Have you oxidized the CNT's to CO2?

Dear Craig Mcbeth  

Temperature, concentration of used acid, treatment time and type of carbon nanotube (length and diameter) are important factors to chemical purification and functionalization of CNTs and you should choose the best optimized condition in your work. .

Generally, HCl treatment is used to CNTs purification and mixture of HNO3 / H2SO4 treatment is used to simultaneously purification and specially COOH functionalization of CNTs.

Nanotubes can be damaged by being sonicated or refluxed in a long period of time, so you must be careful while doing so.

I believe  your CNTs are damaged in first step,when CNTs were refluxed in the presence of 3M HCl for 2 days. I think you can remove this step and repeat your experience in the milder conditions.

Good Luck

Please see :

https://iopscience.iop.org/article/10.1088/0957-4484/20/22/225608/meta

https://link.springer.com/content/pdf/10.1007/s10853-007-1941-y.pdf

You can use only HCl because nitric-sulfuric mixture oxidizes CNTs to CO2. If you want to obtain functionalized CNTs, only HNO3 treatment can be also appropriate. Additionally, the degree of oxidation depends on CNT defectiveness and diameter, so, you need to do some rutine to reveal the optimal conditions.

The recommendations of colleagues on the need for no oxidative treatment to avoid removing part of the defective tube are absolutely correct. In addition, I draw your attention to the need to use the same batch of nanotubes when optimizing and checking the detected modes for another source. There may be sharp differences.  Most importantly, do you need thorough washing and to what extent to solve your subsequent task?

I apologize for the inaccuracy of automatic translation.