Highly versatile and efficient process for CNT oxidation in vapor phase by means of Mg(NO3)2-HNO3-H2O ternary mixture

Nitric acid vapor phase oxidation of carbon nanotubes using Mg(NO3)2-HNO3-H2O ternary mixture is proposed as a further development and significant improvement of the method employing the sole azeotropic HNO3 solution. Based on the so-called “salt effect”, the addition of Mg(NO3)2 to HNO3-H2O solution allows varying the acid vapor concentration within a wide range, even beyond the azeotropic one (> 68 wt.%). Ternary mixture containing 20 wt.% Mg(NO3)2 and variable amount of liquid HNO3 (30 to 58 wt.%) is used for the functionalisation process, achieving nitric acid vapor concentrations in the range 25–93 wt.%. The increase of acid concentration in vapor phase produces a significant augment of the functionalisation degree and a linear increase of the concentration of carboxylic acids groups, which can be thus be simply tuned.