change during a cyclic potential sweep (Figure S1). Both electrolytes show similar oil recovery behavior, while the threshold values for initiating water flow are quite different (−0.23 V for 1 M KCl and −0.26 V for Na 2 SO 4 , respectively). Other parameters such as salt
(2)
The right-hand side of eq 2 contains basic properties of liquid and structural characteristics of the porous sponge, and thus, d(mo−L/A)/dt is a constant, which is consistent with experimental data in Figure 3A (i.e., linear relationship between m−o L/A and t). It also shows that for a particular type of porous material (e.g., CNT sponge), in order to achieve high oil recovery rate, the sample length (L) should be decreased while the cross-sectional area (A) should be enlarged. According to eq 2 and Figure 3A, we have plotted the effective capillary C
DOI: 10.1021/acsami.6b01623 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 4. Recovery of different oils by electrocapillary imbibition. (A) Percentage of oil being displaced (relative to the saturated oil mass in the sponge) over the time (t) of imbibtion, for several selected oils including alkane, gasoline and diesel. Inset shows a CNT sponge in which the oil in the bottom part has been displaced by electrolyte. (B) Time span of complete oil recovery (T) depending on the oil viscosity (ηo) and theoretical calculation.
concentration in these solutions may also influence the oil displacing process by affecting the electrocapillary phenomenon31 and surfactant adsoprtion,32 which has been studied in detail previously. We have mentioned that adding surfactant into the electrolyte is necessary for enabling electrocapillary imbibition. Here, we have studied the effect of surfactant concentration under the same potential (−1.2 V), and found that the oil recovery rate was enhanced when the concentration increased from 0 to 1 wt %, and then stabilized at 2 wt % DTAB (Figure 3C). Surfactants adhering on the solid surface can change the wetting property and have been widely used in oil production to improve the oil recovery from carbonate rocks.33−35 Most recently, potential-controlled adsorption/desorption of charged surfactant on substrates was used to achieve rapid and reversible control of bubble formation during boiling by tuning the surface wettability.36 Here we found that the addition of surfactant changed the contact angle of water droplets sitting on the CNT sponge surface. With increasing DTAB concentration (from 0 to 1 wt %), the CNT sponges changed from superhydrophobic (with apparent contact angle ∼150°) to hydrophilic, as evident from the rapid infiltration of the droplet containing 1 wt % DTAB into the sponge (Figure S2). The wetting transition with increasing surfactant concentration was also demonstrated by spontaneous imbibition of the surfactant solution into empty CNT sponges without applying a potential (Figure S3). Thus, the presence of surfactants enhances the affinity between water and CNTs by physical adsorption at the interface. The combination of surfactant adsorption and electrocapillary effects even renders oilsaturated CNT sponges hydrophilic, enabling the direct oil recovery observed in Figure 1D. On the other hand, the adsorption/desorption of positively charged surfactant DTAB on electrodes is controllable through electrode potential. As evident from Figure 2, the surfactant adsorption is triggered in the potential range of (−1.2 V, −0.24 V), in which oil recovery happens, and a potential within (−0.24 V, 0.1 V) promotes the surfactant desorption and stops the oil recovery. With an eye on the adsorption mechanisms of surfactant on CNT surfaces under potential control and open circuit conditions, we inspect the variation of capillary pressure with surfactant concentration. A two-step mechanism for adsorption of surfactants at solid−liquid interfaces has been proposed to interpret various types of adsorption isotherms: the first step is controlled by the interaction, and the second step is due to the
formation of hemimicelles through asssociations or hydrophobic interaction between the surfactant molecules.37 According to the Gibbs adsorption theory,38,39 the surfactant adsorption regulates the solid-electrolyte interface tension, which enhances the capillary pumping pressure for oil recovery in CNT sponges by electrolyte. In the presence of surfactants, the capillary pressure (Pc) is dependent on the surfactant concentration (C) by the following equation (see details in the Supporting Information) Pc(C) = Pc0 +
2 RT Γ∞ln[1 + k1C(1 + k 2C n − 1)] rc n
(3)
P0c
where is a constant, representing the capillary pressure in a pure oil−water system without surfactant, Γ∞ is the limiting adsorption at high concentration, k1 and k2 are the equilibrium constants involved in the two adsorption steps, and n is the aggregation number of surface hydrophobic aggregates or hemimicelles. Based on the eq 3 and Figure 3C, we plotted the capillary pressure versus surfactant concentration at a constant potential of −1.2 V and obtained n = 1.31 by fitting the experimental data (Figure 3D). The apparent “Langmuir-Type” adsorption isotherm implies the surfactant adsorption on highly polarized electrode surfaces at −1.2 V is controlled by a single step. In contrast, a “plateau-type” surfactant adsorption isotherm was observed at open circuit conditions (Figure S3B), indicating the process is controlled by a two-step mechanism as discovered in ref 37. As a result of the surfactant adsorption on CNT surfaces (Figure 1A), the affinity between the CNTs and incoming water in the sponge is promoted, which increases the capillary pumping pressure and enables electrocapillary water imbibition even the sponge has been prefilled by oil. Among various common surfactants, we also tested SDS (sodium dodecyl sulfate, sodium salt, a kind of anionic surfactant) and studied the concentration effects compared with currently studied DTAB. The result with SDS is similar to that with DTAB, whereas the recovery rate based on the SDS-involved electrolyte solution is much lower (Figure S4). The difference in the recovery efficiency is probably caused by the different adsorption capacity of surfactants to charged surface. As SDS is an anionic surfactant, the amount of molecules adsorbed to negatively charged CNT walls (under negative applied bias) will be less than the cationic surfactant (DTAB). Because water imbibition simultaneously displaces the oil adsorbed in the pores, our next question is how much oil can be D
DOI: 10.1021/acsami.6b01623 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 5. Repeated oil recovery by the CNT sponge. (A) The process of the sponge regeneration. An original sponge was used for oil adsorption/ desorption and then cleaned by vacuum drying, after which the surface hydrophobicity was recovered and the sponge was ready for second oil adsorption. (B) Oil adsorption and desorption (recovery) specific capacities (oil mass relative to the sponge mass) over 8 cycles. (C) SEM image showing the inside of an as-grown CNT sponge. (D) SEM image of a cleaned CNT sponge maintaining the porous structure after vacuum drying.
extruded from the CNT sponge, and whether complete oil recovery is possible? To find the answer, we have used CNT sponges to be saturated by several selected oils (e.g., alkane, gasoline, and diesel) and then subjected to electrocapillary imbibition to remove oil from inside (all under a potential of −1.2 V with 1 wt % DTAB in electrolyte). The tests were continued until there was no net mass increase; at this stage the water front has passed through the entire sponge length (L) and reached the top surface of the sponge, indicating the finish of oil displacement. Then, based on the mass of originally adsorbed oil in the saturated sponge (m+o ), and the measured mass of totally extruded oil (m−o ) by electrocapillary imbibition, we know how much oil has been recovered through a single imbibition process (Figure 4A). We have obtained the percentages of oil displacement (i.e., m−o /m+o ) for different types of oils, all of which locate in a narrow range (95−96 wt %). In our sponge, the oil was mainly adsorbed and stored among the inter-CNT pores, which are formed by the spacing between stacked CNTs. During the imbibition process, water continuously flows in and fills those pores, forcing the oil out. Thus, the highly interconnected open-porous structure ensures such high oil displacing percentage and recovery. The remaining few percentage of oil might be encapsulated within the places with less interconnectivity (for example, small pores enclosed by relatively dense CNT packing) or some oil residue may stick to the CNT surface. Although successful oil recovery can be achieved in our sponges for all the above oils, the time needed until equilibrium (no net mass change) is different, ranging from about 25 min (cyclohexane) to nearly 100 min for high viscosity diesel. Because of the presence of oil which impedes the water
infiltration, the electrocapillary imbibition process for an oilsaturated sponge is much slower than that in an empty sponge (less than 10 min), as reported earlier.27 Here, the oil viscosity is the main factor that influences how fast will the oil be displaced, using the same electrolyte. Generally, higher viscosity results in longer oil displacing period (Figure 4B). The time span of displacing period (T) is related to the oil and electrolyte viscosities which can be roughly estimated by eq 1, T=
2(ηo + ηw ) rcσwocos θ
L2
(4)
Equation 4 depicts a linear relationship between T and oil viscosity (ηo) with a slope of 2L2/(σworc cos θ) (which only depends on the structural parameters, surface tension, and the sponge size), producing a reasonable fitting of the experimental data in the tested oils (Figure 4B). The deviation may be due to the looseness of the PTFE tape cover surrounding the sample in longer time, which increased the effective cross-sectional area for liquid flow. Oils with even higher viscosities (e.g., food oil at 67.80 mPa s) also can be recovered, although the process takes longer time. After oil recovery, the remaining CNT sponge filled with electrolyte and surfactant can be cleaned and reused by simple methods. The sponge was rinsed by purified water and isopropanol, and then subjected to vacuum pyrolysis treatment (at 200 °C for 5 h) which could effectively remove the hydrophilic groups from CNTs and recover the hydrophobic property of the original sponge.40 Vacuum pyrolysis resulted a wrinkled sponge with reduced volume, but subsequent oil adsorption expanded the sponge back to original shape (Figure 5A). Through this regeneration process, we were able to use a E
DOI: 10.1021/acsami.6b01623 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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CNT sponge for oil adsorption and desorption (recovery) repeatedly, for 8 cycles. The sponge maintained high adsorption capacities (100−110 g g−1) as well as desorption capacities (90−100 g g−1) during those cycles, indicating a stable performance and the potential for cyclic use (Figure 5B). We have examined the microstructure of CNT sponges by scanning electron microscopy (SEM) images (Figure 5C, D). Although the sponge shrank slightly after the vacuum pyrolysis treatment, the internal three-dimensional CNT framework was well-maintained, and without bundling or forming aggregations of CNTs. This is favorable for keeping the highly porous sponge structure and thus high specific capacities for both oil adsorption and desorption as shown in Figure 5D. In addition, the surfactant we used here is mild and nontoxic, which can be completely recycled from water by solvent extraction as well.41 Thus, our process in displacing adsorbed oil by surfactantassisted electrocapillary may be carried out in an environmentfriendly manner.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01623. Movie S1, showing the oil displacement process of an oilsaturated CNT sponge completely immersed in the electrolyte (AVI) Electrocapillary imbibition into an oil-saturated CNT sponge using other electrolyte systems during a cyclic potential sweep, pictures of electrolyte droplets containing surfactant placed on the surface of a CNT sponge, spontaneous imbibition of surfactant-added electrolyte into an empty CNT sponge, electrocapillary imbibition of SDS-added electrolyte into an oil prefilled CNT sponge, and theory derivation (PDF)
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AUTHOR INFORMATION
Corresponding Authors
CONCLUSIONS We developed a simple and efficient electrocapillary method to recover adsorbed oil from saturated CNT sponges in controlled manner. A key point is to add an appropriate amount of surfactant to facilitate water imbibition into the sponge pores (so as to displace oil) under a low voltage. More than 95 wt % of adsorbed oil could be directly recovered, and the CNT sponges can be reused for many cycles with high capacity. The oil recovery method demonstrated here may also be applicable in other carbon-based porous materials such as graphene aerogels. Controlled oil-displacing by water is not only useful in environmental areas such as oil adsorption and recovery, but also has potential applications in microfluidic and other microelectromechanical systems.
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Research Article
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded by National Natural Science Foundation of China (NSFC) under Grants 11225208, 11521202, 51325202, and the key subject “Computational Solid Mechanics” of China Academy of Engineering Physics.
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EXPERIMENTAL SECTION
Electrocapillary Imbibition. The experimental setup for surfactant-assisted electrocapillary imbibition of aqueous electrolyte into oil-saturated CNT sponges for direct oil recovery is schematically shown in Figure 1A. A CNT sponge was suspended from a microbalance (ME36S, Sartorius, Germany) over a liquid reservoir containing 1 M KOH aqueous solution (analytically pure, SigmaAldrich, Shanghai) with certain concentration of surfactant DTAB (Sigma-Aldrich, Shanghai), together with an immiscible oil layer on top (e.g., cyclohexane). The electrocapillary imbibition was controlled by a three-electrode system, with a CNT sponge on copper wire as the working electrode (WE), a commercial Ag/AgCl in 3 M KCl as the reference electrode (RE), and another CNT sponge as the contour electrode (CE). All the sponge samples were covered with a thin PTFE tape in order to maintain a constant imbibition cross-sectional area for easy quantitative and analytical interpretation of the experimental results, exception those for visualization experiments in Figure 1. The liquid level in the reservoir was controlled by a motorized lab stage. The sponge was first made in contact with oil, and then totally immersed and soaked for oil saturation (i.e., no significant mass change observed). Subsequently, the oil−water interface was raised until it contacted the sponge for electrocapillary imbibition under potential control. The mass change of the CNT sponge was recorded by a frequency of 1 s−1. Cleaning of the CNT Sponge and Its Characterization. We rinsed the CNT sponge after electrocapillary imbibition by purified water and isopropanol alternately. After that we immersed the sponge in isopropanol for 12 h. Then the CNT sponge was subject to vacuum pyrolysis treatment (at 200 °C for 5 h) (DZF 6050, Beijing) to remove the hydrophilic groups from CNTs. We characterized the inner structure of the as-grown sponge and cleaned sponge using SEM (Leo 1505, Germany). F
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ACS Applied Materials & Interfaces
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DOI: 10.1021/acsami.6b01623 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
