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 includ....

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 ....

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

List of materials used in this article

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