Purification and functionalization of carbon nanotubes

INTRODUCTION

Carbon nanotubes synthesized by the above-mentioned methods may contains a lot of impurities such as metal catalyst particles and amorphous carbon and these impurities increase as we try to degrees the diameter of carbon nanotubes. The utilization of carbon nanotubes is not possible according to their desired properties due to the presence of these impurities. In unpurified CNTs the presence of impurities interferes with the desired properties of CNTs. The excellent mechanical and electrical properties of CNTs are severely reduced by these impurities. To tackle this problem different method has been developed for the purification of CNTs namely gas phase oxidation, electrochemical oxidation, liquid phase oxidation (acid treatment), centrifugation, filtration, ultrasonication, magnetic and microwave purification. Schematic representation off acid treatment is presented in Fig.

Figure Different routes for purification and oxidation of MWCNTs, which results in generation of oxygenated groups on the end tips and side walls

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Utilization of CNTs in many fields renders due to the inertness of CNTs. To improve the dispersion of carbon nanotubes it is necessary to modify the CNTs with different functional groups and create defects on side walls and tips. The creation of defects and functionalization different functional groups not only increase the dispersion but also enhance the chances of doping of nanoparticles on CNTs. Defects and functional groups provide sites for connection between the matrix (CNTs) and reinforcement (nanoparticles, polymers etc.). Oxidation of CNTs is one of the most widely used techniques to functionalize and purify the CNTs. In this approach CNTs are dispersed in the H₂SO_{4 ­}and HNO₃ in the ratio 3:1 v/v and sonicate or reflux at 40-70 ^oC.

Purification/Functionalization procedure

Chemical required

·         Pure Ethanol

·         Distilled water

·         De-ionized water

·         Sulfuric acid H₂SO₄ (98%)

·         Nitric acid HNO₃ (70%)

Method

·         Acid treatment

Procedure

The as obtained raw MWCNTs weighted 250mg were taken in 100 mL round neck bottom flask. With the help of pipette (10 mL) concentrated H₂SO₄ and HNO₃ were added into the flask in the % ratio 3:1 v/v. The flask contained reactants covered properly with aluminum foil. Then the round bottom flask was placed in ultrasonic bath and sonicated for 3h at 40℃. The reaction mixture allowed to cool down to room temperature naturally.

Filtration of Precipitates

Filtration of precipitates was carried out by using centrifuge (Eppendorf 5424R).

Procedure

            The obtained cooled mixture was diluted 5 times with cold distilled water. By using the pipette (10 mL) suck the mixture carefully with sucker and add into the falcon tubes. Fill the falcon tubes in such a way that all falcon tubes should be equally weighted (equal volumes may differ in weightage). Unequally weighted falcon tubes placed in the Centrifuge may cause damage of the instrument. Equally weighted falcon tubes were placed in centrifuge then run the centrifuge at 5000 rpm for 15 min. The black precipitates settled down, the acidic filtrate evacuated by pipette and sucker. The precipitate washed first with distilled water several times to neutralize the pH and then with pure ethanol to remove the impurities. Moistures black precipitates were taken in into china dish with iron spatula and placed in an electrical oven and dried at 60 ℃ for 4 h. Finally, the blackish purified/oxidized powdered multi walled carbon nanotubes were obtained which used for further preparations.

Figure 3.1 Flow sheet diagram of MWCNTs acid treatment process