Article pubs.acs.org/JPCC
Sidewall Chlorination of Carbon Nanotubes by Iodine Trichloride Víctor K. Abdelkader,† María Domingo-García,† M. Dolores Gutiérrez-Valero,‡ Rafael López-Garzón,‡ Manuel Melguizo,‡ Celeste García-Gallarín,‡ F. Javier Lopez-Garzon,*,† and Manuel J. Pérez-Mendoza† †
Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain Departamento de Química Inorgánica y Orgánica, Facultad de Ciencias Experimentales, Universidad de Jaén, 23071 Jaén, Spain
‡
ABSTRACT: The halogenation of carbon nanotubes is of great interest, as it allows the modification of the tube properties or it can serve as primary functionalization for a subsequent covalent fixing. Large degrees of fluorination can be reached although chlorine fixing is preferred, if halogenation is used as primary functionalization, as it is a better leaving group. Here, we report an extensive chlorination of multiwalled carbon nanotubes (MWCNTs) by I2Cl6. The reaction has been studied in acetic acid (AC) and in carbon tetrachloride (TC). The influence of the solvent is critical, as the functionalization is not relevant when the reaction is carried out in AC, while it reaches up to 13 atom % when the solvent is TC. Moreover, chlorine functionalization occurs in defects and borders of the tubes when the solvent is AC, while chlorine is largely fixed on the tube walls when TC is the reaction solvent. These different results are due to iodine trichloride coming out in different chemical species depending on the solvent. The correlation of the TPD and XPS spectra has allowed assignment of the fixed chlorine atoms to five chemical environments. None of the samples show intercalation of chlorine as deduced from X-ray diffraction data.
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the fixation of chlorine on amorphous carbon materials and on active carbons by using chlorine gas or PCl518−22 has also been described. The latter reacts with the amorphous carbon substrates which have been previously treated with hydrogen plasma. Nevertheless, the chlorination of CNTs is more difficult than that of amorphous carbon or active carbons due to their lower reactivity. Treatments with Cl2 gas and Cl2 aqueous solution have been reported as procedures to remove metal catalyst and amorphous carbon residues of multiwalled carbon nanotubes (MWCNTs), with simultaneous introduction of relatively low amounts of chlorine.23,24 Shortening of single-walled carbon nanotubes (SWCNTs) by grinding in a ball mill in the presence of alkyl-halides was reported to introduce up to 6.7 atom % of chlorine, likely into the defects and new nanotube tips formed after pristine SWCNT breakage.25 Modest results of chlorine introduction (up to 2.5 atom %) on SWCNTs have been achieved by electrochemical functionalization in HCl and KCl aqueous solution at potentials above the chloride ion oxidation potential26 and by radical reactions of MWCNTs with Cl2 under cryochemical (77−240 K) conditions.27 Similar results (up to 2.9 atom %) have recently been reported by treatment with SOCl2, although under harsh conditions, involving high pressure and temperature and prolonged reaction times of up to 14 days.28 2.1 atom % of covalently bonded bromine has been reported by treatment with Br2 for seven days.29 Moreover, RF cold plasma of bromine, bromoform, and allyl bromide has been used for bromine fixing of several carbon materials.30 In addition,
INTRODUCTION The chemical functionalization of carbon nanotubes (CNTs) is a procedure to modify their characteristics. Thus, functionalization often consists of the fixation of heteroatoms as oxygen, halogens, nitrogen, and so forth. The properties of these functionalized CNTs (solubility, conductivity, bonding in CNT/polymer composites, etc.) are clearly changed due to these heteroatoms being more electronegative than carbon. On some occasions this primary functionalization is a first stage for the subsequent fixation of more complex functions.1−4 The primary functionalization with oxygen-containing groups is usually carried out by oxidants in liquid or gas phase.5−10 These treatments are far from being selective and result in several types of oxygen containing groups, i.e., carboxyl, carbonyl, phenol, ether, ester, and aldehyde, among others.7,10 The new properties of the functionalized CNTs are a mean value of the effect of all these chemical functions. Carboxyls are frequently the intermediate group2,12,13 when the oxygen containing groups are used to fix other more complex functionalities, so that not all of the oxygen groups are suitable for this purpose. These facts can be important drawbacks if the functionalization aims to specifically modify the properties of carbon-based materials with well-defined surfaces, as in the case of CNTs. For these reasons a specific primary functionalization resulting in only one type of chemical group is much more desirable. Halogens are a good alternative to fix onto CNTs as it is expected they will render only one type of chemical group for every halogen. The covalent fixation of fluorine has been reported by treating several carbon materials with gaseous fluorine or with CF4 cold plasma,14−17 although chlorine is more desirable than fluorine to act as the primary function. Moreover, © 2014 American Chemical Society
Received: December 5, 2013 Revised: January 16, 2014 Published: January 17, 2014 2641
dx.doi.org/10.1021/jp411935g | J. Phys. Chem. C 2014, 118, 2641−2649
The Journal of Physical Chemistry C
Article
Figure 1. Survey XPS spectra of (a) MW-AC-1 and (b) MW-TC-24.
process. The tubes have an average diameter of 9.5 nm, an average length of 1.5 μm, and 0.6% ash content. According to the manufacturer, the amount of amorphous carbon as detected by HRTEM is negligible. Moreover, this is supported by thermogravimetric experiments in air which do not show any significant weight loss below 520 °C. The chlorination was carried out by using a 0.1 M iodine trichloride solution in glacial acetic acid and in carbon tetrachloride. The aim was to analyze the influence of the solvent in the functionalization process, as the former is a polar molecule while carbon tetrachloride is nonpolar. The time of treatment was between 20 min and 168 h depending on the solvent. The reactions were carried out under a nitrogen atmosphere to minimize interhalogen hydrolysis. The carbon nanotube (500 mg) suspensions in iodine trichloride solutions (50 mL) were sonicated for one hour and further maintained at room temperature under stirring until reaction time completion. The nanotube samples so obtained were filtered and repeatedly washed with hexane until no halogen was detected in the filtrate. The nomenclature of the samples is MW followed by an acronym which accounts for the solvent (AC = acetic acid and TC = carbon tetrachloride) and the time of treatment (0.3, 1, 2, 8, 24, or 168 h). For instance, the sample MW-AC-8 has been obtained
photochlorination of MWCNTs results not only in relevant degrees of functionalization, but also in large increases of oxygen containing groups.31 We have recently reported32 the chlorination of MWCNTs by carbon tetrachloride cold plasma. The procedure results in the fixation of chlorine to the borders and defects of the nanotubes. Thus, the lack of effective procedures for the CNT wall chlorination prompted us to explore new procedures. Here we propose a chemical method to covalently bond chlorine to MWCNT walls by using iodine trichloride. The procedure has been studied in two nonaqueous solvents to analyze their influence on the degree of chlorination. The final proposed method attaches a notable amount of chlorine onto the MWCNT surface (up to 13.0 atom % based on XPS analysis) in only two hours of treatment. This amount is far superior to those previously reported and can be easily scaled up to multigram amounts. The effect of chlorination on the characteristics of the MWCNTs is also considered.
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EXPERIMENTAL SECTION Multiwalled carbon nanotubes (Nanocyl-3100) were used. They were produced by the catalytic carbon deposition (CCVD) 2642
dx.doi.org/10.1021/jp411935g | J. Phys. Chem. C 2014, 118, 2641−2649
The Journal of Physical Chemistry C
Article
Table 2. Atomic Percentage of Heteroatomsa
by treatment of MWCNTs with iodine trichloride in acetic acid for 8 h. The nature of the surface chemical groups has been analyzed by XPS in a Kratos Axis Ultra-DLD system. Monochromatic Al/ Mg Kα radiation of twin anode in the constant analyzer energy mode with pass energy of 160 and 20 eV for the survey and high resolution spectra, respectively, was used. The binding energies were determined by setting up the C1s transition at 284.6 eV. The spectra after the background correction were fitted to Lorentzian and Gaussian curves by using XPS CASA software. Moreover, TPD measurements were carried out by heating the samples at 10 °C min−1 in helium flow (50 mL min−1) up to 900 °C. The gases evolved in the process were analyzed by using a quadrupole mass spectrometer Omnistar mod GSD 320. The textural characteristics of the samples were determined from the nitrogen adsorption isotherms at 77 K, which were obtained in ASAP 2020 equipment. The possible changes in crystallinity were determined by X-ray diffraction measurements by using a Bruker D2 PHASER system.
Sample
Cl (%)
I (%)
O (%)
MW-AC-ICl-1 MW-AC-ICl-8 MW-AC-ICl-24 MW-TC-ICl-24 MW-TC-ICl-168
0.2 ± 0.1 0.2 ± 0.1 0.2 ± 0.1 1.6 ± 0.1 4.3 ± 0.2
0.0 ± 0.1 0.0 ± 0.1 0.0 ± 0.1 0.3 ± 0.1 0.8 ± 0.1
2.6 ± 0.2 2.2 ± 0.1 2.3 ± 0.3 0.9 ± 0.1 0.8 ± 0.1
a
Reagent: iodine monochloride. Solvents: acetic acid (-AC- series) and carbon tetrachloride (-TC- series).
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RESULTS AND DISCUSSION Effect of the Solvent on the Chlorination. The first reaction tests were carried out using acetic acid as solvent. The XPS spectra (Figure 1a shows a selected sample) have been used to determine the atomic surface concentrations which are collected in Table 1 (-AC-series). Table 1. Atomic Percentage of Heteroatomsa Sample
Cl (%)
I (%)
O (%)
MWCNTs MW-AC-1 MW-AC-8 MW-AC-24 MW-TC-0.3 MW-TC-2 MW-TC-24 MW-TC-168
0.0 ± 0.1 0.6 ± 0.1 0.4 ± 0.2 0.6 ± 0.1 2.9 ± 0.1 13.0 ± 1.1 11.8 ± 0.6 8.5 ± 0.5
0.0 ± 0.1 0.0 ± 0.1 0.0 ± 0.1 0.0 ± 0.1 0.0 ± 0.1 0.1 ± 0.1 0.1 ± 0.1 0.1 ± 0.1
1.5 ± 0.1 2.2 ± 0.1 2.4 ± 0.1 2.5 ± 0.3 0.7 ± 0.1 0.7 ± 0.1 1.0 ± 0.1 2.3 ± 0.8
a
Reagent: iodine trichloride. Solvents: acetic acid (-AC-series) and carbon tetrachloride (-TC- series).
These data show that the proposed procedure is capable of attaching chlorine to the surface of MWCNTs, although the amounts of chlorine are not very high and those of iodine are negligible (
