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Hydrogen peroxide (H2O2) is an alternative oxidant and green method for the purification and functionalization of carbon nanotubes (CNTs), which works more efficiently in combination with UV radiation. In this paper, we investigated and monitored the oxidation mechanism of ethanol-synthesized and standard multi-walled carbon nanotubes. Characterization was carried out by high-resolution transmission electron microscopy (HRTEM), X-ray photo electron spectroscopy (XPS), acid/base titrations, thermo-gravimetric analysis (TGA) and Raman spectroscopy. Interestingly, we found a close correlation between oxidation time and the nature of the nanotubes. For the first time, a unique cyclic peeling of layers within MWNTs, termed "onion effect" was found. In this process, the degree of crystallinity of the outer layers significantly changed for the different peeling stages. We explain the process and how the number of layers within nanotubes gradually decreases when reaching critical acid concentrations. This method now allows the synthesis of MWCNTs with specific numbers of walls, diameter and controlled degree of crystallinity that could be effectively used for applications in biology and composites. (C) 2018 Published by Elsevier Ltd.