Growth Mechanism of Carbon Nanotubes Deposited by Electrochemical Technique

Indian Journal of Pure & Applied Physics Vol. 43, October 2005, pp. 765-771

Growth mechanism of carbon nanotubes deposited by electrochemical technique

S K Mandal, S Hussain & A K Pal* Department of Instrumentation Science, Jadavpur University, Calcutta 700 032 Received 25 July 2005; accepted 25 August 2005

Understanding the nucleation and growth of carbon nanotubes at room temperature by a novel electrochemical process using acetonitrile as the electrolyte is investigated here. Microscopic insight into the nucleation process clearly reveals coalescence of amorphous carbon clusters in a branched network instigated mainly by dangling-bond induced relaxation and subsequent rearrangement of carbon atoms through a quasi-liquid state leading to the formation of multi-walled carbon nanotubes. Such a direct conversion of amorphous carbon into nanotubes is not abrupt, rather driven by a slow thermodynamical process that is inherent to the electrochemical process. Keywords: Carbon nanotubes, Growth, Electrodeposition IPC Code: C25,B82B

1 Introduction In contrast to the conventional high temperature The fascinating structural and electronic properties (~2000-4000°C) catalytic growth, we have recently of carbon in its various forms like amorphous carbon, reported an off-the-shelf approach to synthesis carbon diamond like carbon, graphite, diamond etc. led to a nanotubes by a simple electrochemical process29. A flurry of research activities over the years1-6. The key aspect of this novel approach was the growth of versatility of carbon in various phases stems from its carbon nanotubes at room temperature without the ability to rehybridize between sp, sp2 and sp3 bonds. presence of any metal catalysts and could be directly Amongst its various forms, the most fascinating one is deposited onto suitable substrates. The process was the tubule one, called carbon nanotube, which is not abrupt one, rather involved a number of phase primarily thought of as a rolled-on graphene sheet transformations of carbon with electrodeposition time with sp2 bonded carbon atoms. Since the pioneering leading to the growth of carbon nanotubes which work done by Iijima7, intense research activities were generated much curiosity. In this paper, an attempt is witnessed so far to understand the growth, structural made to understand this unique growth process of the and electronic aspects of carbon nanotubes having a tubes through microscopic and spectroscopic variety of applications8-15. Generally, the synthesis of investigations. It was observed that the initial these tubes involved high temperature catalytic nucleation layers consisted of amorphous carbon that growth of carbon (graphite) achieved through various subsequently converted into tubular form through a processes like carbon arc discharge11,16,17, laser quasi-melting process. In the intermediate steps, ablation18,19, CVDs20,22 etc. Depending on the growth various carbon structures like network of carbon conditions, the tubes would evolve as single-walled, flakes, aggregated carbon particles in cage like form multi-walled, bundles with distribution in diameters, were also found to occur. We describe here different chiralities, defects and kinks which in turn systematically how the coordination of carbon atoms controlled their electronic features in metallic as well results into sp2 bonded carbon network forming as in semiconducting phase. Despite a large number carbon tubes. The local fluctuations in thermal of theoretical and experimental investigations10,23-28 on environment due to the dissociation of C-H bonds the microscopic insight to the growth of single-walled might possibly have a strong consequence in as well as multi-walled carbon nanotubes, the issue is determining the growth of various carbon structures. still not clearly understood and wide open for critical investigations. Nevertheless, there is a large 2 Experimental Details consensus between theoretical models and In the electrochemical approach29, acetonitrile experimental results. (CH3CN, 1% in volume) in distilled water was used as ______the electrolyte. The deposition took place at room *E mail: [email protected] temperature under the application of a significantly 766 INDIAN J PURE & APPL PHYS, VOL 43, OCTOBER 2005

lower voltage of 16 V between the substrate and [Fig. 1(a-d)]. The microstructures obtained at regular counter electrode (graphite). Both tin-oxide-coated intervals are quite revealing and throw a clear insight glass and Si (001) were used as the substrate placed into the growth process of the carbon nanotubes. The on the working electrode. The post-deposition growth initial nucleation layer consisted of carbon structures of the films on the substrate was monitored by uniformly distributed in granular form throughout the electron microscopy at an interval of one hour till the films [Fig. 1(a)] obtained at t=1 h. This layer formation of carbon nanotube after a deposition primarily consisted of carbon clusters in its period of four hours. The films obtained at amorphous phase as clearly evidenced by the FTIR intermediate duration were found to have carbon in its spectrum [Fig. 2(a)]. The SEM image for the film amorphous phase (a-C) which subsequently converted [Fig. 1(b)] obtained after 2 hour (t=2 h) depicted that into carbon nanotube structures. We discuss this some of the clusters tended to coalesce in a branched systematic growth of carbon nanotube from initial network. The network structure became prominent amorphous carbon layers at room temperature based with the increase in deposition time t [Fig. 1(c) on electron microscopic investigations. The in situ corresponding to t=3 h] and finally converted into growth and nucleation of carbon nanotube films were carbon nanotubes interconnected in a web-like form characterized by Raman (Horiba U-1000), Fourier as clearly indicated in Fig. 1(d) (t=4 h). XPS analysis Transform Infrared (FTIR) Spectroscopy (Nicolet, also clearly indicated the presence of carbon only, no MAGNA-IR-750), X-Ray Photoelectron trace of other elements was found (Fig. 2). The Spectroscopy (XPS) (Perkin-Elmer, PHI-1257), detailed characterizations of the carbon nanotubes Scanning Electron Microscope (SEM, Hitachi S- obtained in the electrodeposition technique were 2300) and Transmission Electron Microscope (TEM, presented in our earlier paper29. JEOL 2010F, UHR) respectively. The conversion of a-C into carbon nanotube could also be clearly evidenced by the FTIR spectra. The 3 Results and Discussion FTIR spectrum given in [Fig. 3(a)] (t=1 h) clearly Scanning electron microscopic (SEM) images of indicated the formation of amorphous carbon with carbon structures on Si obtained at intervals of one characteristic peaks between 3100 to 2700 cm−1 and hour since the deposition started (t=0) are presented in 1470 cm−1. The peaks observed at high frequency

Fig. 1—SEM microstructures of the films on Si deposited for (a) t=1 h (b) t=2 h (c) t=3 h and (d) t=4 h MANDAL et al.: GROWTH MECHANISM OF CARBON NANOTUBES 767

Fig. 2—XPS spectrum obtained for films on Si after a period of t= 4 h indicating the presence of carbon only

(2931-3100 cm−1) were typical of sp2 hybridized carbon and were more intense than the peak at 2870 cm−1 corresponding to sp3 hybridized carbon. With the progress in deposition time, the peaks at the low frequency range becomes prominent corresponding to the vibrational modes of carbon nanotubes. The FTIR spectrum obtained for film after a deposition of t=4 h [Fig. 3(b)] indicated main features centered around 1060, 1411 and 1583 cm−1 in the lower frequency range. The broad peak at ~ 1060 cm−1 is a 30 characteristic to Si-O-Si stretching vibrations . The peaks centered at 1411 cm−1 and 1583 cm−1 along Fig. 3—FTIR spectra obtained for films on Si after a period −1 −1 of (a) t= 1 h and (b) t= 4 h with other satellite peaks at 1475 cm , 1478 cm , 1606 cm−1 and 1630 cm−1 respectively may be glass differed in morphology to some extent from that attributed to the vibrational modes for carbon on silicon substrate. The whole process of conversion nanotubes31. Small peaks at 2854 and 2918 cm−1 in was not abrupt, rather involved a very slow the higher frequency region of the spectra correspond thermodynamically or chemically motivated kinetics to the C-H stretching vibrations of chemisorbed leading to the various carbon microstructures and hydrogen and are quite weak in comparison to carbon finally to carbon nanotubes. The observed growth of nanotubes. The broad band centered at ~ 3400 cm−1 carbon nanotubes from amorphous carbon clusters could be attributed to the presence of -OH groups and would raise some interesting issues on the aspects of molecular water in the films. carbon structures and its phase transitions into various The growth was also critically found to be allotropes: (i) what is the source of carbon here? dependent on the substrate used and to ascertain the (ii) How the amorphous carbon phase is converted above, the films were deposited both on Si and SnO2 into tubule structure even at room temperature? and coated glass substrates retaining the other (iii) How the tubes nucleate even without the presence experimental conditions invariant. We present the of any metal catalysts?. SEM images of the growth of carbon structures on In this case, the growth of carbon nanotubes SnO2 coated glass substrate obtained at a regular occurred solely from the amorphous carbon clusters interval of 1 hour as given in [Fig. 4(a-d)]. Although as the electrodeposition progressed. This clearly the initial nucleation layer is amorphous carbon for suggested a different growth mechanism. Even though films deposited on Si and SiO2 substrates, the the melting point of any type of carbon phase is quite intermediate structures of the coatings on SnO2 coated high, we presumed that a quasi-melting process of the 768 INDIAN J PURE & APPL PHYS, VOL 43, OCTOBER 2005

Fig. 4—SEM microstructures of the films on tin-oxide coated glass deposited for (a) t=1 h (b) t=2 h (c) t=3 h and (d) t=4 h carbon clusters might lead to the formation of the revealed the formation of the multi-walled nature of tubes. This presumption is supported here by in situ the tubes with typical diameters 100-250 nm after 4h microscopic investigations during the growth of of deposition. Thus, the microstructural information carbon nanotubes. The size of the clusters and their obtained from the SEM studies (Figs 1 and 4) are in distribution obviously played the critical role in conformity with those depicted from the TEM studies determining the diameter and its distribution, (Fig. 5). uniformity over the length of the carbon nanotubes In conventional approach, carbon nanotubes are etc. synthesized by enriching the graphite source with To unveil the intriguing kinetics of the growth of single metal or mixture of transition metal catalysts carbon nanotubes from amorphous carbon clusters, and the growth takes place from amorphous carbon of HRTEM images of the films grown at very early condensed phase or small carbon clusters of gas stages of the nucleation process came to our phase. It is conceived that the single-walled nanotubes advantage. In Fig. 5, early nucleation phases are are produced in the presence of transition metal illustrated with the TEM images of the films obtained catalysts, however, not required for producing multi- after t=1 h [Fig. 5(a)], t=1.5 h [Fig. 5(b)] and t=2 h wall nanotubes. The exact role of catalysts although [Fig. 5(c)] respectively. Figure 5(a) clearly indicated had been discussed by various researchers, still the dispersed carbon clusters of typical size ~ 10 nm remains controversial. The issue of open- or close- distributed over the entire film. Quite interestingly, as ended growth of nanotubes had been investigated by the electrodeposition progressed, the coalescence of different theoretical models32-34. The longitudinal the carbon clusters in branched network is clearly growth of tubes by addition of C2 dimers or C3 trimers visible in [Fig. 5(b)]. Expansion of this network with was largely influenced by pentagon or heptagon- time is further evidenced by the microstructure pentagon defects in controlling termination of tube obtained after 2 h of deposition as shown in growth and also plays a crucial role in determining the [Fig. 5(c)]. A more clear insight of the formed size and distribution of the tubes. network is also presented in [Fig. 5(d)]. Termination In electrodeposition technique described here, the + of the branch at any instant of time basically controls source of carbon is obviously CH3 radicals the length of nanotubes and the extension of the dissociated from acetonitrile in the electrochemical network. High resolution TEM images (Fig. 6) further process giving rise to the formation of amorphous MANDAL et al.: GROWTH MECHANISM OF CARBON NANOTUBES 769

Fig. 5—TEM microstructures of the films in the electrochemical process deposited for (a) t=1 h (b) t=1.5 h (c) t= 2 h and (d) t=3 h

Fig. 6—HRTEM image of (a) a portion of multi-walled carbon nanotube (b) showing parallel fringes of the tube carbon in the form of very small clusters with induced by surface diffusion, dangling-bond driven unsaturated dangling bonds. The dehydrogenation of relaxation and topological defects etc. could relax to 2 carbon from CH3 radicals could be predicted through an undistorted sp configuration even at low the following reactions35,36: temperature37 as shown schematically in Fig. 7. Reconfiguration of two distorted sp3 sites into a π- + - + − bonded pair was most likely to occur giving rise to the CH3CN+H2O = CH3 + CN + H + OH + + formation of graphite networks. CH + CH +2e → CH =CH +H 3 3 2 2 2 The hydrogen may have an important role to nCH2=CH2 → [−CH2CH2 −]n facilitate the growth of nanotubes here. Transport of + [−CH2CH2 −]n → Cn+2nH2 H ions to the cathode played an important role in the formation of initial layer of a-C phase during the The carbon atoms could form chains of highly growth. The presence of hydrogen at the cathode distorted carbon atoms with four-fold coordination would facilitate the separation of chemically different (sp3). This highly distorted four-fold site possibly bonded carbon regions minimizing the internal stress. 770 INDIAN J PURE & APPL PHYS, VOL 43, OCTOBER 2005

amorphous carbon (2273 K), due to the low free energy, 2-D graphitic layers could be formed38. Conversion of hydrogen-rich carbon solid into multi- walled nanotubes or amorphous fiber-like nanostructures is previously investigated by Sarangi et al39. In our case, the formation of multi-walled tubes is believed to be instigated by the quasi-melting of the a-C through a phase transformation into sp2 bonded hexagonal networks. The graphitization initiated at the external surface would move towards the center forming parallel layers of graphitic carbons. The transformation here would lead to multi-walled tubes, rather than carbon fibers as clearly evidenced by HRTEM images (Fig. 6). As the density of highly graphitized carbon (~ 2.2 g-cm−3) is higher than the amorphous carbon (~1.5 g-cm−3), the core obviously becomes hollow and the hence the formation of multi- walled tubes rather than carbon fibers. The required thermal energy to stimulate the quasi-melting process is obviously inherent in the growth process itself and not externally supplied (because of the nucleation at room temperature). The possible source of thermal energy is the dissociation C-H bonds (~400 3 Fig. 7—Schematic view of the conversion of distorted sp sites kJoule/mol) and H+ ion bombardment, giving rise to into a sp2 bonded sites (ref. 37) the quasi-liquid phase of carbon40.

The fluctuation in the local thermal environment These hydrogen atoms would possibly segregate played the key role for the growth of carbon without substantial energy barrier facilitating nanotubes and various nanostructures in the adsorbed carbon atoms to be inserted into the surface electrochemical process described here. The dimer bond. Thus, following segregation of hydrogen, consequence of the fluctuation in local temperature graphene sheet might grow quasi-epitaxially leading during the growth of multi-walled carbon nanotubes towards the formation of multi-wall carbon in the conventional high temperature process was nanotubes. The most plausible reactive sites triggering theoretically considered by Crespi et al.41 and had a the coalescence of the a-C clusters could be ascribed large consequence on the formation of cap at the apex to the elimination of unsaturated dangling bonds of the tubes. From the viewpoint of the kinetics of the leading to the rearrangement of atoms and is supposed tube formation, it can be inferred that the formation of to be the main driving force to induce coalescence of various nanostructures in the electrochemical process clusters. Because of the variation in cluster size and described here are governed by the thermodynamical their reactive sites, the coalescence of the clusters and state of the carbon clusters inherent to the growth the effective cluster-cluster interaction would give process itself, interpretation of which is not very rise to a variety of carbon nanostructures including straightforward. Nevertheless, direct conversion of tubes. The substrate-induced strain might also have a-C into various carbon nanostructures including played an important role in rearrangement of clusters carbon nanotubes at room temperature is quite and their bonding environment and subsequent remarkable in contrast to the conventional high growth. We found that under similar electrochemical temperature approach and probably throws new light conditions, the rearrangement of a-C clusters may into the ongoing research of carbon structures. lead to the growth of different tube morphologies depending on the substrate. The growth of various carbon structures including tubes from a-C clusters 4 Conclusion was previously reported by various authors utilizing In summary, an attempt to understand the growth different PVD and CVD techniques. Even at mechanism of carbon nanotubes obtained through a temperature quite lower than the crystallization of novel electrochemical process at room temperature is MANDAL et al.: GROWTH MECHANISM OF CARBON NANOTUBES 771

presented here. The microscopic investigations at 14 Kazaoui S, Minami N, Jacquemin R, Kataura H & Achiba Y, various stages of the growth clearly revealed a direct Phys Rev B, 60 (1999) 13339. 15 Postma H W Ch, Jonge M de, Yao Z & Dekker C, Phys Rev conversion of amorphous carbon into nanotubes and B, 62 (2000) R10633. various nanostructures, which was solely instigated by 16 Iijima S& Ichihashi T, Nature, 363 (1993) 603. the coalescence and subsequent quasi-melting of the 17 Bethune D S, Klang C H, Vries M S de, Gorman G, et al., carbon clusters. The quasi-liquid phase of the carbon Nature, 363 (1993) 605. 18 Thess A, Lee R, Nikolaev P, Dai H, et al., Science, 273 clusters and kinetics of the consequent growth of (1996) 483. carbon nanotubes even at room temperature revealed 19 Cheng H M, Li F, Su G, Pan H Y, et al., Appl Phys Lett, 72 a new phenomenon from the thermodynamical point (1998) 3282. of view. The experimental observations and 20 Jong J, Cassell A M & Dai H, Chem Phys Lett, 292 (1998) inferences would be more robust by theoretical 567. 21 Ge M & Sattler K, Science, 260 (1993) 515. understanding of the kinetics of the growth process, 22 Yudasaka M, Tasaka K, Kikuchi R, Ohki Y & Yoshimura S, and hence to predict and control the microstructures J Appl Phys, 81 (1997) 7623. to be obtained in the electrochemical technique. 23 Charlier J C & Iijima S, Topics Appl Phys, 80 (2001) 55 and references therein. Acknowledgement 24 Gavillet J, Loiseau A, Journet C, Willaime F, et al., Phys Rev The authors wish to thank the Defence Research Lett, 87 (2001) 275504. 25 Kiang C –H & Goddard III W W, Phys Rev Lett, 76 (1996) and Development Organization (DRDO), Ministry of 2515. Defence, Government of India, for sanctioning 26 Maiti A, Brabec C J & Bernholc J, Phys Rev B, 55 (1997) financial assistance for executing this programme. R6097. Thanks are due to Prof. P V Satyam, Institute of 27 Charlier J -C, De Vita A, Blasé X & Car R, Science, 275 Physics, Bhubaneswar, India for his assistance in (1997) 646. 28 Fan X, Buczko R, Puretzky A A, Geohegan D B, et al., Phys recording the HRTEM micrographs. Rev Lett, 90 (2003) 145501. 29 Pal A K, Roy R K, Mandal S K, Gupta S & Deb B, Thin References Solid Films, 2004 (in press). 1 Shimakawa K & Miyake K, Phys Rev Lett, 61 (1988) 994. 30 Socrates G, Infrared Characteristics Group Frequencies: 2 Fink J, Muller-Heinzer-ling T, Pfluger J, Scheerer B, et al., Tables and Charts (John Wiley Chichester 1994). Phys Rev B, 30 (1984) 4713. 31 Chen J, Hamon M A, Hu H, Chen Y, et al., Science, 282 3 Galli G, Martin R M, Car R & Parrinello M, Phys Rev Lett, (1994) 95. 62 (1989) 555. 32 Smalley R E, Mater Sci Eng B, 19 (1993) 1. 4 Geis M G, Proc IEEE, 79 (1991) 669. 33 Saito R, Fujita M, Dresselhaus G & Dresselhaus M S, Mater 5 Stephan U, Frauenheim Th, Blaudeck P & Jungnickel G, Sci Eng B, 19 (1993) 185. Phys Rev B, 49 (1994) 1489. 34 Buongiorno Nardelli M, Brabec C, Maiti A, Toland C, 6 Chakrabarti K, Chakrabarti R, Chaudhuri S & Pal A K, Dia Bernholc J, Phys Rev Lett, 80 (1998) 313. Rel Mater, 7 (1998) 1227. 35 Fu Q, Jiu J -T, Wang H, Cao C –B & Zhu H S, Chem Phys 7 Iijima S, Nature, 354 (1991) 56. Lett, 301 (1999) 87. 8 Saito S, Science, 278 (1997) 77. 36 Yan X B, Xu T, Yang S R & Xue Q J, J Phys D: Appl Phys, 9 Bachtold A, Hadley P, Nakanishi T & Dekker C, Science, 37 (2004) 2416. 294 (2001) 1317. 37 Tamor M A, Applications of Diamond Films and Related 10 Kwon Y -K, Lee Y H, Kim S -G, Jund P, et al., Phys Rev Materials (Eds Feldman A, Tzeng Y, Yarbrough W A, Lett, 79 (1997) 2065. Yoshikawa M, Murakawa M, NIST, Gaithensburg, p-691, 11 Journet C, Maser W K, Bernier P, Loiseau A, et al., Nature, 1995). 388 (1997) 756. 38 Abrahamson J, Carbon, 19 (1973) 217. 12 Ajayan P M, Stephan O, Colliex C & Trauth D, Science, 265 39 Sarangi D, Godon C, Granier A, Moalic R, et al., Appl Phys (1994) 1212. A, 73 (2001) 765. 13 Kazaoui S, Mandal S K & Minami N, AIP Conf Proc, 633 40 Murooka Y & Hearne K R, J Appl Phys, 43 (1972) 2656. (2002) 318. 41 Crespi V H, Phys Rev Lett, 82 (1999) 2908.