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

Summary

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.

Abstract

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.

Introduction

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.

Protocol

1. Carbon Nanotube (CNT) Array Growth

1. 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 <100> n-type silicon wafer doped with phosphorous, with a diameter of 3 inch, a thickness of 381 μm, and a resistivity of 5-10 Ωcm. Usually this silicon wafer has a thermal oxide layer with a thickness of 300 nm.
2. 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.
3. 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 Å/sec is preferred. Use aluminum oxide pellets with purity of 99.99% or higher.
4. 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 Å/sec or less is preferred. Use iron pellets with purity of 99.95% or higher.
5. Cut and dice the catalyst coated silicon wafer into multiple smaller chips, preferably into 1x1 cm samples.
6. Load several catalyst coated silicon chips into a 1 inch diameter quartz tube furnace (Figure 2).
7. Increase the temperature of the furnace to 750 °C under a constant flow of 400 sccm argon (Ar) gas at a pressure of 600 Torr.
8. Once the growth temperature of 750 °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.
9. 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 °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
10. 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.
11. Characterize the overall growth characteristics, including growth quality, length, diameter, and packing density, by electron microscopy.

2. Oxygen Adsorption Induced by UV/Ozone Treatment

1. 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).
2. 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 μm tall CNT array from superhydrophobic to superhydrophilic.
3. 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.
4. Re-expose the CNT arrays to another round of UV/ozone treatment if they are not hydrophilic enough.
5. Characterize the surface chemistry of the UV/ozone treated CNT array by x-ray photoelectron spectroscopy.

3. Oxygen Adsorption Induced by Oxygen Plasma Treatment

1. 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.
2. Set the oxygen flow rate to 150 sccm and the chamber pressure to 500 mTorr. Set the RF power to 50 Watts.
3. 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.
4. 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.
5. Re-expose the CNT arrays to another round of oxygen plasma treatment if they are not hydrophilic enough.
6. Characterize the surface chemistry of the oxygen plasma treated CNT array by x-ray photoelectron spectroscopy.

4. Oxygen Desorption Induced by Vacuum Annealing Treatment

1. Place several samples of CNT array in the chamber of a vacuum oven (Figure 5).
2. Reduce the chamber pressure to at least 2.5 Torr.
3. Increase the chamber temperature to 250 °C or higher.
4. 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 μm tall CNT array from superhydrophilic to superhydrophobic and more than 24 hr to convert a one millimeter tall CNT array from superhydrophilic to superhydrophobic.
5. 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.
6. Re-expose the arrays to another round of vacuum annealing treatment if they are not hydrophobic enough.
7. Characterize the surface chemistry of the vacuum annealed CNT array by x-ray photoelectron spectroscopy.

5. Wetting Properties Characterization

1. 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.
2. Place a sample of CNT array on the contact angle goniometer sample table. Make sure this sample is not tilted toward one direction.
3. Bring the microneedles assembly closer to the sample and slowly dispense a 5 μl water droplet on top surface of the CNT array.
4. 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.
5. Calculate the contact angle by processing the captured image with a dedicated software such as DROPimage by ramé-hart or LBADSA.²⁴

Results

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...

Discussion

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...

Materials

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