Y. Hyun, J. Park, J. Goak and N. Lee
Keywords: carbon nanotubes, gas phase purification, metallic impurities, battery applications
Summary:Carbon nanotubes (CNTs) have been produced in a large quantity by catalytic chemical vapor deposition using metallic catalysts. Thus the metallic impurities inevitably remains in the raw CNTs. Extremely pure CNTs, which are free of metallic impurities, are sometimes required for high-end applications such as lithium-ion batteries, high-voltage cables, biological sensors, etc., otherwise, causing failure of their device performances. There have been many studies on elimination of metallic impurities: mainly, acid purification, high-temperature vacuum purification and chlorine purification. These methods have their own pros and cons. The acid purification are simple but may damage CNTs while removing metallic impurities and furthermore gives rise to environmental pollution. The vacuum purification is able to eliminate most of metallic impurities without damage to CNTs but requires a sophisticated equipment and high cost. The chlorine purification is a gas-phase method to reduce effectively metallic impurities but needs a high-cost scrubber system of toxic chlorine gas. We have developed an environmentally benign and cost-effective method to eliminate metallic impurities of CNTs using nitrogen-bubbled chloroform (CHCl3) . CNTs were produced using Fe and Co embedded on Al2O3 and MgO. The metallic impurities of Fe, Co, Al and Mg were measured totally 5,133 ppm by ICP-AES. After the raw CNTs were loaded inside a glass tube furnace, liquid-phase chloroform was bubbled by nitrogen gas and carried into the CNTs heated at high temperature. Chloroform was decomposed to produce chlorine radicals, atomic hydrogen and hydrochloric acid, which reacted with metallic impurities to form volatile metal chlorides. The exhaust gases were passed through a cold trap and sodium hydroxide trap to prevent any emission of harmful substances such as metal chloride and unreacted chloroform to environment. Chloroform had a higher purification efficiency of metallic impurities in CNTs than other chlorine-containing solvents, dichloromethane and tetrachloromethane. As the temperature increased, the concentration of metallic impurities monotonically decreased and then was saturated to be 12 ppm at 1,050 ℃. The metallic impurities did not further decreased after 15 min at 1,050 ℃. For comparison, we also purified the CNTs using acid-reflux purification and high-temperature vacuum purification, which showed the metallic impurities as low as 50 and 40 ppm, respectively. Our purification method demonstrated a higher purification efficiency than other methods, by reducing the metallic content from 5,133 to 12 ppm at the optimum condition using chloroform at 1050 ℃ for 15 min. During the chloroform treatment, the CNT surface was modified with chlorine functional groups, but they were easily removed by nitrogen-bubbled water vapors at 1050 ℃ for 15 min, characterized by XPS. Electrical properties and dispersion of CNTs subject to the chloroform and water vapor treatments were characterized using the CNT solutions in N-Methyl-2-pyrrolidone (NMP). The purified CNTs showed a poor dispersion in NMP than raw CNTs but the dispersion was easily restored by the water vapor treatment to remove the chlorine functional groups.