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Surface modification of MWCNTs by O2 plasma treatment and its exposure time dependent analysis by SEM, TEM and vibrational spectroscopy
Superlattices and Microstructures 64 (2013) 399–407
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Surface modification of MWCNTs by O2 plasma treatment and its exposure time dependent analysis by SEM, TEM and vibrational spectroscopy Prabhash Mishra, Harsh, S.S. Islam ⇑ Nano-Sensor Research Laboratory, F/O Engineering and Technology, Jamia Millia Islamia, Jamia Nagar, New Delhi, India
a r t i c l e
i n f o
Article history: Received 10 July 2013 Received in revised form 23 September 2013 Accepted 7 October 2013 Available online 14 October 2013 Keywords: Carbon nanotubes Electron microscopy Raman spectroscopy FTIR Surface energy
a b s t r a c t In this paper, we report the efficient surface modification of multiwall carbon nanotubes (MWCNTs) by oxygen (O2) plasma treatment while in situ monitoring of its sidewall surface degradation as a function of plasma exposure time. The nanostructure and surface morphology of pre- and post plasma treated CNTs were investigated by using Field Emission Electron microscopy (FESEM), High resolution Transmission Electron microscopy (HRTEM), and Raman spectroscopy (RS). A systematic study was carried out by FTIR and surface energy measurements on the analysis of functional groups attached on the CNTs surface leading to the transformation of surface nature from hydrophobic to hydrophilic with the increase of plasma treatment time. The results confirmed that O2 plasma treatment time greatly enhanced oxygen content on CNTs surface, and modified structural properties while inducing more surface damages and defects. Surface defects are sometimes desirable for sensing purpose and it is shown that the extent of defect generation can be easily tuned with the plasma exposure time without damaging the integrity of the nanotubes pattern. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction After the discovery of carbon nanotubes in 1991 [1], the researchers have been striving for better understanding of their basic properties and applications in various fields of science and ⇑ Corresponding author. Tel.: +91 11 26980532; fax: +91 11 26988846. E-mail address: safi[email protected] (S.S. Islam). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.10.010
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technology. But the major hurdles for exploring the possibility of CNTs commercial applications are many, such as – no control on its chirality – a decisive factor whether the CNT will be metallic or semiconductor, its insolubility nearly in all solvents, growth related impurities and defects, lump/ bundle formation leading to poor dispersion [2]. To remove all these drawbacks researchers have taken various approaches, for example, tailor the physical and chemical properties by covalently attach chemical groups through wet treatment oxidation [3,4], air oxygen [5], ozone oxidation [6], and plasma oxidation [7,8] leading to formation of hydroxyl and carboxyl groups on the surface of the nanotubes. But all these treatments involve very harsh conditions and create severe damages on the sidewalls, open the nanotube tips, chop the nanotubes into small pieces, and also change the orientation and alignment of CNTs. Interestingly, these treatments are surface specific so that the bulk properties are preserved. In comparison, plasma treatment has the advantages of shorter reaction time, non-polluting processing, and providing a wide range of different functional groups depending on plasma parameters. It is a valuable technique to engineer surface properties without alteration of the bulk compositions [9]. Over the years plasma treatment has been employed by many researchers on CNTs. Kang et al. [10] has demonstrated a simple and efficient in situ H2 and O2 plasma purification process for removal of the residual carbonaceous particles and metallic impurities without significant structural damage to individual CNTs. Felten et al. [11] investigated the impact on the electronic structure of CNTs upon using an O2 plasma to graft oxygen containing surface groups and showed that the plasma treatment improves the uniformity of the distribution of surface defects. Another study on surface modification of MWCNTs by O2 plasma was carried out by Tao Xu et al. [12] for large scale purification of MWCNTs powder where the authors claimed successful achievement of damage free complete purification within 30 min. C. Chaglun et al. [13] has carried out grafting oxygen containing functional groups by microwave-excited Ar/H2O surface wave plasma and estimated the effect of gas partial pressure, treatment time, and microwave power on the introduction of oxygen-containing group onto the surfaces of MWCNTs. The effect of plasma parameters on surface morphology, atomic composition and structure of vertically aligned carbon nanotube forests by RF Ar/O2 plasma was recently reported by Bin Zhao et al. [14]. the authors demonstrated the post treatment effect on the preservation of vertical alignment and change in bonding structure due to surface functionalization by CAO and OAC@O groups. In most of the studies as mentioned, it was found that the authors emphasized the potentiality of plasma treatment as a nonpolluting method either for purification or grafting functional groups in CNTs (measured by FTIR), surface damage studies by SEM, TEM and Raman, and quantification of elements attached on nanotube surface by XPS measurements. There is hardly any report available done good analysis how the surface damage, surface functionalization and change in surface nature are correlated with the plasma processing time. In this work, we report a systematic study to understand the changes that results from the surface modification strategy. Accurate characterization of the surface chemistry of MWCNTs structure is done before and after HF (high frequency) O2 plasma treatment with respect to treatment time. FTIR studies has been carried out to estimate the surface functional group introduced on nanotube surface by plasma treatment and simultaneously the wettability properties of the modified CNTs are carried out by surface energy measurements. Both FTIR and surface energy measurement are reported in this paper with the duration of O2 plasma exposure.
2. Experimental Prior to CNTs growth, iron (Fe) layer with thickness of 3 nm were sputtered on N-type Si (100) with 500 nm oxide layer. The CNTs were grown on SiO2/Si substrate by using self design thermal CVD with mixture of H2 + Ar and C2H2 at flow rate of 30 sccm and 60 sccm respectively. C2H2 was used as a source gas and mixture of Ar + H2 used as a dilution. An electric feed through was used for rapid increase of temperature. CNTs were grown at 800 °C for 3 min and total pressure of the chamber was kept constant at 10 Torr. After growth of CNTs, Ar was flowed continuously in the chamber till the chamber attained room temperature.
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The microstructure and surface morphology of as-grown and post plasma treated CNTs were examined by Field Emission Scanning Electron Microscope (FESEM, FEI Nova Nano SEM 450) and Transmission electron microscope (TEM, FEI Tecani F20). Structural qualities and degree of defects of CNTs were analyzed by micro-Raman spectrometer (LabRAM HR 800, Horiba JY) fitted with a peltier cooled CCD detector and an Olympus BX-41 microscope. The excitation of the samples was performed with an aircooled Ar+-laser (Spectra Physics) tuned at 488 nm. Measurements were carried out in the back scattering geometry using a 50 LWD microscope objective. The beam was focused at a spot size of 1.19 lm and the power density (lW/cm2) was kept low to avoid any excessive heating of the probe region. FTIR spectra were recorded using Vertex V70 spectrophotometer (BRUKER) in the range of 400–2000 cm 1. Oxygen plasma treatment of CNTs was done in a plasma system (Diener electronic GmbH Germany) equipped with a HF generator (High frequency Generator) under 80 W power and 40 kHz frequency. Oxygen gas at 25 sccm flow rate was introduced into the chamber. The treatment time of the sample was varied from 0 min to 15 min. The working pressure was adjusted at 0.8 mbar during the plasma processing. The water contact angle and the surface energy of pre- and post plasma treated CNTs were measured with Phoenix 300 goniometer (Surface Electro Optics, Korea) equipped with CCD camera. Apparatus was also used in conjunction with specialized software for determination of surface energy. The Girifalco–Good–Flowkes–Young method (for 1 contact angle of 1 fluid on 1 solid) was used to calculate the average surface energy proving at three different spots.
Fig. 1. SEM images of the oxygen plasma treated MWCNTs (a) untreated, (b) after 5 min, (c) after 10 min, and (d) after 15 min.
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3. Results and discussion 3.1. Morphological surface analysis of MWCNTs: SEM and TEM studies The morphology of MWCNTs was characterized by FESEM. Fig. 1(a) shows the surface morphology of as-grown carbon nanotubes film before and after exposing to the oxygen plasma and the differences were observed on the surface structure at different intervals of processing time. The untreated CNTs present a typical morphology with random network structure as shown in Fig. 1(a). Plasma treatment on CNTs was carried out at fixed O2 concentration and exposure time was varied from 0 to 15 min keeping the plasma power at 80 W. Plasma treated nanotube structure shows significant change on their surface morphology. Fig. 1(b) shows some damages on the surface of carbon nanotubes for 5 min plasma treatment. We observed an increase in defects/damages on the carbon nanotube surface for 10 min and 15 min plasma treatment time respectively and these are shown in Figs. 1(c) and 2(d). During the plasma treatment, more charge ions and electron in plasma exhibited the effect on surface morphology physically and chemically. As a whole, structural damage (e.g. sidewall damage) was caused by plasma treatment and surface modifications were induced and the degree of damage got enhanced with plasma processing time which is evident from TEM measurements. Fig. 2 shows HRTEM images recorded on as-grown- and oxygen plasma treated CNTs with respect to treatment time (Fig. 2(a)–(d)). As grown CNTs have some amorphous carbon layer that is deposited on the outer surface of the walls. During the short period of plasma treatment time, such as 5 min, amorphous- and edge carbons were etched away from outer layers present on the untreated nanotube surface. With increase in O2 plasma treatment time, surface of the nanotubes evidently changed. Within 10 min duration of plasma treatment time, the tube surface got damaged to a certain extent. When treated for 15 min, more surface degradation and damages occurred and is shown in Fig. 2. Generally in oxygen plasma treatment, etching by oxygen active species or the interaction of oxygen active species and nanotubes surface create defects on the carbon nanotubes surface and the degree of defect concentration is increased with plasma treatment time. We have observed that etching of nanotubes surface created more defects and damages on the nanotube surface. The HRTEM images confirm damages on the concentric curved/sidewall of carbon nanotubes in some regions of the sidewalls of carbon nanotubes.
3.2. Defect/surface damage level on CNTs: Raman studies The structural properties and quality of CNTs are investigated by Raman spectroscopy. It is used to analyze the presence of amorphous in crystalline phases corresponding to differences in graphitizations. Graphitization of pre- and post plasma treated was comparatively analyzed based on destruction level due to plasma treatment time. Raman spectra of CNTs typically consists of a graphitic or G-band from highly ordered CNT sidewalls, whereas disorder in the sidewall structure results in a D-band [8]. Fig. 3 represents the Raman spectra of CNTs and a comparison of qualitative study of as- prepared and plasma treated CNTs. They relatively have two major peaks at 1578 cm 1 as G-band associated with the E2g in plane CAC stretching vibrations, that indicates the crystalline graphitic carbon. Other D-band peak at 1347 cm 1, shows amorphous carbon and the imperfection on the CNTs. The ratio intensity of D and G band, (Id/Ig) are very important factors to distinguish the structural imperfections, i.e. chemical change of CNTs sidewalls and the degree of defects on CNTs [15]. From Fig. 4, the Id/Ig ratio was calculated to around 0.71, suggesting as-grown CNTs are crystalline in nature. Increase in the D band intensity was observed after plasma treatment. Although the CNTs were treated under different time intervals, there was an increase in Id/Ig ratio with the increase of treatment time. The value could be attributed to smaller amount of amorphous carbon and the changes taking place in the structure of CNTs. The Id/Ig ratio allow us to estimate the relative extent of structural defect using the relation L = 4.4 * (Id/Ig) 1 nm [16], where L is the graphitic crystalline size. For as grown CNTs, Id/Ig is 0.71, where the plane graphitic crystalline size is L = 6.19 nm. For plasma treated CNTs, crystalline size linearly decreased to 4.25 nm, 3.56 nm and 3.08 nm for 5 min, 10 min, and 15 min respectively.
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Fig. 2. HRTEM images of nanotube treated by oxygen plasma of a fix power and varing the plasma treatment time (a) 0 min, (b) 5 min, (c) 10 min, and (d) 15 min.
Fig. 3. Effect of O2 plasma treatment on Raman spectra of MWCNTs.
Raman results also suggested that the disorder and defect density in O2 plasma treated CNTs increased linearly. Besides, an increase in the number of polar group took place with plasma treatment duration time and this is in good agreement with the FTIR analysis.
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Fig. 4. Change in Id/Ig ratio of MWCNTs as a function of plasma treatment time.
Fig. 5. FT-IR spectra of MWCNTs treated with different oxygen plasma treatment time: (a) pristine MWCNTs, (b) 5 min, (c) 10 min, and (d) 15 min.
3.3. Identification of functional groups on MWCNTs surface: FTIR studies FTIR is used to analyze the surface composition of as grown CNTs and its changes after plasma treatment. Fig. 5(a)–(d) shows the FTIR spectra in the range of 2000–400 cm 1 of (a) pristine carbon
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Fig. 6. Photograph of water droplet with contact angle of MWCNTs treated under different processing time: (a) untreated, (b) 5 min, (c) 10 min, and (d) 15 min.
nanotubes, and plasma treated nanotubes for (b) 5 min, (c) 10 min, and (d) 15 min. duration. FTIR results indicate that sp2-hybrized C@C bond decreased significantly and oxygen containing group CAOAC and CAO bond increased over the period of processing time. Skeletal vibrations representing C@C stretching show absorbance in 1650–1430 cm 1 range [17]. Peak of the plasma treated CNTs (Fig. 5) at 990 cm 1 and 1265 cm 1 correspond to symmetric CAOAC stretching and aromatic CAO stretching respectively. After O2 plasma treatment, deterioration/damage on nanotube sidewalls linearly increased with treatment time; and p bond (C@C) in graphitic
Fig. 7. Contact angle and surface energy as a function of processing time of MWCNTs presence of oxygen plasma treatment.
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structure within the carbon nanotubes became chemically unstable due to unsaturated structure of C@C bonds, making it more reactive for the active species of O atom in O2 plasma atmosphere. Compared with the results of as grown CNTs, we presume that plasma etching of CNTs in the presence of O2 plasma treatment, may form an active CNTs lattice for functinalization of oxygen containing functional group onto CNTs surface [18]. It is clear from Fig. 5(d) that with the increase in treatment time from 0 to 15 min, the peak in the region of oxygen containing compounds gets increased due to the strengthening of CAOAC and CAO bond over the period of time. 3.4. Estimation of wettability of CNTs surface: contact angle and surface energy measurements In static contact angle measurements, de-ionized water and CNTs (as grown CNTs on silicon substrate surface) were used for measurements. The contact angle of untreated CNTs was observed to be 138(±2)° and corresponding estimated surface energy was 1.48 mJ/m2. Fig. 6 shows the photograph of water droplet taken on untreated and plasma treated CNTs where treatment time was varied from 0 to 15 min. The oxygen plasma treated samples showed a decrease in the contact angle or increase in surface energy with increasing treatment time. After oxygen plasma treatment, highly reactive oxygen radicals and ablation by energetic oxygen ions interact with CNTs, and attach variety of oxygencontaining functional groups such as CAO, and OACAO at the CNTs surface. When the treatment time increased beyond 10 min, the surface got severely deteriorated. A similar effect was observed up to 15 min. This clearly shows that the plasma parameters has definite effect on the surface modification of CNTs. Fig. 7 shows that the surface energy of all the samples increased rapidly after the plasma processing. 4. Conclusion In conclusion, oxygen plasma treatment is an effective approach for surface modification of carbon nanotubes. In this paper, reactive ions of oxygen species reacted with the surface of nanotubes which in turn changed the surface properties. Analysis of structural change of CNT sidewalls after plasma treatment was performed by scanning electron microscopy and transmission electron microscopy. Raman results confirmed the defect on the surface structure that linearly increased as a function of plasma treatment time. FTIR results indicate that sp2-hybrized C@C bond decreased significantly and oxygen containing group CAOAC and CAO bond increased over the period of time. This, in turn, made the surface area more activated. In contact angle measurements, water wettability indicates high surface energy with more sensitive activated surface area. We presume that oxygen plasma treatment of carbon nanotubes will be used to enhance CNTs surface reactivity for various potential device applications such as sensors. Acknowledgements The authors gratefully acknowledge the financial support provided by Ministry of Communication and Information Technology (MCIT), Govt. of India through its sanctioned project Ref. No. 20(14)/ 2007-NANO (Vol. III) to carry out this work. References [1] S. Iijima, Helical microtubules of graphitic carbon, Nature (London) 35 (1991) 456. [2] K.E. Geckeler, T. Premkumar, Carbon nanotubes: are they dispersed or dissolved in liquids?, Nanoscale Res Lett. 6 (2011) 136. [3] C. Bussy, M. Pinault, J. Cambedouzou, M. Julie Landry, P. Jegou, M.M. hermite, P. Launois, J. Boczkowski, S. Lanone, Critical role of surface chemical modifications induced by length shortening on multi-walled carbon nanotubes-induced toxicity, Particle Fibre Toxicol. 9 (2012) 46. [4] J.M. Simmons, B.M. Nichols, S.E. Baker, M.S. Marcus, O.M. Castellini, C.S. Lee, R.J. Hamers, M.A. Eriksson, Effect of ozone oxidation on single-walled carbon nanotubes, J. Phys. Chem. B 110 (2006) 7113. [5] N. Dementev, S. Osswald, Y. Gogotsi, E. Borguet, Purification of carbon nanotubes by dynamic oxidation in air, J. Mater. Chem. 19 (2009) 7904. [6] O. Byl, J. Liu, J.T. Yates, Etching of carbon nanotubes by ozone a surface area study, Langmuir 21 (2005) 4200.
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[7] C. Chen, A. Ogino, X. Wang, M. Nagatsu, Oxygen functionalization of multiwall carbon nanotubes by Ar/H2O plasma treatment, Diamond Relat. Mater. 20 (2011) 153. [8] W.H. Wang, B.C. Huang, L.S. Wang, D.Q. Ye, Oxidative treatment of multi-wall carbon nanotubes with oxygen dielectric barrier discharge plasma, Surf. Coat. Technol. 205 (2011) 4896. [9] H. Kwon, M. Estili, K. Takagi, T. Miyazaki, A. Kawasaki, Combination of hot extrusion and spark plasma sintering for producing carbon nanotube reinforced aluminum matrix, Composites 47 (2009) 570. [10] M. Kang, Y. Kim, H. Jeon, In-situ hydrogen and oxygen plasma purification of carbon nanotubes, J. Korean Phys. Soc. 39 (2001) 1072. [11] A. Felten, J. Ghijsen, J.-J. Pireaux, R.L. Johnson, C.M. Whelan, D. Liang, G.V. Tendeloo, C. Bittencourt, Effect of oxygen rfplasma on electronic properties of CNTs, J. Phys. D Appl. Phys. 40 (2007) 7379. [12] T. Xua, J. Yanga, J. Liub, Q. Fu, Surface modification of multi-walled carbon nanotubes by O2 plasma, Appl. Surf. Sci. 253 (2007) 8945. [13] C. Chen, A. Ogino, X. Wang, M. Nagatsu, Oxygen functionalization of multiwall carbon nanotubes by Ar/H 2O plasma treatment, Diam. Relat. Mater. 20 (2011) 153. [14] B. Zhao, L. Zhang, X. Wang, J. Yang, Surface functionalization of vertically-aligned carbon nanotube forests by radiofrequency Ar/O2 plasma, Carbon 50 (2012) 2710. [15] Z. Shang, S. Huang, X. Xu, J. Chen, Mo/MgO from avalanche-like reduction of MgMoO4 for high efficient growth of multiwalled carbon nanotubes by chemical vapor deposition, Mater. Chem. Phys. 114 (2009) 173. [16] F. Tuinstra, J.L. Koenig, Raman spectrum of graphite, J. Chem. Phys. 53 (1970) 1126. [17] J. Coates, Encyclopedia of Analytical Chemistry, Interpretation of Infrared Spectra, in: R.A. Meyers (Ed.), A Practical Approach, John Wiley & Sons Ltd., Chichester, 2000, pp. 10815–10837. [18] Z. Bin, Z. Lei, W. Xianying, Y. Junhe, Surface functionalization of vertically-aligned carbon nanotube forests by radiofrequency Ar/O2 plasma, Carbon 50 (2012) 2710.
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Surface modification of MWCNTs by O2 plasma treatment and its exposure time dependent analysis by SEM, TEM and vibrational spectroscopy Prabhash Mishra, Harsh, S.S. Islam ⇑ Nano-Sensor Research Laboratory, F/O Engineering and Technology, Jamia Millia Islamia, Jamia Nagar, New Delhi, India
a r t i c l e
i n f o
Article history: Received 10 July 2013 Received in revised form 23 September 2013 Accepted 7 October 2013 Available online 14 October 2013 Keywords: Carbon nanotubes Electron microscopy Raman spectroscopy FTIR Surface energy
a b s t r a c t In this paper, we report the efficient surface modification of multiwall carbon nanotubes (MWCNTs) by oxygen (O2) plasma treatment while in situ monitoring of its sidewall surface degradation as a function of plasma exposure time. The nanostructure and surface morphology of pre- and post plasma treated CNTs were investigated by using Field Emission Electron microscopy (FESEM), High resolution Transmission Electron microscopy (HRTEM), and Raman spectroscopy (RS). A systematic study was carried out by FTIR and surface energy measurements on the analysis of functional groups attached on the CNTs surface leading to the transformation of surface nature from hydrophobic to hydrophilic with the increase of plasma treatment time. The results confirmed that O2 plasma treatment time greatly enhanced oxygen content on CNTs surface, and modified structural properties while inducing more surface damages and defects. Surface defects are sometimes desirable for sensing purpose and it is shown that the extent of defect generation can be easily tuned with the plasma exposure time without damaging the integrity of the nanotubes pattern. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction After the discovery of carbon nanotubes in 1991 [1], the researchers have been striving for better understanding of their basic properties and applications in various fields of science and ⇑ Corresponding author. Tel.: +91 11 26980532; fax: +91 11 26988846. E-mail address: safi[email protected] (S.S. Islam). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.10.010
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technology. But the major hurdles for exploring the possibility of CNTs commercial applications are many, such as – no control on its chirality – a decisive factor whether the CNT will be metallic or semiconductor, its insolubility nearly in all solvents, growth related impurities and defects, lump/ bundle formation leading to poor dispersion [2]. To remove all these drawbacks researchers have taken various approaches, for example, tailor the physical and chemical properties by covalently attach chemical groups through wet treatment oxidation [3,4], air oxygen [5], ozone oxidation [6], and plasma oxidation [7,8] leading to formation of hydroxyl and carboxyl groups on the surface of the nanotubes. But all these treatments involve very harsh conditions and create severe damages on the sidewalls, open the nanotube tips, chop the nanotubes into small pieces, and also change the orientation and alignment of CNTs. Interestingly, these treatments are surface specific so that the bulk properties are preserved. In comparison, plasma treatment has the advantages of shorter reaction time, non-polluting processing, and providing a wide range of different functional groups depending on plasma parameters. It is a valuable technique to engineer surface properties without alteration of the bulk compositions [9]. Over the years plasma treatment has been employed by many researchers on CNTs. Kang et al. [10] has demonstrated a simple and efficient in situ H2 and O2 plasma purification process for removal of the residual carbonaceous particles and metallic impurities without significant structural damage to individual CNTs. Felten et al. [11] investigated the impact on the electronic structure of CNTs upon using an O2 plasma to graft oxygen containing surface groups and showed that the plasma treatment improves the uniformity of the distribution of surface defects. Another study on surface modification of MWCNTs by O2 plasma was carried out by Tao Xu et al. [12] for large scale purification of MWCNTs powder where the authors claimed successful achievement of damage free complete purification within 30 min. C. Chaglun et al. [13] has carried out grafting oxygen containing functional groups by microwave-excited Ar/H2O surface wave plasma and estimated the effect of gas partial pressure, treatment time, and microwave power on the introduction of oxygen-containing group onto the surfaces of MWCNTs. The effect of plasma parameters on surface morphology, atomic composition and structure of vertically aligned carbon nanotube forests by RF Ar/O2 plasma was recently reported by Bin Zhao et al. [14]. the authors demonstrated the post treatment effect on the preservation of vertical alignment and change in bonding structure due to surface functionalization by CAO and OAC@O groups. In most of the studies as mentioned, it was found that the authors emphasized the potentiality of plasma treatment as a nonpolluting method either for purification or grafting functional groups in CNTs (measured by FTIR), surface damage studies by SEM, TEM and Raman, and quantification of elements attached on nanotube surface by XPS measurements. There is hardly any report available done good analysis how the surface damage, surface functionalization and change in surface nature are correlated with the plasma processing time. In this work, we report a systematic study to understand the changes that results from the surface modification strategy. Accurate characterization of the surface chemistry of MWCNTs structure is done before and after HF (high frequency) O2 plasma treatment with respect to treatment time. FTIR studies has been carried out to estimate the surface functional group introduced on nanotube surface by plasma treatment and simultaneously the wettability properties of the modified CNTs are carried out by surface energy measurements. Both FTIR and surface energy measurement are reported in this paper with the duration of O2 plasma exposure.
2. Experimental Prior to CNTs growth, iron (Fe) layer with thickness of 3 nm were sputtered on N-type Si (100) with 500 nm oxide layer. The CNTs were grown on SiO2/Si substrate by using self design thermal CVD with mixture of H2 + Ar and C2H2 at flow rate of 30 sccm and 60 sccm respectively. C2H2 was used as a source gas and mixture of Ar + H2 used as a dilution. An electric feed through was used for rapid increase of temperature. CNTs were grown at 800 °C for 3 min and total pressure of the chamber was kept constant at 10 Torr. After growth of CNTs, Ar was flowed continuously in the chamber till the chamber attained room temperature.
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The microstructure and surface morphology of as-grown and post plasma treated CNTs were examined by Field Emission Scanning Electron Microscope (FESEM, FEI Nova Nano SEM 450) and Transmission electron microscope (TEM, FEI Tecani F20). Structural qualities and degree of defects of CNTs were analyzed by micro-Raman spectrometer (LabRAM HR 800, Horiba JY) fitted with a peltier cooled CCD detector and an Olympus BX-41 microscope. The excitation of the samples was performed with an aircooled Ar+-laser (Spectra Physics) tuned at 488 nm. Measurements were carried out in the back scattering geometry using a 50 LWD microscope objective. The beam was focused at a spot size of 1.19 lm and the power density (lW/cm2) was kept low to avoid any excessive heating of the probe region. FTIR spectra were recorded using Vertex V70 spectrophotometer (BRUKER) in the range of 400–2000 cm 1. Oxygen plasma treatment of CNTs was done in a plasma system (Diener electronic GmbH Germany) equipped with a HF generator (High frequency Generator) under 80 W power and 40 kHz frequency. Oxygen gas at 25 sccm flow rate was introduced into the chamber. The treatment time of the sample was varied from 0 min to 15 min. The working pressure was adjusted at 0.8 mbar during the plasma processing. The water contact angle and the surface energy of pre- and post plasma treated CNTs were measured with Phoenix 300 goniometer (Surface Electro Optics, Korea) equipped with CCD camera. Apparatus was also used in conjunction with specialized software for determination of surface energy. The Girifalco–Good–Flowkes–Young method (for 1 contact angle of 1 fluid on 1 solid) was used to calculate the average surface energy proving at three different spots.
Fig. 1. SEM images of the oxygen plasma treated MWCNTs (a) untreated, (b) after 5 min, (c) after 10 min, and (d) after 15 min.
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3. Results and discussion 3.1. Morphological surface analysis of MWCNTs: SEM and TEM studies The morphology of MWCNTs was characterized by FESEM. Fig. 1(a) shows the surface morphology of as-grown carbon nanotubes film before and after exposing to the oxygen plasma and the differences were observed on the surface structure at different intervals of processing time. The untreated CNTs present a typical morphology with random network structure as shown in Fig. 1(a). Plasma treatment on CNTs was carried out at fixed O2 concentration and exposure time was varied from 0 to 15 min keeping the plasma power at 80 W. Plasma treated nanotube structure shows significant change on their surface morphology. Fig. 1(b) shows some damages on the surface of carbon nanotubes for 5 min plasma treatment. We observed an increase in defects/damages on the carbon nanotube surface for 10 min and 15 min plasma treatment time respectively and these are shown in Figs. 1(c) and 2(d). During the plasma treatment, more charge ions and electron in plasma exhibited the effect on surface morphology physically and chemically. As a whole, structural damage (e.g. sidewall damage) was caused by plasma treatment and surface modifications were induced and the degree of damage got enhanced with plasma processing time which is evident from TEM measurements. Fig. 2 shows HRTEM images recorded on as-grown- and oxygen plasma treated CNTs with respect to treatment time (Fig. 2(a)–(d)). As grown CNTs have some amorphous carbon layer that is deposited on the outer surface of the walls. During the short period of plasma treatment time, such as 5 min, amorphous- and edge carbons were etched away from outer layers present on the untreated nanotube surface. With increase in O2 plasma treatment time, surface of the nanotubes evidently changed. Within 10 min duration of plasma treatment time, the tube surface got damaged to a certain extent. When treated for 15 min, more surface degradation and damages occurred and is shown in Fig. 2. Generally in oxygen plasma treatment, etching by oxygen active species or the interaction of oxygen active species and nanotubes surface create defects on the carbon nanotubes surface and the degree of defect concentration is increased with plasma treatment time. We have observed that etching of nanotubes surface created more defects and damages on the nanotube surface. The HRTEM images confirm damages on the concentric curved/sidewall of carbon nanotubes in some regions of the sidewalls of carbon nanotubes.
3.2. Defect/surface damage level on CNTs: Raman studies The structural properties and quality of CNTs are investigated by Raman spectroscopy. It is used to analyze the presence of amorphous in crystalline phases corresponding to differences in graphitizations. Graphitization of pre- and post plasma treated was comparatively analyzed based on destruction level due to plasma treatment time. Raman spectra of CNTs typically consists of a graphitic or G-band from highly ordered CNT sidewalls, whereas disorder in the sidewall structure results in a D-band [8]. Fig. 3 represents the Raman spectra of CNTs and a comparison of qualitative study of as- prepared and plasma treated CNTs. They relatively have two major peaks at 1578 cm 1 as G-band associated with the E2g in plane CAC stretching vibrations, that indicates the crystalline graphitic carbon. Other D-band peak at 1347 cm 1, shows amorphous carbon and the imperfection on the CNTs. The ratio intensity of D and G band, (Id/Ig) are very important factors to distinguish the structural imperfections, i.e. chemical change of CNTs sidewalls and the degree of defects on CNTs [15]. From Fig. 4, the Id/Ig ratio was calculated to around 0.71, suggesting as-grown CNTs are crystalline in nature. Increase in the D band intensity was observed after plasma treatment. Although the CNTs were treated under different time intervals, there was an increase in Id/Ig ratio with the increase of treatment time. The value could be attributed to smaller amount of amorphous carbon and the changes taking place in the structure of CNTs. The Id/Ig ratio allow us to estimate the relative extent of structural defect using the relation L = 4.4 * (Id/Ig) 1 nm [16], where L is the graphitic crystalline size. For as grown CNTs, Id/Ig is 0.71, where the plane graphitic crystalline size is L = 6.19 nm. For plasma treated CNTs, crystalline size linearly decreased to 4.25 nm, 3.56 nm and 3.08 nm for 5 min, 10 min, and 15 min respectively.
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Fig. 2. HRTEM images of nanotube treated by oxygen plasma of a fix power and varing the plasma treatment time (a) 0 min, (b) 5 min, (c) 10 min, and (d) 15 min.
Fig. 3. Effect of O2 plasma treatment on Raman spectra of MWCNTs.
Raman results also suggested that the disorder and defect density in O2 plasma treated CNTs increased linearly. Besides, an increase in the number of polar group took place with plasma treatment duration time and this is in good agreement with the FTIR analysis.
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Fig. 4. Change in Id/Ig ratio of MWCNTs as a function of plasma treatment time.
Fig. 5. FT-IR spectra of MWCNTs treated with different oxygen plasma treatment time: (a) pristine MWCNTs, (b) 5 min, (c) 10 min, and (d) 15 min.
3.3. Identification of functional groups on MWCNTs surface: FTIR studies FTIR is used to analyze the surface composition of as grown CNTs and its changes after plasma treatment. Fig. 5(a)–(d) shows the FTIR spectra in the range of 2000–400 cm 1 of (a) pristine carbon
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Fig. 6. Photograph of water droplet with contact angle of MWCNTs treated under different processing time: (a) untreated, (b) 5 min, (c) 10 min, and (d) 15 min.
nanotubes, and plasma treated nanotubes for (b) 5 min, (c) 10 min, and (d) 15 min. duration. FTIR results indicate that sp2-hybrized C@C bond decreased significantly and oxygen containing group CAOAC and CAO bond increased over the period of processing time. Skeletal vibrations representing C@C stretching show absorbance in 1650–1430 cm 1 range [17]. Peak of the plasma treated CNTs (Fig. 5) at 990 cm 1 and 1265 cm 1 correspond to symmetric CAOAC stretching and aromatic CAO stretching respectively. After O2 plasma treatment, deterioration/damage on nanotube sidewalls linearly increased with treatment time; and p bond (C@C) in graphitic
Fig. 7. Contact angle and surface energy as a function of processing time of MWCNTs presence of oxygen plasma treatment.
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structure within the carbon nanotubes became chemically unstable due to unsaturated structure of C@C bonds, making it more reactive for the active species of O atom in O2 plasma atmosphere. Compared with the results of as grown CNTs, we presume that plasma etching of CNTs in the presence of O2 plasma treatment, may form an active CNTs lattice for functinalization of oxygen containing functional group onto CNTs surface [18]. It is clear from Fig. 5(d) that with the increase in treatment time from 0 to 15 min, the peak in the region of oxygen containing compounds gets increased due to the strengthening of CAOAC and CAO bond over the period of time. 3.4. Estimation of wettability of CNTs surface: contact angle and surface energy measurements In static contact angle measurements, de-ionized water and CNTs (as grown CNTs on silicon substrate surface) were used for measurements. The contact angle of untreated CNTs was observed to be 138(±2)° and corresponding estimated surface energy was 1.48 mJ/m2. Fig. 6 shows the photograph of water droplet taken on untreated and plasma treated CNTs where treatment time was varied from 0 to 15 min. The oxygen plasma treated samples showed a decrease in the contact angle or increase in surface energy with increasing treatment time. After oxygen plasma treatment, highly reactive oxygen radicals and ablation by energetic oxygen ions interact with CNTs, and attach variety of oxygencontaining functional groups such as CAO, and OACAO at the CNTs surface. When the treatment time increased beyond 10 min, the surface got severely deteriorated. A similar effect was observed up to 15 min. This clearly shows that the plasma parameters has definite effect on the surface modification of CNTs. Fig. 7 shows that the surface energy of all the samples increased rapidly after the plasma processing. 4. Conclusion In conclusion, oxygen plasma treatment is an effective approach for surface modification of carbon nanotubes. In this paper, reactive ions of oxygen species reacted with the surface of nanotubes which in turn changed the surface properties. Analysis of structural change of CNT sidewalls after plasma treatment was performed by scanning electron microscopy and transmission electron microscopy. Raman results confirmed the defect on the surface structure that linearly increased as a function of plasma treatment time. FTIR results indicate that sp2-hybrized C@C bond decreased significantly and oxygen containing group CAOAC and CAO bond increased over the period of time. This, in turn, made the surface area more activated. In contact angle measurements, water wettability indicates high surface energy with more sensitive activated surface area. We presume that oxygen plasma treatment of carbon nanotubes will be used to enhance CNTs surface reactivity for various potential device applications such as sensors. Acknowledgements The authors gratefully acknowledge the financial support provided by Ministry of Communication and Information Technology (MCIT), Govt. of India through its sanctioned project Ref. No. 20(14)/ 2007-NANO (Vol. III) to carry out this work. References [1] S. Iijima, Helical microtubules of graphitic carbon, Nature (London) 35 (1991) 456. [2] K.E. Geckeler, T. Premkumar, Carbon nanotubes: are they dispersed or dissolved in liquids?, Nanoscale Res Lett. 6 (2011) 136. [3] C. Bussy, M. Pinault, J. Cambedouzou, M. Julie Landry, P. Jegou, M.M. hermite, P. Launois, J. Boczkowski, S. Lanone, Critical role of surface chemical modifications induced by length shortening on multi-walled carbon nanotubes-induced toxicity, Particle Fibre Toxicol. 9 (2012) 46. [4] J.M. Simmons, B.M. Nichols, S.E. Baker, M.S. Marcus, O.M. Castellini, C.S. Lee, R.J. Hamers, M.A. Eriksson, Effect of ozone oxidation on single-walled carbon nanotubes, J. Phys. Chem. B 110 (2006) 7113. [5] N. Dementev, S. Osswald, Y. Gogotsi, E. Borguet, Purification of carbon nanotubes by dynamic oxidation in air, J. Mater. Chem. 19 (2009) 7904. [6] O. Byl, J. Liu, J.T. Yates, Etching of carbon nanotubes by ozone a surface area study, Langmuir 21 (2005) 4200.
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