pubs.acs.org/Langmuir © 2009 American Chemical Society
Superhydrophobic Conductive Carbon Nanotube Coatings for Steel Sunny Sethi and Ali Dhinojwala* Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909
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Received January 12, 2009. Revised Manuscript Received February 23, 2009 We report the synthesis of superhydrophobic coatings for steel using carbon nanotube (CNT)-mesh structures. The CNT coating maintains its structural integrity and superhydrophobicity even after exposure to extreme thermal stresses and has excellent thermal and electrical properties. The coating can also be reinforced by optimally impregnating the CNT-mesh structure with cross-linked polymers without significantly compromising on superhydrophobicity and electrical conductivity. These superhydrophobic conductive coatings on steel, which is an important structural material, open up possibilities for many new applications in the areas of heat transfer, solar panels, transport of fluids, nonwetting and nonfouling surfaces, temperature resilient coatings, composites, water-walking robots, and naval applications.
Superhydrophobicity is important for fabricating selfcleaning surfaces, controlling flow in microfluidics, filtration, robotics, and naval applications.1-3 Conventionally, superhydrophobic surfaces are generated by conjunction of high surface roughness and low surface energy materials.4-8 These low surface energy materials, generally being organic in nature, make the surface thermally and electrically insulating. Here, we report a unique and versatile technique to impart superhydrophobicity to stainless steel by surface modification using a carbon nanotube (CNT)-mesh structure. These coatings are environmentally stable, thermally robust, and conductive. These coatings show a water contact angle of 167° ( 3° and can withstand high thermal shocks, exposure to boiling water, and high temperature. The versatility of the process enables coating objects of various shapes and sizes, thus making this process suitable for industrial applications. In addition, this coating can further be reinforced and functionalized with polymers without significantly compromising on superhydrophobicity and conductivity. To prepare these unique coatings, mill-finished stainless steel 304 was used as the substrate. The steel surface was pretreated by etching in 9 M H2SO4 at 90 °C. The exposure to acid at high temperatures removes passive layers9 on the steel surfaces and creates many big and small pits on the surface of steel (SEM pictures before and after etching are provided as Supporting Information). CNTs are grown on this treated steel surface by the floating catalyst method. In this method, xylene was used as the carbon source and ferrocene as the catalyst. The catalyst was introduced as a solution of 1 g of *Corresponding author. E-mail:
[email protected]. (1) Daniel, S.; Chaudhury, M. K.; Chen, J. C. Science 2001, 291, 633–636. (2) Nakajima, A.; Hashimoto, K.; Watanabe, T. Monatsh. Chem. 2001, 132, 31–41. (3) Genzer, J.; Efimenko, K. Biofouling 2006, 22, 339–360. (4) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 1377–1380. (5) Ma, M.; Mao, Y.; Gupta, M.; Gleason, K. K.; Rutledge, G. C. Macromolecules 2005, 38, 9742–9748. (6) Huang, L.; Lau, S. P.; Yang, H. Y.; Leong, E. S. P.; Yu, S. F.; Prawer, S. J. Phys. Chem. B 2005, 109, 7746–7748. (7) Zhu, Y.; Zhang, J.; Zheng, Y.; Huang, Z.; Feng, L.; Jiang, L. Adv. Funct. Mater. 2006, 16, 568–574. (8) Bhushan, B.; Nosonovsky, M.; Jung, Y. C. J. R. Soc. Interface 2007, 4, 643–648. (9) Masarapu, C.; Wei, B. Langmuir 2007, 23, 9046–9049.
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Figure 1. Nonaligned CNT-coated stainless steel substrates: (A) SEM image showing CNTs on stainless steel substrate. (B) TEM image showing that the fibrillar structures on stainless steel substrate are nanotubes and not solid carbon nanofibers. The diameter of the CNT is 10 nm. (C) Optical image showing surface modification of a 0.5 m long stainless steel tube coated with nonaligned CNT. (D) Optical image showing different geometries of stainless steel substrates (rings, pipes, mesh, wires, and plates) can be coated with nonaligned CNTs. ferrocene in 100 mL of xylene. The substrate was heated up to 750 °C in a tube furnace in an argon-hydrogen (85:15 v/v) atmosphere.10 The xylene-ferrocene mixture was sublimed at 190 °C, and the vapors were injected in the furnace for a time period of 1 h. The resulting carbon nanotube structure was characterized using several techniques. Figure 1A shows a scanning electron microscope (SEM) image of CNT-mesh grown on a steel surface. The growth process forms a highly entangled structure unlike the CNT grown on silicon wafers.10,13 (10) Talapatra, S.; Kar, S.; Pal, S. K.; Vajtai, R.; Ci, L.; Victor, P.; Shaijumon, M. M.; Kaur, S.; Nalamasu, O.; Ajayan, P. M. Nat. Nanotechnol. 2006, 1, 112–116.
Published on Web 3/12/2009
DOI: 10.1021/la9001187
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Letter
Figure 2. Superhydrophobicity of samples and illustrations of possible applications. (A) A 10 μL deionized water droplet on stainless steel
substrate. A water contact angle of 167° ( 3° was observed with a very small hysteresis. (B) Optical image of an ice droplet on the surface of steel. (When cooled to subzero temperature, keeping the air at room temperature, the CNT-coated surface retards ice formation as compared to more hydrophilic surfaces, showing its potential for use in cryogenic devices.) (C) SEM image of the CNT-coated surface after boiling in water and quenching in ice. The image shows that the structure remains intact and that there is no delamination. Inset shows an optical image taken on a Rame Hart goniometer. It shows that the surface is still superhydrophobic after the harsh treatment. (D) Optical image showing maximum water displaced by CNT-coated steel plates before they sink in water. (E) Measurements of force (mN) as the plate is pushed into the water. The bottom red line corresponds to the density of water multiplied by the volume of the plate (buoyancy force). The top red line shows the maximum force supported by plate before sinking. (F) Optical image of 0.5 mm thick stainless steel plate floating on the water surface.
Transmission electron microscope (TEM) images at high resolution show that these structures are CNTs with a diameter of 10 nm. The use of vapor phase chemical vapor deposition (CVD) and the use of the simple process of etching as a surface treatment allows us to grow nonaligned CNTs on objects with different shapes and sizes, with curved surfaces, and inside of tubes and pores as shown in Figure 1C and 1D. To characterize the superhydrophobic nature of this coating, several water contact angle measurements were made. A water droplet forms a high contact angle of 167° ( 3° (shown in Figure 2A) and rolls off the surface very easily when the steel plate is tilted slightly. The superhydrophobicity is also retained when the plates are cooled below 0 °C. At low temperatures, the water droplet freezes with contact angles of ∼170° (Figure 2B). Upon heating the steel plates back to room temperature, the ice droplets melt and the resulting puddle of water rolls off the surface, leaving the surface completely dry. We find that these modified steel surfaces are very stable under extreme environmental conditions. Four tests are chosen to demonstrate the stability of these coatings. The first test is a low temperature test in which the coated steel plates are put into liquid N2 until the temperature of the plates has equilibrated with the temperature of liquid N2. The second test, a high temperature test, involves heating the plates in air at 300 °C for 2 h. In the third test, the plates are immersed in boiling water for 1 h. For the final test, the plates are immersed in boiling water for 5-10 min and then immediately transferred to an ice-water bath at 0 °C. SEM micrographs are taken after each test to study changes in the structure of the CNTs after each of these harsh treatments. Thereafter, water contact angles are measured. Figure 2C shows a representative SEM micrograph taken after the quench test which illustrates that the CNTs are still intact. The inset in Figure 2C shows a water droplet on the steel surface indicating that the surface maintains its superhydrophobicity. After each 4312
DOI: 10.1021/la9001187
of these thermal tests, SEM analysis indicated that the carbon nanotube structure remained intact, and high water contact angle measurements (155-170°) confirmed this observation. These nonaligned CNT-coated steel surfaces can be used for a number of applications. Here, we report two such very unique examples. The first example is the use of CNT-coated steel plates to mimic the ability of water striders and water spiders to float on water. Water striders and water spiders use surface tension and superhydrophobic legs to resist gravity. For example, water striders have long thin legs which show a water contact angle of around 170° and can stand on the surface of water using surface tension forces.11,12 The CNTcoated steel plates, being superhydrophobic, allow them to float on water. Figure 2D shows a steel plate coated with CNT-mesh being pushed on a water surface. Due to superhydrophobicity, a large dimple is formed on the surface of water as the plate resists going under water. The force required to submerge the plate was measured as shown in Figure 2E. This force of 100 mN is approximately 10 times larger than 8 mN determined using buoyancy force (corresponding to the weight of the water equivalent to the volume of the plate). In addition, Figure 2F shows a 0.5 mm (weight of 1.3 g) thick stainless steel plate floating on water, which illustrates the advantages of using nonaligned CNT coatings on steel in designing water walking robots and miniature floating devices. The second application takes the advantage of both CNTs and polymers to form hybrid coatings with better scratch resistance. To illustrate this process, a thin layer of poly (dimethylsiloxane) (PDMS) was spin-coated on CNT-coated steel using a dilute solution of Sylgard 184 in xylene (1 g of (11) Feng, X.-Q.; Gao, X.; Wu, Z.; Jiang, L.; Zheng, Q.-S. Langmuir 2007, 23, 4892–4896. (12) Hu, D. L.; Chan, B.; Bush, J. W. M. Nature 2003, 424, 663–666. (13) Zhang, Z. J.; Wei, B. Q.; Ramanath, G.; Ajayan, P. M. Appl. Phys. Lett. 2000, 77, 3764–3766.
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Figure 3. PDMS reinforced CNT-coated steel. (A) SEM image of PDMS coated CNT. The sample retained high conductivity and superhydrophobicity after coating. (B) Higher magnification SEM image showing PDMS binding to the CNTs. (C) Illustration describing the features observed in the SEM images in Figure 1A and B. The PDMS coating anchors the CNTs at a few places without forming a film on top. (D and E) Optical images of the pressure sensitive tapes after the adhesion test (based on ASTM D3359-02) done on a pristine CNT-coated steel plate and a CNT-coated steel sample reinforced with PDMS, respectively. Sylgard in 2 mL of xylene). The sample was then heated at 70 °C for 4 h to cross-link the PDMS chains. The conditions are optimized such that the PDMS coating does not cover up all the CNT. However, there is enough PDMS to cement the CNT network on the surface of the steel (Figure 3A-C). This hybrid coating improves scratch resistance while maintaining superhydrophobicity (water contact angle of 147° ( 8°) and electrical conductivity. The pencil scratch test showed that the hardness after reinforcing with PDMS is between 4H and 6H in comparison to hardness between B and HB for the CNTcoated steel (Supporting Information). This increase in scratch resistance not only makes this coating more durable but also makes it usable in more abrasive environments. However, there is a trade off between scratch resistance and electrical conductivity. For the given concentration of PDMS solution, the electrical resistance was 1-10 kΩ as compared to 2-3 Ω for the CNT-coated steel plate (Supporting Information). Nevertheless, the conductivity of PDMS reinforced coatings is many orders of magnitude better than PDMS coatings on steel. The adhesion of these coatings with the substrate was also tested using a Scotch tape test based on ASTM standard (D3359-02). In this test, the pressure sensitive tape (PSA) is pressed against the coating and peeled off. The amount of coating transferred to the PSA tape after peeling is indicative of the adhesion between the coating and the substrate. Figure 3D shows the image of the PSA tape after peeling off from the pristine CNT-coated steel surface. It can be seen that
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a layer of CNT is peeled off the steel substrate. On the other hand, Figure 3E shows the optical image of the PSA tape after peeling off from the PDMS reinforced CNT surface. We observe significantly less transfer of the coating to the PSA tape after the CNTs are impregnated with PDMS, indicating better adhesion of the coating with the steel substrate. In summary, we showed a simple and versatile method to impart superhydrophobicity to a stainless steel surface. This superhydrophobic coating can withstand extreme environmental shocks and is electrically conducting. We showed that these coatings on steel open up many novel and exciting applications in the areas of coatings for heat exchangers, fluid transportation, electrodes for fuel cells, solar panels, nonfouling surfaces, temperature resilient coatings, composites, robotics, airplanes, and ships. These areas go beyond the applications anticipated for aligned CNTs in electronics and displays. Acknowledgment. We thank Mike Heiber for his help in programming the force measurements. This work was supported by National Science Foundation Awards (DMR0512156 and DMR-0609077). Supporting Information Available: SEM images of steel before and after the etching treatment, and experimental procedure for scratch and conductivity tests. This material is available free of charge via the Internet at http://pubs.acs. org.
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