A study on the effect of different chemical routes on functionalization of MWCNTs by various groups (-COOH, -SO3H, -PO3H2)
© Kumar et al; licensee Springer. 2011
Received: 3 August 2011
Accepted: 7 November 2011
Published: 7 November 2011
Pristine multiwall carbon nanotubes [MWCNTs] have been functionalized with various groups (-COOH, -SO3H, -PO3H2) using different single- and double-step chemical routes. Various chemical treatments were given to MWCNTs using hydrochloric, nitric, phosphoric, and sulphuric acids, followed by a microwave treatment. The effect of the various chemical treatments and the dispersion using a surfactant via ultrasonication on the functionalization of MWCNTs has been studied. The results obtained have been compared with pristine MWCNTs. Scanning electron microscopy, energy dispersive X-ray [EDX] spectroscopy, and transmission electron microscopy confirm the dispersion and functionalization of MWCNTs. Their extent of functionalization with -SO3H and -PO3H2 groups from the EDX spectra has been observed to be higher for the samples functionalized with a double-step chemical route and a single-step chemical route, respectively. The I D/I G ratio calculated from Raman data shows a maximum defect concentration for the sample functionalized with the single-step chemical treatment using nitric acid. The dispersion of MWCNTs with the surfactant, Triton X-100, via ultrasonication helps in their unbundling, but the extent of functionalization mainly depends on the chemical route followed for their treatment. The functionalized carbon nanotubes can be used in proton conducting membranes for fuel cells.
Keywordsfunctionalization carbon nanotubes dispersion surfactant
Currently, carbon nanotubes [CNTs] are the state-of-the-art materials actively studied by both experimentalists and theoreticians because of their versatile structural, electronic, mechanical, optical properties [1–3]. The pristine CNTs generally exist in bundled form due to the presence of strong Van der Waals interactions between them. In particular, these intermolecular forces of attraction are based on the pi [π] bond stacking phenomena between adjacent nanotubes, and there can be at least hundreds of π stacking sites between two CNTs. Hence, intermolecular forces are very strong. CNTs should be unbundled prior to their use for any application. Dispersion of nanotubes can be achieved using various surfactants, polymers, biomolecules, etc. via a physical or chemical method. In the case of surfactants, the surfactant groups get adsorbed onto the CNT surface without disturbing the π stacking system of the graphene sheet and result in dispersion. Out of the different surfactants being used for the dispersion of CNTs like sodium dodecylbenzenesulfonate [SDS], dodecyltrimethyl ammonium bromide, Tween 20 (Sigma-Aldrich, St. Louis, MO, USA), Tween 80 (ICI Americas, Inc., Wilmington, DE, USA), Triton X-100 (Dow Chemical Company, Midland, MI, USA), etc., the SDS and Triton X-100 have been reported to result in the minimum and the maximum dispersions of nanotubes, respectively . Triton X-100 is mainly used to disperse CNTs due to its number of advantages including a non-covalent approach for dispersion, and the presence of a benzene ring in its chemical structure can be easily removed by washing. The most common approach is to disperse the CNTs in an aqueous surfactant solution, which is then subjected to ultrasonication in order to mechanically break the aggregation and eventually yield fully separated CNTs. The surfactant molecules are adsorbed onto the surface of CNTs (as shown in Scheme 1, see Additional file 1), have repulsion between them, and hence help to disperse the CNTs .
Dispersion of CNTs depends upon a number of factors, such as the type of CNTs, their geometry, the relative ratio of CNTs, and the type of surfactant being used. After dispersing the nanotubes, it is desirable to functionalize them with various chemical groups depending upon the application for which we want to use them.
Various chemical groups can be attached physically or chemically to the side walls or end caps of nanotubes, without significantly changing their desirable properties . This process is called functionalization of nanotubes. A large number of methods are being used for the functionalization of CNTs, which can be broadly divided into the endohedral and exohedral methods. We have followed the exohedral mode in which the chemical groups are attached to the outer wall of the CNTs. Exohedral functionalization can be further subdivided into the covalent and non-covalent approaches. In the covalent approach, functionalization has been achieved by attaching the functional group on the side walls, end caps, or defect sites of nanotubes with a covalent bond, whereas in the non-covalent approach, chemical groups are attached by the wrapping of polymers, biomolecules, etc. on nanotubes.
In the present study, MWCNTs have been covalently functionalized with different chemical groups (-COOH, -SO3H, -PO3H2) using various single- and double-step chemical routes. The effect of dispersion using Triton X-100 via ultrasonication, before the functionalization of CNTs, has also been studied. The defect concentration has been determined from Raman studies. The extent of functionalization with different groups has been determined from the EDX results and chemical routes which results in the identification of sulfonation and phosphonation of higher extents.
Amount of MWCNTs
Dispersion before functionalization
Forty milligrams of MWCNTs had been taken, and 20 mL HNO3 was added to it. The sample was refluxed for 240 min at 100°C. Furthermore, the sample was given multiple washings via centrifugation at 12, 000 rpm for 6 min (six times) and dried overnight in an oven at 60°C. For the second step of functionalization, a 1:1 v/v ratio of HNO3 and H2SO4 (15 mL each) was added to the dried sample. Microwave treatment was given for 5 min on an on/off basis. After this, 20 mL of HCl was added slowly to the sample, and it was refluxed for 60 min at an ambient temperature. In order to give the sample multiple washings, centrifugation was done at 12, 000 rpm for 6 min (six times). The functionalized sample was dried overnight in an oven at 60°C.
Fifty milligrams of MWCNTs had been taken and dispersed with 1.9% Triton X-100 and 200 mL of deionized [DI] water via ultrasonication for 120 min. After this, the sample had been given multiple washings through centrifugation at 7, 000 rpm for 10 min (six times) and dried overnight in an oven at 60°C. For the functionalization, a 1:1 v/v ratio of HNO3 and HCl (25 mL each) was added to the dried dispersed sample, and it was refluxed for 90 min at 80°C and then centrifuged at 12, 000 rpm for 10 min (six times). The functionalized sample was dried overnight in an oven at 60°C.
Fifty milligrams of MWCNTs had been taken and dispersed with 1% Triton X-100 and 200 mL DI water via ultrasonication for 60 min. After this, the sample had been given multiple washings through centrifugation at 12, 000 rpm for 6 min (six times) and dried overnight in an oven at 60°C. Furthermore, a 1:1 v/v ratio of HNO3 and HCl (25 mL each) was added to the dried dispersed sample, and it was refluxed at 80°C for 90 min. The sample had been given multiple washings via centrifugation at 12, 000 rpm for 6 min (six times) and dried overnight in an oven at 60°C. For the second step of functionalization, a 1:1 v/v ratio of HNO3 and H2SO4 (25 mL each) was added to the dried sample, and microwave treatment was given for 5 min on an on/off basis. After this, 30 mL HCl was added slowly to the above mixture. The sample was then refluxed for 60 min at an ambient temperature, followed by centrifugation at 12, 000 rpm for 6 min (six times). The sample was dried overnight in an oven at 60°C.
Twenty milligrams of MWCNTs had been taken, and 10 mL of H3PO4 was preheated at 60°C for 20 min and then added to the CNTs. Furthermore, 10 mL of HNO3 was added to the above mixture. It was mixed and refluxed at 130°C for 60 min. In order to give multiple washings, the sample was centrifuged at 12, 000 rpm for 6 min (six times) and dried overnight in an oven at 60°C.
Transmission electron microscopy
Transmission electron microscopy [TEM] (Libra 120, Carl Zeiss AG, Oberkochen, Germany) at an acceleration voltage of 120 kV was used to examine the size and distribution of the CNT surface of various samples. The TEM specimens were prepared by placing a few drops of the sample solution on a lacey carbon grid.
Scanning electron microscopy
Scanning electron microscopy [SEM] micrographs were obtained with a Hitachi S-4800 field-emission SEM (Hitachi High-Tech, Minato-ku, Tokyo, Japan) at an acceleration voltage of 0.5 to 30 kV. Specimens for high-resolution imaging were coated with Osmium.
Energy dispersive X-ray
The energy dispersive X-ray [EDX] (X-Max 50011, HORIBA Ltd., Minami-Ku, Kyoto, Japan) spectra were obtained to determine the elemental information on the CNT at 16 kV and 15 μA.
Raman spectroscopy was carried out at room temperature using a FRA 106/S (BRUKER OPTIK GMBH, Ettlingen, Germany) Raman spectrometer, with a 1006-nm Nd-YAG laser and a 4-cm-1 resolution.
Results and discussion
Pristine CNTs are generally chemically inert and insoluble in many solvents. In order to make them suitable for various applications, they have to be functionalized with different groups. The functionalized CNTs are soluble in various organic solvents. The functionalization of CNTs strongly depends upon the chemical route followed. In the present study, different chemical routes have been used for the functionalization of CNTs with the -COOH, -SO3H, and -PO3H2 groups, and their effects on the functionalization have been studied. MWCNTs have been functionalized with the -COOH, -SO3H, and -PO3H2 groups using various chemical routes given in Scheme 2 (see Additional file 2):
Single-step process (FCNT03 and PhCNT01)
Double-step process (FPCNT01 and DFCNT03)
Without dispersion with surfactant (FPCNT01 and PhCNT01)
After dispersion with surfactant (DFCNT03 and FCNT03).
Concentration of different elements from EDX data
Intensities of the G and D bands and intensity ratio (I D/I G) calculated from Raman data
I D/I G
Position of peak (cm -1 )
Position of peak (cm -1 )
MWCNTs have been functionalized with different groups using various single- and double-step chemical routes. The maximum sulfonation (functionalization with -SO3H groups) has been achieved for sample FPCNT01 which was functionalized using a double-step chemical route, whereas the maximum phosphonation (functionalization with -PO3H2 groups) has been achieved for sample PhCNT01. The highest defect concentration (ID/IG) has been observed for sample FCNT03, which has been functionalized with a single-step process using HNO3. The dispersion of CNTs using a surfactant helps in their unbundling, but the more important step is the chemical route followed for their functionalization as observed from EDX results. A proper choice of the chemical route and the amount of acid used can be helpful to control the extent of functionalization with various chemical groups. The incorporation of CNTs functionalized with the -SO3H and -PO3H2 groups in sulfonated polymers can be used as high temperature fuel cell membranes.
PR thanks CSIR, New Delhi for the award of SRF. This research is supported in part by the 2011 Basic Research Program of Korea Institute of Energy Research (KIER) and in part by the New and Renewable Energy of Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government's Ministry of Knowledge Economy (no. 2010T100100838).
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