Abstract:
Efficient methods for producing a superhydrophobic carbon nanotube (CNT) array are set forth. The methods comprise providing a vertically aligned CNT array and performing vacuum pyrolysis on the CNT array to produce a superhydrophobic CNT array. These methods have several advantages over the prior art, such as operational simplicity and efficiency.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 61/321,831, filed Apr. 7, 2010. The contents of this priority document and all other references disclosed herein are incorporated in their entirety for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     Wetting properties of materials have interested researchers for decades, due to their relevance to numerous applications. The wetting properties of a material are dictated by its surface chemistry (Emsley, J.,  Chemical Society reviews,  9(1):91-124 (1980); Wenzel, R. N.,  Industrial  &amp;  Engineering Chemistry,  28(8):988-994 (1936)) and its topographic structure (Bhushan, B. et al.,  Philosophical transactions—Royal Society. Mathematical, Physical and engineering sciences,  367(1894):1631-1672 (2009); Gao, L. and McCarthy,  T. Langmuir,  23(18):9125-9127 (2007); Gao, L. and McCarthy, T.,  Journal of the American Chemical Society,  128(28):9052-9053 (2006); Krupenkin, T. et al.,  Langmuir,  20(10):3824-3827 (2004)). 
     Many investigations have been conducted to understand the surface properties of superhydrophobic materials. A superhydrophobic surface is extremely difficult to wet; it typically has a static contact angle higher than 150° and a contact angle hysteresis less than 10°. Wang, S, and Jiang, L.,  Advanced materials,  19(21):3423-3424 (2007); Men, X. et al.,  Applied physics. A, Materials science  &amp;  processing,  98(2):275-280 (2010); Bhushan, B. et al.,  Philosophical transactions—Royal Society. Mathematical, Physical and engineering sciences,  367(1894):1631-1672 (2009). 
     Superhydrophobic materials can be utilized as a protective coating for creating a self-cleaning, nonstick surface (e.g., for solar panels) and for preventing biofouling. Scardino, A. J. et al.,  Biofouling: The Journal of Bioadhesion and Biofilm Research,  25(8):757-767 (2009). They can be used as electrodes to store charge energy in a non-aqueous supercapacitor. They can also be employed to reduce hydrodynamic skin friction drag in laminar and turbulent flow. Rothstein, J.,  Annual Review of Fluid Mechanics,  42(1):89-109 (2010). Without intending to be bound by theory, the existence of a thin layer of trapped air at the liquid-solid interface is believed to allow a slip velocity at the wall of superhydrophobic material, reducing shear stress or momentum transfer from the flow to the wall. Ou, J. et al.  Physics of Fluids,  16:4635-4643 (2004); Min, T.; Kim, J.  Physics of Fluids  16:L55-L58 (2004); Daniello, R. J. et al.  Physics of Fluids  21, online publ. no. 085103 (2009). This effect can produce advantages at macro- or micro-scale. For example, superhydrophobic materials could reduce fuel consumption of marine vessels and the efficiency of liquid pipelines. They also could be used in drug delivery devices to protect the device or drug from contact with blood, and they could be used to alter the mechanical response of cells. 
     In recent years, production of synthetic materials that exhibit superhydrophobic behavior has been reported. Among these materials, vertically aligned, multi-walled carbon nanotube arrays have gained enormous attention, due to their simple fabrication process and inherent two-length scale topographic structure. Efforts have been made to modify the surface chemistry of the carbon nanotube arrays so that their wetting properties can be tuned precisely. The carbon nanotube arrays can be made hydrophilic by functionalizing their surfaces with oxygenated surface functional groups that allow hydrogen bonds with water molecules to form or hydrophobic by removing those oxygenated surface functional groups from their surfaces. 
     Various oxidation processes can be used to functionalize the surface of carbon nanotube arrays, such as high-temperature annealing in air, UV/ozone treatment, oxygen plasma treatment, and acid treatment. Processes like high-temperature annealing in air and oxygen plasma treatment would be very costly to implement in large scale, not to mention highly probable to over-oxidize the carbon nanotube if an incorrect recipe were used. The acid treatment is generally hazardous, making it inconvenient to work with. On the other hand, the UV/ozone treatment is a simple, safe, and cost-efficient method of producing more hydrophilic carbon nanotubes. 
     However, no analogous simple, safe, cost-efficient process has yet been identified for producing superhydrophobic carbon nanotubes. Previously reported studies suggest that complicated processes are always involved in producing superhydrophobic carbon nanotube arrays. In order to make these arrays superhydrophobic, they have to be coated with non-wetting chemicals such as poly(tetrafluoroethylene) (PTFE), zinc (II) oxide, and fluoroalkylsilane, (Huang, L. et al.,  The journal of physical chemistry, B,  109(16):7746-7748 (2005); Lau, K. et al.,  Nano Lett.,  3(12):1701-1705 (2003); Feng, L. et al.,  Advanced materials,  14(24):1857-1860 (2002)) or be modified by plasma treatments, such as CF4, CH4, and NF3. (Hong, Y. and Uhm, H.,  Applied physics letters,  88(24):244101 (2006); Cho, S. et al.,  Journal of materials chemistry,  17(3):232-237 (2007)); Balu, B. et al.  Langmuir,  24:4785-4790 (2008). However, no prior art has reported a method for producing a superhydrophobic CNT array surface from pure CNTs grown by a simple self-assembly process. 
     In view of the foregoing, there is a need for a simple, safe, cost-efficient process for producing superhydrophobic carbon nanotubes. Such a process could help to speed the investigation and the commercial application of superhydrophobic carbon nanotubes. The present invention satisfies these and other needs. 
     BRIEF SUMMARY OF THE INVENTION 
     In one embodiment, the present invention presents a method for producing a hydrophobic carbon nanotube (CNT) array, the method comprising: 
     providing a vertically aligned CNT array; and 
     performing vacuum-pyrolysis on the CNT array to produce the hydrophobic nanotube array. Preferably, the hydrophobic nanotube CNT array is superhydrophobic (i.e., a superhydrophobic CNT array). 
     Preferably, the vacuum-pyrolysis step is performed under reduced pressure of about 0.5 torr to about 10 torr. More preferably, the vacuum-pyrolysis step is performed under reduced pressure of about 1 torr to about 5 torr. 
     Preferably, the vacuum-pyrolysis step is performed at a reaction temperature of about 100° C. to about 500° C. More preferably, the vacuum-pyrolysis step is performed at a reaction temperature of about 125° C. to about 300° C. 
     Preferably, the vacuum-pyrolysis step has a duration of about one hour to about five hours. 
     Preferably, the vertically aligned CNT is anchored on a surface. Preferably, the vertically aligned CNT array is a member selected from a single-wall CNT array, a multiwall CNT array, and a mixture of a single-wall CNT array and a multiwall CNT array. 
     Preferably, the vertically aligned CNT array is synthesized using a synthesis technique that is selected from chemical vapor deposition (CVD), laser ablation, and arc discharge. Preferably, the vertically aligned CNT is provided by a CVD process. In one aspect of the invention, the CVD process is continuous with the vacuum-pyrolysis step. 
     Preferably, the method for producing a hydrophobic CNT array further comprises an oxidation step before the vacuum pyrolysis step to remove amorphous carbon. 
     Preferably, the method for producing a hydrophobic CNT array further comprises removing contamination using the vacuum-pyrolysis step. 
     Preferably, an outer surface of the superhydrophobic CNT array is at least 85% free from oxygen-containing impurities. More preferably, the outer surface is at least 95% free from oxygen-containing impurities. 
     Preferably, the CNT array&#39;s static water droplet contact angle increases between about 5% to 45% after the vacuum-pyrolysis step. Preferably, the water droplet roll-off angle decreases by at least twofold. Preferably, more than one method is used to assess the array&#39;s superhydrophobicity (e.g., static water droplet contact angle and water droplet roll-off angle). Preferably, the static water droplet contact angle is between about 160° to 180°. Preferably, the water droplet roll-off angle is from about 1° to 5°, which means that a water droplet would not maintain a stable position on the surface of the array when the surface is tilted more than the roll-off angle. 
     Preferably, an outer surface of the superhydrophobic CNT array is at least 85% free from oxygen-containing impurities. More preferably, the outer surface is at least 95% free from oxygen-containing impurities. Still more preferably, the outer surface is at least 97% free from oxygen-containing impurities. 
     In another embodiment, the present invention presents a hydrophobic CNT array, wherein the hydrophobic CNT array is produced by any of the methods claimed herein. Preferably, the hydrophobic CNT array is superhydrophobic. 
     These and other aspects, objects and embodiments will become more apparent when read with the detailed description and drawings that follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 . ( a ) Low-magnification scanning electron mircroscope (SEM) image of vertically aligned carbon nanotube array. ( b ) High-magnification SEM image of the same array showing the presence of some entanglements on the array&#39;s top surface. 
         FIG. 2 . ( a ) Water droplet on a superhydrophobic carbon nanotube array exhibiting an almost spherical shape with a 170° (±2°) static contact angle. ( b ) Time-lapse image of a water droplet bouncing off the surface of a superhydrophobic carbon nanotube array that was tilted 2.5°. Each frame was taken with a 17 ms interval. 
         FIG. 3 . Dispersion of carbon nanotubes with various wetting properties in industrial deionized (DI) water. The degree of CNT hydrophobicity is decreasing from left to right. The four tubes from left to right are: the dispersion of superhydrophobic CNTs (contact angle about 170°); hydrophobic CNTs (contact angle about 143°); hydrophilic CNTs (contact angle about 75°); and strongly hydrophilic CNTs (contact angle about 30°). 
         FIG. 4 . A typical Fourier-transform infrared (FTIR) spectra from superhydrophobic and hydrophilic carbon nanotube arrays showing strong peaks at 810-1320 cm −1 , 1340-1600 cm −1 , 1650-1740 cm −1 , and 2800-3000 cm −1 , which indicate the presence of C—O, C═C, C═O, and C—H x  stretching modes respectively. 
         FIG. 5 . Electrochemical impedance modulus and phase-angle spectra of carbon nanotube arrays with various wetting properties in 1 M NaCl aqueous solution. Superhydrophobic and hydrophilic arrays are indicated by triangle and square markers respectively. 
         FIG. 6 . A process diagram for one embodiment of the present method for making superhydrophobic carbon nanotubes. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     I. DEFINITION OF TERMS 
     The terms “a,” “an,” or “the” as used herein not only includes aspects with one member, but also includes aspects with more than one member. For example, an embodiment including “a vertically aligned CNT array” should be understood to present certain aspects with at least a second vertically aligned CNT array. 
     The term “about” as used herein to modify a numerical value indicates a defined range around that value. If “X” were the value, “about X” would generally indicate a value from 0.95X to 1.05X. Any reference to “about X” specifically indicates at least the values X, 0.95X, 0.96X, 0.97X, 0.98X, 0.99X, 1.01X, 1.02X, 1.03X, 1.04X, and 1.05X. Thus, “about X” is intended to teach and provide written description support for a claim limitation of, e.g., “0.98X.” When the quantity “X” only includes whole-integer values (e.g., “X carbons”), “about X” indicates from (X−1) to (X+1). In this case, “about X” as used herein specifically indicates at least the values X, X−1, and X+1. 
     When “about” is applied to the beginning of a numerical range, it applies to both ends of the range. Thus, “from about 5 to 45%” is equivalent to “from about 5% to about 45%.” When “about” is applied to the first value of a set of values, it applies to all values in that set. Thus, “about 7, 9, or 11%” is equivalent to “about 7%, about 9%, or about 11%.” 
     A “hydrophobic” surface indicates a surface that is difficult to wet because of its chemical composition or geometric microstructure. A hydrophobic surface has a static contact angle greater than 90°. 
     The term “or” as used herein should in general be construed non-exclusively. For example, an embodiment of “a composition comprising A or B” would typically present an aspect with a composition comprising both A and B. “Or” should, however, be construed to exclude those aspects presented that cannot be combined without contradiction. 
     The term “outer surface of the carbon nanotube array” as used herein includes a side or face of an array that is not directly affixed to its support. Typically, the outer surface would be more likely to contact the surrounding environment. For example, typical tests for roll-off angles would place the drop of liquid in contact with the outer surface of the array, not the inner surface, which would be the side of the array affixed to the support. 
     A “superhydrophobic” surface indicates a surface that is extremely difficult to wet because of its chemical composition or geometric microstructure. A superhydrophobic surface has at least one of the following characteristics: a static contact angle greater than 150°, a contact angle hysteresis less than 10°, or a roll-off angle less than 5°. Preferably, a superhydrophobic surface has two of these characteristics; more preferably, all three characteristics. 
     II. EMBODIMENTS 
     In one embodiment, the present invention presents a method for producing a hydrophobic carbon nanotube (CNT) array, the method comprising: 
     providing a vertically aligned CNT array; and 
     performing vacuum pyrolysis on the vertically aligned CNT array to produce the hydrophobic nanotube array. Preferably, the product CNT array is a superhydrophobic CNT array. 
     In one aspect, the present invention provides a vacuum pyrolysis process to render carbon nanotube arrays superhydrophobic. Without being bound by theory, such processes are believed to reverse the effects of oxidation by removing the oxygenated functional groups from the surface of the carbon nanotube, while maintaining the macroscopic structures and packing density of the arrays. Therefore, no deposition of any non-wetting foreign material (e.g., polyfluorocarbons such as poly(tetrafluoroethylene); metal salts, such as zinc (II) oxide) on the array is needed to make them superhydrophobic. 
     The temperature, pressure, and duration of the vacuum pyrolysis can affect the process&#39;s efficiency. Typically, a vacuum pyrolysis process that is performed at a moderate vacuum of 2.5 Torr and a temperature of 250° C. for three hours is sufficient to completely deoxidize the array. 
     Preferably, the vacuum-pyrolysis step is performed under reduced pressure of about 0.5 torr to about 10 torr, such as 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 torr. More preferably, the vacuum-pyrolysis step is performed under reduced pressure of about 1 torr to 5 torr or about 1 torr to 3 torr. Alternatively, the vacuum-pyrolysis step is performed under reduced pressure of about 1 torr to 3 torr. In general, lower pressure is preferable. Without intending to be bound by theory, a lower pressure during the reaction favors the oxygen-containing impurities&#39; dissociation from the surface. Higher pressures disfavor the reaction, prolonging reaction times or even preventing production of superhydrophobic CNT arrays. 
     At sufficiently low pressure, however, further decrease in pressure produces only minor improvement in the reaction. For example, reaction pressures of 1 torr and 0.5 torr produces similar results in the vacuum pyrolysis (e.g., the process produced a similar superhydrophilic surface after approximately the same total reaction time). 
     When oxygen is present in the ambient gas, however, it can oxidize the surface of the starting CNT array, especially during pyrolysis at high temperatures and relatively high pressures (e.g., &gt;10 torr). Preferably, the pyrolysis is free from oxygen. Alternatively and more preferably, the pyrolysis is substantially free of oxygen, thereby avoiding oxidation of the superhydrophobic CNT surface. 
     Preferably, the vacuum-pyrolysis step is performed at a reaction temperature of about 100° C. to about 500° C. such as 100° C., 150° C., 200° C., 250° C., 300° C., 350° C., 400° C., 450° C., or 500° C. More preferably, the vacuum-pyrolysis step is performed at a reaction temperature of about 125° C. to 300° C. (e.g., about 250° C.). Low temperatures (e.g., &lt;100° C., &lt;75° C., or &lt;50° C.) are disfavored because they may not provide sufficient energy for the reaction to proceed efficiently. At high temperatures (e.g., &gt;500° C., &gt;575° C., &gt;625° C., &gt;700° C., &gt;800° C., or &gt;900° C.), the nanotubes or their support (especially if the support is an organic polymer) may partially or completely decompose. Generally, higher temperatures produce a higher chance of decomposition and a faster rate of decomposition. 
     Preferably, the vacuum-pyrolysis step has a duration of about one hour to about five hours. In general, higher temperature and lower pressures during the pyrolysis step tend to decrease the time required to produce a superhydrophobic CNT array. In some aspects, the vacuum pyrolysis can be continuous. In some aspects, the vacuum pyrolysis includes one or more periods of heating (e.g., two, three, or four heating cycles). In certain aspects, the results of the procedure are dependent on the total heating time rather than the number of heating cycles. In one preferred aspect, the present invention provides an iterative process in which the array is subjected to vacuum pyrolysis, assayed for superhydrophobicity, and re-exposed to the vacuum-pyrolysis conditions if the array were not found to be superhydrophobic. 
     CNT arrays are characterized by the orientation of the individual nanotubes composing the array. In a vertically aligned array, the axis running through the central point of a carbon nanotube&#39;s inner diameter is perpendicular to the array&#39;s base (i.e., if the nanotubes were pulled straight our from their bases, they would be oriented like the teeth of a comb or the hair on a head). This is in contrast to a horizontal array (e.g., like beads on a string) or a disordered array. Preferably, the CNT arrays of the present invention are vertically aligned arrays. Without intending to be bound by theory, this vertical alignment minimizes each CNT&#39;s contact area with water, reducing possible van der Waals forces. 
     Preferably, the CNT array is anchored on a surface. Non-anchored tubes can be scraped off, which makes it harder for them to maintain their superhydrophobic properties. Preferably, the CNT array is anchored to a silicon wafer base. Alternatively, the CNT array is anchored to a polymeric base (e.g., silicone). Sansom, E.; Rinderknect, D.; Gharib, M.  Nanotech.,  19, online publ. no. 035302 (2008). Procedures for making anchored, aligned nanotubes and nanotube devices are known to the skilled artisan (e.g., U.S. Patent Application 2009/0130370; U.S. Pat. No. 7,491,628; U.S. Patent Application 2008/0145616; U.S. Patent Application 2010/0196446; Han, Z. J. et al.  Appl. Phys. Lett.  94, online publ. no. 223106 (2009); Men, X.-H. et al.  Appl. Phys. A , DOI 10.1007/s00339-009-5425-6 (2009); Li, S. et al.  J. Phys. Chem. B.  106, 9274-9276 (2002); and Zhang, L. et al.  Langmuir  25:4792-4798 (2009), which are incorporated by reference in their entirety). 
     Individual carbon nanotubes within the array can be single-wall or multiwall. Single-wall nanotubes include one layer of carbon separating the inside and outside of the nanotube. The layer may include different patterns of carbon-carbon bonds depending on its two-dimensional bond geometry. Multiwall nanotubes include more than one layer of carbon separating the inside and outside. The multiple layers may be from a sheet wrapping over itself or from separate, concentric nanotubes. Preferably, the vertically aligned CNT array is a member selected from a single-wall CNT array, a multiwall CNT array, and a mixture of a single-wall CNT array and a multiwall CNT array. 
     A CNT array is also characterized by the packing density of the individual nanotubes composing the array. The packing density is the number of carbon nanotubes in an area; it is determined by the average distance between the different nanotubes in the array. In certain aspects of the present invention, a typical packing density is about 10 6  CNT/mm 2 . At this packing density, the distance between nanotubes at this density is about three to four times the diameter of the nanotube. A higher packing density is generally preferred because more closely associated nanotubes should make the array&#39;s surface more hydrophobic. 
     In certain preferred aspects, a major advantage of the present invention is its ability to make even very short superhydrophobic CNT arrays. Previous studies have suggested that short CNT arrays cannot become superhydrophobic. Lau, K. K. S. et al.  Nano Lett.  3:1701-1705 (2009); Liu, H. et al.  Soft Matter,  2:811-821 (2006). However, by using vacuum-pyrolysis methods, CNT arrays can be made superhydrophobic regardless of length. For example, a CNT array as short as 10 μm can be converted into a superhydrophobic array. 
     Preferably, the vertically aligned CNT array is synthesized using a synthesis technique that is selected from chemical vapor deposition (CVD), laser ablation, and arc discharge, using procedures commonly known to the skilled artisan. Preferably, the vertically aligned CNT is provided by a CVD process (e.g., Seo, J. W. et al.  New J. Physics,  5, 120.1-120.22 (2003)). 
     Carbon nanotube arrays can also be prepared using other procedures known to the skilled artisan, such as those set forth in U.S. Pat. No. 7,491,628; U.S. Patent Application No. 2008/0145616; U.S. Patent Application No. 2003/0180472; and U.S. Patent Application 2010/0247777. 
     In some aspects, the CVD process is continuous with (or at least partially continuous with) the vacuum-pyrolysis step. For example, if the CVD process is continuous with the vacuum-pyrolysis step, the vacuum-pyrolysis process can be merged with the CNT growth process to form a continuous process (e.g., if there is no need to anchor the CNT array). During the cool-down from CVD synthesis of nanotubes, a vacuum is applied rather than a flowing inert gas. In some aspects, this modification eliminates a need for inert gas purging. 
     Some CNT arrays can contain residual catalyst particles or amorphous carbon, e.g., from the CNT synthesis. These impurities may create defects in the array. In certain aspects, the process set forth in the present invention further comprises an oxidation step before the vacuum-pyrolysis step to remove amorphous carbon. Preferably, if analytical techniques indicated a significant amount of catalyst particle leftovers or amorphous carbon in the CNT array, the array could be treated with ozone to oxidize the impurities before the vacuum pyrolysis (e.g., by exposure to 185 nm UV radiation in air for 1 hr). 
     Various other oxidation processes can be used to remove catalyst particles leftovers or amorphous carbon other than the ozone treatment. These other processes include hot air annealing, oxygen plasma treatment, and acid (usually a mixture of nitric acid and hydrochloric acid) treatment (e.g., Tohji, K. et al.  Nature,  383:679 (1996)). While hot air annealing, oxygen plasma treatment, and acid treatment are each more effective in removing the catalyst particles leftovers and amorphous carbon than the ozone treatment, these processes are harsher so that the chance to over-oxidize the CNT array is high. 
     The easiest way to find catalyst particle leftovers and amorphous carbon is by performing electron microscopy analysis on the CNT samples; preferably, by using transmission electron microscopy (TEM) on the CNT samples. If the catalyst particles are only found inside the CNTs and if the thickness of amorphous carbon is much less than the diameter of the CNTs, the preliminary oxidation is unnecessary. 
     In one aspect, a preliminary oxidation is performed if (i) there is any sign of more than one catalyst particle on the average (e.g., preferably, the mean) found on the outer surface of each CNT or (ii) the thickness of amorphous carbon is more than the diameter of the CNT. For example, if TEM indicated 76 surface catalyst particles in a sample comprising 75 nanotubes, the array would be oxidized, but if TEM indicated only 75 or fewer particles in the sample, the array would not be oxidized. Alternatively, if the average number of outer surface catalyst particles is at least one, the array is oxidized to remove them. In some preferred aspects, about 25 to 250, about 50 to 200, or about 60 to 200 CNTs are examined by TEM to make this determination. 
     Some CNT arrays can contain other impurities or contaminants that may adversely affect the array&#39;s properties. These impurities may be volatile or may decompose into volatile products under vacuum-pyrolysis conditions. In certain aspects, the process set forth in the present invention further comprises removing contamination using the vacuum-pyrolysis step. 
     Preferably, the vacuum-pyrolysis step removes oxygen-bearing impurities from an outer surface of the CNT array. More preferably, the oxygen-bearing impurities are organic (i.e., carbon-containing). Oxygen-bearing, organic impurities can include organic compounds containing hydroxyl, carbonyl (e.g., aldehyde or ketone), or carboxyl (e.g., carboxylic acid) groups. Alternatively, the impurities can be organic, oxygen-bearing groups chemically bonded to the surface of the CNT array (e.g., a carboxy group with a covalent, carbon-carbon bond attaching it to a carbon nanotube). 
     Preferably, the water droplet roll-off angle decreases at least two-fold; preferably, the angle decreases from two- to twenty-fold. This is the general assay for superhydrophobicity, but others can be used. Preferably, more than one method is used (e.g., static water droplet contact angle and water droplet roll-off angle). Preferably, the static water droplet contact angle increases between about 5% to about 45%, such as 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, or 45% after the vacuum-pyrolysis step. 
     Preferably, the static water droplet contact angle is between about 160° to 180° (alternatively, the static water droplet contact angle is at least 150°; preferably, at least 160°; and more preferably, at least 170°). Surfaces with static water droplet contact angles of at least 160° are extremely hydrophobic, making them particularly useful (i.e., they are not subject to the “petal effect” allowing water to be pinned to the surface). Preferably, the water droplet roll-off angle is from about 1° to 5°, such as about 1°, 2°, 3°, 4°, or 5°; more preferably, the roll-off angle is from about 1° to 3° (e.g., about 1°). Preferably, the contact angle hysteresis is at most 10°, such as between about 1° to 10° (e.g., about 1°, 2°, 3°, 4°, 5°, 6°, 7°, 8°, 9°, or 10°); more preferably, the contact angle hysteresis is at most 5°. 
     The outer surface of the CNT array can also be monitored for oxygen-bearing bonds as a way to identify the method&#39;s progress. Such monitoring can be carried out with conventional methods (e.g., quantitative FTIR). Preferably, an outer surface of the superhydrophobic CNT array is at least 85% free from oxygen-containing impurities. More preferably, the outer surface is at least 95% free from oxygen-containing impurities. Still more preferably, the outer surface is at least 97% free from oxygen-containing impurities. Alternatively, the outer surface can be free from oxygen-containing impurities to the instrument&#39;s effective limit of detection. 
     In another embodiment, the present invention presents a superhydrophobic CNT array, wherein the hydrophobic CNT array is produced by any of the methods claimed herein. Preferably, the hydrophobic CNT array is superhydrophobic. 
     In certain preferred aspects, a major advantage of the present invention is the use of a simple, high-yielding procedure (vacuum pyrolysis) to produce superhydrophobic CNT arrays. Known methods of generating superhydrophobic CNT arrays are generally low-yielding, may involve corrosive reagents (e.g., the corrosive gases used in plasma treatment), and may change other properties of the CNT array&#39;s surface (e.g., treatment with metal oxide, which makes a continuous metal oxide surface). The present invention presents an alternative method for generating superhydrophobic arrays that is simpler and more efficient. In addition, it better preserves the microstructure of the CNT array. 
     In certain preferred aspects, another advantage of the present invention is the effects of the removal of oxygen-containing impurities from the CNT array&#39;s outer surface to produce superhydrophobic CNT arrays. Known methods of generating superhydrophobic CNT arrays are generally low-yielding, may involve corrosive reagents (e.g., the corrosive gases used in plasma treatment), and may change other properties of the CNT array&#39;s surface (e.g., treatment with metal oxide, which makes a continuous metal oxide surface). The present invention&#39;s removal of oxygen-containing impurities is a simpler and more efficient method of producing superhydrophobic arrays. In addition, it better preserves the microstructure of the CNT array. 
     III. EXAMPLES 
     It is understood that the examples and embodiments described herein are for illustrative purposes only. Various modifications or changes thereof will be suggested to persons skilled in the art, and they are to be included within the purview of this application and the scope of the appended claims. In addition, each reference provided herein is incorporated by reference in its entirety to the same extent as if each reference was individually incorporated by reference. 
     Example 1 
     Preparation of Superhydrophobic CNT Arrays 
     Carbon nanotube arrays used in this study were grown by the standard chemical vapor deposition (CVD) technique on a silicon substrate, using hydrogen and ethylene as the precursor gas. Sansom, E. et al.,  Nanotechnology,  19(3):035302 (2008). The average length of all the arrays was chosen to be about 14±4 μm ( FIG. 1   a ), which was about the minimum length that can be made using CVD techniques while preserving the overall vertical alignment and high packing density of the arrays ( FIG. 1   b ). The main reason this length was chosen is for the difficulties in producing a superhydrophobic surface out of short carbon nanotube arrays reported in the previously reported studies. Lau, K. et al.,  Nano letters,  3(12):1701-1705 (2003). 
     The CNT arrays were subjected to vacuum pyrolysis, typically at a vacuum of 2.5 Torr and a temperature of 250° C. for three hours. After the pyrolysis, the array&#39;s static contact angle was tested by conventional methods to determine its hydrophobicity. If conventional analytical methods indicated that the array was not superhydrophobic (e.g., if the static contact angle were less than 160°), the array was re-subjected to vacuum pyrolysis for another three hours (or longer if re-analysis after the second pyrolysis indicated that the array was still not superhydrophobic). 
     After being subjected to the vacuum-pyrolysis process, the carbon nanotube arrays exhibited extreme water repellency. Their superhydrophobicity was demonstrated by their ultra-high static contact angle of 170°(±2°) ( FIG. 2   a ) and very low contact angle hysteresis of 3°(±1°). These arrays also exhibit a very low roll-off angle of 1° (cf.  FIG. 2   b , though  FIG. 2   b  shows a roll-off angle of 2.5°). The static contact angle, contact angle hysteresis, and roll-off angles were measured using standard techniques known by the skilled artisan (e.g., contact angles were measured with a contact angle goniometer). 
     Example 2 
     Comparison of Post-Vacuum-Pyrolysis CNTs with Control CNTs 
     Comparison of water-based dispersions of the pre- and post-vacuum pyrolysis carbon nanotubes provides further evidence of the vacuum pyrolysis products&#39; superhydrophobicity. Superhydrophobic CNT arrays were prepared by the method of Example 1. These were compared with non-superhydrophobic control arrays prepared by the same initial procedure, but not subjected to vacuum pyrolysis (contact angle about 143°) as well as hydrophilic CNTs (contact angle about 75°); and strongly hydrophilic CNTs (contact angle about 30°). The water-based dispersions are obtained by scraping the nanotube arrays from their growth substrates and ultrasonically dispersing them in standard industrial deionized water for at least two hours. 
     The experiment demonstrated that nanotubes that have been subjected to vacuum-pyrolysis were not dispersed in water even after being sonicated for more than two hours ( FIG. 3 ). In contrast, the more hydrophilic nanotubes can be dispersed easily in water. From this finding, one can conclude that the vacuum-pyrolysis treatment is capable of completely deoxidizing individual nanotubes within the array. 
     Example 3 
     FTIR and Electrochemical Characterization of Superhydrophobic CNT Surface Chemistry 
     To study the effect of the vacuum-pyrolysis process on the surface chemistry of the hydrophilic CNTs, FTIR spectrometry analysis was conducted on array samples using standard methods for the skilled artisan. The superhydrophobic samples were compared with hydrophilic samples (contact angle 30°, as per Example 2&#39;s strongly hydrophilic CNTs). A small portion of the CNT array (&lt;1 mm 2 ) was scraped from the growth substrate, dispersed in 50 ml deuterated dichloromethane, drop-cast onto a KBr window, and then dried overnight under mild vacuum (&gt;5 torr) and without heating to remove the solvent. The FTIR spectrometry analysis was subsequently performed on the sample using an infrared laser with a wavelength of 2500-12500 nm. 
     Four strong bands were detected on the hydrophilic arrays at 810-1320 cm −1 , 1340-1600 cm −1 , 1650-1740 cm −1 , and 2800-3000 cm −1 , which indicate the presence of C—O, C═C, C═O and C—H x  stretching modes respectively ( FIG. 4 ). The peaks at 970, 1028, 1154 and 1201 cm −1  correspond to C—O stretching modes (Kuznetsova, A. et al.,  Chemical Physics Letters,  321(3-4):292-296 (2000)), and the broad shoulder band at 810-1320 cm −1  suggests the existence of C—O—C bonds from ester functional groups. Sham, M. and Kim, J.,  Carbon,  44(4):768-777 (2006); Socrates, G.,  Infrared and Raman characteristic group frequencies: tables and charts,  3rd ed. ed., Wiley: Chichester (2001); Mawhinney, D. et al.,  Journal of the American Chemical Society,  122(10):2383-2384 (2000); Kim, U. et al.,  Physical Review Letters,  95(15):157402 (2005). The peaks at 1378, 1462, 1541 and 1574 cm −1  indicate the presence of C═C stretching vibration modes of the carbon nanotube walls. Kuznetsova, A. et al.,  Chemical Physics Letters,  321(3-4):292-296 (2000); Sham, M. and Kim, J.,  Carbon,  44(4):768-777 (2006); Socrates, G.,  Infrared and Raman characteristic group frequencies: tables and charts,  3rd ed. ed., Wiley: Chichester (2001); Mawhinney, D. et al.,  Journal of the American Chemical Society,  122(10):2383-2384 (2000). The narrow band at a peak of 1703 cm −1  corresponds to C═O stretching modes of either quinone or carboxylic acid ester groups. Kuznetsova, A. et al.,  Chemical Physics Letters,  321(3-4):292-296 (2000); Sham, M. and Kim, J.,  Carbon,  44(4):768-777 (2006); Mawhinney, D. et al.,  Journal of the American Chemical Society,  122(10):2383-2384 (2000); Kim, U. et al.,  Physical Review Letters,  95(15):157402 (2005). 
     These FTIR spectra show that the strength of all peaks associated with the C—O and C═O stretching modes of the superhydrophobic array is significantly lower than that of the hydrophilic one, suggesting that the oxygen desorption process does take place during vacuum-pyrolysis treatment. The strength of the C═C stretching modes also seems to decrease slightly, implying that the graphitic structures of the carbon nanotubes were still intact after the vacuum-pyrolysis treatment. The triplet with peaks at 2848, 2915 and 2956 cm −1  indicate C—H x  bonds from the hydrocarbon functional group. Kim, U. et al.,  Physical Review Letters,  95(15):157402 (2005). This hydrocarbon triplet peaks seems to be unaffected by vacuum-pyrolysis process, implying that these peaks may be associated with contaminations in the FTIR instrument (Kim, U. et al.,  Physical Review Letters,  95(15):157402 (2005)) and have nothing to do with the wetting properties of the arrays. 
     Just like their wetting properties, the electrochemical properties of carbon nanotube arrays are dictated by their surface chemistry. As shown by the measured impedance modulus and phase angle spectra, carbon nanotube arrays with different wetting properties exhibit different electrochemical properties ( FIG. 5 ). For the superhydrophobic array, the frequency of constant impedance spans for three decades from 1 kHz to 1 MHz. On the other hand, the frequency of constant impedance for the hydrophilic arrays spans for six decades from 1 Hz to 1 MHz. At a low frequency of 10 mHz, the impedance modulus of the superhydrophobic array is about two orders of magnitude higher than that of the hydrophilic one. The impedance of the hydrophilic and the superhydrophilic CNT arrays were found to be about 650Ω and 162 kΩ respectively at frequency of 12 mHz in 1 M NaCl solution. This finding implies that the specific capacitance for hydrophilic and the superhydrophilic CNT array is about 3.3 F/g and 9.1 mF/g respectively. 
     Without being bound by theory, these findings are the result of a thin film of air on the interface between the surface of the superhydrophobic array and the aqueous electrolyte. This air film inhibits electrons transfer from the arrays and obstructs protons in the electrolyte to approach the surface of the array. On the other hand, the hydrophilic array is completely wetted by the aqueous electrolyte such that there is no air film that may inhibit electron transfer from the arrays. Because of the air film&#39;s presence film, the impedance of the superhydrophobic array was measured to be two orders of magnitude higher than that of the hydrophilic one. 
     Example 4 
     Flow-Diagram of One Embodiment of the Present Invention 
     This example illustrates a flow diagram of one embodiment of the present invention ( FIG. 6 ). The embodiment provides a vacuum pyrolysis process ( 100 ) to render carbon nanotube arrays superhydrophobic. In this instance, beginning with a vertically aligned CNT array ( 110 ), the array is analyzed for any catalyst particles or amorphous carbon contamination ( 117 ). If either or both of these are present, an oxidation process is performed to remove the contamination ( 121 ). Next, a vacuum-pyrolysis step is performed at a reaction temperature and duration as indicated herein (e.g., a temperature selected from about 100° C. to about 500° C. and a duration selected from about one hour to five hours) ( 125 ). After the vacuum-pyrolysis step is performed, the static contact angle is determined. In certain embodiments, if the static contact angle is within specification ( 136 ), the roll-off angle is determined. In one aspect, if the roll-off angle is within specification ( 147 ), the superhydrophobic CNT array is produced ( 163 ). If either of the static angle or the roll-off angle are not within specification, the vacuum-pyrolysis step ( 125 ) may be performed iteratively to produce the superhydrophobic CNT array ( 163 ). 
     CONCLUSIONS 
     In conclusion, the discoveries reported herein show that the wetting properties of carbon nanotube arrays can be altered by controlling the amount of oxygenated functional groups that are bonded to their surface. The CNT arrays can be made hydrophilic by oxidizing with, e.g., hot air, strong acids, UV/ozone, or oxygen plasma. The CNT arrays can be made superhydrophobic by deoxidizing with vacuum-pyrolysis treatment at moderate vacuum and temperature. Such vacuum-pyrolysis treatment is capable of removing the oxygenated functional groups that are attached to the CNTs&#39; surfaces. 
     All publications and patent applications cited in this specification are herein incorporated by reference as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it will be readily apparent to those of ordinary skill in the art in light of the teachings of this invention that certain changes and modifications may be made thereto without departing from the spirit or scope of the appended claims.