2.2. Functionalization of MWNTs
In this study, r-MWNTs were treated with acid to be purified and have carboxyl and hydroxyl functional groups on their surface [48] and [50]. For the functionalization of the nanotubes, r-MWNTs were added to the concentrated acid mixture consisting of 3:1 volume ratio of H2SO4 (98 vol.%):HNO3 (70 vol.%) solution. A 1000 mL round-bottomed flask was charged with 5 g of r-MWNTs, 100 mL of nitric acid, and 300 mL of concentrated sulfuric acid. Next, the mixture was sonicated at 50 °C for 3 h with an ultrasonic bath. A probe type ultrasound apparatus KS-750F (Kesheng Company, China) with a fixed frequency of 40 kHz was utilized. The input ultrasonic power to the solution was 200 W and operated at 45% amplitude. It must be noted that through the ultrasonication, no mechanical stirring was performed since a perfectly efficient mixing was provided from cavitation phenomena [51]. After acid treatment, the solution was diluted with deionized water and cooled by ice to reduce the reaction rate. The mixture was then washed with deionized water several times until the acidity of the mixture decreased significantly. Later, the solution was filtered through a PTFE membrane filter with 0.2 μm pore size by vacuum filtration and washed with deionized water until pH value of the decantate was the same as deionized water and finally the f-MWNTs sample was obtained. Subsequently, the f-MWNTs were dried under vacuum at 60 °C for 24 h and were milled in a mortar and pestle. The f-MWNTs were characterized to confirm the presence of carboxyl and hydroxyl functional groups using Fourier transform infrared (FTIR) spectroscopy.