This work was supported by The Charyk Foundation and The Fletcher Jones Foundation under grant number 9900600. The authors gratefully acknowledge the Kavli Nanoscience Institute at the California Institute of Technology for use of the nanofabrication instruments, the Molecular Materials Research Center of the Beckman Institute at the California Institute of Technology for use of the XPS and contact angle goniometer, and the Division of Geological and Planetary Sciences of the California Institute of Technology for use of SEM.

1. Carbon Nanotube (CNT) Array Growth
<ol>
	<li>Prepare a silicon wafer with at least one polished side. There is no specific requirement on the size, crystalline orientation, doping type, resistivity, and oxide layer thickness. We typically use a &lt;100&gt; n-type silicon wafer doped with phosphorous, with a diameter of 3 inch, a thickness of 381 &mu;m, and a resistivity of 5-10 &Omega;cm. Usually this silicon wafer has a thermal oxide layer with a thickness of 300 nm.</li>
	<li>If the prepared silicon wafer does not have an oxide layer, add an oxide layer with a thickness of 300 nm on the polished side of the wafer. This oxide layer can be grown thermally or deposited by physical vapor deposition (PVD), preferably using e-beam evaporator.</li>
	<li>Deposit an aluminum oxide (Al2O3) buffer layer on the polished side of the wafer with an average thickness of 10 nm. Deposition using e-beam evaporator at an average deposition rate of 0.5 &Aring;/sec is preferred. Use aluminum oxide pellets with purity of 99.99% or higher.</li>
	<li>Deposit an iron (Fe) catalyst layer on the polished side of the wafer with an average thickness of 1 nm. Since the uniformity of this buffer layer is extremely critical, deposition using e-beam evaporator at an average deposition rate of 0.3 &Aring;/sec or less is preferred. Use iron pellets with purity of 99.95% or higher.</li>
	<li>Cut and dice the catalyst coated silicon wafer into multiple smaller chips, preferably into 1x1 cm samples.</li>
	<li>Load several catalyst coated silicon chips into a 1 inch diameter quartz tube furnace (Figure 2).</li>
	<li>Increase the temperature of the furnace to 750 &deg;C under a constant flow of 400 sccm argon (Ar) gas at a pressure of 600 Torr.</li>
	<li>Once the growth temperature of 750 &deg;C is reached, begin the pretreatment process by flowing a mixture of 200 sccm argon gas and 285 sccm hydrogen (H2) gas, while keeping the pressure constant at 600 Torr. Run the pretreatment process for 5 min.</li>
	<li>Once the pretreatment process is completed, begin the growth process by flowing a mixture of 210 sccm hydrogen gas and 490 sccm ethylene (C2H4) gas, while keeping the pressure constant at 600 Torr. Run the growth process for up to one hour while keeping the growth temperature constant at 750 &deg;C. The length of the CNT arrays is determined by the growth time. CNT arrays with an average length of one millimeter can be achieved by growing them for one hour.2</li>
	<li>Bring the temperature of the furnace back to room temperature under a constant flow of 400 sccm argon gas at a pressure of 600 Torr. Unload the samples once the temperature of the furnace reaches room temperature.</li>
	<li>Characterize the overall growth characteristics, including growth quality, length, diameter, and packing density, by electron microscopy.</li>
</ol>
2. Oxygen Adsorption Induced by UV/Ozone Treatment
<ol>
	<li>Place several samples of CNT array under a high intensity mercury vapor lamp that generates UV radiation at a wavelength of 185 nm and 254 nm. These samples need to be placed at a distance of 5 - 20 cm from the lamp. A commercial UV/ozone cleaner can be used as an alternative (Figure 3).</li>
	<li>Expose these arrays to UV radiation in air at standard room temperature and pressure. The total exposure time depends on their physical properties, the power of UV radiation, and the degree of wettability that wants to be achieved. As an approximation, it takes about 30 min of UV irradiation at 100 mW/cm2 to completely switch a 15 &mu;m tall CNT array from superhydrophobic to superhydrophilic.</li>
	<li>Measure the static contact angle of the UV/ozone treated CNT arrays for water using contact angle goniometer. Protocol to perform this measurement is described in section 5.</li>
	<li>Re-expose the CNT arrays to another round of UV/ozone treatment if they are not hydrophilic enough.</li>
	<li>Characterize the surface chemistry of the UV/ozone treated CNT array by x-ray photoelectron spectroscopy.</li>
</ol>
3. Oxygen Adsorption Induced by Oxygen Plasma Treatment
<ol>
	<li>Place several samples of CNT array in the chamber of an oxygen plasma cleaner/asher/etcher (Figure 4). A remote oxygen plasma cleaner/asher/etcher is preferable than the direct one because of its isotropic nature.</li>
	<li>Set the oxygen flow rate to 150 sccm and the chamber pressure to 500 mTorr. Set the RF power to 50 Watts.</li>
	<li>Expose these arrays to oxygen plasma for several minutes. The total exposure time depends on their physical properties and the degree of wettability that wants to be achieved. Care has to be taken because oxygen plasma is very capable of completely oxidizing the CNT into CO and CO2 molecules. As an approximation, it should take less than 30 min to completely switch a one millimeter tall CNT array from superhydrophobic to superhydrophilic.</li>
	<li>Measure the static contact angle of the oxygen plasma treated CNT arrays for water using contact angle goniometer. Protocol to perform this measurement is described in section 5.</li>
	<li>Re-expose the CNT arrays to another round of oxygen plasma treatment if they are not hydrophilic enough.</li>
	<li>Characterize the surface chemistry of the oxygen plasma treated CNT array by x-ray photoelectron spectroscopy.</li>
</ol>
4. Oxygen Desorption Induced by Vacuum Annealing Treatment
<ol>
	<li>Place several samples of CNT array in the chamber of a vacuum oven (Figure 5).</li>
	<li>Reduce the chamber pressure to at least 2.5 Torr.</li>
	<li>Increase the chamber temperature to 250 &deg;C or higher.</li>
	<li>Expose these arrays to vacuum annealing treatment for several hours. The total exposure time depends on their physical properties and the degree of wettability that wants to be achieved. As an approximation, it takes at least 3 hr to completely switch a 15 &mu;m tall CNT array from superhydrophilic to superhydrophobic and more than 24 hr to convert a one millimeter tall CNT array from superhydrophilic to superhydrophobic.</li>
	<li>Measure the static contact angle of the vacuum annealed CNT arrays for water using contact angle goniometer. Protocol to perform this measurement is described in section 5.</li>
	<li>Re-expose the arrays to another round of vacuum annealing treatment if they are not hydrophobic enough.</li>
	<li>Characterize the surface chemistry of the vacuum annealed CNT array by x-ray photoelectron spectroscopy.</li>
</ol>
5. Wetting Properties Characterization
<ol>
	<li>Prepare a contact angle goniometer. Fill the microsyringe assembly with deionized water. This syringe has to be equipped with a 22 gauge flat-tipped straight needle or a smaller needle. Turn on the illuminator.</li>
	<li>Place a sample of CNT array on the contact angle goniometer sample table. Make sure this sample is not tilted toward one direction.</li>
	<li>Bring the microneedles assembly closer to the sample and slowly dispense a 5 &mu;l water droplet on top surface of the CNT array.</li>
	<li>Capture a picture of the water droplet once it has come to rest on the top surface of the CNT array. Make sure an equilibrium condition has been achieved before taking the image.</li>
	<li>Calculate the contact angle by processing the captured image with a dedicated software such as DROPimage by ram&eacute;-hart or LBADSA.24</li>
</ol>

<ol>
	<li>Sansom, E., Rinderknecht, D., Gharib, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Controlled+partial+embedding+of+carbon+nanotubes+within+flexible+transparent+layers.">Controlled partial embedding of carbon nanotubes within flexible transparent layers.</a> Nanotechnology. 19, 035302 (2008).</li><li>Aria, A. I., Gharib, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Reversible+Tuning+of+the+Wettability+of+Carbon+Nanotube+Arrays:+The+Effect+of+Ultraviolet/Ozone+and+Vacuum+Pyrolysis+Treatments.">Reversible Tuning of the Wettability of Carbon Nanotube Arrays: The Effect of Ultraviolet/Ozone and Vacuum Pyrolysis Treatments.</a> Langmuir. 27, 9005-9011 (2011).</li><li>Lee, S. W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-power+lithium+batteries+from+functionalized+carbon-nanotube+electrodes.">High-power lithium batteries from functionalized carbon-nanotube electrodes.</a> Nat. Nano. 5, 531-537 (2010).</li><li>Aria, A. I., Gharib, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+Dry+Oxidation+on+the+Performance+of+Carbon+Nanotube+Arrays+Electrochemical+Capacitors.">Effect of Dry Oxidation on the Performance of Carbon Nanotube Arrays Electrochemical Capacitors.</a> MRS Proceedings. 1407, (2012).</li><li>Bianco, A., Kostarelos, K., Prato, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Applications+of+carbon+nanotubes+in+drug+delivery.">Applications of carbon nanotubes in drug delivery.</a> Current Opinion in Chemical Biology. 9, 674-679 (2005).</li><li>Scardino, A. J., Zhang, H., Cookson, D. J., Lamb, R. N., Nys, R. d. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+role+of+nano-roughness+in+antifouling.">The role of nano-roughness in antifouling.</a> Biofouling: The Journal of Bioadhesion and Biofilm Research. 25, 757-767 (2009).</li><li>Rothstein, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Slip+on+Superhydrophobic+Surfaces.">Slip on Superhydrophobic Surfaces.</a> Annual Review of Fluid Mechanics. 42, 89-109 (2010).</li><li>Emsley, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Very+strong+hydrogen-bonding.">Very strong hydrogen-bonding.</a> Chemical Society Reviews. 9, 91-124 (1980).</li><li>Bhushan, B., Jung, Y., Koch, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Micro-+nano-+and+hierarchical+structures+for+superhydrophobicity,+self-cleaning+and+low+adhesion.">Micro- nano- and hierarchical structures for superhydrophobicity, self-cleaning and low adhesion.</a> Philosophical Transactions - Royal Society. Mathematical, Physical and Engineering Sciences. 367, 1631-1672 (2009).</li><li>Krupenkin, T., Taylor, J., Schneider, T., Yang, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=From+rolling+ball+to+complete+wetting:+The+dynamic+tuning+of+liquids+on+nanostructured+surfaces.">From rolling ball to complete wetting: The dynamic tuning of liquids on nanostructured surfaces.</a> Langmuir. 20, 3824-3827 (2004).</li><li>Sun, T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Control+over+the+Wettability+of+an+Aligned+Carbon+Nanotube+Film.">Control over the Wettability of an Aligned Carbon Nanotube Film.</a> Journal of the American Chemical Society. 125, 14996-14997 (2003).</li><li>Ebert, D., Bhushan, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Transparent,+Superhydrophobic,+and+Wear-Resistant+Coatings+on+Glass+and+Polymer+Substrates+Using+SiO2,+ZnO,+and+ITO+Nanoparticles.">Transparent, Superhydrophobic, and Wear-Resistant Coatings on Glass and Polymer Substrates Using SiO2, ZnO, and ITO Nanoparticles.</a> Langmuir. 28, 11391-11399 (2012).</li><li>Feng, X., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Reversible+Super-hydrophobicity+to+Super-hydrophilicity+Transition+of+Aligned+ZnO+Nanorod+Films.">Reversible Super-hydrophobicity to Super-hydrophilicity Transition of Aligned ZnO Nanorod Films.</a> Journal of the American Chemical Society. 126, 62-63 (2003).</li><li>Xu, L., Karunakaran, R. G., Guo, J., Yang, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Transparent,+Superhydrophobic+Surfaces+from+One-Step+Spin+Coating+of+Hydrophobic+Nanoparticles.">Transparent, Superhydrophobic Surfaces from One-Step Spin Coating of Hydrophobic Nanoparticles.</a> ACS Appl. Mater. Interfaces. 4, 1118 (2012).</li><li>Lau, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Superhydrophobic+carbon+nanotube+forests.">Superhydrophobic carbon nanotube forests.</a> Nano Letters. 3, 1701-1705 (2003).</li><li>Hong, Y., Uhm, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Superhydrophobicity+of+a+material+made+from+multiwalled+carbon+nanotubes.">Superhydrophobicity of a material made from multiwalled carbon nanotubes.</a> Applied Physics Letters. 88, 244101 (2006).</li><li>Lee, C. H., Johnson, N., Drelich, J., Yap, Y. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+performance+of+superhydrophobic+and+superoleophilic+carbon+nanotube+meshes+in+water-oil+filtration.">The performance of superhydrophobic and superoleophilic carbon nanotube meshes in water-oil filtration.</a> Carbon. 49, 669-676 (2011).</li><li>Hummers, W. S., Offeman, R. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Preparation+of+Graphitic+Oxide.">Preparation of Graphitic Oxide.</a> Journal of the American Chemical Society. 80, 1339 (1958).</li><li>Park, S., Ruoff, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chemical+methods+for+the+production+of+graphenes.">Chemical methods for the production of graphenes.</a> Nature Nanotechnology. 4, 217-224 (2009).</li><li>Peng, Y., Liu, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effects+of+Oxidation+by+Hydrogen+Peroxide+on+the+Structures+of+Multiwalled+Carbon+Nanotubes.">Effects of Oxidation by Hydrogen Peroxide on the Structures of Multiwalled Carbon Nanotubes.</a> Industrial &amp; Engineering Chemistry Research. 45, 6483-6488 (2006).</li><li>Huang, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stable+superhydrophobic+surface+via+carbon+nanotubes+coated+with+a+ZnO+thin+film.">Stable superhydrophobic surface via carbon nanotubes coated with a ZnO thin film.</a> The Journal of Physical Chemistry. B. 109, 7746-7748 (2005).</li><li>Feng, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Super-Hydrophobic+Surfaces:+From+Natural+to+Artificial.">Super-Hydrophobic Surfaces: From Natural to Artificial.</a> Advanced Materials. 14, 1857-1860 (2002).</li><li>Cho, S., Hong, Y., Uhm, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hydrophobic+coating+of+carbon+nanotubes+by+CH4+glow+plasma+at+low+pressure,+and+their+resulting+wettability.">Hydrophobic coating of carbon nanotubes by CH4 glow plasma at low pressure, and their resulting wettability.</a> Journal of Materials Chemistry. 17, 232-237 (2007).</li><li>Stalder, A., Kulik, G., Sage, D., Barbieri, L., Hoffmann, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+snake-based+approach+to+accurate+determination+of+both+contact+points+and+contact+angles.">A snake-based approach to accurate determination of both contact points and contact angles.</a> Colloids and Surfaces. A, Physicochemical and Engineering Aspects. , 286-2892 (2006).</li><li>Naseh, M. V., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fast+and+clean+functionalization+of+carbon+nanotubes+by+dielectric+barrier+discharge+plasma+in+air+compared+to+acid+treatment.">Fast and clean functionalization of carbon nanotubes by dielectric barrier discharge plasma in air compared to acid treatment.</a> Carbon. 48, 1369-1379 (2010).</li><li>Mawhinney, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Infrared+spectral+evidence+for+the+etching+of+carbon+nanotubes:+Ozone+oxidation+at+298+K.">Infrared spectral evidence for the etching of carbon nanotubes: Ozone oxidation at 298 K.</a> Journal of the American Chemical Society. 122, 2383-2384 (2000).</li><li>Sham, M., Kim, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Surface+functionalities+of+multi-wall+carbon+nanotubes+after+UV/Ozone+and+TETA+treatments.">Surface functionalities of multi-wall carbon nanotubes after UV/Ozone and TETA treatments.</a> Carbon. 44, 768-777 (2006).</li><li>Banerjee, S., Wong, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rational+sidewall+functionalization+and+purification+of+single-walled+carbon+nanotubes+by+solution-phase+ozonolysis.">Rational sidewall functionalization and purification of single-walled carbon nanotubes by solution-phase ozonolysis.</a> The Journal of Physical Chemistry. B. 106, 12144-12151 (2002).</li><li>Xu, T., Yang, J., Liu, J., Fu, Q. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Surface+modification+of+multi-walled+carbon+nanotubes+by+O2+plasma.">Surface modification of multi-walled carbon nanotubes by O2 plasma.</a> Applied Surface Science. 253, 8945-8951 (2007).</li><li>Felten, A., Bittencourt, C., Pireaux, J. J., Van Lier, G., Charlier, J. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Radio-frequency+plasma+functionalization+of+carbon+nanotubes+surface+O2,+NH3,+and+CF4+treatments.">Radio-frequency plasma functionalization of carbon nanotubes surface O2, NH3, and CF4 treatments.</a> Journal of Applied Physics. 98, 074308 (2005).</li><li>Chen, C., Liang, B., Ogino, A., Wang, X., Nagatsu, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Oxygen+Functionalization+of+Multiwall+Carbon+Nanotubes+by+Microwave-Excited+Surface-Wave+Plasma+Treatment.">Oxygen Functionalization of Multiwall Carbon Nanotubes by Microwave-Excited Surface-Wave Plasma Treatment.</a> The Journal of Physical Chemistry C. 113, 7659-7665 (2009).</li></ol>

All authors declare that we have no conflict of interests.

We consider UV/ozone treatment as the most convenient oxidation technique because it can be performed in air at a standard room temperature and pressure for up to several hours, depending on the length of the CNT array and the power of the UV radiation. UV radiation, generated by a high intensity mercury vapor lamp at 185 nm and 254 nm, breaks the molecular bonds on the outer wall of CNT allowing ozone, converted simultaneously from air by UV radiation, to oxidize their surface.26,27 The oxidation proces...

The introduction of synthetic materials with tunable wetting properties has enabled many applications including self-cleaning surfaces and hydrodynamic drag reduction devices.6,7 Many reported studies show that to successfully tune the wetting properties of a material, one have to be able to vary its surface chemistry and topographic surface roughness.8-11 Among many other available synthetic materials, nanostructured materials have attracted most of the attention due to their inherent multi-scaled surface roughness and their surfaces can be readily functionalized by common methods. Several examples of these nanostructured materials include ZnO,12,13 SiO2,12,14 ITO,12 and carbon nanotubes (CNT).15-17 We believe that the ability to reversibly tune the wetting properties of CNT has its own virtue since they are considered as one of the most promising materials for future applications. 
CNT can be turned hydrophilic by functionalizing their surfaces with oxygenated functional groups, introduced during an oxidation treatment. To date, the most common method to introduce oxygen adsorbates to the CNT is the well-known wet oxidation techniques, involving the use of strong acids and oxidizing agents such as nitric acid and hydrogen peroxide.18-20 These wet oxidation techniques are difficult to be scaled up to industrial level because of safety and environmental issues and the considerable amount of time to complete the oxidation process. In addition, a critical point drying method may need to be employed to minimize the effect of capillary forces that may destroy the microscopic structure and overall alignment of the CNT array during the drying process. Dry oxidation treatments, such as UV/ozone and oxygen plasma treatments, offer a safer, faster, and more controlled oxidation process compared to the aforementioned wet oxidation treatments.
CNT can be made hydrophobic by removing the attached oxygenated functional groups from their surfaces. Thus far, complicated processes are always involved in producing highly hydrophobic CNT arrays. Typically, these arrays have to be coated with non-wetting chemicals, such as PTFE, ZnO, and fluoroalkylsilane,15,21,22 or be pacified by fluorine or hydrocarbon plasma treatment, such as CF4 and CH4.16,23 Although the abovementioned treatments are not too difficult to be scaled up to industrial level, they are not reversible. Once the CNT are exposed to these treatments, they can no longer be rendered hydrophilic by using common oxidation methods.
The methods presented herein show that the wettability of CNT arrays can be tuned straightforwardly and conveniently via a combination of dry oxidation and vacuum annealing treatments (Figure 1). Oxygen adsorption and desorption processes induced by these treatments are highly reversible because of their non-destructive nature and the absence of other impurities. Hence, these treatments allow CNT arrays to be repeatedly switched between hydrophilic and hydrophobic. Further, these treatments are very practical, economical, and can be easily scaled up since they can be performed using any commercial vacuum oven and UV/ozone or oxygen plasma cleaner. 
Note that the vertically aligned CNT arrays used here are grown by the standard thermal chemical vapor deposition (CVD) technique. These arrays are typically grown on catalyst coated silicon wafer substrates in a quartz tube furnace under a flow of carbon containing precursor gasses at an elevated temperature. The average length of the arrays can be varied from a few micrometers to a millimeter long by changing the growth time.

<table border="1">
<tbody>
 <tr>
 <td> Material Name</td>
 <td> Company</td>
 <td> Catalogue Number</td>
 <td> Comments (optional)</td>
 </tr>
 <tr >
 <td>Lindberg Blue M Mini-Mite tube furnace</td>
 <td>Thermo Scientific</td>
 <td>TF55030A</td>
 <td>1" tube furnace for CNT array growth</td>
 </tr>
 <tr >
 <td>Electronic mass flow controllers</td>
 <td>MKS</td>
 <td>PFC-50 &pi;MFC</td>
 <td>Max flow rate of 1000 sccm </td>
 </tr>
 <tr >
 <td>Electronic pressure controller</td>
 <td>MKS</td>
 <td>PC-90 &pi;PC</td>
 <td>Max pressure of 1000 Torr</td>
 </tr>
 <tr >
 <td>1" quartz tube</td>
 <td>MTI Corp.</td>
 <td>>EQ-QZTube-25GE-610</td>
 <td>1" D x 24" L</td>
 </tr>
 <tr >
 <td>Hydrogen gas</td>
 <td>Airgas</td>
 <td>HY UHP200</td>
 <td>CNT array growth precursor gas, 99.999% purity</td>
 </tr>
 <tr >
 <td>Ethylene gas</td>
 <td>Matheson</td>
 <td>G2250101</td>
 <td>CNT array growth precursor gas, 99.999% purity</td>
 </tr>
 <tr >
 <td>Argon gas</td>
 <td>Airgas</td>
 <td>AR UHP200</td>
 <td>CNT array growth precursor gas, 99.999% purity</td>
 </tr>
 <tr >
 <td>Silicon wafer</td>
 <td>El-Cat</td>
 <td>2449</td>
 <td>With 300 nm polished thermal oxide layer</td>
 </tr>
 <tr >
 <td>Iron pellets</td>
 <td>Kurt J Lesker</td>
 <td>EVMFE35EXEA</td>
 <td>99.95% purity</td>
 </tr>
 <tr >
 <td>Aluminum oxide pellets</td>
 <td>Kurt J Lesker</td>
 <td>EVMALO-1220B</td>
 <td>99.99% purity</td>
 </tr>
 <tr >
 <td>E-beam evaporator</td>
 <td>CHA Industries</td>
 <td>CHA Mark 40</td>
 <td>For buffer and catalyst layer deposition</td>
 </tr>
 <tr >
 <td>UV/ozone cleaner</td>
 <td>BioForce Nanosciences</td>
 <td>ProCleaner Plus</td>
 <td>For oxidizing CNT array</td>
 </tr>
 <tr >
 <td>Oxygen plasma cleaner</td>
 <td>PVA TePla</td>
 <td>M4L</td>
 <td>For oxidizing CNT array</td>
 </tr>
 <tr >
 <td>Vacuum oven</td>
 <td>VWR</td>
 <td>97027-664 </td>
 <td>For deoxidizing CNT array</td>
 </tr>
 <tr >
 <td>SEM</td>
 <td>Zeiss</td>
 <td>1550 VP</td>
 <td>For CNT array growth characterization</td>
 </tr>
 <tr >
 <td>XPS</td>
 <td>Surface Science </td>
 <td>M-Probe</td>
 <td>For surface chemistry characterization</td>
 </tr>
 <tr >
 <td>Contact angle goniometer</td>
 <td>ram&eacute;-hart</td>
 <td>Model 190</td>
 <td>For wetting properties characterization</td>
 </tr>
 </tbody>
</table>

dry oxidation and vacuum annealing treatments for tuning the wetting properties of carbon nanotube arrays

In this article, we describe a simple method to reversibly tune the wetting properties of vertically aligned carbon nanotube (CNT) arrays. Here, CNT arrays are defined as densely packed multi-walled carbon nanotubes oriented perpendicular to the growth substrate as a result of a growth process by the standard thermal chemical vapor deposition (CVD) technique.1,2 These CNT arrays are then exposed to vacuum annealing treatment to make them more hydrophobic or to dry oxidation treatment to render them more hydrophilic. The hydrophobic CNT arrays can be turned hydrophilic by exposing them to dry oxidation treatment, while the hydrophilic CNT arrays can be turned hydrophobic by exposing them to vacuum annealing treatment. Using a combination of both treatments, CNT arrays can be repeatedly switched between hydrophilic and hydrophobic.2 Therefore, such combination show a very high potential in many industrial and consumer applications, including drug delivery system and high power density supercapacitors.3-5
The key to vary the wettability of CNT arrays is to control the surface concentration of oxygen adsorbates. Basically oxygen adsorbates can be introduced by exposing the CNT arrays to any oxidation treatment. Here we use dry oxidation treatments, such as oxygen plasma and UV/ozone, to functionalize the surface of CNT with oxygenated functional groups. These oxygenated functional groups allow hydrogen bond between the surface of CNT and water molecules to form, rendering the CNT hydrophilic. To turn them hydrophobic, adsorbed oxygen must be removed from the surface of CNT. Here we employ vacuum annealing treatment to induce oxygen desorption process. CNT arrays with extremely low surface concentration of oxygen adsorbates exhibit a superhydrophobic behavior.

The CVD method described above results in densely packed vertically aligned multi-walled CNT arrays with a typical diameter, number of wall, and inter-nanotube spacing of about 12 - 20 nm, 8 - 16 walls, and 40 - 100 nm respectively. The average length of the arrays can be varied from a few micrometers long (Figure 6a) to a millimeter long (Figure 6b) by changing the growth time from 5 min to 1 hr respectively. Typically the vertical alignment is good at larger length scale and some...

Watch this Scientific Journal Video about Dry Oxidation and Vacuum Annealing Treatments for Tuning the Wetting Properties of Carbon Nanotube Arrays at JoVE.com

Dry Oxidation and Vacuum Annealing Treatments for Tuning the Wetting Properties of Carbon Nanotube Arrays

This article describes a simple method to fabricate vertically aligned carbon nanotube arrays by CVD and to subsequently tune their wetting properties by exposing them to vacuum annealing or dry oxidation treatment.

In this article, we describe a simple method to reversibly  tune the wetting properties of vertically aligned carbon nanotube (CNT) arrays.  Here, CNT ...

dry-oxidation-vacuum-annealing-treatments-for-tuning-wetting

Research

JoVE Journal

Engineering

14.9K Views.  California Institute of Technology. This article describes a simple method to fabricate vertically aligned carbon nanotube arrays by CVD and to subsequently tune their wetting properties by exposing them to vacuum annealing or dry oxidation treatment.

Video: Dry Oxidation and Vacuum Annealing Treatments for Tuning the Wetting Properties of Carbon Nanotube Arrays

Fabrication, Densification, and Replica Molding of 3D Carbon Nanotube Microstructures

The introduction of new materials and processes to microfabrication has, in large part, enabled many important advances in microsystems, lab-on-a-chip devices, and their applications. In particular, capabilities for cost-effective fabrication of polymer microstructures were transformed by the advent of soft lithography and other micromolding techniques 1, 2, and this led a revolution in applications of microfabrication to biomedical engineering and biology. Nevertheless, it remains challenging to fabricate microstructures with well-defined nanoscale surface textures, and to fabricate arbitrary 3D shapes at the micro-scale. Robustness of master molds and maintenance of shape integrity is especially important to achieve high fidelity replication of complex structures and preserving their nanoscale surface texture. The combination of hierarchical textures, and heterogeneous shapes, is a profound challenge to existing microfabrication methods that largely rely upon top-down etching using fixed mask templates. On the other hand, the bottom-up synthesis of nanostructures such as nanotubes and nanowires can offer new capabilities to microfabrication, in particular by taking advantage of the collective self-organization of nanostructures, and local control of their growth behavior with respect to microfabricated patterns. 
Our goal is to introduce vertically aligned carbon nanotubes (CNTs), which we refer to as CNT "forests", as a new microfabrication material. We present details of a suite of related methods recently developed by our group: fabrication of CNT forest microstructures by thermal CVD from lithographically patterned catalyst thin films; self-directed elastocapillary densification of CNT microstructures; and replica molding of polymer microstructures using CNT composite master molds. In particular, our work shows that self-directed capillary densification ("capillary forming"), which is performed by condensation of a solvent onto the substrate with CNT microstructures, significantly increases the packing density of CNTs. This process enables directed transformation of vertical CNT microstructures into straight, inclined, and twisted shapes, which have robust mechanical properties exceeding those of typical microfabrication polymers. This in turn enables formation of nanocomposite CNT master molds by capillary-driven infiltration of polymers. The replica structures exhibit the anisotropic nanoscale texture of the aligned CNTs, and can have walls with sub-micron thickness and aspect ratios exceeding 50:1. Integration of CNT microstructures in fabrication offers further opportunity to exploit the electrical and thermal properties of CNTs, and diverse capabilities for chemical and biochemical functionalization 3.

We present methods for fabrication of patterned microstructures of vertically aligned carbon nanotubes (CNTs), and their use as master molds for production of polymer microstructures with organized nanoscale surface texture. The CNT forests are densified by condensation of solvent onto the substrate, which significantly increases their packing density and enables self-directed formation of 3D shapes.

The introduction of new materials and processes to microfabrication has, in large part, enabled many important advances in microsystems, lab-on-a-chip ...

Synthesis and Functionalization of Nitrogen-doped Carbon Nanotube Cups with Gold Nanoparticles as Cork Stoppers

Nitrogen-doped carbon nanotubes consist of many cup-shaped graphitic compartments termed as nitrogen-doped carbon nanotube cups (NCNCs). These as-synthesized graphitic nanocups from chemical vapor deposition (CVD) method were stacked in a head-to-tail fashion held only through noncovalent interactions. Individual NCNCs can be isolated out of their stacking structure through a series of chemical and physical separation processes. First, as-synthesized NCNCs were oxidized in a mixture of strong acids to introduce oxygen-containing defects on the graphitic walls. The oxidized NCNCs were then processed using high-intensity probe-tip sonication which effectively separated the stacked NCNCs into individual graphitic nanocups. Owing to their abundant oxygen and nitrogen surface functionalities, the resulted individual NCNCs are highly hydrophilic and can be effectively functionalized with gold nanoparticles (GNPs), which preferentially fit in the opening of the cups as cork stoppers. These graphitic nanocups corked with GNPs may find promising applications as nanoscale containers and drug carriers.

We discussed the synthesis of individual graphitic nanocups using a series of techniques including chemical vapor deposition, acid oxidation and probe-tip sonication. By citrate reduction of HAuCl4, the graphitic nanocups were effectively corked with gold nanoparticles due to the chemically reactive edges of the cups.

Nitrogen-doped carbon  nanotubes consist of many cup-shaped graphitic compartments termed as  nitrogen-doped carbon nanotube cups (NCNCs).  These ...

Fine-tuning the Size and Minimizing the Noise of Solid-state Nanopores

Solid-state nanopores have emerged as a versatile tool for the characterization of single biomolecules such as nucleic acids and proteins1. However, the creation of a nanopore in a thin insulating membrane remains challenging. Fabrication methods involving specialized focused electron beam systems can produce well-defined nanopores, but yield of reliable and low-noise nanopores in commercially available membranes remains low2,3 and size control is nontrivial4,5. Here, the application of high electric fields to fine-tune the size of the nanopore while ensuring optimal low-noise performance is demonstrated. These short pulses of high electric field are used to produce a pristine electrical signal and allow for enlarging of nanopores with subnanometer precision upon prolonged exposure. This method is performed in situ in an aqueous environment using standard laboratory equipment, improving the yield and reproducibility of solid-state nanopore fabrication.

A methodology for preparing solid-state nanopores in solution for biomolecular translocation experiments is presented. By applying short pulses of high electric fields, the nanopore diameter can be controllably enlarged with subnanometer precision and its electrical noise characteristics significantly improved. This procedure is performed in situ using standard laboratory equipment under experimental conditions.

Solid-state nanopores have emerged as a versatile tool for the characterization of single biomolecules such as nucleic acids and proteins1. However, the ...

Ambient Method for the Production of an Ionically Gated Carbon Nanotube Common Cathode in Tandem Organic Solar Cells

A method of fabricating organic photovoltaic (OPV) tandems that requires no vacuum processing is presented. These devices are comprised of two solution-processed polymeric cells connected in parallel by a transparent carbon nanotubes (CNT) interlayer. This structure includes improvements in fabrication techniques for tandem OPV devices. First the need for ambient-processed cathodes is considered. The CNT anode in the tandem device is tuned via ionic gating to become a common cathode. Ionic gating employs electric double layer charging to lower the work function of the CNT electrode. Secondly, the difficulty of sequentially stacking tandem layers by solution-processing is addressed. The devices are fabricated via solution and dry-lamination in ambient conditions with parallel processing steps. The method of fabricating the individual polymeric cells, the steps needed to laminate them together with a common CNT cathode, and then provide some representative results are described. These results demonstrate ionic gating of the CNT electrode to create a common cathode and addition of current and efficiency as a result of the lamination procedure.

A method of fabricating, in ambient conditions, organic photovoltaic tandem devices in a parallel configuration is presented. These devices feature an air-processed, semi-transparent, carbon nanotube common cathode.

A method of fabricating organic photovoltaic (OPV) tandems that requires no vacuum processing is presented. These devices are comprised of two ...

Fabrication of Low Temperature Carbon Nanotube Vertical Interconnects Compatible with Semiconductor Technology

We demonstrate a method for the low temperature growth (350 &#176;C) of vertically-aligned carbon nanotubes (CNT) bundles on electrically conductive thin-films. Due to the low growth temperature, the process allows integration with modern low-&#954; dielectrics and some flexible substrates. The process is compatible with standard semiconductor fabrication, and a method for the fabrication of electrical 4-point probe test structures for vertical interconnect test structures is presented. Using scanning electron microscopy the morphology of the CNT bundles is investigated, which demonstrates vertical alignment of the CNT and can be used to tune the CNT growth time. With Raman spectroscopy the crystallinity of the CNT is investigated. It was found that the CNT have many defects, due to the low growth temperature. The electrical current-voltage measurements of the test vertical interconnects displays a linear response, indicating good ohmic contact was achieved between the CNT bundle and the top and bottom metal electrodes. The obtained resistivities of the CNT bundle are among the average values in the literature, while a record-low CNT growth temperature was used.

A method for the growth of low temperature vertically-aligned carbon nanotubes, and the subsequent fabrication of vertical interconnect electrical test structures using semiconductor fabrication is presented.

We demonstrate a method for the low temperature growth (350 &#176;C) of vertically-aligned carbon nanotubes (CNT) bundles on electrically conductive ...

Development of an Experimental Setup for the Measurement of the Coefficient of Restitution under Vacuum Conditions

The Discrete Element Method is used for the simulation of particulate systems to describe and analyze them, to predict and afterwards optimize their behavior for single stages of a process or even an entire process. For the simulation with occurring particle-particle and particle-wall contacts, the value of the coefficient of restitution is required. It can be determined experimentally. The coefficient of restitution depends on several parameters like the impact velocity. Especially for fine particles the impact velocity depends on the air pressure and under atmospheric pressure high impact velocities cannot be reached. For this, a new experimental setup for free-fall tests under vacuum conditions is developed. The coefficient of restitution is determined with the impact and rebound velocity which are detected by a high-speed camera. To not hinder the view, the vacuum chamber is made of glass. Also a new release mechanism to drop one single particle under vacuum conditions is constructed. Due to that, all properties of the particle can be characterized beforehand.

The coefficient of restitution is a parameter that describes the loss of kinetic energy during collision. Here, a free-fall setup under vacuum conditions is developed to be able to determine the coefficient of restitution parameter for particles in micrometer range with high impact velocities.

The Discrete Element Method is used for the simulation of particulate systems to describe and analyze them, to predict and afterwards optimize their ...

Seedless Growth of Bismuth Nanowire Array via Vacuum Thermal Evaporation

Here a seedless and template-free technique is demonstrated to scalably grow bismuth nanowires, through thermal evaporation in high vacuum at RT. Conventionally reserved for the fabrication of metal thin films, thermal evaporation deposits bismuth into an array of vertical single crystalline nanowires over a flat thin film of vanadium held at RT, which is freshly deposited by magnetron sputtering or thermal evaporation. By controlling the temperature of the growth substrate the length and width of the nanowires can be tuned over a wide range. Responsible for this novel technique is a previously unknown nanowire growth mechanism that roots in the mild porosity of the vanadium thin film. Infiltrated into the vanadium pores, the bismuth domains (~ 1 nm) carry excessive surface energy that suppresses their melting point and continuously expels them out of the vanadium matrix to form nanowires. This discovery demonstrates the feasibility of scalable vapor phase synthesis of high purity nanomaterials without using any catalysts.

A protocol for seedless and high yield growth of bismuth nanowire arrays through vacuum thermal evaporation technique is presented.

Here a seedless and template-free technique is demonstrated to scalably grow bismuth nanowires, through thermal evaporation in high vacuum at RT. ...

Electroactive Polymer Nanoparticles Exhibiting Photothermal Properties

A method for the synthesis of electroactive polymers is demonstrated, starting with the synthesis of extended conjugation monomers using a three-step process that finishes with Negishi coupling. Negishi coupling is a cross-coupling process in which a chemical precursor is first lithiated, followed by transmetallation with ZnCl2. The resultant organozinc compound can be coupled to a dibrominated aromatic precursor to give the conjugated monomer. Polymer films can be prepared via electropolymerization of the monomer and characterized using cyclic voltammetry and ultraviolet-visible-near infrared (UV-Vis-NIR) spectroscopy. Nanoparticles (NPs) are prepared via emulsion polymerization of the monomer using a two-surfactant system to yield an aqueous dispersion of the polymer NPs. The NPs are characterized using dynamic light scattering, electron microscopy, and UV-Vis-NIR-spectroscopy. Cytocompatibility of NPs is investigated using the cell viability assay. Finally, the NP suspensions are irradiated with a NIR laser to determine their effectiveness as potential materials for photothermal therapy (PTT).

A protocol is presented for the synthesis and preparation of nanoparticles consisting of electroactive polymers.

A method for the synthesis of electroactive polymers is demonstrated, starting with the synthesis of extended conjugation monomers using a three-step ...

Two-way Valorization of Blast Furnace Slag: Synthesis of Precipitated Calcium Carbonate and Zeolitic Heavy Metal Adsorbent

The aim of this work is to present a zero-waste process for storing CO2 in a stable and benign mineral form while producing zeolitic minerals with sufficient heavy metal adsorption capacity. To this end, blast furnace slag, a residue from iron-making, is utilized as the starting material. Calcium is selectively extracted from the slag by leaching with acetic acid (2 M CH3COOH) as the extraction agent. The filtered leachate is subsequently physico-chemically purified and then carbonated to form precipitated calcium carbonate (PCC) of high purity (&lt;2 wt% non-calcium impurities, according to ICP-MS analysis). Sodium hydroxide is added to neutralize the regenerated acetate. The morphological properties of the resulting calcitic PCC are tuned for its potential application as a filler in papermaking. In parallel, the residual solids from the extraction stage are subjected to hydrothermal conversion in a caustic solution (2 M NaOH) that leads to the predominant formation of a particular zeolitic mineral phase (detected by XRD), namely analcime (NaAlSi2O6&#8729;H2O). Based on its ability to adsorb Ni2+, as reported from batch adsorption experiments and ICP-OES analysis, this product can potentially be used in wastewater treatment or for environmental remediation applications.

A protocol for the parallel production of precipitated calcium carbonate and zeolitic material from blast furnace slag via mineral carbonation and alkaline hydrothermal conversion, respectively, is presented. The performance of the zeolitic material towards nickel adsorption is tested.

The aim of this work is to present a zero-waste process for storing CO2 in a stable and benign mineral form while producing zeolitic minerals with ...

Precision Milling of Carbon Nanotube Forests Using Low Pressure Scanning Electron Microscopy

A nanoscale fabrication technique appropriate for milling carbon nanotube (CNT) forests is described. The technique utilizes an environmental scanning electron microscope (ESEM) operating with a low pressure water vapor ambient. In this technique, a portion of the electron beam interacts with the water vapor in the vicinity of the CNT sample, dissociating the water molecules into hydroxyl radicals and other species by radiolysis. The remainder of the electron beam interacts with the CNT forest sample, making it susceptible to oxidation from the chemical products of radiolysis. This technique may be used to trim a selected region of an individual CNT, or it may be used to remove hundreds of cubic microns of material by adjusting ESEM parameters. The machining resolution is similar to the imaging resolution of the ESEM itself. The technique produces only small quantities of carbon residue along the boundaries of the cutting zone, with minimal effect on the native structural morphology of the CNT forest.

Low pressure scanning electron microscopy in a water vapor ambient is used to machine nanoscale to microscale features in carbon nanotube forests.

A nanoscale fabrication technique appropriate for milling carbon nanotube (CNT) forests is described. The technique utilizes an environmental scanning ...