Enhancement of Congo red adsorption using oxidated nitrogen-doped carbon nanotubes

This work reports the synthesis of nitrogen-doped multilayer carbon nanotubes (N-MWCNTs). The N-MWCNTs were subjected to mild oxidation in air at different temperatures and exposure times to obtain a variety of composites from N-MWCNTs with iron oxide nanoparticles up to mainly nanostructured hematite in its alpha phase (α-Fe₂O₃). X-ray diffraction allowed us to track the transformation path of the N-MWCNTs have a cross oxidation time. Each composite was fully characterized. FTIR analysis revealed that the oxidated N-MWCNTs exhibited a high content of carboxylic and carbonyl functional groups. Raman spectroscopy characterization shows a decrease in the D-band intensity and an increase in the 2D-band intensity as the oxidation temperature and oxidation time increase. For instance, the 5 min oxidized N-MWCNTs at 620 °C show a 3 % decrease in both the I_D/I_G and I_2D/I_G ratios. At 10 min at 620 °C, a 10 % decrease in the I_D/I_G ratio and a 15 % increase in the I_2D/I_G ratio were observed. The former results indicate the opening of the CNTs tips and the exposure of the graphitic layers as oxidation time increases. The TEM micrographs can easily corroborate the former trends. The performance of the pristine and oxidized N-MWCNTs was evaluated for the adsorption of Congo red (CR) as a model pollutant. The results indicate that mild oxidation temperatures (620 °C) and short oxidation times favor CR adsorption with a faster adsorption equilibrium than pristine N-MWCNTs. The percentage removal of pristine N-MWCNTS was 33 %. The oxidized samples at 620 °C had a higher adsorption removal of CR up to 36 % and 43 % when oxidation time was 5 and 10 min. The results demonstrate that oxidation thermal treatment reduces amorphous carbon and promotes the opening of the CNTs, increasing the active sites on the surface of the material, which facilitates interaction with CR.