Carboxylation treatment of multiwalled carbon nanotubes monitored by infrared and ultraviolet spectroscopies and scanning probe microscopy

The effect of acid treatment on multiwalled carbon nanotubes (MWCNTs) was investigated through analysis of their morphologies followed by atomic force microscopy (AFM). The chemical changes were monitored by Fourier transform infrared (FTIR) and ultraviolet (UV/Vis) spectrophotometries. Treatments with nitric acid as well as with a mixture of nitric and sulfuric acids (1:3 by volume) were analyzed. Both acid treatments applied during a short ultrasonication time do not show any relevant effect. However, an additional peak can be observed at 1200 cm− 1 in the FTIR spectrum of the MWCNTs treated with the acid mixture after 2 h but their UV/Vis spectrum did not change. These results could indicate that new C–O groups appear in the open ends of the nanotubes without modifying the structure of their sidewalls. When the treatment with the acid mixture was prolonged, the sidewall of the nanotubes began to be destroyed up to their nearly complete destruction as observed by both UV/Vis and AFM. The results reported in this work show that three simple and quick techniques can help to control the carboxylation process often carried out before the functionalization of MWCNTs.