Abstract
Double-walled carbon nanotubes (DWCNTs) are fluorinated using (1) fluorine F2 at 200 °C, (2) gaseous BrF3 at room temperature, and (3) CF4 radio-frequency plasma functionalization. These have been comparatively studied using transmission electron microscopy and infrared, Raman, X-ray photoelectron, and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. A formation of covalent C–F bonds and a considerable reduction in the intensity of radial breathing modes from the outer shells of DWCNTs are observed for all samples. Differences in the electronic state of fluorine and the C–F vibrations for three kinds of the fluorinated DWCNTs are attributed to distinct local surroundings of the attached fluorine atoms. Possible fluorine patterns realized through a certain fluorination technique are revealed from comparison of experimental NEXAFS F K-edge spectra with quantum-chemical calculations of various models. It is proposed that fluorination with F2 and BrF3 produces small fully fluorinated areas and short fluorinated chains, respectively, while the treatment with CF4 plasma results in various attached species, including single or paired fluorine atoms and –CF3 groups. The results demonstrate a possibility of different patterning of carbon surfaces through choosing the fluorination method.
Highlights
Even after surface chemical functionalization, due to their inner shell double-walled carbon nanotubes (DWCNTs) display many advantages characteristic of single-walled carbon nanotubes (SWCNTs), small diameter, high strength and flexibility [1]
The radial breathing modes (RBM) peaks below ca. 250 cm−1 are usually attributed to the outer DWCNT shells, while the RBM peaks above ca. 250 cm−1 are assigned to the inner DWCNT shells [22]
The average number of bare carbon atoms surrounding a CF group progressively grows from 1 to 3 with the use of F2, BrF3, and CF4 plasma. These numbers are close to the C–(CF)/CF ratios in the models selected to describe the dominating fluorination patterns for each case by comparing the near-edge X-ray absorption fine structure (NEXAFS) spectra measured at the F K-edge of fluorinated DWCNTs with theoretical spectra from quantum-chemical calculations
Summary
Even after surface chemical functionalization, due to their inner shell double-walled carbon nanotubes (DWCNTs) display many advantages characteristic of single-walled carbon nanotubes (SWCNTs), small diameter, high strength and flexibility [1]. The preferable fluorine distributions on the DWCNT surface are proposed from quantumchemical modelling of the fluorine near-edge X-ray absorption fine structure (NEXAFS) spectra, which showed substantial differences for the samples prepared using elemental F2 at elevated temperature, BrF3 at room temperature, and CF4 rf plasma.
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