Carbon Nanotube Arrays with Tunable Wettability and Their Applications Adrianus Aria * , and Morteza Gharib ** * Graduate Aeronautical Laboratories, California Institute of Technology 1200 E California Blvd, Pasadena, California 91125, USA, indrat@caltech.edu ** Graduate Aeronautical Laboratories, California Institute of Technology 1200 E California Blvd, Pasadena, California 91125, USA, mgharib@caltech.edu ABSTRACT A combination of UV/ozone and vacuum pyrolysis treatments is introduced to tune the wetting properties of vertically aligned multiwalled carbon nanotube arrays. The UV/ozone treatment renders these arrays superhydrophilic by functionalizing them with oxygenated functional groups. These superhydrophilic arrays have a high affinity to water such that they have a very high specific capacitance in aqueous electrolytes. The vacuum pyrolysis treatment renders these arrays superhydrophobic by deoxidizing them. The presence of a thin film of air at the surface of superhydrophobic arrays when they are submerged in water creates a slip condition that is useful for drag reduction. Keywords: carbon nanotube array, superhydrophobic, superhydrophilic, supercapacitor, drag reduction. 1 BACKGROUND Wetting properties of materials have been the topics of interest among researchers for decades, due to their relevance to numerous industrial applications. A lot of investigations have been conducted in the past to understand all the parameters that influence the wettability of a material. Two of these parameters are the surface chemistry [1, 2] and the nanometer-scale surface roughness [3, 4] of the material. It is well known that the surface chemistry of a material can be altered by many ways; some of them involve surface oxidation, non-wetted materials deposition, and electric field application (electrowetting). Quite the opposite, there are not so many ways to modify the nanometer-scale surface roughness of a material. Among many synthetic materials, vertically aligned multiwalled carbon nanotube (MWNT) arrays capture a lot of attentions, due to their exceptional properties, durability, simple fabrication process, and surface chemistry versatility. The MWNT array can be made superhydrophilic straightforwardly by oxidizing them, which results in the presence of oxygenated functional groups on their surface. However, in order to produce superhydrophobic MWNT array, much more complicated processes involving polymer [5] or ceramic [6] deposition as well as plasma treatments [7, 8] are needed. Here, a combination of UV/ozone [9, 10] and vacuum- pyrolysis treatments to tune the wetting properties of MWNT arrays is introduced. The amount of oxygenated functional groups bonded to the MWNT arrays can be easily varied by these treatments. Using a combination of these treatments, the MWNT arrays can be repeatedly switched between superhydrophilic and superhydrophobic. There are actually many oxidation processes that can be used to oxidize the MWNT array, such as high temperature annealing in air, UV/ozone treatment, oxygen plasma treatment, and acid treatment [11, 12]. The acid treatment is generally hazardous and the hot air annealing and oxygen plasma treatment are costly and the probability to over oxidize the MWNT array is quite high. On the other hand, the UV/ozone treatment is safe, simple, cost efficient and can be scaled up for industrial use. Likewise, the vacuum pyrolysis treatment allows a simple, safe, cost efficient process to deoxidize the MWNT arrays. Since a vacuum pyrolysis treatment performed at a mild vacuum and a moderate temperature reverses the effect of surface oxidation, it eliminates the need to deposit foreign materials onto the arrays in order to make them superhydrophobic. 2 EXPERIMENTAL METHODS Vertically aligned MWNT arrays (Fig. 1a and Fig. 1b) were grown by chemical vapor deposition technique on silicon substrates, using hydrogen and ethylene as the precursor gas [13]. The typical diameter and inter-nanotube spacing is about 12-20 nm and 40-100 nm respectively. The length of the array used in this study was about 14 μm. Basic characterization of the MWNT growth was done using scanning electron microscope (Zeiss Leo 1550VP). The hydrophobic MWNT arrays used in this study were produced by oxidizing the as-grown MWNT in UV/ozone treatment (Bioforce Nanosciences UV/Ozone Procleaner Plus) for 21 minutes. The superhydrophobic MWNT arrays were produced by subjecting the superhydrophilic MWNT arrays to vacuum pyrolysis treatement at a mild vacuum of 2.5 Torr and a moderate temperature of 250°C for 3 hours. The quality of the graphitic structure of the MWNT was assessed using transmission electron microscope (FEI Tecnai F30) and Raman spectrometer (Renishaw M1000) with E excit = 2.41 eV. NSTI-Nanotech 2011, www.nsti.org, ISBN 978-1-4398-7142-3 Vol. 1, 2011 149