Subscriber access provided by UNIV OF AKRON

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W.,Washington, DC 20036

Letter

Superhydrophobic Conductive Carbon Nanotube Coatings for SteelSunny Sethi, and Ali Dhinojwala

Langmuir, 2009, 25 (8), 4311-4313• DOI: 10.1021/la9001187 • Publication Date (Web): 12 March 2009

Downloaded from http://pubs.acs.org on April 28, 2009

More About This Article

Additional resources and features associated with this article are available within the HTML version:

• Supporting Information• Access to high resolution figures• Links to articles and content related to this article• Copyright permission to reproduce figures and/or text from this article

pubs.acs.org/Langmuir

Superhydrophobic Conductive Carbon Nanotube Coatings for Steel

Sunny Sethi and Ali Dhinojwala*

Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909

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 extremethermal stresses and has excellent thermal and electrical properties. The coating can also be reinforced byoptimally impregnating the CNT-mesh structure with cross-linked polymers without significantly compromisingon superhydrophobicity and electrical conductivity. These superhydrophobic conductive coatings on steel, whichis 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 self-cleaning surfaces, controlling flow in microfluidics, filtration,robotics, and naval applications.1-3 Conventionally, super-hydrophobic surfaces are generated by conjunction of highsurface roughness and low surface energymaterials.4-8 Theselow surface energy materials, generally being organic innature, make the surface thermally and electrically insulating.Here, we report a unique and versatile technique to impartsuperhydrophobicity to stainless steel by surface modifica-tion using a carbon nanotube (CNT)-mesh structure. Thesecoatings are environmentally stable, thermally robust, andconductive. These coatings show a water contact angle of167�( 3� and canwithstand high thermal shocks, exposure toboiling water, and high temperature. The versatility of theprocess 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 func-tionalized with polymers without significantly compromisingon superhydrophobicity and conductivity.

To prepare these unique coatings, mill-finished stainlesssteel 304 was used as the substrate. The steel surface waspretreated by etching in 9MH2SO4 at 90 �C. The exposure toacid at high temperatures removes passive layers9 on the steelsurfaces and creates many big and small pits on the surface ofsteel (SEM pictures before and after etching are provided asSupporting Information). CNTs are grown on this treatedsteel surface by the floating catalyst method. In this method,xylene was used as the carbon source and ferrocene as thecatalyst. The catalyst was introduced as a solution of 1 g of

ferrocene in 100mL of xylene. The substrate was heated up to750 �C in a tube furnace in an argon-hydrogen (85:15 v/v)atmosphere.10 The xylene-ferrocenemixture was sublimed at190 �C, and the vapors were injected in the furnace for a timeperiod of 1 h.

The resulting carbon nanotube structure was character-ized using several techniques. Figure 1A shows a scanningelectron microscope (SEM) image of CNT-mesh grownon a steel surface. The growth process forms a highly en-tangled structure unlike theCNTgrownon siliconwafers.10,13

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 steelsubstrate are nanotubes and not solid carbon nanofibers. Thediameter of the CNT is 10 nm. (C) Optical image showing surfacemodification of a 0.5 m long stainless steel tube coated with non-aligned CNT. (D) Optical image showing different geometries ofstainless steel substrates (rings, pipes, mesh, wires, and plates) can becoated with nonaligned CNTs.

*Corresponding author. E-mail: ali4@uakron.edu.(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.

(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

© 2009 American Chemical Society

DOI: 10.1021/la9001187Langmuir 2009, 25(8), 4311–4313 4311

Transmission electron microscope (TEM) images at highresolution show that these structures are CNTs with a dia-meter of 10 nm. The use of vapor phase chemical vapordeposition (CVD) and the use of the simple process of etchingas a surface treatment allows us to grow nonaligned CNTs onobjects 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 coat-ing, several water contact angle measurements were made. Awater droplet forms a high contact angle of 167�( 3� (shownin Figure 2A) and rolls off the surface very easily when thesteel plate is tilted slightly. The superhydrophobicity is alsoretained when the plates are cooled below 0 �C. At lowtemperatures, the water droplet freezes with contact anglesof ∼170� (Figure 2B). Upon heating the steel plates back toroom temperature, the ice droplets melt and the resultingpuddle of water rolls off the surface, leaving the surfacecompletely dry.

We find that these modified steel surfaces are very stableunder extreme environmental conditions. Four tests are cho-sen to demonstrate the stability of these coatings. The first testis a low temperature test in which the coated steel plates areput into liquid N2 until the temperature of the plates hasequilibrated with the temperature of liquid N2. The secondtest, a high temperature test, involves heating the plates in airat 300 �C for 2 h. In the third test, the plates are immersed inboilingwater for 1 h.For the final test, the plates are immersedin boiling water for 5-10 min and then immediately trans-ferred to an ice-water bath at 0 �C. SEM micrographs aretaken after each test to study changes in the structure of theCNTs after each of these harsh treatments. Thereafter, watercontact angles are measured. Figure 2C shows a representa-tive SEM micrograph taken after the quench test whichillustrates that the CNTs are still intact. The inset inFigure 2Cshows awater droplet on the steel surface indicatingthat the surfacemaintains its superhydrophobicity. After each

of these thermal tests, SEManalysis indicated that the carbonnanotube structure remained intact, and high water contactangle measurements (155-170�) confirmed this observation.

These nonaligned CNT-coated steel surfaces can be usedfor a number of applications. Here, we report two such veryunique examples. The first example is the use of CNT-coatedsteel plates to mimic the ability of water striders and waterspiders to float on water.Water striders and water spiders usesurface tension and superhydrophobic legs to resist gravity.For example, water striders have long thin legs which show awater contact angle of around 170� and can stand on thesurface of water using surface tension forces.11,12 The CNT-coated steel plates, being superhydrophobic, allow them tofloat on water. Figure 2D shows a steel plate coated withCNT-mesh being pushed on a water surface. Due to super-hydrophobicity, a large dimple is formed on the surface ofwater as theplate resists going underwater. The force requiredto submerge the plate was measured as shown in Figure 2E.This force of 100 mN is approximately 10 times larger than8mNdetermined using buoyancy force (corresponding to theweight 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 illustratesthe advantages of using nonaligned CNT coatings on steelin designing water walking robots and miniature floatingdevices.

The second application takes the advantage of both CNTsand polymers to form hybrid coatings with better scratchresistance. To illustrate this process, a thin layer of poly(dimethylsiloxane) (PDMS) was spin-coated on CNT-coatedsteel using a dilute solution of Sylgard 184 in xylene (1 g of

Figure 2. Superhydrophobicity of samples and illustrations of possible applications. (A) A 10 μL deionized water droplet on stainless steelsubstrate. 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 morehydrophilic surfaces, showing its potential for use in cryogenic devices.) (C) SEM image of the CNT-coated surface after boiling in water andquenching in ice. The image shows that the structure remains intact and that there is no delamination. Inset shows an optical image taken on aRameHart goniometer. It shows that the surface is still superhydrophobic after the harsh treatment. (D) Optical image showing maximumwaterdisplaced by CNT-coated steel plates before they sink in water. (E)Measurements of force (mN) as the plate is pushed into the water. The bottomred line corresponds to the density of water multiplied by the volume of the plate (buoyancy force). The top red line shows the maximum forcesupported by plate before sinking. (F) Optical image of 0.5 mm thick stainless steel plate floating on the water surface.

(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.

DOI: 10.1021/la9001187 Langmuir 2009, 25(8),4311–43134312

Letter

Sylgard in 2 mL of xylene). The sample was then heated at70 �C for 4 h to cross-link the PDMS chains. The conditionsare optimized such that the PDMS coating does not cover upall the CNT. However, there is enough PDMS to cement theCNT network on the surface of the steel (Figure 3A-C). Thishybrid coating improves scratch resistance while maintainingsuperhydrophobicity (water contact angle of 147� ( 8�) andelectrical conductivity. The pencil scratch test showed that thehardness after reinforcing with PDMS is between 4H and 6Hin comparison to hardness between B and HB for the CNT-coated steel (Supporting Information). This increase inscratch resistance not only makes this coating more durablebut also makes it usable in more abrasive environments.However, there is a trade off between scratch resistance andelectrical conductivity. For the given concentration of PDMSsolution, the electrical resistance was 1-10 kΩ as comparedto 2-3 Ω for the CNT-coated steel plate (Supporting Infor-mation). Nevertheless, the conductivity of PDMS reinforcedcoatings is many orders of magnitude better than PDMScoatings on steel.

The adhesion of these coatings with the substrate was alsotested using a Scotch tape test based on ASTM standard(D3359-02). In this test, the pressure sensitive tape (PSA) ispressed against the coating and peeled off. The amount ofcoating transferred to the PSA tape after peeling is indicativeof the adhesion between the coating and the substrate.Figure 3D shows the image of the PSA tape after peeling offfrom the pristine CNT-coated steel surface. It can be seen that

a layer of CNT is peeled off the steel substrate. On the otherhand, Figure 3E shows the optical image of the PSA tape afterpeeling off from the PDMS reinforced CNT surface. Weobserve significantly less transfer of the coating to the PSAtape after the CNTs are impregnated with PDMS, indicatingbetter adhesion of the coating with the steel substrate.

In summary, we showed a simple and versatile method toimpart superhydrophobicity to a stainless steel surface. Thissuperhydrophobic coating can withstand extreme environ-mental shocks and is electrically conducting. We showed thatthese coatings on steel open up many novel and excitingapplications in the areas of coatings for heat exchangers, fluidtransportation, electrodes for fuel cells, solar panels, nonfoul-ing surfaces, temperature resilient coatings, composites, ro-botics, airplanes, and ships. These areas go beyond theapplications anticipated for aligned CNTs in electronics anddisplays.

Acknowledgment. We thank Mike Heiber for his helpin programming the force measurements. This work wassupported by National Science Foundation Awards (DMR-0512156 and DMR-0609077).

Supporting Information Available: SEM images ofsteel before and after the etching treatment, and experimen-tal procedure for scratch and conductivity tests. This materialis available free of charge via the Internet at http://pubs.acs.org.

Figure 3. PDMS reinforced CNT-coated steel. (A) SEM image of PDMS coated CNT. The sample retained high conductivity and super-hydrophobicity after coating. (B) Higher magnification SEM image showing PDMS binding to the CNTs. (C) Illustration describing the featuresobserved 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 ASTMD3359-02) done on a pristine CNT-coated steel plate anda CNT-coated steel sample reinforced with PDMS, respectively.

DOI: 10.1021/la9001187Langmuir 2009, 25(8), 4311–4313 4313

Letter