Surface modification of carbon nanotubes with ethylene glycol plasma

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Surface modification of carbon nanotubes with ethylene glycol plasma ´ vila-Ortaa,*, V.J. Cruz-Delgadoa, M.G. Neira-Vela´zqueza, E. Herna´ndez-Herna´ndeza, C.A. A M.G. Me´ndez-Padillaa, F.J. Medellı´n-Rodrı´guezb a

Centro de Investigacio´n en Quı´mica Aplicada Blvd. Enrique Reyna 140, Saltillo, Coahuila, 25253, Mexico CIEP/FCQ Universidad Auto´noma de San Luis Potosı´, Salvador Nava 6, San Luis Potosı´, SLP. 78210, Mexico

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A R T I C L E I N F O

A B S T R A C T

Article history:

Multi-walled carbon nanotubes (MWCNTs) were modified using plasma polymerization

Received 21 October 2008

with ethylene glycol (EG) as monomer. Conditions of the EG plasma process in a specially

Accepted 28 February 2009

designed reactor and the plasma-polymerized ethylene glycol (PPEG) coating were studied.

Available online 19 March 2009

The study involved varying the plasma powers of 10 and 20 W at a constant process time of 60 min, and EG flow rate of 0.15 cm3/min. Dispersion of the modified MWCNTs was evaluated in several solvents, showing hydrophilic behavior. Morphology of the PPEG coating and the functional groups (hydroxyl) on its surface were characterized by both transmission electron microscope and FTIR spectroscopy. Characterization by thermogravimetric analysis and FTIR suggested that the hydroxyl groups of the PPEG coating residing inside the nanotubes possessed higher thermal stability than the ones outside.  2009 Elsevier Ltd. All rights reserved.

1.

Introduction

Carbon nanotubes (CNTs) are considered the strongest in tensile strength and stiffest in elastic modulus materials but are hydrophobic and inert when unfunctionalized. Their chemical bonding, which provides their unique strength, is composed entirely of sp2 bonds between individual carbon atoms, similar to those of graphite. Carbon nanotubes, categorized as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), naturally align themselves into ‘‘ropes’’ strongly held together by van der Waals forces. A MWCNT consists of multiple layers rolled in on themselves to form either a ‘‘Russian Doll’’ tube shape or a ‘‘Parchment’’ tube shape. Both shapes have interlayer dis˚ . MWCNTs tances or narrow channels of approximately 3.4 A resist reaction with chemicals due to their intertube van der Waals attraction, thus, impeding their many potential applications [1]. Many approaches have been used to graft functional groups, non-covalently or covalently, at the surface of

MWCNTs to add new properties like dispersion in organic and aqueous media [2–6] or their dispersion in polymer matrixes aiming to enhance properties such as tensile strength, Young modulus, thermal stability and electrical conductivity [7]. It should be noted that the covalent functionalization of SWCNTs will break some C@C bonds leaving ‘‘holes’’ in the structures which would adversely affect their mechanical and electrical properties. This structural damage by covalent functionalization is not evident in the case of MWCNTs. Covalent functionalization of CNTs is often used to provide dispersability and nucleation sites for further derivation reactions. For hydrophobic substances, among others, carboxylation is desired. Nevertheless, hydroxylation is preferred for hydrophilic substances. The formation of polymers having mainly functionalized hydroxyl groups (like with EG monomer) is quite favorable using plasma polymerization [8]. Plasma polymerization is a relatively simple, rapid, and dry method that has been used to change the surface chemistry of different materials. The technique was originally implemented to modify the surface

* Corresponding author: Fax: +52 844 4389839. ´ vila-Orta). E-mail address: [email protected] (C.A. A 0008-6223/$ - see front matter  2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.02.033

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of polymers [9–12] but in the last few years has been also used for plasma treatment of nanoparticles such as zinc oxide, aluminum oxide, nanoclays, and carbon nanofibers as well as nanotubes [13–20]. The main principle of plasma polymerization technique is that the ionized and excited molecules and radicals created by the electrical field bombard and react with the surface of the substrate. These activated molecules may etch, sputter, or deposit on the substrate surface. As a result, the surface properties of substrates are modified. The mechanism of plasma polymerization tends to be a radical polymerization process, [21,22] especially when the plasma power is high and an ionic polymerization process when the power is low [19]. The aim of this study is to evaluate the effect of the plasma reactor conditions on the surface modification of carbon nanotubes by plasma polymerization of ethylene glycol and the effect of this modification on thermal stability of hydroxyl functional groups of the PPEG coating. Low plasma powers of 10 and 20 W are used in this study to minimize damage of the nanotube walls.

2.

Experimental

2.1.

Materials

MWCNTs were purchased from Nano-Lab, Inc. (USA), research grade PD30L15, with 30 ± 15 nm in diameter and 1–5 lm in length with purity over 95%, and were used as received. The EG monomer used to modify the carbon nanotubes was supplied by Sigma–Aldrich. The solvents, acetone and methanol, were also from Aldrich.

2.2.

Methodology

The surface of carbon nanotubes was treated to deposit an ultrathin polymer coating using plasma polymerization of ethylene glycol. Fig. 1 provides a schematic diagram of the plasma reactor, designed by the authors, employed for the plasma polymerization described in this work. It consists of four main parts: 1. the reaction chamber includes a 50 mL Pyrex glass tube, 2. an electrical excitation system for generating plasma comprised of a power controller coupled to a radiofrequency (rf) generator of 13.56 MHz, 3. an EG monomer gas delivery system, and 4. a vacuum system. A pneumatic stirrer was used in the glass tube to constantly agitate the MWCNTs in order to achieve a homogeneous PPEG coating on the nanotubes. A copper wire, acting

Fig. 1 – Schematic of the plasma reactor used for the MWCNTs modification.

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as an electrode, was coiled around the glass tube with one end connected to the rf generator (Advance Energy RFX600). To carry out the vapor phase polymerization of PPEG, about 50 mg of MWCNTs were placed inside the reactor and put under vacuum to achieve an internal pressure of 4.5 · 10 3 mbar. The EG monomer was then introduced into the reactor at a constant flow pressure of 1.3 · 10 2 mbar. The MWCNTs were treated 60 min, using two different plasma powers: 10 and 20 W. Treated samples were then placed in separate glass bottles.

2.3.

Characterization

A TITAN-300 kV field emission gun microscope, which has a symmetrical condenser-objective lens type S-TWIN (Cs = 1.3 mm) was used to analyze the morphology of untreated and treated samples. The HRTEM images have been registered in a CCD camera near of the Scherzer focus. FTIR (NICOLET Magna 550) was used to characterize the PPEG coatings on carbon nanotubes. To carry out this characterization, 1 mg of treated samples were mixed with 10 mg of potassium bromide (KBr) to prepare KBr disks of 0.5 cm in diameter and 0.05 cm in thickness. The spectra were obtained at 30 scans and resolution of 4 cm 1 between 400 and 4000 cm 1. Two test specimens were used for each case. The untreated and treated MWCNTs were tested for dispersion in three solvents with different polarity indexes: water, methanol, and ethylene glycol. 1 mg each of the samples was immersed into 10 mL of solvent and agitated. After standing for 24 h, photographs were taken to evaluate the degree of dispersion. The thermogravimetric analyses (TGA) were performed using a thermoanalyser TA Q500. TGA curves were obtained according to the following conditions: sample size is about 10 mg, temperature range of 30 to 800 C, heating rate of 10 C/min and atmosphere of N2 (dynamic) of 50 cm3/min.

3.

Results and discussion

3.1.

Morphology by HRTEM

Fig. 2 shows HRTEM micrographs of untreated and treated MWCNTs. A smooth and homogeneous surface can be seen for the untreated MWCNTs as shown in the micrograph of Fig. 2a; nevertheless small deposits of amorphous carbon are shown. On the other hand, the surface of the treated MWCNTs is rough, surrounded with a fuzzy mass (Fig. 2b). The nature of this fuzzy mass can be related to plasma-polymerized ethylene glycol (PPEG) deposition during plasma treatment, which would result in an ultrathin polymer coating (this is further supported in Section 3.4; it is worth mentioning that the fuzzy mass is only present on the MWCNTs surface and nowhere else). The presence of the PPEG ultrathin polymer coating on the MWCNTs surface was marked with arrows (Fig. 2b), with thickness in the range of 1–3 nm, which is similar in size to those reported by Shi et al., for acrylic acid in ZnO nanoparticles [16] and styrene in carbon nanofibers [18]. Finally, the PPEG ultrathin polymer coating does not necessarily cover the whole MWCTs surface; it was deposited

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Fig. 3 – FTIR spectra of untreated and treated MWCNTs at 10 and 20 W at 60 min.

sample could be attributed to the hydroxyl OH group from PEG overlapping with the water OH from the untreated MWCNT as to be explained in the next paragraph. Spectrum of the untreated MWCNTs shows bands around 3430 and 1735 cm 1 that could be due to the presence of hydroxyl (–OH) and carboxyl (C@O) groups, respectively, on the nanotubes surface [23,24]. Colthup et al. [24] attested that both of these groups were actually due to contamination, very possibly being introduced during the purification process applied by the supplier. Therefore, the broad band at 3000– 3700 cm 1, also present on the spectra of the treated MWCNT, could be attributed to the contaminated hydroxyl (–OH) from the water OH.

3.3.

Fig. 2 – HRTEM micrographs of uncoated MWCNTs (a) and coated with PEG (b) for 60 min at 10 W.

randomly in different portions of the MWCNTs due to their continuous movement in the reactor during treatment. The nature of an ultrathin coating can change the surface properties of MWCNTs as discussed below.

3.2.

FTIR analysis

FTIR was used to further investigate the surface structure of the PPEG coating on carbon nanotubes. The spectra of untreated and treated samples at 10 and 20 W for 60 min are shown in Fig. 3. After modification, a new peak at 1090 cm 1 is quite evident and its size at 20 W is much larger than at 10 W. This peak is due to the C–O bond stretching of CH2–OH units. The broad band (hydroxyl group) at 3000–3700 cm 1 is very interesting as it also appears on the spectrum of the untreated MWCNT. We believe that this band from the treated

Dispersion test

Dispersion tests give a fair idea whether the modification on the carbon nanotubes has been achieved or not. Fig. 4 presents a photograph of six vials containing untreated and treated CNT dispersed in water, methanol, and ethylene glycol. The untreated MWCNTs are highly hydrophobic, hence, their dispersion is nil in these high polarity solvents. The results clearly show that the untreated carbon nanotubes sedimented almost immediately. The treated samples (for 60 min at 10 W), remain fairly dispersed in the same three solvents after standing for 24 h, suggesting a strong interaction between the solvents and modified MWCNTs. Water and methanol are solvents that can dissolve PEG up to a certain extent. The dispersion test clearly indicates the hydrophilic character of the PPEG coating due to the presence of the hydroxyl groups determined by FTIR.

3.4.

Thermal stability

Fig. 5 shows the TGA traces for untreated and treated samples at 10 and 20 W for 60 min heated from 30 to 600 C at a rate of 10 C min 1. The weight loss for the untreated MWCNTs was observed to be 6%, but 7% and 8% weight loss was observed for treated samples at 10 and 20 W, respectively. The weight losses of the latter could be attributed to the dissociation of

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Fig. 4 – Dispersion of the untreated and treated MWCNTs in water (a), methanol (b) and ethylene glycol (c) at 10 W for 60 min.

Fig. 5 – TGA traces of untreated and treated MWCNTs at 10 and 20 W for 60 min heated from 30 to 600 C at a rate of 10 C min 1.

the hydroxyl groups on the CNT [23,25,26] as could be verified by the FTIR spectra of carbon nanotube samples after the TGA test in Fig. 6. Fig. 6a shows the FTIR spectra of the untreated nanotubes maintained at 25, 50, 100, 200 and 300 C for 1 min. There is essentially no change in the spectrum at 25 C, same as with the one taken at room temperature (refer back to Fig. 3) while significant changes were noticeable at above 50 C indicating that both the amorphous carbon and impurity ash and the appended –OH group, introduced during purification at supplier’s lab at 3430 cm 1, are mostly cleaved. The partial hydroxyl group still existent above 50 C is explained later along with the results of the treated MWCNTs.

Fig. 6b and c shows the FTIR spectra of MWCNT treated at 10 and 20 W heated also from 25 to 300 C. The results are quite similar in that the C–O stretch of CH2–OH units at 1090 cm 1 is significantly reduced or cleaved at above 50 C, most probably due to the dissociation of the hydroxyl groups of the coating on the surface of CNT. Nevertheless, above 50 C the presence of the peak at 1090 cm 1 up to 300 C evidences the presence of non-monomeric EG units, i.e. oligoor polymeric EG units, since the boiling temperature of the EG used is ca. 190 C. This fact coupled with the evidence that the coating still prevails at 300 C (see HRTEM inset in Fig. 5) supports the idea of an ultrathin polymer coating (PPEG) on the surface of MWCNTs. Besides, the dissociation of the OH group with increasing temperature is not as severe as compared to the results of the untreated MWCNTs in Fig. 3 at the 3430 cm 1. These interesting results at 3430 cm 1 for both the untreated and treated samples could be explained this way: there is less dissociation of the appended OH groups from the treated MWCNTs because more energy is required to cleave both the overlapping hydroxyl OH from the PPEG and the contaminated water OH groups. Another interesting possibility of the observation of the OH and CH2–OH groups at 3430 and 1090 cm 1, respectively. They are present even after the TGA test at 300 C, indicating the stability of these two groups. To explain this phenomenon, we consulted the work of Chen et al. [26] with hydroxyethylated SWCNTs. These researchers found that many alkyl and hydroxyethyl groups were able to locate themselves on the narrow channels of the inner surface of the nanotubes having an unusual high thermal stability. Their survival was due to the groups being encapsulated in the nanotubes, thus protecting them from cleavage by forming a protective cage. As a result, during the heating of the TGA test, radicals

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4.

Conclusions

A PPEG ultrathin polymer coating (thickness 1–3 nm) was deposited on the surface of carbon nanotubes by means of the plasma polymerization process. This polymer had hydroxyl functional groups as confirmed by FTIR. The modified MWCNTs showed very stable dispersion with water, methanol, and ethylene glycol, confirming a hydrophilic behavior of the treated MWCNTs. TGA and FTIR results suggested that the functional groups of PPEG coating inside the nanotubes had a high thermal stability. The functionalized carbon nanotubes with PPEG may be a very promising reinforcement in polymers and other matrices to produce nanocomposite materials of unique physical properties in some demanding engineering applications, such as the automotive or aeronautic industries.

Acknowledgements The support of the Mexican Council of Science and Technology (CONACyT) under Grant J49551-Y for this work and granting a scholarship to V. J Cruz-Delgado is acknowledged and greatly appreciated. Technical assistance from Sion Ng for reviewing this paper is also appreciated and to Dr. Arturo Ponce for his helpful assistance for TEM measurements.

R E F E R E N C E S

Fig. 6 – FTIR spectra of the untreated MWCNTs (a), treated MWCNTs at 10 W (b) and treated MWCNTs at 20 W (c) kept at 25, 50, 100, 200 and 300 C for 1 min for each temperature.

generated by thermolysis could not escape from within the tubes and would quickly recombine and reattach themselves to nanotube carbon atoms of the confined channel. However, the unprotected functional groups outside the tubes would experience easy cleavage during thermolysis. It has been strongly suggested by theoretical work based on molecular dynamics simulation as well as analytic methods that substances within the narrow confines of MWCNTs might exhibit remarkable thermal stability features that do not occur in bulk material or outside the tubes [27–29].

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