Preparing Nitrogen-Doped Multiwalled Carbon Nanotubes with Regionally Controllable Heterojunction Structure by Nondestructive Postdoping with the Assistance of Heating Fluorination

This paper presents a feasible and universal approach to fabricate regionally controllable nitrogen-heteroatom-doped multiwalled carbon nanotubes with a skin-core heterostructure (N-o-FMWCNT) with the assistance of defluorination in outer-layered fluorinated MWCNTs (o-FMWNCT). First, o-FMWCNT with outer fluorinated tubes encapsulating intact inner tubes was prepared through well-designed direct heating fluorination with temperatures from 25 to 180 degrees C. Subsequently, nitrogen-doping was implemented at a mild temperature under NH3 atmosphere (400 degrees C). According to the results of X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and elemental analysis, about 4.97 atom % nitrogen-heteroatom with primary pyridine-nitrogen configuration was successfully doped into the carbon skeleton at the outer tube of o-FMWCNT with the recombination of C-N bonds. Because of the integrity of its inner tubes, the N-o-FMWCNT was endowed with a relatively high-level conductivity (5.46 x 10(-2) S.m(-1)) in contrast to other reported highly N-doped carbon materials. Correspondingly, the mass-specific capacitance of N-o-FMWCNT as a supercapacitor electrode material was spectacularly improved by 437% at the current density of 0.5 A/g in comparison to that of pristine MWCNTs (p-MWCNT). Meanwhile, nitrogen-doped MWCNTs with a homogeneous doping structure (N-i-FMWCNT) were also prepared through direct isothermal fluorination for comparison. Our work proposes a nondestructive postdoping strategy for preparing functionalized CNTs with a regionally controllable heteroatom-doping structure.