On the Mechanism of Electrochemical Functionalization of Carbon Nanotubes with Different Structures with Aminophenylphosphonic Acid Isomers. An Experimental and Computational Approach

Electrochemical functionalization of Single-Walled Carbon Nanotubes and Herringbone Carbon Nanotubes (SWCNTs and hCNTs, respectively) has been successfully performed with aminophenylphosphonic acid isomers by potentiodynamic treatment in oxidative conditions. Exceptional selectivity and control of the functional species embedded into the Carbon Nanotubes (CNTs) have been achieved by carefully optimizing the electrochemical conditions in the functionalization. X-ray photoelectron spectroscopy (XPS) analysis determined a maximum heteroatom incorporation of ca. 2.3-2.6 at. % N and 1.4-1.6 at. % P when 2-aminophenylphosphonic acid (2APPA) is used with both carbon materials, probably because of the linear polymeric growth comparable to that of oligomers derived from aniline (ANI). However, the polymeric product obtained with 4-aminophenylphosphonic acid (4APPA) is much more limited, giving rise to a branched structure. In addition, electrochemical characterization, Raman spectroscopy and electron microscopy suggest that SWCNTs structure facilitates the oligomerization of longer but less stable chains through π-π interactions with the walls of CNTs, while hCNTs promote a more stable attachment according to their heterogeneity and higher reactivity. Computational calculations have confirmed the experimental results obtained, which allow us to shed more light on the mechanisms of the electrochemical functionalization of CNT with phosphorus and nitrogen-containing polymers. B.M.-S. thanks the Ministry of Science, Innovation and Universities of Spain for the FPU grant (FPU18/05127). The authors would like to thank MICINN and FEDER (projects PID2019-105923RB-100 and RTI2018-095291-B-I00) for the financial support.