Name: dry oxidation and vacuum annealing treatments for tuning the wetting properties of carbon nanotube arrays
Uploaded: 2013-04-15T12:00:00
Description: Watch this Scientific Journal Video about Dry Oxidation and Vacuum Annealing Treatments for Tuning the Wetting Properties of Carbon Nanotube Arrays at JoVE.com

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...

     

<ol>
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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 ...

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