Part II. Effect of Surface Properties on Sensitivity of Gas Sensor Using Carbon Nanotube and Graphene
Chapter 4. Carbon Nanotube-based Gas Sensor with Functionalization
4.5 Functionalization with Plasma Treatment
4.5.3 Effect of Circulation on Uniformity of Functionalization
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repetitive circulation is important because it is the key point to uniform functionalization. The MWCNTs attached to the electrode are not the same distance from the electrode. That is the degree of functionalization becomes different during plasma functionalization. However, if the nanotubes are removed from the electrode, randomly mixed again, and then functionalized and repeated many times, all the nanotubes can be uniformly functionalized regardless of the number of MWCNTs. When there is no the repetitive circulation, the degree of MWCNTs functionalization is not homogeneous and the functionalization of MWCNTs would be depend on the depth of the electrode [205, 206].
Figure 4.8 (b) shows the ID/IG ratio of Raman spectroscopy and the oxygen ratio after plasma treatment with the number of circulations. The functionalization was carried out by using RF plasma at 50W for 40 minutes at one treatment. When the MWCNTs was first functionalized, the ratio of oxygen is 11.7%. The number of circulations increases, the proportion of oxygen also increases. The circulation at 10 times shows an oxygen atomic ratio of 18.1%. This proportion of oxygen atoms is proportional to the degree of functionalization. When the plasma power is 100W, too much defect will
0 1 5 10
60 70 80 90 100
Circulation number
Carbonwt.% Oxygen wt.%
0 10 20 30
Circulation number
Nitrogen wt.%
0 2 4 6 8 10
0 1 2 3 4
Acid O2plasma (50 W)
O2plasma (25 W)
(d) (e) (c)
(a) (b)
Figure 4.11 (a) EDS elemental analysis of carbon and oxygen in O2/air plasma-treated MWCNTs.
The weight percent of C (black) and O (blue) indicate degree of functionalization of MWCNTs. (b) EA analysis of plasma-treated MWCNT after nitrogen functionalization using ammonia gas. Pristine MWCNTs powders were pre-treated with hydrochloric acid (blue), or O2/air plasma at RF power of 25 W (red) and 50 W (black). SEM images (x 150,000) and inset (x 50,000) of pristine (a) and O2/air plasma-treated MWCNTs with circulation number of 5 (d), and 10 (c). The power for treatment of functionalization is 50 W for 40 min. (Inset scale: 500 nm)
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10 100 1000 10000
0 15
Bare silica Plasma 50 W, 20 min, 1 cycle
Size (nm)
Relative frequency (%)
Plasma 50 W, 20 min, 10 cycle Plasma 100 W, 40 min, 1 cycle Average diameter: 524.9 nm
Average diameter: 485.7 nm
Average diameter: 493.0 nm
Average diameter: 813.6 nm
Silane treatment Silane and plasma treatment Average diameter: 618.5 nm
Average diameter: 869.8 nm
0 15
Treatment
RF pow er (W)/
Time (min)
Circulation Avg.
diameter (nm)
Polydispersity Zeta potential
(mV)
Diffusivity (cm2/sec)
Silane, plasma 50 / 20 1 618.5 0.232 -26.81 8.175 x 10-9
Silane - - 869.8 0.226 -12.4 5.813 x 10-9
100 / 40 1 524.9 0.281 -26.23 9.633 x 10-9
10 485.7 0.214 -12.99 1.041 x 10-8
1 493 0.25 -33.01 1.025 x 10-8
- - - 813.6 0.249 -8.97 6.215 x 10-9
plasma
50 / 20
1 μm
(a)
(b)
(c)
0 15
0 15
0 15
0 15
Figure 4.12 Application of our approach to functionalizing micro/nanoparticles of silica. (a) High resolution SEM image of untreated silica. (b) Analysis of diameter, polydispersity, zeta potential and diffusivity from silane- and plasma-treated silica. (c) The diameter distribution of the silica particles. It shows that the diameter decreases by the O2/air plasma. Functionalization of powdery nanomaterials that can be captured and circulated within our plasma reactor is not significantly affected by their size. Any non-uniformity during functionalization, which might be caused in large particles only partially placed in the active plasma zone, can be reduced by the sample circulation.
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be generated on the surface of the nanotube. When the plasma power is 50W, however, functionalization of MWCNTs will increase gradually. If the number of circulations is less than 10, serious defect would not occur. The D/G ratio in Raman spectroscopy shows that the D/G ratio increases with increasing circulation. G-mode comes from sp2 carbon-carbon vibration (at 1580 cm-1) and D-mode (at 1375 cm-1) comes from sp3 carbon bond. It meant that the higher the D / G ratio, the less sp2 binding and the greater the sp3 binding. The results show that the more functionalization becomes as the D-mode increases. The D-mode on the wall of the nanotube is increased, and this D- mode means degree of functionalization. Before functionalization, the pristine MWCNTs shows D / G ratio is 1.43. After the plasma is functionalized by circulation 10 times, the D / G ratio increases to 1.80. Previous studies have reported that the degree of functionalization increases with flow rate, plasma power, and exposure time. In addition, it has been reported that D-mode and G-mode become wider when surface functionalization of fluorine or oxygen occurs.
The extent of the functionalization can be confirmed using FTIR. Figure 4.8 (c) compares the FTIR of pristine nanotubes and the plasma treated nanotubes with number of circulations. Pristine nanotubes show a broad peak around 3450 cm-1, which is related with O-H or -COOH vibration. The peaks are the result of the fact that the moisture in the oxygen is strongly adsorbed to nanotubes or the purification during oxidation process [112]. The 2923 cm-1 peak was caused by C-H stretching, and 1384 cm-1 was caused by the bending mode of C-H or by bending mode of O-H groups in carboxylic acid [207]. Another peak, 1635 cm-1, means the carbon-carbon double bond [207, 208], and this double bond is one of the basic bonds of the nanotube. The 1729 cm-1 peak is due to a carbon-oxygen double bond, which means a double bond of carbon and oxygen in the carboxyl group. Finally, the 1579 cm-1 peak appears from a carbon-carbon single bond or from COO-stretching [202]. Comparing the peaks of the pristine MWCNTs with the plasma-treated MWCNTs, the peaks at 1384 cm-1, 1579 cm-1, 1729 cm-1 and 3450 cm-1 increased respectively. The peak at 1635 cm-1 was the only peak, which means a carbon-carbon double bond. As the functionalization proceeded, the surface of the nanotube was damaged, and the carbon-carbon double bond was reduced. The carbon-carbon double bonds were converted into functionalized hydroxyl, carboxyl and oxygen-contained species.
Figure 4.9 shows sheet resistance and absorbance data for pristine MWCNTs and plasma- treated MWCNTs with the number of circulation. The pristine and plasma-treated MWCNTs were sonicated for 10 minutes in water and compared between the first and seventh day. As see the sample images, the pristine sample showed some CNTs bundle on the ground on the first day, and after 7 days the picture showed a lot of CNTs bundle on the bottom. It is expected that nanotubes with hydrophobic properties are not well dispersed in water. However, plasma-treated CNTs showed good dispersibility in water after 7 days without any other surfactant. As the number of circulation increases, the intensity of absorbance at 550 nm increased and absorbance increased even after 1 week. In the
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case of plasma-treated CNTs, it was well dispersed in water and stable in water. As see the sheet resistance results, the sheet resistance results also increased with increasing the circulation number.
The reason for this is that the more circulation, the more functionalization. The polar groups that are formed by the functionalization, carboxyl and hydroxyl group, have the interaction of hydrogen bonding with water molecules. This hydrogen bonding increases the resistance. The resistance increased from 942 Ω/sq (Circulation 1) to 7432 Ω/sq (circulation 10) because the plasma treatment by the circulation damages the CNTs themselves.
EDS analysis of C and O was performed to confirm the uniformity of functionalization using O2/air plasma with increasing circulation number (Figure 4.10). Fe, Al, O and C elements were detected by EDS analysis and measured according to the number of plasma treatment. The EDS analysis was performed by 5 samples according to the number of circulation. The Fe and Al are catalysts used in the synthesis of nanotubes and are ignored because they have a small portion. The ratio of oxygen obtained from five pristine nanotube samples is 4.8 ± 1.5 wt% (Figure 4.11 (a)). The ratio of oxygen in the five samples of plasma treated CNT (circulation number: 1) increases to 11.5 ± 5.6 wt%. The ratio of oxygen obtained from five plasma-treated CNTs samples (circulation number: 5) was 15.1 ± 5.9 wt%. The oxygen ratio of five functionalized samples (circulation number: 10) is 18.2
± 2.3 wt%. It should be noted that as the circulation increases, the standard deviation decreases. The results show that as the circulation increases, the degree of functionalization increases, and the uniformity of functionalization also increases. Usually, nanotubes are nanomaterials with high aspect ratios and are difficult to functionalization due to their inert surfaces. However, using plasma treatment with such circulation, uniform functionalization of nanotubes is possible. The uniformity of functionalization is possible using by an increase in circulation.
Plasma treatment was used to study not only oxygen functionalization but also nitrogen functionalization. The nitrogen functionalization on the surface of the nanotube has many advantages [209]. The advantage is that it is well dispersed in organic solvents or water, and the anchoring ability is increased. The nitrogen functionalization can be accomplished by filling the chamber with nitrogen and plasma treatment. Previously, plasma treatments with NH3, N2 / H2, and Ar/ N2 gas were reported.
The research used the method of pre-treatment heat treatment or acid. These treatments would help surface oxidize or fluorinate with mixed nitrogen gas. However, there is no report yet on pre- treatment and nitrogen-functionalization using plasma. In this research, oxygen plasma is used for pre-treatment of nitrogen functionalization because it is environmentally friend and it does not use chemical. And nitrogen functionalization is still environmentally friendly as it proceeds under solvent free conditions. Figure 4.11 (b) is the EA result of a nanotube with nitrogen functionalization using ammonia. I confirmed the effect of several pre-treatments before nitrogen functionalization.
hydrochloric acid (blue), O2/air plasma at RF power 25 W (red), O2/air plasma at RF power 50 W total.
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As expected, the degree of functionalization increased with increasing circulation. The degree of nitrogen functionalization was highest in pre-treatment with 50 W RF plasma. The nitrogen- functionalized MWCNTs with pre-treatment using O2/air at 50 W RF power contained up to 3.28 wt%
nitrogen element (50W, circulation number: 10). Without pre-treatment, nitrogen functionalization with plasma treatment is difficult. To our knowledge, no studies have been reported on nitrogen- functionalized MWCNTs using plasma. These results suggest that oxygen and nitrogen functionalization of MWCNTs as well as other micro / nanomaterials can be applied through plasma circulation.
SEM images of plasma-treated nanotubes using O2/air are in Figure 4.11 (c). The average diameter of the MWCNT is 20 nm and the length are approximately 22 μm. The SEM images in
Time (s) ΔR/R0(%)
0 600 1200 1800
0 0.15
0 0.15
0.3
0.3 Plasma-treated
1.9 ppm 3.4 ppm
5.1 ppm 6.9 ppm
8.6 ppm Pristine
1.9 ppm 3.4 ppm 5.1 ppm
6.9 ppm 8.6 ppm
(b)
Plasma-treated
Pristine 0.2
0.15
0.1
0.05
0
0 2 4 6 8 10
Concentration (ppm)
(c)
ΔR/R0(%) Pristine
50 μm
Plasma-treated
50 μm
(a)
500 nm 500 nm
Figure 4.13 Gas sensor made of pristine and plasma-treated MWCNTs for the detection of DMMP.
(a) optical images of pristine and plasma-treated MWCNTs. There is an Au electrode on both sides and a middle 100 μm x 60 μm MWCNT film. The SEM images is zoomed- in MWCNTs film. (b) Response curve to various concentration of DMMP vapor from pristine (top) and plasma-treated (bottom) sensors. (c) Response (ΔR/R0) from pristine (black) and plasma-treated (red) CNTs.
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Figure 4.11 (d) were plasma-treated nanotubes (circulation number: 5). As see the images, overall diameter of nanotube is smaller than pristine. The reduction in diameter after plasma treatment was the same in silica as well as nanotubes in Figure 4.12. The plasma treatment using water vapor showed the same change [210]. The advantage of this phenomenon is that the possibility of becoming a bundle is reduced with decreasing diameter. This is useful when using nanotubes with various solvents. Figure 4.12 (e) is SEM images of a plasma-treated nanotube with 10 circulations. Here, nanotubes are very curled and bundled as if they were susceptible to damage by plasma. This prediction is consistent with the results of high D/G ratios using Raman spectroscopy.