Evolution of single-wall carbon nanotubes during hydrothermal treatment

Solid State Ionics 151 (2002) 205 – 211 www.elsevier.com/locate/ssi

Evolution of single-wall carbon nanotubes during hydrothermal treatment Jose M. Calderon-Moreno a, S. Srikanta Swamy b,1, Masahiro Yoshimura a,* a

Materials and Structures Laboratory, Center of Materials Design, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8503, Japan b Research Institute for Solvothermal Technology, Takamatsu, Japan Received 5 June 2001; accepted 22 December 2001

Abstract The evolution of single-wall carbon nanotubes (SWNT) under hydrothermal treatment was investigated. Single-wall carbon nanotubes were used as the starting material and treated at temperatures between 200 and 800 jC and pressure of 100 MPa. The microstructural evolution was studied by means of transmission electron microscopy and Raman spectroscopy. After treatment at 800 jC, the single-wall nanotubes transformed completely to shorter multiwall carbon nanotubes and graphitic particles. The mechanism by which the observed evolution occurs is discussed. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Single-wall carbon nanotubes; Hydrothermal treatment; Phase stability

1. Introduction The discovery, synthesis and characterization of different nanocarbon structures: fullerenes, nanotubes and related carbon crystals [1,2], have received much attention owing to their unique crystal structure and properties. They have potential applications as high strength reinforcing materials, lubricants, point field emitters, nanowires, etc. [3]. In addition, the potential use of more disordered carbon structures has been recognized. Activated carbon made of disordered graphitic shells is one of the most promising materials * Corresponding author. Tel.: +81-45-924-5323; fax: +81-45924-5358. E-mail address: [email protected] (M. Yoshimura). 1 Now at Mysore University, India.

for achieving a critical hydrogen storage capacity [4]. New, cheap, nontoxic and recyclable materials with a critical hydrogen storage capacity are necessary for the development of new clean energy technologies based on hydrogen combustion out of laboratory research tests, which is of general interest for sustainable economic growth. The widespread application of nanocarbon materials requires clean synthesis routes with reduced energy input in less severe conditions and in higher yields than the very expensive below-gram quantities used for laboratory research nowadays. A new process has to be technologically friendly, and involves a different growing mechanism that is more energetically efficient than in carbon evaporation methods currently in use. Hydrothermal routes may lead to a reproducible fabrication method of crystalline nano-

0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 7 11 - 7

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carbons made of curled graphitic layers arranged concentrically [5– 8]. In fact, natural multiwall carbon nanotubes (MWNT) appear associated with rocks of hydrothermal origin [9]. Formation of MWNT in hydrocarbon fluids in supercritical conditions has been previously observed to be associated with the presence of substantial amounts of amorphous carbon in the condensed solids [10]. However, the mechanism of carbon crystal growth and phase stability, either with sp2 or sp3 bonding and whether grown hydrothermally or in inert atmosphere from the gas phase, is not yet fully understood. Cao et al. [11] reported recently the direct conversion of carbon nanotubes to diamond at 4.5 GPa and 1300 jC in the presence of Ni– Mn –Co catalyst. Suchanek et al. [12] reported the behavior of fullerenes in hydrothermal conditions. However, no study has focused on the structural evolution and phase transformation of carbon nanotubes in hydrothermal conditions. The present study aims to determine the evolution of single-wall carbon nanotubes in pure water at temperatures up to 800 jC and pressure of 100 MPa.

Their Raman spectra were recorded by using a spectrometer in a ‘micro’ mode (T64000, Atago-Jobin Yvon, France –Japan; Ar + laser with the excitation wavelength of 514.5 nm), the diameter of the analyzed region being f 1 Am.

3. Results The characteristic Raman spectrum of untreated SWCNT is shown in Fig. 1, with the strongest Raman bands at f 186, 1565 and 1587 cm 1 [13 –16]. A peak at 186 cm 1 corresponds to the A1g radial breathing mode of the SWNT, indicating the enrollment of graphene sheets in the SWNT. The frequency

2. Experimental procedure Single-wall carbon nanotubes (SWNT) from Bucky, USA were used in the present study. The experiments were carried out using conventional Tuttle-type autoclaves made of stellite superalloy. SWNT ( c 0.02 g) were taken in the golden capsules of 3 mm in diameter and 5 cm in length, which were subsequently filled with double distilled water ( c 0.3 g). The capsules were then sealed and placed inside the autoclave. Experiments were carried out in the desired temperature range between 200 and 800 jC and pressure of 100 MPa for the duration of 30 min to 48 h. The materials obtained before and after the hydrothermal treatments were characterized by high-resolution transmission microscopy (HRTEM) carried out using an apparatus H9000NAR, Hitachi TEM at operating voltages between 100 and 300 kV. Samples were prepared adding methanol and dispersing the suspended tubes in an ultrasonic bath. After ultrasonication, a drop of the suspension was placed on a microgrid and dried in air before TEM observation.

Fig. 1. Raman spectra of (A) untreated SWNT, (B) after treatment at 600 jC for 48 h and (C) after treatment at 800 jC for 48 h.

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of this peak is dependent on the nanotube diameter [17]. The 186 cm 1 frequency corresponds to a radius of curvature f 1.2 nm. Two peaks at f 1565 and f 1587 cm 1 correspond to E2g and A1g modes characteristic of SWNT, respectively. The peak at f 1565 cm 1 disappeared in the Raman spectra of the sample treated in pure water at 600 jC for 48 h, and the peak at f 186 cm 1 decreased its intensity. A very weak feature at f 1350 cm 1 in the untreated sample corresponds to the D-band, typical of glassy carbons or disordered graphite. The D-band, at 1347 cm 1, increases significantly its intensity in the samples treated at 600 jC for 48 h. In the Raman spectrum of SWNT hydrothermally treated at 800 jC for 48 h, the D-band becomes extinct and only the G-band of graphite ( f 1582 cm 1) is observed with a reduced peak width. After hydrothermal treatment at 800 jC, all the Raman peaks characteristic of SWNT become extinct. The evolution of the D-band suggests that the disordering of the SWNT structure occurs at 600 jC, and at higher temperatures, a reordering of carbon atoms is taking place [18]. The spectrum of sample treated at 800 jC indicates a complete phase transformation, where nanotubes convert to a graphitic form of carbon

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with a well-ordered lattice without plane termination or amorphous parts within the analyzed region. HRTEM observation of untreated SWNT (Fig. 2) shows a homogeneous sample; tube diameters are f 1 nm, as indicated by the Raman shift of the radial breathing mode. Nanotube bundles have diameters ranging from 5 to 15 nm and length exceeding tens of micrometers. The ratio between the diameter and length is higher than 1000. The presence of a few nanoparticles was also observed in the as-received purified sample. The morphology of the material changed completely after the hydrothermal treatment in pure water at 600 jC for 48 h (Fig. 3). The sample converts to a mixture of amorphous and graphitic carbon, with nanotube-like structures and crystalline clusters embedded in an amorphous carbon carbon matrix. Fig. 4 shows the presence of multiwall nanotubes and crystalline clusters, covered by an amorphous layer. After treatment at 800 jC for 48 h, no amorphous carbon remained. Instead, crystallization of graphitic particles and growth of nanotubes with multilayered walls occurred. Fig. 5 is a TEM micrograph of the treated sample. The formed multiwall nanotubes have lengths from a few hundreds of nanometers to 1 Am,

Fig. 2. TEM micrograph showing untreated SWNTs.

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Fig. 3. TEM micrograph showing SWNTs treated at 600 jC for 48 h.

Fig. 4. TEM micrograph showing multiwall nanotubes and crystalline graphitic clusters, covered by amorphous carbon, in the sample treated at 600 jC for 48 h.

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Fig. 5. TEM micrograph showing SWNTs treated at 800 jC for 48 h.

and diameters from 20 to 50 nm. Internal diameters are in every case higher than SWNT diameters. Results indicate that the multiwall nanotubes are not formed by the addition of new layers on preexistent single-wall nanotubes. Thus, the mechanism of nucleation of graphitic crystals at the lower temperatures is unclear and will be the subject of further study. At 600 jC, the strongly curved graphene sheets forming the SWNT were disordered and the nanotubes were mainly decomposed. However, at the higher temperature, 800 jC, formation of additional graphitic layers and growth of crystals occur, leading to the disappearance of the amorphous layers covering the sample and the survival of only multiwall carbon nanotubes and graphitic nanoparticles. The conversion of amorphous carbon into ordered crystals in this temperature has been only observed in hydrothermal conditions [5– 7,19]. At 800 jC, the sample has crystallized completely in the form of closed carbon nanocrystals. The graphitic structure of the nanotube walls after treatment was revealed by high-resolution lattice fringe images. Fig. 6 is a HRTEM lattice image of the observed nanocrystals, showing the lattice of several

nanotubes imaged at the same focal point. Some of the hollow cores are superimposed with the layered multiwall of other nanotubes. All nanotubes have the same ˚ . This interlayer spacing in the multiwalls, f 3.4 A interspacing corresponds to the 002 distance of graphitic carbon, in the direction perpendicular to the hexagonal graphene layer. The first nanotube, from the left of the micrograph, is the bigger one, formed by f 40 layers, with a diameter >30 nm and an inner core f 4 nm. The smaller nanotube in Fig. 6 appears in full length and both ending caps can be observed. The ending cap is formed by the bending of graphene sheets due to the presence of pentagon defects in the hexagonal lattice. The lattice fringes of a carbon polyhedral nanoparticle can be seen in the upper left side corner of the micrograph. The polyhedral geometry results in straight parts that have a less pronounced curvature of the graphitic layers. When seeing aligned in parallel with the beam, the lattice of the latter parts appears more clearly defined. The latter is revealed in Fig. 6 by the sharper definition of the lattice image of the particle compared to the nanotube lattices in the same figure. Our observations confirmed that the polygonal par-

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Fig. 6. HRTEM lattice image of some of the particles in the sample treated at 800 jC for 48 h. The spacing corresponds to the 002 distance of a graphitic lattice.

ticles have polyhedral shapes with a hollow core and a multiwall of concentric graphene layers. Thus, they are carbon nanopolyhedrons. They can be also considered as very short multiwall nanotubes. The graphene sheets forming the walls of several nanotubes and graphitic nanoparticles have the same lattice or distance between graphene layers, typical of the 002 spacing of a graphitic lattice. Therefore, the multiwall nanotubes and graphitic nanoparticles cannot be practically distinguished using XRD or Raman because of very similar atomic bonding. However, TEM observation allows distinguishing the different morphology. Detailed high-resolution electron microscopy study shows that the sample treated at 800 jC is formed by hollow multiwall nanotubes and nanopolyhedrons with a graphitic multilayered structure. Thus, graphitic multiwalls were formed as a result of the hydrothermal treatment at 800 jC at the expense of an amorphous carbon layer. The growth of multiwall carbon nanotubes in the absence of metal catalyst has been only observed in the hot hydrothermal fluids [5,6]. It is challenging to find an explanation of how hydrothermal multiwall formation occurs through the rearrangement of solid amorphous carbon. The mechanisms by which the mixture of amorphous carbon seeded with nanotubes evolved to hydrothermal multiwall graphitic particles and the kinetics of the under-

going changes are of remarkable interest. To determine exactly the transformation path of carbon atoms in hydrothermal conditions is not the scope of the present communication, but we can conclude that multiwall growth does not take place in the gas phase or by the dissolution – reprecipitation of carbon atoms. We consider that it involves local rearrangement of atomic bonds of amorphous carbon induced by the reactivity of hot hydrothermal fluids and enhanced mobility of carbon clusters. The mechanism is complex and its study will be the subject of a later work, as well as the hydrothermal behavior of other carbon materials with more complex structures [20].

4. Conclusions We report here the microstructural evolution and phase transformation of single-wall carbon nanotubes under hydrothermal conditions. The morphology of the material changed completely after treatment in pure water. After treatment at 600 jC for 48 h, the disordering of SWNT into graphitic carbon nanoparticles and nanotubes with an amorphous cover was observed. Only multiwall carbon nanotubes and polyhedral graphitic nanoparticles form as a result of the hydrothermal treatment at 800 jC for 48 h. Clear

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evidence from TEM and Raman spectra indicates that the growth of sp2-bonded natural crystals of carbon, MWNT and nanopolyhedrons is favored in hydrothermal conditions in the studied experimental range.

Acknowledgements This work has been carried out under a sponsored project of the Research Institute for Solvothermal Technology, Takamatsu, Kanagawa, Japan. Financial support (No. 96RO6901) from Japan Society for the Promotion of Science (JSPS) is also acknowledged.

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