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

The overall goal of this procedure is to produce carbon nano tube arrays with tunable wetting properties. This is accomplished by first growing vertically aligned carbon nano tube arrays using a common thermal chemical vapor deposition method. The second step is to turn the as grown carbon nanotube arrays super hydrophobic by exposing them to a vacuum ling treatment.

Next, these carbon nano tube arrays can be turned hydrophilic by exposing them to UV ozone or oxygen plasma treatment. The final step is to repeat the UV ozone or oxygen plasma treatment if the arrays are not hydrophilic enough or to repeat the vacuum and kneeling treatment to make them hydrophobic again. Ultimately, water immersion is used to show qualitatively the change in wetting properties of the carbon nano tube arrays.

Samples are further characterized by contact angle goniometry and x-ray spectroscopy. The main advantage of this technique over the existing methods, like wet chemical oxidation are fluorocarbon coating, is that this technique is highly reversible, safer, and can be easily scaled up to an industrial level. This technique is simple, doesn't involve oxidizing agents or assets, and doesn't destroy the structure of the carbon negative race.

Demonstrating the procedure will be BR lion aggressively from our lab. As a first step, prepare a silicon wafer with at least one polished side to serve as a substrate. It should have an oxide layer on the polished side.

Hear about 300 nanometers. Other details such as crystal size, doping and resistivity are unimportant. Complete the preparation by depositing a 10 nanometer buffer layer of highly purified aluminum oxide, and then a one nanometer thick, highly purified iron catalyst layer on the polished side.

Once the silicon is prepared, cut the wafer into multiple smaller chips of about one centimeter by one centimeter. Load several of these chips into a one inch diameter quartz tube furnace. Start a constant flow of argon gas at 400 standard cubic centimeters per minute or 400 SCCM with a pressure of 600 tor through the furnace.

Then set the temperature of the furnace to 750 degrees Celsius when the growth temperature of 750 degrees Celsius is reached. Ideally in 15 to 30 minutes, adjust the flow of Argonne gas to 200 SCCM and introduce molecular hydrogen gas at 285 SCCM. Keep the pressure at 600 tor.

Continue this pretreatment process for five minutes with pretreatment completed. Start the growth process by stopping the flow of Argonne, changing the flow rate of hydrogen to 210 SCCM and starting the flow of ethylene gas at 490 SCCM into the chamber. The pressure should remain at 600.

Tor run the growth process for up to one hour while maintaining a temperature of 750 degrees Celsius. The length of the carbon nano tube arrays is determined by the growth time. When the growth processes ended, stop the flow of hydrogen and ethylene gas.

Restart the flow of argon gas at 400 SCCM and 600 Tor. Turn off the furnace. Once the furnace is at room temperature.

Stop the argon gas flow and unload the samples to increase hydrophilicity. Use UV induced oxygen absorption. Place several samples of carbon nano tube arrays under a commercial UV ozone cleaner with wavelengths of 185 nanometers and 254 nanometers.

The samples should be five to 20 centimeters from the lamp. Expose the arrays to UV radiation in air at standard room temperature and pressure. The total exposure time will vary with the experimental setup and the desired wet ability.

Once exposure is complete, turn off the UV source and retrieve the samples for testing. If the treated samples are determined not to be hydrophilic enough, re-expose them to the UV ozone treatment to achieve more hydrophilic samples. Using oxygen plasma treatment, place several samples of carbon nano tube arrays in the chamber of an isotropic oxygen plasma cleaner.

Set the oxygen flow rate to 150 SC cm and the chamber pressure to 500 militar. Set the RF power to 50 watts. Expose the arrays to oxygen plasma for several minutes.

Again, the total exposure time will depend on the samples and the desired degree of wet wettability Care should be taken since oxygen plasma can destroy the arrays. Repeat the treatment on the arrays if they are not hydrophilic enough. Begin oxygen desorption with a vacuum oven that has reached 250 degrees Celsius under vacuum.

Fill the oven with nitrogen gas until it reaches ambient pressure. Then maintain the nitrogen flow while placing several array samples in the oven. Next, turn off the nitrogen flow, close the oven door and evacuate the oven.

The oven should be kept at a pressure of 2.5. Tor or less s keep the arrays in this environment for several hours, potentially more than a day, as dictated by the samples and the wet ability to be achieved. When the treatment is done.

Fill the oven with nitrogen gas until the oven pressure has reached the ambient pressure. Then remove the samples from the oven. Re-expose the arrays to another round of vacuum eeling if they're not hydrophobic enough.

This image of two carbon nano tube arrays fully submerged in water demonstrates their different wet properties. On the left is a highly hydrophilic array showing its naturally black surface. On the right is a super hydrophobic array that appears reflective due to a thin air film on its surface.

The oxygen to carbon atomic ratio is a measure of the degree of oxidation of the carbon nano tube array. A plot of this ratio as a function of the static contact angle for water shows the oxygen to carbon ratio decreases as the static contact angle increases. The shaded region at right corresponds to super hydrophobic behavior and is associated with very limited oxygen on the surface.

The oxygen to carbon atomic ratio is calculated from x-ray photo electron spectroscopy data for the oxygen one s and carbon one s peaks. This plot shows the deconvolution of the spectrum of a mildly hydrophobic array. The behavior of water in contact with this array is shown in the inset.

Compare this with the spectrum of the highly hydrophilic array, shown it right note the relative intensities of the oxygen containing bonds to the carbon bonds. Greater wetting is possible in the highly hydrophilic array as shown in the inset. Finally, compare the spectrum of the mildly hydrophilic array on the left.

With that of the S hydrophobic array at right. The presence of oxygen in the array is greatly reduced and the inset shows that water on its surface beads. After watching this video, you should have a good understanding on how to turn carbon additive arrays super hydrophobic by exposing them to a vacuum annually treatment and how to make them hydrophilic using PHE or oxygen plasma treatment.