Water‐Dispersible Multiwalled Carbon Nanotubes Obtained from Citric‐Acid‐Assisted Oxygen Plasma Functionalization

Abstract

A new and safe method has been developed to functionalize multiwalled carbon nanotubes (MWCNTs) with fewer surface defects, which significantly increases their dispersibility in water. MWCNTs are pretreated in pure ethanol by a supersonic homogenizer. Then, the mixture is dried and soaked in weak citric acid solution. Finally, the MWCNTs in the citric acid solution are treated with radio frequency (13.56 MHz) oxygen plasma. As a result, many carboxyl functional groups are attached onto the MWCNT surfaces and stable dispersion of the MWCNTs in water is obtained. The treatment conditions are optimized in this study.

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“…An absorption band at 2907 cm −1 corresponds to C–H stretching, indicating an increase in the number of sp 3 carbon bonds. These bonds are assumed to originate from the breakage of C=C bonds during functionalization [45]. The remaining peaks are the same as described for pure MWCNTs.…”

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“…The enhancement of dispersion stability is confirmed from visual observation of the mixer of CNTs in pure water and measuring the settling speed for a dispersion using the absorbance data of the UV-visible spectroscopy as shown in Figures 6(a) and (b), respectively [48]. Attachment of the functional groups onto the CNTs is studied by a Fourier transform infrared (FT-IR) spectrometer (Shimadzu Co., 8700, 100 scans averaged).…”

“…Here, an environmentally friendly approach to functionalizing CNTs has been described, which is developed to attach −COOH groups onto their surfaces, and carried out under a wet condition using citric acid solution in RF (13.56 MHz) oxygen plasma [48,49]. CNTs are first pretreated supersonically in ethanol.…”

“…The dispersibility of the CNTs is also observed by a TEM (JEOL JEM-1400 Plus, acceleration voltage of 120 kV), and their structural quality is measured by a Raman spectrometer (JASCO Co., NR-1800, λ = 532 nm). For thermal analyses a thermogravimetric analyzer (Rigaku Thermo plus TG8120) is used, which is operated under air ambient at a heating rate of 10 K/min from room temperature to 973 K [48]. To obtain the cotton composite, f-CNTs are dispersed into water to produce stable [49].…”

“…1713 cm −1 , which corresponds to the C=O stretching bonds, and the broad band at ca. 3200 cm −1 corresponding to the O−H/−OH bonds [48]. Therefore, carboxyl groups are considered to be attached on the CNTs after functionalization.…”