Vertically-aligned carbon nano-tube membrane filters with superhydrophobicity and superoleophilicity

CARBON

4 8 ( 2 0 1 0 ) 2 1 9 2 –2 1 9 7

available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/carbon

Vertically-aligned carbon nano-tube membrane filters with superhydrophobicity and superoleophilicity Cheesung Lee, Seunghyun Baik

*

SKKU Advanced Institute of Nanotechnology (SAINT), Department of Energy Science and School of Mechanical Engineering, Sungkyunkwan University, Suwon, Gyeonggi-do 440-746, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Article history:

A membrane filter possessing both superhydrophobicity and superoleophilicity is of great

Received 29 October 2009

interest for the possible separation of oil and water. Such a filter was realized in this study

Accepted 12 February 2010

by synthesizing vertically-aligned multi-walled carbon nano-tubes on a stainless steel

Available online 17 February 2010

mesh. The dual-scale structure, nano-scale needle-like tubes on the mesh with micro-scale pores, combined with the low surface energy of carbon amplified both hydrophobicity and oleophilicity. For the tests, diesel was selected as a representative of low viscosity oils. The contact angles for diesel and water were 0 and 163 ± 4. The nano-tube filter could separate diesel and water layers, and even surfactant-stabilized emulsions. The successful phase separation of the high viscosity lubricating oil and water emulsions was also carried out. The separation mechanism can be readily expanded to a variety of different hydrophobic and oleophilic liquids. The simple nano-tube filter might be practically employed in environmental and chemical separation processes including oil spill cleanup.  2010 Elsevier Ltd. All rights reserved.

1.

Introduction

The separation by porous membranes is of great interest in environmental and chemical engineering processes [1–3]. Especially, there is a growing need for the effective separation of oil and water for various applications including oil spill cleanup [4–7]. Conventional oil–water separation methods such as coagulation–flocculation, skimming, centrifugation and gravity separation exhibit shortcomings including low separation efficiency, high operation costs, large size and the generation of secondary pollutants. To address these hurdles the construction of membrane filters with both superhydrophobicity, a water contact angle (CA) greater than 150, and superoleophilicity, an oil CA smaller than 5, has received considerable attention [5,8–10]. Wettability of a solid surface, which is quantitatively defined as CA, can be modulated by the surface energy and geometric structure [8,11,12]. Surface energy is an intrinsic property of a material depending on

chemical composition. Hydrophobicity can be amplified by increasing the roughness of low surface energy material since the contact area between the surface and water droplet is minimized by trapped air [8,9,13–19]. A number of different materials and approaches have been investigated to achieve this goal including anodic aluminum oxide film [20], kapok filters [21,22], composite polymer films [8–10] and carbon fibers [23]. The non-polar covalent bond of carbon with needle-like structures makes vertically-aligned multi-walled carbon nano-tubes (VAMWNTs) suitable to create a membrane with superhydrophobicity [24,25] since the surface roughness can be significantly increased. The oleophilicity of carbonaceous materials such as graphite and carbon fibers has been reported [23,26] and this can also be significantly amplified by the superwetting capillary action [5,27] in the interstitial space of aligned, nano-scale tubes. The attempts of employing nano-tube-based structures in the separation technology

* Corresponding author. E-mail address: [email protected] (S. Baik). 0008-6223/$ - see front matter  2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.02.020

CARBON

4 8 ( 20 1 0 ) 2 1 9 2–21 9 7

have been reported previously including the elimination of hydrocarbons from petroleum, the removal of bacterial contaminants from water and the filtering of particulate pollutants [28–32]. Here we report for the first time on the use of VAMWNTs synthesized on a stainless steel mesh for the separation of oil and water. Both superhydrophobicity and superoleophilicity could be obtained due to the dual-scale structure, needle-like nano-tube geometry on the mesh with micro-scale pores, combined with the low surface energy.

2.

Experimental

2.1.

Materials

Contact angle measurements were carried out under ambient conditions using a digital camera (Canon, G10). A Raman microscope (Kaiser Optics, RamanRxn1 Microprobe) was used at the excitation wavelength of 632.8 nm. The power of the laser was 17 mW, and the spot size was 30 lm in diameter. The water content in diesel oil after the filtration was precisely measured using the coulometric titration method (Metrohm, 831 KF Coulometer). Absorbance was characterized using the UV–Vis–NIR spectrometer (Shimadzu, UV3600).

3.

Stainless steel meshes with different sizes (50–400) were purchased from HanKook Metal. The mesh size is defined as the number of pores in 25.4 mm. Diesel (SK ENERGY) was selected as a representative of low viscosity oils (5.0 ± 0.1 cP) [33]. Lubricating engine oil (Castrol, Power RS R4) with a higher viscosity of 3500 cP was also tested. Deionized water (Human Science, HIQ I) was used for all experiments. The filter holder and container were commercially obtained and modified in the laboratory. The load was applied using A4 size papers (Advance Paper, Double A) and the mass of a single sheet of paper was 5.12 g. The test emulsion was prepared by ultrasonicating, and additionally shaking, diesel and water at a volume ratio of 4:1 (Bath Sonicator, 135 W, 10 min). A 1.73 · 10 3 molar additive of sodium dodecyl sulfate surfactant (Fluka, 71725) was used as an interfacially active stabilizing agent. The high viscosity lubricating oil and water emulsion was also prepared using the identical procedure.

2.2.

Synthesis of VAMWNT filters

Vertically-aligned multi-walled carbon nano-tubes were synthesized on stainless steel meshes by the thermal chemical vapor deposition method [34]. Sandwich-like catalyst layers were deposited on the mesh by an electron beam evaporator (chamber pressure 1 · 10–6 Torr). Aluminum was deposited as a buffer layer (40 nm thick) followed by a 3 nm thick iron catalyst layer. The synthesis was carried out in a horizontal quartz tube with an inner diameter of 29 mm. Firstly, the furnace temperature was increased to 750 C with Ar flow (200 sccm) for 14 min. In the next step, an Ar (150 sccm) and H2 (55 sccm) mixture was introduced for 10 min to form iron nano-particles. C2H4 (25 sccm), H2 (55 sccm), Ar (150 sccm) were then introduced to grow carbon nano-tubes (0.5–3 h). Finally, the reactor was slowly cooled down to room temperature under Ar flow (200 sccm). As a control, the stainless steel mesh with a size of 400 was also spray-coated using polytetrafluoroethylene (Teflon PTFE 35: Dupont).

2.3.

Characterization

Surface morphology of specimens was examined using a scanning electron microscope (JEOL, JSM7401F) at 15.0 kV. The amount and rate of change in the weight of nano-tubes were measured as a function of temperature using a thermogravimetric analyzer (Seiko Exstar 6000, TG/DTA6100).

2193

Results and discussion

Typical samples of VAMWNTs grown on the meshes are shown in Fig. 1. Fig. 1a shows a scanning electron microscopy (SEM) image of the stainless steel mesh with a size of 400. The micro-porosity of the VAMWNT filter could be controlled by the selection of different mesh sizes. Optical images of the mesh, before and after the synthesis of nano-tubes, are compared in Fig. 1b. Fig. 1c shows a top view of the synthesized nano-tubes with an average length of about 970 lm. A rough surface with both micro- and nano-scale structures is clearly shown. The size of the square-shaped pore was decreased due to the long nano-tubes synthesized on the mesh. Nanotubes with different lengths were also synthesized as shown in Fig. 1d. The square-shaped pore was more evident when the tube length was shorter. The nano-scale roughness at the tip of nano-tubes is magnified in a tilted view (Fig. 1e). A cross-sectional view of aligned nano-tubes also revealed dual-scale roughness (Fig. 1f). Magnified SEM images of a thin section extracted from the membrane are shown in Fig. 1g and h. The boundary between pillars of nano-tubes was clearly shown (Fig. 1g). The interstitial space among nanotubes, with an averaged distance smaller than 100 nm (Fig. 1h), can induce superwetting capillary action for oil [5] leading to an immediate spreading and premeating behavior as will be discussed shortly. Also, the hierarchical surface structure, originated from the micro-scale porosity of the mesh and the nano-scale inter-tubular space, can enhance superhydrophobicity because the structure makes larger ‘air sack’ along the droplet’s contact with the VAMWNT filter [15]. A high resolution transmission electron microscopic image of a nano-tube and thermogravimetric analysis are provided in Supplementary data. The CA measurements for water and diesel were carried out as shown in Fig. 2. The water droplet was unstable on the filter and easily rolled off. The superhydrophobicity with a static water CA of 163 ± 4 was achieved as shown in Fig. 2a. This superhydrophobicity was attributed to the increased surface roughness. The dynamic behavior of a diesel droplet is shown in Fig. 2b. Diesel was selected as a representative of low viscosity oils [33]. The diesel droplet immediately spread on the filter, with a contact angle of 0, aided by the capillary action and extremely large surface area [5,9,35]. The penetration through the filter was completed within 0.4 s. The effects of mesh size and nano-tube length on superhydrophobicity were also investigated (see Supplementary data). The superhydrophobicity was achieved for the mesh sizes equal or greater than 100. The micro-scale roughness

2194

CARBON

4 8 ( 2 0 1 0 ) 2 1 9 2 –2 1 9 7

Fig. 1 – Structural characterization of VAMWNTs synthesized on stainless steel meshes (a) SEM image of the stainless steel mesh with a size of 400. (b) Optical images of the mesh before and after the synthesis of nano-tubes. (c) Top view of the synthesized nano-tubes on the mesh (SEM). (d) Top view of the nano-tubes with an average length of about 100 lm. (e) Magnified, tilted view of the tip of nano-tubes. (f) Cross-sectional image of the VAMWNT filter. (g, h) Magnified SEM images.

Fig. 2 – The VAMWNT filter can be used to separate diesel (dyed red) and water (dyed blue) layers. The nano-tube length was about 100 lm, and the mesh size was 400. (a) The static water CA (100 lL, 163 ± 4). (b) The dynamic behavior of diesel droplet (100 lL). (c) Intrusion pressures for diesel and water. The inset shows a filter holder. (d) Diesel was selectively permeated through the filter. The arrow indicates the location of the filter membrane. (e) The container was flipped over, and the applied hydrostatic pressure of water was 626.6 Pa. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

induced from the small pores of the mesh was important. Amorphous carbons were deposited at the tip of nano-tubes as the length was increased reducing the surface roughness and hydrophobicity. The well-graphitized structure at the tip of nano-tubes was important rather than the tube length to induce superhydrophobicity. This is consistent with the Cassie–Baxter model where the contact angle is independent of the height of pillars since the whole droplet is supported on the surface [15]. As a proof-of-concept experiment, three VAMWNT membranes (2 cm · 3 cm each) were mounted on a filter holder and passed through diesel and water layers. The filter holder assembly has three major openings (see inset, Fig. 2c), and

the VAMWNT membranes were fixed by a silicone pad. The silicone pad also has three openings with a total area of 0.000151 m2 (see Supplementary data). Liquid can break through the VAMWNT membrane above an intrusion pressure, Pint  2clvcosh/d [36]. clv is the liquid–vapor surface tension, h is the CA on a flat surface and d is the distance between two adjacent geometric peaks which represents surface roughness. The average intrusion pressures for diesel and water from 10 repeated experiments are shown in Fig. 2c. The vertical range bars indicate standard deviations. The intrusion pressure for water was almost 50 times greater than that for diesel. The flux data for diesel and water were obtained at above the intrusion pressures. The pressure-normalized flux

CARBON

4 8 ( 20 1 0 ) 2 1 9 2–21 9 7

for diesel measured at 80 Pa was 1186.6 kg/m2 h Pa. The flux for diesel was further increased as the applied pressure was increased (4692 kg/m2 h Pa at 400 Pa and 8415 kg/m2 h Pa at 800 Pa) while the flux for water was zero (see Supplementary data). This demonstrates feasibility for the oil and water separation. The flux for water was 85.6 kg/m2 h Pa which could be obtained at a significantly higher pressure (1820 Pa). The exceptionally high flux for oil may be originated from the nano-scale superoleophilic channels. Previous investigations also reported fast mass transport through nano-scale channels [1,3]. Fig. 2d and e shows the separation of diesel and water layers. Images depicting detailed separation processes are provided in Supplementary data. Diesel was dyed red using an oil-based ink, and water was dyed blue using a water-based ink. Diesel was permeated through the filter while repelling water at an applied load of 35.6 Pa. In the next step, diesel was poured into another beaker while keeping water under the filter membrane (Fig. 2d). Water did not leak through the filter membrane even when the container was flipped over (Fig. 2e). This demonstrates excellent characteristics of the VAMWNT membrane as a separation filter for diesel and water layers. A successful phase separation of the surfactant-stabilized diesel and water emulsion was also demonstrated in Fig. 3. The test emulsion was prepared by mixing diesel and water at a volume ratio of 4:1. A 1.73 · 10 3 molar sodium dodecyl sulfate surfactant was added as an interfacially active agent. The emulsion was stable for months. The average volumetric fractions and standard deviations of the phase separated diesel, as a function of the number of iterations, from five experiments are shown in Fig. 3a. The VAMWNT filter with 100 lm tube length was primarily used to obtain the separation data and could be re-used more than 10 times with high separation efficiency. Nano-tubes with millimeter scale lengths showed less adhesion, smaller water contact angle and higher flow resistance. The interface between nano-tubes and stainless

2195

steel meshes needs to be further improved for harsh operating conditions. The separated oil fraction rapidly increased by the iterative filtering procedure with the VAMWNT filter, and almost pure diesel could be obtained after the 3rd filtering. The phase separated diesel flux from the emulsion was 72.4 kg/m2 h Pa which could be measured at 1020 Pa. The flux is smaller than that for pure diesel probably due to the binding energy of surfactant. The inset to Fig. 3a shows the test emulsion before and after filtering. After the 1st filtering, the separated phase of red diesel could be clearly observed on top of the blue-violet layer where the mixed diesel and water existed. The permeate was re-shaked and immediately passed through the filter multiple times increasing the volumetric fraction of the phase separated diesel. The phase separation of the surfactant-stabilized high viscosity lubricating oil and water emulsion was also successfully performed (Fig. 3a). The viscosity of lubricating oil was 3500 cP which is approximately 700 times greater than that of diesel. The lubricating oil–water emulsion was prepared using the identical procedure. Pure lubricating oil could be obtained after the 5th filtering. This demonstrates feasibility of employing the VAMWNT filters for the separation of various grades of oil and water although more iteration procedures were required for the higher viscosity oil. As a control, the phase separation experiment was also carried out using a polytetrafluoroethylene (PTFE)-coated stainless steel mesh (see Supplementary data). The separation efficiency was not as good as the VAMWNT membrane, and pure diesel could be obtained after the 5th iteration. The absorption spectra of the pure water, pure diesel and separated diesel are shown in the visible range, 400–700 nm (Fig. 3b). The absorbance of water was negligible around 400–490 nm. Photons in the violet-blue wavelength region were scattered while other wavelengths of the light were absorbed. The absorbance of diesel was essentially zero around 590–700 (orange-red colour). The phase separated diesel did not show any peaks in the orange-red wavelength region,

Fig. 3 – The phase separation process of the surfactant-stabilized oil and water emulsions. (a) The volumetric fraction of phase separated diesel and lubricating oil. Diesel and water were dyed red and blue respectively. (b) The absorption spectra of the pure water (dyed blue), pure diesel (dyed red) and separated diesel. The spectra are shifted for comparison, and the zero absorption baseline is shown as a dotted line. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

2196

CARBON

4 8 ( 2 0 1 0 ) 2 1 9 2 –2 1 9 7

demonstrating the successful operation of the filter. The water content in the phase separated diesel was also quantitatively measured by the coulometric titration method. The value was 85.0 ppm confirming the high separation efficiency of the filter.

4.

Conclusion

In summary, a novel filter with both superhydrophobic and superoleophilic properties was developed by synthesizing needle-like, aligned multi-walled carbon nano-tubes on a stainless steel mesh with micro-scale pores. The CAs for diesel and water were 0 and 163 respectively. The pristine graphene structure at the tip of nano-tubes with appropriate micro-scale roughness was necessary to achieve superhydrophobicity. The macro-scale separation of diesel and water layers, and even surfactant-stabilized emulsions, was very efficient. The successful phase separation of the high viscosity lubricating oil and water emulsion was also demonstrated. The system represents a new class of carbon nano-tube membrane filters for the separation of oil and water, providing advantages over chemically intensive treatment methods which generate secondary pollutants. The simple mechanism can be readily expanded to a variety of different hydrophobic and oleophilic liquids.

Acknowledgments This work was supported by a grant from the Center for Nanoscale Mechatronics and Manufacturing (one of the 21st Century Frontier Research Programs) and WCU program through the NRF of Korea funded by the MEST (R31-2008-000-10029-0).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2010.02.020.

R E F E R E N C E S

[1] Peng X, Jin J, Nakamura Y, Ohno T, Ichinose I. Ultrafast permeation of water through protein-based membranes. Nat Nano 2009;4:353–7. [2] Shannon MA, Bohn PW, Elimelech M, Georgiadis JG, Marinas BJ, Mayes AM. Science and technology for water purification in the coming decades. Nature 2008;452(7185):301–10. [3] Holt JK, Park HG, Wang Y, Stadermann M, Artyukhin AB, Grigoropoulos CP, et al. Fast mass transport through sub-2nanometer carbon nanotubes. Science 2006;312(5776):1034–7. [4] Hoffmann S, Nistch W. Membrane coalescence for phase separation of oil-in-water emulsions stabilized by surfactants and dispersed into smallest droplets. Chem Eng Technol 2001;24(1):22–7. [5] Yuan J, Liu X, Akbulut O, Hu J, Suib SL, Kong J, et al. Superwetting nanowire membranes for selective absorption. Nat Nano 2008;3(6):332–6. [6] Rosemarie B. Koaleszenzprobleme in chemischen Prozessen. Chem Ing Tech 1986;58(6):449–56.

[7] Adebajo MO, Frost RL, Kloprogge JT, Carmody O, Kokot S. Porous materials for oil spill cleanup: a review of synthesis and absorbing properties. J Porous Mater 2003;10(3):159–70. [8] Feng L, Zhang Z, Mai Z, Ma Y, Liu B, Jiang L, et al. A superhydrophobic and super-oleophilic coating mesh film for the separation of oil and water. Angew Chem Int Ed 2004;43(15):2012–4. [9] Zhang J, Huang W, Han Y. A composite polymer film with both superhydrophobicity and superoleophilicity. Macromol Rapid Commun 2006;27(10):804–8. [10] Qin F, Yu Z, Fang X, Liu X, Sun X. A novel composite coating mesh film for oil–water separation. Front Chem Eng China 2009;3:112–8. [11] Gau H, Herminghaus S, Lenz P, Lipowsky R. Liquid morphologies on structured surfaces: from microchannels to microchips. Science 1999;283(5398):46–9. [12] Shull KR, Karis TE. Dewetting dynamics for large equilibrium contact angels. Langmuir 1994;10(1):334–9. [13] Nakajima A, Fujishima A, Hashimoto K, Watanabe T. Preparation of transparent superhydrophobic boehmite and silica films by sublimation of aluminum acetylacetonate. Adv Mater 1999;11(16):1365–8. [14] Herminghaus S. Roughness-induced non-wetting. Europhys Lett 2000;52:165–70. [15] Cassie ABD, Baxter S. Wettability of porous surfaces. Trans Faraday Soc 1944;40:546–51. [16] Chen Y, He B, Lee J, Patankar NA. Anisotropy in the wetting of rough surfaces. J Colloid Interface Sci 2005;281(2):458–64. [17] He B, Patankar NA, Lee J. Multiple equilibrium droplet shapes and design criterion for rough hydrophobic surfaces. Langmuir 2003;19(12):4999–5003. [18] McHale G. Cassie and Wenzel: were they really So wrong? Langmuir 2007;23(15):8200–5. [19] McHale G, Shirtcliffe NJ, Newton MI. Super-hydrophobic and super-wetting surfaces: analytical potential? Analyst 2004;129(4):284–7. [20] Tang K, Yu J, Zhao Y, Liu Y, Wang X, Xu R. Fabrication of super-hydrophobic and super-oleophilic boehmite membranes from anodic alumina oxide film via a two-phase thermal approach. J Mater Chem 2006;16:1741–5. [21] Huang X, Lim T-T. Performance and mechanism of a hydrophobic–oleophilic kapok filter for oil/water separation. Desalination 2006;190(1–3):295–307. [22] Lim T-T, Huang X. In situ oil/water separation using hydrophobic–oleophilic fibrous wall: a lab-scale feasibility study for groundwater cleanup. J Hazard Mater 2006;137(2):820–6. [23] Inagaki M, Kawahara A, Nishi Y, Iwashita N. Heavy oil sorption and recovery by using carbon fiber felts. Carbon 2002;40(9):1487–92. [24] Ci L, Vajtai R, Ajayan PM. Vertically aligned large-diameter double-walled carbon nanotube arrays having ultralow density. J Phys Chem C 2007;111(26):9077–80. [25] Ci L, Manikoth SM, Li X, Vajatai R, Ajayan PM. Ultrathick freestanding aligned carbon nanotube films. Adv Mater 2007;19(20):3300–3. [26] Zheng YP, Wang HN, Kang FY, Wang LN, Inagaki M. Sorption capacity of exfoliated graphite for oils-sorption in and among worm-like particles. Carbon 2004;42(12– 13):2603–7. [27] Wei QF, Mather RR, Fotheringham AF, Yang RD. Evaluation of nonwoven polypropylene oil sorbents in marine oil-spill recovery. Mar Pollut Bull 2003;46(6):780–3. [28] Srivastava A, Srivastava ON, Talapatra S, Vajtai R, Ajayan PM. Carbon nanotube filters. Nat Mater 2004;3(9):610–4.

CARBON

4 8 ( 20 1 0 ) 2 1 9 2–21 9 7

[29] Hinds BJ, Chopra N, Rantell T, Andrews R, Gavalas V, Bachas LG. Aligned multiwalled carbon nanotube membranes. Science 2004;303(5654):62–5. [30] Casavant MJ, Walters DA, Schmidt JJ, Smalley RE. Neat macroscopic membranes of aligned carbon nanotubes. J Appl Phys 2003;93(4):2153–6. [31] Miller SA, Young VY, Martin CR. Electroosmotic flow in template-prepared carbon nanotube membranes. J Am Chem Soc 2001;123(49):12335–42. [32] Park SJ, Lee DG. Performance improvement of micron-sized fibrous metal filters by direct growth of carbon nanotubes. Carbon 2006;44(10):1930–5. [33] Lim T-T, Huang X. Evaluation of hydrophobicity/oleophilicity of kapok and its performance in oily water filtration:

2197

comparison of raw and solvent-treated fibers. Ind Crop Prod 2007;26(2):125–34. [34] Kim HG, Lee CS, Choi JB, Chun KY, Kim YJ, Baik SH. Effects of a sandwich-like catalyst on the vertical growth of carbon nanotubes synthesized by using chemical vapor deposition. J Korean Phys Soc 2009;54:1006–10. [35] Shirtcliffe NJ, Mchale G, Newton MI, Perry CC, Roach P. Porous materials show superhydrophobic to superhydrophilic switching. Chem Commun 2005(25):3135–7. [36] Journet C, Moulinet S, Ybert C, Purcell ST, Bocquet L. Contact angle measurements on superhydrophobic carbon nanotube forests: effect of fluid pressure. EPL 2005;71(1):104–9.