Abstract
A comprehensive study of a less harmful functionalization of multiwalled carbon nanotubes (MWCNT) was performed. The influence of ultrasonic cavitation and solid/liquid interface was indirectly evaluated by statistical characterization of MWCNT aspect ratio and degree of functionalization. MWCNT aspect ratio was determined through hundreds of electron microscopy images. Degree of functionalization of MWCNT was characterized by elemental analysis and thermogravimetry. Distribution of different oxygen-containing functional groups was assessed by potentiometric titration. MWCNT aqueous dispersions were evaluated by electrophoretic mobility and dynamic light scattering. An acid volume over 90% lower than usually reported in literature guaranteed MWCNTs with significantly larger aspect ratio, higher proportion of carboxylic groups and a more negative zeta potential in aqueous suspension. These results can be related to a more localized ultrasonic cavitation in MWCNTs/acid interface for reduced liquid volumes. This distinguished influence of ultrasonic cavitation may be a valuable contribution to more eco-friendly processes with nanomaterials.
Highlights
A moderate temperature increase to 70 °C allowed an expressive functionalization in just 20 min of reaction via ultrasound, which represents a reduction of almost 80% in time spent on the process
The raise of temperature is a known strategy to increase functionalization of multiwalled carbon nanotubes (MWCNT),[8,29] but it can compromise their aspect ratio and cause yield product decrease.[8]. These results indicate that acid treatment with moderate temperature and reduced time of exposure to ultrasonic cavitation time is an effective strategy for obtaining expressive functionalization of MWCNTs with better preservation of its length
The literature[9] reports that the oxidation in liquid phase preferentially occurs on the existing defects of MWCNTs and on their edges, but erosion of their walls is possible. These results indicate that the functionalization of MWCNTs in acid medium via ultrasound did not promoted erosion of MWCNT walls in a sufficient level for the decrease in the diameter
Summary
A more efficient functionalization via ultrasound in comparison to reflux was observed in some studies.[17,21,22] The use of ultrasound is related to the phenomenon of acoustic or ultrasonic cavitation, which occurs by the interaction of microbubbles with the generated acoustic field. This cavitation involves mechanisms of stable and inertial types.[21,34,35,36,37] Stable cavitation is based on bubble oscillation, without necessarily promoting their collapse.[35,36,37] Inertial cavitation is related to the increase of microbubbles and their subsequent implosion, which
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