The most widely used method treats CNTs with concentrated nitric acid, sulfuric acid, or mixtures of both at elevated temperature. This oxidizes the tube surfaces and ends, introducing carboxylic acid (-COOH) and hydroxyl (-OH) groups primarily at defect sites and tube ends (which are more reactive than intact sidewalls). Acid functionalization also removes residual catalyst metals from CVD synthesis, reducing metal content to <1 wt%. The drawback is that it introduces additional lattice defects and shortens the tubes, which reduces aspect ratio and can lower mechanical properties in composites.
