Multiwalled carbon nanotubes (MWCNTs) were synthesized on austenitic stainless steel foils (Type 304) using a home-built thermal chemical vapor deposition (CVD) under atmospheric pressure of hydrogen (H2) and acetylene (C2H2). During the growth, the stainless steel substrates were heated at different temperatures of 600, 700, 800, and 900°C. It was found that MWCNTs were grown on the stainless steel substrates heated at 600, 700, and 800°C while amorphous carbon film was grown at 900°C. The diameters of MWCNTs, as identified by scanning electron microscope (SEM) images together with ImageJ software program, were found to be 67.7, 43.0, and 33.1 nm, respectively. The crystallinity of MWCNTs was investigated by an X-ray diffractometer. The number of graphitic walled layers and the inner diameter of MWCNTs were investigated using a transmission electron microscope (TEM). The occurrence of Fe3O4 nanoparticles associated with carbon element can be used to reveal the behavior of Fe in stainless steel as catalyst. Raman spectroscopy was used to confirm the growth and quality of MWCNTs. The results obtained in this work showed that the optimum heated stainless steel substrate temperature for the growth of effective MWCNTs is 700°C. Chemical states of MWCNTs were investigated by X-ray photoelectron spectroscopy (XPS) using synchrotron light.
Multiwalled carbon nanotubes (MWCNTs) have been widely studied by many researchers. MWCNTs can be synthesized by different methods such as arc discharge [
In this work, a home-built thermal CVD system was designed and constructed for growing MWCNTs. The austenitic stainless steel foil (Type 304) was used as flexible substrate. It also behaves as catalyst and, hence, no need for an addition of extra catalyst for synthesizing MWCNTs. Furthermore, during the growth of MWCNTs, the stainless steel substrates were heated at different temperatures of 600, 700, 800, and 900°C under atmospheric pressure of hydrogen (H2) and acetylene (C2H2). The effects of heated stainless steel substrate temperatures on the growth of MWCNTs were also reported.
Figure
Schematic illustration of thermal CVD system.
The tubular furnace was used as a heating source in CVD system. It consists of an alumina tube with an inner diameter of 6 cm and a length of 150 cm. The dimension of the furnace is 35 cm × 100 cm × 60 cm. The heating source is electrical wire winding around the alumina tube with a power of 3.6 kW. The furnace can be heated up to a maximum temperature of 1200°C using a programmable temperature controller.
Argon (Ar), hydrogen (H2), and acetylene (C2H2) gases were used as raw materials for the gas system. Three flow meters (Cole-Parmer, TW03227-12) were used to control the flow rate of gases from 0 to 200 sccm. The pressure inside the tube was measured by vacuum gauges (ENFM, 7011). The ball valves (Swagelok, B-42S4) were used to open/close all three gases. A rotary pump was used to evacuate and remove the oxygen out of the tube before supplying argon, hydrogen, and acetylene gases into the alumina tube.
The water system consists of a glass tube with an inner diameter of 3.5 cm and a length of 10 cm. The glass tube was filled with water of about 100 cm3. The drain valve (Swagelok, B-42S4) was connected between the high temperature gases and glass tube. The drain valve was managed to open when the pressure within the alumina tube was higher than the atmospheric pressure to protect the water feed back into the alumina tube. The exhausted gas was cooled down by water before it was drained out.
In this study, to protect the contamination of alumina tube inner surface, a stainless steel tube with an inner diameter of 4.9 cm, 1 mm thick and 150 cm long, was fabricated and placed in the alumina tube of tubular furnace. Figure
Schematic illustration of stainless steel tube with the brass coupling holder inserted in the alumina tube of the tubular furnace.
The austenitic stainless steel foils (Type 304) with a thickness of approximately 50
The substrates were placed in the stainless steel tube and evacuated with a rotary pump until the pressure inside the stainless steel tube was less than 10−2 mbar. To prevent the oxidation of surface stainless steel substrate surface, Ar was fed into the stainless steel tube with a flow rate of 50 standard cubic centimeters per minute (sccm). At the same time, the stainless steel tube was heated up to 600°C. Then, the valve of a rotary pump was closed until the pressure in the stainless steel tube was higher than atmospheric pressure and the drain valve was opened for gas exit. The exhausted gas was cooled down by the water and drained afterwards. H2 used as reductive gas was injected into the stainless steel tube with a flow rate of 200 sccm for 30 min to etch the oxide on the substrate surface. After that, C2H2 was fed into the stainless steel tube with a flow rate of 160 sccm for 30 min. Finally, the MWCNTs were formed at 600°C and they were cooled down using Ar with a flow rate of 50 sccm until it reached the room temperature. The above process was repeated for the heating temperatures of 700, 800, and 900°C, respectively.
The black carbon films grown on a stainless steel foil substrate were scraped off by a plastic sheet and divided into two equal parts. The first part was used to determine the crystallinity, chemical bonding and morphology by X-ray diffractometer (XRD), Fourier transform Raman (FT-Raman) spectrometer, and scanning electron microscope (SEM), respectively. The second part was used to verify the hollow tube using a transmission electron microscope (TEM).
Scanning electron microscopy (SEM, Quanta 450 FEI) was used to investigate the morphology of the MWCNTs. The operating voltage and current were 30 kV and 10
X-ray diffraction (XRD, Bruker D8 Advance) was used to determine the crystallinity of MWCNTs. The MWCNTs were scanned over the diffraction angle
The MWCNTs were sonicated with ethanol for 15 min and dropped on a copper grid. Then, they were taken into the holder of electron microscope. Transmission electron microscopy (TEM, Hitachi HT 7700) was conducted using an accelerating voltage of 120 kV for low resolution and 200 kV for high resolution with a current of 60
X-ray photoelectron spectroscopy (XPS, ULVAC-PHI) with Al K radiation as the excitation source at the Synchrotron Light Research Institute (SLRI), Thailand, was used for the characterization of chemical composition and chemical states of MWCNTs. The binding energy was calibrated using the C1s peak (284.6 eV) as a reference. Gaussian-Lorentzian function, after performing Shirley background subtraction, was used for XPS peak deconvolution.
Fourier transform Raman spectroscopy (Perkin Elmer Spectrum GX) was also used to confirm the growth of MWCNTs and identify the crystallinity. The laser with a power of 80 mW and a working wavelength of 1024 nm was used as a light source. The laser beam was focused onto the sample surface with a spot area of 1 mm2.
Figure
SEM images of carbon films grown on heated substrates at different temperatures of 600–900°C.
Figures
Size distribution of: MWCNTs (a)–(c) and amorphous carbon films (d).
The crystallinity and purity of MWCNTs as investigated using a powder X-ray diffractometer (XRD) are shown in Figure
X-ray diffraction patterns of MWCNTs and amorphous carbon films (blue line) grown on heated substrate temperatures of 600, 700, 800, and 900°C.
Figure
TEM images of synthesized MWCNTs and Fe3O4 catalytic particle grown on heated substrate temperatures of (a) 600°C, (b) 700°C, and (c) 800°C, respectively.
Figure
XPS survey scan of MWCNTs and amorphous carbon films.
The binding energies and atomic concentrations of C1s and O1s for MWCNTs and amorphous carbon films are listed in Table
Binding energies and atomic concentrations of C1s and O1s for MWCNTs and amorphous carbon films.
Heated temperature (°C) | Binding energy (eV) | Atomic concentration (at. %) | ||
---|---|---|---|---|
C1s | O1s | C1s | O1s | |
600 | 284 | 532 | 92.49 | 7.51 |
700 | 284 | 533 | 96.33 | 3.67 |
800 | 284 | 532 | 98.27 | 1.73 |
900 | 284 | 533 | 98.37 | 1.63 |
It is seen from Table
The C1s spectra in Figure
XPS deconvolution of MWCNTs and amorphous carbon films.
Atomic concentrations of C1s and O1s for MWCNTs and amorphous carbon films.
The binding energies and atomic concentrations of chemical bonds are given in Table
Atomic concentrations of sp2 C=C and sp3 C-C for MWCNTs and amorphous carbon films.
Heated temperature (°C) | sp2 C=C | sp3 C-C | sp3/sp2 ratio |
---|---|---|---|
Atomic concentration (at. %) | Atomic concentration (at. %) | ||
600 | 77.09 | 12.23 | 0.159 |
700 | 74.07 | 12.52 | 0.169 |
800 | 71.24 | 12.58 | 0.177 |
900 | 72.55 | 13.03 | 0.179 |
Figure
Raman parameters of MWCNTs and amorphous carbon films grown on heated stainless steel substrate temperatures of 600–900°C.
Heated temperature (°C) | Raman shift (cm−1) | Intensity (a.u.) |
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|
|
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600 | 1340 | 1582 | 0.2659 | 0.2295 | 1.16 |
700 | 1338 | 1582 | 0.1759 | 0.1364 | 1.29 |
800 | 1335 | 1582 | 0.1230 | 0.0933 | 1.32 |
900 | 1341 | 1578 | 0.1365 | 0.0990 | 1.38 |
Raman spectra of MWCNTs and amorphous carbon films (blue line) grown on heated stainless steel substrate temperatures of 600, 700, 800, and 900°C.
Therefore, the degree of crystallinity of MWCNTs grown at higher heated substrate temperature is very low because of the occurrence of amorphous carbons. Interestingly, these results are strongly consistent with the results obtained from XRD and XPS.
To understand the effects of heated substrate on the formation of amorphous carbon films, the pure substrates heated at 900°C were further investigated. For comparison, the substrate heated at 700°C was also carried out. Both samples were mounted with epoxy resin followed by the polishing procedure of SiC grinding papers. The cross-sectional SEM images and energy dispersive spectroscopy (EDS) line scan profiles in the range of 3.75
Cross-sectional SEM images of the substrates heated at (a) 700°C and (b) 900°C.
EDS line scan profiles of the substrates heated at (a) 700°C and (b) 900°C.
Growth mechanism of MWCNTs and amorphous carbon films grown on the substrates heated at (a) 700°C and (b) 900°C.
In summary, the MWCNTs and amorphous carbon films were synthesized on austenitic stainless steel substrates using a home-built thermal CVD system. The surface of the flexible stainless steel was activated by hydrogen at different temperatures of 600, 700 800, and 900°C. The effects of the heated substrate temperatures on the crystalline structure and the size of MWCNTs were investigated. The results show that the diameter of MWCNTs decreased with increasing the temperature of heated substrate. The better uniformity of MWCNTs was observed on the substrate heated at 700°C. Furthermore, the amorphous carbon instead of MWCNT was observed on the substrate heated at 900°C. The degree of crystallinity of MWCNTs grown at higher heated substrate temperature is very low because of the occurrence of amorphous carbons. The observation of Fe3O4 nanoparticles in XRD patterns and TEM images strongly confirms that among Fe, Ni, Mn, and Cr elements contained in stainless steel, only Fe element acted as catalysts. The XPS results showed the chemical states of the purely carbon films as C1s and O1s. The deconvoluted XPS spectra of C1s revealed the covalent bonds of carbon-carbon bonds (sp2 and sp3) and carbon-oxygen bonds (C=O and O-C=O). As a result, the C1s core level spectra of the carbon films were a function of heated substrate temperatures. The results obtained from this work showed that the simple method can be used effectively for the growth of MWCNT on flexible stainless steel at a suitable temperature. The obtained results are possible for widely used in some applications such as flexible electrodes and flexible batteries due to their flexible characters.
The authors declare that they have no conflicts of interests regarding the publication of this paper.
This work was supported by Department of Physics, Faculty of Science, Kasetsart University, Thailand. The authors are grateful for the grant supported by the SUT-NANOTEC-SLRI Joint Research Facility, Synchrotron Light Research Institute, Thailand, for the XPS analysis.