Rapid and enhanced functionalization of MWCNTs in a dielectric barrier discharge plasma in presence of diluted CO2
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Multiwalled carbon nanotubes (MWCNTs) have been functionalized in dielectric barrier discharge (DBD) plasma in presence of a mixture of carbon dioxide and a diluent gas at room temperature and atmospheric pressure. He, Ar, and N2 were examined as the diluent gases. Temperature-programmed desorption was used to investigate the influence of various plasma parameters and type of diluent gas as well as the amount of diluent in the plasma gas. It is found that the quantity of functional groups on the surface of MWCNTs is a maximum when He is used as diluent gas. It also passes through a maximum when He content is 60%. The presence of He improves the reactivity of the plasma, which leads to an increase in the quantity of functional groups. However, high percentages of He decrease the CO2 content, which in turn decreases the number of functional groups. FTIR and Raman spectroscopy showed the presence of Oxygen-containing functional groups on MWCNTs surfaces.
KeywordsPlasma Treatment Dielectric Barrier Discharge Dielectric Barrier Discharge Plasma Excited Species Dielectric Barrier Discharge Reactor
Carbon nanotubes (CNTs) are promising materials for a variety of potential applications due to their excellent electrical, optical, thermal, mechanical, and chemical properties. Since the landmark paper by Iijma  in 1991, CNTs have been increasingly used in different applications such as sensing, catalysis, composites, adsorption processes, drug delivery, biology, and nanoelectronics [2, 3, 4, 5]. For instance, adding CNTs to polymeric matrices as filler (even less than or equal to 1 wt.%) improve dramatically various properties of polymers, or they can be used as an efficient tool for therapeutic molecule transportation and translocation [2, 3]. The high surface area of CNTs also makes them an ideal candidate as supports for catalytic metal nanoparticles . Likewise, CNTs can be employed as new adsorbent for removing many kinds of pollutants such as volatile organic compounds and heavy metal ions .
Functionalization of CNTs improves significantly their performance in most applications . When nanotubes are used as filler in polymer composites, their homogeneous dispersion in a polymer matrix is necessary. Pristine nanotubes are severely bundled and are insoluble in organic solvents or water, due to strong van der Waals interactions. Functionalization of CNTs increases interaction of nanotubes with matrix materials and also increases their solubility in water and organic solvents. Therefore, this surface modification of CNTs improves their dispersion in polymer matrices .
The current methods to functionalize nanotubes covalently, are acid treatment (wet chemistry) and high-temperature vapors exposure . These approaches may damage the structure of nanotubes depending on temperature and treatment time. In acid treatment, it appears that nanotubes are not only shortened, but also their graphitic structure is destroyed to some extent [11, 12]. It has been accepted [13, 14, 15] that a wet treatment can improve the dispersion of nanotubes in a polymer matrix; however, due to shortening of nanotubes and destroying their structure, it may severely affect the electrical properties of CNT-polymer nanocomposites and decrease the electrical percolation threshold.
An alternative to these methods is the use of plasma method which can have considerable advantages [16, 17]. The nanotube surface functionalization under plasma exposure can occur during a short period (even less than a minute) and at low temperature, even as low as room temperature; therefore, it has less damaging effect on nanotube structure. When the plasma-treated nanotubes are incorporated in polymer matrices, they lead to the formation of nanocomposites with higher dispersion state of the nanotubes and more highly improved electrical properties as compared to the acid-treated ones [13, 18].
Dielectric barrier discharge (DBD) plasma treatment is known as an effective method for the surface modification of materials. The atmospheric pressure DBD plasma also offers advantages over vacuum-based plasmas .
In our previous works [13, 19], multiwall carbon nanotubes (MWCNTs) were functionalized at atmospheric pressure by a He DBD plasma followed by exposure to NH3 leading to formation of nitrogen-containing functional groups and by an air DBD plasma resulting in formation of oxygen-containing functional groups. A CO2 plasma, which is normally used for surface modification of materials such as polymers , can be also used to functionalize CNTs; however, it has been rarely used for this purpose. In this study, we investigate functionalization of MWCNTs by a CO2 DBD plasma in presence of a diluent gas (helium (He), argon (Ar), or nitrogen (N2)) at atmospheric pressure. The main emphasis is on the influence of the type and concentration of the diluent gases on level of functionalization which has been rarely addressed in literature. We also optimized the voltage and time of plasma exposure. Temperature-programmed desorption (TPD) technique was used to compare the quantity of functional groups on the nanotube surface formed at different conditions. Fourier transform infrared (FTIR) technique was used to confirm the formation of oxygen-containing functional groups on the nanotube surface. Raman spectroscopy was also used to indicate the formation of defect sites in the structure of nanotubes due to their functionalization with the plasma treatment.
2.1 Functionalization of nanotubes
MWCNTs (pure: >95%) were provided by Shenzhen Nanotech Port Co. Ltd (China). Prior to exposing to plasma, MWCNTs were pretreated. They were exposed to a flow of He and then heated to 1000∘C at a rate of 5∘C/min and retained at 1000∘C for 30 minutes. The temperature was then reduced to room temperature under He flow. This pretreatment led to the removal of most of the defects and functional groups formed on the surface of the nanotubes during synthesis and purification processes. In this case, it can be said that after plasma treatment, the observed functional groups on the nanotube surface were mostly formed by plasma treatment. Pretreated MWCNT is called annealed MWCNT and denoted by A-MWCNT.
A HAMEG 20-MHz oscilloscope was used along with high-impedance and high-voltage probe for monitoring the generated current and phase shift (Fig. 1).
About 35 mg of A-MWCNTs was used in each experiment. The molar percentage of the gas diluent in the plasma gas was adjusted by two UFC-7300 MFCs. In order to compare the effect of various diluents on the extent of functionalization, He, Ar, and N2, as the diluent gases, were used. Various mole percentages of He, i.e., 20, 60, and 85%, were also investigated.
Microdischarges, passing through the MWCNTs sample in the DBD reactor, with lifetimes of a few nanoseconds , and the high-frequency AC electric current used in the plasma treatment experiments ensure homogeneous functionalization of the MWCNTs surfaces. In addition, during the plasma treatment, the sample was expanded and slightly fluidized.
2.2 Characterization of nanotubes
Based on the TPD results, the sample with the largest quantity of functional groups was selected for more detailed characterization, i.e., FTIR and Raman spectroscopy. This sample was functionalized under the following conditions: plasma voltage=9 kV, frequency=2.6 kHz, plasma duration=20 s, and He as the diluent gas with mole percent of 60. The sample prepared as such is denoted as F-MWCNT.
Raman spectra were obtained on an Almeg Thermo Nicolet Dispersive Raman Spectrometer by using the second harmonic at 532 nm of an Nd:YLF laser. FTIR was performed using a Bruker Vector22 spectrometer with a resolution of 5 cm−1. FTIR spectroscopy with diffuse reflectance (DRIFTS) accessory was used because of the high absorbance of nanotubes. Prior to DRIFTS-FTIR, the samples were purged with He at 100∘C for 30 min. In order to determine the gases evolved through thermal desorption of functional groups, TPD was also done on 70 mg of F-MWCNTs using an online FTIR instrument (Fig. 2).
3 Results and discussion
3.1 FTIR spectra of nanotubes
F-MWCNTs spectrum shows several peaks, in addition to the 1520 cm−1 one, that are missing in the A-MWCNTs spectrum. The weak peak at 1015 cm−1 may be assigned to functional groups containing C–O single bond, e.g., lactone. The peaks observed at 1650–1730 cm−1 are attributed to carbonyl group of the oxygen-containing groups such as lactone, quinone, and carboxylic anhydride [19, 23]. These indicate that exposing A-MWCNTs to CO2+He plasma led to the formation of oxygen-containing functional groups on their surface.
As is observed in Fig. 3, the intensity of the peak assigned to C=C stretching band of the nanotubes in the spectrum of F-MWCNTs (at 1520 cm−1) is larger than that in the spectrum of A-MWCNTs. It appears that the increase in the quantity of functional groups and defects on the nanotube surface due to the plasma treatment has intensified this peak .
3.2 Raman spectra of nanotubes
3.3 Effects of plasma parameters on the extent of functionalization
A plasma consists of highly energetic and excited species, the density of which depends upon the plasma parameters, i.e., voltage, treatment time, and atmosphere . The extent of functional groups formed on MWCNT surface by plasma treatment, depends upon the density of the excited species. The effects of these parameters are presented as follows.
3.4 Effect of the diluent gas on the extent of functionalization
The discharge power of the DBD plasma system for different He percentages. Voltage=9 kV, frequency=2.6 kHz
Phase shift (°)
The discharge power of the DBD plasma system for each He percentage was calculated, based on the voltage, current, and phase shift, and are reported in Table 1. The increase in He percentage from 20 to 60 and 85% has led to an increase in current respectively from 20 to 25 and 28 mA while the phase shift remained almost constant. An increase in current in the presence of higher He concentration indicates an increase in the density of the excited species in the plasma atmosphere [28, 29, 30].
As is seen, not only the exposure time for maximum degree of functionalization, but also the quantity of each maximum, depends on the He concentration. The exposure time for maximum degree of functionalization for 20, 60, and 85% He is 40, 20, and 10 s, respectively. By increasing the He percentage from 20 to 60 the maximum quantity of functional groups increases. This is unexpected from the reaction engineering point of view, since both the exposure time (similar to reaction time) and the CO2 (as a potential reactant) concentration simultaneously decrease. This may indicate that the presence of He cause the ionization and excitation of more CO2 molecules. This in turn results in an increase in the reactivity of the plasma because the ionized or excited CO2 molecules act as a reactant [28, 29, 30]. However, further increase in He percentage to 85% leads to a decrease in the maximum quantity of functional groups. It seems that this significant decrease in the quantity of CO2 (as a potential reactant) caused the decrease in the maximum quantity of functional groups.
Therefore the type of the diluent gas affects the extent of functionalization. Another advantage of the presence of a diluent gas is that the diluent gas prevents perfect decomposition of CO2 and formation of coke on the nanotube surface .
3.5 Thermal desorption behavior of F-MWCNTs
As is observed (Fig. 10), the CO2 evolution beginning at about 430∘C has a small shoulder, and a major peak at about 760∘C, correspondingly the CO evolution commences at about 530∘C and shows a shoulder at about 620–730∘C followed by a major peak at 850∘C. These profiles are almost similar to the profiles of CO2 and CO evolution during TPD of the air plasma-treated MWCNTs reported by Vesali et al. . However, for the CO2+He plasma-treated MWCNTs, the CO2 and CO evolution begins at higher temperatures. This indicates that the functional groups formed by the CO2+He plasma treatment have a higher thermal stability than those formed by the air plasma treatment.
CO is mainly formed due to the decomposition of anhydrides at temperatures lower than about 600∘C and carbonyls, quinines, and ethers at higher temperatures. The evolution of CO2 takes place as a result of decomposition of anhydrides at temperatures below 600∘C and lactones at higher temperatures [19, 31, 32]. Moreover, CO2 can be produced during secondary reactions. When the evolved gases slowly pass through narrow micropores, the CO molecules can react with surface oxygen atoms and convert to CO2 [19, 23]. However, Zhou et al.  have shown that the influence of the secondary reactions on the TPD profiles of carbon material that have a graphitic structure and very small micropore volumes is negligible. Thus, CO and CO2 are mostly evolved at temperatures higher that 600∘C (see Fig. 10), indicating formation of a minor amount of anhydrides.
Multiwall carbon nanotubes (MWCNTs) are functionalized in a dielectric barrier discharge (DBD) plasma reactor in the presence of CO2 and an inert gas including He, Ar, or N2. The effects of plasma treatment voltage and exposure time are also investigated. Oxygen containing functional groups are chemically bond on the surface of MWCNTs in the DBD plasma in the presence of diluted CO2 gases.
Dilution of CO2 in an inert gas, up to a certain percentage, increases the amount of functional groups formed on the functionalized MWCNTs surface, due probably to an enhanced charge and energy transfer mechanism to reactive species. The enhancement effect increases in the order of N2<Ar<He inert gases. The amount of functional groups is increased up to 3.6 times as He is used instead of N2.
Both the percentage of the inert gas and the exposure time, interrelated with each other, have optimum values on the enhancement of the functional groups, which may detach from the functionalized MWCNTs surface at excessive amount of the inert gas and/or exposure time. The amount of functional groups reached a maximum when He percentage was 60%.