The H2SO4/HNO3 mixture at your mentioned ratio has widely been used for oxidising CNTs. From my practical experience I can tell you that should you use only HNO3 (5-8M Soln) without sulfuric acid would definitely serve your purpose. But the strength and volume of acid you use depend on how much CNT you want to oxidise and which process you want to follow. 

You must to use functionalization  agent based desired size and surface properties of the CNTs .After functionalization  of carbon nanotubes with 3:1 acid mixtures(H2SO4:HNO3,usually 98% sulfuric, 70% for nitric),may be  found oxygen functionalities such as (-COOH, -OH =C-O,etc.The importance of functionalization  may be due to increase the oxidation properties of CNTs. Also, with functionalized those groups , dispersion and conductivity properties are changed. The extent of oxidation is relates with diameter, size ,distribution and defects  level .FTIR and Raman spectroscopy are suitable tools to evaluate oxidation and defects levels.

To avoid expectable destroying, it is better to oxidation of CNTs  on refluxing at a  lower time and use dilute solutions of this mixture and stirring overnight or  

heating mixture  at low temperature under sonication.

Thanks guy

Any kind of characterization method other than FTIR, Raman spectroscopy and SEM to prove that my CNTs are purified?

TGA also work to check on purification before and after process.