We report the fabrication of vertically aligned carbon nanotubes (CNT) composite using thermal chemical vapor deposition (CVD). A forest of vertically aligned CNTs was grown using catalytic CVD. Fluorocarbon polymer, films were deposited in the spaces between vertically aligned MWCNTs using thermal CVD apparatus developed in-house. The excessive polymer top layer was etched by exposing the sample to water plasma. Infrared spectroscopy confirmed the attachment of functional groups to CNTs. Alignment of CNTs, deposition of polymer and postetched specimens were analyzed by field emission scanning electron microscope (FE-SEM). Uniform distribution of monomodel vertically aligned CNTs embedded in the deposited polymer matrix was observed in the micrograph. Observed uniform distribution otherwise is not possible using conventional techniques such as spin coating.
The unique physical properties (electrical, thermal, mechanical, and magnetic) of carbon nanotubes (CNTs) and their high aspect ratio make them attractive candidates for the fabrication of the nanostructures. Anisotropic properties of CNTs require their alignment or patterning in a manner that these properties can be efficiently exploited for a variety of applications like nanoelectromechanical systems (NEMS), microelectromechanical systems (MEMS), and CNT-membranes [
Various methods using electrophoresis deposition (EPD) [
For fabricating the isoporous CNT membrane, a polymer is deposited in the spaces between the vertically aligned CNTs. These membranes have the potential to separate the challenging liquid-liquid and gaseous mixtures [
The present study describes the fabrication of isoporous CNT composite for membrane application by depositing the polymer in the spaces between vertically aligned CNTs using CVD.
Hexaflouropropylene oxide (C3F6O) and methyl viologen dichloride hydrate (98%) were supplied by Sigma-Aldrich Chemicals (UK). Multiwalled carbon nanotubes (MWCNTs) forest was grown at University of Surrey, UK.
Vertically aligned multiwalled carbon nanotubes of 1 micron length with an inner diameter of nearly 9.5 nm were grown using temperature catalytic CVD. A nickel layer of 5 nm was sputtered on n-type silicon substrate. This nickel layer was then plasma-activated to form nickel catalyst particles. Catalytic CVD reactor was evacuated to 10−6 Torr and heated to 1023 K. Ammonia at a rate of 100 standard cubic centimeters per minute (sccm) was purged in CVD system. Subsequently, acetylene gas at a rate of 25 sccm is introduced to the reactor for a duration of 10 minutes. The grown and vertically aligned CNTs forest was finally subjected to fluorocarbon polymer deposition by TCVD method.
A forest of 1 micron long MWCNTs was subjected to fluorocarbon polymer deposition by TCVD. Deposition of fluorocarbon polymer by TCVD involves (i) decomposition of the precursor gas by heating. (ii) transport of the decomposition species towards substrate held at room temperature (iii) initiation of polymerization of the decomposed species.
Thermal CVD apparatus was developed in-house using quartz tubular reactor equipped with nichrome coil as a heating source according to the scheme given in Figure
Schematic of CVD apparatus developed in-house.
Precursor gas was introduced through a nozzle at one end of the reactor while silicon substrate was placed at the opposite end. Flow rate of precursor was measured and controlled by a mass flow controller. Temperature of the substrate having aligned CNTs was maintained at 298 K by back face water cooling.
Hexaflouropropylene oxide (C3F6O) was used as a precursor to deposit the fluorocarbon polymer films. TCVD system equipped with nichrome coil was first evacuated, and then dry nitrogen was purged thoroughly into the reactor to make it oxygen free. Forest of aligned carbon nanotubes grown by CVD was placed in the TCVD system and kept at ambient temperature.
Nichrome coil was heated to 723 K. C3F6O gas was purged into TCVD reactor from inlet port in such a manner that the whole gas had a contact with the coil. The precursor gas flow was maintained at a rate of 15 standard cubic centimeters per minute (sccm). The pressure of TCVD reactor was maintained at 100 mTorr. After a process time of 40 min, carbon nanotubes were completely masked by a white layer of polymer. Thereafter, precursor gas flow was cutoff and heating terminated. Reactor was again evacuated, and the nitrogen was purged until the system approached near to atmospheric pressure [
Silicon substrate was etched by dipping the sample in 20% KOH solution for 2 h at 80°C. The excessive polymer was etched by exposing the sample to water plasma under a vapor pressure of 0.3–0.5 Torr for 80 min. The power needed for the plasma was supplied by a radio frequency at 250 kHz.
Alignment of functionalized SWCNTs was analyzed by Field-Emission Scanning Electron Microscope (FE-SEM) Quanta-200. FTIR was used to analyze the functional groups at the surface of SWCNTs. The FTIR spectrum was taken using Perkin Elmer Spectru-100 series instrument.
Figures
(a) Cross-sectional SEM image of vertically grown MWCNTs. (b) Top view of grown vertically aligned MWCNTs.
Fourier transform infrared spectroscopy (FTIR) confirms that deposited PTFE is identical to bulk PTFE (Figure
FTIR spectrum of (i) PTFE and (ii) PTFE coated CNTs.
The polymerization mechanism for the said process is given by Limb et al. [
Figures
(a) Surface morphology of a sample after deposition of polymer by thermal CVD (after 15 minutes). (b) Cross-sectional view of deposited fluorocarbon polymer between vertically aligned MWCNTs (after 40 minutes).
After 15 min of the deposition process, the black colour of sample turned to whitish. Gaps between vertically aligned MWCNT were observed after 15 min of deposition. MWCNTs are also still visible, but their blackish surface turned whitish (Figure
Figure
Surface morphology of Etched membrane.
Although the thrust of the paper resides in fabrication of a composite membrane by embedding vertically aligned CNTs in a polymer using in situ polymerization, however, preliminary evaluation reveals some characteristics related to diffusion parameters of the fabricated membrane as reflected in Table
Membrane parameters and formulae used for calculation.
Membrane parameters | Values | Method |
---|---|---|
Molar flux of MV+2, |
166.02 × 10−11 | Determined experimentally |
Diffusivity of MV+2, |
7.74 × 10−10 | Reference [ |
Concentration, |
5 | Concentration of feed solution |
Membrane thickness, |
10−6 | From cross section of SEM |
Pore area, |
4.29 × 10−7 |
|
Membrane area exposed to solution, |
3 × 10−5 | Exposed area of membrane to solution. |
Porosity of membrane, |
1.43% |
|
Permeable pore density, (no. of CNTs/m2) | 2.3 × 1014 | Calculated from top view of SEM. |
Calculated core diameter of CNTs, |
8.9 | Permeable pore density = ( |
The core diameter of CNTs inferred from these data works out to be ~8.9 nm which favourably compares with actual core diameter of ~9.5 nm of CNTs grown by catalytic CVD. Further diffusion experiments are planned, and the result will be reported in a companion paper.
The thermal CVD is an effective technique to deposit fluorocarbon polymer between intertube gaps of vertically aligned CNTs to fabricate CNT membranes using relevant precursor. It is a more efficient and convenient method compared to the spin coating. Defect-free deposition without affecting the alignment of carbon nanotubes is achieved. The contention is adequately supported by far field and high resolution SEM cross-sectional images of the fabricated and etched membranes. FTIR analysis indicates the attachment of CF2 groups with host CNT membrane walls. These films are anticipated to have superior properties quite akin to bulk PTFE.
The work was supported by an overseas Grant by the Higher Education Commission (HEC) of Pakistan. Their support is gratefully acknowledged.